Over 14% Efficiency of Directly Sputtered Cu(In,Ga)Se2 Absorbers without Postselenization by Post‐Treatment of Alkali Metals

Direct sputtering of a single quaternary Cu(In,Ga)Se₂ (CIGS) target without postselenization is a promising approach to fabricating CIGS absorbers. However, the device efficiency of the quaternary‐sputtered CIGS is limited to 10%–11% due to the low and uncontrollable Se supply during the quaternary sputtering process. Here, an enhanced efficiency of 14.1% is reported by directly sputtering from a CIGS target without extra Se supply followed by sequential postdeposition treatments (PDT) of NaF and KF. The effects of different post‐treatments of alkali metals on quaternary‐sputtered CIGS thin films are discussed in detail. A Cu‐depleted surface is not observed in the quaternary‐sputtered CIGS thin films after KF‐PDT, different from the observation in the coevaporated CIGS, in which the Cu‐depleted surface layer induced by KF‐PDT enhances the efficiency. On the other hand, it is found that KF‐PDT reduces Se vacancies more effectively than NaF‐PDT, which could be another electrically benign behavior of KF‐PDT. The effective passivation of Se vacancies after KF‐PDT overcomes the Se‐poor nature of the quaternary sputtering process without postselenization. Therefore, KF‐PDT combined with Na doping, which is known to annihilate In_Cu defects, significantly improves minority carrier lifetime and cell performance.     

Chalcopyrite Semiconductors for Quantum Well Solar Cells

The possibilities of using highly absorbing chalcopyrite semiconductors of the type Cu(In,Ga)Se₂ in a quantum well solar cell structure are explored. Thin alternating layers of 50 nm CuInSe₂ and CuGaSe₂ were grown epitaxially on a GaAs(100) substrate. The optical properties of a resulting structure of three layers indicate charge carrier confinement in the low band gap CuInSe₂ layer. By compositional analysis interdiffusion of In and Ga at the interfaces was found. The compositional profile was converted into a conduction‐band diagram, for which the quantization of energy levels was numerically confirmed using the effective‐mass approximation. The results provide a promising basis for the future development of chalcopyrite‐type quantum well structures and their application, i.e. in quantum well solar cells.     

Non‐Hydrazine Solutions in Processing CuIn(S,Se)2 Photovoltaic Devices from Hydrazinium Precursors

We report an approach for the fabrication of CuIn(S,Se)₂‐based photovoltaic devices from hydrazinium precursors in non‐hydrazine solvents, specifically a ethanolamine/dimethyl sulfoxide (EA/DMSO) mixture. For the first time, both Cu hydrazinium precursor and Cu‐In hydrazinium precursor are found with good solubility in non‐hydrazine solvents, producing molecular‐level blending of metal precursors. Sulfur loss in Cu hydrazinium precursor is compensated for by either introduction of excessive S/Se or the formation of S/Se‐bridged Cu‐In compounds. The success of dissolving Cu‐In hydrazinium precursor is ascribed to the coordinated S group and strong intramolecular interaction within non‐hydrazine solvents. X‐ray diffraction (XRD) and Raman characterization indicate the formation of the CuIn(S,Se)₂ phase after annealing. Through introducing different amounts of excess S/Se, the ratio between CuInS₂ and CuInSe₂, as well as the morphology of the resulted CuIn(S,Se)₂ film can be controlled. Optimized devices exhibit a power conversion efficiency of 3.8% with a CISS absorber layer of only around 300 nm thickness, which is comparable to N₂H₄‐based devices of similar thickness.     

