Material Characteristics and Electrical Performance of Perovskite Solar Cells with Different Carbon-Based Electrodes Mixed with CuSCN

Perovskite solar cells are the most cutting-edge photovoltaic technology having high efciency and short fabrication time. In recent decades, there has been a signifcant rise in the study of the usage of carbon materials in perovskite solar cells because of low cost and earth abundance. Several studies have been conducted to mix hole transport materials with carbon materials to improve the hole extraction capability. Nevertheless, no research has reported using CuSCN on diferent carbon electrodes on perovskite solar cells. In this research, various carbon materials, including carbon nanotubes (CNT), graphite, activated carbon, and reduced graphene oxide (rGO), are mixed with CuSCN. Te carbon materials and CuSCN were mixed by ball mill and then deposited using the doctor blading method to become an electrode layer. Te existence of CuSCN in carbon materials was proved by conducting the energy dispersive X-ray test. CNT mixed with CuSCN material exhibits the highest electrical conductivity indicated by I D / I G ratio of 1.22 using Raman spectroscopy. Perovskite solar cell with a mix of CNTand CuSCN electrode exhibits the lowest series resistance of 76.69 Ω , resulting in the optimum solar cell performance such as a short-circuit current density ( J SC ) of 0.199mA/cm 2 , open-circuit voltage ( V OC ) of 0.52 V, fll-factor (FF) of 0.369, and efciency of 0.0735.


Introduction
Te development of photovoltaics began with the introduction of the frst generation of photovoltaics using silicon material founded by Bell Labs in 1954. Even though the efciency of silicon solar cells is relatively high at 25%, this solar cell production is relatively expensive and complex [1]. After that, the second generation of photovoltaics emerged, thin-flm solar cells. Tough thin-flm solar cells can be produced more efciently and have lower production costs, second-generation solar cells' efciency is not greater than that of the frst generation. Terefore, the third generation emerged, namely, perovskite solar cells, which meet the record efciency and short fabrication time [2].
Perovskite solar cells (PSCs) have an active layer made of perovskite. Perovskite materials have attracted a lot of attention because they can be implemented in several applications, such as solar cells [3], energy storage devices [4], and light-emitting diodes [5]. Perovskite is a material discovered by Russian mineralogist L. A. Perovski with an ABX 3 crystal structure. An organic cation, such as methylammonium (MA) and formamidinium (FA), is denoted by the letter A. B is a metal cation, such as lead (Pb 2+ ), tin (Sn 2+ ), and germanium (Ge2 + ). Te halide anion (X) includes iodine (I − ), bromine (Br − ), and chlorine (Cl − ) [6]. PSCs were frst discovered by Akihiro Kojima in 2009 with an efciency of 3.8% and rocketed to 25% in 2022 [7,8]. People are giving a lot of attention to perovskite solar cells because of their low-priced production, easy manufacturing, and high efciency [9].
Although the rise in the efciency value of perovskite solar cells is accelerating, some challenges must be addressed, such as stability and upscaling [10]. In recent decades, there has been a signifcant rise in the study of the usage of carbon materials in PSCs [11]. Carbon materials have the potential to be applied to PSCs because of their superior characteristics, namely, low cost and earth abundance [12,13]. Carbon materials can be inserted into various layers of perovskite solar cells, such as the electrode layer, hole transport layer (HTL), perovskite layer, and electron transport layer (ETL) [14].
Several reports have revealed that perovskite solar cells with carbon-based electrodes can produce good stability [15] and withstand moisture [16]. Ku et al. originally proposed using carbon materials in perovskite solar cells in 2013. Tey used the deposition printing technique to apply carbon material to the counter electrode of PSCs [17]. Along with the developing solar cell technology, perovskite solar cells with carbon counter electrodes are also known as carbonperovskite solar cells (C-PSCs) [18].
Eliminating HTL in perovskite solar cells can enhance stability, reduce costs, and simplify fabrication [19]. However, several unresolved issues exist because of the absence of HTL, such as bad surface contact between the electrode layer and the perovskite layer, inefective hole mobility, and a mismatch of energy level arrangements [20]. Terefore, mixing hole transport material (HTM) and carbon on perovskite solar cell electrodes is a potential efort to improve the hole extraction capability due to the shorter charge transport path [21]. Ball milling is a mixing technique that is ecofriendly and reliable [22]. Several studies have been conducted to mix carbon with HTM, such as adding CuS [23] and CuPc [24] in carbon black and graphite counter electrodes of PSCs. However, no research has reported the performance of perovskite solar cells when afected by the use of CuSCN on various carbon electrodes. CuSCN is used because it is an HTM having the highest conduction band energy level [25].
In this research, we investigate the efect of using various carbon materials, including carbon nanotubes (CNTs), graphite, activated carbon, and reduced graphene oxide mixed with CuSCN-based HTM at the counter electrode on the material characteristics and the performance of perovskite solar cells.

