Circular Design Principles Applied on Dye-Sensitized Solar Cells

Abstract

In a world with growing demand for resources and a worsening climate crisis, it is imperative to research and put into practice more sustainable and regenerative products and processes. Especially in the energy sector, more sustainable systems that are recyclable, repairable and remanufacturable are needed. One promising technology is dye-sensitized solar cells (DSSCs). They can be manufactured with low energy input and can be made from non-toxic components. More than 70% of the environmental impact of a product is already determined in the design phase of a product, which is why it is essential to implement repair, remanufacturing and recycling concepts into the product design. In this publication, we explore appropriate design principles and business models that can be applied to DSSC technology. To realize this, we applied the concept of Circo Track, a method developed by the Technical University of Delft, to DSSCs and investigated which design concepts and business models are applicable. This method enables companies to transform a product that is disposed of after its useful life into one that can be used for longer and circulates in material cycles. The most important result is the description of a performance-based business model in which DSSCs are integrated into the customer’s building and green energy is provided as a service. During the operational phase, data is collected for product improvement and maintenance, and repair is executed when necessary. When the contract expires, it can be renewed, otherwise the modules are dismantled, reused, remanufactured or recycled.

1. Introduction

Resources and the climate crises are major societal problems worldwide [1]. The effects are intensifying, as can be seen, for example, in extreme weather conditions or bottlenecks in supply chains [2]. Renewable energy technologies are playing an important role in tackling the climate crisis [3,4]. However, the materials that will make up these regenerative systems will have to be mined and manufactured and the energy transition will not be realized without significant material and energy costs [5]. For example, to limit global warming to less than 2 °C, 60 TW of photovoltaic systems need to be installed by 2050, which will require a large amount of aluminum (480 million tons) but aluminum has high global warming potential [6]. Even so, currently photovoltaic systems as well as wind energy plants have difficulties in the end-of-life phase; recycling of materials is mostly not possible, so the materials have to be disposed of [7]. These technologies run through a linear economy.

The linear economy is the most concerning problem in context of the resource crisis [8] and is also often referred to as the “take, make and waste” economy [9]. The idea of the circular economy offers solutions to that issue. The circular economy has been highlighted by the Ellen MacArthur Foundation, among others, and is already being applied in industries as a policy objective, in particular within the EU Green Deal and Circular Economy Action Plan [10,11]. With this concept, products are made to last to be reparable, remanufacturable and recyclable [12]. Essential in this context are design principles, because at least 70% of the environmental impact of a product is already predetermined in the design phase of a product [13].

A promising technology which can potentially be made from non-toxic materials and with low energy demand in the production process is dye-sensitized solar cells (DSSC) [14]. DSSCs could not only replace conventional photovoltaic technology but supplement it. Possible applications include transparent versions in windows of buildings or in vehicles and indoor applications, e.g., for Internet of Things devices, as they also work in diffuse-light conditions in the evening or morning hours as well as indoors [15,16,17,18]. Since DSSCs can also be used for aesthetic purposes, the design of colored facades in architecture could also be a niche for this technology [19].

The operating principle of a DSSC is briefly explained as follows: a DSSC consists of two conductive glass plates; usually, fluorine-doped tin oxide (FTO) is applied on the surface of the glass to enable conductivity. On the front electrode, a layer of semiconductor material is applied, generally titanium dioxide (TiO₂). On the porous layer of TiO_2, the photoactive material, in our case natural dye, is incorporated. On the counter electrode, a catalyst is applied, typically graphite or platinum. Finally, front and counter electrodes are connected by an electrolyte, also described as the redox couple, and iodide/triiodide (I⁻/I³⁻) is often used. If light excites electrons in the photoactive material, the dye and the electrons pass the semiconductor layer and enter an external circuit. Here, the electrical energy can be used. The electrons enter the DSSC through the counter electrode and the catalyst layer. The electrolyte transports the electrons to the dye molecules and complete the circuit [20].

This technology has not yet reached the stage of large-scale industrial manufacturing, and therefore approaches to circular design need to be explored to minimize the environmental impact of this technology from the outset. Of course, the processes for the circular economy, such as remanufacturing or recycling, need to be flexible and adaptable in the initial phase, as DSSC technology is expected to undergo several transformations before reaching an industrial scale.

So far, a main focus in research was to improve PCE as well as its long-term stability [21]. These aspects are fundamental, but a technology for renewable energies with high efficiency, which causes strong environmental impacts, is not suitable to cope with the acute crises.

Work in the area of sustainable DSSCs ranges from life cycle assessment (LCA) to eco-design approaches and the 12 principles of Green Chemistry [22,23,24].

LCAs can be used to analyze the environmental impact and energy balance of products. Several LCAs for DSSC describe that the cumulative energy demand (CED) for DSSCs is comparable with amorphous, silicon-based photovoltaics (a-Si PV)—760.21 MJ/m² for a-Si PV and 863.2 MJ/m² for DSSC modules [22,25,26]. Since a-Si PV already is produced on industrial scale, in contrast to DSSC modules, big improvements in energy efficiency can be expected in DSSC production [26]. The energy payback time for DSSCs is between 0.6 and 3.3 years, and for crystalline silicon photovoltaics (c-Si PV) between 0.6 and 1.5 years [22,27]. However, the niche application at lower light intensity is not considered. This setup would give the DSSC technology advantages over c-Si PV in the LCA.

Mustafa et al. investigated a window-integrated DSSC system without the end-of-life stage due to a lack of appropriate information. In their case, the production phase caused most of the greenhouse gas (GHG) emissions due to the Malaysian energy mix, which consists of over 80% coal and gas [28]. That is why they emphasize improving the production process with, for example, low-temperature approaches [28].

De Wild-Scholten et al. pointed out that the main environmental impact is due to energy consumption during the manufacturing process, in particular the production of the glass substrate [29]. This was also confirmed by Parisi et al. adding that glass, silver and ruthenium are causing around 90% of the global impact during a DSSC life time [22].

To mend this issue, they suggest using thin-glass, metal or polymer substrates. They mention that recycling could improve the environmental impact, but no practical experiments have been conducted so far [29]. In previous work, we investigated the recycling and remanufacturing potential of non-toxic DSSCs and found promising opportunities to use FTO-glass longer and integrate DSSCs in a glass recycling process [21,30,31].

Ansanelli et al. stated that for c-Si PV recycling, two sequential steps of recycling have to be realized. First, the recovery plant to separate and recover raw materials such as aluminum, copper, glass, silver and silicon from the c-Si PV modules. Secondly, the glass reuse line for the use of the recovered glass in construction elements is needed. In their LCA, they describe a downcycling scenario, where only building materials such as predalles slabs are manufactured [32]. Nevertheless, they found that the process is environmentally friendly, even taking into account the environmental impact resulting from the implementation of recycling treatments [33,34]. Challenges of c-Si module recycling are the variety of modules and cell structures and the separation of cells from the glass [35]. The electricity consumed for heat treatment can be considered as the largest load and thus causes the largest environmental impact [32]. For this reason, the energy used significantly determines the price and carbon emissions of the recycling process. If it is green energy, the carbon footprint is low; if the energy for heating and transportation comes from fossil sources, it is high. An LCA study has shown that the production of c-Si cells from recycled material has a 58% lower environmental impact than c-Si modules produced from virgin material, mainly due to energy savings, as the energy-intensive processing of raw silicon is not required [36]. In Germany, for every kWh saved, 0.42 kg of CO₂ and EUR 0.17–0.23 could be saved [37,38].

Anctil et al., stated that in the case of thin-film technologies, such as CdTe and CIGS, a-Si/thin-film Si, the use of abundant material could lead to a waste problem in the future [39,40]. Reasons for that are that the manufacturer has no incentive to take back end-of-life modules and that abundant material has little recovery value. Possible solutions they describe are on the one hand political incentives and on the other hand that the costs for end-of-life management should be included in the PV module price [39].

If non-toxic DSSCs are used, they can potentially be used for conventional glass recycling [30]. When recycled cullet from DSSCs is used, indirect energy savings can be achieved because the extraction, mining, processing and transportation of raw materials are not required. Direct energy savings can be achieved during the melting process in glass furnaces, as the cullet melts at lower temperatures than the pure raw materials..

Every kg of raw materials replaced by DSSC cullet could save between 1.9–2.35 MJ [41].

