Abstract
Nonreciprocal photonic management can shift the absorption-emission balance in favor of absorption and enhance the conversion efficiency beyond the detailed balance Shockley - Queisser limit. Nonreciprocal photovoltaic (PV) cells can provide the conversion of the entire exergy (Helmholtz free energy) of quasi-monochromatic radiation into electric power. Recent discoveries in electromagnetics have demonstrated the ability to break Kirchhoff’s reciprocity in a variety of ways. The absorption-emission nonreciprocity may be realized via dissipationless one-way optical components as well as via the greenhouse-type electron-photon kinetics that traps the low-energy near-bandgap photons in the cell. We calculate the limiting performance of the nonreciprocal dissipationless monochromatic converter and discuss the limiting efficiency of the nonreciprocal converter based on the greenhouse effect. We also perform detailed modeling of the greenhouse effect in the GaAs PV converter and determined its PV performance for conversion of 809 nm laser radiation. In perovskite PV cells the greenhouse filter establishes a sharp absorption edge and reduces conversion losses related to the distributed PV bandgap and laser-cell matching losses.
Keywords
- photovoltaic conversion
- photon exergy
- absorption-emission no reciprocity
- power beaming
- greenhouse effect
1. Introduction
Power delivery by a laser beam is an emerging technology with numerous potential applications. The capabilities of unmanned aerial vehicles, various robotic platforms, and sensor networks will be strongly enhanced due to remote charging. Currently all technological components for power beam delivery and conversion are commercially available. Integration of lasers with photovoltaic converters requires matching laser quanta to semiconductor material characteristics. In traditional design, the optimal bandgap value is determined by a tradeoff between the low near-bandgap absorption and thermalization losses [1, 2]. The optimal bandgap depends on laser power, optoelectronic properties of semiconductor material, and cell design [1]. Usually, the optimal bandgap wavelength exceeds the semiconductor bandgap by 20–80 nm. Even in optimized converters, the photoelectron thermalization and weak near-bandgap absorption produce notable losses [2]. Greenhouse-type filter that traps photons with wavelengths above the laser wavelength is a tool of choice to eliminate the laser-cell matching losses [3, 4]. Moreover, as it is shown in this work, the greenhouse filter can generate the greenhouse effect in the cell, which strongly reduces the emission from the cell and, in this way, increases the conversion efficiency beyond the detailed-balance Shockley - Queisser limit. The greenhouse PV effect is a special case of nonreciprocal photonic management, which violates Kirchhoff’s reciprocity between absorption and emission [5, 6].
The atmospheric greenhouse effect was discovered by Joseph Fourier, who calculated the balance between the incoming solar power and the outgoing power of emitted radiation and found the Earth’s average temperature near of
Recently proposed greenhouse design [3] mimics the greenhouse effect and nonequilibrium greenhouse processes, such as trapping of near bandgap radiation and reusing it for conversion into electricity. Greenhouse filter is placed at the front surface of a cell and the back surface mirror. It establishes a photonic bandgap above the semiconductor bandgap and traps the photons with energies below the photonic bandgap (see Figure 1). The mirror may be a wideband Bragg reflector, or photonic crystal reflector, or metallic reflector with small absorption. In the greenhouse design, the photon emission from the converter is limited by recombination processes of hot photo carriers that emit photons with energy above the photonic bandgap. Bandgap photons emitted by the near-bandgap photocarriers are recycled by the filter and reused in the cell for PV conversion. In the PV greenhouse effect, the nonradiative recombination plays a role of a greenhouse media, which heats the greenhouse. The radiative processes play a role in photosynthesis, which convert solar energy into biochemical energy used by plants. Therefore, the greenhouse PV design requires high-quality PV materials with weak nonradiative recombination, i.e. materials with high internal quantum (radiative) efficiency. Fast progress in traditional semiconductor materials and the development of novel optoelectronic materials raises principle questions about photonic management for PV conversion.
