Photoconductive Photodetectors Research Articles

High‐Performance Polymer Photodetector Using the Non‐Thermal‐and‐Non‐Ultraviolet–Ozone‐Treated SnO2 Interfacial Layer

In organic or perovskite photovoltaic‐type devices, to improve device performance, the thermal and ultraviolet–ozone (UVO) treatments are usually critical for the metal oxide film fabrication by removing the interfacial layer induced defects. However, the defects might play the opposite role in photoconductive photodetectors, where the defect‐induced photogenerated charge accumulation can lead to a tunneling current. Herein, a high‐performance solution‐processed broadband organic photodetector (OPD) in the visible wavelength is realized and characterized by introducing the non‐thermal‐and‐non‐UVO‐treated SnO2 nanoparticle film between the indium tin oxide (ITO) electrode and the active layer of the poly(3‐hexylthiophene) (P3HT):phenyl‐C61‐butyric‐acid‐methyl‐ester (PC61BM) blend. The untreated SnO2 nanoparticle layer can efficiently block the external charge injection, which considerably reduces the dark current density. Under illumination, the photogenerated charges are accumulated at the interface between the SnO2 layer and the active layer, and thus contribute a tunneling injection, resulting in a significantly high photocurrent. Therefore, the OPD shows a high performance in every figure of merit: external quantum efficiency (EQE) of 1430%, responsivity of 6.97 A W−1, detectivity of 2.29 × 1013 Jones, and −3 dB bandwidth of 2.8 MHz. The solution‐processed high‐performance photodetectors without thermal and UVO treatments are highly compatible with low cost, flexible, and large‐area electronics.

Mechanisms behind slow photoresponse character of Pulsed Electron Deposited ZnO thin films

Semiconducting Zinc Oxide (ZnO) is ideal candidate for ultraviolet (UV) photodetector due to its promising optoelectronic properties. Photoconductive type ZnO photodetectors, which is fabricated in metal-semiconductor-metal configuration, show mostly very high photoconductivity under UV light, but they are plagued by slow photoresponse time as slow as several tens of hours, even more. Most of the studies claimed that atmospheric adsorbates such as water and oxygen create charge traps states on the surface and remarkably increase both the photoconductivity and response time. There are also limited studies, which claim that the defect states acting as hole trap centers prolong response time significantly. However, the underlying physical mechanism is still unclear. Here we study the effects of both adsorbates and defect-related states on the photoresponse character of Pulsed Electron Deposited ZnO thin films. In order to distinguish between these two mechanisms, we have compared the time-dependent photoresponse measurements of bare-ZnO and SiO2 encapsulated-ZnO thin film samples taken under UV light and high vacuum. We show that the dominant mechanism of photoresponse in ZnO is the adsorption/desorption of oxygen and water molecules even when the measurement is performed in high vacuum. After the encapsulation of sample surface by a thin SiO2 layer, the adsorption/desorption rates can significantly improve, and the effects of these molecules partially removed.

(Invited) Mid-Infrared Colloidal Quantum Dot Based Nanoelectronics and Nano-Optoelectronics

Electronic and optoelectronic devices fabricated from colloidal quantum dots (CQD) provide a promising path toward realizing low-cost devices with greatly simplified device fabrication procedure, owing to their solution-processability. The impact that CQD technology would bring is expected to be significant, especially in the mid-infrared application areas, which is currently dominated by epitaxial semiconductor technologies. In this work, we introduce a new generation of infrared CQDs, namely Ag2Se CQDs, which has been recently uncovered to show distinct optical absorption in the mid-infrared. We report on the fabrication of solution-processed photoconductive photodetectors and discuss our analyses on the electronic and optoelectronic characteristics of our devices. We also demonstrate the feasibility of mid-infrared photodetection with a measured peak responsivity of 0.16 mA/W at 4 μm under room temperature operation.

High bandwidth and responsivity mid-infrared graphene photodetector based on a modified metal-dielectric-graphene architecture.

