Despite the incredible progress made, the highest efficiency perovskite solar cells are still restricted to small areas (<1 cm2). In large part, this stems from a poor understanding of the widespread spatial heterogeneity in devices. Conventional techniques to assess heterogeneities can be time consuming, operate only at microscopic length scales, and demand specialized equipment. We overcome these limitations by using luminescence imaging to reveal large, millimeter-scale heterogeneities in the inferred electronic properties. We determine spatially resolved maps of “charge collection quality”, measured using the ratio of photoluminescence intensity at open and short circuit. We apply these methods to quantify the inhomogeneities introduced by a wide range of transport layers, thereby ranking them by suitability for upscaling. We reveal that top-contacting transport layers are the dominant source of heterogeneity in the multilayer material stack. We suggest that this methodology can be used to accelerate the development of highly efficient, large-area modules, especially through high-throughput experimentation.

Two-dimensional Ruddlesden-Popper perovskites (2D-LRPPs) are an interesting material class with potential applications in the field of optoelectronics [1]. The 2D-LRPP semiconductor includes a metal-halide inorganic octahedral layer separated by large organic cations. This configuration provides quantum confinement and strong dielectric effects [2]. In 2D-LRPPs, the decreased mobility of halide anions in contrast to 3D counterparts limits the interdiffusion across heterostructures, which ultimately could lead to more stable heterostructures even during external perturbations like light and heat [3]. This report presents a scalable solution-based two-step methodology to fabricate microcrystalline PEA₂PbX₄-PEA₂PbX₄ (X = Br, I) lateral heterojunctions at room temperature. The fabrication of the high band gap PEA₂PbBr₄ semiconductor at the edges of the low band gap PEA₂PbI₄ semiconductor, and vice versa, lead to a band alignment that could enable a directional energy and charge flow from the high band gap material toward the low band gap material. Different anion sources, solvent mixtures, and anion concentrations influence the quality of the formed heterojunction. The results indicate that the formation of the lateral heterojunction is based on an anion exchange for PEA₂PbI₄-PEA₂PbBr₄ and dissolution-recrystallization processes for PEA₂PbBr₄-PEA₂PbI₄. The fabrications of such 2D-LRPPs heterostructure offer an opportunity to investigate charge- or energy-transfer phenomena at the heterojunction and the interchange between these materials, which could lead to their applications in optoelectronics, and among others.

Despite the meteoric rise in the development of a variety of perovskite electronic and optoelectronic devices, the phenomenon of ion migration remains a common and longstanding Achilles’ heel limiting their performance and operational stability. In particular, ionic screening of the applied gate potential especially near room temperature reduces the gate modulation of carriers in the semiconducting channel of lead (Pb) perovskite thin film transistors (PeTFTs), resulting in inferior carrier mobilities and non-idealities in device characteristics [1]. Similarly, ionic movements under light and/or bias have been shown to result in current-voltage hysteresis [2], open circuit voltage gain [3] and short circuit current losses [4] in operating Pb perovskite solar cells (PSCs). Despite several efforts in the literature to mitigate such effects through compositional and additive engineering, there appears no clarity on the impact of tin (Sn) substitution on the resulting ion dynamics of Pb halide perovskites.

In this talk, I will share our research on understanding the ionic transport in methylammonium (MA)-free mixed Pb-Sn perovskites using PeTFTs and PSCs as two different platforms. Firstly, we demonstrate that mixed Pb-Sn based PeTFTs do not suffer from ion migration effects as significantly as their pure-Pb counterparts, thereby reliably exhibiting hysteresis-free p-type transport with high mobility reaching 5.4 cm²/Vs, which is one of the highest reported TFT mobilities for hybrid perovskite thin films [5]. The reduced ionic migration in mixed Pb-Sn PeTFTs is manifested in the relative invariance of the current-voltage hysteresis as well as ON-current for a range of scan rates and improved temporal stability of the ON-current with time. Moreover, the observation of an activated temperature dependence of the field-effect mobility with low activation energy (< 50 meV for T > 200 K) in the case of mixed Pb-Sn compositions (as opposed to the negative temperature coefficient resulting from ionic screening effects in the pure-Pb composition) is consistent with the presence of shallow defects present in these materials, thereby reinforcing the suppressed role of ionic transport in dictating the charge transport in these materials. In fact, to the best of our knowledge, such activated temperature dependence of FET mobility is also reported for the first time in the field of 3D halide perovskites. Furthermore, by performing photoluminescence microscopy under bias on lateral two-terminal devices, we visualize the suppressed in-plane ionic migration in Sn-containing perovskite compositions compared to their pure-Pb counterparts.

Next, we performed scan-rate dependent current-voltage (and hysteresis) measurements and temperature dependent impedance spectroscopy measurements on optimized MA-free Pb and Pb-Sn perovskite solar cells, which demonstrate the suppressed motion of ions in Pb-Sn devices as compared to their Pb-only analogues, thereby generalizing our earlier observations from PeFETs [6]. We also obtain mechanistic insights into these experimental observations by conducting first-principles calculations, which reveal the key role played by Sn vacancies (with low formation energy) in increasing the migration barrier for iodides due to severe local structural distortion in the lattice.

All in all, our results emphasize the relatively unexplored bright prospect of tin substitution in obtaining improved lateral charge transport and suppressed ionic migration effects in Pb halide perovskite devices.

References

* denotes equal-contributing first author.

[1] Senanayak, S. P. et al. Sci. Adv. 2017, 3, e1601935.

[2] Snaith, H.J. et al. J. Phys. Chem. Lett. 2014, 5, 1511-1515.

[3] Herterich, J. et al. Energy Technol. 2021, 9, 2001104.

[4] Thiesbrummel, J. et al. Adv. Energy Mater. 2021, 11, 2101447.

