Photocurrent Sensitivity Research Articles

Shapes of polarization sensitive photocurrent pulses in Ag/Pd nanocomposite films: influence of firing temperature

The results of investigation of the shapes of polarization-sensitive photocurrent pulses in nanocomposite Ag/Pd films with different temperatures of firing are presented. Ag/Pd nanocomposite films were produced in accordance with thick film technology at temperatures T = 773 and 1013 K. Photocurrent excitation was carried out by femtosecond laser pulses. The obtained polarization photocurrent dependencies are characteristic for photocurrent due to photon drag and surface photogalvanic effects. It was shown that longitudinal photocurrent pulse duration in 773 K films (26 ns) is significantly longer than in 1013 K films (3 ns).

In situ incorporation of laser ablated PbS nanoparticles in CH3NH3PbI3 films by spin-dip coating and the subsequent effects on the planar junction CdS/CH3NH3PbI3 solar cells

This is the first report on in situ incorporation of lead sulfide (PbS) nanoparticles in methyl ammonium perovskite thin films (CH3NH3PbI3:PbS) by the two step spin-dip method and the successive effects on the photovoltaic properties of HTM free Glass/FTO/CdS/CH3NH3PbI3 planar junctions. PbS nanocolloids of different concentrations were prepared by pulsed laser ablation of PbS target in isopropanol. CH3NH3PbI3:PbS thin films were synthesized by two step process of dipping spin coated PbI2 thin films in a dissolution of CH3NH3I in isopropanol containing PbS nanocolloids. Analysis on the structure, composition, morphology and optoelectronic properties of CH3NH3PbI3:PbS films with varying PbS nanoparticle concentrations were done and the results were compared with that of pristine CH3NH3PbI3 films. The CH3NH3PbI3:PbS films were of higher crystallinity with improved photocurrent sensitivity. The thin films were incorporated to HTM free p-n junction solar cells using chemical bath deposited CdS as window layer. The champion cell, Glass/FTO/CdS/CH3NH3PbI3:PbS showed Voc of 0.9 V, the highest value ever reported for HTM free p-n junction solar cell. Also, the device yielded Jsc of 7.76 mA/cm2, FF of 0.4 and a power conversion efficiency of 2.9%. The stability studies for 40 days evidenced that the champion device maintained the open circuit voltage unchanged.

Study of exciton-polaron interaction in pentacene field effect transistors using high sensitive photocurrent measurements

Luminescence quenching in the presence of polarons is one of the major challenges in organic light emitting devices. In this work, exciton quenching in the presence of polarons is studied using phase sensitive photocurrent measurements on pentacene field effect transistors. The enhancement of conduction in the organic field effect transistors on light illumination is studied using photocurrent spectral response measurements and corresponding optical simulations. The photocurrent is shown to be governed by the polaron mobility and the exciton quenching efficiency, both of which depend on the polaron density in the channel. Two models are proposed on the exciton dynamics in the presence of gate induced polarons in the transistor channel. The first model simulates the steady-state exciton concentration profile in the presence of exciton-polaron interaction. The second one is a three-dimensional steady state exciton-polaron interaction model, which supports the findings from the first model. It is shown that the excitons quench by transferring its energy to polarons, thereby promoting the latter to high energy states in the density of states manifold. The polarons move in the higher energy states with greater microscopic mobility before thermalizing, thereby leading to an enhancement of conduction. It is observed that for the present system, where charge carrier transport is by hopping, all polarons interact with excitons. This implies that for low mobility systems, the interaction is not limited to deep trapped polarons.

