Jakub Holovský

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.

Experimental quantification of useful and parasitic absorption of light in plasmon-enhanced thin silicon films for solar cells application

A combination of photocurrent and photothermal spectroscopic techniques is applied to experimentally quantify the useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating optimized metal nanoparticle arrays, located at the rear surface, for improved light trapping via resonant plasmonic scattering. The photothermal technique accounts for the total absorptance and the photocurrent signal accounts only for the photons absorbed in the μc-Si:H layer (useful absorptance); therefore, the method allows for independent quantification of the useful and parasitic absorptance of the plasmonic (or any other) light trapping structure. We demonstrate that with a 0.9 μm thick absorber layer the optical losses related to the plasmonic light trapping in the whole structure are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. An average useful absorption of 43% and an average parasitic absorption of 19% over 400–1100 nm wavelength range is measured for μc-Si:H films deposited on optimized self-assembled Ag nanoparticles coupled with a flat mirror (plasmonic back reflector). For this sample, we demonstrate a significant broadband enhancement of the useful absorption resulting in the achievement of 91% of the maximum theoretical Lambertian limit of absorption.

Effect of the thin-film limit on the measurable optical properties of graphene

The fundamental sheet conductance of graphene can be directly related to the product of its absorption coefficient, thickness and refractive index. The same can be done for graphene’s fundamental opacity if the so-called thin-film limit is considered. Here, we test mathematically and experimentally the validity of this limit on graphene, as well as on thin metal and semiconductor layers. Notably, within this limit, all measurable properties depend only on the product of the absorption coefficient, thickness, and refractive index. As a direct consequence, the absorptance of graphene depends on the refractive indices of the surrounding media. This explains the difficulty in determining separately the optical constants of graphene and their widely varying values found in literature so far. Finally, our results allow an accurate estimation of the potential optical losses or gains when graphene is used for various optoelectronic applications.

Attenuated total reflectance Fourier-transform infrared spectroscopic investigation of silicon heterojunction solar cells

Silicon heterojunction solar cells critically depend on the detailed properties of their amorphous/crystalline silicon interfaces. We report here on the use of attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy to gain precise insight into the vibrational properties of the surfaces and ultrathin layers present in such solar cells. We fabricate ATR prisms from standard silicon wafers similar to those used for device fabrication. In this fashion, we acquire very-high sensitivity FTIR information on device-relevant structures. Our method has no requirement for minimum layer thickness, enabling the study of the impact of the different fabrication process steps on the film microstructure. We discuss the necessary requirements for the method implementation and give a comprehensive overview of all observed vibration modes. In particular, we study vibrational signatures of Si-H(X), Si-H(X)(Si(Y)O(Z)), B-H, hydroxyl groups, and hydrocarbons on the Si(111) surface. We observe subtle effects in the evolution of the chemical state of the surface during sample storage and process-related wafer handling and discuss their effect on the electronic properties of the involved interfaces.

Photocurrent Spectroscopy of Perovskite Layers and Solar Cells: A Sensitive Probe of Material Degradation

Optical absorptance spectroscopy of polycrystalline CH3NH3PbI3 films usually indicates the presence of a PbI2 phase, either as a preparation residue or due to film degradation, but gives no insight on how this may affect electrical properties. Here, we apply photocurrent spectroscopy to both perovskite solar cells and coplanar-contacted layers at various stages of degradation. In both cases, we find that the presence of a PbI2 phase restricts charge-carrier transport, suggesting that PbI2 encapsulates CH3NH3PbI3 grains. We also find that PbI2 injects holes into the CH3NH3PbI3 grains, increasing the apparent photosensitivity of PbI2. This phenomenon, known as modulation doping, is absent in the photocurrent spectra of solar cells, where holes and electrons have to be collected in pairs. This interpretation provides insights into the photogeneration and carrier transport in dual-phase perovskites.

Measurement of doping profiles by a contactless method of IR reflectance under grazing incidence

An improved contactless method of the measurement and evaluation of charge carrier profiles in polished wafers by infrared reflectance was developed. The sensitivity of optical reflectance to the incidence angle was theoretically analyzed. A grazing incident angle enhances sensitivity to doping profile parameters. At the same time, the sensitivity to experimental errors sharply increases around the Brewster angle. Therefore, the optimal angle of 65° was chosen. Experimental errors such as unintentional polarization of the measurement beam were minimized by division by reference spectra taken on an undoped sample and further by normalization to a fixed value in the region of 4000 cm-1 to 7000 cm-1. The carrier profile in boron-doped samples was parametrized by 3 parameters and that in phosphorous-doped samples was parametrized by 4 parameters, using additional empirically determined assumptions. As a physical model, the Drude equation is used with two parameters assumed to be concentration-dependent: relaxation time and contribution from band-to-band excitations. The model parameters were calibrated independently by infrared ellipsometry. The presented method gives results in satisfactory agreement with the profiles measured by the electrochemical capacitance-voltage method.

Direct measurement of optical losses in plasmon-enhanced thin silicon films (Conference Presentation)

Plasmon-enhanced absorption, often considered as a promising solution for efficient light trapping in thin film silicon solar cells, suffers from pronounced optical losses i.e. parasitic absorption, which do not contribute to the obtainable photocurrent. Direct measurements of such losses are therefore essential to optimize the design of plasmonic nanostructures and supporting layers. Importantly, contributions of useful and parasitic absorption cannot be measured separately with commonly used optical spectrophotometry. In this study we apply a novel strategy consisting in a combination of photocurrent and photothermal spectroscopic techniques to experimentally quantify the trade-off between useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating self-assembled silver nanoparticle arrays located at their rear side. The highly sensitive photothermal technique accounts for all absorption processes that result in a generation of heat i.e. total absorption while the photocurrent spectroscopy accounts only for the photons absorbed in the μc-Si:H layer which generate photocarriers i.e. useful absorption [1]. We demonstrate that for 0.9 μm thick μc-Si:H film the optical losses resulting from the plasmonic light trapping are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. For the films deposited on nanoparticle arrays coupled with a flat silver mirror (plasmonic back reflector), we achieved a significant broadband enhancement of the useful absorption resulting from both surface texturing and plasmonic scattering, and achieving 91% of the theoretical Lambertian limit of absorption. [1] S. Morawiec et al. Experimental Quantification of Useful and Parasitic Absorption of Light in Plasmon-Enhanced Thin Silicon Films for Solar Cells Application. Scientific Reports 6 (2016)