This plenary conference presentation was prepared for the Organic, Hybrid, and Perovskite Photovoltaics XXIII conference at SPIE Optics + Photonics, 2022.

This plenary conference presentation was prepared for the Organic, Hybrid, and Perovskite Photovoltaics XXIII conference at SPIE Optics + Photonics, 2022.

The absorption of a photon by Organic Semiconductors (OSCs) results in the formation of a bound electron-hole pair quasiparticle or Frenkel Exciton. The energy required to separate an exciton into noninteracting electron and hole, Exciton Binding Energy (Eb), is a critical parameter for the purpose of improving the efficiency of optoelectronic devices such as solar cells and light emitting diodes. In the last two decade there have been many efforts to measure the Eb of OSCs using different techniques. However, there are discrepancies in the literature and the reported values are scattered over a large range between a few meV to 1.5eV, even for a specific material. Eb of Frenkel excitons can be estimated as the difference between the transport (Eg) and the optical gap (Eopt), a traditional definition borrowed from the language of Wannier Exciton in inorganic SCs. Here, we explore the Eb of different variants of PBnDT-FTAZ polymer. We focus on the two most common methods used to measure Eg: combination of Ultraviolet Photoelectron Spectroscopy and Inversed Photoelectron Spectroscopy (UPS-IPES) and Solid-State Cyclic Voltammetry (CV). We show that Eb measured by the abovementioned methods are not consistent or correlated with each other. The Eg measured by UPS-IPES technique is comparable with (or even smaller than) Eopt leading to small Eb. On the other hand, CV usually measures larger Eg compared to the Eopt resulting in larger values of Eb that are scattered between 200meV-1eV depending on the molecular structure of the materials. This discrepancy is the result of lack of both an exclusive theoretical and a functional definition of Eb that includes the relaxation effects, an important characteristic of Frenkel excitons. Moreover, due to the nature of each measurement method, they might measure different parameters than the actual properties of the bulk in a photovoltaic device. Our results elucidate the current conundrum on determination of Eb in OSCs and emphasize the importance of establishing standard theoretical and practical guidelines on how to properly estimate Eb.

In bulk heterojunction organic solar cells, the energetic landscape at the donor-acceptor interface provides the driving force for charge separation. In this presentation, I will discuss our latest insights into the photophysical processes governing charge separation, recombination, and energetic (voltage) losses in novel NFA-based systems studied by steady-state and advanced transient spectroscopy techniques. I will primarily address the question, how the interfacial energy offsets control exciton dissociation and charge separation in binary and ternary blends of polymer or small molecular donors with novel NFAs, including photoactive layers using state-of-the-art Y-type acceptors.

Third-generation solar cells, such as organic and perovskite solar cells are all relying on a semiconducting thin-film active layer to harvest the solar energy. The bulk morphology of the active layer in terms of crystal structure, orientation, grain size and nanophase separation behaviors is known to be critical to the solar cell device performance. Here, we will present our recent studies on the process-structure-device correlation of organic and perovskite solar cells. In these studies, state-of-art grazing incidence scattering techniques using X-rays and neutrons were employed for various purposes, such as wide-angle/small-angle X-ray/Neutron scattering (GIWAXS/GISAXS/GISANS) and transmission small-angle X-ray scattering (GTSAXS).

The composition dependence of OSCs based on PM6 and Y6 was studied. Surprisingly, a power conversion efficiency (PCE) over 10% was achieved in the OSCs with 10 wt % PM6. Study indicates that 10 wt % PM6 is sufficient to provide efficient hole transport.[1] Furthermore, vacuum free solution-processed semitransparent OSCs (ST-OSCs) containing 10% - 45% PM6 were studied by employing PEDOT:PSS as an electrode. The light utilization efficiency (LUE) of 2.72% (defined as a product of PCE and AVT) was realized in ST-OSCs with 20 wt % PM6, which is among the best reported values for solution-processed ST-OSCs. This work provides a straightforward approach to achieve high LUE solution-processed ST-OSCs by combining small fraction visible-absorbing donors with more near-infrared acceptors, which opens applications of ST-OSCs as BIPVs.

