Abstract
The objective of this program is to investigate the use of Cu/sub 2-x/Se to produce low cost, high efficiency photovoltaic solar cells. The Cu/sub 2-x/Se films are produced by coevaporation of Cu and Se from separate, individually controlled vapor sources onto heated glass substrates. This method gives greater composition controllability and is readily adaptable to large scale production efforts. Two quartz crystal microbalances are used to separately monitor the Cu and Se deposition rates. The structural, electrical, and optical properties of the Cu/sub 2-x/Se films have been measured for deposits made on 250/sup 0/C substrates. The optical absorption measurements show the material having an indirect band gap of 1.4 eV and a direct gap of 2.2 eV. These values are for stoichiometric indices in the range of 0.17 less than or equal to x less than or equal to 0.26. Hall and conductivity measurements give hole mobilities in the range of 3 to 7 cm/sup 2//Vsec and hole densities of the order of 4 x 10/sup 22/ cm/sup -3/. For deposits made on substrate at 160/sup 0/C, the mobility is in the range of 3 to 10 cm/sup 2//Vsec and hole densities on the order of 10/sup 18/ to 10/sup 21/ cm/sup -3/ for 0.1 less than or equal to x less than or equal to 0.3. To date, the best cell has photovoltaic characteristics of J/sub sc/ = 11.6 mA/cm/sup 2/, V/sub oc/ = 460 mV, F.F. = 0.62 and eta = 3.3% when tested under simulated AM1 illumination. In an effort to improve cell performance, low resistance CdS was used. Cell performance degraded considerably with the low resistance CdS resulting in substantially lower values for both V/sub oc/ and I/sub sc/. It is believed in part that this difficulty can be traced to pinhole defects in the ITO electrode. When the low resistance CdS is deposited on ITO, regions in the CdS appear to be high in Cd where pinholes in the ITO were observed.
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