The development of advanced photovoltaic devices, including those that might overcome the single junction efficiency limit, as well as the design of new materials, rely heavily on advanced characterization techniques. Among all the existing methods, optical ones are well suited to probe quantitatively optoelectronic properties, and luminescence-based ones feature preeminently for this purpose. We here present the use of multidimensional imaging techniques that record spatially (with up to 2 µm spatial resolution), spectrally (5 nm), and time-resolved (50 ps) luminescence images. We will discuss the benefits and challenges of looking into energy conversion systems from a multidimensional perspective. We will use some examples, mostly drawn from halide perovskite and III-V materials and device, which will help revisit questions related to efficient transport and conversion in solar cells.
KEYWORDS: Multijunction solar cells, Absorption, Sun, Solar cells, Solar radiation models, Solar energy, Energy efficiency, Instrument modeling, Solar radiation, Thermodynamics
The resilience against non-idealities of hot-carrier multijunction solar cells (HCMJSCs) is assessed and compared with two references, namely a multijunction solar cell (MJSC) and a hot carrier solar cell (HCSC). We investigate the impact on the efficiency of three deviations from the ideal case: nonoptimal design, internal limitations, and nonstandard operation conditions. We show that the HCMJSC maintains a high efficiency even when materials with nonoptimal bandgaps are considered, broadening the range of candidate materials for its implementation. We also show that the requirement for hot carriers’ thermalization is much less stringent than with the standard HCSC architecture, allowing to surpass the best MJSC efficiency with currently achievable thermalization coefficients. Finally, we estimate the influence of nonstandard illumination by varying the AM spectrum and estimate numerically the yearly averaged efficiency of devices installed in two different locations. Preliminary results on temperature dependence are also presented.
Perovskite-based solar cells are the subject of intense study today because of their promise in terms of high efficiency, easy and low-cost fabrication. To gain insight on the behavior of carriers inside the perovskite layer, time resolved photo luminescence (TR-PL) and time resolved fluorescence imaging (TR-FLIM) are used. However, owing to their long lifetimes (~1µs) and slow diffusion (D~1e-2cm2s-1) the acquired signals require specific care for interpretation.
In a previous work, we showed how these properties can be exploited to derive a scaling-law for the normalized time derivative of the TR-PL signal just after the laser pulse 1/tau. This scaling links the derivative to the material parameters: interface and bulk non radiative recombination, radiative recombination, and diffusion. Our previous focus showed the impact of the laser fluence on the derivative and its use to obtain among others the external radiative recombination coefficient.
In this work we extend the possibility of our previous technique to separate surface and bulk contributions using the impact of the laser wavelength on the scaling of 1/tau through its impact on the spatial distribution of photogenerated carriers. The absorption coefficient of the material at the laser wavelength plays a crucial role in the scaling. We use theoretical computations as well as drift-diffusion simulation to analyze the range of applicability of our technique. We apply experimentally our methodology on perovskite samples with a pulsed laser of varying wavelength. The aim is to determine quantitatively the bulk, front and bottom surface non radiative. We show experimental validation of the scaling on perovskite material and analyze how it can be combined with drift-diffusion simulation. We investigate both interfaces (bottom and front) by varying the illumination side and show how this technique allows for the quantitative comparison of non-radiative recombination at both interfaces. We discuss the experimental uncertainty.
This work describes the relative contribution of intervalley scattering and phonon bottleneck effects in type-II InAs/AlAsSb quantum well solar cells. Moreover, recent predictions also suggest that altering the QW to barrier thickness ratio in these structures enables control of the phonon scattering rate, and therefore hot carrier relaxation may be inhibited by design. Experimental analysis of these predictions is presented in solar cell architectures, as well as, their effects upon both the optical and electrical performance of these devices.
In photovoltaic devices, thermalization of hot carriers generated by high energy photons is one of the major loss mechanisms, which limits the power conversion efficiency of solar cells. Hot carrier solar cells are proposed to increase the efficiency of this technology by suppressing phonon-mediated thermalization channels and extracting hot carriers isentropically. Therefore, designing hot carrier absorbers, which can inhibit electron-phonon interactions and provide conditions for the re-absorption of the energy of non-equilibrium phonons by (hot) carriers, is of significant importance in such devices. As a result, it is essential to understand hot carrier relaxation mechanisms via phonon-mediated pathways in the system. In this work, the properties of photo-generated hot carriers in an InGaAs multi-quantum well structure are studied via steady-state photoluminescence spectroscopy at various lattice temperatures and excitation powers. It is observed that by considering the contribution of thermalized power above the absorber band edge, it is possible to evaluate hot carrier thermalization mechanisms via determining the thermalization coefficient of the sample. It is seen that at lower lattice temperatures, the temperature difference between hot carriers and the lattice reduces, which is consistent with the increase of the quasi-Fermi level splitting for a given thermalized power at lower lattice temperatures. Finally, the spectral linewidth broadening of multiple optical transitions in the QW structure as a function of the thermalized power is investigated.
Short time carrier dynamics of transient photoluminescence decays contain valuable information on the optoelectronic properties of photovoltaic materials. We perform a theoretical analysis on short time dynamics to provide scaling laws for the time derivative of the transient photoluminescence signal as a function of both laser excitation power and wavelength . This innovative approach allowed us to extract in a simple and effective manner the external radiative recombination rate and was tested on different absorbers such as state-of-the-art triple cation mixed halide perovskite and III-V materials. Moreover, by coupling this analysis with the fitting of the whole PL decay, we have quantified different transport parameters and precisely estimated their uncertainties.
