Morphology Of Blend Films Research Articles

Synthesis of Bithiophene-Based D-A₁-D-A₂ Terpolymers with Different A₂ Moieties for Polymer Solar Cells via Direct Arylation.

A series of bithiophene (2T)-based D-A1-D-A2 terpolymers with different A2 moieties were prepared via direct arylation reaction. In these terpolymers, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) was selected as the first electron-accepting (A1) moiety, 2,1,3-benzothiadiazole (BT) or fluorinated benzothiadiazole (FBT) or octyl-thieno[3,4-c]pyrrole-4,6-dione (TPD) or 2,1,3-benzoselendiazole (SeT) was selected as the second electron-accepting (A2) moiety, while bithiophene with hexyl side chain was used as the electron-donating moiety. The UV-vis absorption, electrochemical properties, blend film morphology, and photovoltaic properties were studied to explore the effects of the A2 moiety. It is shown that these terpolymer films exhibit broad absorption (350–1000 nm), full width at half-maximum of more than 265 nm and ordered molecular packing. Varying the A2 moiety could affect the energy levels and blend film morphology leading to different polymer solar cell (PSC) performances of these (2T)-based D-A1-D-A2 terpolymers. As a result, the highest Jsc of 10.70 mA/cm2 is achieved for Polymer 1 (P1) with BT as A2 moiety, while the higher highest occupied molecular orbital (HOMO) level limits the open circuit voltage (Voc) and leads to a power conversion efficiency (PCE) of 3.46%.

Significant enhancement of responsivity of organic photodetectors upon molecular engineering

Upon incorporating S⋯O noncovalent conformational locks and side chain engineering, the optical absorption and blend film morphology of molecular acceptors are effectively tuned, resulting in an enhanced photocurrent and thus a higher responsivity (from 70.5 mA W−1 to 291.3 mA W−1) for P3HT-based OPDs.

Si-Bridged Ladder-Type Small-Molecule Acceptors for High-Performance Organic Photovoltaics.

In this study, we synthesized 5,11-dihexyl-4,4,10,10-tetraoctylbenzo[1,2- b:4,5- b']bisthieno[4″,5″- b″:4‴,5‴- b‴]silolo[2″,3″- d:2‴,3‴- d']thiophene (ArSi) as a ladder-type electron-rich core for the preparation of three acceptor-donor-acceptor-type nonfullerene acceptors (NFAs)-ArSiID, ArSiID-F, and ArSiID-Cl-featuring (3-oxo-2,3-dihydro-1 H-inden-1-ylidene)malononitrile (ID), 2-(5,6-difluoro-3-oxo-2,3-dihydro-1 H-inden-1-ylidene)malononitrile (ID-F), and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1 H-inden-1-ylidene)malononitrile (ID-Cl) as peripheral electron-poor units, respectively. These molecules exhibit strong absorption covering the region of 600-850 nm. The incorporation of the halogen atoms onto the terminal units adjusted the energy levels and light-harvesting ability of these materials. We employed the conjugated polymers J51 and PBDB-T, having middle optical energy gaps as donor together with these ArSi derivativesas acceptor to study the blend film morphology and the corresponding organic photovoltaic (OPV) performances. After optimization with device engineering works, aPBDB-T:ArSiID-F-based device with a power conversion efficiency up to 9.4% was achieved. This study is the first case to examine the effects of various halogen modifications on the performance of ArSiderivatives that serve as NFAs for OPVs. Our findings should encourage further investigations on this rarely studied core structure for optoelectronic applications.

Fused-ring acceptor with a spiro-bridged ladder-type core for organic solar cells

Side-chain engineering could tune the crystallinity, solubility and aggregation styles of fused-ring electron acceptors (FREAs), and thus could influence the morphology and charge carrier mobility of blend films as well as the final device performances. Herein, we developed a novel FREA called spiro-IDT-O-IC, which is composed of a spiro-bridged IDT core and two 1,1-dicyanomethylene-3-indanone (IC) terminal units. Similar acceptor molecule IDT-O-IC with four 4-octyloxyphenyl side chains is also prepared for the control experiment. The absorptions, energy levels, molecular configurations, and electronic wave functions of these two acceptor molecules have been comparatively studied. Polymer solar cells (PSCs) fabricated with PBDB-T:spiro-IDT-O-IC as the active layer demonstrated a PCE of 6.23% with a Jsc of 11.76 mA/cm2, a Voc of 0.84 V and an FF of 0.63. To the best of our knowledge, this is the first report that a spiro-bridged ladder type core was used for the fabrication of fused ring acceptors.

