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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.

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.

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 (MAPbI(3), MAPbBr(3), FAPbBr(3)) 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 FAPbBr(3) crystal surface.

Funding Information: The authors acknowledge SOLEIL for the provision of synchrotron radiation (proposal numbers: 20161265, 20171063, 20180483, 20181721, and 20191506) facilities at the GALAXIES beamline. Some measurements were carried out at the CoESCA endstation at the BESSY-II electron storage ring operated by the Helmholtz-Zentrum Berlin für Materialien und Energie. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. The authors acknowledge research funding from the Swedish Research Council (Grant Nos. VR 2016-04590, VR 2018-04125, VR 2018-04330, VR 2018-06465), Swedish Energy Agency (P50626-1, P43549-1), the Göran Gustafsson foundation, the Swedish Foundation for Strategic Research (project nr. RMA15-0130), and the Carl Tryggers foundation (Grant No. CTS 18:59). | openaire: EC/H2020/730872/EU//CALIPSOplus 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.

[Abstract] In this work we focus in setting the limits of the tolerance factor and the size of the A-cations that stabilize the perovskite structure in hybrid dicyanamide compounds [A][Mn(dca)3]. For this purpose, we propose an alternative, simple approach to calculate a more realistic effective ionic radius for the large and anisotropic A-cations often present in these type of compounds. We test the proposed procedure by analysing the crystal structures of [A][Mn(dca)3] dicyanamide hybrids reported in the literature and recalculating the tolerance factors of such compounds, as well as by preparing five new [A][Mn(dca)3] members, discussing also the influence of the A-cation shape in the stability limits of the perovskite structure. Interestingly, such methodology is not only useful to develop new compounds of the emerging family of (multi)functional multi(stimuli)-responsive dicyanamide materials but can also be applied to other hybrid organic-inorganic perovskites and related materials. As for financial support, the authors thank Ministerio de Economía y Competitividad MINECO and EU-FEDER (projects MAT2017-86453-R and PDC2021-121076-I00), and Xunta de Galicia for the collaboration agreement “Development of research strategic actions Universidade da Coruña I ​+ ​D ​+ ​i 2021–2022: CICA-Disrupting Projects 2021SEM-A3 (NanoCool). J.G.-B. and J.M.B.-G. acknowledge Xunta de Galicia for Predoctoral and Postdoctoral Fellowships, respectively. I.D.-F thanks Ministerio de Universidades for a FPU Predoctoral Fellowship. A. G.-F. and U.B.C thank the Carl Tryggers foundation and the Göran Gustafsson foundation for financial support

A detailed examination of the electronic structures of methylammonium lead triiodide (MAPI) and methylammonium iodide (MAI) is performed with

Metal halide perovskites have emerged as attractive materials for use in solar cells, light emitting diodes and other optoelectronic devices, mainly due to their impressive charge transport properties, strong light absorption, long carrier diffusion lengths and long excited state lifetime. The extensive research on these materials has paved the way for a new class of materials: metal halide perovskite nanocrystals (NCs). Due to their high photoluminescence quantum yield and narrow emission that can be tuned by size and compositional variations, perovskite NCs are considered to be ideal candidates compared to traditional quantum dots. With the growing interest in these materials and the current challenges in their commercialization, this review aims mainly to provide the necessary understanding of the influence of capping ligands on the synthesis and application of perovskite NCs. The different synthetic approaches and the role of ligands in determining the morphological and optical properties of the resulting NCs will be discussed. Thereafter, we review the advances in understanding the surface chemistry and ligation in the metal halide perovskite NCs. Lastly, we review the ligand exchange and management processes that are shown to be beneficial in improving the performance and stability of perovskite nanocrystal films for optoelectronic applications.

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.

Scalable methods for deposition of lead halide perovskite thin films are required to enable commercialization of the highly promising perovskite photovoltaics. Here, we have developed a slot-die coating process under ambient conditions for methylammonium lead iodide (MAPbI(3)) perovskite on heated substrates (about 90 degrees C on the substrate surface). Dense, highly crystalline perovskite films with large grains (100-200 mu m) were obtained by careful adjustment of the deposition parameters, using solutions that are similar but more dilute than those used in typical spin-coating procedures. Without any further after treatments, such as antisolvent treatment or vapor annealing, we achieved power conversion efficiencies up of 14.5% for devices with the following structure: conducting tin oxide glass (FTO)/TiO2/MAPbI(3)/spiro-MeOTAD/Au. The performance was limited by the significant roughness of the deposited films, resulting from the hot-casting method, and the relatively high deposition temperature, which led to a defect-rich surface due to loss of MAI.

