Superior incident photon-to-current conversion efficiency of Mo-doped activated carbon supported CdS-sensitized solar cells

In this study, the performance of activated carbon (AC) produced from defatted black cumin (Nigella sativa L.) by chemical activation with zinc chloride (ZnCl2) activator in photovoltaic application is evaluated. It is of great importance to increase the photovoltaic efficiency of cadmium sulfide (CdS)-based solar cells, which are widely used in photovoltaic applications, with AC support. The main purpose of the study is to determine how the incident photon-to-current conversion efficiency (IPCE) of undoped and Mo-doped AC supported CdS semiconductor materials changes in the presence of AC support material and to interpret the observed effect in the light of literature. For these reasons, in the study, AC supported CdS (CdS/AC) (5%, 10%, and 15% by weight) and Mo-doped CdS/AC semiconductor materials with different molybdenum (Mo) concentrations (0.33%, 1%, 3%) are produced by chemical precipitation method. Produced CdS/AC and Mo-doped CdS/AC semiconductor materials are characterized by IPCE, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) measurements. Based on the result values, the optimum CdS concentration with the highest IPCE (%) value is determined as 10% (for CdS/AC). As a result of the experimental measurements, the optimum Mo concentration with the maximum IPCE (%) value is found as 1% (for Mo-doped CdS/AC). In particular, it is clear that an appreciable increase (from 4.70 to 39.00%) in IPCE (%) of 1% Mo-doped 10% CdS/AC semiconductor material is achieved when compared to pure CdS. Thus, the ability to increase the photovoltaic efficiency of CdS-based solar cells, which are widely used in photovoltaic applications, with AC support has been clearly demonstrated. This study presents a new strategy to increase the solar cell efficiency of semiconductor-based solar cell structures using biowaste-based AC supported CdS semiconductor materials.


Introduction
The conversion of solar energy into usable electricity via a solid-state p-n junction-based photovoltaic (PV) device holds great opportunities in our quest to reduce our current dependence on fossil fuels and subsequently reduce harmful greenhouse gas emissions (Najm et al. 2021;Zhuk et al. 2017). Currently, global PV installations consist of monocrystalline silicon (c-Si), polycrystalline silicon (mc-Si), and thin-film technologies (Gulkowski et al. 2019). The real trend in solar cell technology has to do with multicomponent thin film materials, which are believed to replace expensive crystalline silicon cell technology. Research has focused on identifying and improving the optimum light absorber, buffer, and window layers (Islam and Thakur 2020;Vigil-Galán et al. 2015).

