The emergence of new organic semiconductors has driven the continuous development of organic solar cells, with the power conversion efficiency of single-junction devices having passed 19%. The strong intermolecular interactions between organic semiconductors lead to the self-assembly of them into hierarchical aggregates, which exhibits vastly different optoelectronic properties compared to those of the single molecules. Revealing and controlling the complex aggregation structure of organic semiconductors, and establishing the key relationship between structure and the power conversion process, is vitally important toward high performance organic solar cells, but remains as a grand challenge.   

   Dedicating to this field, we have developed a number of physical and chemical approaches to tune the hierarchical aggregates of organic photovoltaic molecules: We developed the heating-induced aggregation strategy to suppress the large-size aggregation of crystalline semiconductors, realizing the conversion from large-size aggregation to small aggregation, which broadens the light absorption range and enhances exciton splitting; we developed the solution-induced aggregation strategy, realizing the conversion from random aggregation to ordered aggregation, which increases the light absorption intensity and improves charge transport; we also developed the small-molecular fibrillization strategy, realizing the conversion from short-range aggregation to long-range aggregation, which resolves the serious charge recombination issue during charge hoping and achieves a device efficiency of over 19%. The correlation between aggregates and light absorption, exciton dissociation and charge transport processes is eventually established to direct the future development of organic solar cells.

Organic solar cells (OSCs) are one of the most promising cost-effective options for utilizing solar energy in high energy-per-weight or semi-transparent applications. Recently, the OSC field has been revolutionized through synthesis and processing advances, primarily through the development of numerous novel non-fullerene small molecular acceptors (NFA) with efficiencies now reaching >19% when paired with suitable donor polymers. The device stability and mechanical durability of these  non-fullerene OSCs have received less attention and developing devices with high performance, long-term morphological stability, and mechanical robustness remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OSC commercialization. Here, we discuss our current understanding of the phase behavior of OSC donor:acceptor mixtures and the relation of phase behavior and the underlying hetero- and homo-molecule interactions to performance, processing needs (e.g., kinetic quenches), and morphological and mechanical stability. Characterization methods range from SIMS and DSC measurements to delineate phase diagrams and miscibility to x-ray scattering to determine critical morphology parameters and molecule packing and dynamic mechanical analysis (DMA) to assess specifically the hetero-interactions. The results presented and its ongoing evolution are intended to uncover fundamental molecular structure-function relationships that would allow predictive guidance on how desired properties can be targeted by specific chemical design. Comparative studies show that the molecular hetero-interactions between the donor and NFA are not always the geometric mean of the homo-interactions. This underscores the limited success often encountered when Hanson Solubility Parameters and surface energies are used to estimate molecular interactions. – We will also present a vignette detailing some work at NCSU regarding the integration of OPV into greenhouses [1-4]. 

[1] “Achieving net zero energy greenhouses by integrating semitransparent organic solar cells”, E Ravishankar, RE Booth, C Saravitz, H Sederoff, HW Ade, BT O’Connor, Joule 4, 490-506 (2020)

[2] “Balancing crop production and energy harvesting in organic solar-powered greenhouses”, E Ravishankar, M Charles, Y Xiong, R Henry, J Swift, J Rech, J Calero, et al. Cell Reports Physical Science 2, 100381 (2021)

[3] “Organic solar powered greenhouse performance optimization and global economic opportunity”, E Ravishankar, RE Booth, JA Hollingsworth, H Ade, H Sederoff, et al. Energy & Environmental Science 15, 1659-1671(2021)

[4] “Beyond energy balance in agrivoltaic food production: Emergent crop traits from color selective solar cells”, M Charles, B Edwards, E Ravishankar, J Calero, R Henry, J Rech, et al, bioRxiv (2022) doi: https://doi.org/10.1101/2022.03.10.482833

The rapid growth of the Internet of Things (IoT) has created a demand for energy-efficient, autonomous devices capable of operating over extended periods. Indoor photovoltaic (PV) systems offer a promising solution to power such IoT devices, providing a sustainable and renewable energy source. However, accurate evaluation of the performance of indoor PV systems in real-world conditions is essential to optimize their design and ensure reliable long-term operation.

