Shuixing Li

A spirobifluorene and diketopyrrolopyrrole moieties based non-fullerene acceptor for efficient and thermally stable polymer solar cells with high open-circuit voltage

In this study, we design and synthesize a new non-fullerene electron acceptor, SF(DPPB)4, in which a spirobifluorene (SF) core is installed with four benzene endcapped diketopyrrolopyrrole (DPP) arms. SF(DPPB)4 exhibits energy levels matching perfectly with those of the commonly used poly(3-hexyl thiophene) (P3HT) donor in polymer solar cells (PSCs). Furthermore, a designed cross-shaped molecular geometry helps in suppressing strong intermolecular aggregation in the P3HT : SF(DPPB)4 blend, leading to efficient non-fullerene PSCs. The resultant devices give a maximum power conversion efficiency (PCE) of 5.16% with an extremely high open-circuit voltage (Voc) of 1.14 V. In contrast, the devices based on P3HT : PC61BM blends provide a PCE of 3.18% with a Voc of 0.62 V. Finally, we observe that the P3HT : SF(DPPB)4 devices exhibit significantly improved thermal stability from that of the P3HT : PC61BM devices; upon thermal treatment at 150 °C for 3 h, the PCEs of P3HT : SF(DPPB)4 devices remain unchanged, whereas those of the P3HT : PC61BM devices drop drastically to below 1%. The abovementioned results demonstrate that the new design strategy of employing a high-performance non-fullerene acceptor, SF(DPPB)4, is promising for the future practical application of PSCs.

A simple perylene diimide derivative with a highly twisted geometry as an electron acceptor for efficient organic solar cells

Perylene diimide (PDI), which features intense absorption, a low-lying energy level, and high electron mobility, is a promising building block for electron acceptors in organic solar cells (OSCs). However, this planar molecule has a strong tendency to form large aggregates during film formation which strongly limits its OSC performance. Herein, we report a new and simple PDI derivative, B(PDI)3, in which a central benzene unit is employed to connect three PDI arms. This compact arrangement of sterically bulky PDI moieties leads to a twisted molecular geometry of the resultant structure. This suppresses the strong crystallization tendency of PDI chromophores, owing to the broken molecular coplanarity and symmetry. Therefore, B(PDI)3 is applied as a non-fullerene acceptor in OSCs, providing a good power conversion efficiency of 5.65% when blended with the PTB7-Th donor.

Nonfullerene Tandem Organic Solar Cells with High Open‐Circuit Voltage of 1.97 V

Small-molecule nonfullerene-based tandem organic solar cells (OSCs) are fabricated for the first time by utilizing P3HT:SF(DPPB)4 and PTB7-Th:IEIC bulk heterojunctions as the front and back subcells, respectively. A power conversion efficiency of 8.48% is achieved with an ultrahigh open-circuit voltage of 1.97 V, which is the highest voltage value reported to date among efficient tandem OSCs.

Efficient Organic Solar Cells with Non‐Fullerene Acceptors

Fullerene-free OSCs employing n-type small molecules or polymers as the acceptors have recently experienced a rapid rise with efficiencies exceeding 12%. Owing to the good optoelectronic and morphological tunabilities, non-fullerene acceptors exhibit great potential for realizing high-performance and practical OSCs. In this Review, recent exciting progress made in developing highly efficient non-fullerene acceptors is summarized, mainly correlating factors like absorption, energy loss and morphology of new materials to their correspondent photovoltaic performance.

A non-fullerene acceptor with a fully fused backbone for efficient polymer solar cells with a high open-circuit voltage

Herein, we design and synthesize a perylene diimide derivative with a fully fused backbone, FITP, which possesses an elevated lowest unoccupied molecular orbital level and high electron mobility. Consequently, polymer solar cells with FITP as the acceptor can provide the best efficiency of 7.33% with a high voltage of 0.99 V.

