Chemical Bath Deposition Of CdS Research Articles

Optical transmission and photoconductivity of chemical bath-deposited CdS thin films for optoelectronic applications

This paper presents a comprehensive study of the optical transmission, photoconductivity, and morphology of CdS thin films deposited via the chemical bath deposition (CBD) method. The films deposited for 60 min exhibit an optical energy band gap value of 2.42 eV and the highest optical transmission of 75 % in the wavelength range of 500–900 nm. For these films, the transmission electron microscopy imaging shows a distribution of particle sizes around 10 nm. The wavelength dependence of the photoconductivity, extracted from photo resistivity data, shows that the maximum photoconductivity occurs at 492 nm wavelength. This corresponds to an electrical energy band gap of 2.52 eV, which is greater than the optically measured energy gap of 2.42 eV. Based on these results, the electron-hole pair binding energy for CdS films is estimated as 100 meV. According to the electrical resistivity vs. temperature measurements, the activation energy is 1.26 eV which agrees with the electrical energy band gap of 2.52 eV. CdS films annealed under nitrogen gas at 200 °C for one hour exhibited an energy gap of 2.32 eV. These films displayed a conductivity of 60 × 10−4 Scm−1, a carrier concentration of 6.38 × 1014 cm−3, and a mobility of 7.46 cm2 V−1 s−1. These impressive characteristics suggest the suitability of CBD-grown CdS films annealed at 200 °C in nitrogen gas, to be used as the window material in CdS/CdTe thin film solar cells and other optoelectronic applications.

Rear Surface Passivation for Ink‐Based, Submicron CuIn(S, Se)2 Solar Cells

AbstractA N, N‐dimethylformamide and thiourea‐based route is developed to fabricate submicron (0.55 and 0.75 µm) thick CuIn(S,Se)2 (CISSe) thin films for photovoltaic applications, addressing challenges of material usage, throughput, and manufacturing costs. However, reducing the absorber film thickness below 1 µm in a regular CISSe solar cell decreases the device efficiency due to losses at the highly‐recombinative, and mediocre‐reflective Mo/CISSe rear interface. For the first time, to mitigate the rear recombination losses, a novel rear contacting structure involving a surface passivation layer and point contact openings is developed for solution processed CISSe films and demonstrated in tangible devices. An atomic layer deposited Al2O3 film is employed to passivate the Mo/CISSe rear surface while precipitates formed via chemical bath deposition of CdS are used to generate nanosized point openings. Consequently, Al2O3 passivated CISSe solar cells show an increase in the open‐circuit voltage (VOC) and short‐circuit current density when compared to reference cells with equivalent absorber thicknesses. Notably, a VOC increase of 59 mV contributes to active area efficiencies of 14.2% for rear passivated devices with 0.75 µm thick absorber layers, the highest reported value for submicron‐based solution processed, low bandgap CISSe solar cells.

12.84% Efficiency Flexible Kesterite Solar Cells by Heterojunction Interface Regulation

AbstractDue to their environmental friendliness, low cost, and versatility in applications, flexible Cu2ZnSn(S, Se)4 (CZTSSe) solar cells have garnered significant attention in recent years. However, the lack of alkali metal elements in the flexible substrate has posed a challenge to the advancement of flexible CZTSSe solar cells. In this study, a post‐treatment strategy is proposed involving Rb ions to simultaneously regulate the surface properties of the CZTSSe film and the reactions during the CdS chemical bath deposition (CBD) process. Material and chemical characterization results confirm that Rb ions effectively passivate the detrimental Se0 defect, leading to a more active surface for the CdS epitaxial growth. Additionally, the coordination between Rb ions and thiourea in the CBD solution improves the ion‐by‐ion deposition of the CdS layer. These beneficial effects significantly improve the heterojunction interface and consequently enable the flexible CZTSSe solar cell to achieve a record total‐area efficiency of 12.84% and excellent bending performance. Overall, the heterojunction interface regulation strategy employed in this work, along with the obtained remarkable results, offer promising prospects for the development of flexible CZTSSe solar cells.

Visible Light Reductive Photocatalysis of Azo-Dyes with n-n Junctions Based on Chemically Deposited CdS.

