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.     

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.     

Non‐destructive optical analysis of band gap profile,crystalline phase,and grain size for Cu(In,Ga)Se2 solar cells deposited by 1‐stage, 2‐stage,and 3‐stage co‐evaporation

Cu(In,Ga)Se₂ (CIGS) thin films co‐evaporated by 1‐stage, 2‐stage, and 3‐stage processes have been studied by spectroscopic ellipsometry (SE). The disappearance of a Cu_2‐xSe optical signature, detected by real time SE during multistage CIGS, has enabled precise endpoint control. Band gap energies determined by SE as depth averages show little process variation for fixed [Ga]/([In] + [Ga]) atomic ratio, whereas their broadening parameters decrease with increasing number of stages, identifying successive grain size enhancements. Refined SE analysis has revealed band gap profiling only for 3‐stage CIGS. Solar cells incorporating these absorbers have yielded increased efficiencies in correlation with phase control, grain size, and band gap profiling. Copyright © 2013 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.     

Dynamic radiative properties of the Cu(In,Ga)Se2 layer during the co‐evaporation process

A study of the wavelength‐integrated emissivity has been performed on the optical stack Cu_xSe/Cu(In,Ga)Se₂/Mo. The wavelength interval used in the study was 2–20 µm, which covers 95% of the radiated heat from a black body heated to 500°C. Substrate temperatures around 500°C are commonly used in production of Cu(In,Ga)Se₂ thin films for solar cells. The integrated emissivity was obtained from directional reflectivity measurements of experimental samples with different thicknesses of the Cu_xSe layers. It was subsequently compared to the emissivity from numerical simulations based on newly obtained values of the refractive index values for Cu(In,Ga)Se₂ and Cu_xSe at these wavelengths. Good agreement was found between the measured and simulated values. At a Cu(In,Ga)Se₂ thickness of 1.8 µm and a Mo thickness of 400 nm, a maximum in the integrated emissivity was found for a Cu_xSe thickness of 30 nm. The results are valuable input into understanding the dynamics of the change in emissivity between Cu‐rich Cu(In,Ga)Se₂ with segregated Cu_xSe and Cu‐poor single phase Cu(In,Ga)Se₂ at temperatures around 500°C. In co‐evaporation of Cu(In,Ga)Se₂, this emissivity change is often monitored and used as a process control (end‐point detection). Copyright © 2010 John Wiley & Sons, Ltd.     

The structural evolution of Cu(In,Ga)Se2 thin film and device performance prepared through a three-stage process

Cu(In,Ga)Se₂ (CIGS) absorber layer is grown on Mo-coated soda-lime glass (SLG) substrates using co-evaporation deposition technique. The growth characteristics of the CIGS films deposited through a three-stage process are examined by interrupting the deposition along the reaction pathway. In the three-stage process, the absorber layer undergoes several phase transformations with Cu content. The γ-(In,Ga)₂Se₃ layer is formed first and is then converted to α-Cu(In,Ga)Se₂ via β-Cu(In,Ga)₃Se₅. When α-Cu(In,Ga)Se₂ stoichiometry is reached, Cu_2−xSe segregation at the surface and at grain boundaries begins to occur. The Cu_2−xSe improved the densification and grain growth of the absorber layer. Then, as the absorber layer reverts to substoichiometric composition, the Cu_2−xSe phase disappears and the depleted server Cu near the surface instead. This paper reports several types of defects found in absorber layers that act as non-radiative recombination centers, such as impurity phases (Cu_2−xSe and Cu(In,Ga)₃Se₅), deep point defects (In_Cu), grain boundaries, and voids. The highest efficiency at 10.97% was achieved when the bulk [Cu]/([In]+[Ga]) ratio was 0.98 at the third stage of the process. This result is attributed to the low-concentration deep-level defects that act as recombination centers and to the denser structure with larger grain size.     

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%.     

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.     

