Roland Schmechel

For perovskite solar cells (PSCs), despite the remarkable advancements in power conversion efficiency and environmental stability, degradation under operational conditions remains a barrier between research and commercialization. Understanding and monitoring the underlying physics of electronic band gap states during degradation as a result of ambient conditions is essential for further improvements in device stability. Here, advanced defect spectroscopy utilizing thermally stimulated currents (TSC) coupled to a metal‐insulator‐semiconductor (MIS) structure is used to determine the energetic landscape of band gap states in relevant state‐of‐the‐art triple cation mixed halide (TCMH) perovskite samples to demonstrate the capabilities of this method. MIS TSC is used to resolve the electrical effect of degradation induced by ambient conditions and natural illumination. Coupling these findings to optoelectronic and structural analysis of individual perovskite layers and functional PSCs, a correlation between material degradation, device performance and trap state evolution can be obtained. We find that with aging under ambient conditions and exposure to light, in TCMH the electron trap density is reduced, while the density of detected hole states increases. For this unexpected behavior, we propose a change of trap type and polarity during degradation.

Investigating thermal material properties such as thermal conductivity (κ), specific heat capacity (C_p) and decomposition temperature of Ta- and Al-substituted solid state electrolyte Li₇La₃Zr₂O₁₂ (LLZO) in dependency of temperature and grain size.

This article investigates the stochastic Schottky barrier variations of printed distributed Schottky diodes consisting of a self-assembled arrangement of crystalline silicon microcones onto a metal layer. The microcone formation emerges from an inkjet printed Si nanoparticle film after laser sintering, yielding a Schottky diode when a corresponding top metallization is applied. The I-V characteristic and the voltage-dependent impedance of such diode is measured. By using the simulation software Advanced Design System (ADS), we develop a new scalable circuit model consisting of many different elementary diodes, which can explain the measured behavior. The elementary microcone diodes differ electrically in their barrier height, which is modeled as a stochastic process with a Gaussian distribution. A comparison between this model and a single diode model based on the thermionic field emission theory is conducted. We show that the distributed model outperforms the single-diode model in every regard and allows a prediction of the power levels of the harmonic frequency generation. Through more in-depth research, we find that a distributed barrier height leads to a smoother I-V curve, which in turn can lead to higher second and third harmonic power levels. By adjusting the barrier height distribution, the desired harmonics can be increased.

The charge carrier mobility is a crucial parameter determining the device performance for numerous different semiconductor applications. Consequently, an accurate measurement of this quantity is crucial. For this purpose, the transient space charge limited current (SCLC) method is commonly applied and is preferable over, for example, Hall or field effect measurements, as the analyzed current direction is in line with typical device architectures. For the transient SCLC method, a voltage step is applied and the transit time of injected charge carriers is determined using displacement currents. Consequently, the difficulty of this method is the use of an adequate RC time constant for the sample charging, as it needs to be much shorter than the transit time. This parameter generally limits the application of transient SCLC strongly, in terms of obtainable charge carrier mobility or minimum required film thickness. Here, we demonstrate a measurement circuit with a low RC time constant, which works in a wide current range (1 A–0.5 A) and thus allows for significant flexibility in terms of minimum film thickness or detectable charge carrier mobility. The circuit is fast enough to measure, for example, charge carrier mobilities of up to for a 76 nm thick 4,4’,4”-Tris[phenyl(m-tolyl)amino]triphenylamine (MTDATA) layer, without using limited bridge circuitry. For this purpose, a capacitor coupled fast transistor switch generates a voltage step to avoid voltage oscillations and a fast operational amplifier is used for amplification of the voltage over a variable measurement resistor. We demonstrate the circuit working principle by measuring benchmarked MTDATA diodes and discuss its range of application.

Introducing a modal system approach for the analytical perovskite thin-film trap physics evaluation. Our study confirms existing models for trap formation in MAPI, substantiating different defect states in the grain boundary and bulk regions.

In this paper we investigate the stochastic Schottky barrier variations of printed distributed Schottky diodes consisting of a self-assembled arrangement of crystalline silicon microcones onto a metal layer. The microcone formation emerges from an inkjet printed Si nanoparticle film after laser sintering yielding a Schottky diode when a corresponding top metallization is applied. The elementary microcone diodes differ electrically in their barrier height, which is modelled as a gaussian distribution. The circuit simulation software Advanced Design System (ADS) is used to analyze the rectification abilities of the overall structure. The results show that a distributed barrier height leads to a smoother IV-characteristic, which can also be interpreted as an aggregated diode with a smaller turn on voltage. A Fourier analysis of the rectified time-domain signal shows an amplification of the frequency components up to the third harmonic in comparison to a non distributed single diode.

