Effect of Ring‐Fusion on Miscibility and Domain Purity: Key Factors Determining the Performance of PDI‐Based Nonfullerene Organic Solar Cells

Compared to the rapid development of nonfullerene organic solar cells (OSCs) based on the state‐of‐the‐art indacenodithiophene (IDT)‐based small molecule acceptors (SMAs), the progress for perylene diimide (PDI)‐based electron acceptors has lagged behind owing to the lack of understanding on the structure–morphology–performance relationship of PDI SMAs. Given the ease of synthesis for PDIs and their high intrinsic electron mobility, it is crucial to identify key material parameters that influence the polymer:PDI blend morphology and to develop rational approaches for molecular design toward high‐performance PDI‐based SMAs. In this study, three pairs of PDI‐based SMAs with and without ring‐fusion are investigated and it is found that ring‐fusion and domain purity are the key structural and morphological factors determining the fill factors (FFs) and efficiencies of PDI‐based nonfullerene OSCs. This data shows that nonfullerene OSCs based on the ring‐fused PDI‐based SMAs exhibit much higher average domain purity and thus increased charge mobilities, which lead to enhanced FFs compared to those solar cells based on nonfused PDIs. This is explained by higher Florry Huggins interaction parameters as observed by melting point depression measurements. This study suggests that increasing repulsive molecular interactions to lower the miscibility between the polymer donor and PDI acceptor is the key to improve the FF and performance of PDI‐based devices.     

Bis(Naphthalene Imide)diphenylanthrazolines: A New Class of Electron Acceptors for Efficient Nonfullerene Organic Solar Cells and Applicable to Multiple Donor Polymers

Unlike universally applicable fullerene derivatives, current nonfullerene electron acceptors are rarely effective with more than one donor polymer in bulk heterojunction (BHJ) solar cells. A novel class of nonfullerene electron acceptors, bis(naphthalene imide)‐3,6‐diphenyl‐trans‐anthrazolines (BNIDPAs), that is applicable and yields efficient photovoltaic devices with multiple donor polymers, including a thiazolothiazole–dithienosilole copolymer (PSEHTT) and benzodithiophene copolymers (PBDTT‐FTTE and PTB7) is reported. Photovoltaic devices composed of the BNIDPA‐butyloctyl (BO) acceptor with PSEHTT, PBDTT‐FTTE, and PTB7, respectively, have power conversion efficiencies of 3.0%–3.1% with high open‐circuit voltages of ≈1.0 V. In contrast, BHJ devices composed of BNIDPA‐DT acceptor with larger 2‐decyltetradecyl chains and the same donor polymers have substantially reduced bulk electron mobility and reduced photovoltaic efficiencies of 1.3%–1.7%, which highlight the critical role of the size of alkyl chains appended onto nonfullerene electron acceptors. The present results provide a rare example of nonfullerene electron acceptors that are capable of pairing with multiple donor polymers to achieve efficient BHJ solar cells.     

Significant Influence of the Methoxyl Substitution Position on Optoelectronic Properties and Molecular Packing of Small‐Molecule Electron Acceptors for Photovoltaic Cells

Molecular engineering of nonfullerene electron acceptors is of great importance for the development of organic photovoltaics. In this study, a series of methoxyl‐modified dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐based small‐molecule acceptor (SMA) isomers are synthesized and characterized to determine the effect of substitution position of the terminal group in these acceptor–donor–acceptor‐type SMAs. Minor changes in the substitution position are demonstrated to greatly influence the optoelectronic properties and molecular packing of the isomers. Note that SMAs with planar molecular backbones show more ordered molecular packing and smaller π–π stacking distances, thus dramatically higher electron mobilities relative to their counterparts with distorted end‐groups. By utilizing polymer poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophen)‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione)] (PBDB‐T) as an electron donor, an optimum power conversion efficiency (PCE) of 11.9% is achieved in the device based on PBDB‐T:IT‐OM‐2, which is among the top efficiencies reported as of yet. Moreover, the PCE stays above 10% as the film thickness increases to 250 nm, which is very advantageous for large‐area printing. Overall, the intrinsic molecular properties as well as the morphologies of blends can be effectively modulated by manipulating the substituent position on the terminal groups, and the structure–property relationships gleaned from this study will aid in designing more efficient SMAs for versatile applications.     

