Defect Density Profiling in Light-Soaked and Annealed Hydrogenated Amorphous Silicon Solar Cells
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DEFECT DENSITY PROFILING IN LIGHT-SOAKED AND ANNEALED HYDROGENATED AMORPHOUS SILICON SOLAR CELLS Richard S. Crandall1, Jeffrey Yang2, and Subhendu Guha2 1 National Renewable Energy Laboratory, Golden, CO 80401, [email protected] 2 United Solar Systems Corp., Troy, MI, 48084 ABSTRACT The fundamental ingredient lacking in solar cell modeling is the spatial distribution of defects. To gain this information, we use drive-level capacitance profiling (DLCP) on hydrogenated amorphous silicon solar cells. We find the following: Near the p-i interface the defect density is high, decreasing rapidly into the interior, reaching low values in the central region of the cell, and rising rapidly again at the n-i interface. The states in the central region are neutral dangling-bond defects whose density agrees with those typically found in similar films. However, those near the interfaces with the doped layers are charged dangling bonds in agreement with the predictions of defect thermodynamics. We correlate the changes in solar cell efficiency owing to intense illumination with changes in the defect density throughout the cell. Defects in the central region of the cell increase to values typically found in companion films. We describe the measurements and interpretation of DLCP for solar cells with the aid of a solar cell simulation. INTRODUCTION Knowledge of the defects and their distribution in space and energy is a fundamental ingredient in hydrogenated amorphous-silicon (a-Si:H) solar cell understanding and modeling. This information is often lacking, as the defect density is obtained from measurements on homogeneous films. In a device, the structure is far from homogeneous. Theoretical arguments using defect thermodynamics predict that charged dangling bonds are distributed from the interfaces with the doped layers into the bulk of a solar cell[1, 2]. Preliminary measurement [3] of the defect density profile in a superstrate a-Si:H solar cell using the drive-level capacitance profiling (DLCP) technique were consistent with the theoretical predictions[1, 2]. Near the p-i side of the device the defects are positively charged lying above the Fermi level and in the n-i side they are negatively charged lying below the Fermi level. A wealth of useful information has been obtained about deep defects and their changes due to light-soaking using DLCP. However, the way that the defect density changes in an actual device is still an open question. At best one can correlate defect density changes in films with efficiency changes in companion devices [4]. The good correlation between defect density change and fill factor may still be questioned as to what actually takes place in a device. What is needed is a measurement of changes in the defect density profile in an actual device. In this paper we describe DLCP measurements of defect density throughout the i layer of substrate n-i-p and superstrate p-i-n a-Si:H solar cells as function of light-soaking. To aid in interpreting the DLCP measurements we use AMPS solar cell simulations [5].
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