This PDF file contains the editorial “Introducing JPE Special Series” for JPE Vol. 6 Issue 02

Thermally activated delayed fluorescence (TADF) is an emerging hot topic. Even though this photophysical mechanism itself has been described more than 50 years ago and optoelectronic devices with organic matter have been studied, improved, and even commercialized for decades now, the realization of the potential of TADF organic light-emitting diodes (OLEDs) happened only recently. TADF has been proven to be an attractive and very efficient alternative for phosphorescent materials, such as dopants in OLEDs, light-emitting electrochemical cells as well as potent emitters for chemiluminescence. In this review, the TADF concept is introduced in terms that are also understandable for nonchemists. The basic concepts behind this mechanism as well as state-of-the-art examples are discussed. In addition, the future economic impact, especially for the lighting and display market, is addressed here. We conclude that TADF materials are especially helpful to realize efficient, durable deep blue and white displays.

Organo-metal halide perovskite–based solar cells have been the focus of intense research over the past five years, and power conversion efficiencies have rapidly been improved from 3.8 to >21%. This article reviews major advances in perovskite solar cells that have contributed to the recent efficiency enhancements, including the evolution of device architecture, the development of material deposition processes, and the advanced device engineering techniques aiming to improve control over morphology, crystallinity, composition, and the interface properties of the perovskite thin films. The challenges and future directions for perovskite solar cell research and development are also discussed.

Planar heterojunction perovskite solar cell is one of the most competitive photovoltaic technologies, while charge transport materials play a crucial role. We successfully demonstrated an effective electron transport material, namely chemical bath deposited cadmium sulphide (CdS) film under low temperature, in perovskite-based solar cells. Power conversion efficiency of 16.1% has been achieved, which is comparable to that of devices based on TiO₂ film prepared via low-temperature processes. Electronic impedance spectra reveal that the CdS-based device presents a higher recombination resistance than TiO₂-based devices, which reduces carrier recombination and increases the open circuit voltage. The interface between CdS and perovskite was characterized with improved characteristics when compared to TiO₂, e.g., efficient carrier extraction and reduced surface defect–associated degradation in the devices, which help to alleviate anomalous hysteresis and long-term instability. Furthermore, the entire device was fabricated via solution process with a processing temperature below 100°C, suggesting a promising method of further development of perovskite solar cells and commercial manufacturing.

Seebeck nanoantennas, which are based on the thermoelectric effect, have been proposed for electromagnetic energy harvesting and infrared detection. The responsivity and frequency dependence of three types of Seebeck nanoantennas is obtained by electromagnetic simulation for different materials. Results show that the square spiral antenna has the widest bandwidth and the highest induced current of the three analyzed geometries. However, the geometry that presented the highest temperature gradient was the bowtie antenna, which favors the thermoelectric effect in a Seebeck nanoantenna. The results also show that these types of devices can present a voltage responsivity as high as 36  μV/W for titanium–nickel dipoles resonant at far-infrared wavelengths.

A two-step process is used to grow crystalline silicon (c-Si) on glass at low temperatures. In the first step, nanocrystalline seed layers are formed at temperatures in the range of 230 to 400°C by either metal-induced crystallization or by direct deposition on heated substrates. In the second step, c-Si is grown on the seed layer by steady-state liquid phase epitaxy at a temperature range of 580 to 710°C. Microcrystalline Si layers with grain sizes of up to several tens of micrometers are grown from In and Sn solutions. Three-dimensional simulations of heat and convective flow in the crucible have been conducted and give valuable insights into the growth process. The experimental results are promising with regard to the designated use of the material in photovoltaics.

