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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.

Lead halide perovskitesolar cells have reached power conversionefficiencies during the past few years that rival those of crystallinesilicon solar cells, and there is a concentrated effort to commercializethem. The use of gold electrodes, the current standard, is prohibitivelycostly for commercial application. Copper is a promising low-costelectrode material that has shown good stability in perovskite solarcells with selective contacts. Furthermore, it has the potential tobe self-passivating through the formation of CuI, a copper salt whichis also used as a hole selective material. Based on these opportunities,we investigated the interface reactions between lead halide perovskitesand copper in this work. Specifically, copper was deposited on theperovskite surface, and the reactions were followed in detail usingsynchrotron-based and in-house photoelectron spectroscopy. The resultsshow a rich interfacial chemistry with reactions starting upon depositionand, with the exposure to oxygen and moisture, progress over manyweeks, resulting in significant degradation of both the copper andthe perovskite. The degradation results not only in the formationof CuI, as expected, but also in the formation of two previously unreporteddegradation products. The hope is that a deeper understanding of theseprocesses will aid in the design of corrosion-resistant copper-basedelectrodes.

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.

A series of heteroleptic Cu(I) diimine complexes with different ancillary ligands and 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid (dbda) as the anchoring ligand were selfassembled on TiO2 surfaces and used as dyes for dye-sensitized solar cells (DSSCs). The binding to the TiO2 surface was studied by hard X-ray photoelectron spectroscopy for a brominecontaining complex, confirming the complex formation. The performance of all complexes was assessed and rationalized on the basis of their respective ancillary ligand. The DSSC photocurrent-voltage characteristics, incident photon-to-current conversion efficiency (IPCE) spectra, and calculated lowest unoccupied molecular orbital (LUMO) distributions collectively show a push-pull structural dye design, in which the ancillary ligand exhibits an electron-donating effect that can lead to improved solar cell performance. By analyzing the optical properties of the dyes and their solar cell performance, we can conclude that the presence of ancillary ligands with bulky substituents protects the Cu(I) metal center from solvent coordination constituting a critical factor in the design of efficient Cu(I)-based dyes. Moreover, we have identified some components in the I-/I-3(-)-based electrolyte that causes dissociation of the ancillary ligand, i.e., TiO2 photoelectrode bleaching. Finally, the detailed studies on one of the dyes revealed an electrolyte-dye interaction, leading to a dramatic change of the dye properties when adsorbed on the TiO2 surface.

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 (MAPbI(3) and Cs(x)FA(1-x)PbI(3)) 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 MAPbI(3) 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.

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.

Renewable energy is an important topic as global energy consumption continues to rise. Because the sun emits an enormous amount of energy, solar energy is a promising source. However, most of the commercial solar cell technology is manufactured in an energy demanding process and there is a need for new, easily processed materials. This thesis concerns quantum dots, which are nanoparticles that can absorb light of different energies depending on their size. They can be synthesised by solution-based chemistry and turned into solid thin films to harvest sunlight. The fundamental properties of quantum dots need to be better understood before production on large scales may commence. The aim of this thesis was to investigate the fundamental properties of lead sulfide quantum dots. The methods used in this thesis are based on photoelectron spectroscopy. They allowed investigation of materials as-is, but also changes upon excitation by laser or X-rays. Using a laser, dynamics on pico- to microsecond timescales were studied by time-resolved photoelectron spectroscopy. Using a range of X-rays, the probability of charge transfer in the attosecond range was investigated. Steady-state investigation showed that different surface treatment of the quantum dots caused different resistance towards surface oxidation and X-ray damage. Different layers in the structure of solar cells can influence the photovoltage, an important parameter in achieving high power conversion efficiencies. Time-resolved photoelectron spectroscopy was developed and used to investigate the contributions of the layers to photovoltage generation. We observed photovoltage dynamics on a timescale covering six orders of magnitude. The mechanism of charge transfer in quantum dots of different sizes was studied by core-hole clock spectroscopy in the attosecond regime. Our results show that quantum confinement affects the charge transfer only at low excitation energies., Förnybar energi är viktig då den globala energikonsumtionen fortsätter öka. Solenergi är en lovande energikälla eftersom solen strålar ut otroligt mycket energi. Då kommersiell solcellsteknologi använder mycket energikrävande tillverkningsprocesser finns det behov av nya material som är enkla att tillverka. Denna avhandling handlar om kvantpunkter som är nanopartiklar vilka kan absorbera ljus av olika energi, beroende på partiklarnas storlek. Lösningsbaserad syntes är en teknik för att tillverka tunna filmer av kvantpunkter. Grundläggande egenskaper hos kvantpunkter behöver kartläggas ytterligare innan storskalig produktion av sådana kan påbörjas. Målet med denna avhandling var att undersöka grundläggande egenskaper hos kvantpunkter bestående av blysulfid. Analysmetoderna i denna avhandling baseras på fotoelektron-spektroskopi. Dessa möjliggör studier av material utan ytterligare bearbetning samt hur de förändras när de påverkas av laserljus eller röntgenstrålning. Med laserpåverkan har dynamik på pico- till mikrosekundsskala kunnat studeras med tidsupplöst fotoelektron-spektroskopi och genom att använda röntgenstrålning med olika energi har sannolikheten för laddningsöverföring studerats på attosekundtidsskalan. Tidsoberoende studier visade att olika ytbehandlingar på kvantpunkterna gav upphov till olika motstånd mot oxidation och strålskador. Olika lager i solcellers struktur kan påverka fotospänningen, vilken är en viktig parameter för att uppnå hög energiomvandlingseffektivitet. Tidsupplöst fotoelektronspektroskopi utvecklades och användes för att undersöka olika lagers bidrag till fotospänningen. Vi observerade tidsberoendet hos fotospänningen på en tidskala som spände över sex storleksordningar. Laddningsöverföringsmekanismen hos kvantpunkter med olika storlek studerades på attosekundstidsskalan med kärnhålsklocks-spektroskopi. Våra resultat visar att kvanteffekter orsakade av instängning påverkar laddningsöverföringen enbart vid låga excitationsenergier., QC 2022-10-11

