Silaindacenodithiophene‐Based Fused‐Ring Non‐Fullerene Electron Acceptor for Efficient Polymer Solar Cells

In this work, a new A‐D‐A type nonfullerene small molecular acceptor SiIDT‐IC, with a fused‐ring silaindacenodithiophene (SiIDT) as D unit and 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (INCN) as the end A unit, was design and synthesized. The SiIDT‐IC film shows absorption peak and edge at 695 and 733 nm, respectively. The HOMO and LUMO of SiIDT‐IC are of −5.47 and −3.78 eV, respectively. Compared with carbon‐bridging, the Si‐bridging can result in an upper‐lying LUMO level of an acceptor, which is benefit to achieve a higher open‐circuit voltage in polymer solar cells (PSCs). Complementary absorption and suitable energy level alignment between SiIDT‐IC and wide bandgap polymer donor PBDB‐T were found. For the PBDB‐T:SiIDT‐IC based inverted PSCs, a D/A ratio of 1: 1 was optimal to achieve a power conversion efficiency (PCE) of 7.27%. With thermal annealing (TA) of the blend film, a higher PCE of 8.16% could be realized due to increasing of both short‐circuit current density and fill factor. After the TA treatment, hole and electron mobilities were elevated to 3.42 × 10⁻⁴ and 1.02 × 10⁻⁴ cm²·V⁻¹·s⁻¹, respectively. The results suggest that the SiIDT, a Si‐bridged fused ring, is a valuable D unit to construct efficient nonfullerene acceptors for PSCs.     

A Three‐dimensional Non‐fullerene Small Molecule Acceptor for Solution‐processed Organic Solar Cells

An acceptor‐donor‐acceptor (A‐D‐A) three‐dimensional (3D ) small molecule acceptor (SFTTIC ), using spirobifluorene as the core unit linking with four thieno[3,2‐b ]thiophenes (TT ) and end‐capped with 2‐(3‐oxo ‐2,3‐dihydro‐1H ‐inden‐1‐ylidene)malononitrile (INCN ) was developed for solution processed organic solar cells. SFTTIC has a high absorption coefficient up to 3.12 × 10⁵ mol⁻¹•cm⁻¹, good thermal stability and appropriate energy levels. The optimized power conversion efficiency (PCE ) of 5.66% and 4.65% was achieved for the devices with PBDB ‐T:SFTTIC and PTB7 ‐Th:SFTTIC , respectively.     

小分子给体/高分子受体型有机太阳能电池研究进展

有机太阳能电池具有低成本、柔性和质量轻等优势,是一种有应用前景的光伏技术,受到人们的广泛关注.有机太阳能电池的光敏活性层通常由p-型有机半导体(包括小分子和高分子)与n-型有机半导体(包括小分子和高分子)共混而成.小分子给体/高分子受体型有机太阳能电池具有形貌热稳定性优异的特点,值得深入研究.本综述旨在总结小分子给体/高分子受体型有机太阳能电池的研究进展,分别介绍了基于酰亚胺基、氰基和含硼氮配位键(B←N)的高分子受体的活性层材料体系的发展状况.在器件性能方面,通过分子设计、相分离形貌调控,改善了小分子给体/高分子受体的匹配性,将该类电池的能量转换效率从最初的0.29%提升至目前的9.51%,为性能的进一步提升总结了经验;在稳定性方面,基于该体系形貌热稳定性优异的特点,开发出高温耐受型有机太阳能电池器件.最后,展望了小分子给体/高分子受体型有机太阳能电池的未来发展方向和前景.     

Photodegradation of C‐PCPDTBT and Si‐PCPDTBT: Influence of the Bridging Atom on the Stability of a Low‐Band‐Gap Polymer for Solar Cell Application

The kinetics of photodegradation and the reactivity of different sites of the low‐band‐gap polymers poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (C‐PCPDTBT) and poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (Si‐PCPDTBT) are investigated as thin films and are compared to those of poly(3‐hexylthiophene) (P3HT). The decay kinetics are monitored with UV/Vis spectroscopy and the reactivity and product evolution are investigated with X‐ray photoelectron spectroscopy (XPS). Both polymers exhibit higher stability than P3HT. The bridging atom in the cyclopentadithiophene (CPDT) subunit has a significant influence on the stability. Varying oxidation rates for the different elements were observed. In the case of Si‐PCPDTBT, the silicon atom is oxidized primarily, whereas the photooxidation rates of the other elements are reduced relative to C‐PCPDTBT. Additionally, XPS experiments with varying excitation energies reveal a significant reaction gradient within a few nanometers of the surface of degraded thin films of C‐PCPDTBT.     

