FLASH ANNEALING - A NEW PROCESS FOR ENHANCING THE CONVERSION EFFICIENCY OF N-WSE2 SINGLE-CRYSTAL-BASED PHOTOELECTROCHEMICAL SOLAR-CELLS

The present paper reports a new process called 'flash annealing' for t he enhancement of solar to electrical conversion efficiency (eta) for n-WSe2 single-crystal-based photoelectrochemical (PEC) solar cells. Th e flash annealing process involves annealing of the as-grown n-WSe2 cr ystals at a predetermined high temperature (750 +/- 10-degrees-C) for very short times (1 2 min) in a dynamic vacuum followed by a rapid que nching to room temperature. Improvements in photovoltage (e.g. from 18 0 mV to 600 mV) and fill factor (e.g. from 0.33 to 0.60) have been inv ariably observed. These results have been compared with another mode o f annealing in which n-WSe2 crystals are subjected to long-time furnac e annealing (at 750 +/- 10-degrees-C for 1 h). Evaluation Of XRD patte rns before and after flash annealing revealed an increase in the inten sity of the 001 lines and a decrease in peak full width at half maximu m (FWHM). However, in the case of long-time annealing, the intensity o f the 001 lines is found to decrease. On the other hand, the FWHM Of X RD peaks remained almost the same as before annealing. These results h ave been explained on the basis of Fermi level depinning for n-WSe2 si ngle-crystal photoelectrodes (n-WSe2/I-, I3-) after flash annealing, T he flash annealing process is expected to decrease the density of bulk imperfections because thermal stresses (due to non-uniform heating) a re set up which act to drive away the bulk imperfections. In the case of prolonged annealing, however, due to uniform heating, thermal stres ses are absent and consequently the density of bulk imperfections is n ot expected to change.