Low External Quantum Efficiency Research Articles

Ultrahigh EQE (15%) Solar‐Blind UV Photovoltaic Detector with Organic–Inorganic Heterojunction via Dual Built‐In Fields Enhanced Photogenerated Carrier Separation Efficiency Mechanism

AbstractA new strategy of constructing an additional heterojunction on the surface of epitaxially grown Ga2O3 film with a distorted lattice is proposed to solve the problem of low external quantum efficiency (EQE) in traditional Ga2O3 heterojunction photovoltaic devices. Experimentally, an organic–inorganic hybrid poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate/Ga2O3/p‐type Si solar‐blind ultraviolet (SBUV) photovoltaic detector is constructed to achieve an ultrahigh EQE of ≈15% at 0 V bias, which is 1–2 orders of magnitude higher than that of the Ga2O3 photovoltaic devices reported previously. Here, an enhanced mechanism of photogenerated carrier separation efficiency induced by dual built‐in fields is proposed to explain the high EQE of Ga2O3 SBUV photovoltaic devices. In addition, the organic–inorganic hybrid detector displays a high SBUV–visible rejection ratio (R255 nm/R405 nm of ≈450) and fast response speed (rise time of 60 ms and decay time of 88 ms). All these results indicate that the strategy proposed could provide reference for the fabrication of high‐performance Ga2O3 SBUV photovoltaic detectors.

Hole-Transporting Side-Chain Polymer Bearing a Thermally Crosslinkable Bicyclo[4.2.0]octa-1,3,5-trien-3-yl Group for High-Performing Thermally Activated Delayed Fluorescence OLED.

A new side-chain polymer (X-TPACz) bearing hole-transporting pendant groups accompanying a thermally crosslinkable entity was synthesized using N-([1,1'-biphenyl]-4-yl)- N-(4-(9-(4-vinylbenzyl)-9 H-carbazol-3-yl)phenyl)bicyclo[4.2.0]octa-1(6),2,4-trien-3-amine (6) via addition polymerization. The X-TPACz could be spontaneously crosslinked without using any further reagents and showed a good film-forming property upon low-temperature thermal treatment. The thermal curing temperature for the X-TPACz film was optimized to be 180 °C based on a differential scanning calorimetry thermogram. Moreover, the thermal degradation temperature of X-TPACz measured to be over 467 °C using thermogravimetric analysis demonstrated that it shows excellent thermal stability. In particular, X-TPACz exhibits the highest occupied molecular orbital (HOMO) energy level to be -5.26 eV, which is beneficial for facile hole injection and transportation. Consequently, the thermally activated delayed fluorescence organic light-emitting diodes fabricated using X-TPACz as the hole-transporting material showed state-of-the-art performances with a low turn-on voltage ( Von) of only 2.7 V and a high external quantum efficiency (EQE) of 19.18% with a high current efficiency (CE) of 66.88 cd/A and a high power efficiency (PE) of 60.03 lm/W, which are highly superior to those of the familiar poly(9-vinylcarbazole) (PVK)-based devices ( Von = 3.9 V, EQE of 17.42%, with CE of 58.33 cd/A and PE of 33.32 lm/W). The extremely low turn-on voltage and high EQE were found to be due to the higher-lying highest occupied molecular orbital energy level ( EHOMO = -5.23 eV) and better hole-transporting property of X-TPACz than those of PVK.

