Photovoltage Generation Research Articles

Photovoltage Generation at p-Type Semiconducting Polymer/Electrolyte Interfaces

Photovoltage Generation at p-Type Semiconducting Polymer/Electrolyte Interfaces

Wireless pulsed nanophotoelectrochemical cell for the ultrafast degradation of organic pollutants

An urgent demand exists for advanced-technologies to efficiently remove persistent organic pollutants from water, while minimizing energy consumption. Here, we introduce an innovative wireless nanophotoelectrochemical (nPEC) cell using pulsed light for the ultrafast degradation/mineralization of organic pollutants. The nPEC cell comprises a nanostructured Si-pn photodiode that monolithically integrates: (i) a Si-n/Au nanowire-based-photocathode for effective light absorption and photovoltage generation, and (ii) a Si-p/mesoporous-NiPt photoanode serving as catalyst to wirelessly amplify the sulfate radical production by low-intensity light without any bias voltage. The efficacy of the nPEC cell was shown by ultrafast degradation (>99 %) and mineralization (>98 %) of three emerging pollutants (tetracycline, levofloxacin and anatoxin-A). Notably, reaction kinetics were boosted by more than one order of magnitude when exposed to light intensities ca. 5-fold lower than sunlight. Remarkably, pulsed light beams in the 100–500 Hz range provided an additional enhancement in the degradation/mineralization efficiencies, reducing energy-input by half, while enhancing the catalyst's oxidation state and durability.

Ultrafast, self-powered monolithic pressure sensing technology induced by piezo-pyrophototronics

Pressure sensing is a pivotal aspect of modern technology that has been extensively explored, but still presents significant challenges. This work introduces an innovative ultrafast self-powered pressure sensing technology by harnessing the lateral photovoltaic effect (LPE) in a CIGS heterojunction via piezo-pyrophototronics. By utilizing non-uniform irradiation, a surface gradient of charge carriers is induced, resulting in the generation of a lateral photovoltage (LPV) that contributes to the excellent self-powered characteristic of the sensor. This gradient is then coupled with external pressure through the piezo-phototronic effect, enabling the sensing capability of the device. The enhancement of LPV, which is dependent on the irradiation position, power and wavelength, exhibits an extremely linear relationship with the applied external pressure ranging from 0 to 2.0 MPa. Besides, the pyroelectric field generated by transient temperature variations introduces an additional modulation to the pressure sensing effect, leading to an impressive pressure sensitivity of 64.25 mV/MPa, and a rapid response speed of 7.8/8.9 μs. This work highlights the benefits of the lateral photovoltaic effect, piezo-phototronic effect, and pyroelectric modulation in developing highly sensitive and ultrafast self-powered pressure sensors.

Interface-resolved photovoltage generation dynamics and band structure evolution in a PbS quantum dot solar cell.

For directed development of solar cells using nanomaterials such as quantum dots, there is a need to understand the device function in detail. Understanding where photovoltage is generated in a device and where energy losses occur is a key aspect of this, and development of methods which can provide this information is needed. We have previously shown that time-resolved photoelectron spectroscopy of core levels can be used to follow the photovoltage dynamics at a specific interface of a lead sulfide quantum dot solar cell. Here, we use the method's selectivity and sample design to investigate the photovoltage generation in different parts of this solar cell and determine how the different layers (including the absorber layer thickness) contribute to charge separation. We show that all layers contribute to photovoltage generation and that a gold contact deposited on the quantum dots is necessary for full photovoltage generation and slow charge recombination. By combining the information obtained, we are able to experimentally follow the time evolution of the solar cell band structure during the charge separation process. Furthermore, we can identify which specific layers need to be optimized for an overall improvement of quantum dot cells. In the future, this methodology can be applied to other types of devices to provide insights into photovoltage generation mechanisms.

A photoelectrocatalytic system as a reaction platform for selective radical-radical coupling.

The selection of electrode material is a critical factor that determines the selectivity of electrochemical organic reactions. However, the fundamental principles governing this relationship are still largely unexplored. Herein, we demonstrate a photoelectrocatalytic (PEC) system as a promising reaction platform for the selective radical-radical coupling reaction owing to the inherent charge-transfer properties of photoelectrocatalysis. As a model reaction, the radical trifluoromethylation of arenes is shown on hematite photoanodes without employing molecular catalysts. The PEC platform exhibited superior mono- to bis-trifluoromethylated product selectivity compared to conventional electrochemical methods utilizing conducting anodes. Electrochemical and density functional theory (DFT) computational studies revealed that controlling the kinetics of anodic oxidation of aromatic substrates is essential for increasing reaction selectivity. Only the PEC configuration could generate sufficiently high-energy charge carriers with controlled kinetics due to the generation of photovoltage and charge-carrier recombination, which are characteristic features of semiconductor photoelectrodes. This study opens a novel approach towards selective electrochemical organic reactions through understanding the intrinsic physicochemical properties of semiconducting materials.

