Investigation of Cu(In,Ga)Se2 thin‐film formation during the multi‐stage co‐evaporation process

In order to transfer the potential for the high efficiencies seen for Cu(In,Ga)Se₂ (CIGSe) thin films from co‐evaporation processes to cheaper large‐scale deposition techniques, a more intricate understanding of the CIGSe growth process for high‐quality material is required. Hence, the growth mechanism for chalcopyrite‐type thin films when varying the Cu content during a multi‐stage deposition process is studied. Break‐off experiments help to understand the intermediate growth stages of the thin‐film formation. The film structure and morphology are studied by X‐ray diffraction and scanning electron microscopy. The different phases at the film surface are identified by Raman spectroscopy. Depth‐resolved compositional analysis is carried out via glow discharge optical emission spectrometry. The experimental results imply an affinity of Na for material phases with a Cu‐poor composition, affirming a possible interaction of sodium with Cu vacancies mainly via In(Ga)_Cu antisite defects. An efficiency of 12.7% for vacancy compound‐based devices is obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Re‐investigation of preferential orientation of Cu(In,Ga)Se2 thin films grown by the three‐stage process

The present contribution deals with the rather longstanding issue of the preferential orientation of Cu(In,Ga)Se₂ polycrystalline thin films. We investigate both the influence of the growth process parameters and that of the presence of Na on the competition between [112] and [220] orientations. The influence of the presence of Na is studied through the comparison of CIGSe layers co‐evaporated on our laboratory standard Mo‐coated soda lime glass (SLG/Mo) and on substrates with a sodium diffusion barrier (SLG/barrier/Mo); the process dependence of the orientation is evaluated through the comparison of films grown by the standard bithermal three‐stage (400–630°C) and the derived isothermal three‐stage process (620°C). For all the process/substrate combinations, the properties of the films (preferential orientation, grain size and morphology) have been determined at key steps of the growth. From these experimental results, it can be concluded that, as already suggested in the literature, the final layer orientation is strongly related to the texturation of the (In,Ga)₂Se₃ precursor; however, the amount of Na available when the film becomes Cu‐rich (recrystallization at the end of the 2nd‐stage) can also strongly impact the film orientation. Such a phenomenon is herein interpreted by mean of the grain boundary migration model of recrystallization. In agreement with this new interpretation of the experimental data, processes have been designed in order to grow [220] textured CIGSe layers. Copyright © 2011 John Wiley & Sons, Ltd.     

Texture and morphology variations in (In,Ga)2Se3 and Cu(In,Ga)Se2 thin films grown with various Se source conditions

Texture and morphology variations in co‐evaporated (In,Ga)₂Se₃ and Cu(In,Ga)Se₂ (CIGS) films grown with various Se source conditions during growth were studied. The Se species of simply evaporated, large molecular Se (E‐Se, low‐sticking coefficient), and RF‐plasma cracked atomic Se (R‐Se, high sticking coefficient) were used in the present work. (In,Ga)₂Se₃ precursor films, which were prepared during the first stage of CIGS film growth by the three‐stage process, showed systematic variations in texture and Na distribution profile with varying evaporative Se (E‐Se) flux. The properties of CIGS films and solar cells also showed systematic variations, and the open‐circuit voltage (V_oc) and fill factor were found to be especially sensitive to the E‐Se flux. R‐Se grown (In,Ga)₂Se₃ precursor films featured granular morphology with strong (105) and (301) peaks in the diffraction pattern, and the texture was very similar to an E‐Se grown film fabricated with a Se to group III metal (In + Ga) flux ratio (P_{[Se]/[In + Ga]}) of about 6, although the nominal P_{[Se]/[In + Ga]} used for an R‐Se source was very small and less than 0.5. The R‐Se grown CIGS films displayed, however, highly dense surfaces and larger grain sizes than E‐Se grown CIGS films. The controllability of film morphology and the Na diffusion profile in (In,Ga)₂Se₃ and CIGS films with various Se source conditions are discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

Adjusting the Ga grading during fast atmospheric processing of Cu(In,Ga)Se2 solar cell absorber layers using elemental selenium vapor

