Major growth in the image sensor market is largely as a result of the expansion of digital imaging into cameras, whether stand-alone or integrated within smart cellular phones or automotive vehicles. Applications in biomedicine, education, environmental monitoring, optical communications, pharmaceutics and machine vision are also driving the development of imaging technologies. Organic photodiodes (OPDs) are now being investigated for existing imaging technologies, as their properties make them interesting candidates for these applications. OPDs offer cheaper processing methods, devices that are light, flexible and compatible with large (or small) areas, and the ability to tune the photophysical and optoelectronic properties − both at a material and device level. Although the concept of OPDs has been around for some time, it is only relatively recently that significant progress has been made, with their performance now reaching the point that they are beginning to rival their inorganic counterparts in a number of performance criteria including the linear dynamic range, detectivity, and color selectivity. This review covers the progress made in the OPD field, describing their development as well as the challenges and opportunities.

Organic Photodiodes: The Future of Full Color Detection and Image Sensing

Wavelength-selective light detection is crucial for many applications, including imaging and machine vision. Narrowband spectral responses are required for colour discrimination, and current systems use broadband photodiodes combined with optical filters. This approach increases the architectural complexity and limits the quality of colour sensing. Here we report a method for tuning the spectral response to give filterless, narrowband red, green and blue photodiodes. The devices have simple planar junction architectures with the photoactive layer being a solution-processed mixture of either an organohalide perovskite or lead halide semiconductor and an organic (macro)molecule. The organic (macro)molecules modify the optical and electrical properties of the photodiode and facilitate charge collection narrowing of the device's external quantum efficiency. These red, green and blue photodiodes all possess full-width at half-maxima of <100 nm and performance metrics suitable for many imaging applications. Photodiodes with an intrinsic narrow spectral response make it possible to discriminate between red, green and blue light without the need for any optical filters.

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Filterless narrowband visible photodetectors

Spectrally selective light detection is vital for full-colour and near-infrared (NIR) imaging and machine vision. This is not possible with traditional broadband-absorbing inorganic semiconductors without input filtering, and is yet to be achieved for narrowband absorbing organic semiconductors. We demonstrate the first sub-100 nm full-width-at-half-maximum visible-blind red and NIR photodetectors with state-of-the-art performance across critical response metrics. These devices are based on organic photodiodes with optically thick junctions. Paradoxically, we use broadband-absorbing organic semiconductors and utilize the electro-optical properties of the junction to create the narrowest NIR-band photoresponses yet demonstrated. In this context, these photodiodes outperform the encumbent technology (input filtered inorganic semiconductor diodes) and emerging technologies such as narrow absorber organic semiconductors or quantum nanocrystals. The design concept allows for response tuning and is generic for other spectral windows. Furthermore, it is material-agnostic and applicable to other disordered and polycrystalline semiconductors.

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Narrowband light detection via internal quantum efficiency manipulation of organic photodiodes

Recently, color generation in resonant nanostructures have been intensively studied. Despite of their exciting progresses, the structural colors are usually generated by the plasmonic resonances of metallic nanoparticles. Due to the inherent plasmon damping, such plasmonic nanostructures are usually hard to create very distinct color impressions. Here we utilize the concept of metasurfaces to produce all-dielectric, low-loss, and high-resolution structural colors. We have fabricated TiO2 metasurfaces with electron-beam lithography and a very simple lift-off process. The optical characterizations showed that the TiO2 metasurfaces with different unit sizes could generate high reflection peaks at designed wavelengths. The maximal reflectance was as high as 64% with full width at half-maximum (fwhm) around 30 nm. Consequently, distinct colors have been observed in bright field and the generated colors covered the entire visible spectral range. The detailed numerical analysis shows that the distinct colors wer...

All-Dielectric Full-Color Printing with TiO2 Metasurfaces

Perovskite photodetectors (PPDs), which combine the advantages of perovskite semiconductor materials with superior optical and electronic properties and solution-processed manufacturing, have emerged as a new class of revolutionary optoelectronic devices with potential for various practical applications. Encouraged by the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of perovskite materials with interesting optoelectronic properties, increasing research attention is focused on the development of high performance PPDs. In this review, the recent progress on emerging PPDs is comprehensively summarized from the perspective of device physics and materials science. The strategies for extending the spectral response range of PPDs and improving the performance of devices are investigated. Furthermore, the methods for realizing narrowband photodetectors are also discussed, where filter-free and self-filter narrowband PPDs are achieved based on the concept of charge collection narrowing. Meanwhile, the promising future directions in this research field are proposed and discussed, including multifunctional PPDs, perovskite–organic hybrid photodetectors, flexible and transparent PPDs, self-powered PPDs, and photodetector systems and arrays. This review provides valuable insights into the current status of highly sensitive PPDs and will spur the design of new structures and devices to further enhance their photo-detection performances and meet the need of versatility in practical application.