Disorder-induced bulk photovoltaic effect in a centrosymmetric van der Waals material

Sunlight is widely seen as one of the most abundant forms of renewable energy, with photovoltaic cells based on pn junctions being the most commonly used platform attempting to harness it. Unlike in conventional photovoltaic cells, the bulk photovoltaic effect (BPVE) allows for the generation of photocurrent and photovoltage in a single material without the need to engineer a pn junction and create a built-in electric field, thus offering a solution that can potentially exceed the Shockley–Queisser efficiency limit. However, it requires a material with no inversion symmetry and is therefore absent in centrosymmetric materials. Here, we demonstrate that breaking the inversion symmetry by structural disorder can induce BPVE in ultrathin PtSe2, a centrosymmetric semiconducting van der Waals material. Homogenous illumination of defective PtSe2 by linearly and circularly polarized light results in a photoresponse termed as linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE), which is mostly absent in the pristine crystal. First-principles calculations reveal that LPGE originates from Se vacancies that act as asymmetric scattering centers for the photo-generated electron-hole pairs. Our work emphasizes the importance of defects to induce photovoltaic functionality in centrosymmetric materials and shows how the range of materials suitable for light sensing and energy-harvesting applications can be extended.

Engineering the directionality of hot carrier tunneling in plasmonic tunneling structures

Tunneling metal–insulator–metal (MIM) junctions can exhibit an open-circuit photovoltage (OCPV) response under illumination that may be useful for photodetection. One mechanism for photovoltage generation is hot carrier tunneling, in which photoexcited carriers generate a net photocurrent that must be balanced by a drift current in the open-circuit configuration. We present experiments in electromigrated planar MIM structures, designed with asymmetric plasmonic properties using Au and Pt electrodes. Decay of optically excited local plasmonic modes preferentially creates hot carriers on the Au side of the junction, leading to a clear preferred directionality of the hot electron photocurrent and hence a preferred polarity of the resulting OCPV. In contrast, in an ensemble of symmetric devices constructed from only Au, polarity of the OCPV has no preferred direction.

Impact of charge-compensated Fe and Nb co-substitution on BaTiO3: Bandgap and grain size reduction and enhanced bulk photovoltaic power of Al/BFNT/Ag solar cell

The generation of above bandgap photovoltage using bulk ferroelectric materials has become a subject of great interest, however, their photocurrent density is limited by a broad bandgap and poor conductivity. To overcome this limitation, we replaced aliovalent metal ions (Fe3+and Nb5+)at the B-site of robust ferroelectric BaTiO3and fabricated an Al/BaTi1-2xFexNbxO3/Ag photovoltaic device. Both the experimental and the theoretical studies showed that bandgap was lowered to ∼2.55 eV and hence absorption of wide energy range of the solar spectrum was attained. An apt top electrode, reduced bandgap and domain size resulted in greater photocurrent density of 1.46 μA/cm2 and photovoltage of 8.31 V for Al/0.075BFNT/Ag solar cell in unpoled condition. This research suggest that reduced band gap, mixed structural phases and nano-sized domains suffices greatest PV power output while the large polarization and poling are not necessary prerequisites.

Accelerating defect analysis of solar cells via machine learning of the modulated transient photovoltage

Fast and non-destructive analysis of material defect is a crucial demand for semiconductor devices. Herein, we are devoted to exploring a solar-cell defect analysis method based on machine learning of the modulated transient photovoltage (m-TPV) measurement. The perturbation photovoltage generation and decay mechanism of the solar cell is firstly clarified for this study. High-throughput electrical transient simulations are further carried out to establish a database containing millions of m-TPV curves. This database is subsequently used to train an artificial neural network to correlate the m-TPV and defect properties of the perovskite solar cell. A Back Propagation neural network has been screened out and applied to provide a multiple parameter defect analysis of the cell. This analysis reveals that in a practical solar cell, compared to the defect density the charge capturing cross-section plays a more critical role in influencing the charge recombination properties. We believe this defect analysis approach will play a more important and diverse role for solar cell studies.

