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Lead-free double-perovskites (A₂M⁺M³⁺X₆) have been recently proposed as a stable and environmental friendly alternative to lead-based perovskites. Among the vast number of possible double-perovskite compositions, Cs₂AgBiBr₆ has been the most investigated material, both from a theoretical and experimental point of view. To date, the majority of fundamental studies on this material have been conducted on single crystals or powders, which is due to the poor solubility of the material precursors and to difficulties in obtaining smooth and uniform films. Vapour deposition offers to be a promising alternative for the formation of Cs₂AgBiBr₆ films, overcoming the drawbacks of traditional solution chemistry methods and time-consuming growth of single crystals. In particular, physical vapour deposition can produce high-quality films with various thicknesses and smooth surfaces. In this way, numerous spectroscopic techniques which require non-scattering surfaces (like TRPL or THz spectroscopy), as well as thickness-dependent measurements can be conducted achieving an insight in the optoelectronic properties of the material. Here, we disclose our recent findings on vapour-deposited Cs₂AgBiBr₆, demonstrating not only efficient double-perovskite solar cells, but also presenting a fundamental understanding of the carriers dynamics in the double-perovskite thin films, not achievable otherwise.

Mesostructured materials characterized by an open void network with narrow pore size distribution can be employed as matrices in which to synthesize perovskite nanocrystals of controlled size displaying quantum confinement effects. In this talk, the different approaches that can be taken within this methodology to obtain nanostructured perovskite films of high optical quality will be described and the properties of the resulting materials analyzed.[1,2] Electronic band gap tuning arising from quantum size effects are put into practice to controllably modify photoluminescence, electroluminescence and light harvesting properties of these nanostructures. Also, the interaction of the nanocrystals with the different types of matrices employed to host them will be studied, as well as their stability versus different environments[3] and their thermal stability. Actual integration of these films within optoelectronic devices in different configurations will be proposed and their performance and prospective applications discussed. Finally, the possibility to integrate these nanostructured films in different photonic architectures aiming at enhancing the performance of the devices will also be addressed.

Despite the outstanding optoelectronic properties of lead halide perovskites and the impressive performances achieved in perovskite-based devices, several concerns still hinder the widespread application of these materials. Two of the main bottlenecks today are: (i) the toxicity of Pb²⁺ cations, and (ii) the instability of hybrid organic-inorganic perovskites coupled with the difficulty in synthesizing fully-inorganic ones due to poor solubility of cesium halide salts in common solvents.

Here, we have used a fully-dry mechanical approach via ball-milling of different precursors to synthesize a wide range of phase-pure halide perovskites and related compounds with suitable optoelectronic properties for different applications from photovoltaics to light-emission. In particular, we have synthesized fully-inorganic CsPbX₃ (X = I, Br, and Cl) compounds and investigated the structural, chemical, and optical effects of adding potassium halides (KX). We have also synthesized lead-free hybrid and inorganic perovskites based on Sn₂₊ as well as other related compounds such as A₂Sn(IV)X₆ “vacancy-ordered perovskites” and A₃Bi₂X₉ (A = Cs, formamidinium, and methylammonium).

Eventually, we have demonstrated that the so-formed high quality dry powders can be used to make thin films via single-source vacuum deposition, thus enabling their use in different devices such as solar cells or light-emitting diodes.

Metal halide perovskites (MHPs) are promising emitting materials for next-generation light-emitting diodes (LEDs) because MHPs have the advantages of easy solution processing, high color purity with a narrow full width at half maximum, easy synthesis and color tunability, and inexpensive material cost. Compared to the low dimensional 0D nanoparticles or 2D Ruddlesden-Popper MHPs, polycrystalline MHPs have fundamental electroluminescent (EL) limitations in terms of low exciton binding energy, long exciton diffusion length, and severe exciton quenching routes at the interface between conducting oxide anode and MHP emitting layer and in the MHP bulk. Here, we present the efficient strategies of the fine stoichiometry method, a self-organized conducting polymeric anode, nanograin engineering, and core-shell-mimicked polycrystal to achieve highly efficient polycrystalline MHP LEDs (PeLEDs). First, we suggest the highly efficient polycrystalline MAPbBr₃ PeLEDs by controlling the precursor stoichiometry, improving surface morphology, and decreasing the grain size of the MAPbBr₃ film. As a result, the current efficiency was improved from 0.002 cd/A to 42.9 cd/A. By unraveling the additive-based nanocrystal pinning (A-NCP) method, the limitations of EL efficiency in MHP films and PeLEDs were investigated so as to achieve an external quantum efficiency of 8.79% ph el^-1. By adopting these strategies to the flexible graphene, we developed graphene-based MAPbBr₃ PeLEDs that is beneficial to the out-coupling effect and exciton quenching due to In and Sn ions from ITO electrode. Then, we investigated a molecularly-decoupled ideal conducting polymeric anode for high-efficiency PeLEDs. Conducting polymers have a trade-off between a conductivity and a work function; i.e., the work function of conducting polymers decreases when the conductivity increases. Therefore, we introduce an effective molecular scale control strategy to decouple the work function with conductivity in conducting polymer anodes while maintaining the blocking capability of exciton quenching. Thereby, the high device efficiencies of 52.86 cd A^-1 and 10.93% ph el^-1 were achieved in MAPbBr₃ PeLEDs. In addition, we developed kinetically-controlled core-shell-mimicked nanograins by incorporating a molecular semiconducting additive to shield the nanograins for suppressing defects at grain boundary region. The organic-shielded polycrystalline nanograins achieved improved photophysical and electroluminescence properties, and reduced free carrier density; and thus external quantum efficiency of 11.7% ph el^-1 was achieved. Our various strategies for high-efficiency polycrystalline PeLEDs will provide broad interests to the research fields of perovskite electronics.

Halide perovskite are receiving a huge attention in the recent few years. Undoubtedly this attention is mainly due to the outstanding power conversion efficiencies, surpassing 23%, reported for photovoltaic devices, fabricated with polycrystalline films from low cost techniques. This high performance is due to the high potentiality of the halide perovskite material but in order to take full advantage of it proper selective contacts have to be developed. The possibility of low temperature and solution process make that this technology can be easily integrated with very different kinds of materials and the interface properties can be tailored with a broad range of possibilities, including organic and inorganic materials and nanoparticles. In this talk we show how systems with very different nature can be used to engineering the perovskite interface in perovskite optoelectronic devices. The different systems have been systematically characterized in order to determine in which way interface modifications affect their ultimate performance.

Following a surge of interest in lead halide perovskites, research on colloidal nanocrystals of these materials has gathered momentum in the last years. In such a narrow time span, several properties/features of halide perovskite nanocrystals have been investigated, among them electroluminescence, lasing, anion-exchange, as well as control of size and shape such that nanocrystals in the quantum confinement regime can now be easily fabricated, with narrow size distributions. One aspect that remains relatively less explored is the surface chemistry of these nanocrystals. Ligand binding in this case is very dynamic and can be strongly influenced by the external environment. The present talk will highlight the research activities of our group on understanding surface ligand binding and how this influences the synthesis, stability, optical properties, and post-synthesis transformations of colloidal lead halide perovskite nanocrystals. The various techniques that are used to investigate the surface of nanocrystals will also be critically discussed.

