Characterization of the Bulk Recombination in Hydrogenated Amorphous Silicon Solar Cells
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CHARACTERIZATION OF THE BULK RECOMBINATION IN HYDROGENATED AMORPHOUS SILICON SOLAR CELLS J. Deng, J.M. Pearce, V. Vlahos, R.W. Collins, and C.R. Wronski Center for Thin Film Devices, the Pennsylvania State University, University Park, PA 16802
ABSTRACT Dark forward bias current, JD-V, characteristics offer a probe for characterizing carrier recombination and the defect states in the intrinsic layers of hydrogenated amorphous silicon (a-Si:H) solar cells. Detailed studies were carried out on such characteristics for the cells with optimized p/i interfaces and high quality i-layers in which the current transport is bulk recombination dominated. It was found that the diode quality factor n is not a constant with bias voltages as has been generally considered. Instead, it can be best described through the bias dependent differential diode quality factors, n(V) = [kT/q]-1[d(lnJD)/dV]-1, whose magnitude and shape reflect the gap state distribution in the corresponding bulk i-layers. The n(V) characteristics obtained on cell structures with both hydrogen diluted and undiluted i-layers have been utilized in characterizing the differences in the distribution of defect states in the two i-layers both in annealed state as well as after creating light induced defects. In the characterization of the Staebler-Wronski Effect (SWE) using JD-V characteristics, a new phenomenon is observed – relaxation of light induced defect states created by 1 sun illumination at 25oC, which is also found in the follow-on studies on the photo-conductivities of corresponding thin films. INTRODUCTION Previous studies have shown that it is possible in forward bias current-voltage (JD-V) characteristics of a-Si:H p-i-n and n-i-p solar cells to separate the carrier recombination in the bulk of the i-layers from that in the p/i interface regions [1-4]. In cell structures having appropriate protocrystalline a-Si:H interface regions bulk recombination dominates the characteristics over a wide range of voltage. Results have also been obtained which clearly establish the absence of the high densities of defect states at the p/i, n/i interfaces predicted by the defect pool model [3]. The absence of such highly non-uniform distributions of gap states across the i-layer reduces the large complexity in the interpretation of JD-V characteristics in terms of carrier recombination and gap states in the i-layers and allows them to be addressed with an analysis based on first principles [5]. In this treatment account is taken of (i) the homogenous densities of defect states across the entire i-layer; (ii) the potential barriers Vn, Vp adjacent to the n, p contacts due to the high concentration of carriers there and (iii) ShockleyReed-Hall (SRH) diffusion/recombination in the bulk and at the p/i interfaces. As a consequence the contribution to the currents of recombination in the bulk is most pronounced at low biases with those of the p/i interface at the higher voltages. As the voltages approach the built-in potential, Vbi, the currents become limited by car
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