The role of nanoparticle inks in determining the performance of solution processed Cu2ZnSn(S,Se)4 thin film solar cells

Cu₂ZnSnS₄ (CZTS) nanoparticle inks synthesized by the injection of metal precursors into a hot surfactant offer an attractive route to the fabrication of Earth‐abundant Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film photovoltaic absorber layers. In this work it is shown that the chemical reaction conditions used to produce CZTS nanoparticle inks have a fundamental influence on the performance of thin film solar cells made by converting the nanoparticles to large CZTSSe grains in a selenium rich atmosphere and subsequent cell completion. The reaction time, temperature and cooling rate of the nanoparticle fabrication process are found to affect doping level, secondary phases and crystal structure respectively. Specifically, prolonging the reaction offers a new route to increase the concentration of acceptor levels in CZTSSe photovoltaic absorbers and results in higher device efficiency through an increase in the open circuit voltage and a reduction in parasitic resistance. Quenching the reaction by rapid cooling introduces a wurtzite crystal structure in the nanoparticles which significantly degrades the device performance, while elevating the reaction temperature of the nanoparticle synthesis introduces a secondary phase Cu₂SnS₃ in the nanoparticles and results in the highest cell efficiency of 6.26%. This is correlated with increased doping in the CZTSSe absorber and the results demonstrate a route to controlling this parameter. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons, Ltd.     

CuSCN Modified Back Contacts for High Performance CZTSSe Solar Cells

Optimization of the back contact interface is crucial for improving the performance of Cu₂ZnSnS₄ (CZTS) thin film solar cells. In this paper, self-depleted CuSCN is deployed as an intermediate layer at the Mo/CZTS interface to improve the quality of the back contact. This CuSCN layer, obtained via aqueous solution processing, reduces the thickness of Mo(S,Se)₂ and eliminates multi-layer crystallization of the absorber by suppressing the undesirable reaction between Mo and Se during the selenization process. By regulating the selenium infiltration into the CZTS precursor films during the selenization process, highly crystalline, single-layer Cu₂ZnSn(S,Se)₄ (CZTSSe) absorber layers are realized. The single-layer CZTSSe absorber exhibits reduced carrier recombination, enhanced carrier density and increased work function. The improved back contact and absorber layer enables 11.1% power-conversion-efficiency to be achieved.     

Improved performance of Ge‐alloyed CZTGeSSe thin‐film solar cells through control of elemental losses

Nanocrystal‐based Cu₂Zn(Sn_yGe_1‐y)(S_xSe_4‐x) (CZTGeSSe) thin‐film solar cell absorbers with tunable band gap have been prepared. Maximum solar‐conversion total area efficiencies of up to 9.4% are achieved with a Ge content of 30 at.%. Improved performance compared with similarly processed films of Cu₂ZnSn(S_xSe_4‐x) (CZTSSe, 8.4% efficiency) is achieved through controlling Ge loss from the bulk of the absorber film during the high‐temperature selenization treatment, although some Ge loss from the absorber surface is still observed following this step. Despite limitations imposed by elemental losses present at the absorber surface, we find that Ge alloying leads to enhanced performance due to increased minority charge carrier lifetimes as well as reduced voltage‐dependent charge carrier collection. Copyright © 2013 John Wiley & Sons, Ltd.     

All‐Solution‐Processed Cu2ZnSnS4 Solar Cells with Self‐Depleted Na2S Back Contact Modification Layer

The thin‐film photovoltaic material Cu₂ZnSnS₄ (CZTS) has drawn worldwide attention in recent years due to its earth‐abundant, nontoxic element constitution, and remarkable photovoltaic performance. Although state‐of‐the‐art power conversion efficiency is achieved by hydrazine‐based methods, effort to fabricate such devices in a high throughput, environmental‐friendly way is still highlydesired. Here a hydrazine‐free all‐solution‐processed CZTS solar cell with Na₂S self‐depleted back contact modification layer for the first time is demonstrated, using a ball‐milled CZTS as light absorber, low‐temperature solution‐processed ZnO electron‐transport layer as well as silver‐nanowire transparent electrode. The inserting of Na₂S self‐depleted layer is proven to effectively stabilize the CZTS/Mo interface by eliminating a detrimental phase segregation reaction between CZTS and Mo‐coated soda lime glass, thus leading to a better crystallinity of CZTS light absorbing layer, enhanced carrier transportation at CZTS/Mo interface as well as a smaller series resistance. Furthermore, the self‐depletion feature of the Na₂S modification layer also averts hole‐transportation barrier within the devices. The results show the vital importance of interfacial engineering for these CZST devices and the Na₂S interface layer can be extended to other optoelectronic devices using Mo contact.     