Broadband polarized photodetector based on p-BP/n-ReS2 heterojunction

Two-dimensional (2D) atomic crystals, such as graphene, black phosphorus (BP) and transition metal dichalcogenides (TMDCs) are attractive for use in optoelectronic devices, due to their unique crystal structures and optical absorption properties. In this study, we fabricated BP/ReS2 van der Waals (vdWs) heterojunction devices. The devices realized broadband photoresponse from visible to near infrared (NIR) (400–1800 nm) with stable and repeatable photoswitch characteristics, and the photoresponsivity reached 1.8 mA/W at 1550 nm. In addition, the polarization sensitive detection in the visible to NIR spectrum (532–1750 nm) was demonstrated, and the photodetector showed a highly polarization sensitive photocurrent with an anisotropy ratio as high as 6.44 at 1064 nm. Our study shows that van der Waals heterojunction is an effective way to realize the broadband polarization sensitive photodetection, which is of great significance to the realization and application of multi-functional devices based on 2D vdWs heterostructures.

Ultrafast Photo‐Response by Surface State‐Mediated Optical Transitions in Topological Insulator Bi2Te3 Nanowire

AbstractTopological insulators are a new class of materials with highly interesting optoelectronic properties such as strong light absorption, polarization‐dependent surface photocurrent, and topological phase transitions. In the present study, anomalous characteristics of the topological surface state (TSS) are discovered in a single‐crystalline Bi2Te3 nanowire (NW) through mediated optical transitions by utilizing an optical pump‐THz probe. As a result, ultra‐fast carrier recombination occurs owing to the presence of TSS, and a phonon frequency shift occurs due to enhanced electron–phonon interaction. In particular, the large optical absorption of the second TSS is closely related to high optical conductivity. The photoresponse for visible light in Bi2Te3 NW with TSS represents a remarkable improvement. The result indicates that the presence of (first and second) TSS and quantum well 2D electron gas effectively contributes to the significant improvement in photocurrent sensitivity by enhancement of the photocurrent generation. In summary, TSS‐assisted optical transitions affect the efficiency of the optoelectronic device.

Binary Self-Assembly of Conjugated Block Copolymers and Quantum Dots at the Air-Liquid Interface into Ordered Functional Nanoarrays.

Controlling the nanoscale morphology of conducting polymer/nanoparticle hybrid films is a highly desired but challenging task. Here, we report that such functional hybrid films with unprecedented structural order can be formed through the self-assembly of conjugated block copolymers and CdSe quantum dots at the air-water interface. The one-step assembly of quantum dots and block copolymers composed of polythiophene and polyethylene glycol (P3HT-b-PEG) at the fluidic interface generated a highly ordered assembly structure of P3HT nanowires and one-dimensional quantum dot arrays. Structure analyses revealed a unique self-assembly behavior and size dependency, which are distinct from the conventional self-assembly of coil-type polymers on solid substrates. Interestingly, hydrophobic quantum dots reside at the interface between P3HT and PEG domains without disrupting the P3HT packing structure, which is advantageous for the optoelectronic properties. Furthermore, large particles bridge the P3HT nanowires at both ends, while small particles decorate each P3HT/PEG interfaces, thus forming tight p-n junctions for a broad size range of nanoparticles. The nanoparticle-incorporated hybrid films showed more than an order of magnitude higher photocurrent and light sensitivity compared to polymer-only films, consistent with the assembly structure with close contact between the organic and inorganic semiconductors.

Generation of a Polarization Sensitive Photocurrent in a CuSe/Se Nanocomposite Thin Film

CuSe-based structures are widely used in various fields of photonics and optoelectronics. It has been shown for the first time that a photocurrent depending on the direction of the wave vector and polarization of the incident radiation can be excited in thin films consisting of amorphous Se and CuSe nanocrystallites. Films on glass substrates have been obtained by the successive thermal deposition of selenium and copper in vacuum at room temperature. The photocurrent has been excited by radiation of a femtosecond laser at a wavelength of 795 nm at room temperature. It has been found that the longitudinal photocurrent measured in the direction of the plane of incidence is maximal at p-polarization and vanishes at s-polarization. The transverse pho-tocurrent perpendicular to the plane of incidence is an odd function of the polarization angle and is absent at p- and s-polarizations. In both cases, the photocurrent is an odd function of the angle of incidence of light on the film surface. The results obtained are in qualitative agreement with the theory of generation of the surface photogalvanic effect.

