Showing papers on "p–n junction published in 2021"

Construction of hydroxide pn junction for water splitting electrocatalysis

Abstract: NiFe oxyhydroxide (NiFe−OH) has shown promising electrocatalytic oxygen evolution reaction (OER) activity. Here we suggest that the performance of NiFe−OH can be further enhanced by constructing pn junction. Using MnCo carbonate hydroxide (MnCo−CH)@NiFe−OH pn junction as a demonstration, we show that upon the construction of pn junction, the electrons flow from n-type NiFe−OH to MnCo−CH, which consequently generates a positively charged region on NiFe−OH. The density function theory calculation reveals that such an electronic property change results in an improved OER energetics. As a result, the MnCo−CH@NiFe−OH pn junction shows significantly enhanced OER performance that is ∼10 and ∼500 times that of NiFe−OH and MnCo−CH (in terms of the OER currents at the overpotential of 270 mV), respectively. Moreover, the pn junction also shows a greatly boosted hydrogen evolution reaction (HER) and therefore the overall water electrolysis activity that outperforms the Pt/C||RuO2 catalysts.

Theory of the nonreciprocal Josephson effect

Abstract: The heterojunction between different materials often exhibits a rectifying effect; e.g., pn junction is used for diode. On the other hand, the Josephson junction between two different superconductors is assumed to show symmetric response between two directions of the current, i.e., the voltage drop $V$ is antisymmetric with respect to the sign change of the current $I$. However, there should be an asymmetry between the states of charge accumulation on the right and left sides of the Josephson junction, which can lead to the nonreciprocal responses. Here we demonstrate theoretically that nonreciprocal $I\text{\ensuremath{-}}V$ characteristic appears due to this charging energy difference both in the classical and quantum regimes.

1D p–n Junction Electronic and Optoelectronic Devices from Transition Metal Dichalcogenide Lateral Heterostructures Grown by One-Pot Chemical Vapor Deposition Synthesis

Abstract: 2101086 (1 of 9) © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH G. Q. Ngo, T. Bucher, Prof. I. Staude, Dr. T. Vogl, Dr. F. Eilenberger Institute of Applied Physics Friedrich Schiller University Albert-Einstein-Str. 15, 07745 Jena, Germany Prof. I. Staude Institute of Solid State Physics Friedrich Schiller University Max-Wien-Platz 1, 07743 Jena, Germany Dr. U. Hübner Leibniz Institute of Photonic Technology (IPHT) Albert-Einstein-Str. 9, 07745 Jena, Germany Dr. A. George, T. Bucher, Prof. I. Staude, Dr. F. Eilenberger, Prof. A. Turchanin Abbe Center of Photonics Albert-Einstein-Straße 6, 07745 Jena, Germany Dr. T. Vogl Cavendish Laboratory University of Cambridge JJ Thomson Avenue, Cambridge CB3 0HE, UK Dr. F. Eilenberger Fraunhofer-Institute for Applied Optics and Precision Engineering IOF Albert-Einstein-Str. 7, 07745 Jena, Germany ReseaRch aRticle

p-n Junction Based Direct-Current Triboelectric Nanogenerator by Conjunction of Tribovoltaic Effect and Photovoltaic Effect.

Abstract: Triboelectric nanogenerators (TENGs) have attracted much interest in recent years, due to its effectiveness and low cost for converting high-entropy mechanical energy into electric power. The traditional TENGs generate an alternating current, which requires a rectifier to provide a direct-current (DC) power supply. Herein, a dynamic p-n junction based direct-current triboelectric nanogenerator (DTENG) is demonstrated. When a p-Si wafer is sliding on a n-GaN wafer, carriers are generated at the interface and a DC current is produced along the direction of the built-in electric field, which is called the tribovoltatic effect. Simultaneously, an UV light is illuminated on the p-n junction to enhance the output. The results indicate that the current increases 13 times and the voltage increases 4 times under UV light (365 nm, 28 mW/cm2) irradiation. This work demonstrates the coupling between the tribovoltaic effect and the photovoltaic effect in DTENG semiconductors, promoting further development for energy harvesting in mechanical energy and photon energy.

