Minority Carrier Diffusion Research Articles

Van der Waals heterostructure of Bi2O2Se/MoTe2 for high-performance multifunctional devices

Two-dimensional van der Waals heterostructures exhibit distinctive electronic and optoelectronic properties, making them promising structures for constructing advanced multifunctional devices. However, devices based on conventional charge-carrier transport mechanisms often perform only a single function, which limits its integration and performance. Here, we present a vertical van der Waals heterostructure made of Bi2O2Se and MoTe2, allowing it to act as high-performance backward diode, forward diode, photodetector and photovoltaic device at various working conditions. The applications are enabled by band-alignment switching between p–n heterostructure controlled by minority carrier diffusion and n–n heterostructure governed by the thermionic emission and tunneling-mediated processes. As a backward diode, the device displays a high reverse rectification ratio of 5.0 × 104. As a photodetector, the device demonstrates a broad spectral photoresponse ranging from ultraviolet (365 nm) to near-infrared (1050 nm). When irradiated by 532 nm laser, the photodetector shows a responsivity of up to 11.6 A/W and achieves quick response/recovery speed of 19.6/8.8 μs. As a photovoltaics device, an external quantum efficiency of 78% and a responsivity of 0.33 A/W are observed. This study showcases the potential for high-performance multifunctional devices utilizing Bi2O2Se/MoTe2 heterostructures and provides comprehensive insights into the designed band alignment and its applications.

Analysis of plasma-elastic component of time-domain photoacoustic response

The plasma-elastic component of the photoacoustic response in the time-domain of thin semiconductor samples excited by long electromagnetic radiation pulses is analyzed in detail. The plasma-elastic component model assumes that ambipolar diffusion can be approximated by the minority carrier diffusion. The results obtained show that the plasma-elastic component in thin semiconductor samples affects photoacoustic measurements in the time domain, which is important for the photoacoustic determination of semiconductor electronic properties.

Trap-mediated bipolar charge transport in NiO/Ga2O3 p+-n heterojunction power diodes

The construction of p-NiO/n-Ga2O3 heterojunction becomes a popular alternative to overcome the technological bottleneck of p-type Ga2O3 for developing bipolar power devices for practical applications, whereas the identification of performance-limiting traps and the bipolar transport dynamics are still not exploited yet. To this end, the fundamental correlation of carrier transport, trapping and recombination kinetics in NiO/β-Ga2O3 p+-n heterojunction power diodes has been investigated. The quantitative modeling of the temperature-dependent current-voltage characteristics indicates that the modified Shockley-Read-Hall recombination mediated by majority carrier trap states with an activation energy of 0.64 eV dominates the trap-assisted tunneling process in the forward subthreshold conduction regime, while the minority carrier diffusion with near-unity ideality factors is overwhelming at the bias over the turn-on voltage. The leakage mechanism at high reverse biases is governed by the Poole-Frenkel emissions through the β-Ga2O3 bulk traps with a barrier height of 0.75 eV, which is supported by the identification of majority bulk traps with the energy level of EC − 0.75 eV through the isothermal capacitance transient spectroscopic analysis. These findings bridge the knowledge gap between bipolar charge transport and deep-level trap behaviors in Ga2O3, which is crucial to understand the reliability of Ga2O3 bipolar power rectifiers.

20% efficiency mg/PCBM/p-type silicon hybrid solar cells

Silicon/organic hybrid solar cells, with relatively simple and low-energy consumption fabrication processes, hold the promise for over-all cost reduction. However, their performances are still not comparable to the commercial crystalline silicon (c-Si) solar cells. Here, hybrid devices with organic phenyl-C61-butyric acid methyl ester (PCBM) layer inserted between metal electrode and textured p-type crystalline silicon (c-Si(p)) substrate are investigated. The quality of the PCBM/c-Si(p) contact is much improved after a SiOx thin layer is formed on the a-Si:H(i) surface and a Voc of 706 mV and an efficiency of 20.0% are achieved on the hybrid solar cells. Analysis of the dark J-V-T characteristics of the hybrid device shows that the junction current is dominated by the minority-carrier diffusion current.

