Ultrafast Terahertz Spectroscopy for Measuring Carrier Dynamics in Nanoscale Photovoltaic Materials

Published

February 15, 2010

Author(s)

Okan Esenturk, Paul A. Lane, Joseph S. Melinger, Edwin J. Heilweil

Abstract

Femtosecond pump-probe methods are useful tools for investigating transient electronic and vibrational states of conducting materials and molecular photochemistry. Ultraviolet and visible excitation pulses (<150 fs, <20 mJ, 266-800 nm) with time-delayed broadband terahertz (500 GHz to 3 THz) probing pulses (Time-Resolved Terahertz Spectroscopy; TRTS) are used to measure linear spectroscopic transmission changes resulting from exciton and free carrier population in organic semiconducting thin films. Picosecond timescale exciton geminate recombination and longer-time free-carrier conduction in semiconductor polymers and nanolayered donor-acceptor films are discussed. Systems investigated with terahertz probe pulses include thiophene-based polymers (P3HT, PBTTT) studied as drop and spin-cast films on transparent quartz substrates. The relative conductivity of these films increases with increasing P3HT polymer molecular weight, structural regularity, and the fused rings in PBTTT further increases conduction. Recent studies of composite and nanolayered films (by vapor deposition) containing alternating Zn-phthalocyanine (ZnPc) and buckminsterfullerene (C60) also yield high conduction that scales linearly with the number of interfaces and total film thickness. We find evidence for a short-lived charge transfer state of C60 that decays within several picoseconds of excitation. In contrast, both composite and multilayered films exhibit long-lived THz dynamics that depends on the composition and structure of the films. The optimum composition for charge transfer within composite films is observed for a 1:1 blend of ZnPc with C60 and a 4:1 blend of P3HT with Phenyl C61 Butyric Acid Methyl Ester (PCBM) while an increase in charge photo-generation with decreasing layer thickness (2 nm) exhibits the strongest THz signal. These findings parallel results for transistor devices pointing to the advantage of optically measuring material properties before device test.

Citation

SPIE proceedings series

Volume

7600

Pub Type

Journals

Download Paper

https://doi.org/10.1117/12.843636

Local Download

Keywords

Buckminsterfullerene, Femtosecond, Organic photovoltaics, PCBM, P3HT, PBTTT, Phthalocyanine, Terahertz, THz Spectroscopy, Time-Resolved

Citation

Esenturk, O. , Lane, P. , Melinger, J. and Heilweil, E. (2010), Ultrafast Terahertz Spectroscopy for Measuring Carrier Dynamics in Nanoscale Photovoltaic Materials, SPIE proceedings series, [online], https://doi.org/10.1117/12.843636, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=904838 (Accessed April 18, 2024)