Theory of Recombination in Non-Crystalline Junctions
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1165-M03-02
Theory of Recombination in Non-Crystalline Junctions Marco Nardone1, Victor G. Karpov1, and Diana Shvydka2 1 University of Toledo, Dept. of Physics and Astronomy, Toledo, OH 43606 2 University of Toledo, Dept. of Radiation Oncology, Toledo, OH 43606 ABSTRACT A theory of non-crystalline recombination junctions is developed and compared to experimental data. Junction transport is represented as hopping in both real and energy spaces, dominated by rare yet exponentially effective optimum channels having favorable configurations of localized states. Our work correlates the current-voltage characteristics of non-crystalline devices with material parameters and predicts large non-ideality factors increasing under light. INTRODUCTION The purpose of this paper is to develop a theory of recombination junctions based on noncrystalline materials. Our approach is based on the concept of a mobility gap, rather than a forbidden gap, wherein a quasi-continuous spectrum of energy levels can exist [1]. This concept applies to important photovoltaic (PV) materials: a-Si:H [2] polycrystalline CdTe/CdS [3], and CuIn(Ga)Se2 (CIGS) [4] exhibiting quasi-continuous spectra of localized states. We show how recombination via such localized states plays an important role in thin-film PV. This paper conceptually follows earlier work [5] on transversal hopping through thin amorphous films, and other work [6], in which hopping in the energy space was shown to be an effective recombination mechanism. Our model deals with a combination where hopping occurs simultaneously in the real space (through the junction) and the energy space (through the gap). We recall the standard current voltage characteristic where the current density is given by I = I 0 [exp (qV AkT ) − 1] − I L
(1)
where q is the electron charge, A is the non-ideality factor, and IL is the photogenerated component. According to the classical model [7,8], the forward current is dominated by thermal activation over the junction barrier W0 (see figure 1) with a probability proportional to exp(W0/kT). Because in equilibrium the corresponding forward current I00exp(-W0/kT) is balanced by the reverse current I0, IV takes its standard form in Eq. (1) with A = 1 where qV = DW is the bias induced change in junction barrier. Another known mechanism [7,9] sketched in figure 1 includes recombination via a single energy level, so that the electron and the hole overcome comparable activation barriers º W0/2 corresponding to A = 2 in Eq. (1). In more recent work [10] the latter mechanism was extended to include possible tunneling of charge carriers to the localized states in the space charge region and to account for the energy distribution of those states (the idea of tunneling was put forward earlier [11] in connection with the mechanism of thermionic emission through Schottky barriers). However, there are empirical arguments making the classical models insufficient, such as, observations of A > 2, noticeable variations in IV curves between nominally identical devices, and others th
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