Since the seminal work on the solid-state perovskite solar cell (PSC) demonstrating a power conversion efficiency (PCE) of 9.7% and stability for 500 h in 2012, perovskite photovoltaics surged swiftly. As a result, a certified PCE of 25.5% was achieved in 2020. According to Web of Science, publications on PSC increase exponentially since 2012, leading to 18,000 as of September 27, 2020, which indicates that PSC is considered as a very promising photovoltaic technology. In this talk, history, progress, and perspective of PSC will be covered in terms of efficiency, stability and upscaling. High photovoltaic performance was realized by compositional engineering, device architecture and coating methodologies for the past 10 years. Toward theoretical efficiency over 30% and commercialization of long-term stable PSC, further studies on suppression of recombination and developments of scalable technologies are required. Importance of interface and grain boundary engineering is emphasized to reach the theoretical efficiency with voltage over 1.3 V and fill factor over 0.9. For commercialization, materials may be issued because the current precursor mixture is problematic due to the underlying aging effect. We developed cost-effective materials based on delta FAPbI₃ powder for reproducibly high efficiency PSC. The best PCE of 21.1% was certified using the synthesized perovskite powder. Large-area uniform perovskite coating is pre-requisite in upscaling PSC. Solvent- and/or additive-engineering-based solutions were developed for large-area perovskite films. Regarding stability, except for the encapsulation, a paradoxical approach using a hydrophilic passivation layer was developed, which demonstrated over 90% of the initial PCE after 1000 h.

In this talk I will overview the virtue of nanomaterials for solar energy conversion using as examples dye-sensitized (DSSC) and perovskite solar cells (PSC).

Some years back we introduced alkoxy functionalized donor groups as a building block in organic dyes as light harvesters for DSSC. This donor group provides a desirable 3-dimensional structure that aids in surface protection of electrons injected into the semiconductor from oxidants in the electrolyte, allowing for record-setting cobalt- and copper-based redox shuttles to be utilized more frequently. With these systems we recently set the world record efficiency for DSSC of 12.25%. DSSCs are ideally suited for ambient light and indoor applications where efficiencies up to 35% have been reached calculated with respect to the fluorescent light source.

In our work on perovskite solar cells (PSC) we have achieved efficiencies above 23% with a mixed composition of iodide/bromide and organic and inorganic cations. With the use of SnO2 compact underlayers as electron acceptor contacts we have constructed planar perovskite solar cells with a hysteresis free efficiency above 22%. Through compositional engineering, larger perovskite grains grown in a monolithic manner are observed and reproducibility and device stability are improved. With regards to lifetime testing, we have shown a promising stability at 85 oC for 500 h under full solar illumination and maximum power point tracking (95% of the initial performance was retained). Our present main directions of developing FAPbI3 perovskites and passivation interface layers will be discussed in the lecture.

Hybrid perovskites persist as one of the leading materials in photovoltaics due to remarkable solar-to-electric power conversion efficiencies.^[1-2] Their limited stability under device operation conditions, however, remains challenging.^[1-2] This stimulates the development of layered perovskite analogs based on the assemblies of organic and inorganic components featuring higher stabilities under operating conditions.^[3-7] To this end, supramolecular chemistry provides a powerful tool for controlling the properties of hybrid materials by tailoring noncovalent interactions. We demonstrate its utility through molecular modulation based on fine-tuning various noncovalent interactions (i.e. supramolecular engineering),^[7-10] such as metal coordination,^[10]   hydrogen^[6,9-10] or halogen bonding,^[8] and π-interactions,^[7] among others,^[11] in a manner that has been uniquely assessed by solid-state NMR spectroscopy.^[5-6,8-10] As a result, perovskite solar cells that exhibit superior performances can be obtained, which is accompanied by enhanced operational stabilities.^[6-10] Moreover, the underlying molecular design can be extended into creating layered perovskite architectures to enable further stability enhancements.^[3-7] This has been investigated by using a combination of techniques complemented by solid-state NMR to unravel the design principles and highlight the supramolecular approach in advancing hybrid photovoltaics.

We demonstrate the critical role of surface recombination in mixed-cation, mixed-halide perovskite, FA_0.83Cs_0.17Pb(I_0.85Br_0.15)₃. By passivating non-radiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS) we transform these thin films. We demonstrate average minority carrier lifetimes > 4 {\mu}s, nearly single exponential monomolecular PL decays, and concomitantly high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We confirm both the composition and valence band edge position of the FA_0.83Cs_0.17Pb(I_0.85Br_0.15)₃ perovskite using multi-institution, cross-validated, XPS and UPS measurements. We extend the APTMS surface passivation to higher bandgap double cation (FA,Cs) compositions (1.7 eV, 1.75 eV and 1.8 eV) as well as the widely used triple cation (FA,MA,Cs) composition and observe significant PL and PL lifetime improvements after surface passivation. Finally, we demonstrate that the average surface recombination velocity (SRV) decreases from ~1000 cm/s to ~10 cm/s post APTMS passivation for FA_0.83Cs_0.17Pb(I_0.85Br_0.15)₃. Our results demonstrate that surface-mediated recombination is the primary non-radiative loss pathway in MA-free mixed-cation mixed-halide films with a range of different bandgaps, which is a problem observed for a wide range of perovskite active layers and reactive electrical contacts. This study indicates that surface passivation and contact engineering will enable near-theoretical device efficiencies with these materials.

The bandgap tunability of mixed-halide perovskites makes them promising candidates for light emitting diodes and tandem solar cells. However, illuminating mixed-halide perovskites results in the formation of segregated phases enriched in a single-halide. This segregation occurs through ion migration, which is also observed in single-halide compositions, and whose control is thus essential to enhance the lifetime and stability of the devices. Using pressure-dependent transient absorption spectroscopy, we find that the formation rates of both iodide- and bromide-rich phases in MAPb(Br_xI_1-x)₃ reduce by two orders of magnitude on increasing the pressure to 0.3 GPa. We explain this reduction from a compression-induced increase of the activation energy for halide migration, which is supported by first-principle calculations. A similar mechanism occurs when the unit cell volume is reduced by incorporating a smaller cation. These findings reveal that stability with respect to halide segregation can be achieved either physically through compressive stress or chemically through compositional engineering.

