Surface Potential and Surface Photovoltage of Oxide and Nitride coated multicrystalline Silicon Solar Cells using a Scan
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Surface Potential and Surface Photovoltage of Oxide and Nitride coated multicrystalline Silicon Solar Cells using a Scanning Kelvin Probe Iain D. Baikie, Kelvin Research Laboratory, KP Technology Ltd, Milton House, Wick, KW1 5LE, UK. email: [email protected] ABSTRACT We have applied a high resolution scanning Kelvin probe to perform dark surface potential topographies of multicrystalline silicon solar cells having thin coatings of Si3N4 and SiO2. We clearly observe the electrical characteristics of the screen printed bus-bar and associated fingers, grain boundaries, together with characteristic structures on the oxide and nitride, coupled to significant surface potential variations across larger sections of the wafer. Associated surface photovoltage measurements can be unambiguously decoded to show coating and bulk contributions. The nitride coating exhibits carrier trapping lifetimes in excess of 13 minutes at 300K. INTRODUCTION The development of multicrystalline silicon (mc-Si) with screen printed contacts is essential for mass production of inexpensive solar cells. Already production volumes of such cells are twice that of single crystal silicon (sc-Si). Current mc-Si solar cell efficiencies are about 12-15% compared with up to 29% for sc-Si. The main reason for this difference are the defects introduced during the manufacturing process, including grain boundaries, dislocations, interface trapped charge, metal impurities, resistance losses, lattice strain [1]. Various techniques have been used to characterize solar cells including thermography, electron and laser beam induced current, microwave photoconductance, all of which have been reviewed by Istratov et al [2]. Another recently developed approach to detect parallel resistances is the ShuntScan technique developed by van der Heide [3]. Silicon nitride (Si3N4) coatings have led to improved efficiencies over silicon oxide (SiO2) passivation due to, for example, improved anti-reflection properties and hydrogen gettering of bulk defects. Problems remain however with the electrical performance of such cells including the series and parallel resistance's caused by imperfections during growth. The surface and interface regions play an important role in the overall solar cell efficiency since, if carriers recombine at surface or interface traps, they are removed from the load circuit. Passivation films can display a variety of charge trapping mechanisms however surface potential (sp) measurements offer a valuable insight into device operation: spatially resolved dark measurements contain information concerning the surface structures such as the bus-bars, metal fingers and the work function of the passivated silicon surfaces. Illumination of the substrate by photons of greater than band-gap light energy produces the surface photovoltage (SPV). A thorough review of this technique has been recently published by Kronik [4]. SPV can be used to non-invasively characterize both surface and buried interfaces. We report a procedure allowing unambiguous deconvolut
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