A two-dimensional finite-element model was developed to simulate the optoelectronic performance of thin-film, p-i-n junction solar cells. One or three p-i-n junctions filled the region between the front window and back reflector; semiconductor layers were made from mixtures of two different alloys of hydrogenated amorphous silicon; empirical relationships between the complex-valued relative optical permittivity and the bandgap were used; a transparent-conducting-oxide layer was attached to the front surface of the solar cell; and a metallic reflector, either flat or periodically corrugated, was attached to the back surface. First, frequency-domain Maxwell postulates were solved to determine the spatial absorption of photons and thus the generation of electron–hole pairs. The AM1.5G solar spectrum was taken to represent the incident solar flux. Second, drift-diffusion equations were solved for the steady-state electron and hole densities. Numerical results indicate that increasing the number of p-i-n junctions from one to three may increase the solar-cell efficiency by up to 14%. In the case of single p-i-n junction solar cells, our simulations indicate that efficiency may be increased by up to 17% by incorporating a periodically corrugated back reflector (as opposed to a flat back reflector) and by tailoring the bandgap profile in the i layer.

The paper reports on the scattering properties of different submicron mesoporous TiO₂ structures and their correlation with the efficiency of photovoltaic devices. Bruggeman’s effective medium theory was used to calculate the effective refractive index. T-matrix and Mie theory were used to evaluate scattering parameters, such as scattering coefficient and asymmetry factor. The parameters presented here can be used either to understand the suitability of these TiO₂ structures for photovoltaic and photocatalysis applications or as inputs for full optoelectronics simulation of devices. Scattering coefficients of the structures were found to mainly depend on their volume and porosity rather than shape. The dimensions and refractive index of scattering structures commonly used for photovoltaic and photocatalytic application are generally within a range of dimensions and refractive indices quite similar to that discussed in this paper, and hence, results discussed will also be indicative for other scattering structures for the same application. The effects of shape, size, and porosity on scattering parameters have also been discussed.

In the last years, hybrid organic/silicon solar cells have attracted great interest in photovoltaic research due to their potential to become a low-cost alternative for the conventionally used silicon pn-junction solar cells. This work is focused on hybrid solar cells based on the polymer poly(3-hexylthiophene-2,5-diyl), which was deposited on n-doped crystalline silicon via spin-coating under ambient conditions. By employing an anisotropic etching step with potassium hydroxide (KOH), the reflection losses at the silicon surface were reduced. Hereby, the short-circuit current density of the hybrid devices was increased by 31%, leading to a maximum power conversion efficiency (PCE) of 13.1% compared to a PCE of 10.7% for the devices without KOH etching. In addition, the contacts were improved by replacing gold with the more conductive silver as top grid material to reduce the contact resistance and by introducing a thin (∼0.5  nm) lithium fluoride layer between the silicon and the aluminum backside contact to improve electron collection and hole blocking. Hereby, the open-circuit voltage and the fill factor of the hybrid solar cells were further improved and devices with very high PCE up to 14.2% have been realized.

Analysis of randomly textured interface in solar cells is an ambitious effort considering the numerous structures and variation in the texture. We use the Fourier transform based method, which takes randomly textured interface as initial input and manipulates its frequency spectrum to synthesize interface texture with the desired texture behavior. Afterward, the synthesized interfaces are applied to simulate flexible thin-film silicon tandem (aSi/μcSi) solar cells. Using the simulation, we have shown how interface between the back and front areas of the solar cell evolves, and the effect of the back contact; the front area roughness is analyzed and interference fringes observed on experiments are discussed. For the simulations, a finite integration technique with time-harmonic inverse iterative method is used. All simulations are performed on high-performance computers, which allows simulation of a big simulation domain (<5  μm×5  μm) with fine mesh size (<10  nm). The analysis performed shows that the interface roughness at the front contact remains similar to the initial back contact roughness. Furthermore, a solar cell with flat back contact can be as efficient as a solar cell with rough back contact when the front area roughness is well optimized.