Responsible Editor: Philippe Garrigues
Cadmium sulfide (CdS) is an important II-VI compound semiconductor with high transparency, direct band gap transition (Eg ~ 2.4 eV), high electron affinity (4.2 eV), and n-type conductivity (Horoz and Sahin 2017;Rajendra and Kekuda 2012). CdS also improves lattice heterojunction interface coupling, extends excess carrier lifetime, and optimizes band alignment of devices in which it is used. CdS thin films are considered one of the most important materials for solar cell applications. It has a wide band gap with high optical transmittance and high electrical conductivity. For these reasons, it can be used as a window layer alongside other semiconductors such as CdS (Arun et al. 2022;Liu et al. 2020b;Venkatesh et al. 2020), CdTe (Bosio et al. 2020), CIGS (Ahmadpanah et al. 2021), and ZnO (Liu et al. 2020a;Tüzemen et al. 2021).
As a derivative of dye-sensitized solar cells, semiconductor-based solar cells attract extensive research due to their low cost-effectiveness and ease of manufacture. Compared with conventional organic dyes, semiconductor materials offer some attractive advantages as light absorbers, including high molar extinction coefficient, tunable bandgap, large intrinsic dipole moment, and hot electron injection. In particular, the multi-exciton generation effect of semiconductor materials pushes the theoretical maximum power conversion efficiency of semiconductor-based solar cells up to 44%, which exceeds the Shockley and Queisser limit for conventional semiconductor solar cells (Goodwin et al. 2018). So far, various semiconductor materials such as CdS (Lee et al. 2017), CdSe (Horoz et al. 2012), PbS (Huang and Zou 2015), and CdTe (Li et al. 2020) have been used as sensitizers to build semiconductor-based solar cells. However, due to the limited light absorption capacity or incomplete charge separation, all power conversion efficiencies are much lower than the maximum theoretical value.
Activated carbon (AC) is a promising carbonaceous material with superior properties such as high porosity and internal surface area (Batur and Kutluay 2022;İzgi et al. 2019;Kutluay et al. 2019;Şahin et al. 2021). Chemical and physical activation methods of AC allow materials to produce a controlled pore distribution and surface area that defines the electrode/electrolyte interface for photovoltaic applications (solar cells) (Arbab et al. 2016;Vispute et al. 2011). AC is incorporated into the electrodes of photovoltaic devices as follows: electroconductive additives, supports for active materials, electron transfer catalysts, intercalation hosts, and substrates for current leads. For these reasons, AC is of course also very suitable as electrode materials for solar cells (Mehmood et al. 2016).
In a CdS-based solar cell, CdS semiconductor materials play a very important role as sensitizers. It is of great importance to increase the photovoltaic efficiency of CdSbased solar cells, which are widely used in photovoltaic applications, with AC support (Shao et al. 2019). Also, AC is included in the electrodes of photovoltaic devices as electro-conductive additives, support for active materials. For these reasons, AC is used to improve efficiency in solar cells. In a related study, it was observed that the efficiency of the solar cell designed in the absence of AC was 3.38%, while the efficiency of the same solar cell with AC support was 5.45%. This result shows that the use of AC as a support material in photovoltaic applications has a great impact (Mehmood et al. 2016).
To the authors' knowledge, there is no known production of Mo-doped AC (defatted black cumin (Nigella sativa L.) waste biomass-based) supported CdS semiconductor materials, and this presents an innovation aspect of our proposed study. Here, instead of undoped-doped CdS, which is frequently used in the literature, AC-doped CdS (CdS/AC) (5%, 10%, and 15% by weight) and molybdenum (Mo)-doped CdS/AC semiconductor materials with different concentrations (0.33%, 1%, and 3% by weight) are produced. The main purpose of the study is to determine how the incident photon-to-current conversion efficiency (IPCE) of undoped and Mo-doped AC supported CdS semiconductor-based solar cell structures change and to interpret the observed effect in the light of the literature.

Raw material and chemicals
Defatted black cumin (Nigella sativa L.) waste (DBCW), which is used as a precursor (raw material) in activated carbon (AC) production, was obtained from a local black cumin oil mill in Siirt province. DBCW samples were first dried in an oven at 80 °C for 48 h. The dried DBCW samples were sieved to a particle size range of 500-850 µm after grinding. Later, these DBCW samples were used as lead material in AC production. Zinc chloride (ZnCl 2 ) used in AC production was obtained from Shanghai Sinopharm Chemical Reagent Company. Sodium hydroxide (NaOH), ethanol (C 2 H 5 OH), hydrogen chloride (HCl), sodium thiosulfate pentahydrate (Na 2 S 2 O 3 ·5H 2 O), and potassium iodide (KI) chemicals were purchased from the Scharlau Company. Cadmium acetate dihydrate (Cd(CH 3 COO) 2 .2H 2 O) as a source of cadmium (Cd), sodium sulfide (Na 2 S) as a source of sulfur (S), and ammonium molybdenum tetrahydrate ((NH 4 ) 6 Mo 7 O 24 .4H 2 O) as a source of molybdenum (Mo) were obtained from Sigma-Aldrich. Solvents in the study were prepared using deionized water. All chemicals/reagents used in this study were used without purification in the experiments as they were of standard analytical purity.

Production of AC
In AC production, approximately 3 g of dried DBCW was mixed with ZnCl 2 activator at room temperature at an impregnation rate of 105% (DBCW/ZnCl 2 , mass ratio), and this mixture was allowed to soak for 1 min (impregnation time). Then, the impregnated sample was placed in a heatresistant container and subjected to carbonization and activation process under N 2 flow for 60 min in a horizontal tubetype high-temperature reactor at a heating rate of 10 °C/ min from room temperature to the desired final temperature (550 °C). The product obtained as a result of this process was cooled to room temperature under N 2 atmosphere in the reactor. This product was then washed first with 0.5 M HCl, then repeatedly washed with C 2 H 5 OH and deionized hot water until the pH of the rinse water was neutral, and finally dried at 80 °C for 24 h. The resulting AC was then stored in an airtight container to be used as a support material.

