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After graphene ignited the field of two-dimensional (2D) van der Waals (vdW) materials, other 2D materials, such as transition metal dichalcogenides (TMDs), have emerged as attractive integration elements for novel heterostructures and offered extraordinary optoelectronic application potential. In addition, plasmonic resonances typically play a unique role in efficiently harvesting incident light due to their ability to manipulate electromagnetic waves on a deep-subwavelength scale. These capabilities are utilized in nano-electronic devices to further enhance their exceptional performances, such as high speed, high sensitivity, and low power consumption. To exploit the full potential of a variety of photodetector/photoconversion devices, it is crucial to clarify the complex nature of the photoresponse mechanisms at the nanoscale. In addition, a rapid and high-throughput characterization method for plasmonic metal nanoparticles is required for application design.This thesis covers two of my most significant contributions:1. Photocurrent mechanisms in a van der Waals metal-semiconductor junctionIn this part, we report the photocurrent generation mechanism distribution in a Schottky barrier photodiode consisting of a defect-free molybdenum disulfide-gold (MoS2-Au) interface. We find that the in-plane PTE current generated by the near fields of a nanoscale light source can be four-folds stronger than the PV current, which is the working horse of a conventional Schottky barrier photodiode. Furthermore, we show that the spatial variation of the PTE effect in amplitude and polarity can be controlled through nanoscale heat management, by adding a thin hexagonal boron nitride layer (h-BN, 5 nm) onto the MoS2. This study points out a promising way for high-efficiency integrated photodetectors.2.Machine learning-enhanced rapid size quantification for nanoparticleswe develop a rapid optical characterization method that integrates through-focus scanning microscopy (TSOM) with machine learning (ML) to directly measure the size of individual nanoparticles and the size distribution of a nanoparticle ensemble. With silver nanocubes (AgNC) as a model system, we achieved an estimation error of 5% and a root mean square error (RMSE) of 8.2 nm on individual particles, as well as an error of 1.6 % on the average particle size and 0.4 nm on the size distribution on particle ensembles. The single particle level measurement allows the measurement of complex mixtures with broad size distributions, which have size-dependent features smeared in other rapid measurements such as UV-vis absorption spectroscopy. This work describes a ML-assisted single particle-level optical characterization for the size information and can be potentially extended to more complicated nano-materials such as anisotropic and dielectric nanoparticles.

Photocatalysts for harvesting solar energy to either electricity or chemical fuels have attracted much attention recently, but they have big obstacles such as wide bandgaps and rapid charge recombinations to overcome for final applications. In this study, we investigates a useful method to utilize vanadium redox pairs, which are commonly applied for vanadium redox flow batteries, to diminish charge recombinations and thus to enhance photocurrent response in regenerative solar energy storage. The results reveal significant improvements in photocurrent density by forming cuprous and cupric oxides in TiO2/CuxO electrodes under solar AM 1.5 illuminations using the vanadium photoelectrochemical storage cell at 0.025M of vanadium redox species in the acid electrolytes. In addition, the stabilized photocurrent density of the copper content optimized TiO2/CuxO electrodes is almost tripled from the TiO2 only electrode because the charge recombinations can be mitigated with the content optimized TiO2/CuxO electrodes. Therefore, the optimized TiO2/CuxO electrode results in the highest charge storing performance in the catholyte chamber, and the roles of vanadium redox species are also clearly demonstrated.

Small and low-cost chlorophyll sensors are popular in agricultural sector and food-quality control.Combining such sensors with silicon CMOS electronics is challenged by the absence of silicon-integrated light-sources.We experimentally achieve optical absorption sensing of chlorophyll based pigments with silicon (Si) micro light-emitting diodes (LED) as light-source, fabricated in a standard SOI-CMOS technology.By driving a Si LED in both forward and avalanche modes of operation,we steer its electroluminescentspectrum between visible (400-900 nm) and near-infrared (1120 nm). For detection of chlorophyll in solution phase, the dualspectrum light from the LED propagates vertically through glycerol micro-droplets containing sodium copper chlorophyllin at varying relative concentrations. The transmitted light is detected via an off-chip Si photodiode. The visible to near-infrared color ratio (COR) of the photocurrent yields the effective absorption coefficient. We introduce the LED-specificmolar absorption coefficient as ametric to compute the absolute pigment concentration (0.019 ± 0.006 mol L-1) and validate the results by measurements with a hybrid spectrophotometer. With the same sensor, we also show noninvasive monitoring of chlorophyll in plant leaves. COR sensitivities $\sim 3.9 \times 10^{4}$ mol-1L and $\sim 5.3 \times 10^{4}$ mol-1L are obtained for two leaf species, where light from the LED propagates diffusely through the thickness of the leaf prior to detection by the photodiode. Our work demonstrates the feasibility of realizing fully CMOS-integrated optical sensors for biochemical analyses in food sector and plant/human health.

Understanding the fundamental charge carrier dynamics is of great significance for photodetectors with both high speed and high responsivity. Devices based on two-dimensional (2D) transition metal dichalcogenides can exhibit picosecond photoresponse speed. However, 2D materials naturally have low absorption, and when increasing thickness to gain higher responsivity, the response time usually slows to nanoseconds, limiting their photodetection performance. Here, by taking time-resolved photocurrent measurements, we demonstrated that graphene/MoTe2 van der Waals heterojunctions realize a fast 10 ps photoresponse time owing to the reduced average photocurrent drift time in the heterojunction, which is fundamentally distinct from traditional Dirac semimetal photodetectors such as graphene or Cd3As2 and implies a photodetection bandwidth as wide as 100 GHz. Furthermore, we found that an additional charge carrier transport channel provided by graphene can effectively decrease the photocurrent recombination loss to the entire device, preserving a high responsivity in the near-infrared region. Our study provides a deeper understanding of the ultrafast electrical response in van der Waals heterojunctions and offers a promising approach for the realization of photodetectors with both high responsivity and ultrafast electrical response.

