Small Fill Factor Research Articles

Green‐Solvent‐Processed 17% Efficient Polymer Solar Cell Achieved Synergistically by Aligning Energy Levels and Improving Morphology with the Quaternary Strategy

Low and unbalanced charge mobilities result in low short‐circuit current density (JSC) and small fill factor (FF), which greatly limits the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) processed with green solvents. Herein, a unique quaternary material system (PM6:BTP‐BO‐4F:BTP:PhI‐Se) is reported, which uses an upshifted highest occupied molecular orbital (HOMO) acceptor guest (BTP) and a deep‐HOMO, ultra‐wide bandgap polymer donor guest (PhI‐Se) as quaternary strategy to align energy levels and improve morphology, leading to open‐circuit voltage (VOC) (from 0.812 to 0.851 V), FF (from 66.1% to 76.7%), and JSC (from 24.4 to 26.1 mA cm−2) increased simultaneously, hence obtaining PCEs of 17.0% processed with toluene. When processed with chlorobenzene (CB), 18.2% efficiency is obtained. Adding BTP and PhI‐Se as the third component increases hole and electron mobilities, respectively, going from 1.32/0.63 × 10−4 cm2 V−1 s−1 for the host binary to 1.56/2.56 and 2.26/2.86 × 10−4 cm2 V−1 s−1 for the BTP and PhI‐Se ternary. With both adding, the values shift to 3.69/3.20 × 10−4 cm2 V−1 s−1 for the quaternary blends. The crystalline coherence length (CCL) increases from 18.8 nm to 20.2 nm and 23.6 nm, respectively, for the two ternaries, and then 25.7 nm for the quaternary blend.

Influence of SnO2, ZnO and TiO2 layer on the performance of CIGS and CdTe solar cells

As a type electron transmission layer, SnO2, ZnO and TiO2 layer were applied as a buffer between transparent conductive oxide and n-CdS layer in CIGS and CdTe solar cells. J-V curve of device with three different buffer layer were simulated theoretically through Poisson equation and electron-hole continuity equation. Open-circuit voltage, short-circuit current, fill factor and conversion efficiency were derived. It’s shown that for CdTe solar cells, performance of device with three kind of buffer layers SnO2, ZnO and TiO2 did not differ two much. While for the CIGS solar cells, device with TiO2 showed a small fill factor, then leading to a lower conversion efficiency. Transparent oxide SnO2 and ZnO were suggested for the buffer layer for optimization CIGS and CdTe solar structure depending on the art of manufacturing.

Full-field quantum imaging with a single-photon avalanche diode camera

Single-photon-avalanche diode (SPAD) arrays are essential tools in biophotonics, optical ranging and sensing and quantum optics. However, their small number of pixels, low quantum efficiency and small fill factor have so far hindered their use for practical imaging applications. Here, we demonstrate full-field entangled photon pair correlation imaging using a 100-kpixels SPAD camera. By measuring photon coincidences between more than 500 million pairs of positions, we retrieve the full point spread function of the imaging system and subsequently high-resolution images of target objects illuminated by spatially entangled photon pairs. We show that our imaging approach is robust against stray light, enabling quantum imaging technologies to move beyond laboratory experiments towards real-world applications such as quantum LiDAR.

Synthesis and characterization of Co and Ga co-doped ZnO thin films as an electrode for dye sensitized solar cells

Using a time efficient and cost effective approach so called sol‒gel dip coating route, we deposited thin films of ZnO and 1% Co & (0%, 0.5%, 1%, and 1.5%) Ga co-doped ZnO on the fluorine doped tin oxide substrates. All the films exhibited hexagonal wurtzite structure. X-rays study revealed that crystallite size increased with increase of doping. Optical parameters like absorbance, transmittance, refractive index, band gap, extinction coefficient and dielectric constants were measured using UV–Vis spectroscopy and it has been noticed that doping resulted in reduction of band gap. It has also been observed that, the films prepared with 1% Co & 1% Ga co-doped ZnO have comparatively smaller band gap and thus have high refractive index and high transmittance in visible region. The calculation of different electrically important factors for instance, high current density, small open circuit voltage, efficiency and fill factor revealed that co-doped ZnO with 1% Co and 1% Ga has relatively high efficiency of 2.43% and thus shows the potential of this composition as an electrode for solar cell devices.