Recent advancements in perovskite solar cell performance were achieved by stabilizing the α-phase of FAPbI3 perovskites in nip-type cell architectures, enabling power conversion efficiencies (PCEs) of 25.2%. However, these advancements could not be directly translated into similar PCE improvements in pin-type perovskite cells which limits the possibilities to create highly efficient and stable perovskite photovoltaic cells. We fabricated a high-quality double-cation perovskite (MA0.07FA0.93PbI3) with low bandgap energy (1.54 eV) using a two-step approach on a standard p-type polymer (PTAA). The fabricated neat perovskite films exhibit large grains ( 1 µm) allowing to reach external photoluminescence quantum yields (PLQYs) of 20% with an unprecedented charge carrier lifetime of over 18 µs without further passivation. The exceptional opto-electronic quality of the neat material was translated into high efficiency pin-type cells with PCEs of up to 22.5% with improved stability under illumination. The low-gap cells (1.54 eV) stand out by their exceptional fill factors of 83% due to reduced charge transport losses and high short-circuit currents ( 24 mAcm-2). Using intensity dependent QFLS measurements, we quantify an implied PCE of 28.4% in the neat material which can be realized upon minimizing interfacial recombination and a further improvement of charge extraction.

Non-radiative recombination processes are the biggest hindrance to approaching the radiative limit of the open-circuit voltage for wide-band gap perovskite solar cells.

Matched energy levels of charge transport layer are crucial to minimize non-radiative recombination pathways. By tuning the lowest-unoccupied molecular-orbital of electron transport layers via the use of different fullerenes and fullerene mixtures, we demonstrate open-circuit voltages exceeding 1.35 V in CH₃NH₃Pb(I_0.8Br_0.2)₃ device.

Further optimization of mobility in binary fullerenes electron transport layer can boost the power conversion efficiency as high as 18.6%. We note in particular that the V_oc-fill factor product is > 1.085 V, which is the highest value reported for halide perovskites with this band gap.

All-inorganic perovskite compositions are highly interesting for solar cell application due to their outstanding thermal stability and tandem-relevant band gap. Although efficiencies over 19 % have been achieved[1], all-inorganic perovskite solar cells still lag behind their organic-inorganic counterparts. These lower efficiencies are largely due to lower open-circuit voltages compared to organic-inorganic perovskites with the same band gap.

This study investigates the efficiency potential of all-inorganic perovskite layers using intensity-dependent photoluminescence (PL) measurements. This contact-less measurement of a neat absorber film or a layer stack allows for the construction of a pseudo JV curve, providing potential performance metrics such as open-circuit voltage (V_OC) and fill factor[2]. Based on realistic assumptions on photocurrent generation, an efficiency potential of the film or stack can be calculated. This is a powerful tool to compare the potential of perovskite compositions and transport layers and to identify the dominating loss mechanisms.

For this presentation, DMAI-CsPbI₃ films[1] with a band gap of 1.73 eV are fabricated and analysed. Neat films on quartz glass show a PL-derived efficiency potential of 24.2 %, a remarkable value that for the first time quantifies a potential that can be achieved with ideal transport layers. Films on top of a bilayer of compact and mesoporous TiO₂ still show an efficiency potential of 22.9 %.

In addition, CsPbI₂Br films with a band gap of 1.9 eV are fabricated and compared to CsPbI₃ films. Films on top of the self-assembled monolayer (SAM) molecule 2PACz show a very high PL quantum yield (PLQY) of above 12 %. In a previous study on organic-inorganic perovskites, only potassium-passivated triple cation perovskite films showed a higher PLQY[2]. With the measured quasi-Fermi level splitting (QFLS) of 1.54 eV, 97 % of the radiative V_OC limit was reached, which is on par with the best organic-inorganic films we measured[2]. The resulting efficiency potential of 22.8 % is surprisingly close to the efficiency potential of CsPbI₃ on TiO₂ (22.9 %), which has a much lower band gap. These findings suggest very low defect density both in the CsPbI₂Br perovskite bulk and at the interface between perovskite and contact layer.

This study shows very high efficiency potentials that can be achieved with ideal transport layers. By comparing the most relevant compositions and transport layers, this study helps to identify the most promising route for all-inorganic perovskites and the main loss-inducing interfaces.

Recently, perovskite solar cells showed power conversion efficiencies (PCE) over 25%. However, commercialization of perovskite solar cells is hampered by environmental concerns due to the toxicity of the used lead. Hence, there is a high interest to substitute the lead by less toxic elements. Tin is a promising candidate, since it has similar electronic properties as lead and so can be easily replaced in the ABX₃ perovskite structure. Tin based solar cells also showed an incredible increase in solar cell efficiency of up to 13 % in the last years.[1][2] But those solar cells still lag behind the lead based ones, mainly due to high recombination losses.[3] In order to achieve less recombination losses and thus higher power conversion efficiencies, we modified the hole transport as well as the electron transport layer in our FASnI3 perovskite solar cells (PSC).

We altered the hole transport interface by introducing nanoparticles in between the HTL and perovskite absorber. An improvement in efficiency from 3 % to 4 % could be achieved. It seems, that the nanoparticles reduce the back reflection at the hole transport-perovskite-interface and lead to a smoother perovskite layer with less pinholes, which is beneficial for further depositions from solution and the prevention of shunts.

To further minimize V_OC losses, we evaluated three different C₆₀ derivatives (PCBM, ICBA, bis-PCBM) as electron transport layer to optimize the band alignment and interface towards the perovskite absorber. By this we could reach a nearly doubling of the open circuit voltage with ICBA from 380 mV to 650 mV and 480 mV for bis-PCBM in comparison to the commonly used PCBM in our Sn-based perovskite solar cells. This leads to an improvement in efficiency from an average of 3.6 % for PCBM to 5.6 % for ICBA as ETL. Detailed analysis about this phenomenon will be presented.

To sum up by modifying the charge transport layers, we could reach nearly 6 % for FASnI₃ devices without further additives except of the generally used SnF₂.

Perovskite bulk materials and inorganic quantum dots, are the most competitive materials for the future photovoltaics. Their outstanding properties in terms of photoconversion efficiency (PCE) led to a big progress, reaching an impressive PCE of 25.1%.^[1] Formamidinium based perovskite solar cells present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. ^[2] Here PbS (quantum dots and nanoplatelets) are employed to control the morphological and optoelectronic properties and a lot of efforts are dedicated in the understanding the chemical interactions and the physical process involved in the stabilization of the perovskite material. The use of PbS nanoplatelets with (100) preferential crystal orientation potentiates the effects on the crystal growth of perovskite grains and improves the stability of the material and the solar cell. A stable incident photon to current efficiency in the infrared region of the spectrum for 4 months have been obtained, one of the best stability achievement for planar perovskite solar cells. On the other hands the energetic trends revealed by our DFT calculations make clear that the origin of thermodynamically favoured black FAPbI₃ upon inclusion of PbS is due mainly to two distinct, but both needed, mechanisms: first, the structure stabilization that destabilizes the yellow phase due to its large surface energy and, second, the crucial chemical stabilization by chemical bonds between the PbS and FAPbI₃ that stabilizes the black phase.^[3]

Lead halide perovskite nanocrystals (NCs) have emerged as a potential material for LED and solar cell applications [1]. However, despite of their promising performance, the band gap of lead halide perovskite NCs remains large that limit the absorption in infrared (IR) part of spectrum [2]. In this work, we explore the possibility to harvest the IR spectrum using formamidinium lead iodine (FAPI) perovskite nanocrystals by doping of PbS NCs. As a short wave IR absorber, PbS NCs has attracted much attention yet it suffer with high dark current eventually that limit the device performance. By mixing the two compounds with an optimum ratio, it is possible to preserve most of the IR absorption while the transport driven by the wider band gap of the perovskite, this enabling a dark current reduction. In addition to understand the electronic structure of FAPI/PbS hybrid, we fabricated an FET using high capacitance solid state gating. Using this strategy, we show that the hybrid material has an n-type nature with a charge carrier mobility of 2 x 10^-3 cm² V^-1s^-1.

