Tom Markvart

Thermodynamics of losses in photovoltaic conversion

This letter presents a thermodynamic analysis of losses in an ideal solar cell. It is shown that the maximum voltage—corresponding to the voltage produced by a hot-carrier solar cell—is equal to the energy of the incident solar photon multiplied by the appropriate Carnot factor. Voltage generated by the usual p-n junction cell is lower on account of entropy generation through kinetic losses, photon cooling, and etendue expansion of the incident beam. Simple expressions can be obtained by an approximation where the energy and entropy changes are modeled by the corresponding expressions for a two-dimensional ideal photon gas.

The thermodynamics of optical étendue

The concept of etendue is applied to the propagation of luminescent radiation, and to the transformation of such radiation in absorbing and luminescent media. Central to this analysis is the notion of etendue as a measure of the number of rays in the beam which permits the definition of entropy and transition to the formalism of statistical mechanics. When considered from the statistical viewpoint, etendue conservation along the path of a beam in clear and transparent media then implies the conservation of entropy. The changes in thermodynamic parameters of a beam upon absorption and re-emission can then be determined in terms of the corresponding changes resulting from the addition or removal of photons from the incident and emitted beam. The thermodynamic theory which follows gives the rate of entropy generation in this process. At moderate light intensities, the results resemble the thermodynamics of a two-dimensional gas. The formalism allows an extension to absorption/emission processes where a high-temperature incident light beam is transformed reversibly into low-temperature luminescent radiation, corresponding to a potential increase in the open-circuit voltage of a solar cell.

Thermodynamics and reciprocity of solar energy conversion

A model is developed which describes the useful conversion of radiation in different systems: solar cells, photochemical and photobiological systems (as in photosynthesis). The connection with irreversible thermodynamics is emphasised for cases when the differences in temperature and potential are small. Some conjectures are made as to the extension of this work to systems with larger fluxes of heat and photons.

Harnessing High-Altitude Solar Power

As an intermediate solution between Glasers satellite solar power (SSP) and ground-based photovoltaic (PV) panels, this paper examines the collection of solar energy using a high-altitude aerostatic platform. A procedure to calculate the irradiance in the medium/high troposphere, based on experimental data, is described. The results show that here a PV system could collect about four to six times the energy collected by a typical U.K.-based ground installation, and between one-third and half of the total energy the same system would collect if supported by a geostationary satellite (SSP). The concept of the aerostat for solar power generation is then briefly described together with the equations that link its main engineering parameters/variables. A preliminary sizing of a facility stationed at 6 km altitude and its costing, based on realistic values of the input engineering parameters, is then presented.

Microgrids: Power systems for the 21st Century?

Microgrids – de-centralised electricity generation combined with on-site production of heat - bear the promise of substantial environmental benefits, brought about by higher energy efficiency and by facilitating the integration of renewable sources such as photovoltaic arrays or wind turbines. With the use of modern control technologies, microgrids can achieve a good match between generation and load, resulting in a low impact on the electricity network despite a potentially significant level of generation by intermittent energy sources. Tom Markvart, University of Southampton, UK reports.

Beyond the Yablonovitch limit: Trapping light by frequency shift

It is shown that randomizing the photon distribution over the frequency as well as orientation variables dramatically improves the efficiency of optical confinement in a weakly absorbing material such as crystalline silicon. The enhancement in average optical path length over the Yablonovitch limit [E. Yablonovitch, J. Opt. Soc. Am. 72, 899 (1982)] is given by an inverse Boltzmann factor of the frequency shift, making it possible to manufacture, for example, efficient crystalline silicon solar cells of thickness barely 1 μm.

Principles of solar cell operation

The two steps in photovoltaic energy conversion in solar cells are described using the ideal solar cell, the Shockley solar cell equation, and the Boltzmann constant. Also described are solar cell characteristics in practice; the quantum efficiency of a solar cell; the optical properties of solar cells, including antireflection properties, transmission, and light trapping; typical solar cell structures, including the p–n junction, uniform emitter and base, diffused emitter, heterojunction cells, p–i–n structure, and series resistance.

Large surface photovoltages observed at methyl-terminated silicon surfaces synthesised through a two-step chlorination-alkylation method

Methylation of the silicon surface through a chlorination-alkylation method has been used to improve the electronic properties of silicon. Upon alkylation of the surface, an increase in the minority carrier recombination lifetime and the surface photovoltage is observed, in line with an increase in surface charge. A likely explanation of this unusually large band bending is charge accumulation during the removal of chlorine from the surface.

Impact of Small Phonon Energies on the Charge-Carrier Lifetimes in Metal-Halide Perovskites

Metal-halide perovskite (MHP) solar cells exhibit long nonradiative lifetimes as a crucial feature enabling high efficiencies. Long nonradiative lifetimes occur if the transfer of electronic into vibrational energy is slow due to, e.g., a low trap density, weak electron-phonon coupling, or the requirement to release many phonons in the electronic transition. Here, we combine known material properties of MHPs with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI3 lead to a strong dependence of recombination rates on trap position, which we deduce from the underlying physical effects determining nonradiative transitions. This is important for nonradiative recombination in MHPs, as it implies that they are rather insensitive to defects that are not at midgap energy, which can lead to long lifetimes. Therefore, the low phonon energies of MHPs are likely an important factor for their optoelectronic performance.

What is the useful energy of a photon

The fundamental upper bound on the efficiency of photovoltaic conversion continues to attract interest of the research community. By considering the conversion efficiency of a monochromatic photon gas at constant pressure, we show that this limit is equal to the availability (or exergy), as defined in textbooks on classical thermodynamics. The application of this result to the full spectrum of black-body radiation yields the Petela-Press-Landsberg efficiency. Generalization to include entropy generation on account of the kinetic nature of the conversion process, by drawing a parallel with the efficiency of an infinite tandem converter, yields a theoretical efficiency limit of 85.2%.