Balanced Partnership between Donor and Acceptor Components in Nonfullerene Organic Solar Cells with >12% Efficiency

Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near‐infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high‐performance fullerene‐free OSCs based on a combination of a medium‐bandgap polymer donor (FTAZ) and a narrow‐bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single‐junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.     

Design of a New Fused‐Ring Electron Acceptor with Excellent Compatibility to Wide‐Bandgap Polymer Donors for High‐Performance Organic Photovoltaics

Fused‐ring electron acceptors (FREAs) have recently received intensive attention. Besides the continuing development of new FREAs, the demand for FREAs featuring good compatibility to donor materials is becoming more and more urgent, which is highly desirable for screening donor materials and achieving new breakthroughs. In this work, a new FREA is developed, ZITI , featuring an octacyclic dithienocyclopentaindenoindene central core. The core is designed by linking 2,7‐dithienyl substituents and indenoindene with small methylene groups, in which the indeno[1,2‐b]thiophene‐2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile part provides a large and unoccupied π‐surface. Most notably, ZITI possesses an excellent compatibility with commercially available polymer donors, delivering very high power conversion efficiencies of over 13%.     

Highly Efficient Organic Solar Cells Consisting of Double Bulk Heterojunction Layers

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double‐BHJ OSCs is constructed via stamp transferring of low bandgap BHJ atop of mediate bandgap active layers. Such devices allow a large gain in photocurrent to be obtained due to enhanced photoharvest, without suffering much from the fill factor drop usually seen in thick‐layer‐based devices. Overall, double‐BHJ OSC with optimal ≈50 nm near‐infrared PDPP3T:PC₇₁BM layer atop of ≈200 nm PTB7‐Th:PC₇₁BM BHJ results in high power conversion efficiencies over 12%.     

Two Well‐Miscible Acceptors Work as One for Efficient Fullerene‐Free Organic Solar Cells

High‐performance ternary organic solar cells are fabricated by using a wide‐bandgap polymer donor (bithienyl‐benzodithiophene‐alt‐fluorobenzotriazole copolymer, J52) and two well‐miscible nonfullerene acceptors, methyl‐modified nonfullerene acceptor (IT‐M) and 2,2′‐((2Z ,2′Z )‐((5,5′‐(4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydros‐indaceno[1,2‐b :5,6‐b ′]dithiophene‐2,7‐diyl)bis(4‐((2‐ethylhexyl)oxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H ‐indene‐2,1‐diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short‐circuit current density of 19.7 mA cm^?2 and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.     

Hidden Structure Ordering Along Backbone of Fused‐Ring Electron Acceptors Enhanced by Ternary Bulk Heterojunction

Fused‐ring electron acceptors (FREAs), as a family of non‐fullerene (NF) acceptors, have achieved tremendous success in pushing the power conversion efficiency of organic solar cells. Here, the detailed molecular packing motifs of two extensively studied FREAs—ITIC and ITIC‐Th are reported. It is revealed for the first time the long‐range structure ordering along the backbone direction originated from favored end group π–π stacking. The backbone ordering could be significantly enhanced in the ternary film by the mutual mixing of ITIC and ITIC‐Th, which gives rise to an improved in‐plane electron mobility and better ternary device performance. The backbone ordering might be a common morphological feature of FREAs, providing explanations to previously observed small open circuit voltage loss and superior performance of FREA‐based devices and guiding the future molecular design of high‐performance NF acceptors.     

Over 14% Efficiency in Organic Solar Cells Enabled by Chlorinated Nonfullerene Small‐Molecule Acceptors

To make organic solar cells (OSCs) more competitive in the diverse photovoltaic cell technologies, it is very important to demonstrate that OSCs can achieve very good efficiencies and that their cost can be reduced. Here, a pair of nonfullerene small‐molecule acceptors, IT‐2Cl and IT‐4Cl, is designed and synthesized by introducing easy‐synthesis chlorine substituents onto the indacenodithieno[3,2‐b]thiophene units. The unique feature of the large dipole moment of the C? Cl bond enhances the intermolecular charge‐transfer effect between the donor–acceptor structures, and thus expands the absorption and down shifts the molecular energy levels. Meanwhile, the introduction of C? Cl also causes more pronounced molecular stacking, which also helps to expand the absorption spectrum. Both of the designed OSCs devices based on two acceptors can deliver a power conversion efficiency (PCE) greater than 13% when blended with a polymer donor with a low‐lying highest occupied molecular orbital level. In addition, since IT‐2Cl and IT‐4Cl have very good compatibility, a ternary OSC device integrating these two acceptors is also fabricated and obtains a PCE greater than 14%. Chlorination demonstrates effective ability in enhancing the device performance and facile synthesis route, which both deserve further exploitation in the modification of photovoltaic materials.     