Comparison of performances of three active layers cascade OPVCs with those obtained from corresponding bi-layers

In this study, organic photovoltaic cells based on planar heterojunctions using small-molecules were fabricated. Two cell configurations were used, planar heterojunctions based on a classical electron donor/electron acceptor couple and planar heterojunctions using three active layers, one donor, one acceptor and a central ambipolar layer. The donor is a thiophene derivative called BSTV, the acceptor is the fullerene while the ambipolar material is a phthalocyanine dye, the SubPc. After studying the performances of cells based on the couples SubPc/C60, BSTV/C60 and BSTV/SubPc, we explore the dependence of the short circuit current and the external quantum efficiency on the SubPc interlayer thickness in cells with three active layers. By comparison with the optimum couple SubPc/C60 the ternary structure BSTV/SubPc/C60 exhibits higher short circuit current but smaller fill factor. Nevertheless, for an optimum interlayer thickness of 12.5 nm, the ternary structure allows an improvement of the cell efficiency of 11% by comparison with the highest result obtained with the best couple. As shown by optical measurements, long range Föster energy transfer from the larger band gap donor, BSTV, to the smaller band gap, SubPc, is possible. Therefore, the increase of Jsc is attributed to two-step exciton dissociation process. Excitons produced in the outer BSTV layer are transferred to the SubPc central layer and then dissociated at the interface.

Effects of different small molecule hole transporters on the performance and charge transfer dynamics of perovskite solar cells

Perovskite solar cells with popular 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) as the hole transporting material are compared to those prepared with 2,2′,7,7′-tetrakis(N,N-di(4-methoxyphenyl)amino)-spiro-(fluorene-9,9′-xanthene) (X60), its recently proposed new and potentially cheaper alternative, and with a mixture of both materials. The cells with both small-molecule transporters have similar photocurrent and photovoltage, but the cells with X60 exhibit about 13% smaller fill factor than those with spiro-OMeTAD. The dynamics of the interfacial charge transfer occurring on the time scales from femtoseconds to seconds is explored. The overall hole injection is slower from methylammonium lead iodide to X60 than that to spiro-OMeTAD by about 0.04ns−1. Similarly, slower hole recombination is found for the cells with X60, taking place on the millisecond time scale in the dark. The recombination under illumination is accelerated more for ×60 material, which might explain the differences in fill factor values. The cells with the 1:1 mixture of spiro-OMeTAD and X60 show intermediate behavior in all aspects, revealing that a simple combination of small molecule transporters can be used to tune the properties of HTM.

Compact SPAD-Based Pixel Architectures for Time-Resolved Image Sensors.

This paper reviews the state of the art of single-photon avalanche diode (SPAD) image sensors for time-resolved imaging. The focus of the paper is on pixel architectures featuring small pixel size (<25 μm) and high fill factor (>20%) as a key enabling technology for the successful implementation of high spatial resolution SPAD-based image sensors. A summary of the main CMOS SPAD implementations, their characteristics and integration challenges, is provided from the perspective of targeting large pixel arrays, where one of the key drivers is the spatial uniformity. The main analog techniques aimed at time-gated photon counting and photon timestamping suitable for compact and low-power pixels are critically discussed. The main features of these solutions are the adoption of analog counting techniques and time-to-analog conversion, in NMOS-only pixels. Reliable quantum-limited single-photon counting, self-referenced analog-to-digital conversion, time gating down to 0.75 ns and timestamping with 368 ps jitter are achieved.

