Tin Sulfide (SnS) Films Deposited by an Electric Field-Assisted Continuous Spray Pyrolysis Technique with Application as Counter Electrodes in Dye-Sensitized Solar Cells

The deposition of tin sulfide (SnS) nanostructured films using a continuous spray pyrolysis technique is reported with an electric field present at the nozzle for influencing the atomization and the subsequent film deposition. In the absence of the electric field, the X-ray diffraction pattern shows the orthorhombic phase of SnS with a crystallographic preferred orientation along the (040) plane. The application of the electric field results in significant improvement in the morphology and a reduction in surface roughness (28 nm from 37 nm). The direct optical band gap of the films deposited with and without the electric field is estimated to be 1.5 and 1.7 eV, respectively. The photothermal deflection spectroscopy studies show a lower energetic disorder (no Urbach tail), which indicates an annealing effect in the SnS films deposited under the electric field. The improvement in the film properties is reflected in the expected improvement in the power conversion efficiency (PCE) of dye-sensitized solar cells fabricated using the SnS film as a counter electrode. An enhancement of PCE from 2.07% for the film deposited without the electric field to 2.89% for the film deposited with the electric field shows the role of the electric field in the fabrication of improved SnS film

Morphology, structure and optical properties of low bandgap organic-inorganic halide perovskite based on CH₃NH₃SnxPb₁₋ₓI₃

Herein, we investigate morphology, structure and optical properties of low band gap organic-inorganic halide perovskite based on a mixture of lead and tin as the divalent cation in ABX3 structure. A significant change in morphology of CH3NH3SnxPb1−xI3 perovskite with x as well as an alteration in crystal structure from I4cm (β-phase) to pseudocubic P4mm (α-phase) space groups is observed when Sn is the dominant divalent cation (x ≥ 0.5). Photo thermal defection optical absorption spectroscopy (PDS) and photoluminescence (PL) of CH3NH3SnxPb1−xI3 show the non-linear change in the band edge of perovskite. The bandgap as low as 1.17 eV and the most red-shifted PL at 1035nm is achieved for perovskite with x=0.8. In addition, higher electronic disorder is measured for CH3NH3SnxPb1−xI3 compounds with higher x

Exponential Family Trend Filtering on Lattices

Trend filtering is a modern approach to nonparametric regression that is more adaptive to local smoothness than splines or similar basis procedures. Existing analyses of trend filtering focus on estimating a function corrupted by homoskedastic Gaussian noise, but our work extends this technique to general exponential family distributions. This extension is motivated by the need to study massive, gridded climate data derived from polar-orbiting satellites. We present algorithms tailored to large problems, theoretical results for general exponential family likelihoods, and principled methods for tuning parameter selection without excess computation.Comment: 53 pages; 6 figures; 3 table

Slow Carrier Cooling in Hybrid Pb-Sn Halide Perovskites

Low bandgap perovskites, such as mixed Pb-Sn systems, are essential to make bottom cells in all perovskite tandem photovoltaics. However, currently the fundamental dynamics of carriers in these materials are not well explored. Here we use ultrafast broadband pump-probe spectroscopy to probe the two-stage carrier cooling dynamics in CH3NH3Pb1-xSnxI3 perovskites and show that the cooling is slower than in pure Pb systems. The first stage of cooling is associated with Fröhlich interactions between carriers and metal-iodide bonds that slows down monotonically with increasing Sn content. The second stage of cooling involves the decay of optical phonon into acoustic phonons and is slowest for 60% Sn content, due to an increasing gap between the optical and acoustic phonon branches. Our results provide first insights into carrier cooling dynamics in hybrid Pb-Sn halide perovskites and pave the way for further understanding of the fundamental nature of Fröhlich interactions and the decay of optical phonons in perovskite systems.EPSR

Fabrication of ZnO/Cu<inf>2</inf>O heterojunctions in atmospheric conditions: Improved interface quality and solar cell performance

Zn_1-xMg_xO/Cu_2O heterojunctions were successfully fabricated in open-air at low temperatures via atmospheric atomic layer deposition of Zn_1-xMg_xO on thermally oxidized cuprous oxide. Solar cells employing these heterojunctions demonstrated a power conversion efficiency exceeding 2.2% and an open-circuit voltage of 0.65 V. Surface oxidation of Cu_2O to CuO prior to and during Zn_1-xMg_xO deposition was identified as the limiting factor to obtaining a high quality heterojunction interface. Optimization of deposition conditions to minimize Cu_2O surface oxidation led to improved device performance, tripling the open-circuit voltage and doubling the short-circuit current density. These values are the highest reported for a ZnO/Cu_2O interface formed in air, and highlight atmospheric ALD as a promising technique for inexpensive and scalable fabrication of ZnO/Cu_2O heterojunctions.This is the final published version. It is also available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0927024814005005#

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO.

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.The authors acknowledge the support of the Cambridge Overseas and Commonwealth Trust, the Rutherford Foundation of New Zealand, Girton College Cambridge. This work has been funded by ERC Advanced Investigator Grant, Novox, ERC-2009-adG247276 and by the EPSRC (under RGS3717)

Charge Generation and Electron-Trapping Dynamics in Hybrid Nanocrystal-Polymer Solar Cells

We investigate the charge-trapping dynamics in hybrid nanocrystal-polymer systems and their effect on performance in photovoltaic devices. Employing various steady-state spectroscopy techniques and ultrafast, three-pulse transient absorption methods, we identify the depth of electron trap states in the nanocrystal band gap and measure their population dynamics. Our findings show that photogenerated electrons are trapped at midgap states on the nanocrystal within hundreds of picoseconds. The trapping of the majority of charge carriers before charge extraction results in a lowering of the quasi-Fermi level of the electrons which limits the device open-circuit voltage, thereby underlining the significance of these processes in conjugated polymer/nanocrystal hybrid photovoltaics.Engineering and Physical Sciences Research Council (Grant IDs: EP/M005143/1, EP/G060738/1, EP/G037221/1), Worshipful Company of Armourers and Brasiers (Gauntlet Trust award), German National Academic Foundation (Studienstiftung)This is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/acs.jpcc.6b0759

Interface engineering of mesoscopic hybrid organic-inorganic perovskite solar cells

We report on the optimization of the interfacial properties of titania in mesoscopic CH3NH3PbI3 perovskite solar cells (PSCs). Modification of the mesoporous titania (mp-TiO2) film by TiCl4 treatment substantially reduced the surface traps, as is evident from the sharpness of the absorption edge with a significant reduction in Urbach energy (from 320 to 140 meV) determined from photothermal deflection spectroscopy, and led to an order of magnitude enhancement in the bulk electron mobility and corresponding decrease in the transport activation energy (from 170 to 90 meV) within a device. After optimization of the photoanode–perovskite interface using various sizes of TiO2 nanoparticles, the best photovoltaic efficiency of 16.3% was achieved with the mesoporous TiO2 composed of 36 nm sized nanoparticles. The improvement in device performance can be attributed to the enhanced charge collection efficiency that is driven by improved charge transport in the mesoporous TiO2 layer. Also, the decreased recombination at the TiO2–perovskite interface and better perovskite coverage play important roles