Can the greenhouse filter increase the solar light conversion efficiency beyond the SQ limit? The answer substantially depends on a form of the nonequilibrium distribution function of photo-generated carriers. In the greenhouse converter, we have two characteristic bandgaps. The accumulation and collection of photocarriers occur near the semiconductor bandgap. The photonic bandgap controls the absorption-emission balance in the cell. In this design, only photons emitted by hot electrons can leave the converter. The population of hot electrons and emission from the cell are controlled by the cooling of the hot electrons. If photocarriers near the semiconductor bandgap and hot energy photocarriers above the photonic bandgap have the same photo-induced chemical potential, the greenhouse filter just shifts the PV bandgap from the semiconductor bandgap to the photonic bandgap. In other words, in this quasi-equilibrium case (actually, in a chemical equilibrium between low energy and high energy photocarriers), the limiting conversion efficiency is the SQ efficiency with the PV bandgap equaled to the photonic bandgap. However, hot photocarriers usually rapidly lose their energy and, as a result, the density of hot photocarriers decreases, and their chemical potential is significantly reduced in comparison with the chemical potential of carriers near the semiconductor bandgap. In this nonequilibrium regime with fast photocarrier cooling, the conversion efficiency may exceed the SQ limit. To realize the nonequilibrium regime, the difference between photonic and semiconductor bandgaps should substantially exceed the thermal energy. Therefore, the photonic bandgap should at least be
Let us highlight that in the traditional cell design all conversion processes occur in the narrow energy interval above the semiconductor bandgap. Therefore, the conversion efficiency turns out to be insensitive to details of electron, photon, and phonon processes. In particular, the detailed modeling of PV conversion as a function of characteristic photoelectron relaxation and extraction times has shown that optimization of traditional design and operating regimes does not allow for surpassing the SQ limit [9]. The greenhouse design provides a splitting of key conversion processes. The photocarrier accumulation and collection take place near the semiconductor bandgap, while photocarrier recombination with photon escape occurs above the photonic bandgap. The filter provides an effective tool to reduce cell emission and shift the absorption-emission balance in favor of absorption.
To avoid cell heating, the greenhouse design requires photovoltaic materials with low nonradiative losses, i.e. high quantum efficiencies. Among traditional photovoltaic materials, GaAs has the highest radiative efficiency. In particular, high-quality GaAs with internal quantum efficiency (also termed internal radiative efficiency) of 99.7% provides solar cells with external radiative efficiency (ERE) above 30% [10, 11, 12]. In silicon, the Auger recombination is stronger than radiative recombination and the corresponding ERE of 1% substantially limits the conversion efficiency. Recent progress in emerging photovoltaic materials, − organic materials, dye-sensitized, CuInGaSe (CIGS), and lead-halide perovskites, − has demonstrated strong improvements in ERE. In particular, the ERE of CIGS currently exceeds 24%. ERE is the integral characteristic, which may be calculated via the special averaging of external quantum efficiency (EQE) in the relatively narrow spectral range around the absorption threshold, which in traditional semiconductors coincides with the bandgap [13]. The perovskite materials demonstrate EQE values very close to unity in wide spectral ranges and gradual reduction of EQE near threshold energy [14, 15], which substantially limits the conversion efficiency of traditional PV cells.
In this work, we investigate and optimize greenhouse photonic management for photovoltaic conversion of monochromatic radiation. Recent progress in electromagnetics has demonstrated the ability to break the absorption-emission reciprocity in a variety of ways. To understand the advantages and limitations of greenhouse photonic management, in Section 2 we derive the monochromatic detailed-balance efficiency and in Section 3 we consider the limiting nonreciprocal monochromatic efficiency realized via dissipationless nonreciprocal optical components. In Section 4, we investigate the greenhouse photonic management and present results of simulations of conversion of
2. Detailed-balance limiting efficiency for monochromatic radiation
Let us start with the SQ detailed balance approach, which is based on two assumptions. The first assumption is the reciprocity of photonic (radiative) processes. In the equilibrium, the emitted radiation is exactly given by the absorbed radiation reversed in time. In other words, following Kirchhoff’s law, emissivity,
The generalized SQ model is described by three parameters. The detailed balance is taken into account by the ratio of the absorbed flux to the equilibrium emitted flux,
where e = 2.71828, and LW(
The SQ efficiency (Eq. 2) is the efficiency for conversion of the radiation power. The thermodynamic efficiency of energy conversion at zero output power is reached in the open circuit regime (negligible current). In the quasi-monochromatic limit, the cell is eliminated by photons within a narrow bandwidth,
As expected, for the ideal cell
Analytical solution for the monochromatic energy conversion may be found, when the photons in the incoming flux may be described by the Boltzmann statistics, i.e.