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.

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.

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.

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 photovoltai ...

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 TiIV-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 °C for half an hour. PSCs with the composition of the light-absorbing material FA0.85MA0.10Cs0.05Pb(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 °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 °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.

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.

High-end organic–inorganic lead halide perovskite semitransparent p–i–n solar cells for tandem applications use a phenyl-C61-butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnOx electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I)3 and (Cs,FA)Pb(Br,I)3 and in this study on Cs0.05FA0.79MA0.16PbBr0.51I2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnOx exposure was therefore suggested to form a nonideal perovskite/SnOx interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnOx growth on the perovskite, the mass dynamics of monitor crystals coated by partial p–i–n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20–50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnOx on the perovskite. Although measurements on the perovskite bulk below and the SnOx film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.

High-end organic–inorganic lead halide perovskite semitransparent p–i–n solar cells for tandem applications use a phenyl-C61-butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnOx electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I)3 and (Cs,FA)Pb(Br,I)3 and in this study on Cs0.05FA0.79MA0.16PbBr0.51I2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnOx exposure was therefore suggested to form a nonideal perovskite/SnOx interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnOx growth on the perovskite, the mass dynamics of monitor crystals coated by partial p–i–n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20–50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnOx on the perovskite. Although measurements on the perovskite bulk below and the SnOx film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.

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.

Lead halide perovskite solar cells have significantly increased in both efficiency and stability over the last decade. An important aspect of their long-term stability is the reaction between the perovskite and other materials in the solar cell. This includes the contact materials and their degradation if they can potentially come into contact through, e.g., pinholes or material diffusion and migration. Here, we explore the interactions of silver contacts with lead halide perovskites of different compositions by using a model system where thermally evaporated silver was deposited directly on the surface of the perovskites. Using X-ray photoelectron spectroscopy with support from scanning electron microscopy, X-ray diffraction, and UV-visible absorption spectroscopy, we studied the film formation and degradation of silver on perovskites with different compositions. The deposited silver does not form a continuous silver film but instead tends to form particles on a bare perovskite surface. These particles are initially metallic in character but degrade into AgI and AgBr over time. The degradation and migration appear unaffected by the replacement of methylammonium with cesium but are significantly slowed down by the complete replacement of iodide with bromide. The direct contact between silver and the perovskite also significantly accelerates the degradation of the perovskite, with a significant loss of organic cations and the possible formation of PbO, and, at the same time, changed the surface morphology of the iodide-rich perovskite interface. Our results further indicate that an important degradation pathway occurred through gas-phase perovskite degradation products. This highlights the importance of control over the interface materials and the use of completely hermetical barrier layers for the long-term stability and therefore the commercial viability of silver electrodes.

This paper focuses on the development of 2-terminal CIGS-perovskite tandem solar cells by exploring a range of stack sequences and synthetic procedures for depositing the associated layers. In the end, we converged at a stack sequence composed of SLG/Mo/CIGS/CdS/i-ZnO/ZnO:Al/NiO/PTAA/Perovskite/LiF/PCBM/SnO2/ITO. With this architecture, we reached performances only about 1% lower than the corresponding 4-terminal tandem cells, thus demonstrating functional interconnects between the two sub-cells while grown monolithically on top of each other. We go through the stack, layer-by-layer, discussing their deposition and the results, from which we can conclude what works, what does not work, and what potentially could work after additional modifications. The challenges for a successful 2-terminal tandem device include: how to deal with, or decrease, the surface roughness of the CIGS-stack, how to obtain uniform coverage of the layers between the CIGS and the perovskite while also obtaining a benign interface chemistry, and how to tune the band gaps of both the CIGS and the perovskite to obtain good optical matching. The investigation was based on CIGS with a power conversion efficiency around 14%, and perovskites with an efficiency around 12%, resulting in 2-terminal tandem cells with efficiencies of 15–16%. The results indicate that by using higher performing CIGS and perovskite sub-cells, it should be possible to manufacture highly efficient 2-terminal CIGS-perovskite tandem devices by using the protocols, principles, and procedures developed and discussed in this paper. (Less)

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

Inverted perovskite solar cells (PSCs) have been introduced as better candidate for roll-to-roll printing and scaleup than their conventional configuration counterparts, while their fabrication is ...

