Recently molecular photovoltaics, such as dye sensitized cells (DSCs) and perovskite solar cells (PSCs) have emerged as credible contenders to conventional p-n junction photovoltaics. Their certified power conversion efficiency currently attains 25.5 %, exceeding that of the market leader polycrystalline silicon. This lecture covers the genesis and recent evolution of DSCs and PSCs, describing their operational principles and current performance. DSCs have meanwhile found commercial applications for ambient light harvesting and glazing producing electric power from sunlight. The scale up and pilot production of PSCs are progressing rapidly but there remain challenges that still need to be met to implement PSCs on a large commercial scale. PSCs can produce high photovoltages rendering them attractive for applications in tandem cells, e.g. with silicon and as power source for the generation of fuels from sunlight. Examples to be presented are the solar generation of hydrogen from water and the conversion of CO2 to chemical feedstocks such as ethylene, mimicking natural photosynthesis.

Halide perovskite semiconductors are generating enormous exciting for light-harvesting and light-emission technologies. One of the keys steps to maximising optoelectronic performance is through eliminating non-radiative recombination mechanisms and hence maximising luminescence yields. Here, I will detail recent work in our group exploring how to best make each photon count in absorbers, emitters and full device stacks. I will present a combination of modelling and experimental results investigating how photons interact within and between layers in device stacks such as tandems, including photon recycling effects and light coupling between layers -- leading to key device design parameters. Furthermore, I will present results on both solar cells and light-emitting diodes (LEDs) exploring routes towards suppressing non-radiative recombination and therefore maximising performance.

Starting from solutions, the formation of the perovskite structure for hybrid organic-inorganic components used for solar cells depends strongly on a multitude of parameters during processing. Similar to other known structure formation processes, also the perovskite formation can be divided into different stages. Additives can for example be used to suppress structure formation in a certain stage [1]. Equally the timing available of the various stages is relevant for the final structure as can be systematically shown by changing the processing atmosphere [ACS Appl. Mater. Interfaces, accepted] or processing method [2] and even individual processing parameters. Exploring the relevance of the individual steps in structure formation will help us understand how to achieve reproducible nanostructures and hence reliable material properties.

Halide perovskites have demonstrated exceptional potential for both light harvesting and light emission applications spanning photovoltaics, light-emitting devices, photodetectors, radiation detectors, memory devices etc. In particular, brightly luminescent perovskite colloidal nanocrystals with their fascinating tunable optical and electronic properties have garnered significant attention across broad disciplines. In this talk, I will focus on their exceptional photophysics such as hot carrier properties, long exciton diffusion lengths as well as nonlinear properties. I will distill their underlying mechanisms and highlight our group’s efforts in some of these areas. A succinct overview of the state-of-the-art as well as the prospective outlook will also be presented. 

Keywords: halide perovskite, colloidal nanocrystals, photophysics, nonlinear, carrier dynamics

References

M. J. Li et. al., “Slow Cooling and Highly Efficient Extraction of Hot Carriers in Colloidal Perovskite Nanocrystals”, Nature Communications 8:14350 (2017)

M. Righetto et al., “Hot carriers perspective on the nature of traps in perovskites”, Nature Communications 11: 2712 (2020),

J. Fu et. al., “Hot carrier cooling mechanisms in halide perovskites”, Nature Commununications 8:1300 (2017)

D. Giovanni et. al., “Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping”, Light: Science & Applications 10: 2 (2021)

W. Chen et. al., “Giant five-photon absorption from multidimensional core-shell halide perovskite colloidal nanocrystals”, Nature Communications 8: 15198 (2017)

Huajun He and Tze Chien Sum, “Halide perovskite nanocrystals for multiphoton applications”, Dalton Trans., 49, 15149-15160 (2020)

Inverted p-i-n perovskite solar cells (PSCs) are pivotal for fabricating perovskite-based tandem photovoltaics with the highest performance. However, severe non-radiative recombination at the perovskite/electron transport layer and at the grain boundaries (GBs) still limits their open-circuit voltage (V_OC) and fill factor (FF) as compared to their n-i-p counterparts. We introduce a novel dual passivation approach using phenethylammonium chloride (PEACl) to simultaneuosly passivate the GBs and the perovskite/C₆₀ interface by using PEACl:PbCl₂ as the additive and PEACl for surface treatment, respectively. [1] Employing the self-assembled monolayer [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) as hole-transporting layer and a methylammonium-free Cs_0.18FA_0.82PbI₃ perovskite absorber (bandgap ~1.57 eV), we achieve a substantially enhanced charge carrier lifetime and quasi-Fermi level splitting (~63 mV enhancement) compared to reference devices. By analyzing X-ray diffraction, cathodoluminescence, scanning electron microscopy, X-ray/ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy measurements, we attribute the positive effects to the formation of a thin heterogeneous 2D Ruddlesden Popper (PEA)₂(Cs_1−xFA_x)_n−1Pb_n(I_1−yCl_y)3_n+1 phase with n ~1-2 at the GBs and film surface. This leads to efficient chemical passivation of GB and surface defects as well as additional electronic passivation at the perovskite/C₆₀ interface due to hole-blocking. Our dual passivation strategy results in one of the highest reported PCEs for p-i-n PSCs of 22.7% with a remarkable V_OC and FF of 1.162 V and 83.2%, respectively. In addition, the dual passivated PSCs exhibit a strongly enhanced activation energy for ion migration together with an enhanced stability under light illumnation and prolonged heat treatment.