New composite photocatalysts have been obtained by chemical bath deposition of CdS on top of either nanostructured crystalline ZrO2 or TiO2 films previously deposited on conductive glass FTO. Their morphological, photoelectrochemical and photochemical properties have been investigated and compared. Time resolved spectroscopic, techniques show that in FTO/TiO2/CdS films the radiative recombination of charges, separated by visible illumination of CdS, is faster than in FTO/ZrO2/CdS, evidencing that carrier dynamics in the two systems is different. Photoelectrochemical investigation evidence a suppression of electron collection in ZrO2/CdS network, whereas electron injection from CdS to TiO2 is very efficient since trap states of TiO2 act as a reservoir for long lived electrons storage. This ability of FTO/TiO2/CdS films is used in the reductive cleavage of N=N bonds of some azo-dyes by visible light irradiation, with formation and accumulation of reduced aminic intermediates, identified by ESI-MS analysis. Needed protons are provided by sodium formate, a good hole scavenger that leaves no residue upon oxidation. FTO/TiO2/CdS has an approximately 100 meV driving force larger than FTO/ZrO2/CdS under illumination for azo-dye reduction and it is always about 10% more active than the seconds. The films showed very high stability and recyclability, ease of handling and recovering.

Effect of annealing temperature on p–n junction formation in Cu2SnS3 thin-film solar cells fabricated via the co-evaporation of elemental precursors

Cu2SnS3 (CTS) thin-film solar cells were fabricated by the co-evaporation of the precursors, and the effect of annealing in N2 atmosphere on their photovoltaic properties was investigated by varying the annealing temperature after the chemical bath deposition of CdS. The characteristics of the solar cells improved as the annealing temperature was increased in the 250 °C–275 °C range (annealing time: 30 min). However, annealing temperatures exceeding 275 °C caused the deterioration of the device characteristics. Therefore, annealing in the 250 °C–275 °C range after CdS deposition is important for forming an optimum p–n junction at the CTS/CdS interface for manufacturing the CTS solar cells evaluated in this study. The best-performing solar cell fabricated using a CTS film annealed at 275 °C after CdS deposition exhibited an open circuit voltage of 0.181 V, with a short circuit current density of 20.9 mA cm−2, fill factor of 0.462, and power conversion efficiency of 1.74%.

Efficiency enhancement of Sb2Se3 solar cells based on electron beam evaporation CdS film with variable deposition temperature

In our previous work, the detailed crystal structure, optical and electrical properties of evaporated CdS (hexagonal CdS) and chemical bath deposition CdS (cubic CdS) were discussed. The hexagonal CdS film is more suitable as the buffer layer for Sb2Se3 film solar cells compared with its cubic counterpart. However, in our experiments, the influence of high annealing temperature on hexagonal CdS is difficult to control. Moreover, hexagonal CdS is unsuitable for large-area industrial production of Sb2Se3 solar cells. In this study, an electron beam evaporation-prepared CdS buffer layer (hexagonal CdS) with high quality, uniformity and low-temperature treatment was introduced in Sb2Se3 thin film solar cells. The influence of different substrate temperatures during electron beam evaporation on the crystal structure, surface morphologies, and optical properties of CdS and Sb2Se3 films was investigated. The device performance improvement from 3.52% to 5.89% was obtained after the surface roughness of the CdS film and the heterojunction interface were optimised. Lastly, the efficiency of the Sb2Se3 solar cells was obtained to be 6.71% based on the structure of FTO/EB-CdS/Sb2Se3/MoO3/Au.