A calculation of these direct energy savings, costs and CO₂ savings from the recycling process is presented below:

1 MJ = 0.278 kWh = 0.278 × 10⁻³ MWh

Natural gas causes 0.201 tCO₂/MWh emissions [42]. The following calculates how much CO₂ can be saved if one ton of raw material is replaced by cullet:

(2.35 × 0.278 × 10⁻³)MWh/kg × 0.201 tCO₂/MWh × 1.000 kg = 0.13 tCO₂

In total, 0.13 tCO₂ can be saved per ton of cullet, replacing the raw material. Glass production mainly uses natural gas. The gas price now is 129.80 EUR/MWh (3 November 2022), and the highest price was 339.00 EUR/MWh (26 August 2022) [43]. The cost savings of using one ton of cullet are calculated as follows:

(2.35 × 0.278 × 10⁻³) MWh/kg × 129.80 EUR/MWh × 1.000 kg= EUR 84.80

A total of EUR 84.80 per ton of cullet could therefore be saved in a recycling process for the energy costs of the glass furnace alone.

In summary, energy savings can be achieved both from glass furnaces (direct savings) and in the supply chain and processing of raw materials (indirect savings). Most importantly, fossil fuels and energy, and thus CO₂ emissions, are reduced through the use of DSSC cullet [41]. In addition, waste can be reduced and scarce resources are not required for production.

Eco-design takes a holistic approach to design. It considers manufacturing efficiency, product lifetime, return on energy invested, availability, recyclability and hazardousness of materials. The goal of eco-design is to design an environmentally friendly product without neglecting important aspects such as performance and costs [23].

In their work, Miettunen et al. focus on an eco-design approach for recycling and investigate different material choices and their influence on the recycling process [44]. They look at a standard DSSC design including silver. They described that, when switching from a thick glass substrate to thin flexible substrates, the silver recovery in an existing pyrometallurgic recycling process is economically viable [44].

Mariotti et al. also discuss the latest trends in DSSC development and eco-design. They come to the conclusion that the use of alternative substrates (for example plastic), prevent the use of precious metals and applying the principles of green chemistry can lead to more sustainable DSSCs [19].

The 12 principles of Green Chemistry range from preventing waste to increasing efficiency or design for degradation [24]. Furthermore, they point out that often solvents make it difficult to evaluate if a process is green, since they are used as a reaction medium and for purification [19]. Mariotti et al. investigated the sustainability of all components of DSSCs and suggested the introduction of guidelines for sustainable materials extraction and using non-toxic, abundant, low cost and recycled materials [16]. These suggestions are made for three upcoming photovoltaic technologies: DSSCs, perovskite solar cells and organic solar cells. In the following, a brief overview of the technologies as well as the manufacturing processes is given in order to understand the life cycle assessments of the technologies.

1.1. Dye-Sensitized Solar Cells

DSSCs with toxic materials achieve a power conversion efficiency (PCE) of above 14% while using metal-based dyes, such as ruthenium and platinum counter electrodes [45,46,47]. DSSCs made from non-toxic material, as produced in our laboratory, based on natural, plant-based dyes with graphite counter electrodes reach efficiencies below 1% [48,49,50]. Another problem remains long-term stability as the liquid electrolyte evaporates. If the electrolyte is refilled, the DSSCs can be used for at least four months. [51]. Solid state or gel electrolytes are the subject of research to improve this long-term stability [52,53,54]. Gel electrolyte-based DSSCs are stable for at least 140 days [55]. Further research in long-term stability and efficiency is needed to develop competitive products. For indoor applications, a lifetime of 5 years and for outdoor applications a lifetime up to 25 years are required to be competitive [22,56].

For conventional DSSCs, component production and assembling following process steps have to be applied [57]:

• Glass preparation (Laser scribing and cleaning);
• Printing and drying of silver paste;
• TiO₂ screen printing on glass substrates;
• Sintering in furnaces at 500 °C;
• Printing Platinum (Pt) on counter electrode substrates;
• Sintering platinum in furnaces at 450 °C;
• Dye dipping;
• Assembling thermoplastics (PE);
• Drying for curing in convection ovens;
• Electrolyte injections;
• Patch sealing;
• Ultrasonic soldering;
• Panel assembly.

1.2. Perovskite Solar Cells

Perovskite solar cells and organic solar cells are other technologies researchers are focusing on [58,59]. Perovskite solar cells show high power conversion efficiencies of 25.5% [60]. A downside, from an environmental point of view, is that they contain lead. Perovskite solar cells without lead are the subject of research, but do not achieve the same long-term stability and PCEs [61]. Factors for a rapid market entry of these technologies are scaling-up, stability and efficiency, which are the focus of active research in this field [62]. Recent developments show an efficiency of 15.27%, which remains at 80% of the original level after 800 h of storage in ambient air [63]. Companies that work on the commercialization of perovskite solar cells are for instance: Oxford PV GmbH (Brandenburg, Germany), Swift solar (Sancarlos, CA, USA), Solaronix (Aubonne, Switzerland), Saule Technology (Wroclaw, Poland) and Microquanta Semiconductor (Hangzhou, China) [64].

The manufacturing steps for perovskite solar cells are listed as follows [65]:

• Front glass preparation;
• Indium tin oxide (ITO) sputtering;
• Printing and heat treatment of nickel oxide (NiO), perovskite layer (Lead(II) iodide (PbI₂₎, methylammonium iodide (MAI), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)) and zinc oxide (ZnO) layers;
• Al sputtering;
• Lamination;
• Back glass;
• Edge Sealing;
• Junction boxes;
• Testing.

1.3. Organic Solar Cells

Lower production costs are an advantage of organic solar cells and PCEs of over 18% were achieved [59,66]. The major problem of organic solar cells is the lack of long-term stability [66]. Organic solar cells can potentially be used in tandem cells to increase energy yields because they can operate at different light intensities than, for example, perovskite solar cells [67].

The manufacturing process for organic solar cells is described in the following list [68]:

• Cleaning of glass-ITO substrates;
• Preparation of active layer blends (PTB7:PC₇₁BM);
• Fabrication of hole-only devices;
• Addition of 1,8-Diiodooctance to the active layer blend;
• UV-Ozone treatment of glass-ITO substrates;
• Deposition of hole transport layers (PEDOT:PSS);
• Deposition of active layer blends (PTB7:PC₇₁BM);
• Deposition of electron transport layers (Ca);
• Cathode deposition (Ag);
• Fabrication of electron-only devices;
• Testing.

1.4. Crystalline Silicon Photovoltaics

For c-Si PV modules, there are already some suggestions on how the modules can be designed with the circular economy in mind. Moreover, this technology is widely in use, and the opportunity should be taken to draw lessons from it for the design of upcoming technologies.

Various design aspects of conventional c-Si PV modules are problematic with respect to circularity. These include the irreversible fixing of the frame, the poor recyclability of the junction boxes, the use of toxic materials such as lead-containing connections, a large mix of materials and the fusion of wafers with the ethylene vinyl acetate (EVA) film [69]. Initial approaches to solving these problems include alternative encapsulation materials such as polyvinyl butyral (PVB) or alternative encapsulation structures such as Apollon Solar’s “New Industrial Solar Cell Encapsulation” (NICE) technology with a gas encapsulation. The alternative materials can possibly enable easier recycling [70]. Other solutions include modular components and reversible fixation so that the components can be repaired and reused. These involve modular junction boxes and module frames without fixed bonding [69]. Other design approaches relate to contacting. For example, the contacts containing lead can be replaced by components made of silver or conductive plastics. To reduce the material mix and eliminate fluorine-containing back sheets, glass sheets can be used instead of plastic sheets on the back [71].

The well-established manufacturing process for c-Si PV is listed below [72,73]:

• Silica sand;
• Metallurgical silicon production;
• Poly-crystalline silicon production;
• Ingot molding and silicon wafer production;
• Solar cell production;
• Module production.

In summary, the LCA approach focuses on the environmental impact of a product, whereas eco-design focuses mainly on sustainability at the component level, at least in the studies investigating DSSCs.

Eco-design should also consider the economic viability of a product or process, but to date, only the economics of recycling silver from DSSCs in a pyrometallurgical process have been mentioned in research [44], to the best of the authors’ knowledge. Materials that make up a common c-Si module and are therefore potentially of interest for recycling include: silicon, aluminum, silver, copper, tin and lead. In the case of thin-film modules, there are also other materials such as cadmium. The dopants, such as boron and phosphorus, are present in very small amounts and are therefore probably not of interest for the recycling process [32].