Revealing the Impact of F4‐TCNQ as Additive on Morphology and Performance of High‐Efficiency Nonfullerene Organic Solar Cells

AbstractFluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells.

Thieno[3,2‐b]thiophene‐Bridged Conjugated Polymers Based on Dithieno[3,2‐b:2′,3′‐d]silole and Thieno[3,4‐c]pyrrole‐4,6‐dione for Polymer Solar Cells: Influence of Side Chains on Optoelectronic Properties

AbstractTwo new D‐π‐A copolymers combining dithieno[3,2‐b:2′,3′‐d]silole as donor unit and thieno[3,4‐c]pyrrole‐4,6‐dione as acceptor unit are designed and synthesized. The influence of different side chains on the optical properties, the energy levels of the highest occupied molecular orbitals and lowest unoccupied molecular orbitals, electronic structures, morphologies of blend films, and the photovoltaic properties of the conjugated polymers are investigated. As a result, the polymers reveal different light absorption and energy levels, which contributes to the short‐circuit current and the open‐circuit voltage, respectively. The dihedral angles between the donor unit and the π bridge are greatly increased due to the steric effect caused by introducing the alkyl chain to the π bridge. Eventually, a maximum power conversion efficiency of 2.65% is obtained.

Experimental investigation of charge transfer, charge extraction, and charge carrier concentration in P3HT:PBD-DT-DPP:PC70BM ternary blend photovoltaics

A sensitizer consisting of D-A structured polymer (PBD-DT-DPP) is assessed with P3HT:PC70BM solar cell. This polymer possesses a narrow energy band gap, which complements in absorption of P3HT:PC70BM binary blend. Use of PBD-DT-DPP as a sensitizer in the ternary blend OPV is reported earlier, which showed enhanced power conversion efficiency (PCE) in the conventional device structure. In this work, charge transfer and charge transport properties of the ternary blend (inverted device) for its improved performance are discussed in detail. Ternary blend system demonstrated an improved device performance (∼3.35%) as compared with the binary of P3HT:PC70BM devices (∼2.58%). Ternary blend devices showed enhanced short circuit current density (JSC) of ∼4 mA/cm2 relative to P3HT:PC70BM. To understand the reason for the decrease in fill factor (FF), mobility analysis using charge extraction with increasing linear voltage (CELIV) measurements were carried out. However, there is not much change in the open circuit voltage (VOC). Results indicate increased bimolecular recombination for the ternary blend, suggesting increased disorder in the morphology of ternary blend films. Time-resolved microwave conductivity (TRMC) measurement was utilized to understand the photoconductance properties of the ternary blend films. Interestingly, TRMC results showed binary PBD-DT-DPP:P3HT and PBD-DT-DPP:PC70BM mixtures possess similar transient photo-conductance to that of P3HT:PC70BM. Further, photoluminescence (PL) measurements were carried out to probe the charge transfer process in the blend systems. Capacitance-voltage results showed enhanced carrier concentration in ternary system. The detailed analysis showed that ternary system helps in improving the charge generation and charge transport and hence improved device performance.

Efficient Non-fullerene Organic Solar Cells Enabled by Sequential Fluorination of Small-Molecule Electron Acceptors.

Three small-molecule non-fullerene electron acceptors containing different numbers of fluorine atoms in their end groups were designed and synthesized. All three acceptors were found to exhibit relatively narrow band gaps with absorption profiles extending into the near-infrared region. The fluorinated analog exhibited enhanced light-harvesting capabilities, which led to improved short-circuit current densities. Moreover, fluorination improved the blend film morphology and led to desirable phase separation that facilitated exciton dissociation and charge transport. As a result of these advantages, organic solar cells based on the non-fullerene acceptors exhibited clearly improved short-circuit current densities and power conversion efficiencies compared with the device based on the non-fluorinated acceptor. These results suggest that fluorination can be an effective approach for the molecular design of non-fullerene acceptors with near-infrared absorption for organic solar cells.

Enhanced Organic Solar Cell Performance by Lateral Side Chain Engineering on Benzodithiophene-Based Small Molecules

The three novel acceptor–donor–acceptor (A–D–A) conjugated small molecules were synthesized, each featuring a benzodithiophene (BDT) core presenting lateral flexible side chains: TB-BDT6T substituted with 2-ethynyl-5-octylthiophene, TS-BDT6T substituted with 2-(octylthio)thiophene, and TT-BDT6T substituted with 2-(2-ethylhexyl)thieno[3,2-b]thiophene groups. The lateral incorporation of functionalized π-conjugated flexible side chains, without altering the end-capped acceptor (cyanoacetate) moieties, amended the optoelectronic properties of these BDT-based small molecules. X-ray diffraction spectroscopy revealed that these small molecules possess high crystallinity; moreover, the optimized blend film morphologies, recorded using atomic force microscopy, revealed miscibility with PC61BM, and turn out nanoscale phase separations. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of these small molecules were allowed, leading to high open-circuit voltages (Voc) for their solar...