2D-σ-2A type cruciform host material with silane core for highly efficient solution-processable green thermally activated delayed fluorescence organic light emitting diodes

A new solution processable cruciform tetraphenylsilane-based bipolar host material, bis(4-(9H-carbazol-9-yl)phenyl)bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)silane (SiCT), was designed and synthesized. Two carbazoles as donors and two diphenyl triazine moieties as acceptors were tethered to the tetraphenylsilane. SiCT showed great solubility in common organic solvents, high thermal stability, high triplet energy (T1 ∼3.0 eV), and a superior bipolar electronic nature, which are sufficient for it to function as a host material in thermally activated delayed fluorescence organic light emitting diodes (TADF-OLEDs). Consequently, the solution-processed green TADF-OLEDs employed SiCT as a bipolar host exhibited a low turn-on voltage and high external quantum efficiency (EQE) of 19.17% with a high power efficiency (PE) of 41.13 lm W−1, while control device based-on SiC as unipolar host showed a low EQE of 12.74% with a low PE of 16.13 lm W−1. This work highlights the potential of a bipolar tetraphenylsilane derivative as a host material in solution-processable green TADF-OLEDs.

The emergence and prospects of deep-ultraviolet light-emitting diode technologies

By alloying GaN with AlN the emission of AlGaN light-emitting diodes can be tuned to cover almost the entire ultraviolet spectral range (210–400 nm), making ultraviolet light-emitting diodes perfectly suited to applications across a wide number of fields, whether biological, environmental, industrial or medical. However, technical developments notwithstanding, deep-ultraviolet light-emitting diodes still exhibit relatively low external quantum efficiencies because of properties intrinsic to aluminium-rich group III nitride materials. Here, we review recent progress in the development of AlGaN-based deep-ultraviolet light-emitting devices. We also describe the key obstacles to enhancing their efficiency and how to improve their performance in terms of defect density, carrier-injection efficiency, light extraction efficiency and heat dissipation. This Review covers recent progress in AlGaN-based deep-ultraviolet light-emitting devices. The key technologies of how to improve their performance, carrier-injection efficiency, light extraction efficiency and heat dissipation are discussed.

High Performance Plasmonic Nanolasers with External Quantum Efficiency Exceeding 10.

Plasmonic nanolasers break the diffraction limit for an optical oscillator, which brings new capabilities for various applications ranging from on-chip optical interconnector to biomedical sensing and imaging. However, the inevitably accompanied metallic absorption loss could convert the input power to heat rather than radiations, leading to undesired low external quantum efficiency and device degradation. To date, direct characterization of quantum efficiency of plasmonic nanolasers is still a forbidden task due to its near-field surface plasmon emissions, divergent emission profile, and the limited emission power. Here, we develop a method to characterize the external quantum efficiency of plasmonic nanolasers by synergizing experimental measurement and theoretical calculation. With systematical device optimization, we demonstrate high performance plasmonic nanolasers with external quantum efficiency exceeding 10% at room temperature. This work fills in a missing yet essential piece of key metrics of plasmonic nanolasers. The demonstrated high external quantum efficiency of plasmonic nanolasers not only clarifies the long-standing debate, but also endorses the exploration of them in various practical applications such as near-field spectroscopy and sensing, integrated optical interconnects, solid-state lighting, and free-space optical communication.

Stable, Efficient Red Perovskite Light‐Emitting Diodes by (α, δ)‐CsPbI3 Phase Engineering

AbstractRecently, inorganic cesium–lead halide perovskites with high thermal stability have attracted much attention as promising light‐emitting material for research of perovskite‐based light‐emitting diodes (PeLEDs) toward high‐definition displays. However, the CsPbI3‐based red PeLEDs still suffer low external quantum efficiency (EQE) and poor device stability due to the spontaneous phase transition from cubic CsPbI3 (α‐CsPbI3) to nonradiative orthorhombic phase (δ‐CsPbI3) under ambient conditions. Here, a feasible approach is reported on phase engineering by incorporating the long‐chain cation (e.g., 2‐(naphthalene‐1‐yl)ethanamine (NEA)) in CsPbI3 for stable and high‐performance CsPbI3‐based red light‐emitting diodes (LEDs). A high EQE of 8.65% is successfully achieved for the characteristic red emission at ≈682 nm representing the highest value among Cs‐based red PeLEDs up to now. More importantly, the corresponding PeLEDs exhibit outstanding stability with EQE retaining 90% after 3 months of storage. These results verify the potential of using cesium‐based inorganic perovskite as viable alternatives to methylammonium (MA)‐ or formamidinium (FA)‐based perovskite for desirable practical applications.