Polymer Main‐Chain Substitution Effects on the Efficiency of Nonfullerene BHJ Solar Cells

“Nonfullerene” acceptors are proving effective in bulk heterojunction (BHJ) solar cells when paired with selected polymer donors. However, the principles that guide the selection of adequate polymer donors for high‐efficiency BHJ solar cells with nonfullerene acceptors remain a matter of some debate and, while polymer main‐chain substitutions may have a direct influence on the donor–acceptor interplay, those effects should be examined and correlated with BHJ device performance patterns. This report examines a set of wide‐bandgap polymer donor analogues composed of benzo[1,2‐b:4,5‐b′]dithiophene (BDT), and thienyl ([2H]T) or 3,4‐difluorothiophene ([2F]T) motifs, and their BHJ device performance pattern with the nonfullerene acceptor “ITIC”. Studies show that the fluorine‐ and ring‐substituted derivative PBDT(T)[2F]T largely outperforms its other two polymer donor counterparts, reaching power conversion efficiencies as high as 9.8%. Combining several characterization techniques, the gradual device performance improvements observed on swapping PBDT[2H]T for PBDT[2F]T, and then for PBDT(T)[2F]T, are found to result from (i) notably improved charge generation and collection efficiencies (estimated as ≈60%, 80%, and 90%, respectively), and (ii) reduced geminate recombination (being suppressed from ≈30%, 25% to 10%) and bimolecular recombination (inferred from recombination rate constant comparisons). These examinations will have broader implications for further studies on the optimization of BHJ solar cell efficiencies with polymer donors and a wider range of nonfullerene acceptors.     

High‐Efficiency Nonfullerene Organic Solar Cells: Critical Factors that Affect Complex Multi‐Length Scale Morphology and Device Performance

Organic solar cells (OSCs) made of donor/acceptor bulk‐heterojunction active layers have been of widespread interest in converting sunlight to electricity. Characterizing of the complex morphology at multiple length scales of polymer:nonfullerene small molecular acceptor (SMA) systems remains largely unexplored. Through detailed characterizations (hard/soft X‐ray scattering) of the record‐efficiency polymer:SMA system with a close analog, quantitative morphological parameters are related to the device performance parameters and fundamental morphology–performance relationships that explain why additive use and thermal annealing are needed for optimized performance are established. A linear correlation between the average purity variations at small length scale (≈10 nm) and photovoltaic device characteristics across all processing protocols is observed in ≈12%‐efficiency polymer:SMA systems. In addition, molecular interactions as reflected by the estimated Flory–Huggins interaction parameters are used to provide context of the room temperature morphology results. Comparison with results from annealed devices suggests that the two SMA systems compared show upper and lower critical solution temperature behavior, respectively. The in‐depth understanding of the complex multilength scale nonfullerene OSC morphology may guide the device optimization and new materials development and indicates that thermodynamic properties of materials systems should be studied in more detail to aid in designing optimized protocols efficiently.     

Modulation of End Groups for Low‐Bandgap Nonfullerene Acceptors Enabling High‐Performance Organic Solar Cells

The field of nonfullerene organic solar cells (OSCs) has seen an impressive progress, largely due to advances in high‐performance small molecule acceptors (SMAs). As a large portion of the solar energy is located in the near‐infrared region, it is important to develop ultralow‐bandgap SMAs that have extended absorption in the spectral range of 800–1000 nm to maximize light absorption and efficiencies. In this work, three low‐bandgap SMAs, namely, IXIC, IXIC‐2Cl, and IXIC‐4Cl, are designed and synthesized with same fused terthieno[3,2‐b]thiophene donor unit and different end groups (EGs). The three SMAs all have low optical bandgap (E_g) of 1.35, 1.30, and 1.25 eV, respectively. The chlorination on EGs can lower the energy level and broaden absorption range of the SMAs. As a result, the V_oc of the devices is reduced but the J_sc is significantly increased. In addition, the addition of chlorine atoms can enhance π–π stacking and crystallinity of the SMAs, which result in high fill factors. Overall, the optimum EGs are monochlorine‐substituted IC and OSCs based on PBDB‐T:IXIC‐2Cl that can achieve remarkable power conversion efficiencies (PCEs) of 12.2%, which is one of the highest PCEs for nonfullerene organic solar cells based on low‐bandgap SMAs.     