A Bi-Anti-Ambipolar Field Effect Transistor

Abstract: Multistate logic is recognized as a promising approach to increase the device density of microelectronics, but current approaches are offset by limited performance and large circuit complexity. We here demonstrate a route toward increased integration density that is enabled by a mechanically tunable device concept. Bi-anti-ambipolar transistors (bi-AATs) exhibit two distinct peaks in their transconductance and can be realized by a single 2D-material heterojunction-based solid-state device. Dynamic deformation of the device reveals the co-occurrence of two conduction pathways to be the origin of this previously unobserved behavior. Initially, carrier conduction proceeds through the junction edge, but illumination and application of strain can increase the recombination rate in the junction sufficiently to support an alternative carrier conduction path through the junction area. Optical characterization reveals a tunable emission pattern and increased optoelectronic responsivity that corroborates our model. Strain control permits the optimization of the conduction efficiency through both pathways and can be employed in quaternary inverters for future multilogic applications.

Single nanoflake-based PtSe2 p–n junction (in-plane) formed by optical excitation of point defects in BN for ultrafast switching photodiodes

Abstract: Here, novel lateral PtSe2 p–n junctions are fabricated based on the PtSe2/BN/graphene (Gr) van der Waals heterostructures upon the illumination of visible light via the optical excitation of the mid-gap point defects in hexagonal boron nitride (h-BN). A stable photo doping effect was achieved for tuning the polarity of PtSe2-based field-effect transistors (FETs). The constructed diodes display excellent rectifying performance, with a rectification ratio of up to ∼1.0 × 105 and an ideality factor of ∼1.3. Distinctive self-biased photovoltaic behavior was detected, specifically in the positive open-circuit voltage (Voc = 0.32 V) at zero source–drain current (Ids), and also the negative short-circuit current (Isc = 16.2 nA) at zero source–drain voltage (Vds) generated for the p–n diode state upon the illumination of incident light (600 nm, 40 mW cm−2). Moreover, output Voc switching behavior was achieved for the p–n diode state by switching the input light signal on and off, with a photoresponse over the broadband spectral range of 200–1200 nm. Various photovoltaic parameters were also measured. Also, using this elegant approach, homoinverters were fabricated that reached a maximum gain of ∼30 (VDD = 2 V). These findings pave the way to developing self-biased photovoltaic devices by exploiting 2D noble metal dichalcogenide materials.

Boosting the efficiency of GeSe solar cells by low-temperature treatment of p-n junction

Abstract: Germanium monoselenide (GeSe) is an emerging promising photovoltaic absorber material due to its attractive optoelectronic properties as well as non-toxic and earth-abundant constitutes However, all previously reported GeSe solar cells rely on a superstrate configuration coupled with a CdS buffer layer, and suffer from unsatisfactory performance Here we demonstrate that this low efficiency arises from the inevitable high-temperature treatment of p-n junction in superstrate configuration This results in the diffusion of Cd atoms from CdS layer into GeSe film that introduces detrimental deep trap states inside the bandgap of GeSe (∼034 eV below conduction band minimum) We adopt therefore a substrate configuration that enables the deposition of CdS atop pre-deposited polycrystalline GeSe film at room temperature, avoiding the Cd diffusion By optimizing the annealing temperature of complete devices via a high-throughput screening method, the resulting substrate solar cells annealed at 150°C achieve an efficiency of 31%, two times that of the best previously reported superstrate GeSe results

High-Performance p-n Junction Transition Metal Dichalcogenide Photovoltaic Cells Enabled by MoOx Doping and Passivation.

Abstract: Layered semiconducting transition metal dichalcogenides (TMDs) are promising materials for high-specific-power photovoltaics due to their excellent optoelectronic properties. However, in practice, contacts to TMDs have poor charge carrier selectivity, while imperfect surfaces cause recombination, leading to a low open-circuit voltage (VOC) and therefore limited power conversion efficiency (PCE) in TMD photovoltaics. Here, we simultaneously address these fundamental issues with a simple MoOx (x ≈ 3) surface charge-transfer doping and passivation method, applying it to multilayer tungsten disulfide (WS2) Schottky-junction solar cells with initially near-zero VOC. Doping and passivation turn these into lateral p-n junction photovoltaic cells with a record VOC of 681 mV under AM 1.5G illumination, the highest among all p-n junction TMD solar cells with a practical design. The enhanced VOC also leads to record PCE in ultrathin (<90 nm) WS2 photovoltaics. This easily scalable doping and passivation scheme is expected to enable further advances in TMD electronics and optoelectronics.

