Abstract-Effective detection of weak terahertz pulses in electro-optic sampling at kilohertz repetition rate

Philipp Krauspe, Natalie Banerji, and Julien Réhault

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-37-1-127

The standard terahertz (THz) detection mechanism known as electro-optic sampling can be improved in sensitivity by biasing the polarization of the sampling field. In this work, we show theoretically and experimentally how weak signals can be amplified without inducing distortions. Our study identifies the influence of THz field strength, the polarization quality, and biasing amplitude on signal amplification and distortion. Here we present a distortion-free amplification of a factor of 28 while at the same time reducing the measurement time significantly.

© 2019 Optical Society of America

Abstract-Terahertz short-range mobilities in neat and intermixed regions of polymer:fullerene blends with controlled phase morphology

Philipp Krauspe,a  Demetra Tsokkou,  Martina Causa',  Ester Buchaca-Domingo, Zhuping Fei,  Martin Heeney,Natalie Stingelin,  Natalie Banerji



https://pubs.rsc.org/en/Content/ArticleLanding/2018/TA/C8TA08061E#!divAbstract

The molecular-level arrangement of the donor and acceptor (i.e. the local morphology) in organic solar cells governs charge separation and charge transport via its effect on the mobility of charges. However, the nanometre-scale mobility in such systems, which can be measured using terahertz (THz) spectroscopy, has been little investigated at relevant low excitation densities, due to extremely weak signals. Here, we study the mobility over short distances and at ultrashort times using time-resolved optical-pump-THz-probe (OPTP) spectroscopy on pBTTT:PCBM blends. This complements our previous results obtained with transient absorption (TA) and electro-modulated differential absorption (EDA) techniques. In the pBTTT:PCBM system, the co-existence of fullerene-/polymer-rich (‘neat’) and co-crystalline (‘intermixed’) regions can be controlled through choice of composition (weight ratio of the two components, use of processing additive). We demonstrate high short-range mobilities that help charges separate, and we show how this mobility of photogenerated charges develops in time, in particular as the charges move between different phase regions of the blend. By reducing the pump fluence below the threshold for nonlinear recombination mechanisms, we access these properties at solar cell operating conditions. Overall, we explain the necessity of different local phases through their influence on charge lifetime and mobility.