We have conducted a comprehensive study on the in-situ evaluation of indoor PV performance for autonomous, long-range IoT applications. The objective is to assess the feasibility and effectiveness of indoor PV systems as a reliable power source for IoT devices, considering the unique challenges posed by indoor environments.

The study employs a practical approach, involving the deployment of a prototype indoor PV system in a real-world setting. The system is designed to capture and convert ambient light into electrical energy, which is then utilized to power an array of autonomous, long-range IoT devices. By measuring and analyzing the system's performance metrics, including power generation, energy storage, and device autonomy, we gain valuable insights into the capabilities and limitations of indoor PV systems.

To ensure accurate evaluation, the study also investigates the impact of different indoor lighting conditions on the PV system's performance. This includes analyzing the effects of artificial lighting sources, variations in light spectra, and dynamic light levels encountered in typical indoor environments. The data collected is used to develop predictive models and optimization algorithms that enable efficient energy harvesting and utilization.

The findings from this research contribute to the understanding of indoor PV system performance, enabling the design and implementation of sustainable, autonomous IoT solutions. By evaluating the system's energy generation, storage, and usage in real-world conditions.

The wide variations in bandgaps offered by metal halide perovskites provide a unique opportunity to develop multijunction thin-film solar cells with an efficiency that surpass the limits of a single-absorber photovoltaic devices. Additionally, perovskite-based multijunction solar cells enable lower the levelized cost of energy. Recent results on perovskite-perovskite and perovskite-copper indium gallium selenide (CIGS) multijunction cells will be presented. Often wide-bandgap mixed-halide perovskites and narrow-bandgap mixed-metal perovskites suffer from non-radiative recombination due to bulk traps and interfacial recombination centers that limit the open-circuit voltage in wide-bandgap sub-cells and restrict photocurrent for narrow-bandgap materials. We studied the origin of these traps with photocurrent spectroscopy and absolute photoluminescence spectroscopy and employ passivation strategies to eliminate losses and increase stability. Combined with reducing parasitic absorption losses using optical simulations as guide, this allows fabricating efficient multi-junction cells in all-perovskite configurations and in combination with other thin-film semiconductors such as CIGS. Collectively, these strategies enable monolithic tandem solar cells with a power-conversion efficiency of  26%.

Several different theoretical concepts promise to allow going beyond the efficiency limitations of single junction solar cells, tandem devices being arguably the most promising approach, in many cases already exceeding the best performance of the constituting single junction devices. Among the many different possible combinations of semiconductors, halide perovskites in conjunction with crystalline silicon in a monolithic tandem configuration are currently the most advanced contender in the race for the technology that will most likely gain market maturity in the near future. In this contribution, I’ll share several of our recent key developments that led us to exceed 30% efficiency on a small area for different types of monolithically integrated two-terminal perovskite-silicon tandem solar cells. Beyond that, I will discuss which obstacles will have to be overcome to further enhance the efficiency and critically the operational stability under accelerated indoor and prolonged outdoor conditions along several of our most recent findings.

A tremendous increase in the efficiency of organic and perovskite solar cells has been observed over the past few years, where single-junction OPVs (organic photovoltaics) and wide bandgap PSCs (perovskite solar cells) demonstrate efficiencies of over 19% and 20%, respectively.^1–3 This has enabled the fabrication of perovskite-organic tandem solar cells where it is possible to overcome the fundamental efficiency limits of single-junction devices. However, several challenges related to the choices of recombination layers remain. Most of the perovskite-perovskite and perovskite-organic tandem solar cells in the literature utilize metallic recombination junctions consisting of an ultrathin layer of Ag or Au. However, despite their ultrathin nature, such metallic layers can significantly reduce the light transmission to the back subcell, minimizing the device's performance.⁴ Here, we report the fabrication of perovskite-organic tandem solar cells with a remarkable power conversion efficiency (PCE) of 23.6% in a 2T (two-terminal) configuration, utilizing different combinations of material systems for the recombination layer. We show that using a bilayer recombination layer composed of an ultrathin Ag and a subsequently deposited PEDOT:PSS layer exhibits significant parasitic absorption and reduces the device's overall performance. Replacing the hybrid Ag/PEDOT:PSS recombination layer with an inorganic bilayer consisting of a thin indium zinc oxide (IZO) layer and conformal thin molybdenum oxide (MoO_x), leads to significant cell performance improvements with the ensuing tandem solar cells exhibiting a Voc of 2.1 V, Jsc of 14.56 mA/cm², and a FF of 77% leading to an overall PCE of 23.6%. Our work highlights the critical role of recombination layers in tandem perovskite-organic solar cells while expands the range of interlayer systems that could lead to further performance improvements.