Wide Band Gap Non‐Fullerene Small Molecular Acceptors Containing Spirobifluorene and Benzotriazole with Three Different End‐Capped Groups for P3HT‐Based Organic Solar Cells

Spirofluorene (SF) and benzo[d][1,2,3]triazole (BTA) have been considered as promising building blocks to construct n‐type photovoltaic materials. Herein, three new small molecule acceptors (SMAs) named BTA21 , BTA23 and BTA27 with the structure of A₂ = A₁‐D‐A₁ = A₂ have been designed, in which SF and BTA were used as a central unit of D and bridged acceptor unit of A₁, respectively. In addition, 3‐ethylrhodanine, 2‐(3‐ethyl‐4‐ oxothiazolidin‐2‐ylidene)malononitrile and malononitrile were chosen as terminal acceptor units to modulate the properties of the final SMAs. Three SMAs show wide optical band gaps (E_g) of 2.19, 2.15 and 2.21 eV, respectively, with gradually down‐shift of the lowest unoccupied molecular orbital (LUMO) levels in the order of BTA21 , BTA23 and BTA27 depending on the electron‐withdrawing capability of terminal acceptor units. BTA21 shows great advantages with respect to donor poly(3‐hexylthiophene) (P3HT) over BTA23 and BTA27 , such as well energy‐level matching, complementary absorption and proper morphology. Concequently, P3HT: BTA21 shows the best power conversion efficiency (PCE) value of 3.28% with an open‐circuit voltage (V_OC) of 1.02 V, a short‐circuit current (J_SC) of 5.45 mA·cm^–2 and a fill factor (FF) of 0.59. These results indicate that the terminal acceptor group end‐capped in SMAs plays a significant role in controlling their optical, electronic, and photovoltaic properties.     

Anthracene‐Containing Wide‐Band‐Gap Conjugated Polymers for High‐Open‐Circuit‐Voltage Polymer Solar Cells

The synthesis, characterization, and photophysical and photovoltaic properties of two anthracene‐containing wide‐band‐gap donor and acceptor (D–A) alternating conjugated polymers ( P1 and P2 ) are described. These two polymers absorb in the range of 300–600 nm with a band gap of about 2.12 eV. Polymer solar cells with P1 :PC₇₁BM as the active layer demonstrate a power conversion efficiency (PCE) of 2.23% with a high V_oc of 0.96 V, a J_sc of 4.4 mA cm⁻², and a comparable fill factor (FF) of 0.53 under simulated solar illumination of AM 1.5 G (100 mW cm⁻²). In addition, P2 :PC₇₁BM blend‐based solar cells exhibit a PCE of 1.42% with a comparable V_oc of 0.89 V, a J_sc of 3.0 mA cm⁻², and an FF of 0.53.

     

Polymer Acceptor Based on B←N Units with Enhanced Electron Mobility for Efficient All‐Polymer Solar Cells

We demonstrate that polymer electron acceptors with excellent all‐polymer solar‐cell (all‐PSC) device performance can be developed from polymer electron donors by using B←N units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain B←N units, the π–π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all‐PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all‐PSCs with state‐of‐the‐art polymer acceptors.     

Side‐Chain Engineering of Benzodithiophene‐Fluorinated Quinoxaline Low‐Band‐Gap Co‐polymers for High‐Performance Polymer Solar Cells

A new series of donor–acceptor co‐polymers based on benzodithiophene and quinoxaline with various side chains have been developed for polymer solar cells. The effect of the degree of branching and dimensionality of the side chains were systematically investigated on the thermal stability, optical absorption, energy levels, molecular packing, and photovoltaic performance of the resulting co‐polymers. The results indicated that the linear and 2D conjugated side chains improved the thermal stabilities and optical absorptions. The introduction of alkylthienyl side chains could efficiently lower the energy levels compared with the alkoxyl‐substituted analogues, and the branched alkoxyl side chains could deepen the HOMO levels relative to the linear alkoxyl chains. The branched alkoxyl groups induced better lamellar‐like ordering, but poorer face‐to‐face packing behavior. The 2D conjugated side chains had a negative influence on the crystalline properties of the co‐polymers. The performance of the devices indicated that the branched alkoxyl side chains improved the V_oc, but decreased the J_sc and fill factor (FF). However, the 2D conjugated side chains would increase the V_oc, J_sc, and FF simultaneously. For the first time, our work provides insight into molecular design strategies through side‐chain engineering to achieve efficient polymer solar cells by considering both the degree of branching and dimensionality.     