In this paper, a novel graphene-based mid-infrared photoconductive photodetector is designed that is composed of nanophotodetectors in series and parallel forming an array. The whole structure utilizes a single "unpatterned" graphene layer in a modified metal-dielectric-graphene (MDG) architecture that is composed of a conventional MDG structure combined with additional nanoelectrodes for photocurrent guiding and absorption enhancement. Finite difference time domain and finite element methods are utilized to obtain optical and electrostatic characteristics of the photodetector. Specifically, responsivity, quantum efficiency, dark current, bandwidth, noise equivalent power (NEP), and specific detectivity (D*) are extracted by employing realistic graphene as well as graphene-metal characteristics. For an optimized device, maximum absorption efficiency is as much as 70% at a wavelength of λ=6.77 μm; however, the peak absorption wavelength can be effectively tuned between λ=6-7 μm. It is shown that increasing the drain-source voltage enhances the responsivity and bandwidth with a side effect of increasing the dark current. Responsivity of the 2×2 photodetector is RA=0.63 AW-1 for V DS =0.5 V with I Dark =350 μA, NEP=16.91 pW/√Hz, and D*=1.53×105 Jones.

(Invited) Mid-Infrared Colloidal Quantum Dot Based Nanoelectronics and Nano-Optoelectronics

Electronic and optoelectronic devices fabricated from colloidal quantum dots (CQDs) provide a promising path toward realizing low-cost devices with greatly simplified device fabrication procedure, owing to their solution-processability. The impact that CQD technology would bring is expected to be significant, especially in the mid-infrared application areas, which is currently dominated by epitaxial semiconductor technologies. In this work, we introduce a new generation of infrared CQDs, namely Ag2Se CQDs, which has been recently uncovered to show distinct optical absorption in the mid-infrared. We report on the fabrication of solution-processed photoconductive photodetectors and discuss our analyses on the electronic and optoelectronic characteristics of our devices. We also demonstrate the feasibility of mid-infrared photodetection with a measured peak responsivity of 0.16 mA/W at 4 μm under room temperature operation.

Effective electron extraction from active layer for enhanced photodetection of photoconductive type detector with structure of Au/CH3NH3PbI3/Au

Abstract Increasing the carrier concentration as well as carrier life time could be effective ways to increase the photoresponse of a photoconductive type photodetector. In this paper, layer-structure photodetectors have been fabricated by connecting CH3NH3PbI3 film with and without electron transport layer (SnO2 and SnO2/PCBM bilayer). We found that the electron extraction at the electron transport layer and MAPbI3 interface will boost the photocurrent and responsivity, and faster electron extraction will result in a higher photocurrent improvement. At the same time, the closer is the distance of the interface to the photogenerated electron-hole pairs, the larger is the photocurrent enhancement. The largest photocurrent increasement is almost 3 times the initial value, which increases from 1.54 μA to 6.10 μA.

Oxides/graphene heterostructure for deep-ultraviolet photovoltaic photodetector

Compared with photoconductive photodetectors (PDs), photovoltaic PDs not only have higher response speed and stability in theory, but also have the advantage of operating under zero bias. However, the current research of MgZnO (MZO) based deep-ultraviolet (DUV) PD is still mainly based on photoconductive PD. The reason for this situation is the lack of a conductive window layer that is unobstructed for DUV light in the vertical structure of the photovoltaic PD. In this work, a high-quality single hexagonal MZO thin film was grown on 4H-SiC substrate by magnetron sputtering, and the first vertical-structure (Gr/MZO/4H-SiC) DUV photovoltaic PD based on Gr/MZO van der Waals heterojunction was constructed by utilizing the single-layer graphene (Gr) which is high transmittance to DUV light. The device exhibited good photovoltaic response characteristics. Under 255 nm DUV light, the device had a stable open circuit voltage of 184 mV. Under zero bias, the photoresponsivity of the device is 2.64 mA/W, and the external quantum efficiency is 1.29%. While the response rise time is 0.0782 s, the decay time is 0.162 s, and the photoresponse cut off edge is 302 nm. More importantly, this result provides a new reference method for the development of high-performance MZO-based photovoltaic PD.