Enhancement of photo‐conversion efficiency in Cu2ZnSn(S,Se)4 thin‐film solar cells by control of ZnS precursor‐layer thickness

CZTSSe thin‐film absorbers were grown by stacked ZnS/SnS/Cu sputtering with compound targets, and the precursors were annealed in a furnace with a Se atmosphere. We controlled the thickness of the ZnS precursor layer for the CZTSSe thin films in order to reduce the secondary phases and to improve the performance of the devices. The optimal value of the ZnS precursor thickness was determined for the CZTSSe absorbers, and this configuration showed an efficiency of up to 9.1%. In this study, we investigated the depth profiles of the samples in order to determine the presence of secondary phases in the CZTSSe thin films by Raman spectroscopy and Kelvin probe force microscopy. Cu₂SnSe₃, ZnSe, and MoSe₂ secondary phases appeared near the back contact, and the work function distribution of the CZTSSe thin‐film surface and the secondary phase distribution were different depending on the depths of the absorber layer. This phase characterization allows us to describe the effects that changes in the thickness of the ZnS precursor can have on the performance of the CZTSSe thin‐film solar cells. Although it is important to identify the phases, the effects of secondary phases and point defects are not yet fully understood, even in optimal devices. Therefore, phase identification that is based on the work function and the results obtained from the Raman spectra in terms of the depth profile are instrumental to improve the surface and interface of CZTSSe thin‐film solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of Na and MoS2 on Cu2ZnSnS4 thin‐film solar cell

Cu₂ZnSnS₄ (CZTS)‐based materials have a useful band gap and a high absorption coefficient; however, their power conversion efficiency is low compared with that of CdTe and Cu(In,Ga)Se₂‐based solar cells. Two of the factors that strongly affect CZTS solar cell characteristics are the MoS₂ layer and the presence of defects. In this study, Mo back‐contact layers were annealed to control MoS₂ layer formation and the Na content in the Mo layer before the absorber precursor layer was deposited. The increase in oxygen content in the Mo layer suppressed MoS₂ layer formation. In addition, the increase in Na diffusion during the initial stage of the absorber precursor deposition decreased the defect density in the absorber layer and in the absorber–buffer interface. These results were verified through measurements of the external quantum efficiency, the temperature dependence of the open‐circuit voltage (V_OC), and admittance spectra. The current densities (J_SC) and V_OC, as well as the power conversion efficiencies, improved as the annealing temperature of the Mo layer increased, which suggests that CZTS solar cell characteristics can be improved by suppressing MoS₂ layer formation and increasing Na content in the Mo layer before deposition of the absorber precursor layer. Copyright © 2014 John Wiley & Sons, Ltd.     

Cu2ZnSnS4 thin film solar cells by fast coevaporation

Solar cells based on kesterite‐type Cu₂ZnSnS₄ (CZTS) were fabricated on molybdenum coated soda lime glass by evaporation using ZnS, Sn, Cu, and S sources. The coevaporation process was performed at a nominal substrate temperature of 550°C and at a sulfur partial pressure of 2–3 × 10⁻³ Pa leading to polycrystalline CZTS thin films with promising electronic properties. The CZTS absorber layers were grown copper‐rich, requiring a KCN etch step to remove excess copper sulfide. The compositional ratios as determined by energy‐dispersive X‐ray spectroscopy (EDX) after the KCN etch are Cu/(Zn + Sn): 1.0 and Zn/Sn: 1.0. A solar cell with an efficiency of 4.1% and an open‐circuit voltage of 541 mV was obtained. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrodeposited Cu2ZnSnSe4 thin film solar cell with 7% power conversion efficiency

High performance Cu₂ZnSnSe₄ (CZTSe) photovoltaic materials were synthesized by electrodeposition of metal stack precursors followed by selenization. A champion solar cell with 7.0% efficiency is demonstrated. This is the highest efficiency among all of the CZTSe solar cells prepared from electrodeposited metallic precursors reported to‐date. Device parameters are discussed from the perspective of material microstructure and composition in order to improve performance. In addition, a high performance electrodeposited CZTS (S only) solar cell was demonstrated and its device characteristics were compared against the CZTSe (Se only) cell. Using secondary ion mass spectrometry for the analysis of the chemical composition of the absorber layer, a higher concentration of oxygen in the electrodeposited absorber is thought to be the root cause of the lower performance of the electrodeposited CZTS or CZTSe solar cells with respect to a solar cell fabricated by evaporation. The grain boundary areas of Sn‐rich composition are thought to be responsible for the lower shunt resistance commonly observed in CZTSe devices. We measured the longest minority carrier lifetime of 18 ns among all reported kesterite devices. This work builds a good baseline for obtaining higher efficiency earth‐abundant solar cells, while it highlights electrodepositon as a low cost and feasible method for earth‐abundant thin film solar cell fabrication. Copyright © 2013 John Wiley & Sons, Ltd.     