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.     

Over 15.7% Efficiency of Ternary Organic Solar Cells by Employing Two Compatible Acceptors with Similar LUMO Levels

Efficient organic solar cells (OSCs) are fabricated using polymer PM6 as donor, and IPTBO‐4Cl and MF1 as acceptors. The power conversion efficiency (PCE) of IPTBO‐4Cl based and MF1 based binary OSCs individually arrive to 14.94% and 12.07%, exhibiting markedly different short circuit current density (J_SC) of 23.18 mA cm^?2 versus 17.01 mA cm^?2, fill factor (FF) of 72.17% versus 78.18% and similar open circuit voltage (V_OC) of 0.893 V versus 0.908 V. The two acceptors, IPTBO‐4Cl and MF1, have similar lowest unoccupied molecular orbital levels, which is beneficial for efficient electron transport in the ternary active layer. The PCE of optimized ternary OSCs arrives to 15.74% by incorporating 30 wt% MF1 in acceptors, resulting from the simultaneously increased J_SC of 23.20 mA cm^?2, V_OC of 0.897 V, and FF of 75.64% in comparison with IPTBO‐4Cl based binary OSCs. The gradually increased FFs of ternary OSCs indicate the well‐optimized phase separation and molecular arrangement with MF1 as morphology regulator. This work may provide a new viewpoint for selecting an appropriate third component to achieve efficient ternary OSCs from materials and photovoltaic parameters of two binary OSCs.     

Donor Polymer Can Assist Electron Transport in Bulk Heterojunction Blends with Small Energetic Offsets

Conventional organic solar cell (OSC) systems have significant energy offsets between the donor and acceptor both at the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Because of this, in a bulk heterojunction (BHJ) system, electrons typically transport in acceptors, whereas holes typically transport in donors. It is not favorable for electrons to hop back and forth between the donor and acceptor because the hopping is energetically disfavored. In such conventional OSC systems, the addition of donor polymer to acceptor films should typically reduce the electron mobility. In this study, a surprisingly large increase (up to 30×) in electron mobility is observed in an OSC blend when introducing a polymer donor into small molecular acceptor. By ruling out morphology reasons, it is shown that the donor polymer can assist the electron transport by providing “bridges” or a “shortcut” for electron transport across the domains of small molecular acceptors. This can happen because, for these systems, the LUMO offset is small. The study shows the benefits of donor‐assisted electron transport in BHJ systems with small energetic offsets. This finding could be also applied to other fields to tune the optimized charge transport property of organic materials or slush blends.     

Single‐Junction Binary‐Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency

A new fluorinated nonfullerene acceptor, ITIC‐Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end‐capping group 1,1‐dicyanomethylene‐3‐indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2‐b]thiophene and the acceptor unit IC due to electron‐withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short‐circuit current density (J _SC). On the other hand, incorporation of F would improve intermolecular interactions through C? F···S, C? F···H, and C? F···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing J _SC and fill factor. Indeed, the results show that fluorinated ITIC‐Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC‐Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC‐Th1 electron acceptor and a wide‐bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC‐Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene‐based single‐junction binary‐blend OSCs. Moreover, the OSCs based on FTAZ:ITIC‐Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC₇₁BM (PCE = 5.22%).     

Low-Bandgap Porphyrins for Highly Efficient Organic Solar Cells: Materials,Morphology, and Applications

With developments in materials, thin-film processing, fine-tuning of morphology, and optimization of device fabrication, the performance of organic solar cells (OSCs) has improved markedly in recent years. Designing low-bandgap materials has been a focus in order to maximize solar energy conversion. However, there are only a few successful low-bandgap donor materials developed with near-infrared (NIR) absorption that are well matched to the existing efficient acceptors. Porphyrin has shown great potential as a useful building block for constructing low-bandgap donor materials due to its large conjugated plane and strong absorption. Porphyrin-based donor materials have been shown to contribute to many record-high device efficiencies in small molecule, tandem, ternary, flexible, and OSC/perovskite hybrid solar cells. Specifically, non-fullerene small-molecule solar cells have recently shown a high power conversion efficiency of 12% using low-bandgap porphyrin. All these have validated the great potential of porphyrin derivatives as effective donor materials and made DPPEZnP-TRs a family of best low-bandgap donor materials in the OSC field so far. Here, recent progress in the rational design, morphology, dynamics, and multi-functional applications starting from 2015 will be highlighted to deepen understanding of the structure–property relationship. Finally, some future directions of porphyrin-based OSCs are presented.     