. Materials and Methods
2.1. Device Architecture. Te perovskite solar cell structure is illustrated in Figure 1. It uses the n-i-p architecture. FTOcoated glass (Sigma-Aldrich) is applied as a top contact of perovskite solar cells because of its good characteristics, including high transparency, low sheet resistance, and easy deposition [26]. We used TiO 2 (Sigma-Aldrich) as the electron transport material (ETM) because it has several excellent properties, such as a large band gap (3.7 eV), good optical transparency, good electron mobility, and good environmental stability [25].
For the active layer, chlorine-based perovskite material (CH 3 NH 3 PbCl 3 ) from Sigma-Aldrich is used due to its good properties, namely, the high light absorption coefcient, good electron transport characteristics, and good stability [25,27]. Te counter electrode consists of two materials, namely, HTM and carbon. For the HTM, we used CuSCN due to its outstanding characteristics, namely, high hole mobility, good chemical stability, broad band gap (3.8 eV), and high energy level of the conduction band [27,28]. For the carbon, we used activated carbon provided by Megah Abadi Chemical, graphite and carbon nanotubes (CNTs) from Sigma-Aldrich, and reduced graphene oxide (rGO), which was synthesized by using Hummer and chemical reduction methods. Lastly, the cell's rear contact is made of FTO-coated glass.
In perovskite solar cells, the mobility of electrons and holes explains how the device works. Te movement of electrons and holes can be described by using an energy level diagram. Figure 2 illustrates a representation of the energy levels of the manufactured perovskite solar cells. Te energy level of FTO-coated glass is −4.4 eV [29]. Te valence and conduction energy levels for TiO 2 are −8.0 eV and −4.3 eV, respectively [25]. Te energy levels of both valence and conduction bands of the perovskite layer are −5.43 eV and −3.75 eV [27]. Te proposed devices have a counter electrode made of CuSCN and carbon, which causes a 0.2 eV energy shift [24]. As a result, the electrode with a mixture of carbon and CuSCN has valence and conduction energy levels of −5.1 eV and −1.3 eV [20,25].

Reduced Graphene Oxide Synthesis.
Tis research synthesizes graphene material from graphite powder with Hummer and chemical reduction methods. Generally, oxidation and reduction are two essential processes in synthesizing reduced graphene oxide (rGO) [30,31].
Te oxidation process is carried out to create graphene oxide (GO) from graphite powder. First, 3 g of graphite powder and 1.5 g of NaNO 3 are dispersed in 69 ml of H 2 SO 4 and stirred while sitting on an ice bath with a solution temperature of 10°C. After that, 9 g of KMNO 4 were added while maintaining the temperature below 20°C. Once the mixture had been heated to 35°C and stirred for 30 minutes, 138 ml of water was added and maintained at 67°C for 15 minutes. Ten, the mixture was rinsed with 200 ml of HCl, distilled water, and ethanol, followed by a fltering process. After that, the mixture was then heated at a temperature of 70°C for 24 hours [32].
Te reduction process begins with dissolving 10 g of ascorbic acid and 0.25 g of GO into 100 ml of aquadest. Ten, the combination was heated at 95°C for 60 minutes and ultrasonicated for 900 seconds. After that, the mixture was twice rinsed in 200 ml of distilled water and ethanol, followed by a fltering process. Once the mixture had been fltered, it was then heated at a temperature of 70°C for 24 hours until reduced graphene oxide (rGO) was obtained [32].