Production of CdS
In the CdS production process, 0.05 M Cd(CH 3 COO) 2 .2H 2 O (0.67 g) and 0.05 M Na 2 S (0.19 g) were dissolved in two clean beakers containing 50 mL of distilled water using chemical precipitation technique. Then, these two solutions were transferred to a different beaker and mixed with a magnetic stirrer for about 1 h. At the end of this period, the mixing process was stopped and the solution obtained was allowed to settle to the bottom, and then the precipitated sample was separated from the solvent with the filter paper. After this process, the obtained sample was washed several times with distilled water and C 2 H 5 OH to remove unwanted components. After washing, the wet precipitate was dried in an oven at 80 °C for 10 h. CdS sample obtained after drying was ground in a mortar to produce CdS semiconductor material.

Production of AC supported CdS
Different ratios such as 5%, 10%, and 15% CdS/AC were used in the production of AC supported CdS (CdS/AC). For this purpose, in the CdS/AC production process, 0.05 M Cd(CH 3 COO) 2 .2H 2 O and 0.05 M Na 2 S were dissolved in two clean beakers containing 50 mL of distilled water using chemical precipitation technique. Then, these two solutions were transferred to a different beaker containing the calculated amount (3.24 g) of AC for different ratios (5%, 10%, and 15% CdS/AC) and mixed with a magnetic stirrer for about 1 h. At the end of this period, the mixing process was stopped and the solution obtained was allowed to settle to the bottom, and then the precipitated sample was separated from the solvent with the filter paper. After this process, the obtained sample was washed several times with distilled water and C 2 H 5 OH to remove unwanted components. After washing, the wet precipitate was dried in an oven at 80 °C for 10 h. CdS/AC sample obtained after drying was ground in a mortar to produce CdS/AC semiconductor material. This process was also carried out separately for the synthesis of 5%, 10%, and 15% CdS/AC semiconductor materials.

Design of undoped and Mo-doped AC supported CdS semiconductor material-based solar cells
How the Zn 2 SnO 4 (ZTO) oxide semiconductor, which is used as a photoanode in a semiconductor-based solar cell, is obtained and how it is coated on F-doped SnO 2 conductive glasses are explained in detail below. Nano-sized ZTO was obtained in powder form by the solvothermal method. In the solvothermal method, zinc acetate (ZnC 4 H 6 O 4 ) and tin chloride (SnCl 2 ) were placed in a certain amount of C 2 H 5 OH in accordance with the stoichiometer and mixed with a magnetic stirrer. A certain amount of NaOH aqueous solution was added to the resulting mixture and mixed until a homogeneous mixture was obtained (average 30 min). The homogeneous mixture was transferred into the autoclave and ZTO was obtained in powder form by heating in an oven at a certain temperature. ZTO paste was obtained by mixing ZTO in powder form with a small amount of C 2 H 5 OH. A certain amount of ZTO paste was coated on F-doped SnO 2 conductive glasses with the doctor blade technique so that it spreads evenly. The obtained samples were annealed at high temperature in the furnace to obtain the desired substrates. After the desired ZTO anodes were obtained, these ZTO anodes were immersed in the produced undoped and Mo-doped CdS/AC semiconductor materials and left for 24 h. Thus, a good coating of the semiconductors on the ZTO was achieved.
Cu 2 S counter electrode, another important part of the solar cell, was obtained by following the steps given below.
Cu 2 S counter (counter) electrodes were obtained on F-doped SnO 2 conductive glasses by doctor blade method. In short, 0.5 M copper (II) nitrate (Cu(NO 3 ) 2 ) chemical was dissolved in 100 µl (µL) methanol (CH 3 OH) and some of the solution obtained was dropped on the F-doped SnO 2 conductive glass. After the dripping process, the Cu(NO 3 ) 2 solution was homogeneously distributed on the conductive glass by the doctor knife method. Then, 1 M sodium sulfide (Na 2 SO 3 ) was mixed in distilled water and CH 3 OH solvent 1:1 by volume, and a few drops of the obtained solution were dropped onto the conductive glass containing Cu(NO 3 ) 2 . Thus, the color of the conductive glass changed from blue to brown. The conductive glass was washed with pure water and dried in nitrogen environment. This was applied a second time by repeating the abovementioned processes in the same way, and the resulting film was calcined at 450 °C for 30 min in an ash oven and then left to cool. Thus, the Cu 2 S counter (counter) electrode was obtained. These counter electrodes were obtained for each sample produced. Thus, semiconductor-based solar cells were designed by connecting the photoanodes (ZTO) and Cu 2 S counter (counter) electrodes with a coupler clamp. While taking IPCE measurements, 10 µL of polysulfide electrolyte solution, the synthesis method of which is given below, was dropped between the photoanode/electrode obtained in order to ensure electron transfer between the photoanode and the electrode. In the synthesis of a polysulfide electrolyte, it was obtained by mixing the chemicals 5 M Na 2 SO 3 , 2 M sulfur (S), and 0.2 M potassium chloride (KCl) in a solvent consisting of distilled water and CH 3 OH in a 1:1 volumetric ratio.
In order to calculate both the power conversion efficiency of the produced undoped and Mo-doped CdS/AC semiconductor materials and to examine their photovoltaic properties, IPCE measurements were made for the semiconductorbased solar cells designed as a result of the abovementioned processes, and the data were interpreted using the curves obtained.

