Solar cells based on organic semiconductors and metal halide perovskites

Prof. Feng Gao
Lecture of Institute guest
7.10.2022 10:00, Lecture room A

Solar cells based on organic semiconductors and metal halide perovskites have attracted significant attention in the past decade. For organic solar cells (OSCs), recent advances based on non-fullerene acceptors (NFAs) come along with reduced non-radiative voltage losses. We show that the non-radiative voltage losses in these state-of-the-art donor:NFA OSCs show no correlation with the energies of charge-transfer electronic states at donor:acceptor interfaces, different from conventional fullerene-based OSCs. We have been able to rationalize the low voltage losses in these devices, where we highlight the critical role of the thermal population of local exciton states in decreasing the non-radiative losses. An important finding is that the molecular photoluminescence properties of the pristine materials define the limit of non-radiative voltage losses in OSCs, indicating that it is critical to design high-luminescence-efficiency donor and acceptor materials with complementary optical absorption bands extending into the near-infrared region. For perovskite solar cells (PSCs), spiro-OMeTAD has been frequently employed as the hole transport layer to achieve high open-circuit voltage (and hence high efficiencies). Spiro-OMeTAD is conventionally doped by hygroscopic lithium salts with the assistance of volatile 4-tert-butylpyridine, which, however, brings a time-consuming doping process as well as poor device stability. We successfully develop an instantly efficient and clean doping strategy to replace the conventional spiro-OMeTAD doping. As demonstrated by experimental and theoretical investigations, the radical dopant leads to significant increase of the conductivity through efficient hole polarons generation. The ionic salts can further modulate the work function with negligible effects on the film conductivity, critical for reaching optimal open-circuit voltage values by a favorable energetic level alignment. Spiro-OMeTAD based on our new doping strategy enables PSCs with a high power conversion efficiency over 25% and an excellent stability against moisture, heat and illumination. Our findings pave the way for achieving PSCs with high efficiencies and excellent stability at the same time.

The lecture is presented in English