Ever since organic-inorganic halide perovskites have been employed in electronics and optoelectronics, their unique and excellent photophysical and electrical properties^[1–3] have guaranteed their versatile applications in solar cells, LEDs, lasers, photodetectors and beyond^[4,5]. Sn-based perovskites along with their surface and interface functionalization after assembling with charge transport layers (CTLs) are considered promising ways to achieve a win-win between environmental friendliness and cell performance^[6–10]. In view of the sophisticated chemical and physical properties of Sn-based perovskites, the theoretical calculation based on density functional theory (DFT) has been employed to feasibly and flexibly model the interplay between absorbers and CTLs. In order to understand the fundamental physico-chemical mechanisms of that interplay, the influence of surface termination on structural and electronic properties at FASnI₃/C₆₀ (as ETL) interfaces, in particular, the dependence of vacuum level (and by that work function) within surface alone and interface integrated frameworks, thereupon constructing the heterostructure energy level alignment, has been investigated thoroughly. The resulting type-II heterostructure as well as the increasing surface dipole and charge carrier mobility tell an effective charge transport from FASnI₃ to C₆₀. Our findings contribute to discovering other promising alternatives as ETLs for lead-free perovskites in the light of surface and interface engineering.

The spectacular rise of the photovoltaic power conversion efficiency of hybrid organic-inorganic perovskites to over 25% has conferred them a promising position in the field of photovoltaics and optoelectronics[1]. To further improve their device performance, there is a considerable ongoing research effort on tuning their interfacial energetics with charge selective contacts along with passivating and/or functionalizing their surfaces and interfaces[2]. Among possible charge selective contact materials, nickel oxide (NiO) is commonly used as a hole transporting layer in thin-film optoelectronic technologies based on organic or hybrid materials. Here[3], we computationally scrutinize the interfacial properties of the prototype MAPbI₃/NiO heterostructure, which has shown excellent photovoltaic performance and in particular a large open-circuit voltage[4]. We study the valence band energy level alignment between MAPbI₃ and NiO considering i) the defect-free system ii) the role of defects and iii) doping. To further highlight the influence of surface dipoles on work functions and absolute valence energy alignment, we present a theoretical methodology, bridging classical electromagnetism and modern atomistic approaches, to show their intimate connection[5]. We demonstrate the potentials of the methodology, through a variety of cases such as surface termination and passivation and/or functionalization. Our approaches to inspect the properties of heterojunctions, interfaces and surface dipoles transcend the limits of halide perovskites and provide computational strategies to fine-tune energy level alignments for optimizing the performance of broader families of optoelectronic devices.

To develop a detailed understanding about halide perovskite processing from solution, the crystallization processes are investigated during spin coating and slot-die coating of MAPbI3 at different evaporation rates by simultaneous in situ photoluminescence, light scattering, and absorption measurements. Based on the time evolution of the optical parameters it is found that for both processing methods initially solvent-complex-structures form, followed by perovskite crystallization. The latter proceeds in two stages for spin coating, while for slot-die coating only one perovskite crystallization phase occurs. For both processing methods, it is found that with increasing evaporation rates, the crystallization kinetics of the solvent-complex structure and the perovskite crystallization remain constant on a relative time scale, whereas the duration of the second perovskite crystallization in spin coating increases. This second perovskite crystallization appears restricted due to differences in solvent-complex phase morphologies from which the perovskite forms. The work emphasizes the importance of the exact precursor state properties on the perovskite formation. It further demonstrates that detailed analyses of multimodal optical in situ spectroscopy allows gaining a fundamental understanding of the crystallization processes that take place during solution processing of halide perovskites, independent from the specific processing method.

Highly efficient perovskite-based optoelectronic devices require the preparation of high-quality perovskite thin films. However, detailed understanding and rationalization of the solution-based perovskite processing are still lacking. Detailed process rationalization will lead to reliability and reproducibility in perovskite thin-film preparation, also on a large scale. Thus, here we rationalize the influence of the precursor solution concentration on the formation kinetics, which determines, e.g., the process window during the preparation process.

To rationalize the influence of the precursor solution concentration on the formation kinetics, the series of 0.5 M, 0.8 M, and 1.2 M Cs_0.05FA_0.85MA_0.10PbBr₃ (3CatPbBr₃) solutions is spin-coated. A home-built optical in-situ setup^[1] is utilized to follow the formation process of this series via in-situ UV-vis and PL measurements. A bromide-based perovskite composition is chosen since those form directly from solution without intermediate steps.^[2]

In-situ UV-vis measurements show an increase in the transflectance signal over the entire wavelength range instead of a clear absorption edge expected to form during spin-coating. However, during in-situ PL measurements, a defined PL peak increases fast in intensity while the peak position shifts to a longer wavelength. Over time the PL signal vanishes. For each concentration, the signal onsets correlate in time for both in-situ measurements, and their appearance is delayed increasing the concentration. Thus, complementary in-situ UV-vis and PL measurements reveal a strong concentration dependence of the 3CatPbBr₃ perovskite formation kinetics. Namely, a delayed crystallization onset accompanied by slowed-down crystallite growth is determined for higher concentrated solutions.

