Room-temperature multiple ligands-tailored SnO2 quantum dots endow in situ dual-interface binding for upscaling efficient perovskite photovoltaics with high VOC.

The benchmark tin oxide (SnO2) electron transporting layers (ETLs) have enabled remarkable progress in planar perovskite solar cell (PSCs). However, the energy loss is still a challenge due to the lack of "hidden interface" control. We report a novel ligand-tailored ultrafine SnO2 quantum dots (QDs) via a facile rapid room temperature synthesis. Importantly, the ligand-tailored SnO2 QDs ETL with multi-functional terminal groups in situ refines the buried interfaces with both the perovskite and transparent electrode via enhanced interface binding and perovskite passivation. These novel ETLs induce synergistic effects of physical and chemical interfacial modulation and preferred perovskite crystallization-directing, delivering reduced interface defects, suppressed non-radiative recombination and elongated charge carrier lifetime. Power conversion efficiency (PCE) of 23.02% (0.04 cm2) and 21.6% (0.98 cm2, VOC loss: 0.336 V) have been achieved for the blade-coated PSCs (1.54 eV Eg) with our new ETLs, representing a record for SnO2 based blade-coated PSCs. Moreover, a substantially enhanced PCE (VOC) from 20.4% (1.15 V) to 22.8% (1.24 V, 90 mV higher VOC, 0.04 cm2 device) in the blade-coated 1.61 eV PSCs system, via replacing the benchmark commercial colloidal SnO2 with our new ETLs.

PMMA Thin Film with Embedded Carbon Quantum Dots for Post-Fabrication Improvement of Light Harvesting in Perovskite Solar Cells.

Perovskite solar cells (PSCs) with a standard sandwich structure suffer from optical transmission losses due to the substrate and its active layers. Developing strategies for compensating for the losses in light harvesting is of significant importance to achieving a further enhancement in device efficiencies. In this work, the down-conversion effect of carbon quantum dots (CQDs) was employed to convert the UV fraction of the incident light into visible light. For this, thin films of poly(methyl methacrylate) with embedded carbon quantum dots (CQD@PMMA) were deposited on the illumination side of PSCs. Analysis of the device performances before and after application of CQD@PMMA photoactive functional film on PSCs revealed that the devices with the coating showed an improved photocurrent and fill factor, resulting in higher device efficiency.

A Cryogenic Process for Antisolvent-Free High-Performance Perovskite Solar Cells.

A cryogenic process is introduced to control the crystallization of perovskite layers, eliminating the need for the use of environmentally harmful antisolvents. This process enables decoupling of the nucleation and the crystallization phases by inhibiting chemical reactions in as-cast precursor films rapidly cooled down by immersion in liquid nitrogen. The cooling is followed by blow-drying with nitrogen gas, which induces uniform precipitation of precursors due to the supersaturation of precursors in the residual solvents at very low temperature, while at the same time enhancing the evaporation of the residual solvents and preventing the ordered precursors/perovskite from redissolving into the residual solvents. Using the proposed techniques, the crystallization process can be initiated after the formation of a uniform precursor seed layer. The process is generally applicable to improve the performance of solar cells using perovskite films with different compositions, as demonstrated on three different types of mixed halide perovskites. A champion power conversion efficiency (PCE) of 21.4% with open-circuit voltage (VOC ) = 1.14 V, short-circuit current density ( JSC ) = 23.5 mA cm-2 , and fill factor (FF) = 0.80 is achieved using the proposed cryogenic process.

Characterization of Low-Frequency Excess Noise in CH3NH3PbI3-Based Solar Cells Grown by Solution and Hybrid Chemical Vapor Deposition Techniques.

In this study, detailed investigations of low-frequency noise (LFN) characteristics of hybrid chemical vapor deposition (HCVD)- and solution-grown CH3NH3PbI3 (MAPI) solar cells are reported. It has been shown that LFN is a ubiquitous phenomenon observed in all semiconductor devices. It is the smallest signal that can be measured from the device; hence, systematic characterization of the LFN properties can be utilized as a highly sensitive nondestructive tool for the characterization of material defects in the device. It has been demonstrated that the noise power spectral densities of the devices are critically dependent on the parameters of the fabrication process, including the growth ambient of the perovskite layer and the incorporation of the mesoscopic structures in the devices. Our experimental results indicated that the LFN arises from a thermally activated trapping and detrapping process, resulting in the corresponding fluctuations in the conductance of the device. The results show that the presence of oxygen in the growth ambient of the HCVD process and the inclusion of an mp-TiO2 layer in the device structure are two important factors contributing to the substantial reduction in the density of the localized states in the MAPI devices. Furthermore, the lifetimes of the MAPI perovskite-based solar cells are strongly dependent on the material defect concentration. The degradation process is substantially more rapid for the devices with higher initial defect density compared to the devices prepared under optimized conditions and structure that exhibit substantially lower initial trap density.

