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.     

Microstructure and Raman Scattering of Cu<Subscript>2</Subscript>ZnSnSe<Subscript>4</Subscript> Thin Films Deposited onto Flexible Metal Substrates

Cu₂ZnSnSe₄ thin films are produced by selenizing electrochemically layer-by-layer deposited and preliminarily annealed Cu–Zn–Sn precursors. For flexible metal substrates, Mo and Ta foils are used. The morphology, elemental and phase compositions, and crystal structure of Cu₂ZnSnSe₄ films are studied by scanning electron microscopy, X-ray spectral microanalysis, X-ray phase analysis, and Raman spectroscopy.     

Structure and properties of electrodeposited films and film stacks for precursors of chalcopyrite and kesterite solar cells

A comparative analysis of the structure and surface morphology of copper, indium, tin, and zinc films and their layered compositions fabricated by electrochemical deposition in the galvanostatic steady-state mode, galvanostatic mode with the ultrasonic agitation of electrolytes, and forward and reverse pulsed modes with rectangular potential pulses is made. The effect of the electrodeposition modes on the structure, optical properties, and surface morphology of amorphous and crystalline selenium films is studied. Cu/In/Se and Cu/Sn/Zn/Se film compositions, which are model precursors of, respectively, chalcopyrite CuInSe₂ and kesterite, are fabricated by successive electrochemical deposition. Upon being converted by subsequent annealings to CuInSe₂ and Cu₂ZnSnSe₄ semiconductor materials, these precursors will be used as the base layers for a new generation of inexpensive high-efficiency thin-film solar cells.     

Growth mechanisms of co‐evaporated kesterite: a comparison of Cu‐rich and Zn‐rich composition paths

Earth abundant kesterite solar cells have achieved 7–10% cell efficiency mostly by processes that separate the film deposition and the annealing into two sequential steps. In contrast, co‐evaporation onto a high‐temperature substrate, demonstrating previous success in chalcopyrite (Cu(In,Ga)Se₂) solar cells, allows real‐time composition control. Chalcopyrite research widely supports the model that Cu‐rich growth conditions assist grain growth, and subsequently, the endpoint composition can be adjusted back to Cu‐poor via monitoring the surface emissivity of the film. On the basis of the same intentions, the recent development of co‐evaporated kesterite (Cu₂ZnSnSe₄) adapts the concept and achieves 9.2% efficiency. To understand the effect of growth strategies, this study examines the phase evolution, grain morphology, and device performance in Cu‐rich growth and other strategies (Zn‐rich and close‐to‐stoichiometric). By characterizing films obtained from interrupted depositions and also interpreting the variation in surface emission during growths, this study found a subtle hindrance in the reaction of Cu_xSe_y and ZnSe possibly caused by the volatile nature of SnSe_x. The hindrance explains why, distinctive from chalcopyrite, little difference in grain size is observed between kesterite films made by Cu‐rich versus Zn‐rich growth at these deposition rates. At last, a Zn‐rich growth 9.1% device, certified by the National Renewable Energy Laboratory, is presented, which equals the performance of the previously‐reported Cu‐rich growth device. At the present stage, we believe the Cu‐rich and Zn‐rich growth share equal promise for the optimization of kesterite solar cells. Copyright © 2013 John Wiley & Sons, Ltd.     

Cu2ZnSnSe4 thin film solar cells produced via co‐evaporation and annealing including a SnSe2 capping layer

Cu₂ZnSnSe₄ (CZTSe) thin film solar cells have been produced via co‐evaporation followed by a high‐temperature annealing. In order to reduce the decomposition of the CZTSe, a SnSe₂ capping layer has been evaporated onto the absorber prior to the high‐temperature treatment. This eliminates the Sn losses due to SnSe evaporation. A solar cell efficiency of 5.1% could be achieved with this method. Moreover, the device does not suffer from high series resistance, and the dominant recombination pathway is situated in the absorber bulk. Finally, different illumination conditions (white light, red light, and yellow light) reveal a strong loss in fill factor if no carriers are generated in the CdS buffer layer. This effect, known as red‐kink effect, has also been observed in the closely related Cu(In,Ga)Se₂ thin film solar cells. Copyright © 2013 John Wiley & Sons, Ltd.     

