Ultrafast Broadband Time-Resolved Photoluminescence Spectroscopy for Advanced Molecular Optoelectronic Materials

Abstract

Understanding the fundamental physical processes in functional materials is essential for designing new materials and device structures. Tracking photoexcitation dynamics from sub-picosecond timescales is particularly important for advanced optoelectronic materials and devices, including photovoltaic cells. Because many next generation optoelectronic materials, including semiconductor nano-crystals and organic semiconductors, show strong excitonic character, the study of ultrafast exciton dynamics is of central importance.

Time-resolved photoluminescence (TRPL) spectra contain rich signatures of exciton dynamics. Ultrafast laser-based nonlinear optical gating makes it possible to obtain TRPL spectra with excellent time resolution and sensitivity. However, conventional upconversion TRPL spectroscopy is constrained by a narrow detection bandwidth, which is disadvantageous in studying the nano- and molecular materials characterized by broad PL spectra. In this thesis, we developed and applied ultrafast TRPL spectroscopy techniques based on broadband phase matched third-order nonlinear optical gating.

The home-built TRPL system based on an optical Kerr gate shutter provides 200 fs time resolution and broad colour resolution to illuminate the processes of exciton formation, dissociation and energy relaxation on the femto- to picoseconds timescale in polymer photovoltaics. By measuring the dependence of initial PL amplitude on excitation intensity in polymer thin films, we can estimate the volume of the initial excitations on the order of 10 nm³, corresponding to substantial delocalization along polymer chains. We also showed that charge carrier generation, studied by transient absorption spectroscopy, is strongly correlated with exciton PL dynamics in a polymer: fullerene bulk heterojunction. We conclude that charge carriers are predominantly formed from initially delocalized and non-relaxed excitations, suggesting that circumventing exciton relaxation may be crucial to the photon-to-carrier conversion process.

A novel TRPL technique, transient grating photoluminescence spectroscopy (TGPLS), is developed to address the problem of high PL background that is inherent to the Kerr gating technique. TGPLS exploits a transient phase grating induced by the interference of femtosecond laser pulses in a Kerr medium as the optical gate. TGPLS features a compact setup, ultra-broad detection bandwidth (covering the whole visible region, and potentially beyond), ultrafast time resolution (200 fs) and extremely low background – a combination that outperforms other TRPL methods.

A study of intramolecular energy transfer in donor/acceptor multichromophore arrays based on perylene derivatives shows the advantages of TGPLS. We are able to capture TRPL spectra with both high temporal and spectral resolution. Moreover, not only the fast dynamics from the donor, but also the slow dynamics from the acceptor are well resolved, with excellent background suppression. This enables us to apply global and target analysis to resolve the energy transfer process on subpicosecond timescales and assess the validity of Förster energy transfer model in this moderate coupling regime.

Finally, we investigate the sub-picosecond TRPL dynamics for the first time in organometal halide perovskites - a promising material for new generation solar cells. These measurements employ the optical Kerr gate system with infrared femtosecond laser pulse as the gate beam to enable TRPL detection in the near IR region. From the picosecond rise time for PL and the lack of initial polarization anisotropy, we conclude that the PL dynamics are dominated by non-geminate photocharge recombination at room temperature. The PL spectral dynamics are consistent with transient absorption measurements, with both revealing signatures of thermalizing free charge carriers. Furthermore, we also present the first TRPL dynamics measurements of these samples undergoing amplified spontaneous emission at high excitation density. These measurements reveal that the onset gain is delayed by relaxation of the mobile free charge carriers to the emissive states at the band edge. These novel measurements illustrate the unique properties of organometal halide perovskites for high performance optoelectronic devices.