Chlorinated Narrow Bandgap Polymer Suppresses Non-Radiative Recombination Energy Loss Enabling Perylene Diimides-Based Organic Solar Cells Exceeding 10% Efficiency

The scarcity of narrow bandgap donor polymers matched with perylene diimides (PDI)-based nonfullerene acceptors (NFAs) hinders improvement of the power conversion efficiency (PCE) value of organic solar cells (OSCs). Here, it is reported that a narrow bandgap donor polymer PDX, the chlorinated derivative of the famous polymer donor PTB7-Th, blended with PDI-based NFA boosts the PCE value exceeding 10%. The electroluminescent quantum efficiency of PDX-based OSCs is two orders of magnitude higher than that of PTB7-Th-based OSCs;therefore, the nonradiative energy loss is 0.103 eV lower. This is the highest PCE value for OSCs with the lowest energy loss using the blend of PTB7-Th derivatives and PDI-based NFAs as the active layer. Besides, PDX-based devices showed larger phase separation, faster charge mobilities, higher exciton dissociation probability, suppressed charge recombination, elevated charge transfer state, and decreased energetic disorder compared with the PTB7-Th-based OSCs. All these factors contribute to the simultaneously improved short circuit current density, open circuit voltage, and fill factor, thus significantly improving PCE. These results prove that chlorinated conjugated side thienyl groups can efficiently suppress the non-radiative energy loss and highlight the importance of fine-modifying or developing novel narrow bandgap polymers to further elevate the PCE value of PDI-based OSCs.     

Naphthodithiophene‐Based Nonfullerene Acceptor for High‐Performance Organic Photovoltaics: Effect of Extended Conjugation

Naphtho[1,2‐b:5,6‐b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron‐withdrawing 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile to yield a fused‐ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene‐based IHIC2, naphthodithiophene‐based IOIC2 with a larger π‐conjugation and a stronger electron‐donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: ?3.78 eV vs IHIC2: ?3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10^?3 cm² V^?1 s^?1 vs IHIC2: 5.0 × 10^?4 cm² V^?1 s^?1). Thus, IOIC2‐based OSCs show higher values in open‐circuit voltage, short‐circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2‐based counterpart. In particular, as‐cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8‐diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2 ‐ based devices, higher than that of the FTAZ:IHIC2 ‐ based devices (7.31%). These results indicate that incorporating extended conjugation into the electron‐donating fused‐ring units in nonfullerene acceptors is a promising strategy for designing high‐performance electron acceptors.     

A Narrow-Bandgap n-Type Polymer with an Acceptor–Acceptor Backbone Enabling Efficient All-Polymer Solar Cells

Narrow-bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow-bandgap polymer acceptor L14, featuring an acceptor–acceptor (A–A) type backbone, is synthesized by copolymerizing a dibrominated fused-ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A–A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low-lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open-circuit voltage (V_oc), which is attributed to a small nonradiative recombination loss (E_loss,nr) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and V_oc, an excellent efficiency of 14.3% is achieved, which is among the highest values for all-polymer solar cells (all-PSCs). The results demonstrate the superiority of narrow-bandgap A–A type polymers for improving all-PSC performance and pave a way toward developing high-performance polymer acceptors for all-PSCs.     

Fused Hexacyclic Nonfullerene Acceptor with Strong Near‐Infrared Absorption for Semitransparent Organic Solar Cells with 9.77% Efficiency

A fused hexacyclic electron acceptor, IHIC, based on strong electron‐donating group dithienocyclopentathieno[3,2‐b ]thiophene flanked by strong electron‐withdrawing group 1,1‐dicyanomethylene‐3‐indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST‐OSCs). IHIC exhibits strong near‐infrared absorption with extinction coefficients of up to 1.6 × 10⁵m ^?1 cm^?1, a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10^?3 cm² V^?1 s^?1. The ST‐OSCs based on blends of a narrow‐bandgap polymer donor PTB7‐Th and narrow‐bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single‐junction and tandem ST‐OSCs reported in the literature.     

