Amorphous thin films for solar-cell applications. Quarterly report No. 2, 11 December 1979-10 March 1980

Measurements of both primary and secondary photocurrents for photon energies between 0.58 and 2.0 eV in a-Si:H solar-cell structures have been used to provide information about the density of states above the valence band as well as show that holes are mobile deep in the bandgap. The drift mobility is independent of excitation energy, indicating that the majority carriers are excited to the same states, irrespective of the excitation energy. Deposition studies in the dc proximity system have shown that the conductivity and photoconductivity of doped films (both boron- and phosphorus-doped) increase with substrate temperature. Mass spectroscopy studies have shown that the larger Si/sub x/H/sub y//sup +/ clusters are favored by operating rf discharges at low rf powers and high pressures. The Hall mobility is roughly constant below 360 K but exhibits a thermal activation energy of approx. 0.13 eV at higher temperatures. These observations rule out simple extended-state transport as the conduction mechanism. The photoelectromagnetic effect has been used to estimate the hole lifetime (approx. 3.4 x 10/sup -7/s) in undoped a-Si:H; the electron lifetime is approx. 1.7 x 10/sup -6/s. An a-Si:H monolithic solar panel consisting of 16 cells in series has been fabricated on a 4'' x 4'' glass substrate (63 cm/sup 2/ of active area), and the panel exhibits a conversion efficiency of 3.6%. In another series of experiments, the thicknesses and doping levels of p-i-n cells made in a dc(P) discharge were optimized. The best cell made in these experiments had an efficiency of 5.3% with an area of 1.19 cm/sup 2/

Amorphous thin films for solar-cell applications. Quarterly report No. 1, 11 September-10 December 1979

Research progress on amorphous Si:H solar cells is described. Tasks include theoretical modeling, deposition and doping studies, experimental characterization of a-Si:H, formation of solar cell structures, and evaluation of solar cell parameters. A new method for determining the drift mobility of majority carriers in doped a-Si:H is discussed. Deposition and doping studies have been performed in an rf magnetron discharge system. Mass spectroscopy has been used to show that the major impurities in the SiH/sub 4/ discharge occur at m/e values of 45, 47, and 49 at concentrations 10/sup -4/-10/sup -5/ times that of the principal ion, SiH/sub 3//sup +/. Boron implantation of an i-n structure produces a p-i-n cell with an enhanced V/sub oc/ but reduced J/sub sc/ as compared to cells in a nonimplanted region. Laser annealing at power densities up to 60 MW/cm/sup 2/ (30-ns pulse) causes partial crystallization of the a-Si:H, but there are no significant changes in the photoluminescence spectrum or the hydrogen content. the photo-Hall effect in undoped a-Si:H has been measured as a function of wave-length and temperature. The photoelectromagnetic spectrum for the short-circuit current has been used to estimate a hole diffusion length of approx.0.1-0.3 ..mu..m in undoped a-Si:H. Recently p-i-n cells have been fabricated with conversion efficiencies up to approx. 4.5%. Both boron and phosphorus concentrations were found to vary inversely with rf power for doped a-Si:H films made in an rf capacitive discharge. Solar cells have also been fabricated with a-(Si,Ge):H alloys. (WHK