A. Fejfar

Optical absorption and light scattering in microcrystalline silicon thin films and solar cells

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 α from transmittance, reflectance and absorptance (with the help of constant photocurrent method) measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of α 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.

Raman Spectroscopy of Organic–Inorganic Halide 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.

Direct measurement of the deep defect density in thin amorphous silicon films with the ‘‘absolute’’ constant photocurrent method

Direct measurement of the deep defect density in thin amorphous silicon films with the help of the ‘‘absolute’’ constant photocurrent method is demonstrated here. We describe in detail how the optical (photocurrent) absorption spectrum can be measured directly in absolute units (cm−1) without additional calibration and undisturbed by interference fringes. Computer simulation was performed to demonstrate absolute precision of the measurement and to explain residual interferences which are sometimes observed. The residual interferences are shown to be direct fingerprints of an inhomogeneous defect distribution.

Local characterization of electronic transport in microcrystalline silicon thin films with submicron resolution

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.

Basic features of transport in microcrystalline silicon

Charge transport in microcrystalline silicon is strongly influenced by its heterogeneous microstructure composed of crystalline grains and amorphous tissue. An even bigger effect on transport is their arrangement in grain aggregates or possibly columns, separated by grain boundaries, causing transport anisotropy and/or depth profile of transport properties. We review special experimental methods developed to study the resulting transport features: local electronic studies by combined atomic force microscopy, anisotropy of conductivity and diffusion length and also their thickness dependence. A simple model based on the concept of changes of transport path for description of the observed phenomena is reviewed and its consequences for charge collection in microcrystalline based solar cells are discussed.

Microcrystalline silicon thin films studied by atomic force microscopy with electrical current detection

Hydrogenated microcrystalline silicon (μc-Si:H) layers with thickness from 100 to 540 nm were prepared in situ by plasma enhanced chemical vapor deposition. The growth of μc-Si:H on various substrates [NiCr, device quality, and laser annealed amorphous silicon (a-Si:H)] was studied in ultrahigh vacuum by atomic force microscope using a conductive cantilever which enabled simultaneous measurement of morphology and local current with lateral resolution below 5 nm. The effect of barriers, voltage, and time on contrast in local current map is discussed in detail. Coexistent amorphous and microcrystalline regions are clearly identified due to their different conductivity. Laser annealing of the a-Si:H substrate significantly increases the crystalline fraction at the same layer thickness. Grains as small as 10–30 nm separated by less conductive grain boundaries were revealed in microcrystalline regions.

Size and Purity Control of HPHT Nanodiamonds down to 1 nm

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.

Model of transport in microcrystalline silicon

Abstract Large complexity of microstructure in hydrogenated microcrystalline silicon and existence of at least two different sizes of crystallites is demonstrated by combined atomic force microscope topography/local current map. We correlate activation energy and prefactor of the simplest transport property – dark conductivity, measured parallel to the substrate – with the crystallinity and roughness in wide range of microcrystalline silicon samples. This allowed us to formulate a simple model of transport based on the idea that, contrary to small grains, the formation of their aggregates (large grains/columns) dramatically changes the mechanism of transport from band like to hopping.

Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.

Transport study of self‐supporting porous silicon

We have measured dark DC conductivity and time‐of‐flight (TOF) of carriers in self‐supporting porous silicon films in the temperature range 298–480 K. The dark I‐V curves show superlinear behavior with activation energies of 0.38–0.67 eV. The TOF measurements allowed us to evaluate the drift‐length of non‐equilibrium carriers and revealed a significant decrease of the collected charge with increasing delay (tdel≥1 ms) of the exciting 3 ns laser pulse after the voltage application, probably due to field redistribution in the Si crystallites.