Self-Organized Heterocyclic Amines Films on Carbon Substrates for Photovoltaic Applications

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Presently, the main trends in photovoltaics are high efficiency and cheapness. The main solutions for the first item are: (i) the presence of several junctions (there are four of them); (ii) ideal solar cell architecture, such as hybrids; (iii) new materials, for example, perovskite; (iv) new substrates; and (v) new physical insights, such as plasmonic efficiency enhancement. In the cheapness stage, the following approaches are proposed: (i) simplification of the production technology of solar cells; (ii) temperature reduction during the manufacturing process; (iii) inexpensive materials; (iv) use of simple equipment. Among the promising fields of application are organic on silicon hybrids. However, the high cost of the silicon substrate reduces the commercial potential of this solution. One of the candidates to replace silicon substrate is carbon. This article takes an important step towards understanding the formation of heterocyclic amines on a carbon substrate by self-organization. The regimes of the corresponding morphology of thin organic films have been determined. This brings the technology of self-organization closer to the formation of inexpensive and easily manufactured solar cells.

Abstract

Future technologies for organic photovoltaics include self-organization and self-assembly. Heterocyclic amines, namely sodium sulfacyl, clonidine, and cyanocobalamin, were deposited on four types of carbon-on-paper substrates by the self-organization assembly method. Each organic film was deposited in the chemical bath for 10, 20, 40, 60, and 90 min. Carbon substrates were thin layers of carbon composites deposited on Maestro paper. Compositions of carbon films of thicknesses about 20 mcm included graphitized carbon black “PureBlack@”and graphite “KGPS-1” as the permanent components, as well as activated carbon, magnetite, nanotubes, and needle graphite DBX-010 as variable components. Polyvinyl butyral (PVB) served as a binder for all of these composites. Morphological features of organic-carbon hybrids were investigated using optical microscopy MII-4 of 500 nm resolution with a SLR camera. The injection properties of the obtained hybrids were studied on standard equipment for current–voltage characteristics measuring. The thin organic films demonstrated the possibility of self-organization on various carbon substrates. The best grid morphology was determined for the optimal deposition time between 20 and 40 min with circular-type cells. The best injection properties correlated with the best morphology. These heterocyclic amines-on-carbon hybrids are promising structures for the formation of non-expensive and easily-fabricated solar cells.

1. Introduction

Self-organization and self-assembly concepts for the preparation of thin films have been developed in the last decades [1,2,3]. They allow the development of self-assembled architectures for organic photovoltaics, electrochemical energy storage in pseudocapacitors, and metal−organic framework materials for CO₂ capture [4]. Simultaneously during the self-organization process, there is functionalization and sensibilization of substrate surfaces [5,6]. Along with silicon substrates, carbon substrates have recently been investigated [7,8,9,10]. Carbon substrates are among the most suitable for non-expensive devices [11], including solar cells [12,13]. They have also been used for fuel cells [14], catalysis [15,16], microarray fabrication in biosensing [17] (in particular, the authors made an important conclusion that the carbon substrates themselves, as well as chemical compounds based on carbon–carbon bonds, provide significantly increased chemical stability), Si ribbon technology for solar cells [18], and other applications, such as deicing [19]. This is due to: (i) unique properties of both the isolated molecule and self-organized molecular assemblies or aggregations; (ii) the combination of high absorption coefficient of organics and good C transport properties; (iii) hybrid compatibility with C planar technology.

The carbon substrates can usually be functionalized by some modified carbon [20,21] or other additions [22].

On the other hand, the chemical bath technological process for the preparation of organic films from solutions, including aqueous solutions, is very simple and not expensive [23,24,25]. Self-organization and self-assembly occur during this process. Thanks to self-organization, a clonidine-silicon hybrid with an efficiency of about 8.5% was created [2].

Copper phthalocyanine is one of the organic substances that has been used in organic solar cells in recent decades [26,27,28,29,30]. It was interesting to look at other phthalocyanines, such as cobalt phthalocyanine or cyanocobalamin (vitamin B12). In another classification, copper and cobalt phthalocyanines are aromatic heterocyclic amines. Aromatic heterocyclic amines with shorter chains are sulfacyl sodium and clonidine. Previously, these heterocyclic amines were used with silicon substrates to create hybrids for organic solar cells [2,25].