In addition, the concentration-dependent chemical characteristics of the solution concentration series are investigated by SAXS, NMR, and UV-Vis measurements. Concentration-dependent changes in the solution chemistry reason those slowed-down formation kinetics. Namely, the colloidal size, their interaction, and the individual chemical surrounding of the Pb²⁺ ion modify with the solution concentration.

Thus, this application example demonstrates the significance and importance of in-situ measurements during film formation. Overall, this connected study investigating the precursor solution chemistry and the corresponding formation kinetics identifies that slight modifications in the precursor solution impact the perovskite formation dramatically. Hence, even for slight variations in the precursor solution, preparation procedures need to be adjusted to achieve high-quality thin films. Thus, for complete process control, each detail influencing the perovskite formation, such as the precursor solution concentration, needs to be understood to fabricate high-quality thin-film by, e.g., aimed induction of the crystallization during an open process window.

Hybrid perovskite photovoltaics is a very promising, emerging thin-film technology that has not only rapidly increased in power conversion efficiencies of perovskite solar cells featuring a large range of bandgaps, but also promises low-cost industrial-scale fabrication due to its compatibility with solution processing and the abundancy of the precursor materials. However, the fabrication of perovskite modules on the industrial scale still faces major challenges. Besides the material’s toxicity due to lead incorporation and its instability with respect to stress factors like humidity, temperature and light (which could be addressed in the future by employing suitable encapsulation and product cycle strategies), the scalability and reproducibility of large-scale solution processing does not yet meet industrial standards. For one thing, morphological defects are very likely to occur in large-area deposition techniques such as slot-die coating, spray coating and inkjet printing. For another thing, the opto-electronic functionality of solution processed perovskite thin-films varies from batch to batch due to their sensitivity to the processing parameters such as temperature, humidity, lighting, etc.

In response to these issues, we propose a combined toolkit of modeling and monitoring. First, we in situ characterize the perovskite formation process starting from the precursor solution thin-films with high spatial and temporal resolution, that is, we evaluate multidimensional series of reflectance and (spectral) photoluminescence signals. Second, we correlate these properties with sophisticated models of the perovskite thin-film drying and crystallization. That is to say, we vary the processing parameters over a wide range and evaluate the respective response signals from the in situ characterization techniques in reference to the predictions of our models. In this way, we show that the drying, as measured by reflectance oscillations, is mainly controlled by the temperature and the local mass transfer coefficient (as well as the choice of solvents). Likewise, we demonstrate that the crystallization dynamics, as correlated with the photoluminescence response, depend on the drying rate and temperature (as well as the precursor materials). To sum up, we manage to understand and surveil the morphology formation in perovskite thin-films both on the lateral and on the temporal scale. This methodology could enable direct process control in the future, meaning that both spatial and temporal non-homogeneities in solution processing can be directly identified and corrected instantly by feedback controlling the processing parameters.

The key point in fabricating homogenous and pinhole-free metal halide perovskite films is controlling the nucleation and crystal growth process. For scalable coating and printing techniques, vacuum and gas flow-assisted drying processes turn out to be the most promising methods to induce favorable nucleation and crystallization. But, the exact interplay and nature of these processes are mostly unclear.

Our work provides the first deep insights into the crystallization dynamics of inkjet-printed metal halide perovskite thin films by optical in-situ monitoring [1,2]. We shed light on the so-called vacuum drying process, which enabled us to identify a novel process to print high-quality perovskite thin films. Contrary to the previously accepted notion that vacuum drying is the main contributor to perovskite crystallization, a decisive crystallization process is induced by the additional flow of gas over the sample. Moreover, we can show that this gas flow induces oriented crystallization during layer formation employing grazing incidence x-ray diffraction. The controlled preferential layer formation provides an effective route to fabricate high-efficiency perovskite solar cells. Utilizing this gas flow-assisted vacuum drying process, we find that regular, optically dense and pinhole-free MHP layers can be fabricated via inkjet printing, which yields solar cells with a power conversion efficiency of 16%, as compared to 4.5% for vacuum drying.

Nevertheless, further detailed analysis is necessary to fully understand the formation and nucleation process, and with this, establish the process to achieve performing larger area solar cells.

