Zn Buffer Research Articles

Influence of Deposition Parameters on Performance of Zn(O,S) Buffer Layer Deposited by Solution-Based Continuous Flow Reactor for CIGS Solar Cells

CdS thin films are commonly used as buffer layers in hetero-junction structured CIGS thin film solar cells. However, the toxicity of Cd has raised concerns regarding environmental safety and alternative materials have been explored. ZnS is a leading candidate to replace Cd, with the presence of Zn(O,S) impurities known to be required for efficient CIGS solar cell fabrication. In this study, we synthesized Zn(O,S) films using a solution-based deposition method combining a continuous flow reactor (CFR) with another solutionbased deposition method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structural and physicochemical properties of the films, confirming the efficacy of the CFR process for depositing Zn(O,S) thin films in a short period of time. We successfully deposited Zn(O,S) thin films using the combined CFR process. Cd-free B:ZnO/Zn(O,S)/CIGS solar cells were fabricated on Mo-coated glass substrates to investigate the effects of experimental parameters, such as deposition time and method, on the performance of the Zn(O,S) buffer layer in the devices.

Influence of oxygen content on the properties of In2(OxS1−x)3 used as buffer material in Cu(In,Ga)Se2 solar cells

We investigate magnetron-sputtered In2(OxS1−x)3 compounds acting as an alternative buffer system to the solution-grown CdS or Zn(O,S) buffer layers in Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The influence of the oxygen content on the solar cell performance, microstructure of the mixed systems, bandgap, and band offsets to CIGS is investigated experimentally and also characterized by calculations based on density functional theory. Samples in a series with different chemical compositions ranging from In2S3 to In2O3 are either directly deposited from ceramic targets or from a pure In2S3 target by reactive sputtering by adding O2 in the Ar sputtering gas. The binary compounds In2S3 and In2O3 sputtered at 220 °C substrate temperature from ceramic targets exhibit a crystalline structure, whereas the ternary In2(O,S)3 compounds are either nanocrystalline in the case of In2(O0.25S0.75)3 or amorphous for In2(O0.5S0.5)3 and In2(O0.75S0.25)3. For [O]/([O] + [S]) ratios above 0.25, the cell efficiencies decrease drastically, mainly due to lower open-circuit voltages (VOC). This behavior can be explained by an increase of the negative conduction band offset between the CIGS absorber and the oxygen-rich In2(OxS1−x)3 or In2O3 buffer, resulting in pronounced VOC losses. Adding oxygen to In2S3 with optical bandgap energies of around 2 eV results in a bowing of the values to below 2 eV and finally reaching values of around 2.7 eV for In2O3 if an indirect band transition is assumed. In summary, our results reveal that pronounced oxygen incorporation in In2S3 is not beneficial in terms of CIGS device efficiency because oxygen is electronically inactive and poorly miscible.

Numerical Investigation of Copper Zinc Tin Sulphide Based Solar Cell with Non‐Toxic Zn (O, S) Buffer Layer

Zinc oxide sulphide Zn (O, S) buffer layer has attracted enormous attention due to its composition of non‐toxic, low cost, and earth abundant elements, unlike cadmium sulphide (CdS) used in copper zinc tin sulphide (CZTS) solar cells. Zn (O, S) possesses a direct optical bandgap that's adjustable between 2.7 and 3.6 eV related to the mole fraction , making it a potential candidate as a buffer layer with CZTS based solar cells. In this work, the Zn (O, S)/CZTS‐based solar cell structure is numerically studied and compared with CdS/CZTS. The existence of defects and traps along the layers and interfaces is taken in consideration. To achieve optimal results, this research has focused on; effects of the element composition (x = S/(S + O)) ratio of Zn (O, S) buffer layer, the addition of a back surface field (BSF) layer, the influence of Zn (O, S)/CZTS and CdS/CZTS interfaces defect density, varying thicknesses and doping concentrations of the layers (absorber, buffer, BSF) on open‐circuit voltage (Voc), short‐circuit density (Jsc), fill factor (FF), and power conversion efficiency. Findings illustrate that the highest conversion efficiency of the Zn (O, S)/CZTS solar cell reached 14.65% with Voc = 0.9972 V, Jsc = 18.37 mA cm−2 and FF = 79.96% which is better than the CdS/CZTS structure.