This article deals with the morphology of self-organized aromatic heterocyclic organics on the carbon patterned substrate and some of their injection properties. The technology used for the fabrication of organic–silicon hybrids is simple enough and differs by preparation at room temperature, using water solutions of organic components, and a lack of vacuum equipment. To our knowledge, this is the first of such a study in the scientific literature.

2. Materials and Methods

Carbon-on-paper substrates were prepared by mechanical mixing of the powder of the active material with a solution of polyvinyl butyral (PVB)—(SDW 3A grade from China Qingdao Hong Jin Chemical Co. LTD, Qingdao, China) in ethanol (96% v/v from HLR Co., Kiev, Ukraine), which acted as a binder. The polymer content in the composite was 20%.

We used the following initial carbon materials for preparation of composites: graphite KGPS-1—dry colloidal graphite preparations S-1 from Ltd. Zavallivsky graphite plant, Kirovohrad region, Hayvoronskyi district, Zavallia town, Ukraine; activated carbon—high-quality activated granulated coconut carbon for filters from Jacobi Carbon Co., Kalmar, Sweden; carbon black—grade “PureBlack@” from Superior Graphite Co., Chicago, IL, USA; carbon nanotubes—Baytubes C150P from Bayer Material-Science AG, Leverkusen, Germany; DBX-010—rod-shaped graphite from EnerGraph Inc., Coral Springs, FL, USA.

Composites were deposited on Maestro paper (MAESTRO^® extra 160 g/m² from Mondi group, United Kingdom). For better homogenization of the composition during its mixing, ethanol was added as a solvent at the rate of 1 g of solvent per 1 g of solid substance. To eliminate clumping of carbon particles, the pre-mixed composition was placed in an ultrasonic bath (Codyson PS-40A Ultrasonic Cleaner, China). After ultrasonic treatment, the composition was once again subjected to mechanical stirring for 45 min. Using the laboratory electric heat film coating machine (TMAX-TMH from Xiamen Tmax Battery Equipments Limited, Xiamen, China), Figure 1a, compositions were applied to a copper or paper subtract (metal foil 18 μm thick, Hohsen Corp., Osaka, Japan).

Preliminary drying of the composition was carried out at room temperature for 60 min. Such processing led to the hardening of the composite material. In order to equalize the thickness and compact the working mass, the samples were passed through rolling mill (8” width heat calendering machine from Xiamen Tmax Battery Equipments Limited, Xiamen, China), Figure 1b. The samples were cut with a round hole with a diameter of 48 mm. Successive weighing of the substrate and the manufactured sample allowed one to determine the composite mass and calculate the density of the material.

Three commercial aromatic pharmaceutical drugs: sulfacyl sodium, Darnitsa company, Kyiv, Ukraine, clonidine hydrochloride, Darnitsa company, Kyiv, Ukraine and cyanocobalamin (vitamin B12), Hebei Huarong Pharmaceutical Co., Ltd., Shijiazhuang, China, used in this study, were purchased at a pharmacy, in tablet form and ampoule [31] (see

Table 1. Pharmaceutical materials, their abbreviations, chemical compositions, molecular formulas, and additional substances [26].

N	Title	Molecular Formula	Additional Substances
1	Sulfacyl sodium, sulfanilamide, C8H10N2O3S		H2O
2	Clonidine hydrochloride, C9H9Cl2N3, (N-(2,6-dichlorophenyl)-4,5-dihydro-1H-imidazol-2-amine)	HCl	Tablet: lactose monohydrate, starch, polyvidone, magnesium stearate. H2O
3	Cyanocobalamin (Vitamin B12), C63H88CoN14O14P, Coα-[α-(5.6 dimethyl benzimidazolile-Coβ-cobamidcyanide		H2O

).

Solid B12, Hebei Huarong Pharmaceutical Co., Ltd., Shijiazhuang, China, was purchased in a chemist’s shop to obtain the concentrated B12 solution (to 1.25%) to the contrary pharmaceutical 0.05% solution. It should be noted that the liquid organic medium did not relate to the homogenous medium and its identification as a solution was conditional. These were the mixtures of water and medical materials. The medical materials were in tablet form and contained some additional excipients and ligands, for example, starch, lactose, etc. These components were absent in the molecular formula and are shown in column 4 of Table 1. Due to these factors, some of the solution could be referred to as the suspension.