Vacuum-deposition is one of the most technologically relevant techniques for perovskite solar cells fabrication. The intrinsically additive nature of vapor-based methods allows the design of multiple device architectures, circumventing solvent compatibility considerations. This can be a tool to maximize the photovoltaic conversion efficiency, but it is also a key factor to enable the fabrication of tailored systems for the characterization of the solar cell working mechanisms.

This work presents the progress on vapor phase deposited perovskites for single and multijunction devices. Multiple source co-deposition is employed for the fabrication of multi-cations/anions perovskite compounds, including low bandgap Pb-Sn and wider bandgap versions. We carry out a detailed study of the device interfaces, with the focus on the influence of thin organic charge extraction layers, strong dopants, ionic compounds, and conjugated polymers. With this approach, we can identify the main factors limiting the device performance, which is crucial to develop the full potential of the technology.

Hybrid Organic-Inorganic Perovskites (HOIPs) have been widely researched over the last
decade, with great success in increasing their solar cell efficiency. HOIPs offer a viable
alternative to silicon solar cells, primarily in their promise for large-scale fabrication via solution
processing. Unfortunately, too much remains a mystery regarding a fundamental
understanding of the self-assembly process and precisely which solution processing variables
would be ideal (reagents, solvents, and processing conditions). This makes it a challenge to
create a member of the HOIP class with pre-specified characteristics beyond simply maximizing
device efficiency. There exists an overwhelming combinatorial problem due to the variety of
possible species: Many choices of A and B site cations (including blends of each), the choice of
halide (again, potentially a blend), choice of solvent blend, and aspects of their processing
(timing of adding anti-solvents, etc.). Exploring all possible candidates would be insurmountable
using a standard Edisonian approach. To redress this situation, we have developed a model
tailored to be used within a Bayesian optimization Gaussian Process Regression (GPR) scheme.[1]
Bayesian optimization is, arguably, the best method of maximizing an expensive objective, like
ours, and is ideal for this scenario. We tested this approach by maximizing the solvation of
HOIP salts (the smallest APbX3 molecule) using a simple computational ersatz for the enthalpy
of solvation, namely the intermolecular binding energy between the salt and solvent obtained
from DFT calculations, which served as our objective function. We hypothesized a connection
between good solvation and slow crystallization kinetics, and hence high quality thin films. We
studied two test cases for which we were able to compute the DFT intermolecular binding
energy of all possible candidates. The first was the best species composition for a pure halide in
a pure solvent (72 possible combinations). The second was for a mixed halide in a pure solvent
(240 possible combinations). We found we were able to achieve global maximization in under
half the number of iterations needed for the next best model (a Bayesian optimization
approach without using our model), and significantly better than all other common models.
We were able to identify the experimentally preferred “gold standard” compositions for pure
halides and mixed halide systems in a variety of pure solvents. Further, we were also able to
identify unexpected highly ranked predictions of high performers that deserve experimental
confirmation.

Conducting polymers have gained the rising attention of the scientific community, due to their thermoelectric efficiencies that enable the design of room temperature flexible heat harvesters, that can potentially cover the energy demands of the nomadic modern society. Their thermoelectric efficiency is usually optimized by tuning their oxidation levels, considering though, that the Seebeck coefficient (S) is decreasing with the electrical conductivity (σ), as both are antagonistically related to the carrier concentration. In this work we present a concurrent increase of S and σ and we experimentally derive the dependence of Seebeck coefficient on charge carrier mobility, for the first time in organic electronics. Through detailed control of the polymer synthesis, we enabled the formation of a more dense percolation path that facilitated the charge transport and the thermodiffusion of the charge carriers inside the conducting polymer, while the material shifted from a fermi glass to a semimetal, as its crystallinity increased. With our work not only we provide a solid understanding on the origin of the thermoelectric properties of conducting polymers, but we also highlight the importance of enhanced charge carrier mobility for the design of efficient thermoelectric polymers.

Organic near-infrared (NIR) to visible upconversion devices (OUCs) are receiving interest due to manifold applications in remote sensing, night vision, NIR-imaging and biomedicine. In an OUC, an organic photodetector (OPD) is series connected with an organic light emitting component. When NIR light is absorbed by the OPD, electron-hole pairs are formed. Under an appropriate bias and for conditions of energetic interface matching, charges are easily driven in the light-emitting component where they recombine leading to visible light emission. The outcome is a NIR-to-visible upconverted image that can be captured using a conventional camera. OUCs benefit from the numerous advantages of organic electronics in general and offer the potential to convert a NIR scene into a visible image using large area devices that can be realized at low cost by high-throughput coating and printing techniques on flexible substrates. However, no all-solution processed OUC has been demonstrated so far. Here we report on the fabrication and optimization of all-solution processed OUCs based on a NIR squaraine dye OPD [1] and an emitting phenyl-substituted poly(para-phenylenevinylene) copolymer termed Super Yellow (SY). The key for multilayer solution processing is to have a layer structure which can withstand solvents used in subsequent processing. To cope with layer dissolution, poor wettability and imperfect film morphology, several approaches such as surface treatment, orthogonal solvents and cross-linkable layers were successfully used. In a first step, we fabricated OUCs with a SY/Ca/Al OLED and found that these devices are surprisingly long-term stable both during storage and in the presence of NIR light. The device converts NIR light at 980 nm efficiently to visible light at 590 nm with a high NIR light on/off ratio (800 at 4.5 V). To circumvent the vacuum thermal evaporation step of Ca, we then added a salt (Li⁺CF₃SO₃^-) to the SY layer and thereby transformed the OLED into an organic light-emitting electrochemical cell (OLEC). OUCs comprising an OLEC were fabricated with top electrodes composed of Al (evaporated) or Ag (screen printed). Due to the presence of mobile ions in OLECs, such OUCs are temporal dynamic devices. Device operation involves the formation of electric double layers at the SY interfaces that facilitate the injection of electrical charges, the formation of n- and p-doped regions and an intrinsic region in between, where electron-hole recombination and light emission takes place [2]. We characterized the dynamic behavior of such all-solution processed OUCs in detail and found that an appropriate pre-conditioning voltage step results in a low turn-on voltage (~2.7 V) and a NIR-to-visible photon-to-photon conversion efficiency on a par with the SY/Ca/Al OLED. OPD-OLEC upconverters can pave the way to simplified manufacturing of low-cost and large-area NIR imagers entirely via solution-based techniques.

