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.     

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%).     

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.     

Fused‐Ring Acceptors with Asymmetric Side Chains for High‐Performance Thick‐Film Organic Solar Cells

A kind of new fused‐ring electron acceptor, IDT‐OB, bearing asymmetric side chains, is synthesized for high‐efficiency thick‐film organic solar cells. The introduction of asymmetric side chains can increase the solubility of acceptor molecules, enable the acceptor molecules to pack closely in a dislocated way, and form favorable phase separation when blended with PBDB‐T. As expected, PBDB‐T:IDT‐OB‐based devices exhibit high and balanced hole and electron mobility and give a high power conversion efficiency (PCE) of 10.12%. More importantly, the IDT‐OB‐based devices are not very sensitive to the film thickness, a PCE of 9.17% can still be obtained even the thickness of active layer is up to 210 nm.     

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.     

Enhancing the Performance of Polymer Solar Cells via Core Engineering of NIR‐Absorbing Electron Acceptors

In order to utilize the near‐infrared (NIR) solar photons like silicon‐based solar cells, extensive research efforts have been devoted to the development of organic donor and acceptor materials with strong NIR absorption. However, single‐junction organic solar cells (OSCs) with photoresponse extending into >1000 nm and power conversion efficiency (PCE) >11% have rarely been reported. Herein, three fused‐ring electron acceptors with varying core size are reported. These three molecules exhibit strong absorption from 600 to 1000 nm and high electron mobility (>1 × 10^?3 cm² V^?1 s^?1). It is proposed that core engineering is a promising approach to elevate energy levels, enhance absorption and electron mobility, and finally achieve high device performance. This approach can maximize both short‐circuit current density ( J_SC) and open‐circuit voltage (V_OC) at the same time, differing from the commonly used end group engineering that is generally unable to realize simultaneous enhancement in both V_OC and J_SC. Finally, the single‐junction OSCs based on these acceptors in combination with the widely polymer donor PTB7‐Th yield J_SC as high as 26.00 mA cm^?2 and PCE as high as 12.3%.     

Carbon–Oxygen‐Bridged Ladder‐Type Building Blocks for Highly Efficient Nonfullerene Acceptors

Recently, acceptor–donor–acceptor (A–D–A) small molecules have emerged as promising nonfullerene acceptors (NFAs) for organic solar cells and have attracted great attention. The carbon‐bridged (C‐bridged) ladder‐type D unit plays a crucial role in developing high‐performance A–D–A NFAs. However, the medium electron‐donating capability of C‐bridged units is unfavorable for making NFAs with strong light‐harvesting capability. In this regard, carbon–oxygen‐bridged (CO‐bridged) ladder‐type units present advantages in developing strong light‐absorbing NFAs. Here, recent progress in the newly emerging CO‐bridged NFAs is highlighted. The synthetic methods for the polycyclic CO‐bridged building blocks are introduced. The photovoltaic performance for CO‐bridged NFAs is summarized and discussed. Perspectives on developing high‐performance CO‐bridged‐NFA‐based solar cells are made.     

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.     

High‐Performance Additive‐/Post‐Treatment‐Free Nonfullerene Polymer Solar Cells via Tuning Molecular Weight of Conjugated Polymers

In recent years, rapid advances are achieved in polymer solar cells (PSCs) using nonfullerene small molecular acceptors. However, no research disclosing the influence of molecular weight (M_n) of conjugated polymer on the nonfullerene device performance is reported. In this work, a series of polymers with different M_ns are synthesized to systematically investigate the connection between M_n and performance of nonfullerene devices for the first time. It is found that the device performance improves substantially as the M_n increases from 12 to 38 kDa and a power conversion efficiency (PCE) as high as 10.5% is realized. It has to be noted this PCE is achieved without using any additives and post‐treatments, which is among the top efficiencies of additive‐ and post‐treatment‐free PSCs. Most importantly, the variation trend of the optimal active layer thickness and morphology is significantly different from the device with fullerene as acceptor. The findings clarify the effect of M_n on the performance of nonfullerene PSCs, which would benefit further efficiency improvement of nonfullerene PSCs.     

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.     

Nonfullerene Tandem Organic Solar Cells with High Performance of 14.11%

Fabricating solar cells with tandem structure is an efficient way to broaden the photon response range without further increasing the thermalization loss in the system. In this work, a tandem organic solar cell (TOSC) based on highly efficient nonfullerene acceptors (NFAs) with series connection type is demonstrated. To meet the different demands of front and rear sub‐cells, two NFAs named F‐M and NOBDT with a whole absorption range from 300 to 900 nm are designed, when blended with wide bandgap polymer 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) and narrow bandgap polymer PTB7‐Th, respectively, the PBDB‐T: F‐M system exhibits a high V_oc of 0.98 V and the PTB7‐Th: NOBDT system shows a remarkable J_sc of 19.16 mA cm^?2, which demonstrate their potential in the TOSCs. With the guidance of optical simulation, by systematically optimizing the thickness of each layer in the TOSC, an outstanding power conversion efficiency of 14.11%, with a V_oc of 1.71 V, a J_sc of 11.72 mA cm^?2, and a satisfactory fill factor of 0.70 is achieved; this result is one of the top efficiencies reported to date in the field of organic solar cells.     

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.     

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.     

High‐Performance Nonfullerene Polymer Solar Cells based on Imide‐Functionalized Wide‐Bandgap Polymers

High‐performance nonfullerene polymer solar cells (PSCs) are developed by integrating the nonfullerene electron‐accepting material 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophne) (ITIC) with a wide‐bandgap electron‐donating polymer PTzBI or PTzBI‐DT, which consists of an imide functionalized benzotriazole (TzBI) building block. Detailed investigations reveal that the extension of conjugation can affect the optical and electronic properties, molecular aggregation properties, charge separation in the bulk‐heterojunction films, and thus the overall photovoltaic performances. Single‐junction PSCs based on PTzBI:ITIC and PTzBI‐DT:ITIC exhibit remarkable power conversion efficiencies (PCEs) of 10.24% and 9.43%, respectively. To our knowledge, these PCEs are the highest efficiency values obtained based on electron‐donating conjugated polymers consisting of imide‐functionalized electron‐withdrawing building blocks. Of particular interest is that the resulting device based on PTzBI exhibits remarkable PCE of 7% with the thickness of active layer of 300 nm, which is among the highest values of nonfullerene PSCs utilizing thick photoactive layer. Additionally, the device based on PTzBI:ITIC exhibits prominent stability, for which the PCE remains as 9.34% after thermal annealing at 130 °C for 120 min. These findings demonstrate the great promise of using this series of wide‐bandgap conjugated polymers as electron‐donating materials for high‐performance nonfullerene solar cells toward high‐throughput roll‐to‐roll processing technology.