$ per W metrics for thermoelectric power generation: beyond ZT

Thermoelectric materials for power generation are typically compared using the dimensionless figure-of-merit ZT because it relates directly to the device efficiency. However, for practical applications, the cost of power generation – as governed by material, manufacturing, and heat exchanger costs – is also a critical factor which is not captured in ZT alone. The necessary analysis, derived herein, optimizes the coupled thermoelectric and economic problem for the leg length, L, and system fill factor, F, as functions of these costs. Fuel, operating, and maintenance costs are excluded. This optimization yields the minimum $ per W value for thermoelectric power generation and a framework for comparing materials beyond ZT. This analysis shows that even very expensive thermoelectric materials have the potential to be the most cost effective at the system level, if incorporated with sufficiently short legs and small fill factor. An approximate scaling analysis, verified using numerical calculations, gives the first closed-form, analytical expressions for optimal L and F to minimize $ per W. The analysis also delineates various cost-dominant regimes with different priorities for materials development, including: (i) a heat exchanger cost dominated regime, where ZT should be increased regardless of material or manufacturing costs; (ii) an areal cost, C′′, dominated regime at fixed F, where ZT/C′′ should be maximized, and (iii) a volumetric cost, C′′′, dominated regime at fixed F, where ZT/(kC′′′) should be maximized, reinforcing the need for low thermal conductivity, k. The cost–performance framework derived here will be applied to a number of real materials and applications in a separate manuscript.

Co-optimized design of microchannel heat exchangers and thermoelectric generators

Designs of heat exchangers have mostly been disconnected to the performance of thermoelectric generator (TEG) systems. The development work, mostly focused on thermoelectric materials, required a significant amount of engineering parametric analysis. In this work, a micro plate-fin heat exchanger applied to a TEG is investigated and optimized to maximize the output power and the cost performance of generic TEG systems. The cost per performance is counted by a measure of price per power output ($/W). The channel width, channel height, fin thickness of heat exchanger, and fill factor of TEG are theoretically optimized for a wide range of pumping power. In conjunction with effective numeric tests, the model discusses the optimum size of the system components' dimensions at two area sizes of the substrate plate of heat exchanger. Results show that at every pumping power, there are particular values of channel width and fin thickness that provide maximum output power in the TEG. In addition, for producing maximum cost performance at lower pumping power, larger channel width and channel height and smaller fill factor are required. The results also illustrate that there is a unique pumping power for fixed thickness of fin and ceramic substrates that provides minimum cost per performance for the TEG systems. The theoretical results of the micro heat exchanger are in a good agreement with the experimental investigation data.

Back-Side Readout Silicon Photomultiplier

We present a novel structure for the back-side readout silicon photomultipler (SiPM). Current SiPMs are front-illuminated structures with front-side readout, which have relatively small geometric fill factor leading to degradation in their photon detection efficiency (PDE). Back-side readout devices will provide an advantageous solution to achieve high PDE. We designed and investigated a novel structure that would allow back-side readout while creating a region of high electric field optimized for avalanche breakdown. In addition, this structure has relatively high fill factor and also allow direct coupling of individual micro-cell of the SiPM to application-specific integrated circuits. We will discuss the performance that can be attained with this structure through device simulation and the process flow that can be used to fabricate this structure through process simulation.

Development of rotary infrared micro-scanner

Poor resolution and lower modulation thansfer function(MTF) value in infrared imaging system have been remaining for many years because of its large Airy disc of optical lens and the small fill-factor of focal plane array(FPA),respectively.This paper reported a popular way to improve the resolution and MTF of infrared images using micro-scanning technique.The principles were briefly described.The designing rotary IR micro-scanner was discussed.The rotary micro-scanner imaging system using a 240×320 pixels α-siuncooled FPA,and got higher resolution and MTF value of IR videos obviously.

