Organic photovoltaics based on 5-hexylthiophene-fused porphyrazines

Organic photovoltaics based on 5-hexylthiophene-fused porphyrazines as the donor material were investigated. Bilayer solar cells consisting of a solution deposited donor layer and a vapor deposited C₆₀ acceptor layer showed decent photovoltaic performances of up to PCE of 1.2%, which is comparable to the corresponding CuPc/C₆₀ bilayer solar cells. On the other hand, bulk-heterojunction solar cells with PC₆₁BM gave much reduced photovoltaic performances with maximum PCE of 0.70%, which can be ascribed to poor film morphology.     

Donor–acceptor–acceptor–donor small molecules for solution processed bulk heterojunction solar cells

We report on the optical and electrochemical characterization (experimental and theoretical) of two donor substituted benzothiadiazole with different cyano based acceptor π-linkers, tetracyanobutadiene (TCBD) SM1 and dicyanoquinomethane (DCNQ) SM2, and explore them as the donor component for solution processed bulk heterojunction organic solar cells, along with PC₇₁BM as the electron acceptor. The solution bulk heterojunction (BHJ) solar cells based on dichloromethane (DCM) processed active layer with SM1 and SM2 as donor and PC₇₁BM as acceptor achieve power conversion efficiency (PCE) of 2.76% and 3.61%, respectively. The solar cells based on these two small molecules exhibit good Voc, which is attributed to their deep HOMO energy level. The higher PCE of the device based on SM2 compared to SM1 is attributed to the its small bandgap, broader absorption profile and enhanced hole mobility. Additionally, the PCE of the SM2:PC₇₁BM based solar cells processed with 1-chloronaphthalene CN (3 v%)/DCM is further improved reaching upto 4.86%. This increase in PCE has been attributed to the improved nanoscale morphology and more balanced charge transport in the device, due to the solvent additive.     

High‐Yield Synthesis and Electrochemical and Photovoltaic Properties of Indene‐C70 Bisadduct

[6, 6]‐Phenyl‐C₆₁‐butyric acid methyl ester (PC₆₀BM) is the widely used acceptor material in polymer solar cells (PSCs). Nevertheless, the low LUMO energy level and weak absorption in visible region are its two weak points. For enhancing the solar light harvest, the soluble C₇₀ derivative PC₇₀BM has been used as acceptor instead of PC₆₀BM in high efficiency PSCs in recent years. But, the LUMO level of PC₇₀BM is the same as that of PC₆₀BM, which is too low for the PSCs based on the polymer donors with higher HOMO level, such as poly (3‐hexylthiophene) (P3HT). Here, a new soluble C₇₀ derivative, indene‐C₇₀ bisadduct (IC₇₀BA), is synthesized with high yield of 58% by a one‐pot reaction of indene and C₇₀ at 180 °C for 72 h. The electrochemical properties and electronic energy levels of the fullerene derivatives are measured by cyclic voltammetry. The LUMO energy level of IC₇₀BA is 0.19 eV higher than that of PC₇₀BM. The PSC based on P3HT with IC₇₀BA as acceptor shows a higher V_oc of 0.84 V and higher power conversion efficiency (PCE) of 5.64%, while the PSC based on P3HT/PC₆₀BM and P3HT/PC₇₀BM displays V_oc of 0.59 V and 0.58 V, and PCE of 3.55% and 3.96%, respectively, under the illumination of AM1.5G, 100 mW cm^?2. The results indicate that IC₇₀BA is an excellent acceptor for the P3HT‐based PSCs and could be a promising new acceptor instead of PC₇₀BM for the high performance PSCs based on narrow bandgap conjugated polymer donor.     