The strong anisotropic electrical properties of one-dimensional (1D) nanostructure semiconductors, especially the anisotropic carrier transport, have a negative and significant influence on the performance of solar cells if the nanostructures have random orientation. Considering the advantages of nanorod solar cells in carrier transport, we have achieved growth of vertically aligned Sb2Se3 nanorod array with highly (hk1) orientation on CdS substrate, and constructed superstrate nanorod solar cells for the first time. The Sb2Se3 nanorod array solar cells exhibit the more efficient and long-range carrier transport in vertical direction. Furthermore, in order to suppress interface recombination, a CuInSe2 quantum dots (QDs) sensitizer has been applied to fill the volume between the nanorods completely, thus forming an interpenetrating nanocomposite structure. The CuInSe2 QDs can harvest additional light by absorption of visible light and contribute photocurrent. Meantime, the QDs function as a hole transport material and thus reduce the dependence of lateral transport. Consequently, the interpenetrating nanocomposite CuInSe2/Sb2Se3 solar cells display a power conversion efficiency of 7.54% with significant enhancements in the short-circuit current density and open-circuit voltage over pure Sb2Se3 nanorod cells. This is the highest efficiency for superstrate solar cells based on Sb2Se3 nanorod arrays.

In this study, we demonstrate for the first time the construction of a ternary BiVO4/FeVO4/Cu2O nanorod array (NRA) photoelectrode via seed-assisted refluxing, hydrothermal and successive ionic-layer adsorption and reaction (SILAR) method and its photoelectrochemical (PEC) water oxidation performance. The energy band structures of the BiVO4, FeVO4 and Cu2O show that two type-II band alignments with a ladder-like structure are formed in the constructed ternary BiVO4/FeVO4/Cu2O NRA photoelectrode, enabling the photoelectrode to display substantially enhanced PEC performance. The constructed ternary photoelectrode loaded with the optimized Cu2O nanoparticles achieves a zero-bias photocurrent density of 50 μA cm−2, 5 times higher than that of the pure BiVO4 NRA photoelectrode. The measurements of optical property reveal that the constructed BiVO4/FeVO4/Cu2O nanorod arrays (NRAs) demonstrate improved light absorption capability. The electrochemical impedance and photoluminescence measurements confirm that the charge carriers of the BiVO4/FeVO4/Cu2O heterojunction NRAs are transferred and separated effectively. The substantially boosted PEC water oxidation activity of the constructed ternary BiVO4/FeVO4/Cu2O heterojunction NRA photoelectrode is ascribed to the improved light harvesting and effective charge separation.

Photodiodes (PDs) which convert optical signals to electrical signals have been widely used in many fields. In contrast to pin PDs, in uni-traveling carrier (UTC) PDs, the absorption layer is p type doped and only photo-generated electrons are transported, which results in several superior characteristics including super high speed operation, high saturation current and low voltage operation. In this work, high speed InP based waveguide UTC-PDs have been fabricated by Zn diffusion doping for the first time to our knowledge, in contrast to the in situ doping of previous UTC-PDs. Diffusion doping will make it much easier for the UTC-PDs to be integrated with high quality passive waveguide and multi-quantum well (MQW) based active devices for photonic integrated circuit (PIC) applications. UTC-PDs with 3 × 15, 3 × 25 and 3 × 50 µm2 absorption areas are fabricated, all having over 50 GHz bandwidth at -1V bias voltage. At -3 V bias, the RF power at 40 GHz shows no saturation at up to 17 mA photocurrent for the 3 × 25 µm2 PD.

Recently, high-performance GeSn photodiodes (PDs) with external light illuminated on the device's top surface have been demonstrated on various platforms. However, for image sensing systems with a focal-plane array (FPA), the front-illuminated sensors usually suffer from area limitations. Here, we report high-performance back-illuminated Ge0.92Sn0.08/Ge multiple-quantum-well (MQW) p-i-n PD on 300-mm silicon substrate, which was realized entirely by complementary metal-oxide-semiconductor (CMOS) compatible processes. A broadband photoresponse between 1,000-2,100 nm was observed, and the responsivity is 0.2850 and 0.0085A/W at 1,550 and 2,000 nm, respectively. A specific detectivity larger than 109 cm·Hz1/2/W was achieved between 1,050 and 1,900 nm, covering all the conventional telecommunication bands (O to U band). Furthermore, the influence of the anti-reflective layers also was studied in detail. The result shows the black Si (b-Si) surface enhances more photocurrent between 1,000-1,500 nm while the SiO2 layer (400-nm-thickness) increases more current beyond 1,500 nm. The 3-dB bandwidth was calculated to be up to 8 GHz for a mesa with a diameter of 20 μm at −2 V. Our experiments demonstrated the high-detectivity and high-speed back-illuminated GeSn/Ge MQW PD with the potential applications in image sensing systems operated in the short-wave infrared (SWIR) range.

Schottky-contacted nanosensors have attracted extensive attention due to their high sensitivity and fast response time. In this article, we proved that the construction of Schottky contact by Pt nanoparticles (NPs) decoration can effectively improve the performance of V2O5 nanobelts photodetectors. After modified by Pt NPs, the photocurrent of V2O5 nanobelts is increased by more than two orders of magnitude, and the photoresponse speed is improved by at least three orders of magnitude. Detailed studies have shown that the performance enhancement is attributed to the formation of the Schottky contact at the electrode-semiconductor interface due to the decrease of surface gas adsorption and the increase of V2O5 work function after Pt NPs modification. The strong built-in field in the Schottky barrier region will quickly separate photogenerated carriers, thereby reducing the electron-hole recombination rate, resulting in the fast response time and an increase in the free carrier density. Moreover, it is found that this enhancement effect can be regulated by controlling the pressure to modulating the Schottky barrier height at the interface. Overall, the Pt NPs-modified V2O5 nanobelts photodetector exhibits a broad response spectrum (visible to near infrared), fast rise/fall response time (less than 6.12/6.15 ms), high responsivity (5.6 A/W), and high specific detectivity (6.9 × 108 Jones). This study demonstrates the feasibility of building a Schottky barrier to enhance the photodetection performance, which provides a general and effective strategy towards the construction and its practical application of supersensitive and fast-response nanosensors.

