Improving the stability of amorphous silicon solar cells by chemical annealing
Wang, Nanlin (2005) Improving the stability of amorphous silicon solar cells by chemical annealing. PhD thesis, Iowa State University.
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Amorphous silicon solar cells have been extensively used for many years due to the low cost, easy fabrication and special properties. However, it is found that the properties of a-Si:H solar cells degrade upon light illumination, and this limits the application of a-Si:H solar cells and brings the stability concern. Recently, it has been shown that this instability is correlated with the presence of multiple bonded Si-H bonds (i.e., SiH2), and a technique, namely chemical annealing, was suggested to improve the stability of a-Si:H. Although a number of results have been reported to improve the stability of a-Si:H films based on chemical annealing technique, no chemical annealed devices with good quality were reported. In this work, chemical annealed a-Si:H films and devices, and non chemical annealed films and devices were produced in a remote, low pressure ECR plasma enhanced CVD, and systematic experiments were carried out to study the role of chemical annealing in enhancing the stability of a-Si:H solar cells. It is found that the structure of the films grown using chemical annealing technique depended critically upon whether the annealing was done with hydrogen or plasma. When the annealing was done in hydrogen plasma, the films remained amorphous; in contrast, when in helium plasma, and the annealing time is equal or more than 20 seconds, the films became crystalline. These unusual results show that it is not necessary to have a high hydrogen dilution to obtain nanocrystalline films, and contradict the generally accepted assumption that high hydrogen dilutions are needed to crystallize the amorphous films based on the reactive etching of H ions. Instead, it might be the case that not only the reactive etching effect from the H ions, but also the ion bombardment play a role in crystallizing the amorphous films. Comparable helium plasma annealed a-Si films and non helium plasma annealed a-Si films were produced. Upon light soaking, the chemical annealed films show a much lower degradation in photoconductivity than that of non chemical annealed films. Comparable helium plasma annealed a-Si solar cells and non helium plasma annealed a-Si solar cells were prepared as well. Upon light soaking, the degradation in the most sensitive device factor, the fill factor, showed that chemical annealed devices possess a much better stability than non chemical annealed devices. FTIR data of chemical annealed films provide a much less SiH2 bond content than that of non chemical annealed films. Clearly, it indicates that chemical annealing technique leads to significant improvement in the stability of both films and devices in a-Si:H by reducing the SiH2 bond density inside. Similar results happened in hydrogen plasma annealed films and devices. Comparable hydrogen plasma annealed films and non annealed films; comparable chemical annealed devices and non chemical annealed devices could be produced as well. Hydrogen plasma annealed films performed lower degradation in photoconductivity than non annealed films. Chemical annealed devices showed a significantly less degradation in fill factor than non annealed devices. FTIR data demonstrated that the stability of a-Si:H was closely related to the SiH2 bond content and the chemical annealing technique did reduce the SiH2 bond intensity
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