Growth and Characterization of Seed-Assisted, EDTA-Treated, Chemical Bath-Deposited CdS

Growth and Characterization of Seed-Assisted, EDTA-Treated, Chemical Bath-Deposited CdS

Electrically activated chemical bath deposition of CdS on carbon nanotube arrays

Electrically stimulated depositions of CdS nanoparticles on the surface of aligned multi-walled carbon nanotube (CNT) arrays were investigated by applying a voltage of 3 V to the electrodes placed into a chemical bath with an aqueous ammonium solution of CdCl2 and thiourea. The CNT arrays were fixed on positive, negative, and neutral electrodes. Morphology, composition and structure of CdS-CNT hybrids were studied using scanning electron microscopy, Raman scattering and X-ray photoelectron spectroscopy. CdS nanocrystallites in the hybrids were found to exhibit synthesis-dependent luminescence properties. S-vacancy enriched CdS grown on the positively charged CNT array possessed red photoluminescence (PL). Purple emission was attributed to small-size CdS nanocrystals synthesized on the negatively charged CNTs. The CNT array located on a neutral electrode was uniformly covered by CdS nanoparticles with a highly intense blue PL. Our results indicate a promising effect of the electrical field for enhancement of the growth rate of CdS nanocrystals.

Front passivation of Cu(In,Ga)Se2 solar cells using Al2O3: Culprits and benefits

In the past years, the strategies used to break the Cu(In,Ga)Se2 (CIGS) light to power conversion efficiency world record value were based on improvements of the absorber optoelectronic and crystalline properties, mainly using complex post-deposition treatments. To reach even higher efficiency values, further advances in the solar cell architecture are needed, in particular, with respect to the CIGS interfaces. In this study, we evaluate the structural, morphological and optoelectronic impact of an Al2O3 layer as a potential front passivation layer on the CIGS properties, as well as an Al2O3 tunneling layer between CIGS and CdS. Morphological and structural analyses reveal that the use of Al2O3 alone is not detrimental to CIGS, although it does not resist to the CdS chemical bath deposition. The CIGS optoelectronic properties degrade when the CdS is deposited on top of Al2O3. Nonetheless, when Al2O3 is used alone, the optoelectronic measurements reveal a positive impact of this inclusion such as a very low concentration of interface defects while keeping the same CIGS recombination channels. Thus, we suggest that an Al2O3 front passivation layer can be successfully used with alternative buffer layers. Depth-resolved microscopic analysis of the CIGS interface with slow-muons strongly suggests for the first time that low-energy muon spin spectroscopy (LE-µSR) is sensitive to both charge carrier separation and bulk recombination in complex semiconductors. The demonstration that Al2O3 has the potential to be used as a front passivation layer is of significant importance, considering that Al2O3 has been widely studied as rear interface passivation material.

The effect of growth technique on the characteristic properties of CdS layers for solar cell applications

The effects of two different growth methods including electrodeposition (ED) (two-electrode configuration) and chemical bath deposition (CBD) on the characteristic properties of CdS thin-films were explored and reported. The electrodeposited CdS (ED-CdS) layers were grown on glass/fluorine-doped tin oxide (FTO) substrates using acidic and aqueous solution containing 0.3 M of thiourea (SC(NH2)2) and 0.2 M of cadmium chloride hydrate (CdCl2.xH2O). The chemical bath deposited CdS (CBD-CdS) layers were also grown on glass/FTO substrates using alkaline and aqueous solution containing 4 × 10–3 M of cadmium acetate dihydrate (Cd(CH3CO2)2.2H2O), 20 × 10–3 M of thiourea (TU) and 6 × 10–2 M ammonium acetate (NH4C2H3O2). The deposited CdS thin-films were characterised using X-ray diffraction (XRD), UV–Visible spectrophotometer (UV–Vis), scanning electron microscopy (SEM), and photoelectrochemical (PEC) cell measurement to study their structural, optical, morphological, and electrical properties, respectively. The structural study shows the polycrystalline nature of the ED-CdS and CBD-CdS thin-films with stable hexagonal phase after heat treatment. The preferred orientation for both ED-CdS and CBD-CdS layers was along (002) hexagonal plane. The average crystallite size of CdS thin-films grown by both deposition methods were in the range ~ (11–38) nm and ~ (22–53) nm before and after heat treatment, respectively. Optical studies reveal the direct bandgap value of 2.42 eV for the heat-treated ED-CdS and CBD-CdS layers which correspond to the bulk bandgap of CdS (hexagonal phase). Morphological studies depict the average grain sizes in the range ~ (90–260) nm for the CdS thin-films. The PEC cell measurements show that the CdS layers grown by both ED and CBD methods were n-type in electrical conduction before and after heat treatment. No visible precipitations of elemental S or CdS particles were observed in the deposition electrolyte of ED-CdS showing a stable bath using TU precursor during the growth. The solar cells fabricated using CBD-CdS showed better performance as compared to the devices fabricated using ED-CdS due to the uniform coverage of FTO surface and better fill factor (FF).

