prof. Tomáš Markvart, Ph.D.

Combination of affordable organic dyes of high quantum yield with silicon can be an interesting way fordevelopment of highly efficient thin film photovoltaic cells utilizing silicon sensitisation. This work is focused on investigating the energy transfer processes including photon tunnelling from photosensitive molecules of BASF R305 high quantum yield dye to silicon substrate. Energy transfer from dye molecules to silicon substrate is evaluated by measuring the quenching of molecular photoluminescence lifetime using time-correlated single photon counting (TCSPC) technique. Energy transfer is further studied in dependence on dye layer thickness. The results can be useful for further studies leading to design of ultrathin silicon solar cells.

The use of the electrical energy storage (EES) plays an important role in the transition of energy generation towards renewable energy sources (RESs). An effective sizing of EES systems is very important in order to cope with the volatility of RESs and to ensure a reliable energy supply. The present paper provides a methodology which helps to determine the minimum required EES size for conceiving a fully standalone system. Its approach is based on the evaluation of the energy balance for a given design period, and it can also be applied for sizing the EES system in grid-connected applications. The methodology was validated using measurement data obtained from two different systems corresponding to: a) a near-zero energy building with local generation sources, and b) a large-scale battery energy storage system (BESS) installed in a factory and used for peak-shaving. The obtained results confirmed the effectiveness of the proposed methodology by estimating the required size of the EES system. A good correlation was found between the estimated and the installed BESS size in the considered systems. The deviation between real and estimated BESS capacities was found to be less than 5%.

The year 2021 marks the 60th anniversary of the Shockley-Queisser paper which laid the theoretical foundations for the fundamentals of solar cell operation. This paper reviews the principal results of this papers and the developments that followed. Starting with links to the earlier radiative balance laws of Kirchhoff and Planck, we discuss the nature of detailed balance between the incident and emitted photon beams. The resulting efficiency limit is juxtaposed with another popular limit due to Trivich and Flinn. The dependence of the Shockley-Queisser limit on the intensity and sizes (étendues) of the two beams – in other words, on the concentration of sunlight – is discussed in some detail. The paper then takes a look at a broader view of the detailed balance with the help of the generalised Planck law and the refinements that this implies for the efficiency and the current-voltage characteristic of the cell. A natural extension of the Shockley-Queisser detailed balance into the realms of thermodynamics is outlined by considering the photon entropy in the two beams. This produces the detailed balance limit in a typical thermodynamic form, similar to the result for a heat engine, where the efficiency losses are expressed in terms of entropy generation. The paper concludes with a brief discussion of how the results can be extended to the operation of practical solar cells with a more realistic description of light absorption and nonradiative recombination. A brief overview is also given of mechanisms for how the Shockley-Queisser limit can be exceeded.

In a recent paper, Guillemoles et al attempt to clarify and explain the often cited paper by Shockley and Queisser (SQ) which defines the limits to photovoltaic conversion by a single-junction solar cell. The SQ paper is not easy to read and is therefore easily misunderstood. As modern solar cells approach theoretical efficiency limits, the fundamentals become particularly important and the effort by Guillemoles et al is therefore to be welcome. However, in doing so, the authors have fallen into several pitfalls, and the aim of the present note is to clarify a number of misconceptions and correct some errors in that paper for specialists and non-specialists alike to help disentangle the complexities of the SQ paper.

Energy transfer from a submonolayer of rhodamine 6G molecules to a 130 nm thick crystalline silicon (Si) waveguide is investigated. The dependence of the fluorescence lifetime of rhodamine on its distance to the Si waveguide is characterized and modeled successfully by a classical dipole model. The energy transfer process could be regarded as photon tunneling into the Si waveguide via the evanescent waves. The experimentally observed tunneling rate is well described by an analytical expression obtained via a complex variable analysis in the complex wavenumber plane.

Silicon photosensitisation via energy transfer from dye molecular layers is a promising area of research for excitonic silicon photovoltaics. We present the synthesis and photophysical characterisation of vinyl and allyl terminated Si(111) surfaces decorated with perylene molecules. The functionalised silicon surfaces together with Langmuir-Blodgett (LB) films based on perylene derivatives were studied using a wide range of steady-state and time resolved spectroscopic techniques. Fluorescence lifetime quenching experiments performed on the perylene modified monolayers revealed energy transfer efficiencies to silicon up to 90 per cent. We present a simple model to account for the near field interaction of a dipole emitter with the silicon surface and distinguish between the ‘true’ FRET region (<5 nm) and a different process, photon tunneling, occurring for distances between 10 nm - 50 nm. The requirements for a future ultra-thin crystalline solar cell paradigm include efficient surface passivation and keeping a close distance between the emitter dipole and surface. These are discussed in the context of existing limitations and questions raised about the finer details of the emitter-silicon interaction.