Organic emitters exhibiting thermally activated delayed fluorescence (TADF) are rare-earth-metal-free dyes which can theoretically realize 100% internal quantum efficiency (IQE) of organic light emitting diodes (OLEDs). They were demonstrated to be promising candidates for highly efficient OLEDs [1]. TADF emitter-based light-emitting layers characterized by both high photoluminescence quantum yields and small singlet-triplet energy splitting (ΔEST) have to be developed for fabrication of efficient OLEDs. Small ΔEST (typically less than 0.1 eV) can be easily obtained in exciplex-based systems. Since electron and hole are positioned on two different molecules, exciplex-based emitters are characterized by small exchange energy and efficient TADF [1]. If exciplexes are formed between layers of donating and accepting materials in OLED structures, energy barrier free interface for charges is obtained. This results in low operating voltages of OLEDs and enhanced power efficiencies (PE). Aiming to obtain both high IEQ and PE of OLEDs, exciplex-based OLED approaches demonstrated promising results [1].

Recently developed exciplex-forming donor-acceptor solid mixtures will be presented as either emitters or hosts with TADF properties for highly efficient OLEDs including interface exciplex-based OLEDs. For instance, exciplex-forming system consisting of 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole as donor and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine as acceptor allowed to achieve very high value of of maximum external quantum efficiency (18%) for OLEDs based on interface exciplex emitter [2]. In addition, new approach for the development of the color-tunable OLEDs with the minimal number of the functional layers will be presented [3]. Guest-host free OLEDs combining emission from both excitons and exciplexes based on aggregation-induced emission enhancement of carbazole and tetraphenylethylene derivatives will be reported [4]. Extremely efficient warm-white OLEDs exploiting interface exciplex emission with harmless for the human eyes electroluminescence spectra will be presented [5]. Utilization of interface exciplex-host will be demonstrated for optimization and enhancement of performance of non-doped OLEDs based on new emitters showing aggregation induced emission enhancement [6].

Perovskite solar cells continue to gain momentum in research and gain in efficiency, and multi-junction perovskite solar cells offer a roadmap to performance levels well beyond that ever possible with single junction silicon or other thin-film technologies. However, operational stability is an area which is understood to be important by the community, but consistently takes a back seat in contemporary perovskite research.    
I will present different approaches we have adopted to improving the efficiency, and fundamental stability of the perovskite absorber materials and devices, and highlight degradations which can occur in the bulk of the perovskite absorber materials and induced by the charge extraction heterojunction. I will give further insight into what factors influence stability, and how to mitigate degradation.  
Beyond stability, I will highlight how moving from a single absorber layer, to a multijunction cell should lead to much higher efficiencies, and I will show experimental realisation of progress along such a road map.  
Beyond laboratory based research, I will highlight our progress towards manufacturing scaleup through the technology company, Oxford PV Ltd., and the key challenges which need to be overcome to deliver an industrialised perovskite PV technology.

We describe the results of analysis of kinetic phenomena using frequency modulated techniques. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases.¹ The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We also shows the identification of the impedance of ionic diffusion by measuring single crystal samples.² Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in deffects of the structure.³ We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent. We describe important insights to the measurement of EQE in frequency modulated conditions, which shows that the quantum efficiency can be variable at very low frequencies.⁴

Organic–inorganic lead halide perovskites have shown photovoltaic performances above 20% in a range of solar cell architectures while offering simple and low-cost processability. Despite the multiple ionic compositions that have been reported so far, the presence of organic constituents is an essential element in all of the high-efficiency formulations, with the methylammonium and formamidinium cations being the sole efficient options available to date. We present a perovskite composition based on a combination of guanidinium/methylammonium cations that exhibits superior photovoltaic performance and material stability compared with MAPbI₃. We demonstrate that the incorporation of large guanidinium (Gua) cations unexpectedly forms a highly stable 3D crystalline structure, plausibly mediated by the increased number of H bonds within the inorganic framework. The prepared MA_1–xGua_xPbI₃ perovskite preserved the good optoelectronic properties associated with the organic lead halide materials, leading to a high PCE surpassing 20% for a Gua content of 14% and stabilized performance for 1,000 h under continuous light illumination, a fundamental step within the perovskite field.

Even though hybrid perovskites have undergone a remarkable development within the last years, state of the art processing approaches such as solution processing or evaporation suffer from an intrinsically high complexity, as the actual perovskite crystallization and its film processing happen simultaneously and are inextricably interconnected.

Here we present an alternative, entirely dry processing approach, which decouples perovskite crystallization and film formation, by using readily prepared perovskite powders and produce films by appropriate mechanical pressure treatment. We show how a mechanochemical synthesis approach by ball milling allows to produce a wide range of phase pure and exceptionally stable hybrid perovskite powders with a high flexibility in processing and address the impact of milling parameters on the powder properties. Using these powders, we demonstrate how the used pressure and the powder microstructure, i.e. particle size and stoichiometry affect the mechanical stability, compactness and surface roughness of the pressed layers. We further address how specific temperature treatment during the pressing step can improve the properties of the pressed layer, and show their capability to be used in perovskite based optoelectronic devices.

Perovskite semiconductors have recently emerged as promising materials for optoelectronic applications, with photovoltaic efficiencies that have now reached over 23%. However, the poor stability of the material is still a major challenge for commercial application. One possible strategy for improving environmental stability is the incorporation of large organic hydrophobic cations in the perovskite structure, which results in a layered two-dimensional (2D) material where the organic molecules form a dielectric barrier between the semiconductor layers. 2D perovskites show enhanced stability in contrast to the conventional 3D compositions,^[1] however the presence of dielectric confinement results in higher exciton binding energies, wider bandgaps and limited charge carrier diffusion lengths.^[2]  Mixed phase compositions can be a viable approach for obtaining films that combine the enhanced environmental stability of 2D perovskite and good charge transport properties of the 3D semiconductor for device application. We have investigated the charge transport properties of mixed-phase 2D-3D perovskite thin films deposited from solution using time resolved Photoluminescence (PL) and Optical-Pump Terahertz-Probe (OPTP) spectroscopy. We demonstrate the preferential formation of 2D domains close to the substrate and predominance of 3D crystallites on the front surface, consequently creating an energy cascade with a preferential direction across the semiconductor film.^[3,4] Evidence for the charge and energy transfer through the cascade could be observed in unusual OPTP transients. Based on the experimental observations we proposed a model that reproduces the recombination and transfer dynamics, combining the effects of charge diffusion and photon reabsorption. This allows us to identify the most relevant transfer mechanisms in play in such self-assembled energy cascades. Our findings show that the presence of 2D domains does not hinder the charge transport properties of the semiconductor film with respect to the pure 3D structures, demonstrating the suitability of the mixed phase 2D-3D perovskite films for optoelectronic applications.

