57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

This project involves sophisticated spectroscopic and electrical measurements of optoelectronic devices based on high-purity organic molecular single crystals.  The spectral response of these high-performance organic semiconductors to photoexcitation is followed by complementary electrical characterization of devices, such as the measurements of surface conductivity, photo-voltage or charge carrier mobility.  This comprehensive characterization, performed according to the novel methods developed in our group, should provide a general picture of the exciton generation, dynamics and dissociation/recombination in molecular semiconductors important for understanding such processes as exciton fission, fusion, diffusion and carrier generation.  These are the key phenomena in operation of excitonic solar cells, and thus the knowledge obtained in this project would be crucial for understanding and improvement of organic solar cells.  Both the device fabrication and opto-electronic characterization in this project are conducted at the state-of-the-art level that requires highly skilled personnel.

Since the beginning of the project, we have obtained very interesting preliminary results that hint very strongly on:

(a) triplet exciton nature of photoconductivity in certain molecular crystals;

(b) surface photo-conductivity in highly ordered molecular crystals;

(c) unusually long-range exciton diffusion in pure molecular crystals (micrometers);

(d) transformation from a triplet fission dominated to (no-fission) singlet photo-luminescence observed as a function of temperature in single crystalline rubrene;

(e) One of our recent experiments related to this project shows that photoluminescence of crystalline organic semiconductors depends strongly on the exciton dynamics and transformations, such as exciton fission (a singlet exciton splits into two long-lived triplets) and exciton fusion (two triplets combine into one singlet that can emit light). It shows that the excitonic PL depends sensitively on the level of disorder in the organic crystal, because diffusing excitons interact with disorder. This study has been recently published in Advanced Materials with an acknowledgment to this ACS PRF grant for a partial support (Y. Chen, B. Lee, D. Fu and V. Podzorov, “The origin of a 650 nm PL band in rubrene” Adv. Mater. 23, 5370 (2011)).

All these excitonic processes are extremely important for photovoltaic effect in organic semiconductors. The fundamental insights obtained in this project will be very important for building a solid foundation for organic photo-voltaics. Our preliminary results will be developed into more solid and systematic experiments coupled to theoretical modeling.  The hiring of a new qualified postdoc, who is an expert on excitons in molecular crystals, will greatly improve the efficiency of implementation of this project.