Tin oxide as an emerging electron transport medium in perovskite solar cells

Electron transport medium (ETM) is one of the most important components determining the photovoltaic performance of organic-inorganic halide perovskite solar cells (PSCs). Among the metal oxide semiconductors, anatase (TiO2) is the most common material used as ETM in PSCs to facilitate charge collection as well as to support a thin perovskite absorber layer. Production of conductive crystalline TiO2 requires relatively higher temperatures (400–500 °C) which limits its application to glass substrates coated with fluorine tin oxide (FTO) as other tin oxides (e.g. indium tin oxide) degrade at temperatures above 300 °C. Furthermore, this renders it unsuitable for flexible devices, often based on low-temperature flexible plastic substrates. Pure tin oxide, one of the earliest metal oxide semiconductors, is often used in myriad electronic devices and has shown outstanding characteristics as an ETM in PSC systems. Thus, tin oxide can be considered a viable alternative to TiO2 due to its excellent electron mobility and higher stability than other alternatives such as zinc oxide. This review article gives a brief history of ETMs in PSC systems and reviews recent developments in the use of tin oxide in both pure and composite form as ETMs. Efficiencies of up to 21% have been reported in tin oxide based PSCs with photovoltages of up to ~1214 mV

Humidity Versus Photo-Stability of Metal Halide Perovskite Films in a Polymer Matrix

Despite the high efficiency of over 21% reported for emerging thin film perovskite solar cells, one of the key issues prior to their commercial deployment is to attain their long term stability under ambient and outdoor conditions. The instability in perovskite is widely conceived to be humidity induced due to the water solubility of its initial precursors, which leads to decomposition of the perovskite crystal structure; however, we note that humidity alone is not the major degradation factor and it is rather the photon dose in combination with humidity exposure that triggers the instability. In our experiment, which is designed to decouple the effect of humidity and light on perovskite degradation, we investigate the shelf-lifetime of CH3NH3PbI3 films in the dark and under illumination under high humidity conditions (Rel. H. > 70%). We note minor degradation in perovskite films stored in a humid dark environment whereas upon exposure to light, the films undergo drastic degradation, primarily owing to the reactive TiO2/perovskite interface and also the surface defects of TiO2. To enhance its air-stability, we incorporate CH3NH3PbI3 perovskite in a polymer (poly-vinylpyrrolidone, PVP) matrix which retained its optical and structural characteristics in the dark for ∼2000 h and ∼800 h in room light soaking, significantly higher than a pristine perovskite film, which degraded completely in 600 h in the dark and in less than 100 h when exposed to light. We attribute the superior stability of PVP incorporated perovskite films to the improved structural stability of CH3NH3PbI3 and also to the improved TiO2/perovskite interface upon incorporating a polymer matrix. Charge injection from the polymer embedded perovskite films has also been confirmed by fabricating solar cells using them, thereby providing a promising future research pathway for stable and efficient perovskite solar cells

SnO2–TiO2 Hybrid Nanofibers for Efficient Dye-Sensitized Solar Cells

Pristine SnO2 nanostructures typically result in low open circuit voltage (VOC) <500 mV due to the lower Fermi energy (EF) when employed as a photoanode materials in dye sensitized solar cells (DSSCs). On the other hand, the most successful photoanode material, i.e., TiO2 nanoparticle although provides a high VOC ⩾ 800 mV result in poor charge collection owing to their inferior electron mobility (μn). Herein, we employ nanofiber–nanoparticle composite of SnO2–TiO2 which showed similar VOC and short circuit current density (JSC) to a reference TiO2 based DSSCs. The nanocomposite developed here involves multi-porous SnO2 nanofibers characterized by a lower EF; however, with higher μn and TiO2 nanoparticles of higher EF and lower μn. The TiO2 particles in the pores of SnO2 nanofibers were developed by TiCl4 treatment, whose concentration is optimized for the saturated JSC and VOC. The best performing DSSCs fabricated using the composite electrodes deliver power conversion efficiency (PCE) of ≈7.9% (VOC ≈ 717 mV; JSC ≈ 21 mA cm−2), which is significantly higher than pure SnO2 photoanode with PCE ≈ 3.0% (JSC ≈ 14.0 mA cm−2 and VOC ≈ 481 mV) at similar experimental conditions

Advances in stable and flexible perovskite solar cells

Roll-to-roll (R2R) production is an innovative approach and is fast becoming a very popular industrial method for high throughput and mass production of solar cells. Replacement of costly indium tin oxide (ITO), which conventionally has served as the transparent electrode would be a great approach for roll to roll production of flexible cost effective solar cells. Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are brittle and ultimately limit the device flexibility. Perovskite solar cells (PSCs) have been the centre of photovoltaic research community during the recent years owing to its exceptional performance and economical prices. The best reported PSCs fabricated by employing mesoporous TiO2 layers require elevated temperatures in the range of 400–500 °C which limits its applications to solely glass substrates. In such a scenario developing flexible PSCs technology can be considered a suitable and exciting arena from the application point of view, them being flexible, lightweight, portable, and easy to integrate over both small, large and curved surfaces

Prognostic model to predict postoperative acute kidney injury in patients undergoing major gastrointestinal surgery based on a national prospective observational cohort study.