Preparing an anatase TiO2(101) surface with a high density of oxygen vacancies and associated reduced Ti species in the near-surface region results in drastic changes in the water adsorption chemistry compared to adsorption on a highly stoichiometric surface. Using synchrotron radiation excited photoelectron spectroscopy, we observe a change in the water growth mode, from layer-by-layer growth on the highly stoichiometric surface to bilayer growth on the reduced surface. Furthermore, we have been able to observe Ti3+ enrichment at the surface upon water adsorption. The Ti3+ enrichment occurs concomitant with effective water dissociation into hydroxyls with a very high thermal stability. The water bilayer on the reduced surface is thermally more stable than that on the stoichiometric surface, and it is more efficient in promoting further water dissociation upon heating. The results thus show how the presence of subsurface defects can alter the wetting mechanism of an oxide surface.

Charge separation efficiency is a crucial parameter for photovoltaic devices-polymers consisting of alternating electron-rich and electron-deficient parts can achieve high such efficiencies, for instance, together with a fullerene electron acceptor. This offers a viable path toward solar cells with organic bulk heterojunctions. Here, we measured the charge-transfer times in the femtosecond and attosecond regimes via the decay of sulfur is X-ray core excited states (with the core-hole clock method) in blends of a low-band gap polymer {PCPDTBT [poly[2,6-(4,4-bis (2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-1/1 dithiophene)-alt-4,7- (2,1,3-benzothiadiazole)]]} consisting of a cyclopentadithiophene electron-rich part and a benzothiadiazole electron-deficient part. The constituting parts of the bulk heterojunction were varied by adding the fullerene derivative PCBM ([6,6]-phenyl-C-61-butyric acid methyl ester) (weight ratio of polymer/PCBM as 1:0, 1:1, 1:2, and 1:3). For low-energy excitations, the charge-transfer time varies to the largest extent for the thiophene donor part. The charge-transfer time in the 1:2 blend is reduced by 86% compared to that of pristine PCPDTBT. At higher energy excitations, the charge-transfer time does not vary with the chemical environment, as this regime is dominated by intramolecular conduction that yields ultrafast charge-transfer times for all blends, approaching 170 as. We thus demonstrate that the core-hole clock method applied to a series with changing composition can give information about local electron dynamics (with chemical specificity) at interfaces between the constituting parts the crucial part of a bulk heterojunction where the initial charge separation occurs.

A trend in high performing lead halide perovskite solar cell devices has been increasing compositional complexity by successively introducing more elements, dopants, and additives into the structure; and some of the latest top efficiencies have been achieved with a quadruple cation mixed halide perovskite CsxFAyMAzRb1-x-y-zPbBrqI3-q. This paper continues this trend by exploring doping of mixed lead halide perovskites, FA0.83MA0.17PbBr0.51I2.49, with an extended set of alkali cations, i.e., Cs+, Rb+, K+, and Na+, as well as combinations of them. The doped perovskites were investigated with X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, hard X-ray photoelectron spectroscopy, UV–vis, steady state fluorescence, and ultrafast transient absorption spectroscopy. Solar cell devices were made as well. Cs+ can replace the organic cations in the perovskite structure, but Rb+, K+, and Na+ do not appear to do that. Despite this, samples doped with K and Na have substantially lon...

CsyFA1−yPb(IxBr1−x)3 perovskite solar cells were prepared using non-hazardous solvents and metallic nitrate precursor films for the material synthesis.

Dye-sensitized solar cells (DSCs) have been widely studied in the last two decades and start to be commercialized in the photovoltaic market. Comprehensive characterization is needed to fully understand and optimize the device performance and stability. In this review, we summarize different characterization methods for dye-sensitized solar cells with liquid redox electrolytes or solid state hole transporting materials, most of which can also be used for similar devices such as perovskite based thin film solar cells. Limitations and advantages of relevant methods for studying the energy levels and time scales involved in charge transfer processes as well as charge transport related characteristic lengths are discussed. A summary of recent developments in DSCs and the importance of measured parameters for the device optimization procedure are mentioned at the end.