Organic semiconductors and hybrid/organic materials have attracted interest for electronic applications due to their potential for use in low-cost, large-area, flexible electronic devices. Here we will report on recent developments pertaining to surface modifiers and both n- and p-dopants that could impact the charge injection/collection processes in organic light emitting diodes, organic field effect transistors, organic photovoltaics and hybrid organic/inorganic perovskite devices. We will also discuss the development of organic and metallo-organic-based dimers as n-dopants and very briefly described metal dithiolene complexes as p-dopants for organic semiconductors including their impact on device performance. The application of n-doping for the development of electron injection layers for organic light emitting diodes (OLEDs) will be highlighted, including their use for doping of electron transport materials which result in high conductivities and in some cases good thermoelectric performance.  In the case of OLEDs, it will be shown that photoactivation can lead to stable doping of materials (i.e. the doping-induced conductivity remains relatively constant over hundreds of hours) beyond the expected thermodynamic limit, which would be predicted based on an assessment of the effective reduction potential of the n-dopant and the reduction potential of the electron transport material.

Selected References:

“n-Doping of Organic Electronic Materials using Air-Stable Organometallics,” Adv. Mater. 24 (5), 699-703 (February 2012, DOI: 10.1002/adma.201103238)

“Electrode Work Function Engineering with Phosphonic Acid Monolayers and Molecular Acceptors: Charge Redistribution Mechanisms.” Adv. Funct. Mater. 1704438/1-12 (2018, DOI:10.1002/adfm.201704438)

“Solution doping of organic semiconductors using air-stable n-dopants.” Appl. Phys. Lett. 100, 083305 (February 2012)

“A universal method to produce low work function electrodes for organic electronics,” Science 336 (6079), 327-332 (April 2012, DOI: 10.1126/science.1218829)

“Surface modified fullerene electron transport layers for stable and reproducible flexible perovskite solar cells.” Nano Energy 49, 324-332 (2018, DOI:10.1016/j.nanoen.2018.04.068)

“Ultralow doping in organic semiconductors: Evidence of trap filling.” Phys. Rev. Lett. 109 (17), 176601/1-5 (November 2012, DOI: 10.1103/PhysRevLett.109.176601)

“Reduction of contact resistance by selective contact doping in fullerene n-channel organic field-effect transistors.” Appl. Phys. Lett. 102, 153303-153307 (April 2013, DOI: 10.1063/1.4802237)

“Modification of the fluorinated tin oxide/electron-transporting material interface by a strong reductant and its effect on perovskite solar cell efficiency.” Mol. Syst. Des. Eng. Advance Article (2018, DOI:10.1039/C8ME00031J)

“Production of Heavily n- and p-Doped CVD Graphene with Solution-Processed Redox-Active Metal-Organic Species," Materials Horizons Advance Article (September 2013, DOI: 10.1039/C3MH00035D

“Controllable, Wide-Ranging n-Doping and p-Doping of Monolayer Group 6 Transition-Metal Disulfides and Diselenides.” Adv. Mater. 30, 1802991 (2018, DOI: 10.1002/adma.201802991)

  Quantum dot (QD) based light-emitting diodes (QLEDs) are competitive alternatives to purely organic light-emitting diodes (OLEDs) in terms of color purity, luminescence intensities, and external quantum efficiencies (EQEs). Hybrid QD/organic polymer light-emitting diodes combine the advantageous emitting properties of the QDs with the flexibility in device construction of polymeric materials. In QD/polymer hybrid LEDs, adding QD emitters in the polymer host usually leads to strong charge trapping or charge transfer from the polymer host to the QDs, which adversely affects device performance. In a host-guest system comprising a blue-emitting donor polymer and a red-emitting QD acceptor, the emission process should preferably occur via long-range Förster energy transfer (FRET), without charge trapping or charge transfer. Therefore, the design challenge for such a system is to optimize energy transfer, while at the same time separating donor and acceptor sufficiently far to avoid short-range processes such as charge transfer and trapping.^1,2

  To address this challenge, we designed a novel QD/semiconducting polymer hybrid material in which the surface of the QDs is covered with an electrically insulating shell with an optimized thickness. We separate donor and acceptor far enough to prevent charge transfer and trapping, while still allowing FRET by i) shielding the emitting CdSe core of the QDs with a ZnS shell having a wider bandgap than the core and ii) grafting polystyrene (PS) chains onto the ZnS surface. The PS layer furthermore enhances miscibility with the polymeric host. We studied the performance of QLEDs and single charge carrier devices based on the red-emitting CdSe/ZnS/PS core shell QDs (acceptor) blended with the blue emitter poly(dioctylfluorene) (PFO) as the host (donor). For optimal QD size, PS molecular weight, grafting density and particle loading, FRET from PFO to the QDs is the dominant process, with charge trapping being only marginal compared to active layers containing non-PS-grafted QDs.  

In the last few years co-crystals of conjugated organic compounds have attracted much attention as next-generation composite materials for organic optoelectronics. The vivid interest is driven by targeted materials design concepts inter alia via the isometric approach,¹ creating segregated or mixed-stacked co-crystal systems. Reversible segregated-mixed stack conversion can be induced by external multi-stimuli, generating large color changes by switching between localized exciton vs. charge-transfer (CT) emission.² Combined pump-probe, low-temperature PL and atomistic quantum-chemical studies reveal the spectral signatures and kinetics of exciton generation, transport and deactivation.^3,5 Switching from 1D to 2D mixed-stacking motif gives rise to well-balanced ambipolar charge transport materials, being a prerequisite for optimized electrically driven light emission, and allowing for the first CT-crystal-based organic light emitting transistors (OLET).⁴ Systematic opening new pathways in the field. Targeted crystal design allows for systematic color variation and intermolecular TADF characteristics while keeping the structural characteristics.⁵

With the considerable progress in efficiency, stability and manufacturability, perovskite solar cells are now well along the path to reaching commercialisation. While single junction silicon photovoltaic technology has reached an impressive 26.7 percent power conversion efficiency, it will be forever limited to efficiencies below 30 percent. We will discuss how in a few short years, Oxford PV’s perovskite technology in tandem with silicon has achieved a certified 28 percent efficiency, and the clear roadmap to efficiencies extending well beyond 30 percent. Additionally, this talk will explain the importance of a fundamental understanding of the perovskite material properties and its relationship with the development of a stable technology; sharing long-term reliability test data based on the IEC61215 standards. We will also reflect on the challenges and progress that has been made to scale this perovskite-silicon tandem solar cell technology from the lab to commercial sized solar cells at pilot production and initial manufacturing levels.

The ability to precisely control the equilibrium carrier concentration in organic semiconducting devices is of great interest. The ability to solution process doped layers is of extreme importance for high throughput production of organic electronic devices via roll-to-roll or ink-jet printing. In this talk, I will discuss doping in conjugated polymers containing Lewis basic sites and the effects of doping on optical property and conductivity. The pyridine moiety on the polymer provides an accessible lone-pair of electrons that can associate with the Lewis acid. Under these circumstances, Lewis acid withdraws electron density from the pyridine moiety leading to a lower energy charge-transfer band. Addition of pyridine regenerates the spectra of pristine polymer due to the formation of the pyridine-Lewis acid adduct and liberation of the parent polymer, demonstrating that the binding is reversible. Addition of the Lewis acid effectively p-dopes the hole transport in the parent polymer, leading to increases in the free hole density and conductivity. This methodology is advantageous since the polymer, Lewis acid, and the adduct have excellent solubility in organic solvents, negating the need for co-solvents that uses in molecular dopant such as F4TCNQ.