[5] Senanayak, S.P.* and Dey, K.* et al. Nat. Mater. 2023, 22, 216–224.

[6] Dey, K. et al. arXiv 2023.

Nanocrystals (NCs) have become a key component in lead halide perovskite-based devices due to their outstanding optoelectronic properties. While colloidal synthesis has reached an impressive level of maturity in less than a decade allowing for the fabrication of stable NCs with near unity quantum yield (QY), other fabrication approaches have emerged over the past few years. Among them, the synthesis within nanoporous matrices allows obtaining stable and ligand-free pNCs within thin films with optical quality perfectly suited for integration into devices. [1] Beyond the possibility of employing these films as active layers for solar cells with good efficiency [2] they represent an ideal test bench for exploring light matter interaction at the nanoscale in lead halide perovskites without ligand or solvent induced effects characteristic of colloidal NCs. [3]

In this work, we present a study on charge carrier recombination in FAPbBr₃ NCs assemblies synthesized within metal-oxide porous matrices where interparticle distance is controlled via changes in the pore filling fraction. A broad photophysical characterization comprising time resolved absorption and emission spectroscopies as a function of temperature and fluence is employed to study the different recombination pathways of photogenerated carriers. We observe a transition from isolated emitters to interconnected NCs that preserve confinement effects while allowing for long range electronic transport. We show how interconnectivity determines charge carrier dynamics as it affects trap filling, radiative recombination and multiparticle interactions. These results are discussed in terms of recombination and diffusion models for both the isolated and interconnected case and study the transition between these scenarios. These findings provide an insight into the photophysical properties of an exciting nanostructured material in a configuration suited to be incorporated into a device.

The need for self-powered electronics is progressively growing in parallel to the flourishing of the Internet of Things (IoT). Although batteries are dominating as powering devices, other small systems are attracting attention, such as piezoelectrics, thermoelectrics and photovoltaics. These last ones can be adapted from their classical outdoor configuration to work preferentially under indoor illumination, i.e. through the harvesting of the spectrum emitted by LEDs and/or fluorescent lamps. However, crystalline silicon, the classical photovoltaic material for solar panels, has a bad gap not suitable for ensuring good efficiency with such spectra. Other semiconductors, with wider band gaps, can come into play for this task. Still, the materials of choice, having to be integrated within households, should also satisfy the criterion of non-toxicity, as well as maintain low-costs of production. While lead- based halide perovskites cannot represent a valuable solution for this scope, due to the strong environmental and health concerns associated to the presence of Pb, analogous compounds based on the heaviest pnictogen, i.e. bismuth, could work as sustainable light-harvesters for indoor photovoltaic devices.

In this contribution, we will show our most recent results obtained from the integration of the double perovskite Cs2AgBiBr6 in carbon-based perovskite solar cells, devices characterized by a high degree of sustanaibility, also due to the use of recycled materials within the carbon eletrodes. Similar technologies might have interesting applications for powering the IoT within private households or other indoor environments.

We will also show our studies on the use of a perovskite-inspired material, the oxyhalide BiOI, produced from a fully aqueous-based deposition method, as photoanodes for the oxygen reduction reaction as well as for the light-driven charging of liquid electrolytes for flow cell batteries.

 Growing demand for ultralow detection limits in medical radiography, high-energy physics, and security screening has spurred extensive research on X-ray imaging scintillators and detectors. Current market offerings of high-performance ceramic scintillators require harsh and costly fabrication techniques. In contrast, perovskite nanosheets and copper nanoclusters offer unique optical properties and high X-ray absorption cross-sections, making them promising alternatives. This presentation will discuss the room-temperature synthesis of colloidal CsPbBr3 nanosheets, exhibiting superior scintillation performance due to efficient energy transfer processes between stacked thin and thick nanosheets. Moreover, by combining CsPbBr3 perovskite nanosheets with thermally activated delayed fluorescence (TADF), reabsorption-free organic X-ray imaging scintillators were developed, offering a low detection limit of 38.7 nGy/s and exceptional imaging resolution. Cu-based halide nanoclusters composed of Cu4I6 nanoparticles and nanorods exhibit ultrahigh photoluminescence quantum yields, low detection limits of 96.4 and 102.1 nGy/s - about 55 times lower than standard medical diagnosis doses (5.5 μGy s–1) - and extraordinary X-ray imaging resolutions exceeding 30 lp/mm, more than doubled compared to conventional CsI:Tl and Ga2O2S:Tb scintillators. This research paves new avenues for developing high-resolution X-ray imaging screens based on Cu-based halide nanoclusters for applications in medical radiography and non-destructive detection.

Perovskite semiconductors are emerging low-cost materials for photovoltaics, light emitting devices and detectors. Because of the inclusion of high atomic numbered elements, perovskites are promising candidates for high efficiency X-ray sensing. In this talk, I will discuss the properties of perovskite semiconductors for X-ray and visible photon sensing. Firstly, we report a long carrier diffusion length in 2D perovskite single crystals, assisted by the shallow trap and de-trapping process. Next, we show that such a long diffusion length ensures a full charge collection after charge ionization, which is beneficial for detectors for X-ray and other photons. In addition, we have found the shallow trap also extend the carrier transport lifetime that facilitate a charge multiplication in the detector driven under high voltages. Such a process introduces a photo conductivity gain, leading to an unusually high X-ray and visible photon sensing efficiency. A high gain can be also achieved by building a hetero-structured device. Interfacing perovskites with a high mobility graphene channel can also multiplicate the photo-generated carriers. With a hetero-structured device, we show a high X-ray sensitivity over 108 µCGy-1cm-2.

