Multifunctionality of Organic Devices: Light Emission, Photovoltage Generation, and Photodetection Satyajit Sahu and Amlan J. Pal* Indian Association for the CultiVation of Science, Department of Solid State Physics and Centre for AdVanced Materials, Kolkata 700 032, India ReceiVed: March 13, 2008; ReVised Manuscript ReceiVed: April 17, 2008 We present multifunctionality in heterostructure devices based on poly(3-hexylthiophene) and copper phthalocyanine layers. Under solar light illumination, the devices yield photovoltage in an open-circuit condition. They, in addition, emit light in the forward bias and can also detect a trace of light in the reverse-bias direction. Highest occupied molecular orbitals and lowest unoccupied molecular orbitals of the two materials, along with the metal work functions, are suitable for the three functionalities at different bias modes. While the electroluminescence spectrum of the device has been compared with the photoluminescence spectra of the components, the spectral response of the photocurrent matched well with the electronic absorption spectrum of the poly(3-hexylthiophene)/copper phthalocyanine heterojunction. 1. Introduction Organic semiconductors have shown multidimensional in- terests ranging from basic research to several electronic and optoelectronic devices. The studies of carrier mobility and conductivity have primarily led to an understanding of the new class of semiconductors. With in-depth knowledge of such a wide range of semiconducting materials, several of the electronic and optoelectronic devices have been revisited in the last couple of decades. Some of the devices that match the performance of the ones based on the inorganic counterpart are light-emitting devices, 1 photodetectors, 2–7 photovoltaic solar cells, 2,7,8 sensors, 9 field-effect transistors, 10,11 etc. Though the working principles of the devices are diverging in nature, they often possess identical device architecture, namely, a heterostructure, where two layers of organic (semi- conducting) materials favoring movement of the two types of carriers are sandwiched between two dissimilar metal electrodes. While a forward-biased structure emits electroluminescence (EL), a reverse-biased one is used to detect any trace of light (as a photodetector). Parameters of photovoltaic solar cells, such as the short-circuit current (I SC ) and open-circuit voltage (V OC ), are measured without any external bias. The mechanism involved in these devices in terms of exciton formation (or dissociation) and carrier transport is often dictated by the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMO) of the two materials, along with the work function of the two metal electrodes. In spite of the contrasting nature of functioning of light- emitting diodes (LEDs) and photovoltaic solar cells (or photo- detectors), the identity of their device architecture has been intriguing and hence prompted researchers to achieve dual functionality in single devices. One of the extensively studied dual-function devices is light-emitting organic solar cells based on single-molecule or donor-acceptor heterostructures. 3,12–16 There are a couple of examples where electroluminescent and photodetecting properties have been achieved in devices based on a layer of either an organic material 17,18 or a composite system of nanoparticles and conjugated organics. 19,20 In the present work, we report multifunctionality (light emission, photovoltage generation, and photodetection) in one hetero- structure device. We have used traditional materials [poly(3- hexylthiophene) or P3HT and copper phthalocyanine or CuPc] to avoid any radical effects like exciton quenching (as in fullerenes) or a too feeble blocking contact with the electrodes (as in aluminum quinoline). 2. Experimental Section Materials. Regioregular poly(3-hexylthiophene) (P3HT) and copper(II) phthalocyanine (CuPc) were purchased from Aldrich Chemical Co. P3HT was further purified by dissolving it in chloroform and filtering off the undissolved components. Substrates. Indium-tin oxide (ITO)-coated glass substrates with a resistivity of 10-15 Ω m were purchased from Optical Filters, U.K. A part of the ITO was etched using HCl and Zn powder. The stripped ITO substrates were soaked and washed repeatedly in deionized water. The slides were finally treated with methanol and acetone in sequence. Thin Film Formation. A chloroform solution of P3HT (2.5 mg/mL) was spun on the precleaned ITO substrates at speeds * To whom correspondence should be addressed. E-mail: sspajp@ iacs.res.in. Figure 1. Current density versus voltage plots of an ITO/P3HT/CuPc/ Al device (25 nm/25 nm) under dark and 100 mW/cm 2 solar illumi- nation conditions. J. Phys. Chem. C 2008, 112, 8446–8451 8446 10.1021/jp802194e CCC: $40.75  2008 American Chemical Society Published on Web 05/03/2008