Acceptor ITIC Research Articles

High performance ternary organic photomultiplication detectors based on non-fullerene acceptor ITIC

To extend response spectrum to near infrared region, one non-fullerene acceptor ITIC was doped into P3HT:PC61BM blends, and the organic photomultiplication detectors (OPMDs) with structure of ITO/PEDOT:PSS/P3HT:ITIC:PC61BM(100:x:1, wt/wt/wt)/Al were prepared. The absorption and photoluminescence spectra of the active layer films and the current density-voltage characteristics of the devices were measured and analyzed, and the photomultiplication principle of the devices was studied. The results showed that a wide spectral response of 400–850 nm is realized in the ternary P3HT:ITIC:PC61BM active layer, and the external quantum efficiency, responsivity and specific detectivity of the ternary OPMDs based on P3HT:ITIC:PC61BM are all larger than those of the binary ones based on P3HT:PC61BM, and a maximum external quantum efficiency of 1113.71 %, specific detectivity of 6.42 × 1013 Jones and photoresponsivity of 762.99 A/W are obtained at 850 nm and under −16 V bias when the ITIC mass ratio in the ternary active layer is 4 % (i.e., x = 4). Such a considerable performance can be due to the fact that doping ITIC into P3HT:PC61BM can widen absorption spectrum of active layer to near infrared area, and increase electron trap density and exciton dissociation interfaces in active layer and create cascade energy levels for better carrier transport.

Wide band tri-receptor quaternary organic photodetector based on PC61BM, ITIC, and IICO-4F

In this paper, we report a wide band spectrum tri-receptor quaternary organic photodetector (OPD) based on the additional two non-fullerene acceptor materials, ITIC and IEICO-4F, with the polymer material P3HT as donor and the fullerene PC61BM as the first acceptor. The second acceptor ITIC and the third accptor IEICO-4F, with different molecular structures and absorption, were introduced into the conventional system P3HT:PC61BM to improve narrow response spectral range and the uneven response of OPDs. The experiments results show, that proposed non-fullerene acceptor materials of the narrow optical band gap were successful to broaden the absorption spectral range to near infrared. Respectively, the responses (R) and external quantum efficiency at 630, 530, and 460 nm, are up to 0.293 A W−1, 0.288 A W−1, 0. 243 A W−1, and 58%, 67%, and 66%. The quaternary OPDs achieve significant response spectrum across the 400–950 nm range with the specific detection rate (D*) exceeding 1012 Jones. It demonstrates that expanding spectrum non-fullerene materials ITIC and IEICO-4F can improve the mixing degree of the mixed materials, decrease agglomeration brought on by single widening excess, and increase spectrum absorption and moderate non-uniform response. The exciton dissociation efficiency is considerably increased by simultaneous charge transfer between the receptors and the receptors in the active layer. Moreover, the carrier mobility is improved, bimolecular recombination is suppressed.

Phenoxazine-based small molecule heterojunction nanoparticles for photocatalytic hydrogen production.

A phenoxazine-based small organic molecular donor POZ-M is designed and synthesized to prepare organic heterojunction nanoparticles (NPs) with a small molecular acceptor ITIC for photocatalytic hydrogen production, giving a reaction rate of up to 63 mmol g-1 h-1. A beneficial molecular design strategy highlights the role of miscibility between POZ-M and ITIC, which is necessary to achieve satisfactory charge separation at the donor/acceptor interface.

Effects of charge injection barrier on the dark current of organic photodiodes

In this work, we investigated the effect of charge injection barrier on the dark current density of organic photodetectors. The photosensitive layer of these photodetectors contains an electron-donating polymer PNTOD, and the derivatives of the widely used non-fullerene acceptor, namely ITIC, IT-2F and IT-4F. All devices showed similar absorption profiles and external quantum efficiencies, while different dark current density and thus specific detectivity. It is interesting to note that both electron and hole injection barriers of devices decreased with increased fluorine substituents in end groups of the non-fullerene acceptor ITIC. The electron injection barriers reduced from 1.37 eV for ITIC to 1.02 eV for IT-4F, and the corresponding hole injection barriers slightly dropped from 0.75 eV for ITIC to 0.57 eV for IT-4F, leading to an increased dark current at reversed bias. These findings verified the importance of rational energy level alignment of photosensitive materials for achieving high-performance organic photodetectors. • The effects of charge injection barrier on the dark current density of organic photodetectors were investigated. • OPDs based on ITIC derivatives showed similar photocurrent and responsivity, while obviously different dark current density and detectivity. • Increasing F-substituents of ITIC derivatives leads to decreased charge injection barrier and thus increased dark current density.

