Effect of Granularity on CuInSe 2 Solar Cell Response
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EFFECT OF GRANULARITY ON CuInSe2 SOLAR CELL RESPONSE JAMES R. SITES Department of Physics, Colorado State University,
Fort Collins, CO 80523
ABSTRACT Polycrystalline CuInSe 2 solar cells, fabricated by evaporation or by selenization of metal films, are granular and relatively porous. Grains are a few hundred nanometers in dimension, and hence about 1% of the atoms are at a surface. Despite the granularity, quantum efficiency is quite high and implies a diffusion length exceeding the grain dimension. The primary photovoltaic loss is excessive forward recombination current. The proposed model consists of single crystal CuInSe 2 granules with an indium rich surface layer. When properly passivated, the otherwise uncoordinated indium bonds are terminated by oxygen. However, residual non-passivated crystalline surface states distributed throughout the depletion region provide the paths for enhanced recombination.
INTRODUCTION Thin-film polycrystalline solar cells with a p-type CuInSe 2 absorber and n-CdS or ZnO window have achieved conversion efficiencies of 14.1% for the terrestrial solar spectrum [1]. This value is about 60% of the theoretical maximum for a 1.02 eV bandgap material at room temperature. In contrast, crystalline silicon cells (bandgap 1.12 eV) have achieved 23% efficiency [2], over 90% of their theoretical maximum. Scanning electron microscopy shows a granular structure for CuInSe 2 with typical grain dimension slightly under 1 pm [3]. The purposes of this work are to identify the primary loss mechanisms and show how they are physically related to the granularity.
CURRENT-VOLTAGE The active area short-circuit current, normalized to a standard solar 2 spectrum [41, has reached 41 mA/cm for both evaporated [5] and selenized [1] CuInSe 2 cells. This value is about 88% of the theoretical maximum, compared to 93% for crystalline silicon with textured surface and two-layer antireflective coating [2]. The CuInSe 2 quantum efficiency can exceed 0.9 for the wavelength range from 450 to 90 nm [1]. Thus, its photocurrent is not significantly reduced by the granular structure, and the effective diffusion length is greater than 1 pm. The CuInSe 2 forward diode current, however, is generally much higher than that of a comparable bandgap crystalline cell. Figure I is a log plot of current-density vs. voltage. The light curves are translated by the photocurrent JL so the plot shows the forward diode current. In the ideal case the solid curves would superimpose, and in fact for better cells they come closer than shown here. At the far right is the target curve based on the best crystalline silicon cells adjusted for bandgap. At any voltage the CuInSe 2 forward current is orders of magnitude above the target. With the photocurrent and maximum-power current approximately fixed, the horizontal 2 differences near 30 and 4 mA/cm give the open circuit and maximum power voltage losses due to the polycrystallinity and additionally due to illumination and series resistance. These losses account for most of the efficiency diff
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