Mapping Charge Recombination and the Effect of Point-Defect Insertion in GaAs Nanowire Heterojunctions

Electronic devices are extremely sensitive to defects in their constituent semiconductors, but locating electronic point defects in bulk semiconductors has previously been impossible. Here we apply scanning transmission electron microscopy (STEM) electron-beam-induced current (EBIC) imaging to map electronic defects in a $\mathrm{GaAs}$ nanowire Schottky diode. Imaging with a nondamaging 80 or 200 kV STEM acceleration potential reveals a minority-carrier diffusion length that decreases near the surface of the hexagonal nanowire, thereby demonstrating that the device's charge collection efficiency (CCE) is limited by surface defects. Imaging with a 300 keV STEM beam introduces vacancy-interstitial (or Frenkel) defects in the $\mathrm{GaAs}$ that increase carrier recombination and reduce the CCE of the diode. We create, locate, and characterize a single insertion event, determining that a defect inserted 7 nm from the Schottky interface broadly reduces the CCE by $10\mathrm{%}$ across the entire nanowire device. Variable-energy STEM EBIC imaging thus allows both benign mapping and pinpoint modification of a device's electron-hole-recombination landscape, enabling controlled experiments that illuminate the impact of both extended (one- and two-dimensional) and point (zero-dimensional) defects on semiconductor device performance.

Evaluation of Damage Coefficient for Minority-Carrier Diffusion Length of Triple-Cation Perovskite Solar Cells under 1 MeV Electron Irradiation for Space Applications

Organo-halide perovskite solar cells (PSCs) are lightweight and low cost, and they offer high power conversion efficiencies. PSCs have proven to be useful in terrestrial applications. In addition, they are particularly attractive for space applications because they can offer a higher radiation tolerance than GaAs and Si solar cells. This paper evaluates the damage coefficient for minority-carrier diffusion length KL of perovskite crystals after 1 MeV electron irradiation by time-resolved photoluminescence measurements to investigate the reason for their high radiation tolerance. Results show that perovskite crystals have a lower damage coefficient KL than that of InP crystals with a high radiation tolerance. On the other hand, first-principles calculations indicate that the displacement energy of perovskite crystals is as low as that of Si, which does not have a high radiation tolerance. The present results suggest that the annealing effect occurs for PSCs at room temperature.