Understanding How Processing Additives Tune the Nanoscale Morphology of High Efficiency Organic Photovoltaic Blends: From Casting Solution to Spun‐Cast Thin Film

Adding a small amount of a processing additive to the casting solution of photoactive organic blends has been demonstrated to be an effective method for achieving improved power conversion efficiency (PCE) in organic photovoltaics (OPVs). However, an understanding of the nano‐structural evolution occurring in the transformation from casting solution to thin photoactive films is still lacking. In this report, the effects of the processing additive diiodooctane (DIO) on the morphology of the established blend of PBDTTT‐C‐T polymer and the fullerene derivative PC₇₁BM used for OPVs are investigated, starting in the casting solution and tracing the effects in spun‐cast thin films by using neutron/X‐ray scattering, neutron reflectometry, and other characterization techniques. The results reveal that DIO has no observable effect on the structures of PBDTTT‐C‐T and PC₇₁BM in solution; however, in the spun‐cast films, it significantly promotes their molecular ordering and phase segregation, resulting in improved PCE. Thermodynamic analysis based on Flory‐Huggins theory provides a rationale for the effects of DIO on different characteristics of phase segregation due to changes in concentration resulting from evaporation of the solvent and additive during film formation. Such information may help improve the rational design of ternary blends to more consistently achieve improved PCE for OPVs.     

Synthesis of a Modified PC70BM and Its Application as an Electron Acceptor with Poly(3‐hexylthiophene) as an Electron Donor for Efficient Bulk Heterojunction Solar Cells

A simple and effective modification of phenyl‐C₇₀‐butyric acid methyl ester (PC₇₀BM) is carried out in a single step after which the material is used as electron acceptor for bulk heterojunction polymer solar cells (PSCs). The modified PC₇₀BM, namely CN‐PC₇₀BM, showed broader and stronger absorption in the visible region (350–550 nm) of the solar spectrum than PC₇₀BM because of the presence of a cyanovinylene 4‐nitrophenyl segment. The lowest unoccupied molecular energy level (LUMO) of CN‐PC₇₀BM is higher than that of PC₇₀BM by 0.15 eV. The PSC based on the blend (cast from tetrahydrofuran (THF) solution) consists of P3HT as the electron donor and CN‐PC₇₀BM as the electron acceptor and shows a power conversion efficiency (PCE) of 4.88%, which is higher than that of devices based on PC₇₀BM as the electron acceptor (3.23%). The higher PCE of the solar cell based on P3HT:CN‐PC₇₀BM is related to the increase in both the short circuit current (J_sc) and the open circuit voltage (V_oc). The increase in J_sc is related to the stronger light absorption of CN‐PC₇₀BM in the visible region of the solar spectrum as compared to that of PC₇₀BM. In other words, more excitons are generated in the bulk heterojunction (BHJ) active layer. On the other hand, the higher difference between the LUMO of CN‐PC₇₀BM and the HOMO of P3HT causes an enhancement in the V_oc. The addition of 2% (v/v) 1‐chloronapthalene (CN) to the THF solvent during film deposition results in an overall improvement of the PCE up to 5.83%. This improvement in PCE can be attributed to the enhanced crystallinity of the blend (particularly of P3HT) and more balanced charge transport in the device.     

A Versatile Fluoro‐Containing Low‐Bandgap Polymer for Efficient Semitransparent and Tandem Polymer Solar Cells

The versatility of a fluoro‐containing low band‐gap polymer, poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b’]dithiophene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothia‐diazole)] (PCPDTFBT) in organic photovoltaics (OPVs) applications is demonstrated. High boiling point 1,3,5‐trichlorobenzene (TCB) is used as a solvent to manipulate PCPDTFBT:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) active layer morphology to obtain high‐performance single‐junction devices. It promotes the crystallization of PCPDTFBT polymer, thus improving the charge‐transport properties of the active layer. By combining the morphological manipulation with interfacial optimization and device engineering, the single‐junction device exhibits both good air stability and high power‐conversion efficiency (PCE, of 6.6%). This represents one of the highest PCE values for cyclopenta[2,1‐b;3,4‐b’]dithiophene (CPDT)‐based OPVs. This polymer is also utilized for constructing semitransparent solar cells and double‐junction tandem solar cells to demonstrate high PCEs of 5.0% and 8.2%, respectively.     