Background: Acute illness, existing co-morbidities and surgical stress response can all contribute to postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery. The aim of this study was prospectively to develop a pragmatic prognostic model to stratify patients according to risk of developing AKI after major gastrointestinal surgery. Methods: This prospective multicentre cohort study included consecutive adults undergoing elective or emergency gastrointestinal resection, liver resection or stoma reversal in 2-week blocks over a continuous 3-month period. The primary outcome was the rate of AKI within 7 days of surgery. Bootstrap stability was used to select clinically plausible risk factors into the model. Internal model validation was carried out by bootstrap validation. Results: A total of 4544 patients were included across 173 centres in the UK and Ireland. The overall rate of AKI was 14·2 per cent (646 of 4544) and the 30-day mortality rate was 1·8 per cent (84 of 4544). Stage 1 AKI was significantly associated with 30-day mortality (unadjusted odds ratio 7·61, 95 per cent c.i. 4·49 to 12·90; P < 0·001), with increasing odds of death with each AKI stage. Six variables were selected for inclusion in the prognostic model: age, sex, ASA grade, preoperative estimated glomerular filtration rate, planned open surgery and preoperative use of either an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. Internal validation demonstrated good model discrimination (c-statistic 0·65). Discussion: Following major gastrointestinal surgery, AKI occurred in one in seven patients. This preoperative prognostic model identified patients at high risk of postoperative AKI. Validation in an independent data set is required to ensure generalizability

Nanostructured tin oxide materials as photoanode for dye sensitized solar cells

Photoanode plays vital role in the operation of dye sensitized solar cells (DSSCs) owing to its many fold functions: (i) as a substrate to anchor dye, (ii) enable the bound electron-hole pair formed in the dye to be separated into mobile charge carriers, and (iii) provide a medium to accept and transport the photoinduced electrons. Although TiO2 nanoparticles (NPs) is the most successful employed photoanode material in DSSCs delivering efficiency (η) ~14.3% due to its high specific surface area (~150 m2/g), it is characterized by slow electron diffusion and mobility (µe). On the other hand, SnO2 shows an outstanding performance in many electronic devices owing to its higher µe. The high µe could transport electrons much faster and minimize the charge recombination in the device. From this point of view, SnO2 is an interesting alternative to that of TiO2. In this research, NPs and multifunctional NPs-Nanospheres (NPs-NSs) were synthesized hydrothermally by simply varying the hydrothermal annealing temperature from 150 to 200 °c. The multifunctional (NPs-NSs) shows higher crystallinity and superior light scattering in comparison to NPs analogue because of its unique morphology i.e., the size of the spheres in the range of ~800 nm – 1 m which are comparable to the visible light wavelength. The photovoltaic (PV) parameters include short circuit density (JSC), open circuit voltage (VOC) and were higher for the multifunctional (NPs-NSs) i.e., 17 mA cm–2, 500 mV and ~4.0% vs. 7, 370 and ~1.3 for NPs analogue, respectively. For the superior charge transport, on the other hand, various one dimensional electrospun nanomaterials were synthesized. A strategy was adopted by just keeping the precursor concentration changing while other constituents of the solution constant. Among them, multiporous nanofibers (MPNFs) resulted in a significantly higher JSC ~18 mA cm–2 compared to that of porous NFs (PNFs) ~10 mA cm–2 due to 8-10-folds larger surface area of the former, which consequently adsorbed large amount of dye. However, the fill factor (FF) and VOC were limited to (~45-50%) and (~450-500 mV) in pure SnO2 based DSSCs, respectively. Finally, the lower VOC and FF were enhanced using a composite by incorporating TiO2 NPs into SnO2 photoanode via TiCl4 treatment. This technique considerably improved the VOC (from ~500 to 720 mV) and similarly JSC from (~17 to 22 mA cm–2). Though VOC and JSC increased dramatically, the FF was still limited to ~50%. In the second approach, an electrospun composite (SnO2-TiO2) NFs was used as photoanode, which yielded VOC (~730 mV) and FF (~60%) simultaneously, closer to that of TiO2 (Voc ~700-800 mV and FF ~60-68%). Results of this research provide promising directions such as increasing surface area and surface roughness of the composite nanofibers so that efficiency levels for practical applications could be realized

