Klaus Lips

Helmholtz-Zentrum Berlin für Materialien und Energie | HZB · Institute for Silicon Photovoltaics

Electron paramagnetic resonance (EPR) spectroscopy provides information about the physical and chemical properties of materials by detecting paramagnetic states. Conventional EPR measurements are performed in high Q resonator using large electromagnets which limits the available space for operando experiments. Here we present a solution toward a po...

Electron paramagnetic resonance (EPR) spectroscopy is an essential tool to investigate the effects of ionizing radiation, which is routinely administered for reducing contaminations and waste in food products and cosmetics as well as for sterilization in industry and medicine. In materials research, EPR methods are not only employed as a spectrosco...

Defect centres in crystals like diamond or silicon find a wide application in quantum technology, where the detection and control of their quantum states is crucial for their implementation as quantum sensors and qubits. The quantum information is usually encoded in the spin state of these defect centres, but they also often possess a charge which...

Electron paramagnetic resonance–on-a-chip (EPRoC) devices use small voltage-controlled oscillators (VCOs) for both the excitation and detection of the EPR signal, allowing access to unique sample environments by lifting the restrictions imposed by resonator-based EPR techniques. EPRoC devices have been successfully used at multiple frequencies (7 t...

The Vanadium redox flow battery (VRFB) is considered a promising candidate for large-scale energy storage in the transition from fossil fuels to renewable energy sources. VRFBs store energy by electrochemical...

Graphitic carbon nitride (gCN) materials have been shown to efficiently perform light-induced water splitting, carbon dioxide reduction, and environmental remediation in a cost-effective way. However, gCN suffers from rapid charge-carrier recombination, inefficient light absorption, and poor long-term stability which greatly hinders photocatalytic...

Electrically detected magnetic resonance (EDMR) is a spectroscopic technique that provides information about the physical properties of materials through the detection of variations in conductivity induced by spin-dependent processes. EDMR has been widely applied to investigate thin-film semiconductor materials in which the presence of defects can...

Halide perovskite-based photon upconverters utilize perovskite thin films to sensitize triplet exciton formation in a small-molecule layer, driving triplet-triplet annihilation upconversion. Despite having excellent carrier mobility, these systems suffer from inefficient triplet formation at the perovskite/annihilator interface. We studied triplet...

Electron paramagnetic resonance (EPR) is the gold standard for studying paramagnetic species. As an example, in structural biology, it allows to extract information about distance distributions on the nanometer scale via site-directed spin labeling. Conventional pulsed EPR of biological samples is currently limited to relatively large sample concen...

In this paper, we present an in-depth analysis of a voltage-controlled oscillator (VCO)-based sensing method for electron spin resonance (ESR) spectroscopy, which greatly simplifies the experimental setup compared to conventional detection schemes. In contrast to our previous oscillator-based ESR detectors, where the ESR signal was encoded in the o...

Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, novel asp...

Among all pigments found in nature, melanin draws considerable attention for bioelectronics applications due to a set of physicochemical properties such as antioxidant activity, metal chelation, biocompatibility, broad-band absorbance throughout the UV-visible region, hydration-dependence conductivity and intrinsic and extrinsic free radical charac...

We have employed state-of-the-art cross-correlation noise spectroscopy (CCNS) to study carrier dynamics in silicon heterojunction solar cells (SHJ SCs). These cells were composed of a light absorbing n-doped monocrystalline silicon wafer contacted by passivating layers of i-a-Si:H and doped a-Si:H selective contact layers. Using CCNS, we are able t...

In this paper, we present an in-depth analysis of a voltage-controlled oscillator (VCO) based sensing method for electron spin resonance (ESR) spectroscopy, which greatly simplifies the experimental setup compared to conventional detection schemes. In contrast to our previous oscillator-based ESR detectors, where the ESR signal was encoded in the o...

Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, we presen...

The nanostructure of hydrogenated amorphous silicon (a-Si∶H) is studied by a combination of small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS) with a spatial resolution of 0.8 nm. The a-Si∶H materials were deposited using a range of widely varied conditions and are representative for this class of materials. We identify t...

We prepared triplet–triplet annihilation photon upconverters combining thin-film methylammonium lead iodide (MAPI) perovskite with a rubrene annihilator in a bilayer structure. Excitation of the perovskite film leads to delayed, upconverted photoluminescence emitted from the annihilator layer, with triplet excitation of the rubrene being driven by...

