Field-Assisted Exciton Dissociation in Highly Efficient PffBT4T-2OD:Fullerene Organic Solar Cells

Abstract

Understanding the photophysics of charge generation in organic semiconductors is a critical step toward the further optimization of organic solar cells. The separation of electron–hole pairs in systems with large energy offsets is relatively well-understood; however, the photophysics in blends with low driving energy remains unclear. Herein, we use the material system PffBT4T-2OD:PC71BM as an example to show that the built-in electric field plays a critical role toward long-range charge separation in high-performance devices. By using steady-state and time-resolved spectroscopic techniques, we show that in neat films an energetic barrier impedes polymer exciton dissociation, preventing charge transfer to the fullerene acceptor. In complete devices, this barrier is diminished due to the built-in electric field provided by the interlayers/contacts and accompanying space-charge distribution. The observed behavior could also be relevant to other systems with low driving energy and emphasizes the importance of using complete devices, rather than solely films, for photophysical studies.

Efficient non-fullerene organic solar cells employing sequentially deposited donor–acceptor layers

“…4b). 70 The signal after 100 ps in D-A blends cannot come from excitons which are expected to decay completely (as seen in measurements on pure NCDBT film), but possibly from polarons. The smaller amplitude in c-BHJ films thus might come from faster BR due to its finer phase separation.…”

The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation

“…A notable feature here is the slow formation of charges which was also observed before in TA for the same blend. 31 This is in contrast with other well intermixed fullerene-based OPV systems where charge generation is much faster (∼200 fs). [32][33][34] The slow generation here may be associated with the relatively large domain size of PffBT4T-2OD aggregates in the PffBT4T-2OD:PC 71 BM blend as shown in the TEM (Fig.…”

“…However, the dynamics at both 1300 nm and 2000 nm push appeared to be similar as shown in our previous work. 31 Therefore, we assign the signal to the optical activation of localized immobile charged states that can be CTS or separated carriers in the shallow traps. As a result of this push beam, extra photocurrent (∆J) is generated.…”

Energy Transfer to a Stable Donor Suppresses Degradation in Organic Solar Cells

“…Transient Absorption (Pump‐probe) Spectroscopy : Samples for TA measurements were prepared identically to the active layers of solar cells. Since this system does not exhibit electric‐field‐induced charge generation, all measurements were performed on active layer films and reference films of the BHJ components. Seed pulses (800 nm, < 100 fs) were generated at a repetition rate of 4 kHz by a Ti:Sapphire regenerative amplifier (Spectra‐Physics Solstice, Newport Corporation), and routed toward an optical parametric amplifier (TOPAS, Light Conversion) to provide the pump.…”