Structural Engineering of Si/TiO2/P3HT Heterojunction Photodetectors for a Tunable Response Range

First-Principles Band Alignments at the Si:Anatase TiO2 Interface

TiO₂ has been identified as a promising electron transport layer in Si solar cells. Experiments have revealed that the Si:TiO₂ interface undergoes structural changes depending on how it was fabricated. However, less is understood about the sensitivity of electronic properties, such as band alignments, to these changes. Here, we present first-principles calculations of band alignments between Si and anatase TiO₂, investigating different surface orientations and terminations. By calculating vacuum-level alignments, we observe a large band offset reduction of 2.5 eV for the O-terminated Si slab compared to other terminations. Furthermore, a 0.5 eV increase is found for the anatase (101) surface compared to (001). We compare the band offsets obtained through vacuum alignment with four different heterostructure models. Even though the heterostructure models contain an excess of oxygen, their offsets agree well with vacuum-level alignments using stoichiometric or H-terminated slabs, and the reduction in band offsets seen for the O-terminated Si slab is not observed. Additionally, we have investigated different exchange-correlation treatments including PBE + U, postprocessing GW corrections, and the meta-GGA rSCAN functional. We find that rSCAN provides more accurate band offsets than PBE, but further corrections are still required to achieve <0.5 eV accuracy. Overall, our study quantifies the importance of surface termination and orientation for this interface.

High responsivity of VIS-NIR photodetector based on Ag2S/P3HT heterojunction

Ag₂S quantum dot (QD) photodetectors (PDs) have attracted a lot of attention in the field of imaging system and optical communication. However, the current Ag₂S PDs mainly works in the near-infrared band, and its detection ability in the visible band remains to be strengthened. In this paper, we used poly(3-hexylthiophene) (P3HT) with high carrier mobility and Ag₂S QDs to construct heterojunction PD. Stronger absorption in blends with polymer P3HT compared to single Ag₂S QDs. The optical absorption spectra show that the Ag₂S/P3HT has strong light absorption peak at 394 and 598 nm. The results show that P3HT significantly enhances the absorption of Ag₂S QDs from the visible to near-infrared band. The output characteristics, transfer characteristics and fast switching capability of the device at 405 nm, 532 nm and 808 nm were tested. The device has the responsivity of 6.05 A W^-1, 83.72 A W^-1and 37.31 A W^-1under 405 nm, 532 nm and 808 nm laser irradiation. This work plays an important role in improving the detection performance of Ag₂S QDs and broadening its applications in photoelectric devices for weak light and wide spectrum detection.

Facilely Achieved Self-Biased Black Silicon Heterojunction Photodiode with Broadband Quantum Efficiency Approaching 100

Photodiodes are fundamental components in modern optoelectronics. Heterojunction photodiodes, simply configured by two different contact materials, have been a hot research topic for many years. Currently reported self-biased heterojunction photodiodes routinely have external quantum efficiency (EQE) significantly below 100% due to optical and electrical losses. Herein, an approach that virtually overcomes this 100% EQE challenge via low-aspect-ratio nanostructures and drift-dominated photocarrier transport in a heterojunction photodiode is proposed. Broadband near-ideal EQE is achieved in nanocrystal indium tin oxide/black silicon (nc-ITO/b-Si) Schottky photodiodes. The b-Si comprises nanostalagmites which balance the antireflection effect and surface morphology. The built-in electric field is explored to match the optical generation profile, realizing enhanced photocarrier transport over a broadband of photogeneration. The devices exhibit unprecedented EQE among the reported leading-edge heterojunction photodiodes: average EQE surpasses ≈98% for wavelengths of 570-925 nm, while overall EQE is greater than ≈95% from 500 to 960 nm. Further, only elementary fabrication techniques are explored to achieve these excellent device properties. A heart rate sensor driven by nanowatt faint light is demonstrated, indicating the enormous potential of this near-ideal b-Si photodiode for low power consuming applications.