Electronic excitation plays a key role in the operation of organic-based semiconductor devices such as organic solar cells, photodetectors and light-emitting diodes. Thus, developing our understanding of the processes that dictate the fate of primary photoexcitations is essential in the design, optimization and longevity of such devices. In the field of organic solar cells, there has been an intense search for low energy band gap systems, based upon π-conjugated polymers, for more efficient collection of solar energy in the near IR. In our recent work,[1] three low energy bandgap polymers have been demonstrated to exemplify a significantly higher extinction coefficient compared to other conjugated polymer systems, either homopolymers or donor-acceptor polymers, to date, especially in higher molecular weights, due to their extended persistence length.

Using ultrafast Time-Resolved Infrared spectroscopy we have studied the excited state structural evolution in solution in these three polymers systems; two copolymers of diketopyrrolopyrrole and thiophene (DPP-3T) or thienothiophene (DPP-TT-T) and one a copolymer of indacenodithiophene with benzothiadiazole (IDTBT). In all three cases, vibrational features appear promptly after excitation. The vibrational features appear on a broad absorption background that decays, which is tentatively assigned to polaron absorption due to interchain interactions. For DPPTT-T, both the broad background and IR bands decay within 200 ps. However, for IDTBT we observe not only longer decay times but also significant differences in the kinetics between the broad background and IR bands, indicative of an initial intramolecular charge-transfer state formation which then evolves to the charge separated species that subsequently recombines. We will discuss the possible origin of these phenomena with the aid of calculations of vibrational spectra and models for excited state.

The ultrafast optical processes in a self-doped conjugated polyelectrolyte, where any complications related to dopant diffusion, formation of partial charge transfer complexes or structural disruption by external dopant molecules are absent, are presented. Conjugated polyelectrolytes have conjugated backbones with ionic side chains. Their numerous applications include biosensing, cell imaging and interlayers in electronic devices. The ability to conduct both electronic and ionic charge, as well as their favorable doping, make conjugated polyelectrolytes particularly interesting. CPE-K is a narrow-bandgap conjugated polyelectrolyte, which becomes self-doped upon dialysis treatment. The doping is directly evident in the absorption spectrum, where the P₂ polaron band appears around 1200 nm. By carefully determining the extinction coefficient of this band, we estimate the doping density in the polyelectrolyte doped at different levels. We carried out transient absorption (TA) spectroscopy in CPE-K solutions and thin films, with pumping in either the neutral S₀-S₁ or the polaronic P₂ transition. We show that there is electronic coupling of polarons to nearby neutral sites, which share the same ground state for their optical transitions (both are depleted in the TA experiments, no matter which transition is excited). Similar, correlated and very short-lived dynamics are observed at both excitation wavelengths. This result contrasts with the conventional picture of localized intragap polaron states and warrants a revised model for the electronic structure and optical transitions in doped organic systems. We also show that inter-site Coulomb interactions are present (i.e. the positive polarons cause a Stark shift in the transitions of nearby neutral sites). The electronic coupling and electrostatic effects of the polarons occur independently on doping concentration and on whether the self-doped material forms a thin film or is in solution. Finally, we present the terahertz (THz) conductivity of those doped thin films, establishing the local carrier mobility.

The optoelectronic landscape of conjugated polymers is highly dependent on their molecular arrangement and packing, with minute changes in local order, such as chain conformation and torsional backbone order/disorder, frequently having a substantial effect on macroscopic properties.[1-3] Here, we show that blending semiconducting polymers with insulating commodity plastics, an approach previously shown to benefit charge transport[4,5] and improve flexibility,[6] enables controlled manipulation of semiconductor backbone planarity. The key is to create a polarity difference between the semiconductor backbone and its side chains, while matching the polarity of the side chains and the additive. We demonstrate the applicability of this approach by judicious comparison of regioregular poly(3-hexylthiophene) (P3HT) with two of its more polar derivatives, namely the diblock copolymer poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) and the graft polymer poly[3-but(ethylene oxide)thiophene] (P3BEOT), as well as their blends with poly(ethylene oxide) (PEO). Proximity between polar side chains and a similarly polar additive reduces steric hindrance between individual chain segments by essentially ‘expelling’ the side chains away from the semiconducting backbones.  This process, shown to be facilitated via exposure to polar environments such as humid air/water vapor, facilitates backbone realignment towards specific chain arrangements, thereby enabling control of backbone planarity graft polymers with aqueous/ionic compatiblity that are usually torsionally disordered.[7]

Aggregates – that is short-ranged ordered moieties in the solid-state of π-conjugated polymers – play an important role in the photophysics and performance of various optoelectronic devices. It is often not clear what controls their formation during the spin-coating process of a film. Previously, we have shown that many polymers change from a disordered to a more ordered conformation when cooling a solution below a characteristic critical temperature. Here, I adress how we can use this knowledge to control the formation of aggregates during the deposition of films.

Using in situ time-resolved absorption spectroscopy on the prototypical semiconducting polymers P3HT, PFO, PCPDTBT, and PCE11 (PffBT4T-2OD), we show that spin-coating at a temperature below the critical temperature enhances the formation of aggregates with strong intra-chain coupling. An analysis of their time-resolved spectra indicates that the formation of nuclei in the initial stages of film formation for substrates held the critical temperature is responsible for this. We observe that the growth rate of the aggregates during film formation is thermally activated with an energy that exceeds the activation energy of the solvent viscosity . From this we conclude that the rate controlling step is the planarization of a chain that is associated with its attachment to a nucleation center.

Further, it is well known that the film formation process during spin-coating as well as the subsequent long-time film drying process differ significantly when DIO is added to a solution of P3HT. Here we show how the addition of DIO increases the time until the disorder-order transition sets in, yet it accelerates the actual transition, which impacts on the nature of the resulting aggregates. Moreover, aggregates form through an all-over solidification throughout the entire film rather than the spreading of a solidfication front in the absence of DIO in the solution.

For many semiconductor applications photoemission spectroscopy (PES) play the role of an invaluable tool to determine key electronic parameters for the description of the device functionality, such as band offsets and interface dipoles. With the rise of photovoltaic technologies based on halide perovskite (HaP) absorber layers, the use of direct and inverse PES methods has been pursued to monitor the formation of interfaces between halide perovskite films and adjacent functional layers, that are employed for charge transfer in perovskite-based optoelectronic devices.

We reported on the energy level alignment between HaP films and organic charge transport layers [1], oxide substrates [2], carbon nanotube thin-films [3] and high-work function MoO₃ overlayers [4]. In this talk I will summarize these findings and describe the challenges to generalize these trends as the complex chemistry between HaP layer and adjacent semiconductor often lies at the root of the observed interfacial alignment processes and band bending.

