Plastic Near-Infrared Photodetectors Utilizing Low Band Gap Polymer**

Organic photodetectors (PDs) have been the subject of extensive research in the past decade due to several inherent advantages: large-area detection, wide selection of materials, and low-cost fabrication on flexible substrates. High external quantum efficiency (EQE), full-color, fast-response, and position-sensitive PDs have been reported in the past. However, there are few reports on organic near-infrared photodetectors (NIR-PDs) in spite of their tremendous potential in industrial and scientific applications, such as remote control, chemical/biological sensing, optical communication, and spectroscopic and medical instruments. S. Meskers and co-workers reported an infrared PD in which doped poly(2, 4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) was used as the active material. More recently, G. Konstantatos and coworkers fabricated NIR-PDs by spin-coating colloidal quantum dots from solution onto gold interdigitated electrodes. The device showed a large photoconductive gain and high detectivity at 1.3 lm. However, 3-dB bandwidth was only about 18 Hz and the working voltage was as high as 40 V. These characteristics strongly restrict their applications in the fields of imaging and communication where high-speed and low-power PDs are desired. Thus, there is a strong need for the development of fast response and low working voltage NIR-PDs while simultaneously maintaining the benefit of low-cost solution process. Here we report an organic near-infrared photodetector using a new low band gap polymer. By utilizing an ester group modified polythieno[3,4-b]thiophene, we have successfully lowered the highest occupied molecular orbital (HOMO) energy level of the low band gap (LBG) polymer, so that it can match the energy level of (6,6)-phenyl C61-butyric acid methyl ester (PCBM), and has good solubility and easy processing ability. In this communication, we report a device which has a donoracceptor type energy structure whose operation shows excellent NIR detection capability. Reports on LBG polymers for solar energy conversion have emerged recently. The preparation of LBG, high mobility, solution-processable polymers is not trivial and requires judicious design. Among several band gap tuning strategies for conjugated polymers, polymerization of fused heterocyclic rings has been known to yield polymers with very low band gaps. Polythieno[3,4-b]thiophene (PTT) is one kind of LBG polymers in which the fused thiophene moieties can stabilize the quinoid structure of the backbone, thereby reducing the band gap of the conjugated system. Several PTTs without side chains have been reported previously, but the poor solubility makes them difficult to process and limits their use in electro-optical and electronic devices. Synthesis of alkyl chains substituted thieno[3,4-b]thiophenes monomers have been reported and the resulting polymers exhibit better solubility, but poor oxidative stability. It was found that the HOMO levels of these polymers are too high to match the energy levels of the commonly used electron acceptor, PCBM. We report a new type of ester group modified PTT polymer (Scheme 1). The introduction of an ester group at the 2-position of thieno[3,4-b]thiophene has two effects. First, the electron withdrawing ester group can stabilize the electron-rich thienothiophene and lower the HOMO energy level of the polymer to match the energy level of PCBM. Second, a long tertiary alkyl side chain from the ester group can increase the solubility of the polymer. Polymer was synthesized by Stille polycondensation reaction between the bisbrominated thieno[3,4-b]thiophene and bis-stannylated thiophene. (Scheme 1; see Supporting Information for details) The resulting polymer has good solubility in chloroform and chlorobenzene. In contrast to inorganic semiconductors, photoexcitation of organic semiconductors generates strong bound excitons rather than free charge carriers. To dissociate excitons efficiently, the donor/acceptor bulk heterojunction approach is typically used. The active layer in our PD comprises of PTT and PCBM (Fig. 1a), forming interpenetrating donor/acceptor networks. Details of the device fabrication process are given in the Experimental section. Figure 1b shows the absorption spectra of PTT and PTT: PCBM films. Pure PTT thin film abC O M M U N IC A IO N