Dye-sensitised solar cells (DSSCs) have been recognised and rapidly developed as one of the third generation of photovoltaic technologies over the past three decades. This chapter describes the scientific history, working principles, development and mechanistic understanding of each component in the device, in addition to some of the key techniques used to study DSSCs, such as transient absorption spectroscopy, photoelectron spectroscopy and electrochemical impedance spectroscopy. We hope that this chapter will be useful for people who are interested in the applications of chemistry in renewable energy conversion.

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.

The authors thank the Engineering and Physical Sciences Research Council (EPSRC) for support (EP/R023980/1). Z.A.-G. acknowledges funding from a Winton Studentship, and ICON Studentship from the Lloyd’s Register Foundation. S.D.S acknowledge the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (HYPERION, Grant Agreement No. 756962), and the Royal Society and Tata Group (UF150033). M.A.-J. thanks Cambridge Materials Limited, Wolfson College, University of Cambridge, and EPSRC for their funding and technical support. F.U.K. thanks the Jardine Foundation and Cambridge Trust for a doctoral scholarship. This work was carried out with the support of the Diamond Light Source, instrument I09 (proposal SI22668-1). The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. H.R., G.J.M. and U.B.C. acknowledge research funding from the Swedish Research Council (Grant nos. VR 2018-04125, 2018-06465, and 2018-04330), the Swedish Foundation for Strategic Research (Project no. RMA15-0130) and the Swedish Energy Agency (Grant no. P43549-1).

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 Gratzel [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.

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.

X-ray photoelectron spectroscopy is a powerful tool for the characterization of molecular and hybrid solar cells. This technique allows for atomic-level characterization of their components as well as for the determination of the electronic structure that governs the key conversion processes. In this chapter, we introduce the basic concepts of electronic structure in molecules and semiconducting materials followed by a description of the concepts of photoelectron spectroscopy and how they relate to electronic structure. Finally, we give examples of the application of photoelectron spectroscopy to different types of molecular and hybrid solar cell materials demonstrating the type of information that can be obtained, to gain fundamental understanding and to further develop such devices.

The anatase TiO2(101) surface and its interaction with water is an important topic in oxide surface chemistry. Firstly, it benchmarks the properties of the majority facet of TiO2 nanoparticles and, secondly, there is a controversy as to whether the water molecule adsorbs intact or deprotonates. We have addressed the adsorption of water on anatase TiO2(101) by synchrotron radiation photoelectron spectroscopy. Three two-dimensional water structures are found during growth at different temperatures: at 100 K, a metastable structure forms with no hydrogen bonding between the water molecules. In accord with prior literature, we assign this phase to chains of disordered molecules. Growth 160 K results in a metastable structure with expressed hydrogen bonding between the water molecules. At 190 K, the water molecules become disordered as the thermal energy is too high and hence the hydrogen bonds break. The result is a structure with isolated monomers. Partial dissociation is observed for all three growths, with the molecular state only slightly favored in energy (20–40 meV) over the dissociated state. Heating of a thick film leads to more dissociation compared to a bilayer, when formed at 100 K. Thus, extending the water network facilitates proton transport and hence dissociation. The results reconcile apparent conflicting experimental results previously obtained by scanning tunneling microscopy (STM) and core level photoelectron spectroscopy.

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.

In dye-sensitized solar cells (DSSCs) photogenerated positive charges are normally considered to be carried away from the dyes by a separate phase of hole-transporting material (HTM). We show that there can also be significant transport within the dye monolayer itself before the hole reaches the HTM. We quantify the fraction of dye regeneration in solid-state DSSCs that can be attributed to this process. By using cyclic voltammetry and transient anisotropy spectroscopy, we demonstrate that the rate of interdye hole transport is prevented both on micrometer and nanometer length scales by reducing the dye loading on the TiO2 surface. The dye regeneration yield is quantified for films with high and low dye loadings (with and without hole percolation in the dye monolayer) infiltrated with varying levels of HTM. Interdye hole transport can account for >50% of the overall dye regeneration with low HTM pore filling. This is reduced to about 5% when the infiltration of the HTM in the pores is optimized in 2 μm th...

