Morphologic improvement of the PBDTTT-C and PC71BM blend film with mixed solvent for high-performance inverted polymer solar cells.

Tracing the evolution of the bulk heterojunction structure, a dramatic promotion in the efficiency of polymer solar cells has been obtained in recent years. The active layer morphology of low-bandgap polymer solar cells is one of the critical factors for high-efficiency performance. In the past, the relationship between morphology improvement and the device's characteristics (such as efficiency, fill factor and short-circuit current) in low-bandgap polymer solar cells has been studied intensively with regards to the conventional structure. Here we demonstrate the morphologic improvement of the poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl]/[6,6]-phenyl C71 butyric acid methyl ester (PBDTTT-C/PC71BM) blend film for inverted solar cells. By utilizing a mixed solvent of dichlorobenzene/chlorobenzene with (1,8-diiodooctane) additives, the device efficiency can be significantly enhanced, from 0.92% to 4.43%. This enhancement is attributed to active layer morphologic improvement promoting carrier transport. Furthermore, the thickness optimization of the active layer and the electron blocking layer MoO3 further contributes to efficiency. The device performance could be achieved with an efficiency as high as 5.35%, an open-circuit voltage of 0.70 V, a short-circuit current density of 13.5 mA cm(-2), and a fill factor of 57%.

Overcoming the Fermi-Level Pinning Effect in the Nanoscale Metal and Silicon Interface.

Silicon-based photodetectors are attractive as low-cost and environmentally friendly optical sensors. Also, their compatibility with complementary metal-oxide-semiconductor (CMOS) technology is advantageous for the development of silicon photonics systems. However, extending optical responsivity of silicon-based photodetectors to the mid-infrared (mid-IR) wavelength range remains challenging. In developing mid-IR infrared Schottky detectors, nanoscale metals are critical. Nonetheless, one key factor is the Fermi-level pinning effect at the metal/silicon interface and the presence of metal-induced gap states (MIGS). Here, we demonstrate the utilization of the passivated surface layer on semiconductor materials as an insulating material in metal-insulator-semiconductor (MIS) contacts to mitigate the Fermi-level pinning effect. The removal of Fermi-level pinning effectively reduces the Schottky barrier height by 12.5% to 16%. The demonstrated devices exhibit a high responsivity of up to 234 µA/W at a wavelength of 2 µm, 48.2 µA/W at 3 µm, and 1.75 µA/W at 6 µm. The corresponding detectivities at 2 and 3 µm are 1.17 × 108 cm Hz1/2 W-1 and 2.41 × 107 cm Hz1/2 W-1, respectively. The expanded sensing wavelength range contributes to the application development of future silicon photonics integration platforms.

Polyvinyl Butyral Polymeric Host Material-Based Fluorescent Thin Films to Achieve Highly Efficient Red and Green Colour Conversion for Advanced Next-Generation Displays.

Rare-earth element-free fluorescent materials are eco-friendlier than other traditional fluorescent precursors, such as quantum dots and phosphors. In this study, we explore a simple and facile solution-based technique to prepare fluorescent films, which are highly stable under ordinary room conditions and show hydrophobic behaviour. The proposed hybrid material was designed with hybrid composites that use polyvinyl butyral (PVB) as a host doped with organic dyes. The red and green fluorescent films exhibited quantum yields of 89% and 80%, respectively, and both are very uniform in thickness and water resistant. Additionally, PVB was further compared with another polymeric host, such as polyvinylpyrrolidone (PVP), to evaluate their binding ability and encapsulation behaviour. Next, the effect of PVB on the optical and chemical properties of the fluorescent materials was studied using UV spectroscopy and Fourier transform infrared spectroscopy. The analysis revealed that no new bond was formed between the host material and fluorescent precursor during the process, with intermolecular forces being present between different molecules. Moreover, the thickness of the fluorescent film and quantum yield relation were evaluated. Finally, the hydrophobic nature, strong binding ability, and optical enhancement by PVB provide a powerful tool for fabricating a highly efficient fluorescent film with enhanced stability in an external environment based on its promising encapsulation properties. These efficient fluorescent films have a bright potential in colour conversion for next-generation display applications.

Ultra-thin Ag/Si heterojunction hot-carrier photovoltaic conversion Schottky devices for harvesting solar energy at wavelength above 1.1 µm.