Taking advantage of the superflat surface of ultrananocrystalline diamond (UNCD), highly precise UNCD nanowire (NW) arrays were fabricated to develop high-performance UV photodetectors. The large surface-to-volume ratio of a nanowire significantly increases the number of surface trap states, and the reduced dimensionality effectively confines the active area of the charge carrier and shortens the transit time, which results in an enhanced photoconductivity and response speed. In this paper, the zero-biased UV photodetectors based on nitrogen-incorporated ultrananocrystalline diamond nanowire arrays have been demonstrated and characterized. The estimated responsivity was 2.0 A/W for 350 nm incident light when the device operated at room temperature. The UVA and UVB photocurrent signals from this visible blind photodetector were well defined with a rise and decay time of less than 1 s.

In this paper, a multifunctional ultraviolet-C (UVC) micro-LED with monolithically integrated photodetector (PD) based on AlGaN multiple quantum wells (MQWs) was fabricated. We designed and integrated UVC LED with sizes of 40 and 300 m emitted at 277 nm on a single chip which also serve functions as PDs and further studied the performance of on-chip real-time photodetection. The photocurrent generated from one LED can well reflect the light output power of the adjacent LED. Furthermore, both 40 m UVC micro-LED and 300 m UVC LED play important roles in high speed UVC communication with a record bandwidth of 497.58 MHz for 40 m micro-LED and 467.38 MHz for 300 m LED, offering each other as backup light source in high speed UVC communication. And the data transmission rates of 1.6 Gbps for 40 m micro-LED and 1.21 Gbps for 300 m LED over 1 m link based on the 16-ary quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM-OFDM) with pre-equalization were achieved. The multifunctional UVC micro-LED monolithically integrated PD, without the need for an external PD to monitor the fluctuations in the luminescence, providing high stability for diverse applications, such as spatial wireless communication for 6G communication.

In this paper, a lead-free halide perovskite CsCu2I3 film with high stability was prepared by the anti-solvent assisted crystallization method. Then, we coupled it with Ga2O3 to prepare a corresponding heterojunction deep ultraviolet (UV) photodetector. After testing, we concluded that the photodetector is sensitive to 254 nm UV light. The photodetector has good reproducibility, and has an ultra-high photo-to-dark current ratio (PDCR) of more than 105. In addition, under a bias of 10 V and an illuminated intensity of 200 μW/cm2, the responsivity (R) and specific detectivity (D*) reached 20 mA/W and 107 cm Hz1/2 W−1 (Jones), and the external quantum efficiency (EQE) is 10%. Meanwhile, the prepared photodetector could operate at zero bias, i.e., self-powered operation, along with a photocurrent of about 1 nA under illumination with UV light intensity of 200 μW/cm2.

The conversion efficiency of energy-harvesting devices can be increased by utilizing hot-carriers (HCs). However, due to ultrafast carrier-carrier scattering and the lack of carrier injection dynamics, HC-based devices have low efficiencies. In the present work, we report the effective utilization of HCs at the nanoscale and their transfer dynamics from a non-noble metal to a metal oxide interface by means of real-space photocurrent mapping by using local probe techniques and conducting femtosecond transient absorption (TA) measurements. The photocurrent maps obtained under white light unambiguously show that the HCs are injected into the metal oxide layer from the TiN layer, as also confirmed by conductive atomic force microscopy. In addition, the increased photocurrent in the bilayer structure indicates the injection of HCs from both layers due to the broadband absorption efficiency of TiN layer, passivation of the surface states by the top TiN layer, and smaller barrier height of the interfaces. Furthermore, electrostatic force microscopy and Kelvin probe force microscopy provide direct evidence of charge injection from TiN to the MoOx film at the nanoscale. The TA absorption spectra show a strong photo-bleaching signal over wide spectral range and ultrafast decaying behavior at the picosecond time scale, which indicate efficient electron transfer from TiN to MoOx. Thus, our simple and effective approach can facilitate HC collection under white light, thereby achieving high conversion efficiency for optoelectronic devices.

Broadband photodetectors (PDs) have great applications in both industrial and scientific sectors. In this study, solution-processed broadband PDs with an “inverted” vertical photodiode device structure without incorporating transparent conductive oxides electrodes, fabricated by bulk heterojunction (BHJ) composites composed of a low optical gap conjugated polymer blended with highly electrically conductive PbS quantum dots (QDs), operated at room temperature, are reported. The low optical gap conjugated polymer incorporated with PbS QDs contributes to the spectral response from the ultraviolet (UV)-visible to the infrared (IR) range. To realize the IR spectral response and to circumvent the weak IR transparency of the transparent oxide electrodes, the implementation of a photodiode with an “inverted” vertical device structure with the Au anode and the Ba/Al bilayer semitransparent cathode passivated with the MgF2 layer is demonstrated. Photoinduced charge carrier transfer occurring within the BHJ composite gave rise to decent photocurrent, resulting in detectivities greater than 1012 Jones (cm Hz1/2/W) over the wavelength from the UV-visible to the IR range under low applied bias. Thus, our findings of the utilization of the BHJ composites and an “inverted” vertical photodiode without the incorporation of the transparent conductive oxide electrodes provide a facile way to realize broadband PDs.

The rapid progress in the development of non-fullerene electron acceptors has led to key breakthroughs in the power conversion efficiency of organic solar cells (OSCs). Nevertheless, the development of non-fullerene OSCs are still hindered by the short exciton diffusion length in both donor and acceptor domains, which is paramount in determining the charge generation properties in OSCs. Reducing the domain size to match the diffusion length via bulk heterojunction (BHJ) structure has been an essential strategy to overcome this limitation, but will inevitably compromise charge collection due to the intermixing of donor and acceptor. Here, we use a solution-based contact film transfer method to fabricate planar heterojunction (PHJ) device, which serves as an excellent platform to investigate exciton diffusion and charge collection owing to the clearly defined interface. By reconstructing external quantum efficiency spectra and device modeling in PHJ cells based on a large bandgap polymer J52 and a low bandgap small molecule IEICO-4F, exciton diffusion lengths of 9.2 nm and 7.5 nm are extracted for J52 and IEICO-4F, respectively. The experimentally obtained values for exciton diffusion length confirm that BHJ structure is an important prerequisite for efficient organic solar cells. Nevertheless, as revealed by the carrier dynamic studies, BHJ devices could suffer from severe recombination losses as a result of energetic disorder and traps. By considering the recombination of free and trapped carriers and intermediates in the photocurrent generation processes, we show that the stronger recombination of BHJ compared to PHJ is also related to its shorter CT states lifetime and lower CT states density based on the reduced Langevin recombination analysis. The inherent limitation in BHJ devices is thus a major disadvantage that need to be overcome for further improvement. On the basis of the results of exciton diffusion length measurement and carrier dynamic study, we propose that finely tuning domain size in non-fullerene OSCs still play a crucial role in controlling the device performance.

