Abstract
Photochromic dye-sensitized solar cells (DSSCs) are novel semi-transparent photovoltaic devices that self-adjust their optical properties to the irradiation conditions, a feature that makes them especially suitable for building integrated photovoltaics. These novel solar cells have already achieved efficiencies above 4%, and there are multiple pathways to improve the performance. In this work, we conduct a full characterization of DSSCs with the photochromic dye NPI, combining electrical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS). We argue that the inherent properties of the photochromic dye, which result in a modification of the functioning of the solar cell by the optical excitation that also acts as a probe, pose unique challenges to the interpretation of the results using conventional models. Absorption of light in the visible range significantly increases when the NPI dye is in the activated state; however, the recombination rate also increases, thus limiting the efficiency. We identify and quantify the mechanism of enhanced recombination when the photochromic dye is activated using a combination of EIS and IMPS. From the comparison to a state-of-the-art reference dye (RK1), we were able to detect a new feature in the IMPS spectrum that is associated with the optical activation of the photochromic dye, providing a useful tool for assessing the electronic behavior of the device under different conditions of light excitation. This study provides guidelines to adequate characterization protocols of photochromic solar cells and essential insights on the interfacial electronic processes.
Highlights
Dye-sensitized solar cells (DSSCs) are an emerging photovoltaic technology that has initiated its industrial development.[1]
We present a clear and simple procedure to measure and analyze photochromic dye solar cells by optoelectronic smallsignal perturbation techniques, distinguishing between the activated and deactivated states of the dye molecule
We have shown that the general behavior of the photochromic NPI dye in an electrical impedance spectroscopy (EIS) study is fundamentally similar to any other dye previously studied
Summary
Dye-sensitized solar cells (DSSCs) are an emerging photovoltaic technology that has initiated its industrial development.[1]. The opposite process is much slower, as it takes several hours before the photochromic dye reaches the fully deactivated state.[13] To tackle the challenge of developing new photochromic materials with higher efficiencies and faster selfadjustable optical properties, a better understanding of the interfacial processes, as well as the recombination and transport properties of the devices is needed In this regard, a thorough fundamental study and the development of a reliable measurement procedure that takes into account the changing behavior of the dye under illumination are required. On the other hand, combined with EIS, intensity-modulated photocurrent spectroscopy (IMPS),[22−33] where a small light intensity modulation is superimposed on a constant illumination intensity, is a powerful tool to study and separate transport and recombination processes. We intend to cast light on the origin of the decrease of the photovoltage observed in DSSCs prepared with the NPI photochromic dye when the cell is optically activated
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