Highly Efficient Fullerene-Free Organic Solar Cells Operate at Near Zero Highest Occupied Molecular Orbital Offsets

Abstract

Herein, we investigated a series of fullerene-free organic solar cells (OSCs) based on six different donor:acceptor (D:A) blends with varied highest occupied molecular orbital (HOMO) offsets from −0.05 to 0.21 eV. First, to verify the energetic compatibility of a specific D:A pair, especially for HOMO offsets, we established a simple method to estimate the hole transfer tendencies between D and A by using bilayer hole-only devices. It reveals that the asymmetrical diode effect of the bilayer hole-only devices can correlate with the FF and J sc of the relevant OSCs. Second, to find out whether HOMO offset is the main restriction of hole transfer, we measured transient absorption spectra and examined the hole transfer behavior in the blends, revealing that the occurrence of hole transfer is independent of the HOMO offsets and ultrafast in the time scale of ≤4.6 ps for those blends with ≥0 eV HOMO offsets. In contrast, a negative HOMO offset can significantly slow down the hole transfer with a half-time of ∼400 ps. Furthermore, we compare the device parameters under varied light intensities and discover that the bimolecular recombination should be one of the main restrictions for high device performance. Surprisingly, small HOMO offsets of 0 and 0.06 eV can also enable high PCEs of 10.42% and 11.75% for blend 2 (PTQ10:HC-PCIC) and blend 3 (PBDB-TF:HC-PCIC), respectively. Overall, our work demonstrates not only the validity of high-performance OSCs operating at the near zero HOMO offsets but also the charge dynamic insights of these blends, which will help gain understanding on the further improvement of OSCs.

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

“…In the framework of Marcus theory, the efficiency of the first step, CT formation via interfacial charge transfer, is related to the so‐called driving force for charge generation, Δ E S1,CT , which is the difference in energy of the intramolecular singlet (S 1 ) excited state on the donor or acceptor and the CT state . Various recent publications deal with the efficiency and dynamics of charge generation in NFA blends, revealing efficient interfacial charge transfer also for small Δ E S1,CT . Efficient charge generation has been rationalized by, e.g., favorable microelectrostatics .…”

Fused‐Ring Core Engineering for Small Molecule Acceptors Enable High‐Performance Nonfullerene Polymer Solar Cells

“…In addition, the Δ E HOMO s between polymer donor (PM6) and SMAs (T6Me, T7Me, and T8Me) are calculated to be 0.07, 0.02, and −0.04 eV, respectively, and a high PCE of 12.09% for T6Me‐based PSC can still be realized. As previous work shows, hole transfer occurs even with a slightly negative HOMO offset but with a much slower rate and lower efficiency, and a slightly positive or near‐flat HOMO offset is sufficient for fast and efficient hole transfer . PM6:T8Me blend films suffered from severe bimolecular recombination due to the negative HOMO offset of PM6 and T8Me, thus leading to the lowest PCE.…”

“…The HOMO and LUMO energy levels of T6Me, T7Me, and T8Me were calculated to be −5.50/−3.94, −5.45/−3.93, and −5.39/−3.89 eV, respectively, indicating that extending the hexacyclic‐fused conjugated core could upshift both HOMO and LUMO energy levels. Previous work have revealed that minimal positive HOMO offsets (0–0.1 eV) are sufficient for hole transfer from A to D, whereas a negative HOMO offset retards hole transfer and results in severe charge recombination . Obviously, the HOMO offset of PM6 and T8Me was negative (−0.04 eV), and thus the corresponding PM6:T8Me blend film was subjected to fearful bimolecular recombination, which was detrimental to J SC and PCE.…”

“…To better figure out the structure−property relationship, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐fluorothiophen‐2‐yl)‐benzo[1,2‐ b :4,5‐ b ′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐ c :4′,5′‐ c ′] dithiophene‐4,8‐dione))] (PM6) was selected as donor to fabricate PSC devices in view of complementary absorption spectrum and correspondingly well‐matched energy level. In spite of the fact that the high HOMO energy offset (Δ E HOMO ) between polymer donor and SMA plays an important role in device performance during exciton dissociation and recombination process, high PCEs can be achieved at near zero Δ E HOMO . In addition, the energy loss ( E loss ) can be defined as the difference between the optical bandgap ( E opt ) of the blend and the V OC of the device ( E loss = E opt − e V OC ), thereby a decreased HOMO offset benefits the enhancement of V OC and mitigation of energy loss for PSCs …”

“…In spite of the fact that the high HOMO energy offset (Δ E HOMO ) between polymer donor and SMA plays an important role in device performance during exciton dissociation and recombination process, high PCEs can be achieved at near zero Δ E HOMO . In addition, the energy loss ( E loss ) can be defined as the difference between the optical bandgap ( E opt ) of the blend and the V OC of the device ( E loss = E opt − e V OC ), thereby a decreased HOMO offset benefits the enhancement of V OC and mitigation of energy loss for PSCs …”