Michael Powalla

Comparative study of the influence of LiF, NaF, and KF on the performance of polymer bulk heterojunction solar cells

Based on the well-known beneficial effect of a thin LiF layer underneath Al contacts for organic solar cells, a comparative study of interlayers made from the alkaline fluorides LiF, NaF, and KF is presented for polymer bulk heterojunction solar cells. The overall suitability of these materials and the underlying mechanisms are discussed. While an improvement in cell efficiency up to a factor of 2 can be reached with all three fluorides, the necessary thickness of the interlayer for maximum improvement is smallest for NaF and largest for LiF, suggesting the alternative use of NaF instead of LiF.

Highly efficient organic solar cells with printable low-cost transparent contacts

Highly efficient organic solar cells with all-solution-processed low-cost transparent front contacts based on highly conductive formulations of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) are reported, demonstrating the feasibility of replacing the costly conventional vacuum-deposited indium-tin-oxide contacts by means of simple printing techniques. For small cell areas, the relatively high sheet resistances were found to be less detrimental to the photovoltaic performance than the deficiencies in the transparency of the contact layer.

Influence of Cu content on electronic transport and shunting behavior of Cu(In,Ga)Se2 solar cells

The paper presents the structural and electrical characterizations of Cu(In,Ga)Se2 thin films and thin-film solar cells realized with different Cu contents in the absorber material. It is shown that the bulk resistivity of the Cu(In,Ga)Se2 thin films (measured in coplanar geometry) dramatically increases with decreasing Cu content. Simultaneously, the shunt resistance Rp of the Cu(In,Ga)Se2 solar cells increases with decreasing Cu content in the absorber material. For a wide range of Cu contents, the resistivity of the thin films is directly proportional to Rp of the solar cell made from the same absorber material. We propose that Rp in Cu(In,Ga)Se2 solar cells originates from highly localized shunt regions in the absorber material. The higher Rp of cells made from Cu-poor material is then due to the high resistivity of the embedding Cu-poor material resistively suppressing extensive current flow towards these shunt regions. Further, we observe an increase of the film resistivity by two orders of magnitud...

Improved Photocurrent in Cu(In,Ga)Se 2 Solar Cells: From 20.8% to 21.7% Efficiency with CdS Buffer and 21.0% Cd-Free

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.

CD-free Cu(In,Ga)Se2 thin-film solar modules with In2S3 buffer layer by ALCVD

The atomic layer chemical vapour deposition (ALCVD) technique allows the deposition of highly homogeneous thin-films with an excellent step coverage. This method has already shown promising results for the deposition of cadmium-free buffer layers in Cu(In,Ga)Se2 (CIGS) thin-film solar cells (13.5% efficiency with indium sulphide buffer). In this work, the process has been up-scaled to module areas of up to 30×30 cm2. The indium sulphide buffer layer was deposited at substrate temperatures between 160 and 220 °C using indium acetylacetonate and hydrogen sulphide precursors. An efficiency of η=10.8% (open-circuit voltage, VOC=592 mV; fill factor, FF=62%; current density, jSC=29.5 mA/cm2) for a module area of 30×30 cm2 has been achieved. For laboratory cells even an efficiency of 14.9% was realised. Damp heat stability testing of CIGS mini-modules indicates a similar behaviour of both devices with ALCVD indium sulphide and solution grown cadmium-sulphide buffer layer.

Diffusion barriers for CIGS solar cells on metallic substrates

Abstract Al 2 O 3 layers of 1–3 μm were deposited as diffusion barriers by RF sputtering from a ceramic target on metal foils of Ti, Kovar ® and Cr steel. Cu(In,Ga)Se 2 (CIGS)-based thin-film solar cells were deposited onto these substrates using a co-evaporation process for CIGS at T ⩾550 °C. CIGS solar cells of 0.25 cm 2 achieved efficiencies of approximately 10–11% without any Na doping. Without barriers, the cell efficiencies were limited to significantly lower values except for Ti substrates. The reduced efficiency values can be attributed mainly to a reduction in fill factors, and secondly, to reduced open-circuit voltages. The different solar cell efficiencies can be correlated with the amount of impurities entering the CIGS layer by diffusion from the substrates, as investigated by simultaneous secondary ion mass spectrometry and sputtered neutral mass spectrometry depth profiling. Without diffusion barriers, Fe and Cr concentrations of several hundred ppm were detected in CIGS layers on Cr steel. Fe, Ni, Co and Ti concentrations from Kovar ® and Ti substrates were much smaller, indicating a reduced diffusion. Using Al 2 O 3 barriers, the concentrations of Fe and Cr in CIGS are reduced proportionally to the barrier thickness by up to a factor of 100 when compared to systems without barriers.

