Martin Ledinsky

Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance

Solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Using highly sensitive photothermal deflection and photocurrent spectroscopy, we measure the absorption spectrum of CH3NH3PbI3 perovskite thin films at room temperature. We find a high absorption coefficient with particularly sharp onset. Below the bandgap, the absorption is exponential over more than four decades with an Urbach energy as small as 15 meV, which suggests a well-ordered microstructure. No deep states are found down to the detection limit of ∼1 cm(-1). These results confirm the excellent electronic properties of perovskite thin films, enabling the very high open-circuit voltages reported for perovskite solar cells. Following intentional moisture ingress, we find that the absorption at photon energies below 2.4 eV is strongly reduced, pointing to a compositional change of the material.

Organic-Inorganic Halide Perovskites: Perspectives for Silicon-Based Tandem Solar Cells

We investigate the efficiency potential of organic-inorganic halide perovskite/crystalline silicon tandem solar cells, a new class of photovoltaic devices targeting long-term cost reductions by ultrahigh conversion efficiencies. Methyl ammonium lead triiodide perovskite solar cells are particularly interesting as the top cell in Si-based tandem devices due to their suitable band gap, high photovoltage, and low sub-bandgap absorption. We derive optical models for a perovskite/Si tandem cell with Lambertian light trapping in the perovskite top cell, as well as for a top cell in the single pass limit. We find that unlike for other thin-film device architectures, light trapping is not required for the triiodide perovskite/Si tandem to reach matched top and bottom cell currents. While a Lambertian top cell could be employed in a four-terminal tandem, a top cell in the single pass limit enables a current-matched monolithic device with realistic top cell thicknesses. We calculate a limiting efficiency of 35.67% for an ideal (no parasitic absorption, ideal contacts) monolithic tandem, assuming a top cell open-circuit voltage of 1100 mV.

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.

High-yield fabrication and properties of 1.4 nm nanodiamonds with narrow size distribution.

Detonation nanodiamonds (DNDs) with a typical size of 5 nm have attracted broad interest in science and technology. Further size reduction of DNDs would bring these nanoparticles to the molecular-size level and open new prospects for research and applications in various fields, ranging from quantum physics to biomedicine. Here we show a controllable size reduction of the DND mean size down to 1.4 nm without significant particle loss and with additional disintegration of DND core agglutinates by air annealing, leading to a significantly narrowed size distribution (±0.7 nm). This process is scalable to large quantities. Such molecular-sized DNDs keep their diamond structure and characteristic DND features as shown by Raman spectroscopy, infrared spectroscopy, STEM and EELS. The size of 1 nm is identified as a limit, below which the DNDs become amorphous.

Discriminating adenocarcinoma from normal colonic mucosa through deconvolution of Raman spectra

In this work, we considered the feasibility of Raman spectroscopy for discriminating between adenocarcinomatous and normal mucosal formalin-fixed colonic tissues. Unlike earlier studies in colorectal cancer, a spectral deconvolution model was implemented to derive spectral information. Eleven samples of human colon were used, and 55 spectra were analyzed. Each spectrum was resolved into 25 bands from 975 to 1720 cm(-1), where modes of proteins, lipids, and nucleic acids are observed. From a comparative study of band intensities, those presenting higher differences between tissue types were correlated to biochemical assignments. Results from fitting procedure were further used as inputs for linear discriminant analysis, where combinations of band intensities and intensity ratios were tested, yielding accuracies up to 81%. This analysis yields objective discriminating parameters after fitting optimization. The bands with higher diagnosis relevance detected by spectra deconvolution enable to confine the study to some spectral regions instead of broader ranges. A critical view upon limitations of this approach is presented, along with a comparison of our results to earlier ones obtained in fresh colonic tissues. This enabled to assess the effect of formalin fixation in colonic tissues, and determine its relevance in the present analysis.

Ultrathin Nanocrystalline Diamond Films with Silicon Vacancy Color Centers via Seeding by 2 nm Detonation Nanodiamonds

Color centers in diamonds have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report chemical vapor deposition (CVD) growth of nanocrystalline diamond (NCD) films as thin as 5-6 nm with photoluminescence (PL) from silicon-vacancy (SiV) centers at 739 nm. Instead of conventional 4-6 nm detonation nanodiamonds (DNDs), we prepared and employed hydrogenated 2 nm DNDs (zeta potential = +36 mV) to form extremely dense (∼1.3 × 1013 cm-2), thin (2 ± 1 nm), and smooth (RMS roughness < 0.8 nm) nucleation layers on an Si/SiOx substrate, which enabled the CVD growth of such ultrathin NCD films in two different and complementary microwave (MW) CVD systems: (i) focused MW plasma with an ellipsoidal cavity resonator and (ii) pulsed MW plasma with a linear antenna arrangement. Analytical ultracentrifuge, infrared and Raman spectroscopies, atomic force microscopy, and scanning electron microscopy are used for detailed characterization of the 2 nm H-DNDs and the nucleation layer as well as the ultrathin NCD films. We also demonstrate on/off switching of the SiV center PL in the NCD films thinner than 10 nm, which is achieved by changing their surface chemistry.

Optical characterisation of organosilane-modified nanocrystalline diamond films

We report on an optical characterisation of nanocrystalline diamond films photochemically functionalised with the organosilane-coupling agent, N1-(3-(trimethoxysilyl)propyl)hexane-1,6-diamine (alternative names: N-(6-aminohexyl)aminopropyl-trimethoxysilane, (3-(6-aminohexylamino)propyl) trimetoxysilane, AHAPS). The presence and homogeneity of the organosilane layers were detected by fluorescence microscopy and infrared reflectance-absorbance spectroscopy. The results indicated that a homogeneous surface coverage with organosilane layers was achieved on diamond surfaces which were modified either by hydrogen or by oxygen plasma treatment. The functionalised nanocrystalline diamonds present a promising solution in future biosensor applications.

Photolithography-free interdigitated back-contacted silicon heterojunction solar cells with efficiency >21%

We report on the development of interdigitated back-contacted silicon heterojunction solar cells with conversion efficiencies well above 21%. Doped hydrogenated amorphous silicon layers, needed for electron and hole collection, are patterned via in-situ shadow masking whereas transparent conductive oxide and metal layers, of the back electrodes, are defined via hot melt inkjet printing of an etch resist and subsequent wet etching. Our technology is therefore photolithography-free and avoids any high-temperature step. The best fabricated solar cell presents a high short-circuit current density of 39.9 mA/cm2, an open-circuit voltage of 724 mV and a fill factor of 74.5% resulting in a conversion efficiency of 21.5%, with a strong upside potential. We report also on a silver-free IBC-SHJ solar cell with conversion efficiency >20%.

Passivating contacts for silicon solar cells with 800 °C stability based on tunnel-oxide and highly crystalline thin silicon layer

Passivating contacts based on nanostructured highly crystalline thin silicon layers are presented. The contact layer stack is optimized towards full crystallinity targeting high transparency. We present an optimization of an electron selective contact and demonstrate excellent surface passivation on n-type and also p-type wafers with such highly crystalline layers. On n-type wafers, the electron selective contact attains an implied open-circuit voltage of 718 mV at an annealing temperature of 925°C. For p-type wafers we find optimum conditions between 850°C and 900°C attaining an implied open-circuit voltage of 723 mV. First tests with hole-selective contacts have yielded an implied open-circuit voltage of up to 676 mV after thermal annealing at 800°C.