5.91%-efficient Sb2Se3 solar cells with a radio-frequency magnetron-sputtered CdS buffer layer

Superstrate ITO/CdS/Sb2Se3/Au thin-film solar cells were fabricated with a CdS film produced by a radio-frequency (RF) magnetron-sputtering method as the buffer layer. The effect of the CdS layer thickness (70–120nm) on the properties of Sb2Se3 thin films and the performance of the solar cells was investigated. The optimal CdS thickness was 90nm, which led to the best power conversion efficiency of 5.91%; to the best of our knowledge, this is the highest efficiency for Sb2Se3 solar cells with a sputtered CdS buffer layer to date. X-ray diffraction and scanning electron microscope results indicated that the Sb2Se3 films grown on 90-nm-thick CdS layers had the optimal grain orientation and the best morphology. The current–voltage (J–V) and admittance results indicated that the Sb2Se3 cell with a 90-nm-thick CdS layer had the lowest saturation current and the smallest defect densities. Moreover, temperature- and light intensity-dependent open-circuit (Voc) measurements revealed that the carrier recombination rates at the CdS/Sb2Se3 interface, space-charge region (SCR), and quasi-neutral region (QNR) all achieved the smallest value for the Sb2Se3 cells with a 90-nm CdS buffer layer. These data account for the optimal cell performance and offer new research directions for solving challenges of chemical bath deposition of CdS in Sb2Se3 solar cells.

Interfacial engineering of oxygenated chemical bath–deposited CdS window layer for highly efficient Sb2Se3 thin-film solar cells

Antimony chalcogenides with quasi-one-dimensional nanoribbon structures represent a new group of cost-effective, non-toxic, and earth-abundant materials for photovoltaics. In this work, interfacial engineering of oxygenated chemical bath–deposited (CBD) CdS buffer layers were used to tailor Sb2Se3 thin-film solar cells using close space sublimation (CSS) deposition process. Sb2Se3 solar cells with oxygenated CBD CdS have demonstrated a champion power conversion efficiency of 6.3% with graphite as back contact. Van der Waals (vdW) gap in Sb2Se3 is regulated via oxygen diffusion, which significantly influences the growth direction of (Sb4Se6)n quasi-one-dimensional nanoribbons. The improved interface quality and ribbon orientation, in turn, enhance the electrical and optical properties as well as device performance of the Sb2Se3 solar cells. The defect level and corresponding device performance associated with the oxygen environment were investigated by both theoretical and experimental approaches. This work provides an effective way to tune the microstructure and electronic structure of non-cubic chalcogenides by tailoring the interfacial area between the window layer and Sb2Se3 absorber.

MgCl2 activation of CdS films: An alternative for CdCl2

A detailed study on MgCl2 solution treatment on chemical bath deposited CdS (CBD-CdS) is reported. MgCl2 treated samples have shown the higher VOC and ISC values in the photoelectrochemical (PEC) cell compared to non-treated samples. The improved PEC cell performance in the MgCl2 treated samples were found to be higher carrier concentration and improved effective surface area as well as grain boundary passivation. The MgCl2 treatment was found to increase the cluster size of CdS thin films and the formation of clusters was identified to be due to coalescence of small clusters. Increasing the duration of MgCl2 treatment led to formation of large and densely packed surface morphology. Additionally, the relative flat band potential (VFB) value was found to be tunable with MgCl2 treatment duration and the best photo-activity and highest density of packing was observed for the CdS films which underwent MgCl2 treatment for 14 min.

Wet Pretreatment-Induced Modification of Cu(In,Ga)Se2/Cd-Free ZnTiO Buffer Interface.