Comparative Studies on Optical,Redox, and Photovoltaic Properties of a Series of D–A–D and Analogous D–A Chromophores

A series of new symmetrical donor‐acceptor‐donor (D?A?D) dyes based on s‐indacene‐1,3,5,7(2H,6H)‐tetraone as an acceptor unit containing varying electron donating moieties and analogous donor‐acceptor (D?A) chromophores with indane‐1,3‐dione as an acceptor are synthesized. By employing these two sets of dyes, the influence of a scaffold change from unsymmetric push‐pull (D?A) to symmetrical (D?A?D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D?A?D dyes compared to those of the reference D?A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC₆₁BM or PC₇₁BM as acceptors afforded significantly improved performance for devices based on D?A?D blends (up to a factor of 4 compared to the respective D‐A reference) with power conversion efficiencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D?A?D chromophores exhibit unexpectedly high fluorescence quantum yields Φ_em of up to unity, in striking contrast to their weakly fluorescent D‐A counterparts.     

An Amidine‐Type n‐Dopant for Solution‐Processed Field‐Effect Transistors and Perovskite Solar Cells

This study reports an effective amidine‐type n‐dopant of 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene (DBU) that can universally dope electron acceptors, including PC₆₁BM, N2200, and ITIC, by mixing the dopant with the acceptors in organic solvents or exposing the acceptor films in the dopant vapor. The doping mechanism is due to its strong electron‐donating property that is also confirmed via the chemical reduction of PEDOT:PSS (yielding color change). The DBU doping considerably increases the electrical conductivity and shifts the Fermi levels up of the PC₆₁BM films. When the DBU‐doped PC₆₁BM is used as an electron‐transporting layer in perovskite solar cells, the n‐doping removes the “S‐shape” of J–V characteristics, which leads to the fill factor enhancement from 0.54 to 0.76. Furthermore, the DBU doping can effectively lower the threshold voltage and enhance the electron mobility of PC₆₁BM‐based n‐channel field‐effect transistors. These results show that the DBU can be a promising n‐dopant for solution‐processed electronics.     

Charge Density Dependent Nongeminate Recombination in Organic Bulk Heterojunction Solar Cells

Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility and protection of trapped charge carriers from recombination by a donor–acceptor phase separation until re‐emission from these deep states. In order to clarify which mechanism is dominant temperature‐ and illumination‐dependent charge extraction measurements are performed under open circuit and short circuit conditions at poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C₆₁ butyric acid methyl ester (P3HT:PC₆₁BM) and PTB7:PC₇₁BM (poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]]) solar cells in combination with current–voltage characteristics. It is shown that the charge carrier density n dependence of the mobility μ and the recombination prefactor are different for P3HT:PC₆₁BM at temperatures below 300 K and PTB7:PC₇₁BM at room temperature. Therefore, in addition to μ(n), a detrapping limited recombination in systems with at least partial donor–acceptor phase separation is required to explain the high recombination orders.     

Efficient organic photovoltaics using solution-processed,annealing-free TiO2 nanocrystalline particles as an interface modification layer

A solution-processed, annealing-free TiO₂ nanocrystalline particles (TiO₂ NPs) as an interface modification layer was inserted in organic photovoltaics (OPVs), in which the widely used polymer poly (3-hexyl thiophene) (P3HT), a low band gap alkoxylphenyl substituted [1,2-b:4,5-b′] dithiophene-based polymer (PBDTPO-DTBO), and a soluble small molecule benzodithiophene derivative (TIBDT) were used as the donor material, respectively. The annealing-free TiO₂ NPs could be easily spin-coated upon the surface of organic active layers, and showed comparable properties to thermal-annealed ones. The power conversion efficiencies (PCEs) of OPV devices could be enhanced dramatically with inserting an annealing-free TiO₂ NPs layer. The PCEs of OPV devices based on P3HT:PC₆₁BM, PBDTPO-DTBO:PC₇₁BM and TIBDT:PC₆₁BM bulk heterojunctions were improved by 28%, 15% and 27%, respectively, with an annealing-free TiO₂ NPs layer, in which the highest PCE of 5.76% was achieved in PBDTPO-DTBO:PC₇₁BM OPVs. The solution-processed, annealing-free TiO₂ NPs thin films show great potential applications in the fabrication of large-area OPVs by printing or coating techniques on flexible polymer substrates. In particularly, it would promote to fabricate solution-processed, annealing-free OPV devices with suitable hole transport layer and organic/polymer active materials.     