We study the sequential fabrication of Cu(In,Ga)Se₂ (CIGSe) absorber layers by using an atmospheric pressure selenization with a process duration of only a few minutes and the utilization of elemental selenium vapor from independent Se sources. This technology could proof to be an industrially relevant technology for the fabrication of thin‐film solar cells. Controlling the amount of Se provided during the selenization of metal precursors is shown to be an effective measure to adjust the Ga in‐depth distribution. A reduced Se supply for CIGSe formation leads to a more homogeneous Ga distribution within the absorber. The underlying growth dynamics is investigated by interrupting the selenization at different times. At first, CIGSe formation occurs in accordance with previously suggested growth paths and Ga segregates at the Mo back contact. Between 520 and 580 °C, the growth dynamics differs distinctly, and In and Ga distribute far more uniformly within the absorber depth. We also studied the impact of the precursor architecture. The best performing precursor in terms of efficiency of the respective solar cells was a multilayer with 22 In/CuGa/In triple layers. Simple bilayers stacks lead to films of higher roughness and correlated shunting. By optimizing the precursor architecture and the Ga in‐depth distribution in the CIGSe layer, a conversion efficiency of up to 15.5% (active area) could be achieved. To our knowledge, this is the highest reported efficiency for sulfur free CIGSe‐based solar cells utilizing fast (few minutes) atmospheric processes and elemental Se vapor. Copyright © 2017 John Wiley & Sons, Ltd.     

Flexible Cu(In,Ga)Se2 solar cells with reduced absorber thickness

Reduction of the absorber thickness combined with deposition on a flexible substrate is a technically viable strategy to allow lower cost manufacturing of Cu(In,Ga)Se₂ solar modules. Flexible plastic substrates, however, require a low‐temperature deposition process and appropriate control of the band gap grading for achieving high efficiencies. In this work, we developed solar cells on polyimide films using evaporated Cu(In,Ga)Se₂ absorbers with thickness of 0.8–1.3 µm. The double Ga‐grading profile across the absorber thickness was modified by varying the maximum Cu excess at the end of the second stage or by adapting the In and Ga evaporation flux profiles during the growth process. By minimizing the Cu excess during the intermediate stage of the growth process, no loss in open circuit voltage and fill factor is observed compared with a device having a thicker absorber. Efficiency of 16.3% was achieved for cells with an absorber thickness of 1.25 µm. Insufficient absorption of photons in the long wavelength region is mainly responsible for current loss. By changing the In and Ga evaporation profiles, the shape and position of the Ga notch were effectively modified, but it did not lead to a higher device performance. Modifications of the Ga compositional profile could not help to significantly reduce absorption losses or increase charge carrier collection in absorbers with thickness below 1 µm. Changes of open circuit voltage and fill factor are mostly related to differences in the net acceptor density or the reverse saturation current rather than changes of the double Ga grading. Copyright © 2013 John Wiley & Sons, Ltd.     

Time‐resolved investigation of Cu(In,Ga)Se2 growth and Ga gradient formation during fast selenisation of metallic precursors

Ga segregation at the backside of Cu(In,Ga)Se₂ solar cell absorbers is a commonly observed phenomenon for a large variety of sequential fabrication processes. Here, we investigate the correlation between Se incorporation, phase formation and Ga segregation during fast selenisation of Cu–In–Ga precursor films in elemental selenium vapour. Se incorporation and phase formation are analysed by real‐time synchrotron‐based X‐ray diffraction and fluorescence analysis. Correlations between phase formation and depth distributions are gained by interrupting the process at several points and by subsequent ex situ cross‐sectional electron microscopy and Raman spectroscopy. The presented results reveal that the main share of Se incorporation takes place within a few seconds during formation of In–Se at the top part of the film, accompanied by outdiffusion of In out of a ternary Cu–In–Ga phase. Surprisingly, CuInSe₂ starts to form at the surface on top of the In–Se layer, leading to an intermediate double graded Cu depth distribution. The remaining Ga‐rich metal phase at the back is finally selenised by indiffusion of Se. On the basis of a proposed growth model, we discuss possible strategies and limitations for the avoidance of Ga segregation during fast selenisation of metallic precursors. Solar cells made from samples selenised with a total annealing time of 6.5 min reached conversion efficiencies of up to 14.2 % (total area, without anti‐reflective coating). The evolution of the Cu(In,Ga)Se₂ diffraction signals reveals that the minimum process time for high‐quality Cu(In,Ga)Se₂ absorbers is limited by cation ordering rather than Se incorporation. Copyright © 2014 John Wiley & Sons, Ltd.     

