Reversible electron-hole separation in a hot carrier solar cell

Hot-carrier solar cells are envisioned to utilize energy filtering to extract power from photogenerated electron-hole pairs before they thermalize with the lattice, and thus potentially offer higher power conversion efficiency compared to conventional, single absorber solar cells. The efficiency of hot-carrier solar cells can be expected to strongly depend on the details of the energy filtering process, a relationship which to date has not been satisfactorily explored. Here, we establish the conditions under which electron-hole separation in hot-carrier solar cells can occur reversibly, that is, at maximum energy conversion efficiency. We thus focus our analysis on the internal operation of the hot-carrier solar cell itself, and in this work do not consider the photon-mediated coupling to the sun. After deriving an expression for the voltage of a hot-carrier solar cell valid under conditions of both reversible and irreversible electrical operation, we identify separate contributions to the voltage from the thermoelectric effect and the photovoltaic effect. We find that, under specific conditions, the energy conversion efficiency of a hot-carrier solar cell can exceed the Carnot limit set by the intra-device temperature gradient alone, due to the additional contribution of the quasi-Fermi level splitting in the absorber. We also establish that the open-circuit voltage of a hot-carrier solar cell is not limited by the band gap of the absorber, due to the additional thermoelectric contribution to the voltage. Additionally, we find that a hot-carrier solar cell can be operated in reverse as a thermally driven solid-state light emitter. Our results help explore the fundamental limitations of hot-carrier solar cells, and provide a first step towards providing experimentalists with a guide to the optimal configuration of devices.Comment: 31 pages, 5 figure

Thermoelectric efficiency at maximum power in low-dimensional systems

Low-dimensional electronic systems in thermoelectrics have the potential to achieve high thermal-to-electric energy conversion efficiency. A key measure of performance is the efficiency when the device is operated under maximum power conditions. Here we study the efficiency at maximum power of three low-dimensional, thermoelectric systems: a zero-dimensional quantum dot (QD) with a Lorentzian transmission resonance of finite width, a one-dimensional (1D) ballistic conductor, and a thermionic (TI) power generator formed by a two-dimensional energy barrier. In all three systems, the efficiency at maximum power is independent of temperature, and in each case a careful tuning of relevant energies is required to achieve maximal performance. We find that quantum dots perform relatively poorly under maximum power conditions, with relatively low efficiency and small power throughput. Ideal one-dimensional conductors offer the highest efficiency at maximum power (36% of the Carnot efficiency). Whether 1D or TI systems achieve the larger maximum power output depends on temperature and area filling factor. These results are also discussed in the context of the traditional figure of merit $ZT$

Optimal power and efficiency of single quantum dot heat engines: theory and experiment

Quantum dots (QDs) can serve as near perfect energy filters and are therefore of significant interest for the study of thermoelectric energy conversion close to thermodynamic efficiency limits. Indeed, recent experiments in [Nat. Nano. 13, 920 (2018)] realized a QD heat engine with performance near these limits and in excellent agreement with theoretical predictions. However, these experiments also highlighted a need for more theory to help guide and understand the practical optimization of QD heat engines, in particular regarding the role of tunnel couplings on the performance at maximum power and efficiency for QDs that couple seemingly weakly to electronic reservoirs. Furthermore, these experiments also highlighted the critical role of the external load when optimizing the performance of a QD heat engine in practice. To provide further insight into the operation of these engines we use the Anderson impurity model together with a Master equation approach to perform power and efficiency calculations up to co-tunneling order. This is combined with additional thermoelectric experiments on a QD embedded in a nanowire where the power is measured using two methods. We use the measurements to present an experimental procedure for efficiently finding the external load $R_P$ which should be connected to the engine to optimize power output. Our theoretical estimates of $R_P$ show a good agreement with the experimental results, and we show that second order tunneling processes and non-linear effects have little impact close to maximum power, allowing us to derive a simple analytic expression for $R_P$. In contrast, we find that the electron contribution to the thermoelectric efficiency is significantly reduced by second order tunneling processes, even for rather weak tunnel couplings

Heat flow in InAs/InP heterostructure nanowires

The transfer of heat between electrons and phonons plays a key role for thermal management in future nanowire-based devices, but only a few experimental measurements of electron-phonon (e-ph) coupling in nanowires are available. Here, we combine experimental temperature measurements on an InAs/InP heterostructure nanowire system with finite element modeling (FEM) to extract information on heat flow mediated by e-ph coupling. We find that the electron and phonon temperatures in our system are highly coupled even at temperatures as low as 2 K. Additionally, we find evidence that the usual power-law temperature dependence of electron-phonon coupling may not correctly describe the coupling in nanowires and show that this result is consistent with previous research on similar one-dimensional electron systems. We also compare the strength of the observed e-ph coupling to a theoretical analysis of e-ph interaction in InAs nanowires, which predicts a significantly weaker coupling strength than observed experimentally.Comment: 9 pages, 6 figure

Thermoelectric power factor limit of a 1D nanowire

In the past decade, there has been significant interest in the potentially advantageous thermoelectric properties of one-dimensional (1D) nanowires, but it has been challenging to find high thermoelectric power factors based on 1D effect in practice. Here we point out that there is an upper limit to the thermoelectric power factor of non-ballistic 1D nanowires, as a consequence of the recently established quantum bound of thermoelectric power output. We experimentally test this limit in quasi-ballistic InAs nanowires by extracting the maximum power factor of the first 1D subband through I-V characterization, finding that the measured maximum power factors conform to the theoretical limit. The established limit predicts that a competitive power factor, on the order of mW/m-K^2, can be achieved by a single 1D electronic channel in state-of-the-art semiconductor nanowires with small cross-section and high crystal quality

Nonlinear thermoelectric response due to energy-dependent transport properties of a quantum dot

Quantum dots are useful model systems for studying quantum thermoelectric behavior because of their highly energy-dependent electron transport properties, which are tunable by electrostatic gating. As a result of this strong energy dependence, the thermoelectric response of quantum dots is expected to be nonlinear with respect to an applied thermal bias. However, until now this effect has been challenging to observe because, first, it is experimentally difficult to apply a sufficiently large thermal bias at the nanoscale and, second, it is difficult to distinguish thermal bias effects from purely temperature-dependent effects due to overall heating of a device. Here we take advantage of a novel thermal biasing technique and demonstrate a nonlinear thermoelectric response in a quantum dot which is defined in a heterostructured semiconductor nanowire. We also show that a theoretical model based on the Master equations fully explains the observed nonlinear thermoelectric response given the energy-dependent transport properties of the quantum dot.Comment: Cite as: A. Svilans, et al., Physica E (2015), http://dx.doi.org/10.1016/j.physe.2015.10.00

Multi-directional sorting modes in deterministic lateral displacement devices

Deterministic lateral displacement (DLD) devices separate micrometer-scale particles in solution based on their size using a laminar microfluidic flow in an array of obstacles. We investigate array geometries with rational row-shift fractions in DLD devices by use of a simple model including both advection and diffusion. Our model predicts novel multi-directional sorting modes that could be experimentally tested in high-throughput DLD devices containing obstacles that are much smaller than the separation between obstacles

Realization of a feedback controlled flashing ratchet

A flashing ratchet transports diffusive particles using a time-dependent, asymmetric potential. Particle speed is predicted to increase when a feedback algorithm based on particle positions is used. We have experimentally realized such a feedback ratchet using an optical line trap, and observed that use of feedback increases velocity by up to an order of magnitude. We compare two different feedback algorithms for small particle numbers, and find good agreement with simulations. We also find that existing algorithms can be improved to be more tolerant to feedback delay times