Traditional silicon solar cells can only absorb the solar spectrum at wavelengths below 1.1 µm. Here we proposed a breakthrough in harvesting solar energy below Si bandgap through conversion of hot carriers generated in the metal into a current using an energy barrier at the metal-semiconductor junction. Under appropriate conditions, the photo-excited hot carriers can quickly pass through the energy barrier and lead to photocurrent, maximizing the use of excitation energy and reducing waste heat consumption. Compared with conventional silicon solar cells, hot-carrier photovoltaic conversion Schottky device has better absorption and conversion efficiency for an infrared regime above 1.1 µm, expands the absorption wavelength range of silicon-based solar cells, makes more effective use of the entire solar spectrum, and further improves the photovoltaic performance of metal-silicon interface components by controlling the evaporation rate, deposition thickness, and annealing temperature of the metal layer. Finally, the conversion efficiency 3.316% is achieved under the infrared regime with a wavelength of more than 1100 nm and an irradiance of 13.85 mW/cm2.

Mid-Infrared Response from Cr/n-Si Schottky Junction with an Ultra-Thin Cr Metal.

Infrared detection technology has been widely applied in many areas. Unlike internal photoemission and the photoelectric mechanism, which are limited by the interface barrier height and material bandgap, the research of the hot carrier effect from nanometer thickness of metal could surpass the capability of silicon-based Schottky devices to detect mid-infrared and even far-infrared. In this work, we investigate the effects of physical characteristics of Cr nanometal surfaces and metal/silicon interfaces on hot carrier optical detection. Based on the results of scanning electron microscopy, atomic force microscopy, and X-ray diffraction analysis, the hot carrier effect and the variation of optical response intensity are found to depend highly on the physical properties of metal surfaces, such as surface coverage, metal thickness, and internal stress. Since the contact layer formed by Cr and Si is the main role of infrared light detection in the experiment, the higher the metal coverage, the higher the optical response. Additionally, a thicker metal surface makes the hot carriers take a longer time to convert into current signals after generation, leading to signal degradation due to the short lifetime of the hot carriers. Furthermore, the film with the best hot carrier effect induced in the Cr/Si structure is able to detect an infrared signal up to 4.2 µm. Additionally, it has a 229 times improvement in the signal-to-noise ratio (SNR) for a single band compared with ones with less favorable conditions.

Enhanced Efficiency of Semitransparent Perovskite Solar Cells via Double-Sided Sandwich Evaporation Technique for Four Terminal Perovskite-Silicon Tandem Application.

Halide perovskite based solar cells (PSC's) have shown tremendous potential based on its facile fabrication technique, and the low cost of perovskite thin film formation with efficiency passing through an unmatched growth in recent years. High quality film along with morphology and crystallinity of the perovskite layer influences the efficiency and other properties of the perovskite solar cell (PSC). Furthermore, semitransparent perovskite solar cells (ST-PSC) are an area of attraction due to its application in tandem solar cells, although various factors like suitable transparent rear electrodes and optimized technique limit the power conversion efficiency (PCE). In this article, we fabricated perovskite film using a technique termed Double-sided sandwich evaporation technique (DS-SET) resulting in high quality perovskite film (MAPbI3 and MAPbIxCl3-x). Using this fabrication approach as compared to the traditional spin-coating method, we reported an enhanced photovoltaic performance of the PSC with a better surface morphology and homogeneity. The best parameter via DS-SET was found to be SET 30 min, which demonstrated a PCE (%) up to 14.8% for MAPbI3 and 16.25% for MAPbIxCl3-x, respectively. Addressing the tandem solar cell, incorporating thin Ag as a transparent electrode with a thickness of 20 nm onto the PSC's as the top cell and further combining with the Si solar cell results in the four terminal (4T) tandem solar cell exhibiting a PCE (%) of 24.43%.

Low photoactive phase temperature all-inorganic, tin-lead mixed perovskite solar cell.

CsPbI3 films have recently attracted significant attention as efficient absorbers for thermally stable photovoltaic devices. However, their large bandgap and photoactive black phase formation at high temperature impede their use for practical applications. Using the concept of lattice contraction, we demonstrate a low bandgap (≤1.44 eV) cesium-based inorganic perovskite CsPb x Sn1-x I3 that can be solution processed at low temperature for photovoltaic devices. The results from systematic measurements imply that the partial substitution of lead (Pb) with tin (Sn) results in crystal lattice contraction, which is essential for realizing photoactive phase formation at l00 °C and stabilizing photoactive phase at room temperature. These findings demonstrate the potential of using cesium-based inorganic perovskite as viable alternatives to MA- or FA-based perovskite photovoltaic materials.

The relationship between basic group resonance and quantum yield of high efficiency red light fluorescent solutions.