An integrated n-Si/BiVO4 photoelectrode with an interfacial bi-layer for unassisted solar water splitting.

Integrated n-Si/BiVO4 is one of the most promising candidates for unbiased photoelectrochemical water splitting. However, a direct connection between n-Si and BiVO4 will not attain overall water splitting due to the small band offset as well as the interfacial defects at the n-Si/BiVO4 interface that severely impede carrier separation and transport, limiting the photovoltage generation. This paper describes the design and fabrication of an integrated n-Si/BiVO4 device with enhanced photovoltage extracted from the interfacial bi-layer for unassisted water splitting. An Al2O3/indium tin oxide (ITO) interfacial bi-layer was inserted at the n-Si/BiVO4 interface, which promotes the interfacial carrier transport by enlarging the band offset while healing interfacial defects. When coupled to a separate cathode for hydrogen evolution, spontaneous water splitting could be realized with this n-Si/Al2O3/ITO/BiVO4 tandem anode, with an average solar-to-hydrogen (STH) efficiency of 0.62% for over 1000 hours.

Recent Advances in Rolling 2D TMDs Nanosheets into 1D TMDs Nanotubes/Nanoscrolls.

Transition metal dichalcogenides (TMDs) van der Waals (vdW) 1D heterostructures are recently synthesized from 2D nanosheets, which open up new opportunities for potential applications in electronic and optoelectronic devices. The most recent and promising strategies in regards to forming 1D TMDs nanotubes (NTs) or nanoscrolls (NSs) in this review article as well as their heterostructures that are produced from 2D TMDs are summarized. In order to improve the functionality of ultrathin 1D TMDs that are coaxially combined with boron nitride nanotubes and single-walled carbon nanotubes. 1D heterostructured devices perform better than 2D TMD nanosheets when the two devices are compared. The photovoltaic effect in WS2 or MoS2 NTs without a junction may exceed the Shockley-Queisser limit for the above-band-gap photovoltage generation. Photoelectrochemical hydrogen evolution is accelerated when monolayer WS2 or MoS2 NSs are incorporated into a heterojunction. In addition, the photovoltaic performance of the WSe2 /MoS2 NSs junction is superior to that of the performance of MoS2 NSs. The summary of the current research about 1D TMDs can be used in a variety of ways, which assists in the development of new types of nanoscale optoelectronic devices. Finally, it also summarizes the current challenges and prospects.

Temperature Coefficients of Photoelectrochemistry: A Case Study of Hematite-Base Water Oxidation

The behavior of photoelectrochemical (PEC) water splitting under temperature is fundamentally important yet less investigated, because of the lack of a suitable analysis method to decouple and quantify the contributions of each elemental step (charge carrier generation and transport, photovoltage generation, and surface catalysis) to overall PEC efficiency. Herein, by using hematite as the prototype electrode, its performance dependence on temperature is well resolved, and the efficiency loss/gain of each elemental step during PEC water splitting is also quantitatively analyzed. The deteriorated temperature coefficient of open-circuit voltage on hematite for water oxidation (from −6.50 mV K–1 to −3.50 mV K–1) than sulfite oxidation (from −3.52 mV K–1 to −2.60 mV K–1) reveals that slow water oxidation kinetics retards photovoltage generation. The proposed analysis approach and fundamental understanding of the influences of each elemental step on the temperature coefficients would provide valuable insight into the future development and implementation of PEC devices.

Transparent photo-electrochromic capacitive windows with a Bi-dopant redox ionic liquids

A transparent self-powered photoelectrochromic capacitive window (PECW) was developed by combining a high color contrast electrochromic polymer, a transparent energy-harvesting layer, and a redox ionic liquid (RIL). The RILs were made of a 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) moiety connected to an imidazolium cation via a pyrimidine bridge, which was counterbalanced with a bis(trifluoromethanesulfonyl)imide (TFSI) anion. The potential at the PECW electrodes was precisely controlled by the RILs to match the redox reaction for electrochromic switching and photovoltage generation that triggers autonomous photocoloration. The capacitance of the PECW was enhanced by using a bi-dopant system of RILs, which provided p- and n-doping by TFSI and the RIL cation. The PECW with a photosensitizing dye showed high transparency (>90 %) for allowing a high electrochromic color contrast (>85 %) with a long electrochromic cyclability over 4,000 cycles of the window. The optimized PECW recorded the highest photocoloration efficiency (65cm2W−1min−1) among known photo electrochromic windows. The photocharged energy in the PECW could be transferred to power other electronics, such as electrochromic windows or LEDs. With the new RILs, PECWs provide an integrated model that consists of electrochromic and energy-harvesting functions in one device. Thus, the working principle of the PECW can be widely applied to energy-saving smart windows and multifunctional electrochemical devices.

