Selenium is a p-type semiconductor with a band gap of >1.8 eV and has an absorption coefficient higher than silicon by one order of magnitude in the visible range. Its amorphous form (a-Se) has long been used as a photoconductor for many photoconduction applications due to the high multiplication gain and a low dark current. Crystalline Se (c-Se) has a high absorption coefficient over the visible spectrum and a high theoretical photoconversion efficiency up to 20%, making it an appropriate photon absorber for solar cells. Moreover, in theory, c-Se based solar cells have a higher open circuit voltage (V_OC) than Si based cells because of the higher band gap of c-Se. In this study, we prepared c-Se thin film under various a-Se crystallization conditions and studied the photovoltaic performance of c-Se Schottky junction (SJ) solar cells.

Simple c-Se solar cells without the electron and hole transport layers were first studied (i.e., metal-semiconductor-metal (MSM) device structure). The fabrication begins with an acetone-cleaned ITO glass substrate, on which a ~1.0 nm-thick tellurium seed layer was deposited by RF-magnetron sputtering. An a-Se layer of 0.5-1.0 µm in thickness was then thermal evaporation-deposited at room temperature. The a-Se layer was converted into the crystalline phase by thermal anneal in atmosphere at temperatures between 100-200^oC for various times. To complete the SJ solar cell, a 80 nm-thick Pt film electrode was sputter-deposited on the c-Se layer. SEM, XRD, Raman spectroscopy and XPS were used to characterize the a-Se and c-Se thin films. From the material characterizations, a-Se can be completely converted into c-Se if a proper thermal budget for a-Se crystallization is provided. Grains in the c-Se layer prepared at the crystallization temperature (T_c) of 200^oC has a size ranging from 200 to 400 nm, which increases with the annealing time. The c-Se layer with larger grains has a rougher interface with the top Pt electrode.

Before examining the photovoltaic property of the c-Se solar cells, we studied their photoconduction performance. Compared with their initial amorphous form, the c-Se thin films exhibit an improved photoconduction efficiency in the red light region by a factor of 3-10. The c-Se thin film prepared at T_c= 200^oC demonstrates a larger photocurrent than the one at T_c= 110^oC. This can be ascribed to the larger grain size, which increases with T_c. We thereafter focus the discussion on the photovoltaic characteristic of the c-Se solar cells prepared at T_c= 200^oC because of its better photoconduction property.

The photovoltaic measurement was performed with a solar simulator under illumination at AM 1.5 (100 mW/cm²). Table 1 presents the V_OC , short circuit current, fill factor (FF) and photoconversion efficiency (η) of the solar cells prepared at T_c = 200^oC for three annealing times. The η increases with the size of c-Se grains, most likely resulting from the smaller recombination loss of photocarriers due to less grain boundaries. The larger V_OC and FF for a longer annealing time can also attributed to less defects present in the c-Se layer of larger grain. Because of the lack of the charge transport layers between the absorber layer and the electrodes, the MSM cell structure exhibits poor photovoltaic performance. To improve the photoconversion efficiency, we are undertaking the fabrication of c-Se solar cells with a MISIM structure, in which ZnO (or TiO₂) and MoO_x ultra thin dielectric films are inserted as the electron and hole transport layers, respectively. In addition, the introduction of the ZnO or TiO₂ layer may improve the wetting of the Te seed layer on the substrate, and thereby result in a better microstructure for the c-Se absorber layer. We can, therefore, reduce the thickness of the c-Se layer from 1.0 µm to 0.3 µm so that the film resistance can be greatly decreased. Preliminary results have shown that the photovoltaic performance of the a-Se solar cell can be significantly improved by the introduction of the two charge transport layers.