The high-efficiency perovskite solar cells (PSCs) are mostly fabricated in a glovebox under controlled inert environments.[1] Even though there are many advancements have been made in the PSCs, it still limits the operational longevity under ambient atmospheric conditions.  The formamidinium lead iodide (FAPI) perovskite, is the most appealing Pb-based 3D halide perovskite for solar cell application because of its narrow bandgap.[1,2] However, the FAPI perovskite a-black phase is not stable at room temperature and is challenging to stabilize in an ambient environment due to its thermodynamic stability limitation.[2] We show that pure FAPI PSCs show significantly higher stability when prepared under ambient air compared to FAPI PSCs fabricated under nitrogen. The concomitant use of the N-methylpyrrolidone (NMP) and the preparation under air conditions is demonstrated as an effective combination to structurally stabilize the a-phase of the material and to improve the film quality, which is handled in the air for the fabrication of the solar cell. However, the most dramatic change observed among cells fabricated in a different environment is in the long term stability of unencapsulated devices where the T80 parameter, increases from 21 (in N2) to 112 days (in ambient) to 145 days if PbS quantum dots (QDs) are introduced as additives in air-prepared FAPI PSCs.[3] Furthermore, by adding methylammonium chloride (MACl) the PCE reaches 19.4% and devices maintain 100% of the original performance during two months of storage at ambient conditions. The increase of stability for air fabricated FAPI PSCs is correlated to the presence of Pb-O bonds only in the FAPI films prepared in ambient conditions, thus opening the way to a new strategy for the stabilization in the air towards perovskite solar cells commercialization.

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. In a decade of intensive research it has been a huge improvement in the performance of these devices, however, the two main drawbacks of this system, the use of hazardous Pb and the long term stability, still to be open questions that have not been fully addressed. The photoconversion performance of perovskite solar cells containing alternative metals to Pb is significantly lower than the reported for devices containing Pb, where Sn-based perovskite solar cells is the alternative reporting higher photovoltaic performance close to 14%. Nevertheless, Sn-based perovskite solar cells exhibit a long term stability lower than their Pb containing counterparts, making stability their main problem. In this talk, we highlight how the use of proper additives can increase significantly the stability of formamidinium tin iodide (FASnI3) solar cells, and discuss about the different mechanism affecting this stability, beyond the oxidation of Sn2+, and how they can be countered.

Progress in perovskites solar cells has shown that internal conversion efficiency can be as high as 26% in 2021. The commercialization of these solar cells is hampered by stability, reproducibility, and scalability. Through sequential physical vapor deposition (SPVD), we have demonstrated reproducible and stable halide perovskites thin films for solar cells. Thin films such as methylammonium lead tri-iodide (MAPbI₃), methylammonium lead tri-bromide (MAPbBr₃), methylammonium lead iodide-bromide (MAPb(I_1-xBr_x)₃), and cesium lead tri-iodide (CsPbI₃) have been synthesized. The MAPbBr₃ films were found to be more durable on certain metal surfaces than on others [1]. Gold-zinc (Au-Zn) substrates exhibit the highest degree of film stability, while aluminum (Al) substrates display the lowest degree of film stability [1]. Besides, previous results revealed that MAPbI₃ produced a more efficient solar cell on zinc oxide (ZnO) electron transport layer (ETL) than on titanium dioxide [2]. However, MAPbI₃ rapidly degrades to lead iodide during post-annealing [3]. By SPVD, no post-annealing step was required to promote the degradation of the MAPbI₃. Thus, SPVD presents a promise towards large-scale, reproducible, and stable halide perovskite thin-film materials for solar cells.

Keywords:  methylammonium lead tri-iodide, methylammonium lead tri-bromide, sequential physical vapor deposition, methylammonium lead iodide-bromide, cesium lead tri-iodide, stability, perovskite solar cells.

The operational stability of perovskite solar cells (PSCs) is imperative for their commercialization. Although PSCs have almost reached the climax of device performance, their long-term operational stability remains a primary challenge for real-world applications. Despite significant progress through the engineering of the bulk and the interface layer in device configuration, the mechanisms underlying the degradation of HaPSCs under continuous illumination and heat stresses are still ambiguous. We report on the operational stability of devices made with PTAA; (PCE ≈19.32%) or sputtered NiO_x (PCE≈15.60%) as a hole-transport layer (HTL) under light (for >1000 h) at 20, 60, and 85 °C to unravel the degradation mechanisms. Our work suggests that degradation is initiated mainly by the deterioration of the HaP bulk at columnar inter-grains and the interfacial junction with the release of I₂ gas, which worsens the interface quality. Degradation of the PTAA device was accelerated by the interface deterioration and bulk decomposition initiated by the formation of voids and PbI₂ via iodine migration from defective regions at the columnar grain boundaries with the release of I₂ gas. The NiOx devices significantly improved the device stability with suppression of the HaP bulk degradation by alleviating internal defect dynamics. Capacitance–voltage analysis showed that the PTAA device develops a much wider defective interface layer than the NiO_x device, driven mainly by the chemical reaction of iodine with the interfacial layer. Thus, our results reveal that although the cracking of columnar inter-grains and defective spots in the perovskite bulk is the main origin of device degradation, the nature of the HTL also partly contributes catalysing bulk degradation.

Metal-halide perovskite semiconductors have received tremendous attention in research due to their excellent optoelectronic properties, making them interesting materials for solar cells and light emitting diodes (LEDs). It is fascinating that these perovskites are highly tolerant against electronic defects and at the same time show pronounced ionic conductivity mediated through mobile lattice defects.

