Yingzhi Jin

A series of PBDB-TTn random donor copolymers is synthesized, consisting of an electron-deficient benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) unit and different ratios of two electron-rich benzo[1,2-b:4,5-b′]dithiophene (BDT) and thieno[3,2-b]thiophene (TT) units, with intention to modulate the intrachain and/or interchain interactions and ultimately bulk-heterojunction morphology evolution. A comparative study using 4 × 2 polymer solar cell (PSC) performance maps and each of the [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) and the fused-aromatic-ring-based molecule (m-ITIC) acceptors are carried out. Given the similarities in their absorption ranges and energy levels, the PBDB-TTn copolymers clearly reveal a change in the absorption coefficients upon optimization of the BDT to TT ratio in the backbone. Among the given acceptor combination sets, superior performances are observed in the case of PBDB-TT5 blended with PC₇₁BM (8.34 ± 0.10%) or m-ITIC (11.10 ± 0.08%), and the dominant factors causing power conversion efficiency differences in them are found to be distinctly different. For example, the performances of PC₇₁BM-based PSCs are governed by size and population of face-on crystallites, while intermixed morphology without the formation of large phase-separated aggregates is the key factor for achieving high-performance m-ITIC-based PSCs. This study presents a new sketch of structure–morphology–performance relationships for fullerene- versus nonfullerene-based PSCs.

BDD-based four copolymers PBDD-TTn which contained BDT, TT, and BDD are synthesized and operated with two acceptors, PC₇₁BM and m-ITIC. Two systems have different operating mechanisms, and simultaneously high-performances 8.44% for PC₇₁BM and 11.18% for m-ITIC are obtained.

Employing a layer of bulk-heterojunction (BHJ) organic semiconductors on top of perovskite to further extend its photoresponse is considered as a simple and promising way to enhance the efficiency of perovskite-based solar cells, instead of using tandem devices or near infrared (NIR)-absorbing Sn-containing perovskites. However, the progress made from this approach is quite limited because very few such hybrid solar cells can simultaneously show high short-circuit current (J_SC) and fill factor (FF). To find an appropriate NIR-absorbing BHJ is essential for highly efficient, organic, photovoltaics (OPV)/perovskite hybrid solar cells. The materials involved in the BHJ layer not only need to have broad photoresponse to increase J_SC, but also possess suitable energy levels and high mobility to afford high V_OC and FF. In this work, a new porphyrin is synthesized and blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to function as an efficient BHJ for OPV/perovskite hybrid solar cells. The extended photoresponse, well-matched energy levels, and high hole mobility from optimized BHJ morphology afford a very high power conversion efficiency (PCE) (19.02%) with high V_oc, J_SC, and FF achieved simultaneously. This is the highest value reported so far for such hybrid devices, which demonstrates the feasibility of further improving the efficiency of perovskite devices.

A highly efficient organic photovoltaics/perovskite hybrid solar cell is demonstrated by blending a new conjugated porphyrin-based small molecule with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to function as an efficient bulk-heterojunction layer. The extended photoresponse, matched energy levels, and high hole mobility derived from the optimized bulk-heterojunction morphology contribute to the record-high efficiency of 19.02% in these hybrid devices.

A novel small-molecule acceptor, (2,2′-((5E,5′E)-5,5′-((5,5′-(4,4,9,9-tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-(2-ethylhexyl)thiophene-5,2-diyl))bis(methanylylidene)) bis(3-hexyl-4-oxothiazolidine-5,2-diylidene))dimalononitrile (ITCN), end-capped with electron-deficient 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile groups, is designed, synthesized, and used as the third component in fullerene-free ternary polymer solar cells (PSCs). The cascaded energy-level structure enabled by the newly designed acceptor is beneficial to the carrier transport and separation. Meanwhile, the three materials show a complementary absorption in the visible region, resulting in efficient light harvesting. Hence, the PBDB-T:ITCN:IT-M ternary PSCs possess a high short-circuit current density (J_sc) under an optimal weight ratio of donors and acceptors. Moreover, the open-circuit voltage (V_oc) of the ternary PSCs is enhanced with an increase of the third acceptor ITCN content, which is attributed to the higher lowest unoccupied molecular orbital energy level of ITCN than that of IT-M, thus exhibits a higher V_oc in PBDB-T:ITCN binary system. Ultimately, the ternary PSCs achieve a power conversion efficiency of 12.16%, which is higher than the PBDB-T:ITM-based PSCs (10.89%) and PBDB-T:ITCN-based ones (2.21%). This work provides an effective strategy to improve the photovoltaic performance of PSCs.

