(Invited) Polymer Wrapped Carbon Nanotubes As Highly Effective Hole Transporting Layers for New Perovskite and Quantum Dot Photovoltaic Devices

Abstract

Our recent work (1) has shown that polymer wrapped carbon nanotubes can be used to transform the properties of the hole transporting layer in a number of different perovskite based solar cell configurations. This presentation will report on two examples where the CNT hole transporter produces enhanced device performance, firstly for tin oxide/perovskite alloy/CNT devices and secondly in PbS colloidal quantum dots (CQD) where improved charge extraction by the CNT is particularly important. Halide perovskite solar cells remain to generate a lot of interest and enthusiasm among researchers, largely due to the prospect of readily transitioning from a purely lab based technology to real-world scales. This expectation is largely based on the rapid increase in efficiency over the past few years to values above 22%. An essential component of this device architecture are the two charge selective contacts. The n-type contact in the majority of all devices is TiO2, either as a thin compact layer, or a mesostructured configuration, but recently, SnO2 has moved into the spotlight as n-type contact with recent studies reporting excellent steady-state performances. The p-type contact on the other side of the absorber has been investigated even more intensively. The best-performing devices still use the “original” hole-transport material, spiro-OMeTAD, which provided the crucial breakthrough for the perovskite solar cell in 2012, but which suffers from the fact that it is not sufficiently conductive for efficient charge-transport, and requires extrinsic doping, typically with Li-TFSI, which has been shown to detrimentally impact the device stability. This presentation will demonstrate that for FA0.83MA0.117Pb(I0.87Br0.17)3 based solar cells we can achieve steady-state efficiencies of up to 18.8%, by using polymer-wrapped single-walled carbon nanotubes (SWNTs) as an inert conductive element in undoped spiro-OMeTAD, exceeding the performance of their fully doped counterparts. In a completely different example we can use the same polymer wrapped SWNTs as a hole collection electrode for PbS based CQD photodetectors which are capable of operating over a spectral range extending beyond one micron. We find that the use of a polymer wrapped CQD combined with PMMA filler more than doubles the current collection in such devices and enables efficiencies of over 7%. In addition the SWNT layer produces a substantial improvement in long term device stability in normal ambient conditions. (1) Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J. Nano Lett. 2014, 14, 5561–5568.