Silver Selenide Colloidal Quantum Dots for Mid-Wavelength Infrared Photodetection

Lately discovered silver selenide (Ag2Se) colloidal quantum dots with tetragonal crystal structure exhibit promising optical properties in the mid-wavelength infrared. Although colloidal synthesis of uniform sizes and shapes as well as detailed phase transformation and photoluminescence properties have been studied recently, investigations of their optoelectronic properties as an active layer in photodetector devices remain scarce. Herein, we present the fabrication and characterization of Ag2Se colloidal quantum dot-based photoconductive photodetectors. We investigate the effect of ligand exchange as well as temperature and spectral-dependent photoresponses. Our results suggest that further enhancement in performance could be achieved through accurate control of carrier concentration. With this improvement, Ag2Se colloidal quantum dots may serve as a promising mid-wavelength infrared absorber for the development of thermal infrared sensors and imagers with low size, weight, power consumption, and cost.

Ultrafast Photovoltaic-Type Deep Ultraviolet Photodetectors Using Hybrid Zero-/Two-Dimensional Heterojunctions.

Deep ultraviolet (DUV) photodetectors have wide-range applications in satellite communications, air purification, and missile-plume detection. However, the critical barriers for the currently available wide-band gap semiconductor film-based DUV photodetectors are their low efficiency, complicated processes, and lattice mismatch with the substrate. Quantum dot (QD) devices prepared using solution-based methods can solve these problems. However, so far, there are no reports on photovoltaic-type DUV photodetectors using QDs. In this study, we propose a novel methodology to construct a hybrid zero-/two-dimensional DUV photodetector (p-type graphene/ZnS QDs/4H-SiC) with photovoltaic characteristics. The device exhibits excellent selectivity for the DUV light and has an ultrafast response speed (rise time: 28 μs and decay time: 0.75 ms), which are much better than those reported for conventional photoconductive photodetectors.

A tunable positive and negative photoconductive photodetector based on a gold/graphene/p-type silicon heterojunction

Fast switching of positive and negative photocurrents of a device under the modulation of different incident laser power densities.

Plasmonic Enhancement of Colloidal Quantum Dot Infrared Photodetector Photosensitivity

Infrared photodetector based on lead sulfide (PbS) colloidal quantum dot has been shown to be a promising candidate for infrared detectors, due to the low-cost process of fabrication and their extremely high sensitivity. Moreover, these photodetectors have successfully achieved ultrahigh detectivity—exceeding the indium gallium arsenide-based photodetectors—at room temperature. In this paper, PbS colloidal quantum dots have been synthesized through an all-chemical solution process and their X-ray diffraction patterns have been analyzed to verify the quality of the product. Transmission electron micrograph microscopy image confirms the production of 10-nm PbS nanoparticles. The as-synthesized PbS colloidal quantum dots were mixed with MEH-PPV to form a hybrid nanocomposite. PbS/MEH-PPV hybrid nanocomposite was used as an active material to detect infrared photons and convert into electrical current. The photoconductive photodetector was fabricated by drop casting the hybrid nanocomposite on interdigitated electrodes and tested under different conditions. To study the effect of silver nanoparticles on the performance of the device, different concentrations of Ag/PbS composition were used in the structure of the photodetector. The results show that the Ag additive reduces dark current due to band bending and increases photocurrent through plasmonic effect. Thus, a photoconductive photodetector with improved photosensitivity and enhanced performance has been achieved.

Perovskite–Gold Nanorod Hybrid Photodetector with High Responsivity and Low Driving Voltage

AbstractThe optical and electronic properties of noble metallic nanoparticles can be exploited to enhance the performance of inorganic/organic photodetectors. In this work, a uniformly distributed layer of Au nanorods (AuNRs) is integrated into vertically structured perovskite photoconductive photodetectors and, as a result, perovskite–AuNR hybrid photodetectors that exhibit significant photocurrent enhancements are reported. Ultimately it achieves a responsivity of ≈320 A W−1 at a low driving voltage of −1 V. This is an improvement of 60% compared to the responsivity of pristine devices (≈200 A W−1). The high responsivity and low driving voltage place this device among the highest performing perovskite‐based thin‐film photoconductive photodetectors reported. The stability and linearity of the photoresponse following repeated light/dark cycles are characterized. The hybrid device also shows a fast response (with the decay time of ≈95 ns) compared to pristine devices (≈230 ns). The improvements in photodetection performance are attributed to plasmon‐enhanced optical absorption, as well as advances in charge extraction and transport.