Furthermore, I will give examples for typical pitfalls that occur, when characterizing HaP surfaces and interfaces with PES methods. Most importantly, beam damages effects have been identified as the perovskite layers can exhibit distinct signs of degradation under vacuum conditions and concomitant irradiation with high-energy photons [5]. While we generally intend to avoid these transient effects in our measurements, we can extract additional physical and chemical parameters from the evolution of energy level positions and stoichiometry during the PES measurements [6-8], with direct implications on the material stability. Finally, I will conclude my talk with a view on the perspectives of interfacial analysis for halide perovskites by PES techniques and an outlook on future experiments [9].

Metal halide perovskite solar cells are now effectively competing with their inorganic counterparts in terms of power conversion efficiencies. However, state of the art perovskite solar cells still suffer from limited fill factor (FF) and open circuit voltage (V_oc), due to non-radiative recombination processes happening in the device. We found that selective charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, the CTL/perovskite interfaces induce additional non-radiative recombination pathways which limit the V_oc of the cell. In order to harvest the full thermodynamic potential of the perovskite absorber the interfaces of both the electron and hole transport layers (ETL/HTL) must be properly addressed and improved. Here, we show a significant improvement of the V_oc and FF of pin-type perovskite solar cells by employing a novel surface treatment to a of triple cation perovskite Cs₅(MA_0.17FA_0.83)₉₅)₉₅Pb(I_0.83 Br_0.17)₃ using a polyionic liquid (PIL) material. The resulting solar cell devices show outstanding FF values of up to 83% and V_oc of 1.17V, which lead an extraordinarily PCE of 21.5%. Through combined photoluminescence and electroluminescence studies we found that the PIL reduces the non-radiative recombination at perovskite surface by acting as a defect passivating agent of the bare perovskite surface and limiting the recombination of charges across the perovskite/C₆₀ interface. Photoemission spectroscopy (XPS/UPS) and conductive atomic force microscopy (CAFM) show how the ionic nature of the PIL induces a specific charge redistribution at the perovskite surface, going along with improved extraction of the electrons at the perovskite/C₆₀ interface. Ultimately, the hydrophobic nature of the PIL provides a shielding coverage of the perovsktie which reduces degradation due to moisture and air. The PIL modified devices show exceptionally long dark storage stability and enhanced maximum power point tracking (MPP) lifetimes. In conclusion, our work proposes a novel approach to efficiently suppress non-radiative recombination of charges, promote the charge extraction and improve the stability at the same time. Additionally, given the simplicity of this post treatment, our results can be representative of a more general methodology for device modification, and therefore potentially applicable to other compositions and cell architecture, opening doors for e new class of materials to be implemented in perovskite solar cells.

Most efficient organic photovoltaic (OPV) cells comprise a polymeric donor and a small-molecule acceptor blended in a so-called bulk heterojunction (BHJ) thin film architecture. The BHJ morphology is complex; it is now, for instance, well accepted that most BHJ blends used in OPVs are at least to a certain extent finely intermixed, rarely featuring entirely phase-pure domains. This renders elucidating the nature of molecularly mixed phases, and how these relate to device characteristics, a major challenge in the field. Recently, Ade et al. [1] have introduced the so-called domain-composition-variation concept, which accounts for the relative purity of domains as compared to a reference sample and can be experimentally determined by means of soft X-ray scattering experiments. Despite the fact that it represents a great advance because it allows for a phenomenological correlation between the domain purity and device characteristics, the lack of quantitative information about the absolute domain composition in real devices is still hindering a more refined understanding of how mixed phases impact the device function. Here, we present a new methodology based on fast scanning calorimetry that allows to readily determine the absolute composition of intermixed domains in BHJs processed using the same parameters as those employed for device fabrication (e.g. spin-casting a sub-200-nm thin film on PEDOT:PSS). The method exploits the well-known fact that the devitrification temperature, i.e. the Tg, of a finely intermixed blend in the glassy state is coherently affected by the composition of the blend. Thus, by conducting a careful characterization of the Tgof donor:acceptor thin film blends as well as to OPV cell replicas – by means of physical-ageing experiments [2] –, we are able to determine the composition of the intermixed domains of the OPV cells replicas. We demonstrate the applicability of our method for various donor:acceptor systems and discuss the impact of the knowledge gained on materials selection, device fabrication and long-term stability

It is well known that charge transport layers, selective for electrons (ETL) or for holes (HTL), have a profound effect on the performance and stability of polymer solar cells. The most common HTL material in polymer solar cells, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), has a high conductivity and transparency and is solution-processable. Knowing that PEDOT:PSS layer is also a significant cause of degradation due to its hygroscopic nature, alternative HTL materials, such as, for instance, thermally evaporated Molybdenum oxide (MoO3), are needed to obtain longer device life time. Here we explored solution-processed interfacial layers of Copper thiocyanate (CuSCN) and phosphomolybdic acid (PMA) as HTLs and found similar to improved device performance compared to the conventional HTLs. We present the surface morphology, optical properties, and surface electronic properties of thin films of these HTL materials using Atomic Force Microscopy (AFM), UV-Vis spectroscopy and Kelvin probe. The HTL layers were applied in polymer solar cells with the conventional architecture ITO/HTL/AL/LiF/Al devices, where the active layer (AL) is the polymer:fullerene blend, TQ1:PC70BM (1:3). The performance and charge carrier recombination behaviourof these solar cells with different HTL were compared.

Exciton transport dictates the design and operation of a variety of optoelectronic devices based on organic semiconductors.  This is particularly true in organic photovoltaic cells (OPVs), where excitons diffuse to a dissociating interface to realize a photocurrent.  The sensitivity of OPV performance to the exciton diffusion length (L_D) has made these devices a useful platform for the characterization of this important material property.  While device photocurrent spectroscopy is a straightforward method to extract L_D, it is frequently limited by unknown recombination losses at dissociating interfaces, making the extracted L_D a lower bound. We resolve this limitation and demonstrate a general, device-based photocurrent-ratio measurement to extract the intrinsic L_D. Since interfacial losses are not active layer specific, a ratio of the donor and acceptor internal quantum efficiencies cancels this quantity, allowing L_D to be extracted.  The generality of this method is demonstrated by extracting L_D for both luminescent and dark organic semiconductors, as well as for both small molecule and polymer active materials.  We demonstrate the broader applicability of this approach by also examining semiconductor quantum dots.  Finally, we show that with intrinsic measurements of L_D, additional device-relevant information can be extracted regarding the efficiency of exciton relaxation and interfacial charge separation processes.