Organic–inorganic Pb-free layered perovskites are promising green solution for lighting applications. In particular, Sn-based layered structures are highly efficient photoluminescent materials, but their emission color is limited to the yellow-orange range of the visible spectrum and their synthesis requires a controlled atmosphere and long preparation time. Here, we developed a synthesis protocol, which is performed in three key steps under ambient conditions and at low temperatures (4 °C). This simple synthesis approach enabled to investigate of a set of organic cations with different molecular structures, including both conventional alkylammonium and cyclic cations with heteroatoms, which resulted in stable structures showing different color coordinates, from yellowish-orange to orange-red, to blue-green. The observed variations in color chromaticity are attributed to the distinct connectivity of Sn–Br octahedra, ranging from fully disconnected to face-sharing, while retaining the intercalation of organic-inorganic layers, as confirmed by X-ray and 3D electron diffraction analyses. Our findings can inspire further research into the tunability of the emission of Pb-free layered perovskites via the use of organic cations to promote their use in solid-state lighting.[1]

Halide perovskite (HaP) semiconductors, commonly synthesized from room temperature solutions, possess exceptional optoelectronic properties that rival those achieved by more complex fabrication methods used for conventional semiconductors. The absence of detrimental defects in HaPs is a topic of debate, often attributed to their defect tolerance or self-healing ability.

To contribute to this discussion, we conducted an experimental investigation focused on determining the absolute volume deformation potential (AVDP) of CsPbBr3. The AVDP is a crucial physical parameter that characterizes the energy level shift of a semiconductor in response to volume changes. It therefore allows to quantify the amount of energy necessary to for example rearrange a crystal lattice locally to efficiently screen the electrical energy barrier of defects. Furthermore, it provides insights into the inherent molecular orbital bonding nature of a semiconductor material.

In our study, synchrotron radiation-based X-ray photoelectron spectroscopy was employed to measure the VBM (valence band maximum) energy of CsPbBr3 across a temperature range from room temperature to 125 K. Our experimental findings demonstrate that the AVDP of CsPbBr3 is negative and relatively small compared to conventional semiconductors. This observation suggests that electronic defects can be easily screened through lattice rearrangement. Moreover, the negative sign indicates that the valence band maximum primarily consists of anti-bonding type molecular orbitals. The disruption of these bonds typically generates defect energy levels near or within the bands. Both the magnitude and sign of the AVDP support the notion of defect tolerance in Halide perovskites.

Additionally, we conducted measurements of transient photoluminescence and employed a comprehensive interpretation based on a kinetic rate equation encompassing various recombination processes of different orders. This allowed us to quantify the defect density in CsPbBr3 at similar temperatures, providing a deeper understanding of the evolution of defect properties under volumetric changes.

Our results offer valuable insights into the origins of defect tolerance in Halide perovskites, shedding light on their unique optoelectronic characteristics.

Narrow bandgap perovskite solar cells based on mixed lead-tin perovskites tend to suffer from poor stability under operating conditions. This impedes the successful development of all-perovskite tandems.  We explore the causes of this instability under extended periods of combined 65°C thermal and 1 sun illumination stressing, using a range of structural, optical, and electronic characterization techniques on lead-tin perovskite films, half-stacks and devices.

We show that the phase, absorbance, morphology and mobility of lead-tin perovskite films are stable on timescales that exceed those of device degradation, although we reveal an interesting pattern of phase segregation after stressing for much longer amounts of time. Additionally, we observe only a slight increase in background carrier density and a moderate decrease in charge lifetime over the first few hundred hours of stressing. We argue that these changes likely only partially account for the observed device degradation.

Investigating the EQE and J-V characteristics of devices reveals the formation of a charge extraction barrier in aged devices. We find that the impact of this barrier is hugely decreased in very fast J-V scans, suggesting that mobile ions contribute significantly to device degradation. Device simulations enable us to weigh the impact of all of these changes on photovoltaic performance. We are able to identify more closely the various processes that limit the stability of lead-tin perovskite solar cells. Finally, we propose solutions related to both bulk perovskite composition and device architecture to overcome these challenges.

Tin-lead (Sn-Pb) halide perovskites stand out as top candidates for future near-infrared (NIR) optoelectronics, being particularly promising in all-perovskite tandem solar cells and NIR photodetectors. However, their facile degradation under operational conditions (e.g., ambient-mediated oxidation of Sn²⁺) remains the main impediment towards their widespread deployment. To enable the design of Sn-Pb perovskite compositions with higher innate stability, it is critical to unravel how the constituent ions of perovskite besides Sn²⁺ participate in the degradation of these materials. Specifically, the inconspicuous role of A-site cations in the decomposition of technologically relevant ASn_0.5Pb_0.5I₃ perovskite structures has been largely overlooked.

In this talk, I will describe the effect that A-site cation tuning has on the degradation mechanism under ambient air of Sn-Pb perovskite compositions commonly employed in solar cells, i.e., Cs_xMA_0.3-xFA_0.7Sn_0.5Pb_0.5I₃ (where x = 0, 0.15 and 0.3; MA = methylammnonium; FA = formamidinium). By employing thermogravimetric and spectroscopic techniques, we track the formation of molecular iodine (I₂) and tin(IV) iodide (SnI₄) versus time as indicators of perovskite degradation caused by exogenous O₂ and endogenous I₂, respectively. We detect the rate of I₂ and SnI₄ generation to be approximately one order of magnitude lower for Cs-rich compositions relative to their MA-rich counterparts, clearly indicating that MA replacement by Cs leads to enhanced resilience against oxidative stress. Consequently, this translates into higher stability of optical, electrical and structural properties in Cs-rich perovskite thin films upon air exposure. The talk will conclude with details on the origin of the lower stability of MA-rich compositions, which we ascribe to stronger I₂ adsorption at the surface of perovskite mediated by the polarising power of the MA cation. This work provides key insights on the role of A-site choice on perovskite degradation that serve as valuable guidelines for the design of Sn-Pb perovskite optoelectronics with enhanced stability.

