Hongkun Tian

Dithienocarbazole and Isoindigo based Amorphous Low Bandgap Conjugated Polymers for Efficient Polymer Solar Cells

Three highly rigid and planar low-bandgap conjugated polymers comprising alternate isoindigo and dithienocarbazole groups are synthesized for the fabrication of high performance polymer solar cells. Power conversion efficiencies of up to 7.2% for conventional devices and 8.2% for inverted devices are demonstrated.

Novel NIR-absorbing conjugated polymers for efficient polymer solar cells: effect of alkyl chain length on device performance

Three low bandgap conjugated polymers, i.e., PDTPBT-C8, PDTPBT-C6 and PDTPBT-C5, which consist of alternating N-alkyl dithieno[3,2-b:2′,3′-d]pyrrole and 2,1,3-benzothiadiazole units and carry 1-octylnonyl, 1-hexylheptyl and 1-pentylhexyl as side chains, respectively, were synthesized. These polymers show strong absorption in the wavelength range of 600–900 nm with enhanced absorption coefficient as the length of alkyl chain decreases. The film morphology of the polymers and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PCBM) blends is also dependent on the alkyl chain length. As the length decreases, the film becomes more uniform and the domian size decreases from 400–900 nm for PDTPBT-C8 to ∼50 nm for PDTPBT-C5. Bulk heterojunction photovoltaic solar cells (PSCs) were fabricated based on the blend of the polymers and PCBM with a weight ratio of 1:3. The device performance is dramatically improved as the length of the side chain decreases, due to enhanced film absorption coefficient and improved film morphology. With the polymer PDTPBT-C5, which carries the shortest alkyl chain, power conversion efficiency (PCE) up to 2.80% has been achieved. This result indicates that optimizing the structure of the solublizing alkyl chain is also crucial for the design and synthesis of high performance PSC polymeric materials.

Bulk Heterojunction Photovoltaic Cells with Low Donor Concentration

High-effi ciency organic photovoltaic (OPV) cells are mostly based on a bulk heterojunction [ 1 ] (BHJ) structure, which is essentially a thin fi lm of mixed electron donor and acceptor. With few exceptions, the acceptor component is a fullerene-based material such as C 60 [ 2 ] and PCBM, [ 3 ] whereas a large variety of materials has been found to be useful as the donor component. [ 4 ] In order to achieve high power conversion effi ciency, the donor–acceptor composition of the BHJ needs to be optimized with respect to light absorption and charge generation, including for instance the use of a low bandgap donor, [ 5 ] to complement the absorption of the acceptor and selecting a donor with proper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels to match that of the acceptor. In this work, we show that it is possible to achieve a large open-circuit voltage ( V oc > 1.0 V) in a fullerene-based OPV with almost any donor, provided that the donor is present in a small concentration and that MoO x is used as the Schottky barrier contact [ 6 ] to the BHJ. With fi ne tuning of the donor concentration to overcome the hole-transport limitation in the BHJ, high power conversion effi ciency ( η PCE > 5%) has been realized. In Figure 1 the schematic of the OPV cell layer sequence, indium tin oxide (ITO)/MoO x /BHJ/bathophenanthroline (Bphen)/Al, is shown along with the molecular structures of 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC), Bphen, C 60 , and C 70 . In this sequence, the BHJ layer is a mixed fi lm of TAPC [ 7 ] (donor) and C 60 (acceptor) with various ratios. The concentration of TAPC is indicated in volume fraction. As the major focus of this study was concerned with the composition of the BHJ layer, we kept the thickness and composition of all the other layers constant: ITO (90 nm), MoO x (2 nm), Bphen (8 nm), and Al (100 nm). First, we studied the effect of varying the TAPC concentration in the BHJ layer with a constant thickness of 40 nm. In Figure 2 a the current density–voltage ( J – V ) characteristics are

High Mobility Ambipolar Diketopyrrolopyrrole‐Based Conjugated Polymer Synthesized Via Direct Arylation Polycondensation

A diketopyrrolopyrrole-based conjugated polymer, PDPP-4FTVT, which exhibits ambipolar transport behavior in air with hole and electron mobilities up to 3.40 and 5.86 cm(2) V(-1) s(-1), respectively, is synthesized via direct arylation polycondensation. Incorporation of F-atoms in β-positions of thiophene rings dramatically improves the efficiency of direct arylation polycondensation.

