Abstract-Sign inversion in the terahertz photoconductivity of single-walled carbon nanotube films

Peter Karlsen, Mikhail V. Shuba, Polina P. Kuzhir, Albert G. Nasibulin, Patrizia Lamberti, Euan Hendry

https://arxiv.org/abs/1804.11113

In recent years, there have been conflicting reports regarding the ultrafast photoconductive response of films of single walled carbon nanotubes (CNTs), which apparently exhibit photoconductivities that can differ even in sign. Here, we observe explicitly that the THz photoconductivity of CNT films is a highly variable quantity which correlates with the length of the CNTs, while the chirality distribution has little influence. Moreover, by comparing the photo-induced change in THz conductivity with heat-induced changes, we show that both occur primarily due to heat-generated modification of the Drude electron relaxation rate, resulting in a broadening of the plasmonic resonance present in finite-length metallic and doped semiconducting CNTs. This clarifies the nature of the photo-response of CNT films and demonstrates the need to carefully consider the geometry of the CNTs, specifically the length, when considering them for application in optoelectronic devices.

Abstract-Transient terahertz photoconductivity of insulating cuprates

J. C. Petersen, A. Farahani, D. G. Sahota, Ruixing Liang, and J. S. Dodge

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.115133

We establish a detailed phenomenology of photocarrier transport in the copper oxide plane by studying the transient terahertz photoconductivity of ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_6$. The peak photoconductivity saturates with fluence, decays on multiple picosecond timescales, and evolves into a state characterized by activated transport. The time dependence shows little change with fluence, indicating that the decay is governed by first-order recombination kinetics. We find that most photocarriers make a negligible contribution to the dc photoconductivity, and we estimate the intrinsic photocarrier mobility to be 0.6–0.7 ${\text{cm}}^2/\text{V}\text{s}$ at early times, comparable to the mobility in chemically doped materials.

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Abstract-Exciton localization in solution-processed organolead trihalide perovskites

• Haiping He,
• Qianqian Yu,
• Hui Li,
• Jing Li,
• Junjie Si,
• Yizheng Jin,
• Nana Wang,
• Jianpu Wang,
• Jingwen He,
• Xinke Wang,
• Yan Zhang
• & Zhizhen Ye
• http://www.nature.com/ncomms/2016/160321/ncomms10896/abs/ncomms10896.html

Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium–lead–halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

Abstract-Hot photocarrier dynamics in organic solar cells

• P. A. Lane,
• P. D. Cunningham,
• J. S. Melinger,
• O. Esenturk
• & E. J. Heilweil

http://www.nature.com/ncomms/2015/150716/ncomms8558/full/ncomms8558.html

Photocurrent in an organic solar cell is generated by a charge transfer reaction between electron donors and acceptors. Charge transfer is expected to proceed from thermalized states, but this picture has been challenged by recent studies that have investigated the role of hot excitons. Here we show a direct link between excess excitation energy and photocarrier mobility. Charge transfer from excited donor molecules generates hot photocarriers with excess energy coming from the offset between the lowest unoccupied molecular orbital of the donor and that of the acceptor. Hot photocarriers manifest themselves through a short-lived spike in terahertz photoconductivity that decays on a picosecond timescale as carriers thermalize. Different dynamics are observed when exciting the acceptor at its absorption edge to a thermalized state. Charge transfer in this case generates thermalized carriers described by terahertz photoconductivity dynamics consisting of an instrument-limited rise to a long-lived signal.