Reactive Sputtering Process for CuIn1‐xGaxSe2 Thin Film Solar Cells

CuIn_1‐xGa_xSe₂ (CIGS) thin films are grown on Mo/soda lime glass using a reactive sputtering process in which a Se cracker is used to deliver reactive Se molecules. The Cu and (In_0.7Ga_0.3)₂Se₃ targets are simultaneously sputtered under the delivery of reactive Se. The effects of Se flux on film composition are investigated. The Cu/(In+Ga) composition ratio increases as the Se flux increases at a plasma power of less than 30 W for the Cu target. The (112) crystal orientation becomes dominant, and crystal grain size is larger with Se flux. The power conversion efficiency of a solar cell fabricated using an 800‐nm CIGS film is 8.5%.     

Impact of compositional grading and overall Cu deficiency on the near‐infrared response in Cu(In,Ga)Se2 solar cells

Highly efficient thin film solar cells based on co‐evaporated Cu(In,Ga)Se₂ (CIGS) absorbers are typically grown with a [Ga]/([Ga] + [In]) (GGI) gradient across the thickness and a Cu‐poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased absorption in the near‐infrared (NIR), diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the NIR response. In this contribution [Cu]/([In] + [Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometric point. In order to obtain comparable Ga contents at the interface for proper band alignment, the front GGI gradings were actively modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. By characterizing the modified absorber properties by reflection‐transmission spectroscopy, we attribute the observed behavior to changes in the optical properties rather than to improved carrier collection. Cu‐dependent modifications of the NIR‐absorption coefficients are likely to be responsible for the variations in the optical properties, which is supported by device simulations. Adequate re‐adjustments of the co‐evaporation process and of the alkali‐fluorides post‐deposition treatments allow maintaining V_oc and FF values, yielding an overall increase of efficiency as compared to a reference baseline. © 2016 The Authors. Progress in Photovoltaics : Research and Applications published by John Wiley & Sons Ltd.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Diode characteristics in state‐of‐the‐art ZnO/CdS/Cu(In1−xGax)Se2 solar cells

We report a new state of the art in thin‐film polycrystalline Cu(In,Ga)Se₂‐based solar cells with the attainment of energy conversion efficiencies of 19·5%. An analysis of the performance of Cu(In,Ga)Se₂ solar cells in terms of some absorber properties and other derived diode parameters is presented. The analysis reveals that the highest‐performance cells can be associated with absorber bandgap values of ∼1·14 eV, resulting in devices with the lowest values of diode saturation current density (∼3×10⁻⁸ mA/cm²) and diode quality factors in the range 1·30 < A < 1·35. The data presented also support arguments of a reduced space charge region recombination as the reason for the improvement in the performance of such devices. In addition, a discussion is presented regarding the dependence of performance on energy bandgap, with an emphasis on wide‐bandgap Cu(In,Ga)Se₂ materials and views toward improving efficiency to > 1;20% in thin‐film polycrystalline Cu(In,Ga)Se₂ solar cells. Published in 2005 John Wiley & Sons, Ltd.     

Spray‐ILGAR indium sulfide buffers for Cu(In,Ga)(S,Se)2 solar cells

Thin‐film indium sulfide buffer layers have been prepared using the Spray‐ILGAR technique for use in chalcopyrite solar cells. Buffers deposited on commercially grown Cu(In,Ga)(S,Se)₂ absorbers have produced cells reaching a certified efficiency of 14·7% and average efficiencies matching the reference solar cells prepared with a conventional cadmium sulfide buffer layer. The process parameters have been optimized and the resulting cells have been studied using current–voltage and temperature–illumination‐dependent current–voltage analysis as well as quantum efficiency measurements. Copyright © 2005 John Wiley & Sons, Ltd.     