Preparation of Precursors and Carbon
Paste. TiO 2 precursor, perovskite precursor, and carbon paste should be prepared before the fabrication process. In the process of making TiO 2 precursor, 0.3 ml of titanium (IV) isopropoxide (TTIP) was mixed with 0.1 ml of hydrochloric acid (HCl) and 5 ml of ethanol (C 2 H 6 O). After that, the precursor was stirred for 15 minutes at ambient temperature. Besides, the process of making perovskite precursor begins with mixing 240 mg of CH 3 NH 3 I powder, 140 mg of lead (II) chloride (PbCl 2 ) powder, and 1 ml of N, N-dimethylformamide (DMF) in a vial. Ten, it was stirred for 60 minutes at a temperature of 80°C with a rotating speed of 800 rpm. Materials used in TiO 2 and perovskite precursors are obtained from Sigma-Aldrich.
Te carbon paste comprises several materials, such as carbon powder, CuSCN, ethyl cellulose, and chlorobenzene. In this research, the carbon materials used are activated carbon derived from coconut shells, graphite, carbon nanotubes (CNTs), and reduced graphene oxide (rGO). Te frst step in making carbon paste is mixing 1 g of carbon material with 0.1 g of CuSCN [33] and 0.1 g of ethyl cellulose in a ball mill jar and stirring with a ball milling machine for 30 minutes. Te ball milling technique was used because it efectively prepares materials with a large surface area, such as carbon [34,35]. Subsequently, the mixed carbon powder was dissolved in 0.3 ml of C 6 H 5 Cl (chlorobenzene) until it became a carbon paste. Figure 3 explains the procedure of fabrication of carbon-based electrode perovskite solar cells. Before the perovskite solar cell fabrication process is carried out, there is a stage called precleaning. Precleaning is a stage that involves using an ultrasonic cleaner to wash FTO glass in distilled water, acetone, and ethanol for fve minutes. Te frst stage in the device fabrication process is TiO 2 deposition. On top of the cleaned FTO glass, a drop of a TiO 2 precursor was placed, and then it was spin coated for half a minute at a speed rate of 2000 rpm. After that, the deposited TiO 2 layer was annealed at a temperature of 425°C for half an hour.

Device Fabrication.
Te next stage is perovskite deposition by dripping 0.13 ml of perovskite precursor onto the FTO glass deposited with TiO 2 . Ten, it was spin coated for 30 seconds with a rotating speed of 1000 rpm. Subsequently, it was annealed at a temperature of 130°C for 15 minutes, as shown in Figure 4(a). Afterwards, the carbon paste was deposited on a cleaned FTO glass using the doctor blading method, often used to deposit carbon materials [25]. Ten, it was annealed at a temperature of 100°C for twenty minutes, as shown in Figure 4(b). Te fnal step is assembling two half cells by clips into a complete solar cell having an active area of 2.25 cm 2 .

Material Characterization and Photovoltaic Parameter
Measurement. Material characteristics of the carbon-based electrode, such as surface morphology, element composition, and element distribution, were measured by scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and EDX mapping using the ZEISS EVO MA 10 with a magnifcation of 1000. In addition, the carbon structure was investigated by Raman spectroscopy (Bruker). In order to determine the D and G bands' peak values, the Raman spectra were deconvoluted using the Gaussian function [36].
Te fabricated devices were measured by the HP 4145B semiconductor parameter analyzer in a dark condition to obtain the dark I-V curve. After that, they were placed outdoors and got direct sunlight. Te intensity of sunlight was measured using a solar power meter. A multimeter measured the generated current (I L ). Te dark I-V curve was subtracted by the I L using equation (1) to shift downward [27].
Te shifted dark I-V curve was then rotated on the xaxis, becoming a typical I-V curve. Te I-V curve can show the short circuit current (I SC ), open circuit voltage (V OC ), maximum power voltage (V MP ), and maximum power current (I MP ). Te fll factor (FF) was calculated using equation (2). Furthermore, the efciency was calculated using equation (3) [27]. Te resistance measurement was carried out using the PalmSens, EmStat4S. Te Randles equivalent circuit was used in the resistance measurement connecting the series resistance (R s ) in series and the transfer resistance (R trans ) as well as recombination resistance (R rec ) Journal of Electrical and Computer Engineering in parallel, as shown in Figure 5. Te R s , R trans , and R rec are values that represent the phenomenon of charge transport in carbon, perovskite, and electron transport layers.

Results and Discussion
3.1. Surface Morphology. Te surface morphology of each carbon material deposited on the FTO glass can be seen in Figures 6(a)-6(f ). Figure 6(a) shows the bare activated carbon, while Figure 6(b) represents the combined activated carbon and CuSCN. Both Figures 6(a) and 6(b) illustrate that electrodes made from activated carbon have particles that are random orientation and bulky [37,38]. Figure 6(c) describes that CNTs have a one-dimensional structure with a shape like a very long cylindrical tube and can cross one another [39]. Terefore, this structure can provide a highspeed transportation route, speeding up the charge transport process [40]. Figure 6(d) shows that graphite-based electrodes have a surface morphology like carbon sheets stacked on each other or often referred to as faky graphite [21]. Figure 6(e) illustrates the surface morphology of graphene oxide (GO), and Figure 6(f ) shows the surface morphology of reduced graphene oxide (rGO). Figures 6(e) and 6(f ) illustrate that the surface of the electrode made from graphene-derived carbon has a smoother and more even surface. Terefore, the contact surface area between the electrode layer and the electrode layer in contact becomes larger [12]. Table 1 shows the elemental composition data for each electrode with variations in the carbon material used. Te sample without CuSCN contains only 92.5% carbon and 7.5% oxygen and does not contain copper. On the contrary, the mixture of CuSCN and activated carbon has a carbon content of 91.87% and a copper content of 1.08%, indicating that the two materials' mixing process has been successfully carried out.