Effects of ZnSO4 concentration on the high-rate deposition of CBD-Zn(O,S) buffer layer for Cu(In,Ga)Se2 thin-film solar cells

To utilize eco-friendly materials in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, a Zn-based buffer layer has been adopted to replace the conventional CdS buffer layer. Buffer layers can be grown using a variety of techniques, with chemical bath deposition (CBD) generally used for the preparation of the Zn(O,S) buffer layer. The growth rate of the CBD-Zn(O,S) buffer layer is determined by the dissociation rate of S during chemical reaction between Zn and S ions. Thiourea (TU) is a conventional precursor of S, but it suffers from poor dissociation rate of S, leading to prolonged layer deposition times. In this study, we replaced TU with thioacetamide (TAA), thereby drastically shortening the deposition time from 80 min (TU) to 7 min (TAA). Furthermore, incorporating Zn with S ions is known to be difficult owing to the high-speed release of S ions from TAA. However, by adjusting the molar concentration of ZnSO4, we discovered a process to effectively form the Zn–S bonds for the CBD-Zn(O,S) buffer layer. The efficiency of the CIGS thin-film solar cell shows the best performance at a ZnSO4 concentration of 0.05 M, which rivals the performance of the TU base process. The physical and chemical properties of CBD-Zn(O,S) grown at a high deposition rate using TAA were examined as a function of ZnSO4 molar concentration. We proposed a growth model that accounts for the changes in film thickness and surface morphology evolution, both of which depend on the ZnSO4 concentration. Additionally, to investigate the p–n junction properties in terms of device operation, we elucidated the band alignment at the CIGS/CBD-Zn(O,S) interface, concentrating on the interfacial barrier height. It is empirically demonstrated that a lower ΔEC value improves the solar cell operation.

Enhanced photoconversion efficiency of Cu2MnSnS4 solar cells by Sn-/Zn-based oxides and chalcogenides buffer and electron transport layers

Copper Manganese Tin Sulfide, Cu2MnSnS4 (CMTS), has emerged as a potential candidate for cost-competitive, eco-friendly single- or multiple p-n junction kesterite quaternary photovoltaics (PVs) due to its distinct optical and electrical properties, with earth-abundant as well as non-toxic constituents. In this article, Cu2MnSnS4 absorber-based heterostructures with ZnMgO, WS2, SnO2, CdS electron transport (window) and SnS2, ZnSe, ZnSnO, Zn(O,S) buffer layers have been investigated by executing a systematic study and proposing the mostly efficient device structures containing the favourable band alignment using the SCAPS-1D simulator. Initially, an effective buffer and electron transport (window) layers have been optimized from Al:ZnO/window (ZnMgO, WS2, SnS2, or CdS)/buffer (SnS2, ZnSe, ZnSnO, Zn(O,S))/Cu2MnSnS4/Pt structures. Subsequently, the impact of different properties of each active layer of optimized structures, such as bandgap, layer thickness, carrier concentration, bulk and interface defect densities, and working temperature, was determined by measuring PV parameters and corresponding quantum efficiency. The present study exhibited the maximum photocurrent, JSC of 23.69 mA/cm2, open circuit voltage, VOC of 1.236 V, fill factor, FF of 85.49% and photoconversion efficiency, PCE of 25.03% with optimized SLG/Mo/Cu2MnSnS4/SnS2/ZnMgO/ZnO:Al/Pt structure under AM1.5G irradiation. Furthermore, a detailed comparison among recent studies has been carried out to demonstrate the significance of the proposed structure, challenges, and future prospects. Thus, obtained results pave significant resources and emancipate a new pathway to fabricate enviro-benign, cost-competitive, high-efficiency CMTS-thin film photovoltaics.

