Max Noack

Defect density and dielectric constant in perovskite solar cells

We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH3NH3PbIxCl1−x perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 1011/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

High efficiency sequentially vapor grown n-i-p CH3NH3PbI3 perovskite solar cells with undoped P3HT as p-type heterojunction layer

Photovoltaic devices with perovskite materials as light absorbing material were fabricated through sequential vapor deposition of lead iodide and methylammonium iodide with undoped poly3hexylthiophene (P3HT) as a hole transporting layer. The sequential vapor deposition process produced films and devices with the large grains and low defect densities, very small values of dark current, and high open circuit voltages. The thickness of the P3HT layer was a critical parameter for achieving high solar conversion efficiencies of 13.7%. The vapor deposition process also produced devices with a tight distribution of performance characteristics and very high open circuit voltages (0.99 V).

Auger study of grain boundaries in large‐grained YBa2Cu3Ox

A scanning Auger microscopy study has been carried out on fractured surfaces of YBa2Cu3Ox samples prepared by standard sintering procedures from powders. It is concluded that in the large‐grained samples examined, a majority of the grain boundaries are sufficiently ‘‘open’’ that standard metallographic preparation causes them to become contaminated with C. This suggests that an actual loss of contact at the grain boundaries during sample preparation is responsible for the low Jc values observed in bulk‐sized large‐grained YBa2Cu3Ox.

An arc floating zone technique for preparing single crystal lanthanum hexaboride

An arc floating zone technique has been developed for producing LaB6. The technique consistently produces single crystals which nucleate and grow with essentially random crystallographic orientations. Chemical analyses show that very effective purification is obtained, contamination from the tantalum electrode material does not occur, and stoichiometry of the LaB6 is maintained.

Electron mobility in nanocrystalline silicon devices

Electron mobility in the growth direction was measured using space charge limited current techniques in device-type nin structure nanocrystalline Si:H and nanocrystalline Ge:H structures. The films were grown on stainless steel foil using either hot wire or remote plasma enhanced chemical vapor deposition techniques. Grain size and crystallinity were measured using x ray and Raman spectroscopy. The size of grains in films was adjusted by changing the deposition conditions. It was found that large ⟨220⟩ grain sizes (∼56nm) could be obtained using the hot wire deposition technique, and the conductivity mobility at room temperature was measured to be 5.4cm2∕Vs in films with such large grains. The plasma-grown films had smaller grains and smaller mobilities. The mobility was found to increase with increasing grain size and with increasing temperature.

The chemical characterization of zone refined lanthanum hexaboride

Abstract A complete chemical analysis has been out on zone refined hot pressed LaB 6 crystals. It is shown that 3 pass zone refined crystals are highly purified with C being the major remaining impurity. The B/La ratio of the zone refined crystals is around 5.75 and evidence is presented to show that this low value results from C present in the starting hot pressed material. Density and lattice parameter measurements show that the probable defect structure consists of a loss of boron octahedrons from the lattice with a partial filling of these sites by interstitial La atoms.

Influence of crystallography and purity on brightness of LaB6 cathodes

The brightness of single‐crystal cathodes of LaB6 oriented in the 〈100〉, 〈110〉, and 〈111〉 directions and six other random directions has been measured. The 〈111〉 cathodes produced currents and brightnesses significantly lower than all other orientations. The other orientations studied produced maximum brightnesses in the range 0.7–1.0 MA/cm2 sr, although the 〈100〉 orientation consistently produced currents higher than other orientations and brightnesses as high or higher than most orientations. A change in impurity level from 1540 ppm to less than 1 ppm had no statistical effect on brightness for crystals of the same orientation.

Growth and Electronic Properties of Nanocrystalline Si

Nanocrystalline Silicon is an important electronic materials for solar cells, for display devices and for sensors. In this paper, we discuss the influence of ions on the growth and properties of thenanocrystalline Si:H material. Using a unique growth geometry, combination of hot wire and ECR plasma growth, we show that low energy ion bombrdment is beneficial for growing high quality materials. Ion bombardment by H is shown to etch the films during growth and also promote crystallinity. The results on film growth are compared with simulations of growth using the SRIM program. The electronic properties measured include mobilities of both electrons and holes in device-type structures, carrier lifetimes, diffusion lengths, defect densities and capture cross-sections for holes. Electron mobility is found to increase with grain size, with a minimum mobility being in the range of 1 cm2/V-s. The hole mobility is also in this range, and three different methods of measuring it give approximately the same value. The capture cross-section for holes is of the order of 1-2 × 10 -16 cm 2 . The lifetime of carriers is found to depend inversely on the defect density, implying that the recombination is trap controlled.

The physics of photon induced degradation of perovskite solar cells

Lead-trihalide perovskite solar cells are an important photovoltaic technology. We investigate the effect of light induced degradation on perovskite solar cells. During exposure, the open-circuit voltage (Voc) of the device increases, whereas the short-circuit current (Isc) shows a decrease. The degradation can be completely recovered using thermal annealing in dark. We develop a model based on light induced generation of ions and migration of these ions inside the material to explain the changes in Isc, Voc, capacitance and dark current upon light exposure and post-exposure recovery. There was no change in defect density in the material upon exposure.

Material properties and growth chemistry of nanocrystalline silicon films used for photovoltaic devices

We report on the growth chemistry and properties of nanocrystalline Si:H (nc-Si:H) films and devices grown using three different growth techniques ECR plasma, VHF plasma and hot wire. It is found that the films grown using ECR plasma deposition are predominantly <111> oriented, whereas the orientation depends upon pressure, growth rate and ion bombardment in films grown using VHF plasma. Remote hot wire grown films also show both <111> and <220> orientations. It is also found that all the films are stressed, with both compressive and tensile stress being present in the same film. The stress depends upon the grain size. Stress can explain the shift in Raman peak from its nominal value of 520 cm/sup -1/. Devices with good minority carrier diffusion lengths can be fabricated in both <111> and <220> materials. Electron mobilities are in the range of 0.5-1.0 cm/sup 2//Vs.