| AVS 61st International Symposium & Exhibition | |
| Applied Surface Science | Thursday Sessions |
| Session AS-ThP |
| Session: | Applied Surface Science Poster Session |
| Presenter: | Xerxes Steirer, National Renewable Energy Laboratory |
| Authors: | K.X. Steirer, National Renewable Energy Laboratory R.L. Garris, National Renewable Energy Laboratory J. Li, National Renewable Energy Laboratory M. Dzara, Rochester Institute of Technology P.F. Ndione, National Renewable Energy Laboratory K. Ramanathan, National Renewable Energy Laboratory I. Repins, National Renewable Energy Laboratory G. Teeter, National Renewable Energy Laboratory C.L. Perkins, National Renewable Energy Laboratory |
| Correspondent: | Click to Email |
Thin film solar cells rely upon the efficient transfer of photocharge from the absorbing photovoltaic material to an external circuit via thin buffer junction layers. Problems such as low fill factor and current loss arise when the electron collecting contact (emitter) exhibits a large positive conduction band discontinuity with the absorber. This study shows evidence for one approach to circumvent this problem by taking advantage of electronic defects prominent with chemical bath deposition (CBD) of buffer layers. X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) of heterojunctions formed between CZTSe and CBD-ZnOS with two different buffer preparations exhibit clear differences in electronic properties yet we observe no discernable differences in composition. Structure of CBD-ZnOS made using the different preparation methods is measured with grazing incidence X-ray diffraction (GIXRD) and also exhibits no discernable differences in the peak positions or full widths. XPS/UPS derived band energy diagrams are presented for quasi-in-situ prepared CZTSe/CBD-ZnOS interfaces with both preparation methods yielding valence band offsets equal to -1.0 eV and conduction band offsets equal to 1.1 eV. However, comparison between water only and a water/dimethyl sulfoxide solvent mixture in device characterization and band offset measurements show increased band bending in accordance with higher n-type carrier density with water as the only solvent. Seemingly incongruous, the more strongly n-type buffer layer performs worse in solar cells and exhibits inflected current-voltage response under one-sun illumination. A proposed electron transport band for these buffer layers that seems to circumvent the large conduction band spike is estimated to have energies about 0.6 eV below the conduction band of the CBD-ZnOS. Hence, these defects appear to enable adequate band-lineup with the low-band gap absorber, CZTSe (Eg = 0.96 eV). These findings suggest that cosolvation approaches may allow for the manipulation of the electronic structure of ZnOS and enable a wider range of electronic applications where larger electron affinities are required.