Photocurrent Microscopy Research Articles

Diffusion length in nanoporous TiO2 films under above-band-gap illumination

We determined the carrier diffusion lengths in TiO2 nanoporous layers of dye-sensitized solar cells by using scanning photocurrent microscopy using an ultraviolet laser. Here, we excited the carrier directly in the nanoporous layers where the diffusion lengths were found to 140 μm as compared to that of visible illumination measured at 90 μm. The diffusion length decreased with increasing laser modulation frequency, in which we determined the electron lifetimes and the diffusion coefficients for both visible and UV illuminations. The diffusion lengths have been studied in terms of the sintering temperatures for both cells with and without binding molecules. We found a strong correlation between the diffusion length and the overall light-to-current conversion efficiency, proving that improving the diffusion length and hence the interparticle connections, is key to improving cell efficiency.

Scaling in the quantum Hall regime of graphene Corbino devices

The scaling behavior of graphene devices in Corbino geometry was investigated through temperature dependent conductivity measurements under magnetic field. Evaluation of the Landau level width as a function of temperature yielded a relatively low temperature exponent of κ = 0.16 ± 0.05. Furthermore, an unusually large value close to 7.6 ± 0.9 was found for the universal scaling constant γ, while the determined inelastic scattering exponent of p = 2 is consistent with established scattering mechanisms in graphene. The deviation of the scaling parameters from values characteristic of conventional two-dimensional electron gases is attributed to an inhomogeneous charge carrier distribution in the Corbino devices. Direct evidence for the presence of the latter could be gained by spatially resolved photocurrent microscopy away from the charge neutrality point of the devices.

Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors

We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field effect transistors exhibiting high-quality ambipolar transport. By properly biasing the gate electrode, we can invert the sign of the photocurrent showing that the minority photocarriers are either electrons or holes. Both in the electron- and hole-doping regimes the photocurrent decays exponentially as a function of the distance between the illumination spot and the nearest contact, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on a same sample. Interestingly, the diffusion length of the minority carriers is several times larger in the hole accumulation regime than in the electron accumulation regime, pointing out an electron-hole asymmetry in WS2.

High intensity induced photocurrent polarity switching in lead sulfide nanowire field effect transistors

We report an optoelectronic investigation of lead sulfide nanowires (NWs) by scanning photocurrent microscopy. The photocurrent in p-type lead sulfide NW field effect transistors has demonstrated unusually nonlinear dependence on the intensity of local excitation. Surprisingly, the photocurrent polarity can be reversed under high illumination intensity on the order of 100 W cm−2. The origin of this photocurrent polarity switching is that the photo-injected carriers flip the direction of the electric field near the contact. These observations shed light on the nonlinear optoelectronic characteristics in semiconductor nanostructures and may provide an innovative method for optically tailoring local band structures.

Correlation of Morphology with Photocurrent Generation in a Polymer Blend Photovoltaic Device

Morphological effects on photovoltaic (PV) properties are studied through scanning photocurrent (PC) and photoluminescence (PL) microscopy of a solution processed, polymer blend PV device composed of PFB [poly(9,9'-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine] and F8BT [poly(9,9'-dioctylfluorene-co-benzothiadiazole]. As PFB and F8BT have unique absorbance bands, it is possible to selectively excite only F8BT (488 nm) or both PFB and F8BT (408 nm). Local voltage-dependent photocurrent (LVPC) measurements from particular regions of interest in the PV show that the diode characteristics between different morphologies are essentially the same, except in regard to the magnitude of PC generated. A local PL spectrum is measured simultaneously with PC generation at each pixel in the image maps. Through integration of the local PL spectrum over particular wavelength ranges, PL image maps are created of PFB-PL (435 to 475 nm), F8BT-PL (530 to 570 nm), exciplex-PL (620 to 685 nm) and total-PL (entire spectrum). These data allow direct correlation of PC generation with local chemical composition variations within the PV device. PL image maps show morphological variations on the order of 0.5 to 1 µm of alternating PFB-rich and F8BT-rich phases. While illuminating only F8BT (488 nm light), the PFB-rich phases produce the most PC, however, while illuminating both polymers but mostly PFB (408 nm light), the F8BT-rich phases produce the most PC. These results show that in the morphology where the light absorbing material is less concentrated, the PC generation is increased. Additionally, the exciplex-PL is found to not be a significant radiative loss mechanism of charge carriers for PC generation.

