Enhanced Detectivity and Suppressed Dark Current of Perovskite-InGaZnO Phototransistor via a PCBM Interlayer

A comprehensive review of the current progresses and material advances in perovskite solar cells

Recently, perovskite solar cells (PSCs) have attracted ample consideration from the photovoltaic community owing to their continually-increasing power conversion efficiency (PCE), viable solution-processed methods, and inexpensive materials ingredients. Over the past few years, the performance of perovskite-based devices has exceeded 25% due to superior perovskite films achieved using low-temperature synthesis procedures along with evolving appropriate interface and electrode-materials. The current review provides comprehensive knowledge to enhance the performance and materials advances for perovskite solar cells. The latest progress in terms of perovskite crystal structure, device construction, fabrication procedures, and challenges are thoroughly discussed. Also discussed are the different layers such as ETLs and buffer-layers employed in perovskite solar-cells, seeing their transmittance, carrier mobility, and band gap potentials in commercialization. Generally, this review delivers a critical assessment of the improvements, prospects, and trials of PSCs.

Solution-processed transparent p-type orthorhombic K doped SnO films and their application in a phototransistor

The exploitation of p-type oxide semiconductors with excellent optoelectrical properties as well as a simple preparation process is still challenging owing to the difficulty in producing hole carriers which results from strong hole localization in p-type oxide semiconductors. In this work, we succeeded in using ethylene glycol as a reductant to prepare orthorhombic structure SnO films using a sol-gel method and through K doping the optical and electrical properties of the films were improved. When the orthorhombic K doped SnO (K-SnO) films were applied in a phototransistor, it presented ultra-broadband photosensing from the ultraviolet to infrared region (300-1000 nm), demonstrating a photoresponsivity of 349 A W^-1 and a detectivity of 5.45 × 10¹² Jones at 900 nm under a light intensity of 0.00471 mW cm^-2. In particular, infrared photosensing was for the first time reported in the SnO based phototransistors. This work not only provides a simple method to fabricate high-performance and low-cost p-type K-SnO films and phototransistors, but may also suggest a new way to improve the p-type characteristics of other oxide semiconductors and devices.

Self-biased wavelength selective photodetection in an n-IGZO/p-GeSe heterostructure by polarity flipping

Transparent semiconductor oxides with two-dimensional (2D) heterostructures have been extensively studied as new materials for thin-film transistors and photosensors due to their remarkable photovoltaic characteristics, making them useful for newly developed optoelectronics. Here we demonstrate the fabrication and characterization of an ITO/n-IGZO/p-GeSe transparent selective wavelength photodetector. The wavelength-dependent photovoltaic behavior of the n-IGZO/p-GeSe heterostructure under UV-Visible laser light shifts the I-V curves down with positive V_oc and negative I_sc values of about 0.12 V and -49 nA and 0.09 V and -17 nA, respectively. Interestingly, when an NIR laser irradiated the device, the I-V curves shifted up with negative V_oc and positive I_sc values of about -0.11 V and 45 nA, respectively. This behavior is attributed to the free carrier concentration induced by photogenerated carriers across the device at different points that varied with the wavelength-dependent photon absorption. Consequently, the direction of the electric field polarity across the junction can be flipped. This study demonstrates a zero-bias near-infrared (NIR) photodetector with a high photoresponsivity of 538.9 mA W^-1, a fast rise time of 25.2 ms, and a decay time of 25.08 ms. Furthermore, we observed a detectivity (D) of 8.4 × 10⁹ Jones, a normalized photocurrent to dark current ratio (NPDR) of 2.8 × 10¹⁰ W^-1, and a noise equivalent power (NEP) of 2.2 × 10^-14 W Hz^-1/2. Our strategy opens alternative possibilities for scalable, low-cost, multifunctional transparent near-infrared photosensors with selective wavelength photodetection.