Device Architectures for Low Voltage and Ultrafast Graphene Integrated Phase Modulators

Abstract: The atomic layer thin geometry and semi-metallic band diagram of graphene can be utilized for significantly improving the performance matrix of integrated photonic devices. Its semiconductor-like behavior of Fermi-level tunability allows graphene to serve as an active layer for electro-optic modulation. As a low loss metal layer, graphene can be placed much closer to active layer for low voltage operation. In this work, we investigate hybrid device architectures utilizing semiconductor and metallic properties of the graphene for ultrafast and energy-efficient electro-optic phase modulators on semiconductor and dielectric platforms. (1) Directly contacted graphene-silicon heterojunctions. Without the oxide layer, the carrier density of graphene can be modulated by direct contact to silicon layer, while silicon intrinsic region stays mostly depleted. With doped silicon as electrodes, carriers can be quickly injected and depleted from the active region in graphene. The ultrafast carrier transit time and small RC constant promise ultrafast modulation speed (3 dB bandwidth of 67 GHz) with an estimated Vπ·L of 1.19 V·mm. (2) Graphene integrated lithium niobite modulator. As a transparent electrode, graphene can be placed close to integrated lithium niobate waveguide for improving coupling coefficient between optical mode profile and electric field with minimal additional loss (4.6 dB/cm). Numerical simulation indicates a 2.5× improvement of electro-optic field overlap coefficient, with an estimated Vπ of 0.2 V.

Ternary Co3O4/CdS/SrTiO3 core-shell pn junctions toward enhanced photocatalytic hydrogen production activity

Abstract: A ternary Co3O4/CdS/SrTiO3 core-shell pn junction is fabricated via a simple hydrothermal-chemical-photodeposition method. Herein, a CdS shell is deposited on the surface of a SrTiO3 nanosphere, and then p-type Co3O4 lamellas are grown on the surface of the CdS/SrTiO3 core-shell heterojunction. Evaluated by its hydrogen evolution ability, the as-prepared ternary Co3O4/CdS/SrTiO3 core-shell pn junction exhibits an obvious photocatalytic enhancement of ~15-fold compared to single SrTiO3 nanospheres. This improvement can be mainly ascribed to the narrow band gaps of CdS and Co3O4 increasing the visible light response, and the ternary core-shell pn junction quickly transferring/diffusing photogenerated charge carriers into water. Additionally, the core-shell structure can provide sufficient active sites, and the ultrathin Co3O4 lamellas can shorten the photoelectron transport path to improve the photocatalytic stability.

Harvesting Sub-Bandgap IR Photons by Photothermionic Hot Electron Transfer in a Plasmonic p-n Junction.

Abstract: Plasmonic semiconductors are an emerging class of low-cost plasmonic materials, and the presence of a bandgap and band-bending in these materials offer new opportunities to overcome some of the limitations of plasmonic metals. Here, we demonstrate that in a plasmonic p-n heterojunction (Cu2-xSe-CdSe) the near-IR excitation (1.1 eV) of the hole plasmon in the p-Cu2-xSe phase results in rapid hot electron transfer to n-CdSe, with an energy 2.2 eV above the Fermi level. This hot electron generation and energy upconversion process can be well-described by a photothermionic mechanism, where the presence of a bandgap in p-Cu2-xSe facilitates the generation of energetic photothermal electrons. The lifetime of the transferred electrons in Cu2-xSe-CdSe can reach ∼130 ps, which is nearly 100× longer than that of its metal-semiconductor counterpart. This result demonstrates a novel approach for harvesting the sub-bandgap near IR photons using plasmonic p-n junctions and the potential advantages of plasmonic semiconductors for hot carrier-based devices.

Comprehensive Study and Optimization of Implementing p-NiO in β-Ga2O3 Based Diodes via TCAD Simulation