In this talk an overview on the effect of mobile ions on solar cells and LEDs operated in pulsed mode is given. It is commented on the consequences, the interplay between electronic and ionic charges has on the interpretation of common characterization techniques such as photoluminescence and impedance spectroscopy.

An example discussed in more detail are lead-free Cs₂AgBiBr₆ double perovskite solar cells, whose efficiency-limiting processes remain a matter of research.

Hybrid organic-inorganic halide perovskite materials are promising for light emitting applications. In this talk, I will discuss our recent work on perovskite-based LEDs, where we have established a general protocol for preparing ultrathin, smooth, passivated, and pinhole free films of metal halide perovskites with various compositions, by incorporating bulky organoammonium halide additives to the stoichiometric 3D perovskite precursors. In addition, we have found that a major factor contributing to roll-off of perovskite LEDs is heating. By avoiding heating through multiple strategies, we are able to reduce roll-off and report record-bright perovskite LEDs, pushing toward display, lighting, and even lasing-relevant current densities.

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers.  Remarkably the organic spacers play crucial role in optoelectronic properties of these compounds. The characteristic for ionic crystal coupling of excitonic species to lattice vibration became particularly important in case of soft perovskite lattice. The nontrivial mutual dependencies between lattice dynamics, organic spacers and electronic excitation manifest in a complex absorption and emission spectrum which detailed origin is subject of ongoing controversy. First, I will discuss electronic properties of 2D perovskites with different thicknesses of the octahedral layers and two types of organic spacer.  I will demonstrate that the energy spacing of excitonic features depends on organic spacer but very weakly depends on octahedral layer thickness. This indicates the vibrionic progression scenario which is confirmed by high magnetic fields studies up to 67T. Finally, I will show that in 2D perovskites, the distortion imposed by the organic spacers governs the effective mass of the carriers.  As a result, and unlike in any other semiconductor, the effective mass in 2D perovskites can be easily tailored.    

Layered perovskites are promising materials for applications beyond solar cells in view of their extreme versatile architecture and simple processing.¹ The self-intercalation of organic moieties and inorganic slabs opens a door toward the engineering of efficient emitters from a single material.² In this work, we rationalize the design of the organic layer made by cations from the amine family and study how the accommodation of the functional group at the anchor side, as well as, the length of the tail affect the material lattice dynamic and light emission.³ We discover a new set of single layer white and blue-emitting platelets and we demonstrate that the careful selection of the organic cations allows tuning of the colour emission of the resulting materials, from blue to warm white and pure white colours. Our work provides experimental guidelines for the selection of organic cations in these materials that can be extended to other organic molecules and unravel new functionalities.

References:

Avi Mathur, Hua Fan, and Vivek Maheshwari. Mater. Adv., 2021,2, 5274-5299.

Xiaotong Li, Justin M. Hoffman, and Mercouri G. Kanatzidis. Chem. Rev. 2021 121 (4), 2230-2291

Dhanabalan, B., Biffi, G., Moliterni, A., Olieric, V., Giannini, C., Saleh, G., Ponet, L., Prato, M., Imran, M., Manna, L., Krahne, R., Artyukhin, S., Arciniegas, M. P., Adv. Mater. 2021, 33, 2008004.

Beyond the use in home and office-based printers, inkjet printing (IJP) has become a popular structuring and selective deposition technique across many industrial sectors. More recently great interest also exists in new industrial areas like in the manufacturing of printed circuit boards, solar cells, flexible organic electronic and medical products. In all these cases IJP allows for a flexible (digital), additive, selective and cost-efficient material deposition, which can be used in an in-line production process. Due to these advantages, there is the prospect that up to now used standard processes can be replaced through this low-cost innovative material deposition technique. However, using IJP as a production process in manufacturing, beyond the use in research laboratories, still requires rigorous development of cost and performance optimised functional electronic inks and processes, in particular those allowing for the fabrication on low-cost flexible substrates polyethylene terephthalate. By this means this important aspect also addresses the trend in industry for high-throughput, roll-to-roll device processing, where the use of common plastic substrates instead of glass poses problems concerning the thermal stability of the substrate and the mechanical stability of the deposited device layers, including the transparent conductive electrode (TCEs) against damages caused by substrate bending during the production and operation lifetime of the flexible devices. In this contribution we report on the design, realisation and characterization of printed TCEs as well printing processes for solution-processable metal halide perovskites in perovskite light emitting diodes and solar cells.

Light emitting materials are nowadays important in various applications and for many of them the development of graftable POOH emissive organic molecules combined with inorganic nanoparticle have opened new opportunities for future technologies. A possible way to enhance the emission of organic fluorophores and to control the size of solution-processable nanoparticles at the same time is their co-grafting with Oleic acid onto inorganic nanoparticle surfaces. In order to avoid the classical aggregation-caused quenching effect when high concentrations of organic ligands are present on nanoparticle surface, the opposite effect, namely aggregation-induced emission is a promising approach to generate strong light emission. Indeed, in this case, the high concentration helps to freeze the motion of the molecules to reduce the non-radiative deactivations. However, even though this new nanohybrid materials show strong emission, their solution-processing remains challenging and morphology control as nanoparticles suffer from large distributions over size and shape. Therefore to go towards efficient reproductible devices the transport and morphology limitations related to nanohybrids have to be controled. In this work we thus present the incorporation of nanohybrids in polymer host matrix PVK and oxadiazole. In order to have efficient LED devices, but also to control the emitted light by changing the organic part. By the co-grafting of different organic emitters on the nanoparticle surface we target to reach the white display on LED device.

