BULK SENSITIZATION OF INORGANIC SEMICONDUCTORS WITH ORGANIC GUEST MOLECULES: ZINK PHTHALOCYANINES EMBEDDED IN µc-Si AND ZnSe HOST MATRICES Thomas Mayer, Ulrich Weiler, Eric Mankel and Wolfram Jaegermann Darmstadt University of Technology ABSTRACT Concepts of bulk sensitization of inorganic semi- conductors by organic dye molecules either to increase the absorptivity of indirect semiconductors as µc-Si or to expand in a two photon process the absorption spectrum of wide gap semiconductors to photons of energy below the band gap are introduced. Hybrid composites have been prepared in vacuum by combining CVD and PVD thin film growth methods. Raman and optical spectroscopy have been used to prove the stability of the organic molecules ZnPc and F16ZnPc for the applied growth conditions of µc-Si and ZnSe, respectively. Photoelectron spectroscopy has been used to determine the frontier orbital line up of ZnPc and F16ZnPc versus the energy gaps of Si or ZnSe. INTRODUCTION Functional hybrids based on organic/inorganic composites offer a wide range of possibilities to tailor their chemical and physical properties. Hybrid materials formed by sol-gel techniques have been developed for optical applications as e.g. coloured glass ware, dielectric mirrors and lasers [1]. Incorporated nanocrystals of inorganic semiconductors improve the performance of polymer based solar cells [2, 3]. But the given performance limitations of such hybrid systems may still be dictated by the low mobility of the organic phase. In addition, stability at ambient conditions is still an issue. Organic dye layers have proved to be sufficiently stable for practical use in dye sensitized solar cells [4] when the excited electron is transferred efficiently from the dye LUMO to the conduction band of the TiO2 substrate and subsequently the hole is transferred from the dye HOMO to the redox couple (I2/I3 - ) of the electrolyte. However, the liquid electrolyte still causes problems for practical applications. With this contribution we propose a new concept for photovoltaic applications by combining organic dyes as highly absorbing materials with inorganic semiconductors as good charge carrier transport layers. In contrast to previous suggestions we introduce bulk sensitization of inorganic semiconductors by organic generation centres. Bulk sensitization may be used to enhance the absorptivity of weakly absorbing materials or may even be extended to multi-photon absorption for third generation concepts. To prove the applicability of the concept, hybrid films with increasing organic to inorganic ratios have been grown by CVD respectively PVD. The films have been characterized by optical and Raman spectroscopy. Photoelectron spectroscopy has been used to determine the frontier orbital line up of ZnPc and F16ZnPc versus the energy gaps of Si and ZnSe, respectively. BULK SENSITIZATION OF THE INDIRECT SEMICONDUCTOR µc-Si As indirect semiconductor Si shows low absorptivity for visible light but excellent carrier transport properties. Organic dye molecules on the other hand show high absorptivity but poor transport properties. We propose therefore to incorporate organic dye molecules as generation centres into microcrystalline Si separating the functionalities of electron-hole pair generation from transport making use of two specialized materials. For a continuous process the generated electron-hole pair must be injected from the dye LUMO and HOMO states into the host semiconductor conduction and valence bands. Excited electron-hole pairs may be injected either subsequently or simultaneously by the so called Dexter transfer process or via dipole-dipole interaction by the so called Förster process [5, 6]. The charges will then drift (diffuse) within the intrinsic Si host matrix to the p+ and n+ doped Si contact layers sandwiching the sensitized intrinsic matrix as sketched in Fig.1a. In order to create driving forces for electron as well as hole injection the frontier orbitals should be lined up as sketched in Fig 1b: the HOMO below the valence band maximum of the Si matrix and the LUMO above the conduction band minimum. The goal of the sensitization is to decrease the absorber thickness of thin film Si solar cells thereby reducing production time and cost. a) b) Fig. 1 Sketch of the morphology and proposed p-i-n device structure a) and electronic structure b) of a solar cell with µc- Si/dye composite absorber. 190 1-4244-0016-3/06/$20.00 ©2006 IEEE