Silicon heterojunction (SHJ) solar cells represent a promising technological approach towards higher photovoltaics efficiencies and lower fabrication cost. While the device physics of SHJ solar cells have been studied extensively in the past, the ways in which nanoscopic electronic processes such as charge-carrier generation, recombination, trappin...

We have employed state-of-the-art cross-correlation noise spectroscopy to study carrier dynamics in silicon heterojunction solar cells, complimented by SENTARUS simulations of the same devices. These cells were composed of a light absorbing n-doped crystalline silicon layer contacted by passivating layers of i-a-Si:H and doped a-Si:H electrode laye...

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), a SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-r...

The nanostructure of hydrogenated amorphous silicon (a Si:H) is studied by a combination of small-angle X-ray (SAXS) and neutron scattering (SANS) with a spatial resolution of 0.8 nm. The a-Si:H materials were deposited using a range of widely varied conditions and are representative for this class of materials. We identify two different phases whi...

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), a SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-r...

In this paper, we provide a review of the recent advances in miniaturizing nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectrometers for portable magnetic resonance (MR) applications. We focus the discussion on the application of integrated circuit technology for the miniaturization of the NMR and EPR spectrometer har...

Germanium (Ge) nanoparticles are gaining increasing interest due to their properties that arise in the quantum confinement regime, such as the development of the band structure with changing size. While promising materials, significant challenges still exist related to the development of synthetic schemes allowing for good control over size and mor...

Crystalline surface-anchored metal-organic framework (SURMOF) thin films made from porphyrin-based organic linkers have recently been used in both photon upconversion and photovoltaic applications. While these studies showed promising results, the question of photostability in this organic-inorganic hybrid material has to be investigated before app...

In this paper, we present a new architecture for a voltage-controlled oscillator (VCO)-based electron spin resonance (ESR) detection for a future use in portable, point-of-care ESR spectrometers. The proposed architecture is centered around an application-specified integrated circuit (ASIC) containing a VCO-based ESR detector with two distinct tuni...

Singlet exciton fission is an exciton multiplication process that occurs in certain organic materials, converting the energy of single highly-energetic photons into pairs of triplet excitons. This could be used to boost the conversion efficiency of crystalline silicon solar cells by creating photocurrent from energy that is usually lost to thermali...

In this paper, a transimpedance amplifier (TIA) is presented that utilizes a modified pseudo-resistor (PR) with improved robustness against temperature and process variations, enhanced linearity, and reduced parasitics. Using a biasing scheme named pseudo current mirror, the conventional dependence on absolute process parameters is reduced to a dep...

X-band rapid-scan EPR was implemented on a commercially available Bruker ELEXSYS E580 spectrometer. Room temperature rapid-scan and continuous-wave EPR spectra were recorded for amorphous silicon powder samples. By comparing the resulting signal intensities the feasibility of performing quantitative rapid-scan EPR is demonstrated. For different hyd...

We show that the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) can successfully be applied as a hole selective front contact in silicon heterojunction (SHJ) solar cells. In combination with a superior electron selective heterojunction back contact based on amorphous silicon (a-Si), mono-crystalline n-...

The Energy Materials In-Situ Laboratory Berlin (EMIL) at BESSY-II is currently under construction. Two canted undulators for soft- and hard X-rays will be installed into the BESSY II storage ring in one straight section, complex beamlines with more than twenty optical elements will be set up and a new laboratory building attached to BESSY II will h...

This chapter introduces the basic principles of Electron-Spin Resonance (ESR) and its application to Hydrogenated Amorphous Silicon (a-Si:H). The main advantage of ESR is that almost any kind of sample geometry can be studied nondestructively, provided that the sample fits into the resonator of the ESR spectrometer. When studying thin films or inte...

Electrically detected magnetic resonance (EDMR) spectroscopy is employed to study the influence of triplet excitons on the photocurrent in state-of-the-art microcrystalline silicon thin-film solar cells. These triplet excitons are used as sensitive spin probes for the investigation of their electronic and nuclear environment in this mixed-phase mat...