Vacuum evaporation is proven as a low cost method for the large area deposition of metal films and so is widely used in the food packaging industry. At the same time silver (Ag) films with a thickness of 6-10 nm are promising as the basis for flexible transparent electrodes for organic photovoltaics and displays. However, due to the high surface energy of silver and its slow rate of oxidation in air, very thin evaporated Ag films are morphologically unstable even at room temperature. Consequently, for practical application there is a need to identify an easily implementable and versatile means of imparting long term stability. This talk will describe how a single layer of a bifunctional small molecule deposited from the vapor phase can greatly enhance the morphological and chemical stability of optically thin Ag film electrodes. Due to its very low thickness (~1 nm) this organic layer does not electrically isolate the electrode. It is shown that in 10% efficient inverted, top-illuminated and semi-transparent OPVs substantial improvements in device stability are achieved by inclusion of this layer, which is remarkable give its very low thickness.

In the field of organic electronics, rising research attention has been dedicated to the investigation of novel interlayers. A family of materials that has been deserving increasing interest for such purposes is the one of transition metal oxide nanoparticles, which can provide high quality interlayers for both charge injection or extraction. Metal oxide nanoparticles are suitable for solution-processed electronics thanks to many advantages: they can be dissolved in organic solvents and used as printable inks, can be processed at relatively low temperatures and can offer an increased stability compared to conjugated polymers. However, they also present certain technical challenges due to the higher surface to bulk ratio (i.e. surface trap states) or present problems during film formation (i.e. agglomeration).[1] For this reason, various kinds of polymers can be employed to offer a hybrid solution to these issues.

We present here the improvement of the processability of non-stoichiometric nickel oxide (NiO_x) nanoparticle ink by blending with high molecular weight polyethylene oxide (PEO). Recently, NiO_x has attracted increasing attention as a hole extraction layer in organic and perovskite photovoltaics due to its excellent optical transparency, p-type conductivity and good electron blocking properties. Nonetheless, the fabrication of highly efficient NiO_x thin films is challenging due to the low viscosity of the inks or the high sintering temperatures of the precursor approaches. Here, we show how PEO can help dispersing the nanoparticles hindering their aggregation after deposition without compromising film functionality. Through Kelvin Probe, Contact Angle measurement, X-ray Photoelectron Spectroscopy and Transmission Electron Microscopy we observe that the presence of PEO is beneficial for a better tunability of the NiO_x film thickness and morphology. We also show that such effect on the film formation is observed to be beneficial when the NiO_x:PEO blends are applied as a hole extraction layer on OPV devices, improving device performance. Moreover, the inclusion of the polymer in the nanoparticle ink allows, for the first time, the inkjet-printing of the NiO_x layer without requiring high temperature post-treatment. [Manuscript in preparation]

We developed a method to stabilize and transfer organic semiconductor nanofilms. The method is based on crosslinking of the topmost layers of organic films by low energy electron irradiation. The irradiated films, which are deposited on a dissoluble interlayer, are detached from their original substrates and subsequently deposited onto a new substrate [1]. This allows for the fabrication of highly ordered interfaces of organic 3D films and 2D materials. We demonstrate the versatility of this approach by the fabrication of ambipolar MoS2-pentacene field effect transistors. First we study transport properties in order to confirm ambipolar behaviour. Second we apply raster scanning techniques such as photocurrent microscopy and photoluminescence spectroscopy in order to study the local characteristics of such devices with submicron resolution. This technique allowed us previously to identify in unipolar devices injection barriers at contacts and trapping effects in the channel [2]. In ambipolar devices, we can probe also the generation of free carriers by exciton splitting and charge separation at the 2D/3D heterojunction. In order to excite the two materials separately, we have implemented several laser lines which match the respective absorption maxima and minimal of the 3D and 2D materials. Our initial experiments focus on pentacene and MoS2 as 3D and 2D material, however, the transfer method should also be applicable also to C60, which would open a wide range of possible hybrid p/n junctions and related devices.

1) S . J. Noever, M. Eder, F. del Giudice, J. Martin, F. Werkmeister, S. Hallwig, S. Fischer, O. Seeck, N.-E. Weber, C. Liewald, F. Keilmann, A. Turchanin, B. Nickel
Transferable organic semiconductor nanosheets for application in electronic devices
Advanced Materials (2017)

2) C. Liewald, S. Strohmair, H. Hecht, E. D. Głowackic, B. Nickel
Scanning photocurrent microscopy of electrons and holes in the pigment semiconductor epindolidione
Organic Electronics 60, 51 (2018) 

Hybrid materials open new perspectives towards the design and tailoring of materials with desired features and functions for applications in opto-electronics. From a theory point of view, the ab initio description of organic materials and, in particular, their interfaces to inorganic counterparts remains an exciting though challenging issue. Only having proper theoretical concepts and numerical tools in hand which consistently capture the features of molecular materials, from single molecules to hybrid interfaces, allows for getting insight into the leading excitation processes. As many-body effects play a dominant role forefront methodology, that is capable of reliably predicting the electronic structure and optical excitations, is a must. Theoretical spectroscopy, combining Green-function based methods and density-functional theory provide powerful tools for the in-depth understanding of the response of such materials to excitations by electromagnetic radiation. I’ll discuss recent progress and critical challenges in understanding level alignment and light-matter interaction in selected examples, including organic/inorganic interfaces and hybrid perovskites.

Fabrication of the devices from solvents is the essence of future printed electronics which embodies easy processability, low cost and flexibility.  One particularly interesting device is Light Emitting Field Effect Transistor (LET), as it integrates both the switching and emitting property in a single optoelectronic device, which can significantly simplify the design of active matrix displays.[1] Moreover, as the emission zone is relatively small,  LET can act as future nano light sources.[2] More importantly, LET can render very high current density,[3] a prospective for realizing electrically pumped lasers which still remains as one of the biggest challenges in flexible electronics.

One of the major issues in achieving high performance LET is obtaining both high carrier mobility and high electroluminescence from the active semiconducting layer, requiring complex device engineering.  Here in this work, we present a number of approaches that were applied to decrease optical switch on voltage in hybrid LETs. Firstly, solution processed indium oxide (In₂O₃) was used as electron transport layer to achieve high current density. Interlayer tungsten polyoxometalate (POM) between the indium oxide and the source-drain electrodes improved the overall performance of the device. Electron mobility reached 11 cm²/Vs, which is 3 times higher than that of the control device. Current on/off ratio increases by 2 orders of magnitude reaching 10⁷.[4] Subsequently, organic emissive layer (Super Yellow) was introduced, and light emission visible for naked eye was observed using asymmetric source/drain electrodes.  In the next step, p-channel organic transport layer (C₈-BTBT) was further integrated after improving hole injection. For that, various metal and oxide combinations were applied to determine the optimum hole injection electrode. Finally, we applied hole transport layer and emissive layer blend approach, which considerably decreased the optical switch on voltage as well as enhanced the luminance.  

When everything is said and done, the most remarkable property of Halide Perovskites is that they can have defect densities that approach those that can be estimated from thermodynamics, i.e, a near-absence of kinetically stabilized defects, which naturally is  the kind on which (external and internal) doping is based. This is especially surprising if we consider the quick and “chimie douce” way of preparation of films as well as of most single crystals. Note though that the experimentally "determined"  defect densities are all deduced from  common (indirect) measurements for charged or neutral defects, using mosels that include assumptions, explicitly and/or implicitly.