Secondary phase formation in Zn‐rich Cu2ZnSnSe4‐based solar cells annealed in low pressure and temperature conditions

Zn‐rich Cu₂ZnSnSe₄ (CZTSe) films were prepared by a two‐step process consisting in the DC‐magnetron sputtering deposition of a metallic stack precursor followed by a reactive anneal under a Se + Sn containing atmosphere. Precursor composition and annealing temperature were varied in order to analyze their effects on the morphological, structural, and optoelectronic properties of the films and solar cell devices. Raman scattering measurements show the presence of ZnSe as the main secondary phase in the films, as well as the presence of SnSe at the back absorber region of the films processed with lower Zn‐excess values and annealing temperatures. The ZnSe phase is found to accumulate more towards the surface of the absorber in samples with lower Zn‐excess and lower temperature annealing, while increasing Zn‐excess and annealing temperature promote its aggregation towards the back absorber region of the devices. These measurements indicate a strong dependence of these process variables in secondary phase formation and accumulation. In a preliminary optimization of both the composition and reactive annealing process, a solar cell with 4.8% efficiency has been fabricated, and potential mechanisms limiting device efficiency in these devices are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Cu2ZnSnSe4 thin film solar cells produced by selenisation of magnetron sputtered precursors

Polycrystalline thin films of Cu₂ZnSnSe₄ (CZTSe) were produced by selenisation of Cu(Zn,Sn) magnetron sputtered metallic precursors for solar cell applications. The p‐type CZTSe absorber films were found to crystallize in the stannite structure (a = 5·684 Å and c = 11·353 Å) with an electronic bandgap of 0·9 eV. Solar cells with the indium tin oxide structure (ITO)/ZnO/CdS/CZTSe/Mo were fabricated with device efficiencies up to 3·2% measured under standard AM1·5 illumination. Copyright © 2009 John Wiley & Sons, Ltd.     

Highly conductive ZnO films with high near infrared transparency

We present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n‐type window of low band gap solar cells. We demonstrate that low‐voltage radio frequency (RF) biasing of growing ZnO films during their deposition by non‐reactive sputtering makes them as conductive as when doped by aluminium (ρ≤1·10⁻³Ω cm). The films prepared with additional RF biasing possess lower free‐carrier concentration and higher free‐carrier mobility than Al‐doped ZnO (AZO) films of the same resistivity, which results in a substantially higher transparency in the near infrared region (NIR). Furthermore, these films exhibit good ambient stability and lower high‐temperature stability than the AZO films of the same thickness. We also present the characteristics of Cu(InGa)Se₂, CuInSe₂ and Cu₂ZnSnSe₄‐based solar cells prepared with the transparent window bilayer formed of the isolating and conductive ZnO films and compare them to their counterparts with a standard ZnO/AZO bilayer. We show that the solar cells with nominally undoped ZnO as their transparent conductive oxide layer exhibit an improved quantum efficiency for λ > 900 nm, which leads to a higher short circuit current density J_SC. This aspect is specifically beneficial in preparation of the Cu₂ZnSnSe₄ solar cells with band gap down to 0.85 eV; our champion device reached a J_SC of nearly 39 mAcm⁻², an open circuit voltage of 378mV, and a power conversion efficiency of 8.4 %. Copyright © 2015 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.     

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.     

