Highly performing mixed Sn/Pb-perovskite solar cells (PSCs) are among the most promising options to reduce Pb content in perovskite devices and enable the fabrication of all-perovskite tandem solar cells owing to their reduced band gap. Here, I will show that the introduction of 2,3,4,5,6-pentafluorophenethylaminium cations in a perovskite active layer of composition (FASnI₃)_0.5(MAPbI₃)_0.5 enhances the crystallinity of the active layer, reduces the voltage losses and increases the material stability. The addition of the fluorinated cations allows the fabrication of highly oriented films with improved thermal stability. Moreover, the treated films exhibits merged grains with no evidence of 2D structures, which we believe leads to the passivation of trap states at the grain boundaries. Solar cells fabricated adding to the active layer the fluorinated cation display reduced trap-assisted recombination losses and lower background carrier density, which led to enhanced open-circuit voltage than the reference sample using phenethylammonium cations. The best perovskite solar cell showed an efficiency of 19.13%, with an open-circuit voltage of 0.84 V, which is substantially improved respect to the reference sample which exhibits 17.47% efficiency and 0.77 V as open-circuit voltage. More importantly, the fluorinated cations' addition help to improve the device's thermal stability, maintaining 90.3 % of its initial efficiency after 90 min of thermal stress at 85 C° in Nitrogen atmosphere.

Nowadays, overcoming the stability issue of perovskite solar cells (PSCs) while keeping high efficiency has become an urgent need for the future of this technology. We will show that getting stable PSC begins with the preparation of high-quality perovskite layers and that employing additives in the perovskite precursor solution allows to reach this objective. Two families of additives will be developed in the talk: halides and nanoparticles. We have highlighted the effect of chloride additives on the formation mechanism of methylammonium-free Cs0.1FA0.9PbI3 films upon annealing. By composition profile evolution tracking, two formation steps are distinguished: first the superficial residual solvent is eliminated, second, the solvent in the depth of the film is evaporated. The elimination profile signs the final morphology of the layer. The downward (top-down) growth has been encountered for the pristine and potassium chloride additive cases. It led to the formation of multiple boundaries and middle-sized grain morphology. We unveil that employing both potassium chloride and ammonium chloride additives forces the homogeneous elimination of the solvent across the layer, and then the lateral growth of the grains. It results in large size, monolithic and defect-poor grains with good coverage of the substrate which are the targeted properties for high efficiency and stability. Similar film formation steps and growth direction control have been highlighted by comparing the formation mechanism of MAPbI3 films with and without gold nanoparticles. The pristine layer grows in the top-down direction, while the presence of nanoparticles results in a lateral growth and formation of a monolithic structure.

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted much attention due to their high power conversion efficiency (>25%) and low-cost fabrication. Yet, improvements are still needed for more stable and higher performing solar cells. In this presentation, three engineering approaches are proposed to enhance the photovoltaic efficiency and stability of PSCs: (1) On the first method, a series of highly oriented vertical TiO₂ nanocolumn electron-transporting photonic structures were intentionally fabricated on half of the compact TiO₂-coated fluorine-doped tin oxide substrate by glancing angle deposition with magnetron sputtering. These vertically aligned nanocolumn arrays were then applied as the electron transport layer into triple-cation lead halide perovskite solar cells based on Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. (2) On the second method, we investigated the effect of removing the excess PbI₂ at the interface between the triple-cation Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite and the Spiro-OMETAD hole-transport layer. For this purpose, four different organic salts, including methylammonium iodide (MAI), formamidinium iodide (FAI),  methylammonium bromide (MABr) and methylammonium chloride (MACl) were applied and compared. (3) The last aspect that will be presented involves the application of the downshifting optical property of colloidal carbon quantum dots to enhance the stability of perovskite solar cells against UV degradation.

On the above-mentioned engineering methods, different characterization methods, including far-field and near-field optical experiments, structural and spectroscopic investigations, impedance spectroscopy, together with solar cell efficiency and in  particular device stability measurements are presented together in order to understand the underlying origins of the efficiency and stability enhancement observed in triple-cation perovskite solar cells.

