Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have been demonstrated for the visible regime. However, near-infrared (NIR) photodetection has proven to be challenging and, to date, the true potential of organic semiconductors in this spectral range (800–2500 nm) remains largely unexplored. In this talk, I will introduce a new device concept for organic NIR detectors, based on charge-transfer absorption, enhanced by a resonant optical cavity device architecture. Design rules and optimization strategies will be discussed, yielding wavelength selective devices (20 nm resolution) with a tunability of the detection wavelength over several hundreds of nanometers, allowing the printing of miniature NIR spectrometers. In a second part of the talk, we explore the performance limitations of organic NIR detectors: A relation between open-circuit voltage, dark current, and noise current is demonstrated for OPDs with detection wavelengths beyond 1100 nm. Based on these findings we estimate an upper limit of achievable specific detectivity values for organic photodiodes as a function of their longest NIR detection wavelength.

Near-infrared photodetectors based on narrow-gap organic semiconductor blends are attractive candidates for light-sensitive applications in industry and consumer electronics. However, these photodetectors currently suffer from large dark current densities, strongly limiting their sensitivity. Organic photodiodes typically show dark saturation current levels that are several orders of magnitude higher than expected from radiative band-to-band transitions only, suggesting the presence of a large non-radiative recombination channel – the origin of which is still debated. In this work we conduct ultra-sensitive external quantum efficiency and temperature dependent dark current measurements on organic photodiodes based on narrow-gap organic semiconductors. The thermal activation energy of the dark current at small reverse bias voltages is found to equal half of the effective bandgap energy revealing that the dominant recombination channel is trap-mediated via mid-gap states. By taking Shockley-Read-Hall statistics into account, we derive an analytical expression which accurately describes the dark current in the reverse bias, allowing for the upper limit of the specific detectivity in organic photodiodes to be calculated. Finally, upon comparing the light-to-dark current ratio of a large number of reported organic photodiodes from the literature, we find that the dark current is universally limited by trap-assisted recombination via mid-gap states in narrow gap systems. Our findings shed new light on the origin of noise in organic light-harvesting applications fundamentally limiting the low light performance and sensitivity in photodiodes and detectors.

A strategy to artificially widen the linear dynamic range (LDR) of an organic photodiode (OPD) by introducing a light-intensity-dependent transition of its operation mode, such that a low saturation photocurrent can be overcome by additional operation mechanism, is suggested. The active layer of OPD is doped with a strategically designed and synthesized molecular switch (1,2-bis-(2-methyl-5-(4-cyanobiphenyl)-3-thienyl)tetrafluorobenzene; DAB), exhibiting typical OPD performances with an EQE < 100% under weak light and photoconductive behaviors with an EQE > 100% under strong light, which leads to an artificially extended LDR up to 225 dB. Such unique and reversible transition of the operation mode by light intensity self-recognition of molecular-switch-embedded OPDs can be explained by the unbalanced quantum yield of the cyclization-cycloreversion of the molecular switch. Details of the operation mechanism are discussed in conjunction with various photophysical analyses. A prototype image sensor based on wide-LDR OPDs is suggested to demonstrate superior sensitivity against strong light illumination.

Organic semiconductors have attracted tremendous attention in the past few years, thanks to their excellent flexibility, solution-processability, low-cost and chemical versatility. In particular, organic photodetectors (OPDs) have demonstrated state-of-the-art device performance [1,2]. More recently, photomultiplication-type OPDs have merged as promising candidates for next-generation solution-processed photodetectors, thanks to the ultrahigh responsivity and low-dark current and noise. [3] However, this photomultiplication effect was mainly characterized with enhanced device performance, the underlying mechanism was attributed to the trap-mediated charge transport, and lacks of detail analysis. In this work, we characterized the devices with multiple transient techniques; the charge transport and accumulation of photo-generated carriers were fully analyzed, and we found that ultra-long carrier lifetime and imbalanced charge transport are required for the photomultiplication process.

Inherited retinal dystrophies and late-stage age-related macular degeneration, for which treatments remain limited, are among the most prevalent causes of legal blindness. Retinal prostheses have been developed to stimulate the inner retinal network; however, lack of sensitivity and resolution, and the need for wiring or external cameras, have limited their application. Here I report on the use of conjugated polymer nanoparticles (P3HT NPs), showing that they mediate light-evoked stimulation of retinal neurons and persistently rescue visual functions when subretinally injected in a rat model of retinitis pigmentosa. I will then discuss the Photophysics of the NPs and the open questions related on their functioning in vivo. 

