Novel Nitrogen-Containing Heterocyclic Non-Fullerene Acceptors for Organic PhotovoltaicCells: Different End-Capping Groups Leading to a Big Difference of Power Conversion Efficiencies

Theoretical designing of non-fullerene derived organic heterocyclic compounds with enhanced nonlinear optical amplitude: a DFT based prediction

In current era, non-fullerene (NF) chromophores have been reported as significant NLO materials due to promising optoelectronic properties. Therefore, a series of NF based chromophores abbreviated as TPBD2-TPBD6 with D–π–A architecture was designed from the reference compound (TPBR1) by its structural tailoring with an efficient donor and various acceptor groups for the first time. First, the structures of said compounds were optimized at M06-2X/6-311G (d,p) level. Further, the optimized structures were utilized to execute frontier molecular orbitals (FMOs), UV–Visible (UV–Vis) absorption, density of states (DOS) and transition density matrix (TDM) analyses at the same level to understand the non-linear (NLO) response of TPBR1 and TPBD2-TPBD6. Promising NLO results were achieved for all derivatives i.e., the highest amplitude of linear polarizability ⟨α⟩, first (βtotal) and second ($$\gamma$$ γ total) hyperpolarizabilities than their parent molecule. The compound TPBD3 was noted with the most significant NLO properties as compared to the standard molecule. The structural modeling approach by utilizing the acceptor molecules has played a prominent role in attaining favorable NLO responses in the molecules. Thus, our study has tempted the experimentalists to synthesize the proposed NLO materials for the modern optoelectronic high-tech applications.

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.

Non-Fullerene Acceptors with an Optical Response over 1000 nm toward Efficient Organic Solar Cells

Non-fullerene acceptors (NFAs) with near-infrared (NIR) absorption show promising advantages in organic solar cells (OSCs). However, only a few NFAs can extend the absorption spectra over 1000 nm, and their photovoltaic performance has been unsatisfactory so far. To address this issue, three new NFAs, namely, 6-IFIC, 6-IF2F, and 6-IF4F, were synthesized by simultaneously introducing π-bridge units and different end groups. The π-bridge unit enlarges the conjugation and planarizes the molecular geometry, leading to intense absorption in the NIR range. The asymmetric configuration provides a large dipole moment, enhances the intermolecular interaction, and tunes the miscibility, consequently being beneficial for achieving a favorable morphology in OSCs. When blended with a donor polymer PTB7-Th, the 6-IF2F-based OSC yields the best power conversion efficiency (PCE) of 11.20%, which is among the highest PCEs based on NFAs with absorption over 1000 nm. More importantly, the absorption of the blend film provides a transparency window in the visible range from 400 and 650 nm. Therefore, the semitransparent OSCs based on these three NFAs can achieve over 28% average visible transmittance.

Regioregular Narrow-Bandgap n-Type Polymers with High Electron Mobility Enabling Highly Efficient All-Polymer Solar Cells

Narrow-bandgap n-type polymers with high electron mobility are urgently demanded for the development of all-polymer solar cells (all-PSCs). Here, two regioregular narrow-bandgap polymer acceptors, L15 and MBTI, with two electron-deficient segments are synthesized by copolymerizing two dibrominated fused-ring electron acceptors (FREA) with distannylated aromatic imide, respectively. Taking full advantage of the FREA and the imide, both polymer acceptors show narrow bandgap and high electron mobility. Benefiting from the more extended absorption, better backbone ordering, and higher electron mobility than those of its regiorandom analog, the L15-based all-PSC yields a high power conversion efficiency (PCE) of 15.2% when blended with the polymer donor PM6. More importantly, MBTI incorporating a benzothiophene-core FREA segment shows relatively higher frontier molecular orbital levels than L15, forming a cascade-like energy level alignment with L15 and PM6. Based on this, ternary all-PSCs are designed where MBTI is introduced as a guest into the PM6:L15 host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 16.2%, which is among the highest values for all-PSCs. The results demonstrate that combining an FREA and an aromatic imide to construct regioregular narrow-bandgap polymer acceptors provides an effective approach to fabricate highly efficient all-PSCs.