However, as FAPI is introduced into the PbS NCs array, the benefit of the reduced dark current is partly mitigated by a reduced IR absorption. This problem is address by introducing a plasmonic resonator. The latter relies on a grating that generate a multi passes of the light into the absorbing layer thus enhancing the IR absorption [3]. The resonant electrode enhances the light-matter coupling within the NCs film that enhance the IR absorption up to 3 times [4]. In addition, the reduction of the interelectrode spacing enable photoconduction gain leading to an improved responsivity and detectivity by two order of magnitude in comparison to pristine PbS.

Perovskite solar cells have improved drastically over the past decade, overcoming hurdles of temperature- and water-induced instability to achieve efficient, stable devices. Three-dimensional (3D) perovskites have excellent properties including high charge-carrier lifetimes and mobilities, strong absorption and good crystallinity – ideal for photovoltaic devices. However, 3D perovskite materials struggle especially with moisture-induced degradation¹. The addition of large, hydrophobic organic cations can lead to the formation of two-dimensional (2D) perovskite structures. Devices made with 2D perovskites show much greater stability, but with far lower power conversion efficiencies than their 3D cousins². Materials combining 2D and 3D structures have thus recently become one the most promising candidates for use in solar cells^3,4. In order to fully understand the optoelectronic properties of these 2D-3D hybrid systems we look at BA_x(FA_0.83Cs_0.17)_1-xPb(I_0.6Br_0.4)₃ across the composition range 0 ≤ x ≤ 80 %⁵. We find that small amounts of butylammonium (BA) help to improve crystallinity and passivate grain boundaries, thus reducing monomolecular charge-carrier recombination, and boost charge-carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous films forming, greatly reducing charge-carrier transport capabilities. For low amounts of BA the benevolent effects of reduced recombination and enhanced mobilities lead to outstanding diffusion lengths.

Perovskites offer exciting opportunities to realize efficient multijunction photovoltaic devices. This requires high-V_OC and often Br-rich perovskites, which currently suffer from halide segregation. Here, we study triple-cation perovskite cells over a wide bandgap range (∼1.5−1.9 eV). While all wide-gap cells (≥1.69 eV) experience rapid phase segregation under illumination, the electroluminescence spectra are less affected by this process. The measurements reveal a low radiative efficiency of the mixed halide phase which explains the V_OC losses with increasing Br content. Photoluminescence measurements on nonsegregated partial cell stacks demonstrate that both transport layers (PTAA and C₆₀) induce significant nonradiative interfacial recombination, especially in Br-rich (>30%) samples. Therefore, the presence of the segregated iodide-rich domains is not directly responsible for the V_OC losses. Moreover, LiF can only improve the V_OC of cells that are primarily limited by the n-interface (≤1.75 eV), resulting in 20% efficient 1.7 eV bandgap cells. However, a simultaneous optimization of the p-interface is necessary to further advance larger bandgap (≥1.75 eV) pin-type cells.

Segregation of illuminated mixed-halide perovskites into iodide- and bromide-rich domains presents a critical bottleneck in the application of wide-bandgap absorbers in single- and multi-junction architectures. And while its occurrence has been well-studied in thin films, the influence on operational solar cells lacks sufficient understanding.

This work employs a multimodal characterization procedure to observe the slow progression of halide segregation in efficient solar cells prepared in the p-i-n architecture using sequentially processed perovskite absorbers. Photoluminescence spectroscopy is used to identify the stages that demixing of halide ions entails while simultaneous tracking of photovoltaic parameters allows correlating performance degradation to the migration of ionic species in each stage. A new stage of the process is thus observed upon prolonged illumination. Characterization of sub-bandgap features reveals the occurrence of photo-induced defect formation whose suppression through cationic substitution provides a strategy for stabilization of wide-bandgap compositions against halide segregation.

Perovskite solar cells have been attracting the scientific world attention since 2009 due to outstanding absorption and charge transport properties. The possible ion migration, several ion types content and non-mono crystal structure make them really intriguing as well as challenging system to investigate by optical techniques. Electrons in the perovskite system, after light absorption, are promoted from the valence to the conduction band. First few hundreds of femtosecond after absorption are governed by cooling of hot carriers. When the process is finished, sharp absorption bleach due to the band filling phenomena occurs which decay correlates with photoluminescence kinetics and represents the excited carrier lifetime [1,2]. That decay proceeds by several paths such as the recombination (first-, second- and third-order) and charge injection to an electron transporting material (ETM) or a hole transporting material (HTM).        

We focused on a triple cation perovskite FA_0.76MA_0.19Cs_0.05(I_0.81Br_0.19)₃ sandwiched between a spiro-OMeTAD (HTM) and mesoporous TiO₂ (ETM) layers prepared under drybox (w/o oxygen and water) or ambient (in the presence of oxygen and ambient room humidity) conditions [3]. We performed femtosecond to nanosecond transient absorption as well as picosecond to nanosecond time-resolved emission studies of the prepared cells. We probed the cells from different sides to obtain information about the charge dynamics at ETM or HTM interfaces. The investigation was also supported by the electrochemical impedance and x-ray diffraction measurements.

The morphological studies indicate that the content of unreacted PbI₂ phase in the perovskite structure is much higher near the interface with titania than near the interface with spiro-OMeTAD. The stationary emission spectra and transient bleach peaks of perovskites show additional long-wavelength features close to the titania side. Time-resolved techniques reveal further differences in charge dynamics at both interfaces. The population decay is significantly faster at the ETM side than that at the HTM side for the cells prepared under ambient conditions, and the hole injection is faster for the solar cells with higher photocurrent in the cells prepared under drybox conditions. The charge recombination loss on the millisecond time scale is found to be slower at the interface with titania than with spiro-OMeTAD. The ideality factor of the cells is found to increase with increasing DMSO content in the precursor solution indicating a change in recombination mechanism from bulk to surface recombination [3].

