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Organic materials are suitable for light sensing devices showing unique features such as low cost, large area, and flexibility. Moreover, transparent photodetectors are interesting for smart interfaces, windows, and display-integrated electronics. The ease of depositing ultrathin organic films with simple techniques enables low light absorbing active layers, resulting in the realization of transparent devices. Here, we demonstrate a strategy to obtain high efficiency organic photodetectors and phototransistors based on transparent active layers with a visible transmittance higher than 90%. The photoactive layer is composed of two phases, each a few nanometers thick. First, an acceptor polymer, which is a good electron-transporting material, on top of which a small molecule donor material is deposited, forming noncontinuous domains. The small molecule phase acts as a trap for holes, thus inducing a high photoconductive gain, resulting in a high photoresponsivity. The organic transparent detectors proposed here can reach very high external quantum efficiency and responsivity values, which in the case of the phototransistors can be as high as ∼74000% and 340 A W-1at 570 nm respectively, despite an absorber total thickness below 10 nm. Moreover, frequency dependent 2D photocurrent mapping allows discrimination between the contribution of a fast but inefficient and highly spatially localized photoinduced injection mechanism at the electrodes, and the onset of a slower and spatially extended photoconductive process, leading to high responsivity

International audience; Recently we have improved the efficiency and the output power of our optically pumped continuous-wave THz sources. These sources are based on the beating of two laser lines in a wide bandwidth photodetector. Its intrinsic nonlinear behaviour is used to produce a beatnote at the frequency difference between the two laser lines (photomixing). These photomixers are continuously tunable THz sources working at room temperature. We have developed two kinds of photomixers: GaAs-based for 0.8 mu m pumping and InP-based for 1.5 mu m pumping. On GaAs the best results has been obtained thanks to low-temperature-grown GaAs (LTG-GaAs) photoconductors (PC). Efficiency and power were optimized by designing a new type of thin PC placed in a Fabry-Perot resonator. The high impedance of the PC is a well-known limitation of this device but with our approach it was possible to reduce its impedance by a factor 100. Moreover by designing an impedance matching network it was possible to obtain 1.8 mW at 252 GHz with a total efficiency of 0.5 %. On InP the best results are obtained with uni-travelling-carrier photodiodes (UTC-PD). The device was improved by designing a new heterostructure and new semi-transparent contacts with sub-wavelength apertures. The active layer was also bonded to a silicon substrate thanks to metal thermocompression. It is demonstrated that with this approach it is possible to obtain a power of 0.7 mW at 300 GHz with a total efficiency of 0.7 %. More generally the efficiency of optically pumped terahertz sources will be discussed.

This work reports on the development and characterization of bi-dimensional deep-UV sensor arrays based on synthetic diamond to address the requirements of space-born astrophysical experiments. The material was synthesized at the University of Rome “Tor Vergata” where both heteroepitaxial polycrystalline diamond films and homoepitaxial single-crystal diamonds are grown using a tubular MWCVD reactor. The quality of chemical vapour deposited diamond was characterized by cathodoluminescence, photoluminescence, Raman spectroscopy and thermally stimulated currents. Then, suitable samples were selected and used to fabricate photoconductive single-pixel and 2D array devices by evaporating metal contacts on the growth surface. The electro-optical characterization of the devices was carried out in a wide spectral region, ranging from 120 to 2400 nm. A deuterium lamp and a 0.5 m vacuum monochromator were used to measure the detector responsivity under continuous monochromatic irradiation in the 120–250 nm spectral range, while an optical parametric oscillator tunable laser producing 5 ns pulses was used as light source from 210 up to 2400 nm. Time response, signal-to-noise ratio, responsivity and visible rejection factor were evaluated and the results are hereafter summarized.

New pin-photodiode and metal–semiconductor–metal (MSM) photoconductor devices based on diamond material have been produced showing high responsivity around 200 nm. LYRA, the Large Yield RAdiometer, will use such detectors for the first time for a solar physics space instrument. A set of measurement campaigns was carried out to obtain their XUV-to-VIS characterization (responsivity, linearity, stability and homogeneity). The diamond pin and MSM photodetectors exhibit a high responsivity of 27 mA/W around 200 nm and demonstrate a visible rejection ratio (200 nm versus 500 nm) of six and four orders of magnitude, respectively. We show that these diamond photodetectors are sensitive sensors for the wavelength range of interest (1 nm to 220 nm), stable within a few percent, with a good linearity and moderate homogeneity.

In this paper, two kinds of colloidal quantum dots, PbS and HgTe, are explored for SWIR photodetectors application. The colloidal dots are prepared by hot injection chemical synthesis, with organic ligands around the dots keeping them stable in solution. For the purpose of achieving efficient carrier transport between the dots in a film, these long organic ligands are replaced by shorter, inorganic ligands. We report uniform, ultra-smooth colloidal QD films without cracks realized by dip-coating and corresponding ligand exchange on a silicon substrate. Metal-free inorganic ligands, such as OH- and S2-, are investigated to facilitate the charge carrier transport in the film. Both PbS and HgTe-based quantum dot photoconductors were fabricated on interdigitated gold electrodes. For PbS-based detectors a responsivity of 200A/W is measured at 1.5μm, due to the large internal photoconductive gain. A 2.2μm cut-off wavelength for PbS photodetectors and 2.8μm for HgTe quantum dot photodetectors are obtained.