The ability to three-dimensionally pattern semiconducting electronic and optoelectronic materials could provide a transformative approach to creating active electronic devices without the need for a cleanroom or conventional microfabrication facility. This could enable the generation of active electronics on-the-fly, using only source inks and a portable 3D printer to enable electronics anywhere, including directly on the body. Indeed, interfacing active devices with biology in 3D could impact a variety of fields, including biomedical devices, regenerative biomedicines, bioelectronics, smart prosthetics, and human-machine interfaces. Indeed, developing the ability to 3D print various classes of materials possessing distinct properties could enable the freeform generation of active electronics in unique functional, interwoven architectures. Achieving seamless integration of diverse materials with 3D printing is a significant challenge which requires overcoming discrepancies in material properties in addition to ensuring that all the materials are compatible with the 3D printing process. We will present a strategy for three-dimensionally integrating diverse classes of materials using a custom-built 3D printer to fully create fully integrated device components with active properties. As a proof of concept for demonstrating the integrated functionality of these materials, we 3D printed quantum dot-based light-emitting diodes (QD-LEDs) that exhibit pure and tunable color emission properties, and polymer-based photodiodes for bionic eye applications, which are conformally printed onto curvilinear surfaces. These results represent a critical initial step toward the large-scale 3D printing of multifunctional materials and devices, without the need for conventional microfabrication processes or facilities.

The first demonstration of mid-infrared photodetection with colloidal quantum dots took place in 2011 and used HgTe. The promise of wafer scale processing of infrared imaging devices was exciting, but the performances were 2-3 orders of magnitude below those provided by commercial single crystal materials or superlattices.  There has been progress, with novel quantum dot synthesis, new spectroscopy results, vastly improved electrical transport properties, and more performant device structures.  The ease of deposition of the quantum dot inks has also allowed to demonstrate potentially useful functions, such as dual band, dual polarization, curved arrays, hyperspectral, etc.  At the same time, focal plane arrays are being developed.  Nevertheless, performances at 5 microns are close but slightly less than microbolometers at room temperature, as well as commercial HgCdTe,  and they are still about 10-fold inferior to state of the art HgCdTe single crystal detectors at any temperatures.   In this contribution, I will discuss the present limitations and basic research avenues to overcome them.  I will also briefly discuss intraband detection and alternative materials.   

In this talk I will present an overview of our activities at ICFO on the development of hybrid 2D-CQD photodetectors. I will start by the first demonstration of a highly sensitive graphene-PbS CQD photodetector, the advances made in increasing external quantum efficiency transforming PbS QDs from a passive sensitizer to an active CQD Photodiode and then showcase some use case examples in its operation in image sensors and wearable applications. Then I will discuss briefly the implications of replacing graphene with semiconducting TMD 2D materials and an overview of our activities in this class of hybrid detectors. In the second part of the talk I will discuss recent progress on efficient SWIR QD LEDs and their use in conjunction with a CQD photodiode to demonstrate an optical transmission link at the eye-safe 1550 nm based entirely on CQD optoelectronics (LEDs and photodetectors). In the last part of my talk, I will discuss recent efforts in extending the spectral coverage of PbS QD photodetectors towards the mid and long wave infrared exploiting intersubband transitions in heavily doped QD films.

Semiconductor nanocrystals are now well established as high-performance photonic materials. The majority of applications are focused on their emissive properties, for example as exploited in bioassay and display applications. However, it is also well-known that these properties lend themselves to photodetection (and photovoltaic) technologies with a number of high performance device demonstrators such as those based on PbS reported within the literature [1]. In this talk I will provide a brief review of the use of semiconductor nanocrystals in photodetector applications. Key principles of materials design, device design and operation, and device characterisation will be reviewed based on existing work. Following this the use of hybrid systems to obtain improved performance will be discussed along with alternative nanocrystal systems that move away from those based on cadmium and lead. 

The wide spectral tunability of colloidal quantum dot (CQD) absorption, combined with good mobility of photogenerated charge carriers, make them competitive candidates for sensing wavelengths beyond the Si bandgap. I will present an overview of the latest achievements in CQD infrared photodetection fueled by advances in chemistry, materials science and device architectures. I will then touch on some of the remaining challenges, such as the realization of efficient infrared photodetectors that do not rely on heavy metals. I will present recent results on the use of InAs CQDs for infrared optoelectronics, showing how crucial surface management is in this materials family to achieve sufficient mobility, prevent heavy doping, and minimize surface defects – all aspects needed to enable photodetectors that combine high sensitivity (low dark current and noise, and high photoresponse) and fast operation.

I will start with a brief introduction about recent results relative to HgTe nanocrystals (NCs) used to design short wave focal plane arrays (FPAs). I will, in particular, demonstrate an all nanocrystal based active imaging setup where both light emission (from electroluminescence) and light detection are obtained from HgTe NCs.

