Internal Photocurrent Research Articles

Multiple exciton generation in tin–lead halide perovskite nanocrystals for photocurrent quantum efficiency enhancement

Multiple exciton generation (MEG), the generation of multiple electron–hole pairs from a single high-energy photon, can enhance the photoconversion efficiency in several technologies including photovoltaics, photon detection and solar-fuel production1,2,3,4,5,6. However, low efficiency, high photon-energy threshold and fast Auger recombination impede its practical application1,7. Here we achieve enhanced MEG with an efficiency of up to 87% and photon-energy threshold of two times the bandgap in highly stable, weakly confined formamidinium tin–lead iodide perovskite nanocrystals (FAPb1–xSnxI3 NCs; x ≤ 0.11). Importantly, an MEG-driven increment in the internal photocurrent quantum efficiency exceeding 100% with a low threshold is observed in such NC-sensitized photoconductors under ultraviolet-light illumination. The MEG enhancement mechanism is found to be mediated by the slower cooling and reduced trapping of hot carriers above the MEG threshold after the partial substitution of Pb by Sn. Our findings corroborate the potential importance of narrow-bandgap perovskite NCs for the development of optoelectronics that could benefit from MEG.

Preparation of Low Grain Boundary Perovskite Crystals with Excellent Performance: The Inhibition of Ammonium Iodide.

For the study, we prepared a low grain boundary three-dimensional CH3NH3PbI3 crystal (3D-MAPbI3) on TiO2 nanoarrays by inhibition of ammonium iodide and discussed the formation mechanism of the crystal. Based on the 3D-MAPbI3 crystal, solar cells showed modified performance with a power conversion efficiency (PCE) of up to 19.3%, which increases by 36.8% in contrast to the counterparts. We studied the internal photocurrent conversion process. The highest external quantum efficiency is up to 92%, and the electron injection efficiency is remarkably facilitated where the injection time decreases by 37.8% compared to the control group. In addition, based on 3D-MAPbI3, solar cells showed excellent air stability, which possesses 78.3% of the initial PCE, even though they were exposed to air for 30 days. Our results demonstrate a promising approach for the fabrication of perovskite solar cells with high efficiency and stability.

Self-powered and high responsivity photodetector based on a n-Si/p-GaTe heterojunction

Heterojunction integrated by two-dimensional/three-dimensional materials has shown great potential applications in optoelectronic devices because of its fast response speed, high specific detectivity and broad spectral response. In this work, the vertical n-Si/p-GaTe heterojunction has been designed and fabricated, which shows a high responsivity up to 5.73 A W−1 and a fast response time of 20 μs at zero bias benifitting from the high efficiency of light absorption, internal photocurrent gain and strong built-in electrical field. A specific detectivity of 1012 Jones and a broad spectral response ranging from 300 to 1100 nm can also be achieved. This work provides an alternative strategy for high-performance self-powered optoelectronic devices.

Integration of MoS2 with InAlAs/InGaAs Heterojunction for Dual Color Detection in Both Visible and Near‐Infrared Bands

AbstractAt present, dual‐channel or even multi‐channel recording is a developing trend in the field of photodetection, which is widely applied in environment protection, security, and space science and technology. This paper proposes a novel MoS2/InAlAs/InGaAs n–i–n heterojunction phototransistor by integrating multi‐layered MoS2 with InGaAs‐based high electron mobility transistors (InGaAs‐HEMTs). Due to the internal photocurrent amplification in the InGaAs channels with a narrow energy bandgap of 0.79 eV, this device exhibits high photoresponsivity (R) of over 8 × 105 A W–1 under near‐infrared illumination of 1550 nm at 500 pW. Furthermore, with the combination of the photoconductance effect in the vertical MoS2/InAlAs/InGaAs n–i–n heterojunction and the photogating effect in the lateral phototransistor, this device possesses a unique characteristic under visible illumination that its photoresponsivity can be tuned by the top gate electrode from 6 × 105 A W–1 to ‐4 × 105 A W–1 by gate voltage. This may lead to a new application as an optically controlled electronic inverter, which needs further study in depth. This MoS2/InAlAs/InGaAs phototransistor builds up a new bridge between 2D materials and conventional ternary compounded semiconductor devices.

