Single-walled carbon nanotubes as charge extraction layer for perovskite solar cells

This thesis describes the development of a charge-extraction layer for perovskite solar cells based on a network of polymer-functionalized single-walled carbon nanotubes (SWNTs). The starting point is the addition of SWNTs as additives to improve the charge trans- port characteristics of a solid-state hole-transporting material - 2,2,7,7-tetrakis-(N,N-di-p- methoxyphenylamine)9,9-spirobifluorene (spiro-OMeTAD). In a double-layer structure this is shown to perform comparably well to devices in which the charge transport of spiro-OMeTAD is enhanced by conventional doping. By means of photo-induced absorption spectroscopy, it could be shown that a significant fraction of photogenerated charges are transferred to the SWNTs. This finding demonstrates that the nanotubes act as charge selective contacts in their own right without requiring the presence of dedicated hole transporter material. In- stead, they can be embedded in a matrix of an inert polymer, which can be chosen according to characteristics related to stability rather than their electronic properties. Such an SWNT- PMMA composite structure is shown to outperform conventional hole-transporter systems during and after thermal stressing cycles. Finally, the issue of hysteresis is addressed. The additive 4-tert-butylpyridine (tBP) is shown to significantly improve the steady-state performance of devices with the SWNT-PMMA composite structure. In order to understand the mechanism and underlying interactions of the additive, the possible interaction interfaces are investigated one by one, leading to the conclusion that a direct interaction between tBP and the perovskite absorber must be responsible for the observed effect. The work in this thesis opens the avenue for SWNT-based hole transporter systems which provide a chemically stable and mechanically resilient charge-selective contact, and show particular promise for improving the overall stability of perovskite devices.