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.     

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%).     

Conformation‐Tuning Effect of Asymmetric Small Molecule Acceptors on Molecular Packing,Interaction, and Photovoltaic Performance

Understanding the conformation effect on molecular packing, miscibility, and photovoltaic performance is important to open a new avenue for small‐molecule acceptor (SMA) design. Herein, two novel acceptor–(donor‐acceptor1‐donor)–acceptor (A‐DA1D‐A)‐type asymmetric SMAs are developed, namely C‐shaped BDTP‐4F and S‐shaped BTDTP‐4F . The BDTP‐4F ‐based polymer solar cells (PSCs) with PM6 as donor, yields a power conversion efficiency (PCE) of 15.24%, significantly higher than that of the BTDTP‐4F ‐based device (13.12%). The better PCE for BDTP‐4F ‐based device is mainly attributed to more balanced charge transport, weaker bimolecular recombination, and more favorable morphology. Additionally, two traditional A‐D‐A‐type SMAs ( IDTP‐4F and IDTTP‐4F ) are also synthesized to investigate the conformation effect on morphology and device performance. Different from the device result above, here, IDTP‐4F with S‐shape conformation outperforms than IDTTP‐4F with C‐shape conformation. Importantly, it is found that for these two different types of SMA, the better performing binary blend has similar morphological characteristics. Specifically, both PM6:BDTP‐4F and PM6:IDTP‐4F blend exhibit perfect nanofibril network structure with proper domain size, obvious face‐on orientation and enhance donor‐acceptor interactions, thereby better device performance. This work indicates tuning molecular conformation plays pivotal role in morphology and device effciciency, shining a light on the molecular design of the SMAs.     

Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Organic solar cell optimization requires careful balancing of current–voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor–donor–acceptor (A–D–A)‐type small‐molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near‐IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT‐ and NFBDT‐based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge‐pair states is observed, followed by effective charge extraction. As a result, the PBDB‐T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%—among the best for solution‐processed organic solar cells.     

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.     

Design of a New Small‐Molecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor

Improving the fill factor (FF) is known as a challenging issue in organic solar cells (OSCs). Herein, a strategy of extending the conjugated area of end‐group is proposed for the molecular design of acceptor–donor–acceptor (A–D–A)‐type small molecule acceptor (SMA), and an indaceno[1,2‐b:5,6‐b′]dithiophene‐based SMA, namely IDTN, by end‐capping with the naphthyl fused 2‐(3‐oxocyclopentylidene)malononitrile is synthesized. Benefiting from the π‐conjugation extension by fusing two phenyls, IDTN shows stronger molecular aggregation, more ordered packing structure, thus over one order of magnitude higher electron mobility relative to its counterpart. By utilizing the fluorinated polymer (PBDB‐TF) as the electron donor, the corresponding device exhibits a high efficiency of 12.2% with a record‐high FF of 0.78, which is approaching the theoretical limit of OSCs. Compared with the reference molecule, such a high FF in the IDTN system can be mainly attributed to the more ordered π–π packing of acceptor aggregates, higher domain purity and symmetric carrier transport in the blend. Hence, enlarging the conjugated area of the terminal‐group in these A–D–A‐type SMAs is a promising approach not only for enhancing the electron mobility, but also for improving the blend morphology, and both of them are conducive to the fill‐factor breakthrough.     

Over 12% Efficiency Nonfullerene All‐Small‐Molecule Organic Solar Cells with Sequentially Evolved Multilength Scale Morphologies

In this paper, two near‐infrared absorbing molecules are successfully incorporated into nonfullerene‐based small‐molecule organic solar cells (NFSM‐OSCs) to achieve a very high power conversion efficiency (PCE) of 12.08%. This is achieved by tuning the sequentially evolved crystalline morphology through combined solvent additive and solvent vapor annealing, which mainly work on ZnP‐TBO and 6TIC, respectively. It not only helps improve the crystallinity of the ZnP‐TBO and 6TIC blend, but also forms multilength scale morphology to enhance charge mobility and charge extraction. Moreover, it simultaneously reduces the nongeminate recombination by effective charge delocalization. The resultant device performance shows remarkably enhanced fill factor and J_sc. These result in a very respectable PCE, which is the highest among all NFSM‐OSCs and all small‐molecule binary solar cells reported so far.     

13.7% Efficiency Small‐Molecule Solar Cells Enabled by a Combination of Material and Morphology Optimization

Compared with the quick development of polymer solar cells, achieving high‐efficiency small‐molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open‐circuit voltage of 0.85 V, a high short‐circuit current of 23.16 mA cm^?2 and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high‐performance SMSCs.     