Photothermoelectric Effect in Suspended Semiconducting Carbon Nanotubes

We have performed scanning photocurrent microscopy measurements of field-effect transistors (FETs) made from individual ultraclean suspended carbon nanotubes (CNTs). We investigate the spatial-dependence, polarization-dependence, and gate-dependence of photocurrent and photovoltage in this system. While previous studies of surface-bound CNT FET devices have identified the photovoltaic effect as the primary mechanism of photocurrent generation, our measurements show that photothermoelectric phenomena play a critical role in the optoelectronic properties of suspended CNT FETs. We have quantified the photothermoelectric mechanisms and identified regimes where they overwhelm the photovoltaic mechanism.

Barrier Height Measurement of Metal Contacts to Si Nanowires Using Internal Photoemission of Hot Carriers

Barrier heights between metal contacts and silicon nanowires were measured using spectrally resolved scanning photocurrent microscopy (SPCM). Illumination of the metal-semiconductor junction with sub-bandgap photons generates a photocurrent dominated by internal photoemission of hot electrons. Analysis of the dependence of photocurrent yield on photon energy enables quantitative extraction of the barrier height. Enhanced doping near the nanowire surface, mapped quantitatively with atom probe tomography, results in a lowering of the effective barrier height. Occupied interface states produce an additional lowering that depends strongly on diameter. The doping and diameter dependencies are explained quantitatively with finite element modeling. The combined tomography, electrical characterization, and numerical modeling approach represents a significant advance in the quantitative analysis of transport mechanisms at nanoscale interfaces that can be extended to other nanoscale devices and heterostructures.

Transient Absorption and Photocurrent Microscopy Show That Hot Electron Supercollisions Describe the Rate-Limiting Relaxation Step in Graphene

Using transient absorption (TA) microscopy as a hot electron thermometer, we show that disorder-assisted acoustic-phonon supercollisions (SCs) best describe the rate-limiting relaxation step in graphene over a wide range of lattice temperatures (Tl = 5-300 K), Fermi energies (E(F) = ± 0.35 eV), and optical probe energies (~0.3-1.1 eV). Comparison with simultaneously collected transient photocurrent, an independent hot electron thermometer, confirms that the rate-limiting optical and electrical response in graphene are best described by the SC-heat dissipation rate model, H = A(T(e)(3) - T(l)(3)). Our data further show that the electron cooling rate in substrate-supported graphene is twice as fast as in suspended graphene sheets, consistent with SC model prediction for disorder.

Spatial Mapping of Efficiency of GaN/InGaN Nanowire Array Solar Cells Using Scanning Photocurrent Microscopy

GaN-InGaN core-shell nanowire array devices are characterized by spectrally resolved scanning photocurrent microscopy (SPCM). The spatially resolved external quantum efficiency is correlated with structure and composition inferred from atomic force microscope (AFM) topography, scanning transmission electron microscope (STEM) imaging, Raman microspectroscopy, and scanning photocurrent microscopy (SPCM) maps of the effective absorption edge. The experimental analyses are coupled with finite difference time domain simulations to provide mechanistic understanding of spatial variations in carrier generation and collection, which is essential to the development of heterogeneous novel architecture solar cell devices.

SCANNING PHOTOCURRENT MICROSCOPY IN SEMICONDUCTOR NANOSTRUCTURES

Scanning photocurrent microscopy (SPCM) is a powerful experimental tool used to investigate spatially resolved optoelectronic properties of semiconductors and their nanostructures. Raster-scanned laser excitation generates a position-dependent photocurrent map from which carrier diffusion length, electric field distribution, doping concentration and more can be explored. In this review, we will briefly discuss the history of the technique, the theory behind locally injected carrier transport in semiconductors, the SPCM experimental setup, and recent applications of SPCM in semiconductor nanostructures. Particularly, we have shown that the minority carrier diffusion length can also be obtained by SPCM in two-dimensional semiconductors and that the local excitation can result in an internal electric field because of the difference in electron and hole mobilities.