The surface area / volume ratio of nanostructured semiconductors favors the efficiency of devices based on nanowires, such as sensors or solar cells. In this perspective, the objective of this study was to synthesize germanium nanowires, with and without the addition of copper dopant, to produce nanowire network devices and to investigate electronic and optoelectronic properties, with a view to their use as light sensors. The first step was the formation of gold nanoparticles by heat treatment and the synthesis of the nanowires by the VLS method; of the twenty-five samples produced in the process, seventeen presented nanowires with dimensions and desired composition. In the next stage, network devices in the metal-nanowire-metal architecture were developed. In the initial characterization, the temperature-voltage dependence showed the semiconductor character of the germanium nanowires and the current–voltage measurements indicated that it is possible to produce both ohmic and rectifier contacts. For the study of optoelectronic properties, the current–voltage dependence of the samples in the dark and under illumination was observed and it can be concluded that those that had less current in the dark had more photoresponde. These samples were further analyzed showing rapid saturation of the photocurrent.

The preparation of the compact and full-coverage AgSbS2 thin films is firstly reported using the pyrolysis of the Ag-butyldithiocarbamate and Sb-butyldithiocarbamate complex solution in DMF. The influence of the preparation temperature on the crystal phase, optical absorption, morphology of the AgSbS2 thin films is systematically investigated. The AgSbS2 thin films at 150 °C for 30 min are composed of the individual nanoparticles with the particle sizes of 30–60 nm. When the AgSbS2 thin films are further heated at 350 °C for 2 min, the individual AgSbS2 nanoparticles are melted together and the grain boundary between the AgSbS2 nanoparticles disappear. The solar cells with the architecture of FTO/compact TiO2/AgSbS2/spiro-OMeTAD/Au achieve the photoelectric conversion efficiency (PCE) of 2.09%, along with the open-circuit voltage of 0.44 V, the short-circuit photocurrent density of 10.49 mA cm−2, the fill factor of 0.45. The PCE of 2.09% is the highest value for the AgSbS2 solar cells.

Formation of p-n junction was an effective method to improve the photocatalytic performance due to its built-in electric field. However, the electric field was distributed only on the interface between two semiconductors, which was insufficient in comparison to the entire material. In this work, a multilayer TiO2–CoTiO3 p-n junction was designed and fabricated by atomic layer deposition. The charge transport and photoelectrochemical properties of multilayer TiO2–CoTiO3 films of various thicknesses are thoroughly investigated. The PL and electrochemical tests demonstrate that TiO2–CoTiO3 exhibits enhanced charge separation and transport. Additionally, the extra visible light response was achieved by introduction of CoTiO3. Also, theoretical calculations indicate that the electrons prefer to immigrate from TiO2 to CoTiO3 in a multilayer TiO2–CoTiO3 structure. Benefited from the boosted light absorption and charge transfer, the multilayer TiO2–CoTiO3 composite film exhibits a significantly increased photocurrent, much higher than pure TiO2 and a single TiO2–CoO p-n junction. This multilayer p-n junction structure opens a rational and novel way for nanostructure construction in the energy conversion region.

In this study, the self-powered photodetector based on photovoltaic effect was obtained from BixFeO3 (x = 1, 1.3, 1.5, 1.7) films. In order to study the origin of the photovoltaic effect, the self-polarized structure and defect distribution of the films were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, and piezoelectric force microscopy. In addition, the responsivity (1515 μA/W) and detectivity (1.35Χ1011 Jones) are achieved in the Bi1.5FeO3 film at zero bias after receiving 0.2 mW/cm2 of irradiation light in the wavelength of 500 nm. The rise and decay time of the photocurrent without bias reaches ∼6 ms and ∼15 ms respectively, and did not weaken after a 11 min cycling test. The self-polarized structure and defect dipoles play an important role in the photovoltaic effect of BixFeO3 self-powered photodetector. This work lays a foundation for applications in BixFeO3 self-powered photodetection.

The production of O2 at TiO2 surface during the photoelectrochemical water splitting is the bottleneck of the reaction process indicating the need for surface engineering of TiO2. Here, 1-D TiO2 was coated with the defect tuned SnO2 nanolayer (via sputtering) for enhanced solar water splitting activity. The sputtering strategy allows oxygen vacancy tuning of SnO2 with complete coverage on TiO2. Importantly, the optimized SnO2/TiO2 system exhibited the record photocurrent of 1.85 mA cm−2 at 1.23 V vs. RHE, incident photon to current efficiency of 95% at 350 nm, and H2 and O2 production with ∼90% of faradaic efficiency. Band structure analysis indicated a decrease in work function along with an upward shift of Fermi level and conduction band at the electrolyte interface after SnO2 coating. The present study demonstrates the utilization of oxygen vacancy in SnO2 for the surface engineered electrodes for facile O2 production during solar water splitting.

Two-dimensional (2D) organic–inorganic hybrid materials have attracted widespread attention for photodetection. Moreover, such materials with broadband photoluminescence and strong photocurrent response are still rare. Here, we report two new semiconducting 2D organic–inorganic hybrid materials, namely {PbII2I6[PbII(TETA)]}n (1Pb) and {PbII3I8[PbII(TETA)]}n (2Pb), where TETA = triethylenetetramine, with broadband yellow-green emission. Both compounds are soluble in DMF and yield high surface coverage films through spin coating. Strikingly, the photocurrent responses of such thin films show ca. 600 and 700 nA cm−2 difference between Ilight and Idark for 1Pb and 2Pb, respectively. To the best of our knowledge, 1Pb and 2Pb showed among the highest current obtained in 2D lead iodide hybrid materials under a low voltage (0.7 V). Moreover, 1Pb and 2Pb are stable under heat, moisture and light, which may provide realistic applications for light detection.