Polymer/non-fullerene acceptor bulk heterojunction nanoparticles for efficient photocatalytic hydrogen production from water

Bulk heterojunction nanoparticles (NPs) composed of p-type polymer donor and non-fullerene acceptor are emerging photocatalysts for water splitting. Herein, bulk heterojunction NPs consisting of donor PBDB-T and non-fullerene acceptor ITIC are produced by micro-emulsion method, and are demonstrated as efficient photocatalyst to generate hydrogen from water splitting upon full spectrum illumination, with ascorbic acid as the sacrificial agent. The photocatalytic efficiency is determined by the nanoscale morphology of these bulk heterojunction NPs, which can be regulated by donor/acceptor ratio, binary solvent volume fraction, as well as surfactant content. The best-performing bulk heterojunction NPs show an outstanding hydrogen evolution rate of 257 mmol h−1 g−1, benefiting from the ideal phase separation, reasonable particle size and efficient charge separation and transfer. This work provides a rational guide for producing bulk heterojunction NPs made of organic semiconductors to reach high photocatalytic efficiency.

The Halogenation Effects of Electron Acceptor ITIC for Organic Photovoltaic Nano-Heterojunctions.

Molecular engineering plays a critical role in the development of electron donor and acceptor materials for improving power conversion efficiency (PCE) of organic photovoltaics (OPVs). The halogenated acceptor materials in OPVs have shown high PCE. Here, to investigate the halogenation mechanism and the effects on OPV performances, based on the density functional theory calculations with the optimally tuned screened range-separated hybrid functional and the consideration of solid polarization effects, we addressed the halogenation effects of acceptor ITIC, which were modeled by bis-substituted ITIC with halogen and coded as IT-2X (X = F, Cl, Br), and PBDB-T:ITIC, PBDB-T:IT-2X (X = F, Cl, Br) complexes on their geometries, electronic structures, excitations, electrostatic potentials, and the rate constants of charge transfer, exciton dissociation (ED), and charge recombination processes at the heterojunction interface. The results indicated that halogenation of ITIC slightly affects molecular geometric structures, energy levels, optical absorption spectra, exciton binding energies, and excitation properties. However, the halogenation of ITIC significantly enlarges the electrostatic potential difference between the electron acceptor and donor PBDB-T with the order from fluorination and chlorination to bromination. The halogenation also increases the transferred charges of CT states for the complexes. Meanwhile, the halogenation effects on CT energies and electron process rates depend on different haloid elements. No matter which kinds of haloid elements were introduced in the halogenation of acceptors, the ED is always efficient in these OPV devices. This work provides an understanding of the halogenation mechanism, and is also conducive to the designing of novel materials with the aid of the halogenation strategy.

Fused-ring electron acceptor as an efficient interfacial material for planar and flexible perovskite solar cells

Perovskite solar cell (PSC) has attracted great attention due to its high power conversion efficiency (PCE), low cost and solution processability. The well-designed interface and the modification of electron transport layer (ETL) are critical to the PCE and long-term stability of PSCs. In this article, a fused-ring electron acceptor is employed as the interfacial material between TiO 2 and the perovskite in rigid and flexible PSCs. The modification improves the surface of TiO 2 , which decreases the defects of ETL surface. Moreover, the modified surface has lower hydrophilicity, and thus is beneficial to the growth of perovskite with large grain size and high quality. As a result, the interfacial charge transfer is promoted and the interfacial charge recombination can be suppressed. The highest PCE of 19.61% is achieved for the rigid PSCs after the introduction of ITIC, and the hysteresis effect is significantly reduced. Flexible PSC with ITIC obtains a PCE of 14.87%, and the device stability is greatly improved. This study provides an efficient candidate as the interfacial modifier for PSCs, which is compatible with low-temperature solution process and has a great practical potential for the commercialization of PSCs. • A fused-ring electron acceptor ITIC is employed as the interfacial material for rigid and flexible perovskite solar cells. • ITIC not only suppresses the surface defects of TiO 2 film but also promotes the growth of perovskite crystals. • ITIC improves the electron extraction and reduces the charge recombination which contributes to the performance of PSCs. • The highest power conversion efficiency of 19.61% is achieved for the rigid PSCs after the introduction of ITIC. • Flexible PSC with ITIC obtains a power conversion efficiency of 14.87%, and the device stability is greatly improved.

