Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan–French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

An efficient metal-free formulation of a hole transport material (HTM) based on an ionic liquid polymer is developed for n-i-p perovskite solar cells (PSCs), to address reproducibility issues related to the use of complex dopant mixtures based on lithium salts and cobalt coordination complexes. The conductivity of the HTM is thus significantly improved by 4 orders of magnitude, up to 1.9 x 10 -3 S.cm -1 , using poly(1-butyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide) (PVBI-TFSI) as dopant.Introduced in the FTO/c-TiO 2 /mp-TiO2/K0.05(MA0.15FA0.85)0.95PbI2.55Br0.45/HTM/Au PSC configuration, PVBI-TFSI-HTM formulation shows power conversion efficiency as high as 20.3 %, versus 18.4 % for the standard lithium salt-HTM formulation, with considerably reduced hysteresis and excellent reproducibility. Mechanistic investigations suggest that PVBI-TFSI acts as a source of protons promoting the HTM oxidation.