A Halogenation Strategy for over 12% Efficiency Nonfullerene Organic Solar Cells

Three acceptor–donor–acceptor type nonfullerene acceptors (NFAs), namely, F–F, F–Cl, and F–Br, are designed and synthesized through a halogenation strategy on one successful nonfullerene acceptor FDICTF (F–H). The three molecules show red‐shifted absorptions, increased crystallinities, and higher charge mobilities compared with the F–H. After blending with donor polymer PBDB‐T, the F–F‐, F–Cl‐, and F–Br‐based devices exhibit power conversion efficiencies (PCEs) of 10.85%, 11.47%, and 12.05%, respectively, which are higher than that of F–H with PCE of 9.59%. These results indicate that manipulating the absorption range, crystallinity and mobilities of NFAs by introducing different halogen atoms is an effective way to achieve high photovoltaic performance, which will offer valuable insight for the designing of high‐efficiency organic solar cells.     

Medium Bandgap Polymer Donor Based on Bi(trialkylsilylthienyl‐benzo[1,2‐b:4,5‐b′]‐difuran) for High Performance Nonfullerene Polymer Solar Cells

A new 2D‐conjugated medium bandgap donor–acceptor copolymer, J81 , based on benzodifuran with trialkylsilyl thiophene side chains as donor unit and fluorobenzothiazole as acceptor, is synthesized and successfully used in nonfullerene polymer solar cells (PSCs) with low bandgap n‐type organic semiconductor (n‐OS) 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′]‐ dithiophene (ITIC) and m ‐ITIC as acceptor. J81 possesses a lower‐lying highest occupied molecular orbital (HOMO) energy level of ?5.43 eV and medium bandgap of 1.93 eV with complementary absorption in the visible–near infrared region with the n‐OS acceptor. The PSCs based on J81 :ITIC and J81 :m ‐ITIC yield high power conversion efficiency of 10.60% and 11.05%, respectively, with high V _oc of 0.95–0.96 V benefit from the lower‐lying HOMO energy level of J81 donor. The work indicates that J81 is another promising polymer donor for the nonfullerene PSCs.     

Tuning Energy Levels without Negatively Affecting Morphology: A Promising Approach to Achieving Optimal Energetic Match and Efficient Nonfullerene Polymer Solar Cells

One advantage of nonfullerene polymer solar cells (PSCs) is that they can yield high open‐circuit voltage (V_OC) despite their relatively low optical bandgaps. To maximize the V_OC of PSCs, it is important to fine‐tune the energy level offset between the donor and acceptor materials, but in a way not negatively affecting the morphology of the donor:acceptor (D:A) blends. Here, an effective material design rationale based on a family of D–A1–D–A2 terthiophene (T3) donor polymers is reported, which allows for the effective tuning of energy levels but without any negative impacts on the morphology of the blend. Based on a T3 donor unit combined with difluorobenzothiadiazole (ffBT) and difluorobenzoxadiazole (ffBX) acceptor units, three donor polymers are developed with highly similar morphological properties. This is particularly surprising considering that the corresponding quaterthiophene polymers based on ffBT and ffBX exhibit dramatic differences in their solubility and morphological properties. With the fine‐tuning of energy levels, the T3 polymers yield nonfullerene PSCs with a high efficiency of 9.0% for one case and with a remarkably low energy loss (0.53 V) for another polymer. This work will facilitate the development of efficient nonfullerene PSCs with optimal energy levels and favorable morphology properties.     

Study of Burn‐In Loss in Green Solvent‐Processed Ternary Blended Organic Photovoltaics Derived from UV‐Crosslinkable Semiconducting Polymers and Nonfullerene Acceptors

This work deals with the investigation of burn‐in loss in ternary blended organic photovoltaics (OPVs) prepared from a UV‐crosslinkable semiconducting polymer (P2FBTT‐Br) and a nonfullerene acceptor (IEICO‐4F) via a green solvent process. The synthesized P2FBTT‐Br can be crosslinked by UV irradiation for 150 s and dissolved in 2‐methylanisole due to its asymmetric structure. In OPV performance and burn‐in loss tests performed at 75 °C or AM 1.5G Sun illumination for 90 h, UV‐crosslinked devices with PC₇₁BM show 9.2% power conversion efficiency (PCE) and better stability against burn‐in loss than pristine devices. The frozen morphology resulting from the crosslinking prevents lateral crystallization and aggregation related to morphological degradation. When IEICO‐4F is introduced in place of a fullerene‐based acceptor, the burn‐in loss due to thermal aging and light soaking is dramatically suppressed because of the frozen morphology and high miscibility of the nonfullerene acceptor (18.7% → 90.8% after 90 h at 75 °C and 37.9% → 77.5% after 90 h at AM 1.5G). The resulting crosslinked device shows 9.4% PCE (9.8% in chlorobenzene), which is the highest value reported to date for crosslinked active materials, in the first green processing approach.     