This analytical solution is illustrated in Figure 2. According to Eq. 5, the detailed-balance SQ conversion efficiency of the monochromatic radiation increases with an increase of the photon energy and approaches the Carnot efficiency at high frequencies.
3. Nonreciprocal monochromatic conversion limit
In this section, we consider the thermodynamic limit and material-determined limit of PV converters with negligible emission realized via nonreciprocal dissipationless photonic management. Let us start with the endoreversible thermodynamics of an engine that receives power from an emitter with temperature
The above consideration in the frame of endoreversible thermodynamics assumes that the hot end of an engine may be described by the light-increased temperature. Therefore, it does not apply to the nonequilibrium states of semiconductors, which are described by the light-induced chemical potential. Let us also note, that the detailed-balance approach also cannot be employed for the nonreciprocal conversion with zero emission from the cell. Formally, Eq. 2 in this limit gives a divergent result. To determine the nonreciprocal conversion limit we will directly employ the second thermodynamic law. In the general form applicable to non-temperature distributions, the distribution function of photons emitted by electrons in the cell cannot exceed the distribution function of incoming photons. In the quasi-monochromatic limit, we are interested in values of these functions in the narrow bandwidth near the energy
where
Taking into account that emission from the nonreciprocal converter is absent and every absorbed photon generates an electron in the output circuit, we see that an ideal nonreciprocal converter provides entire conversion of the photon exergy,
As the nonreciprocal photonic management provides 100% reuse of the emitted photons, it is interesting to compare the nonreciprocal limiting efficiency (Eq. 8) with the efficiency of the thermophotovoltaic conversion, where the emitted photons are reabsorbed by the emitter and the corresponding energy is reused in conversion. For the monochromatic emitter, the output electric power may be presented as [21],
where
Finally, we discuss the material limit of the nonreciprocal (time asymmetric) PV converter. If all emitted photons are reabsorbed by the cell, the radiative recombination lifetime of photocarriers approaches infinity, and photocarrier recombination is realized solely via nonradiative processes. In this case, we can employ the detailed balance SQ approach and corresponding analytical solution given by Eqs. 1 and 2, where the parameter
where
Let us note that emission suppression due to nonreciprocal photonic management strongly enhances photon recycling in the system. The limiting Carnot efficiency of the nonreciprocal converter (Eq. 8) corresponds to the zero-emission and infinite intrinsic photon recycling. Any negligible losses (photon leakage or nonradiative recombination) regularize the solution of the SQ model (Eq. 2). This is a general resolution of thermodynamic paradox related to nonreciprocal power converters and nonreciprocal transfer of electromagnetic energy. In this way, the optical diode paradox was resolved by Ishimaru for the nonreciprocal ferrite-loaded waveguide in 1962 [22]. It was shown that any negligible material loss in ferrites leads to the convergent solution, which does not contradict the second law of thermodynamics [23] (see also a review [5]). The Ishimarus results and our analysis show that such paradoxes appear, when the Maxwell equations or photon balance equations are applied to a completely lossless medium and systems. Any negligible dissipation eliminates divergent solutions.