The novelty of this publication is the holistic approach to add business models to the sustainability concept of DSSCs. We used the Circo method, which is a scientific framework developed by C. Bakker et al. from the Technical University of Delft [74]. This framework helps companies and product designers to transform or redesign a linear product to a circular one and couple it with circular business models [75].

2. The Circo Method—Methodology

A Circo Track, a series of workshops that guides companies through the Circo method, consists of [76]:

• Identification of opportunities in the value chain;
• Investigating possible changes to the design and business model;
• Developing a circular business model and identifying necessary changes;
• Scheduling the necessary actions and pitching and implementing the ideas.

In this work, we apply the Circo method to the DSSC technology from the point of view of a DSSC-module manufacturer and describe possible circular design principles and business models.

2.1. Identification of Opportunities in the Value Chain

The first step of the Circo method is to analyze the product’s supply chain and examine important value losses along the product’s life cycle. Figure 1 shows the value pyramid and at which stage of life value is added to the product and where value is lost.

During the extraction of resources, manufacturing, assembling and marketing, the value of the product is increased. Nonetheless, also during this phase, value losses can appear, for instance as costs for transportation or storage. During the use-phase, the product can be repaired to maintain its value. In the end-of-life phase, the value of the product can be preserved by reusing, refurbishing, remanufacturing or recycling the product. In most cases, however, the products, wind power plants and c-Si-modules for instance, are simply not designed for value retention.

To illustrate the economic loss of value, an example is described below: A new smart phone, in this example the iPhone 6, costs from USD 199 to 499 [77]. The average time of use is 18 months, and at this point, the material that could be harvested from the phone in a recycling process is worth around USD 1.03. If the phone is refurbished, for example, it can be resold for USD 45 up to 70 (25.07.2022) [78]. The same economic correlation can be observed for other products, such as cars or textiles [79]. This shows that even if all materials could be recovered in a recycling process, reselling the product is worth 43 times more in the case of a smart phone. That makes retaining value tangible and emphasizes the economic worth of the process. The same is true for PV modules; the challenge, however, is to find suitable markets for the giga watts of used modules [35].

To identify economic and environmental value losses, the value chain is investigated. Therefore, all key actors are added in the value chain, such as the manufacturer of raw materials, components’ manufacturer, sales organization, user and the recycling company, as can be seen in Figure 2.

In the next step, transactions in the system are added. Not only does money flow through the system, but also materials are processed into components and products, services are provided and, finally, the product is recycled or disposed of. Figure 3 illustrates this step.

In the final step, the value losses and opportunities are added. Figure 4 shows the final value chain built using the Circo method.

Opportunities can be derived from most value losses. For example, the value of the raw material at the end of its life is much lower than in the scenario where the product is reused; therefore, reusing or refurbishing is a good economic and environmental opportunity for value retention. This requires that the products are designed for value retention.

2.2. Investigating Possible Changes to the Design and Business Model

In the book Products that last, Bakker C. and den Hollander M. describe five circular business models and six circular design strategies as a foundation for the circular transformation [74].

Below, the five circular business models are listed and explained:

• The classic long-life model provides for a high-quality product with a long service life, with sales representing the classic source of revenue. After-sales support contributes to quality perception. In this model, brands often have a reputation for high prices and high quality products. A long lifespan usually requires service and repair.
• The hybrid model is based on repeated sales of consumable products such as washing powder or printer cartridges, which are connected to a durable long-lasting product, such as a washing machine or a printer.
• The gap exploiter model is based on repair services or selling old and refurbished products. For example, eBay is a gap exploiter as well as providing computer or bicycle repair services. Another variant is collecting used products and selling the materials or components obtained from them.
• The access model is based on providing the customer access to a product while the company keeps ownership of the product. Car-sharing models or tool-sharing are examples. The goal is to intensify the use of a product that is otherwise not used often or is idle most of the time.
• The performance model focuses on providing a service instead of a product. The customer and the company agree on a goal to be achieved or a service to be provided, and the company must find a solution on how to achieve this goal. An example for this model is the public utility Aachen. They make a contract with the customer, mount a c-Si PV system on the roof of the customer’s building and provide green energy [80].

These five business models have been chosen, because each of them enables the control of material flow through the system while generating profit [74]. They symbolize archetypes of business models, whereby in reality hybrid forms can also be found.

The six design strategies from Bakker, C.et al. are listed and explained below [74]:

• Product attachment and trust focuses on creating an emotional bond between the customer and product. It is used when products are disposed of before their technical lifetime is over—custom designed shoes, self-designed photo books, self-made or self-assembled furniture or an inherited watch are some examples.
• Durability aims to maximize the effective life span of a product. Therefore, high-quality materials are used and reliability is verified in long-term tests.
• Standardization and compatibility focuses on product compatibility with other products or interfaces around the world. Charging cables for smart phones are a good example. This design principle is essential, such as when a product or component should be reused in another product or a future generation of the product and standardization over product evolution.
• Ease for maintenance and repair aims to optimize the product for repair. In low-income countries, the cost of repairs is much lower and repairs are more likely to be possible. This model is especially interesting for service-oriented business models, in which the provider keeps ownership of the product.
• Upgradability and adaptability is essential in areas where the consumer needs are changing and technology advances. A product with a long lifespan must be designed to be upgradeable, with the ability to obtain new features or a new design without having to buy a new product, by just upgrading the old one.
• Dis- and re-assembly focuses on designing a product that can be easily disassembled and reassembled. This makes it possible to replace defective parts, rebuild the product elsewhere or resell it after refurbishment.

To implement circular economy solutions, 9R strategies are often used. They consist of 10 hierarchically sorted recommendations for action to implement a circular economy. Those recommendations range from “refuse”, which is the most effective way to reduce the environmental impact, down to “recycle” and “recover”, which only focuses on reusing the material [81]. The Circo method does not directly include the 9R framework, but it can be combined with the design strategies and business models explored for the specific design changes. Most of the R strategies can be found in the business models described in the Circo method, such as rethink, reduce, reuse, repair, refurbish, remanufacture, repurpose, recycle or recover. However, the first step, “refuse”, is not directly implemented (refuse means do not use an item or function at all or choose an alternative [82]; for example, refuse to buy over-packaged products). In the case of DSSC or green energy generation, this would mean saving energy as sufficiency is key in this R strategy.

The next step In the Circo method is to take the identified opportunities and match them with one or several business models and design strategies. Furthermore, the economic and ecologic benefits from each objective are estimated. The ideas are then overlaid with the existing business model to examine which options the company can implement quickly or which are too complex and require a long-term approach; since only one fictitious company is examined and presented in this publication, this part is integrated to explain the method holistically. Therefore, the following questions can be used to identify changes for the value proposition:

• How much will the change cost?
• When can the return on investment be expected?
• Does the existing company have the skills to realize the idea?
• How much influence does the company have on the entire value chain?
• Is the company (culture, colleagues, management, …) ready and willing to change to more circular models?

Finally, the consequences for the design strategy are defined for each case. Based on all assessments, a preferred approach or combination of approaches should be selected for further investigation.

2.3. Developing a Circular Business Model and Identifying Necessary Changes

In this part, the selected business case is formulated in detail. For this purpose, all actors and their activities in the system must be recorded. Customer interaction needs to be identified, as well as revenue streams.

Derived from the chosen business model, the consequences for the entire company have to be described. The elements that need change are differentiated by the influence the company has on them.

• Core: Both the product design and the service supply are core components of the company’s value proposition and, therefore, the company has the opportunity to actively improve on them.
• Internal: Elements such as marketing, logistics, finances and purchasing can be influenced or controlled by the company, although, more external influences are given on this stage.
• External: Elements that are influenced from the outside and are difficult to change from the company’s point of view are, for instance, the supply chain, new knowledge or politics.

Finally, the impact of the changes to the proposition value have to be identified and described in consideration of customer relationship, value chain influence, requirement of new skills and return on investment. This is a summary of the results gathered in Section 2.1, Section 2.2 and Section 2.3.

2.4. Scheduling the Necessary Actions and Pitching and Implementing the Ideas

The necessary measures are to be entered in a roadmap. This shows the significance or impact of a change and the time required for implementation. The roadmap is especially important for the next steps in the company, when the idea is pitched to colleagues, the management or the chief executive officer (CEO). Section 2.4 is not fully executed in this publication, but is only explained to give a complete overview of the Circo method.