Common thermoelectric generators are based on the Seebeck effect, which describes the thermal diffusion of majority charge carriers within a temperature gradient in a solid. It is a unipolar transport phenomenon that gets suppressed if bipolar charge carriers occur. Here, we demonstrate by experiments that thermally excited bipolar charge carriers can be separated by the built-in field without external bias within a p-n junction. Such a phenomenon has been predicted theoretically before but was never proven experimentally. In the experiment, a nominal intrinsic silicon wafer (doping concentration less than 10¹³cm⁻³) was inserted in a p-i-n structure. It could be shown that electric power can be extracted from the space charge region (i-region), while the conventional thermoelectric contribution from the p- and n-regions is suppressed by short-circuiting. While the measured and simulated overall Seebeck effect of intrinsic silicon predicts a zero crossing of output power with increasing hot-side temperature due to a transition from p-type to n-type transport, the measured and simulated output power of the p-i-n structure increases monotonically with increasing hot-side temperature, indicating clearly the different nature of both mechanisms.

Direct conversion of heat fluxes into electricity is usually done by thermoelectric generators (TEGs). For hot-side temperatures above 1000 K, thermal radiation carries a high energy density, comparable with the energy density extracted from TEGs and therefore a direct conversion of thermal radiation into electricity, named thermophotovoltaics (TPV), would also be an option. This paper compares both methods with respect to efficiency and extractable power density. The physical limits are estimated under simplified but realistic boundary conditions. For TPV the radiative detailed balance limit under black body radiation, which was calculated for different hot-side temperatures from 310 K to 3000 K for an optimized bandgap of the applied material was used. But, since very narrow bandgaps leading to strong non-radiative recombination mechanisms, the bandgap was limited to . The effect of suppressing sub-bandgap radiation as well as an enhanced radiation density in the nearfield (near-field TPV) were also included. The TEG efficiency and power density was calculated under thermal matching conditions with a heat transfer coefficient of and an average device . The results are compared with experimental data for TPV and TEGs from literature. It is shown, that up to 600 K hot-side temperature TEGs are superior to TPV, due to a significant higher power density. Above 1000 K TPV profits from higher efficiency by a similar power density. But above 2000 K TPV suffers from cell heating. The range 600 K to 1000 K is currently captured by high temperature thermoelectrics, but near-field TPV (NF-TPV) has good chances to compete with TEGs in this temperature range in the future.

To determine the density of states distribution of traps within a semiconductor, the thermally stimulated current (TSC) method is often applied. However, the bipolar nature of the typical device structure does not allow for strict unipolar operation, and therefore the method does not allow for the separate evaluation of electron and hole traps. The recombination between electrons and holes makes the interpretation of the data difficult, which becomes an essential drawback of this method. To address these issues, we propose the use of a metal insulator semiconductor (MIS) device structure for TSC measurements, which can be operated strictly unipolar by the sign of the applied voltage during the charging process. Thus, the problem of recombination and bipolar contribution to the measurement signal is avoided. As an additional benefit, the MIS device structure typically results in very low leakage currents, and thus a low noise level for the measurement. This permits precise measurements even below 1 pA, and consequently increases the resolution of the method. This aspect is especially important for fractional TSC, as the measurement time is long and the current low when compared to the envelope measurement. Here, we demonstrate the basic principle of this TSC approach, which we name MIS-TSC, using the well-studied organic semiconductor P3HT as a benchmark.

Zinc oxide (ZnO) has been recognized as one of the most promising metal oxide semiconductor material for processing low-cost thin film transistors (TFTs). Within the scope of this work, we demonstrate a simple, stabilizer free and very efficient chemical solution deposition (CSD) route to grow high quality ZnO layers. The identification of a highly soluble zinc ketoiminate precursor that undergoes hydrolysis under ambient conditions with the facile cleavage of the ligands was the key to develop a simple and straightforward process for ZnO thin films under mild process conditions. Upon heat treatment at moderate temperatures, the precursor decomposes cleanly yielding polycrystalline ZnO thin films, which was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The composition was investigated employing complementary techniques such as X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) which revealed high purity ZnO layers. The functional properties in terms of transparency and optical band gap were determined by ultraviolet-visible (UV-Vis) spectroscopy. The transparent ZnO semiconductor thin films serve as active channel layer of thin film transistors (TFT) which was demonstrated by spin coating of the precursor. Subsequent curing in ambient air, yields a 10 nm film that is sufficient to fabricate working TFTs test structures.