Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction

Abstract

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated.Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ~487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ~2000 times less than predicted by Langevin theory. This in turn leads to the build-up of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films, due to the different scaling of electro-absorption and photo-induced absorption signals in transient absorption spectroscopy.

Putting Order into PM6:Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction

“…Typically, OSCs thicker than 100 nm mostly suffer from reduced FFs with only a handful of exceptions. [3][4][5][6][7][8] While it is now well established that, for the NFA systems, charge generation is very efficient, [9][10][11][12][13] the loss in FF is primarily dictated by a competition between charge carrier extraction and non-geminate recombination.…”

Recent progress in thick‐film organic photovoltaic devices: Materials, devices, and processing

“…Other strategies for controlling morphology include post-annealing treatment 28,30,66,87 and ratio adjustment. 12 Postannealing treatment including thermal annealing (TA) and solvent vapor annealing (SVA), can promote polymer-segment stress relaxation, aggregation, and recrystallization, as well as increase phase separation and domain purity to reduce the trap density.…”

“…The authors speculated that the electric fields originate from the migration of hole polarons from intermixed higherenergy regions to lower-energy BQR aggregates after SVA. 30…”

“…28 Meanwhile, it is also believed that a reduced recombination is equally as essential as high mobility in thick junctions, which will be discussed later. 29,30 Much effort has been put forward to enhance the charge-carrier mobility of BHJ devices and realize high-performance thick-film OSCs. For example, a well-defined face-on orientation with strong π-π stacking in the q z -direction can result in high charge-carrier mobilities in both neat and blend films originating from a vertical charge transport channel.…”

“…McGehee and coworkers proposed that a relatively high charge‐carrier mobility (≈10 −2 cm 2 V −1 s −2 ) was needed to minimize bimolecular recombination and space‐charge buildup for achieving a high FF in thick‐film devices 28 . Meanwhile, it is also believed that a reduced recombination is equally as essential as high mobility in thick junctions, which will be discussed later 29,30 …”