Ambipolar organic field-effect transistors with air stability, high mobility, and balanced transport

Ambipolar organic field-effect transistors (OFETs) based on the organic heterojunction of copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T) were fabricated. The ambipolar OFETs eliminated the injection barrier for the electrons and holes though symmetrical Au source and drain electrodes were used, and exhibited air stability and balanced ambipolar transport behavior. High field-effect mobilities of 0.04cm2∕Vs for the holes and 0.036cm2∕Vs for the electrons were obtained. The capacitance-voltage characteristic of metal-oxide-semiconductor (MOS) diode confirmed that electrons and holes are transported at F16CuPc and BP2T layers, respectively. On this ground, complementary MOS-like inverters comprising two identical ambipolar OFETs were constructed.

Low bandgap conjugated polymers based on mono-fluorinated isoindigo for efficient bulk heterojunction polymer solar cells processed with non-chlorinated solvents

Three low bandgap conjugated polymers based on 7-fluorinated isoindigo (IID1F) and dithieno[3,2-b;6,7-b]carbazole (DTC), i.e., poly[N-dodecyldithieno[3,2-b;6,7-b]carbazole-alt-7-fluoro-N,N-di(2-octyldodecyl)isoindigo] (P1), poly[N-dodecyldithieno[3,2-b;6,7-b]carbazole-alt-7-fluoro-N,N-di(3-octyltridecyl)isoindigo] (P2) and poly[N-dodecyldithieno[3,2-b;6,7-b]carbazole-alt-7-fluoro-N,N-di(4-octyltetradecyl)isoindigo] (P3), were synthesized. All three polymers are soluble in non-chlorinated solvent o-xylene owing to regio-random distribution of F-atoms along the conjugated backbone. The position of the alkyl-branching point has a negligible influence on energy levels and absorption spectra of the polymers, but has a small effect on their charge transport properties. Bulk heterojunction (BHJ) polymer solar cells (PSCs) of the polymers were fabricated with phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor. When o-xylene was used as a solvent, all three polymers delivered power conversion efficiencies (PCEs) above 7%. P2 exhibited the best device performance with a PCE of 7.5%. The devices processed with o-xylene showed higher device efficiency than those fabricated with o-dichlorobenzene (o-DCB) since the films of polymer:PC61BM blends prepared with o-xylene exhibited better morphology and higher and more balanced charge-carrier mobilities, leading to less recombination loss and higher fill factor (FF).

Novel thiophene-aryl co-oligomers for organic thin film transistors

A series of thiophene-aryl co-oligomers with phenyl-based central units, i.e. phenyl, biphenyl, fluorene and phenanthrene, were synthesized, and their optical properties and charge carrier transporting properties were characterized. The co-oligomers with biphenyl and phenanthrene units exhibited significantly improved thermal stability. Absorption measurements and electrochemical characterization revealed that all co-oligomers have broader band gap and lower HOMO levels than the corresponding thiophene oligomer, α,ω-dihexylsexithiophene (DHα6T). Introduction of biphenyl and phenanthrene units afforded co-oligomers with the broadest band gap and lowest HOMO levels in comparison with the counterparts of phenyl and fluorene central units. Depending on the structure of the central units, the field-effect mobility (μTFT) of the co-oligomers from top-contact thin film transistors is in the range of (1–6.7) × 10−2 cm2 V−1 s−1. The rigid and planar central unit is favorable for good device performance. Among the co-oligomers, 2,7-bis(5′-hexyl-2,2′-bithien-5-yl)phenanthrene (DH-TTPhTT) exhibits the highest μTFT of 6.7 × 10−2 cm2 V−1 s−1, comparable to that of DHα6T in the same device configuration and fabrication conditions.