An efficient wide range photodetector fabricated using a bilayer Bi2S3/SnS heterojunction thin film

This study presents a detailed optoelectronic investigation on the bilayer Bi2S3/SnS heterojunction photodetector. The Bi2S3 and SnS nanostructures were synthesized by an ultrasonication method and deposited in the form of thin films by a thermal evaporation system to fabricate the photodetectors. The performed optoelectrical measurements showed that the Bi2S3 and Bi2S3/SnS photodetectors are respectively n- and p-type electrical conductivity. This finding was further confirmed by current–voltage (I–V) analysis, Mott–Schottky plots and photoresponse measurements. According to the obtained gain and sensitivity values, the Bi2S3/SnS heterojunction provides a higher photon-to-electrical carrier conversion relative to the Bi2S3 photodetectord due to the enhanced photogeneration of electron–hole pairs and separation of the photoinduced charge carriers under the influence of the electric field build-in at the heterojunction’s interface. The heterojunction exhibited a remarkable photoresponse from 400 to 800 nm by giving the photocurrent gain (G), sensitivity (S), and external quantum efficiency (EQE) values of 8460%, 450% and 45%, respectively. The obtained results represent the Bi2S3/SnS heterojunction photodetector as a potential candidate for devising high-performance optoelectronic devices.

Mutated Channelrhodopsins with Increased Sodium and Calcium Permeability

(1) Background: After the discovery and application of Chlamydomonas reinhardtii channelrhodopsins, the optogenetic toolbox has been greatly expanded with engineered and newly discovered natural channelrhodopsins. However, channelrhodopsins of higher Ca2+ conductance or more specific ion permeability are in demand. (2) Methods: In this study, we mutated the conserved aspartate of the transmembrane helix 4 (TM4) within Chronos and PsChR and compared them with published ChR2 aspartate mutants. (3) Results: We found that the ChR2 D156H mutant (XXM) showed enhanced Na+ and Ca2+ conductance, which was not noticed before, while the D156C mutation (XXL) influenced the Na+ and Ca2+ conductance only slightly. The aspartate to histidine and cysteine mutations of Chronos and PsChR also influenced their photocurrent, ion permeability, kinetics, and light sensitivity. Most interestingly, PsChR D139H showed a much-improved photocurrent, compared to wild type, and even higher Na+ selectivity to H+ than XXM. PsChR D139H also showed a strongly enhanced Ca2+ conductance, more than two-fold that of the CatCh. (4) Conclusions: We found that mutating the aspartate of the TM4 influences the ion selectivity of channelrhodopsins. With the large photocurrent and enhanced Na+ selectivity and Ca2+ conductance, XXM and PsChR D139H are promising powerful optogenetic tools, especially for Ca2+ manipulation.

Anisotropic Photoresponse of the Ultrathin GeSe Nanoplates Grown by Rapid Physical Vapor Deposition.

Anisotropic materials, especially two-dimensional (2D) layered materials formed by van der Waals force (vdW) with low-symmetry, have become a scientific hot-spot because their electrical, optical, and thermoelectric properties are highly polarization dependent. The 2D GeSe, a typical anisotropic-layered orthorhombic structure and narrow bandgap (1.1-1.2 eV) semiconductor, potentially meets these demands. In this report, the ultrathin elongated hexagonal GeSe nanoplates were successfully synthesized by the rapid physical vapor deposition method developed here. The ultrathin elongated hexagonal GeSe nanoplates have a zigzag edge in the long edge and an armchair edge in the short edge. In addition, the typical Raman mode exhibited 90° periodic vibration, having its maximum intensity between the zigzag direction or the zigzag and armchair direction, indicating an anisotropic electron-phonon interaction. Furthermore, the field effect transistor devices based on the elongated hexagonal GeSe nanoplates were constructed and exhibited the p-type semiconducting behavior with a high photoresponse characteriscs. Finally, the polarized sensitive photocurrent was identified, further revealing the intrinsically anisotropy of the GeSe nanoplate. The results illustrated here may give a useful guidance to synthesize the 2D-layered anisotropic nanomaterials and further advance the development of the polarized photodetector.

