Composition dependence of X-ray stability and degradation mechanisms at lead halide perovskite single crystal surfaces.

The multiple applications of lead halide perovskite materials and the extensive use of X-ray based techniques to characterize them highlight a need to understand their stability under X-ray irradiation. Here, we present a study where the X-ray stability of five different lead halide perovskite compositions (MAPbI3, MAPbCl3, MAPbBr3, FAPbBr3, CsPbBr3) was investigated using photoelectron spectroscopy. To exclude effects of thin film formation on the observed degradation behaviors, we studied clean surfaces of single crystals. Different X-ray resistance and degradation mechanisms were observed depending on the crystal composition. Overall, perovskites based on the MA+ cation were found to be less stable than those based on FA+ or Cs+. Metallic lead formed most easily in the chloride perovskite, followed by bromide, and only very little metallic lead formation was observed for MAPbI3. MAPbI3 showed one main degradation process, which was the radiolysis of MAI. Multiple simultaneous degradation processes were identified for the bromide compositions. These processes include ion migration towards the perovskite surface and the formation of volatile and solid products in addition to metallic lead. Lastly, CsBr formed as a solid degradation product on the surface of CsPbBr3.

Interface Energy Alignment between Lead Halide Perovskite Single Crystals and TIPS-Pentacene.

At present, there is a huge development in optoelectronic applications using lead halide perovskites. Considering that device performance is largely governed by the transport of charges across interfaces and, therefore, the interfacial electronic structure, fundamental investigations of perovskite interfaces are highly necessary. In this study, we use high-resolution soft X-ray photoelectron spectroscopy based on synchrotron radiation to explore the interfacial energetics for the molecular layer of TIPS-pentacene and lead halide perovskite single crystals. We perform ultrahigh vacuum studies on multiple thicknesses of an in situ formed interface of TIPS-pentacene with four different in situ cleaved perovskite single crystals (MAPbI3, MAPbBr3, FAPbBr3, and CsxFA1-xPbBryI3-y). Our findings reveal a substantial shift of the TIPS-pentacene energy levels toward higher binding energies with increasing thickness, while the perovskite energy levels remain largely unaffected regardless of their composition. These shifts can be interpreted as band bending in the TIPS-pentacene, and such effects should be considered when assessing the energy alignment at perovskite/organic transport material interfaces. Furthermore, we were able to follow a reorganization on the MAPbI3 surface with the transformation of the surface C 1s into bulk C 1s.

Experimental and Theoretical Core Level and Valence Band Analysis of Clean Perovskite Single Crystal Surfaces.

A detailed understanding of the surface and interface properties of lead halide perovskites is of interest for several applications, in which these materials may be used. To develop this understanding, the study of clean crystalline surfaces can be an important stepping stone. In this work, the surface properties and electronic structure of two different perovskite single crystal compositions (MAPbI3 and Csx FA1- x PbI3 ) are investigated using synchrotron-based soft X-ray photoelectron spectroscopy (PES), molecular dynamics simulations, and density functional theory. The use of synchrotron-based soft X-ray PES enables high surface sensitivity and nondestructive depth-profiling. Core level and valence band spectra of the single crystals are presented. The authors find two carbon 1s contributions at the surface of MAPbI3 and assign these to MA+ ions in an MAI-terminated surface and to MA+ ions below the surface. It is estimated that the surface is predominantly MAI-terminated but up to 30% of the surface can be PbI2 -terminated. The results presented here can serve as reference spectra for photoelectron spectroscopy investigations of technologically relevant polycrystalline thin films, and the findings can be utilized to further optimize the design of device interfaces.

The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials.

There is a high fundamental interest in the surface and bulk chemistry of quantum dot (QD) solids, as they have proven to be very promising materials in optoelectronic devices. The choice of surface ligands for quantum dots in solid devices determines many of the film properties, as the ligands influence for example the doping density, chemical stability and charge transport. Lead halide ligands have developed as the main ligand of choice for lead sulfide quantum dots, as they have been shown to passivate quantum dot surfaces and enhance the chemical stability. In this study, we successfully varied the ligand composition on the surface of PbS quantum dot films from pure lead iodide to pure lead bromide and investigated its influence on the chemical and electronic structure of the QD solids using hard X-ray photoelectron spectroscopy (HAXPES). Furthermore, we developed a surface treatment to prevent the surface oxidation of a bulk PbS reference sample. Through measurements of this sample and of lead halide reference samples, we were able to assign the contributions of different chemical bonding to the Pb 4f core level and of different atomic orbitals to the valence band spectral shape of the QD materials. Overall, we found that the valence band edge position was very similar for all different iodide:bromide ratios and that all investigated compositions were able to protect the quantum dot surfaces within solid films from oxidation. However, the ligand composition significantly influences the sample stability under X-rays. The iodide rich QD solids showed the highest stability with very little to no chemical changes over several hours of X-ray exposure, while the bromide rich QD solids changed already within the first hour of exposure.