Abstract: In this paper, we carried out a comprehensive study and optimization of implementing p-NiO in the β-Ga2O3 based diodes, including Schottky barrier diode (SBD) with p-NiO guard ring (GR), p-NiO/β-Ga2O3 heterojunction (HJ) barrier Schottky (HJBS) diode, and HJ-PN diode through the TCAD simulation. In particular, we provide design guidelines for future p-NiO-related Ga2O3 diodes with material doping concentrations and dimensions to be taken into account. Although HJ-PN has a ~1 V higher turn-on voltage (Von), its breakdown voltage (BV) is the highest among all diodes. We found that for SBD with p-NiO GRs and HJBS, their forward electrical characteristics and reverse leakage current are related to the total width and the doping concentration of p-NiO, the BV is only related to the doping concentration of p-NiO, and the optimal doping concentration of p-NiO is found to be 4 × 1017 cm−3. Compared with the SBD without p-NiO, the BV of the SBD with p-NiO and HJBS diode can be essentially improved by 3 times. As a result, HJ-PN diode, SBD with p-NiO GRs, and HJ-BS diode achieve a BV/specific on-resistance (Ron,sp) of 5705 V/4.3 mΩ·cm2, 3006 V/3.07 mΩ·cm2, and 3004 V/3.06 mΩ·cm2, respectively. Based on different application requirements, this work provides a useful insight about the diode selection with various structures.

Gate Current Transport in Enhancement-Mode p - n Junction/AlGaN/GaN (PNJ) HEMT

Abstract: In this work, we study the gate leakage mechanisms of E-mode ${p}$ - ${n}$ junction/AlGaN/GaN (PNJ) high electron mobility transistors (HEMTs), which have been shown to deliver low gate leakage and wide safe operating gate-bias range. The intrinsic gate leakage through the PNJ-gate was found to be limited by the transport of holes through the ${p}$ -GaN layer, which occurs via Poole-Frenkel emission and phonon-assisted tunneling in low and high gate bias region, respectively. In addition, lateral leakage current and the role of variable hopping process (VRH) are also discussed. Gate leakage current models based on the revealed mechanisms can quantitatively reproduce the gate-leakage behavior in the entire relevant range of gate biases and temperatures.

CdTe X/γ-ray Detectors with Different Contact Materials

Abstract: Different contact materials and optimization of techniques of their depositions expand the possibilities to obtain high performance room temperature CdTe-based X/γ-ray detectors. The heterostructures with ohmic (MoOx) and Schottky (MoOx, TiOx, TiN, and In) contacts, created by DC reactive magnetron sputtering and vacuum thermal evaporation, as well as In/CdTe/Au diodes with a p-n junction, formed by laser-induced doping, have been developed and investigated. Depending on the surface pre-treatment of semi-insulating p-CdTe crystals, the deposition of a MoOx film formed either ohmic or Schottky contacts. Based on the calculations and I-V characteristics of the Mo-MoOx/p-CdTe/MoOx-Mo, In/p-CdTe/MoOx-Mo, Ti-TiOx/p-CdTe/MoOx-Mo, and Ti-TiN/p-CdTe/MoOx-Mo Schottky-diode detectors, the current transport processes were described in the models of the carrier generation-recombination within the space-charge region (SCR) at low bias, and space-charge limited current incorporating the Poole-Frenkel effect at higher voltages, respectively. The energies of generation-recombination centers, density of trapping centers, and effective carrier lifetimes were determined. Nanosecond laser irradiation of the In electrode, pre-deposited on the p-CdTe crystals, resulted in extending the voltage range, corresponding to the carrier generation-recombination in the SCR in the I-V characteristics of the In/CdTe/Au diodes. Such In/CdTe/Au p-n junction diode detectors demonstrated high energy resolutions (7%@59.5 keV, 4%@122 keV, and 1.6%@662 keV).

Ideal PN photodiode using doping controlled WSe2–MoSe2 lateral heterostructure

Abstract: As the tight contact interface of the lateral PN junction enables high responsivity, specific detectivity, and fast response speed, atomic-scale two-dimensional (2D) lateral PN heterostructures are emerging as viable alternatives to silicon-based photodiodes. The optical properties of the current 2D heterostructures depend entirely on the intrinsic properties of 2D materials, which can be greatly improved by forming an ideal PN diode via the doping control of 2D heterostructures. In this study, we propose a high-performance photodiode using a doping-controlled WSe2–MoSe2 PN heterojunction. During the synthesis, the low chemical reactivity of Nb2O5 with WO3 as compared to MoO3 enables sequential growth and prevents niobium (Nb) doping during MoSe2 growth at low temperatures. Conversely, in the WSe2 growth at high temperatures, tungsten (W) to Nb is selectively substituted, resulting in the lateral heterostructure of Nb-doped WSe2–MoSe2. The Nb atoms in WSe2 change the WSe2 type from ambipolar to p-type dominant. Together with intrinsically n-type MoSe2, Nb-doped WSe2 forms a lateral PN heterostructure with a near-unity ideality factor (1.3) and a high forward/reverse current ratio of 104. Our ideal 2D PN photodiode effectively suppresses the dark current in the reverse bias region (∼100 fA at an overall VDS of 0 V to approximately −10 V) and enhances the photocurrent by the high built-in potential at the PN depletion layer (VOC = 0.52 V). Thus, our device exhibits a high Ilight/Idark ratio (105) and a corresponding ultra-high detectivity (5.78 × 1015 Jones), which are approximately 100 times higher than those of reported lateral 2D PN heterostructure photodiodes. These outstanding performances show that the doping-controlled transition metal dichalcogenide PN heterostructures are promising candidates for next-generation optoelectronics.

