RNDr. Branislav Dzurňák, Ph.D.

Nickel oxide (NiOx) has gained attention as a promising inorganic hole transport layer for perovskite solar cells due to its wide bandgap, high transparency, and stability. However, tuning of band alignment by an extra dipole layer is necessary to achieve high efficiencies. Our predictive simulations suggest that NiOx bandgap tuning can also improve solar cell performance. Motivated by these findings, this study experimentally investigates NiOx films with different bandgap fabricated using pulsed laser deposition under varying deposition conditions, including oxygen pressure, substrate temperature and laser frequency.

We present historical context and review recent advances in the realisation of a photosensitised silicon solar cell, highlighting key theoretical and experimental developments. Emphasis is placed on the importance of electrostatic near-field interaction between the transition dipole moments of the luminophore and electron–hole pairs in the silicon crystal at a distance of a few nanometres. The very fast energy transfer then resembles the Förster resonance energy transfer between two molecules. Photon tunnelling via the evanescent field of the excited molecule (‘optical near field’) into optical states in silicon occurs at somewhat larger separation between the molecule and silicon, of the order of tens of nanometres. Accessing the critical Förster regime relies on oxide-free silicon surfaces and the covalent attachment of dyes, enabling efficient passivation and precise control of interface chemistry. Realising a complete photosensitised silicon solar cell remains a challenge and we outline promising directions and review recent progress that brings this goal closer to reality.

Hybrid perovskites are highly attractive for a range of optoelectronic applications. In the case of solar cells, a film thickness close to 1 micrometre is necessary to absorb most of the light. Otherwise, either a back reflector or an efficient light trapping strategy may be desired. Light trapping due to the light scattering on rough surfaces is the most straightforward method to increase optical path of light inside the absorber. Using this approach, we prepared experimentally layers featuring an absorptance enhancement that was corresponding to the photon path length extension several times. In parallel, we measured photoluminescence and recalculated it into absorptance employing reciprocity theorem. We could verify the validity of the theorem even for layers featuring light trapping, however, we identified many cases where the simple reciprocity applied on macroscopic measurable properties does not work.

Hybrid perovskites are a class of thin-film semiconductors with remarkably steep absorption edges and high absorption coefficient. In the case of solar cells, a film thickness of less than a micrometer is usually sufficient to absorb most of the light when combined with a back reflector. Otherwise, an efficient light trapping strategy may be desired, e.g., in the case of tandem or semitransparent cells. Traditionally, light trapping is accomplished by employing randomly nanotextured substrates. In this contribution, absorption enhancements due to not only nanorough but also microrough substrates and with or without additional gold coating are evaluated from the point of gains in photocurrent and from the point of view of valid optical models. We find that light trapping from nanotextured substrates follows mainly the Yablonovitch model, leading to an apparent shift of absorption edge. This contrasts with microrough substrates and also the remarkable efficient light trapping capabilities of bare layers due to their native surface roughness, where the path enhancement in this case is almost uniform, making the layer optically thicker by factor two or more. Light trapping optical models as well as analytical techniques are reviewed, and new insights are presented.

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.

For sustainable energy technologies, MXenes offer unique properties such as high electrical conductivity, hydrophilicity, excellent thermal stability, large interlayer spacing, easily tunable structure, high surface area, and microporous structure that facilitate faster ion transfer. To address limitations of aqueous MXene suspension, Ti3C2-type MXene thin films are prepared from non-aqueous suspensions in N,N-dimethyl formamide (DMF) and N-methyl-2-pyrrolidone (NMP) using solvent exchange method followed by spin coating or drop casting on gold interdigitated electrodes (IDE) and ceramic substrates and their electrical properties are compared. Electrical properties investigated by impedance spectroscopy (4 Hz to 8 MHz) and four-point probe (FPP) measurements show that DMF-MXene layers exhibit higher electrical conductivity than MXene deposited from NMP. The material technology and electrical properties of MXene thin films prepared from non-aqueous solvents may thus be promising for possible use of MXenes in hybrid photovoltaic devices as charge-transporting layers.

Luminescent solar concentrators (LSC) are recently getting attention as promising components for use in building integrated photovoltaics (BIPV). Ongoing efforts to reduce their optical losses result in active search for new luminophores with large Stokes shift in order to reduce the reabsorption probability. Here we present a novel figure of merit for a large group of Gaussian dye luminophores that provides an accurate assessment of optical losses. Our approach originates in simplified analytical solution for spectral overlapping integral of absorption/emission bands and results in assessment of average reabsorption probability. The procedure is then generalized for any Gaussian dye LSC, by considering its basic parameters (Stokes shift, absorbance and size), thus resulting in a simple useful tool for designing practical LSC devices. Calculations are correlated with the experimental results of reabsorption probability obtained by measuring edge fluorescence spectra of various LSCs.

Mixed-halide perovskites are highly promising materials for tandem solar cells. The phenomenon of phase segregation, however hinders their application. Here, we combine Fourier-Transform photocurrent spectroscopy with photoluminescence and current density–voltage (J–V) measurements to study the effect of light soaking on such materials and devices. At first, we observe a gradual formation of an I-rich phase, which correlates with an increase in deep defect level concentration. We attribute these deep defects to charged iodide interstitials and associate phase segregation with iodide migration through interstitial positions. Upon further light soaking, the second less I-rich phase forms, while the deep level concentration simultaneously decreases. An empirical model describing the phase segregation mechanism is proposed to rationalize these observations. Further, we point to an important role of grain size in determining the degree and terminal phase of segregation.

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.

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.

Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectroscopy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect.

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.

In this work, the photoinduced degradation of CH3NH3PbI3 perovskite films under illumination in ambient conditions (relative humidity 30-50%) was studied by Kelvin Probe and Photoluminescence techniques. Using Kelvin Probe techniques, we investigated the effects on the work function of the CH3NH3PbI3 film on fluorine-doped SnO2 (FTO) at various photoinduced degradation states in the dark and under illumination. It was found that the work function of CH3NH3PbI3 film on FTO was gradually increased in the dark after every 10 min illumination step until the film was completely degraded. The gradual increase in work function due to the degradation can be ascribed to modulation doping of the CH3NH3PbI3 by PbI2 phase. It was also found that the contact potential difference (CPD) of CH3NH3PbI3 on FTO was increased under illumination as a result of a positive surface photovoltage relative to the FTO.