Development of procedures for performance measurements and lifetime testing of thin film photovoltaic devices
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Doctoral thesis (monograph)
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Helsinki University of Technology publications in engineering physics. A, 811
AbstractIn this work single junction a-Si, CdTe and CIGS modules and prototype modules and mini-modules were investigated to reliably be able to characterize these technologies for electrical performance and to find out potential failure mechanisms in accelerated lifetime testing. Thin film modules may increase their share of the market in the future. A-Si modules have been manufactured already for several years and CdTe and CIGS modules are shortly entering the market. However, there is a lack of characterization and testing methods to reliably predict the power output and service lifetime of these technologies. In this work physics, material and manufacturing issues related to a-Si, CdTe and CIS thin film technologies were reviewed to understand possible failure mechanisms in reliability testing. The calibration measurement procedures and in particular the choice of the reference device for the thin film photovoltaic materials was investigated. The results enable correctly to define if the modules have passed or failed qualification tests and to predict the power output of the module during its lifetime. A-Si, CdTe and CIGS devices were exposed to accelerated lifetime testing. The potential failure mechanisms were investigated by testing them in thermal cycling and humidity freeze cycles, in damp heat, in dry heat and in light soaking. Laser scanning was used to further investigate the observed failure mechanisms after accelerated lifetime testing. The method enables non-destructive determination of position dependent photocurrent throughout the module. It is suggested to use a reference device with a spectral response similar to that of the sample material to reliably measure the electrical characteristics of a-Si, CdTe and CIGS devices. It is demonstrated that the major cause for failure in accelerated lifetime tests of a-Si, CdTe and CIGS devices is humidity penetration into the module encapsulation. However, the a-Si modules were not very sensitive to humidity penetration. For CdTe prototype modules even small amounts of humidity penetration caused changes in the back contact composition, resulting in electrical degradation. The CIGS prototype mini-modules were not as sensitive to humidity penetration as the CdTe devices were. In case of light soaking the a-Si photovoltaic modules degraded as expected and the degradation stabilized towards the end of the light exposure. In the CdTe, a decrease in maximum power was observed. In the CIGS samples, an increase in maximum power and fill factor was observed which, may however be dependent on material quality. In the laser scanning it is concluded that the method can be used as a powerful non-destructive tool to investigate the short circuit current performance of thin film photovoltaic modules. It is demonstrated that laser scanning can be used to correlate the causes of short circuit current degradation after accelerated lifetime testing with the visual defects observed. It was shown in this work that the current standards for measurement and testing photovoltaic devices may not necessarily be applied as such, especially for CdTe and CIGS thin film technologies. The results also demonstrate that it is important to design proper and reliable encapsulation to increase the service lifetime of the thin film modules.
electrical characterization, reliability testing, reference device, damp heat testing, light soaking, laser scanning