Unlike typical inorganic semiconductors, lead–halide perovskites (LHPs) exhibit significant ionic conductivity, which is believed to affect their performance and stability. Motivated by a recent experimental study that suggested pressure as a means to control ionic conductivity in CsPbBr₃ [1], we present a detailed theoretical study of the atomic scale effects of pressure on anion migration in the low temperature orthorhombic Pnma phase of CsPbBr₃. Using nudged elastic band calculations based on density functional theory, we compute all symmetrically inequivalent activation barriers for anion migration to their closest neighbours, as a function of hydrostatic pressure in the range 0.0–2.0 GPa. We then use those values as parameters in a kinetic model which allows us to connect the atomic scale calculations to the macroscopic anion mobility tensor as a function of applied pressure.

We find that the mobility is enhanced by pressure in the plane spanned by the [100] and [001] lattice directions, while along the [010] direction it is diminished, leading to an effective 3D-to-2D transition of the mobility at elevated pressures. This can be explained by the fact that a network of only a few symmetrically inequivalent paths dominates the mobility at elevated pressures. Our results demonstrate the significant influence of pressure on both the rate and direction of anion migration in CsPbBr₃, which we consider likely to hold for other LHPs.

Lead halide perovskites are becoming subject of intense studies as they are reaching outstanding solar performance figures while they are rather easy to prepare, the main issue with them being their fragility. Our group has carried out theoretical calculations on them using hybrid functionals [1], has dealt with MD studies on them [2] and discussed the effects of ferroelectric domains on the diffusion of electrons and holes [3]. We have in addition tried their substitution with transition metals, trying to obtain the so-called in-gap band structure which has shown promise of enhancing PV solar efficiencies [4].

One case that has led to interesting results is that of partial substitution of Pb²⁺ by Cr²⁺, which has given, using GW-type calculations including spin-orbit effects, a band structure with the desired characteristics: a moderately narrow band, partially occupied and separated enough from both conduction and valence bands [5].

Then we tried the synthesis of such material, achieving the preparation of a perovskite MAPb_1-xCr_xBr_3-2xCl_2x (x=0.25 and 0.5) using mechano-chemical synthesis methods [6]. Its diffractogram agrees with that expected for a perovskite disordered in both halide and metal atoms, with nanocrystal sizes in the 30-50 nm range. Both magnetic measurements and XANES spectra indicate the almost exclusive presence of Cr²⁺ ions as Pb²⁺ substituents, with a magnetic transition to an antiferromagnetic state below 40 K which is confirmed by differential specific heat measurements. Most importantly, UV-Vis-NIR diffuse reflectance spectra show a feature at ca. 1.6 eV of photon energy, absent in the perovskite not substituted with Cr, which is accompanied also by a weaker feature at 0.75 eV. These latter results indicate that the desired in-gap band structure has been achieved, especially since the sum of the energies of these two latter features coincides with the overall bandgap of the perovskite. As far as we know, this is the first time that an isovalent substitution of Pb by Cr in a lead halide perovskite has been achieved.

Transient photoluminescence (tr-PL) measurements are among the most popular characterization techniques to monitor the charge-carrier dynamics and investigate recombination losses in halide perovskite layers and layer stacks.^[1-3] In particular, it is imperative to better understand and characterize interfacial recombination losses that often limit device performance in finished perovskite solar cells. However, interpretation of PL transients on multilayer samples including interfaces is a complex endeavour due to the superposition of various transient effects that modulate the charge-carrier concentration in the perovskite layer and thereby the measured PL. These effects include bulk and interfacial recombination, charge transfer to electron or hole transport layers and capacitive charging or discharging.^[4-6] The combination of these effects leads to substantial deviations from an exponential decay but is rather difficult to describe analytically. Hence, the state-of-the-art of interpretation of tr-PL decays of layer stacks or even full devices is currently still at an early stage. Data interpretation is often restricted to fitting one or several exponential functions to the decay to extract a “lifetime”. This approach causes a loss of information and impedes fully understanding and using the information contained in PL transients.

Here we combine numerical simulations with Sentauraus TCAD and experiments done over ~7 orders of magnitude in dynamic range on a variety of different sample geometries from perovskite films on glass to full devices to present an improved understanding of this method. We propose a presentation of the differential decay time of the tr-PL decay that follows from taking the derivative of the photoluminescence at every time.^[7] Plotting this differential decay time as a function of the time-dependent quasi-Fermi level splitting enables us to distinguish between the different contributions of radiative and non-radiative recombination as well as charge extraction and capacitive effects to the decay.

The excellent optoelectronic properties of metal halide perovskites (MHPs) have attracted extensive scientific interests and boosted their application in optoelectronic devices. Despite their attractive optoelectronic properties, their poor stability under ambient conditions remains the major challenge, hindering their large-scale practical applications. In particular, some MHPs undergo spontaneous phase transitions from perovskites to non-perovskites. Compositional engineering via mixing cations or anions has been widely reported to be effective in suppressing such unwanted phase transition. However, the atomistic and electronic origins of the stabilization effect remain unexplored. Here, by combining Density Functional Theory (DFT) calculations and Crystal Orbital Hamilton Population (COHP) analysis, we provide insights for the undesired phase transition of pristine perovskites (FAPbI₃, CsPbI₃, and CsSnI₃) and reveal the mechanisms of the improved phase stability of the mixed compounds (Cs_xFA_1-xPbI₃, CsSn_yPb_1-yI₃, and CsSn(Br_zI_1-z)₃). We identify that the phase transition is correlated with the relative strength of the M-X bonds as well as that of the hydrogen bonds (for hybrid compositions) in perovskite and non-perovskite phases. The phase transition can be suppressed by mixing ions, giving rise to either increased bond strength for the perovskite or decreased bond strength in their non-perovskite counterparts. Our results present a comprehensive understanding of the mechanisms for the phase instability of metal halide perovskites and provide design rules for engineering phase-stable perovskite compositions.

I will describe our use of multiscale modelling techniques to explore electron and ion dynamics in perovskite materials. My talk will focus firstly on mesoscale modelling of carrier dynamics in halide perovskites using ensemble Monte Carlo methods adapted from inorganic device simulation. I will explain how we can identity the effects of large polaron formation in MAPbI₃ on carrier scattering and mobilities, and explore the evolution of hot carrier distribution functions due to carrier-carrier scattering and its effects on carrier cooling. Secondly, I will describe our studies of the influence of hydrostatic pressure on ion migration in CsPbBr₃, using a transition matrix based microkinetic model parametrised from the results of DFT calculations. The phase diagram of CsPbBr₃ closely resembles the hybrid counterparts, making it a good proxy for studying ion mobility in other halide perovskites.