High‐efficiency Cu2ZnSn(S,Se)4 solar cells fabricated through a low‐cost solution process and a two‐step heat treatment

A method for fabricating high‐efficiency Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is presented, and it is based on a non‐explosive, low‐cost, and simple solution process followed by a two‐step heat treatment. 2‐Methoxyethanol was used as a solvent, and Cu, Zn, Sn, chloride salts, and thiourea were used as solutes. A CZTSSe absorber was prepared by sulfurising and then selenising an as‐coated Cu₂ZnSnS₄ (CZTS) film. Sulfurisation in a sulfur vapour filled furnace for a long time (2 h) enhanced the crystallisation of the as‐coated CZTS film and improved the stability of the CZTS precursor, and selenisation promoted further grain growth to yield a void‐free CZTSSe film. Segregation of Cu and S at the grain boundaries, the absence of a fine‐grain bottom layer, and the large grain size of the CZTSSe absorber were the main factors that enhanced the grain‐to‐grain transport of carriers and consequently the short‐circuit current (J_sc ) and efficiency. The efficiency of the CZTS solar cell was 5.0%, which increased to 10.1% after selenisation. For the 10.1% CZTSSe solar cell, the external quantum efficiency was approximately 80%, the open‐circuit voltage was 450 mV, the short‐circuit current was 36.5 mA/cm², and the fill factor was 61.9%. Copyright © 2016 John Wiley & Sons, Ltd.     

Improving the open‐circuit voltage of Cu2ZnSnSe4 thin film solar cells via interface passivation

This study reports on substantial improvement of the open‐circuit voltage (V _oc) of Cu₂ZnSnSe₄ (CZTSe) thin film solar cells by applying a passivation strategy to both the top and bottom interfaces of the CZTSe absorber, which involves insertion of a thin dielectric layer between the CZTSe and the surrounding layers. The study also presents in‐depth material characterizations using transmission electron microscopy, energy dispersive X‐ray spectroscopy, low‐temperature photoluminescence, and secondary ion mass spectrometry, to reveal the effects of the interface passivation. To passivate the bottom Mo/CZTSe interface, a dielectric layer with patterned local contacts of dimensions down to ~100 nm was prepared using nanosphere lithography. With this, the V _oc, short‐circuit current, and fill factor (FF) were significantly enhanced due to reduction in carrier recombination at the bottom Mo/CZTSe interface. The top CZTSe/CdS interface was passivated by a thin dielectric layer which prevented inter‐diffusion of Cd and Cu at the top interface, thereby improving the junction quality. Application of the top passivation layers resulted in substantial improvement in V _oc and FF, thereby achieving the V _oc deficit of 0.542 V which is the record value among reported CZTSe solar cells. Copyright © 2017 John Wiley & Sons, Ltd.     

Ruthenium Oxide Hydrogen Evolution Catalysis on Composite Cuprous Oxide Water‐Splitting Photocathodes

Photocathodes based on cuprous oxide (Cu₂O) are promising materials for large scale and widespread solar fuel generation due to the abundance of copper, suitable bandgap, and favorable band alignments for reducing water and carbon dioxide. A protective overlayer is required to stabilize the Cu₂O in aqueous media under illumination, and the interface between this overlayer and the catalyst nanoparticles was previously identified as a key source of instability. Here, the properties of the protective titanium dioxide overlayer of composite cuprous oxide photocathodes are further investigated, as well as an oxide‐based hydrogen evolution catalyst, ruthenium oxide (RuO₂). The RuO₂‐catalyzed photoelectrodes exhibit much improved stability versus platinum nanoparticles, with 94% stability after 8 h of light‐chopping chronoamperometry. Faradaic efficiencies of ～100% are obtained as determined by measurement of the evolved hydrogen gas. The sustained photocurrents of close to 5 mA cm^?2 obtained with this electrode during the chronoamperometry measurement (at 0 V vs. the reversible hydrogen electrode, pH 5, and simulated 1 sun illumination) would correspond to greater than 6% solar‐to‐hydrogen conversion efficiency in a tandem photoelectrochemical cell, where the bias is provided by a photovoltaic device such as a dye‐sensitized solar cell.     