Solution‐processed Cu(In,Ga)(S,Se)2 absorber yielding a 15.2% efficient solar cell

The remarkable potential for inexpensive upscale of solution processing technologies is expected to enable chalcogenide‐based photovoltaic systems to become more widely adopted to meet worldwide energy needs. Here, we report a thin‐film solar cell with solution‐processed Cu(In,Ga)(S,Se)₂ (CIGS) absorber. The power conversion efficiency of 15.2% is the highest published value for a pure solution deposition technique for any photovoltaic absorber material and is on par with the best nonvacuum‐processed CIGS devices. We compare the performance of our cell with a world champion vacuum‐deposited CIGS cell and perform detailed characterization, such as biased quantum efficiency, temperature‐dependent electrical measurement, time‐resolved photoluminescence, and capacitance spectroscopy. Copyright © 2012 John Wiley & Sons, Ltd.     

Combinatorial study of NaF addition in CIGSe films for high efficiency solar cells

We report on a sodium fluoride (NaF) thickness variation study for the H₂Se batch furnace selenization of sputtered Cu(In,Ga) films in a wide range of Cu(In,Ga) film compositions to form Cu(In,Ga)Se₂ (CIGSe) films and solar cells. Literature review indicates lack of consensus on the mechanisms involved in Na altering CIGSe film properties. In this work, for sputtered and batch‐selenized CIGSe, NaF addition results in reduced gallium content and an increase in grain size for the top portion of the CIGSe film, as observed by scanning electron microscopy and secondary ion mass spectrometry. The addition of up to 20 nm of NaF resulted in an improvement in all relevant device parameters: open‐circuit voltage, short‐circuit current, and fill factor. The best results were found for 15 nm NaF addition, resulting in solar cells with 16.0% active‐area efficiency (without anti‐reflective coating) at open‐circuit voltage (V_OC) of 674 mV. Copyright © 2013 John Wiley & Sons, Ltd.     

A comparison between thin film solar cells made from co‐evaporated CuIn1‐xGaxSe2 using a one‐stage process versus a three‐stage process

Until this day, the most efficient Cu(In,Ga)Se₂ thin film solar cells have been prepared using a rather complex growth process often referred to as three‐stage or multistage. This family of processes is mainly characterized by a first step deposited with only In, Ga and Se flux to form a first layer. Cu is added in a second step until the film becomes slightly Cu‐rich, where‐after the film is converted to its final Cu‐poor composition by a third stage, again with no or very little addition of Cu. In this paper, a comparison between solar cells prepared with the three‐stage process and a one‐stage/in‐line process with the same composition, thickness, and solar cell stack is made. The one‐stage process is easier to be used in an industrial scale and do not have Cu‐rich transitions. The samples were analyzed using glow discharge optical emission spectroscopy, scanning electron microscopy, X‐ray diffraction, current–voltage‐temperature, capacitance‐voltage, external quantum efficiency, transmission/reflection, and photoluminescence. It was concluded that in spite of differences in the texturing, morphology and Ga gradient, the electrical performance of the two types of samples is quite similar as demonstrated by the similar J–V behavior, quantum spectral response, and the estimated recombination losses. Copyright © 2014 John Wiley & Sons, Ltd.     

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

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

Na‐induced efficiency boost for Se‐deficient Cu(In,Ga)Se2 solar cells

Among different process routes for Cu(In,Ga)Se₂ (CIGS) solar cells, sufficient Se supply is commonly required to obtain high‐quality CIGS films. However, supplying extra Se increases the cost and the complexity. In this work, we demonstrate that extra Na incorporation can substantially increase efficiency of Se‐deficient CIGS solar cells, fabricated by sputtering from a quaternary CIGS target without extra Se supply, from 1.5% to 11.0%. The Se‐deficient CIGS device without extra NaF reveals a roll‐over I–V curve at room temperature as well as significantly reduced J_sc and fill factor at low temperatures. The electrical characteristics of Se‐deficient CIGS films are well explained and modeled by the low p‐type doping due to high density of compensating donors and the presence of deep defects possibly originating from the anti‐bonding levels of Se vacancies. The significant improvement after extra Na incorporation is attributable to the Na‐induced passivation of Se vacancies and the increased p‐type doping. Our result suggests that extra Na addition can effectively compensate the Se deficiency in CIGS films, which provides a valuable tuning knob for compositional tolerance of absorbers, especially for the Se‐deficient CIGS films. We believe that our findings can shine light on the development of novel CIGS processes, distinct from previous ones fabricated in Se‐rich atmosphere. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