Characterizations
Semiconductor materials produced within the scope of the study were characterized by IPCE, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) measurements. At this stage, first of all, photovoltaic properties of pure CdS, CdS/AC, and Mo-doped CdS/AC semiconductor materials were determined by IPCE measurements. Then, the morphological and structural properties of CdS/AC and Mo-doped CdS/AC semiconductor materials, which provide the best efficiency, were investigated by making SEM and XRD measurements, respectively. The basic element components of the produced semiconductor materials were determined by EDX (Bruker, Quantax ED-XS, Germany) analysis. XPS spectra of these materials were obtained using a monochrome Al-Ka X-ray source (1486.71 eV) and a SPECS-Flex mod XPS device.

Evaluation of the performance of AC in CdS semiconductor material-based solar cells
Within the scope of the study, the solar cell efficiency of CdS, CdS/AC, and Mo-doped CdS/AC semiconductor-based solar cell structures was investigated. For this purpose, CdS (CdS/AC) and Mo-doped CdS/AC materials produced by chemical precipitation method on AC were reduced. CdS, CdS/AC, and Mo-doped CdS/AC semiconductor materials obtained as a result of these processes were characterized by IPCE, SEM, XRD, EDX, and XPS measurements.

IPCE measurements
Here, first of all, IPCE measurements were carried out to examine the solar cell efficiency of semiconductor-based solar cell structures obtained by using CdS/AC semiconductors with different CdS concentrations such as 5%, 10%, and 15% and to determine the optimum CdS concentration that gives the best efficiency. The IPCE spectra of CdS/ AC semiconductor-based solar cell structures with different CdS concentrations obtained as a result of the measurements are shown in Fig. 1. According to Fig. 1 Table 1. Based on the values given in Table 1, the optimum CdS concentration with the best IPCE (%) value or the best solar cell efficiency was determined as 10%. Based on the values given in Table 1, two observations are important. First, there is a notable increase in the IPCE (%) efficiency of the CdS semiconductor-based solar cell structure in the presence of AC. The main reason for this is that AC increases electron mobility in the solar cell structure due to its good electronic properties. Another important observation is the decrease in IPCE (%) at high CdS concentrations. Defects at high concentrations reduce solar cell efficiency by allowing light to generate heat instead of electricity, providing new recombination pathways (loss). Furthermore, the defects cause deep energy levels in the semiconductor band gap, which reduces the carrier lifetime and quantum efficiency of solar cells Kutluay et al. 2021;Lin et al. 2017;Yan et al. 2019).

