Amorphous Silicon Photovoltaic Modules on Flexible Plastic Substrates
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Amorphous Silicon Photovoltaic Modules on Flexible Plastic Substrates Yuri Vygranenko1, Miguel Fernandes1,2, Paula Louro1,2, Manuela Vieira1,2, Alireza Khosropour3, Ruifeng Yang3, and Andrei Sazonov3 1 CTS-UNINOVA, 2829-516 Caparica, Portugal 2 Electronics, Telecommunications and Computer Engineering Department, ISEL, Lisbon, Portugal 3 Electrical and Computer Engineering Department, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada ABSTRACT This paper reports on a monolithic 10 cm × 10 cm area PV module integrating an array of 72 a-Si:H n-i-p cells on a 100 μm thick polyethylene-naphtalate substrate. The n-i-p stack is deposited using a PECVD system at 150 oC substrate temperature. The design optimization and device performance analysis are performed using a two-dimensional distributed circuit model of the photovoltaic cell. The circuit simulator SPICE is used to calculate current and potential distributions in a network of sub-cell circuits, and also to map Joule losses in the front TCO electrode and the metal grid. Experimental results show that the shunt leakage is one of the factors reducing the device performance. Current-voltage characteristics of individual a-Si:H p-i-n cells were analyzed to estimate a variation of shunt resistances. Using the LBIC technique, the presence of multiple shunts in the n-i-p cell was detected. To understand the nature of electrical shunts, the change in the surface roughness of all device layers was analyzed throughout fabrication process. It is found that surface defects in plastic foils, which are thermally induced during the device fabrication, form microscopic pinholes filled with highly conductive top electrode material. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) solar cells on thin plastic substrates are of great interest for a wide variety of engineering applications. Their true mechanical flexibility allows the integration with elements of various shapes and sizes enabling the innovative solar products [1]. Lightweight flexible photovoltaic (PV) modules are suitable for mobile devices and various electric appliances to cover part of their power demand from solar energy. Even an integration of photovoltaics in clothes becomes a reality [2]. The radiation hardness of a-Si:H and high powerto-weight ratio of solar cells on thin plastic substrates make them also suitable for extraterrestrial applications [3]. Regardless of the intense research efforts directed to the development of flexible a-Si:H solar cells on low-cost plastic substrates, these devices still exhibit considerably lower performance in comparison to that for glass-based equivalents. The major technological challenge is deposition of doped- and undoped a-Si:H layers with required electronic properties at temperatures lower than that for solar cells on glasses or metal foils [4]. The increased shunt leakage in a-Si:H cells on the plastic substrate is another challenging issue [5,6]. In this contribution, we report on a monolithic a-Si:H-based photovoltaic module utilizing the 100 μm thick polye
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