Hot-carrier solar cells could overcome the Shockley-Queisser limit by having electrons and holes at a higher temperature than the lattice. To generate these hot carriers under concentrated sunlight, the thermalization rate should be as low as possible. Our objective in this presentation is to quantify the influence of different thermalization mechanisms. We determine the carrier temperature in ultrathin GaAs absorbers using continuous-wave photoluminescence and identify distinct surface and volume thermalization contributions. We explain the origin of these contributions using theoretical models involving non-equilibrium LO phonon populations and thermionic emission. We implement these mechanisms in detailed balance calculations for further understanding.
Photoluminescence spectroscopy is a powerful technique to investigate the properties of photo-generated hot carriers in materials in steady state conditions. Hot carrier temperature can be determined via fitting the emitted PL spectrum with the generalized Planck’s law. However, this analysis is not trivial, especially for nanostructured materials, such as quantum wells, with a modified density of states due to quantum confinement effects. Here, we present comprehensively different methods to determine carrier temperature via fitting the emitted PL spectrum with the generalized Planck’s law and discuss under what conditions it is possible to simplify the analysis.
This theoretical study sheds light on questions raised by inter-subband transition in quantum dot intermediate band solar cells. Based on a dedicated analytical model that correctly treats, from a quantum point-of-view, the trade-off between the absorption, the recombination and the electronic transport, we clearly show that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor with a tunnel barrier. Such a barrier, matching the recombination and the tunnel rates, allows to strongly improve the current. On the other hand, by better controlling the retrapping, such a barrier can also improve the voltage. Finally this work, by giving a framework to design efficient inter-subband transitions, opens new opportunities for quantum dot intermediate-band solar cells.
A hot-carrier solar cell (HCSC) is a high-efficiency photovoltaic concept where electrons and holes are at a higher temperature than the lattice, allowing an additional thermoelectric energy conversion. There are two requirements for a HCSC: establishing a hot-carrier population and converting the temperature into extra voltage through energy-selective contacts. We focus on the generation of hot carriers, and the design of absorbers that can make this generation easier. Fundamentally, this requires to increase the power density absorbed per volume unit, so the photocarriers cannot fully thermalize (phonon bottleneck). Beyond simply increasing the light intensity, the main control knobs to favor hot carriers include reducing the thickness of the absorber, increasing its absorptivity, and reducing its bandgap. In this proceeding, we report the fabrication of structures that aim at measuring the influence of these different parameters. We justify our choices for sample structure and fabrication method from the need for high thermal conductivity, in order to prevent lattice heating. We characterize our structures in order to determine precisely the final thickness of all layers, and the absorptivity of the absorber layer. These samples are to be used for an analysis of the temperature with many variable parameters, in order to better understand the thermalization mechanisms and design better absorbers. Ultimately, our objective is to implement all solutions together in order to evidence a hot carrier population under concentrated sunlight illumination.
Hot-carrier solar cells (HCSC) can potentially overcome the Shockley-Queisser limit, by having carriers at a higher temperature than the lattice. To this end, the carriers need to thermalize slower than power is generated by absorbing photons. In thin films, a hot-carrier distribution can only be achieved with very high incident power, by saturating the thermalization channels. Ultra-thin absorbers have a smaller thermalization rate, due to fewer channels. However, they typically absorb only a limited amount of light, which prevents them from reaching high efficiencies. Light trapping is an excellent way to increase significantly the amount of light absorbed in an ultra-thin material. Yet, studies on the coupling between light trapping and hot carriers are still lacking, due to the complexity of the whole system. We analyze numerically and experimentally how light trapping can enable high-efficiency HCSC. This manuscript presents the progress towards the experimental demonstration of the enhancement of the hot-carrier effect with light trapping. 280 nm-thick devices have successfully been reported on a gold mirror using epitaxial lift-off (ELO) and gold-gold bonding. These devices have been characterized by photoluminescence spectroscopy. Hot carriers with a temperature 37 K above lattice temperature were measured, in accordance with theoretical predictions. We are now working towards the ELO of absorbers 10 times thinner, on which we will implement light trapping to increase the carrier temperature.
The Intermediate Band Solar Cell is an advanced concept, which has been predicted to overcome the Shockley-Queisser limit, despite efficiencies remaining below the best single junctions so far. Practical realizations with nanostructures suffer from two intrinsic deficiencies: narrow absorption widths and low radiative efficiencies. We evaluate in this paper the theoretical efficiency expectations with respect to those two properties, and consider in addition the possibility of including an electronic ratchet. We observe that an intermediate band solar cell using a ratchet becomes highly tolerant to non-ideal nanostructures, so that any combination of low absorption and low radiative efficiency becomes compatible with optimized performances above the Shockley-Queisser limit. We conclude that future practical realization may take advantage of quantum wells, which have been less considered so far than quantum dots, due to relatively higher nonradiative recombination rates. Such realizations would take advantage of the higher absorption properties of quantum wells.
Double resonant tunneling barriers are considered for an application as energy selective contacts in hot carrier solar cells. Experimental symmetric and asymmetric double resonant tunneling barriers are realized by molecular beam epitaxy and characterized by temperature dependent current-voltage measurements. The negative differential resistance signal is enhanced for asymmetric heterostructures, and remains unchanged between low- and room-temperatures. Within Tsu-Esaki description of the tunnel current, this observation can be explained by the voltage dependence of the tunnel transmission amplitude, which presents a resonance under finite bias for asymmetric structures. This effect is notably discussed with respect to series resistance. Different parameters related to the electronic transmission of the structure and the influence of these parameters on the current voltage characteristic are investigated, bringing insights on critical processes to optimize in double resonant tunneling barriers applied to hot carrier solar cells.
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