Molecular Electronic Coupling Controls Charge Recombination Kinetics in Organic Solar Cells of Low Bandgap Diketopyrrolopyrrole, Carbazole, and Thiophene Polymers

Low-bandgap diketopyrrolopyrrole- and carbazole-based polymer bulk-heterojunction, solar cells exhibit much faster charge carrier recombination kinetics than that encountered for less-recombining poly(3-hexylthiophene). Solar cells comprising these polymers exhibit energy losses caused by carrier recombination of approximately 100 mV, expressed as reduction in open-circuit voltage, and consequently photovoltaic conversion efficiency lowers in more than 20%. The analysis presented here unravels the origin of that energy loss by connecting the limiting mechanism governing recombination dynamics to the electronic coupling occurring at the donor polymer and acceptor fullerene interfaces. Previous approaches correlate carrier transport properties and recombination kinetics by means of Langevin-like mechanisms. However, neither carrier mobility nor polymer ionization energy helps understanding the variation of the recombination coefficient among the studied polymers In the framework of the charge transfer Marcus theory, it is proposed that recombination time scale is linked with charge transfer molecular mechanisms at,the polymer/fullerene interfaces. As expected for efficient organic solar cells, small electronic coupling existing between donor polymers and acceptor fullerene (V-if < 1 meV) and large reorganization energy (lambda approximate to 0.7 eV) are encountered. Differences in the electronic coupling among polymer/fullerene blends suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based solar, cells. Our approach reveals how to directly connect photovoltaic parameters. as open circuit voltage to molecular properties of blended materials.This work was partially supported by FP7 European Collaborative Project SUNFLOWER (FP7-ICT-2011-7, Contract No. 287594), Ministerio de Educacion y Ciencia (Spain), under Project HOPE CSD2007-00007 (Consolider-Ingenio 2010), and Generalitat Valenciana (Prometeo/2009/058 and ISIC/2012/008 Institute of Nanotechnologies for Clean Energies)