, IPCE (%) values of CdS/AC semiconductor-based solar cell structures
After determining the best CdS concentration value, 10% Cd/AC semiconductor material was doped with Mo element and the effect of doping on the solar cell structure was investigated. In Fig. 2 Table 2. As can be clearly seen in Table 2, Mo-doped 10% CdS/AC semiconductor-based solar cell structure plays an important role in increasing the IPCE (%) value.
Doping with optically active metals like La, Mn, Mo, and Cu can change the electrical and photophysical characteristics of nanoparticles. It is feasible to tailor the electrical and optical characteristics of semiconductor nanocrystals by altering the kind and concentration of the dopant. Furthermore, the dopant alters the charge separation and recombination dynamics by creating electronic states in the quantum dot's midgap region. In the study by Bouich et al. (2020), CdZnS/CdS/CIGS/Mo material was used as an absorber layer in a solar device. They reported that the device performance was measured under AM 1.5 global spectrum for 1000 W/m 2 irradiance, where the obtained efficiencies were in the range of 2-3%. Open-circuit voltage was 345 mV, short circuit current was 16.8 mA/cm 2 , and the fill factor was 38.18%.
One of the most important advantages of the Mo dopant metal is that it forms a highly porous structure in the semiconductor material. Thanks to the porosity of the semiconductor material, it absorbs more light, thus causing a remarkable increase in the efficiency of the solar cell structure (Mehmood et al. 2017). However, the defects formed despite the porous structure in the presence of high Mo dopant metal caused a decrease in IPCE (%). Thus, as a result of the experimental measurement, the Mo concentration with the best IPCE (%) value was determined as 1%. In particular, it is clear that an appreciable increase in IPCE (%) of 10% CdS/AC semiconductor material with 1% Modoped is achieved when compared with pure CdS and 10% CdS/AC materials.

SEM analysis
SEM images of AC, 10% CdS/AC, and Mo (1%)-doped 10% CdS/AC are presented in Fig. 3. As seen in Fig. 3, CdS and Mo-doped CdS materials are homogeneously dispersed on the AC surface. When SEM images of CdS/AC    ), AC supported CdS particle size varies in the range of 10-30 nm and CdS weaves AC pores like a flower; ın other words, it was stated that AC caused the formation of pores on its surface. Moreover, it can be said that CdS and Mo form a homogeneous microporous structure on the surface by settling in the mesopores.

XRD analysis
The XRD diffraction patterns of AC, CdS, 10% CdS/AC, and Mo (1%)-doped 10% CdS/AC materials are shown in Fig. 4. As seen here, pure CdS semiconductor material has a cubic structure, while AC supported undoped and Mo-doped CdS semiconductor materials exhibit a hexagonal structure. The main reason for this situation is that AC, which has a porous structure, plays an active role in changing the structure of the CdS semiconductor material. Another remarkable observation is that the structure of the AC supported CdS semiconductor material does not change, although the AC supported CdS semiconductor material is doped with Mo metal. This result can be accepted as an indication that the production process has been carried out successfully, as it shows that no oxidation or an undesirable compound is formed in the production process. With the help of XRD diffraction patterns, the crystallite sizes of the CdS, CdS/AC, and Mo-doped CdS/AC semiconductor materials produced using the Debye-Scherrer formula shown in Eq.
(1) were calculated as 11.09 nm. It was observed in the calculations that Mo did not change the crystallite size of CdS since the Mo doping concentration was low. Moreover, this numerical Here, D is the crystallite size, k is the Scherrer constant, is the wavelength of the X-ray, is the peak half-width (FWHM) in radians, and B is the Bragg angle of the peak.

EDX analysis
Elemental properties of 10% CdS/AC and Mo (1%)-doped 10% CdS/AC materials were investigated by performing EDX measurements. The EDX spectra of CdS/AC and Modoped CdS/AC semiconductor materials are shown in Fig. 5 and Fig. 6, respectively. In these findings, the detection of the presence of C, Cd, S, and Mo metals is an indication of the successful production of undoped and Mo-doped ACsupported CdS semiconductor materials.

