Summer 2012 Intern Project- Reilly Raab

IMPACT OF CHEMICAL STRUCTURE ON EXCITON DIFFUSION LENGTH IN ORGANIC SOLAR CELLS

Reilly Raab
Physics
UC Santa Barbara

Mentor: Jason Lin
Faculty Advisor: Quyen Nguyen
Department: Chemistry and Biochemistry

In present organic photovoltaic devices, the generation of electric current is dependent on the ability of excitons, or excitations of the solar cell existing as electrostatically bound electron-hole pairs, to diffuse to an interface between electron donor and electron acceptor materials and disassociate into free charge carriers.  Due to the short (~10-20 nm) exciton diffusion length characteristic of most materials used for organic photovoltaics, a significant current loss mechanism arises from the recombination of excitons before they can disassociate. This process often results in the emission of light through photoluminescence. By investigating a set of diketopyrrolopyrrol (DPP) small molecules as donor materials with controlled chemical variations in alkyl chain length or conjugation length and determining their associated exciton diffusion lengths, an empirical relationship between chemical structure and exciton diffusion length may enable a theoretical relationship to be inferred, allowing the synthesis of new materials with superior exciton diffusion to enhance the efficiency of organic solar cells. To determine the exciton diffusion length associated with these different DPP small molecules, photoluminescence measurements were taken for devices constructed with and without a quenching layer (which prevents the reemission of light by excitons that reach the quenching interface) at varying thicknesses of the donor layer during laser excitation of the materials. The resulting dependencies of the photoluminescence data on the thickness of the material for both architectures were compared to a computer model, using exciton diffusion length as a fit parameter to determine the exciton diffusion length of the material.