Efficient non-fullerene polymer solar cells enabled by side-chain conjugated thieno[3,4-c]pyrrole-4,6-dione-based polymer and small molecular acceptors

Abstract

The application of non-fullerene (NF) acceptors in bulk-heterojunction (BHJ) polymer solar cells (PSCs) is a promising approach to overcome the inherent drawbacks of fullerene derivatives-based acceptors. In PSCs, complementary absorption as well as matched molecular energy levels between the low bandgap acceptor-donor-acceptor (A-D-A) small molecular acceptor and medium/wide bandgap polymer donor is crucial to achieve high power conversion efficiency (PCE). Alternating polymers based on benzodithiophene (BDT) electron-donating segment and thieno[3,4-c]pyrrole-4,6-dione (TPD) electron-withdrawing segment own medium bandgap and low-lying highest occupied molecular orbital (HOMO) energy level, leading to presentable photovoltaic properties with fullerene derivatives. To probe into the performances of TPD-based polymers in NF-PSCs, two TPD-based polymers containing alkoxy or alkylthienyl modified benzo[1,2-b:4,5-b′]dithiophene (BDT) were synthesized and adopted as electron-donors and blended with A-D-A-type electron-acceptor 2,2′-[[6,6,12,12-tetrakis(4- hexylphenyl)-s-indacenodithieno[3,2-b]thiophene]methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis(propanedinitrile) (ITIC) to fabricate the corresponding photovoltaic devices. The two-dimensional conjugated polymer PBDTT-TPD shows enhanced extinction coefficient, deeper HOMO energy level and better hole transport performance, resulting in improved PCE of 6.17%. To further boost the performances of the polymers, a small molecular acceptor 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl) bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) with down-shifted energy level was also used to blend with the two polymers in PSCs. Despite the open-circuit voltage (VOC) of the PBDTT-TPD:IDIC-based device is slightly decreased, the short-circuit current density (JSC) and fill factor (FF) are simultaneously improved, yielding an promising PCE of 7.15%. These results indicate that two-dimensional conjugated TPD-based polymers can be potential application as medium bandgap polymeric donor to match with small molecular acceptors having suitable molecular energy levels to get high efficiency in PSCs.