Nanoscale perovskite materials present substantial advantages over large-grain perovskite thin films or bulk, such as high photoluminescence quantum yield, multi-exciton generation or enhanced defect tolerance. The synthetic colloidal approach is one of the most used methods to obtain ABX₃ nanocrystals. However, there are many drawbacks in their processing as thin film (purification and deposition steps, ligand interchange) that require special careful in order to preserve the excellent optoelectronic properties of the colloidal suspension. Here, we prepare ligand-free ABX₃ perovskite q-dots using a pore network of insulating porous matrices as nanoreactors. [1] The infiltration with perovskite precursors followed with a mild thermal treatment leads to nanocrystals embedded in the porous structure. From that way, we achieved strict perovskite size control in the sub 10 nm range that enables the observation of quantum confinement effects in their optical response.[2] Furthermore, the strong optical absorption besides the connectivity of the NCs leads us to integrate them in a solar cell device that reach 9% of photo-conversion efficiency in an alternative configuration to that employed in previously developed QD solar cells.[3] In addition, we demonstrate that the efficient charge transport between q-dots is due to a percolation mechanism and the stability of this type of cell is higher than standard bulk devices.

Continuous-wave optically pumped lasing from quasi-two-dimensional (2D) perovskite has been demonstrated recently. Understanding the key factors limiting the lasing performance is highly relevant for pursuing electrically driven lasing based on such solution-processed quasi-2D perovskite materials. In this work, we compare the photoluminescence (PL) and amplified spontaneous emission (ASE) of quasi-2D perovskite (CsPbBr3 with 80% butylammonium bromide (BABr) spacer) and its 3D counterpart formed by thermal removing the organic spacers. Surprisingly, the superior PL (at low excitation fluence) of quasi-2D perovskite does not translate into a lower ASE threshold (~600 μJ cm^-2). Annealing the quasi-2D perovskite into the 3D perovskite results in a lower ASE threshold (~130 μJ cm^-2) despite the much poorer PL efficiency at low excitation fluence of the 3D perovskite.

Power-dependent time-resolved PL revealed that the excited-state population of the quasi-2D perovskite based on the 80% BA spacer is dominated by excitons. This accounts for the superior PL at low fluence, as the excitonic emission is efficient at low excited-state densities, whereas the free-carrier-recombination-based PL of the 3D materials is a second-order process and needs higher excited-state densities to become efficient. However, at the rather low excited-state density of 10¹⁶ cm^-3, exciton–exciton annihilation sets in for the quasi-2D sample and results in a decreasing radiative efficiency at high excitation power. In contrast, the free-carrier radiative recombination leads to a high radiative efficiency steadily increasing to the transparency carrier density (10¹⁸ cm^-3), which explains its lower ASE threshold.

We further systematically investigate the ASE of a series of quasi-2D perovskites through varying the spacer type and concentration. The experimental results show that the optimum ASE threshold of quasi-2D perovskite films can be achieved by minimizing the surface roughness and thin QWs volume fraction through decreasing the spacer contents or utilizing 1-naphthylmethylamine bromide spacer. Time-resolved photophysical studies manifested that the increase of thick QWs volume fraction correlates with an increased contribution of free carrier radiative recombination to the total emission process of the quasi-2D perovskites. Our work suggests a detailed understanding of the excited-state population(s) in quasi-2D perovskites, and their dynamics is essential for engineering their gain performance. The concrete guidance for material development that our results suggest is that quasi-2D perovskite gain materials should target fast free carrier recombination by engineering the thickness and size of QW, but not maximum PL quantum yields under low power excitation.

Recently, owing to their excellent optoelectronic properties combined with an enhanced material stability, two-dimensional (2D) hybrid halide perovskites have emerged as an attractive alternative to 3D perovskites for photovoltaics [1][2] More, their ability to emit white-light at room temperature with a good color rendering projected this family of materials into the spotlight of potential future low-cost white LED components[3]. However, the underlying physical mechanisms behind these emissive properties are not fully understood. Herein we propose to study  basic optoelectronic properties that might be linked to the broad-band emission observed at different temperatures in a series of (C₆H₁₁NH₃)₂PbX₄ 2D Ruddlesden Popper perovskites (X= Br or I) [4] [5]. We rely on a coupling between advanced structural measurements and an atomistic approach based on the density functional theory (DFT). Indeed, as the temperature is decreased, phase transitions between undistorted and distorted structures were observed for the two compounds of the series by means of temperature dependent X-ray diffraction. While this crystallographic phase transition goes along with a measured polarization-electric field hysteresis for the bromide compound, more complex apparition of satellite incommensurate reflections was instead obtained for the Iodine one. We believe that these satellite reflections might also be connected to a low temperature ferroelectric ordering. In order to set an interplay between the structural distortions and the observed optoelectronic properties, theoretical studies intend to provide insights of the electronic band structures and the polarization characteristics for these compounds.