Tantalum nitride (Ta3N5) is a very promising photoanode material due to its narrow band gap (2.1 eV) and suitable band alignment for solar water splitting. However, it suffers from severe photocorrosion during water oxidation. In this work, it was found that surface passivation by AlOx and TiOx layers results in dramatically different PEC performance of Ta3N5 photoanode for water oxidation. The mechanism study indicates that the negative charges on AlOx can generate additional field to promote separation of photogenerated charges, while the positive charges on TiOx layer show the opposite effect. As a result, the Ta3N5 based photoanode modified with AlOx layer gives a high photocurrent of 12.5 mA cm−2 for 24 h at 1.23 V versus the reversible hydrogen electrode (RHE). Dynamic analysis implies that the hole extraction and transfer are significantly improved by the modification with the AlOx layer. This work reveals the importance of the charges on surface passivation layer in interface engineering of photoelectrodes.

Photoelectrochemical hydrogen generation has been investigated using Yb co-doped (at varied concentration) and 2% Mo doped BiVO4 thin films. Prepared thin films were used as photoanode for photo-assisted PEC water splitting. Doping of Mo6+ and Yb3+ led to extended absorption of light with marginal fall in band gap energy. Significantly high PEC photocurrent (~1.12 mA cm−2 at 1.9 V vs. RHE) is recorded under 1 sun illumination in K2HPO4 electrolyte. The effect is largely attributable to better charge transfer and reduced recombination.

Renewable energy, especially solar energy is a vital alternative solution to power challenge in these present times. More recently, dye-sensitized solar cells (DSSC) play an important role in solar power generation. The performance of DSSC can be increased by adding more cells to a cell stack in a Tandem cell with a separate band gap. A double-layer structured DSSC was analyzed in this study in which ZnO and TiO2 composite photo-anodes were prepared on Fluorine-doped tin oxide. Photovoltaic properties such as current–voltage (I-V) and photocurrent density–voltage curves were investigated for different thickness, absorption coefficients, and wavelengths. DSSC solar cell characterization technique such as I-V measurement was analyzed to evaluate the cells performance. The experimental results show an approximate increase in current of 0.02 A causing the I-V characteristics to change significantly when ZnO and TiO2 are combined. In addition, it was observed that as the J-V characteristics of TiO2 varies with electrode thickness, the current density J sc increases simultaneously and reaches its peak point at 10 μm.

We report a facile and low-cost deposition of Tin sulfide (SnS) nanosheets and its performance for Near Infra-red (NIR) Photodetection under the UV to NIR illumination. Orthorhombic phase of SnS has been observed in the phase identification analysis. The optical absorption edge was observed at 1030 nm, and the band gap was found to be 1.2 eV. Nanosheets morphology along with flakes identified from the scanning electron micrograph analysis. Photoresponse measurement was studied under dark and illumination with a constant bias voltage of 1.5 V and the enhanced photocurrent is observed. The higher spectral responsivity of 0.19 A/W at 1030 nm has been observed in spectral response curve recorded from 300 nm to 1100 nm. The cyclic photoresponse of the SnS photodetectors shows good cyclability with illumination turn on and off alternatively with the time interval of 20 s. The response time (Tr) and recovery time (Tτ) were found to be 2 s and 4 s, respectively. The detectivity Dt of the SnS nanosheets photodetector to be 9.213×1011 Jones. The wavelength-dependent photodetection shows the maximum response at 1030 nm and the higher photosensitivity. Finally, the external quantum efficiency of the fabricated NIR photodetector was found to be 27%.

The stability of perovskite solar cells is an important issue to be addressed for future applications. Perovskite solar cells are vulnerable to exposure to UV light due to promoted chemical reactions. However, preventing UV light from entering solar cells lowers the power conversion efficiency by reducing the photocurrent. The challenge is to improve UV stability without sacrificing efficiency. Here, we demonstrate the reduction of UV light-related negative effects from the perspective of spectral modification. By simultaneously introducing UV–visible downshifting and light trapping, perovskite solar cells can achieve a comparable efficiency of over 21% to that of an unmodified device. The optimized device obtains increased UV stability due to UV–visible downshifting. Different from other strategies, spectral modification externally alters the composition of incident light and improves UV stability without changing the internal device architecture, which is broadly applicable to perovskite solar cells with different structures. The present work may also find applications in other types of solar cells to boost the stability of devices exposed to UV light.

The unsatisfactory solar light absorption of WO3 and poor charge separation of BiVO4 are main limits for their use in photoelectrochemical (PEC) water oxidation. Coupling WO3 with BiVO4 has been considered as a feasible way to improve PEC performance by taking complementary advantages of them. In this work, we obtained nanoflake-structured WO3 by hydrothermal growth with post-annealing. The effect of process variables on morphology and resultant performance were investigated. Electrodeposition growth was utilized to deposit BiVO4 onto WO3 forming WO3/BiVO4 heterojunction thin films. Porous BiVO4 with wormlike morphology was tightly coupled and well-distributed onto WO3 nanoflakes. The optimized best-performing WO3/BiVO4 photoanode exhibits higher photocurrent density than that summation of bare WO3 and BiVO4 over entire range of applied potential. This enhancement is mainly attributed to the effective charge separation at WO3/BiVO4 interface, which is confirmed through electrochemical impedance spectra (EIS) measurements, respectively. Our work provides a referable approach for the growth of WO3/BiVO4 heterojunction photoanode with enhanced PEC performance.