Compared with rare earth elements and heavy metal elements, rare-earth-element-free fluorescent films can greatly reduce environmental hazards. In this study, we use a solution method to produce the fluorescent films. The film thickness is 10 µm, which can maintain fluorescent light intensity in an environment with an average humidity of 55.1 (RH%) after encapsulation. We also find that the type of solvent affects the resonance position of the C[triple bond, length as m-dash]N functional group in DCJTB at a wavenumber of 2196 (cm-1), measured with Fourier transform infrared spectroscopy. The functional group is affected by the polar effect with its displacement decreasing with the quantum yield. Finally, we successfully made a fluorescent solution with a resonance displacement of only 12.8 (cm-1) for the C[triple bond, length as m-dash]N functional group with the quantum yield being as high as 81.3% and a fluorescent film with a quantum yield as high as 84.8%.

Sandwich Evaporation-Solvent Annealing Fabrication of Highly Crystalline MAPbIxCl3-x Perovskite Solar Cells.

Perovskites doped with chlorine (Cl-), which are usually fabricated using the solution process, can effectively improve the stability and carrier mobility. Compared with the low tolerance of the solution process that relies mostly on personal skill, thermal evaporation is an important method for large-scale production of perovskite solar cells but the production cost is high. In this study, the sandwich evaporation-solvent annealing (SE-SA) method is proposed. Using sandwich evaporation with a low-cost chamber of the sandwich evaporation technique (SET) made in the laboratory and with the help of DMSO steam-assisted crystallization, we have successfully produced chlorine-containing perovskite solar cells with a high crystallinity and a high efficiency of 15.1% with Voc = 0.98 V, Jsc = 21.94 mA/cm2, FF = 74.29%, and Rs = 3.66 Ω·cm2, which can greatly reduce the production cost. It is worth mentioning that all the processes are carried out outside a glove box, which makes it possible for large-scale production of chlorine-containing perovskite solar cells by evaporation.

GaAs nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hybrid solar cells.

In this paper, a new type of hybrid solar cell based on a heterojunction between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and vertically aligned n-type GaAs nanowire (NW) arrays is investigated. The GaAs NW arrays are fabricated by directly performing the nano-etching of GaAs wafer with spun-on SiO(2) nanospheres as the etch mask through inductively coupled plasma reactive ion etching. The PEDOT:PSS adheres to the surface of the GaAs NW arrays to form a p-n junction. The morphology of GaAs NW arrays strongly influences the characteristics of the GaAs NW/PEDOT:PSS hybrid solar cells. The suppression of reflectance and the interpenetrating heterojunction interface of GaAs NW arrays offers great improvements in efficiency relative to a conventional planar cell. Compared to the planar GaAs/PEDOT:PSS cells, the power conversion efficiency under AM 1.5 global one sun illumination is improved from 0.29% to 5.8%.

Electroluminescence from ZnO/Si-nanotips light-emitting diodes.

A new and general approach to achieving efficient electrically driven light emission from a Si-based nano p-n junction array is introduced. A wafer-scale array of p-type silicon nanotips were formed by a single-step self-masked dry etching process, which is compatible with current semiconductor technologies. On top of the silicon nanotip array, a layer of n-type ZnO film was grown by pulsed laser deposition. Both the narrow line width of 10 nm in cathodoluminescence spectra and the appearance of multiphonon Raman spectra up to the fourth order indicate the excellent quality of the ZnO film. The turn-on voltage of our ZnO/Si nanotip array is found to be approximately 2.4 V, which is 2 times smaller than its thin film counterpart. Moreover, electroluminescence (EL) from our ZnO/Si nanotips array light-emitting diode (LED) has been demonstrated. Our results could open up new possibilities to integrate silicon-based optoelectronic devices, such as highly efficient LEDs, with standard Si ultralarge-scale integrated technology.

Growth process control produces high-crystallinity and complete-reaction perovskite solar cells.

The growth process control (GPC) method, a new method which is better than thermal evaporation, for producing high-crystallinity perovskites by controlling the growth time in a low vacuum, is explored in this work. Inspired by evaporation technology, GPC is an effective method for modifying traditional thermal evaporation and for controlling the crystal growth of perovskite CH3NH3I3. Compared to fabrication with the process of co-evaporation, the MAPbI3 perovskite solar cell fabricated by GPC has high uniformity and film coverage. All of the manufacturing is carried out outside of the glove box. It provides an easy and effective way for perovskite fabrication for industrialization. Here, after using GPC to form perovskite solar cells, the residual methylammonium iodide (MAI) and PbI2 which is produced by the evaporation process can react completely, observed by time of flight secondary ion mass spectrometry (TOF-SIMS). Finally, formed by GPC, perovskite solar cells exhibit high performance and fewer crystal defects. The electron and hole recombination is greatly reduced. Through the GPC method, the J sc and the filling factor are improved with the increase of time after the fabrication. The power conversion efficiency was increased from 11.12% to 16.4%.