Improving InGaN-LED performance by optimizing the patterned sapphire substrate shape

The epitaxial growths of GaN films and GaN-based LEDs on various patterned sapphire substrates (PSSes) with different values of fill factor (f) and slanted angle (θ) are investigated in detail. The threading dislocation (TD) density is lower in the film grown on the PSS with a smaller fill factor, resulting in a higher internal quantum efficiency (IQE). Also the ability of the LED to withstand the electrostatic discharge (ESD) increases as the fill factor decreases. The illumination output power of the LED is affected by both θ and f. It is found that the illumination output power of the LED grown on the PSS with a lower production of tan θ and f is higher than that with a higher production of tan θ and f.

Plasmonic-Resonant Bowtie Antenna for Carbon Nanotube Photodetectors

The design of bowtie antennas for carbon nanotube (CNT) photodetectors has been investigated. CNT photodetectors have shown outstanding performance by using CNT as sensing element. However, detection wavelength is much larger than the diameter of the CNT, resulting in small fill factor. Bowtie antenna can confine light into a subwavelength volume based on plasmonic resonance, thus integrating a bowtie antenna to CNT photodetectors can highly improve photoresponse of the detectors. The electric field enhancement of bowtie antennas was calculated using the device geometry by considering fabrication difficulties and photodetector structure. It is shown that the electric field intensity enhancement increased exponentially with distance reduction between the CNT photodetector to the antenna. A redshift of the peak resonance wavelength is predicted due to the increase of tip angles of the bowtie antennas. Experimental results showed that photocurrent enhancement agreed well with theoretical calculations. Bowtie antennas may find wide applications in nanoscale photonic sensors.

A hybrid CMOS-imager with a solution-processable polymer as photoactive layer

The solution-processability of organic photodetectors allows a straightforward combination with other materials, including inorganic ones, without increasing cost and process complexity significantly compared with conventional crystalline semiconductors. Although the optoelectronic performance of these organic devices does not outmatch their inorganic counterparts, there are certain applications exploiting the benefit of the solution-processability. Here we demonstrate that the small pixel fill factor of present complementary metal oxide semiconductor-imagers, decreasing the light sensitivity, can be increased up to 100% by replacing silicon photodiodes with an organic photoactive layer deposited with a simple low-cost spray-coating process. By performing a full optoelectronic characterization on this first solution-processable hybrid complementary metal oxide semiconductor-imager, including the first reported observation of different noise types in organic photodiodes, we demonstrate the suitability of this novel device for imaging. Furthermore, by integrating monolithically different organic materials to the chip, we show the cost-effective portability of the hybrid concept to different wavelength regions.

Optical micro-endoscopes for 3D in-vivo imaging

Optical coherence tomography (OCT) is a medical optical imaging technique that can achieve high-resolution 3D images (1 15 m)1 by detecting infrared light scattered by body tissue. It has an imaging depth range of about 3mm in highly-scattered tissue and has already been widely used for external imaging, e.g., for eye diseases and skin cancer diagnosis. It would be useful to apply OCT to internal imaging, since about 85% of all cancers originate from the surface layers of internal organs. As these layers are within its imaging depth range, OCT can detect cancer at an early stage as well as precisely determine tumor margins. However, it is very challenging to apply this technique to internal organs because of stringent size and imaging time constraints. One promising approach is to use microelectromechanical systems (MEMS) scanning micromirrors. Several research groups have demonstrated actuation mechanisms to generate rotational scanning for micromirrors for endoscopic OCT. Among them, electrostatic micromirrors can be very fast and consume low power, but the angular scan range is limited and the required driving voltage is too high ( 100V) to be safe for patients. Electromagnetic micromirrors have also been used for OCT probes, but the assembly process is complicated and the potential electromagnetic interference caused by the use of permanent magnets is another concern. Another problem these mirrors share is that they all have a small fill factor (the ratio of the mirror area to the device area). For a given resolution, the required mirror area is fixed. Thus, a small fill factor leads to a large device, which in turn increases the imaging probe size. Electrothermal mirrors, on the other hand, can achieve a large scan range at low drive voltage, but most of them have large initial tilt angles and significant shift of rotation centers, causing serious optical alignment and coupling problems. Figure 1. Scanning electron micrograph of an electrothermal MEMS mirror. Chip size: 2mm 2mm.

