The Internet of Things (IoT) has recently emerged as a technology with immense potential societal impacts. However, powering IoT devices presents a key challenge. Indoor photovoltaics (IPV), offering the potential for self-powered and low-maintenance IoT devices, may be a promising solution. Concurrently, driven by the need for tandem structure, wide-gap perovskite semiconductors are emerging. The optimal bandgap for IPV devices (1.8 – 1.9 eV) is considerably wider than most traditional solar materials but in accordance with existing wide-gap perovskites.

In this talk we discuss the performance of various wide-gap perovskite IPVs as model systems under spectrophotometrically calibrated indoor light conditions and compares them to thermodynamic performance predictions. Computational simulations indicate trap-assisted recombination as a primary loss mechanism in current perovskite IPVs. To support further advancement in the field, a computational tool is provided to extrapolate calibrated solar (AM1.5G) measurements for benchmark performance estimates at a range of indoor conditions. This work highlights the need for standardized characterization methods and suggests pathways for device optimization to unlock the full potential of perovskite IPVs for a future of sustainable, self-powered IoT devices

Metal halide perovskite semiconductors hold great promise for future photovoltaic applications. The efficiency of metal halide perovskites solar cells (PSCs) is determined by several parameters, one of them is the open circuit voltage (V_OC). In order to achieve high efficiency solar cells, it is important to reach a high V_OC. Consequently, we want to understand the loss mechanisms limiting the V_OC. It is known that the V_OC loss is dominated by non-radiative recombination occurring e.g. in the bulk of the perovskite, or at the interfaces. Measuring the V_OC of a PSC in order to understand what exactly causes the non-radiative recombination and where in PSC, will not provide the answer, because it is just the overall result. Therefore, we try to understand the V_OC loss in perovskite solar cells via absolute photoluminescence intensity analysis. With this analysis, the quasi Fermi level splitting (QFLS) of different components in a PSC can be determined_. Since the QFLS is related to the maximum V_OC achievable for a photovoltaic device, this relation can be used for predicting device properties.

In this work, the effect of passivation on defects and the non-radiative recombination of charge carriers in mid to wide bandgap perovskite solar cells at the perovskite and electron transport layer interface is examined with absolute PL to understand the detrimental processes happening in the PSCs. We begin by analyzing the voltage loss for p-i-n devices and by describing the passivation of surface defects at the interface of the mid-bandgap perovskite with the electron transport layer using a solution-processed quaternary ammonium halide (choline chloride, CCl) or an evaporated ultrathin lithium fluoride (LiF) layer, that translates to solar cells with a high V_OC. We extend this passivation evaluation to an array of wide-bandgap perovskite devices. After this analysis, we demonstrate that non-radiative recombination is primarily located in the film and that it increases with increasing bandgap. The other prominent loss is located near the interface of the perovskite with the electron transport layer and is passivated equally for both passivation techniques and for all bandgaps investigated in this study. This implies that the surface treatment with two different materials result in the same gain. To further understand the working principle of the passivation strategies, we extend the passivation study by increasing the amount of passivation material in between the perovskite-ETL interface. It is found that upon application of increasing thicknesses of LiF or increasing concentration of CCl in between the perovskite/ETL interface, the gain in QFLS and V_OC starts plateauing and an optimum thickness or concentration should be used.

In summary, by combining the absolute photoluminescence spectroscopy results, it is found that by using a LiF or CCl a significant reduction of the non-radiative recombination losses that currently limit wide-bandgap PSCs is achieved. And that this gain is primarily due to minimizing the loss at the perovskite-ETL interface. These results contribute to solving the main challenges of the wide-bandgap perovskite in achieving a high V­_OC.

In conventional flexible perovskite solar cells (FPSCs) produced on plastic substrates such as polyethylene terephthalate (PET) and featuring a conductive transparent oxide layer like indium-doped tin oxide (ITO), the ITO and perovskite layers are susceptible to cracking when the devices are moderately bent. While changes in device series resistance can be indicative of cracks in the ITO layer, characterizing cracks in the perovskite layer is more complex.

At a microscopic level, perovskite layer cracking implies an upsurge in the density of bulk and surface defects, leading to an increase in trap states within the layer. This, in turn, can amplify the loss of photo-generated charge carriers through trap-assisted carrier recombination processes. However, comprehending and quantifying the impact of applied mechanical stress (MS) on device properties and behavior necessitates a sophisticated analysis of charge carrier generation, recombination, and extraction processes both before and after MS application [1].

This study is dedicated to examining the properties and behavior of FPSCs pre- and post-application of MS to unravel the dynamics of recombination losses induced by MS, providing insights into the mechanisms contributing to performance degradation in perovskite-based solar cells. The investigation involves a comprehensive analysis of device properties and behavior through experimental and theoretical approaches. The degradation of photoelectrical parameters in fabricated PFPSCs due to applied MS is elucidated by meticulously scrutinizing various recombination pathways within the perovskite layer.

Interfacial charge carrier recombination is currently one of the major performance bottlenecks in single- and multi-junction metal halide perovskite (MHP) solar cells. In our work, we investigate interfacial charge carrier recombination processes in state-of-the-art MHP thin films and device structures by transient spectroscopies including transient reflection, transient absorption, time-resolved photoluminescence, and time-domain terahertz spectroscopy across a wide dynamic range from femto- to microseconds. The MHPs investigated are multi-cation (mixed) halide perovskites as neat MHP thin films, passivated MHP thin films, perovskite films adjacent to charge transport layers (CTLs), MHP films adjacent to CTLs with additional interlayers (ITLs) and in the presence of electrodes, as well as MHP films on transparent conductive oxides (TCOs) with and without common self-assembled monolayers (SAMs). Organic CTLs are also used, since they allow direct probing of the carrier dynamics in the CTL (not only in the perovskite film), thereby allowing to distinguish between carrier extraction and interfacial recombination. Our spectroscopic experiments are supported by computational studies providing insight into the role of (bulk and surface/interface) defects on carrier recombination, the chemistry of defect passivation at interfaces, and interfacial carrier extraction and recombination dynamics. Our studies provide in-depth insight into interfacial charge carrier recombination processes at various types of interfaces in perovskite devices and reveal pathways to mitigate those losses to enhance the device Voc and quantum efficiency in both single- and multijunction photovoltaic solar cells.