Bismuth-based semiconductors have gained increasing attention as potential nontoxic alternatives to lead-halide perovksites [1]. Whilst early works focussed on the role of defects (particularly defect tolerance) in these materials, recent work has emphasized the important role of electron-phonon coupling [2]. This talk examines electron-phonon coupling in two emerging bismuth-based perovskite-inspired materials: NaBiS₂ and BiOI.

NaBiS₂ is part of a growing family of ternary chalcogenides. We show NaBiS₂ to be phase-stable in ambient air for 11 months, with high absorption coefficients >10⁵ cm^-1 reached from its optical bandgap of 1.4 eV. As a result, a 30 nm thick film has a spectroscopic limited maximum efficiency of 26%, higher than lead-halide perovksites or established thin film solar absorbers. However, we show through ultrafast spectroscopy that the photogenerated charge-carriers in NaBiS₂ slowly decay on a microsecond timescale, and yet the photoconductivity decays within 1 ps. This arises due to carrier localization, and we rationalize this as due to inhomogeneous cation disorder, leading to the formation of S 3p states just above the valence band maximum that facilitate the formation of small hole polarons [3].

The second part of the talk examines BiOI. We show through detailed spectroscopic measurements and state-of-the-art computations that this material is an exception to recent Bi-based perovskite-inspired materials and avoid carrier localization. As a result, mobilities exceeding 80 cm² V^-1 s^-1 are achieved, along with high mobility-lifetime products exceeding 10^-2 cm² V^-1 in single crystals. We show that BiOI photoconductors are highly promising radiation detectors, capable of resolving dose rates down to 22 nGy_air s^-1, which is well below the current medical standard of 5500 nGy_air s^-1.

Layered double perovskites (LDPs) have emerged as a highly promising alternative to traditional single-metal perovskites. Their distinct advantages include improved stability, reduced toxicity, and unexplored chemical space, making them an exciting area of research.

This talk aims to delve into our extensive investigations on the chemistry and diversity of LDPs, also referred to as double metal vacancy-order perovskites. We will discuss our efforts to comprehend their unique properties and photophysics. The scope of our studies encompasses various aspects such as crystal structure analysis, examination of optical properties, evaluation of stability, and exploration of the photoluminescence mechanism.

Furthermore, we will shed light on the current and potential applications of LDPs in the fields of photovoltaics and optoelectronics. The multifaceted nature of LDPs makes them a compelling choice for advancing these technologies. By uncovering their fundamental characteristics and harnessing their unique features, we can pave the way for innovative and sustainable solutions in the realm of energy conversion and light-based applications.

Perovskite solar cells attract attention as promising cost-effective next generation printable photovoltaics.

The power conversion efficiencies (PCEs) have been substantially increased in a short period, based on the improvements of the fabrication protocols for the perovskite layer, and the development of new materials for passivation of the surface or efficient charge collection, etc.

Our research approaches for lead and lead-free perovskite solar cells are as follows.

1) Development of highly purified perovskite precursor materials: We synthesis a series of complexes of lead halides or tin halides as purified precursor for perovskite materials.

2) Development of fabrication methods for perovskite layer by solution process: We focus on the intermediates formed during the solution process and develop efficient fabrication methods [1-3] including surface passivation based on the mechanism.[4-7]

3) Development of organic semiconductors as efficient charge-collection materials from perovskite layer: We design and synthesis novel pi-conjugated materials, including PATAT,[8] in terms of control of the energy level of frontier orbitals, molecular orientation, and interface between perovskite layer.

In this talk, our recent progress on Sn-based perovskite solar cells as well as Pb-based perovskites will be introduced.

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. Among other properties direct and tunable bandgap as well as low non-radiative recombination have been a key factors in this success. Moreover, these properties are also fundamental for the development of other optoelectronic devices, which has also extended the application of halide perovskites to other fields such as LEDs, lasers or photocatalytic systems. Nevertheless beyond the interest of this family of materials, and despite a decade of intensive research with improvement in the performance of perovskite devices, 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. Sn-based perovskite solar cells are the devices presenting the highest performance after Pb-based but significantly below them. In addition, 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 and light soaking for defect engineering 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. As a second topic of the talk, the use of halide perovskite nanocrystals for the preparation of reproducible perovskite LEDs (PeLEDs) will be analyzed as well as the fabrication of Pb-free Sn-based LEDs by inkjet printing. Improvement of both efficiency and stability of the fabricated devices will be analyzed.

The development of high performance lead-free perovskite solar cells (PSCs) is important to address the environmental concern of heavy metal lead. In recent years, tin perovskite solar cell (TPSCs) is developing quickly and emerging as a promising candidate for high efficiency lead-free PSCs. Meanwhile, the narrow bandgap of tin perovskite solar cells enables it to be used for the fabrication of tandem solar cells. In this presentation, I will summarize recent work of our group about quasi-2D tin halide perovskite materials and devices. I will introduce the basic properties of low dimensional tin perovskite, crystal growth kinetic control to manipulate its nanostructure and orientation, as well as device structural engineering to reduce interface carriers recombination. Based on these efforts, the highest efficiency of tin perovskite solar cells is up to 14.6% that is certified in an independent agency. In addition, the quasi-2D structure is also applied for the fabrication of inorganic tin perovskite solar cells. Meanwhile, quasi-2D structure is applied for surface treatment of tin-lead mixed perovskite solar cells to reduce surface defects and ensure effective interface carrier transport. This interface design brings effectively enhanced device performance for both perovskite single junction and tandem solar cells.