High performance metal-insulator-semiconductor-insulator-metal photodetector fabricated on a silicon-on-oxide substrate

We demonstrate a planar metal-insulator-semiconductor-insulator-metal (MISIM) photodetector fabricated on a silicon-on-insulator substrate where the insulator is a stack of SiO2 and HfO2. The detector exhibits an extremely low dark current, as well as a large sensitivity and responsivity for wavelengths in the 365–880 nm range. The current-voltage (I−V) characteristics under moderate illumination intensities are superlinear and saturate at high powers. This behavior is due to a space charge limited current mechanism, which causes a photocurrent amplification process. The current path through the detector is via filament sites induced by a voltage stress of the thermal SiO2 and HfO2 layers. These filaments allow for the internal photocurrent gain. Saturation of the I−V curves is caused by Schottky-type electrodes whose barrier height with silicon controls the thermionic emission of carriers through the filament sites to the silicon depletion region. At a bias of ±4 V, we estimated a maximum sensitivity of 1.25 × 105, a responsivity of 68 A/W, a detectivity of 6.5 × 1013 Jones, and a quantum efficiency of 2.3 × 104% of the photodetectors for an incident optical power of 0.143 μW at 365 nm wavelength.

Suppressing Dark Current in Organic Phototransistors through Modulating Electron Injection via a Deep Work Function Electrode

Organic phototransistors (OPTs) have been extensively investigated as photodetectors because of their internal photocurrent amplification. However, OPTs tend to show high dark current because they ...

Photoconductive properties of polycrystalline selenium based lateral MISIM photodetectors of high quantum efficiency using different dielectrics as the charge blocking layer

We studied the photoconductive performance of polycrystalline selenium (pc-Se) based photodetectors with a lateral metal–insulator-semiconductor-insulator–metal (MISIM) device structure. The insulator layer is a 10 nm-thick Ga2O3, HfO2, or Al2O3 thin film, and used as a charge blocking layer (CBL) to suppress dark current injected from the Al electrodes. The dark current suppression primarily depends on the barrier height of the junctions between the CBLs and electrodes. The Ga2O3 CBL exhibits a poor dark current suppression compared to the HfO2 and the Al2O3 CBLs because of a lower electron barrier at the cathode. The lateral pc-Se photodetectors exhibit a very high internal photocurrent gain due to Fowler–Nordheim tunneling at relatively low applied voltages. The better crystallinity of pc-Se grains formed on the Ga2O3 CBL leads to a higher photoconversion efficiency for the MISIM-Ga2O3 photodetector. Compared with amorphous Se based lateral MISIM photodetectors, the pc-Se photodetectors demonstrate a uniform and much better photoconductive performance over the visible spectrum.

Design of modified InGaAs/InP one-sided junction photodiodes with improved response at high light intensity.

A modified InGaAs/InP one-sided junction photodiode (MOSJ-PD) is presented for the first time. The MOSJ-PD is proposed from the one-sided junction photodiode by inserting a cliff layer into the absorption layer. Compared to the modified uni-traveling carrier photodiode, the MOSJ-PD has the advantages of simpler epitaxial layer structure and lower junction capacitance. In the MOSJ-PD, the space charge effect at high light intensity can be suppressed. Thus, both 3-dB bandwidth and output current can be improved simultaneously. The performance characteristics of the MOSJ-PD, including energy band diagram, internal electric field, frequency response, photocurrent, and responsivity, are carefully studied.

