Xugang Guo

Imide- and Amide-Functionalized Polymer Semiconductors

Xugang Guo,*,† Antonio Facchetti,*,‡,§ and Tobin J. Marks*,‡ †Department of Materials Science and Engineering, South University of Science and Technology of China, No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China ‡Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States

High‐mobility Ambipolar Transistors and High‐gain Inverters from a Donor–Acceptor Copolymer Semiconductor

Ambipolar organic field-effect transistors (OFETs), which are capable of both p- and n-channel operations, are gaining attention as an alternative approach to mimicking complementary metal-oxide semiconductor (CMOS) digital integrated circuits for achieving high-performance and cost-effective circuits in organic electronics. [1‐13] Low power dissipation and high performance are some of the major advantages of CMOS technology over non-complementary ones. [14] Power consumption is minimized in CMOS circuits because the component transistors are selectively turned on only when the circuit is switching, otherwise they are off at the steady state. The better performance of a CMOS circuit in terms of sharp switching and high noise immunity arises because every elemental transistor actively contributes to the function of the circuit. [14] Most efforts towards CMOS-like circuits in organic electronics have focused on utilizing distinct p- and n-type semiconductors. [1,15] However, the necessity of lateral patterning of semiconductors in CMOS circuits makes device fabrication on a common substrate a very complex process. Ambipolar OFETs represent an approach to high-performance CMOS-like circuits that minimize patterning and complex fabrication processes. [1] Ambipolar transistors are also of interest in fundamental studies of charge transport in organic semiconductors [1,6,16] as well as the development of efficient light-emitting transistors. [8,17‐21]

Bithiopheneimide-dithienosilole/dithienogermole copolymers for efficient solar cells: information from structure-property-device performance correlations and comparison to thieno[3,4-c]pyrrole-4,6-dione analogues.

Rational creation of polymeric semiconductors from novel building blocks is critical to polymer solar cell (PSC) development. We report a new series of bithiopheneimide-based donor-acceptor copolymers for bulk-heterojunction (BHJ) PSCs. The bithiopheneimide electron-deficiency compresses polymer bandgaps and lowers the HOMOs--essential to maximize power conversion efficiency (PCE). While the dithiophene bridge progression R(2)Si→R(2)Ge minimally impacts bandgaps, it substantially alters the HOMO energies. Furthermore, imide N-substituent variation has negligible impact on polymer opto-electrical properties, but greatly affects solubility and microstructure. Grazing incidence wide-angle X-ray scattering (GIWAXS) indicates that branched N-alkyl substituents increased polymer π-π spacings vs linear N-alkyl substituents, and the dithienosilole-based PBTISi series exhibits more ordered packing than the dithienogermole-based PBTIGe analogues. Further insights into structure-property-device performance correlations are provided by a thieno[3,4-c]pyrrole-4,6-dione (TPD)-dithienosilole copolymer PTPDSi. DFT computation and optical spectroscopy show that the TPD-based polymers achieve greater subunit-subunit coplanarity via intramolecular (thienyl)S···O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar ordering, but closer π-π spacing than does the TPD-based analogue. Inverted BHJ solar cells using bithiopheneimide-based polymer as donor and PC(71)BM as acceptor exhibit promising device performance with PCEs up to 6.41% and V(oc) > 0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher V(oc) with an enhanced PCE of 6.83%, mainly attributable to the lower-lying HOMO induced by the higher imide group density. These results demonstrate the potential of BTI-based polymers for high-performance solar cells, and provide generalizable insights into structure-property relationships in TPD, BTI, and related polymer semiconductors.

Conjugated polymers from naphthalene bisimide.

Stille coupling of regioisomerically pure dibromonaphthalene bisimides (NBI) with various stannylated thiophene-based monomers yields (very) high molecular weight donor-acceptor conjugated polymers. Electrochemical and optical absorption measurements reveal that LUMO energies are essentially invariant and dictated by the NBI units, while HOMO energies are dictated by the thienyl comonomers. Optical energy gaps ranging from 1.7 to 1.1 eV are thus obtained. The polymers are also characterized by differential scanning calorimetry and fiber WAXD.

Thieno[3,4-c]pyrrole-4,6-dione-based polymer semiconductors: toward high-performance, air-stable organic thin-film transistors.

We report a new p-type semiconducting polymer family based on the thieno[3,4-c]pyrrole-4,6-dione (TPD) building block, which exhibits good processability as well as good mobility and lifetime stability in thin-film transistors (TFTs). TPD homopolymer P1 was synthesized via Yamamoto coupling, whereas copolymers P2-P8 were synthesized via Stille coupling. All of these polymers were characterized by chemical analysis as well as thermal analysis, optical spectroscopy, and cyclic voltammetry. P2-P7 have lower-lying HOMOs than does P3HT by 0.24-0.57 eV, depending on the donor counits, and exhibit large oscillator strengths in the visible region with similar optical band gaps throughout the series (∼1.80 eV). The electron-rich character of the dialkoxybithiophene counits in P8 greatly compresses the band gap, resulting in the lowest E(g)(opt) in the series (1.66 eV), but also raising the HOMO energy to -5.11 eV. Organic thin-film transistor (OTFT) electrical characterization indicates that device performance is very sensitive to the oligothiophene conjugation length, but also to the solubilizing side chain substituents (length, positional pattern). The corresponding thin-film microstructures and morphologies were investigated by XRD and AFM to correlate with the OTFT performance. By strategically varying the oligothiophene donor conjugation length and optimizing the solubilizing side chains, a maximum OTFT hole mobility of ∼0.6 cm(2) V(-1) s(-1) is achieved for P4-based devices. OTFT environmental (storage) and operational (bias) stability in ambient was investigated, and enhanced performance is observed due to the low-lying HOMOs. These results indicate that the TPD is an excellent building block for constructing high-performance polymers for p-type transistor applications due to the excellent processability, substantial hole mobility, and good device stability.