We report a novel Cd-free ZnTiO buffer layer deposited by atomic layer deposition for Cu(In,Ga)Se2 (CIGS) solar cells. Wet pretreatments of the CIGS absorbers with NH4OH, H2O, and/or aqueous solution of Cd2+ ions were explored to improve the quality of the CIGS/ZnTiO interface, and their effects on the chemical state of the absorber and the final performance of Cd-free CIGS devices were investigated. X-ray photoelectron spectroscopy (XPS) analysis revealed that the aqueous solution etched away sodium compounds accumulated on the CIGS surface, which was found to be detrimental for solar cell operation. Wet treatment with NH4OH solution led to a reduced photocurrent, which was attributed to the thinning (or removal) of an ordered vacancy compound (OVC) layer on the CIGS surface as evidenced by an increased Cu XPS peak intensity after the NH4OH treatment. However, the addition of Cd2+ ions to the NH4OH aqueous solution suppressed the etching of the OVC by NH4OH, explaining why such a negative effect of NH4OH is not present in the conventional chemical bath deposition of CdS. The band alignment at the CIGS/ZnTiO interface was quantified using XPS depth profile measurements. A small cliff-like conduction band offset of -0.11 eV was identified at the interface, which indicates room for further improvement of efficiency of the CIGS/ZnTiO solar cells once the band alignment is altered to a slight spike by inserting a passivation layer with a higher conduction band edge than ZnTiO. Combination of the small cliff conduction band offset at the interface, removal of the Na compound via water, and surface doping by Cd ions allowed the application of ZnTiO buffer to CIGS treated with Cd solutions, exhibiting an efficiency of 80% compared to that of a reference CIGS solar cell treated with the CdS.

7.6% CZGSe Solar Cells Thanks to Optimized CdS Chemical Bath Deposition

In this study, CdS chemical bath deposition is investigated to improve the performance of thin film solar cell based on Cu2ZnGeSe4/CdS heterojunction. The influence of both the bath temperature and the dipping duration on the CdS thin film properties are explored thanks to the combination of scanning electron microscopy (SEM) and Raman spectroscopy, while the photovoltaic parameters of the resulting solar cells are discussed from current–voltage (I–V) and external quantum efficiency (EQE) measurements. The highest efficiency achieved herein (without antireflection coating) is 7.6%. Although it represents 35% relative improvement compared to previous best efficiency, this champion device is still limited by interface recombination. Different strategies are finally proposed to further increase the performance of these solar cells.

Atomic layer deposition of Zn(O,S) buffer layers for Cu(In,Ga)Se2 solar cells with KF post-deposition treatment

We investigate the possibility to combine Zn(O,S) buffer layers grown by atomic layer deposition (ALD) with KF post-deposition treated Cu(In,Ga)Se2 (CIGS-KF) in solar cells. It is shown that the beneficial effect on open-circuit voltage from the post-deposition treatment is essentially independent of buffer layer material. However, a wet-chemical surface treatment is required prior to ALD in order to achieve competitive fill factor values. A water rinse is sufficient to create an absorber surface similar to the one formed during a conventional CdS chemical bath deposition process. However, it is observed that CIGS-KF/Zn(O,S) devices made with water-rinsed absorbers systematically result in lower fill factor values than for the corresponding CIGS-KF/CdS references. This effect can be mitigated by decreasing the H2S:H2O precursor ratio during ALD initiation, indicating that the fill factor limitation is linked to the initial Zn(O,S) growth on the modified CIGS-KF surface. The best CIGS-KF/Zn(O,S) devices were fabricated by etching away the KF-modified surface layer prior to ALD, followed by a low-temperature anneal. The thermal treatment step is needed to increase the open-circuit voltage close to the value of the CdS devices. The results presented in this contribution indicate that the main beneficial effects from KF-PDT in our devices are neither associated with the CdS CBD process nor due to the formation of a K-In-Se rich phase on the CIGS surface.

