J.E. Phillips

The design and fabrication of thin-film CdS/Cu 2 S cells of 9.15-percent conversion efficiency

Thin-film polycrystalline CdS/Cu2S cells with energy conversion efficiencies in sunlight of up to 9.15 percent and areas of ∼1 cm2have been developed. The improvement over previously achieved efficiencies is due to the development of techniques to separately measure and minimize fill factor losses. Specific design and fabrication changes based on a detailed quantitative analysis of the cell operation, were introduced to correct series resistance, shunt conductance and field effect losses. Further increases in efficiency can be expected from the development of a planar junction thin-film CdS/Cu2S cell.

High efficiency CuInSe2 based heterojunction solar cells: Fabrication and results

Abstract A process for fabricating CuInSe2/CdS solar cells is described. A data set of 202 substrates each containing 12 cells from 129 separate deposition runs is used to examine the processing parameters for fabricating high efficiency cells. The data show a broad range of CuInSe2 compositions over which high efficiency cells can be made. The use of a thin CdS layer, less than 2 μm in thickness, can increase the short-circuit current by more than 3 mA cm−2. High efficiency devices have also been made on low cost sodalime glass. Air heat treatments at 200 °C for between 8 and 60 h are required to optimize the output of the CuInSe2/CdS cells.

Thin‐film polycrystalline Cu2S/Cd1−xZnx S solar cells of 10% efficiency

Polycrytalline, thin-film Cu/sub 2/S/Cd/sub 1-x/Zn/sub x/S heterojunction solar cells with conversion efficiencies of 10% have been prepared on Cd/sub 1-x/Zn/sub x/S with 0.1< or =x< or =0.2. Light-generated currents of up to 26 mA/cm/sup 2/ (prorated to 100 mW/cm/sup 2/) have been achieved, comparable to the best observed in Cu/sub 2/S/CdS cells of the same design. The improved performance for Cu/sub 2/S-based devices is as a consequence of the higher open-circuit voltage achieved with the addition of zinc.

Thin-film polycrystalline Cu/sub 2/S/Cd/sub 1-x/Zn/sub x/ S solar cells of 10% efficiency

Polycrytalline, thin-film Cu/sub 2/S/Cd/sub 1-x/Zn/sub x/S heterojunction solar cells with conversion efficiencies of 10% have been prepared on Cd/sub 1-x/Zn/sub x/S with 0.1< or =x< or =0.2. Light-generated currents of up to 26 mA/cm/sup 2/ (prorated to 100 mW/cm/sup 2/) have been achieved, comparable to the best observed in Cu/sub 2/S/CdS cells of the same design. The improved performance for Cu/sub 2/S-based devices is as a consequence of the higher open-circuit voltage achieved with the addition of zinc.

Advances in CuInSe2 and CdTe thin film solar cells

Abstract Research on CuInSe2 and CdTe thin film solar cells is discussed. CuInSe2 was deposited by selenization of Cu/In layers and was used to make a 10% efficient CuInSe2/(CdZn)S cell. Characterization of the reaction mechanisms is described. The open-circuit voltage V oc of CuInSe2/(CdZn)S cells is dominated by recombination in the space charge region, so increasing the band gap or decreasing the width of this region should increase V oc . Increasing the band gap with a thin Cu(InGa)Se2 layer at the CuInSe2 surface has demonstrated increased V oc with collection out to the CuInSe2 band gap. A post-deposition treatment and contacting process for evaporated CdS/CdTe cells was developed and high efficiency cells were made. Several steps in the process, including a CdCl2 coating, a 400 °C heat treatment, and a contact containing copper are critical. ZnTe films were deposited from an aqueous solution as a contact to CdTe.

Thin film tandem solar cells based on CuInSe2

Abstract The CuInSe 2 /(CdZn)S heterojunction is the best developed low bandgap solar cell for use in a two-junction tandem device. The potential performance of large area terrestrial systems based on this junction is reviewed. A monolithic tandem cell in which the high bandgap cell is a (CdHg) Te/CdS heterojunction deposited onto a CuInSe 2 /(CdZn)S cell is being developed and progress with this system is described.

The roles of electric fields and illumination levels in passivating the surface of silicon solar cells

The back surface recombination velocity (BSRV) of operating silicon solar cells has been modulated by an externally applied gate voltage and illumination through a semi-transparent MOS structure. All three voltage bias modes (accumulation, depletion and inversion) of this MOS system are examined. In addition, the effects of gate bias voltage, illumination (or carrier injection level) and oxide charge on the BSRV and solar cell performance are investigated. The effects of electric field and illumination are found to play key roles in achieving a low BSRV. Methods of achieving a low BSRV with this structure are described.

Parametric analysis of a-Si solar cells from current voltage measurements

A procedure is presented for accurately representing the current voltage (JV) characteristic of a-Si solar cells. The measured JV data is separated into the forward diode current and photocurrent. The forward diode current is characterized to obtain standard diode parameters A, J/sub 0/, and R. The AM1.5 photocurrent J/sub L/(V) is characterized using the uniform field model to determine the ratio of collection length to thickness L/sub c//D, flat band potential VB, and total photocurrent J/sub LO/. Very good agreement is found between measured and calculated currents from reverse bias to V/sub OC/ in a variety of cells. We find that degradation of FF under light soaking from 69% to 49% can be explained solely by a decrease in a single parameter L/sub C//D, from 14 to 3. By examining cells with D from 0.2 to 0.6 /spl mu/m we find that /spl mu//spl tau/ ranges from 2-4/spl times/10/sup -8/ cm/sup 2//V.

Determining the voltage dependence of the light generated current in CuInSe/sub 2/-based solar cells using I-V measurements made at different light intensities

As the bandgap of CuInSe/sub 2/ is increased by alloying with Ga or S, the loss in efficiency due to the decrease of light generated current with increasing voltage becomes important. The standard technique of quantifying this loss is to analyze spectral response measurements made as a function of applied voltage. Instead, it is shown how to determine the voltage dependence of the light generated current by an analysis of the current-voltage (I-V) measurements made at two different light intensities. By adding an I-V measurement at a third light intensity one can also determine if the analysis technique is valid.

Evidence for amorphous like behavior in small grain thin film polycrystalline solar cells

Solar cells based on CdTe, CuInSe/sub 2/ or the alloys of Cu(In,Ga)Se/sub 2/ have grain sizes on the order of one micron. It has been suggested that materials with these size grains could have an exponential distribution of localized states within the bandgap similar to amorphous materials such as a-Si:H. The current voltage characteristics of a number of CdTe, CuInSe/sub 2/ and Cu(In,Ga)Se/sub 2/ based solar cells made by various processes and manufacturers are measured as a function of light intensity and temperature. These measurements are then analyzed and compared for evidence of this amorphous like behavior.