The most common material for dye-sensitized photocathodes is mesoporous NiO. We transformed the usual brownish NiO to be more transparent by reducing high valence Ni impurities. Two pretreatment methods have been used: chemical reduction by NaBH4 and thermal reduction by heating. The power conversion efficiency of the cell was increased by 33% through chemical treatment, and an increase in open-circuit voltage from 105 to 225 mV was obtained upon heat treatment. By optical spectroelectrochemistry, we could identify two species with characteristically different spectra assigned to Ni3+ and Ni4+. We suggest that the reduction of surface Ni3+ and Ni4+ to Ni2+ decreases the recombination reaction between holes on the NiO surface with the electrolyte. It also keeps the dye firmly on the surface, building a barrier for electrolyte recombination. This causes an increase in open-circuit photovoltage for the treated film.

Low-Temperature Solution Processing of Mesoporous Metal-Sulfide Semiconductors as Light-Harvesting Photoanodes

In this paper, we present the HELIOS (High Energy Laser Induced Overtone Source) laboratory, an in-house high-order harmonic generation facility which generates extreme ultraviolet (XUV) photon pulses in the range of 15-70 eV with monochromatized XUV pulse lengths below 35 fs. HELIOS is a source for time-resolved pump-probe/two-color spectroscopy in the sub-50 fs range, which can be operated at 5 kHz or 10 kHz. An optical parametric amplifier is available for pump-probe experiments with wavelengths ranging from 240 nm to 20 000 nm. The produced XUV radiation is monochromatized by a grating in the so-called off-plane mount. Together with overall design parameters, first monochromatized spectra are shown with an intensity of 2 ⋅ 1010 photons/s (at 5 kHz) in the 29th harmonic, after the monochromator. The XUV pulse duration is measured to be

A perylene sensitizer, ID176, was recently reported which performs much better in solid state dye-sensitized solar cells than in those using liq. electrolytes with iodide/tri-iodide as the redox couple. Here, a characterization is presented of the sensitizer and of the TiO2/dye interface by UV-visible absorption and fluorescence spectroscopy, spectroelectrochem., photoelectron spectroscopy, electroabsorption spectroscopy, photoinduced absorption spectroscopy, and femtosecond transient absorption measurements. The absorption spectrum is reported of the sensitizer red-shifted by addn. of lithium ions to the surface due to a downward shift of the excited state level of the sensitizer, which is of the same order of magnitude as the downward shift of the titanium dioxide conduction band edge. Results from photoelectron spectroscopy and electrochem. suggest that the excited state is largely located below the conduction band edge of TiO2 but that there are states in the band gap of TiO2 which might be available for photoinduced electron injection. The sensitizer was able to efficiently inject into TiO2, when a lithium salt was present on the surface, while injection was much less effective in the absence of lithium ions or in the presence of solvent. In the presence of the hole conductor 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) and LiTFSI, charge sepn. was monitored by the emergence of a Stark shift of the dye in transient absorption spectra, and both injection and regeneration appear to be completed within 1 ps. Regeneration by spiro-MeOTAD is therefore several orders of magnitude faster than regeneration by iodide, and ID176 can even be photoreduced by spiro-MeOTAD.

Solution-processed colloidal quantum dot (CQD) solar cells harvesting the infrared part of the solar spectrum are especially interesting for future use in semitransparent windows or multilayer sola ...

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.

Mesoporous films of highly crystalline Sb2S3 are prepared from a precursor paste of Sb(S2CO-Et)3, hexylamine, and chlorobenzene by annealing at 120 °C.

A new perylene sensitizer, ID176, for dye-sensitized solar cells (DSCs) is presented. The dye has the capability for high photocurrents due to strong absorption from 400 to >700 nm. Photocurrents of up to 9 mA/cm2 were achieved in solid-state DSCs employing the hole conductor 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD), with a conversion efficiency of 3.2%. But the sensitizer did not perform well in conjunction with liq. iodide/tri-iodide electrolytes, suggesting a difference in the injection and regeneration mechanisms in these 2 types of dye-sensitized solar cells.