Çinko oksit, ZnO, birçok optoelektronik cihaz uygulaması için önemli bir malzemedir. Özellikle yüksek eksiton bağlanma enerjisi (60 meV) ile ünlü bir malzemedir. Bu yüksek bağlanma enerjisi onu morötesi ışık-yayan-diyotlar (LEDs) ve lazerler için iyi bir aday malzeme yapar. Yüksek elektriksel taşıyıcı hareketliliği ve geniş enerji bant aralığı (3.37 eV ? 368 nm) sayesinde saydam elektronik aygıtlar ve morötesi ışık sensörlerinde kullanılmak için aday bir malzemedir. Fakat ZnO kristal yapısı içerisinde hata seviyelerine sahiptir (örneğin eksik oksijen ya da fazla çinko atomu). Bu kristal hataları ZnO bazlı morötesi LED, lazer ve ışık sensörlerinin performanslarını düşürmektedir.Bu tez çalışmasında, kristal hatalarını kullanarak band aralığından daha düşük enerjili fotonları emen ve görünür ışığa tepki veren ZnO bazlı foto-ince-film-transistörler (photo-TFT) incelendi. ZnO bazlı foto-ince-film-transistörlerin dizayn, üretim ve karakterizasyonu sunuldu. Üç kutuplu optoelektronik bir cihaz olan photo-TFT?nin yapısı basit olarak iki kutuplu ve ışıkla iletkenliği değişen bir yarı iletkene üçüncü bir kutup eklenmiş halidir. Bu üçüncü kutup (kapı kutbu) yarı iletkenin elektriksel ve optik özelliklerini aktif olarak kontrol eder.Temiz oda ortamlarında, çeşitli boylarda ve kaplama sıcaklıklarında ZnO bazlı photo-TFT?ler üretilmiştir. İlk olarak, üretilen aygıtların transistör karakterizasyonu yapılarak, kapı kutbunun ZnO?ın elektriksel özelliklerini dinamik olarak kontrol edebildiği gösterildi. Aygıt boyutunun ve kaplama sıcaklığının cihaz performansı üzerine etkileri incelendi. 250 °C?de kaplanmış ZnO katmanının emilim ve ışıma karakterizasyonu yapılarak, görünür ışığı absorb etme yeteneği ve hata seviyelerinin ZnO?ın enerji bant aralığındaki enerji seviyeleri araştırıldı. Sonrasında, 250 °C kaplanmış ZnO bazlı photo-TFT?lerin ışığa verdikleri elektriksel tepki (responsivity) ölçülerek aktif olarak ayarlanabilen görünür ışığa tepki mekanizması tartışıldı. Son olarak, ZnO kaplama sıcaklığının ve aygıt boyutunun cihaz performansı üzerine etkileri sunuldu. Zinc oxide, ZnO, is an important material for wide range of optoelectronic device applications. Especially, ZnO is famous with its large exciton binding energy of 60 meV which makes it a good candidate for ultraviolet light emitting diodes and lasers. Moreover, its high carrier mobility and wide band gap of 3.37 eV (368 nm) makes it a promising material for transparent electronics and UV photodetectors. However, ZnO has crystallographic defect states (e.g. oxygen vacancies, zinc interstitials) which degrade the performance of ZnO based LEDs, lasers and UV photodiodes.In this thesis, ZnO based photo-thin-film-transistors (photo-TFTs) with visible light response by using their defect states to absorb subbandgap photons are investigated. The design, fabrication and characterization of ZnO based photo-TFTs are presented. A photo-TFT is a three-terminal optoelectronic device that is a photoconductor structure with an additional gate terminal which actively tunes electrical and optical properties of photoconductive material.In a clean room environment, ZnO based photo-TFTs with various device sizes are fabricated at different ZnO channel layer deposition temperatures (ranging from 80 to 250 °C). Initially, TFT characteristics of fabricated devices are characterized to show that the gate terminal dynamically modulates ZnO?s channel conductivity. Moreover, the effects of the device size and the deposition temperature on device performance are investigated. Then, the optical characterization of ZnO film deposited at 250° C is conducted via absorption and photoluminescence measurements in order to investigate its visible light absorption characteristics and the energy levels of its defect states in the forbidden band gap of ZnO. After that, the responsivity measurements are reported from ZnO based photo-TFTs fabricated at 250 °C and the active tuning mechanism of visible light photoresponse is discussed. Finally, the effects of the deposition temperature and the device size on the visible light responsivity are presented. 72

We report on colloidal PbS quantum dot near infrared photoconductors operating at low voltage. In order to maximize the device sensitivity, we exploited the advantages of different measurement techniques and methods for dark current cancellation and noise reduction including the DC volt-amperometric measurement with offset cancellation, a Wheatstone-bridge configuration and impedance measurements. We demonstrate that even photodetectors with moderate detectivity (10 9 -10 10 cmHz 1/2 /W), exhibit very large SNR.