A new diketopyrrolopyrrole derivative with appropriate energy levels and bipolar charge-transport properties is designed and synthesized. When this molecule is used as either electron donor or acceptor, the resulting organic solar cells both give the power conversion efficiencies over 3%.

We present a ternary strategy to enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (PSCs) with a bipolar small molecule as cascade material. A bipolar diketopyrrolopyrrole small molecule (F(DPP)(2)B-2), as the second electron acceptor, was incorporated into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric-acidmethyl-ester (PC61BM) blend to fabricate ternary blend PSCs. The introduction of the bipolar compound F(DPP) 2B2 can not only broaden the light absorption of the active layer because of its absorption in near infrared region but also play a bridging role between P3HT and PC61BM due to the cascaded energy level structure, thus improving the charge separation and transportation. The optimized ternary PSC with 5 wt% F(DPP) 2B2 content delivered a high PCE of 3.92% with a short-circuit current density (J(sc)) of 9.63 mA cm (2), an open-circuit voltage (Voc) of 0.62 V and a fill factor (FF) of 64.90%, showing an 23% improvement of PCE as compared to the binary systems based on P3HT: PC61BM (3.18%) or P3HT: F(DPP)(2)B-2 (3.17%). The results indicate that the ternary PSCs with a bipolar compound have the potential to surpass high-performance binary PSCs after carefully device optimization. (C) 2015 Elsevier B.V. All rights reserved.

Fully roll-coated ITO-free flexible organic solar cells based on a non-fullerene small molecule acceptor with a PCE of 0.65% were fabricated.

•Three DPP derivatives with different end-groups were designed and synthesized.•Three DPP derivatives exhibit similar energy structures but different photovoltaic properties.•End-groups influence photovoltaic properties of three molecules through the inducing of different morphologies.

To gain ideal conjugated polymers with a low band-gap (E-g) and a deep HOMO (highest occupied molecular orbital) energy level for photovoltaic application, both strong and weak electron withdrawing units were introduced into polymer backbone. A series of -(D-A(1)-D-A(2))(n)- type diketopyrrolopyrrole (DPP) containing polymers P1-P7 were designed and synthesized. The resulting copolymers exhibited both low E-g of 1.27-1.48 eV and deep HOMO energy levels of -5.25to -5.44 eV. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) were investigated. All photovoltaic devices featured high open circuit voltages (V-oc, 0.72-0.91 V). Among the seven copolymers, P5 exhibited the best performance with a V-oc of 0.80 V, a short circuit current density (I-sc) of 3.48 mA/cm(2), a fill factor (FF) of 43.2%, and a power conversion efficiency (PCE) of 1.33%. (C) 2013 Elsevier Ltd. All rights reserved.

Three star-shaped D-A small molecules, (P-DPP)(3)TPA, (4-FP-DPP)(3)TPA, and (4-BuP-DPP)(3)TPA were designed and synthesized with triphenylamine (TPA) as the core, diketopyrrolopyrrole (DPP) as the arm, and unsubstituted or substituted benzene rings (phenyl, P; 4-fluoro-phenyl, 4-FP; 4-n-butyl-phenyl, 4-BuP) as the end-group. All the three small molecules show relatively narrow optical band gaps (1.68-1.72 eV) and low-lying highest occupied molecular orbital (HOMO) energy levels (-5.09∼-5.13 eV), implying that they are potentially good electron donors for organic solar cells (OSCs). Then, photovoltaic properties of the small molecules blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM) as electron acceptor were investigated. Among three small molecules, the OSC based on (P-DPP)(3)TPA:PCBM blend exhibits a best power conversion efficiency (PCE) of 2.98% with an open-circuit voltage (V(oc)) of 0.72 V, a short-circuit current density (J(sc)) of 7.94 mA/cm(2), and a fill factor (FF) of 52.2%, which may be ascribed to the highest hole mobility of (P-DPP)(3)TPA.