Electrical Characteristics and Photoresponse of ZnO/ZnTe Heterojunction Diodes

Heterojunction diodes consisting of n-type ZnO and p-type ZnTe were grown by pulsed laser deposition and molecular beam epitaxy, respectively, on GaAs (001) substrates. Strong diode rectifying behavior was observed in the current–voltage characteristics with a current on/off ratio of J on /J off = 1 × 105 and a diode ideality factor of n = 1.5. A strong photoresponse in the energy range of 2.3 eV to 3.6 eV was observed, corresponding to the bandgap energies of ZnTe and ZnO, respectively. A photovoltaic response was observed with a relatively small fill factor with a short-circuit current J sc ~ 0.8 mA/cm2 and open-circuit voltage of V oc ~ 60 mV subject to illumination by a tungsten lamp. The photovoltaic response and reverse saturation current are believed to be limited by defects related to the mismatch between the ZnTe and ZnO structures and defects in the ZnO layer deposited at low temperature. The spectral response of the diodes is modeled with a close match to experimental measurements.

Global Shutter CMOS Image Sensor With Wide Dynamic Range

A novel concept for global shutter CMOS image sensors with wide dynamic range (WDR) implementation is presented. The proposed imager is based on the multisampling WDR approach and it allows an efficient global shutter pixel implementation achieving small pixel size and high fill factor. The proposed imager provides wide DR by applying adaptive exposure time to each pixel, according to the local illumination intensity level. Two pixel configurations, employing different kinds of a 1-bit in-pixel memory were implemented. An imager, including two different pixels was designed and simulated in 0.18-mum CMOS technology. System architecture and operation are discussed and simulation results are presented.

Observation of low optical overlap mode propagation in nanoscale indium phosphide membrane waveguides

The authors have developed a nanoscale, rib-loaded waveguide that propagates a low optical overlap mode (LOOM) in which less than 1% of the modal field energy resides in the semiconductor material. Because of the small modal fill factor, the potential for extremely low waveguide propagation loss, on the order of 0.001dB or less, is predicted. Elevated membrane waveguides, 50nm thick with a 50nm thick rib, have been fabricated in InP using a multistep microelectromechanical release process. Both transverse electric and transverse magnetic LOOM propagations have been observed and measurements are compared to theoretical predictions.

A numerical simulation of mask shape effect for fabrication of hexagonal microlens array by using excimer laser dragging process

This investigation analysed the characteristics of simulating the fabrication of an aspheric hexagonal microlens array using a laser dragging process. Traditionally, an aspheric microlens array with rectangular coordinates was produced using an excimer laser dragging process. For the hexagonal case, the first step in the proposed dragging process is to generate microchannels with a cross-section shape similar to the mask. Then, the dragging procedure is repeated twice, each time after rotating the work piece by 60° relative to the previous channels. This microlens array can be applied in image processing. Variously shaped masks were computer simulated. The results demonstrate that the parabolic-shaped mask can generate excellent axial symmetry for both rectangular and hexagonal microlens array. The analytical results of this study indicate that a hexagonal microlens array has a better axial symmetry than a rectangular microlens array. But for the parabolic case, a hexagonal microlens array has a smaller fill factor than a rectangular microlens array.

Phase-locking of multicore fibre laser due to talbot self-reproduction

The multicore fibre laser (MCFL), containing an array of single-mode microcores in a circle inside the pump core, is a promising compact laser source. The problem is to synchronize the radiation of microcores with different propagation constants at a given radiation frequency. The theory of phase-locking of an MCFL with an external mirror and an annular waveguide matched to the multicore fibre and having a length some fraction of the Talbot distance is developed. Collective mode selection appears to occur due to spatial filtering at fractional Talbot distances, while the radiation frequency self-adjusts within the spectral gain range to minimize losses. Parallel coupling between microcores is achieved in the limit of a small fill factor. The maximum number of microcores that can be coupled is found. Good agreement is achieved between the theory and previously published experimental results.