Self-organized organic-inorganic hybrids were formed by a chemical bath deposition of the heterocyclic aromatic compounds at room temperature from water solutions under ambient laboratory conditions. The deposition time was varied from 10 min up to 90 min.

The surface morphology of the organic films on carbon substrates was studied using optical microscope MII-4, LOMO, St. Petersburg, Russia, with SLR camera Olympus E-1, Tokyo, Japan.

Ag paint (Silver conductivity paint, SCP< produced by ELECTROLUBE, a division of H.K. Wentworth Limited, Derbyshine, UK), applied to the hybrid as a contact material. Dark and illuminated (30 mW/cm²) current–voltage characteristics (CVCs) were measured by Tester 14 TKS-100 (FORM, Voronezh, Russia) with a measurement error of about 0.25%. Peculiarities of injection properties were evaluated using the injection approach [32].

3. Results

3.1. Carbon Substrates

The deposition conditions for pattern carbon-on-paper substrates are summarized in

Table 2. Conditions for the formation of carbon substrates and their SEM and optical images.

No. of Substrate	Resistivity, Ohm	Composition	Surface Morphology
			Electronic Image	Optical Image
1	4.0	20% carbon black 20% activated carbon (compressed) 40% graphite KGPS-1 20% PVB
2	3.6	16,67% carbon black 50% graphite KGPS-1 16,67% magnetite (BM) 20% PVB
3	26.4	20% carbon black 50% graphite KGPS-1 10% carbon nanotubes 20 % PVB
4	3.0	20% carbon black 40% graphite KGPS-1 20% graphite DBX-010 20% PVB

.

The architecture of the organic–inorganic hybrid solar cell, see Figure 2, consisted of:

(1)

A micro-structured carbon substrate with a surface pattern, with a thickness of about 10 micrometers;

(2)

Organic material: heterocyclic aromatic compounds, with a thickness between 0.1 and 2.0 micrometers;

(3)

Electro-conducting-painted comb type contact, tape thickness, width, and length were 10.0 micrometers, 1 mm, and 3 mm, respectively.

3.2. Surface Morphology

The surface morphology, depending on the deposition time for the three types of organics (sodium sulfacyl, clonidine, and vitamin B12), and four carbon substrates are presented in Figure 3, Figure 4 and Figure 5.

3.3. Current–Voltage Characteristics

The current–voltage characteristics of organic-on-carbon hybrids can be conventionally divided into two large groups: light-sensitive and light insensitive. As an example, the light-sensitive CVCs of 10 min-deposited sulfacyl sodium on carbon substrate 1 in the dark and under illumination are shown in Figure 6a. Figure 6b,c represent the dimensionless sensitivities (DS) [32] of CVCs. The insensitive CVCs of 10 min-deposited vitamin B12 on carbon substrate 2 in the dark and under illumination are shown in Figure 7a. Figure 7b,c represent the dimensionless sensitivities of CVCs. Using DS makes it possible to quantitatively assess the nonlinearity of CVCs and analyze the injection properties of the structures. This is important for further application of organic-on-carbon hybrids for the further creation of solar cells on this basis.

4. Discussion

All compositions on the various carbon substrates presented in Figure 1, Figure 2, Figure 3 and Figure 4 included graphitized carbon black “PureBlack@” and graphite “KGPS-1” as the permanent components, Table 2, column 3. In accordance with [33], graphitized carbon black “PureBlack@” is a nanostructured carbon material with a high internal surface and good conductivity. Graphite “KGPS-1” is a micro-structured flake graphite with very good conductivity and an average particle size of 2 microns. As shown in [34], the combination of both these materials ensures a synergistic effect and high conductivity. It supplies electromagnetic shielding due to great differences in morphology and the existence of tremendous points of phase boundaries.

A particular feature of substrate 1 is the presence of activated carbon as a variable component. Activated carbon has a very high internal surface area of the order of 1500–2000 square meters per gram, which can promote the active formations of crystallization centers. Its electronic and optical images show the roughest morphology, Table 2, columns 4, 5. Most likely, big clusters were formed by micro-structured flake graphite in Graphite “KGPS-1”.