Significant performance enhancement of UV-Vis self-powered CsPbBr3 quantum dot-based photodetectors induced by ligand modification and P3HT embedding

In this work, we report a novel, to the best of our knowledge, strategy to improve the performance of UV-Vis self-powered CsPbBr₃ quantum dot (QD) based photodetectors (PDs) by ligand modification and poly(3-hexylthiophene) (P3HT) embedding. Compared with those based on pure QDs, modified PDs show a shortened response time by nearly ten times, and increases of maximum responsivity and specific detectivity by nearly 45 and 97 times, respectively. Such PDs also show a high stability with 90% of the initial photocurrent being maintained even after storage in ambient air without any encapsulation for 30 days.

Application of Nanostructured TiO2 in UV Photodetectors: A Review

As a wide-bandgap semiconductor material, titanium dioxide (TiO₂ ), which possesses three crystal polymorphs (i.e., rutile, anatase, and brookite), has gained tremendous attention as a cutting-edge material for application in the environment and energy fields. Based on the strong attractiveness from its advantages such as high stability, excellent photoelectric properties, and low-cost fabrication, the construction of high-performance photodetectors (PDs) based on TiO₂ nanostructures is being extensively developed. An elaborate microtopography and device configuration is the most widely used strategy to achieve efficient TiO₂ -based PDs with high photoelectric performances; however, a deep understanding of all the key parameters that influence the behavior of photon-generated carriers, is also highly required to achieve improved photoelectric performances, as well as their ultimate functional applications. Herein, an in-depth illustration of the electrical and optical properties of TiO₂ nanostructures in addition to the advances in the technological issues such as preparation, microdefects, p-type doping, bandgap engineering, heterojunctions, and functional applications are presented. Finally, a future outlook for TiO₂ -based PDs, particularly that of further functional applications is provided. This work will systematically illustrate the fundamentals of TiO₂ and shed light on the preparation of more efficient TiO₂ nanostructures and heterojunctions for future photoelectric applications.

Ferromagnetic CoSe broadband photodetector at room temperature

Broadband infrared photodetectors based on two-dimensional (2D) materials which are the research focus in the infrared field, have wide applications in remote sensing, thermal imaging, and astronomy observation. In this article, the photodetector based on 2D ferromagnetic material CoSe is studied at room temperature, demonstrating the air-stable, broadband, and up to long wavelength properties. The CoSe material is applied to infrared photodetectors for the first time. The 2D material CoSe is synthesized by using the chemical vapor deposition method. The size of the as-grown CoSe is up to 71.8 μm. The photoresponse of the CoSe photodetector ranges from 450 nm to 10.6 μm. The photoresponsivity of this photodetector is up to 2.58 A W^-1 under the 10.6 μm illumination at room temperature. This work provides a new material for broadband photodetector at room temperature and builds a bridge for the magnetoelectronic and broadband photoelectric fields.

Broadband InSb/Si heterojunction photodetector with graphene transparent electrode

Silicon-based Schottky heterojunction photodetectors are promising due to their compatibility with the semiconductor process. However, the applications of these devices are usually limited to wavelengths shorter than 1.1 µm due to the low absorption of electrode materials at infrared. In this report, silicon-based compound semiconductor heterojunction photodetectors with graphene transparent electrodes are fabricated. Due to the high absorption of InSb at infrared, as well as the good transparency and excellent electrical conductivity of the graphene, the as-prepared photodetectors show a broadband photoresponse with high performance which includes a specific detectivity of 1.9 [Formula: see text]10¹² cm Hz^1/2 W^-1, responsivity of 132 mA W^-1, on/off ratio of 1 [Formula: see text]10⁵, rise time of 2 µs, 3 dB cut-off frequency of 172 kHz, and response wavelengths covering 635 nm, 1.55 µm and 2.7 µm. This report proves that graphene as a transparent electrode has a great effect on the performance improvement of silicon-based compound semiconductor heterojunction photodetectors.