Development of n-type Zn(O, S) buffer layer deposited by open-air CVD method for Cu(In, Ga)Se2 solar cells

A Zn(O, S) thin-film is deposited utilizing an open-air CVD method by evaporating zinc-diethyldithiocarbamate, which is a non-vacuum and dry process. In an X-ray diffraction measurement, it is revealed that the films have a wurtzite structure and an [O]/([O] + [S]) ratio of 10%. A bandgap energy of 3.1 eV is estimated from the transmittance and reflectance spectra. By applying the Zn(O, S) as an n-type buffer layer, Cu(In, Ga)Se2 solar cells are fabricated. In the current density–voltage characteristics, distortion is observed at the bias voltages above the open-circuit voltage. It is implied that a large conduction band offset exists at a Zn(O, S)/CIGS interface. A quantum efficiency spectrum in the wavelength region of 380–512 nm is improved compared to a traditional CdS buffer layer. Finally, a 9.2%-efficient CIGS solar cell is demonstrated utilizing the Zn(O, S) buffer layer through an all-dry process.

Chemical phases in the solution-grown Zn(O,S) buffer of post-annealed Cu(In,Ga)Se2 solar cells investigated by transmission electron microscopy and electroreflectance

Thin-film solar cells with Cu(In,Ga)Se2 (CIGS) absorber layers have been intensively studied due to their high power conversion efficiencies. CIGS solar cells with Zn(O,S) buffer layers achieved record efficiencies due to their reduced parasitic absorption compared with the more commonly used CdS buffer. Accordingly, we have studied solution-grown Zn(O,S) buffer layers on polycrystalline CIGS absorber layers by complementary techniques. A bandgap energy Eg of 2.9 eV is detected by means of angle-resolved electroreflectance spectroscopy corresponding to Zn(O,S), whereas an additional Eg of 2.3 eV clearly appeared for a post-annealed CIGS solar cell (250 °C in air) compared with the as-grown state. To identify the chemical phase that contributes to the Eg of 2.3 eV, the microstructure and microchemistry of the Zn(O,S) buffer layers in the as-grown state and after annealing were analyzed by different transmission electron microscopic techniques on the submicrometer scale and energy-dispersive x-ray spectroscopy. We demonstrate that the combination of these methods facilitates a comprehensive analysis of the complex phase constitution of nanoscaled buffer layers. The results show that after annealing, the Cu concentration in Zn(O,S) is increased. This observation indicates the existence of an additional Cu-containing phase with Eg close to 2.3 eV, such as Cu2Se (2.23 eV) or CuS (2.36 eV), which could be one possible origin of the low power conversion efficiency and low fill factor of the solar cell under investigation.

Chemical and Electronic Structure at the Interface between a Sputter‐Deposited Zn(O,S) Buffer and a Cu(In,Ga)(S,Se)2Solar Cell Absorber

The chemical and electronic structure of the interface between a sputter‐deposited Zn(O,S) buffer layer and an industrial Cu(In,Ga)(S,Se)2(CIGSSe) absorber for thin‐film solar cells is investigated with X‐ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy. We find a CIGSSe absorber surface band gap of 1.61 (±0.14) eV, which is significantly increased as compared to the minimal value derived with bulk‐sensitive methods (≈1.1 eV). We find no indication for diffusion of absorber elements into the buffer layer. Surface‐ and bulk‐sensitive measurements of the buffer layer suggest the presence of S‐Zn and S‐O bonds in the Zn(O,S) layer. We find that the naturally existing downward band bending toward the CIGSSe absorber surface is increased by the formation of the interface, likely enhancing carrier separation under illumination. We also derive a flat conduction band alignment, in line with the reported high conversion efficiencies of corresponding large‐area solar cells.