Ferroelectric materials are considered as a promising candidate for photovoltaic devices owing to spontaneous polarization which may affect the photocarrier dynamics and raise photovoltaic effect. However, the interaction mechanism between the ferroelectric polarization and the photocarrier dynamics is still unclear, which limits the practical applications of this kind of materials. Here, we used femtosecond time-resolved reflectance measurements to monitor the photocarrier dynamics of PbTi1-xNixO3 (PTNO) films with different Ni doping concentrations, therefore the samples have different ferroelectric polarizations. We found that in the PTNO films with larger polarizations, the photocurrents are higher and the photocarriers recombined slower. We deduce that the depolarization fields are stronger in films with larger polarizations, and the photogenerated electrons and holes are separated more effectively. The recombination of photocarriers is retarded and the photocurrent is improved. Our study supplies a novel way for the enhancement of photovoltaic performances of ferroelectric material.

Significantly enhanced photoelectrochemical (PEC) performance of the visible light active photoanodes of ferroelectric Bi0.5Na0.5TiO3 nanostructure has been demonstrated by tuning the electrical polarization of the film. The photocurrent density of unpoled Bi0.5Na0.5TiO3 photoanode increases from 0.48 mA cm−2 at 1 V with respect to Ag/AgCl to 1.21 mA cm−2 after negative poling, whereas it decreases to 0.18 mA cm−2 after positive poling. The incident photon-to-current conversion efficiency (IPCE) of the nanostructure photoanodes has been observed to vary from 5% to 28% just by changing the direction of electric polarization. The change in the PEC performance and enhancement in the IPCE value after switching the polarization state of Bi0.5Na0.5TiO3 photoanodes have been attributed to the modification in the band bending at the interface of semiconductor electrode/electrolyte, leading to facilitate the increment in photogenerated charge transfer efficiency. The modulation in the band bending and variation in the charge transfer method after switching the polarization state are confirmed by Mott-Schottky (M-S) and electrochemical impedance spectroscopy (EIS) measurements. Our work provides a new visible light active ferroelectric nanostructure material for efficient visible light active photoelectrochemical performance.

We report for the first time that nonmetal plasmonic TiN nanoparticles act as hot-electron booster to remarkably enhance photocurrent and hydrogen production of CdS nanoroad arrays (NRAs) via a photoelectrochemical cell. The optimized TiN–CdS NRAs produce a bias-free photocurrent of 2.9 mA cm−2. Furthermore, the highest photoconversion efficiency of the TiN–CdS nanorod array (NRA) photoanode with optimized loading of the TiN NPs is up to 2.2%, 1.6-fold that of the pristine CdS NRA photoanode (1.4%) at 0.22 V versus RHE. Most importantly, TiN–CdS NRAs achieve a hydrogen production rate of 101.5 μmol h−1 cm−2, about 2 times higher than that of the CdS NRAs. Experimental evidences of the photoluminescence emission apparently confirm that hot-electron injection from TiN to CdS plays a crucial role for enhancing photocurrent and hydrogen production. This work demonstrates that nonmetal plasmonic TiN can be integrated into new platforms for efficient solar energy conversion.

Here, we report synthesis of Ag2S nanostructures of zig-zag geometry to achieve the improved photoelectrochemical (PEC) water splitting response for hydrogen generation. A two-step process was utilized for the fabrication of working electrodes. The synthesis of zig-zag nanorods was carried out using glancing angle deposition followed by sulfurization. The PEC performance was studied by varying the number of zig-zag arms of Ag2S. The as-prepared four arm Ag2S zig-zag electrodes exhibited superior optical absorption, as well as photocurrent density of 3.04 mA/cm2 (at 1 V vs Ag/AgCl), compared to one arm Ag2S nanorods with minimum charge transfer resistance at the semiconducting electrode/electrolyte interface. The improved photocurrent density of four arm Ag2S zig-zag nanorods electrode was attributed to increased optical trapping and hence, effective absorption of light due to its wavy structure. The theoretical simulations based on rigorous coupled wave analysis were performed to understand the light absorption mechanism for the zig-zag Ag2S nanorods structures. This work provides a simple and effective approach towards the development of an efficient PEC electrode by tuning the morphology of nanostructured materials.

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl62-/IrCl63- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO2 nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

In this work, we investigate the properties of manganese dioxide (MnO2) and graphene oxide (GO) to develop a new photoelectrochemical (PEC) sensor-based MnO2-GO heterojunction on stainless steel electrode for chemical detection of copper (Cu(II)) cation. During the PEC detection process and using chronoamperometry, the photocurrent density increases gradually with the addition of incremental Cu2+ concentration, achieving a relatively low detection limit (0.9120 µM) and a linear interval range from 0.01 to 110 µM with high selectivity and stability under neutral aqueous solution (pH 7.00). In addition, the PEC change of electrode state in presence of Cu(II) cation requires the oxidized state of MnO2-GO heterojunction; this can be reached by applying anodic potential to the electrode (E = + 0.8 V). Thus, a switchable, low-cost, regenerative, and sensitive PEC sensor based on the change of MnO2-GO heterojunction surface state permits the selective detection of copper.

Heterostructures based on new advanced materials offer a cornerstone for future optoelectronic devices with improved photoelectric performance. Band alignment is crucial for understanding the mechanism of charge carrier transportation and interface dynamics in heterostructures. Herein, we grew SnS2/Bi2X3 (X = Se, Te) van der Waals heterostructures by combining physical vapor deposition with chemical vapor deposition. The band alignment, measured by high-resolution X-ray photoelectron spectroscopy, suggested the successful design of type-I SnS2/Bi2Se3 and type-II SnS2/Bi2Te3 heterostructures. The SnS2/Bi2X3 heterostructure greatly improved the photoelectric response of a photoelectrochemical-type photodetector. The photocurrent densities in the type-I SnS2/Bi2Se3 and type-II SnS2/Bi2Te3 heterostructure-based devices were more than one order of magnitude higher than those of SnS2, Bi2Se3, and Bi2Te3. The improved photoelectric properties of the SnS2/Bi2X3 heterostructures can be explained as follows: (i) the photoexcited electrons and holes are effectively separated in the heterostructures; (ii) the charge-transfer efficiency and carrier density at the interface between the SnS2/Bi2X3 heterostructures and the electrolyte are greatly improved; (iii) the formed heterostructures expand the light absorption range. The photoelectric performance was further enhanced by efficient light trapping in the upright SnS2. The photoelectric response is higher in the type-I SnS2/Bi2Se3 heterostructure than in the type-II SnS2/Bi2Te3 heterostructure due to more efficient charge transportation at the type-I SnS2/Bi2Se3 heterostructure/electrolyte interface. These results suggest that suitable type-I and type-II heterostructures can be developed for high-performance photodetectors and other optoelectronic devices.

