Low band gap polymer consisting of quinacridone and diketopyrrolopyrrole and isoindigo units: correlation of ordered structure and intramolecular charge transfer properties

• We synthesized the polymer based on quincaridone, diketopyrrolopyrrole and isoindigo.
• The synthesized PQCDPP, PQCIDG showed low band gap and high molecular weight.
• PQCIDG formed face-on structure on substrate.
• PQCIDG showed PCE of 2.7% with high photo-current (7.4 mA/cm2).

Demand for low band gap polymer materials is driven by the fast growing organic photovoltaic industry. In the present study, to develop highly functional conjugated polymers, poly[quinacridone-diketopyrrolopyrrole] (PQCDPP) and poly[quinacridone-isoindigo] (PQCDIG) were synthesized via the Suzuki coupling reaction. While these two polymers provided high absorption in the region of over 700 nm, PQCIDG had broader absorption across the visible spectrum (300–600 nm) than PQCDPP because a greater intramolecular charge transfer occurs between the quincaridone and isoindigo units. Based on X-ray diffraction (XRD) results, large factions of the two polymers exhibited a face-on orientation with respect to the substrate, and the orientation distributions of 24.5° were more dominant in PQCIDG, compared with PQCDPP. In this study, we fabricated enhanced-polymer-based bulk-heterojunction (BHJ) solar cells using a normal structure of ITO/PEDOT:PSS/active-layer/Al and assessed its performance in terms of photovoltaic characteristics. The PQCIDG-based devices (polymer/PC71BM = 1:3) showed power conversion efficiency (PCE) of 2.7% with open circuit voltage (Voc) of 0.83 V, short circuit current (Jsc) of 7.4 mA/cm2 and fill factor (FF) of 43.1%.

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