Photophysics and Charge Transport in Hybrid Blends of P3AT and beta-SiC Nanocrystals

Hybrid solar cells, whose photoactive layer comprises an organic semiconductor as electron donor and an inorganic semiconductor as electron acceptor, have become of considerable interest as a low- cost solution for solar energy harvesting. They join two very different semiconductor material classes: The inorganic semiconductors, traditionally used in high efficiency solar cells (e.g. Si or GaAs), show highest exciton dissociation and charge generation rates and excellent charge mobility. Organic semiconductors on the other hand, bring solution processability, high absorption coefficients, freedom in synthesis to match materials to the desired optical and electrical properties. However, both systems have their drawbacks. The inorganics suffer from high production costs of pure single crystalline material, the organics from their short range Frenkel excitons and limited charge mobility. In hybrid photovoltaics, one takes advantage from the best characteristics of every system, using a composite structure of organic and inorganic semiconductor. The most prominent representatives at present are cadmium chalcogenides or wide-band gap semiconducting oxides, mixed or infiltrated with conjugated polymer. In this project, we intend to investigate a system using the non-oxidic inorganic wide-band gap semiconductor 3C-SiC (cubic silicon carbide) as the acceptor in an organic poly(3-alkylthiophene) (P3AT) donor matrix. Silicon carbide as acceptor in hybrid cells has been neglected in the past, probably due to its indirect band gap, missing absorption contributions in the visible and expensive production of suitable nanocrystalline material. However, its band energies are suitable to match the HOMO/LUMO of organic donors, which provides a promising outlook for its functionality in hybrid photovoltaics. Further, our first preliminary fast-laser spectroscopy results on P3HT:SiC blends, showed indeed presence of a potential charge transfer state emission, one indicator for a functional donor/acceptor system. Goal of this proposed project is to gain knowledge about the photophysics, charge generation and transport processes in this barely investigated hybrid D/A system. Static and time-resolved fast optical spectroscopy will be used to analyse excitation, exciton dissociation, energy transfer, charge transfer and recombination processes in P3AT: 3C-SiC. This will be correlated with tuning of the P3AT:3C-SiC interface and blend morphology. Finally, charge generation, transport and trapping will be studied in photodiodes with a P3AT:3C-SiC active layer via static I-V and time- resolved photocurrent transient measurements. The potential influence of n- and p-dopants in 3C- SiC on these processes will also be part of the project.

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