Solar Power Can Substantially Prolong Maximum Achievable Airtime of Quadcopter Drones.

Sunlight energy is potentially excellent for small drones, which can often operate during daylight hours and fly high enough to avoid cloud blockade. However, the best solar cells provide limited power, compared to conventional power sources, making their use for aerial vehicles difficult to realize, especially in rotorcraft where significant lift ordinarily generated by a wing is already sacrificed for the ability to hover. In recent years, advances in materials (use of carbon-fiber components, improvement in specific solar cells and motors) have finally brought solar rotorcraft within reach. Here, the application is explored through a concise mathematical model of solar rotorcraft based on the limits of solar power generation and motor power consumption. Multiple solar quadcopters based on this model with majority solar power are described. One of them has achieved an outdoor airtime over 3 hours, 48 times longer than it can last on just battery alone with the solar cells carried as dead weight and representing a significant prolongation of drone operation. Solar-power fluctuations during long flight and their interaction with power requirements are experimentally characterized. The general conclusion is that solar cells have reached high enough efficiencies and can outperform batteries under the right conditions for quadcopters.

Application of Silicon Nanostructure Arrays for 6-inch Mono and Multi-Crystalline Solar Cell.

In this study, we fabricate uniform silicon nanowire (SiNW) arrays on 6-inch mono- and multi-crystalline wafers by employing the improved solution-processed metal-assisted chemical etching (MacEtch) method. Furthermore, the improved MacEtch can be applied to various crystalline orientation wafers. The SiNW arrays are 470 nm in length with high density; they demonstrate a good optical trapping effect and reflectance well below 6% over a broad wavelength range from 300 to 1100 nm. The improved MacEtch shows no difference in reflectance for a pyramid/SiNW mono-crystalline wafer with appropriate uniformity; the average delta from the center to other positions is within 22%. The effective lifetime is lower for SiNW arrays because the higher surface state causes higher surface recombination.Finally, we make the multi-crystalline wafer into an Al-BSF solar cell device with MacEtch SiNW texture, resulting in an averaged power conversion efficiency of 17.83%, which is higher than that of standard acid-textured solar cell devices. Consequently, the improved MacEtch concept is suitable for commercial mass production in the photovoltaic industry.

Photovoltaic Performance Enhancement of Silicon Solar Cells Based on Combined Ratios of Three Species of Europium-Doped Phosphors.

This paper presents a scheme for the enhancement of silicon solar cells in terms of luminescent emission band and photovoltaic performance. The proposed devices are coated with an luminescent down-shifting (LDS) layer comprising three species of europium (Eu)-doped phosphors mixed within a silicate film (SiO2) using a spin-on film deposition. The three species of phosphor were mixed at ratios of 0.5:1:1.5, 1:1:1, or 1.5:1:0.5 in weight percentage (wt %). The total quantity of Eu-doped phosphors in the silicate solution was fixed at 3 wt %. The emission wavelengths of the Eu-doped phosphors were as follows: 518 nm (specie-A), 551 nm (specie-B), and 609 nm (specie-C). We examined the extended luminescent emission bands via photoluminescence measurements at room temperature. Closely matching the luminescent emission band to the high responsivity band of the silicon semiconductor resulted in good photovoltaic performance. Impressive improvements in efficiency were observed in all three samples: 0.5:1:1.5 (20.43%), 1:1:1 (19.67%), 1.5:1:0.5 (16.81%), compared to the control with a layer of pure SiO2 (13.80%).

Plasmonic Light Scattering in Textured Silicon Solar Cells with Indium Nanoparticles from Normal to Non-Normal Light Incidence.

