We report a new β-dicyanodistyrylbenzene (β-DCS)-based polymer (PBDCS), which enables efficient fullerene and non-fullerene organic solar cells with low Eloss and high EQE.

To explore the efficient way of assembling electron-donating (D) and -accepting (A) moieties in small-molecule donors for organic solar cells (OSCs), ADA- and DAD-type triad donor molecules were synthesized and investigated using indolo­[3,2-b]­indole and diketopyrrolopyrrole (DPP) as D and A moieties, respectively. Designing D–A-type donor materials possessing intramolecular charge-transfer (ICT) characteristics is important to facilitate exciton dissociation and retard charge-carrier recombination at the donor and acceptor (PC61BM) interface of bulk heterojunction OSCs. While ADA and DAD triad donors showed similar absorption spectra, their photoinduced ICT nature in the excited state monitored by the transient absorption spectroscopy was quite different. Both molecules exhibit strong electronegativity and abundance of electrons on DPP moieties, facilitating interaction with the neighboring molecules. However, ADA exhibits stronger ICT character than DAD because of the spatially more delocalized lowest unoccupied molecular orbital and abundant electron density at the end-capping DPP moieties. Owing to its stronger ICT character in the excited state, the ADA:PC61BM blend showed more favorable charge separation and reduced charge-carrier recombination at the donor/PC61BM interface. Consequently, ADA:PC61BM devices exhibited higher J SC than DAD:PC61BM OSCs.

To understand the importance of excess energy in the charge separation mechanism in polymer solar cells (PSCs), we focused on the dissociation of the Coulombically bound electron and hole pair at the donor (D) and acceptor (A) interface. A push−pull-type copolymer poly(3-fluorothienothiophenebenzodithiophene) (PTB7) and a homopolymer poly(3-hexylthiophene) (P3HT) were used as model compounds to correlate the chemical structure of the donor materials with the mechanism of photoinduced charge separation at the D/A interface in PSCs. In the case of PTB7, excitons with intramolecular charge-transfer (ICT) characteristics are initially generated due to the push−pull actions between the electron-donating and electron-accepting building units, resulting in electron density displacement to facilitate interfacial charge separation. On the other hand, the delocalized exciton state of P3HT is known to be favorable for the hot exciton dissociation and charge generation while lacking the ICT characteristics. Therefore, understanding the effect of ICT characteristics and delocalization of the exciton state of polymers is important for enhancing the charge separation at the D/A interface and thus the photocurrent in PSCs. Both PTB7:NIDCSEO3 and P3HT:NIDCSEO3 blends exhibited strongly π−π stacked D and A regions with highly crystalline dicyanodistyrylbenzenenaphthalimide-based acceptor, NIDCSEO3, which is beneficial for the exciton delocalization and reduces the effect of blend morphology when comparing the charge separation at the D/A interface. Through the temperature-and pump-dependent femtosecond transient absorption experiments in this work, it was found that the charge separation via the hot CT state is dominant in PTB7:NIDCSEO3 owing to the delocalized ICT-type excitons of PTB7. On the other hand, P3HT:NIDCSEO3 exhibited a complex charge generation mechanism comprising both hot and relaxed states including a polaron pair state within P3HT while preserving the delocalized exciton state based on the highly crystalline homopolymer structure.

Despite the recent rapid development of organic solar cells (OSCs), the low dielectric constant (ϵr=3–4) of organic semiconducting materials limits their performance lower than inorganic and perovskite solar cells. In this work, we introduce oligo(ethylene glycol) (OEG) side chains into the dicyanodistyrylbenzene‐based non‐fullerene acceptors (NIDCS) to increase its ϵr up to 5.4. In particular, a NIDCS acceptor bearing two triethylene glycol chains (NIDCS‐EO3) shows VOC as high as 1.12 V in an OSC device with a polymer donor PTB7, which is attributed to reduced exciton binding energy of the blend film. Also, the larger size grain formation with well‐ordered stacking structure of the NIDCS‐EO3 blend film leads to the increased charge mobility and thus to the improved charge mobility balance, resulting in higher JSC, FF, and PCE in the OSC device compared to those of a device using the hexyl chain‐based NIDCS acceptor (NIDCS‐HO). Finally, we fabricate NIDCS‐EO3 devices with various commercial donors including P3HT, DTS‐F, and PCE11 to show higher photovoltaic performance than the NIDCS‐HO devices, suggesting versatility of NIDCS‐EO3.