A pentacyclic S,N-heteroacene based electron acceptor with strong near-infrared absorption for efficient organic solar cells

Fusion Modes and Number of Fused Rings: Their Impact on Acceptor-Donor-Acceptor Nonfullerene Acceptors in Organic Solar Cells

The power conversion efficiency (PCE) of organic solar cells (OSCs) devices has surpassed 19% owing to the blooming of fused-ring nonfullerene acceptors (NFAs), especially for acceptor-donor-acceptor (A-D-A) type NFAs. However, the structural effect of the angular/linear fusion mode and number of fused rings for A-D-A type NFAs on the photovoltaic performance in OSCs devices remains unclear. Herein, the A-D-A type NFAs (F-0Cl, IDIC8-H, and ITIC) have been selected to obtain the intrinsic role of structural design strategies including the angular/linear fusion mode and the number of fused rings. The results indicate that compared to the linear fusion mode in ITIC, the angular fusion mode in F-0Cl effectively diminishes electronic vibrational coupling within the low-frequency range, leading to lower charge reorganization during the exciton diffusion process. Meanwhile, it facilitates the generation of multiple charge transfer mechanisms at the donor/acceptor (D/A) interface and increases the rates of hole transfer. On the other hand, the decreased number of fused rings of NFAs could inhibit the exciton decay and charge recombination but increase the rates of exciton diffusion and exciton dissociation for individual NFAs and the rates of electron separation for D/A interface. This work provides theoretical insights into structural design strategies, such as linear/angular fusion, and the number of fused rings of NFA, which presents a promising outlook for further enhancing PCE of high-performance A-D-A type NFAs.

A narrow band gap non-fullerene electron acceptor based on a dithieno-3,2-b:2',3'-dlpyrrole unit for high performance organic solar cells with minimal highest occupied molecular orbital offset

Here, a new narrow band gap non-fullerene small molecular acceptor (NFSMA) based on a dithieno-3,2-b:2',3'-dlpyrrole(DTP) unit, namely SNIC-F, was designed and synthesized. Due to the strong electron-donating ability of the DTP-based fused-ring core, SNIC-F showed a strong intramolecular-charge transfer (ICT) effect and thus gave a narrow band gap of 1.32 eV. Benefiting from the low band gap and efficient charge separation, when pairing with a copolymer PBTIBDTT, the device optimized by 0.5% 1-CN gave a high short circuit current (J_sc) of 19.64 mA cm^-2. In addition, a high open-circuit voltage (V_oc) of 0.83 V was obtained due to the near 0 eV highest occupied molecular orbital (HOMO) offset between PBTIBDTT and SNIC-F. As a result, a high power conversion efficiency (PCE) of 11.25% was obtained, and the PCE was maintained above 9.2% as the active layer thickness increased from 100 nm to 250 nm. Our work indicated that designing a narrow band gap NFSMA-based DTP unit and blending it with a polymer donor with small HOMO offset is an efficient strategy for achieving high performance OSCs.

Direct Observations of Uniform Bulk Heterojunctions and the Energy Level Alignments in Nonfullerene Organic Photovoltaic Active Layers

State-of-the-art organic photovoltaic (OPV) cells rely on the engineering of the energy levels of the organic molecules as well as the bulk-heterojunction nanomorphology to achieve high performance. However, both are difficult to measure inside the active layer where the electron donor and acceptor molecules are mingled. While the energy level alignments of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) between the electron donors and acceptors may be altered in the mixed active layer compared to their pure forms, the nanomorphology of the donor and acceptor molecular domains is mostly studied in indirect means. Here, we present the direct observations of the nanomorphology of the molecular domains as well as the energy level alignments in the active layer of a nonfullerene-based OPV (donor: PBDB-T-2F and acceptor: IT-4Cl) using cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). It is revealed that (1) the bulk-heterojunction (BHJ) structures are homogeneous and uniform throughout the ∼1.2 μm thick active layer; (2) the energy alignments between the donor-rich and acceptor-rich domains are directly observed; (3) there exist the intermixing domains at the boundaries of the donor-rich and acceptor-rich domains with thickness in the nm scale; (4) the exciton binding energies in PBDB-T-2F and IT-4Cl are estimated to be 0.74 and 0.32 eV, respectively; and (5) there is an ∼0.7 V loss in the open circuit voltage. The results provide a nanoscale understanding of the OPV active layers to guide further improvement of the OPV performance.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Advanced Acceptor-Substituted S,N-Heteropentacenes for Application in Organic Solar Cells