高效率窄带隙聚合物太阳能电池材料

太阳能电池能够将太阳能直接转化为电能,是利用太阳能资源的一种非常有效的手段。聚合物太阳能电池因成本低、重量轻、制备方便和可制成柔性器件的优点,已经成为该领域的研究热点之一。基于窄带隙共轭聚合物给体/富勒烯受体复合材料体系制得的太阳能电池的最高转换效率已经达到8.3%,而寻找性能更优异的聚合物给体材料是进一步提高光伏性能的关键因素。本文综述了近几年关于高效率窄带隙聚合物太阳能电池给体材料的研究进展,着重介绍了苯并噻二唑类共聚物、稠环噻吩类共聚物和吡嗪类共聚物等窄带隙聚合物给体材料体系及相应光伏器件的性能,分析了各种材料的优点和不足,并对今后这一领域的发展做了展望。     

A High Dielectric N‐Type Small Molecular Acceptor Containing Oligoethyleneglycol Side‐Chains for Organic Solar Cells

We report a new small molecular acceptor, ITIC‐OEG, which is based on indacenodithieno[3,2‐b]thiophene and 1,1‐(dicyanomethylene)‐3‐ indanone including oligoethyleneglycol (OEG) side‐chains. ITIC‐OEG was found to have higher dielectric constant (ε_r=5.6) than that of a reference molecule of ITIC with normal alkyl substituents (ε_r=3.9). The dielectric constant of medium influences significantly the exciton binding energy and the resulting charge separation and recombination. The optical, electrochemical and morphological properties of ITIC‐OEG and its photovoltaic characteristics were investigated by blending with a semi‐crystalline donor polymer, PPDT2FBT, with comparison to those of ITIC. ITIC‐OEG shows more red‐shifted absorption and stronger crystalline packing than ITIC. However, the lower photovoltaic performance (with 1.58% power conversion efficiency, PCE) was measured for PPDT2FBT:ITIC‐OEG, compared to PPDT2FBT:ITIC (5.52% PCE). The incompatibility between PPDT2FBT and ITIC‐OEG (due to high hydrophilic nature of OEG chains) resulted in poor intermixing with large domain separation over 300 nm, showing inefficient charge separation and significant charge recombination. Therefore, to investigate the effect of dielectric constant of the materials on the charge separation and recombination, the blend morphology of the PPDT2FBT:ITIC‐OEG should be optimized first by improving their miscibility and phase separation.     

Rigid Nonfullerene Acceptors Based on Triptycene–Perylene Dye for Organic Solar Cells

Three kinds of nonconjugated rigid perylene bisimide (PBI) derivatives based on a triptycene core were designed, synthesized and characterized. The unique three‐dimensional (3D) conformation of triptycene could enable formation of polymer with the favorable morphology for organic polymer solar cells (PSCs) by relieving the self‐aggregation of rigid PBI units. The low‐lying LUMO energy levels of these compounds demonstrated that they are very suitable for use as acceptors in organic solar cells. A higher power conversion efficiency (PCE) of 6.15 % was obtained for the blend film using the compound with two PBI units ( T‐2 ) as the acceptor and commercial poly[[4,8‐bis[5‐(2‐ethylhexyl)thiophene‐2‐yl]benzo[1,2‐b :4,5‐b ′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)‐carbonyl]thieno[3,4‐b ]thiophenediyl]] (PCE‐10) as the electron donor.     