While organic-inorganic (hybrid) perovskites achieved competitive power conversion efficiency within an impressive short time [1], they still suffer from insufficient stability [2]. Instability of hybrid perovskite thin films under thermal stress and/or in humid air is one of the major obstacles for the commercialization of perovskite-based solar modules [2,3]. One of the most promising approaches to improve the long-term stability is the incorporation of bulky, hydrophobic molecules into the perovskite layer [4]. While the beneficial effects of these post-deposition treatments are generally accepted, there is an ongoing discussion about the mechanism of this passivation effect. Particularly, it is under debate when optimum surface passivation is reached using different concentrations of bulky molecules where a transition is supposed to happen from the surface attached bulky molecule itself to a 2D layer formation with general structure  R₂A_n−1B_nX_3n+1 [5], which forms via intermixing of the bulky molecules and PbI₂ from the 3D perovskite. Here, A denotes the cation, B the metal ion, and X the halogen anion of the 3D perovskite, R is the bulky organic cation.

Here, we use in situ photoluminescence (PL) measurements during spin-coating and annealing to probe the dynamic deposition of 2-thiophenemethylammonium iodide (2-TMAI) and phenylethylammonium (PEAI) with varied concentration on 3D triple cation perovskites. For both molecules, we find the transition from molecular passivation to the formation of an R₂A_n−1B_nX_3n+1 layer at concentrations around 4-10 mmol/L. Using higher concentrations, we see the formation of a distinct surface layer. During spin-coating and annealing of these higher concentrated solutions, we furthermore monitor the transition from a single inorganic layer spaced by the bulky cations (n=1) to mixed 2D layers (n=2 and higher) and, in the case of 2-TMAI, to the formation of a mixed disordered phase [6]. The latter may negatively affect the electronic properties of the perovskite layer [7]. Our results illustrate how in situ PL can be used to gain mechanistic understanding on the 2D layer formation, its interaction with the 3D perovskite, and its transformation to the disordered phase. Therefore, it can be utilized to deliberately optimize the annealing sequence targeting an ideal 2D/3D interface satisfying enhanced charge transport and stability.

Combining abovementioned results with in situ GIWAXS measurements, we propose a model for the surface passivation mechanism either via molecular passivation or 2D layer formation, compare the mechanisms for both investigated molecules, and correlate them with device stability proposing an optimized stabilization treatment.

Lead halide perovskite absorber materials have attracted enormous attention from academia and industry due to their fascinating optoelectronic characteristics featuring multiple possible applications. In particular, the solution-processed perovskite solar cells demonstrated certified power conversion efficiencies of >25.5% thus coming very close to the best crystalline silicon solar cells. Low operational stability of perovskite solar cells is currently the main obstacle to their practical implementation.

Overcoming stability issues requires a very deep understanding of the compositional and structural dynamics of the perovskite absorber materials under the action of stress factors such as light, heat and electric field. Furthermore, the impact of defects and impurities on such dynamics is of crucial importance. These aspects remain insufficiently investigated, whereas many of the proposed aging pathways are still controversial.

               In this lecture, we will discuss compositional and structural inhomogeneities in solution-cast thin films of classical 3D and mixed-dimensional 2D/3D perovskites as revealed by advanced microscopy: SEM, AFM, KPFM and IR s-SNOM. A particular focus will be made on the light-induced phase segregation effects induced by both anion and cation (de)mixing. Furthermore, we will consider typical impurities present in the perovskite absorber films, their localization, effects on the grain boundaries and impact on the operational stability of solar cells.