Toward efficient Cu(In,Ga)Se2 solar cells prepared by reactive magnetron co‐sputtering from metallic targets in an Ar:H2Se atmosphere

In this paper, we show that a reactive co‐sputtering process using metallic CuGa and In targets; an Ar:H₂Se atmosphere is well suited for the deposition of photoactive Cu(In,Ga)Se₂ (CIGSe) absorber layers for thin‐film solar cells in a single process step. The achievement of single‐phase and well‐crystallized layers is thereby no major problem if a sufficiently high H₂Se content and substrate temperatures in the range of 400–500 °C are used. However, in order to achieve the desired Cu‐poor film stoichiometry, which is crucial for the device performance, it has to be considered already that, at moderate substrate temperatures in the range of 400–500 °C, indium has a strong tendency to re‐evaporate from the film surface if the film composition is Cu‐poor. If excess indium is supplied, this effect can lead to a self‐adjustment of the film composition. This allows a very wide process window in a one‐stage process concerning the supply ratio from the two targets of [Cu]/([In] + [Ga])_supply ≈ 0.35–0.8. However, the maximum efficiencies achievable with such a process are limited to 11.7% because an adequate Cu‐poor composition can only be achieved with significant Cu‐poor conditions, which allow only a low material quality. By using an improved process with an intermediate Cu‐rich composition and a final Cu‐poor stage, the absorber quality could be significantly improved; efficiencies of up to 14.3% have been achieved with CIGSe films prepared on Na‐doped Mo back contacts. Copyright © 2015 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.     

Development of gallium gradients in three‐stage Cu(In,Ga)Se2 co‐evaporation processes

We use secondary‐ion mass spectrometry, X‐ray diffraction and scanning electron microscopy to investigate the development over time of compositional gradients in Cu(In,Ga)Se₂ thin films grown in three‐stage co‐evaporation processes and suggest a comprehensive model for the formation of the well‐known ‘notch’ structure. The model takes into account the need for compensating Cu diffusion by movement of group‐III ions in order to remain on the quasi‐binary tie line and indicates that the mobilities of In and Ga ions differ. Cu diffuses towards the back in the second stage and towards the front in the third, and this is the driving force for the movement of In and Ga. The [Ga]/[In + Ga] ratio then increases in the direction of the respective Cu movement because In has a higher mobility at process conditions than has Ga. Interdiffusion of In and Ga can be considerable in the (In,Ga)₂Se₃ film of the first stage, but seems largely to cease in Cu(In,Ga)Se₂ and shows no signs of being boosted by the presence of a Cu₂Se layer. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Electrochemical Synthesis of Magnetostrictive Fe–Ga/Cu Multilayered Nanowire Arrays with Tailored Magnetic Response

Arrays of nanowires are fabricated with alternating segments of the magnetostrictive alloy Fe_1–xGa_x and Cu using electrochemical deposition in nanoporous anodic aluminium oxide (AAO) templates. The difficult nature of Ga‐alloy electrochemistry is overcome by controlling mass‐transfer and hydrodynamic conditions using novel rotating disk electrode templates to obtain highly uniform segment lengths throughout the arrays. Extensive structural characterization by XRD, EBSD and TEM reveals a strong <110> textured Fe_1–xGa_x growth. Furthermore, using vibrating sample magnetometry (VSM), we demonstrate that control of magnetization reversal processes is possible once uniform aspect ratios are obtained for both the Fe–Ga and Cu segments.     

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.     

Composition and bandgap control in Cu(In,Ga)Se2‐based absorbers formed by reaction of metal precursors

The control of composition and bandgap in chalcopyrite thin‐film absorber layers formed by a metal precursor reaction is addressed. Two processes using reaction with either H₂Se or H₂S as the final step of a three‐step reaction process were compared as follows: a three‐step H₂Se/Ar/H₂S reaction and a three‐step H₂Se/Ar/H₂Se reaction. In both processes, significant Ga homogenization was obtained during the second‐step Ar anneal, but the third‐step selenization resulted in Ga depletion near the Cu(InGa)Se₂ surface, whereas the third‐step sulfization did not. Solar cells were fabricated using absorbers formed using each method, and the surface Ga depletion significantly affected device performances. The solar cell incorporating the sulfization yielded a better device performance, with an efficiency of 14.4% (without an anti‐reflection layer) and an open‐circuit voltage of 609 mV. The bandgap control in the metal precursor reaction is discussed in conjunction with the device behavior. Copyright © 2014 John Wiley & Sons, Ltd.     

