Back-Contact Ionic Compound Engineering Boosting the Efficiency and Stability of Blade-Coated Perovskite Solar Cells.

Surface defect passivation, which plays a vital role in achieving high-efficiency perovskite solar cells (PSCs) in a spin-coating process, is rarely compatible with a printing process. Currently, printing PSCs with high efficiency remains a challenge, as only a few laboratories realized an efficiency of over 20%. In this work, zwitterionic compounds 2-hydroxyethyl trimethyl ammonium chloride (HETACl) and butyltrimethylammonium chloride (BTACl) were introduced, both of which can spontaneously adsorb on the surface perovskite and form an ultrathin passivation layer by a dip coating method. The complex formed by the strong interaction of HETACl with MAI on the surface of the perovskite film leads to the formation of a rough perovskite surface, which affects the enhancement of device performance. BTACl with a chemically inert side chain induces a weak interaction with the perovskite. It is demonstrated that BTACl not only passivates surface defects of the perovskite but also heals the grain boundaries and results in more uniform crystallizations. Finally, PSCs upon BTACl treatment were blade-coated in an ambient environment with a relative humidity of <50%, which produced a champion efficiency of 20.5% with negligible hysteresis, and the active area of the cell device was 0.095 cm2. After being stored in air for 30 days, unencapsulated PSCs treated with BTACl retained 95% of their initial efficiency, which is far superior to that of the control and those treated with HETACl.

F-Type Pseudo-Halide Anions for High-Efficiency and Stable Wide-Band-Gap Inverted Perovskite Solar Cells with Fill Factor Exceeding 84.

The quality of wide-band-gap (WBG) perovskite films plays an important role in tandem solar cells. Therefore, it is necessary to improve the performance of WBG perovskite films for the development of tandem solar cells. Here, we employ F-type pseudo-halogen additives (PF6- or BF4-) into perovskite precursors. The perovskite films with F-type pseudo-halogen additives have a larger grain size and higher crystal quality with lower defect density. At the same time, the perovskite lattice increases due to substitution of F-type pseudo-halogen anions for I-/Br-, and the stress distortion in the film is released, which effectively suppresses the recombination of carriers, reduces the charge transfer loss, and inhibits the phase separation. Finally, the power conversion efficiency (PCE) of the inverted 1.67 eV perovskite devices is significantly improved to over 20% with an impressive fill factor of 84.02% and excellent device stability. In addition, the PCE of the four-terminal (4T) perovskite/silicon tandem solar cells reached 27.35% (PF6-) and 27.11% (BF4-), respectively. This provides important guidance for further improving WBG perovskite solar cell performance.

General heterostructure strategy of photothermal materials for scalable solar-heating hydrogen production without the consumption of artificial energy.

Solar-heating catalysis has the potential to realize zero artificial energy consumption, which is restricted by the low ambient solar heating temperatures of photothermal materials. Here, we propose the concept of using heterostructures of black photothermal materials (such as Bi2Te3) and infrared insulating materials (Cu) to elevate solar heating temperatures. Consequently, the heterostructure of Bi2Te3 and Cu (Bi2Te3/Cu) increases the 1 sun-heating temperature of Bi2Te3 from 93 °C to 317 °C by achieving the synergy of 89% solar absorption and 5% infrared radiation. This strategy is applicable for various black photothermal materials to raise the 1 sun-heating temperatures of Ti2O3, Cu2Se, and Cu2S to 295 °C, 271 °C, and 248 °C, respectively. The Bi2Te3/Cu-based device is able to heat CuOx/ZnO/Al2O3 nanosheets to 305 °C under 1 sun irradiation, and this system shows a 1 sun-driven hydrogen production rate of 310 mmol g-1 h-1 from methanol and water, at least 6 times greater than that of all solar-driven systems to date, with 30.1% solar-to-hydrogen efficiency and 20-day operating stability. Furthermore, this system is enlarged to 6 m2 to generate 23.27 m3/day of hydrogen under outdoor sunlight irradiation in the spring, revealing its potential for industrial manufacture.

High-performance broadband position-sensitive detector based on lateral photovoltaic effect of PbSe heterostructure.

