Perovskite-organic tandem solar cells (P-O TSCs) are developing rapidly, igniting tremendous interest in the multijunction community. However, their efficiency still lags behind that of other counterparts, and their current operational stability remains underexplored. This presentation will give a brief introduction into organic based multijunction solar cells and then turn over to hybrid multijunction devices. Two key aspect will be addressed specifically. First, we demonstrate a simplified interconnection layer (ICL) without an additional charge recombination layer (CRL), achieving an average efficiency of 25.12% (with a champion device reaching 25.53%) for P-O TSCs. Second, we present a detailed investigation to uncover how the single-junction sub-cells of multi-junction cells mutually stabilize each other, which leads to excellent long-term lifespan of P-O TSCs, retaining over 91% of their initial performance after 1000 hours of continuous metal-halide lamp illumination without UV filters. Besides, we also highlight the durability of ICLs, providing insights for advancing future tandem configurations.

Shortwave infrared (SWIR) has various applications, including night vision, remote sensing, and medical imaging. SWIR organic photodetectors (OPDs) offer advantages such as flexibility, cost-effectiveness, and tunable properties. In this talk, I will discuss the development of OPD materials processed from green solvents and device engineering to extend the wavelength detection range of OPD to a longer wavelength. We demonstrate a proof-of-concept in PTB7-Th:COTIC-4F blend system, achieving external quantum efficiency (EQE) > 50 % over a broad spectrum  (450 – 1100 nm) with a peak specific detectivity (D*) of 1.1 x 10¹³ Jones at 1100 nm, while cut-off bandwidth, speed, and linearity are preserved. By employing a novel small-molecule acceptor IR6, a record high EQE = 35 % and D* = 4.1 x 10¹² Jones are obtained at 𝝀 = 1150 nm. Additionally, we develop a high-performance non-halogenated-solvent-processed BHJ OPD utilizing a novel ultranarrow-bandgap NFA to achieve an exceptional EQE of ~20 % at λ = 1200 nm while retaining sub-μA.cm⁻² dark current density level and D* > 3 × 10¹¹ Jones over a broad spectrum of 400 – 1200 nm. This work emphasizes the importance of molecular design in optoelectronic devices.

The strategic opportunities of emerging photovoltaics lies in their potential for sustainable large-area manufacturing using scalable solution-processing techniques such as slot-die coating. Perovskite solar cells are, facinatingly, to date the best solution-processable solar cell technology on the verge of commercialization. Mass-production, however, requires the development greener fabrication protocols.

In this talk we will addresses the dual challenges of scalability and environmental sustainability by exploring the development of more environmentally benign ink formulations for perovskite solar cells. Using protic ionic liquids (PIL) as ink additives in water- and alcohol-based precursor inks, we optimized the ink chemistry and process parameters to achieve uniform films with superior optoelectronic properties by slot-die coating. The PIL-based ink underwent a direct liquid-to-solid transition during film formation without forming intermediates, as confirmed by in-situ grazing incidence wide-angle X-ray scattering (GIWAXS). The optimized solvent system, a mixture of water, isopropanol (IPA), and MAP, enabled one-step slot-die coating at ambient conditions, achieving slot-die coated solar cell efficiencies up to 10%. This eco-friendly ink reduces toxicity, extends shelf life, and is stable in ambient environments, presenting a viable pathway for sustainable commercial applications.

This work highlights the potential for green ink formulations in scaling up perovskite solar cell manufacturing. Future efforts will focus on developing data infrastructure to integrate life cycle assessment (LCA) at early ink development stages, enabling the systematic design of environmentally optimized precursor inks and processes.

The rapid advances in increasing the power conversion efficiency (PCE) of organic photovoltaics (OPVs) have been mainly driven by advancements in new materials and the reduction of performance losses associated with traditional cell structures. As the PCE values of state-of-the-art OPVs have now exceeded 20%, the importance of cell engineering—including the active layer, interfaces, and light management—becomes even more crucial and, in most cases, determines the ultimate cell performance. In this talk, I will discuss our recent efforts to increase the PCE of OPVs to well beyond 20%. I will demonstrate how the use of engineered charge-extracting interlayers can help boost the overall performance of the cells as well as their operating lifetime. Moreover, I will highlight our latest advancements in utilizing molecular dopants to enhance the PCE and show how their combination with innovative interlayers and appropriate cell structures can improve material utilization and overall device sustainability, all while maintaining record levels of PCE.