In the field of photovoltaics (PV), an important trend is to increase the aesthetic and creative design aspects of solar cells towards more attractive and customized devices for integration in for instance architecture (e.g. Building Integrated Photovoltaics BIPV). Recent evolutions in this domain are mainly situated in the class of emerging PV such as organic solar cells (OPV), dye sensitized solar cells (DSSC) and perovskite solar cells. These solar cell technologies provide additional degrees of design freedom, as they can be processed by printing and allow the realization of semi-transparent solar cells and the use of various colors. Here we aim to go a step further. In this contribution we report on our aim to develop semi-transparent solar cells with integrated images (photographs, paintings, geometric and graphical patterns, text,..) that generate electricity when illuminated, a concept we termed as “Photovoltaic Photographs”. The proposed concept consists of semi-transparent solar cells with an integrated image (functional as photoactive layer), allowing creative applications such as photovoltaic photographs, paintings, posters, etc. The approach proposed here to obtain a patterned 2D-photoactive layer, is by using direct photo-induced patterning process, i.e. one-step photolithography (with mask / without resist) and direct light writing (i.e. maskless / without resist). Encouraged by our recent realization of a proof-of-principle demonstrator using dye sensitized solar cells, in this project we pursue a thorough understanding and control of the proposed light-induced patterning processes and underlying physicochemical (“bleaching”) mechanisms, and their effects on nanoscale material properties, device characteristics, and stability. These insights will also help us in the exploration of combining this concept and these processes with other PV technologies.

In this paper, we will present our approach to develop a new class of solar cells with dynamic optical properties. We have synthesized new organic photosensitizers showing photochromic properties and incorporated them into dye-sensitized solar cells. We obtained semi-transparent solar cells able to self-adapt autonomously their light transmission and energy production according to daylight variations. We will discuss the synthesis of the new photochromic photosensitizers, their optical and electronic properties, and we will present the development of photochromic semi-transparent mini-modules using them.

Understanding the formation mechanisms of halide perovskites will enable active control of synthesis parameters and increase reproducibility. It is described how the antisolvent influences the crystallization pathway and may lead to the metastable nucleation of perovskite early in the synthesis. On the other hand, a multi-stage synthesis process is found to occur in Br-containing mixed halide perovskites as compared to a single-stage in the iodide reference. In situ photoluminescence measurements during spin coating and annealing reveal that bromide inclusion alters the formation dynamics.

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 25% and further improvements are expected up to 27%. Now, the research on perovskite solar cells (PSCs) has been moving toward commercialization. To facilitate commercialization of these great solar cells, some technical issues such as long-term stability, large scale fabrication process, and Pb-related hazardous materials need to be solved. Also, flexible perovskite solar cell using plastic substrate can be used in niche applications such as portable electrical chargers, electronic textiles, and large-scale industrial roofing. This talk is dealing with our recent efforts to facilitate commercialization of perovskite solar cells. For examples, we introduce a recycling technology of perovskite solar cells, which covers the regeneration process of Pb contained perovskite layer as well as recycling process of Au electrodes and transparent conducting oxide glass. Also, simple fabrication technologies for realizing large scale perovskite module are introduced and recent effort for achieving high efficiency module is going to be presented. Precursor technology is of great importance for yielding high efficiency and reproducibility of PSC, which is one of topics in this talk. Finally, recent interesting results regarding flexible perovskite cells will be discussed.

