Fullerene Sensitized Silicon for Near- to Mid-Infrared Light Detection By Gebhard J. Matt,* Thomas Fromherz, Mateusz Bednorz, Saeid Zamiri, Guillaume Goncalves, Christoph Lungenschmied, Dieter Meissner, Helmut Sitter, N. Serdar Sariciftci, Christoph J. Brabec, and Gu ¨nther Bauer Detection of light in the near- to mid-infrared (IR) spectral range is a technology in demand for many applications such as optical data transmission (1.55 mm), contrast enhancement for imaging systems in foggy environments, and quality control. Most of the optoelectronic devices on the market are based on the rather expensive III–V compound technology and, as a result, the monolithic integration into the well-established and cheap silicon-based complementary metal oxide semiconductor (CMOS) production process is still an unachieved goal. Here, we report on a novel light-sensing scheme based on a silicon/fullerene-derivative heterojunction that allows the realization of optoelectronic devices for the detection of near- to mid-IR light, which is fully compatible with CMOS technology. Despite the absence of light absorption by silicon and the fullerene-derivative in the IR. a heterojunction of these materials absorbs and generates a photocurrent (PC) in the near- to mid-IR. In this spectral range it is proposed that the IR PC is caused by an interfacial absorption mechanism. In essence, an inherent disadvantage of silicon for optoelec- tronic IR applications is its transparency beyond a wavelength of 1.1 mm. To overcome this disadvantage, several technologies such as the heteroepitaxial growth of (polycrystalline) germanium on silicon [1–3] or the usage of near-IR (NIR)-photoconductive and soluble nanoparticles have been developed. [4,5] In the latter case, the facile solution processing of a guest material to the silicon-based host is of particular interest. [5] In this work, the soluble C 60 derivative methano-fullerene [6,6] phenyl-C61 butyric acid methyl ester (PCBM) as guest material has been chosen (see inset of Fig. 1, where the chemical structure is depicted). In contrast to pristine C 60 , PCBM exhibits a solubility of up to 5 wt% in common organic solvents due to functionalization of the fullerene cage with a butyric acid methylester sidegroup. [6] For polycrystalline C 60 thin films processed into field effect devices, the electron mobility is of the order of 1 cm 2 Vs 1 [7,8] and for the spin-cast PCBM thin films it is approximately one to two orders lower in magnitude. [9] The microscopic charge-carrier mobility of PCBM is only weakly temperature dependent and a highly conductive state of PCBM under a filamented current density at 15 K has been reported. [10] In this Communication is shown that a p-Si/PCBM hetero- junction features a PC for photon energies from 0.55 to 1.1 eV (2.25–1.12 mm). The investigated samples have a layered structure (Fig. 1). On top of a boron-doped p-Si wafer (boron concentration 10 15 –10 16 cm 3 ) the PCBM film is deposited by spin-coating, resulting in a PCBM film thickness of 140 nm. By thermal evaporation of Al the electrical front- and back-contacts to the PCBM thin film and to the p-Si wafer are formed. To ensure an Ohmic contact of the Al to the p-Si wafer, the Al/p-Si contact is alloyed at 580 8C in a nitrogen/hydrogen atmosphere. [11] In Figure 2, the current-density–voltage (J–V) characteristics of an Al/p-Si/PCBM/Al heterojunction is presented at room temperature and 77 K. At 297 K a current rectification ratio of 3  10 4 for a bias variation from 1 to þ1 V is observed. Similar values for pristine C 60 /silicon heterojunction diodes have been obtained by Chen et al. [12] Upon cooling, the reverse dark current density at 2 V bias decreases from 3  10 6 A cm 2 at 297 K to the pA cm 2 region at 77 K, while the current rectification increases to 10 7 for a bias variation from 1 to þ1V. From an Arrhenius plot of the dark current in reverse bias at 1 V (see inset in Fig. 2a) an activation energy of 0.45 eV is found for the temperature range from 297 to 240 K. At a sample temperature of 77 K and under broadband-IR illumination with a tungsten lamp that is spectrally restricted by a high-energy-cut-off silicon filter, a J–V characteristic typical for a photovoltaic device is observed (Fig. 2). The light intensity (measured with a calibrated InGaAs detector) behind the Si filter is 3.8  10 3 W cm 1 , which leads to a short-circuit current (J sc ) of J sc ¼ 15 nA cm 2 and a open-circuit voltage of þ0.45 V at 77 K. At room temperature a high-resolution J–V scan around zero bias verifies that a finite short-circuit current of 10 nA cm 2 and an open circuit voltage of 1 mV is still observable (Fig. 2a and inset). Under the same experimental conditions as for the J–V measurements, the PC at various temperatures is spectrally COMMUNICATION www.MaterialsViews.com www.advmat.de [*] Dr. G. J. Matt, Dr. T. Fromherz, M. Bednorz, Prof. H. Sitter, Prof. G. Bauer Institute for Semiconductor and Solid State Physics Johannes Kepler University 4040 Linz (Austria) E-mail: gebhard.matt@jku.at S. Zamiri Christian Doppler Laboratory for Surface Optics Johannes Kepler University 4040 Linz (Austria) G. Goncalves Ecole Nationale Superieure de Chimie et de Physique de Bordeaux (ENSCPB) 33607 Pessac Cedex (France) C. Lungenschmied, C. J. Brabec Konarka Austria 4040 Linz (Austria) Prof. D. Meissner, Prof. N. S. Sariciftci Linz Institute for Organic Solar Cells (LIOS) Johannes Kepler University 4040 Linz (Austria) DOI: 10.1002/adma.200901383 Adv. Mater. 2010, 22, 647–650 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 647