CHCl 3 solution; b) Calculated from the film UV-vis-NIR absorption onset; c) Measured from the CV; d) Calculated by the equation: E LUMO = E HOMO + E g .

Due to the potential advantages of low cost, light weight, flexibility, and large area fabrication, bulk heterojunction (BHJ) solar cells have attracted much attention. [1][2][3][4] The power conversion efficiencies (PCEs) of polymer solar cells (PSCs) based on blends of polymer donors and fullerene acceptors have reached 7-8%. [5][6][7][8][9] Although the highest PCE of solutionprocessed organic small molecule solar cells (OSCs) was still lower than their polymeric counterparts, the advantages of well defined molecular structure, definite molecular weight, and high purity without batch to batch variations render small molecule-based OSCs a promising field. [10][11][12][13] Among the small molecules with high photovotaic performance, the widely used donor building blocks mainly involve triphenylamine (TPA) and thiophene; [14,15] and acceptor building units include diketopyrrolopyrrole (DPP), [16,17] benzothiadiazole (BT), [18][19][20][21] squaraine, [22,23] dicyanovinyl, [24][25][26][27] perylene diimide, [28] etc. [29][30][31] Up to now, solution processed OSCs based on small molecules with PCEs > 4% remain rare. [17,21,[32][33][34] Most of the small molecule-based BHJ solar cells exhibited PCEs lower than 3%.Thiazolothiazole (TT) has a rigid and coplanar fused ring, and thereby ensures highly extended π-electron system and strong π-stacking. As a result, conjugated small molecules and polymers based on thiazolothiazole exhibited high charge carrier mobilities. [35][36][37] Recently, several thiazolothiazole-based copolymers were synthesized for PSC applications, and PCEs up to 5.59% were reported. [38][39][40][41][42][43] However, to our knowledge, there have been no reports on TT-based small molecules for application in organic solar cells. Here we report synthesis and characterization of a new linear, D-A-D organic small molecule with TPA as donor (D) unit, thiophene as bridge, and thiazolothiazole as acceptor unit, (TT-TTPA, Scheme 1). Solution processed solar cells based on blend of TT-TTPA and PC 71 BM afforded a PCE as high as 3.73% after thermal annealing.The synthetic route of compound TT-TTPA is shown in Scheme 1. Stille coupling reaction between 2,5-bis(3-dodecyl-5-trimethylstannyl-thiophen-2-yl)-thiazolo [5,4-d]thiazole [36] and N,N-diphenyl-4-bromoaniline [44] afforded TT-TTPA. The electron-withdrawing building block thiazolothiazole was introduced to lower HOMO level (finally increase open-circuit voltage of OSC) and to extend absorption via intramolecular D-A charge transfer. TT-TTPA was fully characterized by spectroscopic methods and elemental analysis. TT-TTPA is soluble in common organic solvents such as dichlorobenzene, chloroform and THF due to the two solubilizing n-dodecyl substituents. The thermal property of TT-TTPA was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). This D-A-D molecule exhibits excellent thermal stability with decomposition temperature (5% weight loss) at ca. 355 °C ( Figure S1 in Supporting Information). The DSC traces for ...

The rapid advance of fused‐ring electron acceptors (FREAs) has greatly promoted the leap‐forward development of organic solar cells (OSCs). However, the synthetic complexity of FREAs may be detrimental for future commercial applications. Recently, nonfused‐ring electron acceptors (NREAs) have been developed to be a promising candidate to maintain a rational balance between cost and performance, of which the cores are composed of simple fused rings (NREAs‐I) or nonfused rings (NREAs‐II). Moreover, “noncovalently conformational locks”, are used as an effective strategy to enhance the rigidity and planarity of NREAs and improve device performance. Herein, a novel series of NREAs‐II (PhO4T‐1, PhO4T‐2, and PhO4T‐3) is constructed as a valuable platform for exploring the impact of the end group engineering on optoelectronic properties, intermolecular packing behaviors, and device performance. As a result, a high power conversion efficiency of 13.76% is achieved for PhO4T‐3 based OSCs, which is much higher than those of the PhO4T‐1 and PhO4T‐2‐based devices. Compared with several representative FREAs, PhO4T‐3 possesses the highest figure‐of‐merit value of 133.45 based on a cost‐efficiency evaluation. This work demonstrates that the simple‐structured NREAs‐II are promising candidates for low‐cost and high‐performance OSCs.

