Investigation of Charge Trapping at Grain Boundaries in Polycrystalline and Multicrystalline Silicon Solar Cells
- PDF / 514,356 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 10 Downloads / 210 Views
1268-EE07-10
Investigation of charge trapping at grain boundaries in polycrystalline and multicrystalline silicon solar cells Jennifer T. Heath1,2, Chun-Sheng Jiang2, Helio R. Moutinho2, and Mowafak M. Al-Jassim2 Department of Physics, Linfield College, McMinnville, OR 97128. 2 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401. 1
ABSTRACT Scanning capacitance microscopy (SCM) often shows a change in contrast at grain boundaries [1-3]. The origins of this contrast and the efficacy of SCM as a tool to identify band bending at grain boundaries in pc-Si and mc-Si are discussed. Contrast at these grain boundaries could be influenced by different oxide growth rates or by defect states at the oxide interface. In order to determine the influence of such mechanisms on the SCM signal, such effects must be modeled; we show that a simple one-dimensional model agrees well with more detailed models of SCM signal strength and indicates, for example, that very small changes in oxide thickness measurably affect the SCM signal. In our experimental data, the uniformity and quality of the oxide layer are confirmed, and increased contrast consistent with depletion regions is still observed at higher order grain boundaries as identified by electron backscattering diffraction, including Σ9 and Σ27a. Scans of the SCM signal as a function of dc probe voltage allow such regions to be more quantitatively investigated. INTRODUCTION Grain boundaries (GBs) can play an important role in electronic devices, including lowcost solar cell devices with polycrystalline absorber layers. The GBs are generally considered to be detrimental to device performance, harboring both extrinsic and intrinsic defects that act as recombination sites for minority carriers. When GBs are charged or have a large enough density of sub-bandgap defect states to pin the Fermi energy, band bending can occur, causing minority carriers to be attracted to the GB and further increasing the minority carrier recombination rate [4,5]. However, it has also been shown that GBs can play a beneficial role. The surrounding strain fields can cause impurities to be gettered to the GB location, purifying the grains themselves and rendering the impurities electrically inactive [5,6]. It is also possible for GBs to be so heavily doped that they are inverted, which in certain scenarios may allow minority carrier collection from the device to be enhanced [7,8]. Thus, it is desirable to tailor GB properties to enhance the performance of electronic devices based on polycrystalline materials. A number of studies have focused on characterizing the electronic properties of GBs. Scanning Capacitance Microscopy (SCM) is one tool that has been employed to attempt to identify GBs that exhibit band bending, and hence have surrounding depletion regions. Because the SCM signal is larger in regions of low carrier density, it is thought to be enhanced in depletion regions around GBs. While there are experimental and numerical studies of the depletion region near the p-n junction [9],
Data Loading...
