Improving n-i-p Perovskite Solar Cells Stability through Transport Layers
Suer Zhou a, Maryte Daskeviciene b, Matas Steponaitis b, Giedre Bubniene b, Vygintas Jankauskas c, Kelly Schutt d, Philippe Holzhey a, Ashley Marshall a, Pietro Caprioglio a, Grey Christoforo a, James Ball a, Tadas Malinauskas b, Vytautas Getautis b, Henry Snaith a
a University of Oxford, Department of Physics, Clarendon Laboratory, UK, Parks Road, United Kingdom
b Department of Organic Chemistry, Kaunas University of Technology, Kaunas 50254, Lithuania.
c Institute of Chemical Physics, Vilnius University, Vilnius 10257, Lithuania.
d US National Renewable Energy Laboratory (NREL)
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Contributed talk, Suer Zhou, presentation 066
Publication date: 20th April 2022

Since solid-state perovskite solar cells were developed from dye-sensitized solar cells, the commonly used hole-transporting layer (HTM) and electron-transporting layer (ETM) have hardly changed. Despite their high efficiency for n-type-intrinsic-p-type (n-i-p) solar cells, traditional materials impose problems, such as costly synthesis (spiro-OMeTAD), long processing time (SnO2/TiO2), and worse stability compared to inverted p-i-n devices with all-organic transporting layers.

To address these issues, we investigated alternative electron/hole-transporting materials. Working with Kaunas University of Technology (KTU), we have developed a pair of carbazole-based enamine HTMs, MeO5PECz and MeO4PEBCz, which can be synthesized at a fraction of the cost of spiro-OMeTAD. Devices made with these new HTMs have similar performance to that of spiro-OMeTAD even without dopants, such as Li-TFSI and tBP. Furthermore, devices with these HTMs exhibited enhanced stability under 85 °C dark and light aging compared to spiro-OMeTAD devices. Additionally, devices made with these new HTMs show higher quasi-Fermi level splitting (QFLS) compared to doped spiro-OMeTAD, which may help to push the device open-circuit voltage (Voc) closer to the Shockley-Queisser limit.

Having improved the stability of the p-type layer, we investigated improving the Voc and the stability of the n-type contact. The major difference between n-i-p and p-i-n devices is that there is a fullerene transport layer in the p-i-n stack compared to the metal-organic transport layer in the n-i-p stack. Additionally, there are reports of weak n-i-p stability due to the delamination of the perovskite layer from the SnO2 layer.  This suggests that exchanging the SnO2 or TiO2 layer with a functionalized fullerene self-assembling monolayer (SAM) might improve the stability of the n-i-p stack, to a level closer to that of the p-i-n stack.  In collaboration with Georgia Tech, we mainly studied a fullerene-based SAM, C60-POOH-SAM, comparing it to the commercially available C60-COOH-SAM. The C60-POOH-SAM not only increased the stabilized current density (Jsc), but also increased the Voc of devices. We hypothesize that the C60-POOH-SAM simultaneously passivated the interfacial defects between the metal oxide and the perovskite and facilitating electron transport on the n-side.

In this presentation, I will give a summary of the applications of new dopant-free carbazole enamine-based HTMs, and n-type fullerene SAMs, in perovskite devices. Both materials can potentially improve the long-term stability of the n-i-p solar cell as well as lower its fabrication cost.

The research leading to these results had received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 763977 of the PerTPV project and the Marie Skłodowska-Curie grant agreement no. 764787 of the MAESTRO project. V.G., M.S., and T.M. acknowledge funding from the Research Council of Lithuania under grant agreement Nr. 01.2.2-LMT-K-718-03-0040 (SMARTMOLECULES) and thank Dr. E. Kamarauskas for ionization potential measurements. The research from this author has received funding from the Rank prize funding. This work also received funding from the EPSRC UK under project EP/S004947/1. The authors thank Nathan Chang for his helpful input and Nobuya Sakai and Manuel Kober-Czerny for suggestions on analyzing photoluminescence data.

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