Morphology modulation of organic photovoltaics with block copolymer additive based on rational design strategies

Organic photovoltaics are a promising alternative to silicon-based solar cells with benefits of low-cost production and large scalability. However, its performance is restricted by a non-equilibrium phase-separated morphology. Additive compositions of block copolymer P3HT-b-PFTBT are most likely to mix up and form donor and acceptor morphologies. The parallel bulk-heterojunction model was proposed to show the characteristic photovoltaic parameters and the effect of the parallel cascading heterojunction formation made up of isolated PCBM acceptor domains. We demonstrate block copolymer-based stretchable solar cells on plastic foil substrates, with good power conversion efficiency. To compare the efficiency and stretchability, organic photovoltaic devices were constructed using P3HT/PC₆₁BM, PTB7/PC₇₁BM and P3HT/P3HT-b-PFTBT/PCBM active layer combinations. We find that through rational design of the component ratio, the block-copolymer-based solar cell can withstand tensile strain up to 37%.     

Strategy for Enhancing the Dielectric Constant of Organic Semiconductors Without Sacrificing Charge Carrier Mobility and Solubility

Current organic semiconductors for organic photovoltaics (OPV) have relative dielectric constants (relative permittivities, ε _r) in the range of 2–4. As a consequence, Coulombically bound electron‐hole pairs (excitons) are produced upon absorption of light, giving rise to limited power conversion efficiencies. We introduce a strategy to enhance ε _r of well‐known donors and acceptors without breaking conjugation, degrading charge carrier mobility or altering the transport gap. The ability of ethylene glycol (EG) repeating units to rapidly reorient their dipoles with the charge redistributions in the environment was proven via density functional theory (DFT) calculations. Fullerene derivatives functionalized with triethylene glycol side chains were studied for the enhancement of ε _r together with poly(p‐phenylene vinylene) and diketopyrrolopyrrole based polymers functionalized with similar side chains. The polymers showed a doubling of ε _r with respect to their reference polymers in identical backbone. Fullerene derivatives presented enhancements up to 6 compared with phenyl‐C₆₁‐butyric acid methyl ester (PCBM) as the reference. Importantly, the applied modifications did not affect the mobility of electrons and holes and provided excellent solubility in common organic solvents.     

Photoinduced Hole Transfer Becomes Suppressed with Diminished Driving Force in Polymer‐Fullerene Solar Cells While Electron Transfer Remains Active

Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation‐resistant conjugated polymer poly[(4,8‐bis(2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(2,5‐bis(3‐dodecylthiophen‐2‐yl)benzo[1,2‐d;4,5‐d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM); [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM); and the indene‐C₆₀ bis‐adduct IC₆₀BA). Power conversion efficiency improves from 1.52% in IC₆₀BA‐based solar cells to 3.75% in PC₇₁BM‐based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long‐lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC₇₁BM or PC₆₁BM blends also generates long‐lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC₆₀BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC₆₁BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC₆₀BA blend effectively turns off hole transfer from IC₆₀BA excitons to the host polymer, even while electron transfer from the polymer to the IC₆₀BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.     

Revealing the effect of donor/acceptor intermolecular arrangement on organic solar cells performance based on two-dimensional conjugated small molecule as electron donor

A series of solution processed organic solar cells (OSCs) were fabricated with a two-dimensional conjugated small molecule SMPV1 as electron donor and fullerene derivatives PC₇₁BM or ICBA as electron acceptor. The champion power conversion efficiency (PCE) of OSCs arrives to 7.05% for the cells with PC₇₁BM as electron acceptor. A relatively large open circuit voltage (V_OC) of 1.15 V is obtained from cells using ICBA as electron acceptor with an acceptable PCE of 2.54%. The fill factor (FF) of OSCs is 72% or 61% for the cells with PC₇₁BM or ICBA as electron acceptor, which is relatively high value for small molecule OSCs. The relatively low performance of OSCs with ICBA as electron acceptor indicates that ICBA cannot play positive role in photoelectric conversion processes, which is very similar to the phenomenon observed from the OSCs with high efficient narrow band gap polymers other than P3HT as electron donor, the underlying reason is still in debate. The SMPV1 has strong self-assemble ability to form an ordered two dimensional lamellar structure, which provides an effective platform to investigate the effect of electron acceptor chemical structure on the performance of OSCs. Experimental results exhibit that ICBA molecules may prefer to vertical cross-intercalation among side chains of SMPV1, PC₇₁BM molecules may have better miscibility with SMPV1 in the active layer. The different donor/acceptor (D/A) intermolecular arrangement strongly influences photon harvesting, exciton dissociation and charge carrier transport, which may provide a new sight on performance improvement of OSCs by adjusting D/A intermolecular arrangements.     

