D. Hariskos

A novel cadmium free buffer layer for Cu(In,Ga)Se2 based solar cells

Abstract Solar cells based on Cu(In,Ga)Se 2 were prepared replacing the “standard buffer layer” CdS with a In x (OH,S) y thin film. The film is deposited in a chemical bath (CBD) process using an aqueous solution containing InCl 3 and thioacetamide. X-ray photoemission spectroscopy measurements were performed in order to characterize the growth kinetics and the chemical composition. The influence of different concentrations of InCl 3 and thioacetamide in the solution on the electrical properties of the solar cells was studied by measuring the j-V characteristics and the spectral quantum efficiencies. Capacitance-voltage ( C-V ) measurements indicate that the high V ∞ values of devices with the novel buffer layer are correlated with narrower space charge widths and higher effective carrier concentrations in the absorber materials. The achieved conversion efficiency of 15.7% (active area) using the cadmium free In x (OH,S) y buffer demonstrates the potential of this process as an alternative to the standard chemical bath deposition of CdS.

ZnO/CdS/Cu(In,Ga)Se/sub 2/ thin film solar cells with improved performance

This paper reports results from experiments concerning the growth of CuInSe/sub 2/ films on different substrate materials, uncoated, and coated with molybdenum. Specifically the effect on the structure, i.e. preferred orientation, of the polycrystalline films is investigated. It is found that soda-lime float glass results in the most oriented films and also that the highest solar cell conversion efficiency is obtained with devices made from such films. In another set of experiments the effect of various deposition conditions for the ZnO window layer is studied. It is found that optimum performance is not strongly dependent on the deposition process. The highly doped part of the window, ZnO:Al, has been replaced with ITO on some devices and a comparison is made. Finally, ZnO/CdS/CuInSe/sub 2/ and ZnO/CdS/Cu(In,Ga)Se/sub 2/ thin film devices exhibiting active area conversion efficiencies of 15.4% and 16.9%, respectively, are demonstrated.<<ETX>>

Band diagram of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEc) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se2. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band ( ΔEc=80 meV±100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike.

Size-quantized CdS films in thin film CuInS2 solar cells

Semiconductor quantum dots and wires are the subject of great interest, mainly due to their size-dependent electronic structures, in particular increased band gap and therefore optoelectronic properties. We have electrodeposited films of size-quantized CdS (∼4 to 5 nm cross section by 15 nm height) as a buffer layer on CuInS2. The resulting CuInS2/CdS thin-film solar cells gave increased photocurrents and higher light-to-electricity conversion efficiencies (>11%) than those made with conventional nonquantized CdS films. This was due mainly to the increased band gap of the quantized CdS, allowing more light to reach the active CuInS2 layer.

Wet treatment based interface engineering for high efficiency Cu(In,Ga)Se2 solar cells

Using atomic layer epitaxy (ALE) which is a soft deposition technique for ZnO it is possible to avoid the deposition of thick buffer layers by chemical bath deposition (CBD) and wet treatments can be almost reduced to surface treatments. In this work new electrochemical and chemical treatments have been designed with the objective of surface passivation and surface doping by using solutions with different reactivities (via pH, complexing agents, metallic cations). ZnO layers are then deposited by ALE to complete the junctions. The results show relations between the interface treatment and the cell characteristics. Efficiencies comparable and in some cases higher than those of the reference cells made with CBD CdS and sputtered ZnO have been obtained (up to 12.7% with indium treatments).

Effect of air annealing on the electronic properties of CdSCu(In,Ga)Se2 solar cells

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.

High band gap Cu(In, Ga)Se2 solar cells and modules prepared with in-line co-evaporation

Abstract Cu(In 1− x Ga x )Se 2 thin film solar cells were prepared with our standard in-line deposition process with x =0.3, 0.68 and 1. The devices with x >0.3 reach a lower efficiency compared to the standard device with x =0.3 due to the increasing difference between the band gap energy E g and the open circuit voltage V oc . The current–voltage characteristics under illumination show an increasing slope at zero bias with increasing Ga content, indicating an increasing voltage dependence of current collection. The temperature dependence of the photovoltaic output parameters is discussed in respect to whether the wide gap devices could reach higher efficiencies than the standard devices at high temperatures. The influence of chemical bath deposition process parameters is investigated. The time the absorber is exposed to the atmosphere before chemical bath deposition is found to become more important with increasing Ga content of the absorber.

Indium-Based Interface Chemical Engineering by Electrochemistry and Atomic Layer Deposition for Copper Indium Diselenide Solar Cells

The key to achieve better Cu(In, Ga)Se2 (CIGS) cells is through the improvement of the CIGS/ZnO interface. In this work, we illustrate various approaches, wet and dry, to engineer that interface with processes that avoid the use of Cd containing compounds. Wet chemical treatments have been performed so as to test the possibility to improve that interface by surface doping of CIGS. X-ray photoelectron spectroscopy (XPS) and Kelvin probe studies show that such doping is not achieved in the conditions leading to best devices. Rather, the most desirable feature of the surface treatments appears to be surface passivation. We show that this can be achieved via CIGS surface reaction with In(III) ions, leading to 12.5% efficient devices. A well passivated interface can also be achieved directly, using an all dry process, by Atomic Layer Deposition (ALD) of In2S3 buffer layer, yielding to 13.5% efficient devices. The ALD growth of the buffer layers have been studied in situ with the help of a quartz crystal microgravimetry.

Improved open circuit voltage in CuInS/sub 2/-based solar cells

The conversion efficiency of thin film solar cells based on CuInS/sub 2/ (/spl eta/=12%) is mainly limited by a moderate open circuit voltage (/spl ap/720 mV). This limitation can be overcome by modifying the absorber/buffer interface leading to open circuit voltages exceeding 800 mV. The addition of ZnS to the CuInS/sub 2/ as well as adjusting the preparation conditions for the CdS-buffer leads to an increased V/sub oc/. The coevaporation of ZnS or CdS additives and diffusion from precursor layers for two types of fabrication processes has been examined: codeposition of the elements and diffusion of Cu and S into In/sub x/S/sub y/ layers. The addition of ZnS leads to Zn-rich segregations on the CuInS/sub 2/ surface. No shift of the bandgap due to the Zn incorporation could be measured. Additionally, an improved sulfur incorporation using the binary In/sub 2/S/sub 3/ as the In and S source was found.

Small- and large-area CIGS modules by co-evaporation

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.