Influence of Donor Polymer on the Molecular Ordering of Small Molecular Acceptors in Nonfullerene Polymer Solar Cells

Nonfullerene polymer solar cells (PSCs) based on polymer donors and nonfullerene small molecular acceptors (SMAs) have recently attracted considerable attention. Although much of the progress is driven by the development of novel SMAs, the donor polymer also plays an important role in achieving efficient nonfullerene PSCs. However, it is far from clear how the polymer donor choice influences the morphology and performance of the SMAs and the nonfullerene blends. In addition, it is challenging to carry out quantitative analysis of the morphology of polymer:SMA blends, due to the low material contrast and overlapping scattering features of the π–π stacking between the two organic components. Here, a series of nonfullerene blends is studied based on ITIC‐Th blended with five different donor polymers. Through quantitative morphology analysis, the (010) coherence length of the SMA is characterized and a positive correlation between the coherence length of the SMA and the device fill factor (FF) is established. The study reveals that the donor polymer can significantly change the molecular ordering of the SMA and thus improve the electron mobility and domain purity of the blend, which has an overall positive effect that leads to the enhanced device FF for nonfullerene PSCs.     

New Anthracene‐Fused Nonfullerene Acceptors for High‐Efficiency Organic Solar Cells: Energy Level Modulations Enabling Match of Donor and Acceptor

Two new nonfullerene small molecule acceptors (NF‐SMAs) AT‐NC and AT‐4Cl based on heptacyclic anthracene(cyclopentadithiophene) (AT) core and different electron‐withdrawing end groups are designed and synthesized. Although the two new acceptor molecules use two different end groups, naphthyl‐fused indanone (NINCN) and chlorinated INCN (INCN‐2Cl) demonstrate similar light absorption. AT‐4Cl with chlorinated INCN as end groups are shifted significantly due to the strong electron‐withdrawing ability of chlorine atoms. Thus, desirable V_oc and photovoltaic performance are expected to be achieved when polymer PBDB‐T is used as the electron donor with AT‐NC as the acceptor, and fluorinated analog PBDB‐TF with down‐shifted energy levels is selected to blend with AT‐4Cl. Consequently, the device based on PBDB‐TF:AT‐4Cl yields a high power conversion efficiency of 13.27% with a slightly lower V_oc of 0.901 V, significantly enhanced J_sc of 19.52 mA cm^?2 and fill factor of 75.5% relative to the values based on PBDB‐T:AT‐NC. These results demonstrate that the use of a new electron‐rich AT core, together with energy levels modulations by end‐group optimizations enabling the match with polymer donors, is a successful strategy to construct high‐performance NF‐SMAs.     

A High‐Performance D–A Copolymer Based on Dithieno[3,2‐b:2′,3′‐d]Pyridin‐5(4H)‐One Unit Compatible with Fullerene and Nonfullerene Acceptors in Solar Cells

Development of high‐performance donor–acceptor (D–A) copolymers is vital in the research of polymer solar cells (PSCs). In this work, a low‐bandgap D–A copolymer based on dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one unit (DTP), PDTP4TFBT, is developed and used as the donor material for PSCs with PC₇₁BM or ITIC as the acceptor. PDTP4TFBT:PC₇₁BM and PDTP4TFBT:ITIC solar cells give power conversion efficiencies (PCEs) up to 8.75% and 7.58%, respectively. 1,8‐Diiodooctane affects film morphology and device performance for fullerene and nonfullerene solar cells. It inhibits the active materials from forming large domains and improves PCE for PDTP4TFBT:PC₇₁BM cells, while it promotes the aggregation and deteriorates performance for PDTP4TFBT:ITIC cells. The ternary‐blend cells based on PDTP4TFBT:PC₇₁BM:ITIC (1:1.2:0.3) give a decent PCE of 9.20%.     