Solar energy conversion is a fundamental pillar in the ongoing ecological transition towards a sustainable economy and society. Emerging photovoltaic (PV) technologies such as perovskite solar cells (PSCs) inspire a vast scientific community due to their intrinsic advantages with respect to entrenched Si-based PVs: high efficiencies, low materials cost, easy manufacturing, and low energy/mass requirements, all resulting in a lighter environmental footprint. In these systems, the conversion of light into electricity requires charge transport (CT) across several materials. Due to the inherent complexity of the functional interfaces involved, CT key features are difficult to understand holistically solely from experimental outcomes, which hinders a rational design of new devices with better performances. For this reason, the application of computational modeling tools with atomistic resolution represents an ongoing revolution in materials design and optimization for PSCs.

This contribution will discuss how DFT-based approaches are able to assess the CT features across several interfaces between optically active materials and charge-collector layers in different electrochemical environments, which are the core events for PV functioning. In particular, we will discuss heterogenous interfaces between lead halide perovskites and inorganic and organic hole transport materials [1, 2], from the analysis of electronic structures to the determination of CT dynamics and rates [3] and comment on the composition-properties relationships in attractive lead-free perovskite-inspired absorbers [4] targeting indoor PV devices.

[1] Pecoraro, A.; De Maria, A.; Delli Veneri, P; Pavone, M.; Muñoz-García, A. B.  Interfacial electronic features in methyl-ammonium lead iodide and p-type oxide heterostructures: new insights for inverted perovskite solar cells. Phys. Chem. Chem. Phys. 2020, 22, 28401-28413.

[2] Mäkinen, P.; Fasulo, F.; Liu, M,; Grandhi, G. K.; Conelli, D.; Al-Anesi, B.; Ali-Löytty, H.; Lahtonen, K.; Toikkonen, S.; Suranna, G. P.; Muñoz-García, A. B.; Pavone, M.; Grisorio, R.; Vivo, P. Less Is More: Simplified Fluorene-Based Dopant-Free Hole Transport Materials Promote the Long-Term Ambient Stability of Perovskite Solar Cells. Chem. Mater. 2023, 35, 2975-2987.

[3] Pecoraro, A.; Fasulo, F.; Pavone, M.; Muñoz-García, A. B. First-principles study of interfacial features and charge dynamics between spiro-MeOTAD and photoactive lead halide perovskites. Chem. Commun. 2023, 59, 5055-5058.

[4] Lamminen, N.; Grandhi, G. K.; Fasulo, F.; Hiltunen, A.; Pasanen, H.; Liu, M.; Al-Anesi, B.; Efimov, A.; Ali-Löytty, H.; Lahtonen, K.; Mäkinen, P.; Matuhina, A.; Muñoz-García, A. B.; Pavone, M.; Vivo, P. Triple A-Site Cation Mixing in 2D Perovskite-Inspired Antimony Halide Absorbers for Efficient Indoor Photovoltaics. Adv. Energy Mater. 2023, 13, 2203175.

Pure black FAPbI₃ (FAPI) perovskites have recently attained record photovoltaic efficiencies over 26% [1]. To stabilize the a-phase of FAPbI₃ without the incorporation of methylammonium, cesium or bromide in the cubic lattice, methylammonium chloride and (often) excess of PbI₂ are always added in the precursor solution to increase the crystallinity and induce preferred orientation for the perovskite layer. However, the presence of these additives can increase defects and reduce long-term stability of the film [2]. To mitigate defects and enhance the performance of the devices, surface passivation is a common strategy to heal the defective perovskite surface with a large variety of molecules bearing functional groups.

In this work, we have investigated the use of 2-diethylaminoethanethiol hydrochloride (DEAET) as a surface passivator of FAPI. The motivation behind this experiment was the fact that the -SH (thiol) group can bind strongly to Pb²⁺ [3], thus excess PbI₂ could be mitigated and/or undercoordinated Pb²⁺ could be passivated. Simultaneously, the hydrophobic character of the thiols can create a barrier that prevents the infiltration of moisture and oxygen into the perovskite layer [4]. To ensure effective coverage of the FAPbI₃ perovskite film with DEAET, different concentrations and solvents were tested. Upon optimization, we were able to decrease the roughness of the perovskite films by 7 nm and diminished the presence of PbI₂ in the final film after annealing. With these features in hand, the films became more photoluminescent (PL) at the bandgap (1.54 eV), simultaneously increasing the PL lifetimes from 140 to over 190 ns. The enhanced non-radiative recombination led to increased open-circuit potential (from 1.04 to 1.06 V) and efficiencies (from 18.5 to 18.9%) in n-i-p solar cells, accompanied by better reproducibility. Accordingly, DEAET-treated FAPI devices did not lose any efficiency after storage in a desiccator for 1 month. The findings of this study lay the foundation for the utilization of thiol-based salts as efficient agents for interface engineering in pure FAPI devices.

Since their appearance, perovskite solar cells (PSCs) were a promising technology due to their low synthesis temperature and potentially cheap manufacturing process. Besides high potential for conventional photovoltaic applications perovskite solar cells have suitable properties for indoor PV energy harvesting especially under modern LED lighting. Thin, light weight and low-cost energy harvesters are of interest for the “Internet of Things” (IoT) devices and smart home indoor devices requiring uninterrupted operation.

Optimal operation under low intensity and short wavelength LED spectra can be achieved with higher bandgap of the absorber material than it is usually expected for the solar spectrum. In terms of device parameters this implies development of the cells with higher V_OC [1]. Current record efficiency of 40.1% at 0.3W/m² of “White LED” light with 2700K color temperature was achieved by the cell with high V_OC of 1V at this irradiance [2]. Although increasing the band gap of Perovskite absorber layer is required to produce a solar cell with high V_OC, the proper energy alignment between absorber and charge transport layers together with low non-radiation recombination rate are vital [3].

