P. Blösch

Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil

Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.

Review of Progress Toward 20% Efficiency Flexible CIGS Solar Cells and Manufacturing Issues of Solar Modules

Solar cells based on chalcopyrite Cu(In, Ga)Se2 (CIGS) absorber layers show the highest potential for low-cost solar electricity by yielding comparable efficiencies to polycrystalline Si wafer-based cells, while also offering inherent advantages of thin-film technology for cost reduction.Highest efficiency of 20.3% was recently achieved on rigid glass substrate. Deposition of CIGS films onto flexible substrates opens new fields of applications and could significantly decrease production costs by employing roll-to-roll manufacturing and monolithic integration of solar cells to develop modules. Whereas, some years back, it seemed difficult to reach performance levels on flexible substrates similar to that obtained on glass, recent results on flexible polyimide prove that the efficiency gap can be significantly reduced. Different materials, i.e., mostly metals or plastics, have been used as flexible substrates, with highest cell efficiency of 18.7% demonstrated on a polyimide film. Improvements in efficiencies of flexible solar cells and modules achieved over the past few decades are discussed in this paper, addressing the main characteristics of substrate materials. The technology transfer from laboratory research to large-scale industrial production of CIGS modules leads to new manufacturing challenges, mainly for CIGS deposition, interconnections of cells, and long-term performance stability.

Cu(In,Ga)Se

Thin-film solar cells based on the chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber material show high potential for further cost reduction in photovoltaics. Compared with polycrystalline silicon (p-Si) wafer technology, thin-film technology has inherent advantages due to lower energy and material consumption during production but has typically shown lower conversion efficiency. However, in the past two years, new scientific insights have enabled the processing of CIGS solar cells with efficiencies up to 21%, surpassing the p-Si wafer value of 20.4% efficiency for the first time. Now several research groups report record cell efficiency values above 20% using different deposition processes and buffer layers. The presence of potassium was observed in many CIGS devices over the years, but it is only very recently that differences with Na have started being taken into full consideration for device processing and that K was added intentionally to the absorber. In this study, previous reports showing the presence of potassium are reviewed and discussed in more detail. Furthermore, on a scale-up perspective, additional progress has also taken place with CIGS minimodules achieving efficiency up to almost 19% and where further increase can be expected in the near future with the improvements induced by the use of potassium. This shows that the CIGS technology is continuously progressing not only on scientific level but on technological level as well.

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The sodium supply via thermal evaporation of NaF during different stages of a three-stage Cu(In,Ga)Se2 (CIGS) evaporation process has been investigated. Solar cells were processed on soda lime glass with Si3N4 diffusion barrier and on polyimide foils at low substrate temperature of 475°C compatible with the stability of the polyimide foil. Secondary electron micrographs (SEM) of CIGS layers show inhomogeneous microstructure containing regions of small grains near the back contact when sodium is evaporated during the 1st and the 2nd CIGS growth stage, respectively. The CIGS layer structure is affected only to minor extent if sodium is incorporated in the 3rd stage. In order to correlate the layer inhomogeneities with the composition profiles, the CIGS layers were investigated with depth resolved Raman scattering and sputtered neutral mass spectroscopy (SNMS). Both analyzing techniques reveal a strongly graded composition across the CIGS absorber, with an intermediate Ga-poor region and Ga-rich surface and back regions. The performance of resulting solar cells was characterized by means of current-voltage (J-V) and external quantum efficiency (EQE) measurements. It is found that the photovoltaic performance of the cells depends significantly on the NaF incorporation method. Cells developed with a low temperature growth process yielded high efficiencies of up to 16.4% without antireflection coating when NaF was supplied during the 3rd stage of the CIGS growth process.

Thin-Film Solar Cells and Modules—A Boost in Efficiency Due to Potassium

Highest efficiency CIGS solar cells are generally grown with a three-stage co-evaporation process where the absorber layer is in a copper-rich regime for a period of time at the end of the second stage. We investigated the influence of changing the maximum [Cu]/[In+Ga] ratio at the end of stage 2 on the distribution of sodium throughout the absorber layer when sodium is supplied by diffusion from the soda-lime glass substrate. Secondary ion mass spectrometry (SIMS) was used for depth profiling of the Na content, the surface concentration of Na was determined by wavelength dispersive X-ray analysis (WDX) from top view scanning electron micrographs. Raman investigation of the phase composition of the surface and SIMS compositional depth profiles of the investigated absorber layers suggested the possibility of the formation of a Na-rich compound on the absorber layer surface for CIGS grown with low Cu excess while absorbers grown with high excess showed a more evenly distributed Na depth profile. New WDX results further support these claims as a surface [Na]/[Cu+Na] ratio of up to 0.2 for layers grown with low Cu excess was measured while the Na surface values of absorbers grown with high Cu excess are below the detection limit.