Solution-Processed Efficient Nanocrystal Solar Cells Based on CdTe and CdS Nanocrystals

Solution-processed CdTe nanocrystals solar cells have attracted much attention due to their low cost, low material consumption, and potential for roll-to-roll production. Among all kinds of semiconductor materials, CdS exhibits the lowest lattice mismatch with CdTe, which permits high junction quality and high device performance. In this study, high quality CdS nanocrystals were prepared by a non-injection technique with tetraethylthiuram disufide and 2,2′-dithiobisbenzothiazole as the stabilizers. Based on the CdTe and CdS nanocrystals, devices with the architecture of ITO/ZnO/CdS/CdTe/MoOx/Au were fabricated successfully by a solution process under ambient condition. The effects of annealing conditions, film thickness, and detailed device structure on the CdTe/CdS nanocrystal solar cells were investigated and discussed in detail. We demonstrate that high junction quality can be obtained by using CdS nanocrystal thin film compared to traditional CdS film via chemical bath deposition (CBD). The best device had short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) of 17.26 mA/cm2, 0.56 V, and 52.84%, respectively, resulting in a power conversion efficiency (PCE) of 5.14%, which is significantly higher than that reported using CBD CdS as the window layer. This work provides important suggestions for the further improvement of efficiency in CdTe nanocrystal solar cells.

KF post deposition treatment in co‐evaporated Cu(In,Ga)Se2 thin film solar cells: Beneficial or detrimental effect induced by the absorber characteristics

AbstractRecent breakthroughs in Cu(In,Ga)Se2 (CIGS) thin film solar cell energy conversion efficiency are related to the application of a potassium fluoride post‐deposition treatment (KF‐PDT) to the completed absorber. Using X‐ray photoelectron spectroscopy and Raman scattering, we compare CIGS layers prior and after the KF‐PDT in the case of a deterioration and an improvement of the solar cells photovoltaic performance. The purpose is to study and model the modification of the surface in both cases and address some of the required characteristics of the absorber, grown on soda lime glass by 3‐stage process, in order to take advantage of the treatment. We show that, in both cases, KF‐PDT induces the formation of GaF3, which is removed during the subsequent chemical bath deposition of CdS, explaining the Ga depleted absorber surface, already reported in literature. However, the presence or not of an ordered defect compound (ODC), correlated with the third stage duration during the CIGS growth, is shown to be crucial in the modifications of the surface induced by the treatment. When an ODC is present prior the treatment, KF‐PDT leads to the formation of a surface layer of In2Se3 containing K, and the photovoltaic performance of completed solar cells are improved. When no ODC is present prior KF‐PDT, no trace of K is found at the absorber surface after the treatment, copper (Cu) segregates into detrimental CuxSe phases, high amount of elemental Se is formed, and the photovoltaic performance are lowered. The role of the ODC during the KF‐PDT is finally discussed.

Growth and characterization of EDTA assisted CBD-CdS

Chemical bath deposition of CdS (CBD-CdS) thin films with the assistance of a cationic surfactant, ethylenediamine tetraacetic acid (EDTA), is reported in this work. Also the EDTA treated CdS thin films are compared with that of conventional CBD-CdS. Fabricated thin, compact, uniform and adherent EDTA-treated CdS films show enhanced effective surface area and roughness compared to conventional CBD-CdS. The grazing incidence x-ray diffraction analysis shows all the fabricated CdS films are hexagonally crystallized. EDTA-treated CdS films show excellent photo activity compared to conventional CBD-CdS. The flat band potential (Vfb) value was found to be tunable with EDTA concentration.

Zn–Se–Cd–S Interlayer Formation at the CdS/Cu2ZnSnSe4 Thin-Film Solar Cell Interface

The chemical structure of the CdS/Cu2ZnSnSe4 (CZTSe) interface was studied by a combination of electron and X-ray spectroscopies with varying surface sensitivity. We find the CdS chemical bath deposition causes a “redistribution” of elements in the proximity of the CdS/CZTSe interface. In detail, our data suggest that Zn and Se from the Zn-terminated CZTSe absorber and Cd and S from the buffer layer form a Zn–Se–Cd–S interlayer. We find direct indications for the presence of Cd–S, Cd–Se, and Cd–Se–Zn bonds at the buffer/absorber interface. Thus, we propose the formation of a mixed Cd(S,Se)–(Cd,Zn)Se interlayer. We suggest the underlying chemical mechanism is an ion exchange mediated by the amine complexes present in the chemical bath.