Ambifunctional heterpentacenes with the heteroatom sequence SSNSS in the ladder-type backbone were used either as donor or as nonfullerenic acceptor in solution-processed bulk-heterojunction solar cells. Different acceptor moieties and side chains were inserted. Synthesis and characterization of the systematically varied structural motifs provided insight in structure-property relationships. Moreover, a dimeric heteroacene was synthesized, and the optoelectronic properties were compared to those of its monomeric counterpart.

Synthesis and characterization of S,N-heterotetracenes

The synthesis and optoelectronic properties of novel S,N-heterotetracenes consisting of fused heterocyclic thiophene and pyrrole rings are presented. Tetracyclic and benzannulated derivatives with a varying number and sequence of sulfur and nitrogen heteroatoms were synthesized in multistep synthetic routes. A Buchwald-Hartwig amination of brominated precursors, thermolysis of azide precursors, and a Cadogan reaction of nitro-substituted precursors were successfully applied to eventually build-up pyrrole rings to stable and soluble fused systems. The various obtained heteroatom sequences 'SSNS' (SN4), 'SNNS' (SN4''), and 'NSSN' (SN4') allowed for evaluation of structure-property relationships relative to the sulfur analogue tetrathienoacene ('SSSS'). In line with the results for the whole series of S,N-heteroacenes, we find that replacement of sulfur by nitrogen atoms in the tetra- and hexacyclic systems leads to a red-shift in absorption, a decrease in oxidation potential and energy gap. On the other hand, the replacement of a thiophene ring by benzene leads to the opposite effects.

Electron-Deficient and Quinoid Central Unit Engineering for Unfused Ring-Based A1 -D-A2 -D-A1 -Type Acceptor Enables High Performance Nonfullerene Polymer Solar Cells with High Voc and PCE Simultaneously

Here, a pair of A₁ -D-A₂ -D-A₁ unfused ring core-based nonfullerene small molecule acceptors (NF-SMAs), BO2FIDT-4Cl and BT2FIDT-4Cl is synthesized, which possess the same terminals (A₁ ) and indacenodithiophene unit (D), coupling with different fluorinated electron-deficient central unit (difluorobenzoxadiazole or difluorobenzothiadiazole) (A₂ ). BT2FIDT-4Cl exhibits a slightly smaller optical bandgap of 1.56 eV, upshifted highest occupied molecular orbital energy levels, much higher electron mobility, and slightly enhanced molecular packing order in neat thin films than that of BO2FIDT-4Cl. The polymer solar cells (PSCs) based on BT2FIDT-4Cl:PM7 yield the best power conversion efficiency (PCE) of 12.5% with a V_oc of 0.97 V, which is higher than that of BO2FIDT-4Cl-based devices (PCE of 10.4%). The results demonstrate that the subtle modification of A₂ unit would result in lower trap-assisted recombination, more favorable morphology features, and more balanced electron and hole mobility in the PM7:BT2FIDT-4Cl blend films. It is worth mentioning that the PCE of 12.5% is the highest value in nonfused ring NF-SMA-based binary PSCs with high V_oc over 0.90 V. These results suggest that appropriate modulation of the quinoid electron-deficient central unit is an effective approach to construct highly efficient unfused ring NF-SMAs to boost PCE and V_oc simultaneously.