Single-junction halide perovskite solar cells (PSCs) have already achieved a certified power conversion efficiency (PCE) above 25 %, making them one of the most promising emerging photovoltaic technologies. One of the main bottlenecks towards their commercialization is their long-term stability, which should exceed the 20-year mark. Many are the strategies applied to extend device lifetime, among them are the use of additives, the optimization of the fabrication process of perovskite thin films or the replacement of unstable organic transport layers such as Spiro-OMeTAD. Although most of these approaches can effectively improve device efficiency, they frequently fail at providing stable PSCs as defined as those able to display less than 10 % degradation after 1000 h of continuous illumination under 1 sun. In this respect, the understanding of defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on the device efficiency and stability is currently a challenge. In this talk, we will show our most recent results on additive engineering to enhance the stability of highly efficient PSCs. We demonstrate that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous impact on the device stability. We obtained PSCs with ~21 % efficiency that retain ~100 % of the initial efficiency after 1000 h at the maximum power point under simulated AM1.5 illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H^…I and O^…H), results in shallow point defect passivation that has a significant impact on the device stability but not on the nonradiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs.

I will talk about our work on understanding intrinsic stability of perovskites and meta-stability of perovskite solar cells and strategies for boosting perovskite solar cells’ durability against thermal extremes and humidity. Our perovskite solar cells encapsulated by a polymer-based encapsulation scheme were the first to exceed the strict requirements of International Electrotechnical Commission standards for thermal cycling damp heat and humidity freeze. I will then review some of the glass-to-glass bonding technologies that can be carried out at low-temperatures suitable for perovskite solar cells. This will be followed by our recent results of developing polymer-free glass-to-glass bonding technique for encapsulating perovskite solar cells.

Nanometer-sized colloidal metal halide perovskite semiconductors have emerged and brought unique opportunities for photovoltaic application due to the high defect tolerance of perovskite and many features that emerge at the nanoscale. Perovskite quantum dots (QDs) or more broadly, nanocrystals (NCs), show high photoluminescence (PL) quantum yields, spectrally tunable bandgap, flexible compositional control, and crystalline strain benefits. Metal halide perovskite nanocrystals are readily synthesized with exceptional optoelectronic quality, opening a route for next generation light emitters, as well as exploring LHP physics at the nanoscale. CsPbX₃ (X=Cl^-, Br^-, I^-, or mixed halide) QDs exhibit PL tunable from ultraviolet to near-infrared wavelengths by adjusting the halide composition and/or QD size. In 2016, CsPbI₃ QDs also became a point of interest in PV research. Currently CsPbI₃ holds the record efficiency for QD solar cells (16.6%) proving better than any previous QD material composition. This talk will highlight the unique potential of perovskite QD (PQD) solar cells, from synthesis to devices. We will discuss current state of the art and lay out many open opportunities in perovskite QD solar cells, and the related present and future pursuits in QD preparation and device architecture.

Reference:

Protesescu, L. et al. Nanocrystals of cesium lead halide perovskites (CsPbX₃, X = Cl, Br, and I): Novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 15, 3692-3696 (2015).

Swarnkar, A. et al. Quantum dot-induced phase stabilization of α-CsPbI₃ perovskite for high-efficiency photovoltaics. Science 354, 92-95 (2016).

Yuan, J. et al. Band-aligned polymeric hole transport materials for extremely low energy loss α-CsPbI₃ perovskite nanocrystal solar cells. Joule 2, 2450-2463 (2018).

Mixed-dimensional perovskites containing mixtures of organic cations hold great promise to deliver highly stable and efficient solar cells. When used in small amounts, the low-dimensional species can act as a dopant to passivate defects at the grain boundaries of the 3D counterpart, leading to improved performance and stability. However, although a plethora of relatively bulky organic cations have been reported for such purpose, a fundamental understanding of the materials' structure, composition, and phase, along with their correlated effects on the corresponding optoelectronic properties and degradation mechanism remains elusive. Herein, we systematically engineer the structures of bulky organic cations to template the formation of low-dimensional perovskites with contrasting inorganic framework dimensionality (2D to 0D), connectivity (flat vs. corrugated), and coordination deformation. By combining X-ray single-crystal structural analysis with various spectroscopic techniques, such as depth-profiling XPS, solid-state NMR, and femtosecond transient absorption, it is revealed that not all low-dimensional species work equally well as dopants. Instead, it was found that inorganic architectures with lesser structural distortion tend to yield a less disordered energetic and defect landscape in the resulting mixed-dimensional perovskites, augmented in materials with higher PLQY (up to 11%), better solar cell performance (PCE more than 19%) and improved thermal stability (T70 up to 1000 hrs, unencapsulated). Our study highlights the importance of designing templating organic cations that yield low-dimensional materials with much less structural distortion profiles to be used as additives in stable and efficient perovskite solar cells.