Heptacyclic S,N-Heteroacene-Based Near-Infrared Nonfullerene Acceptor Enables High-Performance Organic Solar Cells with Small Highest Occupied Molecular Orbital Offsets

The reduction of energy offsets between donors and acceptors is a direct way to improve the open-circuit voltage (V_OC) and overall performance of organic solar cells (OSCs). In this work, two nonfullerene acceptors (NFAs) (BDTBO-4F and BDTBO-4Cl) were synthesized, which were composed of a heptacyclic S,N-heteroacene core and terminal units with halogen atoms, where the latter modulates the energy level of the frontier molecular orbital. Consequently, BDTBO-4Cl exhibited a deeper highest occupied molecular orbital level (E_HOMO) and lowest unoccupied molecular orbital level (E_LUMO) than BDTBO-4F. Moreover, these two NFAs exhibited high electron mobility and strong absorption at 700-900 nm. The polymer donor PM6 was combined with BDTBO-4F and BDTBO-4Cl, and the resulting OSCs exhibited outstanding power conversion efficiencies of 14.83% for the PM6:BDTBO-4F device and 13.87% for the PM6:BDTBO-4Cl device. More encouragingly, these OSCs exhibited efficient hole transfer from NFAs to PM6, despite small ΔE_HOMO(D-A) values (<0.10 eV). These results prove that modulation of E_HOMO of acceptors to decrease ΔE_HOMO(D-A) is an efficient strategy for high-performance OSCs.

Simple Near-Infrared Electron Acceptors for Efficient Photovoltaics and Sensitive Photodetectors

Although promising progress has been made in near-infrared (NIR) electron acceptors for broadening photoresponse of optoelectronics, there are still strong needs for efficient NIR materials with low synthetic complexities. In this work, three simple NIR acceptors are developed with absorption up to 1000 nm and possessing the same dithiophene cores with varied heteroatom linkages to carbon (C) atom for W1, to silicon (Si) for W2, and to nitrogen (N) for W3. It is found that the tuning of only one atom for simple acceptors can surprisingly lead to a large difference in photoelectric properties and solid stacking, as well as the performance in optoelectronics. Although quite simple, these electron acceptors, especially W1 (C), can also perform quite efficiently as organic photovoltaics (OPVs) as well as sensitive organic photodetectors (OPDs) when blended with PTB7-Th polymer. It is worthy to note that, among the representative NIR acceptors with over 950 nm absorption, W1 possesses one of the best figure-of-merit when considering the photoelectric performance versus synthetic complexity of materials. As a result, the PTB7-Th:W1-based OPDs reach a fast temporal response, ultralow-light intensity detection of 1.70 × 10^-11 W·cm^-2, and a high specific detectivity of 4.28 × 10¹² cm·Hz^1/2·W^-1 at 830 nm, representing a highly sensitive self-powered OPD approach the commercial broadband silicon detectors. These simple structure materials provide a potential example for further application of NIR electron acceptor.

Subtle Morphology Control with Binary Additives for High-Efficiency Non-Fullerene Acceptor Organic Solar Cells

Adding an additive is one of the effective strategies to fine-tune active layer morphology and improve performance of organic solar cells. In this work, a binary additive 1,8-diiodooctane (DIO) and 2,6-dimethoxynaphthalene (DMON) to optimize the morphology of PBDB-T:TTC8-O1-4F-based devices is reported. With the binary additive, a power conversion efficiency (PCE) of 13.22% was achieved, which is higher than those of devices using DIO (12.05%) or DMON (11.19%) individually. Comparison studies demonstrate that DIO can induce the acceptor TTC8-O1-4F to form ordered packing, while DMON can inhibit excessive aggregation of the donor and acceptor. With the synergistic effect of these two additives, the PBDB-T:TTC8-O1-4F blend film with DIO and DMON exhibits a suitable phase separation and crystallite size, leading to a high short-circuit current density (J_sc) of 23.04 mA·cm^-2 and a fill factor of 0.703 and thus improved PCE.