Controllable and innovative preparation of Zn(O,S) buffer layers for CIGS thin film solar cells

The band gap of CdS (Eg = 2.4 eV) was narrow, which made it unable to transmit short-wave sunlight through the CIGS absorber layer. Zn(O,S) thin film, prepared by doping O into ZnS, had a wider band gap, and the raw material was green and non-toxic, making it suitable as a buffer layer for solar cells. However, CBD-Zn(O,S) usually takes longer than CdS deposition. In this work, By shortening the deposition time through the fast hydrolysis of TAA. Na3NTA was added in the solutions to improve the quality of Zn(O,S) thin films. The effect of Na3NTA concentration on the complex ion contents was investigated. When Na3NTA was added at a concentration of 0.04 M along with TAA (0.04 M), the film had a high T% of 88.6%, Eg of 3.76 eV, and a dense grain distribution. ZnS and ZnO grew at (111) and (100) crystals, respectively. Raman and XPS analyses showed the existence of Zn–S and Zn–O bonds, indicating the high crystalline quality of the Zn(O,S) thin films prepared with CBD. Finally, when the optimal concentrations of ZnSO4, NH4OH, Na3NTA, and TAA were 0.04 M, 0.48 M, 0.04 M, and 0.04 M, respectively, the prepared Zn(O,S) thin films could be used as a cadmium-free buffer layer for solar cells.

Enhancement in Efficiency of CIGS Solar Cell by Using a p-Si BSF Layer

Copper–indium–gallium–diselenide Cu(In,Ga)Se2 (CIGS) is a semiconductor compound belonging to group I-III-VI, with a chalcopyrite crystal structure. CIGS is promising for the development of high-performance photovoltaic applications in terms of stability and conversion efficiency. It is one of the main candidates to rival the efficiency and stability of conventional crystalline silicon cells, due to its high light absorption coefficient, lower material cost, and high stability. The limitation of its use is that CIGS integrates indium (In) and gallium (Ga), which are rare and expensive materials. The amount of these materials in the CIGS cell can be reduced by optimizing the thickness of the absorber. We show that the introduction of a layer of highly doped silicon in the structure of the solar cell between the absorber layer and the back surface field layer effectively allows for decreasing the thickness of the absorber. Within the same objective, we focus on the danger of cadmium in the CdS buffer layer. In the first optimizations, we replaced the n-type CdS buffer layer with a n-type Zn(O,S) buffer layer. For this work, we used a one-dimensional simulation program, named Solar Cell Capacitance Simulator in one Dimension (SCAPS-1D), to investigate this new CIGS solar cell structure. After optimization, a maximum conversion efficiency of 24.43% was achieved with a 0.2 μm CIGS absorber layer and a 1 µm Si BSF layer.

Improvement of Quantitative STEM/EDXS Analyses for Chemical Analysis of Cu(In,Ga)Se2 Solar Cells with Zn(O,S) Buffer Layers

Abstract Energy-dispersive X-ray spectroscopy (EDXS) in a transmission electron microscope is frequently used for the chemical analysis of Cu(In,Ga)Se2 (CIGS) solar cells with high spatial resolution. However, the quantification of EDXS data is complicated due to quantification errors and artifacts. This work shows how quantitative EDXS analyses of CIGS-based solar cells with Zn(O,S) buffer and ZnO-based window layers can be significantly improved. For this purpose, CIGS-based solar cells and a reference sample with a stack of Zn(O,S) layers with different [O]/[S] ratios were analyzed. For Zn(O,S), the correction of sample-thickness-dependent absorption of low-energy O–Kα X-rays significantly improves the results of quantitative EDXS. Absorption of characteristic X-rays in CIGS is less relevant. However, for small transmission electron microscopy (TEM) sample thicknesses, artifacts can occur due to material changes by focused-ion-beam (FIB)-based preparation of TEM samples, electron-beam-induced damage, and oxidation of the sample surface. We also show that a Pt-protection layer, deposited on the sample surface before FIB preparation of TEM lamellae, can induce artifacts that can be avoided by first depositing a carbon layer.