Black Phosphorus Quantum Dots Used for Boosting Light Harvesting in Organic Photovoltaics.

Although organic photovoltaic devices (OPVs) have been investigated for more than two decades, the power conversion efficiencies of OPVs are much lower than those of inorganic or perovskite solar cells. One effective approach to improve the efficiency of OPVs is to introduce additives to enhance light harvesting as well as charge transportation in the devices. Here, black phosphorus quantum dots (BPQDs) are introduced in OPVs as an additive. By adding 0.055 wt % BPQDs relative to the polymer donors in the OPVs, the device efficiencies can be dramatically improved for more than 10 %. The weight percentage is much lower than that of any other additive used in OPVs before, which is mainly due to the two-dimentional structure as well as the strong broadband light absorption and scattering of the BPQDs. This work paves a way for using two-dimentional quantum dots in OPVs as a cost-effective approach to enhance device efficiencies.

Crystal Engineering for Low Defect Density and High Efficiency Hybrid Chemical Vapor Deposition Grown Perovskite Solar Cells.

Synthesis of high quality perovskite absorber is a key factor in determining the performance of the solar cells. We demonstrate that hybrid chemical vapor deposition (HCVD) growth technique can provide high level of versatility and repeatability to ensure the optimal conditions for the growth of the perovskite films as well as potential for batch processing. It is found that the growth ambient and degree of crystallization of CH3NH3PbI3 (MAPI) have strong impact on the defect density of MAPI. We demonstrate that HCVD process with slow postdeposition cooling rate can significantly reduce the density of shallow and deep traps in the MAPI due to enhanced material crystallization, while a mixed O2/N2 carrier gas is effective in passivating both shallow and deep traps. By careful control of the perovskite growth process, a champion device with power conversion efficiency of 17.6% is achieved. Our work complements the existing theoretical studies on different types of trap states in MAPI and fills the gap on the theoretical analysis of the interaction between deep levels and oxygen. The experimental results are consistent with the theoretical predictions.

Fabrication of WS2/GaN p-n Junction by Wafer-Scale WS2 Thin Film Transfer.

High quality wafer-scale free-standing WS2 grown by van der Waals rheotaxy (vdWR) using Ni as a texture promoting layer is reported. The microstructure of vdWR grown WS2 was significantly modified from mixture of crystallites with their c-axes both parallel to (type I) and perpendicular to (type II) the substrate to large type II crystallites. Wafer-scale transfer of vdWR grown WS2 onto different substrates by an etching-free technique was demonstrated for the first time that utilized the hydrophobic property of WS2 and hydrophilic property of sapphire. Our results show that vdWR is a reliable technique to obtain type-II textured crystallites in WS2, which is the key factor for the wafer-scale etching-free transfer. The transferred films were found to be free of observable wrinkles, cracks, or polymer residues. High quality p-n junctions fabricated by room-temperature transfer of the p-type WS2 onto an n-type GaN was demonstrated with a small leakage current density of 29.6 µA/cm2 at -1 V which shows superior performances compared to the directly grown WS2/GaN heterojunctions.

Encapsulation of Perovskite Solar Cells for High Humidity Conditions.

We examined different encapsulation strategies for perovskite solar cells by testing the device stability under continuous illumination, elevated temperature (85 °C) and ambient humidity of 65 %. The effects of the use of different epoxies, protective layers and the presence of desiccant were investigated. The best stability (retention of ∼80 % of initial efficiency on average after 48 h) was obtained for devices protected by a SiO2 film and encapsulated with a UV-curable epoxy including a desiccant sheet. However, the stability of ZnO-based cells encapsulated by the same method was found to be inferior to that of TiO2 -based cells. Finally, outdoor performance tests were performed for TiO2 -based cells (30-90 % ambient humidity). All the stability tests were performed following the established international summit on organic photovoltaic stability (ISOS) protocols for organic solar cell testing (ISOS-L2 and ISOS-O1).

Perovskite Photovoltachromic Supercapacitor with All-Transparent Electrodes.

Photovoltachromic cells (PVCCs) are of great interest for the self-powered smart windows of architectures and vehicles, which require widely tunable transmittance and automatic color change under photostimuli. Organolead halide perovskite possesses high light absorption coefficient and enables thin and semitransparent photovoltaic device. In this work, we demonstrate co-anode and co-cathode photovoltachromic supercapacitors (PVCSs) by vertically integrating a perovskite solar cell (PSC) with MoO3/Au/MoO3 transparent electrode and electrochromic supercapacitor. The PVCSs provide a seamless integration of energy harvesting/storage device, automatic and wide color tunability, and enhanced photostability of PSCs. Compared with conventional PVCC, the counter electrodes of our PVCSs provide sufficient balancing charge, eliminate the necessity of reverse bias voltage for bleaching the device, and realize reasonable in situ energy storage. The color states of PVCSs not only indicate the amount of energy stored and energy consumed in real time, but also enhance the photostability of photovoltaic component by preventing its long-time photoexposure under fully charged state of PVCSs. This work designs PVCS devices for multifunctional smart window applications commonly made of glass.

