S. Seyrling

Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films

Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.

Development of multijunction thin film solar cells

Due to the possibility of band gap engineering in Cu(In,Ga)Se2 (CIGS) absorbers and the spectral tunability of dye-sensitized solar cells (DSCs) with suitable choice of the sensitizer, this combination of solar cells could be ideal for the construction of dual junction photovoltaic devices. Factors for performance limitations of mechanically stacked tandem cells have been identified and alternative remedies are developed to overcome efficiency losses. Limiting factors of the efficiency have been determined. In a monolithical configuration, the issue of corrosion of the CIGS cell caused by exposure to the DSC electrolyte has been investigated.

CIGS solar cells grown by a three-stage process with different evaporation rates

Solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) absorber layers have shown high potential for low cost photovoltaic energy conversion. Our group has achieved 18.1% efficient cells on glass substrates and 14.1% on flexible polyimide foils without antireflection (AR) coating. These results were achieved by applying a three-stage evaporation process with common growth rates of about 35 nm/min. Aim of this study was to enhance the growth rates of layers deposited at low substrate temperature (450°C) suitable for polyimide foils and to investigate structural properties of CIGS layers and photovoltaic properties of solar cells. It was found that very high deposition rates of 500 nm/min during the 2nd and 3rd stage of the evaporation process are feasible, maintaining high absorber quality and performance of solar cells.

Optimization of composition grading in Cu(In,Ga)Se2 for flexible solar cells and modules

For the development of high efficiency Cu(In,Ga)Se2 (CIGS) flexible solar cells on polymer films with efficient and robust deposition process the so-called three-stage evaporation processes were modified. CIGS layers were grown by co-evaporation of constituent elements at low substrate temperature compatible with the stability of the polyimide film. The CIGS layers grown with our standard process are compositionally graded, especially the Ga has a strong grading profile across the layer thickness. Different variations of the standard evaporation process were investigated, especially in view of changing the energy band gap profiles in the absorber layer for achieving further improvements in high efficiency solar cells. Further on, the incorporation of sodium during the CIGS deposition was optimized. Structural and chemical composition properties of CIGS layers were characterized with SEM, XRF, SIMS and the photovoltaic properties were characterized with I–V and quantum efficiency measurements. A flexible CIGS solar cell on polyimide film with a record efficiency of 17.6% has been developed.

Na distribution in CIGS solar cells grown with modified three-stage processes

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.