PbSe has attracted considerable attention due to its promising applications in optoelectronics and energy harvesting. In this work, we explore the lateral photovoltaic effect (LPE) of PbSe films with a simple PbSe/Si heterostructure under nonuniform light illumination and zero-bias conditions. The LPE response is strongly dependent on the thickness of the PbSe film, but always shows a linear dependence on the laser spot position in an ultra-large working size of 5 mm and exhibits a wide photoresponse ranging from visible to near-infrared. The maximum position sensitivity can reach up to 190 mV/mm for the 15-nm-thick PbSe device at 1064 nm and nonlinearity is less than 4%, demonstrating its new potential application in novel position sensitive detectors (PSDs). Besides, the device also shows an ultrafast response speed, with the rise and fall time of ∼40 µs and ∼105 µs, respectively, and excellent reproducibility. These results bring great inspirations for developing high-performance broadband and self-powered PSDs based on the PbSe/Si heterostructure.

Additive Engineering for Efficient and Stable MAPbI3-Perovskite Solar Cells with an Efficiency of over 21.

The high density of defects in MAPbI3 perovskite films brings about severe carrier nonradiative recombination loss, which lowers the performance of MAPbI3-based perovskite solar cells (PSCs). Here, methylamine cyanate (MAOCN) molecules were introduced into MAPbI3 solutions to manipulate the crystallizatsion of the MAPbI3 films. MAOCN molecules can slow down the volatilization rate of the solvent and delay the crystallization process of the MAPbI3 film. The crystal quality of the MAPbI3 films is effectively optimized without an additive residue. Perovskite films treated by MAOCN have lower defect density and longer carrier lifetime, which lowers the carrier recombination loss. Meanwhile, the MAPbI3 film based on MAOCN has a more hydrophobic surface. The final MAPbI3-based device efficiency reached 21.28% (VOC = 1.126 V, JSC = 23.29 mA/cm2, and FF = 81.13). After 30 days of storage under atmospheric conditions, the efficiency of unencapsulated MAOCN-based PSCs only dropped by about 5%.

High-Responsivity, Fast, and Self-Powered Narrowband Perovskite Heterojunction Photodetectors with a Tunable Response Range in the Visible and Near-Infrared Region.

In recent years, narrowband photodetectors (PDs) have been widely used in color imaging, spectral detection or discrimination, defense, and scientific research due to their special spectral selective responses. In this work, by combining organic-inorganic hybrid perovskite layers of different band gaps and thicknesses, a series of narrowband perovskite heterojunction PDs with a continuously adjustable spectral range in the visible and near-infrared range are designed and prepared. The PDs can achieve a narrowband photoresponse with a full width at half-maximum (FWHM) of less than 50 nm and a light rejection ratio (between 780 and 532 nm) of over 1100 and exhibit excellent photoresponse performances with an external quantum efficiency (EQE), responsivity (R), and detectivity (D*) as high as 50.3%, 0.331 A/W, and 4.27 × 1010 Jones, respectively. More importantly, the photoresponses of the PDs at zero bias are as good as those at the reverse bias voltages, indicating the outstanding self-powered property. In addition, a fast response time of ∼180/∼200 µs is obtained in the narrowband perovskite heterojunction, and the response speed nearly remains constant for different PDs in the whole tunable wavelength range, demonstrating the suitable and stable structure of the heterojunction, as well as the high crystalline quality of the perovskite layers. This work definitely provides a simple strategy for designing low-cost, high-photoresponsivity, fast speed, and self-powered narrowband PDs with a tunable spectral range.

Ambient sunlight-driven photothermal methanol dehydrogenation for syngas production with 32.9 % solar-to-hydrogen conversion efficiency.

Methanol dehydrogenation is an efficient way to produce syngas with high quality. The current efficiency of sunlight-driven methanol dehydrogenation is poor, which is limited by the lack of excellent catalysts and effective methods to convert sunlight into chemicals. Here, we show that atomically substitutional Pt-doped in CeO2 nanosheets (Pts-CeO2) exhibit excellent methanol dehydrogenation activity with 500-hr level catalytic stability, 11 times higher than that of Pt nanoparticles/CeO2. Further, we introduce a photothermal conversion device to heat Pts-CeO2 up to 299°C under 1 sun irradiation owning to efficient full sunlight absorption and low heat dissipation, thus achieving an extraordinarily high methanol dehydrogenation performance with a 481.1 mmol g-1 h-1 of H2 production rate and a high solar-to-hydrogen (STH) efficiency of 32.9%. Our method represents another progress for ambient sunlight-driven stable and active methanol dehydrogenation technology.