A brief introduction to the Royal Society of Chemistry's extensive range of journals, including Energy & Environmental Science and the EES Family. The EES Family is synonymous with exceptional research in energy and environmental science. Our premier journals are committed to publishing only the highest quality and exceptional work on energy and environmental science. The Royal Society of Chemistry publishes 58 journals with a commitment to rigorous, fair peer review and fast publication times.

As the power-conversion efficiency of organic solar cells exceeds 20%, it becomes a priority to reproduce such performance in systems made from upscaleable materials and processes, and in systems with high operational stability. Small changes in chemical structure, solvent, fabrication process and device scale can all result in disappointing solar cell performance relative to a high-efficiency reference. Methods are needed to interpret changes in device performance in terms of the structural and spectroscopic properties of the active layers, in order to gain insight into how processing affects the basic operational mechanisms of the devices. At the same time, processing approaches that maintain the long term stability of the devices are needed.

In this talk we will present an approach to interpret the luminescence from active layer materials and devices in terms of basic molecular parameters and then analyse the effects of variations in material processing on these parameters in order to rationalise comparative device performance. Temperature dependent luminescence is analysed in terms of the Marcus-Levich-Jortner framework using Bayesian inference to distinguish the influences of different molecular parameters. We show the impact of process parameters such as solvent, additive and blend ratio on model parameters; and also explore the dependence of the quality of the fit on the range of data used in fitting. In a second study we show how the thermodynamic stability of organic heterojunctions can be controlled using novel ternary systems.  

Solution deposition from nanoparticle dispersions allows the use of eco-friendly processing agents such as alcohols or water for the fabrication of organic semiconductor thin-films for optoelectronic applications. Primary application of the eco-friendly device production from organic nanoparticles are solar cells where solvents are evaporated on large scale upon drying. Omitting surfactants to stabilize the dispersions is essential to not jeopardize the device performance. In this work, novel surfactant-free nanoparticle dispersions from high-performance organic semiconductors are synthesized by nanoprecipitation and electrostatically stabilized by electrical doping. The corresponding solar cells achieved power conversion efficiencies beyond 10%, matching the performance of reference solar cells deposited from toxic solvents. When applied to organic photodetectors, the nanoparticle approach allows the fabrication of thick absorber layers which enhance the quantum efficiency in the outer parts of the absorption spectrum, yielding enhanced broad-band detectivity. Aqueous dispersions of the organic semiconductor nanoparticles can be used for catalytic hydrogen generation.

Conjugated polymers (CPs) have garnered significant attention in the fields of photodetectors[1], [2] and photoacoustic due to their tunable electronic properties, ease of processing, and potential for integration with flexible devices. Among these, shortwave infrared (SWIR) conjugated polymers offer a promising avenue for advanced photodetection and imaging applications, thanks to their ability to absorb and emit light in the 1–2 µm wavelength range, which is less susceptible to scattering and absorption by biological tissues compared to visible and near-infrared light. This feature enhances their suitability for deep tissue imaging, as well as for high-performance photodetectors with improved signal-to-noise ratios. In this work, recent advancements in the design and synthesis of new SWIR-responsive conjugated polymers, based on thiadiazolequinoxaline units as electron withdrawing units and various building blocks as electron donating segments will be presented including the incorporation of optimized π-conjugation and side-chain engineering that have led to significant improvements in their absorption, charge transport, and photodetection efficiency. Additionally, their biocompatibility and ease of functionalization open new possibilities for non-invasive bioimaging through photoacoustic application, enabling real-time monitoring of cellular processes and disease progression.

