Photocurrent Imaging Research Articles

Local photocurrent mapping as a probe of contact effects and charge carrier transport in semiconductor nanowire devices

Three types of two-terminal CdS nanowire devices with distinct current versus voltage characteristics were fabricated by forming Schottky and/or Ohmic contacts in a controlled manner. Argon ion bombardment of CdS nanowires increased the carrier concentration allowing the formation of Ohmic Ti–CdS contacts. Scanning photocurrent microscopy (SPCM) was used to explore the influence of the contacts on the spatially resolved photoresponse in two-terminal devices and to analyze charge carrier transport processes. Modeling of the spatial profiles of the local photocurrent images enabled the quantitative extraction of electron and hole mobility-lifetime products in Ohmic devices and the hole mobility-lifetime product in Schottky devices. Analysis of the evolution of SPCM images with bias suggests that the electric field is localized to the optical generation region in the Ohmic devices and localized beneath the contacts in the Schottky devices.

Near‐field scanning optical microscopy and near‐field induced photocurrent investigations of buried heterostructure multiquantum well lasers

SummaryBuried heterostructure multiquantum well laser devices are investigated utilizing a near‐field scanning optical microscope to characterize and correlate the surface topography, optical output and electronic properties of the device. Near‐field photocurrent imaging has been used to accurately measure the unbiased buried heterostructure multiquantum well device in cross‐section, successfully revealing the distribution of pn‐junctions and their associated fields. Moreover, this has been accurately correlated with the physical structure of the device determined by simultaneous shear‐force imaging of the surface. Topographic structure is manifested as a result of strain relaxation (∼10−10 m) of the cleaved cross‐section. These imaging modes are similarly correlated with the optical output of the operational device mapped with 50 nm lateral resolution. The collection‐mode measurements detected electroluminescence external to the active region, highlighting the existence of carrier recombination away from the multiquantum well device region. The combination and correlation of different near‐field scanning optical microscope imaging modes proved powerful in the analysis of the buried heterostructure multiquantum well device, and was shown to assist in the identification of current leakage pathways within the structure.

Chromatographic studies of photodegradation of RuL2(SCN)2 in nanostructured dye-sensitization solar cells

Abstract The claim that ruthenium complexes attached to nanostructured TiO2 can sustain 108 electron transfer cycles is in conflict with photocurrent imaging studies, which clearly show photoinduced degradation of solar cell efficiency. HPLC chromatographic studies of the cis-RuL2(SCN)2 with L = 2,2-bipyridyl-4,4-dicarboxylatoacid (bpca) in solution and extracted from long-term operating solar cells were undertaken to identify the spectrum of possible photochemical products. While a dramatic increase of photochemical stability of the ruthenium complex when attached to TiO2 nanoparticles is confirmed with respect to its photochemical reactivity in solution, photoelectrochemical products of the ruthenium complex can clearly be identified after one to a few months of solar cell operation under simulated solar light. The interpretation is that the ruthenium complex when adsorbed to TiO2 nanoparticles is exposed to different surface states, which warrant different stability conditions to the oxidized ruthenium intermediate. To cite this article: M. Thomalla, H. Tributsch, C. R. Chimie 9 (2006).

Monitoring the reactivity of oxide interfaces in dye solar cells with photocurrent imaging techniques

Abstract By partially dip-coating and processing the F:SnO2 front contact so that a very thin layer of platinum or of ruthenium oxide is formed, a breakdown of solar cell efficiency is observed via photocurrent imaging techniques. This evidences that the front contact functions like a degenerate semiconductor which determines the cell photopotential. Modification of the TiO2 nanosurface with ultra thin alternative oxide layers (ZnO, Al2O3) affects both electron injection and reverse reaction and thus photoefficiency. ZnO- and Al2O3- modified TiO2 interfaces show an increased rate of sensitizer degradation. This result emphasizes the role of interfacial sensitizer bonding for long-term stability. To cite this article: M. Junghanel, H. Tributsch, C. R. Chimie 8 (2005).

