Adrian Chirila

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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In2S3 layers were deposited by flash evaporation technique with varying flash rates. The optical constants of layers based on Tauc–Lorentz model dielectric function were extracted from spectroscopic ellipsometry measurements. X-ray photoelectron spectroscopic investigation revealed the presence of oxygen impurity in as-deposited and air-annealed layers with traces of Na inclusion in the layer grown at high flash rate. The enhancement in crystalline arrangement of as-deposited layer after air annealing was confirmed by Raman spectroscopy. Rutherford backscattering measurements revealed the growth of off-stoichiometric layers at all flash rates. An analytical layer growth model has been proposed supporting the results obtained by various layer characterization techniques. The solar cells were prepared with flash evaporated In2S3 buffer layers and their performances were compared with CdS reference solar cell. A significant gain in short-circuit current was obtained after air annealing of the complete device...

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

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.

Optical, structural, and chemical properties of flash evaporated In2S3 buffer layer for Cu(In,Ga)Se2 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.

Influence of NaF incorporation during Cu(In,Ga)Se 2 growth on microstructure and photovoltaic performance

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.

Development of multijunction thin film solar cells

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.

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

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.