Thin film solar cells have the potential to be an important contributor to the world energy demand in the 21st century. Among all the thin film technologies, CuInGaSe2 (CIGS) thin film solar cells have achieved the highest efficiency. However, the high price of photovoltaic (PV) modules has been a major factor impeding their growth for terrestrial applications. Reduction in cost of PV modules can be realized by several ways including choosing scalable processes amenable to large area deposition, reduction in the materials consumption of active layers, and attaining faster deposition rates suitable for in-line processing. Selenization-sulfurization of sputtered metallic Cu-In-Ga precursors is known to be more amenable to large area deposition. Sputter-deposited molybdenum thin film is commonly employed as a back contact layer for CIGS solar cells. However, there are several difficulties in fabricating an optimum back contact layer. It is known that molybdenum thin films deposited at higher sputtering power and lower gas pressure exhibit better electrical conductivity. However, such films exhibit poor adhesion to the soda-lime glass substrate. On the other hand, films deposited at lower discharge power and higher pressure although exhibit excellent adhesion show lower electrical conductivity. Therefore, a multilayer structure is normally used so as to get best from the two deposition regimes. A multi-pass processing is not desirable in high volume production because it prolongs total production time and correspondingly increases the manufacturing cost. In order to make manufacturing compliant with an in-line deposition, it is justifiable having fewer deposition sequences. Thorough analysis of pressure and power relationship of film properties deposited at various parameters has been carried out. It has been shown that it is possible to achieve a molybdenum back contact of desired properties in a single deposition pass by choosing iv the optimum deposition parameters. It is also shown that the film deposited in a single pass is actually a composite structure. CIGS solar cells have successfully been completed on the developed single layer back contact with National Renewable Energy Laboratory (NREL) certified device efficiencies > 11%. The optimization of parameters has been carried out in such a way that the deposition of back contact and metallic precursors can be carried out in identical pressure conditions which is essential for in-line deposition without a need for load-lock. It is know that the presence of sodium plays a very critical role during the growth of CIGS absorber layer and is beneficial for the optimum device performance. The effect of sodium location during the growth of the absorber layer has been studied so as to optimize its quantity and location in order to get devices with improved performance. NREL certified devices with efficiencies > 12% have been successfully completed.

A rapid thermal processing (RTP) reactor for the preparation of graded CuIn1-xGaxSe2-ySy (CIGSeS) thin-film solar cells has been designed, assembled and is being used at the Photovoltaic Materials Laboratory of the Florida Solar Energy Center. CIGSeS films having the optimum composition, morphology, and semiconducting properties were prepared using RTP. Initially films having various Cu/(In+Ga) ratios were prepared. In the next step selenium incorporation in these films was optimized, followed by sulfur incorporation in the surface to increase the bandgap at the surface. The compositional gradient of sulfur was fine-tuned so as to increase the conversion efficiency. Materials properties of these films were characterized by optical microscopy, SEM, AFM, EDS, XRD, GIXRD, AES, and EPMA. The completed cells were extensively studied by electrical characterization. Current-voltage (I-V), external and internal quantum efficiency (EQE and IQE), capacitance-voltage (C-V), and light beam induced current (LBIC) analysis were carried out. Current Density (J)-Voltage (V) curves were obtained at different temperatures. The temperature dependence of the open circuit voltage and fill factor has been estimated. The bandgap value calculated from the intercept of the linear extrapolation was ~1.1-1.2 eV. Capacitance-voltage analysis gave a carrier density of ~4.0 x 1015 cm-3. Semiconductor properties analysis of CuIn1-xGaxSe2-ySy (CIGSeS) thin-film solar cells has been carried out. The values of various PV parameters determined using this analysis were as follows: shunt resistance (Rp) of ~510 Ohms-cm2 under illumination and ~1300 Ohms-cm2 in dark, series resistance (Rs) of ~0.8 Ohms-cm2 under illumination and ~1.7 Ohms-cm2 in dark, diode quality factor (A) of 1.87, and reverse saturation current density (Jo) of 1.5 x 10-7A cm-2. The efficiency of 12.78% obtained during this research is the highest efficiency obtained by any University or National Lab for copper chalcopyrite solar cells prepared by RTP. CIGS2 cells have a better match to the solar spectrum due to their comparatively higher band-gap as compared to CIGS cells. However, they are presently limited to efficiencies below 13% which is considerably lower than that of CIGS cells of 19.9%. One of the reasons for this lower efficiency is the conduction band offset between the CIGS2 absorber layer and the CdS heterojunction partner layer. The band offset value between CIGS2 and CdS was estimated by a combination of ultraviolet photoelectron spectroscopy (UPS) and Inverse Photoemission Spectroscopy (IPES) to be -0.45 eV, i.e. a cliff is present between these two layers, enhancing the recombination at the junction, this limits the efficiency of CIGS2 wide-gap chalcopyrite solar cells.