High open circuit voltage in regioregular narrow band gap polymer solar cells

The regioregular narrow band gap (E_g ∼1.5 eV) conjugated polymer PIPCP was designed and synthesized. PIPCP contains a backbone comprised of CPDT-PT-IDT-PT repeat units (CPDT = cyclopentadithiophene, PT = pyridyl[2,1,3]thiadiazole, IDT = indacenodithiophene) and strictly organized PT orientations, such that the pyridyl N-atoms point toward the CPDT fragment. Comparison of PIPCP with the regiorandom counterpart PIPC-RA illustrates that the higher level of molecular order translates to higher power conversion efficiencies (PCEs) when incorporated into bulk heterojunction (BHJ) organic solar cells. Examination of thin films via absorption spectroscopy and grazing incidence wide-angle X-ray diffraction (GIWAXS) experiments provides evidence of higher order within thin films obtained by spin coating. Most significantly, we find that PIPCP:PC₆₁BM blends yield devices with an open circuit voltage (V_oc) of 0.86 V, while maintaining a PCE of ∼6%. Comparison against a wide range of analogous narrow band gap conjugated polymers reveals that this V_oc value is particularly high for a BHJ system with band gaps in the 1.4-1.5 eV range thereby indicating a very low E_g - eV_oc loss.