Study and optimization of the growth of a-Si:H on wet-chemically textured c-Si substrates for the enhancement of a-Si:H/c-Si heterojunction solar cells

Abstract : This thesis is the result of the work carried out on the passivation of anisotropically wet-etched crystalline silicon by hydrogenated amorphous silicon deposited by low-temperature RF PECVD. Firstly, the (100) and (111) crystalline orientations were studied to understand the impact thereof on a-Si:H growth. Wet-chemical etching of crystalline silicon was then addressed in order to choose the best recipe to texture silicon wafers and thereby increase their light-trapping properties. Surprisingly, the deposition of a-Si:H on textured wafers was tested and passivation under our standard conditions was not evident at all. Indeed, we experienced huge losses in effective lifetime and implied Voc. We could at first explain this difference by the larger surface and thus larger number of dangling bonds exhibited by textured wafers. However, we imagined that randomly distributed pyramid landscapes could be imperfectly defined in their natural {111} crystallographic orientation and that they rather contained a particular set of specific surface orientations. Such being the case, this would in turn foster particular growth modes locally. Consequently, we decided to use a method that is counter-intuitive for textured c-Si wafers in order to obtain a perfectly abrupt a- Si:H/c-Si interface. The improvements were striking and showed great reproducibility while HR-TEM/STEM analyses clearly indicated that the interface was made more abrupt indeed. This allowed us to procede towards our main goal: improving a-Si:H/c- Si heterojunction (SHJ) solar cell efficiencies on textured c-Si n-type wafers. A proper cleaning routine was defined, the cleanliness of the whole fabrication chain processes (no clean-room conditions) was optimized and adequate layer stacks were developed to enhance SHJ solar cells. As a consequence, a 2x2 cm2 (n)c-Si HJ solar cell was fabricated with a conversion efficiency as high as 20.1% with a Voc of 701 mV, a Jsc of 37.5 mA/cm2 and a FF of 76.3%. This cell contains the two major improvements we have introduced in the conventional HJ solar cell architecture. Implementation of these improvements required a study of their impact on the SHJ solar cell resilience to TCO deposition and on their final J(V) characteristics. A new semi-industrial PECVD cluster tool was installed and new PECVD processes were defined to assess its potential for passivation studies and to check its reproducibility features. As a result, we obtained a 11.3 ms effective lifetime with a 734 mV implied Voc on a symmetrical i/i stack.
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Submitted on : Tuesday, February 24, 2015 - 4:47:37 PM
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Igor Paul Sobkowicz. Study and optimization of the growth of a-Si:H on wet-chemically textured c-Si substrates for the enhancement of a-Si:H/c-Si heterojunction solar cells. Physics [physics]. Ecole Doctorale Polytechnique, 2014. English. ⟨tel-01120054⟩

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