Effect of PTB7 Properties on the Performance of PTB7:PC71BM Solar Cells.

The effect of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) properties on the optical properties, charge transport and photovoltaic performance of PTB7:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) blend films is investigated. We found that the variations in the molecular weight (Mw) and polydispersity index (PDI) of PTB7 mainly affect the phase separation and charge transport (hole mobility) in the blend films. The optical properties are also affected, but the increase in the extinction coefficient does not necessarily imply increased power conversion efficiency. The obtained power conversion efficiency for optimized thickness varied from 4.8% to 7.8% depending on the properties of PTB7. The wide range of obtained power conversion efficiencies illustrates the importance of optimizing the Mw and PDI to optimize bulk heterojunction morphology and achieve high performance solar cells.

Enhanced performance of PTB7:PC71BM solar cells via different morphologies of gold nanoparticles.

The effects of gold nanoparticles (AuNPs) incorporated in the hole transporting layer (HTL) of poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b'] dithiophene-2, 6-diyl] [3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophened iyl]] (PTB7): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) based solar cells are being systematically investigated in terms of the optical properties, electrical properties, and photovoltaic performance. The impacts of AuNPs on the optical response of the devices are modeled by finite-difference time-domain (FDTD) simulation. The size of the AuNPs used in this work is around 50-70 nm, so that 10-20 nm penetrated from the HTL into the active layer. We found that the power conversion efficiencies (PCEs) of the devices with AuNPs are significantly enhanced from 7.5%, for the control device, to 8.0%, 8.1%, and 8.2% for Au nanosphere-, nanorod-, and nanocube-incorporated devices, respectively. Among the photovoltaic parameters of the AuNP devices, the short circuit current density (JSC) exhibits the largest improvement, which can be attributed to the improved optical properties of the devices. On the basis of the calculation results, the scattering cross section for the samples in the presence of AuNPs can be enhanced by a factor of ∼10(10)-10(13) and Au nanocubes exhibit superior scattering cross section compared to the Au nanospheres and nanorods with the same linear dimension. From the experimental impedance spectroscopy results, we found that the addition of AuNPs had little effect on the electrical properties of the device. The device performance is also found to be sensitive to the concentration and morphology of the AuNPs.

Thermal assisted oxygen annealing for high efficiency planar CH3NH3PbI3 perovskite solar cells.

We report investigations on the influences of post-deposition treatments on the performance of solution-processed methylammonium lead triiodide (CH3NH3PbI3)-based planar solar cells. The prepared films were stored in pure N2 at room temperature or annealed in pure O2 at room temperature, 45°C, 65°C and 85°C for 12 hours prior to the deposition of the metal electrodes. It is found that annealing in O2 leads to substantial increase in the power conversion efficiencies (PCEs) of the devices. Furthermore, strong dependence on the annealing temperature for the PCEs of the devices suggests that a thermally activated process may underlie the observed phenomenon. It is believed that the annealing process may facilitate the diffusion of O2 into the spiro-MeOTAD for inducing p-doping of the hole transport material. Furthermore, the process can result in lowering the localized state density at the grain boundaries as well as the bulk of perovskite. Utilizing thermal assisted O2 annealing, high efficiency devices with good reproducibility were attained. A PCE of 15.4% with an open circuit voltage (VOC) 1.04 V, short circuit current density (JSC) 23 mA/cm(2), and fill factor 0.64 had been achieved for our champion device.

AlGaN/GaN heterostructures for non-invasive cell electrophysiological measurements.

Recently, the ability to create bio-semiconductor hybrid devices has gained much interest for cell activity analysis. AlGaN material system has been demonstrated to be a promising cell-based biosensing platform due to a combination of unique properties, such as chemical inertness, optical transparency and low signal to noise ratios. To investigate the potential application of hybrid cell-AlGaN/GaN field effect transistor for cell electrophysiological monitoring, saos-2 human osteoblast-like cells were cultured in high density in non-metallized gate area of a transparent AlGaN/GaN heterostructure field effect transistor. We implemented and characterized the transistor recording of extracellular voltage in the cell-chip junction using the FET chip. The effect of ion channel blocker TEA on transistor signal was explored in order to test the capability of this hybrid chip for in vitro drug screening bioassay. Finally, the effect of cell adhesion on transistor signal was also studied by applying the protein kinase inhibitor H-7.