Influence of imine symmetry and triphenylamine moieties on photovoltaic properties of BHJ organic solar cells

In this paper the influence of imine symmetry and triphenylamine moieties presence on photovoltaic properties of bulk heterojunction organic solar cells were analysed in details. Three imines: (E)-N-(4-((4-((E)-(benzo[d]thiazol-2-ylimino)methyl)phenyl)(phenyl)amino)benzylidene)benzo[d]thiazol-2-imine (PV-BLJ-SC1), (E)-N-(4-diphenylamino)benzylidene)benzo[d]thiazol-2-imine (PV-BLJ-SC2) and (N,N'E,N,N'E)-N,N'-([2,2′:5′,2′'-terthiophene]-5,5′'-diylbis(methanylylidene))bis(benzo[d]thiazol-2-imine) (PV-BLJ-SC3) were investigated as donor or acceptor components in binary and ternary active layer. Moreover, the influence of admixture (1,8-diiodooctane or diphenyl ether), and presence of poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] as cathode buffer layer on the photovoltaic parameters of the constructed imine-containing solar cells was analysed in details. All the conducted experiments confirmed the possibility of using unsymmetrical imine PV-BLJ-SC2, showing the best results, in organic photovoltaics. The HOMO and LUMO levels of the imines were determined by cyclic voltammetry, showing dependance of on the chemical structure and its symmetry, and for PV-BLJ-SC2 gaved −5.07 eV and −2.23 eV, respectively. In the case of PV-BLJ-SC1 and PV-BLJ-SC2 the electron density of HOMO was mainly located on triphenylamine moieties, while in the case of PV-BLJ-SC3 on thiophene moieties, as it was confirmed by the density functional theory studies. Based on the highest conductivity (9 × 10-8 S/cm2) result for PV-BLJ-SC2, this imine was selected to prepare binary and ternary layers with PTB7 or PTB7:PC71BM and thermal imaging studies were performed. Finally, PV-BLJ-SC2 was introduced to the PTB7:PC71BM binary system, where a power conversion efficiency as high as 4.17% was achieved after the addition of 1,8-diiodooctane. It gaved better result than the reference sample containing PTB7:PC71BM and 1,8-diiodooctane (3.11%). In the case of reference solar cells the addition of 1,8-diiodooctane improved the efficiency of about 22 %, while for solar cells containing also PV-BLJ-SC2 imine the efficiency increased by more than 107 %. Our study presents a quite effective and cheap method to improve the efficiency of organic solar cells by adopting a ternary approach.