Simulation of Hydrogenated Amorphous Silicon Germanium Alloys for Bandgap Grading
- PDF / 445,124 Bytes
- 6 Pages / 382.5 x 615.6 pts Page_size
- 53 Downloads / 216 Views
ABSTRACT Computer simulations are reported of hydrogenated amorphous silicon germanium (a-SiGe:H) layers that make up the graded part of the intrinsic layer near the interfaces of a-SiGe:H solar cells. Therefore the graded part is approached with a 'staircase' bandgap profile, consisting of three layers within which the material properties are constant. Calibrated model parameters are obtained by matching simulation results of material properties of intrinsic a-SiGe:H single layers to measurements. Using the obtained model parameter sets subsequent simulations of p-i-n devices with intrinsic material similar to the single layers are matched to measured current-voltage characteristics. The changes in parameter values are evaluated as a function of optical gap. INTRODUCTION Amorphous silicon germanium (a-SiGe:H) has proven to be a suitable low band gap material for the intrinsic layer of the bottom solar cell in a tandem structure [I]. Incorporation of Ge improves the red response, but deteriorates electronic properties. Excellent overviews of experimental data on a-SiGe:H are given by Stutzman et al. [2] and Bauer [3]. Subsequent studies showed that material properties are improved when using strong hydrogen dilution during the deposition [4]. For application in solar cells, band gap grading near the p-i and i-n interfaces - by profiling the Ge content in the intrinsic layer - to accommodate the band offsets, has considerably enhanced the cell characteristics [5]. So far the effect of these graded parts on the electronic behavior of the cell is not fully understood. Computer simulations can be used to give insight in this matter. Guha et al. [5] simulated solar cells with bandgap grading using different profiles and was able to verify trends in experimental observations. Several grading profiles were also studied using computer simulations by Vasanth et al. [6]. In that work the whole i-layer was split into five sub-layers to represent the profiles. Recently, Zimmer et al. [7] simulated bandgap profiling by variation of the band edges, defect density and Urbach energy. However, the dependence of other model parameters on the Ge content has not been taken into account. In our approach we divide the graded part of the device into several layers in which the material properties do not vary with depth, thereby obtaining a 'staircase' bandgap profile. We assume that the properties in the graded part of the solar cell change in the same way as the properties of a series of single layers in which the Ge content is varied. Wronski et al. [8] pointed out that such self-consistent simulation parameters are necessary for a realistic simulation of solar cells, which enables a reliable analysis for optimization. In this study we present how the input parameters of these layers are determined following the calibration strategy proposed by Zeman et al. [9]. To assign proper values to the model input parameters simulation results are matched to experimental data from literature as well as from measurements carried out on single lay
Data Loading...
