Abstract
Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.
Highlights
Bulk heterojunction (BHJ) organic solar cells (OSCs) are being extensively researched as a clean energy source with efficiencies exceeding 10%1–5
PffBT4T-2OD is a crystalline polymeric donor with a broad absorption spectrum in visible region, and OSC devices reported with fullerene acceptors exhibited power conversion efficiencies (PCEs) of 10.8%, one of the highest PCE values recorded for single OSC25
The bis-perylene diimide (PDI) small molecule was synthesized by a modified procedure as described in the supporting information, and used as an acceptor material because of its strong light absorption in visible region (420–600 nm), good electron mobility, and favorable energy level matching with PffBT4T-2OD donor polymer
Summary
Bulk heterojunction (BHJ) OSCs are being extensively researched as a clean energy source with efficiencies exceeding 10%1–5. Despite the beneficial properties of fullerenes in the context of BHJ-OSCs, they are limited in (i) spectral bandwidth, (ii) ambient stability, (iii) tunability of electronic properties, and (iv) relatively high cost. For these reasons, it is not clear whether fullerene derivatives are the best candidate materials for use as OSC acceptors. The BHJ photoactive layer made with PDIs offers a high absorption coefficient in the visible region, good photochemical stability, and good thin film forming properties[7,9,14,15] Despite their attractive features, PDI-based acceptors have met with little success, and few PDI systems have shown PCEs just over 8%16,17. TPV study, photocurrent-effective voltage analysis, and low temperature-dependent characterizations were performed to elucidate the factors that determined the differences among device performances
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