Vacuum-Ultraviolet Photodetection in Few-Layered h-BN.

Over the past 20 years, astro and solar physicists have been working hard to develop a new-generation semiconductor-based vacuum-ultraviolet (VUV, 100-200 nm) photodetector with small size and low power consumption, to replace the traditional microchannel detection system, which is ponderous and has high energy consumption, and finally to reduce the power load and launch costs of explorer satellites. However, this expectation has hardly been achieved due to the relatively low photoresponsivity and external quantum efficiency (EQE) of the reported VUV photoconductive detectors based on traditional wide-band-gap materials and structures. Here, on the basis of few-layer h-BN, we fabricated a high-performance two-dimensional photodetector with selective response to VUV light. Typically, it has high sensitivity (EQE = 2133%, at 20 V) to the extremely weak 160 nm light (3.25 pW). This excellent photoresponsivity can be attributed to the high carrier collection efficiency and existing surface trap states of few-layer h-BN. In addition, this device can maintain a stable performance in a wide temperature range (80-580 K), which is quite favorable for application in deep space with huge temperature fluctuation.

Evaluation of saw damage using diamond-coated wire in crystalline silicon solar cells by photoluminescence imaging

Si ingots were sliced using a diamond-coated wire, and saw damage was observed even after damage removal etching and texturization. Since invisible microscopic damage was observed only under uncontrolled slice conditions, such damage was identified as saw damage. The wafers with saw damage exhibited the degradation of solar cell conversion efficiency (approximately 1–2% absolute). The results of external quantum efficiency (EQE) measurements showed a slight deterioration of EQE in the short wavelength region. Current–voltage characteristic measurements showed similar results that agreed with the EQE measurement results. In addition, EQE mapping measurements were carried out at various irradiation wavelengths between 350 and 1150 nm. Areas with dark contrasts in EQE mapping correspond to saw damage. In the cells with a low conversion efficiency, both EQE mapping and PL images exhibited dark areas and lines. On the other hand, in the cells with a high conversion efficiency, a uniform distribution of saw damage was observed even with the saw damage in the PL images. We believe that sophisticated control to suppress saw damage during sawing is required to realize higher conversion efficiency solar cells in the future.

Influence of Air Exposure on Photocarrier Generation in Amorphous and Phase II Thin Films of Titanyl Phthalocyanine

We examined the influence of atmospheric gases on the photoelectric properties of titanyl phthalocyanine (TiOPc) thin films, using in situ photocurrent measurements under high vacuum without/with exposure to air. Prior to air exposure, the photocurrent of amorphous and Phase II thin films of TiOPc with thicknesses of 24 nm exhibited low external quantum efficiencies (EQE) of 0.05% and 0.52%, respectively, in the Q-band regions, and these values were enhanced to 0.21% and 12.0%, respectively, by air exposure. In contrast, this treatment resulted in a decrease in the photosensitivity, defined as the photo-to-dark current ratio, by 1 or 2 orders of magnitude. Once these thin films experienced air exposure, their photoelectric properties did not recover to the original ones even after restoration to high vacuum.

Effect of Series Resistances on Conversion Efficiency of GaAs/Si Tandem Solar Cells With Areal Current-Matching Technique

