Influence of Annealing and Interfacial Roughness on the Performance of Bilayer Donor/Acceptor Polymer Photovoltaic Devices

Abstract

Through controlled annealing of planar heterojunction (bilayer) devices based on the polyfluorene copolymers poly(9,9‐dioctylfluorene‐co‐bis(N,N′‐(4,butylphenyl))bis(N,N′‐phenyl‐1,4‐phenylene)diamine) (PFB) and poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) we study the influence of interface roughness on the generation and separation of electron–hole pairs at the donor/acceptor interface. Interface structure is independently characterized by resonant soft X‐ray reflectivity with the interfacial width of the PFB/F8BT heterojunction observed to systematically increase with annealing temperature from 1.6 nm for unannealed films to 16 nm with annealing at 200 °C for ten minutes. Photoluminescence quenching measurements confirm the increase in interface area by the three‐fold increase in the number of excitons dissociated. Under short‐circuit conditions, however, unannealed devices with the sharpest interface are found to give the best device performance, despite the increase in interfacial area (and hence the number of excitons dissociated) in annealed devices. The decrease in device efficiency with annealing is attributed to decreased interfacial charge separation efficiency, partly due to a decrease in the bulk mobility of the constituent materials upon annealing but also (and significantly) due to the increased interface roughness. We present results of Monte Carlo simulations that demonstrate that increased interface roughness leads to lower charge separation efficiency, and are able to reproduce the experimental current‐voltage curves taking both increased interfacial roughness and decreased carrier mobility into account. Our results show that organic photovoltaic performance can be sensitive to interfacial order, and heterojunction sharpness should be considered a requirement for high performance devices.

Suppression of geminate charge recombination in organic photovoltaic devices with a cascaded energy heterojunction

“…Models of this type have been shown to agree quantitatively with J-V curves of polymer bilayer OPV devices with abrupt 30 and roughened 31 interfaces. In the model, the OPV was represented as a 3D…”

“…It is noted that this recombination rate is the same as that used in a similar Monte Carlo model which obtained quantitative agreement with experimental J-V data for an amorphous all-polymer OPV 30 . These parameters are expected to mimic the situation which occurs in amorphous all-polymer OPVs, in which field-activated  GS has been reported 30,31,35 . This choice of parameters is deliberate since binary blends of amorphous materials are unlikely to form cascaded energy heterojunctions, therefore one can be reasonably confident that these parameters are free from the potential effects of energy cascades.…”

“…The chosen parameters were similar to those used in other Monte Carlo models to describe charge transport in amorphous conjugated polymers 31 . Note that this approach implies a spatial averaging of the electronic coupling between molecules.…”

Nucleation and growth of thin films of rod-like conjugated molecules

“…This is in particular related to the fact that a lower number of defects at the interface facilitates higher charge carrier mobilities [19][20][21][22]. As we have seen above films formed by upright After reaching a critical coverage, the structure changes to a bulk like molecular arrangement.…”

Simulating charge transport in organic semiconductors and devices: a review

“…Hence for OPVs which rely on conventional exciton dissociation, nano-structured morphologies are required, and device models have been used extensively to understand which forms of morphology are most favourable. As well as bilayers [73,143,377], morphologies examined include interdigitated pillars [48,201,[378][379][380], gyroids [381], idealised BHJs [48, 147,201,380] and non-ideal BHJs [114,123,382,383]. It is shown that interdigitated pillars have highest performance, since the electric field is either parallel or perpendicular to the plane of the heterojunction, while perhaps unexpectedly, gyroid and idealised BHJ morphologies have very similar performance [381].…”