Photoluminescence characterization of wetting layer and carrier dynamics for coupled InGaAs/GaAs surface quantum dot pair structures.

The optical properties are investigated by spectroscopic characterizations for bilayer InGaAs/GaAs quantum dot (QD) structures consisting of a layer of surface quantum dots (SQDs) separated from a layer of buried quantum dots (BQDs) by different GaAs spacers with thicknesses of 7 nm, 10.5 nm and 70 nm. The coupling from the BQDs to SQDs leads to carrier transfer for the two samples with thin spacers, 7 nm and 10.5 nm, in which QD pairs are obtained while not for the 70 nm spacer sample. The carrier tunneling time is measured to be 0.145 ns and 0.275 ns from BQDs to SQD through the 7 nm and 10.5 nm spacers, respectively. A weak emission band can be observed at the wavelength of ∼ 960 nm, while the excitation intensity dependent PL and PLE spectra show that this is from the wetting layer (WL) of the SQDs. This WL is very important for carrier dynamics in bilayer structures of BQDs and SQDs, including for carrier generation, capture, relaxation, tunneling, and recombination. These results provide useful information for understanding the optical properties of InGaAs SQDs and for using such hybrid structures as building blocks for surface sensing devices.

Type-II GaSb quantum dots grown on InAlAs/InP (001) by droplet epitaxy.

GaSb quantum dots (QDs) have been grown by droplet epitaxy within InAlAs barrier layers on an InP (001) substrate. The droplet growth mode facilitates a larger size (average height ∼4.5 nm) and a lower density (∼6.3 × 109 cm-2) for the QDs than would be expected for the 4% lattice mismatch between GaSb and InAlAs. A type-II band alignment between the GaSb QDs and the InAlAs barriers is revealed by photoluminescence (PL) through a prominent blue-shift of ∼0.11 eV resulting from a six orders of magnitude increase in excitation power. Further confirmation of the type-II nature of these QDs is found through time-resolved PL studies showing a biexponential decay with a long carrier lifetime of ∼10.9 ns. These observations reveal new information for understanding the formation and properties of GaSb/InAlAs/InP QDs, which may be an optimum system for the development of both efficient memory cells and photovoltaic devices.

Lateral bipolar photoresistance effect in the CIGS heterojunction and its application in position sensitive detector and memory device.

Cu(In,Ga)Se2 (CIGS) based multilayer heterojunction, as one of the best high efficiency thin film solar cells, has attracted great interest due to its outstanding features. However, the present studies are primarily focused on the structure optimization and modulation in order to enhance the photoelectric conversion efficiency. Here, we exploit another application of this multilayer heterostructure in photoresistance-modulated position sensitive detector by introducing lateral photoresistance effect. The lateral photoresistance measurements show that this multilayer heterojunction exhibits a wide spectral response (~330 to ~1150 nm) and excellent bipolar photoresistance performances (position sensitivity of ~63.26 Ω/mm and nonlinearity <4.5%), and a fast response speed (rise and fall time of ~14.46 and ~14.42 ms, respectively). More importantly, based on the lateral photoresistance effect, the CIGS heterostructure may also be developed as a position-dependent resistance memory device, which can be modulated by changing laser intensity, wavelength, and bias voltage with excellent stability and repeatability, and the position resolution reaches up to 1 µm. These results can be well explained by considering the diffusion and the drift model of carriers in the CIGS multilayer heterojunction. This work provides a new approach of achieving novel photoelectric sensors and memory devices based on the traditional photovoltaic heterostructures.

Enhanced Light-Induced Transverse Thermoelectric Effect in Tilted BiCuSeO Film via the Ultra-thin AuNPs Layer.