3D Device Simulation and First Demonstration of BaSi2 Thin Film Solar Cells

Barium disilicide (BaSi2) shows great promise as a new material for thin film solar cells (Fig. 1) [1]. It has a suitable bandgap of 1.3 eV, a large optical absorption coefficient of 3×104 cm−1 for a photon energy of 1.5 eV, and a large minority-carrier diffusion length of about 10 μm. Furthermore, it is composed of only earth abundant elements and highly stable. Therefore, BaSi2 can be used for future terawatt-class power generation. We have achieved the operation of BaSi2 homojunction solar cells (Fig. 2) [2]. In this structure, an open-circuit voltage V OC beyond 0.8 V and a conversion efficiency (η) beyond 25% are expected [3]. However, the achieved η was very small. In this study, three-dimensional (3D) optical simulation using a 3D pyramid texture that is effective in preventing reflection was performed on a BaSi2 solar cells in order to increase the photocurrent. Futhermore, we suggested a new device structure using Al-doped ZnO (AZO) electron transfer layer (ETL).In this study, we performed a 3D optical simulation using the GenPro4 model developed at TU Delft [4]. In this simulation, the absorptance of each layer is calculated as a function of wavelength λ. We introduced the 3D-texture structure on the front and back surfaces of the Si substrate to investigate the anti-reflection and light trapping effects (Fig. 3(a)). The thickness and complex refractive index n + ik of every layer are given as input (Fig. 3 (b,c)). In addition, to increase light absorption in BaSi2 light absorber layers, we investigated n+-AZO/p-BaSi2 heterojunction solar cells in which the ETL was changed from n+-BaSi2 to n+-AZO.The absorption spectra of the BaSi2 homojunction solar cells are shown Fig. 5. By changing the front surface structure from flat to texture, the reflectance in the λ range 700 – 1200 nm reduced and the J ph in p-BaSi2 increased by 1.2 mA/cm2. On the other hand, the light trapping effect by the texture structure on the back surface could not be observed well. This is probably because almost all the transmitted light was absorbed by the Si substrate before reaching the back surface. Photons with λ between 800 and 1200 nm are absorbed in the Si substrate. In addition, it was found that the largest factor inhibiting light absorption in the p-BaSi2 layer was parasitic absorption in the ETL n+-BaSi2. This parasitic absorption is unavoidable when BaSi2 is used for ETL, because BaSi2 has a very large light absorption coefficient.Other thin film solar cells, e.g. CIGS, Perovskite, are a kind of heterojunction solar cells which consist of p-type absorbers and n-type window layers. The wide bandgap of the window layer allows more photons to reach the absorber layer. Another advantage of a heterojunction is that the recombination in the wide bandgap window layer is quite low in comparison to that in homojunctions. Now based on these basics, we proposed n+-AZO/p-BaSi2 heterojunction solar cell in which the ETL was changed from n+-BaSi2 to n+-AZO (Fig. 6). Because the E g of AZO is very large (~3.3 eV), much light can be transfer to BaSi2 light absorber layer. The absorption spectra of the n+-AZO/p-BaSi2 heterojunction solar cells were shown in Fig. 7. The parasitic absorption in the ETL was significantly reduced. The J ph in the p-BaSi2 layer increased by more than 10 mA/cm2 compared with that of the homojunction solar cells and reached a maximum of 30.23 mA/cm2 using the front textured surface.Based on these simulated results, we tried to form n+-AZO/p-BaSi2 heterojunction solar cells experimentally. p-BaSi2 and p+-BaSi2 films were grown epitaxially on a Cz-p+-Si(111) substrate by molecular beam epitaxy, and ZnO and AZO were deposited by sputtering. The J-V characteristics under AM1.5 illumination and the internal quantum efficiency (IQE) spectrum for an n+-AZO/p-BaSi2 heterojunction solar cell are shown in Fig. 8. It shows η = 0.04%, a short-circuit current density J SC of 3.7 mA/cm2, and an V OC of 50 mV. IQE exceeded 30% at λ = 600 nm. This efficiency is almost the same value as that obtained for BaSi2-pn homojunction solar cells. Therefore, we can state that we succeeded the demonstration of n+-AZO/p-BaSi2 heterojunction solar cells for the first time.[1] T. Suemasu and N. Usami, J. Phys. D: Appl. Phys. 50, 023001 (2017).[2] K. Kodama et al., Appl. Phys. Express 12, 041005 (2019).[3] T. Suemasu, Jpn. J. Appl. Phys. 54, 07JA01 (2015).[4] R. Santbergen et al., IEEE J. Photovoltaics, 7, no. 3, 919–926 (2017). Figure 1

Photoconductive mechanism of IR-sensitive iodized PbSe thin films via strong hole-phonon interaction and minority carrier diffusion.

Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole-phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers (Vk center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3d5/2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron-hole recombination in KI hole traps).

Performance Limits of III–V Barrier Detectors

Minority-carrier lifetimes and diffusion lengths have been deduced from a comparison of band structure simulations and experimental measurements on mid-wave infrared InAsSb XBn and long-wave infrared InAs/GaSb type II superlattice (T2SL) XBp barrier detectors with low diffusion-limited dark current close to mercury cadmium telluride Rule 07 and high quantum efficiency. For the XBn devices, a lifetime of 1.9 μs was observed with a corresponding diffusion length of 14.5 μm. In contrast, the T2SL exhibited a much shorter lifetime of 7.5 ns, but the diffusion length of ∼ 7 μm was long enough to ensure that almost 90% of the photocarriers are collected. The lifetime appears to be Auger limited in the case of n-type InAsSb, but for the p-type T2SL, Shockley–Read–Hall (SRH) traps appear to dominate. In the second case, possible scenarios for the dominance of SRH recombination are discussed to identify pathways for further performance optimization.