Rational Strategy to Stabilize an Unstable High‐Efficiency Binary Nonfullerene Organic Solar Cells with a Third Component

Long device lifetime is still a missing key requirement in the commercialization of nonfullerene acceptor (NFA) organic solar cell technology. Understanding thermodynamic factors driving morphology degradation or stabilization is correspondingly lacking. In this report, thermodynamics is combined with morphology to elucidate the instability of highly efficient PTB7‐Th:IEICO‐4F binary solar cells and to rationally use PC₇₁BM in ternary solar cells to reduce the loss in the power conversion efficiency from ≈35% to <10% after storage for 90 days and at the same time improve performance. The hypomiscibility observed for IEICO‐4F in PTB7‐Th (below the percolation threshold) leads to overpurification of the mixed domains. By contrast, the hypermiscibility of PC₇₁BM in PTB7‐Th of 48 vol% is well above the percolation threshold. At the same time, PC₇₁BM is partly miscible in IEICO‐4F suppressing crystallization of IEICO‐4F. This work systematically illustrates the origin of the intrinsic degradation of PTB7‐Th:IEICO‐4F binary solar cells, demonstrates the structure–function relations among thermodynamics, morphology, and photovoltaic performance, and finally carries out a rational strategy to suppress the degradation: the third component needs to have a miscibility in the donor polymer at or above the percolation threshold, yet also needs to be partly miscible with the crystallizable acceptor.     

Boosting Organic–Metal Oxide Heterojunction via Conjugated Small Molecules for Efficient and Stable Nonfullerene Polymer Solar Cells

Charge events across organic–metal oxide heterointerfaces routinely occur in organic electronics, yet strongly influence their overall performance and stability. They become even more complicated and challenging for the heterojunction conditions in polymer solar cells (PSCs), especially when nonfullerene acceptors with varied energetics are employed. In this work, an effective interfacial strategy that utilizes novel small molecule self‐assembled monolayers (SAMs) is developed to improve the electronic and electric, as well as chemical properties of organic–zinc oxide (ZnO) interfaces for nonfullerene PSCs. It is revealed that the tailored SAMs with well‐controlled energy levels and molecular dipoles can effectively optimize the energetic barrier and work function (WF) of heterointerface for optimal electron extraction. In addition, the introduction of SAMs atop of ZnO facilitates not only acceptor segregation near the n‐contact interface, but also passivation of the photocatalytic activities for ZnO, to improve overall performance and photo stability of the derived nonfullerene PSCs. Overall, the methodology and structure–property relationship revealed herein would be beneficial for a wide range of hybrid electronics.     

Alkyl Chain Length Effects of Polymer Donors on the Morphology and Device Performance of Polymer Solar Cells with Different Acceptors

The development of nonfullerene acceptors has brought polymer solar cells into a new era. Maximizing the performance of nonfullerene solar cells needs appropriate polymer donors that match with the acceptors in both electrical and morphological properties. So far, the design rationales for polymer donors are mainly borrowed from fullerene‐based solar cells, which are not necessarily applicable to nonfullerene solar cells. In this work, the influence of side chain length of polymer donors based on a set of random terpolymers PTAZ‐TPD10‐Cn on the device performance of polymer solar cells is investigated with three different acceptor materials, i.e., a fullerene acceptor [70]PCBM, a polymer acceptor N2200, and a fused‐ring molecular acceptor ITIC. Shortening the side chains of polymer donors improves the device performance of [70]PCBM‐based devices, but deteriorates the N2200‐ and ITIC‐based devices. Morphology studies unveil that the miscibility between donor and acceptor in blend films depends on the side chain length of polymer donors. Upon shortening the side chains of the polymer donors, the miscibility between the donor and acceptor increases for the [70]PCBM‐based blends, but decreases for the N2200‐ and ITIC‐based blends. These findings provide new guidelines for the development of polymer donors to match with emerging nonfullerene acceptors.     

Ternary Organic Solar Cells with >11% Efficiency Incorporating Thick Photoactive Layer and Nonfullerene Small Molecule Acceptor