Element Composition.
Electrodes made from CNT have a carbon content of 93.63% and a copper content of 2.26%. Graphite electrodes have a percentage of 91.77% carbon and 2.66% copper. Te GO electrode has a carbon content of 63.8% and 19.57% oxygen. An electrode made of rGO has a carbon content of 65.86% and an oxygen content of  13.97%. An electrode made of rGO has a lower oxygen content than GO. Tis result indicates that the rGO synthesis process has succeeded in reducing the oxygen content. Te oxidation method is the Hummer method because it is the most commonly used, safest, and fastest method [41,42]. However, the Hummer method has a drawback; the ratio of carbon to oxygen content that can be produced is only 2.25 [43]. Terefore, the carbon content produced is not larger than 70%.   Cu on several sides of the carbon element. Tis result proves that the efort of mixing carbon and CuSCN materials on an electrode has been successfully carried out. Based on Figure 7(e), the electrode made from GO and CuSCN illustrates the larger amount of element Cu in the element carbon. On the other hand, Figure 7(f) describes that the electrode made from a mixture of rGO and CuSCN shows a more dominant carbon element content. Based on these diferences, it can be assumed that the reduction process during the rGO synthesis has succeeded in increasing carbon.

Carbon Structure Analysis.
Raman spectroscopy is one of the most common methods for investigating carbonaceous materials [36]. In this work, the Raman shift was measured between 0 cm −1 and 3500 cm −1 . Generally, two common peaks in the Raman spectrum are D-band and G-band. While the Dband is located at around 1300 cm −1 , the G-band is located at around 1590 cm −1 [44]. D-band refers to the disordered graphitic material. Tis corresponds to the defective graphitic structure of carbon materials. G-band is attributed to the C-C bond in graphitic materials [45]. Figure 8 shows the Raman spectrum of diferent carbon materials. Table 2 defnes the I D /I G ratio of diferent carbon materials. Te intensity ratio between D-band and G-band (I D /I G ) was often measured to estimate the structural defect and structural quality of carbon materials [46,47].
Te CNT and CuSCN sample shows the highest I D /I G ratio value of 1.22, indicating it has many structural defects but has the best electrical conductivity properties. Tis happens because the higher the I D /I G ratio value, the higher the density of the structural defect of its sample [47]. However, it is inversely correlated with the electrical conductivity [45]. It is followed by GO and rGO samples producing I D /I G ratio values of 0.96 and 1.19. Tis trend defnitely corresponds to the values reported by Suragtkhuu et al. [48]. Te minor increase in the I D /I G ratio value from GO to rGO indicates that the defects are introduced to the sp2 carbon during the reduction process. Terefore, the reduction of GO is successfully conducted [49].

Journal of Electrical and Computer Engineering
Graphite and CuSCN produce the lowest I D /I G ratio value of 0.94. Tis sample indicates the highest structural quality because the lower the I D /I G ratio value, the fewer the defects and the better the structural quality. However, it has the lowest electrical conductivity value.