Significant enhancement of light-induced transverse thermoelectric (LITT) effect in tilted BiCuSeO film has been achieved via introduction of an ultra-thin layer of gold nanoparticles (AuNPs) with the thickness of a few nanometers. In both cases of pulsed and continuous light irradiation, about two times increment in the LITT voltage sensitivity is observed for the BiCuSeO film coated with 4-nm-thick AuNPs layer. This can be ascribed to the increased photo-thermal conversion efficiency in the LITT effect owing to the efficient usage of the incident light of AuNPs layer. Thicker AuNPs layer will suppress the voltage sensitivity increment due to the electrical connectivity effect. This work provides an effective strategy for optimizing the performance of thermal-type optical detectors based on the LITT effect.

Interplay Effect of Temperature and Excitation Intensity on the Photoluminescence Characteristics of InGaAs/GaAs Surface Quantum Dots.

We investigate the optical properties of InGaAs surface quantum dots (SQDs) in a composite nanostructure with a layer of similarly grown buried quantum dots (BQDs) separated by a thick GaAs spacer, but with varied areal densities of SQDs controlled by using different growth temperatures. Such SQDs behave differently from the BQDs, depending on the surface morphology. Dedicated photoluminescence (PL) measurements for the SQDs grown at 505 °C reveal that the SQD emission follows different relaxation channels while exhibiting abnormal thermal quenching. The PL intensity ratio between the SQDs and BQDs demonstrates interplay between excitation intensity and temperature. These observations suggest a strong dependence on the surface for carrier dynamics of the SQDs, depending on the temperature and excitation intensity.

Correlation between photoluminescence and morphology for single layer self-assembled InGaAs/GaAs quantum dots.

Single layer self-assembled InGaAs quantum dots (QDs) are manipulated by using different arsenic species on GaAs (100) surface. The As4 molecules are experimentally observed to be more promising than As2 to promote the formation of one-dimensionally-aligned QD-chain arrays. The lateral alignment of QDs and the corresponding formation of dot chains are explained by the anisotropic surface kinetics in combination with the different reactivities of the two molecules with bonding sites on the GaAs (100) surface. Photoluminescence (PL) measurements demonstrate that the spectra of the QD-chains broaden to higher energy and increases in intensity with increasing excitation laser power. The PL band of the QD-chains also exhibits a 9 meV reduction in linewidth as temperature increases starting from 8 K. These observations confirm an efficient lateral coupling between neighboring QDs and thereafter polarized QD emission, whereas the randomly distributed QDs grown with As2 show no preferential polarization. Such QD-chains exhibiting anisotropic properties have the potential for nanophotonics applications like electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation.

Ultrahigh, Ultrafast, and Self-Powered Visible-Near-Infrared Optical Position-Sensitive Detector Based on a CVD-Prepared Vertically Standing Few-Layer MoS2/Si Heterojunction.

MoS2, as a typical transition metal dichalcogenide, has attracted great interest because of its distinctive electronic, optical, and catalytic properties. However, its advantages of strong light absorption and fast intralayer mobility cannot be well developed in the usual reported monolayer/few-layer structures, which make the performances of MoS2-based devices undesirable. Here, large-area, high-quality, and vertically oriented few-layer MoS2 (V-MoS2) nanosheets are prepared by chemical vapor deposition and successfully transferred onto an Si substrate to form the V-MoS2/Si heterojunction. Because of the strong light absorption and the fast carrier transport speed of the V-MoS2 nanosheets, as well as the strong built-in electric field at the interface of V-MoS2 and Si, lateral photovoltaic effect (LPE) measurements suggest that the V-MoS2/Si heterojunction is a self-powered, high-performance position sensitive detector (PSD). The PSD demonstrates ultrahigh position sensitivity over a wide spectrum, ranging from 350 to 1100 nm, with position sensitivity up to 401.1 mV mm-1, and shows an ultrafast response speed of 16 ns with excellent stability and reproducibility. Moreover, considering the special carrier transport process in LPE, for the first time, the intralayer and the interlayer transport times in V-MoS2 are obtained experimentally as 5 and 11 ns, respectively.

High-Efficiency and Stable Organic Solar Cells Enabled by Dual Cathode Buffer Layers.

