This contribution describes an easy and cheap approach to introduce deuterium (D) as an isotopic marker into the commonly used buffer layer materials CdS and Zn(O,S) for Cu(In,Ga)Se2 (CIGS) thin‐film solar cells. D was successfully incorporated during the growth of Zn(O,S) and CdS buffer layers by chemical bath deposition (CBD) with D2O. CIGS solar cells prepared with D‐containing buffers grown by CBD exhibit power conversion efficiencies above 16%, that is, the D content has no detrimental effect on the performance or other solar cell parameters of the devices. With depth profiles obtained by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) we clearly detect the intentionally incorporated D within the solution‐grown Zn(O,S) buffer. Assuming that D is present as OD, we compare the amount of OD within the Zn(O,S) layer with the amount of OH on the surface of the subsequent sputtered (Zn,Mg)O layer. Possible applications and future experiments of the method inserting isotopic markers such as D in functional layers of chalcopyrite‐type thin‐film solar cells and beyond are discussed.

The gallium gradient in Cu(In,Ga)Se-2 (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 mu m 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 (c) 2014 John Wiley & Sons, Ltd.

Zn(O,S) is a promising candidate to replace the commonly used CdS buffer layer for Cu(In,Ga)Se2 (CIGS) thin‐film solar cells due to its non‐toxicity and its potential to enhance the conversion efficiency of the CIGS solar cell. The composition of chemical bath deposited (CBD) and sputtered Zn(O,S) layers with thicknesses well below 100 nm was determined by sputtered neutral and secondary ion mass spectrometry (SNMS and SIMS). Despite numerous mass interferences of double‐charged atoms and dimers with single Zn, O and S isotopes, we developed an evaluation algorithm for quantification of SNMS depth profiles of Zn(O,S) layers. In particular, the superposition of double‐charged S and Zn atoms with O and S isotopes is accounted for numerically in the quantification procedure. For sputtered Zn(O,S) layers, the S/(S + O) atomic ratio and the vertical composition profile can be controlled by the O2 content in the gas flow and the substrate temperature during sputtering whereas for CBD Zn(O,S) the S/(S + O) ratio is constant around 0.7–0.8. A Cu‐depleted layer of about 5 nm on the CIGS surface after buffer deposition was observed for both preparation methods. With negative SIMS, we found more hydroxides and carbon residues in CBD Zn(O,S) as compared to sputtered layers. Best cell performance with sputtered Zn(O,S) layers was achieved for S/(S + O) ratios of 0.25–0.40, yielding efficiencies up to 13%. Our solar cells with CBD Zn(O,S) buffers exhibit higher efficiencies due to an improved open‐circuit voltage. Copyright © 2013 John Wiley & Sons, Ltd.

The influence of annealing in air on high-quality Cu(In,Ga)Se2 (CIGS) thin-film solar cells with Zn(O,S)/(Zn,Mg)O buffers is investigated. We have grown CIGS with an in-line co-evaporation process and the Zn(O,S) buffer layer by chemical bath deposition (CBD). After post-annealing in air, the open-circuit voltage VOC of our cells with CBD Zn(O,S) buffer increases significantly. The enhanced VOC agrees with an increased acceptor density and a reduced space charge width extracted from capacitance-voltage measurements. This finding correlates with a higher sodium concentration after air annealing as measured with secondary ion mass spectrometry (SIMS) at the CIGS/buffer interface and in the Zn(O,S) layer itself. Furthermore, we could detect a decrease of the S/(S+O) ratio in the Zn(O,S) film with sputtered neutral mass spectrometry after different tempering steps. The reduced S/(S+O) ratio implies a decrease in the bandgap energy Eg of Zn(O,S). A reduction of Eg was also extracted from transmission measurements for Zn(O,S) films on quartz glass substrates. This result could explain the observed enhanced short-circuit current density after annealing due to an optimized conduction band offset. In addition, an in-diffusion of zinc from the Zn(O,S) buffer into the CIGS absorber after annealing was detected with SIMS.