Enhanced performance and stability in InGaZnO NIR phototransistors with alumina-infilled quantum dot solid

The optimized ALD infilling process for depositing Al₂O₃ in the vertical direction of PbS QDs enhances the photoresponsivity, relaxation rate and the air stability of PbS QDs hybrid IGZO NIR phototransistors. Infilled Al₂O₃, which is gradually deposited from the top of PbS QDs to the PbS/IGZO interface (1) passivates the trap sites up to the interface of PbS/IGZO without disturbing charge transfer and (2) prevents QDs deterioration caused by outside air. Therefore, an Al₂O₃ infilled PbS QD/IGZO hybrid phototransistor (AI-PTs) exhibited enhanced photoresponsivity from 96.4 A/W to 1.65 × 10² A/W and a relaxation time decrease from 0.52 to 0.03 s under NIR light (880 nm) compared to hybrid phototransistors without Al₂O₃ (RF-PTs). In addition, AI-PTs also showed improved shelf stability over 4 months compared to RF-PTs. Finally, all devices we manufactured have the potential to be manufactured in an array, and this ALD technique is a means of fabricating robust QDs/metal oxide hybrids for optoelectronic devices.

Polyimide-Doped Indium-Gallium-Zinc Oxide-Based Transparent and Flexible Phototransistor for Visible Light Detection

We report a transparent and flexible polyimide (PI)-doped single-layer (PSL) phototransistor for the detection of visible light. The PSL was deposited on a SiO₂ gate insulator by a co-sputtering process using amorphous indium-gallium-zinc oxide (IGZO) and PI targets simultaneously. The PSL acted as both a channel layer and a visible-light absorption layer. PI is one of the few flexible organic materials that can be fabricated into sputtering targets. Compared with the IGZO phototransistor without PI doping, the PSL phototransistor exhibited improved optoelectronic characteristics under illumination with 635 nm red light of 1 mW/mm² intensity; the obtained photoresponsivity ranged from 15.00 to 575.00 A/W, the photosensitivity from 1.38 × 10¹ to 9.86 × 10⁶, and the specific detectivity from 1.35 × 10⁷ to 5.83 × 10¹¹ Jones. These improvements are attributed to subgap states induced by the PI doping, which formed decomposed organic molecules, oxygen vacancies, and metal hydroxides. Furthermore, a flexible PSL phototransistor was fabricated and showed stable optoelectronic characteristics even after 10,000 bending tests.

A Review of Phototransistors Using Metal Oxide Semiconductors: Research Progress and Future Directions

Metal oxide thin-film transistors have been continuously researched and mass-produced in the display industry. However, their phototransistors are still in their infancy. In particular, utilizing metal oxide semiconductors as phototransistors is difficult because of the limited light absorption wavelength range and persistent photocurrent (PPC) phenomenon. Numerous studies have attempted to improve the detectable light wavelength range and the PPC phenomenon. Here, recent studies on metal oxide phototransistors are reviewed, which have improved the range of light wavelengths and the PPC phenomenon by introducing an absorption layer of oxide or non-oxide hybrid structure. The materials of the absorption layer applied to absorb long-wavelength light are classified into oxides, chalcogenides, organic materials, perovskites, and nanodots. Finally, next-generation convergence studies combined with other research fields are introduced and future research directions are detailed.

Enhanced Ultraviolet Photoresponse Characteristics of Indium Gallium Zinc Oxide Photo-Thin-Film Transistors Enabled by Surface Functionalization of Biomaterials for Real-Time Ultraviolet Monitoring