Extreme Carrier Depletion and Superlinear Photoconductivity in Ultrathin Parallel‐Aligned ZnO Nanowire Array Photodetectors Fabricated by Infiltration Synthesis

AbstractUltrathin semiconductor nanowires enable high‐performance chemical sensors and photodetectors, but their synthesis and device integration by standard complementary metal‐oxide‐semiconductor (CMOS)‐compatible processes remain persistent challenges. This work demonstrates fully CMOS‐compatible synthesis and integration of parallel‐aligned polycrystalline ZnO nanowire arrays into ultraviolet photodetectors via infiltration synthesis, material hybridization technique derived from atomic layer deposition. The nanowire photodetector features unique, high device performances originating from extreme charge carrier depletion, achieving photoconductive on–off ratios of >6 decades, blindness to visible light, and ultralow dark currents as low as 1 fA, the lowest reported for nanostructure‐based photoconductive photodetectors. Surprisingly, the low dark current is invariant with increasing number of nanowires and the photodetector shows unusual superlinear photoconductivity, observed for the first time in nanowires, leading to increasing detector responsivity and other parameters for higher incident light powers. Temperature‐dependent carrier concentration and mobility reveal the photoelectrochemical‐thermionic emission process at grain boundaries, responsible for the observed unique photodetector performances and superlinear photoconductivity. The results elucidate fundamental processes responsible for photogain in polycrystalline nanostructures, providing useful guidelines for developing nanostructure‐based detectors and sensors. The developed fully CMOS‐compatible nanowire synthesis and device fabrication methods also have potentials for scalable integration of nanowire sensor devices and circuitries.

Recent progress in organometal halide perovskite photodetectors

Abstract Organometal halide perovskite photodetectors have recently received significant attentions because of their excellent photoelectrical performance. Perovskite photodetectors can be mainly categorized into two types: photovoltaic photodetectors in vertical structure and photoconductive photodetectors in lateral structure. Herein, we review the progress in organometal halide perovskite photodetectors for the past few years, focusing on their key figure-of-merit features and operation mechanisms of different device structures.

Enhanced sensitivity and response speed of graphene oxide/ZnO nanorods photodetector fabricated by introducing graphene oxide in seed layer

Graphene/ZnO nanocomposites photodetectors hold great potential for UV detection because of the combination of photosensitive ZnO and high electron-mobility graphene. In this paper, graphene oxide (GO)/ZnO nanorods photoconductive photodetector with seed layer of GO and ZnO nanocrystals (NCs) hybrids is fabricated via a low-cost solution process. Uniform and oriented GO/ZnO nanorods have been obtained due to the positive role of GO in the growth process of ZnO nanorods, which gives rise to less light scattering and thereby stronger absorption and enhanced photocurrent. When the growth time is 1 h, the optimum photocurrent of GO/ZnO nanorods is about 9.4 times than pure ZnO nanorods, meanwhile, the corresponding detectivity reaches 7.17 × 1011 cm Hz1/2 W−1. In addition, owing to the high carrier mobility of graphene, the response time t 90 of GO/ZnO photodetector beneficially decreases to ~1 s, which is much faster than many other GO/ZnO hybrid photodetectors.

Photoconductive multi-layer graphene photodetectors fabricated on etched silicon-on-insulator substratesr**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0402404), the High-Tech Research and Development Program of China (Grant Nos. 2013AA031401, 2015AA016902, and 2015AA016904), and the National Natural Science Foundation of China (Grant Nos. 61674136, 61176053, 61274069, and 61435002).

