Effects of Grain Boundaries in Amorphous/Multicrystalline Silicon Heterojunction Photovoltaic Cells
- PDF / 202,771 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 26 Downloads / 202 Views
L8.1.1
Effects of Grain Boundaries in Amorphous/Multicrystalline Silicon Heterojunction Photovoltaic Cells M. Farrokh Baroughi and S. Sivoththaman Department of Electrical & Computer Engineering, University of Waterloo 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada. ABSTRACT Spectral response and dark current-voltage characteristics of heterojunctions are used to investigate grain boundary degradation in photovoltaic properties of a-Si/mc-Si heterojunction solar cells. Measured spectral response inside the grain and on the grain boundary shows small but consistent QE degradation due to minority carrier recombination at the grain boundaries. No consistent difference is observed in dark current-voltage characteristics because of large diode area and periphery leakage current in the employed heterojunction diodes. Comparing measurement results and results from device modeling using the simulation software Medici, a recombination velocity of 4900 cm/sec is found at the grain boundaries of employed multicrystalline silicon wafer. The modeling and experimental results can also be used to define an effective grain area that serves as a measure of grain boundary recombination and the influence of grain size. INTRODUCTION Cost reduction is an important issue in the fabrication of silicon (Si) photovoltaic (PV) cells, where the material cost accounts for nearly half of the overall cost. Materials like multicrystalline silicon (mc-Si), silicon ribbons etc., offer a cost effective option for Si PV cells compared to single crystalline Si [1]. In most of those materials however, the presence of large number of grains (mm to cm scale), grain boundaries (GBs), and crystallographic defects necessitates defect passivation. Defect passivation by atomic hydrogen is a very efficient method. However, this imposes a temperature (T) limit for any post-passivation processes such as pn junction diffusion at high-T. Implementation of amorphous Si (aSi)/crystalline Si heterojunctions (HJ) in place of diffused homojunctions in defective Si can keep the process temperature low thereby preserving the defect passivation [2,3]. mc-Si material normally comprises large grains in the range of mm2 to cm2. mc-Si production by casting and Edge-defined Film-fed Growth (EFG) techniques result in very large grain sizes in cm2 range while Electro Magnetic Casting (EMC) and Silicon Sheet from Powder (SSP) techniques results in small grain sizes in mm2 range[4]. Grain boundaries include lots of trap centers that can act as generation-recombination centers. Therefore, grain boundaries can potentially degrade short circuit current by recombining photogenerated carriers; and also fill factor & open-circuit voltage by increasing leakage current of the heterojunction diode in low forward bias region. GB effects in solar cells become very important for small grain mc-Si wafers [5-7]. In this work we have assessed the effects of grain boundaries on spectral response and dark current-voltage (IV) characteristics of aSi/mc-Si HJ solar cells. HJ diodes a
Data Loading...