Novel wide bandgap benzodithiophene-based polymer donors with electron-withdrawing indolin-2-one side chains for efficient organic solar cells with high open circuit voltage

Two new π-conjugated polymers with donor backbone and π-conjugated indolin-2-one side chains, PBDTTI and PBDTTIF, are designed and synthesized as wide bandgap donors for non-fullerene acceptor-based organic solar cells (OSCs). The monomers containing electron accepting indolin-2-one side chains, (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-1-methylindolin-2-one (M1) and (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-5-fluoro-1-methylindolin-2-one (M2), can be easily synthesized via Knoevenagel condensation between 2,5-dibromothiophene-3-carbaldehyde and 2-oxindole or 5-fluoro-2-oxindole, respectively. Stille coupling polymerization of the electron donating benzodithiophene (BDT)-containing monomer 1,1′-[4,8-bis [5-(2-ethylhexyl)-2-thienyl]benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl]bis [1,1,1-trimethylstannane] and M1 or M2 produced PBDTTI or PBDTTIF, respectively. The strong electron accepting π-conjugated 1-methylindolin-2-one and 5-fluoro-1-methylindolin-2-one side chains can achieve low-lying HOMO energy levels of −5.59 eV for PBDTTI and −5.60 eV for PBDTTIF, which is beneficial for realizing high open circuit voltage (VOC) of the resulting OSCs. On the other hand, since the electron acceptor units are on the side chains, the polymer backbone containing only electron donor units could maintain wide bandgaps of 1.91 eV and 1.89 eV for PBDTTI and PBDTTIF, respectively. When PBDTTI and PBDTTIF were used as donors and a small bandgap non-full acceptor ITIC as an acceptor, the resulting OSCs devices achieved VOC of 0.97 and 1.00 V, short circuit current densities (JSC) of 15.60 and 13.70 mAcm-2, and fill factors (FF) of 0.60 and 0.59, resulting in power conversion efficiencies of 8.00 and 7.70%, respectively.

The Role of Silver Nanoparticles in the Hole Transport Layer in Organic Solar Cells Based on PBDB-T:ITIC

In this work, the fabrication and characterization of organic solar cells under the bulk heterojunction approach in a direct architecture, with gold nanoparticles (Ag-NPs) disseminated into the hole transport layer (HTL) based on PEDOT:PSS is carried out. The active layer is a blend of PBDB-T donor with the non-fullerene electron acceptor ITIC. Ag-NPs dispersed in deionized water are synthesized by laser ablation protocols using a pulsed Nd:YAG laser. Results show an open-circuit voltage (Voc) of 0.84 V, a short circuit current density (Jsc) of 15.5 mA/cm2, a fill factor (FF) of 0.49% and a power conversion efficiency (PCE) of 6.4% when 5% of Ag-NPs are incorporated into the PEDOT:PSS layer; for pristine PEDOT:PSS film, the photovoltaic parameters are Voc of 0.85 V, Jsc of 15.4 mA/cm2, FF of 0.45% and PCE of 5.9%; these results mean an 8.5% of PCE enhancement with the Ag-NPs incorporation. Ag-NPs are introduced into the pristine layer PEDOT:PSS to increase light absorption into the active layer through the localized surface plasmonic resonance effect.

Non‐fullerene all small molecule OBHJSCs with profound device characteristics

AbstractIn this work, we have reported two donor materials ONAF and SNAF that complement the widely used non‐fullerene acceptor ITIC, with the aim to develop non‐fullerene all small molecule organic bulk heterojunction solar cells (NF‐ASM‐OBHJSCs). Two small molecule donors A‐D‐D'‐D‐A architecture, namely ONAF and SNAF are synthesized and explored their ability to act as donor material along with the ITIC acceptor. Benzodithiophene (BDT) is used as central donor unit (dialkoxy‐BDT for ONAF and Dialkylthiophene‐BDT for SNAF) along with novel naphthodithiophene (NDT) terminal donor to improve light harvesting and charge conducting ability of the donor materials. N‐Butylrhodanine is employed as terminal acceptor to enhance the intramolecular charge transfer (ICT) and to place energy level of the donor materials. Both the small molecule donor materials have good thermal stability, broad and intense absorption displayed in the UV‐vis region with energy levels suitable for ITIC acceptor. NF‐ASM‐OBHJSCs are fabricated by using all small molecules (ONAF & SNAF) as donors and ITIC as acceptor. OBHJSCs based on SNAF displayed superior photovoltaic properties with power conversion efficiency (PCE) as 9.94 % (Jsc = 15.93 mA cm‐2, Voc = 0.96 V and FF = 0.65) compared to ONAF based devices with PCE as 7.48 % (Jsc = 14.93 mA cm‐2, Voc = 0.82 V and FF = 0.61). The substantial high PCE of SANF is attributed to the broad absorption, relatively well‐matched HOMO‐LUMO energy levels between SNAF – ITIC acceptor, better film morphology for efficient charge transport and exciton dissociation at donor/acceptor (D‐A) junction. The observed PCE is one of the best reported for the NDT‐BDT‐based all small molecule non‐fullerene OBHJSCs. Moreover, simplicity in the device fabrication procedures and low energy loss associated are added advantages with the SNAF based devices. We predict that the HOMO‐LUMO levels (CV) of D/A falling in this category, as in Figure S7, with further close energy levels will have implications on the efficiency of OBHJSCs.