Indium gallium zinc oxide (IGZO) is one of the most promising materials for diverse optoelectronic applications based on thin-film transistors (TFTs) including ultraviolet (UV) photodetectors. In particular, the monitoring of UV-A (320-400 nm) exposure is very useful for healthcare applications because it can be used to prevent various human skin and eye-related diseases. However, the relatively weak optical absorption in the UV-A range and the persistent photoconductivity (PPC) arising from the oxygen vacancy-related states of IGZO thin films limit efficient UV monitoring. In this paper, we report the enhancement of the UV photoresponse characteristics of IGZO photo-TFTs by the surface functionalization of biomolecules on an IGZO channel. The biomaterial/IGZO interface plays a crucial role in enhancing UV-A absorption and suppressing PPC under negative gate bias, resulting in not only increased photoresponsivity and specific detectivity but also a fast and repeatable UV photoresponse. In addition, turning off the device without external bias completely eliminates PPC due to the internal electric field induced by the surface functionalization of biomaterials. Such a volatile feature of PPC enables the fast and repeatable UV photoresponse. These results suggest the potential of IGZO photo-TFTs combined with biomaterials for real-time UV monitoring.

Visible-light-stimulated synaptic InGaZnO phototransistors enabled by wavelength-tunable perovskite quantum dots

Neuromorphic vision sensors are designed to mimic the human visual system, which allows image recognition with low power computational requirements. Photonic synaptic devices are one of the most viable building blocks for constructing neuromorphic vision sensors. Herein, a photonic synaptic sensor based on an inorganic perovskite quantum dot (QD) embedded InGaZnO (IGZO) thin-film phototransistor is demonstrated. The photodetection wavelength ranges of the transistor can be adjusted by changing the halogen ions (Cl, Br) of the perovskite QDs. Under low intensity 450 and 550 nm illumination, the CsPbBr₃ QD embedded phototransistor sensor shows a responsivity of 6.7 × 10² and 4.2 × 10^-2 A W^-1, respectively. The perovskite QD embedded transistor not only presents high responsivity to visible light, but also features excellent synaptic behavior, including an excitatory postsynaptic current (EPSC), pair-pulse facilitation (PPF), long-term memory, and memory erasure through gate voltage regulation. Moreover, the sensor fabrication process in this work is compatible with conventional photolithography processes. Taking these merits into account, the proposed QD embedded IGZO transistor presents a promising route by which to construct artificial visual sensors with color-distinguishable optical signal sensing and processing.

Supercritical Ammoniation-Enabled Interfacial Polarization for Function-Mode Transformation and Overall Optimization of Thin-Film Transistors

Thin-film transistors (TFTs) have drawn widespread applications in the increasingly sophisticated display field. Despite the mature process of fabricating enhancement-mode TFTs, lack of facile methods to realize depletion-mode TFTs restrains the implementation of complementary-type circuits, which in turn leads to relatively high power. Here, the supercritical fluid technique is introduced to elaborately design and tune the interface, providing the opportunity for function-mode transformation of TFTs. By harnessing supercritical-assisted ammoniation (SCA) treatment, interfacial polarization induces negative shift of threshold voltage (from 0.2 to -9.8 V), which allows TFTs to remain normally on-state in the absence of complex capacitor-integrated circuits. This convenient technique, without an additional manufacturing process to achieve function-mode transformation, can thus enable the fabrication of comprehensive-mode TFTs under the same process. Furthermore, comprehensive optimizations in the mobility (increases from 2.08 to 17.12 cm² V^-1 s^-1), leakage current (reduces from 1.33 × 10^-11 to 2.22 × 10^-12 A), hysteresis (reduces from 11.2 to 0.2 V), and on/off current ratio (increases from 9.65 × 10⁴ to 7.98 × 10⁶) are achieved simultaneously. Based on conjoint analysis of electrical and material characterization, a reaction model is established for a clearer understanding of the interfacial polarization process. Overall, this low-temperature SCA treatment offers an environmentally benign strategy to modulate the function mode of electronic devices via interfacial engineering and optimize device performance at the same time, exhibiting promise in promoting the implementation of complementary, low-power circuit.