Screening for artifacts in near-field scanning photocurrent microscopy images of polymer solar cells

We present the results of an investigation to screen for the presence of artifacts in near-field scanning photocurrent microscopy (NSPM) images of polymer solar cells caused by variations in tip-sample separation. We show that by increasing the size of the probe tip we are able to deliberately degrade the surface topographical image without altering the photocurrent image. This work demonstrates that NSPM images of polymer solar cells reliably probe the local charge generation properties of the device and are not significantly influenced by the presence of artifacts. Polyfluorene blend solar cells are used as a model system for this study.

Scanning Photocurrent Imaging and Electronic Band Studies in Silicon Nanowire Field Effect Transistors

We report optical scanning measurements on photocurrent in individual Si nanowire field effect transistors (SiNW FETs). We observe increases in the conductance of more than 2 orders of magnitude and a large conductance polarization anisotropy of 0.8, making our SiNW FETs a polarization-sensitive, high-resolution light detector. In addition, scanning images of photocurrent at various biases reveal the local energy-band profile especially near the electrode contacts. The magnitude and polarity of the photocurrent vary depending on the gate bias, a behavior that can be explained using band flattening and a Schottky-barrier-type change. This technique is a powerful tool for studying photosensitive nanoscale devices.

Photocurrent Imaging of Charge Transport Barriers in Carbon Nanotube Devices

The realization of high-performance electrical devices incorporating single-wall carbon nanotubes critically depends on the minimization of charge transport barriers in the tubes and at the contacts. Herein we demonstrate photocurrent imaging as a fast and effective tool to locate such barriers within individual metallic nanotubes contacted by metal electrodes. The locally induced photocurrents directly reflect the existence of built-in electric fields associated with the presence of depletion layers at the contacts or structural defects along the tubes.

Photoelectrochemical and Raman imaging studies of chemical and mechanical properties of thermally grown titanium oxide scales

The present work focuses on thin TiO2 scales (up to 3 μm). The use of both micro-photoelectrochemistry (MPEC) and Raman imaging aims to get chemical and mechanical information at a micron scale. In particular, the MPEC technique allows images of the history of strain and adhesion levels of the oxide/metal interface to be realised. A set of photocurrent images recorded at different applied potentials allows different images to be constructed, Structural Quality Image (SQI) and Interface Cohesion Image (ICI). In addition, Raman spectroscopy images the chemical nature of the scale, topography of the surface and local stress level in the scale. Combining both techniques is a powerful tool in elucidating the mechanisms of thermal oxidation.

Parameters determining efficiency and degradation of TiO2$vbar;dye$vbar;CuI solar cells

The influence of the micro-morphological structure of the TiO2 film, the distribution of CuI in TiO2 pores and the concentration of added surfactant in the CuI coating solution on the photocurrent of solid-state TiO2|dye|CuI solar cells was examined by space resolved photocurrent imaging technique. Iodine is found to be competing with the oxidized dye molecules in accepting electrons from CuI and decreases the efficiency of the cell. TiO2|dye|CuI cell degrade two hundred times faster than wet sensitization cells. This instability is considered to be due to the decomposition of the electron transfer-bridge between the sensitizer and CuI.

Patterns of efficiency and degradation of composite polymer solar cells

Bulk-heterojunction plastic solar cells (PSC) produced from a conjugated polymer, poly(2-methoxy-5-(3′,7′-dimethyloctyl-oxy)-1,4-phenylenevinylene) (MDMO-PPV), and a methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were investigated using photocurrent imaging techniques to determine characteristic patterns of efficiency and degradation. The solar cells with power efficiencies of up to 2.6% showed significant inhomogeneities and variations depending on the preparation steps (e.g. aluminum deposition), suggesting there is still room for improvements. A characteristic feature of the well-known photoinduced and dark cell degradation is the formation of islands of higher efficiency. Degradation mechanisms appear to have a morphological component. The imaging technique will open opportunities for combinatorial plastic solar cell research.