Photogenerated Carrier Transport Properties in Silicon Photovoltaics

Electrical transport parameters for active layers in silicon (Si) wafer solar cells are determined from free carrier optical absorption using non-contacting optical Hall effect measurements. Majority carrier transport parameters [carrier concentration (N), mobility (μ), and conductivity effective mass (m*)] are determined for both the n-type emitter and p-type bulk wafer Si of an industrially produced aluminum back surface field (Al-BSF) photovoltaic device. From measurements under 0 and ±1.48 T external magnetic fields and nominally “dark” conditions, the following respective [n, p]-type Si parameters are obtained: N = [(3.6 ± 0.1) × 1018 cm−3, (7.6 ± 0.1) × 1015 cm−3]; μ = [166 ± 6 cm2/Vs, 532 ± 12 cm2/Vs]; and m* = [(0.28 ± 0.03) × me, (0.36 ± 0.02) × me]. All values are within expectations for this device design. Contributions from photogenerated carriers in both regions of the p-n junction are obtained from measurements of the solar cell under “light” 1 sun illumination (AM1.5 solar irradiance spectrum). From analysis of combined dark and light optical Hall effect measurements, photogenerated minority carrier transport parameters [minority carrier concentration (Δp or Δn) and minority carrier mobility (μh or μe)] under 1 sun illumination for both n- and p-type Si components of the solar cell are determined. Photogenerated minority carrier concentrations are [(7.8 ± 0.2) × 1016 cm−3, (2.2 ± 0.2) × 1014 cm−3], and minority carrier mobilities are [331 ± 191 cm2/Vs, 766 ± 331 cm2/Vs], for the [n, p]-type Si, respectively, values that are within expectations from literature. Using the dark majority carrier concentration and the effective equilibrium minority carrier concentration under 1 sun illumination, minority carrier effective lifetime and diffusion length are calculated in the n-type emitter and p-type wafer Si with the results also being consistent with literature. Solar cell device performance parameters including photovoltaic device efficiency, open circuit voltage, fill factor, and short circuit current density are also calculated from these transport parameters obtained via optical Hall effect using the diode equation and PC1D solar cell simulations. The calculated device performance parameters are found to be consistent with direct current-voltage measurement demonstrating the validity of this technique for electrical transport property measurements of the semiconducting layers in complete Si solar cells. To the best of our knowledge, this is the first method that enables determination of both minority and majority carrier transport parameters in both active layers of the p-n junction in a complete solar cell.

Degradation prediction of a γ-ray radiation dosimeter using InGaP solar cells in a primary containment vessel of the Fukushima Daiichi Nuclear Power Station

ABSTRACTIndium gallium phosphide (InGaP) solar cell with a superior high-radiation resistance is expected to be a powerful candidate for a dosimeter under a high-radiation dose rate environment. In this study, in order to predict the lifetime as the dosimeter using the InGaP solar cell, we clarify the effect of minority-carrier diffusion length (L) on a radiation-induced current as a dose signal in the InGaP solar cell by irradiation tests and empirical calculations. In the irradiation tests, the short-circuit current density (Jsc) as a function of the γ-ray dose rate is measured to estimate the L for the InGaP solar cell by irradiation tests. The operational lifetime as a detector using the InGaP solar cell under various dose rates is estimated by using the empirical calculations based on the relation between the L and absorbed dose. The results suggest that the dosimeter using InGaP solar cell is able to be used during more than 10 h in the primary containment vessel of the Fukushima Daiichi Nuclear Power Plant and it has a high potential of being a radiation-resistant dosimeter that would contribute to the decommissioning.

20%-efficient epitaxial GaAsP/Si tandem solar cells

We present epitaxial 1.7 eV/1.1 eV GaAs0.75P0.25/Si tandem cells with an NREL-certified efficiency of 20.0%, enabled by a thermally stable tunnel junction interconnect along with a hydrogenated amorphous Si (a-Si:H) carrier-selective contact for the Si bottom cell. Building on these promising tandem results, we also demonstrate a 16.5%-efficient GaAs0.75P0.25 single-junction top cell on Si and a 7.78%-efficient GaAs0.75P0.25-filtered Si bottom cell (both NREL-certified) with improved short-circuit current densities. The quantum efficiency of the GaAs0.75P0.25 single-junction top cell on Si is boosted across the whole wavelength range due to the use of a higher growth temperature, indicating an improved minority-carrier diffusion length. The implementation of random pyramid texturing at the Si back surface enables improved quantum efficiency at wavelengths of 900–1200 nm, corresponding to an increase of 1.42 mA/cm2 in short-circuit current density. The improved short-circuit current densities of the sub-cells together show a pathway to >23% efficiency in a two-terminal tandem configuration.

