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Colloidal semiconductor nanocrystals have been used as building blocks for electronic and optoelectronic devices ranging from field-effect transistors to solar cells. Properties of the nanocrystal films depend sensitively on the choice of capping ligand to replace the insulating synthesis ligands. Thus far, ligands leading to the best performance in transistors result in poor solar cell performance, and vice versa. To gain insight into the nature of this dichotomy, we used time-resolved terahertz spectroscopy measurements to study the mobility and lifetime of PbSe nanocrystal films prepared with five common ligand-exchange reagents. Noncontact terahertz spectroscopy measurements of conductivity were corroborated by contacted van der Pauw measurements of the same samples. The films treated with different displacing ligands show more than an order of magnitude difference in the peak conductivities and a bifurcation of time dynamics. Inorganic chalcogenide ligand exchanges with sodium sulfide (Na2S) or ammonium thiocyanate (NH4SCN) show high mobilities but nearly complete decay of transient photocurrent in 1.4 ns. In contrast, ligand exchanges with 1,2-ethylenediamine (EDA), 1,2-ethanedithiol (EDT), and tetrabutylammonium iodide (TBAI) show lower mobilities but longer carrier lifetimes, resulting in longer diffusion lengths. This bifurcated behavior may explain the divergent performance of field-effect transistors and photovoltaics constructed from nanocrystal building blocks with different ligand exchanges.

Ultrafast pump-probe transient reflectance (TR) spectroscopy was used to study carrier dynamics in an epitaxial perovskite oxide thin film of LaFeO 3 (LFO) with a thickness of 40 unit cells (16 nm) grown by molecular beam epitaxy on (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT). TR spectroscopy shows two negative transients in reflectance with local maxima at $2.5 eV and $3.5 eV which correspond to two optical transitions in LFO as determined by ellipsometry. The kinetics at these transients were best fit with an exponential decay model with fast (5-40 ps), medium ($200 ps), and slow ($ 3 ns) components that we attribute mainly to recombination of photoexcited carriers. Moreover, these reflectance transients did not completely decay within the observable time window, indicating that $10% of photoexcited carriers exist for at least 3 ns. This work illustrates that TR spectroscopy can be performed on thin (<20 nm) epitaxial oxide films to provide a quantitative understanding of recombination lifetimes, which are important parameters for the potential utilization of perovskite films in photovoltaic and photocatalytic applications. V C 2014 AIP Publishing LLC.Perovskite oxides are a class of transition metal oxides with the chemical structure ABO 3 . They have garnered much interest because of the diverse range of their physical and magnetic properties, including ferroelectricity, insulator-tometal transitions, ferromagnetism, and superconductivity. 1 Many perovskite oxides exhibit band gaps in the visible range, leading to growing research interest in utilizing perovskite oxides for photovoltaic (PV) and photocatalytic (PC) applications. 2-9 However, understanding of the underlying ultrafast carrier dynamics in these materials is limited to a few studies, 10-12 despite the critical role that carrier lifetimes play in the design of materials for PV and PC applications. 13 Here, we present a study of the carrier dynamics and recombination lifetimes in an epitaxial LaFeO 3 (LFO) thin film using ultrafast pump-probe spectroscopy. LFO is an interesting material system for PV and PC applications because its band gap is in the visible range (2.1-2.6 eV). 14,15 The relative chemical simplicity of LFO, compared to quaternary oxides, makes it an appealing system with which to begin investigations focused on identifying strategies to enhance carrier lifetimes in perovskites. Ultrafast spectroscopy is an ideal technique to study recombination lifetimes and has already been used for this purpose in other perovskite oxides. [10][11][12]16,17 Much of the previous work, however, was carried out on bulk crystals or polycrystalline films, as opposed to strained epitaxial thin films as reported here. The ability to measure carrier dynamics in ultrathin perovskites is a critical step in understanding how heterostructure-based approaches can be used to control lifetimes. To mitigate contributions from the substrate, the ultrafast pump-probe measurements were performed in a reflective geometry, using transient reflectance (TR) spectroscopy. ...

We report the application of time-resolved terahertz spectroscopy (TRTS) to measure photoexcited carrier lifetimes and mobility, and to determine recombination mechanisms in Cu2ZnSn(S,Se)4 (CZTSSe) thin films fabricated from nanocrystal inks. Ultrafast time resolution permits tracking the evolution of carrier density to determine recombination rates and mechanisms. The carrier generation profile was manipulated by varying the photoexcitation wavelength and fluence to distinguish between surface, Shockley-Read-Hall (SRH), radiative, and Auger recombination mechanisms and determine rate constants. Surface and SRH recombination are the dominant mechanisms for the air/CZTSSe/SiO2/Si film stack. Diffusion to, and then recombination at, the air-CZTSSe interface occurred on the order of 100 picoseconds, while SRH recombination lifetimes were 1–2 nanoseconds. TRTS measurements can provide information that is complementary to conventional time-resolved photoluminescence measurements and can direct the design of efficient thin film photovoltaics.

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The extremely thin absorber (ETA) solar cell architecture can enable higher efficiencies than planar cells for absorbers that have low carrier lifetimes or mobilities. Efficient charge separation requires that interfacial electron and hole transfer proceed much faster than the recombination lifetime of photoexcited carriers. In this work, transient absorption spectroscopy was employed to measure these ultrafast photophysical processes in CdSe-coated ZnO nanowire ETA cells and model planar films. At low pump fluences, carrier lifetime was controlled by Shockley–Read–Hall and surface recombination. Annealing the electrodeposited CdSe films increased the lifetime 50-fold, to >1 ns as measured by transient absorption spectroscopy, which correlated to improved ETA cell performance. Interfacial electron transfer from the CdSe coating into the ZnO nanowires occurred 3 orders of magnitude faster, within 1 ps, independent of the presence or absence of an interfacial CdS buffer layer. Interfacial hole transfer to the ferri­(o)­cyanide redox couple could not be directly measured, but photodegradation of the semiconductor surface in the presence of electrolyte under prolonged light exposure resulted in faster dynamic response due to higher densities of surface electron trap states. This study provides a framework for understanding photophysics and improving performance of nanostructured, semiconductor-sensitized solar cells through a combination of device measurements and ultrafast probes.