Study on the Performance of Oxygen-Rich Zn(O,S) Buffers Fabricated by Sputtering Deposition and Zn(O,S)/Cu(In,Ga)(S,Se)2 Interfaces

We developed a novel process for fabricating oxygen-rich Zn(O,S) buffer layers by magnetron reactive sputtering with a single oxygen-rich Zn(O,S) target, suitable for industrial all-dry production. Then, we successfully fabricated Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells. By varying the oxygen partial pressure during sputtering from 0 to 20%, we precisely controlled the Zn(O,S) composition, then systematically investigated its effects on the quality of oxygen-rich Zn(O,S) films, the properties of formed p-n junctions, and the performance of CIGSSe solar cells with Zn(O,S) buffer. We demonstrated that reactive sputtering with a Zn(O,S) target can generate a homogeneous, high-quality oxygen-rich Zn(O,S) buffer on large-area substrates. We observed a unique and unusual phenomenon: the appropriate content of secondary phase ZnSO4 and ZnSO3 improved the band alignment for oxygen-rich Zn(O,S). Combining our proposed schematic diagram of band alignmentat the Zn(O,S)/CIGSSe interface, we established a crucial correlation between the device performance and the interfacial properties at the p-n junction. For the CIGSSe device performance, the band alignment matching at the heterojunction plays a primary role, and the quality of oxygen-rich Zn(O,S) films plays a secondary role. Consequently, an excellent oxygen-rich Zn(O,S) buffer can be obtained with 10% Zn(O,S) deposition oxygen partial pressure , and the optimized device shows a higher Voc (447 mV) and a similar conversion efficiency (11.2%) than conventional CIGSSe devices with CdS buffer.

Practical Enhancements in Current Density and Power Generation of Bifacial Semitransparent Ultrathin CIGSe Solar Cells via Utilization of Wide Bandgap Zn‐Based Buffer (Adv. Sci. 13/2022)

Semitransparent Ultrathin CIGSe Solar Cells In article number 2105436, Joo Hyung Park and co-workers develop a bifacial and semitransparent ultrathin (STUT) Cu(In1-x,Gax)Se2 (CIGSe) solar cell with a wide bandgap Zn(O,S) buffer, and establish the concept and method to compare power generation ability by defining bifacial compatible efficiency (BCE) to reflect the albedo effect advantage in bifacial application, comparing with monofacial power conversion efficiency (PCE).

Effect of Zn(S,O,OH) Buffer Thin Films Formed on CIGS through Different Stages and Reaction Processes in Chemical Bath Deposition: Interpretations from Mechanisms and Transformation Kinetics Perspective

Herein, Zn(S,O,OH) is used as a buffer material for Cu(In,Ga)Se2 (CIGS) solar cells instead of CdS to increase the quantum efficiency and remove toxicity. To control the fine thickness of the Zn(S,O,OH) thin films in the chemical bath deposition (CBD) method, a quartz crystal microbalance (QCM) system is introduced by measuring the frequency change. As the frequency changes are adjusted according to the S/O ratio of the Zn(S,O,OH) buffer (Zn buffer) produced during the reaction, the formed phase and thickness are cross‐validated several times to obtain the accurate thickness. These different S/O ratios can be modified by different deposition mechanisms and reaction processes depending on the reaction rate of material formation. Therefore, to investigate the associated mechanism and reaction process in detail, Zn buffer thin films are formed and characterized using two different Zn sources (zinc sulfate and zinc acetate) that affect the reaction rate due to the anion effect. In addition, the optimal deposition range is obtained by comparing the performances of the solar cells with Zn thin films of the same thickness, deposited through different stages in a continuous CBD reaction. Finally, the effect of Zn buffers using different Zn sources on the interfaces with CIGS is investigated in the band alignment.

Practical Enhancements in Current Density and Power Generation of Bifacial Semitransparent Ultrathin CIGSe Solar Cells via Utilization of Wide Bandgap Zn-Based Buffer.