Exact analytical expressions for Auger saturation current in diffused Si emitters

If the densities of free minority and majority carriers have a reverse dependence on the Auger recombination enhancement factor, g, and minority-carrier diffusivity has a direct dependence on g, then the minority-carrier transport equations in non-uniformly doped Si regions in the dark can be solved analytically for the case, where lifetime is only determined by Auger recombination, and two new analytical expressions for saturation current density of emitter, J0em, can be derived, one in p-type Si and the other in n-type Si, by imposing boundary conditions including band-gap narrowing (BGN). The new expressions include the integral of emitter doping profile. We calculate curves of majority-carrier mobility versus doping by taking into account BGN and find that majority- and minority-carrier diffusivity of same carrier species can be assigned the same direct dependence on g. The integral of doping profile can be consequently expressed as a function of sheet resistance and the new expressions for J0em can be calculated without necessarily knowing the emitter doping profile. The assumption on diffusivities holds at room temperature for dopings higher than 6 × 1017 cm−3 in both p- and n-type Si. The new J0em-expressions are used to characterize actual reported n+ and p+ metal-coated and n+ passivated emitters with respect to surface doping, diffusion depth, sheet resistance, and surface recombination velocity.

Efficiency Enhancement of CH3NH3SnI3 Solar Cells by Device Modeling

Lead halide perovskite solar cells (LHPSC) have great potential, with conversion efficiency exceeding 20%. However, their toxic nature and difficult fabrication prevent their consideration for commercial applications. To address this, numerical analysis was performed to propose a structure for lead-free perovskite solar cells with MASnI3 as absorber layer. Device modeling for Cd1−xZnxS as electron transport layer (ETL) and methylamine tin halide as hole transport layer (HTL) was carried out using a solar cell capacitance simulator. The simulation results revealed the dependence of the open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and power conversion efficiency on the HTL valence-band offset, absorber layer thickness, absorber layer doping concentration, ETL band offset, minority-carrier diffusion length, and defects at the HTL–absorber and absorber–ETL interfaces. An optimum absorber layer thickness was confirmed, being well consistent with the range for practical absorber layer designs. Moreover, conversion efficiency of 18.71% was found for absorber thickness of 500 nm and doping concentration of 1 × 1016 cm−3. These results will provide important guidelines for design of low-cost perovskite solar cells.

Photoconductivity of acid exfoliated and flash-light-processed MoS2 films

MoS2 has been studied intensively during recent years as a semiconducting material in several fields, including optoelectronics, for applications such as solar cells and phototransistors. The photoresponse mechanisms of MoS2 have been discussed but are not fully understood, especially the phenomenon in which the photocurrent slowly increases. Here, we report on a study of the photoresponse flash-light-processed MoS2 films of different thicknesses and areas. The photoresponse of such films under different light intensities and bias voltages was measured, showing significant current changes with a quick response followed by a slow one upon exposure to pulsed light. Our in-depth study suggested that the slow response was due to the photothermal effect that heats the MoS2; this hypothesis was supported by the resistivity change at different temperatures. The results obtained from MoS2 films with various thicknesses indicated that the minority-carrier diffusion length was 1.36 µm. This study explained the mechanism of the slow response of the MoS2 film and determined the effective thickness of MoS2 for a photoresponse to occur. The method used here for fabricating MoS2 films could be used for fabricating optoelectronic devices due to its simplicity.