Enhance the performance of polymer solar cells via extension of the flanking end groups of fused ring acceptors

Two new fused ring electron acceptors (FREAs) IDT-IC-T and IDT-IC-B with thienyl or phenyl substituents at the terminal INCN unit are synthesized. Theoretical calculations indicate that the two acceptors dominantly favor an intermolecular π-π stacking between the flanking terminal groups. The twist angle between the aryl substituent and INCN unit has a significant influence on the π-π stacking distance of terminal unit. IDT-IC-T with a smaller twist angle has a shorter π-π stacking distance than that of IDT-IC-B with a larger twist angle. In addition, extending the conjugation also affects the blend film morphology. IDT-IC-T and IDT-IC-B based photoactive films show appropriate nanoscale phase separations; whereas, blend films based on the parent compound IDT-IC show large-size acceptor domains. As expected, PBDB-T : IDT-IC-T blend films show higher and more balanced electron and hole mobilities. Moreover, these two acceptors present a good charge-transport connectivity arising from the extended conjugation and the increased intermolecular overlapping. Ultimately, IDT-IC-T demonstrates the highest electron mobility (1.47×10−4 cm2 V−1 s−1) and the best power conversion efficiency (PCE) of 9.43%. As for IDT-IC, which only shows an electron mobility of 7.33×10−5 cm2 V−1 s−1 and a PCE of 5.82%. These findings provide a facile and effective way to improve the photovoltaic performance.

A Chlorinated π-Conjugated Polymer Donor for Efficient Organic Solar Cells

Summary We synthesized a chlorinated benzothiadiazole-T4 polymer donor PBT4T-Cl, in which a chlorine atom had been introduced at the 4 position in the middle thiophene unit to fine-tune the energy level of the final polymers. Compared with its non-chlorinated analog, the PBT4T-Cl-based devices exhibited clear increases in open-circuit voltage and fill factor, achieving power conversion efficiencies (PCEs) up to 11.18% with simple synthesis, which is the highest PCE of the chlorinated polymer-based fullerene polymer solar cells (PSCs) reported to date. Grazing incident wide-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy measurements revealed an optimized morphology of the spin-coated PBT4T-Cl blend films, which supported that chlorine substitution could promote the performance of PSCs. More importantly, the PBT4T-Cl-based devices showed superior stability, with a PCE of 8.16% after 50 days' device storage. Through this research, the chlorination of low-band-gap polymers provides new insight into designing π-conjugated polymer semiconductors and realizing further enhancement of polymer solar cell efficiency as well as stability.

Bar-Coated Ultrathin Semiconductors from Polymer Blend for One-Step Organic Field-Effect Transistors.

One-step deposition of bi-functional semiconductor-dielectric layers for organic field-effect transistors (OFETs) is an effective way to simplify the device fabrication. However, the proposed method has rarely been reported in large-area flexible organic electronics. Herein, we demonstrate wafer-scale OFETs by bar coating the semiconducting and insulating polymer blend solution in one-step. The semiconducting polymer poly(3-hexylthiophene) (P3HT) segregates on top of the blend film, whereas dielectric polymethyl methacrylate (PMMA) acts as the bottom layer, which is achieved by a vertical phase separation structure. The morphology of blend film can be controlled by varying the concentration of P3HT and PMMA solutions. The wafer-scale one-step OFETs, with a continuous ultrathin P3HT film of 2.7 nm, exhibit high electrical reproducibility and uniformity. The one-step OFETs extend to substrate-free arrays that can be attached everywhere on varying substrates. In addition, because of the well-ordered molecular arrangement, the moderate charge transport pathway is formed, which resulted in stable OFETs under various organic solvent vapors and lights of different wavelengths. The results demonstrate that the one-step OFETs have promising potential in the field of large-area organic wearable electronics.

Aggregation Strength Tuning in Difluorobenzoxadiazole-Based Polymeric Semiconductors for High-Performance Thick-Film Polymer Solar Cells.

