Influence of Exciton Diffusion and Charge-Transfer State Dissociation Efficiency on the Short-Circuit Current Densities in Semi-Random Donor/Acceptor Polymer:Fullerene Solar Cells

Four bulk heterojunction (BHJ) polymer:fullerene solar cell systems containing semi-random poly(3-hexylthiophene) (P3HT) based donor/acceptor (D/A) copolymers as donors and phenyl-C-61-butyric acid methyl ester (PC61BM) as an acceptor were studied. Even though all three semi-random D/A polymers have broad absorption profiles and high absorption coefficients, benzothiadiazole (BTD) or thienopyrazine (TP) containing polymers P3HTT-BTD and P3HTT-TP have significantly lower short-circuit current densities (Jsc) than P3HT and P3HTT-DPP-10% (containing 10% diketopyrrolopyrrole (DPP)) in BHJ solar cells. As a result, the power conversion efficiencies (PCE) for P3HTT-BTD and P3HTT-TP are below 1% compared to 2.7% for P3HT:PC61BM and 3.1% for P3HTT-DPP-10%:PC61BM (efficiencies are not mismatch corrected and measured after encapsulation and shipment). Complementary optical and numerical simulations, experimental techniques such as external quantum efficiency (EQE) measurements, and transient photoconductivity studies were used to study the polymer:fullerene blends. The efficiency of each step involved in photocurrent generation was calculated, and it was determined that low exciton diffusion efficiency together with geminate and trap-assisted recombinations in deep trapping sites are responsible for the decrease of Jsc in P3HTT-BTD:PC61BM and P3HTT-TP:PC61BM solar cells, whereas they are negligible in the case of P3HT:PC61BM and P3HTT-DPP-10%:PC61BM. The observed drastic influence of relatively small changes in the chemical structure of the studied polymers (only 10% of the total monomer content changes between polymers) on the exciton diffusion efficiency, charge-transfer (CT) state dissociation efficiency, and free polaron collection efficiency emphasizes the importance of a judicious choice of the acceptor monomer in semi-random D/A polymers.