The Development of Hydrazine‐Processed Cu(In,Ga)(Se,S)2 Solar Cells

The hydrazine‐based deposition of Cu(In,Ga)(S,Se)₂ (CIGS) thin films has attracted considerable attention in recent years due to its potential for the high‐throughput production of photovoltaic devices based on this absorber material. This article provides an introduction as well as presenting a complete picture of the current status of hydrazine‐based CIGS solar‐cell fabrication, including the three major steps of this deposition process: dissolution of the precursor materials in hydrazine, deposition of a film from the resulting precursor solution, and the completion and characterization of a photovoltaic device following absorber deposition. Recent discoveries are then discussed, regarding the dissolution chemistry of the relevant precursor complexes in hydrazine, which together represent the true foundation of this processing method. Recent studies on CIGS film formation are then summarized, including the control and analysis of the crystalline phase, electronic bandgap, and film morphology. Finally, the latest progress in high‐performance device fabrication is highlighted, with a focus on optoelectronic characterization including current–voltage, junction capacitance, and minority carrier lifetime measurements. Finally, a discussion and future outlook is provided.     

Kesterite Thin‐Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4

Quaternary semiconducting materials based on the kesterite (A₂BCX₄) mineral structure are the most promising candidates to overtake the current generation of light‐absorbing materials for thin‐film solar cells. Cu₂ZnSnS₄ (CZTS), Cu₂ZnSnSe₄ (CZTSe) and their alloy Cu₂ZnSn(Se,S)₄ consist of abundant, low‐cost and non‐toxic elements, unlike current CdTe and Cu(In,Ga)Se₂ based technologies. Zinc‐blende related structures are formed by quaternary compounds, but the complexity associated with the multi‐component system introduces difficulties in material growth, characterization, and application. First‐principles electronic structure simulations, performed over the past five years, that address the structural, electronic, and defect properties of this family of compounds are reviewed. Initial predictions of the bandgaps and crystal structures have recently been verified experimentally. The calculations highlight the role of atomic disorder on the cation sub‐lattice, as well as phase separation of Cu₂ZnSnS₄ into ZnS and CuSnS₃, on the material performance for light‐to‐electricity conversion in photovoltaic devices. Finally, the current grand challenges for materials modeling of thin‐film solar cells are highlighted.     

Efficient and Stable TiO2:Pt–Cu(In,Ga)Se2 Composite Photoelectrodes for Visible Light Driven Hydrogen Evolution

Novel thin film composite photocathodes based on device‐grade Cu(In,Ga)Se₂ chalcopyrite thin film absorbers and transparent conductive oxide Pt‐implemented TiO₂ layers on top are presented for an efficient and stable solar‐driven hydrogen evolution. Thin films of phase‐pure anatase TiO₂ are implemented with varying Pt‐concentrations in order to optimize simultaneously i) conductivity of the films, ii) electrocatalytic activity, and iii) light‐guidance toward the chalcopyrite. Thereby, high incident‐photon‐to‐current‐efficiencies of more than 80% can be achieved over the full visible light range. In acidic electrolyte (pH 0.3), the most efficient Pt‐implemented TiO₂–Cu(In,Ga)Se₂ composite electrodes reveal i) photocurrent densities up to 38 mA cm^?2 in the saturation region (?0.4 V RHE, reversible hydrogen electrode), ii) 15 mA cm^?2 at the thermodynamic potential for H₂‐evolution (0 V RHE), and iii) an anodic onset potential shift for the hydrogen evolution (+0.23 V RHE). It is shown that the gradual increase of the Pt‐concentration within the TiO₂ layers passes through an efficiency‐ and stability‐maximum of the device (5 vol% of Pt precursor solution). At this maximum, optimized light‐incoupling into the device‐grade chalcopyrite light‐absorber as well as electron conductance properties within the surface layer are achieved while no degradation are observed over more than 24 h of operation.     

