Dopant-free small-molecule hole-transport material for low-cost and stable perovskite solar cells

Dopant-free hole-transporting materials (HTMs) aim to improve efficiency and stability simultaneously, and are a promising direction for efficient perovskite solar cells (PSCs). To achieve dopant-free and low-cost HTMs, small organic molecules based on imidazole phenanthrene derivatives were easily prepared in a one-pot cycling reaction without the use of a catalyst or harsh conditions. The total cost of 1 g of this new HTM is about $5, which is 1/20th of the cost of the benchmark HTM spiro-OMeTAD (>$100). In particular, the power conversion efficiency (PCE) of PSCs based on the new HTM without using LiTFSI/4-tert-butylpyridine dopants is about 9.11%, which is higher than for PSCs based on spiro-OMeTAD, for which the PCE is 6.21% under the same conditions. Moreover, the light stability of PSCs based on the new additive-free HTM indicated good behaviour over 500 hours, resulting in only 10% loss of initial efficiency. The doped PSC also shows 14% efficiency, maintaining more than 80% of its initial efficiency after 500 hours of light exposure. The exclusive photovoltaic properties of the new HTM can be attributed to its high conductivity and hole mobility, which are related to the small size of the molecules compared to common HTMs known so far. These new HTMs represent a breakthrough in the engineering of additive-free PSCs based on organic HTMs, which will open up new avenues to achieve low-cost and high-stability PSCs.