To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH 3 NH 3 PbI 3 ), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH 3 NH 3 PbI 3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.

Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri-iodide (CH3NH3PbI3), tribromide (CH3NH3PbBr3), and mixed iodide-bromide (CH3NH3PbI3-xBrx) organic-inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH3NH3PbI3 layers and discuss the distinct Raman bands that develop during moisture-induced degradation. We also prove unambiguously that the final degradation products contain pure PbI2. Moreover, we describe CH3NH3PbI3-xBrx Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to-bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio.

High-pressure high-temperature (HPHT) nanodiamonds originate from grinding of diamond microcrystals obtained by HPHT synthesis. Here we report on a simple two-step approach to obtain as small as 1.1 nm HPHT nanodiamonds of excellent purity and crystallinity, which are among the smallest artificially prepared nanodiamonds ever shown and characterized. Moreover we provide experimental evidence of diamond stability down to 1 nm. Controlled annealing at 450 °C in air leads to efficient purification from the nondiamond carbon (shells and dots), as evidenced by X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, and scanning transmission electron microscopy. Annealing at 500 °C promotes, besides of purification, also size reduction of nanodiamonds down to ∼1 nm. Comparably short (1 h) centrifugation of the nanodiamonds aqueous colloidal solution ensures separation of the sub-10 nm fraction. Calculations show that an asymmetry of Raman diamond peak of sub-10 nm HPHT nanodiamonds can be well explained by modified phonon confinement model when the actual particle size distribution is taken into account. In contrast, larger Raman peak asymmetry commonly observed in Raman spectra of detonation nanodiamonds is mainly attributed to defects rather than to the phonon confinement. Thus, the obtained characteristics reflect high material quality including nanoscale effects in sub-10 nm HPHT nanodiamonds prepared by the presented method.

Optical characterization methods were applied to a series of microcrystalline silicon thin films and solar cells deposited by the very high frequency glow discharge technique. Bulk and surface light scattering effects were analyzed. A detailed theory for evaluation of the optical absorption coefficient ␣ from transmittance, reflectance and absorptance ͑with the help of constant photocurrent method͒ measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of ␣ is interpreted in terms of defect density, disorder, crystalline/amorphous fraction and material morphology. The enhanced light absorption in microcrystalline silicon films and solar cells is mainly due to a longer optical path as the result of an efficient diffuse light scattering at the textured film surface. This light scattering effect is a key characteristic of efficient thin-film-silicon solar cells.

We present the results of optical-pump-terahertz probe experiments applied to a set of thin-film microcrystalline silicon samples, with structures varying from amorphous to fully microcrystalline. The samples were excited at wavelengths 800 and 400 nm and studied at temperatures down to 20 K. The character of nanoscopic electrical transport properties markedly change on a subpicosecond time scale. The initial transient photoconductivity of the samples is dominated by hot free carriers with a mobility of ϳ70 cm 2 / Vs. These carriers are rapidly ͑within 0.6 ps͒ trapped into weakly localized hopping states. The hopping process dominates the terahertz spectra on the picosecond and subnanosecond time scales. The saturated high-frequency value of the hopping mobility is limited by the sample disorder in the amorphous sample and by electron-phonon interaction for microcrystalline samples.

We present a novel passivating contact structure based on a nanostructured siliconbased layer. Traditional poly-Si junctions feature excellent junction characteristics but their optical absorption induces current losses when applied to the solar cell front side. Targeting enhanced transparency, the poly-Si layer is replaced with a double-layer stack consisting of a nanostructured silicon oxide capped with a nanocrystalline silicon (nc-Si) layer. The nanostructured silicon oxide layer consists of an amorphous SiOx matrix with incorporated Si filaments connecting one side of the layer to the other, and is referred to as nanocrystalline silicon oxide (nc-SiOx) layer. We investigate passivation quality, measured as saturation current density, and nanostructural changes, characterized by Raman spectroscopy and transmission electron microscopy, carefully studying the influence of annealing dwell temperature. Excellent surface passivation on n-type and also p-type wafers is shown. An optimum annealing temperature of 950 °C is found, resulting in a saturation current density of 8.8 fA cm-2 and 11.0 fA cm-2 for n-type and p-type wafers, respectively. Efficient current extraction is presented with specific contact resistivities of 86 mΩ cm 2 on n-type wafer and 19 mΩ cm 2 on p-type wafers, respectively. Highresolution transmission electron microscopy reveals that the layer stack consists of intermixed SiOx and Si phases with the Si phases being partly crystalline already in the asdeposited state. Thermal annealing at temperatures ≥ 850 °C further promotes crystallization of the Si-rich regions. We show that the addition of the SiOx phase enhances the thermal stability of the contact and we expect it to allow to tune the refractive index and improve transparency while still providing efficient electrical transport thanks to the crystalline Si phase, which extends throughout almost the entire layer.

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Two-dimensional maps of dark conductivity with submicron resolution have been obtained on in situ prepared hydrogenated microcrystalline silicon (μc-Si:H) layers used for solar cells by atomic force microscopy with conductive cantilever. Comparison of the morphology and current image allows clear identification of Si crystallites. Pronounced current decrease has been detected at the grain boundaries. The technique was used to study initial stages of μc-Si:H growth, and we show how the incubation layer, detrimental for solar cells efficiency, can be minimized by pulsed excimer laser crystallization of the initial amorphous layer.