The electrical losses due to the series resistances—the bonding resistance and sheet resistance—in a two-terminal (2T) GaAs/Si tandem solar cell that implements the areal current-matching (ACM) technique was investigated. A 2T GaAs/Si ACM tandem solar cell has a smaller top cell area and a larger bottom cell area, to achieve current matching between the top and bottom cells. The effect of the series resistances on the performance of such an ACM tandem cell was evaluated through device simulations, in which the subcell area ratio (bottom cell area / top cell area) and series resistances were varied under 1-sun and 3-sun conditions. The simulation results revealed that the optimal subcell area ratio at which the tandem cell exhibits the highest efficiency is affected mainly by the bonding resistance, due to a unique characteristic of the fill factor. A larger bonding resistance results in a smaller optimal subcell area ratio, i.e., a larger top cell area, which requires the use of a larger amount of expensive III–V materials. This trend is more prominent under 3-sun conditions. The indoor experiments conducted on an in-house 2T GaAs/Si ACM tandem cell verified the simulated results and showed that a lower external quantum efficiency (EQE) also resulted in a smaller optimal subcell area. Therefore, low bonding resistance and high EQE are of primary importance for realizing high-efficiency low-cost 2T ACM tandem solar cells.

(Invited) The Effects of Zinc-Tin-Oxide Nanosphere Monolayer on Improvement of Light Extraction in Light-Emitting Diodes

Over the last two decades, III-nitride-based light-emitting diodes (LEDs) have been widely used for our daily life, such as illumination, various backlight units for full-color displays, and automotive headlamps. In spite of the versatile advantages of GaN-LEDs, relatively low external quantum efficiency is still a problem to expand their application fields. For the viable solution to unravel pending issues in solid state light-emitting research fields, many efforts have been conducted. In this study, we report on the applications of zinc-tin-oxide (ZTO) nanoparticles and monolayer as a light extraction layer for GaN-based LEDs. The ZTO layers formed on top of an LED epi-structure with a variable Sn-ratio, which was deposited by the spin coating method. The transmission spectra of the ZTO layers with Zn to Sn ratios of 1:1 (ZTO-I) and 1:5 (ZTO-II) exhibited optical transmittances of 98% and 88% in the visible region, respectively. The electroluminescence (EL) and light power-current-voltage (L-I-V) measurements show that double ZTO layers consisting of various Zn to Sn ratios led to enhanced light extraction from blue LEDs. Finally, we will discuss the effects of ZTO monolayer transferred onto the p-GaN surface on the external quantum efficiency of LEDs. The ZTO monolayers were prepared by using the ZTO nanospheres synthesized by precipitation solution method. The transferred ZTO nanosphere monolayers on top surface of LED are very effective way to improve the light extraction efficiency.

Abnormal Radiative Interband Transitions in High-Al-Content AlGaN Quantum Wells Induced by Polarized Orbitals

AlGaN has attracted considerable interest as a wide (direct)-band-gap semiconductor with high thermal and mechanical stability. Thus, it can be used to develop optoelectronic devices operating within the ultraviolet region at high power and under harsh environmental conditions. Despite their recognized prospective applications, Al-rich AlGaN optical devices suffer from low external quantum efficiency. To trace the origin of the said problem, a cathodoluminescence system combined with two scanning probes was set up to investigate the cross-section luminescence of the sample related to application bias. The luminescence from the quantum wells in a deep ultraviolet light-emitting device was identified by layer-resolved spectroscopy. Results show that the primary radiative emission at the band edge exhibits an abnormal behavior, which is different from the other emission that is dependent on external electric fields. First-principles simulations demonstrate that the dispersive crystal field split-off hole (CH...

Sodium bromide additive improved film morphology and performance in perovskite light-emitting diodes

Organometal halide perovskite is a promising material to fabricate light-emitting diodes (LEDs) via solution processing due to its exceptional optoelectronic properties. However, incomplete precursor conversion and various defect states in the perovskite light-emitting layer lead to low luminance and external quantum efficiency of perovskite LEDs. We show here the addition of an optimum amount of sodium bromide in the methylammonium lead bromide (MAPbBr3) precursor during a one-step perovskite solution casting process can effectively improve the film coverage, enhance the crystallinity, and passivate ionic defects on the surface of MAPbBr3 crystal grains, resulting in LEDs with a reduced turn-on voltage from 2.8 to 2.3 V and an enhanced maximum luminance from 1059 to 6942 Cd/m2 when comparing with the pristine perovskite-based device.