One of the most common methods for the deposition of perovskite layers in photovoltaic devices is the antisolvent engineering method. In this method, during the spin-coating of the perovskite precursor solution, an antisolvent is dripped onto the sample, triggering the removal of the host solvent and the crystallization of the perovskite layer. A few selected antisolvents have emerged as the most successful in the deposition of high quality perovskite layers, however, they do not seem to share any common properties with both polar and nonpolar, high and low boiling point solvents employed for device fabrication.

In this talk, I will introduce a general method that allows the fabrication of highly efficient perovskite solar cells by any antisolvent. Through a detailed study of perovskite films and devices fabricated by 14 different antisolvents, we identify the two key antisolvent properties that influence the deposition procedure that would lead to the formation of high quality perovskite films. When taking these properties into account, each antisolvent can be utilized to produce high performance devices with efficiencies up to 22%.

Solar energy can lead a “paradigm shift” in the energy sector with a new low-cost, efficient, and stable technology. Nowadays, three-dimensional (3D) methylammonium lead iodide perovskite solar cells are undoubtedly leading the photovoltaic scene with their power conversion efficiency (PCE) >25%, holding the promise to be the near future solution to harness solar energy [1]. Tuning the material composition, i.e. by cations and anions substitution, and functionalization of the device interfaces have been the successful routes for a real breakthrough in the device performances [2]. However, poor device stability and still lack of knowledge on device physics substantially hamper their take-off. Here, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites have demonstrated high stability, far above their 3D counterparts [3]. However, their narrow band gap limits their light-harvesting ability, compromising their photovoltaic action. Combining 2D and 3D into a new hybrid 2D/3D heterostructure will be here presented as a new way to boost device efficiency and stability, together. The 2D/3D composite self-assembles into an exceptional gradually

organized interface with tunable structure and physics. To exploit new synergistic function, interface physics, which ultimately dictate the device performances, is explored, with a special focus on charge transfer dynamics, as well as long term processing happening during aging. As shown in Fig.1, when 2D perovskite is used on top of the 3D, an improved stability is demonstrated. 2D perovskite acts as a sheath to physically protect the 3D underneath. In concomitance, we discovered that the stable 2D perovskite can block ion movement, improving the interface stability on a slow time scale. The joint effect leads to PCE=20% which is kept stable for 1000 h [3,4]. Incorporating the hybrid interfaces into working solar cells is here demonstrated as an interesting route to advance in the solar cell technology bringing a new fundamental understanding of the interface physics at multi-dimensional perovskite junction. The knowledge derived is essential for a deeper understanding of the material properties and for guiding a rational device design, even beyond photovoltaics.

The surge of perovskite solar cell device performance has been facilitated through breakthroughs in process engineering. Most process optimization has been carried out for spin-coating, which is the most common deposition technique used in metal-halide perovskite solar cell fabrication in labs around the world. Through a series of experiments using a small-footprint optical monitoring setup [1], we were able to gain insight into common perovskite precursor solutions and deposition strategies. For the most commonly used deposition method in perovskite solar cell manufacturing, spin-coating, we were able to distinguish different regimes of spin-coating from optical signatures. Process parameters like the spin-coating speed and precursor solution concentration critically determine wet film thinning and perovskite crystallization. We studied the effect of anti-solvent drip timing on the formation mechanism of metal-halide perovskite semiconductors for two different standard precursor solutions: MAPbI₃ and (Cs,MA,FA)Pb(Br,I)₃.[2] Our results enabled us to rationalize the much better reproducibility of (Cs,MA,FA)Pb(Br,I)₃ perovskite precursor solutions demonstrating that the halide ratio critically affects the fraction of solvate intermediate phase formed during crystallization. Other process parameters rarely considered, such as the timing of moving samples to a hotplate after spin-coating, were found to have a substantial influence on device performance as the compositional homogeneity is strongly affected.[3] Our in-depth insights into spin-coating of metal-halide perovskites prove to be of high value when translating processing strategies to scalable manufacturing methods such as slot-die coating and inkjet printing.

The interest in perovskite photovoltaics has significantly increased over the last few years. High power conversion efficiencies (PCE) and low-cost manufacturing make perovskite PVs a very promising candidate for future applications. Despite the very advantageous features of perovskite materials, several issues still need to be solved before the commercialization of perovskite solar cells (PSCs). The main challenges of bringing perovskite technologies to the market are (i) scaling-up of the cells and modules dimension, (i) usage of lead in the perovskite solar panels, and (iii) stability of the PSC modules. These three challenging topics will be discussed in the presentation, and possible solutions to overcome these issues will be proposed. The implementation of the proposed measures will help to demonstrate the feasibility of high-volume production. It is an important milestone towards the industrial manufacturing of perovskite photovoltaics and their future commercialization.

The talk will review consensus procedures for planning, conducting, and reporting stability testing of perovskite solar cells (PSC) recently formulated by a broad research community [1]. In particular, the suggested protocols highlight the importance of testing for: (1) Redistribution of charged species upon application of electric fields [2]; (2) distinguishing between degradation induced by various stress factors, (3) reversible degradation with qualitatively different recovery dynamics [3,4]. The recommended protocols are not meant to replace existing qualification standards, but rather to contribute to developing an understanding of PSC degradation mechanisms. Acceptance of these protocols and sharing the suggested datasets will facilitate inter-laboratory coordination and assist in the accumulation of PSC stability data acquired under well-defined and comparable conditions. This would allow the application of advanced approaches to analyzing large data sets, such as machine learning methods, and accelerate the development of stable PSC devices.

References

M. V. Khenkin, E. A. Katz, A. Abate, G. Bardizza, J. J. Berry, C. J. Brabec, F. Brunetti, V. Bulović, Q. Burlingame, A. Di Carlo, R. Cheacharoen, Y.-B. Cheng, A. Colsmann, S. Cros, K. Domanski, M. Dusza, C. J. Fell, S. R. Forrest, Y. Galagan, D. Di Girolamo, M. Grätzel, A. Hagfeldt, E. von Hauff, H. Hoppe, J. Kettle, H. Köbler, M. S. Leite, S. (Frank) Liu, Y.-Lin Loo, J. M. Luther, C.-Q. Ma, M. Madsen,  M. Manceau, M. Matheron, M. McGehee, R. Meitzner, M. K. Nazeeruddin, A. F. Nogueira, Ç. Odabaşı, A. Osherov, N.-G. Park, M. O. Reese, F. De Rossi, M. Saliba, U. S. Schubert, H. J. Snaith, S. D. Stranks, W. Tress, P. A. Troshin, V. Turkovic, S. Veenstra, I. Visoly-Fisher, A. Walsh, T. Watson, H. Xie, R. Yıldırım, S. M. Zakeeruddin, K. Zhu and M.  Lira-Cantu. Consensus on ISOS Protocols for Stability Assessment and Reporting for Perovskite Photovoltaics. Submitted.