Solution-processable tetrazine and oligothiophene based linear A–D–A small molecules: Synthesis,hierarchical structure and photovoltaic properties

A series of high coplanar alternative linear small molecules with acceptor–donor–acceptor (A–D–A) structure containing electron-accepting tetrazine (Tz) moiety and electron-donating oligothiophenes (OTs) moiety, alkylated thiophene attached to both sides of the Tz moiety were designed and synthesized. The influences of varied oligothiophene length on small molecules’ optical and electrochemical properties, crystallization, self assembling morphology in blend film with (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), and photovoltaic properties for the application as donor materials in organic solar cells (OSCs) were studied. The optical and electrochemical properties of small molecules showed that the HOMO and LUMO energy levels were determined by the number of OTs moiety and electron-accepting ability of Tz in the alternative small molecules, respectively. Meanwhile, the varied OT moieties can significantly affect the hierarchical structures when mixed with PC₆₁BM. The molecule with intermediate conjugate moity length showed the highest ordering in its crystalline state, as revealed by differential scanning calorimetry (DSC) and X-ray diffraction experiments, and best photovoltaic properties when blended together with PC₆₁BM or (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as active layer in photovoltaic devices. The results indicate that hierarchical structures controlled by adjusting the conjugate moity length of small molecules is an effective way to improve the performance of OSCs. The photovoltaic device based on TT(HTTzHT)₂:PC₇₁BM with 1% DIO additives showed the best performance, with a J_sc of 7.87 mA/cm² and a PCE of 3.24%.     

Acceptor-rich bulk heterojunction polymer solar cells with balanced charge mobilities

In this work, we reported efficient polymer solar cells with balanced hole/electron mobilities tuned by the acceptor content in bulk heterojunction blend films. The photovoltaic cells were fabricated with two new wide band-gap D-A polymers PBDDIDT and PBDDIDTT as the donor material. The molecular conformations of new polymers are carefully evaluated by theoretical calculations. The results of photovoltaic studies show that two devices reach their optimal conditions with rich PC₇₁BM content up to 80% in blend films, which is uncommon with most of reported PSCs. The as-cast devices based on PBDDIDT and PBDDIDTT reveal good photovoltaic performance with PCE of 7.04% and 6.40%, respectively. The influence of PC₇₁BM content on photovoltaic properties is further detailed studied by photoluminescence emission spectra, charge mobilities and heterojunction morphology. The results exhibit that more efficient charge transport between donor and acceptor occurs in rich PC₇₁BM blend films. Meanwhile, the hole and electron mobilities are simultaneously enhanced and afford a good balance in rich PC₇₁BM blend films (D/A, 1:4) which is critical for the improvement of current density and fill factors.     

Highly Efficient Indoor Organic Photovoltaics with Spectrally Matched Fluorinated Phenylene‐Alkoxybenzothiadiazole‐Based Wide Bandgap Polymers

