Effects of SPC temperature on properties of evaporated poly-Si thin films and solar cells

Solid-phase crystallization (SPC) of Si thin films on glass deposited by electron-beam evaporation is compared at different annealing temperatures. Four independent film characterization methods, optical transmission microscopy, Raman and UV reflectance spectroscopy (UV-R), and X-ray diffraction (XRD), reveal that 1.4 micron thick films with n+/p-/p+ solar cell structures have the incubation times of about 285, 32, and 10 min and the full crystallization times of about 825, 94, and 24 min at 600°C, 640°C, and 680°C respectively. The estimated activation energies for the incubation and crystal growth are 3.0 and 3.2 eV respectively. The scanning electron microscopy (SEM) images of the resulting polycrystalline Si (poly-Si) films after Secco etching show that the average grain size gradually decreases with a higher SPC temperature. Similarly, according to the Raman, UV-R, and XRD spectra the crystal quality of the poly-Si films is poorer for the higher SPC temperatures. The sheet resistivities, which are dominated by the properties of the two thin but heavily doped n+ and p+ layers, are similar for all SPC temperatures. The open-circuit voltages (Voc) of about 450 mV measured by Suns-Voc method are also similar for all SPC temperatures and comparable to previously reported values for the best evaporated poly-Si cells. However, the minority carrier diffusion length of about 0.5 micron in the absorber layers for all SPC temperatures extracted from the external quantum efficiency (EQE) data is much shorter than the cell thickness and than previously reported, which can be due to relatively high boron concentration in the absorber found by means of the capacitance-voltage (C-V) analysis.