High-performance polymer solar cells (PSCs) with thick active layers are essential for large-scale production. Polymer semiconductors exhibiting a temperature-dependent aggregation property offer great advantages toward this purpose. In this study, three difluorobenzoxadiazole (ffBX)-based donor polymers, PffBX-T, PffBX-TT, and PffBX-DTT, were synthesized, which contain thiophene (T), thieno[3,2- b]thiophene (TT), and dithieno[3,2- b:2',3'- d]thiophene (DTT) as the π-spacers, respectively. Temperature-dependent absorption spectra reveal that the aggregation strength increases in the order of PffBX-T, PffBX-TT, and PffBX-DTT as the π-spacer becomes larger. PffBX-TT with the intermediate aggregation strength enables well-controlled disorder-order transition in the casting process of blend film, thus leading to the best film morphology and the highest performance in PSCs. Thick-film PSCs with an average power conversion efficiency (PCE) of 8.91% and the maximum value of 9.10% are achieved using PffBX-TT:PC71BM active layer with a thickness of 250 nm. The neat film of PffBX-TT also shows a high hole mobility of 1.09 cm2 V-1 s-1 in organic thin-film transistors. When PffBX-DTT and PffBX-T are incorporated into PSCs utilizing PC71BM acceptor, the average PCE decreases to 6.54 and 1.33%, respectively. The performance drop mainly comes from reduced short-circuit current, as a result of nonoptimal blend film morphology caused by a less well-controlled film formation process. A similar trend was also observed in nonfullerene type thick-film PSCs using IT-4F as the electron acceptor. These results show the significance of polymer aggregation strength tuning toward optimal bulk heterojunction film morphology using ffBX-based polymer model system. The study demonstrates that adjusting π-spacer is an effective method, in combination with other important approaches such as alkyl chain optimization, to generate high-performance thick-film PSCs which are critical for practical applications.

Side‐Chain Optimization of Phthalimide−Bithiophene Copolymers for Efficient All‐Polymer Solar Cells with Large Fill Factors

AbstractA series of phthalimide−bithiophene copolymers with various side‐chains have been used in bulk‐heterojunction all‐polymer solar cells (all‐PSCs) by blending with the well‐known n‐type polymer N2200. Variation in the side‐chains on the bithiophene and N‐imide groups afforded phthalimide polymers with tunable optoelectrical properties and different self‐assembly characteristics. Optimization of the fabrication conditions for the all‐PSCs resulted in a power‐conversion efficiency (PCE) of 3.77 %, with a remarkable fill factor (FF) of 71.21 % for a cell with an active layer thickness of 100 nm. This FF is one of the highest values for all‐PSCs reported to date. Moreover, a greatly improved PCE of 4.7 %, with a good FF of 61.64 %, was obtained by increasing the film thickness to 120 nm. These results indicate that the FFs of all‐PSCs could approach those of polymer:fullerene cells, if a favorable blend film morphology can be realized and efficient charge‐transport/‐extraction can be achieved. These results demonstrate that phthalimide‐based polymers are promising donor semiconductors for the construction of efficient all‐PSCs and that their side‐chains are critical for performance enhancement, in addition to their polymer backbone.

All-Small-Molecule Organic Solar Cells Based on Pentathiophene Donor and Alkylated Indacenodithiophene-Based Acceptors with Efficiency over 8%

Three small molecules IDIC8-M, IDIC8-H, and IDIC8-F with alkylated indacenodithiophene unit as the central building block and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) or its derivations as end groups have been synthesized as nonfullerene acceptors to fabricate solar cells with small molecule donor DRCN5T. The devices based on IDIC8-H incorporating INCN as the end group give a power conversion efficiency (PCE) up to 8.00%. Upon terminal unit fluorination, IDIC8-F based devices exhibit an impressive PCE of 8.42% attributed to the much improved short-circuit current density and fill factor. For DRCN5T:IDIC8-M-based device, a PCE of 6.31% is obtained with high open-circuit voltage over 1 V. The minor structural changes of acceptors have made large difference to the devices performance with the same donor DRCN5T, mainly attributed to the difference of their absorption properties, energy levels and blend film morphologies. These results give a valuable insight on the nonfullerene acceptors based...

Hierarchical Morphology of Polymer Blend Films Induced by Convection-Driven Solvent Evaporation.

