Fig. 22. (a) Energy level diagram of PTB7-Th:PC71BM solar cells. The work-functions of ITO, ITO with PEI, and ITO with CQD-doped PEI were measured by Kelvin probe force microscopy. (b) A diagram of the summarized concept; CQD-doped PEI induced a stronger internal field due to the lower work-function. This strengthened internal field induced better exciton dissociation efficiency. (c) The J–V characteristics of PTB7-Th:PC71BM solar cells with pristine PEI and CQD-doped PEI with various doping ratios. (d) EQE curves of PTB7-Th:PC71BM solar cells with pristine PEI and CQD-doped PEI with various doping ratios (this figure has been adapted/reproduced from ref. 146 with permission from Elsevier, Copyright 2021).