XPS analysis
XPS measurements were performed to evaluate the bonding energies and surface chemical compositions of AC, CdS, 10% CdS/AC, and Mo (1%)-doped 10% CdS/AC materials in more detail. For this purpose, the XPS spectra of the C 1 s, Cd 3d, S 2p, and Mo 3d regions obtained for the mentioned semiconductor materials are shown in Fig. 7. The peaks observed at 282.01, 281.92, and 282.13 eV for AC, CdS/AC, and Mo-doped CdS/AC semiconductor materials (Fig. 7a) show the XPS spectrum of the 1 s region of C (Chen et al. 2020;Kutluay 2021).
For CdS, CdS/AC, and Mo-doped CdS/AC semiconductor materials, it was observed that two corresponding binding energies of Cd 3d 5/2 and Cd 3d 3/2 were found at 405.38, 404.36, and 403.67 eV and 412.08, 411.14, and 410.55 eV, respectively (Fig. 7b). The cleavage energy between the two 3d orbitals of about 6.70 eV observed for these semiconductor materials is indicative of the Cd 2+ ion (Fan et al. 2012).  The XPS spectra in Fig. 7c show the binding energies of the S 2p sites in CdS, CdS/AC, and Mo-doped CdS/AC semiconductor materials at 161.51, 160.89, and 160.16 eV, respectively. These results are in good agreement with the reference values for the S 2− ion (Fantauzzi et al. 2015).
The XPS spectrum of the Mo 3d region given in Fig. 7d for the Mo-doped CdS/AC semiconductor material is attributed to the binding energies of Mo 3d 3/2 and Mo 3d 5/2 at 235.15 and 229.30 eV, respectively (Singer et al. 1996).
Based on the comparison of XPS analysis findings with the literature, it was concluded that CdS, CdS/AC, and Modoped CdS/AC semiconductor materials were successfully produced.

Conclusions
In the current study, CdS/AC and Mo-doped CdS/AC semiconductor materials were produced by using AC as a support material instead of the undoped-doped CdS semiconductor materials, which are frequently used in the literature. The produced CdS/AC and Mo-doped CdS/AC semiconductor materials were characterized by IPCE, SEM, XRD, EDX, and XPS measurements. When the SEM images of AC, CdS/AC, and Mo-doped CdS/AC materials are compared, it can be said that CdS and Mo form a homogeneous microporous structure on the surface by settling in the mesopores. The crystallite size of the Mo-doped CdS/AC semiconductor material was calculated as 11.09 nm. This numerical data can be considered an indication that the semiconductor material is produced at nanoscale. Based on the comparison of EDX and XPS analysis findings with the literature, it was concluded that CdS, CdS/AC, and Mo-doped CdS/AC semiconductor materials were successfully produced. Also, based on the result values, the optimum CdS concentration with the highest IPCE (%) value or the highest solar cell efficiency was determined as 10% (for CdS/AC). As a result of the experimental measurements, the Mo concentration with the maximum IPCE (%) value was found as 1% (for Modoped CdS/AC). In particular, it is clear that a remarkable increase (from 4.70 to 39.00%) in the IPCE (%) value of 1% Mo-doped 10% CdS/AC semiconductor material is achieved when compared to pure CdS. Thus, the ability to increase the photovoltaic efficiency of CdS-based solar cells, which are widely used in photovoltaic applications, with AC support has been clearly demonstrated. In addition, this study presents a new strategy to increase the solar cell efficiency of semiconductor-based solar cell structures using biowaste-based AC supported CdS semiconductor materials. Fig. 7 XPS spectra of C 1 s (a), Cd 3d (b), S 2p (c), and Mo 3d (d) regions of AC (C 1 s peak), CdS/AC (C 1 s, Cd 3d, and S 2p peaks), and Mo-doped CdS/AC (C 1 s, Cd 3d, S 2p, and Mo 3d peaks) materials Author contribution Ömer Şahin: supervision and review. Ebru Batur: conceptualization, investigation, and visualization. Sinan Kutluay: conceptualization, investigation, visualization, writing-review and editing. Sabit Horoz: review and editing. Orhan Baytar: conceptualization, investigation, visualization, writing-review and editing.
Funding This work was supported by Siirt University Scientific Research Projects Coordination Unit under Project Number 2020-SİÜFEB-019.
Data availability Not applicable.

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Competing interests The authors declare no competing interests.