Controllable Double Gradient Bandgap Strategy Enables High Efficiency Solution-Processed Kesterite Solar Cells

The double gradient bandgap absorber has the potential to enhance carrier collection, improve light collection efficiency, and make the performance of solar cells more competitive. However, achieving the double gradient bandgap structure is challenging due to the comparable diffusion rates of cations during high-temperature selenization in kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) films. Here, it has successfully achieved a double gradient bandgap in the CZTSSe absorber by spin-coating the K₂S solution during the preparation process of the precursor film. The K₂S insertion serves as an additional S source for the absorber, and the high-affinity energy of K-Se causes the position of the spin-coated K₂S solution locally Se-rich and S-poor. More importantly, the position of the bandgap minimum (notch) and the depth of the notch can be controlled by varying the concentration of K₂S solution and its deposition stage, thereby avoiding the electronic potential barrier produced by an inadvertent notch position and depth. In addition, the K─Se liquid phase expedites the selenization process to the elimination of the fine grain layer. The champion CZTSSe device achieved an efficiency of 13.70%, indicating the potential of double gradient bandgap engineering for the future development of high-efficiency kesterite solar cells.     

Dual Interfacial Modification Engineering with 2D MXene Quantum Dots and Copper Sulphide Nanocrystals Enabled High‐Performance Perovskite Solar Cells

The performance of perovskite solar cells (PSCs) strongly depends on the electron transport layer (ETL), perovskite absorber, hole transport layer (HTL), and their interfaces. Herein, the first approach to utilize ultrathin 2D titanium‐carbide MXenes (Ti₃C₂T_x quantum dots, TQD) by engineering the perovskite/TiO₂ ETL interface and perovskite absorber and introducing Cu_1.8S nanocrystals to perfect the Spiro‐OMeTAD HTL is represented. A significant hysteresis‐free power conversion efficiency improvement from 18.31% to 21.64% of PSCs is achieved after modifications with the enhanced short‐circuit current density, open‐circuit voltages, and fill factor. Various advanced characterizations, including femtosecond transient absorption spectroscopy, electrochemical impedance spectroscopy, and ultraviolet photoelectron spectroscopy, elucidate that the TQD/Cu_1.8S significantly contribute to the improved crystalline quality of the perovskite film with its large grain size and improved electron/holes extraction efficiencies at perovskite/ETL and perovskite/HTL interfaces. Furthermore, the long‐time ambient and light stability of PSCs are largely boosted through the TQD and/or Cu_1.8S nanocrystals doping, originating from the better crystallization of perovskite, suppressing the film aggregation and crystallization of HTL, and inhibiting the ultraviolet‐induced photocatalysis of the ETL. The findings highlight the TQD and Cu_1.8S can act as a superfast electrons and holes tunnel for the optoelectronic devices.     

Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells

Cu₂ZnSnS₄ (CZTS) is a promising thin‐film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se₂. We introduce a two‐step process for fabrication of CZTS films, involving reactive sputtering of a Cu‐Zn‐Sn‐S precursor followed by rapid annealing. X‐ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high‐temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with J_sc and V_oc both appearing to be limited by interface recombination. Copyright © 2012 John Wiley & Sons, Ltd.     

Cu2ZnSnS4 photovoltaic cell with improved efficiency fabricated by high‐temperature annealing after CdS buffer‐layer deposition

To improve the photovoltaic properties of Cu₂ZnSnS₄ (CZTS) cells, we investigated the effect of both the thickness of the deposited CdS layers and the post‐annealing temperature following CdS deposition on the photovoltaic properties of CZTS cells using a two‐layer CZTS structure. By depositing a thin CdS layer (40 nm) followed by high temperature annealing (603 K), we observed a remarkable increase in the short‐circuit current density because of the enhancement of the external quantum efficiency in the wavelength range of 400–800 nm. The best CZTS cell exhibited a conversion efficiency of 9.4% in the active area (9.1% in the designated area). In addition, we also fabricated a CZTS cell with open‐circuit voltage of 0.80 V by appropriately tuning the composition of the CZTS layers. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of temperature-time profile on Cu2ZnSnS4 films and cells based on sulfur-contained precursors