Conjugation‐Curtailing of Benzodithionopyran‐Cored Molecular Acceptor Enables Efficient Air‐Processed Small Molecule Solar Cells

Small molecule solar cells (SMSCs) lag a long way behind polymer solar cells. A key limit is the less controllable morphology of small molecule materials, which can be aggravated when incorporating anisotropic nonfullerene acceptors. To fine‐tune the blending morphology within SMSCs, a π‐conjunction curtailing design is applied, which produces a efficient benzodithionopyran‐cored molecular acceptor for nonfullerene SMSCs (NF‐SMSCs). When blended with a molecular donor BDT3TR‐SF to fabricate NF‐SMSCs, the π‐conjunction curtailed molecular acceptor NBDTP‐M obtains an optimal power conversion efficiency (PCE) of up to 10.23%, which is much higher than that of NBDTTP‐M of longer π‐conjunction. It retains 93% of the PCE of devices fabricated in a glove box when all spin‐coating and post‐treating procedures are conducted in ambient air with relative humidity of 25%, which suggests the good air‐processing capability of π‐conjunction curtailed molecules. Detailed X‐ray scattering investigations indicate that the BDT3TR‐SF:NBDTP‐M blend exhibits a blend morphology featuring fine interpenetrating networks with smaller domains and higher phase purity, which results in more efficient charge generation, more balanced charge transport, and less recombination compared to the low‐performance BDT3TR‐SF:NBDTTP‐M blend. This work provides a guideline for molecular acceptors' design toward efficient, low‐cost, air‐processed NF‐SMSCs.     

Improved Performance of All‐Polymer Solar Cells Enabled by Naphthodiperylenetetraimide‐Based Polymer Acceptor

A new polymer acceptor, naphthodiperylenetetraimide‐vinylene (NDP‐V), featuring a backbone of altenating naphthodiperylenetetraimide and vinylene units is designed and applied in all‐polymer solar cells (all‐PSCs). With this polymer acceptor, a new record power‐conversion efficiencies (PCE) of 8.59% has been achieved for all‐PSCs. The design principle of NDP‐V is to reduce the conformational disorder in the backbone of a previously developed high‐performance acceptor, PDI‐V, a perylenediimide‐vinylene polymer. The chemical modifications result in favorable changes to the molecular packing behaviors of the acceptor and improved morphology of the donor–acceptor (PTB7‐Th:NDP‐V) blend, which is evidenced by the enhanced hole and electron transport abilities of the active layer. Moreover, the stronger absorption of NDP‐V in the shorter‐wavelength range offers a better complement to the donor. All these factors contribute to a short‐circuit current density (J _sc) of 17.07 mA cm^?2. With a fill factor (FF) of 0.67, an average PCE of 8.48% is obtained, representing the highest value thus far reported for all‐PSCs.     

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.     

Precisely Controlling the Position of Bromine on the End Group Enables Well-Regular Polymer Acceptors for All-Polymer Solar Cells with Efficiencies over 15%

Recent advances in the development of polymerized A–D–A-type small-molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio-isomeric brominated end groups (IC-Br(in) and IC-Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY-IT, PY-OT, and PY-IOT, are developed, where PY-IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY-OT, PY-IOT to PY-IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY-IT, contributing to the enhanced electron transport. Consequently, the PM6:PY-IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY-OT (10.04%) and PY-IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY-IT-based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all-PSCs.     

Efficient Nonfullerene Polymer Solar Cells Enabled by a Novel Wide Bandgap Small Molecular Acceptor

A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flaked with a 2,1,3‐benzothiadiazole (BT) and end‐capped with highly electron‐deficient (3‐ethylhexyl‐4‐oxothiazolidine‐2‐yl)dimalononitrile (RCN) units, has been successfully synthesized as a small molecular acceptor (SMA) for nonfullerene polymer solar cells (PSCs). This SMA exhibits a relatively wide optical bandgap of 2.03 eV, which provides a complementary absorption to commonly used low bandgap donor polymers, such as PTB7‐Th. The strong electron‐deficient BT and RCN units afford SFBRCN with a low‐lying LUMO (lowest unoccupied molecular orbital) level, while the 3D structured spirobifluorene core can effectively suppress the self‐aggregation tendency of the SMA, thus yielding a polymer:SMA blend with reasonably small domain size. As the results of such molecular design, SFBRCN enables nonfullerene PSCs with a high efficiency of 10.26%, which is the highest performance reported to date for a large bandgap nonfullerene SMA.