Despite that perovskite light emitting diodes (PeLEDs) have witnessed remarkable progress in their external quantum efficiency (EQE) reaching values > 20%, and half-lifetimes > 100 hours, a clear understanding of the role of ion migration on the optoelectronics characteristics of the PeLED and its stability is still lacking. Here, we demonstrate that ion migration processes which occur on timescales spanning different orders of magnitude, from milliseconds to hours, dictate PeLED performance. These processes are either reversible such that they are repeated each time the PeLED is measured, or irreversible where the PeLED is permanently activated or degraded compared to its initial state.

we show how the current and electroluminescence (EL) signals vary temporally in response to electrical bias and the relation between the extracted PeLED parameters and those transients.^[1] Then, by combining several experimental techniques such as transient photoluminescence, transient EL, electro-absorption, etc., with theoretical modelling, we were able to reach a simple physical model which explains the transient characteristics of the PeLED on different timescales, and relates them to migrating ionic species, most notable among them are the halides. Moreover, we show how ion migration plays an important role in PeLED degradation. By using several mitigation mechanisms such as using transport layers with good blocking properties, reducing Joule heating and device active area scaling, we demonstrate an operationally stable PeLED with half-lifetimes > 500 hours at 50 mA/cm² and a peak EQE of 11.4% at 330 mA/cm².^{[2, 3]}

[1] Elkhouly, K., Gehlhaar, R., Genoe, J., Heremans, P., Qiu, W., Perovskite Light Emitting Diode Characteristics: The Effects of Electroluminescence Transient and Hysteresis. Adv. Optical Mater. 2020, 8, 2000941. 

[2] Elkhouly, K., Goldberg, I., Boyen, H.-G., Franquet, A., Spampinato, V., Ke, T.-H., Gehlhaar, R., Genoe, J., Hofkens, J., Heremans, P., Qiu, W., Operationally Stable Perovskite Light Emitting Diodes with High Radiance. Adv. Optical Mater. 2021, 9, 2100586.

[3] Goldberg, I., Qiu, W., Elkhouly, K., Annavarapu, N., Mehta, A.N., Rolin, C., Ke, T.H., Gehlhaar, R., Genoe, J. and Heremans, P., Active area dependence of optoelectronic characteristics of perovskite LEDs. Journal of Materials Chemistry C. 2021.

In n-i-p type perovskite solar cells, during the oxidation of Spiro-OMeTAD layer, on the one hand, the diffusion of Li⁺ towards surface of Ag electrode completes the electrochemical cycle and increases conductivity of hole-transporting layer. On the other hand, the migration of Li⁺ through the perovskite layer into SnO₂, which supposedly leads to increase of the built-in voltage. ^[1] This type solar cells suffer an unpredictable catastrophic failure under operation, which is a barrier for commercialization. The fluorescence enhancement at Ag electrode edge and performance recovery after cutting the Ag electrode edge off proved that the shunting position is mainly located at the edge of device. SEM and TOF-SIMS analyses proved the corrosion of the Ag electrode and the diffusion of Ag⁺ ions on the edge for the aged cells. Moreover, much condensed and larger Ag clusters formed on the MoO₃ layer. Such a contrast was also observed while comparing the central and the edge of the Ag/Spiro-OMeTAD film. Hence, the catastrophic failure mechanism can be concluded as: photon-induced decomposition of the perovskite film causes the formation of reactive iodide species, which diffuse and react with the loose Ag clusters on the edge of the cell. The corrosion of Ag electrode and the migration of Ag⁺ ions into Spiro-OMeTAD and perovskite films leads to the forming of conducting filament that shunts the cell. The more condensed Ag cluster on the MoO₃ surface as well as the blocking of holes within the Spiro-OMeTAD/MoO₃ interface successfully prevent the oxidation of Ag electrode and suppress the catastrophic failure.^[2]

Cost-efficient, lightweight solar foils with high power-weight (W/g) values are the dream power source for private driven space exploration, planned satellite mega-constellations, and future habitats on Moon and Mars. Any application outside the earth’s protective atmosphere, however, places enormous demands on material and device stability. While shortwave UV light, atomic oxygen (AtOx), and low-energetic e^-, p⁺ radiation can be shielded easily, high energetic irradiation will damage used semiconductors.

In this presentation, we discuss all-perovskite tandem solar cells that offer low-weight, high-efficiency, and high power-weight attributes, about five times larger than commercially available, industry-standard III-V semiconductor on Ge triple-junction space solar cells. We show that all-perovskite tandem PV possesses a remarkable radiation tolerance. Our tests under 68 MeV proton irradiation revealed negligible degradation (< 6 %) at a dose of 10¹³p⁺cm². Their resilience thus exceeds not only previously tested perovskite/CIGS tandem PV¹ but also commercially available radiation-hardened space PV (> 22%) that we tested under identical conditions.

