Low Temperature Deposition of Amorphous Silicon Based Solar Cells
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749 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
EXPERIMENTAL DETAILS We deposit the single layers and cell structures examined in this study by standaxd PECVD (radio frequency rf = 13.6 MHz) using silane (Sill4) diluted by hydrogen (H2 ), methane (CH4) for the p-layer, and the doping gases phosphine (PH 3) and diborane (B2 H6 ). The hydrogen dilution ratio is defined as rH = [SiH 4]/([SiH4]+[H 2]). The radio frequency power density P (plasma pressure p ) values range from 50 mW/cm2 (150 mbar) in case of undiluted growth, to 200 mW/cm 2 (750 mbar) for highly diluted films to keep the growth rate rd at reasonable values. Light soaking under 2.5 suns conditions is carried out on single layers for 50 hours. We measure the minority carrier diffusion length LD using Steady State Photocarrier Gratings (SSPG) technique and carry out Constant Photocurrent Method (CPM) on doped and undoped single layers to evaluate the Urbach energies (Eu), and subgap absorption coefficients 1l.2eV for defect density (ND) estimations. Optical Transmission Spectroscopy is used to calibrate CPM-data a(hv) and to derive the optical gap T, from the Tauc relation. The cell parameters - efficiency 17,open circuit voltage Voc, short circuit current density jsc , and fill factor FF- are determined under AM 1.5-like illumination at room temperature. We deposit pin-structures on SnO 2 Asahi-glass with Ag back contacts defining active area sizes of 0.5 to 1.0 cm 2 and nip-cells with sputtered Indium Tin Oxide (ITO) front contacts on Ag-evaporated Coming 7059 glass. For the p-layers we use a-SiC:H with Tc= 2.1 eV. The series of nip-cells for our investigations on front contact design have nominal identical structure but different thicknesses of an inserted buffer layer consisting of p-type nc-Si between the absorption layer and p-contact layer deposited at rH = 1:40 and P = 500 mW/cm 2 . Due to the uncertainty in the size of the ITO contact areas, we can indicate the current density and therefore the efficiency of our nip-structures only with an error of about 20%. RESULTS AND DISCUSSION
Single Layers In order to examine the influence of the deposition temperature on the structural characteristics of a-Si:H, we deposit a temperature series under standard conditions. Keeping the pressure at p = 150 mbar and the plasma power at P = 50 mW/cm2 , we vary the deposition temperature from Ts = 200 'C down to Ts = 50'C. Figure 1 shows rd and T, of undiluted and undoped films as a function of the deposition temperature Ts . We observe a strong increase of the growth rate from rd = 1 to rd = 4 A/s towards low temperatures. The tauc gap shows a similar behavior and surpasses T, = 1.9 eV at Ts = 50 'C. In the following, we explain this behavior by the nature of H-bonding in the material. The empirical relationship between T, and the hydrogen content CH reported by Cody et al.[3] Tr(eV)
=
1.53 + 0.015 CH (at.%)
(1)
gives values for CH surpassing 25 % for films deposited at Ts = 50'C. The relation between SiH 2-bonds and the optical ba
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