Preparation of a-Si:H and a-SiGe:H I-Layers for Nip Solar Cells at High Deposition Rates Using a Very High Frequency Tec
- PDF / 489,882 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 54 Downloads / 200 Views
Mat. Res. Soc. Symp. Proc. Vol. 507 0 1998 Materials Research Society
EXPERIMENT The nip solar cell structures with stainless steel substrates were fabricated using a research scale, multi-chamber load locked deposition system. Both the doped layers and the a-Si:H buffer layers, grown between the VHF deposited a-SiGe:H i-layers and the doped layers, were prepared using the conventional PECVD process in which a 13.56 M[Hz rf signal is used. To fabricate the a-Si:H and a-SiGe:H i-layers, a fixed VHF frequency of 70 MHz was used. To improve the cell properties, several buffer layer and i-layer deposition conditions were altered including the substrate temperature, the hydrogen dilution and active gas flows, and the applied power. After fabrication of the nip structure, the devices were completed by depositing Indium Tin Oxide (ITO) conductive layers and then Al collection grids. Both the ITO and Al layers were prepared using standard evaporation techniques. In some cases, Ag/ZnO back reflectors were deposited on the stainless steel substrates prior to nip fabrication in order to enhance the collected current and improve the overall efficiency. The Ag/ZnO back reflectors were prepared in a rollto-roll manufacturing machine using a DC sputtering technique. To characterize the cells, standard J-V and spectral response (quantum efficiency) measurements were made. For the a-Si:H cells, standard white AM1.5 light was used to obtain the J-V data. Since our goal is to use the a-SiGe:H cells as middle and bottom cells for triplejunction cells, the AM1I.5 light for the J-V measurements of the a-SiGe:H cells was filtered using a 530 nm cutoff filter to simulate the absorption due to a top a-Si:H cell. To complete light soaking studies, the cells were subjected to 600-1000 hrs. of one sun light with the cell temperature fixed at 50°C. The i-layer thicknesses were determined using standard capacitance techniques. RESULTS AND DISCUSSION a-Si:H An earlier publication outlines much of the work completed on a-Si:H cells prepared at high ilayer deposition rates by the VHF technique 4. The general conclusion from these studies was that by using the VHF technique and careful selection of deposition conditions, the initial and stable cell efficiencies could be made to remain relatively constant with varying i-layer deposition rate up to 10 A/s. Figure 1 demonstrates this result where the initial and stable efficiencies for a number of cells prepared under a variety of conditions are plotted as a function of deposition rate. All of the cells whose data is shown in the figure had J. near 10 mA/cm 2, i-layers which were roughly 2300 A thick and had no current-enhancing Ag/ZnO back reflectors. The stable efficiencies were obtained by light soaking the cells for 1000 hrs. Below a deposition rate of 10 A/s, the initial and stable cell efficiencies are relatively insensitive to the deposition rate with average values of 6.6 and 5.5%, respectively. The small amount of scatter (+5%) is related to experimental measurement procedures and variation
Data Loading...
