Optical Studies of Materials for Organic Photovoltaic Applications

Abstract

The work summarized in this dissertation comprises of three different sections: I) Vibration modes and electronic excitations in the π-conjugated copolymer PffBT4T and its blend with fullerene molecules designed for applications in organic photovoltaic (OPV) solar cells; II) The introduction of Multiphoton Microscopy (MPM) as a vital tool for mapping the light-matter interactions and function of π-conjugated copolymers and their fullerene blends vs. the film morphology; III) MPM mapping of the first prototype π-conjugated polymers and their blends, namely MEH-PPV and P3HT films and their fullerene blends. A variety of optical spectroscopies were conducted to investigate the charge excitations and correlated infrared (IR)-active and Raman-active vibrations in PffBT4T, a π-conjugated donor–acceptor (DA) copolymer, which, when blended with fullerene PCBM molecules, serves as an active layer in high performance OPV solar cells. The applied optical spectroscopies in films of pristine PffBT4T and PffBT4T/PCBM blend include absorption, photoluminescence, electroabsorption, photoinduced absorption (PA), and resonant Raman scattering. We found that the PffBT4T copolymer chain contains 11 strongly coupled Raman-active vibrational modes, which are renormalized upon photogeneration or doping-induced charge polarons onto the chains. We show that the Raman scattering, doping induced, and photoinduced antiresonances (AR) spectra in PffBT4T are well explained by the amplitude mode model (AMM), where a single vibrational propagator describes the renormalized Raman modes and their related photoinduced AR intensities in detail. Surprisingly, we found that two of the IR active modes in the pristine copolymer must be included in the AMM propagator for explaining the complete photoinduced AR spectrum. This feature is unique to DA-copolymers and indicates that some intrachain C2v symmetry breaking occurs in the chain because of the different electron affinities of the intrachain donor and acceptor moieties. Although a number of imaging microscopies have been applied to films of pi-conjugated copolymers and their fullerene blends, seldom have they been able to detect microscopic defects in the blend films. We have applied multiphoton microscopy (MPM) using a 65 fs laser at 1.56 micron for spectroscopy and mapping of films of various pi-conjugated copolymers and their fullerene blends. All pristine copolymer films have shown third harmonic generation (THG) and two-photon or three-photon absorption induced photoluminescence that could be used for mapping the films with micrometer spatial resolution. Since the fullerene molecules have much weaker THG efficiency than that of the copolymers, we could readily map the copolymer/fullerene blend films that showed interpenetrating micron-sized grains of the two constituents. In addition, we also found second harmonic generation (SHG) from various micron-size defects in the films that are formed during film deposition or light illumination at ambient conditions, which do not possess inversion symmetry. The MPM method is therefore beneficial for organic films and devices of areas ~100 cm2 for investigating the properties and growth of copolymer/fullerene blends for OPV applications. We have applied MPM for spectroscopy and mapping of films of two prototype pi-conjugated polymers, namely MEH-PPV and P3HT with their blends with PCBM fullerene molecules. The pristine polymer films have shown THG and three-photon absorption induced photoluminescence (ThP-PL) emission bands that could be used for mapping the film topography with micrometer spatial resolution. Since the photoluminescence (PL) band of the photogenerated charge transfer excitons at the polymer/fullerene interfaces in films of polymer/fullerene blends is substantially red-shifted compared to that of the pristine polymers, we could readily map the polymer/fullerene grain interfaces using the ThP-PL in the near IR spectral range. From the MPM imaging of the polymer/fullerene films, we show that the polymer grains in MEH-PPV/PCBM are substantially larger than those in P3HT/PCBM, which is detrimental to photovoltaic applications. In addition, we also found in the MEH-PPV/PCBM blend SHG emission bands that originates from micron-size inclusions in the films that are formed during film deposition at ambient conditions, which do not possess inversion symmetry.