Ternary Organic Solar Cells with Efficiency >16.5% Based on Two Compatible Nonfullerene Acceptors

A ternary structure has been demonstrated as being an effective strategy to realize high power conversion efficiency (PCE) in organic solar cells (OSCs); however, general materials selection rules still remain incompletely understood. In this work, two nonfullerene small‐molecule acceptors 3TP3T‐4F and 3TP3T‐IC are synthesized and incorporated as a third component in PM6:Y6 binary blends. The photovoltaic behaviors in the resultant ternary OSCs differ significantly, despite the comparable energy levels. It is found that incorporation of 15% 3TP3T‐4F into the PM6:Y6 blend results in facilitating exciton dissociation, increasing charge transport, and reducing trap‐assisted recombination. All these features are responsible for the enlarged PCE of 16.7% (certified as 16.2%) in the PM6:Y6:3TP3T‐4F ternary OSCs, higher than that (15.6%) in the 3TP3T‐IC containing ternary devices. The performance differences are mainly ascribed to the compatibility between the third component and the host materials. The 3TP3T‐4F guest acceptor exhibits an excellent compatibility with Y6, tending to form well‐mixed phases in the ternary blend without disrupting the favored bicontinuous transport networks, whereas 3TP3T‐IC displays a morphological incompatibility with Y6. This work highlights the importance of considering the compatibility for materials selection toward high‐efficiency ternary organic OSCs.     

Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale‐up. Despite the rapid increase in power conversion efficiency (PCE) of spin‐coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high‐performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT‐M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin‐coated FTAZ:IT‐M devices using a single halogen‐free solvent. More importantly, chlorine‐free, blade coating of FTAZ:IT‐M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X‐ray scattering results reveal that large π–π coherence length, high degree of face‐on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low‐cost, low‐toxicity, and high‐efficiency OSCs by high‐throughput production.     

Highly Efficient Ternary‐Blend Polymer Solar Cells Enabled by a Nonfullerene Acceptor and Two Polymer Donors with a Broad Composition Tolerance

In this work, highly efficient ternary‐blend organic solar cells (TB‐OSCs) are reported based on a low‐bandgap copolymer of PTB7‐Th, a medium‐bandgap copolymer of PBDB‐T, and a wide‐bandgap small molecule of SFBRCN. The ternary‐blend layer exhibits a good complementary absorption in the range of 300–800 nm, in which PTB7‐Th and PBDB‐T have excellent miscibility with each other and a desirable phase separation with SFBRCN. In such devices, there exist multiple energy transfer pathways from PBDB‐T to PTB7‐Th, and from SFBRCN to the above two polymer donors. The hole‐back transfer from PTB7‐Th to PBDB‐T and multiple electron transfers between the acceptor and the donor materials are also observed for elevating the whole device performance. After systematically optimizing the weight ratio of PBDB‐T:PTB7‐Th:SFBRCN, a champion power conversion efficiency (PCE) of 12.27% is finally achieved with an open‐circuit voltage (V _oc) of 0.93 V, a short‐circuit current density (J _sc) of 17.86 mA cm^?2, and a fill factor of 73.9%, which is the highest value for the ternary OSCs reported so far. Importantly, the TB‐OSCs exhibit a broad composition tolerance with a high PCE over 10% throughout the whole blend ratios.     

Optimized Fibril Network Morphology by Precise Side‐Chain Engineering to Achieve High‐Performance Bulk‐Heterojunction Organic Solar Cells

A polymer fibril assembly can dictate the morphology framework, in forming a network structure, which is highly advantageous in bulk heterojunction (BHJ) organic solar cells (OSCs). A fundamental understanding of how to manipulate such a fibril assembly and its influence on the BHJ morphology and device performance is crucially important. Here, a series of donor–acceptor polymers, PBT1‐O, PBT1‐S, and PBT1‐C, is used to systematically investigate the relationship between molecular structure, morphology, and photovoltaic performance. The subtle atom change in side chains is found to have profound effect on regulating electronic structure and self‐assembly of conjugated polymers. Compared with PBT1‐O and PBT1‐S, PBT1‐C‐based OSCs show much higher photovoltaic performance with a record fill factor (FF) of 80.5%, due to the formation of optimal interpenetrating network morphology. Such a fibril network strategy is further extended to nonfullerene OSCs using a small‐molecular acceptor, which shows a high efficiency of 12.7% and an FF of 78.5%. The results indicate the formation of well‐defined fibrillar structure is a promising approach to achieving a favorable morphology in BHJ 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.