Photocatalytic oxidation on nanostructured chalcogenide modified titanium dioxide

Surface mesoscopic titanium dioxide (P25) films deposited onto conducting glass plates (SnO2:F) were modified by colloidal RuxSey nanoparticles (2 nm diameter). A decrease of the photocurrent was found upon modification of TiO2 films. However, interfacial electron transfer kinetics to oxygen was favored. The increase of the catalyst surface concentration onto TiO2, shifts the onset of the photocurrent under UV-illumination, to 0.6 V/RHE in presence of oxygen dissolved in the electrolyte. Concomitant to this, the cathodic current becomes important and shifts to more positive potentials. This phenomenon allows the system to work catalytically under open circuit conditions. On non-modified TiO2, the application of a 0.3 V/RHE potential leads to an enhancement of the photooxidation of formic acid. Photocurrent images revealed a non-homogeneous distribution of the catalyst on the titanium dioxide films.

Parameters determining efficiency and degradation of TiO 2|dye|CuI solar cells

The influence of the micro-morphological structure of the TiO 2 film, the distribution of CuI in TiO 2 pores and the concentration of added surfactant in the CuI coating solution on the photocurrent of solid-state TiO 2|dye|CuI solar cells was examined by space resolved photocurrent imaging technique. Iodine is found to be competing with the oxidized dye molecules in accepting electrons from CuI and decreases the efficiency of the cell. TiO 2|dye|CuI cell degrade two hundred times faster than wet sensitization cells. This instability is considered to be due to the decomposition of the electron transfer-bridge between the sensitizer and CuI.

Direct Photocurrent Mapping of Organic Solar Cells Using a Near-Field Scanning Optical Microscope

Direct photocurrent mapping of organic solar cells (OSCs) using a novel implementation of a near-field scanning optical microscope (NSOM) is described. By rastering the light output from the NSOM through a semitransparent electrode across the OSC surface, it is possible to collect height and photocurrent images simultaneously with a lateral resolution that is governed by the NSOM aperture. The photocurrent images demonstrate that film inhomogeneities and segregation effects strongly influence OSC device performance.

Distinctive photocurrent chemical images on bare SiO2 between continuous metal gates

Distinctive two-dimensional chemical images of hydrogen, ammonia and nitrogen dioxide have been obtained using a scanning light pulse technique (SLPT) on a metal-oxide-semiconductor (MOS) device.The sensor consists of an MOS capacitor with two continuous metal gates of Pd (40nm) and Au (50nm) arranged in a concurrent geometry, leaving a bare silicon dioxide region in between the biased gates. The present arrangement provides a new robust alternative to the existing strategies used to induce spatially distributed chemical responses, required for SLPT and allows to measure the response in a different place than where the biasing electrodes operate.Accordingly, the splitting of biasing and detection coordinates enables to optimize electrical and chemical functions separately providing a higher versatility in the choice of functional sensing materials or stable biasing electrodes.

Chemical images generated by large area homogeneous illumination of metal–insulator–semiconductor structures

A controlled large area illumination, provided by a computer screen, is used to induce distinctive chemical images in field-effect devices upon gas exposure. The present measuring method uses the concurrent optical and chemical modulation of the semiconductor surface potential of a single metal–oxide–semiconductor capacitor to generate two-dimensional photocurrent images, displayed in a bias voltage-color space. Selective patterns for hydrogen, ammonia, and propene are demonstrated. The use of a computer screen as a programable light source allows to simplify a normally complex setup, making the technique more attractive for practical applications.