In this work we evaluate the performance of recently developed high band gap lead halide Perovskite solar cells with fullerene electron transport layer (ETL) under low light LED illumination. These cells were reported [4] to achieve 1.35V V_OC under 1 Sun illumination, so they have high potential under low light LED conditions. Unlike for field applications, characterization procedures for indoor solar cells are still in development and application scenarios vary significantly. Therefore, the cell performance can not be characterized under one specific irradiance. In our work we evaluate the performance of high band gap PSC with CH₃NH₃Pb(I_0.8,Br_0.2)₃ absorber layer and CMC:ICBA (Here CMC is C₆₀­fused N­methylpyrrolidine­m­C₁₂­phenyl and ICBA is the indene­C₆₀ bisadduct) fullerene ETL under wide range of LED lamp (Cree XLamp CXA3050 LED with 3000K color temperature) illumination conditions (200-10000 lx). These perovskite solar cells achieve V_OC of 1.33V, FF of 70.93% and efficiency of 16.4% at 1 Sun, while their performance under 533 lux LED light was outstanding with 1.03V V_OC and 28.6% efficiency. To validate the guidelines of increasing V_OC value for better indoor performance, we compare these high band gap solar cells with CH₃NH₃Pb(I_0.8,Br_0.2)₃ absorber layer and PSCs with CH₃NH₃PbI₃ absorber layer, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) ETL and a V_OC of 1.17V at 1Sun.

Interest in pre-crystallisation of metal halide perovskite has increased in recent years because it allows for easy production of large quantities of powder perovskite. Perovskite thin film devices subsequently made from the powder exhibit high performance [1,2]. In this study we compare methanol, ethanol, 2-propanol and pentanol as solvents for precipitation reactions forming methyl ammonium lead iodide MAPbI₃ and methyl ammonium formamidinium lead iodide MA_0.5FA_0.5PbI₃ perovskites. Using time resolved in-situ small and wide X-ray scattering (SAXS and WAXS), we were able to follow the reaction of the dissolved methylammonium iodide and formamidinium iodide when a suspension of lead iodide was added to the system. The evolution of the diffraction pattern with time was determined whereby the structural assembly from precursors to perovskite could be followed. The perovskite formation was discovered to be, not surprisingly, initially fast on the surface and during small grain formation. However, the rate of total perovskite formation of the bulk was slower and clearly dependent on the polarity of the solvents and progressed via intermediate phases. The measurements were supported with scanning electron microscopy and optical spectroscopy techniques to correlate grain formation with functionality. Our results led to a deeper understanding of the intermediate steps during the reaction. The precipitation method to produce metal halide perovskite described in this work is cost-effective and has fewer safety issues due to the low solvent toxicity. Furthermore, the stoichiometric homogeneity achieved by the pre-crystallisation process in alcohol subsequently simplifies thin film deposition and therefore improves photovoltaic device performance over large areas. This work outlines an alternative processing route for stable photovoltaics based on perovskite powder.

Mixed lead-tin (Pb:Sn) halide perovskites are promising absorbers with narrow bandgaps (1.2-1.3 eV) suitable for high-efficiency all-perovskite tandem solar cells. Currently, the highest efficiency Pb:Sn perovskite solar cells (PSCs) employ methylammonium (MA) as an A-site cation. However, MA is known to be thermally and chemically less stable than formamidinium (FA), therefore it would be favourable to have an MA-free Pb:Sn PSCs that could also deliver high efficiency. Additionally, the solution processing of thick Pb:Sn perovskite films is notoriously difficult in comparison with their neat-Pb counterparts. This is partly due to the rapid crystallization of Sn-based perovskites, resulting in films that have a high degree of roughness. It is more difficult to coat conformal subsequent layers using solution-based techniques on top of rougher films, leading to contact between the absorber and the top metal electrode in completed devices, resulting in a loss of VOC, fill factor, efficiency and stability. Here, we investigate the impact of adding a non-continuous thin layer of alumina nanoparticles inserted in between the thick, rough Pb:Sn perovskite films and the electron transport layers (ETL) in a 'p-i-n' device configuration. This approach leads to enhanced conformality of the subsequent ETL. As a result, devices that employ the thin alumina nanoparticles layer achieved a champion maximum power point tracked efficiency of 15.0% versus 10.3% for the champion control device and the steady-state open-circuit voltage was improved from 0.65 V to 0.75 V. Application of the alumina nanoparticles as an interfacial buffer layer also results in highly reproducible Pb:Sn solar cell devices whilst simultaneously improving device stability at 65 °C under 1 sun illumination. Aged devices showed a 6‑fold improvement in stability over pristine Pb:Sn devices, increasing their lifetime to 120 hours.

One of the major challenges in perovskite research today is the fabrication of stable perovskite materials for optoelectronic applications without performance loss.
To improve the stability of metal halide perovskite films and devices, layered (2D) perovskites are frequently used as a passivating layer. This improved long-term stability, however, often comes at the expense of device performance. More fundamental understanding of the perovskite material is needed to understand this effect. In a recent study, we therefore used pulsed, transient photoconductivity, to estimate the long-range mobility of phase-pure 2D perovskites.^[1,2] To our surprise, we discovered that PEA₂PbI₄, a well-studied 2D material, has an 8 times higher long-range mobility than FA_0.9Cs_0.1PbI₃, a typical three-dimensional perovskite. To gain a better understanding, we used optical probe terahertz spectroscopy to measure short-range mobility and found an identical value for the mobility of the 2D material, indicating superior material quality. We also hypothesize that the main reasons for the underperformance of perovskite device stacks with 2D passivation are the high exciton fraction and the anisotropy of charge carrier transport.
Using our newly acquired knowledge, we attempted to find an improved 2D passivation layer. We begin by screening a variety of candidates using similar methods to those used in previous studies to find one with improved optoelectronic properties as compared to PEA₂PbI₄. Most importantly, we are attempting to modify the exciton binding energy and structural properties of the materials. Our findings show that multiple parameters influence the formation of 2D perovskites, which governs the optoelectronic properties of the final thin films.