Various cathode interface materials have been used in organic solar cells (OSCs) to realize high performance. However, most cathode interface materials have their respective weaknesses in maximizing the efficiency or stability of OSCs. Herein, three kinds of alcohol-soluble cathode interfacial materials are combined with bathocuproine (BCP) to serve as multifunctional bilayer cathode buffers for the regular OSCs, and thus greatly enhanced power conversion efficiencies over 10.11% and significantly improved device stability have been achieved. By utilizing double interlayers, both light absorption and light distribution in active layer are improved. Furthermore, double interlayers offer favorable energy-level alignment, alcohol treatment, and duplicate protection of active layer, resulting in significantly reduced leakage current, suppressed recombination, and efficient charge collection. The improved device stability is related to the blocking effect of the complex formed between BCP and the metal electrode and the additional protection effect of the underlying alcohol-soluble materials. In view of the universal use of alcohol-soluble organic electrolyte as cathode buffer layers and by courtesy of the superiority of the double cathode layers relative to the monolayer controls, the double interlayer strategy demonstrated here opens a new way to fully exploiting the potential of OSCs and is believed to be extended to a wider application.

Sb2S3 thickness-dependent lateral photovoltaic effect and time response observed in glass/FTO/CdS/Sb2S3/Au structure.

As an interesting one dimensional ribbon material, Sb2S3 has recently attracted much attention in recent years due to its exciting optical properties. However, Sb2S3-based photovoltaic or photoelectronic devices are still in research, and there are many things unknown to us and need to be well studied. In this work, the glass/FTO/CdS/Sb2S3/Au structures were successfully prepared with different Sb2S3 thicknesses, and the lateral photovoltaic effect (LPE) was firstly observed in this structure, suggesting its great potential in position sensitivity detectors (PSD). It is demonstrated that the crystallinity of Sb2S3 film increases, and Sb2S3 film tends to be vertical ribbon orientation with increasing thickness. Owing to the strong light absorption of the thicker Sb2S3 film and its one dimensional ribbon like crystal structure, the LPE in the glass/FTO/CdS/Sb2S3/Au structure improves with increasing Sb2S3 thickness from 350 nm to 800 nm, and the glass/FTO/CdS/Sb2S3(800 nm) structure exhibits an unprecedented performance with position sensitivity as large as 2230.4 mV/mm. Moreover, the time response of photovoltage was also firstly measured in this structure, it is observed that both the rise time and the fall time decrease with increasing thickness from 350 nm to 800 nm, and then increase quickly for 1100 nm film, further verifing that the Sb2S3 thickness-dependent LPE is strongly dependent on the carriers' longitudinal transport time. The very large LPE and the relatively fast response speed observed in the glass/FTO/CdS/Sb2S3(800 nm)/Au structure unveils its great potential applications in the optoelectronic detectors and also bring an insight that the suitable thickness is very crucial in Sb2S3-based devices.

Large Lateral Photovoltage Observed in MoS2 Thickness-Modulated ITO/MoS2/p-Si Heterojunctions.

Molybdenum disulfide (MoS2), as a typical two-dimensional (2D) material, has attracted extensive attention in recent years because of its fascinating optical and electric properties. However, the applications of MoS2 have been mainly in photovoltaic devices, field-effect transistors, photodetectors, and gas sensors. Here, it is demonstrated that MoS2 can be found another important application in position sensitive detector (PSD) based on lateral photovoltaic effect (LPE) in it. The ITO/MoS2(3, 5, 7, 9, 10, 20, 50, 100 nm)/p-Si heterojunctions were successfully prepared with vertically standing nanosheet structure of MoS2. Because of the special structure and the strong light absorption of the relatively thick MoS2 film, the ITO/MoS2/p-Si heterojunction exhibits an abnormal thickness-dependent LPE, which can be ascribed to the n- to p-type transformation of MoS2. Moreover, the LPE of ITO/MoS2/p-Si structure improves greatly because of forward enhanced built-in field by type transformation in a wide spectrum response ranging from visible to near-infrared, especially the noticeable improvement in infrared region, indicating its great potential application in infrared PSDs. This work not only suggest that the ITO/MoS2/p-Si heterojunction shows great potential in LPE-based sensors, but also unveils the importance of type transformation of MoS2 in MoS2-based photoelectric devices besides strong light absorption and suitable bandgap.