Fullerene-free ternary polymer solar cells with a high efficiency of 12.16% are fabricated by adding a novel small-molecule acceptor to form a cascaded energy-level structure.

The recent introduction of thermally activated delayed fluorescence (TADF) emitters is regarded as an important breakthrough for the development of high efficiency organic light-emitting devices (OLEDs). The planar D and A groups are generally used to construct TADF emitters for their rigid structure and large steric hindrance. In this work, it is shown that many frequently used nonaromatic (noncontinuous conjugation or without satisfying Hückel's rule) planar segments, such as 9,9-dimethyl-9,10-dihydroacridine, are actually pseudoplanar segments and have two possible conformations–a planar form and a crooked form. Molecules constructed from pseudoplanar segments can thus have two corresponding conformations. Their existence can have significant impact on the performance of many TADF emitters. Two design strategies are presented for addressing the problem by either (1) increasing the rigidity of these groups to suppress its crooked form or (2) increasing the steric hindrance of the linked group to minimize energy of the emitters with the highly twisted form. Following these strategies, two new emitters are synthesized accordingly and successfully applied in OLEDs demonstrating high external quantum efficiencies (20.2% and 18.3%).

A schematic energy level diagram of (2-(9,9-dimethylacridin-10(9H)-yl) thianthrene-5,5,10,10-tetraoxide) shows that molecules constructed from pseudoplanar segments can have two corresponding conformations, which have significant impact on the performance of many thermally activated delayed fluorescence emitters. By either increasing the rigidity of these groups, or by increasing the steric hindrance of the linked group, the problem can be addressed.

Thermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high-performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar–thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

The goal of current studies of thermoelectric materials is to identify novel thermoelectric materials and to enhance their performance by applying appropriate strategies, which targets on manipulating individual effective factors or synergistically optimizing the overall performance, broadens the device application of thermoelectric materials, and boosts the market growth.

Charge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion-controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high-efficiency Shockley-type organic solar cells with junction thicknesses suitable for scaling up.

Strongly suppressed recombination is observed in a polymer:fullerene system resulting in solar cell power conversion efficiencies as high as 9% at a junction thickness of 800 nm. Results indicate that solar cell devices made of this material system with thicknesses as large as 300 nm can exhibit Shockley-type behavior, i.e., the fill factor is unaffected by bimolecular recombination.

A universal method to obtain record-high electronic Seebeck coefficients is demonstrated while preserving reasonable conductivities in doped blends of organic semiconductors through rational design of the density of states (DOSs). A polymer semiconductor with a shallow highest occupied molecular orbital (HOMO) level-poly(3-hexylthiophene) (P3HT) is mixed with materials with a deeper HOMO (PTB7, TQ1) to form binary blends of the type P3HT_x:B_1-x (0 ≤ x ≤ 1) that is p-type doped by F₄TCNQ. For B = PTB7, a Seebeck coefficient S = 1100 µV K⁻¹ with conductivity σ = 0.3 S m⁻¹ at x = 0.10 is achieved, while for B = TQ1, S = 2000 µV K⁻¹ and σ = 0.03 S m⁻¹ at x = 0.05 is found. Kinetic Monte Carlo simulations with parameters based on experiments show good agreement with the experimental results, confirming the intended mechanism. The simulations are used to derive a design rule for parameter tuning. These results can become relevant for low-power, low-cost applications like (providing power to) autonomous sensors, in which a high Seebeck coefficient translates directly to a proportionally reduced number of legs in the thermogenerator, and hence in reduced fabrication cost and complexity.

Record-high Seebeck coefficients are achieved for p-type-doped blends of common conjugated polymers. The method used is based on a rational design of the density of states, such that the characteristic hop occurs from the compound with the shallower HOMO to the compound with the deeper HOMO.

2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) is investigated as a molecular p-type dopant in two hole-transport materials, 2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene (Spiro-TAD) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The electron affinity of F6-TCNNQ is determined to be 5.60 eV, one of the strongest organic molecular oxidizing agents used to date in organic electronics. p-Doping is found to be effective in Spiro-TAD (ionization energy = 5.46 eV) but not in TCTA (ionization energy = 5.85 eV). Optical absorption measurements demonstrate that charge transfer is the predominant doping mechanism in Spiro-TAD:F6-TCNNQ. The host–dopant interaction also leads to a significant alteration of the host film morphology. Finally, transport measurements done on Spiro-TAD:F6-TCNNQ as a function of dopant concentration and temperature, and using a highly doped contact layer to ensure negligible hole injection barrier, lead to an accurate measurement of the film conductivity and hole-hopping activation energy.