Recently, organometallic hybrid perovskite materials are experiencing a real progress for solar cell applications. Due to particularly interesting properties: adaptable band gap, high crystallinity, high charge transport capacity and high thin film efficiency, these materials have the potential to exceed the performance limits of current technologies. They also combine a low cost and processing versatility. Among alternative device structures, carbon-based perovskite solar cells (C-PSCs) look highly promising due to their low cost and abundantly available materials (TiO2, ZrO2, carbon black and graphite powders), cost-efficient scalable fabrication methods and the inherent high stability. A one step (CH3NH3)x(AVA)1-xPbI3 perovskite solution (with AVA= ammonium valeric acid additive) was pipetted to infiltrate mp-TiO2/mp-ZrO2 through a thick porous carbon layer [1][2]. In order to reveal their maximum photovoltaic performance, these devices should be first matured under humidity and temperature. This step lasts of approximately 100-150 h and improves of the cell’s performance. To further investigate their stability, aging campaigns at 85°C/85%RH have been conducted during 1000 h. The macroscopic observations show an inhomogeneous degradation of the perovskite layer, the interfaces and the electrodes, mainly located at the edges. This inhomogeneity probably results from the pipetting process used to infiltrate the perovskite. This was confirmed by the variation of PV parameters during aging, which showed an important decrease in performance close to 50% after 1000 h of aging. In this study, a basic encapsulated system based on glass and a surlyn gasket was used, enabling the humidity permeation up to solar cells and inducing probably an accelerated degradation of devices. Thanks to dedicated characterization techniques, such as laser beam induced current (LBIC) measurements and photoluminescence imaging, the local performances have been correlated to the degradation inhomogeneity. The modifications of the perovskite layer have been evaluated with others more common techniques (X-Ray diffraction, UV-visible absorption and photoluminescence spectroscopy). Thanks to this multiscale approach, a degradation mechanism could be proposed highlighting the role playing by the prior maturation step. Today, others technological solutions are tested such as the inkjet printing for the perovskite infiltration and more advanced encapsulation systems, to improve the stability of these PV devices.

Abstract: Despite of huge success in photovoltaic community, Pb based halide perovskites have energy band gap higher than the optimal band gap required for single junction solar cells, governed by the Shockley–Queisser radiative limit. Sn based perovskites emerge as an alternative with rapidly improving efficiencies. Due to comparable optoelectronic properties as the Pb counterpart and their ability to lead to lower band gaps, Sn based perovskites open a new door to all-perovskite tandem solar cells. Along with efforts for efficiency, it is worth analysing the in-depth recombination dynamics for further development of Sn based perovskite solar cells (PSCs). The long photo-excited charge carrier lifetime (τ) is often attributed for the high performance of PSCs. Herein, we study the effect of ‘B’ cation in ABX₃ (A = MA⁺, FA⁺, and Cs⁺; B = Pb²⁺, Sn²⁺, and X = I⁻) perovskite structure on the charge carrier recombination dynamics to understand the possible recombination mechanism. We fabricated PSCs with p-i-n configuration based on FA_0.95Cs_0.05PbI₃ (pure Pb), MA_0.20FA_0.75Cs_0.05SnI₃ (pure Sn), and (MAPbI₃)_0.4(FASnI₃)_0.6 (Pb-Sn mixed) perovskitres. We optimized the Sn based perovskite thin film (pure Sn) in terms of moisture and thermal stability in order to minimize the error due to perovskite degradation. Further, we compared the charge carrier recombination dynamics of the three devices through transient photovoltage (TPV) measurement.¹ TPV is used to establish the relation between charge carrier density (n) and recombination rate constant (k) under different DC background intensities.² This study establish the rate law of charge carrier decay in the three devices and reveals the nonlinear dependence of k on n in Sn based PSCs, which will be correlated to the slow relaxation lifetime of the charge carrier and their recombination through the defect states in the perovskite thin films.