骨架非稠合的电子受体在聚合物太阳能电池中的应用

Non-fullerene electron acceptors have attracted enormous attention of the research community owing to their advantages of optoelectronic and chemical tunabilities for promoting high-performance polymer solar cells (PSCs). Among them, fused-ring electron acceptors (FREAs) are the most popular ones with the good structural planarity and rigidity, which successfully boost the power conversion efficiencies (PCEs) of PSCs to over 14%. In considering the cost-control of future scale-up applications, it is also worthwhile to explore novel structures that are easy to synthesize and still maintain the advantages of FREAs. In this work, we design and synthesize a new electron acceptor with an unfused backbone, 5, 5'-((2, 5-bis((2-hexyldecyl)oxy)-1, 4-phenylene)bis(thiophene-2-yl))bis(methanylylidene)) bis(3-oxo-2, 3-dihydro-1H-indene-2, 1-diylidene))dimal-ononitrile (ICTP), which contains two thiophenes and one alkoxy benzene as the core and 2-(3-oxo-2, 3-dihydroinden-1-ylidene) malononitrile (IC) as the terminal groups. The synthetic route to ICTP involves only three steps, with high yields. Density functional theory calculations indicate that the non-covalent interactions, O…H and O…S, help reinforce the space conformation between the central core and the terminals. ICTP shows broad and strong absorption in the long-wavelength range between 500 and 760 nm. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of ICTP were measured to be -5.56 and -3.84 eV by cyclic voltammetry. The suitable absorption and energy levels make ICTP a good acceptor candidate for medium bandgap polymer donors. The best devices based on PBDB-T:ICTP showed a PCE of 4.43%, with an open circuit voltage (V_OC) of 0.97 V, a short circuit current density (J_SC) of 8.29 mA∙cm^-2, and a fill factor (FF) of 0.55, after adding 1% 1, 8-diiodooctane (DIO) as the solvent additive. Atomic force microscopy revealed that DIO could ameliorate the strong aggregation in the blended film and lead to a smoother film surface. The hole and electron mobilities of the optimized device were measured to be 9.64 and 2.03 × 10^-5 cm²∙V^-1∙s^-1, respectively, by the space-charge-limited current method. The relatively low mobilities might be responsible for the moderate PCE. Further studies can be performed to enlarge the conjugation length by including more aromatic rings. This study provides a simple strategy to design non-fullerene acceptors and a valuable reference for the future development of PSCs.     

A‐D‐A Type Small Molecules Based on Boron Dipyrromethene for Solution‐Processed Organic Solar Cells

The unique properties of boron dipyrromethene (BODIPY) dyes including facile synthesis, high absorption coefficients, and delocalized molecular orbitals as well as excellent photochemical and thermal stability, make them promising as materials for organic solar cells. Accordingly, in this study three A‐D ‐A structural small molecules of BDTT‐BODIPY, FL‐BODIPY, and TT‐BODIPY have been synthesized, in which two BODIPY acceptor units are symmetrically conjugated to 4,8‐bis(5‐(2‐ethylhexyl) thiophen‐2‐yl)benzo[1,2‐b:4,5‐b]dithiophene (BDTT), 9,9‐dioctyl‐9H‐fluorene (FL), and thieno[3,2‐b]thiophene (TT) donor cores, respectively. The manipulation of the structural parameters significantly improves the performances of the BHJ OSCs, which show power conversion efficiencies of 4.75 %, 1.51 %, and 1.67 % based on [6,6]‐phenyl C₇₁‐butyric acid methyl ester (PC₇₁BM) as the acceptor material and BDTT‐BODIPY, FL‐BODIPY, and TT‐BODIPY as the donor materials, respectively.     

高结晶性小分子给体材料应用于全小分子有机太阳能电池中的研究进展

随着新型小分子给体材料和非富勒烯小分子受体材料的开发和应用, 非富勒烯全小分子有机太阳能电池(NF-ASM OSCs)的光电转换效率已经突破15%, 并逐渐接近聚合物太阳能电池的效率. 相比于聚合物电子给体材料, 小分子电子给体材料拥有其独特的优势, 例如合成批次性差异小、分子量明确和易于提纯等; 但是, 对小分子给体材料的结晶性难于精确调控, 使获得合适的纳米级结构的混合膜仍然是一个挑战. 本综述以给体小分子中心共轭单元的扩展为主线, 从分子设计的角度汇总了近年来对苯并二噻吩、萘并二噻吩和二噻并苯并二噻吩类小分子给体材料的结晶性研究, 并为进一步改善电池活性层形貌和获得更高的光伏性能提供了未来发展的建议.     