2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) is investigated as p-dopant in two hole-transport materials (HTMs). Ultrathin, heavily doped 2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene (Spiro-TAD) injection layers are implemented to ensure a negligible hole injection barrier in transport measurements, allowing accurate determination of conductivity and carrier hopping activation energy in HTMs such as Spiro-TAD.

A novel wide-bandgap conjugated copolymer based on an imide-functionalized benzotriazole building block containing a siloxane-terminated side-chain is developed. This copolymer is successfully used to fabricate highly efficient all-polymer solar cells (all-PSCs) processed at room temperature with the green-solvent 2-methyl-tetrahydrofuran. When paired with a naphthalene diimide-based polymer electron-acceptor, the all-PSC exhibits a maximum power conversion efficiency (PCE) of 10.1%, which is the highest value so far reported for an all-PSC. Of particular interest is that the PCE remains 9.4% after thermal annealing at 80 °C for 24 h. The resulting high efficiency is attributed to a combination of high and balanced bulky charge carrier mobility, favorable face-on orientation, and high crystallinity. These observations indicate that the resulting copolymer can be a promising candidate toward high-performance all-PSCs for practical applications.

A novel wide-bandgap conjugated copolymer PTzBI-Si based on an imide-functionalized benzotriazole unit containing a siloxane-terminated side-chain is developed and used to fabricate all-polymer solar cells (all-PSCs). When processed with a green solvent 2-methyl-tetrahydrofuran, the all-PSC exhibits a power conversion efficiency of 10.1%, which represents the highest efficiency ever reported for all-PSCs.

Triplet population dynamics of solution cast films of isolated polymorphs of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS-Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin-forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS-Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.

Ultrafast spectroscopy of 6,13-bis(triisopropylsilylethynyl)-pentacene polymorphs reveals triplet transfer as the mechanism of correlated triplet pair separation in singlet fission. Crystal structures, solved through both X-ray and computational methods, explain differences in their triplet separation characteristics. Charge transfer integrals form a metric for assessing triplet pair separation, codifying a new approach to a priori screening of emerging singlet fission materials.

Non-fullerene acceptors based on perylenediimides (PDIs) have garnered significant interest as an alternative to fullerene acceptors in organic photovoltaics (OPVs), but their charge transport phenomena are not well understood, especially in bulk heterojunctions (BHJs). Here, charge transport and current fluctuations are investigated by performing correlated low-frequency noise and impedance spectroscopy measurements on two BHJ OPV systems, one employing a fullerene acceptor and the other employing a dimeric PDI acceptor. In the dark, these measurements reveal that PDI-based OPVs have a greater degree of recombination in comparison to fullerene-based OPVs. Furthermore, for the first time in organic solar cells, 1/f noise data are fit to the Kleinpenning model to reveal underlying current fluctuations in different transport regimes. Under illumination, 1/f noise increases by approximately four orders of magnitude for the fullerene-based OPVs and three orders of magnitude for the PDI-based OPVs. An inverse correlation is also observed between noise spectral density and power conversion efficiency. Overall, these results show that low-frequency noise spectroscopy is an effective in situ diagnostic tool to assess charge transport in emerging photovoltaic materials, thereby providing quantitative guidance for the design of next-generation solar cell materials and technologies.

Low-frequency electronic noise is measured in polymer solar cells with fullerene and non-fullerene acceptors. Charge carrier lifetimes deduced from impedance spectroscopy enable the noise data to be fit to the Kleinpenning model. The results establish that low-frequency noise elucidates charge recombination processes that limit power conversion efficiency. This correlated analytical tool provides quantitative guidance to the optimization of emerging photovoltaic materials.

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.

Highly efficient ternary-blend nonfullerene organic solar cells based on two copolymer donors and one electron acceptor are fabricated and evaluated. The multiple energy and charge-transfer pathways in this ternary system enable the power conversion efficiency to reach 12.27%, which is a new record for ternary-blend organic solar cells at present. These devices also exhibit a broad composition tolerance.

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.

By extending the conjugated area of the end-group, a newly designed A–D–A–type small-molecule acceptor, namely IDTN, exhibits dense and ordered packing, and therefore, the electron mobility of the IDTN is over one order of magnitude higher than that of its counterpart. When blended with the donor polymer PBDB-TF, a high efficiency of 12.2% with an outstanding fill factor of 0.78 is achieved.

Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J_SC) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V_OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC₆₁BM), that achieves a high V_OC (0.9 V) with very low energy losses (E_loss = 0.52 eV) from the energy of absorbed photons, a respectable J_SC (13 mA cm⁻²), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC₆₁BM. These results hold promise that given the appropriate morphology, the J_SC, V_OC, and FF can all be improved, even with very low energetic offsets.