In this study, we sought to improve the light trapping of textured silicon solar cells using the plasmonic light scattering of indium nanoparticles (In NPs) of various dimensions. The light trapping modes of textured-silicon surfaces with and without In NPs were investigated at an angle of incidence (AOI) ranging from 0° to 75°. The optical reflectance, external quantum efficiency (EQE), and photovoltaic performance were first characterized under an AOI of 0°. We then compared the EQE and photovoltaic current density-voltage (J-V) as a function of AOI in textured silicon solar cells with and without In NPs. We observed a reduction in optical reflectance and an increase in EQE when the cells textured with pyramidal structures were coated with In NPs. We also observed an impressive increase in the average weighted external quantum efficiency (∆EQEw) and short-circuit current-density (∆Jsc) in cells with In NPs when illuminated under a higher AOI. The ∆EQEw values of cells with In NPs were 0.37% higher than those without In NPs under an AOI of 0°, and 3.48% higher under an AOI of 75°. The ∆Jsc values of cells with In NPs were 0.50% higher than those without In NPs under an AOI of 0°, and 4.57% higher under an AOI of 75°. The application of In NPs clearly improved the light trapping effects. This can be attributed to the effects of plasmonic light-scattering over the entire wavelength range as well as an expanded angle of incident light.

2.2 microm axial resolution optical coherence tomography based on a 400 nm-bandwidth superluminescent diode.

We demonstrate 2.2 microm axial resolution optical coherence tomography (OCT) in 1.1-1.7 microm wavelength regime by using a nonidentical multiple-quantum-well (MQW) superluminescent diode (SLD) with record-bandwidth emission. The compact, low-cost, and reliable light source with extreme broadband emission demonstrates significant potentials for spectroscopic and commercial OCT applications requiring ultrahigh spatial resolution.

Solution-processed carrier selective layers for high efficiency organic/nanostructured-silicon hybrid solar cells.

The reduction of interface minority carrier recombination is regarded as a key performance indicator in improving the power conversion efficiency (PCE) of organic-inorganic hybrid solar cells. In this study, we chose two kinds of carrier-selective layers to be applied in a hybrid solar cell device. A hole selective transporting layer of N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) was added to the interface between Si nanohole structures and PEDOT: PSS, and the electron selective layer cesium carbonate (Cs2CO3) was added to the interface between the backside Si wafer and the rear Ti/Ag electrode. The main process used a clean and low-cost solution process, and the annealed temperature was under 140 °C. In addition, after we inserted these two carrier selective layers, the minority carrier lifetime was prolonged from 29.98 µs to 140.81 µs, indicating its significance in reducing the recombination rate. Eventually, we demonstrated that the PCE of Si/organic heterojunction solar cells can be improved to 13.23%.

Low-Pressure-Assisted Coating Method To Improve Interface between PEDOT:PSS and Silicon Nanotips for High-Efficiency Organic/Inorganic Hybrid Solar Cells via Solution Process.

Nanostructured silicon hybrid solar cells are promising candidates for a new generation photovoltaics because of their light-trapping abilities and solution processes. However, the performance of hybrid organic/Si nanostructure solar cells is hindered because of carrier recombination at surface and poor coverage of organic material poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) on nanostructures. Here we demonstrate low-pressure-assisted coating method of PEDOT: PSS on surface-modified silicon nanotips with broadband light-trapping characteristics to improve interface property and to achieve high-efficiency hybrid solar cells through a solution process. The approach enhances the effective minority-carrier lifetime and the coverage of PEDOT: PSS on the surface of nanostructures. Hybrid solar cells fabricated with surface-modified nanotips exhibit a high fill factor of 70.94%, short-circuit current density of 35.36 mA/cm(2), open-circuit voltage of 0.528 V, and power conversion efficiency of 13.36%. The high efficiency and the high fill factor are achieved because of conformal coating of PEDOT: PSS via a low-pressure-assisted coating process, excellent light harvesting without sacrificing the minority-carrier lifetime, and efficient charge separation/collection of photogenerated carriers.

Optical and Electrical Performance of MOS-Structure Silicon Solar Cells with Antireflective Transparent ITO and Plasmonic Indium Nanoparticles under Applied Bias Voltage.

This paper reports impressive improvements in the optical and electrical performance of metal-oxide-semiconductor (MOS)-structure silicon solar cells through the incorporation of plasmonic indium nanoparticles (In-NPs) and an indium-tin-oxide (ITO) electrode with periodic holes (perforations) under applied bias voltage. Samples were prepared using a plain ITO electrode or perforated ITO electrode with and without In-NPs. The samples were characterized according to optical reflectance, dark current voltage, induced capacitance voltage, external quantum efficiency, and photovoltaic current voltage. Our results indicate that induced capacitance voltage and photovoltaic current voltage both depend on bias voltage, regardless of the type of ITO electrode. Under a bias voltage of 4.0 V, MOS cells with perforated ITO and plain ITO, respectively, presented conversion efficiencies of 17.53% and 15.80%. Under a bias voltage of 4.0 V, the inclusion of In-NPs increased the efficiency of cells with perforated ITO and plain ITO to 17.80% and 16.87%, respectively.