Interaction between ultraviolet (UV) light and carbon nanotube (CNT) networks plays a central role in gas adsorption, sensor sensitivity, and stability of CNT-based electronic devices. To determine the effect of UV light on sorption kinetics and resistive gas/vapor response of different CNT networks, films of semiconducting single-wall nanotubes (s-SWNTs), metallic single-wall nanotubes, and multiwall nanotubes were exposed to O₂ and H₂O vapor in the dark and under UV irradiation. Changes in film resistance and mass were measured in situ. In the dark, resistance of metallic nanotube networks increases in the presence of O₂ and H₂O, whereas resistance of s-SWNT networks decreases. UV irradiation decreases the resistance of metallic nanotube networks in the presence of O₂ and H₂O and increases the gas/vapor sensitivity of s-SWNT networks by nearly a factor of 2 compared to metallic nanotube networks. s-SWNT networks show evidence of delamination from the gold-plated quartz crystal microbalance crystal, possibly due to preferential adsorption of O₂ and H₂O on gold. UV irradiation increases the sensitivity of all CNT networks to O₂ and H₂O by an order of magnitude, which demonstrates the importance of UV light for enhancing response and lowering detection limits in CNT-based gas/vapor sensors.

A methyl ammonium lead iodide (H₃NH₃PbI₃)-based solar cell can have photovoltaic conversion efficiency of more than 20%, primarily because the material shows lower defect density, high carrier mobility-lifetime, and broader absorption spectra. A further improvement in device efficiency can be obtained using light capture and trapping schemes, with textured front surface and back reflector. In order to understand characteristic performance of the device, we used numerical simulation and observed that more than 20% device efficiency can be obtained if defect density of the photosensitive material remains lower than 4×10¹⁴  cm⁻³ and thickness 400 nm or more. Investigation of light trapping scheme shows that the current density (J_sc) can be raised with this scheme, but the most effective increase in the J_sc can be observed for 97-nm thick active layers. Reverse saturation current density of these cells that may be directly related to recombination loss of photogenerated carriers, remains low, but increases linearly with the defect density. A tandem cell with pyramidally textured front surface was investigated with such a perovskite-based top cell and Si heterojunction bottom cell; it shows an efficiency of as high as 29.5%.

Compressive sensing has been widely used in image compression and signal recovery techniques in recent years; however, it has received limited attention in the field of optical measurement. This paper describes the use of compressive sensing for measurements of photovoltaic (PV) solar cells, using fully random sensing matrices, rather than mapping an orthogonal basis set directly. Existing compressive sensing systems optically image the surface of the object under test, this contrasts with the method described, where illumination patterns defined by precalculated sensing matrices, probe PV devices. We discuss the use of spatially modulated light fields to probe a PV sample to produce a photocurrent map of the optical response. This allows for faster measurements than would be possible using traditional translational laser beam induced current techniques. Results produced to a 90% correlation to raster scanned measurements, which can be achieved with under 25% of the conventionally required number of data points. In addition, both crack and spot type defects are detected at resolutions comparable to electroluminescence techniques, with 50% of the number of measurements required for a conventional scan.

One can convert a luminescent solar concentrator to a display by scanning a laser beam on it. When a guest–host system of liquid crystal (LC) and dye materials are incorporated, absorption of excitation light and the radiation pattern of photoluminescence (PL) can be adjusted to changes in lighting condition. The resolution of a displayed image can be degraded by PL spreading in the LC/dye layer. Its contrast can be limited by the PL induced by ambient light. In the experiment, we fabricated a 22×25  mm² cell that contained 0.5 wt. % coumarin 6 in a nematic LC host. The alignment was antiparallel and the gap was 6  μm. Using a blue laser beam of 0.04 mm FWHM, the PL intensity distribution was measured to be 0.20 mm FWHM at zero bias. It became slightly wider at 10 V. For contrast evaluation, we measured PL spectra under two conditions. First, the center of the cell was irradiated by a 1.7-mW blue laser beam. Second, the whole cell was uniformly exposed to light from a fluorescent lamp at illuminance of 800lx. The contrast of luminance was calculated to be 1.4×10⁵. The optical power reaching its edge surfaces was measured and roughly agreed with the prediction by a simple model.