Novel thioxanthone host material with thermally activated delayed fluorescence for reduced efficiency roll-off of phosphorescent OLEDs

2,7-Di(9,9-dimethyl-9H-fluoren-1-yl)-9H-thioxanthen-9-one (DMBFTX) with thermally activated delayed fluorescence (TADF) was well designed and synthesized. The phosphorescent organic light-emitting device (PHOLED) based on this novel TADF host material displays a stable red phosphorescence region, a peak external quantum efficiency (EQE) value of 12.9% and a low EQE roll-off of 38.8% at a luminance of 10000cd/m2, which is benefited from the reverse intersystem crossing (RISC) of TADF host and less populated triplet exitons. Notably, the red device based on the TADF host DMBFTX exhibits superior electroluminescence performance and reduced efficiency roll-off compared with the one hosted by commercially available host 1,3-bis(9-carbazolyl)benzene (mCP), illustrating the high potential of employing the TADF host material with small energy gap to reduce efficiency roll-off in PHOLED.

Ultrathin Si solar cell with nanostructured light trapping by metal assisted etching

We report an 8µm-thick silicon solar cell with an efficiency of 9.60%. Nanostructured silicon surface formed via metal assisted etching shows a broadband reflection below 10%. Despite the excellent optical performance, a moderate short-circuit current (JSC) of 25.44mA/cm2 was collected. Relatively low external quantum efficiency (EQE) at short wavelengths was associated with carrier recombination at the enhanced surface and by the Auger process. Moreover, parasitic absorption at the back contact is the main factor resulting a relatively low EQE in long wavelength region of the spectrum. Our optical simulations show that planarization of the rear Si surface and insertion of a low refractive index dielectric spacer between Si and the rear metal can significantly reduce the parasitic absorption in the metal, resulting in JSC values over 35mA/cm2.

Introduction of AlInAs-oxide current-confinement structure into GaInAsP/SOI hybrid Fabry–Pérot laser

An AlInAs-oxide current-confinement structure was introduced into a III–V/Si-on-insulator (SOI) hybrid laser for realizing an efficient light source composed of III–V/Si hybrid photonic integrated circuits. Lasing of the hybrid Fabry–Pérot laser was confirmed after introduction of the confinement structure providing both optical and electrical confinement. We determined the lasing characteristics of the designed hybrid laser, which showed a low threshold current and high external differential quantum efficiency comparable to that of conventional lasers fabricated on III–V compound semiconductor wafers. Comparison of the photoluminescence spectra of the III–V/SOI hybrid wafers before and after the process proved that oxidation had no influence on the active layer. The threshold current and external differential quantum efficiency of the fabricated laser with a stripe width of 4.5 µm and cavity length of 500 µm were obtained as 50 mA and 11%/facet, respectively.

Fine-Tuned Photoactive and Interconnection Layers for Achieving over 13% Efficiency in a Fullerene-Free Tandem Organic Solar Cell.

Fabricating organic solar cells (OSCs) with a tandem structure has been considered an effective method to overcome the limited light absorption spectra of organic photovoltaic materials. Currently, the most efficient tandem OSCs are fabricated by adopting fullerene derivatives as acceptors. In this work, we designed a new non-fullerene acceptor with an optical band gap (Egopt) of 1.68 eV for the front subcells and optimized the phase-separation morphology of a fullerene-free active layer with an Egopt of 1.36 eV to fabricate the rear subcell. The two subcells show a low energy loss and high external quantum efficiency, and their photoresponse spectra are complementary. In addition, an interconnection layer (ICL) composed of ZnO and a pH-neutral self-doped conductive polymer, PCP-Na, with high light transmittance in the near-IR range was developed. From the highly optimized subcells and ICL, solution-processed fullerene-free tandem OSCs with an average power conversion efficiency (PCE) greater than 13% were obtained.