M. V. Khenkin, K.M. Anoop, E. A. Katz and I. Visoly-Fisher. Bias Dependent Degradation of Various Solar Cells: Lessons for Stability of Perovskite Photovoltaics. Energy & Environmental Science, v. 12, No. 2, p. 550-558 (2019).

M. V. Khenkin, K.M. Anoop, I. Visoly-Fisher, S. Kolusheva, Y. Galagan, F. Di Giacomo, O. Vukovic, B. R. Patild, G. Sherafatipourd, V. Turkovic, H.-G. Rubahnd, M. Madsen, A. Mazanik and E. A. Katz. Dynamics of photoinduced degradation of perovskite photovoltaics: from reversible to irreversible processes. ACS Applied Energy Materials, 1, 799-806 (2018).

M. V. Khenkin, K. M. Anoop, I. Visoly-Fisher, Y. Galagan, F. Di Giacomo, B. R. Patil, G. Sherafatipour, V. Turkovic, H.-G. Rubahn, M. Madsen, T. Merckx, G. Uytterhoeven, J. P. A. Bastos, T. Aernouts, F. Brunetti, M. Lira-Cantu and E. A. Katz. Reconsidering Figures of Merit for the Performance and Stability of Perovskite Photovoltaics. Energy & Environmental Science, 11, 739-743 (2018).

We address a controversy surrounding the luminescence properties of low-dimensional halide perovskites and clarify that an often-observed broad luminescence arises from defect states instead of commonly invoked self-trapped excitons.

Whereas initial research into two-dimensional perovskites was predominantly driven by efforts to employ their narrow emission linewidth for LEDs or to boost the stability of their three-dimensional counterparts in photovoltaics, the latest hotly examined observation is the presence of broad emission bands. This broad emission has the potential for direct white light generation and significant research is currently conducted to find and optimise compounds for this purpose. Crucially, these efforts commonly base on the assumption that the origin of this luminescence is a so-called self-trapped exciton. Whilst this concept is elegant and theoretical calculations have offered some support, experimental evidence for this interpretation is so far scarce.

We therefore studied single-crystals of two-dimensional lead iodide perovskites through a variety of spectroscopic techniques and prove that the broad emission is in fact due to defect states in the bulk of the material. We study two compounds with different A-site cation and further vary the halide to underline the universality of our findings and meticulously exclude all other origins of broad emission bands that have hitherto been proposed.

Our work sheds light on the role of electron-phonon interactions and defect states in lead-based perovskites as well as an improved understanding of the luminescence properties of these compounds.

Perovskite nanostructures have been engineered for LEDs, lasers and photodetectors[1], their reduced dimensionality resulting in quantum confinement of charge carriers which yields dramatically different optoelectronic properties, including enhanced photoluminescence quantum yield[2] and lower thresholds for amplified spontaneous emission[3]. Although the creation of such perovskite nanostructures has clear advantages, it often relies on challenging top-down fabrication methods. It would therefore be highly advantageous if instead nanoscale domains were found to form intrinsically through self assembly in the perovskite.

In this study[4], I report the discovery of intrinsically-occurring nanostructures in FAPbI₃, which exhibit quantum confinement effects manifested as an oscillatory absorption feature above the band gap. These features are present at room temperature but sharpen and become more apparent as the temperature is lowered towards 4 K. I demonstrate that the energetic spacings and temperature-dependence of the peaks vary in a manner consistent with quantum confinement intrinsically associated with the lattice of the material. I suggest the origin of this confinement to be nanodomains with an extent of approximately 10-20 nm. This interpretation is supported by correlating absorption spectra against ab initio calculations based on the bandstructure of FAPbI₃ in the presence of infinite barriers, and simulations for superlattices with moderate barrier heights. I further explore ferroelectricity/ferroelasticity and delta-phase twin boundaries as two possible causes of these domains. Altogether, such absorption peaks present a novel and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying new physical behaviours that may lead to developing new applications. These responses are the outcome of complex interactions of electronic and ionic carriers in the bulk and at interfaces. Based on a systematic application of frequency modulated techniques and time transient techniques to the analysis of kinetic phenomena, we present a picture of the dominant effects governing the kinetic behaviour of halide perovskite devices. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases. Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in defects of the structure. We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent. We apply the dynamic picture to the characterization of perovskite memristors. A memristor is a device that has different metastable states at a voltage V. It has a resistance that depends on the history of the system, and the states can be switched by applied voltage. It is simpler than a transistor in that the control occurs by 2 contacts. As a summary we suggest an interpretation of the effects of charge accumulation, transport, and recombination, how these effects influence the current-voltage characteristics and time transient properties, and we suggest a classification of the time scales for ionic/electronic phenomena in the perovskite solar cells.

Current-voltage measurement and impedance spectroscopy (IS) are two closely related techniques. In this talk, we first show how different external contacts in Perovskite solar cells (PSCs) change the extracted-current transport resistance and the corresponding effects in the IS and the jV curve.[1] Hysteresis is the difference between the jV scan measured form short circuit to open circuit conditions (forward scan) and the opposite direction scan (reverse scan). Normal hysteresis, improved FF and V_oc in the reverse scan compared to the forward scan, is commonly observed in PSCs. This hysteresis has been related to the low-frequency capacitance in the IS response.[2, 3] Inverted hysteresis, which improves FF and V_oc in the forward scan, as well as negative capacitance at the IS low-frequency domain, are also familiar features in PSC, but their origin is still under discussion. In this talk, we show the emergence of these responses in two separate experiments employing different PSCs formulations. By mean of the Surface Polarization Model[4, 5] we expose that these features have a time constant associated with ions/vacancies kinetics interaction with the surface. These interactions increase the interfacial recombination, reducing the recombination resistance obtained by the IS measurements and provoking a flattening of the j-V curve.[6]

Outstanding efficiency (up to 25.2% on glass[1], 19.5% on flexible substrates[2]), low cost, and processability from solution are key advantages of perovskite solar cells (PSC). This technology is rising as a promising candidate for building-integrated photovoltaics, space and automotive applications, and even foldable and lightweight devices for consumer electronics when deployed on flexible substrates[3].

Efforts have been made to up-scale the fabrication of PSC to produce large solar modules[4]: well-known techniques such as screen printing, blade- and slot-die coating, inkjet printing have been adapted and tested as possible manufacturing processes. We have focused on an automated spray coating technique[5], starting from the first fundamental layer for the standard n-i-p structure, the electron transport layer (ETL).

