One-step coating inverted polymer solar cells using a conjugated polymer as an electron extraction additive

We report a facile technique of blending a conjugated polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PCBM[70]) active materials with a conjugated interfacial modification polymer poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) to simplify the coating process and improve the bulk heterojunction (BHJ) polymer solar cell (PSC) performance. The reason for and result of PFN self-organization via a spontaneous vertical delamination onto the ITO surface were investigated by charge transfer state, optical modelling based on transfer matrix formalism, surface energy measurement, scanning Kelvin probe force microscopy and impedance spectroscopy analysis in conjunction with atomic force microscopy and scanning electron microscopy. The relaxed charge transfer state demonstrates that PFN doping has a negligible impact on the donor: acceptor heterojunction interface. The optical simulation of device structures indicates that doping PFN into a BHJ has nearly no influence on the photon absorption profile of the active layer. Very encouraging device performance was achieved in the one-step coating PFN: BHJ PSC with ITO as the cathode, which is comparable to that of the two-step coating PSC. Moreover, for ITO-free inverted PSCs with PEDOT:PSS as the incident light top-electrode, decent device performance can also be obtained, demonstrating the remarkable universality through this facile strategy.