To realize organic photovoltaics with high open-circuit voltages and short-circuit currents, it is necessary to minimize energetic offsets between donor and acceptor semiconductors. This article describes a comprehensive study on charge recombination and generation in a system with very low energetic offsets yet relatively high performance, in order to identify the root cause for the limited fill factor.

In article number 1700566, Min Ju Cho, Dong Hoon Choi, and co-workers report a new conjugated widebandgap donor polymer, 3MT-Th, harmonized with an ITIC acceptor to enable the production of a polymer solar cell (PSC) with high efficiency of 9.73% under eco-friendly conditions using a non-halogenated solvent. This PSC also exhibits excellent shelf-life stability in air and good operational stability under continuous light illumination.

Semitransparent organic solar cells (OSCs) show attractive potential in power-generating windows. However, the development of semitransparent OSCs is lagging behind opaque OSCs. Here, an ultralow-bandgap nonfullerene acceptor, “IEICO-4Cl”, is designed and synthesized, whose absorption spectrum is mainly located in the near-infrared region. When IEICO-4Cl is blended with different polymer donors (J52, PBDB-T, and PTB7-Th), the colors of the blend films can be tuned from purple to blue to cyan, respectively. Traditional OSCs with a nontransparent Al electrode fabricated by J52:IEICO-4Cl, PBDB-T:IEICO-4Cl, and PTB7-Th:IEICO-4Cl yield power conversion efficiencies (PCE) of 9.65 ± 0.33%, 9.43 ± 0.13%, and 10.0 ± 0.2%, respectively. By using 15 nm Au as the electrode, semitransparent OSCs based on these three blends also show PCEs of 6.37%, 6.24%, and 6.97% with high average visible transmittance (AVT) of 35.1%, 35.7%, and 33.5%, respectively. Furthermore, via changing the thickness of Au in the OSCs, the relationship between the transmittance and efficiency is studied in detail, and an impressive PCE of 8.38% with an AVT of 25.7% is obtained, which is an outstanding value in the semitransparent OSCs.

A new nonfullerene acceptor, IEICO-4Cl, is designed to prepare semitransparent organic solar cells (OSCs), yielding a power conversion efficiency of 8.38% with an average visible transmittance of 25.7%, which is among the top results for semitransparent OSCs.

Polymer self-assembly in solution prior to film fabrication makes solution-state structures critical for their solid-state packing and optoelectronic properties. However, unraveling the solution-state supramolecular structures is challenging, not to mention establishing a clear relationship between the solution-state structure and the charge-transport properties in field-effect transistors. Here, for the first time, it is revealed that the thin-film morphology of a conjugated polymer inherits the features of its solution-state supramolecular structures. A “solution-state supramolecular structure control” strategy is proposed to increase the electron mobility of a benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV)-based polymer. It is shown that the solution-state structures of the BDOPV-based conjugated polymer can be tuned such that it forms a 1D rod-like structure in good solvent and a 2D lamellar structure in poor solvent. By tuning the solution-state structure, films with high crystallinity and good interdomain connectivity are obtained. The electron mobility significantly increases from the original value of 1.8 to 3.2 cm² V⁻¹ s⁻¹. This work demonstrates that “solution-state supramolecular structure” control is critical for understanding and optimization of the thin-film morphology and charge-transport properties of conjugated polymers.

A supramolecular self-assembly strategy is used to control the solution-state structure of a conjugated polymer. It is revealed that the thin-film morphology of the conjugated polymer inherits the features of their solution-state supramolecular structures. Through “solution-state supramolecular structure control”, the electron mobility of the polymer is boosted to 3.2 cm² V⁻¹ s⁻¹, nearly doubling the original performance.

With an indenoindene core, a new thieno[3,4-b]thiophene-based small-molecule electron acceptor, 2,2′-((2Z,2′Z)-((6,6′-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl)bis(2-octylthieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NITI), is successfully designed and synthesized. Compared with 12-π-electron fluorene, a carbon-bridged biphenylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene with a centrosymmetry, shows elongated π-conjugation with 14 π-electrons and one more sp³ carbon bridge, which may increase the tunability of electronic structure and film morphology. Despite its twisted molecular framework, NITI shows a low optical bandgap of 1.49 eV in thin film and a high molar extinction coefficient of 1.90 × 10⁵m⁻¹ cm⁻¹ in solution. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which is among the best performance so far reported for fullerene-free organic photovoltaics and is inspiring for the design of new electron acceptors.

A thieno[3,4-b]thiophene-based electron acceptor, NITI, featuring a 14-π-electron indenoindene core is designed and synthesized. Despite its twisted molecular geometry, NITI shows a low optical bandgap and a high molar extinction coefficient. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which represents an exciting progress in the design of new electron acceptors.