Based on weather and electricity demand data for the period 1984–2013, we develop a system model based on energy balance to determine the size of photovoltaic and wind generation combined with energy storage to provide a firm power supply for Great Britain. A simple graphical methodology is proposed where the required wind and PV generation capacities can be read off from a “system configuration diagram” as a function of the available storage size. We show, by way of illustration, that a reliable supply would be produced by a system based on PV and wind generators generating some 30% more electrical energy (approximately 100 TWh p.a.) than the current electricity supply system if supplemented with 30 days of storage. In terms of generation capacities, the current 82 GW of principally thermal generation would then be replaced by about 150 GW of wind turbines and 35 GW of PV arrays.

Thermodynamics has provided a powerful tool to study radiation and its conversion into useful work. Starting from the so-called Shockley's paradox, this article discusses the thermodynamic view of fundamental losses to photovoltaic conversion, and how thermodynamics enters the charge-carrier transport in semiconductors and heat-exchange processes at p-n junctions. Turning to photon flows, considerations based on detailed balance and reciprocity provide a comprehensive picture of the voltage produced by the solar cell in the presence of nonradiative recombination. We shall use these tools to examine several topics under recent discussion, including photon recycling and hot-carrier conversion based on thermoelectricity.

Hot carrier solar cells have attracted interest for many years. Although no working exemplars exist today, the challenges to overcome have become clearer and a substantial research effort has been underway with a focus on inorganic semiconductors, including quantum wells. In this paper we propose a novel strategy to potentially exploit hot photons, based on organic absorbers. Our approach, when combined with photon management structures similar to photonic fluorescent collectors, can potentially enhance the efficiency of complete photovoltaic devices. We present a characterisation method of fluorescent collectors by evaluating the chemical potential and temperature of the emitted fluorescence photon flux. We report on observation of temperatures of the emitted photon flux well above the ambient temperature, indicating the presence of hot photons. We propose a theoretical background to describe how excess thermal energy carried by hot photons can be exploited to increase the chemical potential of the photon flux which is closely related to the open-circuit voltage of the solar cell.

We have developed a methodology to evaluate materials for potential use as absorbers in solar cells without the need for device fabrication. Using our expertise developed for fluorescence collectors we have fabricated relevant structures and characterised them for reabsorption and absolute fluorescence intensity. The latter then form a basis to deduce the chemical potential of the emitted light, closely related to open-circuit voltage of a solar cell where this material would act as an absorber. Detailed analyses are carried out of the relevant losses focusing on interplay between photon recycling, non-radiative quenching and absorptivity/emissivity. Our results present values of open-circuit voltage that can be achieved using easily available laser dyes.

Metal-halide perovskite (MHP) solar cells exhibit long non-radiative lifetimes as a crucial feature enabling high efficiencies. Long non-radiative lifetimes occur if the transfer of electronic into vibrational energy is slow due to, e.g., a low trap density, weak electronphonon coupling or the requirement to release a 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 non-radiative transitions. This is important for non-radiative 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.

The equation for open-circuit voltage of a solar cell based on optoelectronic reciprocity is combined with the standard textbook formula to obtain a general result that includes photon recycling as well as losses by carrier transport. It is shown that, in indirect-gap semiconductors, the general expression reduces to the “conventional” and “optoelectronic” expressions in the limits of low and high radiative efficiency.

We report the photosensitization of crystalline silicon via energy transfer using covalently attached protoporphyrin IX (PpIX) derivative molecules at different distances via changing the diol linker to the surface. The diol linker molecule chain length was varied from 2 carbon to 10 carbon lengths in order to change the distance of PpIX to the Si(111) surface between 6 A and 18 A. Fluorescence quenching as a function of the PpIX-Si surface distance showed a decrease in the fluorescence lifetime by almost two orders of magnitude at the closest separation. The experimental fluorescence lifetimes are explained theoretically by a classical Chance-Prock-Silbey model. At a separation below 2 nm, we observe for the first time, a Forster like dipole-dipole energy transfer with a characteristic distance of R-o = 2.7 nm.