Effect of Se flux on CuIn1‐xGaxSe2 film in reactive sputtering process

CuIn_1‐xGa_xSe₂ (CIGS) thin films were grown on Mo/soda lime glass using a reactive sputtering process in which a Se cracker was used to deliver reactive Se molecules. The Cu_0·6 Ga_0·4 and Cu_0·4In_0·6 targets were simultaneously sputtered under the delivery of reactive Se. The effects of Se flux on CIGS film deposition were investigated. The CIGS film growth rate decreased, and the surface roughness of a film increased as the Se flux increased. The [112] crystal orientation was dominant, and metallic crystal phases such as Cu₉Ga₄ and Cu₁₆In₉ in a film were disappearing with increasing Se flux. A solar cell fabricated using a 900‐nm CIGS film showed the power conversion efficiency of 8·6%, the highest value found in a sub‐micron thick CIGS solar cell related to a reactive sputtering process with metallic targets. Copyright © 2011 John Wiley & Sons, Ltd.     

Low‐Temperature Processable High‐Performance Electrochemically Deposited p‐Type Cuprous Oxides Achieved by Incorporating a Small Amount of Antimony

The development of an electrochemically robust method for the low‐temperature deposition of cuprous oxide (Cu₂O) thin films with reliable and conductive p‐type characteristics could yield breakthroughs in earth abundant and ecofriendly all oxide‐based photoelectronic devices. The incorporation of the group‐V element antimony (Sb) in the solution‐based electrodeposition process has been investigated. A small amount of Sb (1.2 at%) in the Cu₂O resulted in rapid nucleation and coalescence at the initial stage of electrochemical reaction, and finally made the surface morphology smooth in 2D. The growth behavior changed due to Sb addition and produced a strong diffraction intensity, single‐domain‐like diffraction patterns, and low angle tilt boundaries in the Cu₂O:Sb film, implying extremely improved crystallinity. As a result, these films exhibited extraordinary optical transmittance and band‐to‐band photoluminescence emission as well as higher electrical conductivity. The Cu/Cu₂O:Sb Schottky diode showed good rectifying characteristics and more sensible photoresponsibility.     

Electrodeposited zinc grid as low‐cost solar cell front contact

This paper presents an innovative low‐cost electrodeposition process to grow metallic zinc grids as a front contact for Cu(In,Ga)(Se,S)₂ (CIGS) and silicon heterojunction solar cells as an alternative to complex and expensive monolithic integration and silver screen printing techniques respectively. Morphological and electrical properties of the grid have been investigated and compared with a reference evaporated one. High quality and conformal zinc grids have been deposited showing very high growth rates up to 3.3 µm min⁻¹. Zinc grid is successfully deposited as front electrode for CIGS solar cells that are fabricated by a variety of deposition processes. Efficiency (16.3%) is achieved without antireflection coating on a 0.5 cm² co‐evaporated absorber and 14.8% on an electrodeposited one. Using electrodeposition for the growth of the doped ZnO film as well, a 14.1% efficiency is demonstrated on an all‐wet solar cell only composed of layers deposited by atmospheric methods—from absorber to metallic grid. The process is then applied to a 4.2 cm² cell as a first step toward large‐scale application. Finally, a zinc grid is deposited on a 0.5 cm² silicon heterojunction showing a promising 17% efficiency. Copyright © 2016 John Wiley & Sons, Ltd.     