Photoluminescence Study of the Interface Fluctuation Effect for InGaAs/InAlAs/InP Single Quantum Well with Different Thickness.

Photoluminescence (PL) is investigated as a function of the excitation intensity and temperature for lattice-matched InGaAs/InAlAs quantum well (QW) structures with well thicknesses of 7 and 15 nm, respectively. At low temperature, interface fluctuations result in the 7-nm QW PL exhibiting a blueshift of 15 meV, a narrowing of the linewidth (full width at half maximum, FWHM) from 20.3 to 10 meV, and a clear transition of the spectral profile with the laser excitation intensity increasing four orders in magnitude. The 7-nm QW PL also has a larger blueshift and FWHM variation than the 15-nm QW as the temperature increases from 10 to ~50 K. Finally, simulations of this system which correlate with the experimental observations indicate that a thin QW must be more affected by interface fluctuations and their resulting potential fluctuations than a thick QW. This work provides useful information on guiding the growth to achieve optimized InGaAs/InAlAs QWs for applications with different QW thicknesses.

Lateral photovoltaic effect observed in doping-modulated GaAs/Al0.3Ga0.7As.

For photovoltaic effect (PE), both barrier height and carrier lifetime are all very important factors. However, how to distinguish their contributions to the PE is very difficult. In this paper, we prepared a series of GaAs/Al0.3Ga0.7As two dimensional electron gas (2DEG) with typical Al0.3Ga0.7As doping concentration of 0.6 × 1018/cm3, 1.2 × 1018/cm3, and 2.5 × 1018/cm3, respectively (sample number: #1, #2, #3), and studied their lateral photovoltaic effects (LPEs). It is found that their position sensitivities all increase with both laser wavelength and laser power. However, the position sensitivity exhibits a non-monotonic behavior with increasing doping concentration, which can be mainly ascribed to the doping concentration-dependent carrier lifetime, especially in the low power regime. With increasing laser power gradually, the position sensitivity difference between sample #1 and sample #2 is still large and increases a little, while the position sensitivity of sample #3 approaches to that of sample #2, suggesting that the doping concentration-dependent barrier height also starts to play an important role in the high power regime. Our results will provide important information for the design and development of novel and multifunctional PE devices.

Variations of thermoelectric performance by electric fields in bilayer MX2 (M = W, Mo; X = S, Se).

A gate electrode is usually used to controllably tune the carrier concentrations, further modulating the electrical conductivity and the Seebeck coefficient to obtain the optimum thermoelectric figure of merit (ZT) in two-dimensional materials. On the other hand, it is necessary to investigate how an electric field induced by a gate voltage affects the electronic structures, further determining the thermoelectric properties. Therefore, by using density functional calculations in combination with Boltzmann theory, the thermoelectric properties of bilayer MX2 (M = W, Mo; X = S, Se) with or without a 1 V nm-1 perpendicular electric field are comparatively investigated. First of all, the variations of the electrical conductivity (σ), electron thermal conductivity and Seebeck coefficient (S) with the carrier concentration are studied. Due to the trade-off relationship between S and σ, there is an optimum concentration to obtain the maximum ZT, which increases with the temperature due to the enhancement of the Seebeck coefficient. Moreover, N-type bilayers have larger optimum ZTs than P-type bilayers. In addition, the electric field results in the increase of the Seebeck coefficient in low hole-doped MS2 bilayers and high hole-doped MSe2 bilayers, thus leading to similar variations in ZT. The optimum ZTs are reduced from 2.11 × 10-2, 3.19 × 10-2, 2.47 × 10-2, and 2.58 × 10-2 to 1.57 × 10-2, 1.51 × 10-2, 2.08 × 10-2, and 1.43 × 10-2 for the hole-doped MoS2, MoSe2, and WSe2 bilayers, respectively. For N-type bilayers, the electric field shows a destructive effect, resulting in the obvious reduction of the Seebeck coefficient in the MSe2 layers and the low electron-doped MS2 bilayers. In electron-doped bilayers, the optimum ZTs will decrease from 3.03 × 10-2, 6.64 × 10-2, and 6.69 × 10-2 to 2.81 × 10-2, 3.59 × 10-2, and 4.39 × 10-2 for the MoS2, MoSe2, and WSe2 bilayers, respectively.