High quality baseline for high efficiency,Cu(In1−x,Gax)Se2 solar cells

We report on the progress that we have made in the quality of our baseline process for the production of high efficiency soda lime glass/Mo/Cu(In,Ga)Se₂ (CIGS)/CdS/i‐ZnO/ZnO:Al/MgF₂ solar cells. The enhancement of the average performance level has enabled us to reach conversion efficiencies of up to 19·3% (internal measurement). The new quality initiative uses process control, optical and electrical modelling, and the critical revision of all process steps as tools for the attainment of the 19% efficiency level. Our experiments show that the compositional process window for CIGS solar cells that have an efficiency of η ≈ 19% is very wide. Accordingly, we suggest that an efficiency of 19·0–19·5% is achievable in the following compositional process windows: 0·69 ≤ Cu/(Ga + In) ≤ 0·98 and 0·21 ≤ Ga/(Ga + In) ≤ 0·38. In addition, our results show that large CIGS grains are not a necessary requirement for high efficiencies up to 19%. These findings and the partly lacking ability to correlate certain aspects of our progress with experimental parameters lead us to the conclusion that there are still some important process variables undiscovered. From this conclusion and from the evaluation of the available data we infer that there is a potential for the enhancement of CIGS solar cell efficiencies beyond 20%. Copyright © 2007 John Wiley & Sons, Ltd.     

Device characteristics of a 10.1% hydrazine‐processed Cu2ZnSn(Se,S)4 solar cell

A power conversion efficiency record of 10.1% was achieved for kesterite absorbers, using a Cu₂ZnSn(Se,S)₄ thin‐film solar cell made by hydrazine‐based solution processing. Key device characteristics were compiled, including light/dark J–V, quantum efficiency, temperature dependence of V_oc and series resistance, photoluminescence, and capacitance spectroscopy, providing important insight into how the devices compare with high‐performance Cu(In,Ga)Se₂. The record kesterite device was shown to be primarily limited by interface recombination, minority carrier lifetime, and series resistance. The new level of device performance points to the significant promise of the kesterites as an emerging and commercially interesting thin‐film technology. Copyright © 2011 John Wiley & Sons, Ltd.     

Fabrication of Cu(In,Ga)Se2 thin films by sputtering from a single quaternary chalcogenide target

Single‐layered Cu‐In‐Ga‐Se precursors were fabricated by one‐step sputtering of a single quaternary Cu(In,Ga)Se₂ (CIGS) chalcogenide target at room temperature, followed by post selenization using Se vapor obtained from elemental Se pellets. The morphological and structural properties of both as‐deposited and selenized films were characterized by X‐ray diffraction (XRD), Raman spectroscope and scanning electron microscope (SEM). The precursor films exhibited a chalcopyrite structure with a preferential orientation in the (112) direction. The post‐selenization process at high‐temperature significantly improved the quality of the chalcopyrite CIGS. The CIGS layers after post‐selenization were used to fabricate solar cells. The solar cell had an open‐circuit voltage Voc of 0.422 V, a short‐circuit current density J = 24.75 mA, a fill factor of 53.29%, and an efficiency of 7.95%. Copyright © 2010 John Wiley & Sons, Ltd.     

Gallium gradients in Cu(In,Ga)Se2 thin‐film solar cells

The gallium gradient in Cu(In,Ga)Se₂ (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe₂ and CuGaSe₂ were calculated using density functional theory. The migration barrier for the In_Cu antisite in CuGaSe₂ is significantly lower compared with the Ga_Cu antisite in CuInSe₂, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.