A differential feature of substrate 2 is the presence of magnetite [35] as a variable component, Table 2 column 3. This substrate is not as rough as substrate 1, Table 2, column 4, and its optical image shows some crystal aggregates, Table 2, column 5.

A distinctive feature of substrate 3 is the presence of the carbon nanotube as a variable component. Structural features and high electrical conductivity of carbon nanotubes are widely known. The presence of carbon nanotubes can also promote nucleation and the formation of organic structures on the carbon composite. As it can be observed, there are some filaments, Table 2, column 5. This particular substrate demonstrates almost ten time higher resistivity at about 26.4 Ohm.

Finally, a characteristic feature of substrate 4 is the use of the DBH-010 graphite as a variable component, which has a “needle”-like structure [36]. There are big crystals that form big druse, Table 2, column 5.

Changes in surface morphology, depending on deposition time for three types of organics (sodium sulfacyl, clonidine, and vitamin B12) and four carbon substrates, are shown in Figure 2, Figure 3 and Figure 4. The classic genesis of surface morphology is demonstrated by sodium sulfacyl on substrate 1, as can be observed in Figure 3a–e. [2]. At the beginning of a thin film formation (10 min deposition, thickness of about 100 nm), Figure 3a, the film contour repeats the morphology of the substrate. At the next stage of the film growth (20–60 min deposition), circular-type cells are formed with an increase in the thickness of the organic layer, Figure 3b–d. Actually, this is the net-like structure of morphology. Finally (after 90 min of deposition), the shape of a spherulite is formed, Figure 3e. According to previous studies [2,25], a net-like form morphology was recognized as optimal from the point of view of the functionality of hybrids and for the high efficiency of solar energy performances in organic hybrids.

Substrates 2–4 set slightly different dynamics of sodium sulfacyl film formation, as one can see from Figure 3f–j,k–o,p–t. Thus, substrate 2 shows a lower deposition rate and, in fact, the morphology formed in 90 min, Figure 3j, is similar to the morphology formed in 60 min for substrate 1, Figure 3d. There are fractal features for substrate 3 and 4, shown in Figure 3n–o,s–t, when along with the thickening of the walls of the main cells, new cell networks were formed.

Clonidine and substrate 1 also follows the classic genesis of surface morphology, as can be observed in Figure 4a–e. The morphology of clonidine on substrate 2 is significantly different. It has a spongy shape, and there is not even a hint of a circular-type cell structure, see Figure 4f–j. The deposition of clonidine on substrates 3 and 4 clearly show that the substrate controls not only the surface morphology, but also the rate of its deposition. Thus, a 20 min deposition on substrate 4, Figure 4q, and a 40 min deposition on substrate 3, Figure 4m, form a similar morphology as the 60-min deposition on substrate 4, Figure 4s, and 90 min deposition on substrate 3, Figure 4o, which also form a similar morphology. Moreover, only substrate 4 gives the morphology of the globular type after 90 min of deposition, as shown in Figure 4t.

The worst morphology on the substrates 1–4 was displayed by the deposition of vitamin B12. If on substrates 1, 3, and 4 the formation of a circular-type cell structure can still be observed, for example, Figure 5a,k,t, then on substrate 2, the vitamin B12 layer is practically absent, as can be seen in Figure 5f–j.

The analysis of carbon substrate effects on thin film self-organization of sulfacyl sodium, clonidine, and vitamin B12, showed a sharp difference between substrate 2 and the other substrates. This substrate suppresses the ability of self-organization for all three investigated heterocyclic amines.

Regarding the differences in the effects of these three heterocyclic amines on morphology formation, as can be seen from Table 1, the main differences in the chemical structures of the considered heterocyclic amines are the different number of benzene rings: one in sodium sulfacyl with a net morphology, two in clonidine, and at least four in vitamin B12. It is the increase in the number of rings that possibly leads to the formation of a net morphology, especially at short periods of deposition. Thus, after 20 min of sedimentation, sulfacyl and clonidine already form such a morphology, Figure 3b,g,l,q, Figure 4b,l,q, while vitamin B12 forms it only after the film deposition in 60–90 min, Figure 5e,n,s.

By summarizing the investigated morphological properties of heterocyclic amines, namely, sulfacyl sodium, clonidine, and vitamin B12, it is possible to order the sequences of carbon substrate activity in the ability to self-organize the organic films as 1–3–4–2. Very likely, large forms do not facilitate the self-organization process.

