Design, Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra‐Narrow Band Gap

Abstract

The design of narrow band gap (NBG) donors or acceptors and their application in organic solar cells (OSCs) are of great importance in the conversion of solar photons to electrons. Limited by the inevitable energy loss from the optical band gap of the photovoltaic material to the open-circuit voltage of the OSC device, the improvement of the power conversion efficiency (PCE) of NBG-based OSCs faces great challenges. A novel acceptor-donor-acceptor structured non-fullerene acceptor is reported with an ultra-narrow band gap of 1.24 eV, which was achieved by an enhanced intramolecular charge transfer (ICT) effect. In the OSC device, despite a low energy loss of 0.509 eV, an impressive short-circuit current density of 25.3 mA cm is still recorded, which is the highest value for all OSC devices. The high 10.9 % PCE of the NBG-based OSC demonstrates that the design and application of ultra-narrow materials have the potential to further improve the PCE of OSC devices.

A High‐Efficiency Organic Solar Cell Enabled by the Strong Intramolecular Electron Push–Pull Effect of the Nonfullerene Acceptor

“…[36,37] In the various types of NF acceptors, the materials with a so-called A-D-A [38] structure, in which electron-rich central unit is endcapped by two electron-deficient segments and hence ICT can be formed, has exhibited promising photovoltaic properties. [40,41] Especially, introducing fluorine atoms is an efficient way to modulate the ICT effect. Its molecular energy levels including LUMO and highest-occupied-molecular-orbital (HOMO) levels could be elevated or depressed by introducing electron pushing or withdrawing groups into its end-capping segments, by which the V OC of the OSCs could be modulated.…”

“…[22,[33][34][35] In order to realize broad light absorption and suitable molecular energy levels, the conjugated components with the aforementioned groups are often linked with electron-rich building blocks to form intramolecular charge transfer (ICT) effect. [40,41] Especially, introducing fluorine atoms is an efficient way to modulate the ICT effect. For example, 3,9-bis (2-methylene-(3-(1,1-dicyanomethylene)in-danone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC), [39] a well-known A-D-A NF acceptor, has the rigid center donor unit IDTT which can restrict molecular planarity, aggregation, and large phase separation in BHJ blend films.…”

Dithieno[3,2‐b:2′,3′‐d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells

“…[30,[39][40][41][42] Zhan and co-workers [43] and Jen and co-workers [44] have reported a fused-ring thiophenethieno[3,2-b]thiophene-thiophene-based hexacyclic low-bandgap NFA (named IHIC or 4TIC, 1.38 eV), exhibiting a PCE of 9.77% with an relative high visible transmittance. Hou and coworkers [45] have demonstrated an ultranarrow-bandgap NFA (IEICO-4F, 1.24 eV) through utilizing a spacer (alkoxy thiophene) to enhance the ICT effect and electron delocalization, indicating a PCE of 10.9% within a ternary OSC. Designing the narrow bandgap acceptors with NIR absorption matching with mid-bandgap donors could be an ideal case for further improving the PCEs of OSCs.…”

“…[38] Guided by this predictive principle, several methods have been developed to broaden the absorption spectra of NFAs, such as extending the conjugation and enhancing the ICT effect. Hou and coworkers [45] have demonstrated an ultranarrow-bandgap NFA (IEICO-4F, 1.24 eV) through utilizing a spacer (alkoxy thiophene) to enhance the ICT effect and electron delocalization, indicating a PCE of 10.9% within a ternary OSC. Hou and coworkers [45] have demonstrated an ultranarrow-bandgap NFA (IEICO-4F, 1.24 eV) through utilizing a spacer (alkoxy thiophene) to enhance the ICT effect and electron delocalization, indicating a PCE of 10.9% within a ternary OSC.…”

Morphology Control Enables Efficient Ternary Organic Solar Cells

“…[36][37][38] Recent developments in NFAs offer a wider choice of acceptor materials for ternary devices. [39][40][41][42][43][44][45] In terms of ternary blend incorporating two NFAs, impressive progresses have been also obtained. [21,22] Different ternary blends containing one NFA paired with two donors or one donor/one fullerene derivative exhibited high PCEs over 12%.…”