Weak Light-Stimulated Synaptic Hybrid Phototransistors Based on Islandlike Perovskite Films Prepared by Spin Coating

An artificial synaptic device that can provide color discrimination, image storage, and image recognition is highly required to mimic the human vision for biological robots. All-inorganic halide perovskites have attracted extensive attention for the reason of their high stability and favorable photoelectric properties. In this study, a light-stimulated synaptic phototransistor based on a CsPbBr₃/organic semiconductor hybrid film is reported. The fabricated CsPbBr₃ film exhibits an island structure, which reduces the hysteresis effectively and at the same time achieves a high specific detectivity of up to 2 × 10¹⁵ Jones. The decay of the photocurrent can be delayed by changing the gate bias, which is essential for achieving high-performance light-stimulated synaptic devices. Due to the outstanding detectivity of the device, the obvious synaptic functions can be observed when triggered by a light signal with a power of 1.6 nW that is much weaker than previous most perovskite-based hybrid synaptic phototransistors under a low operating voltage of -1 V. The electrical power consumption of the device could be as low as 0.076 pJ when the power of light spike was 7.36 nW. Taking into account this characterization, with changing of light intensity or wavelength, the contrast of the image was enlarged, which can further promote the image recognition accuracy. More significantly, this CsPbBr₃/TIPS hybrid film can be fabricated by facile and low-cost solution processes. This study indicates the great potential of solution-processed perovskite-based light-stimulated synapses for future artificial visual systems.

A visible-light phototransistor based on the heterostructure of ZnO and TiO2 with trap-assisted photocurrent generation

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂). A thin layer of TiO₂ was deposited onto the spin-coated ZnO film via atomic layer deposition (ALD). The electrical characteristics of the TiO₂ layer were optimized by controlling the purge time of titanium isopropoxide (TTIP). The optimized TiO₂ layer could absorb the visible-light from the sub-gap states near the conduction band of TiO₂, which was confirmed via photoelectron spectroscopy measurements. Therefore, the heterostructure of TiO₂/ZnO can absorb and generate photocurrent under visible light illumination. The oxygen-related-states were investigated via X-ray photoelectron spectroscopy (XPS), and the interfacial band structure between TiO₂ and ZnO was evaluated via ultraviolet photoelectron spectroscopy (UPS). Oxygen-related states and subgap-states were observed, which could be used to generate photocurrent by absorbing visible light, even with TiO₂ and ZnO having a wide bandgap. The optimized TiO₂/ZnO visible-light phototransistor showed a photoresponsivity of 99.3 A W⁻¹ and photosensitivity of 1.5 × 10⁵ under the illumination of 520 nm wavelength light. This study provides a useful way to fabricate a visible-light phototransistor based on the heterostructure of wide bandgap oxide semiconductors.

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂).

Enhanced Electro-Optical Performance of Inorganic Perovskite/a-InGaZnO Phototransistors Enabled by Sn-Pb Binary Incorporation with a Selective Photonic Deactivation

Optoelectronic applications using perovskites have emerged as one of the most promising platforms such as phototransistors, photovoltaics, and photodetectors. However, high-performance and reliable perovskite photonic devices are often hindered by the limited spectral ranges of the perovskite system along with the lack of appropriate processing technologies for the implementation of reliable device architectures. Here, we explore a hybrid phototransistor with a heterojunction of a Sn-Pb binary mixed halide perovskite (CsSn_0.6Pb_0.4I_2.6Br_0.4) light absorber and an amorphous-In-Ga-Zn-O (a-IGZO) charge carrying layer. By incorporating Sn-Pb binary components with an all-inorganic base, broadening of light-absorbing spectral ranges with enhanced stability has been achieved, indicating inevitable highly increased conductivity, which triggers a high off-current of the devices. Accordingly, the selectively ultraviolet (UV)-irradiated electrical deactivation (SUED) process is carried out to suppress the high off-current with a reliable device structure. Particularly, it is noted that the selective UV irradiation can facilitate oxidation and distortion of the chemical structure in specific perovskite regions, providing enhanced gate bias modulation of the phototransistor with an increased on/off-current ratio from ∼10³ to ∼10⁶. Finally, the SUED-processed phototransistor exhibits an improvement in the photosensitivity by more than 3 orders of magnitude up to 8.0 × 10⁴ and detects in the spectral range from visible to near-infrared (NIR) light (∼860 nm) with good on/off switching behaviors.

