While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.

We fabricate PbS colloidal quantum dot (QD)-based solar cells using a fullerene derivative as the electron-transporting layer (ETL). A thiol treatment and oxidation process are used to modify the morphology and electronic structure of the QD films, resulting in devices that exhibit a fill factor (FF) as high as 62%. We also show that, for QDs with a band gap of less than 1 eV, an open-circuit voltage (VOC) of 0.47 V can be achieved. The power conversion efficiency reaches 1.3% under 1 sun AM1.5 test conditions and 2.4% under monochromatic infrared (lambda=1310 nm) illumination. A consistent mechanism for device operation is developed through a circuit model and experimental measurements, shedding light on new approaches for optimization of solar cell performance by modifying the interface between the QDs and the neighboring charge transport layers.

groups have recently and independently demonstrated that, by applying an electric fi eld across a pristine fi lm of 3D hybrid perovskites of different chemical composition, a self-sustained fi eld is induced in the semiconductor as a consequence of ion migration toward the electrode regions. [10][11][12][13] The formation of a self-sustained internal fi eld upon device polarization is also in good agreement with the observations reported by Tan et al. when testing perovskite-based light emitting diodes. [ 14 ] This concept has also been the base of the explanation proposed by Tress et al. for the rate-dependent hysteresis seen in current-voltage scans of solar cells. [ 15 ] So far reports suggest that transient electrical characteristics are due to a polarization response of the perovskite active layer that results in changes in the photocurrent extraction effi ciency of the device. [ 11,15 ] However, it must be noted that a variety of dynamics have been reported, which differ in magnitude and time scale, depending both on the specifi c device architecture and, in particular, on the adopted charge extraction layer. [ 7,9 ] This indicates that contact interfaces have a considerable effect on transients in perovskite based devices.In this Communication we investigate the role played by charge extracting layers on the slow transient behavior of CH 3 NH 3 PbI 3 perovskite based solar cells. Such transients, which typically affect both short-circuit currents and open circuit voltage of hysteretic devices, are found to notably modify the open circuit voltage also in the very fi rst J -V scans of so-called "hysteresis-free" devices integrating a phenyl-C61-butyric acid methyl ester (PCBM) charge extraction layer. Here a preconditioning of the device, i.e., a repetition of J -V scans, is needed to achieve completely stable J -V characteristics under illumination. In particular, we fi nd that under device operation, iodide ions migrate to the electron extracting layer. We fi rst show that the use of an organic extraction layer such as PCBM, albeit not hampering ions motion, evidently improves charge extraction with respect to interfaces involving compact TiO 2 , in agreement with what is suggested in other seminal investigations, [ 16,17 ] and makes the short-circuit current density virtually insensitive to the transient phenomena related to ions migration. Moreover, while self-doping of the perovskite fi lm close to the contact has been generally put forward in the study of transient phenomena, [ 12,13 ] here we show that ions can specifi cally interact with the organic electron extracting layer, inducing electronic doping and that such I − /PCBM interaction is at the origin of the preconditioning requirement for stabilizing the device and for improving its open circuit voltage with respect to the fi rst scan.Solution-processable hybrid perovskite semiconductors have risen to the forefront of photovoltaics research, offering the potential to combine low-cost fabrication with high-powerconversion effi ciency. Originally u...

Bulk-heterojunction based organic photodetectors are fabricated by means of drop-on-demand inkjet printing with vertical topology, inverted structure, and small footprint (about 100 μm x 100 μm). Due to optimization of the deposition technique, an external quantum efficiency in excess of 80% at 525 nm and a -3dB bandwidth of a few tens of kHz is achieved.

In this work we demonstrate hyperbranched nanostructures, grown by pulsed laser deposition, composed of one-dimensional anatase single crystals assembled in arrays of high aspect ratio hierarchical mesostructures. The proposed growth mechanism relies on a two-step process: self-assembly from the gas phase of amorphous TiO2 clusters in a forest of tree-shaped hierarchical mesostructures with high aspect ratio; oriented crystallization of the branches upon thermal treatment. Structural and morphological characteristics can be optimized to achieve both high specific surface area for optimal dye uptake and broadband light scattering thanks to the microscopic feature size. Solid-state dye sensitized solar cells fabricated with arrays of hyperbranched TiO2 nanostructures on FTO-glass sensitized with D102 dye showed a significant 66% increase in efficiency with respect to a reference mesoporous photoanode and reached a maximum efficiency of 3.96% (among the highest reported for this system). This result was achieved mainly thanks to an increase in photogenerated current directly resulting from improved light harvesting efficiency of the hierarchical photoanode. The proposed photoanode overcomes typical limitations of 1D TiO2 nanostructures applied to ss-DSC and emerges as a promising foundation for next-generation high-efficiency solid-state devices comprosed of dyes, polymers, or quantum dots as sensitizers.

We demonstrate an organic photodetector showing high detectivity (3.4×1012 Hz0.5 cm/W) at a wavelength of 700 nm, based on squaraine/phenyl-C61-butyric-acid-methyl-ester bulk-heterojunction active material. This result is achieved by suppressing the device dark currents while simultaneously preserving its external quantum efficiency, as high as 15% at 700 nm. To this aim, a thin cross-linked film based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] is exploited to suppress electron injection from the device anode into the organic blend, thus reducing the dark currents by a factor of 30, to the extremely low value of 2 nA/cm2. Also, the detector bandwidth (∼1 MHz) is unaffected by the introduction of a blocking layer.

The working mechanisms of excitonic solar cells are strongly dominated by interface processes, which influence the final device efficiency. However, it is still very challenging to clearly track the effects of inter-molecular processes at a mesoscopic level. We report on the realization of polymer-based hybrid solar cells made of prototypical materials, namely, poly(3-hexylthiophene) (P3HT) finely infiltrated in a TiO 2 scaffold, with power conversion efficiency exceeding 1%. A step-change improvement in the device performance is enabled by engineering the hybrid interface by the insertion of an appropriate molecular interlayer. An unprecedented set of characterization techniques, including time-resolved optical spectroscopy, X-ray photoemission spectroscopy, positron annihilation spectroscopy and atomistic simulations, allows us to rationalize our findings. We show that a suitable chemical structure of the interlayer molecule induces selective intermolecular interactions, and thus a preferential surface energetic landscape and morphological order at the interface which consequently drives a strong improvement in charge generation and a decrease in recombination losses.

Squaraine compounds are currently investigated as high performance active components in both organic and hybrid photovoltaic devices as well as in photodetectors. Their most valuable features include a particularly efficient optical absorption in the Vis-NIR region, high polarizability, and a remarkable chemical stability. Their full exploitation is somewhat limited by a negligible absorption in the UV-Vis region (prototypical squaraines basically do not absorb below 500 nm). The aim of the present paper is the design and synthesis of truly panchromatic squaraines to be effectively employed as the photoactive materials in Vis operating optoelectronic devices. Our strategy involves the design of squaraines that are both nonsymmetric and core-substituted with suitable electron-withdrawing groups. We show the effect of such a design strategy by means of UV-Vis spectroscopy, cyclic voltammetry and prototypical device performances rationalization.