Patterns of efficiency and degradation in dye sensitization solar cells measured with imaging techniques

A larger number of dye sensitization solar cells based on cis-Ru II(LH 2) 2(NCS) 2 with LH 2=2,2′-bipyridyl-4,4′-dicarboxylic acid with an electrolyte consisting of 0.5 M LiI, 50 mM I 2, 0.2 M tert.-butyl pyridine in acetonitrile have been studied, using spatially resolved photocurrent imaging techniques. Measurements have been made after preparation and periodically during a longer period of simulated solar light illumination. The observed phenomena have been grouped into five categories. The first one concerns significant inhomogeneities reflecting the TiO 2-layer preparation technique used. The second category concerns an inhomogeneous deterioration of the dye sensitization cell during illumination. The third phenomenon involves photodegradation itself, which can be visualized by selectively illuminating the dye sensitization solar cell. Changes observed in the composition of the electrolyte, typically indicated by a bleaching of the iodide/iodine solution were also observed. Finally, the fifth category to be considered deals with a loss of electrolyte and the parallel appearance of gas bubbles in the solar cell. All these phenomena may coexist, being responsible for the overall process of degradation. The different mechanisms are discussed and analyzed in an effort to determine parameters critical for increasing efficiency and stability of dye sensitization solar cells.

Near-Field Imaging of the Photocurrent Induced by Light on Boron-Implanted Silicon

We report near-field photocurrent (PC) measurements performed on boron-implanted silicon and acquired at λ = 633, 1330, 1550 nm corresponding to photon energies of 1.96, 0.93, and 0.80 eV, respectively. The second and the third energies are below the silicon energy gap (Egap = 1.12 eV), representing incident radiation to which Si is virtually transparent. The PC images acquired at λ = 1330 and 1550 nm reveal the presence of boron clusters which are a consequence of B-implantation and rapid thermal annealing at 1100 °C. Boron clusters behave as metal clusters embedded into the silicon matrix and introduce gap states which give rise to the observed photocurrent. At energies larger than the Si energy gap the near-field PC images give information about subsurface defects not related to boron and with no correspondence in the surface topography.

Effect of Anisotropic Strain on the Crosshatch Electrical Activity in Relaxed GeSi Films

The physical origin of the crosshatch electrical activity in relaxed GeSi films was studied using a near-field scanning optical microscope (NSOM). The contrast and patterns in the near-field photocurrent images depend on the polarization direction of the NSOM light. These results rule out composition nonuniformity, junction depth variation, and scanning artifacts as dominant sources of the contrast. Numerical calculations show that local changes in band structure due to strain fields of the misfit dislocations are responsible for the experimental observations.

Near-field optics on silicon-electrolyte junctions.

A technique allowing near-field photocurrent (PC) mapping of silicon surfaces in contact with an electrolyte is presented. The illumination source is an optical fibre tip with a 100-nm aperture. A shear force detection system controls the tip-sample distance while scanning the tip across the silicon-electrolyte interface. Topographic and PC images on SiO2/Si mesas both show 300 nm resolution. It is shown that this PC contrast is induced by the tip-topography interaction and hence the PC resolution is limited by the resolution of the topography. Indeed, PC mapping on topography-less patterned porous-silicon/silicon samples shows that the lateral resolution is only limited by the aperture size which is of the order of 100 nm.

Photocurrent mapping with submicron resolution on the silicon-electrolyte junction by using near-field optics

We have developed a technique allowing photocurrent (PC) mapping of silicon surface in contact with an electrolyte which offers an unprecedented spatial resolution. The photocurrent is generated by near-field optics using an optical tip with a 100 nm diameter aperture as an illumination source. The comparison between topographic and photocurrent mapping of SiO2/Si mesas is used to demonstrate the feasibility of such a technique. Topographic and PC images show 300 nm lateral resolution. It is shown that this resolution is topography limited, i.e., determined by the tip-topography interaction. Indeed, PC mapping on topography-less patterned porous silicon/silicon samples shows that the lateral resolution can be as good as 100 nm, limited by the aperture size.