Assessing the effect of surface states of mesoporous NiO films on charge transport and unveiling an unexpected light response phenomenon in tandem dye-sensitized solar cells

What is it about?

Assessing the effect of surface states of mesoporous NiO films on charge transport and unveiling an unexpected light response phenomenon in tandem dye-sensitized solar cells

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Photo by Newpowa on Unsplash

Photo by Newpowa on Unsplash

Why is it important?

In summary, the roles of NiO surface states in tandem dye-sensitized solar cells (t-DSSCs) have been assessed. The NiO surface states affect the charge transport processes in the tandem DSSCs and also participate in the IPCE response from both dyes on different photoelectrodes. By adjusting the photocurrent density of the TiO2 photoanode in combination with the information from our previous work on NiO surface states, we can conclude that there are two pathways for electron transport through the NiO photocathode: (I) the electrons generated from the photoanode are compensated at the photocathode by the injected holes from excited dyes into NiO and (II) electrons generated at the photoanode go through the NiO film and directly reduce the redox couple (I−/I3−), particularly when comparatively more electrons are generated at the photoanode than holes are being generated at the photocathode. Having a matching photocurrent from both photoelectrodes, meaning that an equal number of electrons and holes are generated at their respective electrodes, the sole presence of process I can render a high fill factor for t-DSSCs. If process II is involved to a large degree, the process will lower the FF factor of t-DSSCs due to unwanted resistance caused by the electron transport in or on the NiO film. Furthermore, due to the catalytic behavior of NiO surface states towards the electrolyte and the confirmed energy transfer between desorbed P1 and VG1-C8 dye on TiO2, one can therefore explain the observed unexpected IPCE response from both dyes in a tandem device, despite their complementary optical absorption with little spectral overlap. This study provides new understandings to explain the strange IPCE phenomenon which has been existing in the research community for a long time.