Recent Advancements and Challenges for Low-Toxicity Perovskite Materials

Lead-based organic-inorganic hybrid perovskite materials have been developed for advanced optoelectronic applications. However, the toxicity of lead and the chemical instability of lead-based perovskite materials have so far been demonstrated to be an overwhelming challenge. The discovery of perovskite materials based on low-toxicity elements, such as Sn, Bi, Sb, Ge, and Cu, with superior optoelectronic properties provides alternative approaches to realize high-performance perovskite optoelectronics. In this review, recent advances in the aspects of low-toxicity perovskite solar cells, photodetectors, light-emitting diodes, and thermoelectric devices are highlighted. The antioxidation stability of metal cation and the crystallization process of the low-toxicity perovskite materials are discussed. In the last part, the outlook toward addressing various issues requiring further attention in the development of low-toxicity perovskite materials is outlined.

High Photosensitive Indium-Gallium-Zinc Oxide Thin-Film Phototransistor with a Selenium Capping Layer for Visible-Light Detection

Visible light can be detected using an indium-gallium-zinc oxide (IGZO)-based phototransistor, with a selenium capping layer (SCL) that functions as a visible light absorption layer. Selenium (Se) exhibits photoconductive properties as its conductivity increases with illumination. We report an IGZO phototransistor with an SCL (SCL/IGZO phototransistor) that demonstrated optimal photoresponse characteristics when the SCL was 150 nm thick. The SCL/IGZO phototransistor exhibited a photoresponsivity of 1.39 × 10³ A/W, photosensitivity of 4.39 × 10⁹, detectivity of 3.44 × 10¹³ Jones, and external quantum efficiency of 3.52 × 10³% when illuminated by green light (532 nm). Ultraviolet-visible spectroscopy and ultraviolet photoelectron spectroscopy analysis showed that Se has a narrow energy band gap, in which visible light is absorbed and forms a p-n junction with IGZO so that photogenerated electron-hole pairs are easily separated, which makes recombination more challenging. We show that electrons generated in the SCL flow through the IGZO layer, which enables the phototransistor to detect visible light. Furthermore, the SCL/IGZO phototransistor exhibited excellent durability and reversibility owing to the constant light and dark current and the time-dependent photoresponse characteristics over 8000 s when a red light (635 nm) source was turned on and off at a frequency of 0.1 Hz.

Nanostructured High-Performance Thin-Film Transistors and Phototransistors Fabricated by a High-Yield and Versatile Near-Field Nanolithography Strategy

Thin-film transistors (TFTs) and field-effect transistors (FETs) are basic units to build functional electronic circuits and investigate transport physics. In conventional TFTs or FETs, performance in terms of current level, on-off ratio, and the sensitivity of detection is limited by homogeneous semiconducting layers. In this paper, we develop TFTs with submicron heterostructures by using a strategy based on near-field photolithography. We use an array of total-reflective polydimethylsiloxane pyramids or trenches as a soft photomask in photolithography to induce multiple reflections and diffractions to focus the light. The textured feature enables the generation of gaps, dots, and grids at the nanoscale, with dimensions as small as sub-100 nm on substrates at the centimeter scale. We demonstrated the very high performance oxide TFTs on the nanoscale and periodic degenerately doped heterojunctions, and they yielded a nearly 20-fold increase in transconductance and apparent device mobility. The on-off ratio was higher than 10⁹, with notably enhanced output current and clear scaling effect with channel length. We also built nanostructured wide-gap/narrow-gap heterojunctions to balance the high on-off ratio and sensitive photoresponse in a unidirectional phototransistor. This study shows the viability of programming a variety of nanoscale submicron patterns or interfaces in TFTs and FETs to significantly enlarge the scope of research on multifunctional TFTs and FETs.