Photophysical and photoconductive aspects of donor-acceptor low band gap conjugated copolymer

This article presents the photophysical and photoconductive properties of a newly synthesized low band gap Donor-Acceptor conjugated poly (benzothiadiazole-triphenylamine) co-polymer labelled as P(BTZ-TPA) in which 2,1,3 benzothiadiazole (BTZ) act as the electron deficient moiety and triphenylamine (TPA) is used as donor moiety. Photophysical properties of this synthesized polymer were analyzed by recording steady state absorption and emission spectra of dilute solution and thin films respectively. In addition to this, solvatochromic experiments were carried out using a binary mixture consisting of toluene and acetonitrile to understand the solvent dependence on the ground state and excited states of this polymer. The optical characterization revealed high degree of intramolecular charge transfer through the polymer backbone. The P(BTZ-TPA) thin films exhibit good fluorescence emission with emission peak around 615 nm. Quantum yield of the fluorescence emission is measured in relative method. Also, fluorescence decay parameters were examined using time correlated single-photon counting (TCSPC) technique. Carrier generation and transport of the generated carriers upon photo excitation is studied in pristine polymer films as well as in P(BTZ-TPA):PCBM blend films, by performing photoconductivity studies using broad spectral source and lasers of wavelengths 488 nm and 632 nm. P(BTZ-TPA) thin films exhibited good photoresponse over the entire visible region, with promising internal photocurrent efficiency. Quenching of fluorescence is observed in PCBM blend films. This drastic quenching of fluorescence emission is assigned to the charge transfer interaction between co-polymer and PCBM, and is confirmed by substantial increment in photoconductivity of the blend films. An internal photocurrent efficiency of 17.4% was obtained in the blend films for a biasing field of 10V/μm.

Synthesis and experimental investigations on the photoconductivity of p-aminoazobenzene based non-conjugated polybenzoxazine system

In this paper, we report the study of a novel p-aminoazobenzene based non-conjugated, benzoxazine polymer, poly([4-(6-tert-butyl-4H-benzo[e][1,3]oxazin-3-yl)phenyl]-phenyldiazene) (AZO-PBZ) synthesized using solvent free, thermally activated cationic ring opening polymerization. The thermal, photophysical, electrochemical, and photoconducting properties of the polymer were investigated. The optical absorption band edges of AZO-PBZ thin film was observed at 555 nm. Photoinduced charge transfer nature of AZO-PBZ was studied by analyzing the fluorescence spectra and performing photoconductivity experiments. Incorporation of [6, 6]-phenyl-C61-butyric acid methyl ester denoted as PCBM into the polymer leads to quenching of the fluorescent intensity and exhibits significant photoconductivity. The photocurrent through the PCBM blend films was measured as a function of electric field to recognize the field dependence on carrier generation. Internal photocurrent efficiency of the polymer:PCBM blend samples was found to be in the order of 10−5 and photoconductive sensitivity was of the order of 10−11 S W−1cm, which is adequate for photorefractivity.

Study on the photoconductive properties of charge carriers in donor-acceptor copolymer P(EDOT-FL) and P(EDOT-FL):PCBM blend

The photoinduced charge transfer properties of low band gap donor-acceptor π-conjugated copolymer poly(2,5-(3,4-ethylenedioxythiophene)-alt-2,7-(9,9-dioctylfluorene)) labelled as P(EDOT-FL) has been investigated through solvatochromic and photoconductivity studies. The effect of strong acceptor, Phenyl-C61-butyric acid methyl ester (PCBM) on the photogeneration of free carriers is studied by comparing the fluorescence spectra and photoconductive properties of the polymer-PCBM blend [P(EDOT-FL):PCBM] films with the pristine polymer films. The positive solvatochromism exhibited by the P(EDOT-FL) revealed its photoinduced charge transfer nature of the absorption and emission bands. The blend films exhibited quenching of the fluorescence intensity and showed increased photoconductivity. Internal photocurrent efficiency and photoconductive sensitivity of the pristine P(EDOT-FL) films and P(EDOT-FL):PCBM blend films were calculated as a function of electric field, by measuring the photocurrent generated in the sample. Blend films exhibited higher internal photocurrent efficiency of 86% and photoconductive sensitivity of 5.44×10−7 SW−1cm at 70V/μm. The obtained photoconductive sensitivity at low electric field (<10V/μm) is sufficient for photorefractive applications.