Achieving over 15% Efficiency in Solution-Processed Cu(In,Ga)(S,Se)2 Thin-Film Solar Cells via a Heterogeneous-Formation-Induced Benign p-n Junction Interface.

Abstract: Cu(In,Ga)(S,Se)2 (CIGS) thin-film solar cells have attracted considerable interest in the field of photovoltaic devices due to their high efficiency and great potential for diverse applications. While CdS has been the most favorable n-type semiconductor because of its excellent lattice-match and electronic band alignment with p-type CIGS, its narrow optical band gap (∼2.4 eV) has limited light absorption in underlying CIGS absorber films. Reducing the thickness of CdS films to increase the short-circuit current-density has been less effective due to the following decrease in the open-circuit voltage. To overcome this trade-off between the main parameters, we controlled the formation mechanism of CdS films in chemical bath deposition and established its direct correlation with the properties of p-n junctions. Interestingly, a heterogeneous CdS film formation was found to have a synergetic effect with its ammonia bath solution, effectively reducing charge carrier loss from the shunt paths and interface recombination of CIGS/CdS junctions. With these electrical benefits, the trade-off was successfully alleviated and our best device achieved a power conversion efficiency of 15.6%, which is one of the state-of-the-art CIGS thin-film solar cells prepared using solution-processing techniques.

Effect of NiOx’s film thickness on the electrical properties of Ni/p–NiOx/n-Si structures:

Abstract: In this study, non-stoichiometric NiO films were obtained by using radio frequency sputtering technique to fabricate a heterojunction p–n Schottky diode. The crystal structure and topographical fea...

Temperature Gradient-Dominated Electrical Behaviours in a Piezoelectric PN Junction

Abstract: In this paper, we have systematically investigated the temperature gradient-dependent electrical behaviours in a piezoelectric PN junction. A new iterative computational method is proposed by utilizing the 1-D nonlinear theories of thermo-piezoelectric semiconductors. Coupling between the thermal gradient fields and polarization charges is discussed. It is found that the electromechanical field of a piezoelectric PN junction has a quick response to thermal gradient. Furthermore, gate voltage and carrier transport characteristics can be effectively tuned with thermal-induced and piezoelectric charges. It is shown that a piezoelectric PN junction is highly sensitive to the temperature gradient, which may provide an alternative approach to manipulate the carrier transport in piezotronic devices.

Self-rectifying resistance switching memory based on a dynamic p-n junction

Abstract: Although resistance random access memory (RRAM) is considered as one of the most promising next-generation memories, the sneak-path issue is still challenging for the realization of high-density crossbar memory array. The integration of the rectifying effect with resistance switching has been considered feasible to suppress the sneaking current. Herein, we report a self-rectifying resistance switching (SR-RS) by a newly discovered Li ions migration induced dynamic p-n junction at the Li-doped ZnO and ZnO layer interface. The Au/Li-ZnO/ZnO/Pt structure exhibits a forming-free and stable resistance switching with a high resistance ratio of R OFF/R ON ∼ 104 and a large rectification ratio ∼106. In the Li-ZnO/ZnO bilayer, the electric field drives the dissociation and recombination of the self-compensated [Formula: see text] complex pairs ([Formula: see text] p-type substitutional defect; [Formula: see text] n-type interstitial defect) through the transport of [Formula: see text] between the two layers, thereby induces the formation of a dynamic p-n junction. Using this structure as a memory stacking device, the maximum crossbar array size has been calculated to be ∼16 Mbit in the worst-case scenario, which confirms the potential of the proposed device structure for the selection-device free and high-density resistance random access memory applications.