Using sub-cell selective high-spatial-resolution PL microscopy & intensity dependant absolute PL measurements, we then bring to light the fundamentally different origin of radiation damage in traditional III-V semiconductor-based PV systems compared to halide perovskite-based tandem PV. Pseudo-JV measurements constructed from optically measured quasi-Fermi level (QFLS) splitting of high-and low-gap perovskite absorbers prior to and after proton irradiation reveal no degradation, suggesting that further improvements of their radiation resilience are possible with optimized contact layers.

Metal halide perovskite light-emitting diodes (PeLEDs) show great potentials to be the next-generation lighting technology, with external quantum efficiencies (EQEs) exceeding 20% for infrared, red and green LEDs. However, the efficiencies of blue and white devices severely lag behind. To improve the performance of blue PeLEDs, we employed an integrated strategy combining dimensional engineering of perovskite film and recombination zone modulation in the LED device to obtain an EQE up to 5%.[1] While further incorporating the strategy of interfacial engineering, highly efficient blue PeLEDs with EQEs over 10% have been successfully realized in our group, establishing an excellent platform for white-light emission. In our latest work, we demonstrated efficient white PeLEDs by optically coupling a blue PeLED with a red emitting perovskite nanocrystal layer in an advanced device structure, which allows to extract the trapped optical modes (waveguide and SPP modes) of blue photons in the device to the red perovskite layer via near-field effects. As a result, a white PeLEDs with EQE over 12% is achieved, which represents the state-of-the-art performance for white PeLEDs.[2]

 

Solution-processed metal halide perovskites have emerged as highly promising materials for light-emitting devices such as LEDs and lasers. The external quantum efficiency (EQE) of perovskite LEDs (PeLEDs) has increased rapidly since its first demonstration, and optically pumped perovskite lasers under continuous-wave operation at room temperature has been achieved. Although much research has been devoted to improving the EQE of PeLEDs, their efficiency starts to drop significantly under high injection current density (J). Suppressing efficiency roll-off at high J is required for achieving ultra-bright LEDs, and ultimately, electrically driven lasers since the key parameter of J×EQE or luminance needs to be large enough to exceed a threshold for lasing. The detrimental behavior of efficiency roll-off is likely caused by a combination of charge injection imbalance, Auger-induced luminescence quenching and Joule heating. This phenomenon, also known as efficiency droop, can be quantified by the critical current density (J_c) at which the EQE reduces to half of its peak value. In this talk, we discuss our work in improving J_c and maximum luminance of PeLEDs. We first optimize the hole transport layers for balancing charge injection in quasi-2D PeLEDs, and achieved simultaneously high EQE (16.2%) and luminance (~31,000 cd/m²). Nevertheless, we ran into a limit in increasing J_c of quasi-2D PeLEDs, likely due to the poor charge transport with the insulating long-chain PEABr layer and increased non-radiative Auger recombination rate originated from the enhanced local charge carrier density in the multiple quantum well structures. These motivated us to further contrast the performance of quasi-2D and 3D perovskites. 3D perovskites generally can withstand higher J; however, their photoluminescence quantum yield (PLQY) is usually lower due to small exciton binding energy. We explored surface passivation to improve the PLQY of 3D perovskites by introducing excess KBr during CsPbBr₃ synthesis. The resulting luminance improves by about 4-fold to ~120,000 cd/m² and J_c increases by 20-fold to ~800 mA/cm². Subsequently, we investigated thermal-induced luminescence quenching and fabricated current-focusing devices with nanoscale current injection areas. Using pulsed current injection, we increased J_c up to ~60 A/cm² while operating the device at J values up to ~1 KA/cm² without measurable damage to the device, ultimately leading to a luminance of 7.6 Mcd/m². We further explored the effect of substrate heat conductivity by comparing glass and sapphire substrate based PeLEDs, and showed that the device performance can be further improved using sapphire substrates.

Slot-die technology for scalable solution-processed PV devices.

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Multinary semiconconducting / hybrid materials like perovskites are usually deposited in wet-chemical or evaporation processes. Growth kinetics and reaction paths of these deposition methods generally exhibit a high degree of complexity and are influenced by effects such as diffusion, phase transitions and numerous chemical reactions. Due to this complexity, process optimization for these materials usually takes place in a very empirical instead of analytical, knowledge-based manner. Accordingly, the learning curve bears a great potential for further acceleration. Visualizing and modelling the effects underlying growth and deposition is key for an analytical way of optimization, as it provides the base data for modelling and obtaining in-depth understanding of the processes. At the same time, such data, if obtained “in-situ”, can also be used for controlling and automating the deposition process. Optical metrology methods like in-situ reflectance spectroscopy are ideally suited for these purposes. They are directly linked with the absorption properties of the material, which again directly depend on composition, crystallinity and roughness, and they do not influence the deposition process. In order to obtain the best possible in-situ data for a given process, the metrology system needs to be engineered for optimum performance, (choosing the appropriate optical design, wavelength range, resolution and acquisition speed). In this talk we will demonstrate how, with an appropriately designed system, in-situ metrology can be used for visualizing and analyzing the growth kinetics of wet-chemical and evaporation processes.