Compositional Dependence of the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells Based on Two Donor Polymers

Ternary blend bulk heterojunction (BHJ) solar cells containing as donor polymers two P3HT analogues, high-band-gap poly­(3-hexylthiophene-<i>co</i>-3-(2-ethylhexyl)­thiophene) (P3HT₇₅-<i>co</i>-EHT₂₅) and low-band-gap poly­(3-hexylthiophene–thiophene–diketopyrrolopyrrole) (P3HTT-DPP-10%), with phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an acceptor were studied. When the ratio of the three components was varied, the open-circuit voltage (<i>V</i>_oc) increased as the amount of P3HT₇₅-<i>co</i>-EHT₂₅ increased. The dependence of <i>V</i>_oc on the polymer composition for the ternary blend regime was linear when the overall polymer:fullerene ratio was optimized for each polymer:polymer ratio. Also, the short-circuit current densities (<i>J</i>_sc) for the ternary blends were bettter than those of the binary blends because of complementary polymer absorption, as verified using external quantum efficiency measurements. High fill factors (FF) (>0.59) were achieved in all cases and are attributed to high charge-carrier mobilities in the ternary blends. As a result of the intermediate <i>V</i>_oc, increased <i>J</i>_sc and high FF, the ternary blend BHJ solar cells showed power conversion efficiencies of up to 5.51%, exceeding those of the corresponding binary blends (3.16 and 5.07%). Importantly, this work shows that upon optimization of the overall polymer:fullerene ratio at each polymer:polymer ratio, high FF, regular variations in <i>V</i>_oc, and enhanced <i>J</i>_sc are possible throughout the ternary blend composition regime. This adds to the growing evidence that the use of ternary blends is a general and effective strategy for producing efficient organic photovoltaics manufactured in a single active-layer processing step