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Thomas MacDonald1 Munkhbayar Batmunkh2 Chieh-Ting Lin3 Jinhyun Kim3 James Durrant3 Joe Shapter2 Ivan Parkin1

1, University College London, London, , United Kingdom
2, Flinders University, Bedford Park, South Australia, Australia
3, Imperial College London, London, , United Kingdom

Titanium dioxide (TiO2) metal oxides are currently the most efficient scaffolds for electron transport layers in perovskite solar cells (PCSs). The efficiencies of TiO2 based perovskite solar cells has now exceeded 22% and they can remain stable for over 400 days with negligible hysteresis.[1] In addition, carbon nanotubes (CNTs) have played multifunctional roles in a range of PV cells because of their fascinating properties.[2] In particular, their ability to reduce hysteresis, and improve the stability of TiO2-based PSCs. Our previous work developed a TiO2 nanofiber-based photoelectrode and its performance in a PSC was optimized by tuning the type and amount of CNTs loaded into the mesoporous TiO2.[3] Among the different types of CNTs incorporated into TiO2, single- walled CNTs (SWCNTs) proved to be superior to other types and this was attributed to their highly conductive properties, and higher defect tolerance. Herein, we build on our previous work by studying the origin of performance enhancement in such systems. In doing so, electron injection, and the acquisition of single photons using Transient Absorption Stereomicroscopy (TAS) and Time-Correlated Single Photon Counting (TCSPC) has been investigated. Our studies show that highly conductive SWCNTs incorporated in TiO2 photoelectrodes provided a fast electron transfer within the photoelectrode, and decrease the number of trap states compared to control TiO2. On the basis of our theoretical calculations, the improved open-circuit voltage (Voc) of the cells can be attributed to a shift in energy level of the photoelectrodes after the introduction of SWCNTs. Furthermore, our highest performing photoelectrodes have generated a power conversion efficiency of over 19%, which is amongst the highest ever achieved for methyl-ammonium lead iodide perovskites.

References
[1] W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, et al., Science 2017, 356, 1376–1379.
[2] T. J. Macdonald, D. D. Tune, M. R. Dewi, C. T. Gibson, J. G. Shapter, T. Nann, ChemSusChem 2015, 8, 3396–3400.
[3] M. Batmunkh, T. J. Macdonald, C. J. Shearer, M. Bat Erdene, Y. Wang, M. J. Biggs, I. P. Parkin, T. Nann, J. G. Shapter, Adv. Sci. 2017, 4, DOI 10.1002/advs.201600504.

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