High-speed scanning photocurrent imaging techniques on nanoscale devices

We report the characterization of individual carbon nanotube and Si nanowire field-effect transistors through high-speed scanning photocurrent microscopy with a scanning speed of 1 frame/s and a photocurrent sensitivity of less than 1 pA. This enables us to record photocurrent images that are free from hysteresis effects that modify the field configurations applied by the gate bias voltage. We can clearly resolve the photocurrent signals with polarity inversion near the metallic contacts under gate bias conditions which cause severe hysteresis effects in the nanowire devices. We also studied the dynamics of the hysteresis effects for different gate bias configurations. This high-speed photocurrent imaging technique is particularly useful for obtaining two-dimensional, localized optoelectronic characteristics and their correlation with overall device performance without encountering undesired dynamic responses.

Photocurrent Mapping of 3D CdSe/CdTe Windowless Solar Cells

This paper details the use of scanning photocurrent microscopy to examine localized current collection efficiency of thin-film photovoltaic devices with in-plane patterning at a submicrometer length scale. The devices are based upon two interdigitated comb electrodes at the micrometer length scale prepatterned on a substrate, with CdSe electrodeposited on one electrode and CdTe deposited over the entire surface of the resulting structure by pulsed laser deposition. Photocurrent maps provide information on what limits the performance of the windowless CdSe/CdTe thin-film photovoltaic devices, revealing "dead zones" particularly above the electrodes contacting the CdTe which is interpreted as recombination over the back contact. Additionally, the impact of ammonium sulfide passivation is examined, which enables device efficiency to reach 4.3% under simulated air mass 1.5 illumination.

Spatially Resolved Optoelectronics in Lead Sulfide Nanowires

Spatially resolved optoelectronic investigation can reveal important information on charge transport and recombination in field-effect transistors (FETs) incorporating single nanowires (NWs). Scanning photocurrent microscopy (SPCM) was performed on single PbS NW FETs to extract diffusion length and lifetime of minority carriers as a function of gate voltage and excitation intensity. Interestingly, we observed a reverse of the photocurrent direction in gate-depleted PbS NW FETs under high intensity illumination. We attribute this effect to the back-gate induced carrier concentration gradient.

Development of a multi-wavelength photocurrent mapping system

A novel multi-wavelength near-field scanning photocurrent microscopy (MWNSPM) system has been developed, which directly correlates photocurrent generation at different incident wavelengths with morphology of organic photovoltaic (OPV) cells. The system is based on a near-field scanning optical microscope which generates the topographical map and acts as the nanoscale light source for the OPV cell. The light sources are frequency modulated using optical beam choppers, and the resulting photocurrent signal is demodulated in software via a Fourier transform to recover the signal due to each wavelength. The new MWNSPM system was used to image directly the photocurrent generated from an OPV device based on a 1:1 blend of poly(9,9′-dioctylfluorene-co-bis-N,N′-(4,butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) and poly(9,9′-dioctylfluorene-co-benzo-thiadiazole). A spatial resolution of 170 ± 45 nm was achieved, which is in excellent agreement with the limit of the 200 nm probe aperture and considerably better than the resolution (700 nm) of comparable confocal photocurrent mapping.

Elucidating the Photoresponse of Ultrathin MoS2 Field-Effect Transistors by Scanning Photocurrent Microscopy

The mechanisms underlying the intrinsic photoresponse of few-layer (FL) molybdenum disulphide (MoS2) field-effect transistors are investigated via scanning photocurrent microscopy. We attribute the locally enhanced photocurrent to band-bending assisted separation of photoexcited carriers at the MoS2/Au interface. The wavelength-dependent photocurrents of few layer MoS2 transistors qualitatively follow the optical absorption spectra of MoS2, providing direct evidence of interband photoexcitation. Time and spectrally resolved photocurrent measurements at varying external electric fields and carrier concentrations establish that drift-diffusion currents dominate photothermoelectric currents in devices under bias.