Recent progress on highly sensitive perovskite photodetectors

The organic dye filters of conventional color image sensors achieve the red/green/blue response needed for color imaging, but have disadvantages related to durability, low absorption coefficient, and fabrication complexity. Here, we report a new paradigm for color imaging based on all-silicon nanowire devices and no filters. We fabricate pixels consisting of vertical silicon nanowires with integrated photodetectors, demonstrate that their spectral sensitivities are governed by nanowire radius, and perform color imaging. Our approach is conceptually different from filter-based methods, as absorbed light is converted to photocurrent, ultimately presenting the opportunity for very high photon efficiency.

Filter-Free Image Sensor Pixels Comprising Silicon Nanowires with Selective Color Absorption

A systematic study was conducted into the use of metal-assisted chemical etching (MacEtch) to fabricate vertical Si microwire arrays, with several models being studied for the efficient redox reaction of reactants with silicon through a metal catalyst by varying such parameters as the thickness and morphology of the metal film. By optimizing the MacEtch conditions, high-quality vertical Si microwires were successfully fabricated with lengths of up to 23.2 μm, which, when applied in a solar cell, achieved a conversion efficiency of up to 13.0%. These solar cells also exhibited an open-circuit voltage of 547.7 mV, a short-circuit current density of 33.2 mA/cm(2), and a fill factor of 71.3% by virtue of the enhanced light absorption and effective carrier collection provided by the Si microwires. The use of MacEtch to fabricate high-quality Si microwires therefore presents a unique opportunity to develop cost-effective and highly efficient solar cells.

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Versatile control of metal-assisted chemical etching for vertical silicon microwire arrays and their photovoltaic applications

Silicon microwire arrays have attracted considerable attention recently due to the opportunity they present as highly efficient and cost-effective solar cells. In this study, we report on efficient Si microwire array solar cells with areas of 1 cm2 and Air Mass 1.5 Global conversion efficiencies of up to 10.6%. These solar cells show an open-circuit voltage of 0.56 V, a short-circuit current density of 25.2 mA/cm2, and a fill factor of 75.2%, with a silicon absorption region that is only 25 μm thick. In particular, the maximum overall efficiency of the champion device is improved from 8.71% to 10.6% by conformally coating the wires with a 200 nm thick SiO2 layer. Optical measurements reveal that the layer reduces reflection significantly over the entire visible range.

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Si Microwire Solar Cells: Improved Efficiency with a Conformal SiO2 Layer

Here, a novel microgrid top electrode for highly efficient radial-junction Si microwire solar cells is demonstrated. The microgrid electrode minimizes optical and electrical losses, thus ensuring proper function of the shallow (sheet resistance of ≈100 Ω sq−1) junction emitter. This leads to effective collection of the photocarriers from the shallow junction emitter through the top electrode without severe Auger/surface recombination, improving the overall power conversion efficiency of the Si microwire solar cell. With an optimized microgrid structure, 1 cm2 microwire solar cells show a conversion efficiency of up to 16.5%, with an open-circuit voltage of 565.2 mV and a short-circuit current density of 35.9 mA·cm−2; this conversion efficiency is 72% higher than that of solar cells with an edge electrode (9.6%). Further, an ≈1 μm thick Ni electrode that is formed by electroplating considerably reduces the metal and contact resistances, which reproducibly yields a fill factor of over 80% (max 81.2%). Thus, the use of a novel microgrid to construct an ideal metal/emitter interface presents a unique opportunity to develop highly efficient microwire solar cells.

Microgrid Electrode for Si Microwire Solar Cells with a Fill Factor of Over 80

An optical apparatus, including an optical filter comprising an array of nanowires oriented perpendicular to a light incidence surface of the filter, wherein the optical filter transmits light at a first wavelength that is incident on the incidence surface, wherein the first wavelength is based on a cross-sectional shape of the nanowires. The nanowires are created using a single lithography step. An imaging device and a method of fabricating the same, the device including an array of nanowires formed on a substrate, wherein at least one nanowire in the array of nanowires includes a photoelectric element to produce a photocurrent based, at least in part, on incident photons absorbed by the at least one nanowire.

Young J. Yu

Papers

Filter-Free Image Sensor Pixels Comprising Silicon Nanowires with Selective Color Absorption

Versatile control of metal-assisted chemical etching for vertical silicon microwire arrays and their photovoltaic applications

Si Microwire Solar Cells: Improved Efficiency with a Conformal SiO2 Layer

Microgrid Electrode for Si Microwire Solar Cells with a Fill Factor of Over 80

Multispectral imaging using silicon nanowires