Phthalimide-Based Polymers for High Performance Organic Thin-Film Transistors

The synthesis and characterization of two new thiophene copolymers with backbone phthalimide units is reported. Thin-film optical and wide-angle X-ray diffraction measurements indicate extended electronic conjugation and close intermolecular pi-stacking for both polymers. Ambient carrier mobility of thin-film transistors prepared from these polymers is as high as 0.28 cm(2)/(V s) with an on/off ratio greater than 10(5).

Bithiophene-Imide-Based Polymeric Semiconductors for Field-Effect Transistors: Synthesis, Structure−Property Correlations, Charge Carrier Polarity, and Device Stability

Developing new high-mobility polymeric semiconductors with good processability and excellent device environmental stability is essential for organic electronics. We report the synthesis, characterization, manipulation of charge carrier polarity, and device air stability of a new series of bithiophene-imide (BTI)-based polymers for organic field-effect transistors (OFETs). By increasing the conjugation length of the donor comonomer unit from monothiophene (P1) to bithiophene (P2) to tetrathiophene (P3), the electron transport capacity decreases while the hole transport capacity increases. Compared to the BTI homopolymer P(BTimR) having an electron mobility of 10(-2) cm(2) V(-1) s(-1), copolymer P1 is ambipolar with balanced hole and electron mobilities of ∼10(-4) cm(2) V(-1) s(-1), while P2 and P3 exhibit hole mobilities of ∼10(-3) and ∼10(-2) cm(2) V(-1) s(-1), respectively. The influence of P(BTimR) homopolymer M(n) on film morphology and device performance was also investigated. The high M(n) batch P(BTimR)-H affords more crystalline film microstructures; hence, 3× increased electron mobility (0.038 cm(2) V(-1) s(-1)) over the low M(n) one P(BTimR)-L (0.011 cm(2) V(-1) s(-1)). In a top-gate/bottom-contact OFET architecture, P(BTimR)-H achieves a high electron mobility of 0.14 cm(2) V(-1) s(-1), only slightly lower than that of state-of-the-art n-type polymer semiconductors. However, the high-lying P(BTimR)-H LUMO results in minimal electron transport on exposure to ambient. Copolymer P3 exhibits a hole mobility approaching 0.1 cm(2) V(-1) s(-1) in top-gate OFETs, comparable to or slightly lower than current state-of-the-art p-type polymer semiconductors (0.1-0.6 cm(2) V(-1) s(-1)). Although BTI building block incorporation does not enable air-stable n-type OFET performance for P(BTimR) or P1, it significantly increases the OFET air stability for p-type P2 and P3. Bottom-gate/top-contact and top-gate/bottom-contact P2 and P3 OFETs exhibit excellent stability in the ambient. Thus, P2 and P3 OFET hole mobilities are almost unchanged after 200 days under ambient, which is attributed to their low-lying HOMOs (>0.2 eV lower than that of P3HT), induced by the strong BTI electron-withdrawing capacity. Complementary inverters were fabricated by inkjet patterning of P(BTimR)-H (n-type) and P3b (p-type).

Bithiophene Imide and Benzodithiophene Copolymers for Efficient Inverted Polymer Solar Cells

Bithiophene imide (BTI) and benzodithiophene (BDT) copolymers are synthesized for application in organic photovoltaic (OPV) cells. The electron deficiency of the BTI units leads to polymers with a low-lying HOMOs (∼-5.6 eV). Inverted solar cells are fabricated to investigate the OPV performance of the BTI-based polymers and achieve power conversion efficiencies up to 5.5%, with substantial V(oc)s above 0.9 V which are among the highest V(oc)s reported to date for polymer/PCBM solar cells. The results indicate that the BTI is a promising building block for constructing polymer donors for OPV applications.

Efficient solar cells based on a new phthalimide-based donor–acceptor copolymer semiconductor: morphology, charge-transport, and photovoltaic properties

Bulk heterojunction solar cells based on blends of the new low band gap donor–acceptor copolymer, poly(N-(dodecyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide) (PhBT12), and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were systematically investigated. The PhBT12/fullerene blend films were found to exhibit a crystalline nanoscale morphology with space-charge-limited mobility of holes as high as 4.0 × 10−4 cm2/Vs without thermal annealing, leading to moderately efficient devices. The performance of the solar cells varied significantly with PhBT12/fullerene composition, reaching a power conversion efficiency of 2.0% with a current density of 6.43 mA/cm2 and a fill factor of 0.55 for the 1:1 PhBT12/PC71BM blend devices. However, thermally annealed (120 °C) PhBT12/fullerene blend devices had negligible photovoltaic properties due to micrometer scale phase separation of the blends which is attributed to the long side chains. We expect that better photovoltaic performance can be achieved by modifying the polymer side chain length and the device processing as well. These results show that phthalimide-based donor–acceptor copolymer semiconductors, exemplified by PhBT12, are promising low band gap materials for developing efficient bulk heterojunction solar cells.

Toward Highly Sensitive Polymer Photodetectors by Molecular Engineering

Modified 3,4-ethylenedioxythiophene is employed as the conjugated side chain in conjugated polymers, which can significantly depress the dark current of the polymer photodetectors with little associated decrease in photovoltaic properties, thus enhanceing the detectivities. This approach can be applied to a variety of conjugated polymers covering a photoresponse range from UV to NIR.