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.

In hybrid photovoltaic devices, the photoactive donor/acceptor layer of a solar cell or photodetector does not comprise merely a classical inorganic semiconductor (like silicon) with differently doped zones. Instead, the layer consists of inorganic semiconductor nanocrystals, finely dispersed within an organic semiconductor matrix. Organic semiconductors, known from OLED-based TV displays are bendable and can be simply printed or sprayed onto the desired substrate. Traditional inorganic semiconductors on the other side, show much better charge mobility than their organic counterparts do, but are brittle and expensive. Hybrid-PV utilizes the best properties of both, easy processability of one and better electronic properties of the other. Unfortunately, also this technology has its drawbacks. Because the inorganic semiconductor here is not a continuous large crystal, but an assembly of many nanocrystals, losses by charge trapping at phase boundaries occur, mostly by surface defects of the crystals and incompatibility between the organic and the inorganic. They limit the efficiency of such devices. In the present project, doped silicon carbide (SiC) nanocrystals were used as inorganic acceptor in hybrid solar cells for the first time. Therefore various n- and p-doped nanocrystalline SiC powders were chemically synthesized and each tested in combination with an organic semiconductor donor. Photophysical and device physical properties have been investigated. We found that with our synthesis approach for silicon carbide by carbothermal reduction of sol-gel derived silicon oxycarbide glasses, the wet-chemicallly introduced dopant atom did not only determine the semiconductors defect states, but additionally the polytype composition and surface termination. The latter is an essential finding, since usually SiC is oxide terminated, therefore insulating, which makes post-treatment for electronics inevitable. In our case, it was possible to directly generate a hydrogen-saturated and graphene terminated SiC, highly advantageous for its interface properties. Comparing the photovoltaic behaviour of SiC:N, SiC:Al and SiC:Ga in hybrid matrices with the semiconducting polymer P3HT, graphen-terminated species led to more efficient photo-induced charge transfer and reduced trapping losses. Major reasons are the better compatibility of the polymer with the graphene's surface and the deactivation of SiC's dangling bonds by hydrogen. For oxide-terminated species the usual problems remain. However, at least their surface compatibility with the polymer could also be improved by subsequent coating with fullerenes in ternary blends. These results show that silicon carbide is suitable for hybrid PV and the common loss mechanisms can be overcome by in-situ interface engineering.