Reverse Bias Degradation in Shadowed Devices in TiO 2 Dye-Sensitized Solar Cell Modules
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Reverse Bias Degradation in Shadowed Devices in TiO2 Dye-Sensitized Solar Cell Modules Simone Mastroianni1, Thomas M. Brown1,2 , Alessandro Lanuti 1,2, Lucio Cinà1, Angelo Lembo2, Massimiliano Liberatore1, Andrea Reale1,2, Aldo Di Carlo1,2 1
CHOSE - Centre for Hybrid and Organic Solar Energy, Department of Electronic Engineering University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome (Italy) 2
Dyepower Consortium, viale Castro Pretorio 122, 00185 Rome (Italy)
ABSTRACT Mismatched or shadowed individual cells in a module can operate in the Reverse Bias (RB) regime. Subjecting a dye solar cell (DSC) to an accelerated RB stress by forcing a constant current equal to 2-fold its ISC, produced significant alterations on the current-voltage (I-V) characteristics in RB with time and a severe loss of cell efficiency in 32h. We investigated and identified a key mechanism for RB charge transfer and degradation in DSCs. I-V characteristics in RB were found to be sensitive to the type of dye utilized and to TiCl4 substrate treatment. INTRODUCTION Dye Solar Cells are a low cost photoelectrochemical technology based on a sensitized nanoporous titania layer in contact with an electrolyte 1 which has reached a degree of maturity for the manufacturing of large area modules [2,3]. Under real operating conditions an illuminated cell works in the forward bias regime. The opposite condition occurs when a cell is shadowed in a module. The reverse bias regime is precisely the condition the fully shadowed cell is subjected to. A cell that is mismatched (eg. by shadowing) is inversely polarized by the other wellperforming cells and can suffer ageing when it is stressed over time in this regime [4,5,6,7]. The upscaling from the single cell to the series connection of cells to form modules requires investigating and dealing with this phenomenon. In this work accelerated reverse bias ageing tests were performed on DSCs. We noted an instability of the cells under RB, alterations of the current-voltage characteristics, leading even to breakdown caused by gas formation [6-7]. Amongst the main effects of RB stress we found are a decrease in the diffusion limited current |Ilim| and a shift of the threshold voltage value Vth of the RB curve [6-7] which we here investigate together with the charge transfer mechanisms involved in reverse bias regime with the aim of understanding the principles underlying these. A systematic investigation was performed through current-voltage measurements to analyze the contributions of each different interface/component of the DSC under reverse bias and of the related mechanisms. We noticed a significant role played by the presence of the dye molecule and by pre-treatment on the working electrode, such as that with a TiCl4 solution. Subsequently, we address the electron transfer kinetics at the electrolyte/dye/TCO interface in complete DSCs fabricated with four different sensitizers in order to highlight the contribution of the dye molecules in more depth, especially regarding the ligand types (i.e. thiocya
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