X-ray stability and degradation mechanism of lead halide perovskites and lead halides.

Lead halide perovskites have become a leading material in the field of emerging photovoltaics and optoelectronics. Significant progress has been achieved in improving the intrinsic properties and environmental stability of these materials. However, the stability of lead halide perovskites to ionising radiation has not been widely investigated. In this study, we investigated the radiolysis of lead halide perovskites with organic and inorganic cations under X-ray irradiation using synchrotron based hard X-ray photoelectron spectroscopy. We found that fully inorganic perovskites are significantly more stable than those containing organic cations. In general, the degradation occurs through two different, but not mutually exclusive, pathways/mechanisms. One pathway is induced by radiolysis of the lead halide cage into halide salts, halogen gas and metallic lead and appears to be catalysed by defects in the perovskite. The other pathway is induced by the radiolysis of the organic cation which leads to formation of organic degradation products and the collapse of the perovskite structure. In the case of Cs0.17FA0.83PbI3, these reactions result in products with a lead to halide ratio of 1 : 2 and no formation of metallic lead. The radiolysis of the organic cation was shown to be a first order reaction with regards to the FA+ concentration and proportional to the X-ray flux density with a radiolysis rate constant of 1.6 × 10-18 cm2 per photon at 3 keV or 3.3 cm2 mJ-1. These results provide valuable insight for the use of lead halide perovskite based devices in high radiation environments, such as in space environments and X-ray detectors, as well as for investigations of lead halide perovskites using X-ray based techniques.

Tailoring ultra-fast charge transfer in MoS2.

Charge transfer dynamics are of importance in functional materials used in devices ranging from transistors to photovoltaics. The understanding of charge transfer in particular of how fast electrons tunnel away from an excited state and where they end up, is necessary to tailor materials used in devices. We have investigated charge transfer dynamics in different forms of the layered two-dimensional material molybdenum disulphide (MoS2, in single crystal, nanocrystalline particles and crystallites in a reduced graphene oxide network) using core-hole clock spectroscopy. By recording the electrons in the sulphur KLL Auger electron kinetic energy range we have measured the prevalence of localised and delocalised decays from a state created by core excitation using X-rays. We show that breaking the crystal symmetry of the single crystal into either particles or sheets causes the charge transfer from the excited state to occur faster, even more so when incorporating it in a graphene oxide network. The interface between the MoS2 and the reduced graphene oxide forms a Schottky barrier which changes the ratio between local and delocalised decays creating two distinct regions in the charge transfer dependent on the energy of the excited electron. Thereby we show that ultra-fast charge transfer in MoS2 can be tailored, a result which can be used in the design of emergent devices.

Probing the dye-semiconductor interface in dye-sensitized NiO solar cells.

The development of p-type dye-sensitized solar cells (p-DSSCs) offers an opportunity to assemble tandem photoelectrochemical solar cells with higher efficiencies than TiO2-based photoanodes, pioneered by O'Regan and Grätzel [Nature 353, 737-740 (1991)]. This paper describes an investigation into the behavior at the interfaces in p-DSSCs, using a series of BODIPY dyes, BOD1-3. The three dyes have different structural and electronic properties, which lead to different performances in p-DSSCs. We have applied photoelectron spectroscopy and transient absorption spectroscopy to rationalize these differences. The results show that the electronic orbitals of the dyes are appropriately aligned with the valence band of the NiO semiconductor to promote light-induced charge transfer, but charge-recombination is too fast for efficient dye regeneration by the electrolyte. We attribute this fast recombination, which limits the efficiency of the solar cells, to the electronic structure of the dye and the presence of Ni3+ recombination sites at the NiO surface.