Gate-Dependent Carrier Diffusion Length in Lead Selenide Quantum Dot Field-Effect Transistors

We report a scanning photocurrent microscopy (SPCM) study of colloidal lead selenide (PbSe) quantum dot (QD) thin film field-effect transistors (FETs). PbSe QDs are chemically treated with sodium sulfide (Na2S) and coated with amorphous alumina (a-Al2O3) by atomic layer deposition (ALD) to obtain high mobility, air-stable FETs with a strongly gate-dependent conductivity. SPCM reveals a long photocurrent decay length of 1.7 μm at moderately positive gate bias that decreases to below 0.5 μm at large positive gate voltage and all negative gate voltages. After excluding other possible mechanisms including thermoelectric effects, a thick depletion width, and fringing electric fields, we conclude from photocurrent lifetime measurements that the diffusion of a small fraction of long-lived carriers accounts for the long photocurrent decay length. The long minority carrier lifetime is attributed to charge traps for majority carriers.

Focused-Laser-Enabled p–n Junctions in Graphene Field-Effect Transistors

With its electrical carrier type as well as carrier densities highly sensitive to light, graphene is potentially an ideal candidate for many optoelectronic applications. Beyond the direct light-graphene interactions, indirect effects arising from induced charge traps underneath the photoactive graphene arising from light-substrate interactions must be better understood and harnessed. Here, we study the local doping effect in graphene using focused-laser irradiation, which governs the trapping and ejecting behavior of the charge trap sites in the gate oxide. The local doping effect in graphene is manifested by large Dirac voltage shifts and/or double Dirac peaks from the electrical measurements and a strong photocurrent response due to the formation of a p-n-p junction in gate-dependent scanning photocurrent microscopy. The technique of focused-laser irradiation on a graphene device suggests a new method to control the charge-carrier type and carrier concentration in graphene in a nonintrusive manner as well as elucidate strong light-substrate interactions in the ultimate performance of graphene devices.

High-resolution photocurrent microscopy using near-field cathodoluminescence of quantum dots

We report a fast, versatile photocurrent imaging technique to visualize the local photo response of solar energy devices and optoelectronics using near-field cathodoluminescence (CL) from a homogeneous quantum dot layer. This approach is quantitatively compared with direct measurements of high-resolution Electron Beam Induced Current (EBIC) using a thin film solar cell (n-CdS / p-CdTe). Qualitatively, the observed image contrast is similar, showing strong enhancement of the carrier collection efficiency at the p-n junction and near the grain boundaries. The spatial resolution of the new technique, termed Q-EBIC (EBIC using quantum dots), is determined by the absorption depth of photons. The results demonstrate a new method for high-resolution, sub-wavelength photocurrent imaging measurement relevant for a wide range of applications.

Increased Responsivity of Suspended Graphene Photodetectors

The responsivity of graphene photodetectors depends critically on the elevated temperature of the electronic subsystem upon photoexcitation. We investigate the role of the substrate in providing cooling pathways for photoexcited carriers under ambient conditions by partially suspending few-layer graphene over a trench. Through photocurrent microscopy, we observe p-n junctions near the supported/suspended interfaces that produce photothermoelectric currents. Most importantly, we find the photocurrent in suspended p-n junctions to be an order of magnitude larger than in supported structures. This enhancement is attributed to the elimination of a dominant electronic cooling channel via the surface phonons of the polar substrate. Our work documents this mechanism of energy exchange between graphene and its environment, and it points to the importance of dielectric engineering for future improved graphene photodetectors.

Broadband, Polarization-Sensitive Photodetector Based on Optically-Thick Films of Macroscopically Long, Dense and Aligned Carbon Nanotubes

Increasing performance demands on photodetectors and solar cells require the development of entirely new materials and technological approaches. We report on the fabrication and optoelectronic characterization of a photodetector based on optically-thick films of dense, aligned, and macroscopically long single-wall carbon nanotubes. The photodetector exhibits broadband response from the visible to the mid-infrared under global illumination, with a response time less than 32 μs. Scanning photocurrent microscopy indicates that the signal originates at the contact edges, with an amplitude and width that can be tailored by choosing different contact metals. A theoretical model demonstrates the photothermoelectric origin of the photoresponse due to gradients in the nanotube Seebeck coefficient near the contacts. The experimental and theoretical results open a new path for the realization of optoelectronic devices based on three-dimensionally organized nanotubes.