Isatin-derived non-fullerene acceptors for efficient organic solar cells

Abstract

Geometries optimization, electronic structure, electronic absorption spectra, charge transport rate, morphology, and photovoltaic properties of indacenodithiophene as donor core unit tie with isatin derive different end-capped electron acceptor groups have been performed using Density Functional Theory (DFT) and time dependent density functional theory (TD-DFT). Isatin unit connected with 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile in M1, 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile in M2, 2-thioxothiazolidin-4-one in M3, 2-(4-fluorothiophen-2-yl)acetonitrile in M4 and geometries, opto-electronic properties have been computed at TD-DFT MPW1PW91/6-31G level of theory and compared with reference molecule R. Among all novel structures, M2 has lowest value of energy band gap (1.99 eV) and has maximum absorption (λmax = 824.0 nm) in chloroform by TD-MPW1PW91/6-31G (d,p) using IEFPCM model. The red shift in M2 was due presence of strong electron withdrawing acceptor moiety and more extended conjugation as compared to other structures. Indeed, electron withdrawing functionality i.e fluorine and cyno moieties in M2 structure stabilized its LUMO orbital and hence improved its opto-electronic properties. Transition density matrix (TDM) maps results revealed obvious influence of end-capped acceptor group on the exciton. M2 has lower probability of coupling of electron and hole owing easy exciton dissociation as compared to other designed molecules. Theoretically calculated reorganization energy indicates that M1 has the highest hole transfer rate due to the lowest value of λh (0.0068 eV). Similarly, M4 has the highest electron mobility due to lowest value of λe (0.0053 eV). In short, choice of appropriate electron withdrawing end capped acceptor groups is very important for improving the performance of organic solar cells.