Increasing Quantum Efficiency of Polymer Solar Cells with Efficient Exciton Splitting and Long Carrier Lifetime by Molecular Doping at Heterojunctions

Abstract

Optimizing the photovoltaic processes directly by electric technique attracts the exploration of molecular doping in organic photovoltaics (OPVs). However, the inappropriate and inhomogeneous dopant distribution in the bulk heterojunction (BHJ) film inhibits the performance improvement and mechanism understanding in doped OPVs. A strategy to solve these critical problems is reported here. By employing a planar heterojunction (PHJ) device structure, the role of dopant location and photovoltaic performance is clarified. Dopants tris(pentafluorophenyl)borane (BCF) distributing at the heterojunction rather than in the single component (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) or 3,9-bis(2-methylene(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC)) produces the best performance with quantum efficiency (QE) enhancement. Mechanism studies reveal the facilitated exciton splitting and suppressed bimolecular recombination as the causes for device improvement. The sequential coating procedure is further developed for the doped BHJ cells, and the devices based on this method exhibit a short-circuit current (Jsc) increase of 1.3 mA/cm2, which is an impressive value compared with the previous reports.