Asymmetries in Amorphous Silicon Devices
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ASYMMETRIES IN AMORPHOUS SILICON DEVICES*
Z E. SMITHa) and S. WAGNER Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544
ABSTRACT The consequences of the asymmetries in the density of electronic states of hydrogenated amorphous silicon on the behavior of electronic devices are discussed. Asymmetries in the relative widths of valence and conduction bandtails, the position of the dangling-bond states within the gap, and the occupation statistics of non-correlated defects are shown to affect the performance of p-i-n solar cells, and explain their superior performance and stability when compared with such devices illuminated through the n-layer (n-i-p). The device-modeling concepts which emerge help explain the differences between cell degradation via light exposure under various bias conditions, and cell degradation by dark forward bias.
OVERVIEW Important insights into the materials physics of hydrogenated amorphous silicon (a-Si:H) and its alloys can be gained by examination of the characteristics of devices operated in modes which highlight the asymmetries in the density of states (D.O.S.). Recent experiments [1-3] have focussed on the differences in performance and stability between solar cells illuminated through the p-layer (p-i-n) and those illuminated through the n-layer (n-i-p). These experiments have led to conflicting conclusions concerning the mechanism of the light-induced and forward-bias-current-induced degradation of solar cell efficiency [3-5]. In this work we will discuss how the p-i-n structure, by the sweeping of the Fermi-level across the band from one side of the cell to the other, in effect converts the D.O.S. in energy space into a D.O.S. in real space. We will show how this picture aids in comparing the merits of various numerical device models. We study the implications of this approach for understanding the forward-bias degradation of solar cells. DENSITY OF STATES ASYMMETRIES Bandtails. The most obvious asymmetry in the a-Si:H D.O.S. is the fact that the valence bandtail is much broader than the conduction bandtail. The effects of this asymmetry are relatively well-understood and we only touch on them here. The first is that, even if the band (free-carrier) mobilities were identical for electrons and holes, the greater breadth of the valence bandtail has the consequence of lowering the drift mobility for holes far below that of electrons. This comes about because the wider bandtail has more states, below the mobility edge which are still within the thermalization depth ('-,kT), and so carriers can spend a greater fraction of their time trapped in the tails. A further implication of this asymmetry is that when a-Si:H is operated as a primary photoconductor, there will be more trapped holes than trapped electrons, and the total trapped carrier populations will generally exceed the free carrier populations. Other key bandtail-width effects include the fact that it is more difficult to dope strongly p-type than n-type, in the sense that the wider bandtail
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