Among many building‐integrated semitransparent photovoltaics (BISTPVs), semitransparent ultrathin (STUT) Cu(Inx,Ga1‐x)Se2 (CIGSe) solar cells are distinguishable due to their potential high power conversion efficiency (PCE) among other thin‐film solar cells, versatile applicability based on thin film deposition processes, high stability consisting of all inorganic compositions, and practical expandability to bifacial applications. However, the fundamental trade‐off relationship between PCE and transparency limits the performance of BISTPV because implementing a higher semitransparency lowers the optical budget of incoming light. To expand the available optical budget and to enhance the PCE while maintaining a suitable transparency in STUT CIGSe solar cell with single‐stage coevaporated 500‐nm‐thick absorber, an atomic layer deposited wide bandgap Zn(O,S) buffer is introduced as the replacement of conventional CdS buffer, which partially limits incoming light less than 520 nm in wavelength. As a replacement result, more incoming light becomes valid for power conversion, and the short circuit current density (J sc) has increased comparatively by 17%, which has directly lead to a large increase in PCE up to 12.41%. Furthermore, Zn(O,S) buffer in the STUT CIGSe solar cell also has enhanced the bifacial compatible efficiency (BCE), which has increased to 14.44% at 1.3 sun and 19.42% at 2.0 sun.

Investigation of co-evaporated polycrystalline Cu(In,Ga)S2thin film yielding 16.0 % efficiency solar cell

The interest for pure sulfide Cu(In,Ga)S2chalcopyrite thin films is increasing again because their optical properties make them relevant candidates to be applied as top cell absorbers in tandem structures. Nonetheless, their use as so is still hindered by the level of single-junction cells performance achieved so far, which are far below those demonstrated by selenide absorbers. Amongst the reasons at the origin of the limited efficiency of Cu(In,Ga)S2-based solar devices, one can mention the poor tolerance of S-chalcopyrite to Cu deficiency. In fact, Cu-poor Cu(In,Ga)S2films contain CuIn5S8thiospinel secondary phase which is harmful for device performance. In the present work, we investigate Cu(In,Ga)S2thin films grown by a modified three-stage process making use of graded indium and gallium fluxes during the first stage. The resulting absorbers are single phase and made of large grains extended throughout the entire film thickness. We propose that such a morphology is a proof of the recrystallization of the entire film during the synthesis. Devices prepared from those films and buffered with bath deposited CdS demonstrate outstanding efficiency of 16.0%. Replacing CdS by Zn(O,S) buffer layer leads to increased open circuit voltage and short circuit current; however, performance become limited by lowered fill factor.

Theoretical modeling and optimization: Cd-free CTS/Zn(O,S)/ZnO thin film solar cell

Latterly Zn(O, S) has gained utmost attention as an eco-friendly buffer layer with controllable energy levels for high-performance thin-film solar cell (TFSCs) devices. The article discusses that the conventional detrimental Cd-containing buffer is replaced by the non-toxic Zn(O,S) buffer and yielded a comparable high efficiency. Using SCAPS software, the modeling and numerical simulations of the CTS based solar cell are systematically analyzed. Detail investigations of the band alignment between the CTS absorber and Zn(O, S) buffer layer has been studied as a function of the S/(S+O) ratio along with the effects of thickness, carrier concentration, defect density of buffer layer and absorber layer with interface defect density. The final optimized results of p-CTS/Zn(O,S)/n-ZnO/Al heterojunction revealed that the obtained figure of merit value is comparably high (ɳ=13.65%, VOC = 0.3802 V, JSC = 48.33 mA/cm2, and FF = 74.45%) than the previously reported values. All in all, the present study suggests that Zn(O, S) can be a suitable alternative candidate for the CdS-buffer layer in a CTS-based solar cell.

Evolution of Morphological and Chemical Properties at p-n Junction of Cu(In,Ga)Se2 Solar Cells with Zn(O,S) Buffer Layer as a Function of KF Postdeposition Treatment Time.