Gate-controlled polarization-resolving mid-infrared detection at metal–graphene junctions

The ability to resolve the polarization of light with on-chip devices represents an urgent problem in optoelectronics. The detectors with polarization resolution demonstrated so far mostly require multiple oriented detectors or movable external polarizers. Here, we experimentally demonstrate the feasibility to resolve the polarization of mid-infrared light with a single chemical-vapor-deposited graphene-channel device with dissimilar metal contacts. This possibility stems from an unusual dependence of photoresponse at graphene–metal junctions on gate voltage and polarization angle. Namely, there exist certain gate voltages providing the polarization-insensitive signal; operation at these voltages can be used for power calibration of the detector. At other gate voltages, the detector features very strong polarization sensitivity, with the ratio of signals for two orthogonal polarizations reaching ∼10. Operation at these voltages can provide information about polarization angles, after the power calibration. We show that such unusual gate- and polarization-dependence of photosignal can appear upon competition of isotropic and anisotropic photovoltage generation pathways and discuss the possible physical candidates.

Effect of defect-rich Co-CeOx OER cocatalyst on the photocarrier dynamics and electronic structure of Sb-doped TiO2 nanorods photoanode

This work demonstrates the in-depth mechanism of enhanced photoelectrochemical (PEC) water oxidation of Sb-doped rutile TiO2 nanorods (NRs) photoanode coupled with oxygen vacancy defect-rich Co-doped CeOx (Co-CeOx) oxygen evolution reaction (OER) cocatalyst. The defect-rich Co-CeOx cocatalyst modification improves the conductivity, light absorption, charge transfer efficiency, and surface photovoltage generation of the Co-CeOx/Sb-TiO2 hybrid NRs photoanode. The Co-CeOx cocatalyst also serves as the surface passivating overlayer for the Sb-TiO2 photoanode, which suppresses the surface states mediated recombination of electron-hole pairs in the NRs. The PEC studies further indicate that Co-CeOx cocatalyst induces remarkably large band bending at the semiconductor/electrolyte interface and shortens the carrier diffusion length and depletion layer width, facilitating the rapid separation and transportation of the photocarriers for the surface PEC reactions. The experimental and theoretical studies confirm that the Co-doping in CeOx cocatalyst enhances the surface oxygen vacancy defects, which provides active catalytic sites for OH− adsorption and charge transportation for enhanced OER kinetics. The density functional theory (DFT) calculations demonstrate a higher conductivity of the Co-CeOx cocatalyst, advantageous for rapid charge transfer capability during PEC reactions. The synergy between all these merits helps the optimized Co-CeOx/Sb-TiO2 photoanode to deliver a maximum photocurrent density of 1.41 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE) and an ultra-low turn on potential (Von) of 0.1 VRHE under AM 1.5G solar illumination compared to the Sb-TiO2 NRs (0.96 mA cm−2 at 1.23 VRHE and Von = 0.42 VRHE). This work demonstrates the design of an efficient defect-rich cocatalyst modified photoanode for solar energy harvesting.

Optical Pumping and Electrical Injection of a 3.6 μ m Interband Cascade Laser

We report results from optical pumping of an interband cascade laser (ICL), both with and without a monolayer graphene top contact. Optical pumping wavelengths from a Nd:YAG-pumped optical parametric oscillator were tuned from 1800 nm to 1950 nm. Lasing was demonstrated at a maximum temperature of 240 K for the ICL without graphene and 260 K for the ICL with graphene. The ICL emission wavelength was <inline-formula> <tex-math notation="LaTeX">$3.6~\mu \text{m}$ </tex-math></inline-formula> at 200 K. Overall, threshold intensities were lower and slope efficiencies were higher for an optically-pumped ICL with a monolayer graphene top contact when compared to one without a graphene monolayer. Voltage-current analysis of a narrow-ridge, electrically-injected ICL from the same epitaxial growth evidenced differential resistance at threshold associated with quasi-Fermi-level pinning. Differential resistance values were approximately 10 <inline-formula> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> at low temperature and <inline-formula> <tex-math notation="LaTeX">$2~\Omega $ </tex-math></inline-formula> at 260 K, with corresponding pinning factors that correlate well with the slope efficiencies of both optically-pumped and electrically-injected ICL devices. Optical pumping is understood to result in carrier diffusion and generation of photovoltage and excess electrons to achieve lasing conditions.