A Solution‐Processable Molecule using Thieno[3,2‐b]thiophene as Building Block for Efficient Organic Solar Cells

A solution‐processed acceptor‐π‐donor‐π‐acceptor (A‐π‐D‐π‐A) type small molecule, namely DCATT, has been designed and synthesized for the application as donor material in organic solar cells. The fused aromatic unit thieno[3,2‐b]thiophene (TT) flanked with thiophene is applied as π bridge, while 4,8‐bisthienyl substituted benzodithiophene (BDT) and 2‐ethylhexyl cyanoacetate are chosen as the central building block and end group, respectively. Introduction of fused ring to the small molecule enhances the conjugation length of the main chain, and gives a strong tendency to form π–π stacking with a large overlapping area which favors to high charge carrier transport. Small‐molecule organic solar cells based on blends of DCATT and fullerene acceptor exhibit power conversion efficiencies as high as 5.20 % under the illumination of AM 1.5G, 100 mW cm^?2.     

Synthesis and Characterization of a Soluble A–D–A Molecule Containing a 2D Conjugated Selenophene‐Based Side Group for Organic Solar Cells

A new acceptor–donor–acceptor (A–D–A) small molecule based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) is synthesized via a Stille cross‐coupling reaction. A highly conjugated selenophene‐based side group is incorporated into each BDT unit to generate a 2D soluble small molecule (SeBDT‐DPP). SeBDT‐DPP thin films produce two distinct absorption peaks. The shorter wavelength absorption (400 nm) is attributed to the BDT units containing conjugated selenophene‐based side groups, and the longer wavelength band is due to the intramolecular charge transfer between the BDT donor and the DPP acceptor. SeBDT‐DPP thin films can harvest a broad solar spectrum covering the range 350–750 nm and have a low bandgap energy of 1.63 eV. Solution‐processed field‐effect transistors fabricated with this small molecule exhibit p‐type organic thin film transistor characteristics, and the field‐effect mobility of a SeBDT‐DPP device is measured to be 2.3 × 10⁻³ cm² V⁻¹ s⁻¹. A small molecule solar cell device is prepared by using SeBDT‐DPP as the active layer is found to exhibit a power conversion efficiency of 5.04% under AM 1.5 G (100 mW cm⁻²) conditions.

     

Ternary Organic Solar Cells Based on Two Non‐fullerene Acceptors with Complimentary Absorption and Balanced Crystallinity

The ternary blend structure has been demonstrated as an effective approach to increase the power conversion efficiency of organic solar cells. An effective approach to enhance the power conversion efficiency of ternary solar cells is based on two non‐fullerene acceptors with complimentary absorption range and balanced crystallinity. In this work, we have introduced a high crystallinity small‐molecule acceptor, named C8IDTT‐4Cl with appropriate alkyl side chains into a low crystalline blend of conjugated polymer donor PBDT‐TPD and fused‐ring electron acceptor ITIC‐4F. A ternary device based on the blend PBDT‐TPD:ITIC‐4F:C8IDTT‐4Cl exhibits a best power conversion efficiency of 9.51% with a simultaneous improvement of the short‐circuit current density to 18.76 mA·cm^–2 and the fill factor up to 67.53%. The absorption onset for C8IDTT‐4Cl is located at 900 nm, so that the well complementary light absorption is beneficial to the photocurrent. In addition, the existence of high crystallinity C8IDTT‐4Cl in the ternary device is found helpful to modulate crystallinity, improve heterojunction morphologies and stacking structure, therefore to realize higher charge mobility and better performance.     

Dye‐Incorporated Polynaphthalenediimide Acceptor for Additive‐Free High‐Performance All‐Polymer Solar Cells

All‐polymer solar cells (all‐PSCs) can offer unique advantages for applications in flexible devices, and naphthalene diimide (NDI)‐based polymer acceptors are the widely used polymer acceptors. However, their power conversion efficiency (PCE) still lags behind that of state‐of‐the‐art polymer solar cells, due to low light absorption, suboptimal energy levels and the strong aggregation of the NDI‐based polymer acceptor. Herein, a rhodanine‐based dye molecule was introduced into the NDI‐based polymer acceptor by simple random copolymerization and showed an improved light absorption coefficient, an up‐shifted lowest unoccupied molecular orbital level and reduced crystallization. Consequently, additive‐free all‐PSCs demonstrated a high PCE of 8.13 %, which is one of the highest performance characteristics reported for all‐PSCs to date. These results indicate that incorporating a dye into the n‐type polymer gives insight into the precise design of high‐performance polymer acceptors for all‐PSCs.