3. Circular DSSC—Investigation Inspired by the Circo Method

3.1. Identification of Opportunities in the Value Chain of Building-Integrated DSSCs

Since DSSC production has not yet reached an industrial scale, the value chain of a DSSC is based on laboratory processes and data and is published by companies that manufacture DSSC panels as well as previous research and LCAs in this field [26,83].

LCA is an instrument to measure the environmental impact of a product along its life cycle, including raw material extraction, processing, manufacturing, use phase and end-of-life phase.

The key data of an LCA are explained below:

• Cumulative energy demand (CED) describes the primary energy demand for producing materials, manufacturing of the DSSC system, transport, operation and end-of-life management. However, this can vary according to the scope of the LCA, cradle-to-gate or cradle-to-grave, for instance.
• Energy payback time (EPBT) is a figure that describes the time required until the DSSC system has produced as much energy as was required for the life cycle of the DSSC to produce a return on investment in energy.
• The greenhouse gas (GHG) emission rate is given in CO₂-equivalents/kWh. To obtain this key figure, the total electricity generated by the PV system over its entire life divides all GHG emissions of the PV system life cycle.

To visualize the status quo of the environmental impact of DSSCs, several results of LCAs are given in

Table 1. Overview of LCAs and key data.

	Assumptions					Results
Publication Year and Reference	Setup and Scope	Location (kWh/(m2 Year))	Active Area (%)	Lifetime (Years)	PCE (%)	CED	EPBT (Years)	GHG Emission Rate (gCO2-eq/kWh)
2011 [84]	DSSC/Cradle to gate	Southern Europe 1117	75	20	8	277.4 MJ/m2	1.58	22.38
2014 [57]	DSSC/Cradle-to-gate 1	Southern Europe 1.700	75	20	10.20	0.65 MJ/kWh	1.9	45
2019 [28]	DSSC/Cradle-to-gate 1	Malaysia 1402.82	75	20	5.00	18.75 GJ/kWh	2.42	70.52
2020 [22]	DSSC/Cradle-to-grave 1	Italy 1700	78	5	3.91	6.7 MJ/kWh	3.28	380
2020 [22]	DSSC/Cradle-to-grave 1,2	Italy 1700	88	20	7.20	0.9 MJ/kWh	0.61	50
2021 [85]	c-Si PV/Cradle-to-grave	Chengdu 888.2	85	25	20	2.19 MJ/kWh	13.8	164.41
2016 [86]	Perovskite/Cradle-to-gate	Not given 1.700	65	5	15	504 MJ/m2	1.54	147

¹ Standard system: FTO glass, titanium dioxide, dye (ruthenium), platinum, thermoplastic, electrolyte, silver and solvents. ² Optimistic prognosis for 2050.

. At the end of the list, there is also a c-Si PV and a perovskite example provided for comparison purposes.

When conducting a life cycle assessment, there are complex relationships between geographic conditions, regulatory requirements or the energy mix that vary from country to country, making it difficult to interpret the results. It is recommended to examine the referenced LCA studies to understand the system configurations and assumptions made in the studies. The following discussion can only give brief insights into each study.

Table 1 shows that the environmental impact of a PV system is determined by many parameters, especially the expected lifetime and efficiency of a system. For an industrial DSSC module, lower environmental impact, compared to c-Si PV and perovskite, is expected. The high environmental impact of c-Si PV comes from the energy intensive purification and manufacturing process [87]. DSSCs show shorter EPBT, mainly reached by a simple structure, low cost and eco-compatibility [87].

Mustafa et al., explains the much higher CED in their study with the energy mix of Malaysia, which consists of over 80% coal and gas [28].

Other environmental impacts, for instance human toxicity, eco-toxicity and resource depletion, are predominantly caused by ruthenium, silver and consumed electricity [22].

The performed LCA studies describe many different future scenarios for DSSCs, but the general value chain described is similar. Figure 5 illustrates the value chain of a DSSC module as it is described in several LCA studies [22,25,28].

In contrast to Figure 4, this figure specifically shows typical DSSC stakeholders, for instance, the DSSC component producer, assembler and panel manufacturer and a trade company, which mounts the modules; all differences are highlighted in bold. Raw materials are obtained from manufacturers and processed into components for DSSCs. The panels are assembled, sold and installed on or in the user’s building. Materials flow from left to right. If a recycling process takes place at the end of life, material, heat or components can be returned to the system or to production processes in other industries. Money usually flows from right to left when material or components are purchased. In the step from user to recycler or disposer, the user may have to pay for appropriate disposal of the scrap material. Typical value losses that occur in almost every value chain are costs for energy, transport and storage. During the use phase, the value of the product usually decreases due to aging, but with a maintenance and repair service, value retention can be realized. That also is the first opportunity. During the end-of-life phase, modules could be refurbished, remanufactured or recycled. Since the company keeps ownership of the products during the use-phase, it is often explained as an advantage, whereby the storage of the company is in the customer’s building. Regardless, the material (the assets) would be bound there for around 20 years anyway. Another advantage of service models is that information and data can be collected during the use phase of a product, enabling the product to be improved and adapted to customers’ needs. Green energy could be used for production instead of the general energy mix for green marketing of the company or brand.

During the sourcing phase, the following materials are obtained and processed for the production of DSSCs:

• Substrates: FTO glass or plastic;
• Semiconductor materials: titanium dioxide;
• Dyes: ruthenium (N719 [28], Z907 [22]), organic [59], natural [88];
• Catalyst layers: platinum or graphite;
• Electrolyte: iodine
• For enhancing conductivity: silver [28], carbon solutions [89];
• For sealing: ethylene vinyl acetate copolymer [28], polyethylene, low density resin [28];
• Other process chemicals: deionized water, ethanol, soap [28]; organic solvents [22].

At the sourcing material stage, major environmental and social problems are caused by exploitation and pollution. Value losses and emissions are caused by transportation, refining processes, labor, storage and energy costs. However, these steps are necessary to deliver the necessary components and raw material for further processing and they cannot be designed out completely, unless one refrains from using the product at all (refuse in the 9R strategy). In addition, waste is generated during the extraction and refinement of raw materials.

The greatest environmental impact is in the categories of human toxicity, ecotoxicity and resource depletion, which can be traced back to silver, ruthenium, platinum and electricity, with decreasing relevance in this listing [22].

Over 90% of the embodied energy can be traced back to FTO glass (734.10 MJ/m² = 73.09%) and the metallization paste (190.15 MJ/m² = 18.93%) [28].

For conventional DSSCs, the following component production and assembling process steps have to be applied [57]:

• Glass preparation (Laser-scribing and cleaning);
• Printing and drying of silver paste;
• TiO₂ screen printing on a glass substrate;
• Sintering in a furnace at 500 °C;
• Printing Platinum (Pt) on a counter electrode substrate;
• Sintering platinum in a furnace at 450 °C;
• Dye dipping;
• Assembling the thermoplastic (PE);
• Drying for curing in a convection oven;
• Electrolyte injection;
• Patch sealing;
• Ultrasonic soldering;
• Panel assembly.

An advantage of DSSC in the production process is that they do not need highly energy intensive processes as c-Si PV do [57]. The industrial production process can be compared to other thin-film technologies and has the advantage of not assembling a module from single cells, but from direct fabrication of a DSSC module [57].

A Swiss company, Solaronix SA, is already producing DSSC with similar processes on an pre-industrial level [83]. Even though the energy demand of producing DSSCs is significantly lower compared to c-Si PV, the production phase causes most of the environmental impact of a DSSC due to the energy demand [22,28]. Researchers emphasize developing low temperature processes to make the processes more energy efficient or less energy consuming [57,90]. The greatest amount of energy is used for the preparation of the glass substrate [29]. That is why some LCAs suggest using different substrates, such as thin glass, metal or plastic, without mentioning that the PCE and long-term stability are lower with these alternative substrates [29].

In the installation stage, the DSSC module is integrated into the building or mounted on the roof. Support structures have to be built and other electrical devices have to be used to build up a functioning system, such as a charge controller, possibly a battery, inverter and cables. During the use phase, the system converts light into electrical energy and the EPBT in literature ranges between 0.6–3.28 years [22].