Recently, polymer solar cells (PSCs) exhibit promising potential in the world's renewable energy strategy due to their unique advantages such as low cost, light weight, and large-area fabrication on flexible substrates and have attracted much attention. 1À7 Through the creation of novel donor and acceptor materials and innovation of device fabrication technology, PSCs based on regioregular poly(3-hexylthiophene) (P3HT) have reached power conversion efficiencies (PCEs) over 6%, 8À10 and PCEs of PSCs based on alternating copolymers have been over 7%. 11À16 To further improve the PCE of PSCs, on one hand, electron donors and electron acceptors should have broad absorption, high mobility, and suitable energy levels; on the other hand, a better nanostructural ordering of donor and acceptor blends also facilitates charge generation and transport.Thiazole is a widely used electron-accepting heterocycle due to electron-withdrawing nitrogen of imine (CdN). Small molecules and polymers based on bithiazole were used as semiconductors in organic field-effect transistors (OFETs) and exhibited high electron or hole mobility. 17À20 Recently, conjugated copolymer-based bithiazoles have been used in PSCs as donors, and PCEs up to 3.82% were achieved in combination with PC 71 BM acceptor. 21À31 Most research work focused on main chain engineering of the bithiazole polymers, while there have been no reports on side chain engineering of the bithiazole polymers.Long conjugation length, planar molecular geometry, and rigid structure in π-conjugated polymers often leads to poor solubility or even insoluble in common solvents. For solution processing, the use of long alkyl or alkoxy side chains has been a common approach to improve the solubility of conjugated polymers. However, the side chain nature and position not only affect the molecular weight, solubility, and geometry of the polymers but also affect the absorption, energy levels, and charge transport properties. 32À42 Furthermore, the side chain affects morphology of resulting blends of polymer donors and fullerene acceptors, which has been regarded as a critical factor in determining the PCEs of the photovoltaic devices, and finally affects the photovoltaic performance of devices. 12,43À51Here we demonstrate synthesis and characterization of four structural related copolymers of bithiazole and benzodithiophene with the same backbone but different side chain pattern (P1ÀP4, Figure 1). In particular, we probe into impact of the shape and position of side chains on solubility, absorption, energy levels, and charge transport properties of the polymers as well as on morphology and photovoltaic properties of the donor/acceptor blends. Tiny difference in the side chains of ABSTRACT: Four new copolymers P1ÀP4 containing the same backbone of bithiazole acceptor unit and benzodithiophene donor unit but different side chain pattern were synthesized by Pd-catalyzed Stille coupling. The effect of the side chains on backbone conformation, solubility, absorption spectra, energy levels, charg...

Highly crystalline, well-defined nanowires of a donor–acceptor (D–A) conjugated polymer based on bithiazole-thiazolothiazole (PTz) were successfully prepared by a facile solution self-assembly method. In PTz nanowires, polymer chains align along the long axis of the nanowires forming lamellar structures with close π-stacking perpendicular to the long axis of the nanowires. The nanowires possess a single crystal structure with orthorhombic crystal unit cell in which the lattice parameters are a ≈ 21.05 Å, b ≈ 6.94 Å, and c ≈ 4.64 Å. The intrinsic charge transport property of PTz was characterized by using its single crystal nanowires in field-effect transistors with a mobility up to 0.46 cm2 V–1 s–1. As an example of PTz single crystal nanowires in optoelectronic application, phototransistors of PTz nanowires exhibited a photoresponsivity up to 2531 A W–1 and a photosensitivity up to 1.7 × 104.

An efficient iodination reaction of electron-deficient heterocycles is described. The reaction utilizes KO(t)Bu as an initiator and likely proceeds by a radical anion propagation mechanism. This new methodology is particularly effective for functionalization of building blocks for electron transport materials. Its utility is demonstrated with the synthesis of a new perylenediimide-thiazole non-fullerene acceptor capable of delivering a power conversion efficiency of 4.5% in a bulk-heterojunction organic solar cell.

Two conjugated alternating copolymers of thiazolothiazole with benzodithiophene (P1) or bithiazole (P2) were synthesized by a palladium(0)-catalyzed Stille coupling reaction. The thermal, electrochemical, optical, charge transport, and photovoltaic properties of these copolymers were examined. Compared with P1, P2 exhibits red shifted absorption, a lower band gap, and a lower HOMO level. The field-effect hole mobility of P1 is as high as 2.8 × 10 -3 cm 2 V -1 s -1 , which is 1 order of magnitude higher than that of P2. Polymer solar cells were fabricated based on the blend of the polymers and methanofullerene[6,6]-phenyl C61-butyric acid methyl ester (PC 61 BM). The PSC based on P1:PC 61 BM (1:2, w/w) exhibits a power conversion efficiency of 2.72% under AM 1.5, 100 mW cm -2 , higher than that reported for the thiazolothiazole-based polymers in the literature.

Two new low-bandgap, conjugated donor (D)−acceptor (A) copolymers of porphyrin with 2,3-bis(4-trifluoromethylphenyl)pyrido[3,4-b]pyrazine (P1) and perylene diimide (P2) were synthesized by Sonogashira coupling polymerization and compared with porphyrin−dithienothiophene D−D copolymer (P3). All these polymers possess good thermal stability with decomposition temperatures over 300 °C. Polymers P1 and P2 in films exhibit strong absorption in near-IR (820−950 nm) with optical bandgaps as low as 1.15 eV; their Q-bands red shift 60−190 nm compared to that of P3, while the Soret bands are similar. The HOMO (−5.3 to −5.4 eV) and LUMO (−3.6 to −4.0 eV) of the D−A polymers are lower than that of the D−D polymer. Two-photon absorption (2PA) properties of the polymers were investigated by the femtosecond Z-scan method. The D−A polymer P2 exhibits 2PA cross sections over 7000 GM/repeat unit at telecommunication wavelengths (1320 and 1520 nm), larger than that of P1 and P3, due to the very strong, rigid, and coplanar perylene diimide acceptor and strong D−A intramolecular charge transfer.