Developement of highly efficient large area organic photovoltaic module: Effects of nonfullerene acceptor

Abstract To transfer spin-coating based high performance polymer solar cells (PSCs) to large area using roll-to-roll compatible solution process, it is important to understand effect of the processing method and materials on morphology of bulk-heterojunction (BHJ) films. We developed PBDB-T based PSCs modules using PC71BM or nonfullerene acceptor ITIC with blade coating. It is found that crystallization of ITIC and its following aggregation are significantly affected by solvent drying dynamics which is induced by thermal heating whereas PC71BM shows similar BHJ morphology regardless heating temperature. We studied mechanism and effects of acceptors on BHJ film forming during the coating. ITIC-solvent interaction makes ITIC to form high crystalline structures and but, large aggregated domains during slow solvent evaporation at room temperature. Such aggregated morphology of PBDB-T:ITIC is much improved to optimal morphology for effective charge generation and transporting with a 50 °C heating process. As a result, PBDB-T:ITIC photovoltaic module (active area 58.5 cm2) prepared at 50 °C showed much enhanced efficiency of 9.03% than 5.0% of room temperature processed one. This result is among the highest performance of large active-area PSC modules and provides research guidelines for efficient transfer of spin-coating based small cell technology to roll-to-roll based large-area PSC modules.

Ternary copolymerization strategy reducing the cost of benzodithiophene–benzodithiophenedione polymer, retaining high photovoltaic performance

AbstractThe photovoltaic performance of organic solar cells has been rapidly improved. However, in actual production and application, there are still many challenges for organic photovoltaic devices, and one of the important strategies for facilitating industrialization is reducing the cost of materials. The benzodithiophene (BDT)–benzodithiophenedione (BDD) binary copolymer donor material has good photovoltaic performance. However, the high cost of BDD monomer limits its industrialization. In contrast, lower‐cost benzotriazole (BTA) monomers have an enormous potential to solve this problem. Therefore, this paper reports the synthesis of ternary copolymers by adding BTA units into BDT–BDD copolymer with two different acceptor units. It is found that such a strategy can effectively reduce the cost of BDT–BDD polymer, and increasing the content of BTA units in the polymer to reduce the content of expensive BDD units can still retain the excellent photovoltaic properties of the BDT–BDD polymer. When only 20 mol% of BDD units remained, the bulk heterojunction structure with the synthesized donor polymer and acceptor ITIC can achieve an open circuit voltage of 0.913 V with a short circuit current of 16.87 mA cm−2 and 9.12% maximum photoelectric conversion efficiency, and the material is much cheaper than BDT–BDD polymer, in terms of structural units. Therefore, the results show that the addition of BTA can effectively reduce the material cost (versus BDD only) while retaining the good photovoltaic properties of the material, and has a certain guiding effect on the commercialization of organic solar cells. © 2020 Society of Chemical Industry

Photophysics, morphology and device performances correlation on non-fullerene acceptor based binary and ternary solar cells

Abstract Non-fullerene acceptor (NFA) based organic solar cells (OSCs) are of high efficiency and low energy loss and low recombination features, which is owing to the advantage of non-fullerene acceptors. The photophysics investigation of non-fullerene solar cells, in comparing to fullerene based analogue as well as mixed acceptor ternary blends could help to understand the working mechanism of NFA functioning mechanism. We choose PBDB-T donor, the fullerene derivative PC71BM acceptor, and the non-fullerene acceptor ITIC as the model system, to construct binary and ternary solar cells, which then subject to ultrafast spectroscopy investigation. The charge transfer pathway in binary and ternary blends is revealed. And it is seen that ITIC leads to a faster exciton separation and exciton diffusion. ITIC in blends suppresses the geminate recombination and shows smaller amount of charge transfer states, which is beneficial for the device performance. And the addition of ITIC enhances the crystallinity for both donor and acceptor leads to a morphology change of forming bicontinuous crystalline networks and phase separation. In a consequence, fill factor and JSC, increase dramatically for the related OSC.