Hybrid organo-metal halide perovskite solar cells (PSCs) are promising candidates for next generation photovoltaic device primarily due to their high efficiency, printability and low cost. PSCs have exhibited externally verified power conversion efficiencies (PCE) up to 25.7% in single junction, which have encouraged recent efforts on scalable coating technique in PSCs towards manufacturing. Effective interface passivations of the buried and top perovskite film are among the most important issues to address for reducing energy loss, high efficiency and stability. The benchmark tin oxide (SnO2) electron transporting layers (ETLs) have enabled remarkable progress in planar perovskite solar cell (PSCs). However, the energy loss is still a challenge due to the lack of “hidden interface” control. We report a novel ligand-tailored ultrafine SnO2 quantum dots (QDs) via a facile rapid room temperature synthesis. Importantly, the ligand-tailored SnO¬2 QDs ETL with multi-functional terminal groups in situ refines the buried interfaces with both the perovskite and transparent electrode via enhanced interface binding and perovskite passivation. These novel ETLs induce synergistic effects of physical and chemical interfacial modulation and preferred perovskite crystallization-directing, delivering reduced interface defects, suppressed non-radiative recombination and elongated charge carrier lifetime. Power conversion efficiency (PCE) of 23.02% (0.04 cm2) and 21.6% (0.98 cm2, VOC loss: 0.336 V) have been achieved for the blade-coated PSCs (1.54 eV Eg) with our new ETLs, representing a record for SnO2 based blade-coated PSCs. [1] Combined with 2D/3D graded interfacial passivation, 23.7% PCE was achieved, with significantly enhanced stability. [2]

Perovskite solar cells based on alternatives to lead halide perovskites still present practical drawbacks related to the poor environmental stability (the tin-based ones) and the low power conversion efficiencies that they provide (the bismuth-based ones). The silver-bismuth double perovskite (Ag-Bi DP) is an interesting platform onto which to develop novel concepts for photovoltaic devices since, although presenting very poor optoelectronic properties, it features an excellent stability and is based on relatively a-toxic elements.[1] Its absorption features are hypsochromically shifted in comparison to those of classical lead iodide perovskites and they are more suitable to absorb photons coming from LED lamps, therefore for indoor photovoltaics, which could be exploited to power the IoT, for example.[2] In this context particularly, devices delivering relatively low-power can still have an economical impact, particularly if their costs of production are kept low. Herein we report on the development of Ag-Bi DP solar cells where the classical HTM and top (gold) electrode have been replaced by a carbon-black electrode processed from pure isopropanol though high-throughput ultrasonic spray-coating.[3] The carbon material in the electrode is obtained from the recycling of tires waste through the application of a hydrothermal process, which opens up further relevant perspectives for circularity. Although PCEs in the fabricated devices remain below the 1% threshold, we obtain remarkably high open circuit voltages (up to 1.2 V) from these architectures. This approach may represent a valuable solution for the future large scale production of sustainable photovoltaic devices to power the IoT.

Electrical performance and morphological stability of organic solar cells have a strong dependence on the quenched state of the active layer, driven primarily by the thermal properties of the constituent organic semiconductors. The quenched state of an active layer is impacted by the utilized processing parameters. This makes understanding the thermal behavior of the materials in thin film geometry imperative, which is not possible using conventional DSC. We demonstrate that in situ VASE can characterize both polymorphism in high performance NF-SMAs and the impact of kinetic quenching on density and free volume in cast thin films. Both phenomena impact the performance and stability of the OSCs.

2D perovskites have broad technological appeal because of their tunable mechanical, optical, and electrical properties. For flexible optoelectronic applications, it is necessary to determine how mechanical stresses affect their optoelectronic properties. We compare the impact of strain on the photoluminescence (PL) spectra and charge carrier recombination rates of two different 2D perovskite materials, synthesized using either phenethylammonium or butylammonium cations. Both perovskite materials exhibit strong PL enhancement, redshifts of the PL emission wavelength, and longer recombination lifetimes for compressive strains of ≲1%. These results are discussed in relation to the materials’ band structures and trap states.