Highly oriented CuSbS2 thin films by rapid thermal processing of pre-annealed Sb2S3-Cu layers for PV applications

Copper antimony sulfide (CuSbS2) is an emerging absorber material in solar cells due to its nearly ideal optoelectronic properties, earth abundancy and environmentally benign composed elements. In the present work, highly oriented CuSbS2 thin films have been prepared by annealing layered structures of chemical bath deposited Sb2S3 and thermal evaporated Cu, followed by rapid thermal processing (RTP). A systematic study was performed by varying the processing time. Crystalline structure, morphology, elemental composition, chemical state, optical and electrical properties of the films formed at different conditions were analyzed using various techniques. X-Ray diffraction analysis showed that the post RTP treatment resulted CuSbS2 thin films with preferential orientation along (301) planes perpendicular to the substrate surface, on glass as well as Mo coated glass substrates. Optical properties showed that CuSbS2 has a direct bandgap of 1.6 eV supported by photo-current sensitivity at different wavelengths, which is nearly the ideal bandgap for an absorber layer in solar cell.

Highly flexible and solution-processed organic photodiodes and their application to optical luminescent oxygen sensors

Abstract We present a solution-processed flexible organic photodiode (f-OPD) with a bulk heterojunction (BHJ) structure based on a blend of poly (3-hexylthiophene-2,5-diyl) and 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 (P3HT:PCBM). We used Cs2CO3-doped polyethyleneimine ethoxylated (d-PEIE) as the electron transport layer (ETL) material, which significantly improved the electron injection properties of the f-OPD. Compared with f-OPDs with conventional ETL materials such as Cs2CO3, the external quantum efficiency (EQE) of the d-PEIE-based f-OPD was highly improved. Analytical results showed that the d-PEIE reduced the work function of the cathode, thereby facilitating the efficiency of electron injection from the active layer (AL) to the cathode of the f-OPD. In addition, after 10,000 cycles of tensile bending at a bending radius of 5 mm, the normalized ID variation (ID/ID0) in the d-PEIE-based f-OPD remained above 90%, indicating an excellent device bending stability. Finally, f-OPD-based luminescent oxygen (O2) sensors were successfully fabricated consisting of a photoluminescent O2 sensing film, a light source, and an f-OPD. The O2 sensors based on d-PEIE-based f-OPDs showed the highest photocurrent and O2 sensitivity in relation to the O2 concentration compared with O2 sensors based on f-OPDs with conventional ETL materials.

Preparation and characterization of free-standing BiI3 single-crystal flakes for X-ray detection application

Free-standing BiI3 single-crystal flakes with a rhombohedral appearance and 10–100 µm in thickness were prepared by using physical vapor transport method. The (00l) (l = 3, 6, 9, 12) planes of the as-grown crystal present a distribution of layered structure and approximatively atomically smooth surface with 1.3–1.7 nm step height and roughness average 0.38–0.65 nm. Both planar and coplanar electrode configuration devices for X-ray detection were fabricated with the as-grown BiI3 single crystals and the dark resistivity 5.8–6.4 × 1011 Ω cm and 1.2–1.8 × 1011 Ω cm at room temperature were obtained, respectively. A low dark current or high resistivity for planar device is on account of carrier scattering from the I–Bi–I interlayer van der Waals bonding interface. A high net photocurrent and good sensitivity 1.22–1.36 × 104 µC/Gyair/cm2 under X-ray excitation were also obtained with planar device owing to the uniform electric field distribution and high charge collection efficiency.