Impact of the Cd2+ treatment on the electrical properties of Cu2ZnSnSe4 and Cu(In,Ga)Se2 solar cells

The present contribution aims at determining the impact of modifying the properties of the absorber/buffer layer interface on the electrical performance of Cu₂ZnSnSe₄ (CZTSe) thin‐film solar cells, by using a Cd²⁺ partial electrolyte (Cd PE) treatment of the absorber before the buffer layer deposition. In this work, CZTSe/CdS solar cells with and without Cd PE treatment were compared with their respective Cu(In,Ga)Se₂ (CIGSe)/CdS references. The Cd PE treatment was performed in a chemical bath for 7 min at 70 °C using a basic solution of cadmium acetate. X‐ray photoemission spectroscopy measurements have revealed the presence of Cd at the absorber surface after the treatment. The solar cells were characterized using current density–voltage (J–V), external quantum efficiency, and drive‐level capacitance profiling measurements. For the CZTSe‐based devices, the fill factor increased from 57.7% to 64.0% when using the Cd PE treatment, leading to the improvement of the efficiency (η) from 8.3% to 9.0% for the best solar cells. Similar observations were made on the CIGSe solar cell reference. This effect comes from a considerable reduction of the series resistance (R_S) of the dark and light J–V, as determined using the one‐diode model. The crossover effect between dark and light J–V curves is also significantly reduced by Cd PE treatment. Copyright © 2015 John Wiley & Sons, Ltd.     

Tunable Room‐Temperature Synthesis of ReS2 Bicatalyst on 3D‐ and 2D‐Printed Electrodes for Photo‐ and Electrochemical Energy Applications

The advancement in 3D‐printing technologies conveniently offers boundless opportunities for the customization of a practical substrate or electrode for diverse functionalities. ReS₂ is an attractive transition metal dichalcogenide (TMD), showing strong photoelectrochemical activities. Two advanced systems are merged for the next step in electrochemistry—the limits of the prevailing synthesis techniques of TMDs operating at high temperature or low pressure, which are not compatible with 3D‐printed polymer electrodes that can withstand only comparatively low temperatures, are overcome. A unique NH₄ReS₄ precursor is separately prepared to conduct subsequent ReS₂ electrodeposition at room temperature on 3D‐printed carbon and 2D‐printed carbon electrodes. The deposited ReS₂ is investigated as a dual‐functional electro‐ and photocatalyst in hydrogen evolution reaction and photoelectrochemical oxidation of water. Moreover, the electrodeposition conditions can be adjusted to optimize the catalytic activities. These encouraging outcomes demonstrate the simplicity yet versatility of TMDs based on electrodeposition technique on a rationally designed conductive platform, which creates numerous possibilities for other TMDs and on other low‐temperature substrates for electrochemical energy devices.     

Electrodeposition of ternary CuxSnySz thin films for photovoltaic applications

We exploited alternated electrodeposition of Cu, Sn and S to obtain Cu_xSn_yS_z thin films. These materials are kesterite‐type chalcogenides that have attracted a relevant interest from worldwide researchers as low cost and high conversion efficiency solar cell devices. Films were grown on silver substrate, controlling the growth of the electrodeposited structures at the nanometric level. The obtained films were characterized by diffuse reflectance spectroscopy, voltammetric stripping and atomic force microscopy. Experimental bandgap energies resulted linearly modulated by changes of chemical composition and thickness. On the basis of these results, we candidate electrodeposition as a room temperature method to obtain thin films for solar cell technology with low energy investment and negligible environmental impact. Copyright © 2013 John Wiley & Sons, Ltd.     

The Characteristics of Cu2O Thin Films Deposited Using RF‐Magnetron Sputtering Method with Nitrogen‐Ambient

We investigate the characteristics of Cu₂O thin films deposited through the addition of N₂ gas. The addition of N₂ gas has remarkable effects on the phase changes, resulting in improved electrical and optical properties. An intermediate phase (6CuO·Cu₂O) appears at a N₂ flow rate of 1 sccm, and a Cu₂O (200) phase is then preferentially grown at a higher feeding amount of N₂. The optical and electrical properties of Cu₂O thin films are improved with a sufficient N₂ flow rate of more than 15 sccm, as confirmed through various analyses. Under this condition, a high bandgap energy of 2.58 eV and a conductivity of 1.5×10^?2 S/cm are obtained. These high‐quality Cu₂O thin films are expected to be applied to Cu₂O‐based heterojunction solar cells and optical functional films.