CIGS薄膜(InGa)2Se3-富Cu-富In(Ga)的演变

采用三步共蒸发工艺顺序沉积铜铟镓硒(CuInGaSe2,CIGS)薄膜.薄膜的厚度、组份、晶相结构分别由台阶仪、X射线荧光光谱仪(XRF)和X射线衍射仪(XRD)来表征.在(In,Ga)2Se3预制层-富Cu相的演变过程中,依次发生以下相变:Cu(In,Ga)5Se8、Cu(In,Ga)3Se5、Cu2(In,Ga)4Se7(或Cu(In,Ga)2Se3.5)、Cu(In,Ga)Se2(液相CuxSe).在富Cu相-富In(Ga)相的演变过程中,依次发生以下相变:Cu(In,Ga)Se2(液相CuxSe)、Cu2(In,Ga)4Se7(或Cu(In,Ga)2Se3.5)、Cu(In,Ga)3Se5、Cu(In,Ga)5Se8.对这两个演变过程中薄膜的生长机理和结构特性进行了讨论.     

Structural and electrical properties of co-evaporated In,Garich CIGS thin films

The CIGS thin trims are prepared by co-evaporation of elemental In, Ga and Se on the substrates of Mo-coated glasses at 400℃ followed by co-evaporation of elemental Cu and Se at 550℃. We study the structural and electrical properties using XRD, XRF and Hall effect measurements. In general, Cu（In,Ga）sSes phase exists when Cu/（In＋Ga） ratio is from 0.17 to 0.27, Cu（In,Ga）3Se5 phase exists for Cu/（In＋Ga） ratio between 0.27 and 0.41, Cu2（in,Ga）4Se7 and Cu（In,Ga）2Se3.5 phases exist for Cu/0n＋Ga） ratio between 0.41 and 0.61, and OVC（or ODC） and CuIn0.7Ga0.3Se2 phases exist when Cu/（In＋Ga） ratio is from 0.61 to 0.88. With the increase of Cu/（In＋Ga） ratio, the carrier concentrations of the films gradually increase, but the electrical resistivity gradually decreases.     

Influence of the Cu(In,Ga)Se2 thickness and Ga grading on solar cell performance

Thin‐film solar cells with Cu(In,Ga)Se₂ (CIGS) absorber layers ranging from 1.8 to 0.15 μm in thickness were fabricated by co‐evaporation, with both homogeneous and Ga/(Ga + In) graded composition. The absorption of the CIGS layers was determined and compared with corresponding QE measurements in order to obtain the optical related losses. The material characterization included XRD as well as cross‐sectional SEM analysis. Devices with CIGS layers of all thicknesses were fabricated, and down to 0.8–1 μm they showed a maintained high performance (η ∼ 15%). When the CIGS layer was further reduced in thickness the loss in performance increased. The main loss was observed for the short‐circuit current, although the loss was not only due to a reduced absorbance. The open‐circuit voltage was essentially not affected by the reduction of the CIGS thickness, while the fill factor showed a slight decrease. The fill factor loss was eliminated by introducing a Ga/(Ga+In) graded CIGS, which also resulted in an increased open‐circuit voltage of 20–30 mV for all CIGS thicknesses. Device results of 16.1% efficiency at 1.8 μm CIGS thickness, 15.0% at 1.0 μm and 12.1% at 0.6 μm (total area without anti‐reflective coating) were achieved. Copyright © 2002 John Wiley & Sons, Ltd.     

Preferred orientation of Cu(In,Ga)Se2 thin film deposited on stainless steel substrate

The influences of process parameters and Fe diffusing into Cu(In,Ga)Se₂ (CIGS) films on the orientation of CIGS absorbers grown on the stainless steel (SS) foils are investigated. The structural properties, morphology, and elemental profiles are characterized using X‐ray diffraction, scanning electron microscopy, and second ion mass spectroscopy, respectively. The orientation of CIGS thin films on the SS substrates strongly depends on the texture of the (In,Ga)₂Se₃ precursor, determined by the substrate temperature at the first stage (Ts1) and the flux ratio of Se to (In + Ga). Among these factors, Ts1 is the prerequisite to achieve [300]‐oriented IGS layer, which will yield [200]‐oriented CIGS thin film in the later process. The results indicate that through the comparison of CIGS thin films on the Mo/SS substrates and on the Mo/ZnO/SS substrates and combined with simply calculation, Fe diffusing into the CIGS layer will hinder the growth of the CIGS grains along [112] orientation. The grazing‐incidence X‐ray diffraction results suggest that the surface of the [220]‐textured CIGS thin film on the SS substrate still has [220] predominance, whereas the surface texture of the [220]‐texture CIGS thin film on the Mo/soda‐lime glass substrate became [112] predominant, which is due to the different compensation ability between Fe and Na elements. Finally, the relations between the device parameters and the degrees of the preferred orientation of CIGS absorbers are investigated. Copyright © 2012 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.