For indoor and non-building integrated applications, there are no LCAs. Yet, the expected lifetime for clothing, tents or other devices is much shorter compared to rooftop or building integrated systems. DSSCs with different properties are required for these applications. The use-phase and end-of-life stage will show the largest differences compared to building integrated systems. That is why in this manuscript, these shorter lifetime applications are not further investigated.

In most cases, the recycling process was excluded from the LCA due to a lack of information [29,57]. However, Parisi et al. proposed a recycling process in one of their optimistic LCA scenarios and used approaches for glass and metal recycling described in the literature for c-Si PV [22]. They found that 68.8% of glass and 90% of metals can be recycled; the glass, however, is of poor quality and cannot be used for photovoltaic applications again. All other materials, in their scenario, are incinerated as hazardous waste [22].

Since ruthenium dyes, or in general metal-based dyes, have a considerable environmental impact, it is seen as a key component in recycling [57]. A Korean patent exists, describing a method of how the dye can be recovered [91]. Recovery of natural dyes is less attractive and toxicity and other environmental impacts are negligible. In addition, the recycling of platinum is emphasized, as the material extraction and the manufacturing process have a massive impact on the environment [87].

The following identified value losses and derived opportunities are summarized: Along the value chain, several value losses can be identified, such as costs and emissions for transportation, storage and energy for production processes. In addition, the social aspects along the value chain must not be ignored. Cobalt is considered in DSSC production, as part of the counter electrode or electrolyte for instance, to enhance DSSCs properties [92]. Still, most cobalt mining takes place in the Democratic Republic of Congo, poisoning miners and the environment [93]. Theses aspects have to be dealt with and solutions for more sustainable production and mining phases have to be found.

The LCAs point out that the production process causes most of the environmental impact because of its energy demand. An easy solution to this issue is the use of green energy. At the moment, it might be more expensive, but the plan of the EU is to transform the energy system and be carbon neutral by 2050 [10] and Germany already by 2045 [94]. When all energy needed for production comes from renewable sources, the environmental impact will be almost zero. Nevertheless, the efficiency of the production process can be further improved, especially if production takes place in a country with a carbon-intensive energy mix [22].

Exploitation and pollution along the value chain has to be addressed and first steps in that direction are, for example, the EU supply chain law: “Corporate sustainability due diligence” [95]. In this way, fair working conditions are supported along the supply chain.

Most LCAs do not consider a recycling process for DSSC due to a lack of information [29]. When it was considered, material from c-Si PV modules were used to estimate what amount of material and quality could be recovered [22].

Manufacturing can be made to be more efficient, logistics can be optimized, but the largest value loss happens at the end-of-life phase of a module. The described economic value, which can be gained by intelligent product design for reusing, repairing, refurbishing, remanufacturing or recycling of the product, has not been investigated thoroughly yet [21]. Here, new designs and business models could retain the greatest value of the product and thus, minimize the environmental impact.

Table 2. Overview of identified value losses and derived opportunities.

Value Loss	Opportunity
Energy intense FTO glass production	Reuse the material, use recycling material for production, use green energy
Toxic ruthenium dyes	Use natural non-toxic dyes
Precious metals used (silver and platinum)	Use alternative carbon based solutions
No repair, remanufacture or recycling strategy	Use alternative design approaches
No information during use-phase	Implement service business models
Low efficiency and stability of DSSCs	Research is working on higher PCEs and long-term stability. For instance, gel- or solid-state electrolytes are a topic of research [52,96]. Implement refilling business models use DSSCs not only for harvesting energy but also as design element in buildings.
With c-Si PV, a strong, well-established product might prevent DSSCs from market entry	Use DSSC as supplements; for instance, indoors, integrated in devices, for transparent windows, etc.

gives an overview of the value losses and derived opportunities we identified.

3.2. Investigating Possible Changes to the Design and Business Model

In the next step, the identified opportunities, further called cases, will be matched with one or several business models and design strategies.

Table 3. Overview of cases, design strategies and business models.

Cases	Design Strategies	Business Models	Reference
1. Refilling DSSCs	Upgradability, ease of maintenance and repair	Hybrid	[51,97]
2. DSSCs as design element	Attachment and Trust, Durability	Classic long life Gap exploiter	[98]
3. Long-life DSSC	Durability	Classic long life	[22,56]
4. Short life time for consumables	Dis- and re-assembly	Gap exploiter and access	[99]
5. DSSC can be used in glass recycling	Design for recycling	Gap exploiter	[30]
6. Remanufacturing of used DSSCs	Standardization and compatibility	Gap exploiter	[31]
7. Using recycled material	Dis- and reassembly	Gap exploiter	[100,101]
8. DSSC in a performance contract	Ease of maintenance and repair, upgradability and adaptability	Performance	[80]

shows an overview of the cases matched with the strategies and business models.

The following cases from Table 3 are explained in more detail.

• Refilling DSSCs: Because of having a relative short lifetime, the electrolyte has to be refilled [51,97]. In this way, a comparatively high-priced DSSC could be sold and electrolyte refill pads, comparable to a printer cartridge, could be sold. In the case of a window integrated system, for instance, in winter, a more transparent electrolyte could be filled in to maximize the heat gain through a window. In summer, a more opaque electrolyte could reduce the cooling load.
• DSSCs as design element: DSSCs could be used not only to generate green energy, but also as a design element in architecture. This case could also be used to raise public awareness of renewable energy in the context of the circular economy [102].
• Long life DSSC: Long life through solid-state or gel-electrolyte technology. For DSSC, this would mean a long lasting product that needs a low level of maintenance and works for around 20 years, and is comparable to c-Si PV [22,56].
• Short life time for consumables: Since long-term stability remains an issue, such DSSCs could be used in products which themselves have short lifetimes. Clothing, tents or some electronic devices or backpacks could be possible applications [99]. The recycling process gets much more difficult when different materials are mixed together. That is why the DSSCs should be easy to separate from the product and easy to recycle. Gap exploiter and access, selling or renting consumables with DSSC technology and taking it back for reuse or remanufacturing are some options.
• DSSC can be used in glass recycling: At the end of the useful life, the non-toxic DSSC can be used in glass recycling [30].
• Remanufacturing of used DSSCs: TCO glass is reusable and has high environmental impact [31].
• Using recycled materials: LCD TVs, flat screens or smart phones can be used as conductive substrates for DSSC manufacturing [100,101].
• DSSC in a performance contract: Instead of selling DSSCs, a company delivers electricity with DSSCs integrated into the customer’s roof or building. Data during the use phase can be collected and the product can be appropriately improved. Similar offers are already being used by public utility companies such as Aachen, for example, where c-Si PV modules are used to generate energy [80].

To proceed with the Circo method, the economic and ecologic benefits from each objective are estimated. However, investigating the sustainability of a product is complicated, especially when it comes to electronics that are made of a mixture of different materials, each of which occurs in only a small quantity. As an indicator for a state-of-the-art status, the Cradle to Cradle (C2C) certified registry was investigated. At the moment (19.08.2022), 13 out of 759 products from the C2C product registry are electronic products, comprising around 1.7% of the total.

The products range from relatively simple products such as switches and sockets up to PV modules. Of these 13 products, five are sockets or switches, which are of comparatively low complexity. Only one product, Beosound Level, was certified with the new version 4.0, which is much more difficult to obtain compared to the older version, 3.1. Three glass–glass c-Si modules can be found in the product registry. For c-Si modules, entire series are certified so a variety of applications are possible, for example, roof- mounted or building-integrated.

Table 4. C2C-certified products in context of electronics without sockets and switches.

Manufacturer	Product Description	Standard Version	Level	Expiration Date
Solitek (Vilnius, Lithuania)	Glass–glass solar modules	3.1	Silver	12 December 2022
Solarwatt GmbH (Dresden, Germany)	Panel vision H series and Vision 60/EasyIn series	3.1	Silver	30 June 2024
Maxeon Solar Technologies, Ltd. (Singapore)	Maxeon Solar Panels	3.1	Silver	30 June 2024
Whitecroft Lighting Limited (Ashton-under-Lyne, UK)	Selene 2	3.1	Bronze	30 June 2024
Tridonic GmbH & Co KG (Dornbirn, Austria)	CLE, LLE and QLE LEDModules	3.1	Bronze	30 June 2024
Brandbase BV (Amsterdam, The Netherlands)	Remote control cars	3.1	Bronze	4 November 2022
Brandbase BV	Turbo Rechargeable battery	3.1	Bronze	7 January 2023
Fenoplastica Lights & Electrics (Barcelona, Spain)	Industrial Ventilation Systems	3.1	Bronze	29 November 2023
Bang & Olufsen A/S (Struer, Denmark)	Beosound Level	4.0	Bronze	1 September 2023

gives an overview of the more complex, to-date (19 August 2022) C2C-certified electronic devices.