Electronic Structure Characterization of Cross-Linked Sulfur Polymers.

Cross-linked polymers of elemental sulfur are of potential interest for electronic applications as they enable facile thin-film processing of an abundant and inexpensive starting material. Here, we characterize the electronic structure of a cross-linked sulfur/diisopropenyl benzene (DIB) polymer by a combination of soft and hard X-ray photoelectron spectroscopy (SOXPES and HAXPES). Two different approaches for enhancing the conductivity of the polymer are compared: the addition of selenium in the polymer synthesis and the addition of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) during film preparation. For the former, we observe the incorporation of Se into the polymer structure resulting in a changed valence-band structure. For the latter, a Fermi level shift in agreement with p-type doping of the polymer is observed and also the formation of a surface layer consisting mostly of TFSI anions.

Electronic structure dynamics in a low bandgap polymer studied by time-resolved photoelectron spectroscopy.

Means to measure the temporal evolution following a photo-excitation in conjugated polymers are a key for the understanding and optimization of their function in applications such as organic solar cells. In this paper we study the electronic structure dynamics by direct pump-probe measurements of the excited electrons in such materials. Specifically, we carried out a time-resolved photoelectron spectroscopy (TRPES) study of the polymer PCPDTBT by combining an extreme ultraviolet (XUV) high harmonic generation source with a time-of-flight spectrometer. After excitation to either the 1st excited state or to a higher excited state, we follow how the electronic structure develops and relaxes on the electron binding energy scale. Specifically, we follow a less than 50 fs relaxation of the higher exited state and a 10 times slower relaxation of the 1st excited state. We corroborate the results using DFT calculations. Our study demonstrates the power of TRPES for studying photo-excited electron energetics and dynamics of solar cell materials.

Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology, Charge Generation and Device Performance.

In this work, molecular tuning of metal xanthate precursors is shown to have a marked effect on the heterojunction morphology of hybrid poly(3-hexylthiophene-2,5-diyl) (P3HT)/CdS blends and, as a result, the photochemical processes and overall performance of in situ fabricated hybrid solar cells. A series of cadmium xanthate complexes is synthesized for use as in situ precursors to cadmium sulfide nanoparticles in hybrid P3HT/CdS solar cells. The formation of CdS domains is studied by simultaneous GIWAXS (grazing incidence wide-angle X-ray scattering) and GISAXS (grazing incidence small-angle X-ray scattering), revealing knowledge about crystal growth and the formation of different morphologies observed using TEM (transmission electron microscopy). These measurements show that there is a strong relationship between precursor structure and heterojunction nanomorphology. A combination of TAS (transient absorption spectroscopy) and photovoltaic device performance measurements is used to show the intricate balance required between charge photogeneration and percolated domains in order to effectively extract charges to maximize device power conversion efficiencies. This study presents a strong case for xanthate complexes as a useful route to designing optimal heterojunction morphologies for use in the emerging field of hybrid organic/inorganic solar cells, due to the fact that the nanomorphology can be tuned via careful design of these precursor materials.

Energy level alignment in TiO2/metal sulfide/polymer interfaces for solar cell applications.

Semiconductor sensitized solar cell interfaces have been studied with photoelectron spectroscopy to understand the interfacial electronic structures. In particular, the experimental energy level alignment has been determined for complete TiO2/metal sulfide/polymer interfaces. For the metal sulfides CdS, Sb2S3 and Bi2S3 deposited from single source metal xanthate precursors, it was shown that both driving forces for electron injection into TiO2 and hole transfer to the polymer decrease for narrower bandgaps. The energy level alignment results were used in the discussion of the function of solar cells with the same metal sulfides as light absorbers. For example Sb2S3 showed the most favourable energy level alignment with 0.3 eV driving force for electron injection and 0.4 eV driving force for hole transfer and also the most efficient solar cells due to high photocurrent generation. The energy level alignment of the TiO2/Bi2S3 interface on the other hand showed no driving force for electron injection to TiO2, and the performance of the corresponding solar cell was very low.

The Electronic Impact of Light-Induced Degradation in CsPbBr3 Perovskite Nanocrystals at Gold Interfaces.