Cu₂ZnSnS₄ (CZTS) films with high-quality are necessary to the high-performance CZTS solar cells. The influence of different temperature-time profiles during sulfurization on CZTS films were investigated by characterizing the compactness, roughness and grain size of CZTS films. With slowing the heating-rate during sulfurization process, the grain size of the film is becoming larger and larger, but the large grain size degrades the flatness of the CZTS films at the same time. The formation of films having the target composition (EDX) and phase purity (Raman) was attributed to the optimum choice of composition ration of the precursor and the thermal profile during sulfurization. The optical and electrical properties of corresponding CZTS films were detected by using UV–vis–INR Spectrophotometer and Hall measurement, respectively. Finally, the CZTS film solar cells were fabricated and the best cell demonstrated 3.91% efficiency. A high R_s is the main reason to inhibit the performance of the cells.     

Impact of alloying duration of an electrodeposited Cu/Sn/Zn metallic stack on properties of Cu2ZnSnS4 absorbers for thin‐film solar cells

The impacts of preheating of an electrodeposited Cu/Sn/Zn (CTZ) stack precursor on structural changes of the CTZ precursor and the impact on structural and electric properties of the finally obtained Cu₂ZnSnS₄ (CZTS) films are discussed in detail. We found that preheating for relatively long durations improved the qualities of CZTS films: these films were composed of large grains and had compact and flat surface morphologies. The best solar cell with efficiency of 8.1% was obtained on the basis of a CZTS film derived from the CTZ precursor preheated for 200 min. The external quantum efficiency response of the cell indicated efficient utilization of photons with relatively long wavelength regions because of its good structural and electronic properties. On the other hand, a short circuit current density–temperature property of one of the best cells in this study suggested that the CZTS film had deep acceptor levels and/or an appreciable energy barrier to the Mo back contact. Moreover, an open circuit voltage–temperature property of the corresponding device showed activation energy of 1.18 eV, indicating preferential occurrence of CdS–CZTS interface recombination. Copyright © 2015 John Wiley & Sons, Ltd.     

9.0% efficient Cu2ZnSn(S,Se)4 solar cells from selenized nanoparticle inks

Thin‐film solar cells using Cu₂ZnSn(S,Se)₄ absorber materials continue to attract increasing attention. The synthesis of kesterite Cu₂ZnSnS₄ nanoparticles by a modified method of hot injection is explained. Characterization of the nanoparticles by energy dispersive X‐ray spectroscopy, X‐ray diffraction, Raman, and transmission electron microscopy is presented and discussed. When suspended in an ink, coated, and processed into a device, the nanoparticles obtained by this synthesis achieve a total area (active area) efficiency of 9.0% (9.8%) using AM 1.5 illumination and light soaking. This improvement over the previous efficiency of 7.2% is attributed to the modified synthesis approach, as well as fine‐tuned conditions for selenizing the coated nanoparticles into a dense absorber layer. Copyright © 2014 John Wiley & Sons, Ltd.     

Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu‐ and Ag‐Based Kesterite Compounds

The development of kesterite Cu₂ZnSn(S,Se)₄ thin‐film solar cells is currently hindered by the large deficit of open‐circuit voltage (V_oc), which results from the easy formation of Cu_Zn antisite acceptor defects. Suppressing the formation of Cu_Zn defects, especially near the absorber/buffer interface, is thus critical for the further improvement of kesterite solar cells. In this paper, it is shown that there is a large disparity between the defects in Cu‐ and Ag‐based kesterite semiconductors, i.e., the Cu_Zn or Cu_Cd acceptor defects have high concentration and are the dominant defects in Cu₂ZnSn(S,Se)₄ or Cu₂CdSnS₄, but the Ag_Zn acceptor has only a low concentration and the dominant defects are donors in Ag₂ZnSnS₄. Therefore, the Cu‐based kesterites always show p‐type conductivity, while the Ag‐based kesterites show either intrinsic or weak n‐type conductivity. Based on this defect disparity and calculated band alignment, it is proposed that the V_oc limit of the kesterite solar cells can be overcome by alloying Cu₂ZnSn(S,Se)₄ with Ag₂ZnSn(S,Se)₄, and the composition‐graded (Cu,Ag)₂ZnSn(S,Se)₄ alloys should be ideal light‐absorber materials for achieving higher efficiency kesterite solar cells.     

