Improve the Stability of Doped Spiro-OMeTAD

A key challenge for the practical applications of perovskite solar cells lies in the low stability of the devices, which depends not only on the perovskite materials3 but also on the charge transport layers.^[1,2] Currently, all high-performance perovskite solar cells (PSCs) with > 24% PCE are based on the bench-mark hole transport layer Spiro-OMeTAD, which is doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI) and 4-tert-butylpyridine (tBP), which harms the stability of the devices. ^[3] In addition, the complex in situ oxidation processes make it difficult to understand the mechanism of conventional spiro-OMeTAD doping and further limit the development of stable HTLs with high PCEs.
We have developed a clean and free post-oxidation doping for spiro-OMeTAD by using stable organic radicals as the dopant and ionic salts as the dopant modulator (termed ion-modulated (IM) radical doping). ^[4] The doped Spiro-OMeTAD based on our IM radical doping strategy provided a high PCE of over 25% and excellent stability (T80 for ~ 1200 h under 70±5% relative humidity and T80 for ~ 800 h under 70±3 °C without encapsulation), minimizing the trade-off between efficiency and stability of PSCs. In this doping strategy, the radicals provide hole polarons that immediately increase the conductivity and work function, and the ionic salts further modulate the work function by affecting the energetics of the hole polarons. By modifying the dopants, we can further increase the thermal stability of the Spiro-OMeTAD layer, which can survive at 80 ^oC for more than 2000 hours (Unpublished results).