The understanding of the interfacial properties in perovskite devices under irradiation is crucial for their engineering. In this study we show how the electronic structure of the interface between CsPbBr3 perovskite nanocrystals (PNCs) and Au is affected by irradiation of X-rays, near-infrared (NIR), and ultraviolet (UV) light. The effects of X-ray and light exposure could be differentiated by employing low-dose X-ray photoelectron spectroscopy (XPS). Apart from the common degradation product of metallic lead (Pb0), a new intermediate component (Pbint) was identified in the Pb 4f XPS spectra after exposure to high intensity X-rays or UV light. The Pbint component is determined to be monolayer metallic Pb on-top of the Au substrate from underpotential deposition (UPD) of Pb induced from the breaking of the perovskite structure allowing for migration of Pb2+.

Oxygen-induced doping of spiro-MeOTAD in solid-state dye-sensitized solar cells and its impact on device performance.

Solid state dye-sensitized solar cells (sDSCs) employing the hole conductor 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) require the presence of oxygen during fabrication and storage. In this paper, we determine the concentrations of oxidized spiro-MeOTAD within devices under different operating and storage conditions by UV-vis spectroscopy. Relative concentrations of spiro-MeOTAD(+) were found to be greater than 10% after illumination for standard sDSCs, where no chemical dopant had been used in the solar cell fabrication but oxygen and lithium ions were present. We suggest that oxidized spiro-MeOTAD is created as a byproduct of oxygen reduction at the TiO(2) surface during cell illumination. Furthermore, we studied the effect of light soaking under different conditions and associated changes in spiro-MeOTAD(+) concentration on the solar cell measurements. Our findings give insights to photochemical reactions occurring within sDSCs and provide guidelines for which doping levels should be used in device fabrication in absence of oxygen.

Direct Measurements of Interfacial Photovoltage and Band Alignment in Perovskite Solar Cells Using Hard X-ray Photoelectron Spectroscopy.

A heterojunction is the key junction for charge extraction in many thin film solar cell technologies. However, the structure and band alignment of the heterojunction in the operating device are often difficult to predict from calculations and, due to the complexity and narrow thickness of the interface, are difficult to measure directly. In this study, we demonstrate a technique for direct measurement of the band alignment and interfacial electric field variations of a fully functional lead halide perovskite solar cell structure under operating conditions using hard X-ray photoelectron spectroscopy (HAXPES). We describe the design considerations required in both the solar cell devices and the measurement setup and show results for the perovskite, hole transport, and gold layers at the back contact of the solar cell. For the investigated design, the HAXPES measurements suggest that 70% of the photovoltage was generated at this back contact, distributed rather equally between the hole transport material/gold interface and the perovskite/hole transport material interface. In addition, we were also able to reconstruct the band alignment at the back contact at equilibrium in the dark and at open circuit under illumination.

Comparing spiro-OMeTAD and P3HT hole conductors in efficient solid state dye-sensitized solar cells.

Two hole conductor materials, spiro-OMeTAD and P3HT, were compared in solid-state dye-sensitized solar cells. Two organic dyes containing one anchor unit (D35) or two anchor units (M3) were used in the comparison. Absorbed photon to current conversion efficiency close to unity was obtained for the devices with spiro-OMeTAD. Energy conversion efficiencies of 4.7% and 4.9% were measured for the devices with spiro-OMeTAD and the dyes D35 and M3, respectively. For the devices using the P3HT hole conductor the results were rather different comparing the two dye molecules, with energy conversion efficiencies of 3.2% and 0.5% for D35 and M3, respectively. Photo-induced absorption measurements suggest that the regeneration of the dyes, and the polymer infiltration, is not complete using P3HT, while spiro-OMeTAD regenerates the dyes efficiently. However, the TiO(2)/D35/P3HT system shows rather high energy conversion efficiency and electrochemical oxidation of the dyes on TiO(2) indicates that D35 have a more efficient dye to dye hole conduction than M3, which thereby might explain the higher performance. The dye hole conduction may therefore be of significant importance for optimizing the energy conversion in such hybrid TiO(2)/dye/polymer systems.

Spatial microheterogeneity in the valence band of mixed halide hybrid perovskite materials.