The Role of Sulfur in Solution‐Processed Cu2ZnSn(S,Se)4 and its Effect on Defect Properties

Understanding the electrically active defects in kesterite Cu₂ZnSn(S,Se)₄(CZTSSe) is critical for the continued development of solar cells based on this material, but challenging due to the complex nature of this polycrystalline multinary material. A comparative study of CZTSSe alloys with three different bandgaps, made by introducing different fractions of sulfur during the annealing process, is presented. Using admittance spectroscopy, drive level capacitance profiling, and capacitance‐voltage profiling, the dominant defect energy level present in the low sulfur content device is determined to be 0.134 eV above the valence band maximum, with a bulk defect density of 8 × 10¹⁴ cm^?3, while the high sulfur content device shows a deeper defect energy level of 0.183 eV and a higher bulk defect density, 8.2 × 10¹⁵ cm^?3. These findings are consistent with the current density–voltage characteristics of the resulting solar cells and their external quantum efficiency. It suggests that as the sulfur content increases, the bandgap of the absorber is enlarged, leading to an increasing open‐circuit voltage (V_oc), that is accompanied by stronger recombination due to the higher defect density of the sulfur‐rich absorber. This is reflected in large V_oc deficit and poor carrier collection of the high sulfur content device.     

Thin film solar cell with 8.4% power conversion efficiency using an earth‐abundant Cu2ZnSnS4 absorber

Using vacuum process, we fabricated Cu₂ZnSnS₄ solar cells with 8.4% efficiency, a number independently certified by an external, accredited laboratory. This is the highest efficiency reported for pure sulfide Cu₂ZnSnS₄ prepared by any method. Consistent with literature, the optimal composition is Cu‐poor and Zn‐rich despite the precipitation of secondary phases (e.g., ZnS). Despite a very thin absorber thickness (~600 nm), a reasonably good short‐circuit current was obtained. Time‐resolved photoluminescence measurements suggest a minority carrier‐diffusion length on the order of several hundreds of nanometers and relatively good collection of photo‐carriers across the entire absorber thickness. Copyright © 2011 John Wiley & Sons, Ltd.     

Suppressed Interface Defects by GeSe2 Post-Deposition Treatment Enables High-Efficiency Kesterite Solar Cells

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) has emerged as a promising photovoltaic material not only because of its environmentally benign and earth-abundant constituents, but also its outstanding photoelectronic properties. Unfortunately, the significant open-circuit voltage (V_oc) loss and inferior fill factor (FF) resulting from abundant nonradiative carrier recombination at depletion region has become a major obstacle for further improving device performance. Here, an effective strategy to passivate the deep trap and band-tail states in the heterojunction is proposed, by modifying the CZTSSe absorber layer with GeSe₂ post-deposition treatment. The results reveal that the Ge⁴⁺ can migrate into the front surface of the absorber, which plays an active role in suppressing the Cu_Sn deep defects and [2Cu_Zn+Sn_Zn] defect clusters, accordingly dramatically reducing severe interfacial nonradiative carrier recombination of CZTSSe photovoltaic device. Under optimal treatment conditions, the CZTSSe solar cell efficiency increases from 10.36% to 12.22%, mainly benefitting from the increasement of V_oc and FF.     

High photo‐conversion efficiency in double‐graded Cu(In,Ga)(S,Se)2 thin film solar cells with two‐step sulfurization post‐treatment

Sulfur is extensively used to increase the bandgap of Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells and to improve the open circuit voltage (V_OC ) in order to optimize the characteristics of the devices. This study uses a sulfurization process to obtain a double‐graded bandgap profile. Selenization was carried out on Cu(In,Ga) precursors, followed by one sulfurization process or two consecutive sulfurization processes on top of the CIGSe absorber layer surface. The optimum two‐step sulfurization process provides an increase of V_OC of 0.05 V and an improvement of conversion efficiency of 1.17%. The efficiency of the 30 × 30 cm² monolithic module, which has 64 CIGS cells connected in series (aperture area: 878.6 cm²), is 15.85%. The optical and electrical properties of the phase and the work function distribution were investigated using the depth profiles of the absorber layer as a function of the sulfurization conditions. The CIGSSe thin film formed by two‐step sulfurization with a high sulfur concentration exhibits a single work function peak, better crystallinity, and higher conversion efficiency than those of the thin film formed by two‐step sulfurization at low sulfur concentration. In terms of the Raman spectra depth profile, the phase areas for the CIGSSe thin film that underwent the optimized high sulfur concentration two‐step‐sulfurization appeared to have less of Cu_2‐xSe phase than that with low sulfur concentration. Consequently, surface and interface phase analysis is an essential consideration to improve cell efficiency. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.