Cadmium-free buffer layers deposited by atomic later epitaxy for copper indium diselenide solar cells

As a soft and highly controllable deposition technique, atomic layer epitaxy (ALE) is well suited to deposit buffer and window layers on CIS thin solar films with high interface quality. In this work we have investigated ALE buffer layers of zinc oxysulfide, indium sulfide and aluminum oxide deposited at low temperature (160°C). The most promising results have been obtained with using indium sulfide buffer layers, with a record efficiency of 13.5% (30.6 mA /cm2, 604 mV, FF=0.73 under 100 mW /cm2, without AR coating) achieved on a standard CIGS absorber. This opens a route for a dry cadmium-free buffer process fully compatible with the other vacuum deposition techniques (coevaporation, sputtering).

Dielectric barriers for flexible CIGS solar modules

Abstract Cu(In,Ga)Se 2 (CIGS)-based thin-film solar modules are commonly deposited on float glass substrates at temperatures of approximately 550°C. For the preparation of flexible and monolithically integrated solar modules on metal foils, the substrates first have to be coated by an electrically insulating barrier. In this study, dielectric barrier layers of Al 2 O 3 and SiO x were deposited on metal foils of Ti, Kovar ® and ferritic Cr steel. The insulation properties were tested by sputtering small Mo contacts onto the barriers and measuring the resistance and breakdown voltages of the layers before and after CIGS deposition. Best insulating barriers could be achieved with 6-μm-thick combi layers of SiO x (plasma CVD)/SiO x (sol–gel) and SiO x (plasma CVD)/Al 2 O 3 (sputtered). These layers additionally act as diffusion barriers. Results of solar cell and module characterisation are presented to demonstrate the progress in barrier development.

Sodium co-evaporation for low temperature Cu(In,Ga)Se2 deposition

Abstract Cu(In,Ga)Se2 (CIGS) thin-films were deposited ‘in-line’, i.e. the substrate is moved with respect to the sources on soda-lime glass (SLG). To better understand the role of substrate temperature during CIGS deposition, we successively decreased the substrate temperature. The CIGS layers were analysed using sheet resistance measurements, SEM and XRD; in addition, solar cell devices were prepared. We found significant deterioration in solar cell parameters and a different morphology with decreasing substrate temperature. The results were compared with the results from CIGS layers and devices grown on sodium free glass. In a second step, a sodium evaporation source was added to the deposition system to provide additional sodium during film growth. With this technique, solar cells (0.5 cm2) with efficiencies of 11.7% (SLG) and 10.9% (Na-free glass) were fabricated ‘in-line’ at a substrate temperature of 400°C.

Microstructural and chemical studies of interfaces between Cu(In,Ga)Se2 and In2S3 layers

Microstructural and chemical properties of the interfaces between Cu(In,Ga)Se2 (CIGS) and In2S3 layers in dependence on the In2S3 deposition temperature and Na concentration were investigated. The In2S3 layers were deposited by atomic layer deposition on CIGS layers at substrate temperatures ranging from 140°C to 240°C. Interfaces were investigated by means of scanning electron microscopy, bright-field and high-resolution transmission electron microscopy, electron diffraction, and energy-dispersive x-ray spectrometry. An orientation relationship between CIGS {112) and In2S3 {103) planes was found for the sample deposited at 210°C, whereas no orientation relationship was detected for the 240°C sample. Cu diffusion from CIGS into In2S3 was detected, as well as Cu depletion and In enrichment on the CIGS side of the interface. All three effects are enhanced with increasing deposition temperature. These results indicate the formation of a buried junction in the CIGS layer. In addition, a Na-free solar cell was...