A Novel Alternating Phenylenevinylene Copolymer with Perylene Bisimide Units: Synthesis, Photophysical, Electrochemical, and Photovoltaic Properties John A. Mikroyannidis,* ,† Minas M. Stylianakis, † G. D. Sharma,* ,‡ P. Balraju, ‡ and M. S. Roy § Chemical Technology Laboratory, Department of Chemistry, UniVersity of Patras, GR-26500 Patras, Greece, Physics Department, Molecular Electronic and Optoelectronic, JNV UniVersity, Jodhpur 342005, India, and Defence Laboratory, Jodhpur 342005, India ReceiVed: February 23, 2009; ReVised Manuscript ReceiVed: March 13, 2009 An alternating phenylenevinylene copolymer P bearing perylene bisimide moieties along the backbone was synthesized by Heck coupling. The tert-butyl and hexyloxy side groups enhanced the solubility of the copolymer in common organic solvents. It had glass transition and decomposition temperatures of 72 and 370 °C, respectively. The absorption of P was broad, with longer wavelength maximum around 500 nm and optical band gap of 1.66 eV. The solution of P emitted yellow-orange light with photoluminescence (PL) maximum at 555 nm. Moreover, efficient interaction and charge/energy transfer between the phenylenevinylene-donor and the perylene bisimide-acceptor led to PL quenching in the thin film of P. The current-voltage characteristics of ITO/copolymer P/Al indicated that copolymer P behaved as an n type organic semiconductor. The electron current was found to be a space charge limited current (SCLC), providing a direct measure of mobility as a function of temperature and field. The average electron mobility for this copolymer was 0.85 × 10 -2 cm 2 /Vs. The photogeneration mechanism in blends of copolymer P (electron acceptor) with poly(3- phenyl hydrazone thiophene) (PPHT) (electron donor) were investigated. Annealing of the complete device was found to result in an increase in power conversion efficiency from 1.67% to 2.32%. By studying the dependence of photocurrent on effective bias voltage, annealing was found to increase the charge generation efficiency through an increase in the efficiency of separation of exciton following the charge transfer. Introduction Polymer solar cells (PSCs) have been extensively studied because of their motivation for developing inexpensive, efficient, and renewable energy sources. 1-4 Power conversion efficiencies (PCEs) as high as 5-6% have been reported for bulk hetero- junction PSCs using regioregular poly(3-hexylthiophene) (P3HT) as donor and a solution processable fullerene derivative (PCBM) as acceptor. 5,6 However, there are some drawbacks of PCBM for application in PSCs, including weak absorption in the visible region and the possibility of phase separation from the polymer donor. Therefore, nonfullerene hybrid devices 7,8 and PSCs in which a polymer donor is blended with a polymer acceptor have attracted interest recently. 9-14 However, the PCE of polymer devices remains relatively low at present. One of the reasons for the low efficiency of the PSCs is the lack of good polymer acceptors with high electron affinity, high electron mobility, and good sunlight-harvesting properties. The peak photon intensity of the solar spectrum lies at about 700 nm. To make the absorption spectra of the conjugated polymers match the solar spectrum, their absorption maximum should be near 700 nm, which means that the band gap (E g ) of the conjugated polymer should be lower than 1.74 eV. 15 The most successful approach to obtain the low band gap polymers is a copolymerized D-A structure. 16-19 The copolymerization of the donor (electron-rich) with higher HOMO energy level and the acceptor (electron deficient) with lower LUMO energy level results in a lower band gap polymer due to an interchain charge transfer from donor to acceptor. Perylene bisimides have attracted much interest and been widely investigated because of their unique merits such as thermal stability, inexpensiveness, and especially its large molar absorption coefficient and good electron accepting properties. They are potential candidates as electron-accepting materials in organic photovoltaic solar cells. 20 Both PPV and perylene bisimide chromophores have been applied in bulk-heterojunc- tion-like solar cell configurations as donor and as acceptor materials, respectively. 21,22 Recently, a new class of donor-accep- tor polymers consisting of alternating oligo(p-phenylenevi- nylene) and perylene bisimide connected via saturated spacers was synthesized. 23 Moreover, two copolymers containing perylene bisimide, PPV and triphenylamine segments were synthesized and used for photovoltaic cells. 24 Finally, a series of novel alternating phenylenevinylene or fluorenevinylene copolymers containing perylene bisimide were synthesized in our laboratory and were used for photovoltaic cells. 25 The main objective of the present investigation was the synthesis and characterization of an alternating phenylenevi- nylene copolymer containing perylene bisimide moieties that is a candidate for photovoltaic cells. This copolymer was successfully synthesized by Heck coupling and had the donor (D)-acceptor (A) architecture. In particular, it contained the electron-donating 2,5-dihexyloxyphenylene as the D unit and the electron-withdrawing perylene bisimide as the A unit. The alternating copolymerization of D and A units has been used as an effective way to lower the band gap of conjugated * Corresponding authors. E-mail: mikroyan@chemistry.upatras.gr (J.A.M.); sharmagd_in@yahoo.com (G.D.S.). Phone: +30 2610 997115 (J.A.M.); +91-0291-2720857 (G.D.S.). Fax: +30 2610 997118 (J.A.M.); 91-0291- 2720856 (G.D.S.). † Chemical Technology Laboratory, Department of Chemistry, University of Patras. ‡ Physics Department, Molecular Electronic and Optoelectronic, JNV University. § Defence Laboratory. J. Phys. Chem. C 2009, 113, 7904–7912 7904 10.1021/jp901651z CCC: $40.75  2009 American Chemical Society Published on Web 04/14/2009