Photocurrent Dynamics Research Articles

Time-Resolved Picosecond Photocurrents in Contacted Carbon Nanotubes

We introduce coplanar stripline circuits to resolve the ultrafast photocurrent dynamics of freely suspended carbon nanotubes (CNTs) in the time domain. By applying an on-chip pump-probe laser spectroscopy, we demonstrate that CNTs, contacted by metal electrodes, exhibit a picosecond photocurrent response. We find a combination of an optically induced ultrafast displacement current, transport of photogenerated charge carriers at the Fermi velocity to the electrodes, and interband charge-carrier recombination processes to dominate the ultrafast photocurrent of the CNTs.

Organic semiconductor composites: Influence of additives on the transient photocurrent

We report on the effect of various guest molecules added to a functionalized anthradithiophene (ADT) host on photoexcited charge carrier dynamics in solution-deposited thin films, from ∼100 ps to >100 μs after photoexcitation with 100 fs laser pulses. An addition of 2 and 5 wt % of C60 to a fluorinated ADT derivative, ADT-TES-F, resulted in transient photocurrent amplitude enhancement by a factor of ∼3 and 10, respectively. In contrast, an addition of 10 wt % of another ADT derivative, ADT-TIPS-CN, or a functionalized pentacene derivative (TIPS pentacene) to ADT-TES-F led to a decrease in the amplitude by a factor of ∼3–4 and dramatically different photocurrent dynamics.

Modeling photocurrent transients in organic solar cells

We investigate the transient photocurrents of organic photovoltaic devices in response to asharp turn-on of illumination, by numerical modeling of the drift-diffusion equations. Weshow that the photocurrent turn-on dynamics are determined not only by the transportdynamics of free charges, but also by the time required for the population of geminatecharge pairs to reach its steady-state value. The dissociation probability of ageminate charge pair is found to be a key parameter in determining the deviceperformance, not only by controlling the efficiency at low intensities, but alsoin determining the fate of charge pairs formed by bimolecular recombinationat high intensities. Bimolecular recombination is shown to reduce the turn-ontime at high intensities, since the typical distance traveled by a charge pair isreduced.

Electric field effect in the spin dynamics of self-assembled InAs/GaAs quantum dots

We identify fundamental mechanisms of electron and hole dynamics in self-organized InAs/GaAs quantum dots (QDs) subject to vertical electric fields by photocurrent investigations. We propose a spin–flip mechanism involving a spin exchange between neighboring QDs. The spin–flip process is revealed in the photocurrent dynamics when the exciton population increases unexpectedly with reverse bias.

Photocurrent dynamics in a poly(phenylene vinylene)-based photorefractive composite

All parameters describing the charge carrier dynamics in a poly(phenylene vinylene)-based photorefractive (PR) composite relevant to PR grating dynamics were determined using photoconductivity studies under various illumination conditions. In particular, the values of the coefficients for trap filling and recombination of charges with ionized sensitizer molecules could be extracted independently. It is concluded that the PR growth time without preillumination is mostly determined by the competition between deep trap filling and recombination with ionized sensitizer molecules. Further, the pronounced increase in PR speed upon homogeneous preillumination (gating) as reported recently is quantitatively explained by deep trap filling.

Charge transport due to photoelectric interface activation in pure nematic liquid-crystal cells

We report a study of the crucial role of liquid-crystal–polymer interface on photoinduced transport and redistribution of charges in pure nematic liquid-crystal cells that exhibit a photorefractivelike effect. A stationary photocurrent that is 30% of the dark current has been measured for very low power illumination (few mW) and low applied dc electric field (about 0.1 V/μm). The experimental results indicate a clear dependence of the effect on the light wavelength. The absence of photocurrent in cells with only one component, liquid-crystal, or polymer, suggests that both are not intrinsically photoconductive, rules out light-induced charge injection by the electrodes, and indicates the polymer–liquid-crystal interface as the photoactive element in the effect. The photocurrent dynamics indicate the presence of various mechanisms. We suppose that the effect is due to photoinduced carriers injection through the liquid-crystal–polymer interface and recombination process with the counterions present on the opposite side. Different hypotheses are made and discussed.

The Nature of Electron Migration in Dye-Sensitized Nanostructured TiO2

The relationship between the morphology of nanostructured anatase TiO2 films and electron migration in these electrodes has been investigated. Toward this end, three different TiO2 powders have been studied, all of which have a unique microstructure. Photocurrent dynamics is used to elucidate electron transport in nanostructured TiO2 networks. In particular, intensity-modulated photocurrent spectroscopy proves to be a powerful characterization technique. In all cases the transport of electrons through a TiO2 network can be described with a macroscopic diffusion model. By fitting the data to this model it is possible to determine the absorption coefficient, the diffusion coefficient, and the lifetime of the electrons. Upon illumination, electrons accumulate in the nanostructured film. A fraction of these electrons are stored in deep surface traps. Another fraction of the electrons reside in the conduction band and are free to move. The average concentration of these excess mobile conduction band electrons amounts to one electron per nanoparticle, irrespective of the type of electrode, the film thickness, or the irradiation intensity. Equilibration of the quasi-Fermi level combined with infinite fast extraction of electrons at the substrate explains this saturation effect.

Photocurrent dynamics in single quantum well infrared photodetectors investigated by using free electron laser pulses

We have studied the optical and transport properties of AlGaAs/GaAs single quantum well infrared photodetectors (SQWIPs). The SQWIP shows a narrow band photocurrent at around 9.2 μm, due to intersubband transition in the steady state characteristics, measured using a Fourier transform infrared spectrometer. The bias voltage dependence of the magnitude and the spectral shape of the observed steady state photocurrent indicates that the photocurrent is strongly affected by the tunneling escape process. Furthermore, a transient photoresponse of the SQWIP has been studied using free electron laser (FEL) pulses. A comparison of the steady state photocurrent and the transient photoresponse shows that the charging current induced by the band bending effect contributes to an increased photo sensitivity.