<p>Bulk-heterojunctions (BHJs) comprising mixed phases of donor and acceptor organic semiconductors have propelled the performance of organic photovoltaics (OPVs) and organic photodetecors (OPDs) to performances closing in on those of competing technologies. However, poor control over interfaces and concerns over the long-term stability of such mixed phases mean that alternative architectures, such as bilayers or single-components, are re-gaining in research attention.</p> <p>Here, I will first present a simple and potentially low-cost hybrid organic/inorganic bilayer architecture for photodetectors and colour-tuneable solar cells, comprising the low-cost wide-bandgap inorganic semiconductor copper thiocyanate (CuSCN) and small-molecular acceptors. I will show that photodetectors using this simple bilayer can achieve an extremely low dark-current (10^-8 Acm^-2 @ -2V), which is partly enabled by low non-radiative voltage losses , and an extremely low noise spectral density [1]. Coupled with a light spectral respone that is widely tuneable through the choice of organic acceptor, this enables the fabrication of photodetectors with a specific detectivity in the near-infrared that approaches that of commerical Silicon photodetectors.</p> <p>Secondly, I will report on the characterisation and modelling of a variety of non-fullerene acceptors for single-component light-to-charge generation, which have been proposed to potentially offer a new avenue for opto-electronic devices. We combine experimental characterisation under varying conditions (field, temperature, excitation) with molecular and device-level calculations, to relate exciton and charge dissociation efficiency in single-component devices to molecular parameters, and investigate the potential of&nbsp; using free charge-generation in single organic semicondcutors for optoe-electronic devices.</p>

Sustainably-printed organic semiconductors are cornerstones in the pursue of environmentally-friendly, large-area solution-processed energy generation technologies such as organic photovoltaics. Among them, poly[[6,7-difluoro[(2-hexyldecyl)oxy]-5,8-quinoxalinediyl]-2,5-thiophenediyl]] (PTQ10) shines as semiconducting polymer with an inherently low synthetic cost and adequate compatibility with non-halogenated and non-aromatic solvents, thus meeting the requirements for a low cost, human- and environmentally-safer open-air film production with uncompromised device performance.

In this talk, the microstructural features of the archetypal PTQ10 polymer are thoroughly studied by x-ray diffraction and atomic force microscopy methods for its application onto sustainably printed organic solar cells. PTQ10 films processed by either spin or bar coating in neat and co-solvent formulations that match the up-scaling green processing requirements are showcased and rationalized in terms of characteristic structural features (e.g., d-spacings, degree of paracrystallinity) within the Hansen solubility framework. The use of PTQ10 as donor polymer in a non-halogenated photoactive layer blend is then presented and exploited to form versatile indoor and outdoor organic solar cells [1]. The industrial potential of this organic photovoltaic blend is further demonstrated in all-printed modular form factors, accordingly deployed for agrivoltaic energy harvesting in a domestic greenhouse in Sweden [2]. Unprecedented failure modes related with module delamination are therein observed as a consequence of the harsh humidity and thermal cycling conditions found inside the greenhouse. Overall, the use of PTQ10 and its variants as donor polymer shows enormous potential for efficient and sustainably-printed organic solar cells, photodetectors and thin film transistors.

Solution-processed nanocomposite films comprising small molecule organic semiconductors (OSCs) and inorganic colloidal quantum dots (QDs) are promising systems for low-cost, high efficiency, solar energy harvesting technologies.[1,2] In these systems, OSCs capable of singlet fission (SF) absorb high energy photons and generate triplet excitons which are harvested by inorganic QDs. Here, radiative recombination results in the emission of multiple lower energy photons which could be absorbed by an optically coupled silicon-photovoltaic (Si-PV) module, offering a route to surpass the radiative efficiency limit.  Achieving efficient photon-multiplication requires a precise nanoscale morphology, with QDs uniformly dispersed within the OSC matrix at length scales comparable to the triplet exciton diffusion length. However, mismatches in size, shape, and surface energy between QDs and OSCs typically results in strong aggregation and phase separation. Whilst a proof-of-concept system has been achieved based on a tetracene derivative,[3] the emission of the OSC is spectrally mismatched to Si-PV absorption, limiting its use in practical applications. This work explores the self-assembly of high-triplet energy OSCs based on diphenylhexatriene (DPH) and dithienohexatriene (DTH) derivatives that offer improved spectral matching with Si-PV. In-situ grazing incidence X-ray scattering is employed during high-throughput blade coating to study the self-assembly of promising DPH-/DTH-based systems. Improved QD dispersions are obtained by tailoring QD surface chemistries and optimizing thermal processing conditions. The results provide design rules and scalable processing strategies for developing photon-multiplier films, paving the way for their integration into high-efficiency Si-PVs for enhanced energy harvesting.