In this talk I will show how this behaviour likely reflects a  fundamental property of these materials, with a rather simple basis. In the talk I will combine experimental results from several sources, including our own, for thermodynamic, optical, and electrical data and will ponder the possibility that the conclusions can be generalized to help look for other ultra-low defect density materials.

Two-dimensional hybrid lead halide perovskite derivatives have garnered considerable interest for opto-electronic applications, due to the presence of strongly bound and stable excitons even at room temperature. While the excitons in these systems seem to be analogous to those in semiconductor quantum wells, polar lattice fluctuations and dynamic disorder give rise to strong exciton-phonon coupling effects. We have recently identified clear signatures of polaronic effects on the excitonic correlations via quantitative analysis of linear and non-linear optical spectral[1][2]. Here, we establish that the dynamic structural complexity in a prototypical 2D lead iodide perovskite results in the coexistance of diverse exciton resonances, each with distict degree of polaronic character. We coherently excite and probe vibrational wavepacket dynamics by means of high resolution impulsive stimulated Raman spectroscopy, that evolve along different configurational coordinates defined by the normal vibrational modes of the lattice. Based on density functional theory calculations, we assign the observed vibrational modes to various low-frequency (< 50 cm-1) optical phonons involving the motion within the lead iodide layer. We demonstrate that different excitons induce specific lattice reorganizations, which are signatures of their polaronic binding. 

Organic semiconductors are used in optoelectronic devices, such as organic light-emitting diodes, organic and perovskite solar cells, and organic field-effect transistors. The performance of such devices depends heavily on charge injection and transport. Here, we describe a universal strategy to create Ohmic contacts on organic semiconductors, even with ionization energies of up to 6 eV. The method is based on the use of an interlayer that causes electrostatic decoupling of the electrode from the semiconductor, while establishing alignment of the Fermi level with the energy levels of the organic semiconductor. This interface engineering method to create Ohmic contacts enables us to characterize charge transport in a large range of organic semiconductors. From these measurements we are able to extract important parameters, such as the charge-carrier mobility, energetic disorder and the molecular site spacing. These experimental parameters are then compared to theoretical multiscale simulations, which compute these parameters considering the molecular arrangement and electronic interaction between the molecules. Excellent agreement is found between experiment and theory, which paves the way for predictive charge-transport simulations from the molecular level. Furthermore, the use of this injection startegy will be demonstrated to create efficient organic devices.

In order to improve the understanding of the device physics, specifically charge generation, transport and recombination, of organic and perovskite solar cells, a multitude of experimental techniques is available today. In this work we present an overview of opto-electronic characterization techniques including current-voltage curves, impedance spectroscopy, intensity-modulated photocurrent/photovoltage spectroscopy, and various transient experiments such as charge extraction by linearly increasing voltage, transient photocurrent, transient photovoltage and charge extraction. Using our drift-diffusion model we are able to simulate all of the above techniques and investigate how nonidealities like interface barriers, traps or low mobilities can manifest themselves as specific signatures in the data. The combination of different experiments therefore allows us to provide guidelines for the interpretation of measurement results and to unambiguously identify the limiting processes and dominant loss mechanisms. Furthermore, the simulation can be employed inside global fitting routines in order to determine material and device parameters. Hereby we show that the combination of steady-state, transient and modulated techniques is a key to increase the reliability of the extracted parameter values.

Accurately identifying and understanding the dominant charge carrier recombination mechanism in perovskite solar cells is of crucial importance for further improvements of this already promising photovoltaic technology. Understanding the order, rate and spatial whereabouts of the recombination processes as a function of excess carrier concentration is imperative towards identifying the remaining culprits behind the voltage deficit to the thermodynamic radiative limit (1.33 V for MaPbI3). With this objective, various electrical transient methods based on photovoltage decay have previously been employed. However, these techniques can be strongly influenced by the device capacitive response which overlays with the steady state relevant bulk carrier recombination. Herein, we will first outline the main limiting parameters in the assignment of correct and steady state relevant charge carrier lifetimes in both organic and perovskite thin film solar cells. To ascertain the identification of steady state relevant bulk charge carrier dynamics, we will highlight the benefit of evaluating thicker films at higher light intensity to minimize the impact of spatial capacitance artifacts. We focus our study on the electrical transient response in very efficient planar co-evaporated solar cells with an increased active layer thickness, up to 820 nm. While the increased capacitance for the thin cells leads to longer perceived decay times in the lower voltage regime, the higher voltage regime shows kinetics becoming independent of active layer thickness, allowing us to identify the transition from capacitance affected to the sought-after bulk charge carrier dynamics. Finally, we determine that the recombination order in thicker devices is ranging in between 1.6-2. These values are noticeably lower than what has previously been reported by these or equivalent electrical methods and are, more in line with dynamics observed in pure film, pointing towards trap-assisted and free carrier recombination under operating conditions in complete perovskite photovoltaic devices.

Thin metal oxide films have attracted a lot of attention in the past years due to their unique ability to act as electrode contact layers in novel electronic and optoelectronic devices. Prominent examples are molybdenum oxide (MoO_x) and titanium oxide (TiO_x) thin films used as hole and electron contact layers, respectively, in organic and hybrid photovoltaics. Amongst many different methods used for fabrication of these films, reactive sputtering remains as a promising technique, due to the unique composition tuning and industrial scale processing possibilities[1]. In the work presented here, crystalline MoO_x and TiO_x layers are developed from reactive sputtering and vacuum annealing, and implemented as contact layers in organic and hybrid solar cells.

The film composition is characterized using X-Ray Photoelectron Spectroscopy (XPS), work function using Low Energy Electron Microscopy (LEEM) and Ultraviolet Photoelectron Spectroscopy (UPS), structure using Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD), morphology using Atomic Force Microscopy (AFM) and optical properties using UV-VIS spectroscopy. Importantly, we find that both the structure and work function of the developed thin films can be tuned by the annealing process, spanning an almost 2eV tuning range in the case of MoO_x.[2] Furthermore, due to the formation of the crystalline films with a low defect density, made possible via the reactive sputtering method, more efficient and stable contact layers for photovoltaic devices are developed. Non-encapsulated DBP/C₇₀ solar cell devices based on the sputtered MoO_x are demonstrated to remain with impressive 80% of the initial performance after 240 hours of light soaking under 1 sun (1000W/m²) at ~60°C, which is superior to similar devices based on conventional thermally evaporated MoO_x layers. Fabricated PTB7/PCBM solar cells devices based on the sputtered TiO_x are demonstrated to lead to s-shape-free high performing devices, otherwise typically appearing when employing TiO_x as contact layers. The underlying film properties leading to these appealing device properties are evaluated based on the extensive surface and film characterization performed.

 The work thus demonstrates a viable method for tuning the electronic and optoelectronic properties of metal oxide thin films, which can be applied in combination with a wide range of materials in e.g. organic and hybrid photovoltaics.