MAPbI₃ amongst other OMH perovskites is undergoing tetragonal-to-cubic phase transitions within the operational temperature range of solar cells. Many properties of these hybrid perovskite materials, such as piezoelectricity, pyroelectricity, ferroelectricity and ionic conductivity are directly linked to the crystal phase and temperature of the sample. Notably, polar domains that form in MAPbI₃ thin-films during the solar cell fabrication have been observed,[1,2,3] We have previously shown that poling of these domains can be achieved in an external E-field that is similar to built-in fields in solar cells during operation.[4]
In this work, we use Piezoresponse Force Microscopy to monitor an evolution of these domains that is triggered by common thermal treatment of perovskite solar cell at 100°C.[5] Our findings suggest that control over the domain structure in these light absorbing, semiconducting compounds may be essential in order to maximize performance and stability of hybrid perovskite solar cells.

Misaligned domains can hamper charge carrier transport in a solar cell and temperature cycling during operation will have an immediate impact on degradation processes. In particular, microstructural changes due to stress and strain when the thin-films undergo a tetragonal/cubic phase transition are an often overlooked factor contributing to the instability of perovskite solar cells. We show that local crystal defects can form and vanish both at ferroic domain walls and crystal grains and in turn modulate ionic conductivity and diffusion. Finally, we extend our investigations towards triple-cation perovskites in order to identify design rules for new compositions such as lead-free perovskites and more stable perovskite-inspired solar cell absorber materials.[6]

Metal-halide perovskites (MHPs) are a type of materials that have the potential to greatly impact optoelectronic devices. A significant material’s science challenge has been the presence of electronic trap states, which are difficult to characterize and mitigate. Various attempts have been made to use electronic spectroscopies to study the defect states in bulk crystals of MHPs. However, due to the mixed nature of electronic and ionic conductivity in MHPs, the results of these experiments often have a high level of uncertainty in distinguishing between electronic and ionic charge contributions.

In this study, we employ a method called photo-induced current transient spectroscopy (PICTS), which was previously used in highly resistive inorganic semiconductors, to analyze single crystals of two types of MHPs: lead bromide 2D-like ((PEA)2PbBr4) and standard "3D" (MAPbBr3) perovskites. By applying PICTS, we are able to obtain two distinct outcomes that allow us to differentiate between the electronic and ionic contributions to the photocurrents. This differentiation is based on the ion mobility of the two different materials. Our experimental findings reveal the presence of deep level trap states in the 2D perovskite ((PEA)2PbBr4), which has limited ion mobility. Furthermore, our results establish new boundaries for the use of PICTS in studying 3D perovskites, which exhibit greater ionic diffusion.

With the increasing demand for artificially intelligent hardware systems for brain-inspired in-memory and neuromorphic computing, perovskite-based memristors has emerged as a promising candidate for resistive random-access memory (ReRAM) devices [1]. Metal halide perovskite semiconductors exhibit mixed ionic-electronic conduction resulting to intrinsic memory effects (hysteresis) in the current-voltage (I-V ) response [2-4]. In order to meet the necessary demands in various complex computing frameworks, understanding the underlying mechanisms governing the resistive switching of these perovskite-based memristor devices is of paramount importance. Here, we present the dynamic impedance spectral evolution of the state transition in perovskite-based memristor devices exhibiting significant transformation of the low frequency capacitance to inductance near the threshold voltage [5]. This transition implies that the interfacial reactivity between the migrating ions with the thin Ag metal contact results to the further gradual decrease in the device resistance indicative of a non-filamentary switching mechanism. Moreover, the incorporation of a thin undoped interfacial buffer layer exhibits an abrupt state transition in the characteristic I-V  response [6]. This abrupt state transition is part of a two-step SET process where both drift-related halide migration and diffusion-related filamentary formation is observed. Furthermore, we develop a dynamical model that helps untangle and quantify the switching regimes consistent with the experimental memristive response. This further insight on the complex interplay among mobile ions, vacancies, and metal provides another degree of freedom in device design for versatile applications with varying levels of complexity.

Over the past decade, perovskite-based solar cell efficiency has dramatically increased from 10% to over 25%, but their instability remains a significant challenge. Environmental factors such as humidity, oxygen, light, and heat have been shown to have a negative impact on these devices. In our previous work, we used drift-diffusion device simulations to demonstrate the role of ion penetration in transport layers toward contacts [2, 3]. This study expands on that model by including chemical reactions within or at connections. By using a drift-diffusion-reaction device model, we can account for the efficiency degradation of perovskite solar cells.

In this study, we simulate a MAPbI3 perovskite solar cell with free iodine anions and investigate how these ions affect cell performance. Our study indicates that chemical reactions involving iodine [4] can alter the charge distribution within the device and moreover lead to the formation of recombination centers for the charge carriers participating in these reactions. We also investigate the possibility of iodine ions penetrating the soft organic blocking layers and affecting the device contacts. To gain a deeper understanding of the interplay between different mechanisms, we analyze the impact of light and dark stress conditions. Finally, we examine irreversible processes such as the release of iodine molecules as gas and the immobilization of iodine species by reactive contacts.

Huge compositional space of perovskites make this class of compounds a fertile ground for search of exotic properties that can lead to new technological developments. Especially, the limit of channel length minimization in traditional semiconductor electronics has led to the focus on spintronics and other such efforts where the spin of electrons can be controlled by virtue of the special properties of the host material. One such manifestation of spin control can be seen in Rashba-Dresselhaus effect discovered around 60 years ago in non-magnetic insulators with broken inversion symmetry [1]. This phenomenon of spin splitting in the momentum space if it occurs in the band edges can help in realising the spin field effect transistor proposed by Datta and Das [2]. To achieve the spin control via switching of gate voltage, existence of ferroelectric polarisation in the material is necessary. This has led to a lot of research on ferroelectric Rashba semiconductors (FERSC) [3,4] and formulation of design principles necessary to search for such materials [5].

