Improving the Efficiency of Carbon-Based Perovskite Solar Cells with Passivation of Electron Transport Layer ^†

Abstract

Among perovskite solar cells (PSCs), carbon-based perovskite solar cells (C-PSCs) are regarded as one of the best advantageous designs centered on a number of desirable characteristics, such as outstanding scalability, long-term stability, and cost-effectiveness. In these C-PSCs, titanium oxide (TiO₂) has usually been utilized as the electron transport layer (ETL) because of its simplicity in preparation and low cost. In these hole transport layer-free C-PSCs, the quality of ETLs is essential for the high performance of PSCs. In this paper, we used TiCl₄ post-treatment for the passivation of the titania layer (TiO₂) to improve the quality of ETL. Consequently, after passivation, the charge recombination has been reduced, the efficiency increased from 3.15% to 4.16%, and resulted in a 32.06% improvement in power conversion efficiency (PCE).

1. Introduction

The interesting properties of perovskite materials have attracted the interest of numerous scholars in recent years, leading to the development of PSCs [1]. Among them, PSCs with carbon-based back electrodes have generated considerable concentration due to their exceptional durability, low cost, and resistance to a variety of environmental and operational circumstances [2]. Ku et al. initially demonstrated printable hole-transport layer (HTL)-free carbon-based perovskite solar cells (C-PSCs) in 2013 [3]. The quality of the ETL is critical in HTL-free C-PSCs, with TiO₂ being the most commonly used material. However, due to the presence of structural defects and surface inhomogeneity of TiO₂ ETL, charge recombination can occur at the TiO₂/perovskite contact quite easily, limiting the device’s photovoltaic efficiency significantly [4]. The lack of expensive noble metals and expensive hole transport materials (HTMs) greatly lowers the cost of the device. In addition, the use of hydrophobic carbon electrodes in place of precious metals improves the device’s overall stability while preventing moisture from penetrating the perovskite [5]. It has been discussed in many papers that this device’s functional layers, the electron transporting layer, the insulating layer, and the carbon can be optimized to increase efficiency [6]. The fabricated device in this work, as shown in Figure 1, consists of a fluorine-doped tin dioxide (FTO) conductive glass substrate, a layer of compact titania, a mesoporous titania layer, a mesoporous insulating layer of ZrO₂, and a porous conductive carbon contact. By infiltrating liquid through the stack, the perovskite precursor solution is deposited [7].

In this work, we applied TiCl₄ post-treatment for TiO₂ ETL to increase C-PSCs performance. We dipped the samples in an aqueous TiCl₄ solution for 30 min, 40 min, and 60 min at 70 °C. The optimum results were obtained at 40 min of dipping time. The TiCl₄ post-treatment was first used in dye-sensitized solar cells (DSCs) and then carried over to PSCs [8]. TiO₂ with a small surface area frequently shows severe J-V characteristic hysteresis [9]. It has been shown that the TiCl₄ treatment can increase the surface area of TiO₂ while improving connectivity and passivating surface defect levels [10]. As a result, the surface roughness decreases with TiCl₄ passivation, and advancements in the photovoltaic characteristics of C-PSCs were seen.

2. Experimental Section

Device Fabrication

First, for device fabrication, we take fluorine-doped tin oxide conductive glass substrate (FTO-coated). In an ultrasonic cleaner, distilled water, ethanol, and acetone were used to clean the FTO substrates. After cleaning, the compact titania layer was printed using the screen on the FTO glass substrates and the samples were annealed at 500 °C for 30 min. After that, we dipped some samples for passivation in TiCI₄ aqueous solution at 70 °C for 40 min; then, we rinsed them with distilled water followed by 30 min of annealing at 450 °C. A mesoporous titania layer was then applied to the samples, both passivated and non-passivated, and they were both annealed for 30 min at 500 °C. A mesoporous titania layer was then applied to the samples, both passivated and non-passivated, and they were both annealed for 30 min at 500 °C. Then, zirconia layer was deposited through screen printings and annealed at 500 °C for 30 min. Then, we deposited the carbon layer and hardened it at for 30 min at 400 °C using a hotplate. Next to carbon layer deposition, we infiltrated perovskite (MAPbI₃) into the stack through a micropipette, and waited for 40 min. After 40 min, we heated the samples at 70 °C for 20 min and waited for them to cool to room temperature [11]. Then, we checked the efficiencies of devices with and without passivation using a solar simulator as shown in Figure 2.

3. Results and Discussion

The J-V curves were noted and placed into

Table 1. Average values of photovoltaic parameters for passivated and non-passivated samples.

Device	Voc (mV)	Jsc (mA/cm2)	Isc (mA)	PCE (%)
Non-passivated	756.9	17.03	25.55	3.15
Passivated	802.0	19.38	29.08	4.16

to compare the performance of passivated and non-passivated devices in Figure 3. The non-passivated device exhibits low efficiency with an open-circuit voltage (Voc) of 756.9 mV, a short-circuit current density (Jsc) of 17.03 mA cm⁻², short circuit current (Isc) of 25.55 mA, a fill factor (FF) of 24.41%, and a power conversion efficiency (PCE) of 3.15%; which are improved to 802.0 mV, 19.38 mA cm⁻², 29.08 mA, 26.74% and a PCE of 4.16%, respectively, for the passivated device. This improvement in performance is due to the enhanced quality of TiO₂ ETL.

4. Conclusions

In conclusion, it was possible to produce an efficient ETL to obtain proficient PSCs by modifying the compact TiO₂ surface with the TiCl₄ solution, which resulted in a 32.06% improvement in efficiency. The TiCl₄ treatment improves charge extraction and reduces charge recombination in the ETL layer, due to which efficiency increases. According to our findings, complex interface engineering offers a potentially effective means of obtaining high-performance PSCs.