Revealing shunt origins by temprerature dependence of elctroluminescence under reverse-bias

The electroluminescence (EL) imaging has been investigated extensively to get spatially resolved information of electronic performance of solar cells. The EL under forward-bias represents the total excess minority carrier density in the cells, and all the causes reducing the carrier numbers (intrinsic defects and extrinsic deficiencies) can be detected as dark parts. In contrast, the EL under reverse bias is induced by high electric field mainly located in the depletion layer. The electrically active defects can be detected as hot spots, which are tightly related with harmful current shunt paths. In this report, temperature dependence of EL under reverse-bias was investigated in detail, and various shunt paths with different origins can be revealed with the combined analyses by lock-in Thermography (LIT) under forward-bias and reverse-bias. In addition, we analyzed some depth positions and impurities of defects by using Electron Beam Induced Current (EBIC), Scanning Electron Microscope (SEM) and Electron Probe Micro Analyzer (EPMA). In these results, luminescence under reverse-bias showed that both increasing and decreasing phenomena on crystalline defects. The decreasing region might be caused like `Avalanche breakdown', thus electron mobility should be reduced by a lattice vibration increase. On the other hand, increasing region could be caused that an increase of the tunnel current due to narrowing in the width of a forbidden band in pn junction under reverse-biased basically high electric field and high temperature namely might be caused like `Zener breakdown'. Hot spots were detected an inactive defects in 2 μm, grainboundary and distorted texture with EBIC, and were found some impurities by EPMA. Moreover, we proposed two defect models by combining information of EL image and LIT image under forward-bias and reverse-bias, as new distribution method of defects.