The study of electrophysical characteristics and parameters of the prepared hybrid composites is very important for the future formation of solar cells on these hybrids. It was established that CVCs depend fundamentally on the properties of the substrate. There are two types of CVC behaviors: photosensitive and non-photosensitive. Examples of such behaviors are shown in Figure 6 and Figure 7, respectively. The differences in CVCs are shown in

Table 3. Light-to-dark ratio of organic–carbon hybrids with different organics on various substrates.

Substrate	Deposition Time
	10 min	20 min	40 min	60 min	90 min
Sulfacyl sodium
1	2.04	2.28	1.45	1.16	1.43
2	1.0	1.12	1.03	1.0	1.73
3	1.58	1.25	1.71	2.03	1.28
4	1.99	1.0	1.67	1.02	2.82
Clonidine
1	1.38	3.68	2.25	1.46	1.0
2	1.04	1.06	1.0	1.0	1.0
3	1.15	1.0	1.07	1.16	1.09
4	1.16	1.13	1.26	1.1	1.45
Vitamin B12
1	1.99	1.0	1.14	1.0	1.05
2	1.0	1.01	1.1	1.04	1.13
3	1.04	1.02	1.06	1.0	1.01
4	1.0	1.0	1.0	1.5	1.05

. Usually, CVCs for hybrids formed on substrate 1 are photosensitive, Figure 5a. Table 3 shows data on the ratio of light to dark currents at the voltage of 0.1 V.

Both sulfacyl sodium and clonidine have noticeable photosensitivity on substrate 1. The optimal deposition time of 20–40 min coincides with the time required for the formation of self-organized films on this substrate. At the same time, hybrids on substrate 2 do not show photosensitivity, Figure 7a. Hybrids on substrates 3 and 4 have intermediate values of photosensitivity and optimal values of the ratio of light to dark currents with the time of film formation in the range of 40 to 60 min. The least favorable situation is with vitamin B12. At the same time, the ratio of light and dark currents is significantly lower, but also reaches values of 1.5–2.0 at certain times of deposition on different substrates. In the case of this experimental work, the best substrate, in terms of electrical parameters, as well as in terms of morphological quality, is substrate 1. This corresponds to the efficiency of solar hybrids of these compounds on silicon [2].

Back to the composition of substrates, Table 2, magnetite is included only in substrate 2. Moreover, this substrate does not contribute to the self-organization of heterocyclic amines and their photosensitivity. In addition, substrate 1 contains activated carbon, and this substrate exhibits both the highest ability for the self-organization of heterocyclic amines and better electrical properties.

Analysis of dimensionless sensitivity of CVCs, presented in Figure 6b and Figure 7b, demonstrates Ohmic dependences of all illuminated curves, DS = 1.0. At the same time, some dark CVCs behave as injection characters with a maximum value DS = 1.5. This characterizes the structure as a potential p-type layer in a future solar cell structure. In a two-layer structure with n-type layers, the structures with larger light-to-dark ratios will be able to provide greater efficiency in converting solar energy into electrical. On the basis of CVCs, the possibilities of structures under investigation for the formation of two or more layered hybrids in future solar cells were analyzed.

It is necessary to note that the next step of studies will focus on clonidine-on-carbon-on-copper hybrid that shows photovoltaic properties; this will be the subject of some further research reports.

5. Conclusions

The deposition of heterocyclic amines, namely, sulfacyl sodium, clonidine, and vitamin B12, on carbon substrates functionalized by activated carbon, graphite KGPS-1, magnetite (BM), carbon nanotubes, and graphite DBX-010, has shown different abilities to self-organization. This deposition in a chemical bath from an aqueous solution can be an alternative approach for the formation of organic-on-carbon hybrids. The thin organic films demonstrated the possibility of self-organization on different carbon substrates. The sequence of carbon substrate activity involves the ability to self-organize these films due to the functionalization of carbon is 1 (activated carbon), 3 (carbon nanotubes), 4 (graphite DBX-010), 2 (magnetite). The analysis of morphological and electrical properties of organic-on-carbon hybrids allowed determining the optimal deposition times between 20 and 40 min with circular-type cells. The substitution of silicon substrates by carbon substrates can be the promising due to easy fabrication and a non-expensive process for the formation of solar cells.