Conduction band discontinuity and carrier multiplication at the MgxZn1−xO/MgyZn1−yO interface

The multiplication of photogenerated electrons induced by larger conduction band discontinuity contributes to internal photocurrent increasing.

Surface plasmon excitation in semitransparent inverted polymer photovoltaic devices and their applications as label-free optical sensors

Herein, we report on surface plasmon (SP)-sensitive semitransparent inverted polymer photovoltaic (PV) devices that are based on multilayered material systems consisting of poly(3-hexylthiophene): fullerene-derivative bulk-heterojunction PV layers and thin gold or silver anodes. We demonstrate that these PV devices allow the simultaneous generation of both electrical power and SPs on their anodes for photoexcitation just above the optical absorption edge of the PV layers, resulting not only in attenuated total reflection, but also in attenuated photocurrent generation (APG) under the SP resonance (SPR) condition. Moreover, we also confirm that the biomolecular interaction of biotin–streptavidin on the PV devices can be precisely detected via apparent SPR angle shifts in the APG spectra, even without the need for complex attenuated total reflection configurations. We highlight our view that APG measurements made using these PV devices show great potential for the development of future generations of compact and highly sensitive SPR-based optical sensors. Researchers investigate surface plasmon excitation in photovoltaic devices and explore their application as highly sensitive optical sensors. Byoungchoo Park and co-workers at Kwangwoon University in South Korea have developed semitransparent inverted polymer photovoltaic devices with a planar multilayer structure that are capable of simultaneously generating electrical power and surface plasmons on irradiation. When surface plasmon resonance is induced in these devices, both total internal reflection and photocurrent generation are attenuated. The researchers demonstrated that this property can be exploited to sensitively detect biological compounds. Specifically, they used apparent shifts in the surface plasmon resonance angle in attenuated photocurrent generation to detect the biomolecular interaction of biotin-streptavidin. They conclude that these devices have great potential as next-generation optical sensors that are simple, inexpensive, compact and highly sensitive.

TPD doped polystyrene as charge transporter in DiPBI sensitized photorefractive composites

We incorporate a mixture of polystyrene (PS) and the highly conductive N, N′-diphenyl-N, N′-bis(3-methylphenyl)-[1, 1′-biphenyl]-4, 4′-diamine (TPD) as charge transporting agent into a photorefractive composite, wherein the liquid crystal 4-cyano-4-n-pentylbiphenyl (5CB) is the electro-optical unit and the perylene bisimide dimer DiPBI acts as sensitizing component. Investigation of the photocurrent reveals a strong enhancement of the photoconductivity. Compared to composites, wherein poly-n-vinylcarbazole (PVK) is the charge transporting agent, the internal photocurrent efficiency is enhanced 11 times. This dramatic improvement is attributed to an increase of charge generation and transport and it allows for a reduction of the applied electric field to get a photoconductivity that is comparable to PVK comprising composites.

Does Conjugation Help Exciton Dissociation? A Study on Poly(p‐phenylene)s in Planar Heterojunctions with C60 or TNF

Internal photocurrent quantum yields near 100% can be obtained from the separation of loosely bound geminate pairs when sufficiently large electric fields are applied to organic heterojunctions. The fields needed for complete electron-hole dissociation decrease to those prevailing in organic solar cells under operating conditions when well-conjugated polymers are employed.