Ultrasensitively photoelectronchemical determination of cysteine and coenzyme A with CdSe quantum dots-covered ZnO nanorods photoelectrode

A new photoelectrochemical system was fabricated by incorporating the cysteine or coenzyme A as electron donor into photoelectrochemical reaction of the nanostructured CdSe/ZnO photoelectrode to develop a photoelectrochemical method for the ultrasensitive determination of the cysteine and coenzyme A. The CdSe/ZnO photoelectrode was prepared by covering the CdSe quantum dots on the surface of ZnO nanorods arrays. The property of photoelectrode was investigated as photosensitive interface and electron acceptor, and its photoelectrochemical reaction was study with substrate. Under the 20 mW/cm2 410 nm visible light illuminations, the sensitive photocurrent response to the cysteine or coenzyme A was obtained at bias voltage 0 V. After the optimized experimental conditions, the photocurrent was proportional to the concentration of cysteine or the logarithm of coenzyme A concentration in the range of 1.00 × 10−2–20.0 μmol/L and 2.00 × 10−2–50.0 μmol/L, respectively. The detection sensitivity was 71.7 nA/μmol/L for cysteine. The detection limit was estimated to be 6.00 × 10−3 μmol/L (S/N = 3) and 1.00 × 10−2 μmol/L (S/N = 3) for cysteine and coenzyme A, respectively. The other amino acids or common coenzymes were not interfering with the determination of cysteine and coenzyme A. Compared with other methods for the determination of cysteine and coenzyme A, the proposed method exhibits a wide measurement range, high sensitivity, and low cost.

Surface Modification for High Photocurrent and pH Sensitivity in a Silicon-Based Light-Addressable Potentiometric Sensor

A device structure made from the Si substrate etched from the smooth surface by KOH wet etching is investigated for use as a light-addressable potentiometric sensor (LAPS). Stacked Si3N4/SiO2 layers were used as the sensing membrane on the top side, and a hard mask layer generated by KOH etching comprises the bottom side of the LAPS. A single-side polished P-type Si wafer with a thickness of $350~\mu \text{m}$ was reduced to $175~\mu \text{m}$ by KOH wet etching of the polished side. A high photocurrent was observed in an LAPS sample with increased surface roughness while under backside illumination. Additionally, the increased roughness of the sensing membrane on the top side generated by the unpolished Si surface in the same sample produced higher pH sensitivity than conventional polished Si surfaces. The pH sensitivity and linearity were 45.4 mV/pH and 99.7%, respectively, for pH values ranging from 2 to 12. A 4.4-fold higher photocurrent was achieved using the thin-Si LAPS with a thickness of $175~\mu \text{m}$ measured at a light-modulated frequency of 1 kHz compared with that using a Si substrate with a thickness of $550~\mu \text{m}$ . An acceptable photocurrent was obtained with a modulated illumination frequency up to 20 kHz, which is beneficial for 2-D images acquired by high-speed scanning. This simple device structure consisting of a thin-Si LAPS increases the photocurrent and pH sensitivity. The thickness of the LAPS Si substrate could be reduced in the future to optimize the process stability and sensing performance.

Efficient Flexible White-Light Photodetectors Based on BiFeO3 Nanoparticles

A heterostructured white-light photodetector was fabricated by the sequential deposition of zinc oxide (ZnO), bismuth ferrite (BFO), and poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) onto a flexible poly(ethylene terephthalate) (PET) substrate. Central to this unique structure is an n+–n junction at the ZnO–BFO interface, which allows the device to drive unusually large currents. The intrinsic BFO electric field, arising from the depletion region within the ferroelectric material, is reduced through the combined effect of two barrier fields formed at the BFO–Au and ZnO–BFO interfaces. The combination of these three fields reduces parasitic recombination and causes photoexcited electron–hole pairs to drift to the electrodes. The photocurrent sensitivity and photocurrent gain were determined to be 0.04 A/W and 105, respectively. The decay time extracted from the photoresponse curve was 6 s, whereby the photocurrent drops by a factor of 30. In addition, the device exhibits good flexibili...

Determination of COD based on Photoelectrocatalysis of FeTiO3.TiO2/Ti Electrode

Iron infrastructure technology of (Fe)-doped TiO2 nanotubes arrays (NTAs) was prepared for COD photoelectrocatalysis sensor. Fe-TiO2 NTAs was prepared using sol-gel method and coated with TiO2/Ti electrode by immersion technique. The optimization of COD photoelectrocatalytic sensor against Rhodamine B, Methyl Orange, and Methylene Blue organic dyes using photoelectrochemical system in a batch reactor. The high ordered FeTiO3.TiO2/Ti NTAs to determine COD value showed the high photocurrent response linearity and sensitivity to MO organic dye from the concentration of 5 ppm to 75 ppm with an average RSD value of 3.35. The development in this research is to utilize ilmenite mineral as model applied to COD sensor.