Herein, we thus present the development of uniform large-area (> 120 cm2) compact films of tin oxide nanoparticles (SnO2-NP) on rigid glass/ITO substrates via spray deposition, and we show their subsequent application as ETL in PSCs.

Our work investigated the effect of the spray deposition parameters (such as gas pressure, nozzle aperture, spray deposition velocity, flow rate, spray distance and spray cycle times) on the morphology and electro-optical properties of the SnO2 films, as well as the influence of the substrate temperature: given a set of deposition parameters, we found out that SnO2 layers sprayed at a lower temperature (25-30 °C) performed better as ETL in PSCs than those produced at higher temperatures (60-120°C), opening up to the application to flexible substrates.

Furthermore, PSCs endowed with SnO2 films, sprayed at low-temperature, performed as good as those with standard spin-coated films, delivering up to 16.8% efficiency under 1 sun illumination and demonstrating that automated spray coating at low temperature can be an effective strategy for PSC technology transfer from lab to industry to manufacture large-area devices, even on flexible substrates.

While solution-processable metal halide perovskites have sparked a major interest in (opto)electronic applications, light emitting diodes (PeLEDs) from metal perovskites have not been utilized by inkjet-printing [1]. Our work represents the first demonstration of inkjet-printed PeLEDs by utilizing a modified poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) templating layer. By adding potassium chloride (KCl) to the commonly used hole injection material PEDOT:PSS underneath the inkjet-printed perovskite layer, we significantly influence the crystallization behavior and later device performance of the PeLEDs.

The so-called “salty” PEDOT:PSS acts as a seeding template to induce crystal nucleation of the metal halide perovskites inkjet-printed on top, while having only a minor effect on the electrical properties of PEDOT:PSS. Together with the perovskite polymer composite ink, optimized for inkjet printing, this templating approach eliminates the conventional antisolvent treatment required to induce favorable perovskite nucleation.

PeLEDs utilizing the KCl-induced templating effect in a planar PEDOT:PSS/MAPbBr₃:PEG architecture show greatly improved performance, mainly due to improved crystallization dynamic in contrast to PeLEDs containing pure PEDOT:PSS. Specifically, KCl-modified PEDOT:PSS contact layers enabled the realization of inkjet-printed PeLEDs with a 30-fold increased luminance at the same operating voltage. Impressively, the ink-jet printed PeLEDs shown in this paper have comparable performance parameters to spin-coated reference devices and pave the way to cost-effective, scalable, and printable PeLEDs [2].

Hybrid organic-inorganic metal-halide perovskite solar cells (PSCs) have achieved a tremendous rise in power conversion efficiency (PCE) from 3.8 %[1] to an impressive level of 25.2 %[2]. However, a reliable transfer of solution processing from spin coating to scalable printing techniques and a homogeneous deposition on large substrate sizes is challenging.

Typically, Poly(triaryl amine) (PTAA) is replacing the widely used Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)[3,4] as hole transport layer (HTL) in planar inverted (p-i-n) PSCs because of its efficient carrier transport properties.[5-7] PSCs with PTAA mostly benefit from boosted open circuit voltage[5,8,9] due to a proper energy level alignment.[10]

However, PTAA is a non-polar polymer with a low surface energy and thus is highly hydrophobic, which leads to severe dewetting of the subsequently deposited polar perovskite precursor solutions.[11-13]

Based on our recent publication on a universal nanoparticle (NP) wetting agent for perovskite precursor solutions on non-wetting materials deposited via spin coating[14], we here show the utilization of blade coated non-conductive silicon oxide (SiO₂) NP dispersions to enable the deposition of a homogeneous perovskite layer on the highly hydrophobic HTL. The NPs enhance the HTL surface energy, thus, wetting and homogeneous spreading of the precursor solution is strongly improved so that pinholes in the perovskite layer and thereby short-circuited devices are avoided. In this work, we demonstrate the transfer of this wetting concept to scalable gas stream-assisted blade coating and solution-processed PSCs and modules in the inverted device architecture with PTAA as HTL on large-area substrates for the first time. In order to prevent void formation at the HTL interface of gas stream-assisted blade coated perovskite layers, the effect of blending small amounts of lead chloride (PbCl₂) in the perovskite precursor solution is investigated, which also improves reproducibility and device performance. Following these optimizations, blade coated PSCs with 0.24 cm² active area achieve up to 17.9 % PCE. Furthermore, to prove scalability, we show enlarged substrates of up to 9×9 cm² and analyze the homogeneity of the perovskite layer in blade coating direction. Moreover, by implementing the blade coated NP wetting agent, we fabricate large-area modules with a maximum PCE of 9.3 % on a 49.60 cm² aperture area. This represents a further important step bringing solution-processed inverted PSCs closer to application.

Mixed halide perovskites (MHPs) under photoirradiation phase segregate into I- and Br-rich domains and, if in contact with a solvent, the film will then expel iodine into solution. Hole trapping at the I-site in MHPs dictates the iodine migration. We have now succeeded in modulating the iodide expulsion process in MHPs through externally applied electrochemical bias. At anodic potentials, electron extraction at the electrode interface becomes more efficient, leading to build-up of holes within the MHP film. This in turn favors phase segregation and increases the rate iodine expulsion. Conversely, at cathodic bias we facilitate electron-hole recombination within the MHP film and slow down iodine expulsion. The tuning of EFermi through external bias modulates charge extraction at the perovskite electrode interface and indirectly controls the build-up of holes, which in turn induces iodine expulsion. Suppressing iodine migration in perovskite solar cell is important for attaining greater stability of perovskite solar cells since they operate under internal bias.

Heavy water or deuterium oxide (D₂O) comprises of deuterium, a hydrogen isotope twice the mass of hydrogen. In contrast to the report of shorter charge carrier lifetimes and lower/invariant efficiencies on deuteration of perovskite, we herein uncover the unexpected effect of D2O as solvent additive to enhance the power conversion efficiency and stability in solar cell devices. Here, we demonstrate the PCE increment of triple-A cation (cesium (Cs)/methylammonium (MA)/formaminidium (FA)) perovskite solar cells from approximately 19.2% (reference) to ~21 % (using 1 vol% D₂O) with higher stability in comparison with 1% H₂O (by vol) additive. The in-depth investigation using ultrafast optical spectroscopy divulge the suppression of trap states from 2.5 x 10¹⁷ cm^-3 to 0.7 x 10¹⁷ cm^-3 and increase of PL lifetime from 35 nm to 70 nm. Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy validates N-H₂ group as the preferential isotope exchange site and induced alteration of the FA to MA ratio as a result of perovskite deuteration. Theoretical simulations using first-principles density functional shows a decrease in PbI₆ phonon frequencies in the deuterated perovskite lattice which stabilizes the PbI₆ structures and weakens the electron-LO phonon (Fröhlich) coupling. Herein, our findings of selective isotope exchange in perovskite opens the opportunities for tuning perovskite optoelectronic properties.