Anomalous properties such as operational instability and photocurrent hysteresis in perovskite-based devices present a major obstacle to their future commercialization. Halide ion/defect migration has been widely accepted as one of the main mechanisms behind these limiting properties, but a definitive explanation of this relationship has remained elusive. Here, we present a quantitative multi-scale diffusion framework that fully describes halide diffusion in polycrystalline metal halide perovskites (MHPs). By using time-of-flight secondary ion mass spectroscopy (ToF-SIMS) technique we could simultaneously monitor both the fast grain boundary (GB) diffusivity and three to four orders of magnitude slower volume/bulk diffusivity. Our framework reveals an inverse relationship between the activation energies of GB (EGB) and volume (EV) diffusions, such that MHPs (such as MAPbI3) with a larger EV also possess a smaller EGB. Importantly, this relationship explains some of the most conflicting observations in the literature, namely that MHPs with improved stability typically exhibit reduced hysteresis, thanks to the simultaneous existence of small volume and large GB halide diffusivities, respectively, pointing us to propose a model of grain boundary “strength”. This nontrivial relation between volume and GB halide diffusivities is derived from a wide range of MHP systems, including MA- and FA-based iodide and bromide perovskites. Even when GB passivation approaches are used, GB diffusivity increases reducing hysteresis at the expense of volume diffusion, which enhances stability. The quantitative elucidation of multiscale halide diffusion in polycrystalline MHPs provides an important path toward addressing these outstanding issues.

Excitons --- Coulomb-bound electron-hole pairs --- are the primary photo-excitations in two dimensional Ruddlesden Popper metal halides (RPMH). Spectral signatures of these quasi-particles manifest in the optical spectra as well-defined resonances separated by a characteristic energy. Here, we use these distinct excitonic resonances and their coherent dynamics as a spectroscopic probe of the unique characteristics of these excitations. We observe that the spectroscopic observables reflect the peculiar interactions of excitons with the anharmonic RPMH lattice that can be contextualized within the “exciton-polaron” framework. Unlike conventional 2D Wannier exitons, we consider that the electron-hole pairs in RPMHs intrinsically carry the lattice dressing, which we measure using impulsive vibrational spectroscopy. Moreover, we observe that there are multiple excitons that are dressed distinctly by the lattice phonons. We measure the intrinsic and density-dependent exciton dephasing rates of these multiple excitons and their dependence on temperature by means of two-dimensional coherent excitation spectroscopy. We find that diverse excitons display distinct intrinsic dephasing rates mediated by phonon scattering involving different effective phonons and contrasting rates of exciton-exciton elastic scattering. These findings establish specifically the consequence of anharmonic lattice interactions on the exciton many-body quantum dynamics, which critically define fundamental optical properties that underpin photonics and quantum optoelectronics. We further explore the origin of the plurality of excitons in RPMHs by systematic investigation of other materials variants obtained via substitution of either the organic cation or the metal cation. We find that a complex electronic structure with multiple carrier bands emerges, which may be responsible for the distinct excitons. Such an electronic structure originates from variations in coordination geometries of the metal halide octahedra induced by subtle changes in the organic-inorganic interactions, with measurable consequences on the exciton polaron characteristics.

We introduce the concept that free-charge generation in organic photovoltaic (OPV) materials may best be described by competition between long- and short-range electron transfer events, and that the distribution of rates as a function of distance follows the predictions of Marcus theory. Our results reveal the fundamental connection between solution-phase electron transfer research that has been conducted in the chemistry community over many decades, and the younger materials science effort to develop efficient OPV materials. The model that emerges provides insight into how the microstructure of OPV materials influences the electron transfer process via both entropic and quantum-mechanical mechanisms.

In this talk I shall discuss which factors influence the separation of charge transfer states in organic solar cells, and what are appropriate models to describe this process. I shall in particular focus on the role of order, disorder, exciton delocalision and intermolecular interactions

The efficiency of Organic Solar Cells (OSCs) has surpassed 18%, after a new generation of Non-Fullerene Acceptors (NFAs), the Y-series, was introduced to the field. These materials are characterised by high electron mobility, which is commonly attributed to its 3-dimensional packing motif in the single crystal. However, the bridge that links the NFA packing from single crystals to solar cells has not clearly been shown yet. In this work, we investigate the molecular organisation of a large body of NFAs, following the evolution of their packing motif in single-crystals, powder, and thin films made with pure NFAs and donor:NFA blends. We identified the most relevant packing motifs and polymorphs for the NFAs, discussing their role in the bulk heterojunction morphology, performance and charge transport, by combining experimental and theoretical approaches.

