Effect of Superlattice Doped Layers on the Performance of a-Si:H P-I-N Solar Cells
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EFFECT OF SUPERLATTICE DOPED LAYERS ON THE PERFORMANCE OF a-Si:H P-I-N SOLAR CELLS RAJEEWA R. ARYA, ANTHONY CATALANO AND JAMES O'DOWD Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, PA 18940
Newtown,
ABSTRACT Superlattice doped layers of the type ABABAB... have been prepared where A is the wide bandgap a-Si:C:H doped layer and B is the narrow bandgap a-Si:C:H or a-Si:H doped layer. The bandgaps of the individual layers were modulated by changing the gas phase composition of methane during the plasma deposition. By varying the structure of the films, superlattice p-layers with resistivities in the range of 106 - 107 ohm-cm with optical bandgaps of 2.0 - 2.4 eV, and activation energies of 0.35 - 0.48 eV and superlattice n-layers with resistivities in the range of 104 - 105 ohm-cm with optical bandgaps of 1.86 - 2.03 eV and activation energies of 0.38 - 0.47 eV have been obtained. P-I-N solar cells have been prepared with both p and n layers comprised of superlattice structures. Conversion efficiencies as high as 10.86% have been achieved under simulated AM1.5 Global conditions. Measurements reveal a marked improvement in both built-in voltage and carrier collection length. INTRODUCTION Amorphous silicon-hydrogen alloy thin film solar cells are now widely used in consumer electronics and are increasingly finding use in other terrestrial applications. One major impediment to large scale terrestrial use of a-Si:H solar cells is its relatively low conversion efficiency as compared to crystalline silicon solar cells. In the past decade, from the time Carlson and Wronski [1] first demonstrated a 2.4% efficient p-i-n a-Si:H solar cell, large strides have been made in the improvement of the conversion efficiency of a-Si:H solar cells. The major breakthrough came with the use of P-I-N device structure and the development of wide bandgap amorphous silicon carbide (a-Si:C:H) p-layers [2] which led to the first 10% efficient a-Si:H solar cell [3]. Since then several approaches have been taken to further improve the conversion efficiency. These include the use of high quality i-layers, graded bandgap i-layers and p-layers [4], microcrystalline n-layers [5], and improvements in light-trapping by the use of textured conducting tin oxide (CTO) and Indium tin oxide (ITO) [6,7]. By using combinations of the above mentioned features, several groups have reported conversion efficiences in the 10 to 11.5% range [8,9,10]. In this paper we discuss the use of a novel device structure to achieve high efficiency a-Si:H solar cells. This structure consists of P-I-N solar cells in which the doped layers are deposited as superlattice or multilayered layers rather than as single homogeneous layers. In an a-Si:H P-I-N solar cell with light incident on the p-side, the role of the optical and the electronic properties of doped layers (the p- and the n-layer) are of paramount importance. In this device structure, the p-layer should have (i) wide optical bandgap and (ii) high conductivity. Since only long wavelength light reaches
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