4. Photovoltaic greenhouse effect
The Kirchhoff’s law is valid for opaque bodies in thermodynamic equilibrium with the environment. As it is highlighted in Ref. [24], these two assumptions are often not satisfied. In particular, any photovoltaic converter operates in strongly nonlinear regime far from equilibrium. In this section, we investigate the nonreciprocal photonic management realized due to greenhouse type filter, which mimics the greenhouse operation, where the greenhouse glass/plastic reduces thermal emission and preserves more thermal energy in the greenhouse. The greenhouse filter is placed at the front surface of the cell. It reflects low energy photons in some narrower energy interval (∼ 50–150 meV) above the semiconductor bandgap (see Figure 1). In other words, the filter and back surface mirror establishes the photonic bandgap,
The limiting efficiency of the greenhouse PV converter is smaller than the Carnot efficiency by the factor of
As we discussed above, the efficiency of the greenhouse PV converter strongly depends on cooling mechanisms of hot photocarriers. Let us consider kinetics of the GaAs cell. The photon with energy above the semiconductor band gap creates electron and hole with energies
Usually in semiconductor materials, the whole mass strongly exceeds the electron mass and practically whole photon energy is transferred to photoelectron. In GaAs these effective masses are
Photoelectrons accumulated above semiconductor bandgap may be described by the Boltzmann distribution function with the light-induced chemical potential
These photoelectrons are collected and produce the output voltage
If inter-electron interaction dominates over other processes in photoelectron kinetics, the whole system is described by the same chemical potential. The conversion efficiency is given by Eq. 2 with the absorption-emission balance established above the photonic bandgap. In this case, the greenhouse filter only suppresses the matching losses. If cooling of photoelectrons above the photonic bandgap dominates over phonon-induced thermo-excitation of photoelectrons (see Figure 4), the chemical potential of hot photoelectrons,
Typically, cooling processes of hot photoelectrons are rather fast and, therefore, the chemical potential
where
To illustrate the operation of the greenhouse converter we perform simulations of conversion efficiency of
where
The modeling of PV performance is based on analytical SQ solution (Eqs. 1 and 2), which was described in details for GaAs cell in Ref. cite4 and for various thermophotovoltaic cells in Ref. [33]. In Ref. [3] we investigated the same power beaming conversion in quasi-equilibrium approximation
The main results of our modeling are presented in Figure 5, which shows the increase in the conversion efficiency due to the greenhouse filter as a function of internal quantum efficiency (IQE),
To distinguish the nonequilibrium greenhouse effect in PV conversion, in Figure 6 we present the conversion efficiency calculated in the nonequilibrium model with fast photoelectron cooling, quasi-equilibrium approximation, and for traditional cell design without the greenhouse filter. The efficiency as a function of the dimensionless cell thickness,
Figure 7 demonstrates the conversion efficiency of 809 nm laser radiation as a function of the internal quantum efficiency (Figure 7a) and the laser power (Figure 7b) for the greenhouse GaAs PV converter (green lines), the quasi-equilibrium approximation for the same converter (blue lines), and traditional converter without the greenhouse filter. As it is shown in Figure 7b, all converters have the same, weak (logarithmic) dependence of the efficiency on the laser power. Dependencies of efficiency on the material IQE are substantially different. While the efficiency of the traditional converter has a rather weak, linear dependence on IQE, the performance of the greenhouse converter in the quasi-equilibrium approximation and especially in the model with photoelectron cooling strongly depends on the IQE. Our modeling shows that for suppression of the laser-cell matching losses (the quasi-equilibrium model) we need good PV materials with IQE better than 0.8. The efficiency improvement due to suppression of emission losses via cooling of photoelectrons requires high-quality PV materials with IQE better than 0.99. Otherwise, the nonradiative recombination dominates over the radiative component and determines the converter performance.
Perovskites are low-cost and rather nonhomogeneous materials. These materials show a very smooth absorption edge with the width ∼ 100 nm [14]. While above this range perovskites demonstrate excellent, very close to unity EQE, the smooth absorption drastically reduces the photovoltaic performance [14, 15]. Rau et al. [34] proposed that perovskites may be considered as a semiconductor with the distributed PV bandgap [35]. This model is widely applied to perovskite cells and successfully explains a significant reduction of conversion efficiency with respect to SQ limit [34, 35, 36]. In particular, according to the distributed bandgap model, the 100 meV standard deviation from the mean bandgap reduces the conversion efficiency by 6%. The greenhouse filter is a valuable tool to establish the sharp absorption edge above the smooth material absorption edge. Thus, for the power beaming with perovskite cells, the greenhouse design is expected to increase the conversion efficiency due to the suppression of both the laser-cell matching losses and the distributed bandgap losses.
5. Conclusions
Photonic management of radiative processes is an effective tool to enhance the performance of photovoltaic converters. An ideal nonreciprocal converter provides the entire conversion of the photon exergy,
Acknowledgments
The work is supported by the Army Research Laboratory. Research of AS was accomplished under Cooperative Agreement No. W911NF-18-2-0222. The work of KS is supported by Texas A&M University. The views and conclusions contained in this paper are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the US Government.
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