Confined and Chemically Flexible Grain Boundaries in Polycrystalline Compound Semiconductors

Grain boundaries (GBs) in polycrystalline Cu(In,Ga)Se₂ thin films exhibit only slightly enhanced recombination, as compared with the grain interiors, allowing for very high power‐conversion efficiencies of more than 20% in the corresponding solar‐cell devices. This work highlights the specific compositional and electrical properties of Cu(In,Ga)Se₂ GBs by application of appropriate subnanometer characterisation techniques: inline electron holography, electron energy‐loss spectroscopy, and atom‐probe tomography. It is found that changes of composition at the GBs are confined to regions of only about 1 nm in width. Therefore, these compositional changes are not due to secondary phases but atomic or ionic redistribution within the atomic planes close to the GBs. For different GBs in the Cu(In,Ga)Se₂ thin film investigated, different atomic or ionic redistributions are also found. This chemical flexibility makes polycrystalline Cu(In,Ga)Se₂ thin films particularly suitable for photovoltaic applications.     

Efficiency Improvement of Near‐Stoichiometric CuInSe2 Solar Cells for Application in Tandem Devices

State‐of‐the‐art Cu(In,Ga)Se₂ (CIGS) solar cells are grown with considerably substoichiometric Cu concentrations. The resulting defects, as well as potential improvements through increasing the Cu concentration, have been known in the field for many years. However, so far, cells with high Cu concentrations show decreased photovoltaic parameters. In this work, it is shown that RbF postdeposition treatment of CuInSe₂ solar cells allows for capturing the benefits from the improved absorber quality with increasing Cu content. A reduced defect density and an increased doping level for cells with high Cu concentrations close to stoichiometry are demonstrated. Implementing a high mobility front transparent conductive oxide (TCO), the improved absorbers with 1.00 eV bandgap yield a solar cell efficiency of 19.2%, and combined with a perovskite top cell a 4‐terminal tandem efficiency of 25.0% are demonstrated, surpassing the record efficiency of both subcell technologies.     

Atomic‐Scale Observation of Oxygen Substitution and Its Correlation with Hole‐Transport Barriers in Cu2ZnSnSe4 Thin‐Film Solar Cells

Kesterite‐type Cu₂ZnSn(S,Se)₄ has been extensively studied over the past several years, with researchers searching for promising candidates for indium‐ and gallium‐free inexpensive absorbers in high‐efficiency thin‐film solar cells. Many notable experimental and theoretical studies have dealt with the effects of intrinsic point defects, Cu/Zn/Sn nonstoichiometry, and cation impurities on cell performance. However, there have been few systematic investigations elucidating the distribution of oxygen at an atomic scale and the correlation between oxygen substitution and charge transport despite unavoidable incorporation of oxygen from the ambient atmosphere during thin‐film fabrication. Using energy‐dispersive X‐ray spectroscopy, scanning transmission electron microscopy, and electron energy‐loss spectroscopy, the presence of nanoscale layers is directly demonstrated in which oxygen is substantially substituted for Se, near grain boundaries in polycrystalline Cu₂ZnSnSe₄ films. Density‐functional theory calculations also show that oxygen substitution remarkably lowers the valence band maximum and subsequently widens the overall bandgap. Consequently, anion modification by oxygen can make a major contribution to the formation of a robust barrier blocking the holes from bulk grains into grain boundaries, thereby efficiently attaining electron?hole separation. The findings provide crucial insights into achieving better energy conversion efficiency in kesterite‐based thin‐film solar cells through optimum control of oxidation during the fabrication process.     

Solution‐Processed TiO2 Nanoparticles as the Window Layer for CuIn(S,Se)2 Devices

As a wide‐bandgap semiconductor, titanium dioxide (TiO₂) with a porous structure has proven useful in dye‐sensitized solar cells, but its application in low‐cost, high‐efficiency inorganic photovoltaic devices based on materials such as Cu(InGa)Se₂ or Cu₂ZnSnS₄ is limited. Here, a thin film made from solution‐processed TiO₂ nanocrystals is demonstrated as an alternative to intrinsic zinc oxide (i‐ZnO) as the window layer of CuInS_xSe_1?x solar cells. The as‐synthesized, well‐dispersed, 6 nm TiO₂ nanocrystals are assembled into thin films with controllable thicknesses of 40, 80, and 160 nm. The TiO₂ nanocrystal films with thicknesses of 40 and 80 nm exhibit conversion efficiencies (6.2% and 6.33%, respectively) that are comparable to that of a layer of the typical sputtered i‐ZnO (6.42%). The conversion efficiency of the devices with a TiO₂ thickness of 160 nm decreases to 2.2%, owing to the large series resistance. A 9‐hour reaction time leads to aggregated nanoparticles with a much‐lower efficiency (2%) than that of the well‐dispersed TiO₂ nanoparticles prepared using a 15‐hour reaction time. Under optimized conditions, the champion TiO₂ nanocrystal‐film‐based device shows even higher efficiency (9.2%) than a control device employing a typical i‐ZnO film (8.6%).     

