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.
Inverted perovskite solar cells passivated via organic cations exhibit superior power conversion efficiency compared to un-passivated ones. These record efficiencies have been reached thanks to the use of large organic cations to passivate the interface between the perovskite absorber and the transport layers. Here, we study the optoelectronic properties and chemical structure of interface doped perovskite solar cells where large cations, namely Cl-PEAI and F-PEAI, were incorporated at both front and rear interfaces of the absorber. The effect of the cation addition led to an increase of all the main PV characteristic, reaching PCE values up to 23.7%. We combined steady-state and time-resolved multidimensional photoluminescence imaging techniques to probe the main optoelectronic and transport properties of such devices. We thus obtained quantitative maps for physical parameters such as Quasi Fermi Level Splitting (QFLS), Urbach Energy and surface recombination rate, which proved a homogenous passivation by Cl-PEAI and F-PEAI cations over the 3D surface. For instance, the front and rear surface recombination velocities are reduced by a factor of 6 to 8 for Cl-PEAI based samples and 5 to 10 for F-PEAI based samples. In addition, we identified interfacial passivation as the main mechanism leading to a clear improvement of the Voc which increases from 1.10 to 1.16 eV. Indeed, we noticed a clear increase in terms of QFLS only after the addition of the electron transport layer whereas only an increase in the range of 0.01-0.02 eV was observed for bare perovskite thin film with the cation on top. Mapping the opto-electronical properties showed their good spatial homogeneity, By linking optical and electrical measurements we highlight the benefits of this passivation method in maximising all the main photovoltaic characteristics and in approaching inverted perovskite solar cell theoretical limit.
Absorption coefficient rules photonic transport within solar cells and is key for optical optimization. By probing materials density of states, it offers insights in defects characterization of photovoltaic device. Moreover, obtaining this quantity locally would refine the interpretation of the photoluminescence (PL) maps. Its measurement raise challenges as far as classical spectroscopy signals depends on the whole stack structure. Also, its exponential decrease hinders its accurate determination near bandgap energy.
When describing photoluminescence, light absorption influences both the generation function - at the excitation wavelength - and the photons emission and propagation - close to the bandgap energy. We take advantage of these dependances to explore different techniques to obtain either the absorptivity or the absorption coefficient and discuss the link between these two quantities. Transient photoluminescence excitation (PLE) measurement is used to determine the local absorptivity above the bandgap with multidimensional PL imaging techniques. We assess the impact of pulsed excitation on the measurement by modelling the both transient and continuous regime. In addition, absorptivity can be obtained near the optical bandgap directly from the PL spectrum. In this frame, low temperature measurements have already allowed the determination of absorption coefficient with ultra-low values.
These different approaches are compared to reflection measurements classically used at the global scale. The limitations and application scope of the different absorption determination will be described, as well as how they relate to one another. Finally, application to PL imaging will be discussed.
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.
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.
Analyzing the photoluminescence (PL) maps of semiconductors complementarily in time and wavelength allows to derive their key optoelectronic and transport properties. Up to now, separate acquisitions along time or wavelength had to be acquired for time and wavelength so that a comprehensive study of the dynamics was out of reach. We developed a 4D imaging set-up that allows the simultaneous acquisition of spectral and temporal luminescence intensity with micrometric spatial resolution under the exact same experimental conditions. This novel set-up relies on single pixel imaging, an approach that enables the reconstruction of the spatial information recorded from a higher resolution non-imaging detector. The sample PL signal is spatially modulated with different patterns by a digital micro-mirror device1. We make use of this technique for the first time with a streak camera as a detector, allowing to record the PL intensity decays and spectrum for each pixel with very high temporal (<100ps) and spectral resolutions (<1nm). A patent application has been filled. We demonstrate the use of this setup by characterizing III-V samples. We observe the spatial variations of a red shift occurring during the short time of the decay.
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