Efficient near-infrared light-emitting diodes based on liquid PbSe quantum dots

Recently, near-infrared light-emitting diodes (NIR LEDs) based on PbSe quantum dots (QDs) have attracted considerable attention due to their facilely tunable emission wavelength, as well as high quantum yield. However, the low external quantum efficiency (EQE) of these LEDs has restricted their actual applications because of the non-radiative recombination caused by the aggregation in the solid-state QD films. Therefore, we proposed in this work to employ the liquid-type structure in NIR LEDs base on PbSe QDs, which exhibited the main advantages relying on the fact that the liquid structure could prevent the active layer from self-aggregation and improve the device stability. The emission intensity of these NIR LEDs was optimized by tuning the concentration of PbSe QDs. Besides, the radiation power of PbSe QD-based devices with different emission wavelengths was analyzed under different biases, and the maximum EQE of NIR LEDs was confirmed to be 5.3%. This result represents the highest record among the reported NIR QD-LEDs, indicating this kind of liquid-type NIR LEDs is promising for commercial applications.

Multiple-Color-Generating Cu(In,Ga)(S,Se)2 Thin-Film Solar Cells via Dichroic Film Incorporation for Power-Generating Window Applications.

There are four prerequisites when applying all types of thin-film solar cells to power-generating window photovoltaics (PVs): high power-generation efficiency, longevity and high durability, semitransparency or partial-light transmittance, and colorful and aesthetic value. Solid-type thin-film Cu(In,Ga)S2 (CIGS) or Cu(In,Ga)(S,Se)2 (CIGSSe) PVs nearly meet the first two criteria, making them promising candidates for power-generating window applications if they can transmit light to some degree and generate color with good aesthetic value. In this study, the mechanical scribing process removes 10% of the window CIGSSe thin-film solar cell with vacant line patterns to provide a partial-light-transmitting CIGSSe PV module to meet the third requirement. The last concept of creating distinct colors could be met by the addition of reflectance colors of one-dimensional (1D) photonic crystal (PC) dichroic film on the black part of a partial-light-transmitting CIGSSe PV module. Beautiful violets and blues were created on the cover glass of a black CIGSSe PV module via the addition of 1D PC blue-mirror-yellow-pass dichroic film to improve the aesthetic value of the outside appearance. As a general result from the low external quantum efficiency (EQE) and absorption of CIGSSe PVs below a wavelength of 400 nm, the harvesting efficiency and short-circuit photocurrent of CIGSSe PVs were reduced by only ∼10% without reducing the open-circuit voltage (VOC) because of the reduced overlap between the absorption spectrum of CIGSSe PV and the reflectance spectrum of the 1D PC blue-mirror-yellow-pass dichroic film. The combined technology of partial-vacancy-scribed CIGSSe PV modules and blue 1D PC dichroic film can provide a simple strategy to be applied to violet/blue power-generating window applications, as such a strategy can improve the transparency and aesthetic value without significantly sacrificing the harvesting efficiency of the CIGSSe PV modules.

Highly Narrowband Photomultiplication Type Organic Photodetectors.

Filterless narrowband response organic photodetectors (OPDs) present a great challenge due to the broad absorption range of organic semiconducting materials. The reported narrowband response OPDs also suffer from low external quantum efficiency (EQE) in the desired response window and low rejection ratio. Here, we report highly narrowband photomultiplication (PM) type OPDs based on P3HT:PC71BM (100:1, wt/wt) as active layer without an optical filter. The full width at half-maximum (fwhm) of the PM-type OPDs can be well retained less than 30 nm under different biases. Meanwhile, the champion EQE and rejection ratio approach 53 500% and 2020 at -60 V bias, respectively. The small fwhm should be attributed to the sharp absorption edge of active layer with small amount of PC71BM. The PM phenomenon is attributed to hole tunneling injection from the external circuit assisted by trapped electron in PC71BM near the Al electrode under light illumination. These highly narrowband PM-type OPDs should have great potential applications in sensitively detecting specific wavelength light and be blind to light outside of the desired response window.