Direct photoelectrochemical (PEC) water splitting is a promising solution for solar energy conversion; however, there is a pressing bottleneck to address the intrinsic charge transport for the enhancement of PEC performance. Herein, a versatile coupling strategy was developed to engineer atomically dispersed Ni-N4 sites coordinated with an axial direction oxygen atom (Ni-N4-O) incorporated between oxygen evolution cocatalyst (OEC) and semiconductor photoanode, boosting the photogenerated electron-hole separation and thus improving PEC activity. This state-of-the-art OEC/Ni-N4-O/BiVO4 photoanode exhibits a record high photocurrent density of 6.0 mA cm-2 at 1.23 V versus reversible hydrogen electrode (vs RHE), over approximately 3.97 times larger than that of BiVO4, achieving outstanding long-term photostability. From X-ray absorption fine structure analysis and density functional theory calculations, the enhanced PEC performance is attributed to the construction of single-atomic Ni-N4-O moiety in OEC/BiVO4, facilitating the holes transfer, decreasing the free energy barriers, and accelerating the reaction kinetics. This work enables us to develop an effective pathway to design and fabricate efficient and stable photoanodes for feasible PEC water splitting application.

It is challenging to realize a visible-blind infrared photodetector as the materials that absorb infrared light also absorb visible light. Here, we report the synthesis of IrP2 nanoparticle-embedded few-layer graphene by one-step solid-state pyrolysis and its application in visible-blind infrared sensing. A linear photodetector device was fabricated by drop casting IrP2 nanoparticle-embedded few-layer graphene onto a flexible PET substrate with two gold electrodes separated by ∼16 μm. The photoconductive gain was found to be as high as ∼145% with response and decay times of ∼0.4 and ∼2.8 s, respectively, under 1550 nm irradiation of 800 mW cm-2. The room-temperature responsivity was ∼1.81 A W-1 at 80 mW cm-2 and ∼0.54 A W-1 at a high incident power of ∼2200 mW cm-2 under a bias of 1 V. Interestingly, the device showed response even in the long-wavelength infrared region, but no response was found under visible light. The embedded IrP2 nanoparticles act as trap centers inducing photogating in the device, and the average trap state energy was estimated to be ∼16.5 ± 1.5 meV from the temperature-dependent photocurrent studies. The device was found to be immune to air exposure and bending, suggestive of use a a wearable sensor.

Photovoltaic windows that can be switched between transparent and energy harvesting mode can be realized by using ultrathin solar absorbers embedded in an optical nanocavity. In the present work, we use a 5 nm thick amorphous germanium absorber integrated in a magnesium-based thin film optical cavity, which switches from an absorptive to a transparent state due to hydrogen absorption. We analyze the influence of the mirror layer thickness on the light absorption, photocurrent generation, and transmission as well as color neutrality of the device. The optical properties are studied by 1-D transfer-matrix method by changing Mg thickness between 0 and 100 nm, then compared to the experimental results of fabricated devices. When the thickness of Mg increases, the switchable average transparency varies between 25% and 0%, while the power conversion efficiency rises up to 2.3%. The applicability of the device is tested by modeling the annual power generation in realistic scenarios. The influence of the cardinal orientation and the seasons on the switchable photovoltaic window implemented in a building facade with the abovementioned parameters is analyzed for different switching scenarios.

Using vacuum evaporation and magnetron sputtering technology, with copper phthalocyanine (CuPc) as the active layer, a vertical organic thin-film transistor (VOTFT) with a structure of Cu/CuPc/Al/CuPc/Cu was fabricated. The static photoelectric and dynamic photoelectric characteristics of the VOTFT were experimented, and its photocurrent multiplication mechanism was analyzed. The results of the static electrical characteristics experiment show that the base Al and the adjacent two layers of CuPc form good Schottky contact and the output characteristics show unsaturated characteristics. Illumination lowers the Schottky barrier between the active layer and the base, which is conducive to carrier transport. A dynamic light signal with a frequency of 200 Hz is applied to the VOTFT; according to the experiment, its turn-on time ton = 1.5 ms, and its turn-off time toff = 1.8 ms, due to its shorter conductive channel and smaller junction capacitance.

In this work, UV light activated multi-cycle photoelectric properties of TiO2 and CdS/TiO2 films in formaldehyde were researched. TiO2 film was prepared by screen printing, CdS/TiO2 compounded film was synthesized by SILAR method. XRD and FE-SEM was used to characterize the TiO2 and CdS/TiO2 samples. Multi-cycle photoelectric properties of TiO2 and CdS/TiO2 with uv light on and off were evaluated by testing the photocurrent. On one hand, under the same bias voltage, CdS/TiO 2showed a higher photocurrent than that by TiO2. The reason for this result should be ascribed to the compounded structure in CdS/TiO2, with which the separation and transfer of photogenerated electron-hole pairs could be improved. On the other hand, with the testing cycle number increased, the photocurrent amplitudes of TiO2 and CdS/TiO2 increased. These results suggested that the time to reach a stable photocurrent value for TiO2 and CdS/TiO2 is much longer than one cycle time (300 S). To illustrate the increased photocurrent amplitude value cycle by cycle, the photocurrent of CdS/TiO2 to a much longer time (more than 4000 seconds) was also tested. To explain these results, corresponding possible illustrations were presented.