A novel strategy for the construction of photoelectrochemical sensing platform based on multifunctional photosensitizer

A novel strategy was proposed for the construction of photoelectrochemical (PEC) sensing platform based on multifunctional photosensitizer which can simultaneously achieve photo-sensitization, target-recognition and interface conjugation. Herein, N3 dye, bis(4,4′-dicarboxy-2,2′-bipyrid ine) dithiocyanato ruthenium (II), was employed as a model multifunctional molecule and covalently tethered on amino-functionalized TiO2 nanowires. The obtained photoanode can produce a sensitive photocurrent response towards Hg2+ under visible illumination (∼540 nm). The decrease in photocurrent can be attributed the larger HOMO-LUMO gap, the higher LUMO energy level and the reduced electron cloud density of N3–Hg2+ complex compared with the free N3 dye, which is demonstrated by DFT calculations. Under optimized conditions, the PEC assay displayed a linear dynamic range of 0.5–50 μM for Hg2+ with a detection limit of 0.13 nM. Moreover, the long-term stability of the PEC sensor was achieved due to the covalent conjugation of the photosensitizer to the TiO2 nanowire. The proposed sensing strategy, by utilizing the multifunctional molecule photosensitizers for direct responding to targets, presents a novel methodology for the development of high performance PEC sensors in future.

Cathodic photoelectrochemical detection of PCB101 in environmental samples with high sensitivity and selectivity

A novel cathodic photoelectrochemical (PEC) sensing method was developed for fast and convenient detection of PCB101 taking advantages of the excellent PEC reducibility of Pd quantum dots (QDs) modified molecularly imprinted TiO2 nanorods (NRs). Attributed to the efficient PEC reduction of PCB101 on the cathode surface, sensitive cathodic photocurrent would be produced with increasing PCB101 concentration under a negative bias potential, giving a low PCB101 detection limit of 5×10−14molL−1. Meanwhile, molecular imprinting (MI) technique was integrated by in situ introduction of MI sites on the surface of TiO2 NRs, so that highly specific adsorption and reduction of PCB101 congener could be obtained. The results indicated that the PEC sensor presented excellent selectivity toward PCB101 with the coexistance of 100-fold excess of other pollutants including the PCBs congeners, other aromatic pollutants, and heavy metal ions. The cathodic PEC sensor was sucessfully applied in determination of PCB101 in real water and soil samples, and the results had good consistency with that obtained by the traditional GC–MS. This work provides a new concept and research basis for fabricating cathodic PEC sensor for the environmental pollutants with specific structures that were easily reduced.

Sensitivity of resonant tunneling diode photodetectors

We have studied the sensitivity of AlGaAs/GaAs double barrier resonant tunneling diode photodetectors with an integrated GaInNAs absorption layer for light sensing at the telecommunication wavelength of λ = 1.3 μm for illumination powers from pico- to microwatts. The sensitivity decreases nonlinearly with power. An illumination power increase of seven orders of magnitude leads to a reduction of the photocurrent sensitivity from SI = 5.82 × 103 A W−1 to 3.2 A W−1. We attribute the nonlinear sensitivity–power dependence to an altered local electrostatic potential due to hole-accumulation that on the one hand tunes the tunneling current, but on the other hand affects the lifetime of photogenerated holes. In particular, the lifetime decreases exponentially with increasing hole population. The lifetime reduction results from an enhanced electrical field, a rise of the quasi-Fermi level, and an increased energy splitting within the triangular potential well. The non-constant sensitivity is a direct result of the non-constant lifetime. Based on these findings, we provide an expression that allows us to calculate the sensitivity as a function of illumination power and bias voltage, show a way to model the time-resolved photocurrent, and determine the critical power up to which the resonant tunneling diode photodetector sensitivity can be assumed constant.