Band Discontinuity Effect on a-Si:H and a-SiGe:H Solar Cells
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EXPERIMENTAL The samples for the internal photoemission measurement were deposited on specular stainless steel (ss) substrates without back reflectors. The structure of samples used to measure AE, is ss/p Bc-Si:H (600 A)/i a-Si:H (100 - 400 A)/n a-Si:H (-100 A) (#1 and 2 of Table I). The intrinsic a-Si:H layer was grown by rf plasma-enhanced chemical vapor deposition using a disilane-hydrogen mixture. Hydrogen diluted silane was used for gc-Si:H layer deposition. BF 3 and PH3 were added as dopants for p and n type doping, respectively. Details of deposition parameters have been reported before [8]. A semitransparent Au or Al front electrode was evaporated on top of the sample to form an Ohmic contact. The monochromatic exciting light of varying wavelength was incident on the front metal electrode. A reverse bias of 50 mV was applied for collecting the photo-generated carriers. For cell performance studies, a-Si:H and a-SiGe:H alloy solar cells were deposited on ss substrates without back reflectors. The structure of the cells is ss/n/i a-Si:H (or a-SiGe:H)/p jcSi:H/ITO. The thicknesses of the intrinsic layers are 5000 and 3700 A for a-Si:H and a-SiGe:H cells, respectively. Table I. Sample structures for internal photoemission measurement and observed threshold energies.
I 2 3 4
E, (eV)
Structure
Sample No.
ss/p ss/p ss/n ss/n
Btc-Si:H (600 A)/i a-Si:H (400 A)/n a-Si:H (100 A)/Au Btc-Si:H (600 A)li a-Si:H (100 A)/n a-Si:H (100 A)/Au a-Si:H (200 A)/i a-Si:H (1000 A)/p ic-Si:H (100 A)/ITO gc-Si:H (600 A)/h a-Si:H (100 A)/p gc-Si:H (100 A)/Al
1.68 1.57 1.74 1.85
DETERMINATION OF AE, AND AE, According to Kane's model [9] for indirect transitions from semiconductors, the quantum yield Y of internal photoemission in the vicinity of the threshold energy (Et) can be expressed as
Y - (hv-Ft)5/2
(1)
where hv is the energy of photons. The internal photoemission process in the ss/p Bc-Si:H/i aSi:H/n a-Si:H/Au sample can be described as follows. The photons that are transmitted through the metal electrode and the thin n and i layers are partially absorbed in the Bc-Si:H layer. The electrons are photoemitted from the valence band of Btc-Si:H to the conduction band of a-Si:H (see Fig. ). The threshold energy, Etj, is the sum of the bandgap, E8 (Bc-Si:H), of Bic-Si:H, and AE,. Since both i and n layers are very thin, the predominant contribution to the photocurrent is due to the transition from the valence band of Btc-Si:H to the conduction band of the intrinsic aSi:H alloy.
652
The 2/5 power of the photoemission quantum yield Y is plotted against the photon energy in Fig.2 for two samples with different thicknesses of the intrinsic a-Si:H layer (#1 and 2 of Table I). As seen in Fig.2, (Y) 2"5 becomes more linear with hv when the i a-Si:H layer becomes thinner. For sample #2 with a 100 A thick i layer, a good linear relationship is observed in the range of 1.7 to 2.4 eV. The threshold energy, E l, is obtained by extrapolating the straight line to the intercept; and 1.57 eV is obtained for sample #2. For the sample with
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