Tin Oxide as a Photoanode for Dye-sensitised Solar Cells: Current Progress and Future Challenges

Tin oxide (SnO2) is a candidate for applications requiring high electrical conductivity and optical transparency, such as a photoanode in dye-sensitised solar cells (DSSCs), due to its higher electron mobility and wider optical transparency than many other metal oxide semiconductors (MOS), such as TiO2 and ZnO. However, DSSCs employing SnO2 show significantly lower photoconversion efficiency, compared to that achieved by popular choices, such as TiO2, due to its intrinsic limitations such as lower conduction band energy and isoelectric point. A survey of literature shows a revived interest in SnO2-based DSSCs, for example, strategies to (i) increase the dye uptake, (ii) increase its Fermi energy level, and (iii) reduce the recombination, such as by increasing surface roughness and novel morphologies towards (i), and doping of transition metals for (ii) and (iii). In response to these improvements, SnO2-based DSSCs showed similar open circuit voltage and superior short circuit current to that achieved by TiO2. We have undertaken a critical review on the progress made in overcoming the limitations and capitalising the advantages of SnO2 to fabricate more efficient DSSCs. We identify that more investment is required to reduce the recombination in SnO2 for it to emerge as an efficiency record holder in DSSCs

One Dimensional SnO2/ZnO Nanofiber Composite for Dye Sensitized Solar Cell

One-dimensional nano-morphology SnO2 and composite of ZnO/SnO2 nanofibers were synthesized, and fabricated by electrospinning technique. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (FESEM) techniques to confirm the crystal structure and morphology. Dye-sensitized solar cells (DSSCs) were fabricated using these nanofibers as photoanodes and their performance were compared with to commercial TiO2 paste (P25)-based DSSCs. photoconversion efficiency (PCE) of 4.64 % was realized in composite ZnO/SnO2 nanofibers based DSSCs which comparable to 4.75 % for P25 based DSSCs

Medicinal Plants of Turmic Valley (Central Karakoram National Park), Gilgit-Baltistan, Pakistan

This study was undertaken to enumerate the medicinal plants of the area, find out the conservation status, and record the folk knowledge from the inhabitants of Turmic Valley during 2011-2013. The valley is located in the Rondo division of the District Skardu on the Northeastern side of the Indus River. The detailed information about the local flora regarding medicinal uses was collected from the local herbal healers (Hakeems) and other knowledgeable people. Locally used herbs of the area prevent and cure the people from various diseases such as joint pains, bronchitis, flu and fever, lowering blood pressure, constipation, liver disorders, stomach and abdominal problems, etc. The most common medicinal herbs found in the region belong to the families Gentianaceae, Berberidaceae, Umbelliferae, Labiatae, Rosaceae, Compositae, Urticaceae, and Ranunculaceae. The inhabitants of the valley mostly use the 42 plant species for the treatment of different health problems. Forty-two species of plants (including 4 Gymnosperms, 1 monocotyledon, and 37 dicotyledons) and 35 types of diseases have been identified during the current study. Thymus linearis, Rosa webbiana, Urtica dioca, Pleurospermum candollei, Berberis spp., Delphinium brononianum, and Mentha angustifolia were the commonly used plant species in the valley. The collected baseline data of this study will be helpfulfor young researchers in the fieldof taxonomy, ethnobotany, pharmacology, organic chemistry, and particularly for biodiversity conservation. Over exploitation, habitat destruction, and over grazing are the major threats for the loss of the important flora of the area

Probing Electron Lifetime and Recombination Dynamics in Large Area Dye-Sensitized Solar Cells by Electrochemical Impedance Spectroscopy

A number of nanocrystalline, mesoporous large area (~0.2- 2 cm2) dye-sensitized solar cells (DSSCs) are probed by electrochemical impedance spectroscopy measurements to realize their carrier dynamics such as charge transport resistance (RCT), electron diffusion coefficient (Dη), and electron lifetime (τn) by applying an equivalent electrical model. The experimental upshots reveal that the electron lifetime relates with the device physical parameters which was neglected in previous studies. It is also found that the RCT relates negatively with the device area i.e. it decrease upon increasing photo-exposed area. The observed lowering of current density (JSC) over a series of experiments upon increasing the photoelectrode area is attributed to the decrease in RCT. The charge carriers upon injection into semiconductor layer find increased diffusion pathways and eventually recombine with the hole species when characterized by lower carrier lifetime. The thickness of the electrode film does not play an effective role indicating that the dynamics of larger area DSSCs differs largely from those of a single cell. The experimental results available indicate that a nearly complete collection of charge carriers is possible in large area modules provided the physical dimensions of photoelectrode area are considered. The results from the study hints future directions to build high efficiency DSSC modules and further asserts considering the diffusion length in two-dimensions while fabricating larger area cells