Diverse simulations of time-resolved photoluminescence in thin-film solar cells: A SnO2/CdSeyTe1−y case study

Abstract: Time-resolved photoluminescence (TRPL) is widely used to measure carrier lifetime in thin-film solar cell absorbers. However, the injection dependence of data and frequent non-exponential decay shapes complicate the interpretation. Here, we develop a numerical model to simulate injection-dependent TRPL measurements in a SnO2/CdSeyTe1−y solar cell structure, considering parameters of interest to researchers in industry and academia. Previous simulations have shown that in low injection, excess electrons and holes injected by the laser pulse are rapidly separated in the electric field formed by the pn junction. As a result, at early times, the PL signal can decay faster than the Shockley–Read–Hall lifetime in the absorber bulk ( τbulk). Prior simulations have shown that the charge stored in the junction can slowly leak out to affect decays at late times. However, it has not been clear if and to what degree charge storage can affect the slopes extracted from TRPL decays— τ2—commonly cited as the TRPL-measured lifetime. Here, we show that charge storage can, in some cases, result in τ2 values that substantially overestimate τbulk. Previous simulations indicate that high-injection conditions can screen the junction field and minimize charge separation. Here, we show that continued injection increases can drive down τ2 below τbulk as radiative recombination becomes dominant. We catalog charge storage and radiative recombination impacts for a diverse set of material parameters and compare results to double-heterostructure models.

Vertical Tip-to-Tip Interconnection p–n Silicon Nanowires for Plasmonic Hot Electron-Enhanced Broadband Photodetectors

Abstract: A p–n junction photodetector, formed between the tip-to-tip interconnection of vertical silicon nanowire arrays, was fabricated by using a facile and economic process with remarkable performances

Vertical GaN p+-n junction diode with ideal avalanche capability grown by halide vapor phase epitaxy

Abstract: A vertical GaN p+-n junction diode with an ideal breakdown voltage was grown by halide vapor phase epitaxy (HVPE). A steep p+-n interface was observed even with the use of the HVPE method. No Si-accumulating layer was formed at the p+-n interface because of the continuous HVPE growth from the n-type drift layer to the p-type layer. This method provides improved electrical properties compared with the regrowth of p-type GaN layers. The minimum ideality factor of approximately 1.6 was obtained. The breakdown voltage increased from 874 to 974 V with the increase in the temperature from 25 to 200 °C, which suggests that avalanche multiplication causes the breakdown. The temperature-dependent breakdown voltage was in good agreement with the breakdown voltage calculated using the ideal critical electric field. These results indicate that HVPE is promising for the fabrication of vertical GaN power devices.

Near-Broken-Gap Alignment between FeWO4 and Fe2WO6 for Ohmic Direct p-n Junction Thermoelectrics.

Abstract: We report a near-broken-gap alignment between p-type FeWO4 and n-type Fe2WO6, a model pair for the realization of Ohmic direct junction thermoelectrics Both undoped materials have a large Seebeck coefficient and high electrical conductivity at elevated temperatures, due to inherent electronic defects A band-alignment diagram is proposed based on X-ray photoelectron and ultraviolet-visible light reflectance spectroscopy Experimentally acquired nonrectifying I-V characteristics and the constructed band-alignment diagram support the proposed formation of a near-broken-gap junction We have additionally performed computational modeling based on density functional theory (DFT) on bulk models of the individual compounds to rationalize the experimental band-alignment diagram and to provide deeper insight into the relevant band characteristics The DFT calculations confirm an Fe-3d character of the involved band edges, which we suggest is a decisive feature for the unusual band overlap

Electric field-driven one-step formation of vertical p–n junction TiO2 nanotubes exhibiting strong photocatalytic hydrogen production

Abstract: In this study, vertically aligned p–n junction TiO2 nanotubes can be formed by anodization of the Ti substrate following the addition of a high electric field in a Pd precursor-containing electrolyte. The bottom region of the TiO2 nanotubes with a high concentration of Pd because of the high electric field which was induced to be p-type. In contrast, the region with a low Pd concentration (top of the TiO2 nanotubes) was determined to be n-type, similar to the pristine TiO2. The concentration profile of the dopant in TiO2 nanotubes was investigated via TOF-SIMS and XPS. Defect formation energies in TiO2 nanotubes were estimated using density-functional theory calculation to understand the p–n junction formation. The p–n junction TiO2 nanotubes showed a high photocatalytic hydrogen production rate of 25.2 μL cm−2 h−1 under solar light irradiation as a result of the enhancement of visible light photoactivity.