Solar cells containing a polycrystalline Cu(In,Ga)Se absorber outperform the ones containing a monocrystalline absorber, showing a record efficiency of 22.9%. However, the grain boundaries (GBs) are very often considered to be partly responsible for the enhanced recombination activity in the cell and thus cannot explain the registered record efficiency. Therefore, in the present work, we resolve this conundrum by performing correlative electron beam-induced current-electron backscatter diffraction investigations on more than 700 grain boundaries and demonstrating that 58% of the grain boundaries exhibit an enhanced carrier collection compared to the grain interior. Enhanced carrier collection thus indicates that GBs are beneficial for the device performance. Moreover, 27% of the grain boundaries are neutral and 15% are recombination-active. Correlation with microstructure shows that most of the ∑3 GBs are neutral, whereas the random high-angle grain boundaries are either beneficial or detrimental. Enhanced carrier collection observed for a big fraction of high-angle grain boundaries supports the "type-inversion" model and hence the downward band bending at GBs. The decrease in current collection observed at one of the high-angle grain boundaries is explained by Cu being enriched at this GB and hence by the upward shift of the valence band maximum.

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Alternative buffer layers in CIGSe are deposited mainly using chemical bath deposition because of its benefits like simplicity, good film quality and surface/step coverage. All the layers in CIGSe cell stack such as back contact, absorber and window layers are deposited by vacuum-deposition methods such as coevaporation, sputtering, and sometimes thermal evaporation, except for the buffer layer. Therefore, in the present work we demonstrate the feasibility to deposit In 2 S 3 by RF magnetron sputtering reaching cell efficiencies of 13.6%, which is the highest value available for sputtered In 2 S 3 in literature to date. Absorber surface damage and nonuniform buffer layer thickness are the primary limitations when using sputtering, and hence need to be eliminated for reaching reasonable cell efficiencies. We studied the extent of sputter induced damage on CIGSe absorber as well as the sputtering-and annealing-induced intermixing phenomenon at the In 2 S 3 /Cu(In,Ga)Se 2 interface at the subnanometer level using atom probe tomography. We have also shown that a post deposition annealing not only significantly improves the crystallinity of In 2 S 3 , but also recovers the surface damage caused by sputter-induced intermixing resulting in an improved p-n Junction quality (as shown by the electron beam induced current investigations), and substantially improves cell efficiency. The present work opens a new way for designing efficient and industry-compatible CIGSe cells using sputter-deposited Cdfree buffer layers. Moreover, this work clearly demonstrates that this novel and fully vacuum-deposited CIGSe cell meets the standard requirements, in terms of chemistry, structure, and electrical performance of a working cell for the PV industry.

Graphene oxide−titanium dioxide (GO−TiO 2 ) nanocomposite sheets were transferred onto Si and quartz substrates by Langmuir−Blodgett (LB) technique at different subphase TiO 2 concentrations and pH values. The effects of subphase and heat treatment conditions on the composition, surface morphology, microstructure, and hydrophobic performance of the composite sheets were investigated by XPS, Raman, AFM, HR-TEM, and contact-angle measurements. The wetting behavior of composite sheets before and after vacuum heat treatment was analyzed in conjunction with the extent of TiO 2 uptake, distribution of nanoparticles over GO sheets, and overall surface roughness. The degree of subphase ionization significantly affects the uptake and aggregation behavior of TiO 2 , which tends to be dominated by the interaction of TiO 2 with carboxylic acid functional groups at the edges of the GO sheets. Wettability studies revealed improvement in the hydrophobicity of composite sheets compared to the GO sheets, which is attributed primarily to the larger surface roughness induced by TiO 2 nanoparticles. Heat treatment in vacuum causes formation of a reduced graphene oxide (rGO) wrap on the surface of TiO 2 nanoparticles, which substantially enhances the hydrophobicity of the composite sheets. UV exposure causes deterioration of the hydrophobicity of as-transferred composite sheets, while the heat-treated composites retain their superior hydrophobicity, owing to the presence of a rGO wrap, which hinders the reconstruction of surface −OH groups on TiO 2 . The rGO−TiO 2 composite sheets exhibit high transmittance in the visible region and undeteriorated hydrophobic behavior even after prolonged UV irradiation, which have potential applications in the development of flexible transparent nonwetting electronic devices.

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