Facile synthesis of 9,10-diarylphenanthrenes and poly(9,10-diarylphenanthrene)s

One-pot reduction of 9,10-diaryl-9,10-dihydrophenanthrene-9,10-diols to 9,10-diarylphenanthrenes was achieved with Zn/H+ in acetic acid. Accordingly, various novel phenanthrenes and polyphenanthrenes with efficient blue emission were easily synthesized.

Suzuki–Miyaura catalyst-transfer polycondensation with Pd(IPr)(OAc)2 as the catalyst for the controlled synthesis of polyfluorenes and polythiophenes

Controlled Suzuki–Miyaura catalyst-transfer polycondensations (SCTPs) of fluorene- and thiophene-based AB-type monomers have been demonstrated with a N-heterocyclic carbene (NHC)-based Pd complex, Pd(IPr)(OAc)2, as the catalyst. The number average molecular weights (Mns) of the resulting poly(9,9-dioctyl-9H-fluorene)s (PF8s) were linear to the conversions of the monomer. PF8s with Mns in the range of 10.5–69.2 kDa and polydispersity indices (PDIs) of ∼1.60 were successfully synthesized by tuning the feed ratios ([monomer]0/[Pd]). The protocol is also applicable to the controlled synthesis of poly(3-hexylthiophene) (P3HT). Polymers with Mns of 9.5–63.8 kDa, which were linearly correlated to feed ratios, were obtained when the catalyst loading was tuned to 5–0.5 mol%. The “living” characteristics of the polymerization were also confirmed by monomer-addition and block copolymerization experiments. In addition, PF8 with a moderate molecular weight (MW) was mainly end-capped with Br/H end groups as evidenced by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass and 1H NMR spectra, indicating that this polymerization involves a catalyst-transfer mechanism. The mechanism was further confirmed by the fact that the cross-coupling of dibromofluorene (1 equiv.) and biphenylboronic acid ester (1 equiv.) with Pd(IPr)(OAc)2 as the catalyst preferentially gave a di-substituted product.

Low-Band-Gap Conjugated Polymers of Dithieno[2,3-b:7,6-b]carbazole and Diketopyrrolopyrrole: Effect of the Alkyl Side Chain on Photovoltaic Properties

Four donor–acceptor (D–A) conjugated polymers of dithieno[2,3-b;7,6-b]carbazole (DTC) and diketopyrrolopyrrole, which have different alkyls on the nitrogen atom in the DTC unit and are named as P-C8C8, P-C5C5, P-C12, and P-C10, respectively, have been synthesized for studying the effect of the alkyl side chains on the optoelectronic properties of the polymers. All polymers are soluble in various organic solvents and exhibit identical optical band gaps (E(g)(opt)) of ~1.3 eV and highest occupied molecular orbital energy levels of ~−5.1 eV. Organic thin-film transistors and bulk heterojunction polymer solar cells (BHJ PSCs) with phenyl-C(71)-butyric acid methyl ester (PC(71)BM) as the electron-accepting material were fabricated via solution spin-casting. Compared to the polymers substituted by branched alkyl chains, the polymers with straight alkyl chains show higher hole mobility. Of these polymers, P-C10 exhibits the highest field effect mobility up to 0.011 cm(2)/V·s. The alkyl chain on the DTC unit has a strong impact on the film morphology of polymer:PC(71)BM blends. Severe phase separation was found for polymers containing branched alkyl chains, and those with straight alkyl chains formed uniform films featuring fine phase separation. An open-circuit voltage (V(oc)) of 0.72 V, a short-circuit current density (J(sc)) of 13.4 mA/cm(2), a fill factor (FF) of 62%, and a power conversion efficiency (PCE) of 5.9% were demonstrated for BHJ PSCs based on the P-C10:PC(71)BM [1:3 (w/w)] blend film.