Only for the Beosound Level, some sustainable design hints are given: modular design, high quality materials, built to last, replaceable battery, upgradeable, recirculated material [103]. What must be implemented to obtain the certificate is described in the Cradle to Cradle Certified^® Product Standard [104,105]. After all, what was actually implemented, design changes or business models are not described; only in one case some design hints are given. Therefore, the further estimation of the economic and ecologic benefits is based on previous research rather than C2C-certified products:

• Refilling DSSCs: DSSCs can be refilled manually or in the future by humidity, morning dew or rain [51,97]. If refilling is done manually, a hybrid business model could be applied. A high-quality DSSC can be sold, and electrolyte refill packs could be sold on a regular basis; if the customer is willing to buy the refill packs, this represents an economic value. This already works with razor blades or coffee pads. The question is, if the customer is willing to invest in constant refilling for this technology, which is unlikely, since the c-Si technology functions without refilling. When coupled with the second case “design element”, refilling could be used for changing the color of the DSSC or an upgrade can be sold with a better performing or more stable electrolyte. From an environmental point of view, resources must be invested in packaging, since the electrolyte is likely to be supplied in small quantities and without a lot of packaging.
• Design element: When DSSCs are used not only for power generation but also as art and design elements, the product can be sold for a higher price. For the ecologic advantages, it can be expected that product attachment and trust will influence the customer to use this product longer. Refilling can be used here because it allows the product to be adapted to changing customer requirements. Figure 6 shows a student project by Malin Melzer from the University of Art and Design Halle, in which a concept for a DSSC design element is described and a prototype is manufactured.

3.

Long life DSSC: Has a higher ecological value since the product does not need to be replaced. Even though the product with a long life can be sold for more, the competition with c-Si PV still limits the price and thus the economic value. For all commercially available c-Si PV, the price for one Watt peak is below EUR 0.35 [106].

4.

Short life for consumables: The lifetime of the DSSC should be at least as long as that of the consumable upgraded with the DSSC. The recycling process could be more complex. That is why the ecological challenges are high. From an economic point of view, it is attractive to upgrade tents or clothing with electricity generation, as the products can be sold at a higher cost. An example is the “Gratzel Solar Backpack 2” from GCell, a backpack with an integrated DSSC for charging a phone [99].

5.

Glass recycling: FTO glass production causes has great environmental impact on DSSC production. That is why it has a lot of advantages both ecologically and economically for recycling FTO glass [30].

6.

Remanufacturing: has an even higher value retention, is less energy intensive and therefore offers higher benefits than recycling. The required reverse logistics, however, are more complicated and reduce the economic benefit [31].

7.

Using recycled material: Re-using a TCO substrate from smart phones, for example, reduces the necessary energy during the production process and reduces resource depletion [100,101]. Again, the logistics and the quality of the material is a problem and reduces the economic benefit.

8.

Performance contract: The classic disadvantage of a service-based model is the high initial investment since the product is not sold but the company retains ownership. In the long term, the company has a great interest in using durable, easy-to-maintain and efficient DSSCs, which are ecologic benefits. During the use phase, information about the product can be collected for improving the product. At the end of the contract the product can be taken back for reuse, remanufacturing or recycling. In general, service-oriented business models are the goal of the circular economy, as they bring major benefits both economically and ecologically [107]. On top of that, to acquire new customers is at least five times more expensive than customer retention [108,109].

In Figure 7, the economic and ecologic benefits are summarized.

The estimation of the economic and ecologic value is based on research results and the assumptions listed in the enumeration above, i.e., cases 1 to 8. This is only a qualitative assessment and experiments and prototypes are needed to support the statements with evidence. Usually, in the Circo track, a qualitative assessment is made by experts in the same way. In Section 3.3, the most promising cases, 6, 7 and 8, will be used to build a circular business model around them.

In the next phase of the Circo method, the changes the company has to make in several categories is estimated and described. Therefore, the fictitious business model from this publication must first be explained. In our scenario, the standard DSSC modules are manufactured by the module manufacturer, as described in Section 3.1. The module is then sold to a craftsman who installs the system on the user’s roof. So far, there is no direct contact from module manufacturer to the user. At the end of its life cycle, the DSSC module is fed into similar recycling streams as the c-Si PV modules, with a comparably high recycling rate and the same downcycling; and so, the material cannot be reused for photovoltaic applications.

In further investigations, the number of the design cases is reduced to three by fusing them together as follows:

• Refilling and design: The design of the product must be adapted so that the DSSC can be easily refilled and customized to meet the needs of customers. New employees, likely designers, are required. They can improve the aesthetics of the product. The return on investment can be achieved comparatively quickly as the product continues to be sold and a new revenue stream, the refill packs, generate an additional revenue stream. The new business model of selling refilling packages makes it necessary to build a new branch and to get in contact with the user; so far, the product is only sold to the trade company. That is why more people need to be hired for service and product management. The impact on the value chain for this design change does not pose major challenges, as only contact with the user has to be established.
• Long life and using recycled material: By re-using old material, energy and costs can be reduced. Even so, new supply channels for the material need to be established, as well as good relationships with partners along the value chain. The longer life of the product is coupled to new technology, such as solid-state or gel-electrolytes, which reach better long-term stability to the price of lower PCEs. When using recycled material, the production cost can be reduced. For example, when using cullet in glass production, 2–3% of energy can be saved for every 10% increase in used cullet [110]. Due to the long service life, the product can be sold at a higher price, but is limited by the cheap alternative c-Si PV module. New skills are required for implementing the new technology into the production line and quality investigations of the reused conductive substrate. In addition, the legal requirements and warranty issues must be clarified. The required influence on the value chain focuses on building up the connection with the recycling companies to acquire the used devices or components, flat screen TVs or mobile phones, for instance, for DSSC manufacturing.
• Performance, remanufacturing and recycling: In this model, the largest investment costs will be incurred at the beginning of the changeover, as the products are no longer sold but remain the property of the company. Revenue is earned over time as customers pay their monthly fee for the service provided; the return on investment for this model is lengthy. On the positive side, the material is returned to the company at the end of the contract. Then, the real advantage comes: the old DSSC can be upgraded in a remanufacturing process and be reused as new products. Many new capabilities need to be integrated into this business model, such as service to the customer, compliance management, quality control for used modules, remanufacturing and recycling partners and the development of an upgradable product. The service-oriented model extends the influence on the value chain to new areas, such as direct contact with the user, logistics partners for take-back, craftsmen, etc.

Table 5. Overview of the fused cases, design strategies and business models.

Cases	Design Strategies	Business Models
1. Refilling and design	Upgradability, ease of maintenance and repair, attachment and trust, durability	Hybrid, Classic long life, Gap exploiter
2. Long life and using recycled materials	Durability, Dis- and re-assembly	Classic long life, Gap exploiter
3. Performance, remanufacturing and recycling	Ease of maintenance and repair, upgradability and adaptability, design for recycling, standardization and compatibility	Performance, Gap exploiter

summarizes the merged models and presents the core elements.

The case “Short life for consumables” is not further investigated, since a different technology is used: thin film products with a plastic substrate. The plastic based DSSCs are probably difficult to separate from the product and an appropriate recycling process is not predictable. Figure 8 shows the overview of the business scope of the different cases; the more filled boxes there are, the easier the existing company could implement the changes and realize the case idea.

The following criteria are explained:

• Capital necessary for change describes how much capital is required for the change. No green box means that a lot of capital is required, whereas three green boxes mean that the company could already start the project without investment.
• Return on investment (ROI) describes how quickly the money invested can be earned back by adapting the new business model. Three blue squares represent a very fast ROI, whereas no blue square indicates a long time to ROI.
• Skills to realize the idea stands for the capability of the company and employees. Three yellow squares indicate that employees are able to make all necessary changes independently. No yellow square means that external partners must be engaged, employees must be trained or new people with appropriate skills must be hired.
• Required influence on the value chain stands for the dependency on the value chain. Three gray squares indicate that the company can implement the new business model on its own. No gray square means that the necessary changes depend strongly on the partners along the supply chain.
• The willingness and readiness of companies to change in the area of the circular economy was assessed as high for all business models. It describes how receptive the corporate culture, the knowledge of the employees and the readiness for change are. Change management in the company often plays an important role, as it is difficult to change a corporate culture from selling products to a service-oriented culture. It is a difficult task to anchor the idea of the circular economy in the minds and hearts of all employees.

