• Energy Research

The fabrication of a 11.4% efficient thin film solar cell based on CuInS2 with an Inx(OH,S)y buffer layer is described. The device parameters and performance are compared to heterojunctions with a standard Us buffer layer. A junction breakdown at negative bias under illumination is related to the buffer layer. A simple model implying photoconductive shunting paths is presented.

CuInS2 thin films were prepared by sulfurization of sequentially deposited CuIn stacks with elemental sulfur. Precursors as well as reacted films are characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). Excess copper is required for optimum properties and leads to a CuS secondary phase segregated at the surface. Stoichiometry is regained by etching after which heterojunctions are formed by deposition of a CdSZnO window layer. An active area efficiency of 10.4% has been achieved.

ABSTRACTWe report on a new chemical bath deposition kinetics for the zinc sulfide oxide Zn(S,O) buffer layer as used in Cu(In,Ga)Se2 (CIGS)‐based solar cells. The new approach allows at high rates a better control of the growth kinetics, the step coverage on the rough CIGS surface, and the [S]/([S]+[O]) ratio in the film. Layer thicknesses as needed for buffer layer applications can be grown at moderate temperatures of 60–80 °C within 5–8 min. Applying this high‐rate Zn(S,O) buffer in CIGS/Zn(S,O)/(Zn,Mg)O/ZnO:Al devices, we realized highly efficient small area solar cells, 30 × 30 cm2 submodules, and 60 × 120 cm2 full‐size modules. Copyright © 2012 John Wiley & Sons, Ltd.

ABSTRACTPolycrystalline widegap Cu(In,Ga)Se2 (CIGSe) absorbers for top cells in photovoltaic tandem devices can be synthesized via [Ga]/([Ga] + [In]) (GGI) ratios of > 0.5. However, the power conversion efficiencies of such high‐GGI devices are smaller than those of the record cells with GGI < 0.5. In the present work, the effects of the GGI ratio on various CIGSe material properties were studied and correlated with the radiative and nonradiative open‐circuit voltage (VOC) deficits of the thin‐film solar cells. Average grain sizes, grain boundary (GB) recombination velocities, fluctuations in luminescence energy distribution, barrier heights at GBs, effective electron lifetimes, and Urbach energies were investigated in five solar cells with GGI ratios from 0.13 to 0.83. It was found that the GGI variation affects GB recombination velocities, fluctuations in spatial luminescence distributions, the average grain size, the electron lifetime, and the Urbach energy. In contrast, the detected ranges of barrier heights at GBs are independent of the GGI ratio. Mainly Ga/In gradients give rise to substantial radiative VOC losses in all solar cells. Nonradiative VOC deficits are dominant especially for solar cells with GGI > 0.5, which can be attributed to low bulk lifetimes and enhanced recombination at GBs in CIGSe absorbers in this compositional range.

A system for large-area deposition of Cu(In,Ga)Se/sub 2/ by co-evaporation has been built and taken into operation. The system is designed after a patented approach, to use the kind of in-line process we are using in our small-area research work. Small-area laboratory cells have been made from CuInSe/sub 2/ and Cu(In,Ga)Se/sub 2/ films fabricated by the in-line process reaching active area efficiencies as high as 15.4% (CuInSe/sub 2/) and 17.6% (Cu(In,Ga)Se/sub 2/). Our Cu(In,Ga)Se/sub 2/ module technology is described and the best results are reported. Best obtained aperture area efficiency for small modules (15 cm/sup 2/) is 12.4% made on Cu(In,Ga)Se, films from our small-area deposition system. The large-area deposition system has produced CuInSe/sub 2/ from which small-area cells with conversion efficiencies of 8.3% have been obtained so far.

AbstractA ZnS/Zn1‐xMgxO buffer combination was developed to replace the CdS/i‐ZnO layers in in‐line co‐evaporated Cu(In,Ga)Se2(CIGS)‐based solar cells. The ZnS was deposited by the chemical bath deposition (CBD) technique and the Zn1‐xMgxO layer by RF magnetron sputtering from ceramic targets. The [Mg]/([Mg] + [Zn]) ratio in the target was varied between x = 0·0 and 0·4. The composition, the crystal structure, and the optical properties of the resulting layers were analyzed. Small laboratory cells and 10 × 10 cm2 modules were realized with high reproducibility and enhanced stability. The transmission is improved in the wavelength region between 330 and 550 nm for the ZnS/Zn1‐xMgxO layers. Therefore, a large gain in the short‐circuit current density up to 12% was obtained, which resulted in higher conversion efficiencies up to 9% relative as compared to cells with the CdS/i‐ZnO buffer system. Peak efficiencies of 18% with small laboratory cells and 15·2% with 10 × 10 cm2 mini‐modules were demonstrated. Copyright © 2009 John Wiley & Sons, Ltd.

New processing developments in the Cu(In,Ga)Se2 (CIGS)-based solar cell technology have enabled best cell efficiencies to exceed 21%. The key innovation involves the alkali post-deposition treatment (PDT) of the CIGS film. Furthermore, the range of optimal CIGS growth parameters and the minimal thickness of the CdS buffer layer is affected by the process modifications. In 2013, we reported a 20.8% record device with PDT. Later optimizations, e.g., in the composition profile and CdS buffer layer thickness as discussed in this study, enabled us to increase the photocurrent density with only a slight loss in open-circuit voltage and unchanged fill factor, resulting in the current world record of 21.7% efficiency. Furthermore, a record efficiency of 21.0% could be achieved with a Cd-free Zn(O,S) buffer layer. This contribution presents measurements, simulations, and a discussion of the photocurrent increase.

Abstract The effect of air annealing on state-of-the-art, solar-cell-quality CdS Cu(In,Ga)Se 2 heterojunctions has been studied using contact potential difference and surface photovoltage measurements. The annealing treatment is shown to have no significant effect on the band lineup of the heterojunction. However, the surface photovoltage spectral response increases markedly upon air annealing. These results can be reconciled if air annealing of the junctions leads mainly to elimination of recombination centers, rather than to changes in the built-in voltage or in the band lineup. We also show that ZnO deposition has an effect on the surface photovoltage that is similar to that of air annealing.

The long-term stability of solar cells is a crucial factor for the competitiveness of a technology. In this study, the accelerated aging of CIGS solar cells was studied, and the influence of an applied bias during the endurance test on the open-circuit voltage V oc and fill factor (FF) was investigated. Time constants for parameter drifts of the open-circuit voltage and the associated activation energy were determined. The observed parameter drifts will be discussed, and a model will be proposed based on SCAPS simulations, explaining the observed behavior of the electrical characteristics of the solar cells. Therefore, cells were dark annealed under dry conditions at two different temperatures and different voltage biases were applied to the cells. Our study revealed that the application of a positive bias, which is similar to light soaking, first leads to an improvement and stabilization of the open-circuit voltage and FF followed by a slow decrease of these parameters. This long-term decrease can be explained in terms of a back barrier or phototransistor, as simulated with SCAPS. However, applying a positive bias enhances the long-term stability of these devices. The appearance of a back barrier is associated with a time constant exceeding 30 years. Therefore, this degradation mechanism is not critical.