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., QC 20220318

The anodic electrodeposition method is investigated as an alternative technique for the preparation of a titanium oxide (TiO2) blocking underlayer (UL) for perovskite solar cells (PSCs). Extremely thin Ti-IV-based films are grown from aqueous acidic titanium(III) chloride in an electrochemical cell at room temperature. This precursor layer is converted to the UL (ED-UL), in a suitable state for PSC applications, by undertaking a sintering step at 450 degrees C for half an hour. PSCs with the composition of the light-absorbing material FA(0.85)MA(0.10)Cs(0.05)Pb(I0.87Br0.13)(3) (FA and MA denote the formamidinium and methylammonium cations, respectively) based on the ED-UL are compared with PSCs with the UL of a standard type prepared by the spray-pyrolysis method at 450 degrees C from titanium diisopropoxide bis(acetylacetonate) (SP-UL). We obtain power conversion efficiencies (PCEs) of over 20% for mesoscopic perovskite devices employing both ED-ULs and SP-ULs. Slightly higher fill factor values are observed for ED-UL-based devices. In addition, ED-ULs prepared by the same method have also been applied in planar PSCs, resulting in a PCE exceeding 17%, which is comparable to that for similar PSCs with an SP-UL. The preparation of ED-ULs with a lower sintering temperature, 150 degrees C, has also been examined. The efficiency of a planar PSC incorporating this underlayer was 14%. These results point out to the possibility of applying ED-ULs in flexible planar PSCs in the future.

Surface properties of colloidal quantum dots (CQDs) are critical for the transportation and recombination of the photoinduced charge carrier in CQD solar cells, therefore dominating the photovoltaic performance. Herein, PbS CQD passivated using liquid-state ligand exchange (LSLX) and solid-state ligand exchange (SSLX) strategies are in detail investigated using photoelectron spectroscopy (PES), and solar cell devices are prepared to understand the link between the CQD surface properties and the solar cell function. PES using different energies in the soft and hard Xray regime is applied to study the surface and bulk properties of the CQDs, and the results show more effective surface passivation of the CQDs prepared with the LSLX strategy and less formation of lead-oxide. The CQD solar cells prepared with LSLX strategy show higher performance, and the photoelectric measurements suggest that the recombination of photoinduced charges is reduced for the solar cell prepared with the LSLX approach. Meanwhile, the fabricated solar cells exhibit good stability. This work provides important insights into how to fine-tune the CQD surface properties by improving the CQD passivation, and how this is linked to further improvements of the device photovoltaic performance.

Lead halide perovskite solar cells with multi-cation/mixed halide materials now give power conversion efficiencies of more than 20%. The stability of these mixed materials has been significantly improved through the addition of Cs+ compared to the original methylammonium lead iodide. However, it remains one of the most significant challenges for commercialisation. In this study, we use photoelectron spectroscopy (PES) in combination with visible laser illumination to study the photo-stability of perovskite films with different compositions. These include Br : I ratios of 50 : 50 and 17 : 83 and compositions with and without Cs+. For the samples without Cs and the 50 : 50 samples, we found that the surface was enriched in Br and depleted in I during illumination and that some of the perovskite decomposed into Pb0, organic halide salts, and iodine. After illumination, both of these reactions were partially reversible. Furthermore, the surfaces of the films were enriched in organic halide salts indicating that the cations were not degraded into volatile products. With the addition of Cs+ to the samples, photo-induced changes were significantly suppressed for a 50 : 50 bromide to iodide ratio and completely suppressed for perovskites with a 17 : 83 ratio at light intensities exceeding 1 sun equivalent.

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they arepromising for application in photovoltaics and other optoelectronic devices. The ultrafastcharge transport is intriguing, offering insight into the mechanism of electron tunnelingprocesses within the material. In this study we investigated the charge transfer times of PbSquantum dots of different sizes and non-quantized PbS reference materials by comparing thepropensity 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 andare similar at high excitation energy for quantum dots and non-quantized PbS. However, atlow excitation energies a distinct difference in charge transfer time is observed with thefastest 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 barrierfor charge transfer, which reduces the probability of interparticle transfer at low excitationenergies. The probability of intraparticle charge transfer is limited by the density of availablestates which we describe according to a wavefunction in a quantum well model. The strongerquantum confinement effect in smaller PbS quantum dots is manifested as longer chargetransfer times relative to the larger quantum dots at low excitation energies., QC 20221011

Quantum dot solar cells have not yet achieved optimal device performances and to direct development there is thereforea need to understand the device function of present solar cell structures in more detail. Understanding where photovoltage isgenerated in a device and where energy losses occur is a key aspect of this. We have previously shown that time-resolved core levelphotoelectron spectroscopy can be used to follow the photovoltage rise and decay at a specific interface from pico- to microsecondtimescales. Here, we extend this study and investigate the photovoltage generation in different parts of a PbS quantum dot solar cellthrough sample design. We show that thick absorbing quantum dot layers are required for generating a high photovoltage at theinterface between n-type PbS quantum dots and p-type quantum dots. Furthermore, we show that the full photovoltage is only generatedwhen a gold contact is deposited on the quantum dots and that the presence of this contact also leads to significantly slowercharge recombination., QC 20221011