ABSTRACTThin film Cu(In,Ga)Se2 (CIGSe) solar cells deposited on flexible steel foil substrates by coevaporation are investigated. Iron diffusion from the steel foil substrate material into the CIGSe absorber layers is studied by secondary ion mass spectroscopy (SIMS) for different maximum absorber growth temperatures and solar cell back‐contact configurations. The optimization of the absorber growth temperature, the introduction of a diffusion barrier layer at the back contact, and the adjustment of the sodium doping lead to device efficiencies above 14%. Defect spectroscopy by means of admittance measurements shows a clear correlation of the deep defect concentration with the Fe content in the CIGSe absorber layer and a correlation between the sodium incorporation and the net carrier concentration. It is found that too much Na supports the diffusion of Fe, inducing an increased deep defect concentration. The Fe‐induced deep defects, which form a broad acceptor‐type defect band at 0.44 eV above the valence band maximum, are identified as the cause of bulk recombination, limiting the device performance in these solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

The effect of the amount of Na present during the 3-stage growth of CIGS at very low temperature T2 on polyimide (PI) foils is studied. While at higher growth temperatures Na seems to impede In-Ga interdifussion, at very low temperatures it appears to further the process. An increase in Voc for a higher Na concentration can be explained by a higher net carrier concentration as measured by drive level capacitance profiling. Admittance spectroscopy measurements show shallow defects when the Na concentration increases. These results suggest that the main role of Na could be the passivation of InCu donor deep defect, in agreement with Wei’s theory. Efficiencies of up to 15.1 % (0.5 cm2 active area with antireflection coating) and 13.6%, 14.1% (1 cm2 total and active area respectively without antireflection coating) for nominal T2=420° C were achieved on PI substrates so far.

For the series production of Cu(In,Ga)Se-2 (CIGS)-based thin-film solar cells it is desirable to replace the thin CdS buffer layer between absorber and transparent front contact by non-toxic, low-absorbing semiconductors. In2S3, deposited by atomic layer deposition, has already been qualified as an alternative buffer material. In this work, results of indium sulphide buffer layers deposited by thermal evaporation are presented. Pressed powders with different compositions and morphology were used for evaporation at about 720 degrees C, resulting in different layer compositions and cell performances. The composition of the initial powder material and of the pellets after the deposition steps was determined by XRF. The deposited In2SxOy buffer layers and the buffer/absorber interface region were analysed by SIMS and sputtered neutral mass spectrometry (SNMS) depth profiling.Fine-grained pressed In2S3 powder evaporates rather homogeneously during the entire deposition run, resulting in nearly stoichiometric In2S3 layers. S and Cl are evaporated preferentially from pellets of coarse-grained, S-poor In2S2.4 powder containing 2 at% Cl, leading to excess S and high Cl concentrations in the first deposited layer. The subsequent layers are S-poor and the In/S ratios continue to increase. In all interface regions additional amounts of Se, In, and Ga are detected, which could be attributed to the Cu-poor defect layer on top of the CIGS absorbers. The best solar cell performance with efficiences of about 13% was achieved with the S-rich buffer layers. Stoichiometric In2S3 or S-poor layers yield lower efficiences between 8 and 11%. Copyright (c) 2008 John Wiley & Sons, Ltd.

Culn1-xGaxSe2 (CIGS) thin films with various Ga contents x were characterized by micro-Raman spectroscopy. All samples were investigated using the 532 nm line of a Nd:YAG laser in the back-scattering geometry. A linear increase of the frequency of the chalcopyrite A1 mode from 174 cm-1 to 184 cm-1 with increasing Ga content was observed. Special attention was directed to the interface between the CIGS absorber and the Mo back contact. Sputtered neutral mass spectroscopy (SNMS) and Raman line scans on cross-sections revealed a reduced formation of MoSe2 at the Mo/CIGS interface for higher Ga contents