Unlabelled: We investigated the buried interface between monocrystalline n-type silicon (n-Si) and the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) ( Pedot: PSS), which is successfully applied as a hole selective contact in hybrid solar cells. We show that a post-treatment of the polymer films by immersion in...

Microcrystalline silicon carbide alloys (μc-SiC:H) have attracted attention as a potential window layer for thin film silicon solar cells and silicon heterojunction solar cells. Thanks to the combination of high transparency and conductivity. At present, identification of electronically active impurities and defects is highly relevant for applicati...

Multifrequency electron paramagnetic resonance (EPR, 9.4 and 263 GHz) and X-ray absorption spectroscopy (XAS) were employed to study structural and electrochemical changes of selected Mn complexes in contact with dried Nafion films upon electro-oxidation and after long-term illumination. It was found that when in contact with Nafion the Mn-Me3TACN...

Today, most metal and nitrogen doped carbon catalysts for ORR reveal a rather heterogeneous composition. This can be reasoned by a non-optimized precursor composition and different steps in the preparation process to get the required active material. The significant presence of inorganic metal species interfere the assignment of descriptors related...

Photochemical upconversion based on triplet–triplet annihilation (TTA-UC) is employed to enhance the short-circuit currents generated by two varieties of thin-film solar cells, a hydrogenated amorphous silicon (a-Si:H) solar cell and a dye-sensitized solar cell (DSC). TTA-UC is exploited to harvest transmitted sub-bandgap photons, combine their ene...

A knowledge-based approach towards developing a new generation of solar energy conversion devices requires a fast and direct feedback between sophisticated analytics and state-of-the-art processing/test facilities for all relevant material classes. A promising approach is the coupling of synchrotron-based X-ray characterization techniques, providin...

We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) ( PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate...

The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed depende...

The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed depende...

The heterojunction between amorphous silicon (sub)oxides (a-SiOx:H, 0 < x < 2) and crystalline silicon (c-Si) is investigated. We combine chemical vapor deposition with in-system photoelectron spectroscopy in order to determine the valence band offset ΔEV and the interface defect density, being technologically important junction parameters. ΔEV inc...

Here we describe a new high frequency/high field continuous wave and pulsed electrically detected magnetic resonance (CW EDMR and pEDMR) setup, operating at 263GHz and resonance fields between 0 and 12T. Spin dependent transport in illuminated hydrogenated amorphous silicon p-i-n solar cells at 5K and 90K was studied by in operando 263GHz CW and pE...

The in-operando detection and high resolution spatial imaging of paramagnetic defects, impurities, and states becomes increasingly important for understanding loss mechanisms in solid-state electronic devices. Electron spin resonance (ESR), commonly employed for observing these species, cannot meet this challenge since it suffers from limited sensi...

Photon upconversion (UC) processes result in the emission of photons at higher energies than those absorbed. Among the several recent novel applications of UC, the most widely studied is its use with photovoltaic (PV) cells. Photon UC can sensitize PV cells to portions of the solar spectrum at lower energy than the band gap, which are wasted in a n...

A wide variety of liquid and solid phase crystallized silicon films are investigated in order to determine the performance limiting defect types in crystalline silicon thin-film solar cells. Complementary characterization methods, such as electron spin resonance, photoluminescence, and electron microscopy, yield the densities of dangling bond defec...

The investigation of paramagnetic species (such as point defects, dopants, and impurities) in solid-state electronic devices is of significance, as they affect device performance. Conventionally these species are detected and imaged by electron spin resonance (ESR) technique. Many times ESR is not sensitive enough to deal with miniature devices hav...

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H), known as the Staebler-Wronski effect, has been studied by time-domain pulsed electron-paramagnetic resonance. Electron-spin echo relaxation measurements in the annealed and light-soaked state revealed two types of defects (termed type I and II), which can be discerned by their el...

By employing electron paramagnetic resonance spectroscopy, transmission electron microscopy, and optical measurements, we systematically correlate the structural and optical properties with the deep-level defect characteristics of various tailored periodic Si microhole arrays, which are manufactured in an easily scalable and versatile process on na...

We investigate the characteristics of intra‐grain and grain boundary defects in polycrystalline Si films, by employing quantitative electron paramagnetic resonance measurements on liquid phase crystallized layers with an average grain size of 200 µm and tailored solid phase crystallized Si layers with similar intra‐grain morphology but systematical...