We carried out KF postdeposition treatment (PDT) on a Cu(In,Ga)Se2 (CIGS) layer with a process time varying from 50 to 200 s. The highest CIGS solar-cell efficiency was achieved at a KF PDT process time of 50 s; in this condition, we observed the highest level of K element at the near-surface of the CIGS layer and the perfectly passivated pinholes on the CIGS surface. At process times above 150 s, the oversupplied KF agglomerated into large islands and was subsequently eliminated during the deposition of the chemical bath deposition (CBD)-Zn(O,S) buffer layer owing to the islands' water-soluble characteristics. As a result, the growth mechanism of the CBD-Zn(O,S) layer varied as a function of KF PDT process time. X-ray photoemission spectroscopy (XPS) measurements were used to examine the dependency of the chemical state on the KF PDT process time, and from the results, we formulated a chemical reaction model based on the shift in the elemental binding energy following deposition of the CBD-Zn(O,S) buffer layer. The chemical states of the K-In-Se phase, which have a beneficial effect on the solar-cell performance owing to the formation of durable and improved p-n junctions, are formed only at a KF PDT process time of 50 s. We derived band alignments from the XPS depth profiles by extracting the conduction- and valence-band offsets, and we used optical-pump-THz-probe spectroscopy to measure the ultrafast photocarrier lifetimes related to the defect states following KF PDT. Our key findings can be summarized as follows: (i) photocarrier transport is beneficial at a low barrier height, and (ii) the photocarrier lifetime increases when the K-In-Se phases are formed on the CIGS surface, which allows K+ ions to be effectively substituted into Cu vacancies.

Impact of the Buffer/Absorber Interface on the Metastability of Fill Factor Temperature Coefficients in CIGSSe Solar Cells

AbstractThe metastable behavior of the fill factor temperature coefficient (βFF,rel) in Cu(In,Ga)(S,Se)2 solar cells is investigated for absorbers with a different Ga content in the front surface, and for different buffer materials. The buffer/absorber interface region is suggested to be the location which causes the metastability in βFF,rel. Temperature dependent current–voltage curves, light‐biased external quantum efficiency, and low temperature capacitance–voltage measurements are performed to characterize the electrical properties and metastable defects in the investigated solar cell devices. The presence of Ga in the absorber front surface, together with the buffer layer material plays a significant role in the metastable behavior. Additionally, energy dispersive X‐ray spectroscopy indicates that the InxSy buffer layers allow for elemental interdiffusion from absorbers with a Ga‐richer front surface, whereas a Zn(O,S) buffer shows to be rather resilient to atomic diffusion. The elemental interdiffusion between the buffer and absorber layers is proposed to be the cause of the defects causing the observed βFF,rel metastability.

Efficient Sb2(S,Se)3/Zn(O,S) solar cells with high open-circuit voltage by controlling sulfur content in the absorber-buffer layers

The purpose of this study is the efficiency improvement of the Sb2Se3 solar cells. In this study, an experimental Sb2Se3 solar cell (glass/Mo/MoSe2/Sb2Se3/CdS/ZnO/ZnO:Al/Ag) with an efficiency record of 9.2% has been simulated. Absorber/buffer interface engineering plays a significant role in enhancing the efficiency of Sb2Se3 solar cells. To achieve this purpose, two approaches have been considered: first, adding sulfur to Sb2Se3 absorber, and second, using an alternative buffer with a wider bandgap instead of conventional CdS buffer. The effects of various x = Se/(S + Se) ratios of Sb2(S1−xSex)3 absorber layer on the photovoltaic performance were investigated. For Sb2(S,Se)3/CdS solar cell, optimum Se/(S + Se) mole fraction of 0.6 < x < 0.8 leads to improved efficiency. Also, ZnO1−ySy buffer layer was applied to replace the conventional CdS buffer layer of Sb2Se3 solar cells to reduce parasitic absorption and improve the short-circuit current density (Jsc). Bandgap for ZnO1−ySy semiconductor is higher than CdS. So, this leads to improved external quantum efficiency at short wavelengths. Optimization of band alignment through ZnO1−ySy buffer layer reduces interface carrier recombination and improves open-circuit voltage (Voc). The band alignment at the Sb2(S1−xSex)3/ZnO1−ySy interface is optimized by adjusting the selenium-to-sulfur ratio in the absorber layer and sulfur-to-oxygen ratio in the buffer layer. This work reveals that the most suitable interface for Sb2(S1−xSex)3/ZnO1−ySy heterojunction is formed when sulfur mole fractions ranging 0.5–0.6 in the Zn(O,S) buffer layer and 0.1–0.2 in the Sb2(S,Se)3 absorber layer. The results show that the efficiency improves from 9.2% to 15.65%, which represents a 70% improvement compared with the conventional Sb2Se3/CdS solar cell.