Solid-state showdown: comparing the photovoltaic performance of amorphous and crystalline small-molecule diketopyrrolopyrrole acceptors
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A novel glass-forming diketopyrrolpyrrole (DPP) derivative (DPP-glass) and a crystalline isoelectronic analogue were synthesized and their photophysical, electrochemical and photovoltaic properties were investigated. The compounds were processed into thin films that were further studied by Atomic force microscopy (AFM) and X-ray diffraction (XRD). Solution-processed inverted and regular bulk heterojunction photovoltaic cells were fabricated in different blend ratios with P3HT as donor. Both DPP derivatives show appreciable photovoltaic performance with power conversion efficiencies (PCE) ranging from 0.30 to 2.50%. The regular device with amorphous DPP-glass showed the highest efficiency (2.50%) in the blend ratio 1:2 whereas the device with crystalline DPP derivative shows an efficiency of 1.56% under simulated 1.5 solar irradiation of 100 mWcm-2. The better performance of DPP-glass in comparison to its crystalline analogue is attributed to a more uniform film morphology and fewer defects, leading to improved electron mobility.