Development of photovoltaic cells on electrodeposited copper indium selenide films.

摘要

Using polycrystalline thin films of CuInSe{dollar}sb2{dollar} (thickness 1.5 {dollar}mu{dollar}m) prepared by an electrodeposition method, photovoltaic cells of the form ZnO/CdS/CuInSe{dollar}sb2{dollar} have been fabricated and studied. Before the cell fabrication, properties of the electrodeposited CuInSe{dollar}sb2{dollar} were first studied. Conditions for the preparation of high resistivity CdS thin films (thickness 360 A) by a chemical bath deposition method were then established. This was achieved by varying the deposition temperature and amount of NH{dollar}sb4{dollar}OH in the solution. It was observed that the quality of CdS films deposited at a temperature of 60{dollar}spcirc{dollar}C and an NH{dollar}sb4{dollar}OH concentration of 0.48 M was the best. Thermal stability of low resistivity, In- or Ar-doped, rf-sputtered ZnO thin films (thickness 1{dollar}mu{dollar}m) was also studied. This was done by heat treating the ZnO films in air, O{dollar}sb2{dollar} and N{dollar}sb2{dollar} at temperature in a range from 200 to 350{dollar}spcirc{dollar}C. It was observed that the amount of increase of resistivity at a given temperature decreased as the doping concentration was increased from 0.5 to 5.3 wt.%. For the films containing 2 wt.% In or Al, the resistivity increased as the treating temperature was increased from 200 to 350{dollar}spcirc{dollar}C. For the samples treated at 200{dollar}spcirc{dollar}C, the increase in resistivity was about 1 order of magnitude.; The high resistivity CdS and low resistivity ZnO thin films were then deposited on electrodeposited CuInSe{dollar}sb2{dollar} to fabricate cells of the form ZnO/CdS(high {dollar}rho{dollar})/CuInSe{dollar}sb2{dollar}. For comparison, cells of CdS(low {dollar}rho{dollar})/CdS(high {dollar}rho{dollar})/CuInSe{dollar}sb2{dollar} were also fabricated by evaporation of low resistivity CdS. The CuInSe{dollar}sb2{dollar} films used were treated either in vacuum or Ar. For those treated in vacuum, very poor properties were observed. The properties improved after a post fabrication heat treatment in air, however, the efficiency of these cells was below 2%. The low conversion efficiency was due to the low open circuit voltage. From capacitance-voltage measurements, this was found to be due to a high acceptor concentration on the surface of the vacuum treated CuInSe{dollar}sb2{dollar} films (10{dollar}sp{lcub}17{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar}). For the cells fabricated on the CuInSe{dollar}sb2{dollar} films treated in Ar, photovoltaic effects were present before the air heat treatment. An efficiency of 6.8% was obtained for one of the best cells, sample J8-4 (with low resistivity CdS window). For cells with ZnO window, a conversion efficiency of 6.3% was obtained (cell O51). For these cells, the acceptor concentration in CuInSe{dollar}sb2{dollar} was 10{dollar}sp{lcub}16{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar}, which was one order of magnitude lower than that of CuInSe{dollar}sb2{dollar} films treated in vacuum.; The diffusion length of minority carriers (electrons) in the electrodeposited, p-type CuInSe{dollar}sb2{dollar} was first measured using the photocurrent and capacitance methods. For the vacuum treated CuInSe{dollar}sb2{dollar} films, the electron diffusion length was small (less than 0.1 {dollar}mu{dollar}m). For those treated in Ar, values of the electron diffusion length were about 0.5 {dollar}mu{dollar}m. These values are close to those reported for evaporated CuInSe{dollar}sb2{dollar} thin films.; Some of the fabricated cells were also studied using an electron beam induced current (EBIC) method. From the EBIC experiments, the effective diffusion lengths of electrons with values greater than 1 {dollar}mu{dollar}m were obtained. Considering the surface recombination effect, the electron diffusion length of the electrodeposited CuInSe{dollar}sb2{dollar} was finally found to be 2.4 {dollar}mu{do