A non-fullerene electron acceptor modified by thiophene-2-carbonitrile for solution-processed organic solar cells

Effective electron acceptor materials usually have a deep lowest unoccupied molecular orbital (LUMO) energy level that can split excitons and generate current. A non-fullerene electron acceptor (F8-DPPTCN) was developed, using fluorene as the core with arms of diketopyrrolopyrrole (DPP) having thiophene-2-carbonitrile as the terminal units. The new molecule had a LUMO of −3.65 eV and a narrow bandgap (Eg) of 1.66 eV, owing to the electronegativity of the thiophene-2-carbonitrile group and its conjugation with DPP units. Organic solar cells (OSCs) with F8-DPPTCN as the acceptor and poly(3-hexylthiophene) (P3HT) as the donor were fabricated. A power conversion efficiency (PCE) of 2.37% was obtained with an open-circuit voltage (Voc) of 0.97 V, a short-circuit current (Jsc) of 6.25 mA cm−2, and a fill factor (FF) of 0.39. Structural characterization showed that P3HT and F8-DPPTCN were kinetically trapped in a weakly separated state whereas thermal annealing led to the crystallization of P3HT and the formation of a network structure with a mesh-size of several hundred nanometers. When a solvent additive, diiodooctane, was used and the mixture was thermally annealed, both P3HT and F8-DPPTCN crystallized and a multi-length scale network was formed. Though the PCEs were low, the changes in the PCE could be correlated with the morphological changes, opening pathways to increase performance further.

An Unfused‐Core‐Based Nonfullerene Acceptor Enables High‐Efficiency Organic Solar Cells with Excellent Morphological Stability at High Temperatures

Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.

A Near‐Infrared Photoactive Morphology Modifier Leads to Significant Current Improvement and Energy Loss Mitigation for Ternary Organic Solar Cells

Abstract Herein, efficient organic solar cells (OSCs) are realized with the ternary blend of a medium band gap donor (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PBDB‐T)) with a low band gap acceptor (2,2′‐((2Z,2′Z)‐(((2,5‐difluoro‐1,4‐phenylene)bis(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐6,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (HF‐PCIC)) and a near‐infrared acceptor (2,2′‐((2Z,2′Z)‐(((4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(4‐((2‐ethylhexyl)oxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (IEICO‐4F)). It is shown that the introduction of IEICO‐4F third component into PBDB‐T:HF‐PCIC blend increases the short‐circuit current density (J sc) of the ternary OSC to 23.46 mA cm−2, with a 44% increment over those of binary devices. The significant current improvement originates from the broadened absorption range and the active layer morphology optimization through the introduction of IEICO‐4F component. Furthermore, the energy loss of the ternary cells (0.59 eV) is much decreased over that of the binary cells (0.80 eV) due to the reduction of both radiative recombination from the absorption below the band gap and nonradiative recombination upon the addition of IEICO‐4F. Therefore, the power conversion efficiency increases dramatically from 8.82% for the binary cells to 11.20% for the ternary cells. This work provides good examples for simultaneously achieving both significant current enhancement and energy loss mitigation in OSCs, which would lead to the further construction of highly efficient ternary OSCs.

Enhancement of intra- and inter-molecular π-conjugated effects for a non-fullerene acceptor to achieve high-efficiency organic solar cells with an extended photoresponse range and optimized morphology

In this work, a new A–D–A type non-fullerene electron acceptor, DF-PCNC, which possesses an electron-donating (D) core constructed by linking a 2,5-difluorobenzene ring with two cyclopentadithiophene moieties and two electron-accepting (A) end-groups of 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (NC), is designed and synthesized. Because of the extension of the π-conjugation system, DF-PCNC shows stronger and more red-shifted absorption peaks while compared to those of its counterpart, DF-PCIC, which has the same D core but smaller A terminals of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC). Furthermore, NC groups can enhance the intermolecular π–π stacking of DF-PCNC in the condensed state. Thus, when it is blended with a polymer donor, PBDB-T, to fabricate organic solar cells (OSCs), good morphologies of the blended films are achieved through appropriate optimizations: both donor and acceptor form highly crystalline phase-separation domains with appropriate nanoscaled sizes, which is beneficial to charge generation and transport in OSCs. As a result, the short-circuit current density (JSC) of the PBDB-T:DF-PCNC device is increased by 16% compared with that of the PBDB-T:DF-PCIC one, and a high fill factor (FF) of 72.62% is maintained, leading to a better power conversion efficiency (PCE) of 11.63%, which is the highest value for OSCs based on non-fullerene acceptors adopting decreased fused-ring D cores to date.