Surface Cleaning and Passivation Using (NH4)2S Treatment for Cu(In,Ga)Se2 Solar Cells: A Safe Alternative to KCN

With the aim of developing a safe alternative to the KCN etchant for the removal of Cu_xSe secondary phases at the surface of Cu(In,Ga)Se₂ (CIGSe) absorber, a method based on ammonium sulfide (AS) chemical treatment is proposed. Although lower etching rates are observed compared with the KCN reference solution, the AS solution is found to selectively etch Cu_xSe phases. In addition, it allows modifying the surface chemical state of the CIGSe absorber by incorporation of sulfur. As a consequence, the minority carrier lifetime located close to the surface of the absorber is found to be improved. Furthermore, it is demonstrated that optimizing the AS treatment time induces a remarkable enhancement in the electrical performances of the CIGSe‐based solar cells.     

Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?

Photovoltaic thin film solar cells based on kesterite Cu₂ZnSn(S_x,Se_1–x)₄ compounds (CZTSSe) have reached >12% sunlight‐to‐electricity conversion efficiency. This is still far from the >20% record devices known in Cu(In_1–y,Ga_y)Se₂ and CdTe parent technologies. A selection of >9% CZTSSe devices reported in the literature is examined to review the progress achieved over the past few years. These devices suffer from a low open‐circuit voltage (V_oc) never better than 60% of the V_{oc max}, which is expected from the Shockley‐Queisser radiative limit (S‐Q limit). The possible role of anionic (S/Se) distribution and of cationic (Cu/Zn) disorder on the V_oc deficit and on the ultimate photovoltaic performance of kesterite devices, are clarified here. While the S/Se anionic distribution is expected to be homogeneous for any ratio x, some grain‐to‐grain and other non‐uniformity over larger area can be found, as quantified on our CZTSSe films. Nevertheless, these anionic distributions can be considered to have a negligible impact on the V_oc deficit. On the Cu/Zn order side, even though significant bandgap changes (>10%) can be observed, a similar conclusion is brought from experimental devices and from calculations, still within the radiative S‐Q limit. The implications and future ways for improvement are discussed.     

Extracellular synthesis of cuprous selenide nanospheres by a biological‐chemical coupling reduction process in an anaerobic microbial system

Biosynthesis of metal nanoparticles represents a clean, eco‐friendly and sustainable “green chemistry” engineering. Lately, a number of metal selenides were successfully synthesized by biological methods. Here, cuprous selenide (Cu₂Se) nanospheres were prepared under mild conditions by a novel biological‐chemical coupling reduction process. The simple process takes place between EDTA‐Cu and Na₂SeO₃ in presence of an alkaline solution containing NaBH₄ and a selenite‐reducing bacteria, Pantoea agglomerans. It is noteworthy that the isolated Pantoea agglomerans and Cu⁺ ions, where the latter are obtained from reducing Cu²⁺ ions by NaBH₄, play a key role, and Cu⁺ ions not only can promote the generation of Se^2? ions as a catalyst, but also can react with Se^2? ions to form Cu₂Se. XRD pattern, SEM, and TEM images indicated that Cu₂Se nanoparticles were tetragonal crystal structure and the nanospheres diameter were about 100 nm. EDX, UV–vis, and FTIR spectra show that the biosynthesized Cu₂Se nanospheres are wrapped by protein and have a better stability. This work first proposes a new biosynthesis mechanism, and has important reference value for biological preparation of metal selenide nanomaterials. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1264–1270, 2016     

