Bandgap Narrowing in Non‐Fullerene Acceptors: Single Atom Substitution Leads to High Optoelectronic Response Beyond 1000 nm

Abstract

absorption spectra extending to the NIR region have been designed and applied to the fabrication of OSCs. [6][7][8] A critical challenge arises as one decreases optical bandgaps (E g opt ) with respect to simultaneously achieving a high external quantum efficiency (EQE) and high open-circuit voltage (V OC ). [9] This challenge is due to the counterbalance between the driving force for charge separation, which aids in photocurrent generation, and voltage loss in the cell. [10,11] Finding ways to maximize V OC requires one to reduce the energy loss (E loss = E g opt -eV OC ) that occurs as a result of the multiple states that follow exciton generation. [12] Narrow bandgap (NBG) non-fullerene acceptors (NFAs) have emerged as the next generation of electron acceptors in OSCs. [13][14][15][16][17][18] Tunability of E g opt through molecular design allows one to tailor NIR absorption characteristics. [19,20] Considering that the maximum human photopic sensitivity is 555 nm and the maximum human scotopic sensitivity is 507 nm, [21] transparent photoactive materials should predominantly absorb solar radiation from ≈650 nm into the NIR region for semitransparent solar cell applications. In addition, since ≈50% of solar radiation intensity is in the NIR region, the development of NBG-NFAs with E g opt below ≈1.35 eV is desirable to effectively harvest solar NIR radiation. [1] Another encouraging feature of NFAs is that the energetic offsets that drive charge generation are small (<0.3 eV), [18,[22][23][24] which is beneficial for maintaining low E loss . Despite these desirable features, there has been less consideration for designing NBG-NFAs for transparent/NIR absorbing OSC applications. To address this challenge and expand the design of NIR harvesting acceptor molecules, we demonstrate in this contribution a new molecular design for ultra NBG-NFA materials with strong NIR response and small E loss .The two NBG NFAs described in this contribution are COTIC-4F and SiOTIC-4F (Figure 1a). Their molecular design includes incorporation of a cyclopentadithiophene (CPDT), or dithienosilole (DTS), unit as the central donor (D) fragment, which is flanked by two alkoxythienyl units (D′) to form an electron-rich D′-D-D′ central core. The D′-D-D′ units are end-capped with Two narrow bandgap non-fullerene acceptors (NBG-NFAs), namely, COTIC-4F and SiOTIC-4F, are designed and synthesized for the fabrication of efficient near-infrared organic solar cells (OSCs). The chemical structures of the NBG-NFAs contain a D′-D-D′ electron-rich internal core based on a cyclopentadithiophene (or dithienosilole) (D) and alkoxythienyl (D′) core, end-capped with the highly electron-deficient unit 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (A), ultimately providing a A-D′-D-D′-A molecularconfiguration that enhances the intramolecular charge transfer characteristics of the excited states. One can thereby reduce the optical bandgap (E g opt ) to as low as ≈1.10 eV, one of the smallest values for NFAs reported to date. In bulk-he...

Solution‐Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

“…When replacing alkyl groups with alkoxy groups onto thiophene π‐spacers, an E g opt of the resulting molecule, IEICO, significantly decreases from 1.50 eV to 1.34 eV . PBDTTT‐E‐T:IEICO‐based OSCs show a PCE of 8.4% with a J sc of 17.7 mA cm −2 due to a strong NIR response, while maintaining a high V oc of 0.82 V. Bazan and co‐workers developed a ultra‐narrow‐bandgap NFA, COTIC‐4F, by combining strong electron‐rich CPDT and alkoxy thienyl π‐spacers as the central donor complexes (Figure c) . As a result, COTIC‐4F exhibits the lowest E g opt of 1.10 eV for nonfullerene small molecule acceptors reported thus far while remaining transparent in the visible region and exhibiting a PCE of ≈7.4%.…”

Quantifying the Nongeminate Recombination Dynamics in Nonfullerene Bulk Heterojunction Organic Solar Cells

“…[1][2][3][4][5][6] Most high-performing OSCs use fullerene-based acceptors in a bulk-heterojunction (BHJ) configuration, and integral parameters and the density of charge carriers under different operating conditions. [34,[39][40][41][42] Thus, it is of great importance to develop a general quantitative model that describes total nongeminate recombination losses which originate from different recombination processes. In the case of inverted NFA-OSCs in particular, V OC versus light intensity dependence routinely exhibits additional contributions that are not related to bimolecular recombination, which manifests itself with slopes smaller than kT/q ( Figure S1, Supporting Information).…”

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

“…Efficient charge separation is encouraged by having a small energetic offset (≈0.3 eV) between the LUMO of the ED and EA (Δ E LUMO in Figure a) and between HOMO of EA and ED (Δ E HOMO in Figure a). Conversely, some recent reports indicate that the HOMO/LUMO offset might not be necessary for FREA‐based OSCs and may also reduce charge transfer energy losses …”

“…Recently, Bazan and co‐workers reported two narrow bandgap FREAs, COTIC‐4F ( A96 ) and SiOTIC‐4F ( A97 , Figure ) . In the FREA design, CPT or DTS central “D” unit was attached to two alkoxy‐thiophene which was end‐capped with DFIC terminal units in COTIC‐4F and SiOTIC‐4F , respectively.…”