Resistance Analysis.
In general, resistance analysis is used to analyze the charge transport phenomenon that occurs in perovskite solar cells. Te Randles circuit connects the series resistance (R s ) in series while the transfer resistance (R trans ) and recombination resistance (R rec ) are connected in parallel. Terefore, the analysis of R s is opposite to that of R trans and R rec .
Te R s , R trans , and R rec shown in Table 3 are values that represent the phenomenon of charge transport in carbon, perovskite, and electron transport layers, respectively [50,51]. Tere is no GO electrode because it is an insulator [52]. Terefore, it cannot be applied as an electrode. Table 3 explains that the smallest series resistance value of 76.69 Ω is obtained from the device with a CNT and CuSCN electrode. Ten, the device with an electrode made from pure activated carbon, a combination of CuSCN with activated carbon, graphite, and rGO, produced a series resistance of 283.4 Ω, 194.7 Ω, 633 Ω, and 50000 Ω, respectively. Perovskite solar cells with CNT and CuSCN-based electrodes produce the smallest R s indicating that the charge movement from the perovskite to the carbon layer occurs efectively. Tis happens because CNT has a very high conductivity level compared to conventional carbons such as graphite [39,53,54]. Compared to R trans and R rec , R s is the value afecting solar cell performance the most [55].
Perovskite solar cells with an electrode of pure activated carbon produced a R trans value of 131.6 Ω. Furthermore, perovskite solar cells with an electrode made from a combination of CuSCN with activated carbon, CNT, graphite, and rGO produced a R trans value of 82.48 Ω, 2322 Ω, 3415 Ω, and 0.000001 Ω, respectively. Te low R trans value indicates inefcient charge movement from the perovskite layer to the carbon layer [40]. Tis is because the perovskite layer's surface is of poor quality, which increases the probability that recombination will occur there [21].
PSCs with counter electrodes consisting of a mixture of CuSCN with CNT, graphite, and rGO produced R rec values with insignifcant diferences, namely, 1372 Ω, 1138 Ω, and 1491 Ω, respectively. On the other side, PSCs with an electrode made of pure activated carbon and a mixture of activated carbon with CuSCN resulted in a R rec of 1.09 Ω and 7.583 Ω. In contrast to the series resistance characteristics, the higher the R rec value of a PSC, the lower the recombination that occurs in the PSC [56]. Terefore, the probability of recombination in the ETL layer of PSCs with counter electrodes consisting of a combination of CuSCN with CNT, graphite, and rGO is relatively low. On the other hand, the probability of recombination occurring in the ETL of perovskite solar cells with an electrode made of pure activated carbon and a combination of CuSCN and activated carbon is high.
3.6. Device Performance. Table 4 explains the performance of perovskite solar cells with diferent electrode materials, such as short circuit current density (J SC ), open-circuit voltage (V OC ), and fll factor (FF), which were reprinted with permission from Barruna et al.'s study [57].
Te highest short-circuit current of 0.199 mA/cm 2 was obtained from a perovskite solar cell with a counter electrode consisting of CNT and CuSCN due to several factors such as a high conductivity value of MWCNT (35.6 S/cm) [21], low value of R s , and high value of both R trans and R rec . Perovskite solar cells with mixed electrodes of CuSCN with rGO, graphite, and activated carbon produced J SC of 0.019 mA/cm 2 , 0.070 mA/cm 2 , and 0.000004 mA/cm 2 , respectively. Te small J SC value is the result of the high value of Rs and the low values of both R trans and R rec, illustrating the high recombination rate in the carbon, perovskite, and ETL layers sequentially [56].
Te I-V curve of the fabricated perovskite solar cells with a variety of carbon materials is described in Figure 9. Te perovskite solar cell having an electrode made of CNT and CuSCN produced the largest squareness. As a result, this device shows the largest FF of 0.369. Furthermore, perovskite solar cells with CuSCN mixed electrodes with graphite and rGO resulted in FF values of 0.348 and 0.273, respectively. Te result in this research is supported by the research reported by Cai et al. stating that perovskite solar cells with an electrode made of MWCNT could produce the best FF value of 0.75 compared to carbon black and graphite electrodes showing FF values of 0.65 and 0.64 [58].
Based on the PCE values, the perovskite solar cell with a counter electrode made from a mixture of CNT and CuSCN produced the most signifcant efciency of 0.0735%. Besides, the solar cells with an electrode made of graphite and CuSCN shows a PCE of 0.0168%. Lastly, the solar cell with an electrode made of rGO and CuSCN produced an efciency of 0.0003%. Perovskite solar cell with an electrode made of CuSCN and CNT shows the highest efciency because the V OC and J SC values of this cell are higher than the solar cells with an electrode made of graphite and rGO.
Overall, a perovskite solar cell with a counter electrode made of a mixture between CNT and CuSCN illustrates the best fll factor and efciency. Tis result concurs with the

Conclusion
In conclusion, combining CuSCN-based HTM and several carbon materials such as activated carbon, graphite, CNT, and rGO on the electrode layer of a perovskite solar cell was demonstrated. CNT mixed with the CuSCN material exhibits the highest conductivity indicated by the I D /I G ratio value of 1.22. Based on the resistance measurement, the perovskite solar cell with a CNT-mixed CuSCN electrode has the lowest series resistance of 76.69 Ω, resulting in the optimum solar cell performance such as J SC of 0.199 mA/cm 2 , V OC of 0.52 V, FF of 0.369, and efciency of 0.0735%.

Data Availability
Data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that there are no conficts of interest.  Journal of Electrical and Computer Engineering 9