Separating grain-boundary and bulk recombination with time-resolved photoluminescence microscopy

Two-photon excitation (2PE) microscopy allows contactless and non-destructive cross-sectional analysis of grain-boundary (GB) and grain-interior (GI) properties in polycrystalline solar cells, with measurements of doping uniformity, space-charge field distribution, and carrier dynamics in different regions of the device. Using 2PE time-resolved microscopy, we analyzed charge-carrier lifetimes near the GBs and in the GI of polycrystalline thin-film CdTe solar cells doped with As. When the grain radius is larger than the minority-carrier diffusion length, GI lifetimes are interpreted as the bulk lifetimes τB, and GB recombination velocity SGB is extracted by comparing recombination rates in the GI and near GBs. In As-doped CdTe solar cells, we find τB = 1.0–2.4 ns and SGB = (1–4) × 105 cm/s. The results imply the potential to improve solar cell voltage via GB passivation and reduced recombination center concentration in the GI.

Diffusion of minority carriers against electric field (high injection level)

A one-dimensional analytic model describing the motion of minority carriers against the electric field direction under the conditions of high injection level is developed. The results of the model can also be used to estimate the motion of carriers against the field in the case of an arbitrary injection level. The model makes it possible to describe, in good agreement with the results of computer simulation, the modulation of the collector layer resistance in a high voltage SiC bipolar transistor.

Extraction of Front- and Rear-Interface Recombination in Silicon Double-Heterojunction Solar Cells by Reverse Bias Transients

We present a method, based upon reverse-recovery (RR) transient measurements, for determining the interface recombination parameters of double-sided heterojunction solar cells. A physics-based model is developed, and normalized parameters are used to provide results that can be scaled to arbitrary wafer thickness and minority-carrier diffusion coefficient. In the case of dominant recombination at only one interface, interface recombination velocity can be extracted directly from RR times. In devices with significant recombination at both interfaces, numerical modeling must be used. The effects of minority-carrier current spreading in small devices can be corrected for analytically. The results are then applied to both PEDOT/ n-Si and PEDOT/n-Si/TiO2 heterojunction cells. We find that the PEDOT/n-Si interface, despite favorable band offsets and a significant built-in voltage, is not an ideal hole injector because of recombination at the PEDOT/n-Si interface. We also find that the effective surface recombination velocity at the Si-TiO2 interface in a metallized device is 330 cm/s, confirming that the interface has a low defect density. Finally, we reflect on the significance of these results for the further development of silicon heterojunction cells.

Theoretical study on the influence of photon reabsorption on spatially resolved photocarrier radiometric imaging of silicon wafers

The influence of reabsorption of spontaneously emitted photons within silicon wafers on spatially resolved steady-state photocarrier radiometric (PCR) imaging is theoretically analyzed. A new PCR model with photon reabsorption, in which both band-to-band absorption and free carrier absorption are taken into account, is introduced. It is shown that the influences strongly depend on both the excess carrier density and its distribution, which are sensitive to the doping level, electronic transport properties, sample thickness, and surface topography. If photon reabsorption is ignored in the determination of the electronic transport parameters for high-doping double polished silicon wafers via multi-parameter fitting, the relative errors of the fitted minority carrier lifetime, diffusion coefficient, and front surface recombination velocity could reach 38%, 9.5%, and 24%, respectively.

Near-field transport imaging applied to photovoltaic materials

We developed and applied a new analytical technique—near-field transport imaging (NF-TI or simply TI)—to photovoltaic materials. Charge-carrier transport is an important factor in solar cell performance, and TI is an innovative approach that integrates a scanning electron microscope with a near-field scanning optical microscope, providing the possibility to study luminescence associated with recombination and transport with high spatial resolution. In this paper, we describe in detail the technical barriers we had to overcome to develop the technique for routine application and the data-fitting procedure used to calculate minority-carrier diffusion length values. The diffusion length measured by TI agrees well with the results calculated by time-resolved photoluminescence on well-controlled gallium arsenide (GaAs) thin-film samples. We report for the first time on measurements on thin-film cadmium telluride using this technique, including the determination of effective carrier diffusion length, as well as the first near-field imaging of the effect of a single localized defect on carrier transport and recombination in a GaAs heterostructure. Furthermore, by changing the scanning setup, we were able to demonstrate near-field cathodoluminescence (CL), and correlated the results with standard CL measurements. The TI technique shows great potential for mapping transport properties in solar cell materials with high spatial resolution.