The unique electro‐optical features of organic photovoltaics (OPVs) have led to their use in applications that focus on indoor energy harvesters. Various adoptable photoactive materials with distinct spectral absorption windows offer enormous potential for their use under various indoor light sources. An in‐depth study on the performance optimization of indoor OPVs is conducted using various photoactive materials with different spectral absorption ranges. Among the materials, the fluorinated phenylene‐alkoxybenzothiadiazole‐based wide bandgap polymer—poly[(5,6‐bis(2‐hexyldecyloxy)benzo[c][1,2,5]thiadiazole‐4,7‐diyl)‐alt‐(5,50‐(2,5‐difluoro‐1,4‐phenylene)bis(thiophen‐2‐yl))] (PDTBTBz‐2F_anti)‐contained photoactive layer—exhibits a superior spectrum matching with indoor lights, particularly a light‐emitting diode (LED), which results in an excellent power absorption ratio. These optical properties contribute to the state‐of‐the‐art performance of the PDTBTBz‐2F_anti:[6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM)‐based OPV with an unprecedented high power‐conversion efficiency (PCE) of 23.1% under a 1000 lx LED. Finally, its indoor photovoltaic performance is observed to be better than that of an interdigitated‐back‐contact‐based silicon photovoltaic (PCE of 16.3%).     

Mechanistic Investigation into Dynamic Function of Third Component Incorporated in Ternary Near‐Infrared Nonfullerene Organic Solar Cells

Organic solar cells (OSCs) consisting of an ultralow‐bandgap nonfullerene acceptor (NFA) with an optical absorption edge that extends to the near‐infrared (NIR) region are of vital interest to semitransparent and tandem devices. However, huge energy‐loss related to inefficient charge dissociation hinders their further development. The critical issues of charge separation as exemplified in NIR‐NFA OSCs based on the paradigm blend of PTB7–Th donor (D) and IEICO–4F acceptor (A) are revealed here. These studies corroborate efficient charge transfer between D and A, accompanied by geminate recombination of photo‐excited charge carriers. Two key factors restricting charge separation are unveiled as the connection discontinuity of individual phases in the blend and long‐lived interfacial charge‐transfer states (CTS). By incorporation of a third‐component of benchmark ITIC or PC₇₁BM with various molar ratios, these two issues are well‐resolved accordingly, yet in distinctly influencing mechanisms. ITIC molecules modulate film morphology to create more continuous paths for charge transportation, whereas PC₇₁BM diminishes CTS and enhances electron transfer at the D/A interfaces. Consequently, the optimal untreated ternary OSCs comprising 0.3 wt% ITIC and 0.1 wt% PC₇₁BM in the blend deliver higher J_SC values of 21.9 and 25.4 mA cm^‐2, and hence increased PCE of 10.2% and 10.6%, respectively.     

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC₇₁BM acceptor, the PBDTTS-DFIQ:PC₇₁BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility (μ_h/μ_e), and nice morphology in comparison with PBDTTS-IQ:PC₇₁BM blend film. Encouragingly, the PBDTTS-DFIQ:PC₇₁BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC₇₁BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage (V_oc) of 0.84 V, short current density (J_sc) of 13.26 mA cm⁻² and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs.     

Naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole‐Containing π‐Conjugated Compound: Nonfullerene Electron Acceptor for Organic Photovoltaics

The development of nonfullerene acceptor materials applicable to organic photovoltaics (OPVs) has attracted considerable attention for the achievement of a high power conversion efficiency (PCE) in recent years. However, it is still challenging due to the insufficiency of both the variety of effective electron‐deficient units and certain guidelines for the design of such materials. This work focusses on naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole (NTz) as a key electron‐deficient unit. Therefore, a new electron‐accepting π‐conjugated compound (NTz‐Np), whose structure is based on the combination of NTz and the fluorene‐containing imide‐annelated terminal units (Np), is designed and synthesized. The NTz‐Np compound exhibits a narrow optical energy gap (1.73 eV), a proper energy level (?3.60 eV) of the lowest unoccupied molecular orbital, and moderate electron mobility (1.6 × 10^?5 cm² V^?1 s^?1), indicating that NTz‐Np has appropriate characteristics as an acceptor against poly(3‐hexylthiophene) (P3HT), a representative donor. OPV devices based on NTz‐Np under the blend with P3HT show high photovoltaic performance with a PCE of 2.81%, which is the highest class among the P3HT/nonfullerene‐based OPVs with the conventional device structure. This result indicates that NTz unit can be categorized as a potential electron‐deficient unit for the nonfullerene acceptors.