Currently, constructing ternary organic solar cells (OSCs) and developing nonfullerene small molecule acceptors (n‐SMAs) are two pivotal avenues to enhance the device performance. However, introducing n‐SMAs into the ternary OSCs to construct thick layer device is still a challenge due to their inferior charge transport property and unclear aggregation mechanism. In this work, a novel wide band gap copolymer 4,8‐bis(4,5‐dioctylthiophen‐2‐yl) benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl‐alt‐N‐(2‐hexyldecyl)‐5,5′‐bis(thiophen‐2‐yl)‐2,2′‐bithiophene‐3,3′‐dicarboximide (PDOT) is designed and blend of PDOT:PC₇₁BM achieves a power conversion efficiency (PCE) of 9.5% with active layer thickness over 200 nm. The rationally selected n‐SMA based on a bulky seven‐ring fused core (indacenodithieno[3,2‐b]thiophene) end‐capped with 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene) malononitrile groups (ITIC) is introduced into the host binary PDOT:PC₇₁BM system to extend the absorption range and reduce the photo energy loss. After fully investigating the morphology evolution of the ternary blends, the different aggregation behavior of n‐SMAs with respect to their fullerene counterpart is revealed and the adverse effect of introducing n‐SMAs on charge transport is successfully avoided. The ternary OSC delivers a PCE of 11.2% with a 230 nm thick active layer, which is among the highest efficiencies of thick layer OSCs. The results demonstrate the feasibility of using n‐SMAs to construct a thick layer ternary device for the first time, which will greatly promote the efficiency of thick layer ternary devices.     

Burn‐in Free Nonfullerene‐Based Organic Solar Cells

Organic solar cells that are free of burn‐in, the commonly observed rapid performance loss under light, are presented. The solar cells are based on poly(3‐hexylthiophene) (P3HT) with varying molecular weights and a nonfullerene acceptor (rhodanine‐benzothiadiazole‐coupled indacenodithiophene, IDTBR) and are fabricated in air. P3HT:IDTBR solar cells light‐soaked over the course of 2000 h lose about 5% of power conversion efficiency (PCE), in stark contrast to [6,6]‐Phenyl C61 butyric acid methyl ester (PCBM)‐based solar cells whose PCE shows a burn‐in that extends over several hundreds of hours and levels off at a loss of ≈34%. Replacing PCBM with IDTBR prevents short‐circuit current losses due to fullerene dimerization and inhibits disorder‐induced open‐circuit voltage losses, indicating a very robust device operation that is insensitive to defect states. Small losses in fill factor over time are proposed to originate from polymer or interface defects. Finally, the combination of enhanced efficiency and stability in P3HT:IDTBR increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene‐based acceptor instead.     

Side‐Chain Impact on Molecular Orientation of Organic Semiconductor Acceptors: High Performance Nonfullerene Polymer Solar Cells with Thick Active Layer over 400 nm

A new n‐type organic semiconductor (n‐OS) acceptor IDTPC with n‐hexyl side chains is developed. Compared to side chains with 4‐hexylphenyl counterparts (IDTCN), such a design endows the acceptor of IDTPC with higher electron mobility, more ordered face‐on molecular packing, and lower band gap. Therefore, the IDTPC‐based polymer solar cells (PSCs) with a newly developed wide bandgap polymer PTQ10 as donor exhibit the maximum power conversion efficiency (PCE) of 12.2%, a near 65% improvement in PCE relative to the IDTCN‐based control device. Most importantly, the IDTPC‐based device is insensitive to the thickness of the active layer from 70 to 505 nm, which still gives a PCE of 10.0% with the active‐layer thickness of 400 nm. To the best of the authors' knowledge, a PCE of 10.0% is the highest value for the nonfullerene PSCs with an active layer thicker than 400 nm. These results reveal that the blend of PTQ10 and IDTPC exhibits great potential for highly efficient nonfullerene PSCs and large‐area device fabrication.     

Thermally Durable Nonfullerene Acceptor with Nonplanar Conjugated Backbone for High‐Performance Organic Solar Cells

A nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, based on 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene as an electron‐donating core and 2‐(6‐fluoro‐2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)‐propanedinitrile as electron‐withdrawing end groups, is designed and synthesized. i‐IEICO‐2F has a twist structure in the main conjugated chain, which causes blueshifted absorption and leads to harmonious absorption with a high bandgap donor. The bandgap of i‐IEICO‐2F compliments the bandgap of suitable wide bandgap donor polymers such as J52, leading to complete light absorption throughout the visible spectrum. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance including an open‐circuit voltage of 0.93 V, a short‐circuit current density of 16.61 mA cm^?2, and a fill factor of 73%, and result in a power conversion efficiency (PCE) of 11.28%. The i‐IEICO‐2F‐based devices reach PCEs of >11% without using any additives or post‐treatments. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing (TA) treatment at 150 °C. Based on UV, atomic force microscopy (AFM), and grazing incidence wide angle X‐ray scattering (GIWAXS) results, i‐IEICO‐2F devices show almost identical morphology and molecular orientation throughout the TA treatment and excellent stability compared to other IEICO derivatives.