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Wave-optics analysis is performed to investigate the benefits of utilizing Bragg-reflectors and inverted ZnO opals as intermediate reflectors in micromorph cells. The Bragg-reflector and the inverted ZnO opal intermediate reflector increase the current generated in a 100 nm thick upper a-Si:H cell within a micromorph cell by as much as 20% and 13%, respectively. The current generated in the bottom muc-Si:H cell within the micromorph is also greater when the Bragg-reflector is used as the intermediate reflector. The Bragg-reflector outperforms the ZnO inverted opal because it has a larger stop-gap, is optically thin, and due to greater absorption losses that occur in the opaline intermediate reflectors.

Abstract:In situ Yb-doped amorphous carbon thin films were grown on Si substrates at low temperatures (<200 °C) by a simple one-step RF-PEMOCVD system as a potential photonic material for direct integration with Si CMOS back end-of-line processing. Room temperature photoluminescence around 1 µm was observed via direct incorporation of optically active Yb 3+ ions from the selected Yb(fod) 3 metal-organic compound. The partially fluorinated Yb(fod) 3 compound assists the suppression of photoluminescence quenching by substitution of C-H with C-F bonds. A four-fold enhancement of Yb photoluminescence was demonstrated via deuteration of the a-C host. The substrate temperature greatly influences the relative deposition rate of the plasma dissociated metal-organic species, and hence the concentration of the various elements. Yb and F incorporation are promoted at lower substrate temperatures, and suppressed at higher substrate temperatures. O concentration is slightly elevated at higher substrate temperatures. OPEN ACCESSMaterials 2014, 7 5644Photoluminescence was limited by the concentration of Yb within the film, the concentration of Yb ions in the +3 state, and the relative amount of quenching due to the various de-excitation pathways associated with the vibrational modes of the host a-C network. The observed wide full-width-at-half-maximum photoluminescence signal is a result of the variety of local bonding environments due to the a-C matrix, and the bonding of the Yb 3+ ions to O and/or F ions as observed in the X-ray photoelectron spectroscopy analyses.

Phone: þ00 1 416 946 7372, Fax: þ00 1 416 971 2326The DC saddle-field (DCSF) glow discharge method was used to deposit intrinsic a-Si:H onto c-Si to passivate the c-Si surface. The effective minority carrier lifetime in the heterostructures as a function of the excess minority carrier density in the c-Si wafers was measured. The results were then analyzed in the context of recombination associated with interface defect states using three known recombination models. The defect density and the charge density at the interface are inferred. In addition subsequent annealing of the samples was studied. It is shown that for our intrinsic a-Si:H samples improvements in surface passivation are directly correlated with the reduction of interface defects and not the reduction of minority carrier concentration at the interface due to electric field. We have achieved excellent surface passivation with effective carrier lifetime >4 ms for an intrinsic a-Si:H sample deposited at a process temperature of 200 8C and thickness of about 30 nm. It is also demonstrated that subsequent annealing, at 240 8C, of the samples which were prepared at process temperatures <240 8C greatly increases the effective lifetime.

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