Germanium quantum dot infrared photodetectors addressed by self-aligned silicon nanowire electrodes

Germanium quantum dots (GeQDs), addressed by self-aligned and epitaxial silicon nanowires (SiNWs) as electrodes, represent the most fundamental and the smallest units that can be integrated into Si optoelectronics for 1550 nm wavelength detection. In this work, individual GeQD photodetectors have been fabricated based on a low temperature self-condensation of uniform amorphous Si (a-Si)/a-Ge bilayers at 300 °C, led by rolling indium (In) droplets. Remarkably, the diameter of the GeQD nodes can be independently controlled to achieve wider GeQDs for maximizing infrared absorption with narrower SiNW electrodes to ensure a high quality Ge/Si hetero-epitaxial connection. Importantly, these hetero GeQD/SiNW photodetectors can be deployed into predesigned locations for scalable device fabrication. The photodetectors demonstrate a responsivity of 1.5 mA W^-1 and a photoconductive gain exceeding 10² to the communication wavelength signals, which are related to the beneficial type-II Ge/Si alignment, gradient Ge/Si epitaxial transition and a larger QD/NW diameter ratio. These results indicate a new approach to batch-fabricate and integrate GeQDs for ultra-compact Si-compatible photodetection and imaging applications.

One-Step Coating Processed Phototransistors Enabled by Phase Separation of Semiconductor and Dielectric Blend Film

Fabrication of organic thin-film transistors (OTFTs) via high throughput solution process routes have attracted extensive attention. Herein, we report a simple one-step coating method for vertical phase separation of the poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(methyl methacrylate) (PMMA) blends as semiconducting and dielectric layers in OTFTs. These OTFTs can be used as phototransistors for ultraviolet (UV) light detection, where the phototransistors exhibited great photosensitivity of 597.6 mA/W and detectivity of 4.25 × 10¹⁰ Jones under 1 mW/cm² UV light intensity. Studies of the electrical properties in these phototransistors suggested that optimized P3HT contents in the blend film can facilitate the improvement of film morphology, and therefore form optimized vertical phase separation of the PMMA and P3HT. These results indicate that the simple one-step fabrication method creates possibilities for realizing high throughput phototransistors with great photosensitivity.

Fabrication of a cost effective and broadband self-powered photodetector based on Sb2Te3 and silicon

The excellent conductive surface electronic states of topological insulators make them suitable candidates for the next generation optoelectronic devices. Moreover, their unique semiconducting properties are favorable for building heterojunctions with other semiconductors. Here, we fabricated a low cost and broadband self-powered photodetector based on Sb₂Te₃ and Si. The photolithography and thermal evaporation technique were combined to fabricate a series of asymmetric planar Sb₂Te₃ electrodes on the surface of an n-type silicon substrate. An obvious asymmetrical current voltage curve occurred under dark conditions, which is ascribed to the asymmetry of each electrode. During the photodetection test, self-powered photodetection was obtained upon 940 nm light irradiation. Moreover, the photodetector exhibited fast and broadband photodetection from 365 nm to 940 nm with a response time less than 40 ms.

Strategies toward High-Performance Solution-Processed Lateral Photodetectors

Due to their low cost and ease of integration, solution-processed lateral photodetectors (PDs) are becoming an important device type among the PD family. In recent years, enormous effort has been devoted to improving their performances, and great achievements have been made. A summary of the core progress, especially from the perspective of design principles and device physics, is necessary to further the development of the field, but is currently lacking. Here, to address this need, first, the working mechanism of PDs and the device figures-of-merit are introduced. Second, by classifying the active materials into four categories, including inorganic, organic, hybrid, and perovskite, the developed strategies toward high performance are discussed respectively. To close, the common physical rules behind all these strategies are generalized, and suggestions for future development are given accordingly.