On the Role of Charged Defect States and Deep Traps in the Photocarrier Drift and Diffusion in a-Si:H
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On the Role of Charged Defect States and Deep Traps in the Photocarrier Drift and Diffusion in a-Si:H Paul Stradins and Akihisa Matsuda Thin Film Silicon Solar Cells Super Laboratory, Electrotechnical Laboratory Tsukuba, Ibaraki 305-8568, Japan, email [email protected] ABSTRACT The drift and diffusion in the presence of charged defects and photocarriers trapped in the tail states is re-examined. In continuity equations, diffusive and drift currents are related to free particles while the Poisson equation includes all charges. In order to make use of ambipolar diffusion approximation, the mobilities and diffusion coefficients should be attributed to the total electron and hole populations making them strongly particle-number dependent. Due to the asymmetry of the conduction and valence band tails, almost all trapped electrons reside in negatively charged defects (D-). A simple model of photocarrier traffic via tail and defect states allows to establish the effective mobility values and coefficients in Einstein relations. In a photocarrier grating experiment, grating of D- is counterbalanced by the grating of trapped holes. Nevertheless, electrons remain majority carriers, allowing the measurement of minority carrier diffusion length, but analysis is needed to relate the latter with µτ product. INTRODUCTION Minority carrier measurements are important to characterize materials for a-Si:H based solar cells. Holes diffuse slowly in i-layer and have low mobility-lifetime product causing the loss in the efficiency [1]. Slow hole motion also leads to an accumulation of a space charge and consequent lowering of electron mobility-lifetime product [2]. Minority carrier diffusion and drift measurements under cw illumination are performed using steady state [3,4] and moving [5] photocarrier grating techniques (SSPG and MPG). Since the conductivity of the illuminated sample is high, fast diffusion of the majority carriers is held back by the electric field from the slower minority carriers. As a result, the blurring of carrier grating at small interference grating periods is governed mostly by the minority carrier diffusion. Photocarrier gratings have been analyzed using continuity and Poisson’s equations and considering trapping in exponential tails [3-5]. On the other hand, most of the recombination events in light-exposed samples proceed via defects. Consequently, the effective mobility and diffusion values will be determined by trapping of photocarriers into defects. This work examines the role of defects in diffusion and drift of photocarriers in the steady state. TRAPPED HOLE AND CHARGED DEFECT GRATINGS Under illumination, the quasi-Fermi levels split and move towards the mobility edges with increasing illumination intensity. All electrons available between the trap quasi-Fermi levels (QFL) are redistributed over these gap states. Since the tail of the valence band (VB) is much broader than that of the conduction band (CB), a large number of electrons from VB tail is introduced between QFL. These electrons are accommodated
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