Defects in Cu(In,Ga)Se2 Chalcopyrite Semiconductors: A Comparative Study of Material Properties,Defect States,and Photovoltaic Performance

Understanding defects in Cu(In,Ga)(Se,S)₂ (CIGS), especially correlating changes in the film formation process with differences in material properties, photovoltaic (PV) device performance, and defect levels extracted from admittance spectroscopy, is a critical but challenging undertaking due to the complex nature of this polycrystalline compound semiconductor. Here we present a systematic comparative study wherein varying defect density levels in CIGS films were intentionally induced by growing CIGS grains using different selenium activity levels. Material characterization results by techniques including X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, secondary ion mass spectrometry, X‐ray photoelectron spectroscopy, and medium energy ion scattering indicate that this process variation, although not significantly affecting CIGS grain structure, crystal orientation, or bulk composition, leads to enhanced formation of a defective chalcopyrite layer with high density of indium or gallium at copper antisite defects ((In, Ga)_Cu) near the CIGS surface, for CIGS films grown with insufficient selenium supply. This defective layer or the film growth conditions associated with it is further linked with observed current‐voltage characteristics, including rollover and crossover behavior, and a defect state at around 110 meV (generally denoted as the N1 defect) commonly observed in admittance spectroscopy. The impact of the (In, Ga)_Cu defects on device PV performance is also established.     

Aging Effect of a Cu(In,Ga)(S,Se)2 Absorber on the Photovoltaic Performance of Its Cd‐Free Solar Cell Fabricated by an All‐Dry Process: Its Carrier Recombination Analysis

Cd‐free Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells are fabricated by an all‐dry process (a Cd‐free and all‐dry process CIGSSe solar cell) with aged CIGSSe thin film absorbers. The aged CIGSSe thin films are kept in a desiccator cabinet under partial pressure of oxygen of ≈200 Pa for aging time up to 10 months. It is reported for the first time that aged CIGSSe thin film with increased aging time results in significant enhancement of photovoltaic performance of Cd‐free and all‐dry process CIGSSe solar cells, regardless of the alkali treatment. Based on carrier recombination analysis, carrier recombination rates at the interface and in the depletion region of the Cd‐free and all‐dry process CIGSSe solar cells are reduced owing to avoidance of sputtering damage on CIGSSe absorber surface, which is consistent with the strong electron beam‐induced current signal near CIGSSe surface after the increased aging time. It is implied that the interface and near‐surface qualities are clearly improved through the increased aging time, which is attributable to the self‐forming of In_x(O,S)_y near CIGSSe surface, which acts as a buffer layer. Ultimately, the 22.0%‐efficient Cd‐free CIGSSe solar cell fabricated by all‐dry process is achieved with the aged Cs‐treated CIGSSe absorber with the aging time of 10 months.     

Benign Solutions and Innovative Sequential Annealing Processes for High Performance Cu2ZnSn(Se,S)4 Photovoltaics

Solution‐based earth‐abundant Cu₂ZnSn(Se,S)₄ (CZTSSe) is proven to be a promising tool for thin‐film photovoltaic fabrication. Combining fully dissolved copper (Cu) and zinc/tin (Zn/Sn) hydrazinium constituents in an ethanolamine (EA) and dimethylsulfoxide (DMSO) solution mixture forms the CZTS precursor. All solutes in the precursor solution are intermixed on the molecular scale with excellent homogeneity. Sequential annealing steps under chalcogen vapor allow for enhanced grain growth while preventing the back contact from forming an excessively thick Mo(S,Se)₂ layer. The resulting devices achieve power conversion efficiencies of 7.5% under 1 sun conditions.     