In this work, we look into Iodate perovskites in non-centrosymmetric rhombohedral phases with Iodine as a heavy element giving spin orbit coupling effects. We study the effect of A cation on the Rashba spin splitting in these iodates by comparing the Rashba parameters of (A)IO₃ in R3m phases where A=K, Rb, Cs, and Tl. Through ab-initio density functional theory calculations, electronic band structure of these compounds are obtained and the energy splitting near the conduction and valence band edges are noted. Our study reveals that the Rashba splitting is inversely proportional to the ionic radii of the cation while the band gap is directly proportional. KIO₃ with a band gap of 2.29 eV and Rashba splitting of ~1 eVÅ is the most suitable candidate for FERSC applications.

  Intragap states are among the critical factors that limit the performance and stability of perovskite solar cells (PeSCs). They not only serve as trapping sites for photogenerated carriers and open the dominant non-radiative loss mechanism in PeSCs under sunlight illumination conditions, but also become original sites (such as defects) where degradation starts from. Further PeSCs development requires a comprehensive understanding of trap state properties, including how they are filled and depopulated in a working device. Conventional spectroscopic techniques are not sufficiently selective to specifically follow the dynamics of trapped carriers, particularly at actual PeSC working conditions. Here we apply novel infrared optical activation spectroscopy [i.e., optical pump-IR push-photocurrent (PPPc)], to observe in real time the evolution and properties of trapped carriers in operando PeSCs. In these techniques, band-edge carriers are generated by an optical visible “pump” beam, followed by the carrier trapping processes. Then the photons of IR “push” beam absorbed by the trapped carriers excite them back to the band states. The IR de-trapped carriers contribute to the device photocurrent, therefore the amplitude and behaviour of IR-induced current help to evaluate the concentration and dynamics of trapped carriers in the device.

  We compared the behaviour difference due to trapped holes in pristine and surface-passivated FA_0.99Cs_0.01PbI₃ PeSCs using a combination of temperature-dependent steady-state PPPc, ns time-resolved PPPc, and kinetic models. We found that the trap-filling process occurred in two steps: first, in a few-ns timescale, low-concentration trap states are filled in the bulk of perovskite material; then, in a much longer (~100 ns) timescale, high density of traps at material interfaces is populated. Surface passivation by n-octylammonium iodide dramatically reduces the number of trap states (~10 times) and hence substantially improves the device performance. The activation energy of the dominant hole traps was measured to be in the order of ~280 meV and was not affected by the surface-passivation process.

  Our results successfully demonstrate that PPPc techniques are powerful and highly sensitive to reveal the dynamic, concentration, and activation energy of trapped carriers, facilitating a comprehensive understanding of the role of trap states in PeSCs. We expect that the in-situ measuring of PPPc signals in working PeSCs under different ageing conditions (e.g., heat, illumination, humidity) allows us to trace the change of trapped carriers’ dynamics and properties. Therefore, it becomes another starting point for material scientists to further develop the existing materials and devices for the next-generation PeSCs with excellent stabilities.

Perovskite photophysics in the past decade has been uprooted and revisited from the grounds several times. Initially, Metal halide perovskites were considered as excitonic semiconductors, due to their pronounced excitonic absorption and narrow-band efficient optical emission. Then ultrafast spectroscopy studies have revealed bimolecular recombination dynamics, proving that excitons are dissociated into opposite charge carriers, with beneficial effects for charge separation in solar cells. Perovskite photophysics seemed to have been rationalized: free carriers are favored over exciton by Saha equilibrium, radiative recombination is bimolecular and is the inverse process of optical absorption.

Yet several issues with such a picture started appearing, again from ultrafast spectroscopy measurements. I will review how radiometric time-resolved photoluminescence reveals that the radiative recombination rate is much lower than what expected from the absorption rate. Furthermore, the combination of transient absorption and time resolved photoluminescence in a tandem setup demonstrates that free carriers are majority even at low temperature and in 2D perovskites, when Saha equilibrium predicts instead that bound excitons should prevail.

An extensive debate on the exciton binding energy has ensued, resulting in the realization that the exciton binding energy measured in absorption, when perovskites are in their ground state, is significantly different from the binding energy after excitons have been created.

Clearly, a sound description of photophysics of halide perovskites needs additional ingredients describe the dissociation of excitons in the excited state. Phonon coherences, ultrafast electron diffraction, XAS and XRD are among the techniques that have evidenced a significant distortion of the perovskite lattice upon optical absorption, a phenomenon also known as the formation of a new quasiparticle, the polaron.

I will present a picture of perovskite photophysics that applies to both 3D and 2D materials and consists in the creation of excitons upon optical absorption, their spontaneous dissociation into opposite charge polarons and, finally optical emission by bimolecular recombination of polarons into excitons.

We provide a survey of our experimental studies of coherent spin dynamics of electrons and holes in lead halide perovskite semiconductors: bulk crystals [1-3], nanocrystals [4-5] and 2D structures [6]. Time-resolved Faraday/Kerr rotation techniques was used for that, measurements were performed at cryogenic temperatures and in strong magnetic fields. We measure spin relaxation and spin coherence times, evaluate electron and hole Lande g-factors for a representative set of perovskites with the band gap energy varying from 1.5 to 3.2 eV. We establish universal trends for the g-factors on the band gap energy in bulk materials and show that they are modified by the quantum confinement in perovskite nanocrystals. Strong interaction of hole spins with nuclear spins is found, which is considerably stronger that the one for the electron. Optically detected nuclear magnetic resonance highlights the dominating role of Pb ions in interaction with electrons and holes. Spin mode locking effect based on spin synchronization under periodic laser excitation is found in nanocrystals in glass. Experimental approaches of spin physics give reach information about these materials.