Abstract

Organic-inorganic halide perovskites are promising as the light absorber of solar cells because of their efficient solar power conversion. Power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have already reached a very high level of up to 25.2 %, which is comparable to silicon solar cell technology. Most of high-performing PSCs reported to date contain a small molecular hole transport layer (HTL) material of 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD). An issue frequently occurring in spiro-OMeTAD-based PSCs is quick performance degradation at high temperature. In this study, we discover that post-doping of the spiro-OMeTAD layer by iodine released from the perovskite layer is one possible mechanism of the high-temperature PSC degradation. The iodine doping leads to the highest occupied molecular orbital level of the spiro-OMeTAD layer becoming deeper and, therefore, induces the formation of an energy barrier for hole extraction from the perovskite layer. We demonstrate that it is possible to suppress the high-temperature degradation by employing an iodine-blocking layer or an iodine-free perovskite in PSCs. These findings will guide the way for the realization of thermally stable perovskite optoelectronic devices in the future.

Room temperature ion migration under an external electric field is being increasingly accepted as one of the major origins of the commonly observed current-voltage hysteresis in solar cells and light-emitting diodes, fabricated using three-dimensional (3D) hybrid lead halide perovskites. ^[1] This mixed ionic-electronic nature of perovskite and other perovskite-related materials can now be taken advantage of in resistance-switching memory devices whose performance, in terms of the ON/OFF ratio, depends on the efficiency of the vacancy/ion migration process. However, in the halide perovskites field, a direct link between the average/local structure and the preferred ion migration hopping pathway has yet to be established. In our study, we combined the study of average/local structural characterization and detailed electrical measurements to shine a light on the interrelationships between the structure and efficiency of ion migration in layered methylammonium copper halide materials (MA₂CuX₄). ^[2] We adopted the solvent acidolysis crystallization technique to grow various halide-deficient single crystals and with the help of synchrotron X-ray powder diffraction (XRPD) and pair distribution function (PDF) analyses, we identified the halogen vacancy site in the copper halide octahedra, the octahedra tilting, and the thermal vibrations of the atoms around their average positions. We correlated the variations in these parameters to the hysteresis observed in the current-voltage curves and subsequently to the ON/OFF ratios of proof-of-concept memory devices fabricated using inert Pt electrodes. Furthermore, our best ON/OFF ratio of 10 from our Pb-free devices compares well to the results obtained from two-dimensional Pb-based devices utilizing inert electrodes. Our experimental results made on single crystalline samples highlights the need to study detailed structural factors that could affect ionic migration in perovskite and perovskite-related compounds, which is important for the performance of memristor and optoelectronic devices.

Thanks to their high absorption coefficient and ideal band-gap [1], halide perovskite materials are good candidates for the next generation of solar cells with an impressive certified power conversion efficiency of 25.2 %. Defect-induced trap states are believed to be partially responsible for the instability of perovskite materials through their formation, passivation and subsequent degradation of the material [2]. Extracting information related to these trap states such as their concentration and the trapping rate is thus essential in order to compare samples and devices to help drive the improvement of perovskite solar cells. Time-resolved photoluminescence (TRPL) is a powerful tool to investigate excited charge carrier (electrons and holes) recombinations in semiconductors and molecular systems. However, due to the non-excitonic nature of excited charge carriers in lead halide perovskite materials coupled with the presence of localised trap states in their band-gap, the TRPL of these materials is complicated to interpret. Here we discuss two models used in the literature to simulate charge carrier recombinations and TRPL in perovskite materials. These models consider the bimolecular nature of direct electron-hole recombination but differ in their treatment of trap-mediated recombinations; one describing trapping as a monomolecular process [3] whereas the other considers it to be a bimolecular process between free carriers and the available trap states [4]. The dependency on the excitation fluence of each model is discussed as well as the importance to allow complete recombination of all excited charge carriers between two consecutive excitation pulses. Finally, we compare these models to the commonly used bi-exponential model.

Within the past few years, metal halide perovskites have been attracting significant interest due to their and their versatile use in a wide range of applications. These materials have been used in lasers, photodetectors, and most commonly, in photovoltaic devices and light emitting diodes. Despite the cheap and simple fabrication methods by which these materials are deposited, the resulting perovskite films are effectively high-quality semiconductors, and the power conversion efficiencies of lead halide perovskite solar cells are now exceeding certified values of 23%. However, perovskite-based devices are yet to achieve their full potential. One of the major hindrances to achieving this is an incomplete understanding of perovskite surfaces and interfaces. Deficiencies at these interfaces may be responsible for the largest losses in perovskite-based optoelectronic devices; limiting charge extraction, increasing non-radiative recombination rates and leading to hysteresis, and significantly increasing the voltage loss in perovskite photovoltaics. Herein, I will present interface modification strategies to mitigate these deficiencies. Charge-transfer dopants are used to dope the perovskite at the interface, resulting in the formation of narrow homojunctions. These homojunctions result in reduced interfacial recombination, suppressed hysteresis and improved device performance, yielding steady-state device efficiencies of over 21%. I will also explore the use fluoride-containing ionic liquids at the metal-oxide perovskite interface and show that not only do they affect the work function of the metal oxide, but also interact strongly with the perovskite, significantly improving the quality of the perovskite film. The utility of these defect mitigation strategies can readily be applied beyond perovskite PV and is likely to also improve the performance of a range of other perovskite-based optoelectronic devices

Thin-film perovskite solar cells (PSCs), whose record efficiency has rocketed from under 4% to over 25% (comparable to silicon solar cells) in just ten years, offer unprecedented promise of low-cost, high-efficiency renewable electricity generation. Organic-inorganic halide perovskite (OIHP) materials at the heart of PSCs have unique crystal structures, which entail rotating organic cations inside inorganic cages, imparting them with desirable optical and electronic properties. To exploit these properties for PSCs application, the reliable deposition of high-quality OIHP thin films over large areas is critically important. The microstructures and grain-boundary networks in the resulting polycrystalline OIHP thin films are equally important as they control the PSC performance and stability. Fundamental phenomena pertaining to synthesis, crystallization, coarsening, microstructural evolution, and grain-boundary functionalization involved in the processing of OIHP thin films for PSCs will be discussed with specific examples. In addition, the unique mechanical behavior of OIHPs, and its implication on the reliability of PSCs, will be discussed. The overall goal of our research is to have deterministic control over the scalable processing of tailored OIHP thin films with desired compositions, phases, microstructures, and grain-boundary networks for efficient, stable, and reliable PSCs of the future.