Si‐Doped Cu(In,Ga)Se2 Photovoltaic Devices with Energy Conversion Efficiencies Exceeding 16.5% without a Buffer Layer

In this communication, novel and simplified structure Cu(In,Ga)Se₂ (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si‐doped CIGS films grown by the three‐stage coevaporation method, B‐doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si‐doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer‐free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer‐based CIGS device performance, are found to be also effective in enhancing buffer‐free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer‐free CIGS photovoltaic devices.     

Defect Engineering in Multinary Earth‐Abundant Chalcogenide Photovoltaic Materials

Application of zinc‐blende‐related chalcogenide absorbers such as CdTe and Cu(In,Ga)Se₂ (CIGSe) has enabled remarkable advancement in laboratory‐ and commercial‐scale thin‐film photovoltaic performance; however concerns remain regarding the toxicity (CdTe) and scarcity (CIGSe/CdTe) of the constituent elements. Recently, kesterite‐based Cu₂ZnSn(S,Se)₄ (CZTSSe) materials have emerged as attractive non‐toxic and earth‐abundant absorber candidates. Despite the similarities between CZTSSe and CIGSe/CdTe, the record power conversion efficiency of CZTSSe solar cells (12.6%) remains significantly lower than that of CIGSe (22.6%) and CdTe (22.1%) devices, with the performance gap primarily being attributed to cationic disordering and associated band tailing. To capture the promise of kesterite‐like materials as prospective “drop‐in” earth‐abundant replacements for closely‐related CIGSe, current research has focused on several key directions to control disorder, including: (i) examination of the interaction between processing conditions and atomic site disorder, (ii) isoelectronic cation substitution to introduce ionic size mismatch, and (iii) structural diversification beyond the zinc‐blende‐type coordination environment. In this review, recent efforts targeting accurate identification and engineering of anti‐site disorder in kesterite‐based CZTSSe are considered. Lessons learned from CZTSSe are applied to other complex chalcogenide semiconductors, in an effort to develop promising pathways to avoid anti‐site disordering and associated band tailing in future high‐performance earth‐abundant photovoltaic technologies.     

Solution‐Processed Low‐Bandgap CuIn(S,Se)2 Absorbers for High‐Efficiency Single‐Junction and Monolithic Chalcopyrite‐Perovskite Tandem Solar Cells

A novel molecular‐ink deposition route based on thiourea and N,N‐dimethylformamide (DMF) that results in a certified solar cell efficiency world record for non‐vacuum deposited CuIn(S,Se)₂ (CIS) absorbers and non‐vacuum deposited absorbers with a bandgap of 1.0 eV, is presented. It is found that by substituting the widely employed solvent dimethyl sulfoxide with DMF, the coordination chemistry of InCl₃ could be altered, dramatically improving ink stability, enabling up to tenfold increased concentrations, omitting the necessity for elevated ink temperatures, and radically accelerating the deposition process. Furthermore, it is shown that by introducing compositionally graded precursor films, film porosity, compositional gradients, and the surface roughness of the absorbers are effectively reduced and device conversion efficiencies are increased up to 13.8% (13.1% certified, active area). The reduced roughness is also seen as crucial to realize monolithically interconnected CIS‐perovskite tandem devices, where semitransparent MAPbI₃ devices are directly deposited on the CIS bottom cell. Confirming the feasibility of this approach, monolithic devices with near perfect voltage addition between subcells of up to 1.40 V are presented.     

A Bottom‐Up Approach toward All‐Solution‐Processed High‐Efficiency Cu(In,Ga)S2 Photocathodes for Solar Water Splitting

The development of solution‐processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S₂) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap—wider than their selenide counterparts—which can potentially increase the attainable photovoltage. A straightforward and all‐solution‐processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S₂ is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase‐pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n‐type CdS and TiO₂ by solution‐based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm^–2 and onsetting at ≈0.6 V versus RHE under AM 1.5G illumination for CuInS₂ films. Electrodes prepared by our method rival the state‐of‐the‐art performance for these materials.