The efficiency of polymer solar cells (PSCs) has made impressive progress in the last few years. Still, the synthetic complexity of some of the best performing materials is high and is a possible bottleneck towards scalable commercial applications. In this work we present a new class of donor polymer that can be prepared in just two steps from commercially available starting materials.^[1] The simplicity of the synthesis allowed a library of polymers with differing alkyl-chain lengths and comonomers to be readily prepared and investigated. The thermal, electrochemical and photophysical properties of the resulting polymer library helped towards the development of structure-property design guidelines. The polymers were furthermore investigated as donor materials in solar cell devices with Y6 and L8BO as acceptor, with the best performing material FO6-T showing highly promising power conversion efficiencies (PCEs) of 15.4 %. Recently, we also showed that our polymer FO6-T is compatible with green solvents such as 1,2-xylene and 2-MeTHF.^[2] In order to study the scalability and commercial viability of the newly synthesised donors compared to state-of-the-art materials, I conducted a synthetic complexity (SC) analysis, which takes into account different industrially relevant parameters, such as hazardous chemicals involved and the number of synthetic steps, and we found that our material shows one of the lowest SC for well-performing donor materials to date.

We believe that these materials are highly promising for commercial application in PSCs due to strong device performance and a low cost and truly simple two step synthetic protocol.

As our societies struggle to meet their ambitions for decreasing greenhouse gas emissions and the Paris Climate agreement, research into alternative solar energy conversion and storage technologies becomes even more relevant. Solar fuel production by photocatalysis is an economically promising route, especially when driven by earth abundant and organic materials. While organics’ bottom-up design possibilities promise tailorable structure-function relationships for enhanced activity, the advancement is often hindered by limiting knowledge of interwoven photo-physical processes and properties that lead to recombination losses.[1]

In this talk, I will explain how time-resolved (transient) spectroscopy techniques in combination with varying environmental conditions can be used to provide insights into the very beginning of the solar energy conversion process chain, focussing on exciton generation and separation, and charge stabilization. This enables us to better understand light-matter interactions, and to tailor them to address bottlenecks associated with exciton recombination.

Our recent study on a series of functionalized polymers with varying porosity has revealed that not a maximization of a BET surface area is key, but rather the active interaction area with the photocatalytic environment, if exciton or charge separation is driven by sacrificial agents.[2]

In context of photocatalysis, interactions with aqueous ions, which are highly relevant for enabling applications in sea water conditions, are typically disregarded. Our study on suspended nanoparticles in presence of different salt shows how ions can impact stabilization of excitons and significantly extend their lifetimes, thereby enabling a new way to address excitons’ commonly fast and rate-limiting recombination.[3]

Lastly, I will introduce Terahertz permittivity measurements as convenient technique to probe the complex permittivity, and with that the dielectric properties of organic semiconductors on ps-time scales. The dielectric response defines exciton binding and is hence relevant for charge carrier photogeneration in all solar energy conversion technologies, but its values are highly frequency dependent, and commonly extracted at timescales orders magnitude off the ps-regime. Our study focussing on carbon nitrides now reveals dielectric screening and transport properties at the early time scales of solar energy conversion process chains. At the same time, it shows that also in this ultrafast regime, the environment and ions can matter, and strongly enhance photophysical parameters.[4]

References:

[1] T. Banerji, F. Podjaski, J. Kröger et al.: Polymer photocatalysts for solar-to-chemical energy conversion. Nat. Rev. Mater. 6, 168–190 (2021).

[2] B. Willner, C. M. Aichiston, F. Podjaski et al.: Correlation between the Molecular Properties of Semiconducting Polymers of Intrinsic Microporosity and Their Photocatalytic Hydrogen Production. J. Am. Chem. Soc. (2024), https://doi.org/10.1021/jacs.4c08549.

[3] F. Podjaski*, S. Gonzalez Carrero, C. Aichiston, P. Khlat, K. Steward, L. Hart, S. Hillman, J.-S. Kim, I. McCulloch, J. R. Durrant. In preparation.

[4] R. Jahangir, F. Podjaski*, P. Alimard, S. A. J. Hillman, S. Davidson, S. Stoica, A. Kafizas, M. Naftaly, J. R. Durrant: Terahertz-permittivity of Carbon Nitrides: Revealing humidity-enhanced dielectric properties on the picosecond timescales relevant for charge carrier photogeneration. Submitted. Preprint: https://www.arxiv.org/abs/2411.06226.