The valence band of lead halide hybrid perovskites with a mixed I/Br composition is investigated using electronic structure calculations and complementarily probed with hard X-ray photoelectron spectroscopy. In the latter, we used high photon energies giving element sensitivity to the heavy lead and halide ions and we observe distinct trends in the valence band as a function of the I : Br ratio. Through electronic structure calculations, we show that the spectral trends with overall composition can be understood in terms of variations in the local environment of neighboring halide ions. From the computational model supported by the experimental evidence, a picture of the microheterogeneity in the valence band maximum emerges. The microheterogeneity in the valence band suggests that additional charge transport mechanisms might be active in lead mixed halide hybrid perovskites, which could be described in terms of percolation pathways.

Unravelling the ultrafast charge dynamics in PbS quantum dots through resonant Auger mapping of the sulfur K-edge.

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they are promising for application in photovoltaics and other optoelectronic devices. The ultrafast charge transport is intriguing, offering insight into the mechanism of electron tunneling processes within the material. In this study, we investigated the charge transfer times of PbS quantum dots of different sizes and non-quantized PbS reference materials by comparing the propensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays. We show that charge transfer times in PbS quantum dots decrease with excitation energy and are similar at high excitation energy for quantum dots and non-quantized PbS. However, at low excitation energies a distinct difference in charge transfer time is observed with the fastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots. Our observations can be explained by iodide ligands on the quantum dots creating a barrier for charge transfer, which reduces the probability of interparticle transfer at low excitation energies. The probability of intraparticle charge transfer is limited by the density of available states which we describe according to a wave function in a quantum well model. The stronger quantum confinement effect in smaller PbS quantum dots is manifested as longer charge transfer times relative to the larger quantum dots at low excitation energies.

Thermal degradation of lead halide perovskite surfaces.

Commercial use of lead halide perovskites requires improved thermal stability and therefore a better understanding of their degradation mechanisms. The thermal degradation of three clean perovskite single crystal surfaces (MAPbI3, MAPbBr3, FAPbBr3) was investigated using synchrotron-based photoelectron spectroscopy. Central findings are that the halide has a large impact on thermal stability and that the degradation of formamidnium results in the formation of a new organic species at the FAPbBr3 crystal surface.

The Complex Degradation Mechanism of Copper Electrodes on Lead Halide Perovskites.

Lead halide perovskite solar cells have reached power conversion efficiencies during the past few years that rival those of crystalline silicon solar cells, and there is a concentrated effort to commercialize them. The use of gold electrodes, the current standard, is prohibitively costly for commercial application. Copper is a promising low-cost electrode material that has shown good stability in perovskite solar cells with selective contacts. Furthermore, it has the potential to be self-passivating through the formation of CuI, a copper salt which is also used as a hole selective material. Based on these opportunities, we investigated the interface reactions between lead halide perovskites and copper in this work. Specifically, copper was deposited on the perovskite surface, and the reactions were followed in detail using synchrotron-based and in-house photoelectron spectroscopy. The results show a rich interfacial chemistry with reactions starting upon deposition and, with the exposure to oxygen and moisture, progress over many weeks, resulting in significant degradation of both the copper and the perovskite. The degradation results not only in the formation of CuI, as expected, but also in the formation of two previously unreported degradation products. The hope is that a deeper understanding of these processes will aid in the design of corrosion-resistant copper-based electrodes.

Electronic Structure and Chemical Bonding in Methylammonium Lead Triiodide and Its Precursor Methylammonium Iodide.

A detailed examination of the electronic structures of methylammonium lead triiodide (MAPI) and methylammonium iodide (MAI) is performed with ab initio molecular dynamics (AIMD) simulations based on density functional theory, and the theoretical results are compared to experimental probes. The occupied valence bands of a MAPI single crystal and MAI powder are probed with X-ray photoelectron spectroscopy, and the conduction bands are probed from the perspective of nitrogen K-edge X-ray absorption spectroscopy. Combined, the theoretical simulations and the two experimental techniques allow for a dissection of the electronic structure unveiling the nature of chemical bonding in MAPI and MAI. Here, we show that the difference in band gap between MAPI and MAI is caused chiefly by interactions between iodine and lead but also weaker interactions with the MA+ counterions. Spatial decomposition of the iodine p levels allows for analysis of Pb-I σ bonds and π interactions, which contribute to this effect with the involvement of the Pb 6p levels. Differences in hydrogen bonding between the two materials, seen in the AIMD simulations, are reflected in nitrogen valence orbital composition and in nitrogen K-edge X-ray absorption spectra.