Recently, graphene-based photodetectors have been rapidly developed. However, their photoresponsivities are generally low due to the weak optical absorption strength of graphene. In this paper, we fabricate photoconductive multi-layer graphene (MLG) photodetectors on etched silicon-on-insulator substrates. A photoresponsivity exceeding is obtained, which enables most optoelectronic application. In addition, according to the analyses of the high photoresponsivity and long photoresponse time, we conclude that the working mechanism of the device is photoconductive effect. The process of photons conversion into conducting electrons is also described in detail. Finally, according to the distinct difference between the photoresponses at 1550 nm and 808 nm, we estimate that the position of the trapping energy is somewhere between 0.4 eV and 0.76 eV, higher than the Fermi energy of MLG. Our work paves a new way for fabricating the graphene photoconductive photodetectors.

Application of Hyperbolic Metamaterials for Responsivity Enhancement of Thin Film Photo-Conductive Detectors

A theoretical study on the application of periodically tapered metal-semiconductor layers on the top surface of a GaAs-based photo-conductive photodetector is performed and its influence on the absorbance and responsivity improvement is investigated. To do so, first, the proposed pyramid and cone-shaped hyperbolic metamaterial (HMM) structures consisting of periodic Au/GaAs layers are designed for the 820~840 nm which is the peak absorption wavelength of the thin-film photo-conductive layer. Based on the results around the structural parameters of HMMs, a proper structure is chosen to couple to the photo-conductive layer. The results of simulation in CST Microwave Studio show that the absorbance reaches near unity for the metasurface-enhanced detector and the responsivity enhancement of about 140 can be obtained by using pyramid-shaped structures. The results can be valuable for applications in different types of photodetectors.

Spectra-selective PbS quantum dot infrared photodetectors.

Traditional photoconductive photodetectors (PDs) commonly respond to higher energy photons compared with the bandgaps of PD active materials. Different from the wide detection spectra of traditional PDs, the present reported PbS quantum dot (QD) PDs can detect the spectra-selective light source. Spectra-selective PDs (ss-PDs) of perovskite/QDs and QD/QDs were respectively implemented by integrating two functional layers. The top layer (facing the light) was utilized to filter the non-target spectra and the bottom layer was used for detection. The response spectrum wavelength and the range of ss-PDs can be conveniently tailored by tuning the QD size. The obtained selectivity factor and normalized detectivity ratio from target and non-target illumination can reach at least 10. A narrow detection range with a full width at half maximum (FWHM) ∼100 nm was applied by typical QD/QD based ss-PDs. The prototype ss-PDs were successfully applied in identifying an unknown light source. The convenient tuning and identification capabilities of the present QD based ss-PDs may provide a versatile route to obtain highly spectrum-selective PDs in order to meet the demands for special fields.

Characteristics of photoconductive UV photodetector based on ZnO nanorods grown on polyethylene naphthalate substrate by chemical bath deposition method

Vertically aligned ZnO nanorods were synthesized on a polyethylene naphthalate (PEN) substrate using a chemical bath deposition method at a low temperature. The structural and optical investigations revealed the high quality of the fabricated ZnO nanorods on flexible substrate. A metal-semiconductor-metal UV photodetector based on ZnO nanorods was fabricated on the PEN substrate. The optoelectronic characteristics of fabricated UV photodetector were studied in the dark and under 325 nm UV light illumination at −3 V and 3 V bias voltages. The responsivity and photosensitivity of the ZnO nanorod UV photodetector were 2.856 A/W and 1175% at 3 V bias voltage, respectively. Moreover, the response and the recovery times measured during the turn-on and turnoff of UV illumination were 1.2 s and 1.8 s, respectively.

Room-temperature operation of a titanium supersaturated silicon-based infrared photodetector

We report room-temperature operation of 1 × 1 cm2 infrared photoconductive photodetectors based on silicon supersaturated with titanium. We have fabricated these Si-based infrared photodetectors devices by means of ion implantation followed by a pulsed laser melting process. A high sub-band gap responsivity of 34 mV W−1 has been obtained operating at the useful telecommunication applications wavelength of 1.55 μm (0.8 eV). The sub-band gap responsivity shows a cut-off frequency as high as 1.9 kHz. These Si-based devices exhibit a non-previous reported specific detectivity of 1.7 × 104 cm Hz1/2 W−1 at 660 Hz, under a 1.55 μm wavelength light. This work shows the potential of Ti supersaturated Si as a fully CMOS-compatible material for the infrared photodetection technology.