Homogeneous thin films of polymer blends with a desired morphology are necessary because of their applications in the fields such as optoelectronics, sensors, biomedicine, and so on. The frequently employed approach for the thin film preparation, spin coating is only able to achieve a homogeneous film for a small area because of the overwhelming spin-driven solvent evaporation with increased size. Here, a convection-guided morphology formation for polystyrene:poly(methyl methacrylate) blend films is reported. In situ observation shows that the morphology changed from homogeneous deposition with a scale less than 10 μm to a self-organized cellular pattern with a scale of more than 100 μm after the fluid flow is involved. Selective dissolution of the hierarchical films reveals that the cellular morphology is attributed to the flow-field-guided deposition of sequentially generated precipitates. The coupling of phase separation and fluid convection results in the hierarchical morphology that includes Voronoi cellular division as the primary structure and the detailed heterogeneous inner-cell features as the secondary structure. Isolated modulation of either micro- or mesoscale in the hierarchical morphology could be carried out via adjusting phase interaction or the convection disturbance correspondingly, providing a flexible and straightforward strategy to construct designed hierarchical structures for polymer thin films toward desired function or property.

False flax (Camelina sativa) seed oil as suitable ingredient for the enhancement of physicochemical and biological properties of chitosan films

To overcome the drawbacks of synthetic films in food packaging industry, researchers are turned to natural bio-based edible films enriched with various plant additives. In current study chitosan blend films were produced by incorporating Camelina sativa seed oil at varying concentrations to chitosan matrix. The chitosan blend films were characterized both physicochemically (structural, morphological, thermal, optical and mechanical) and biologically (antimicrobial and antioxidant activity). The incorporation of C. sativa seed oil notably enhanced thermal stability, antioxidative, anti-quorum sensing and antimicrobial activity. Except elongation at break, other mechanical properties of the blend films were not affected by incorporation of C. sativa seed oil. The surface morphology of blend films was recorded as slightly rough, non-porous and fibre-free surface. As it was expected the optical transmittance in visible region was gradually decreased with increasing fraction of seed oil. Interestingly the hydrophilicity of the blend films revealed a swift increase which can be explained by the formation of micelle between glycerol and Tween 40 in blend films. This study provides valuable information for C. sativa seed oil to be used as a blending ingredient in chitosan film technology.

Modulating Molecular Orientation Enables Efficient Nonfullerene Small-Molecule Organic Solar Cells

In bulk-heterojunction organic solar cells (BHJ-OSCs), exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in blend films. Efficient photovoltaic small molecules (SMs) typically possess an acceptor–donor–acceptor structure that causes intrinsic anisotropy, limiting the control over molecular packing because of the lack of an effective method for modulating molecular orientation. In this report, we design a group of model compounds, named DRTB-T-CX (X = 2, 4, 6, and 8), to demonstrate that adjusting the length of the end alkyl chain can be used to modify the molecular orientation. A top-performance power conversion efficiency (PCE) of up to 11.24% is achieved with a DRTB-T-C4/IT-4F-based device, which is the best performance reported for a state-of-the-art nonfullerene SM organic solar cell (NFSM-OSC).

Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells

Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.

Modulating the Symmetry of Benzodithiophene by Molecular Tailoring for the Application in Naphthalene Diimide‐Based N‐Type Photovoltaic Polymers

To investigate the effect of symmetry of copolymerized unit on the properties of naphthalene diimide (NDI)‐based photovoltaic polymers, three n‐type semiconductor polymers, PNDI‐BDT, PNDI‐T‐BTh, and PNDI‐T‐P‐T are synthesized by copolymerizing NDI with benzodithiophene (BDT), thienylene‐benzothiophene (T‐BTh), and thienylene‐phenylene‐thienylene (T‐P‐T) units, respectively. The latter two building blocks could be regarded as the derivatives from BDT by molecular tailoring and only T‐BTh is an asymmetric segment. The light absorption, carrier mobility, film morphology, crystallinity, and photovoltaic properties of three PNDIs are systematically characterized and compared. Although PNDI‐T‐BTh adopts a face‐on molecular orientation, it shows a high field‐effect electron mobility (μe) of 1.06 × 10−2 cm2 V−1 s−1, which is comparable to the μe of PNDI‐BDT (1.37 × 10−2 cm2 V−1 s−1) with edge‐on molecular orientation. The photovoltaic devices with PNDI‐T‐BTh as the acceptor show the highest power conversion efficiency (PCE) of 5.99% with VOC of 0.87 V, JSC of 10.28 mA cm−2, and FF of 0.67, while PNDI‐BDT and PNDI‐T‐P‐T containing the symmetric copolymerized units give lower PCE of 2.06 and 4.12%, respectively. The higher PCE with PNDI‐T‐BTh could be attributed to the favorable face‐on molecular orientation, fiber‐like blend film morphology and relatively balanced hole and electron mobilities. Our results indicate that the introduction of the asymmetric unit could efficiently improve the photovoltaic performance, which provides a feasible and effective strategy to design novel NDI‐based and even other type rylene diimide‐based photovoltaic polymers.