Then, I will discuss a strategy to design improved sensors based on phototransistors, where the gate enables carrier density tuning and thus dark current reduction. I will discuss two type of gating methods: (i) ionic glass based on LaF₃ and (ii) paraelectric gating using SrTiO₃ (STO).  I will show that LaF₃ ionic gates can be used to generate specific operating points leading to the formation of a p-n junction within the transistor channel. The latter is used to enhance charge dissociation. On the other hand, SrTiO₃ is specifically designed for low temperature operation where low noise operations are required. In this regime, conventional high capacitance gating based electrolyte and ionic glass become ineffective due to the freezing of the ions, while STO thanks to the divergence of its dielectric constant is well suited for operation below 100 K which remains typical for mid wave infrared applications. Finally, I will show that phototransistor geometry can be coupled to a light trapping strategy. Here, using a guided mode resonator, I will demonstrate a broad band (>500 nm) enhancement of the light absorption through several resonances.

One of the great interests for colloidal nanocrystals (NCs) is their optical tunability controlled by the synthesis processes. Nonetheless, for NC-based devices, post-fabrication tunability remains very limited.¹ Here, we show a NC-based active photonic device where the optical tunability is achieved thanks to an inhomogeneous absorption obtained by coupling a HgTe nanocrystal² array to a plasmonic cavity. Bias tunability results from hopping transport which enables tuning the charge collection from low-field regions (in the very vicinity of the electrodes) to high-field ones (away from the electrodes).³ As a result, we observe a 15 meV blueshift of the photocurrent spectrum for a device working in the extended short-wave infrared under the application of 3 V bias voltage.³ This shift has the opposite sign with respect to the redshift expected from Stark effect while occurring under much weaker applied electric fields. We show that hopping transport, often seen as a bottleneck to achieve high-performing optoelectronic devices, can play an essential role in the operation of active photonic devices.

Perovskite based on metal halide perovskites is a most promising material system for photovoltaics as well as for photodetectors. However, further advance demands improved device stability. Here we show the importance of perovskite composition and interface engineering by demonstrating long-term stable perovskite composites and devices. Most interestingly, the perovskite composite appears to be significantly more stable than the corresponding device, which is a strong argument that a major part of the instabilities are introduced from the interface. A high-throughput robotic approach is used to screen 160 mixed-cation mixed-halide perovskites based on several optical characterizations, such as UV-vis absorption and photoluminescence spectra. Such automated big data approaches allow to uniquely identify the most photo-thermal-stable perovskites under elevated temperature and illumination. Most interestingly, while several perovskite compositions are found to be stable against high temperatures of up to 85 C, the situation is more complex in working devices, as a stable device requires both - a stable semiconductor layer and stable interfaces. A p-type interface consisting of polymeric multilayers with variable doping is introduced as a most robust anode interface at elevated temperatures of 65 degrees Celsius, but still shows degradation at temperatures beyond 85 C. This work further introduces into the fundamentals how to accelerate the screening for stable perovskites layer and devices for long term operation.

Narrow bandgap Pb-Sn-based perovskite photodiodes offer the possibility of spectral sensitivity from the visible to the near infrared but often suffer from relatively high dark current densities under a reverse bias where they are generally operated. Among the common strategies to reduce dark current density, the inclusion of charge-blocking layers between the electrodes and the perovskite layer has become popular. While these blocking layers are successful in increasing the energy barrier for charge injection, the lower limits of dark current density reached experimentally remain typically orders of magnitude higher than the expected intrinsic bulk thermal-generated dark current density. Hence, another process than bulk thermal generation is responsible for the remaining dark current. To find its origin we carefully analyzed the activation energy of the dark current by studying its temperature dependence in optimized Pb-Sn perovskite photodiodes with different bandgaps, employing a series of electron-blocking layers. The results demonstrate that the dark current activation energy corresponds to the energy offset between the highest occupied molecular orbital of the electron blocking layer and the conduction band minimum of the perovskite. As a conseqeunce, thermal charge generation at that interface is the main cause for the dark current and it is governed by the interfacial energy offset at the that interface. By increasing this energy offset by using an appropriate blocking layer, a perovskite photodiode was fabricated that has a wavelength sensitivity up to 1050 nm, combined with an ultralow dark current density of 5 × 10⁻⁸ mA cm⁻² and noise current of 2 × 10⁻¹⁴ A Hz^−1/2.

The excellent optoelectronic properties of metal halide perovskites, such as strong light absorption, slow charge recombination, defect tolerance, and large carrier mobility, make them good candidates not only for solar cells but also for photodetectors. Their tunable bandgap allows light detection of near-infrared and visible light, which can be extended with combination of low-bandgap organic light absorbers. High-performance photodetectors with sensitivity superior to silicon ones have been quickly demonstrated with the capability to resolve weak light of sub-picowatt per square centimeter and short response speed of sub-nanosecond. In addition, new functionalities, such as wavelength selectivity, can easily be achieved without using a filter. In this talk, I will present the progress in understanding the noise, gain, sensitivity and color selectivity of perovskite photodetectors.  The challenges, particularly stability, will be briefed as well, for applications such as X-ray imaging.