Design Of Non-fullerene Acceptors Research Articles

A series of selenium-containing non-fullerene acceptors with side chain engineering for organic solar cells

Non-fullerene acceptors (NFAs) have been crucial in improving the efficiency of organic solar cells (OSCs) due to their attractive properties, such as strong light-harvesting ability, easily tunable energy levels, and high crystallinity. Nevertheless, general NFAs have limited light absorption, and studies on the design of NFAs that can absorb a broad range of the solar spectrum and that offer complementary absorption to that of donors should be conducted. Because selenium, a chalcogen element, can induce strong intermolecular Se⋯Se interactions, planarity, and extended effective conjugated length in OSC materials, NFAs with selenium usually exhibit redshifted absorption. Here, we synthesized a series of selenium-containing non-fullerene acceptors with different inner alkyl chain lengths, named Se-Y6-BO (2,2'-((2Z, 2′Z)-((12,13-bis(2-butyloctyl)-3,9-dinonyl-12,13-dihydroselenopheno[2″,3'':4′,5']thieno[2′,3':4,5]pyrrolo[3,2-g]selenopheno[2′,3':4,5]thieno [3,23,2-b] [1,2,5]thiadiazolo [3,43,4-e]indole-2,10-diyl)bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) and Se–Y6-HD (2,2'-((2Z, 2′Z)-((12,13-bis(2-hexyldecyl)-3,9-dinonyl-12,13-dihydroselenopheno[2″,3'':4′,5']thieno[2′,3':4,5]pyrrolo[3,2-g]selenopheno[2′,3':4,5]thieno [3,23,2-b] [1,2,5]thiadiazolo [3,43,4-e]indole-2,10-diyl)bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile). The optical absorption of two selenium-containing NFAs was redshifted compared with that of Y6 due to the effect of selenium atoms in the backbone. The PM6:Se-Y6-BO-based devices achieved a power conversion efficiency (PCE) of 13.8% with an open-circuit voltage (VOC) of 0.81 V, short-circuit current density (JSC) of 24.4 mA cm−2, and a fill factor of 0.70. Similarly, the OSCs based on PM6:Se–Y6-HD achieved a PCE of 13.6% and a JSC of 23.7 mA cm−2.

Optimizing spectral and morphological match of nonfullerene acceptors toward efficient indoor organic photovoltaics with enhanced light source adaptability

High-performance indoor organic photovoltaics (IOPV) require large-bandgap material systems to absorb visible light efficiently and reduce energy loss. However, state-of-the-art non-fullerene acceptors (NFAs) have absorptions in the near-infrared region and are thus not suitable for IOPV applications. Herein, we report a series of large-bandgap (>1.70 eV) NFAs named FCC-Cl-C8, FCC-Cl-4Ph and FCC-Cl-6Ph by modifying the alkyl side chains with alkylphenyl chains partially or completely. Results show that the bulky alkylphenyl side chains can finely tune the absorption properties of the NFAs and also affect their morphological properties. Interestingly, the best-performing NFA is the one (named FCC-Cl-4Ph) with partial alkyl and alkylphenyl substitutions, which blue-shift the absorption of the NFAs while minimizing the negative morphological effect of the bulky alkylphenyl chains. As a result, FCC-Cl-4Ph can achieve excellent indoor efficiencies over 29% under a 3000 K LED lamp at 1000 lux and show better solution processability over FCC-Cl-C8. More importantly, FCC-Cl-4Ph can maintain high indoor performance (29.7–26.8% at 1000 lux) under a wide range of indoor lighting spectra (2600, 3000, 4000, and 6500 K LED lamps), which should be due to the blue-shifted spectra of FCC-Cl-4Ph and better matching with various indoor conditions. This work reveals an interesting structure-property relationship and offers useful strategies for the further design of NFAs toward efficient IOPV devices.

The halogen effect of perylene diimide-based non-fullerene acceptors on photovoltaic properties

To develop photovoltaic materials based on perylene diimide, two non-fullerene acceptors namely PCPD-F and PCPD-Cl are rationally designed and readily synthesized. By introducing the cyclopentadithiophene unit with the strong electron-donating ability, PCPD-F and PCPD-Cl exhibit strong and bathochromic absorption compared to other perylene diimide derivatives. Meanwhile, different halogens in PCPD-F and PCPD-Cl significantly affect the physicochemical properties and device performance. With the substitution of chlorine atom, PCPD-Cl shows relatively broad absorption and strong crystallinity. However, the chlorination makes PCPD-Cl less miscible with polymer donors, leading to the unfavorable morphology with large-scale phase separation in organic solar cells. Consequently, the device based on PCPD-F achieves the best power conversion efficiency of 9.03% by blending with the polymer donor PM6, which is among the high-performance organic solar cells based on perylene diimide. This work demonstrates it is crucial to meticulously select the substituted atom and position in the design of non-fullerene acceptors.

Determining the Correlation between Excited State Dynamics and Donor and Acceptor Structure in Nonfullerene Acceptors

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor-acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor-acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where D represents 9,9-di-n-propylfluorene, A1 represents benzothiadiazole, and A2 represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-D or D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1∗ and S1 on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-D and D-A1-A2 in solution and film, and for D-A1-A1-D in solution. In the film, the singlet excited state of D-A1-A1-D predominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOC loss commonly associated with NFA-based OPV devices.

Design of Nonfullerene Acceptors with Near‐Infrared Light Absorption Capabilities

AbstractA series of narrow bandgap electron acceptors is designed and synthesized for efficient near‐infrared (NIR) organic solar cells. Extending π‐conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7‐Th, IOTIC‐2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm−2 with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7‐Th:IOTIC‐2F‐based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC‐2F, into the PTB7‐Th:IOTIC‐2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy‐level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics.

A2–A1–D–A1–A2 type non-fullerene acceptors based on methoxy substituted benzotriazole with three different end-capped groups for P3HT-based organic solar cells

In the last three years, the A2–A1–D–A1–A2 skeleton has become increasingly popular in the design of non-fullerene acceptors (NFAs), and it could match particularly well with the classic p-type polymer of poly(3-hexylthiophene) (P3HT).