Case one and two have an equal challenge for capital that need to be invested, but the third option will come with the biggest and most challenging tasks for the company. However, the gain for switching to a service-based business model are high.

For all cases, the internal drivers for change in the company are making the products more flexible, developing a sustainable product and adapting the product to the customer’s needs. External motivations for the shift to circular business models include tender requirements for construction projects, regulatory compliance and the resource crisis.

The results of Figure 8 are comparatively vague. It only gives indications of where the company’s weaknesses lie, or which idea would be easiest to realize with the available skills and resources. At this point, the Circo method could be improved by performing a cost-utility analysis. In this way, a more precise picture of the business scope of the company could be created. The most important improvements would be quantifiable criteria for a robust basis for action.

At this point of the Circo track, the company must decide which case or business model to tackle first. The performance model is the most difficult to achieve, but also provides the most promising environmental and economic value. The analysis of the existing LCA studies and further literature revealed that the production process and energy demand, as well as the supply chain, have to be optimized. Regardless, improving the end-of-life phase enables new business models and material circulation as well as value retention. That is why, in the below, the opportunity of improving the end-of-life scenario has been chosen. In the following steps, “case 3” (the performance, remanufacturing and recycling model) is further investigated. Next, the design strategies are further evaluated. If the change to the performance model is too costly or not realistic in one step, smaller steps towards it could be implemented. First, the customers could be motivated to return the old product by offering a payback option. That way, a remanufacturing could be implemented without a performance model. Furthermore, additional services could be implemented, such as maintenance or upgrades during the use phase. This way the switch to a performance-based model can be realized in smaller steps. However, it should be mentioned in this context that the public utility company Aachen already applies a performance business model with c-Si PV [80].

Design strategy for remanufacturing and recycling:

For the performance model, the product should be built in a way so that it can be upgraded. The TCO glass and the TiO₂ are state of the art and used in most of the cells [16]. These parts are interesting for remanufacturing, because in the next generation of DSSC, there is a high probability that this will still be used as a base material. The sensitizer and electrolyte, on the other hand, have a high potential for upgrading.

Remanufacturing and recycling is much easier when no harmful substances are added in the system.

Table 6. Overview material use in conventional DSSC and non-toxic DSSC.

Conventional DSSC	Non-Toxic DSSC
FTO Glass	FTO Glass
Dye/ruthenium	Dye/hibiscus
Electrolyte Iodine	Electrolyte iodine
TiO2	TiO2
TCO/fluorine-doped tin oxide	TCO/fluorine-doped tin oxide
Platinum	Graphite
Silver
Plastic

shows the conventional material of a DSSC and a non-toxic alternative.

Even though the efficiency of the conventional DSSC plant is significantly higher, this could be outweighed by the ecological value of the non-toxic DSSC. In addition to efficiency and long-term stability, the sustainability of the product plays an essential role [19]. Nature-based dyes can improve the sustainability considerably [111]. In previous work, we have shown that non-toxic DSSCs are suitable for remanufacturing and are promising for glass recycling [30,31]. Mariotti et al., also emphasize to re-design the DSSCs to enable a dye refurbishment [19]. Silver or metal components in general are causing problems in the glass recycling process [112]. That is why the ruthenium, silver and platinum layers should be replaced with environmental friendly alternatives [19,22,28]. Carbon-based solutions are promising and are investigated in the research [28,87,89,113]. A completely new design approach such as Apollon Solar’s NICE technology, without plastic encapsulation but with insulating gas, could be a future prospect for DSSC production as well [114].

Design for ease for maintenance and repair, upgradability and adaptability:

Since DSSCs are in the early stages of industrial production, the product changes and new technologies are very dynamic. That is why it is not essential to use materials that last forever, but to provide upgradable products that can be easily adapted to changing customer needs or new technologies [74].

3.3. Developing a Circular Business Model and Identifying Necessary Changes

In this section, the circular business model is described in detail. Figure 9 visualizes the circular business model.

Below the roles of the actors in the value chain are explained:

• The raw material manufacturer supplies the materials for the DSSC component manufacturer. The core activities are marketing, sales and contracting.
• The DSSC component producer supplies the DSSC components to the assembler and the panel manufacturer. The core activities are making and delivering parts.
• The manufacturer of DSSC modules (our point of view in this case) assembles the DSSC module from the parts. Furthermore, the service provider branch has been built. With the data from the service provider branch, the technology can be improved. From the collector, old modules are achieved and remanufactured. When possible, recycled TCO glass from the parts manufacturer is bought. The core activities are making and delivering the product, providing service, close cooperation with trade companies and collectors as well as planning the system that will be installed. The system can be customized and focused on self-sufficiency, optimized self-consumption or yield maximization, for instance. Furthermore, the product design and the product improvements, derived from the use phase, are carried out by the DSSC manufacturer.
• The trade companies install the product on the building of the user. If the competences are available, this party could also take over the role for maintenance and repair services as well as the role of the collector. The core activities are installation and logistics.
• The user purchases and uses the service. The core activities are purchasing, limited maintenance and minor repairs.
• The collector dismantles the system from the building of the customer and sell them to the recoverer or the module manufacturer. Core activities: Dismantling DSSC systems and selling them.
• The recoverer separates the old modules and recovers material that can be reused at the DSSC parts manufacturer or, if no higher value retention processes are possible, the materials can still be used for glass recycling.
• The service provider branch carries out maintenance and repair or assigns a craftsmen company to do this. They also collect data to enable product improvements. The core activities are warranty and financing services. Since this service provider is an essential part of the performance business model, it is in the company’s interest to build up the service provider role in-house as quickly as possible and not to rely on external partners.
• The waste processor handles the parts that cannot be recycled or remanufactured. The material is burned or composted. The burning process can be used to produce electricity and heat, which can be used for the manufacturing processes of other players in the product cycle. The core activities are processing waste, burning and composting.

Another very important part is to clarify the customer interaction. In this model, the customer already comes into contact with the service provider at the time of purchase or when signing the contract. The company then assigns a craftsperson to install the system. The system should be automatically monitored and provide data for checking the status of the DSSC system in order to act quickly and appropriately when a defect or malfunction occurs. If new technology is available, an upgrade might be installed. If the customer wants to change the aesthetics of the system, this is also possible for a fee. If the customer wants a new product or wants to cancel the contract, a collector/craftsperson is assigned to disassemble the system.

The next important point of the Circo method is to clarify where revenue can be generated. The greatest value retention can be achieved by reusing the old product. The monthly fee for providing the service is the main source of income. To plan the system individually for the customer is a chance for revenue as well as a close customer relationship. Upgrades or aesthetic changes could generate another revenue stream. Another major benefit comes into play; when the contract ends and the system is disassembled yet reusable, the greatest value retention is obtained. Reuse requires a remanufacturing process, where at least the TCO glass could be reused. The last resort would be glass recycling if the TCO glass cannot be reused.

The newly built closer contact with the consumer enables advertising of new or other products as well and being essential for customer retention.

On the side of the collector’s new activities are managing stock, collecting the product, storing the product, delivery product, packing product, stock management, picking up old product and legal disposal of product.

In Figure 10, a business model canvas of the described business model is depicted for summarizing the ideas and information.

At this point, it is emphasized in the Circo method to look at possible obstacles to be able to prepare counter measures. In the initial phase, there is not enough scrap material for reprocessing or recycling. Therefore, this part of the infrastructure will come into play later, when the first DSSC systems are retrieved. The focus at the beginning is in building up the service infrastructure for performance-based contracting. Another challenge will be to determine the quality of the retrieved DSSCs after the first life. One option could be to measure the conductivity of the TCO glass to determine if the TCO layer is useable. If the quality of the device is too low or the work is labor intense, the generated revenue could be not sufficient. In general, the performance model requires many new and unknown elements in the system: monthly payment, pre-financing, customer service and direct customer relationship, to name just a few.