The power conversion efficiencies (PCEs) of lab-sized organic solar cells (OSCs), usually processed from low-boiling-point and toxic solvents, have reached high values over 18%. However, the key limiting factor for green-processed OSCs is imperfect micro-morphology, which dominating a significant drop of the efficiency. Herein, we obtain record PCEs over 17% in OSCs processed from a green solvent paraxylene (PX) by a guest-assisted assembly strategy, where a third component (guest) is employed to manipulate the molecular interaction of the binary blend. In addition, the high-boiling-point green solvent PX also enables us to deposit uniform large-area module (36 cm2) with a high efficiency of over 14%. The strong molecular interaction between the host and guest molecules also enhances the operational stability of the devices. Our guest-assisted assembly strategy provides a unique approach to develop large-area and high-efficiency OSCs processed from green solvents, paving the way towards industrial development of OSCs.

All-Small-Molecule Organic Solar Cells (ASM-OSCs) have reached 17% efficiency. Unlike polymers, SM synthesis is reproducible with controlled quality, making ASM-OSC highly relevant for commercialization. We perform extensive optoelectronic and microstructure characterization on high-performing ASM-OSC deposited via solution and vacuum thermal evaporation. We quantify absorption, voltage losses, and charge transport – probed via ellipsometry, sensitive EQE, and CELIV, respectively. To rationalize differences in charge carrier mobility, we construct the phase diagram and study phase separation via scattering methods. Example systems include ZnPc:C60, DCV5T-Me:C60, BTR:PCBM, and BTR-Cl:Y6. We find, e.g., that ASM-OSCs can achieve lower voltage losses than corresponding polymer-based OSCs.

Insights into the structure-processing-morphology correlation of PM6:Y-series NFAs (Y6, Y7, Y12, and DTY6) bulk-heterojunction (BHJ) systems processed from a green solvent, ortho-xylene (o-XY), is investigated in comparison with the same blends processed from traditional halogenated solvent, chlorobenzene (CB) and chloroform (CF). The high-performance systems preserved PCEs > 15% all exhibit fast charge carrier extraction, low density of bulk traps as well as long drift length and diffusion length for free charge carriers, which reveals the key factors for OSCs to maintain high PCEs regardless of processing solvents.

Intrinsically photo-stable organic bulk heterojunctions (BHJs) are crucial for achieving long lifetimes of organic photovoltaics.1 However, the intrinsic photodegradation of BHJs is often coupled with various externally induced degradation mechanisms, bringing challenges to optimizing their long term operational reliability. Here, experimental tools including grazing-incidence wide-angle X-ray and resonant soft X-ray scattering are combined with bulk quantum efficiency analysis to quantitatively identify the sources that limit the intrinsic lifetime of non-fullerene acceptor based BHJs under illumination.2 The methodology presented can effectively separate intrinsic degradation from externally induced causes, and thus can be used as a tool for guiding molecular and morphological design of long-lifetime organic BHJs. Furthermore, a model is proposed to attribute the physical origins of photodegradation of organic BHJs to second-order events such as exciton-exciton annihilation.2-3 The model is supported by the observed square law dependence of the degradation rate with light intensity.

A significant concern of perovskite solar cells (PSCs) technology is accidental Pb leaching from damaged devices, due to the well-documented Pb toxicity. We have examined the penetration profile of Pb from aqueous solutions of dissolved perovskite into the soil as a function of perovskite composition, as halide perovskites typically contain organic cations, which may affect the adsorption of Pb cations to soil particles by competition. The penetration profiles showed shallow immobilization of the Pb cations in all soils studied, with negligible odds of reaching and contaminating ground water. The adsorption mechanism was also determined. This suggests that Pb in PSCs and its effect on the environment are not as concerning, as they seem to be.

We report ink formulations for solution-processable near-infrared organic photodiodes (OPDs) to fabricate organic CMOS image sensors on silicon wafers. The ink for the hole transport layer, which consists of cross-linkable semiconducting polymers, fully covers an 8-inch silicon wafer with less than 3 nm of variation in thickness by spin-coating. The ink is stable over several months. For the ink for the active layer, new additives are found to reduce micrometer size phase separation of donor and acceptor semiconductors after thermal annealing, which is fatal to achieve pixel to pixel reproducibility. Both inks are free from halogenated solvents.