Electrostatics of optical rectification in metallic particles

An electrostatic theory of optical rectification is presented here, namely, the static photovoltage or photocurrent generation under light illumination, in metallic particles. The hydrodynamical model for the charge carriers in the metals is employed. By solving the hydrodynamic equation and the Maxwell equation perturbatively, the second-order susceptibility is analytically obtained, from which the optical rectification is explained. Electrostatic potential problems involved in the optical rectification under the local response approximation are formulated in arbitrary geometries and then are solved for simple geometries of metallic planar interfaces, slabs, cylinders, and spheres. The photovoltage and photocurrent spectra, their incident-angle dependence, and the electrostatic potential distribution for an incident plane wave light are demonstrated and discussed in the context of plasmonic resonances.

Voltage generation by photosystem I complexes immobilized onto a millipore filter under continuous illumination

Here we measured the long-term photovoltage (ΔV) generation by photosynthetic pigment-protein complex of photosystem I (PS I) from cyanobacterium Synechocystis sp. PCC 6803 directly. Prior to measurements PS I was immobilized onto a Millipore membrane filter (MF) and sandwiched between two semiconductor indium tin oxide (ITO) electrodes. The ability of ITO|PS I-MF|ITO system to generate a stable ΔV over a 50 min period of irradiation by white light was detected in the presence of cytochrome c6, methyl viologen and disaccharide trehalose. Re-injection of a fresh buffer into ITO|PS I-MF|ITO preserves the immobilized PS I functionality after six months of storage at 23 °C in the dark. The developed system represents a simple bio-hybrid solar converter to obtain voltage signals under long-term, periodic illumination. This approach can also be used to couple photoactive (light-driven enzymes) and catalytic modules to generate H2 under the steady-state conditions.

Ultrasensitive Self-Powered Position-Sensitive Detector Based on n-3C-SiC/p-Si Heterojunctions

Cubic silicon carbide (3C-SiC) on silicon (Si) is an excellent platform for developing sensors that can function in a harsh environment due to its superior mechanical, electrical, chemical, optoelectronic properties and low wafer cost. Here, we report a position-sensitive detector (PSD) based on the heterojunction formed between n-type SiC and p-type Si. The PSD utilizes the lateral photovoltaic effect (LPE) with a linear dependence of LPE on laser spot positions. The position sensitivity is found to be 554.82 mV/mm at zero bias conditions under an illumination of 200 μW (637 nm), which is among the most sensitive LPE-based PSDs to date. The generation of the lateral photovoltage (LPV) under light illumination is investigated by examining the band diagram of the 3C-SiC/Si heterojunction. The influence of the illumination intensity and wavelength on the position sensitivity is also explained. This work demonstrates a potential application for ultrasensitive, self-powered optoelectronic sensing of the n-3C-SiC/p-Si platform.

Engineering Cu2ZnSnS4 grain boundaries for enhanced photovoltage generation at the Cu2ZnSnS4/TiO2 heterojunction: A nanoscale investigation using Kelvin probe force microscopy

Over the past several years, kesterite Cu2ZnSnS4 (CZTS) absorber has been investigated comprehensively; however, the performance is still hampered by a large open-circuit voltage deficit associated with CZTS bulk defects and interface recombination. To overcome this trend, we report a facile approach to passivate both defect prone areas, i.e., bulk of CZTS and CZTS interface with a TiO2 buffer layer, simultaneously. The existence of oxygen ambient during TiO2 deposition has modulated the electrical properties of CZTS grain boundaries (GBs) not only inside the bulk but also at the surface of CZTS. The passivation of surface GBs is favorable for CZTS/TiO2 heterojunction electronic properties, whereas passivated bulk GBs improve the carrier transport inside the CZTS absorber. To directly probe the photovoltage generation at the CZTS/TiO2 heterojunction, Kelvin probe force microscopy is conducted in surface and junction modes. The acquired photovoltage map exhibits higher values at the GBs, which reveals an increment in downward band bending after oxygen diffusion inside the bulk of CZTS. In point of fact, the enhanced diffusion of oxygen accounts for the suppression of carrier recombination and reduction in dark current. Finally, current–voltage and capacitance–voltage measurements performed on the CZTS/TiO2 heterojunction further validate our outcomes. Our findings provide critical insight into the engineering of CZTS GBs to control electronic properties of CZTS and CZTS/TiO2 heterojunctions.