Photovoltaic properties of low band gap copolymers based on phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units

Two novel copolymers P1 and P2 having phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units, were synthesized by heck coupling and employed as electron donor along with PCBM or modified PCBM (F) as electron acceptor for the fabrication of bulk heterojunction (BHJ) photovoltaic devices. These copolymers P1 and P2 showed broad band absorption around 640–700 nm and optical band gap of 1.60 eV and 1.72 eV, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) estimated from cyclic voltammetry measurement reveals that these values are well suitable for the use of these copolymers as electron donor along with PCBM derivatives as electron acceptor for BHJ active layer. The suitable LUMO off set allows efficient photo-induced charge transfer at the donor/acceptor interfaces in the BHJ photovoltaic device and resulting power conversion efficiency (PCE) of 2.8% and 3.29% for P1 and P2, respectively, when PCBM is used as acceptor. This value has been improved up to 3.52% and 4.36% for the devices based on P1 and P2 when F is used as electron acceptor, instead of PCBM. We have also investigated the effect of solvent annealing on the photovoltaic performance of device based on P1: F and P2: F blends and found that the over all PCE of the devices is 4.36% and 4.88%, respectively. The increase in PCE is mainly due to the improvement in the Jsc, which is due to the increased charge transport in the annealed device as compared to as cast device.

The PCE for the BHJ ploymer cells based on solvent annleaed P1:F P2:F active layer is 4.36% and 4.88%, respectively.Figure optionsDownload as PowerPoint slideHighlights
► We have used two low band gap polymers P1 and P2 along with PCBM and modified PCBM i.e. F for the fabrication of BHJ organic solar cells.
► We have achieved the power conversion efficiency of 3.52% and 4.36% for the devices based on P1 and P2 as electron donor and F as electron acceptor.
► The PCE has been further improved up to 4.88% when the cast film is solvent annealed.