Keywords: Pb-free perovskite, rate law of charge carrier decay, transient photovoltage, defects, bimolecular recombination

One of the most promising third generation photovoltaic technologies is Perovskite Solar Cells (PSC). This perovskite family presents outstanding properties such as high absorption coefficient, long diffusion lengths, low-cost technology and ease of scalability. Currently, the efficiency record is set to 25.6 % [1]. On the other hand, the development of new materials combining multiple cations and multiple halides is one of the main research strategies to achieve high performance.

In this work, we have analysed the temperature behaviour of perovskite based solar cells with structure FTO/c-TiOx/m-TiOx/RbCsFAMAPbIBr/Spiro/Au. We have obtained the temperature dependence of the solar cell parameters (Voc, Isc, FF and efficiency) from -20 ºC up to 50 ºC. Impedance Spectroscopy (IS) has been carried out at room temperature and Cole-Cole spectra have been fitted using the Matryoshka circuit. Ideality factor has been obtained both from Voc vs light and from the recombination resistance extracted from IS measurements. In order to determine the dominant recombination mechanism, the activation energy has been estimated from the extrapolation of Voc down to 0 K [2, 3]. 

Hybrid tin halide perovskite solar cells stand as the most promising alternative to their toxic lead-based counterparts, although addressing their poor ambient stability remains as the main challenge to make this technology competitive. Hence, detailed knowledge of their chemical degradation pathways is essential to mitigate their decomposition. This talk will cover our recent findings on the degradation mechanism of tin perovskite thin films (i.e. (PEA)_0.2(FA)_0.8SnI₃, where PEA is phenylethylammonium and FA is formamidinium) via a combination of spectroscopy, diffraction and ab initio simulation techniques [1]. We find that SnI₄, a product that forms as a result of the oxygen-mediated decomposition of perovskite, reacts further to form iodine via the combined action of atmospheric water and oxygen. Iodine is then shown to be a highly aggressive species that rapidly leads to further perovskite degradation to give more SnI₄, establishing a cyclic degradation mechanism. We then find the ambient stability of tin perovskite films to be highly dependent on the hole transport layer chosen as the substrate, which is used to tackle the oxidative degradation of the material and increase its ambient stability. We expect the findings presented herein to provide key design rules towards stable, lead-free tin perovskite solar cells.

In the past decade, metal halide perovskite (MHP)-based solar cells marked a breakthrough in photovoltaic technologies and reach power conversion efficiencies exceeding 25%. While MHPs exhibit a remarkable defect tolerance, film degradation will eventually deteriorate the optoelectronic properties and hence device performance. A key strategy to substantially enhance the stability is to tailor the interfaces in the device.¹

Here, I will discuss the impact of interface formation on device performance also considering the effect of chemical reactions on interface energetics and durability,² particularly for our recent research activities on oxide buffer- and transport layers.³ In particular, I will describe our  use of surface-sensitive photoemission spectroscopy (PES) as a primary tool to provide guidelines for controlling the chemistry and optimize the electronic properties of MHP interfaces. In combination with further advanced characterization techniques, such as Kelvin probe force microscopy (KPFM) to determine surface photovoltage of the investigated layer stacks or quantitative photoluminescence imaging for the analysis of bulk optical properties, we specifically use PES methods to put the role of charge selective contacts in the focus of our research.⁴

Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells. However, the mechanisms that allow for carriers to be transferred across these interfaces are still unknown.

In this presentation, I will discuss the role of crystal surface structural defects on optoelectronic properties of lead halide perovskites through synchrotron-based techniques. The importance of interfaces and their contribution to detrimental recombination will also be discussed. As a result of these contributions to better understanding 2D and 3D defects, the perovskite solar cell field has been able to improve device performance. Albeit the rapid improvements in performance, there is still a need to understand how these defects affect long term structural stability and thus optoelectronic performance over the long term.