Comparative study on the effects of alkylsilyl and alkylthio side chains on the performance of fullerene and non-fullerene polymer solar cells

Two novel high gap donor polymers – PBDTTSi-TzBI and PBDTTS-TzBI , based on imide-fused benzotriazole (TzBI) with asymmetric side chains and alkylsilyl (Si) or alkylthio (S) substituted 4,8-di(thien-2-yl)benzo-[1,2- b :4,5- b ′]dithiophene (BDTT) – are successfully synthesized. The effect of the side chain variation on the photophysical, morphological and photovoltaic properties of blends of these polymers with fullerene and non-fullerene acceptors is investigated. The PBDTTSi-TzBI polymer shows a deeper highest occupied molecular orbital energy level, which results in higher open-circuit voltages. Nevertheless, the polymer solar cells fabricated using PBDTTS-TzBI in combination with PC 71 BM afford a higher power conversion efficiency of 7.3% ( vs 4.0% for PBDTTSi-TzBI :PC 71 BM). By using the non-fullerene acceptor ITIC, the absorption of the blends extends to 850 nm and better device efficiencies are achieved, 6.9% and 9.6% for PBDTTSi-TzBI :ITIC and BDTTS-TzBI :ITIC, respectively. The better performance of the PBDTTS-TzBI :ITIC-based devices is attributed to the strong and broad absorption and balanced charge transport, and is among the best performances reported for non-fullerene solar cells based on TzBI-containing polymer donors. • High gap copolymers based on benzodithiophene and imide fused benzotriazole are synthesized. • The polymers are applied in organic solar cells with both fullerene and non-fullerene acceptors. • Side chain variation (alkylthio vs -silyl) is used to optimize the active layer blend properties. • Alkylsilyl substitution leads to higher open-circuit voltages. • Power conversion efficiencies up to 9.6% are achieved under solar illumination.

Impact of Donor-Acceptor Interaction and Solvent Additive on the Vertical Composition Distribution of Bulk Heterojunction Polymer Solar Cells.

The vertical composition distribution of a bulk heterojunction (BHJ) photoactive layer is known to have dramatic effects on photovoltaic performance in polymer solar cells. However, the vertical composition distribution evolution rules of BHJ films are still elusive. In this contribution, three BHJ film systems, composed of polymer donor PBDB-T, and three different classes of acceptor (fullerene acceptor PCBM, small-molecule acceptor ITIC, and polymer acceptor N2200) are systematically investigated using neutron reflectometry to examine how donor-acceptor interaction and solvent additive impact the vertical composition distribution. Our results show that those three BHJ films possess homogeneous vertical composition distributions across the bulk of the film, while very different composition accumulations near the top and bottom surface were observed, which could be attributed to different repulsion, miscibility, and phase separation between the donor and acceptor components as approved by the measurement of the donor-acceptor Flory-Huggins interaction parameter χ. Moreover, the solvent additive 1,8-diiodooctane (DIO) can induce more distinct vertical composition distribution especially in nonfullerene acceptor-based BHJ films. Thus, higher power conversion efficiencies were achieved in inverted solar cells because of facilitated charge transport in the active layer, improved carrier collection at electrodes, and suppressed charge recombination in BHJ solar cells.

