A deep understanding of environmental effects on perovskite solar cell (PSC) performance is highly desirable for further progress toward large-scale deployment of this technology. We investigate the ...operation of PSCs in the temperature range 15–50 °C and report an unusual bimodal behavior in photovoltaic (PV) performance, with positive and negative temperature coefficients (TCs) below and above room temperature (RT), respectively. Furthermore, the performance metrics exhibit hysteresis, as their values depend on whether the measurements are made during the heating or cooling stages of the experiment. Conventional semiconductor solar cells, in contrast, exhibit a monotonic and nonhysteretic performance decline in this temperature range. The variations in power conversion efficiency primarily follow changes in open-circuit voltage and fill factor. Photoluminescence data suggest that the performance variations below RT are accompanied by a reduction in defect-related traps in the perovskite absorber and a drop in interfacial built-in potential at the perovskite/transport layer interface. The behavior above RT follows the conventional trend and can hence be explained by charge–phonon interactions. Our findings offer significant insight into the salient PV properties and photophysics of perovskite materials that define their performance in the real-world operating temperature range.
We propose a measurement-based method to produce a maximally-entangled state from a partially-entangled pure state. Our goal can be thought of as entanglement distillation from a single copy of a ...partially-entangled state. The present approach involves local two-outcome weak measurements. We show that the application of these local weak measurements leads to a probabilistic amplification of entanglement. In addition, we examine how the probability to find the maximally-entangled state is related to the entanglement of the input state. We also study the application of our method to a mixed initial state. We show that the protocol is successful if the separable part of the mixed initial state fulfils certain conditions.
We derive the Hamiltonian of a superconducting circuit that comprises a single-Josephson-junction flux qubit inductively coupled to an LC oscillator, and we compare the derived circuit Hamiltonian ...with the quantum Rabi Hamiltonian, which describes a two-level system coupled to a harmonic oscillator. We show that there is a simple, intuitive correspondence between the circuit Hamiltonian and the quantum Rabi Hamiltonian. While there is an overall shift of the entire spectrum, the energy level structure of the circuit Hamiltonian up to the seventh excited states can still be fitted well by the quantum Rabi Hamiltonian even in the case where the coupling strength is larger than the frequencies of the qubit and the oscillator, i.e., when the qubit-oscillator circuit is in the deep-strong-coupling regime. We also show that although the circuit Hamiltonian can be transformed via a unitary transformation to a Hamiltonian containing a capacitive coupling term, the resulting circuit Hamiltonian cannot be approximated by the variant of the quantum Rabi Hamiltonian that is obtained using an analogous procedure for mapping the circuit variables onto Pauli and harmonic oscillator operators, even for relatively weak coupling. This difference between the flux and charge gauges follows from the properties of the qubit Hamiltonian eigenstates.
We investigate the use of machine learning for solving analytic problems in theoretical physics. In particular, symbolic regression (SR) is making rapid progress in recent years as a tool to fit data ...using functions whose overall form is not known in advance. Assuming that we have a mathematical problem that is posed analytically, e.g.~through equations, but allows easy numerical evaluation of the solution for any given set of input variable values, one can generate data numerically and then use SR to identify the closed-form function that describes the data, assuming that such a function exists. In addition to providing a concise way to represent the solution of the problem, such an obtained function can play a key role in providing insight and allow us to find an intuitive explanation for the studied phenomenon. We use a state-of-the-art SR package to demonstrate how an exact solution can be found and make an attempt at solving an unsolved physics problem. We use the Landau-Zener problem and a few of its generalizations as examples to motivate our approach and illustrate how the calculations become increasingly complicated with increasing problem difficulty. Our results highlight the capabilities and limitations of the presently available SR packages, and they point to possible modifications of these packages to make them better suited for the purpose of finding exact solutions as opposed to good approximations. Our results also demonstrate the potential for machine learning to tackle analytically posed problems in theoretical physics.
We investigate all-inorganic perovskite CsPb x Sn1–x Br3 thin films to determine the variations in the band gap and electronic structure associated with the Pb/Sn ratio. We observe that the band gap ...can be tuned between 1.86 eV (x = 0) and 2.37 eV (x = 1). Intriguingly, this change is nonlinear in x, with a bowing parameter of 0.9 eV; furthermore, a slight band gap narrowing is found for low Pb content (minimum x ∼ 0.3). The wide tunability of the band gap makes CsPb x Sn1–x Br3 a promising material, e.g., for a wide-gap subcell in tandem applications or for color-tunable light-emitting diodes. Employing photoelectron spectroscopy, we show that the valence band varies with the Pb/Sn ratio, while the conduction band is barely affected.
Hybrid metal-organic halide perovskites have recently attracted a great deal of attention because of their interesting electronic, optical and transport properties, which make them promising ...materials for high-performance, low-cost solar cells. Fundamental understanding of the formation mechanisms and dynamics of photoinduced charge carriers is essential for improving the performance of perovskite solar cell devices. For example, a significant amount of absorbed solar energy is lost as a result of carrier thermalization. This energy could be harnessed by extracting hot carriers before they cool down to the band edges. Although such hot carrier collection is experimentally challenging, theoretical investigations based on time-dependent methods can guide future experimental research by providing insights into the thermalization process. Here, we perform ab initio nonadiabatic molecular dynamics simulations to study non-radiative relaxation dynamics of charge carriers in hybrid halide perovskites. We find that the carrier relaxation time can be considerably increased by mixing halogen atoms in the perovskite materials. These findings show that simple approaches could be adopted to slow down the thermalization process of hot carriers in perovskite materials.
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