Polarized light can provide significant information about objects, and can be used as information carrier in communication systems through artificial modulation. However, traditional polarized light detection systems integrate polarizers and various functional circuits in addition to detectors, and are supplemented by complex encoding and decoding algorithms. Although the in-plane anisotropy of low-dimensional materials can be utilized to manufacture polarization-sensitive photodetectors without polarizers, the low anisotropic photocurrent ratio makes it impossible to realize digital output of polarized information. In this study, we propose an integrated polarization-sensitive amplification system by introducing a nanowire polarized photodetector and organic semiconductor transistors, which can boost the polarization sensitivity from 1.24 to 375. Especially, integrated systems are universal in that the systems can increase the anisotropic photocurrent ratio of any low-dimensional material corresponding to the polarized light. Consequently, a simple digital polarized light communication system can be realized based on this integrated system, which achieves certain information disguising and confidentiality effects., Though low-dimensional (LD) materials are attractive for polarization-type photodetectors, their low anisotropic photocurrent rate hinders widespread application. Here, the authors report an amplification system for enhanced photocurrent in polarization-sensitive LD material-based photodetectors.

Perovskite/silicon tandem solar cell (TSC) is a promising strategy to receive high power conversion efficiency (PCE), exceeding the Shockley-Queisser (S-Q) limit of single junction SC. However, the reflected light still dominates the loss of photocurrent, happened at the surface and interface of two sub cells. Here, a suitable spacer layer is accepted to manage the light harvesting in the four-terminal (4T) perovskite/silicon TSC. The influence of microstructure, refractive index and thickness of spacer layer on the photocurrent has been systematically studied by the photoelectric simulation. And then the designed spacer layer is a double-side textured structure with a refractive index (RI) of 1.4 in 4 μm thickness, receiving the best light-harvesting to being the largest photocurrent. Furthermore, the relationship of light-absorption between perovskite and silicon SCs is investigated as a function of bandgap and thickness of perovskite, yielding the optimal photocurrent of 21.65 mA/cm2 and 19.71 mA/cm2 for top and bottom cells, respectively. Finally, four types of industrialized silicon SCs are integrated into the TSCs to study the photoelectric performance in detail, predicting the theoretical limitation PCE of 33.52%. Therefore, this simulated study provides a theoretical guide for the optical design of high-efficiency 4T TSCs.

Limited light absorption and acute charge recombination on the surface of ZnO nanostructure-based photoelectrodes are the basic challenges to be grappled with for better photoelectrochemical performance. Here, in this study, we attempted to fabricate a methodical and sturdy nano-heterojunction photoelectrode by anchoring chemically stable spinel ferrite (MgFe2O4) nanoparticles (NPs) on ZnO nanorod (NR) arrays. The magnetic ferrites with a low band gap improve the ability of nano-heterojunction photoelectrodes to harvest solar energy in the visible region which further enhances the photocurrent density and photoelectrochemical conversion efficiency (PCE), respectively, when compared to pure ZnO NR. The optimized sample MFZ_3 h exhibited the most intense photocurrent density 0.54 mA/cm2 among all the samples. The band alignment at the MgFe2O4/ZnO nano-heterojunction enables significant charge transfer and separation properties. The photoelectrode composed of hybrid nanostructure provides long term durability. This work denotes an easy yet fruitful strategy of developing cost-effective earth-abundant magnetite-based heterojunction photoelectrodes that act as an effective photocatalyst for photoelectrochemical water splitting (PEC) application.

Vertical Si nanowire (NW) arrays are a promising photoanode material in the photoelectrochemical (PEC) water splitting field because of their highly efficient light absorption capability and large surface areas for PEC reactions. However, Si NW arrays always suffer from high overpotential, low photocurrent density, and low applied bias photon-to-current efficiency (ABPE) due to their low surface catalytic activity and intense charge recombination. Here, we report an efficient oxygen evolution cocatalyst of optically transparent, mesoporous ultrathin (2.47 nm thick) In2O3 nanosheets, which are coupled on the top of Si NW arrays. Combined with a conformal TiO2 thin film as an intermediate protective layer, this Si NW/TiO2/In2O3 (2.47 nm) heterostructured photoanode exhibited an extremely low onset potential of 0.6 V vs reversible hydrogen electrode (RHE). The Si NW/TiO2/In2O3 (2.47 nm) photoanode also showed a high photocurrent density of 27 mA cm-2 at 1.23 V vs RHE, more than 1 order of magnitude higher than that of the Si NW/TiO2 photoanodes. This improvement in solar water splitting performance was attributed to the significantly promoted charge injection efficiency as a result of the In2O3 nanosheet coupling. This work presents a promising pathway for developing efficient Si-based photoanodes by coupling ultrathin 2D cocatalysts.

A UV detector based on the zinc oxide (ZnO) thin film was fabricated by using sol–gel process with zinc acetate dehydrate (Zn(CH3COO)22H2O) as a precursor. The hexagonal crystal structure of the ZnO sample was revealed by X–ray diffraction patterns (XRD). Field emission scanning electron microscopy (FESEM) results indicated that the ZnO film’s surface is smooth and uniform, having grain size of about 50 nm. Photoluminescence spectroscopy (PL) revealed UV light and broadband emissions, which were attributed to near band–edge (NBE) and deep level–edge emissions (DLE), respectively. The metal–semiconductor–metal (MSM) device based on the fabricated ZnO film showed a sensitivity of 43% upon exposure light (0.66 μW/cm2) at 5 V and a response peak of 4.3 A/W upon exposure to UV light, respectively. The UV detector showed excellent stability with time and a strong photocurrent response under 380 nm light. These results show a low-cost method of fabricating a high-performance ZnO MSM UV photodetector with a quick response, fast recovery, and high responsivity.

III–V compound nanowires have electrical and optical properties suitable for a wide range of applications, including photovoltaics and photodetectors. Furthermore, their elastic nature allows the use of strain engineering to enhance their performance. Here we have investigated the effect of mechanical strain on the photocurrent and the electrical properties of single GaAs nanowires with radial p-i-n junctions, using a nanoprobing setup. A uniaxial tensile strain of 3% resulted in an increase in photocurrent by more than a factor of 4 during NIR illumination. This effect is attributed to a decrease of 0.2 eV in nanowire bandgap energy, revealed by analysis of the current–voltage characteristics as a function of strain. This analysis also shows how other properties are affected by the strain, including the nanowire resistance. Furthermore, electron-beam-induced current maps show that the charge collection efficiency within the nanowire is unaffected by strain measured up to 0.9%.