Metal-halide perovskites (MHP) stand out as highly promising and cost-effective optoelectronic materials due to their exceptional optoelectronic properties and versatile fabrication methods [1-6]. These materials find applications in diverse fields such as solar cells, light-emitting diodes, photodetectors, and even quantum emitters. Quantum confinement can unveil unexpected and advantageous characteristics, leading to the development of high-performance devices.

One approach to induce quantum confinement involves creating layers of quantum-confined materials through the deposition of multiple thin films. Thermal evaporation emerges as a particularly promising technique for fabricating halide perovskite films, offering precise control over layer thickness, fine-tuning of composition, stress-free film deposition, and the ability to modify surface properties. Thermal evaporation in perovskite fabrication has expanded the possibilities of thin film production, showcasing its capability to generate ultrathin perovskite films that serve as the foundation for multi-quantum well structures.

This method enables the manipulation of growth properties, influencing the optoelectronic characteristics of nanoscale thin films and inducing quantum confinement effects within the structure. The precise control over photoluminescence through quantum confinement opens up a wide array of possibilities for unconventional optoelectronic properties and novel applications of perovskites [7-10].

1. J. Li et al; , Joule 2020, 4, 1035

2. H.A. Dewi et al., Sust. Energy & Fuels. 2022, 6, 2428

3. E. Erdenebileg, et al Solar RRL, 2022, 6, 2100842

4. HA Dewi, et al., Adv. Funct. Mater. 2021, 11, 2100557

5. J.Li et al., Adv. Funct. Mater. 2021, 11, 2103252

6. E. Erdenebileg et al., Material Today Chemistry, 2023, 30, 101575

7. E. Parrott et al. Nanoscale, 2019, 11, 14276

8. KJ Lee et al., Nano Letters, 2019, 19, 3535

9.  KJ Lee et al., Advanced Materials, 2021, 33, 2005166

10. T. Antrack et al., Adv. Sci. 2022, 9, 2200379

Perovskite photovoltaics stand out as a top contender in the quest for new solar energy harvesting technologies. However, the presence of toxic lead in perovskite devices necessitates exploration of alternatives. The most popular of which has been tin-perovskites, known for their extremely fast and difficult to control crystallization. In this talk, a novel method of printing of tin-perovskite films will be presented, involving a crystallization trigger and subsequent control of nucleation and growth through solvent engineering. It was developed using an in-line optical spectroscopy characterization equipment providing analysis of the crystallization process in-situ. Photoluminescence, Raman and reflectance probes were installed and probed the corresponding signals to give information of film changes after printing and during post-printing treatment. The new method for printing tin-perovskite films enabled the fabrication of the first slot-die coated solar cell device, opening avenues for the development of fully printed, lead-free perovskite photovoltaics at an industrial scale.

Hybrid perovskites are one of the most promising materials for next-generation optoelectronic properties, including solar cells. However, the solution-based fabrication process brings to the formation of polycrystalline films, resulting in defects, which have a detrimental effect on the perovskite solar cell performance because they are expected to trap charge carriers, facilitating nonradiative electron-hole recombination. Hence, the investigation of the impact of defects on fundamental photo-physical properties becomes crucial for the optimization of the device.

In this framework, I have investigated the optoelectronic properties of three differently defective materials. More in details, I employed different amount of antisolvent and the passivation procedure to finely tune the defects concentration. In order to accurately evaluate defects concentration, I performed transient absorption measurement, and I analyzed the results considering the Burnstein-Moss effect. With the aim of investigating the correlation between defects and fluence regime, I used time resolved techniques, i.e. time resolved photoluminescence and transient absorption spectroscopy. In particular, time resolved photoluminescence highlighted that, by increasing the trap states concentration in the material, a shift in the onset for radiative regime arises at progressively higher fluences. On the other hand, transient absorption spectroscopy evidenced the strong correlation between the organic cation used for the passivation and the occurrence of the Auger regime. Therefore, for the material with lower trap states concentration, the range in which radiative recombination dominates is enlarged. Ultimately, this is reflected in the solar cell efficiency which is enhanced by 22% with respect to the higher defective material.

In organic solar cells, the charge-transfer (CT) electronic states are crucial in the charge-generation process, significantly influencing the overall device performance. They are formed at the interface between the electron-donor (D) and electron-acceptor (A) material, which usually exhibit significant electric fields.

In this study, we use a dedicated D-A system to tune intrinsic and extrinsic interface electric fields. It consists of two doped organic layers on top of each other, forming a planar organic p-n junction. By applying increasing voltages up to 6 V and introducing different thicknesses of intrinsic layers between 0 nm and 20 nm at the interface, the electric field at the interface can be deliberately varied. We observe substantial shifts in the CT-state peak emission, approximately 0.5 eV (150 nm), causing a transition from red to green color. This effect can be explained in a classical electrostatic picture, as the interface electric field superimposes the Coulomb interaction between the electron-hole pair. Our investigations illustrate the extent to which CT-state energies are influenced by their immediate electric environment. While CT state energy is often referred to as a fixed quantity, we want to emphasize that this understanding needs to be revised, especially for planar systems with a high degree of interfacial dipole alignment.