It is not realistic to implement all changes within one’s own company at the first step. It is important to identify partners in the value chain and build appropriate relationships. In later stages, more activities and roles could be implemented in one’s own company.

Not mentioned in the Circo method, but a useful tool for further investigation of potential barriers, is the feasibility study and the Political, Economic, Social, Technological, Legal, and Environmental (Pestel) analysis. The feasibility study examines a company’s strengths and weaknesses, investigating profitability, technical feasibility, requirements and core difficulties. The Pestel analysis is used to determine a company’s environment and identify barriers. With these methods, the Circo method could be improved further and the results would be more explicit. It might also be possible to apply these methods after the Circo method to further investigate the business model.

To prepare the roadmap and next steps, the consequences for design changes and possible solutions have to be defined. In

Table 7. Consequences for the product design.

Changed Activity	Resulting Design Question	Possible Solutions	Realization Activities
Repair and upgrade	Modular, easy to replace components, easy to clean, available spare parts	Make electrolyte and dye upgradable. Refilling electrolyte can be an option for prolonging the lifetime and also upgrading a DSSC [51,97]	(A) Define the scope of functions for an upgrade (B) Prepare specifications for product development (C) Contact product development
Remanufacturing	Non-toxic components, compact transportation and storage (reverse logistic), easy to dismantle (minimal labor), use parts in new configurations, indicators of quality, compatibility over generations (combination of old and new parts)	Use non-toxic components, compact module design, reuse the TCO glass	(D) Define characteristics for compatibility and quality (E) Contact product development (F) Contact logistic partners
Recycling	Non-toxic components, compact transportation and storage (reverse logistics), easy to dismantle, knowledge about the used material, material bonds that can be broken, sufficient volumes	Use non-toxic components to make the glass reusable for the conventional glass recycling process or PV applications	(G) In addition to switching to non-toxic material, no big changes are required (H) Contacting recycling companies and run recycling tests with non-toxic-DSSCs

, the major consequences for the product design are summarized. The order in which the changed acticities are given represents the value retention; repair and maintain delivers the highest value retention whereas recycling delivers the lowest.

Table 8. Consequences for the service.

Changed Activity	Added Service Elements	Description of Service	Realization Activities
Performance contracting	Performance-based contracts with customer instead of selling the product	A DSSC system is integrated in the customers building (roof- or window-systems). The customer is supplied with green energy and pays per kWh. When the contract ends, it could be extended or the system is dismantled and reused, remanufactured or recycled	(I) Build up the service department (J) Get partners on board (craftsmen company, collectors, maintenance and repair) (K) Clarify legal requirements
Value retention and upgrade	Maintenance, repair, upgrade services, monitoring and optimizing performance, remote management, installing modules as assets	Data collection During operation: remote monitoring, data storage, data analysis and optimizing performance, minimizing number of visits, collecting and processing user feedback	(L) Build up monitoring and data analysis team (M) Find suitable partners for maintenance and repair

shows the consequences for the service.

Figure 11 shows the required effort and dependency for the required product design and service changes.

The method used in this case is called portfolio analysis. The X-axis describes an external factor: the dependeny on the supply chain or other partners who are not part of the company. The Y-axis describes an internal factor; in this case, the effort required from the company to realize the idea. A weekness of the Circo method at this point is that X-axis and Y-axis are not quantified. Usually, “Small”, “Medium” and “Large” should be quantified. Only then, the position of the task can be understood. To improve the significance of Figure 10.,more experts should be consulted to position the elements in the diagram. In Figure 10, “Dependency” is low if the skills and knowledge for redesigning the product or service are available within the company. If external parties are required, the dependency rises. “Effort” is a parameter for the time and resources that are required for realizing the activity. With product design and the service supply, the core elements have been described. Elements such as marketing, logistics, finances and purchasing (internal elements) as well as the supply chain, new knowledge, law and regulations or politics (external elements) have to be described.

Table 9. Requirements and benefits of internal elements.

Internal Elements	Requirements	Benefit
Marketing and sales	Switch from B2B to B2C marketing	Green corporate image: affordable green energy with sustainable technology
Purchase	Buying recycled material if possible and implement quality control	Savings from buying cheaper recycled materials instead of new materials or components
Logistic	The reverse logistic concept has to be built up with the appropriate partners. A collector is necessary who can dismount the system and ideally test the quality and can decide which modules are usable for remanufacturing or which modules should be recycled	Material and DSSCs can be recovered and the highest possible value retention can be achieved
Finance and capital	Additional costs at the beginning, as the modules are not sold, but the service is provided on a contract basis	Continuous income stream instead of a one-time sale
Operation	The service sector must be regularly expanded and improved	The customer relationship improves. The service offered and the monitoring of the DSSC system operating data allow for a closer relationship. Additional revenue can be generated by selling updates or enhancing the aesthetics of the system
Compliance Management	Appropriate solutions for compliance with laws and regulations must be implemented. In particular, the warranty for used or remanufactured products must be clarified	Mandatory

gives an overview of the requirements and benefits of the internal elements.

Table 10. Requirements and benefits of external elements.

External Elements	Requirements	Benefit
Law and regulations	EU Green Deal, national laws and regulations. These external factors must be taken into account	Mandatory
Influence of the value chain	More parts and components are needed for spare parts. New partners have to be found and a resilient relationship with partners has to be established	Less raw material is needed.
Customer relationship	The service sector needs to be built up and requires regular improvement	A closer relationship should be enabled by monitoring the system, offering upgrades, maintenance and repair services
New knowledge	Missing knowledge and skills must be made up through employee training or hiring new staff	Improvement of the product thanks to the data from the use phase of the product

shows the requirements and benefits of external elements.

The impact of the changes to the proposition value is given in Section 3.1, Section 3.2 and Section 3.3. The Roadmap is drawn based on all the considerations in the final chapter.

3.4. Scheduling the Necessary Actions and Pitching the Idea

The roadmap is the basis for the next steps in the company to realize the change to a circular business model and product. Figure 12 visualizes how such a roadmap could look.

The employee of the company who participates in the Circo track leaves the workshop series with a roadmap of the idea and first steps for implementation. In this case, the next step is to discuss the idea with his/her own department and the Chief Executive Officer (CEO) and win them over to the idea. Furthermore, the feasibility has to be evaluated. The EU Green Deal and climate neutrality goals are important external influences that support the idea. If the company wants to realize the performance business model, further partners have to be acquired for maintenance and repair services as well as for the dismantling and collection of the old modules.

A bar chart could be used to further improve the roadmap. It is more suitable because the length of the bar indicates the time needed for the task. If the CEO can be convinced to support the project, the work breakdown structure (WBS) should be used to break each task into a working package. The WBS is part of the network planning technique, which is used to visualize the dependencies in the chain of actions and to classify the individual tasks of the project in terms of time.

4. Conclusions

In this paper, we applied the Circo method to explore possible DSSC-related business models and sustainable design for a circular economy. The focus is value retention, which is connected to design principles. First solutions from research are reviewed and described. Whereas LCA studies focus on more efficient production processes in their proposals, we emphasize the value pyramid. The recycling and reuse of TCO glass is a core element, as effectively low amounts of energy are required to produce new products. Experiences with c-Si PV and circular design or design failures are highlighted, from which lessons should be learned for the development of new technologies. The Circo method is usually applied in an existing company to transform a linear product into a circular one. However, we wanted to use this method for a scientific investigation of technologies in the context of business models, because in this way, new perspectives, ideas and further sustainable options could be described.

Section 3.1, Section 3.2 and Section 3.3 provide concrete ideas for linking sustainability and business models with DSSC technology. Other aspects of the Circo method are given only as implementation examples to learn about the method and be able to apply it in any other case where a circular product or service is to be developed.

We also describe where the Circo method could be improved. Methods such as the cost-utility analysis, the feasibility study, Pestel analysis, WBS and the network planning technique can help to further improve the Circo method and make the results even more valuable. In general, these methods make the criteria used quantifiable and help set specific project management timelines.

Among other business model ideas, a performance-based contract is described in detail in this paper. Under the performance-based contract, the user pays a monthly fee while receiving green energy provided by the service provider. The service provider installs a building-integrated DSSC system on the consumer’s property and retains ownership of the modules. At the end of the contract, it can be renewed or the modules are taken back and prepared for a second life or for recycling.