Secondary electron emission in extreme-UV detectors: Application to diamond based devices

A study on the effect of secondary electron emission, which strongly affects the detection of extreme-UV radiation, was performed on diamond detectors. Two different structures were compared: interdigitated contacts and a transverse Schottky diode configuration. Both devices were electrically characterized by I-V measurements and their responsivity was measured in the extreme UV spectral region (20–120 nm) by using He-Ne gas discharge radiation sources and a toroidal grating vacuum monochromator. Through an ad-hoc measurement configuration, the contributions of the internal photocurrent and of the photoemission current have been analyzed and separately evaluated. The results showed that secondary electron emission, which clearly depends on the experimental conditions (e.g., external electric field, pressure, etc.), is one of the most relevant processes affecting the spectral responsivity in the extreme UV band. In particular, for interdigitated devices, extreme care must be taken in order to obtain an absolute value of their responsivity, while detectors in the transverse configuration can be shielded in such a way to avoid secondary electron current contribution and therefore provide a more correct and reliable response.

ZnO‐based photodetector with internal photocurrent gain

AbstractThe photoresponsive structures prepared by magnetron sputtering of ZnO on p‐Si substrates followed by vacuum evaporation of semitransparent Ni film on ZnO surface are investigated. The obtained Ni/n‐ZnO/p‐Si structures show high sensitivity that sharply increases with increase in applied voltage. Under a bias voltage of 5 V, the responsivities at λ = 390 nm and λ = 850 nm were equal to 210 and 110 A/W, which correspond to quantum efficiencies of 655 and 165, respectively. It is assumed that the observed strong response is attributed to internal gain in the Ni/n‐ZnO/p‐Si phototransistor structure containing Ni/n‐ZnO Schottky contact as the emitter junction and n‐ZnO/p‐Si heterostructure as the collector junction. The response time of the device is ∼10–7 s. Alternative mechanisms of photocurrent multiplication in such structures are also discussed.

Bias effect in photocurrent response of Si nanocrystals

We report on the photocurrent response of hydrogenated nanocrystalline silicon (nc-Si:H) thin films under external bias voltages. The band gap transition and internal photoemission photocurrent of the nc-Si:H thin films can be enhanced and controlled by adjusting the depletion and inversion layers in the metal-semiconductor junction through the external bias voltage. The photocurrent response from the internal photoemission is found to be able to extend the photodetection wavelength of the Si material to the optical telecommunication range of 1.3–1.6 μm.

Charge‐injection photogate pixel fabricated in CMOS silicon‐on‐insulator technology

AbstractConcept, theoretical analysis, and experimental results obtained from a charge‐injection photogate (CI‐PG) pixel detector fabricated in CMOS silicon‐on‐insulator (SOI) technology are presented. The charge collected in the photodetector during a certain charge collection (integration) time is injected into the substrate for readout. This readout principle presents a huge internal photocurrent amplification (∼104) taking place in the photodetector, obtained through the ‘time‐compression’ approach. Here, the readout circuitry is fabricated on highly doped, 200 nm thick, SOI film, while the photogate detector is fabricated on higher resistivity handle‐wafer. The latter, together with the 30 V biasing possibilities, enhances the quantum efficiency of the pixel, especially for irradiations with wavelengths in the near‐infra‐red part of the spectra. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Internal Electric Field and Photocurrent of Polymer/Perylene Heterojunction Solar Cell

The internal electric field in a heterojunction solar cell made of poly(5-(2'-ethylhexyloxy)-2-methoxy-1,4-phenylenevinylene) (MEH-PPV) as a donor and 3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI) as an acceptor is studied by electroabsorption (EA). The 1ω-EA signal detected at the fundamental electric-field-modulation frequency ω comes from the bulk of the film for MEH-PPV but from the interface of the film for PTCBI. The bias dependence of the bulk electric field for MEH-PPV is correlated with that of the photocurrent generated by MEH-PPV excitation, indicating that the electric field has an essential role in carrier photogeneration. An analysis of the photocurrent action spectrum indicates that the photocurrent-active region in the MEH-PPV layer changes its location depending on the applied bias voltage. This suggests that the region of strong electric field in the MEH-PPV layer changes its location along with the active layer.