1 V high open-circuit voltage fluorinated alkoxybiphenyl side-chained benzodithiophene based photovoltaic polymers

Utilizing two-dimensional (2-D) conjugated structure and extending two-dimensional π-conjugation system with benzene can improve the performance of the BDT-based polymer solar cells (PSCs). In this work, combining with strong electron-drawing ability of fluorine atom, a new monomer BBFBDT with fluorinated alkoxybiphenyl unit as side-chain was designed and synthesized to construct medium band-gap donor (D) - acceptor (A) copolymer P1 with a benzo[1,2- c:4,5-c’]dithiophene-4,8-dione (BDD) acceptor. Blending with a classical non-fullerene acceptor ITIC, the P1-based PSCs reached a power conversion efficiency (PCE) of 4.16% and when coupled with a fullerene acceptor PC71BM, the PCE of PSCs reached 4.66% with an open-circuit voltage (Voc) of 0.93 V, a short-circuit current density (Jsc) of 9.83 mA cm−2 and a fill factor (FF) of 50.97%. The relatively poor Jsc of P1-based devices may be caused by the bad complementarity of absorption spectra. Furthermore, a wide band-gap D-A copolymer P2, with a electron-deficient 4,7-bis(5-bromothiophen-2-yl)-2-((2-ethylhexyl)oxy)-5,6-difluoro-2H-benzo[d][1,2,3]triazole (TZ) as the acceptor unit, was synthesized to match the absorption spectra of ITIC. Finally, the efficiency achieved 6.59% with Voc of 0.99 V, Jsc of 14.37 mA cm−2 and FF of 46.32%.

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

AbstractThe 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.

Diffusion-Limited Crystallization: A Rationale for the Thermal Stability of Non-Fullerene Solar Cells.

Organic solar cells are thought to suffer from poor thermal stability of the active layer nanostructure, a common belief that is based on the extensive work that has been carried out on fullerene-based systems. We show that a widely studied non-fullerene acceptor, the indacenodithienothiophene-based acceptor ITIC, crystallizes in a profoundly different way as compared to fullerenes. Although fullerenes are frozen below the glass-transition temperature Tg of the photovoltaic blend, ITIC can undergo a glass-crystal transition considerably below its high Tg of ∼180 °C. Nanoscopic crystallites of a low-temperature polymorph are able to form through a diffusion-limited crystallization process. The resulting fine-grained nanostructure does not evolve further with time and hence is characterized by a high degree of thermal stability. Instead, above Tg, the low temperature polymorph melts, and micrometer-sized crystals of a high-temperature polymorph develop, enabled by more rapid diffusion and hence long-range mass transport. This leads to the same detrimental decrease in photovoltaic performance that is known to occur also in the case of fullerene-based blends. Besides explaining the superior thermal stability of non-fullerene blends at relatively high temperatures, our work introduces a new rationale for the design of bulk heterojunctions that is not based on the selection of high- Tg materials per se but diffusion-limited crystallization. The planar structure of ITIC and potentially other non-fullerene acceptors readily facilitates the desired glass-crystal transition, which constitutes a significant advantage over fullerenes, and may pave the way for truly stable organic solar cells.

Interaction between the Non-Fullerene Acceptor ITIC and Potassium.

Using density functional theory calculations and photoemission measurements, we have studied the interaction between the non-fullerene small-molecule acceptor ITIC and K atoms (a representative of reactive metals). It is found that the acceptor–donor–acceptor-type geometric structure and the electronic structure of ITIC largely decide the interaction process. One ITIC molecule can combine with more than 20 K atoms. For stoichiometries Kx≤6ITIC, the K atoms are attracted to the acceptor units of the molecule and donate their 4s electrons to the unoccupied molecular orbitals. K–ITIC organometallic complexes, characterized by the breaking of some S–C bonds in the donor unit of ITIC and the formation of K–S bonds, are formed for stoichiometries Kx≥7ITIC. The complexes are still conjugated despite the breaking of some S–C bonds.

A medium-band-gap polymer based alkoxyl-substituted benzoxadiazole moiety for efficient polymer solar cells

A medium-band-gap donor–acceptor (D–A) polymer (PBDTS-DTBO) was designed and synthesized with alkoxyl-substituted benzo[c][1,2,5]oxadiazole (BO) as A unit and benzo[1,2-b:4,5-b']dithiophene (BDT) derivative (BDTTS) as D moiety. The resulting polymer not only possesses a high number-average molecular weight (Mn) of 53.0 kDa, but also exhibits a deep highest occupied molecular orbital (HOMO) energy level of −5.43 eV, which is in favor of a higher VOC of 0.88 V in the photovoltaic devices. Consequently, due to excellent phase separation and weak bimolecular recombination, the efficiency of the fullerene-based solar cells can reach 8.47% with a high current density (JSC) of 14.93 mA cm−2 and FF of 0.661, which is one of highest values for currently reported benzoxadiazole-based materials. In addition, considering the polymer also shows a medium optical bandgap of 1.75 eV, which can be well complementary with the classical non-fullerene acceptor ITIC. The best performance device exhibits a good efficiency of 6.45% in fullerene-free devices, which could be ascribed to the serious bimolecular recombination. This work revealed that alkoxyl-substituted benzoxadiazole could be a promising moiety to construct efficient light-harvesting polymer.