Vacuum-based technologies have proven to be a promising route for industrial scale production of perovskite photovoltaics, but performance is still lagging behind its solution-based counterparts. Currently, vapor-based methods such as co-evaporation struggle to grow complex perovskites (6 or more elements), being limited by the number of sources available and the ability to control them. This further limits the ability of many vacuum-based processes to introduce passivating or stabilizing agents. In particular, these are needed for the synthesis of high-efficiency low-bandgap materials (e.g. Sn/Pb) needed for perovskite-perovskite tandems or silicon-perovskite tandems.We will report on the progress on vapor phase deposited perovskites, including novel low vacuum based deposition methods such as close space sublimation. Using substrate configuration we optimize the incoupling of sunlight which leads to current densities very close to the detailed balance limit. We have prepared semi-transparent with varying perovskite thickness allowing for their use in building integrated PV as well as for bi-facial solar cells.

Indoor photovoltaics (IPV) offer a range of potential applications, from powering the Internet-of-Things to providing clean, renewable energy for residential and commercial settings. These devices can be realized through a variety of materials such as organics and perovskites, both of which demonstrate impressive power conversion efficiencies, due in part to their spectral tunability. The theoretical thermodynamic power conversion efficiency limit for single-junction PV devices under typical indoor spectra is over 50%, significantly higher than the detailed balance limit of 33.7% under 1 Sun, and the optimal bandgap shifts to ∼1.8 eV. However, accurate characterization of IPV devices is essential for enabling device optimization and thus unlocking their full potential. The IPV performance is measured using a bespoke spectrophotometrically calibrated test setup to simulate realistic indoor lighting conditions. In this work, we explore parameters for improving the performance of large-area IPV devices, focusing on organic and perovskite-based devices. We experimentally and theoretically probe and demonstrate the performance of organic and perovskite IPV devices at low light intensities, which are verified by their implementation in IPV-powered wireless sensor nodes (WSNs).

The strive for more efficient buildings is pushing digitalization. With that the need for connected sensors is growing rapidly. These sensors are commonly powered by batteries and require costly maintainace to change batteries with a few years intervall and generate a large amount of waste batteries.

By harvesting ambient energy the battery life time can be extended and in many cases the battery become redundant. In buildings the main source of ambient energy is light that can be harvested by solar cells. Other examples of applications for connected devices suitable for light energy harvesting include electronic shelf labels and asset tracking labels.

Epishine is a Swedish innovative company that has developed a scalable manufacturing process to produce organic solar cells by roll to roll printing and lamination technologies, to meet an increasing demand for high performance indoor solar cells. 

In this talk I will give an overview of Epishine´s technology and manufacturing process, together with potential and requirements for indoor solar cells in selected market segments and applications. I will also discuss our view on the future of scaling indoor solar powered products and manufacturing.

Our in-house developed ITO-free organic photovoltaics (OPV) device stack proved to be well suited both for upscaling to larger area modules as well as for low-light applications as power source for the internet of things. It is based on a reflective (electron) metal back contact, followed by the absorber layer and a transparent (hole) front contact based on PEDOT:PSS with or without an Ag grid, depending on the requirements for the conductivity. With such devices, we were able to achieve efficiencies >15% under '1 sun' and up to more than 20% under cold white LED illumination with 500 lux. The solar cells designed for indoor usage were found to be extremely stable, with lifetimes deduced from accelerated ageing experiments of more than 100 years. Also under full sunlight the stability is very promising if a UV-filter is employed.
In order to achieve high visual transmission with our OPV devices we only had to replace the back electrode by thin Ag layers sandwiched between metal oxides. This type of electrode offers both high visual transparency and strong near infrared reflectivity and thus we were able to achieve promising values of 8.65% power conversion efficiency (PCE) at an average visual transmission (AVT) of 46.3%, giving rise to a light utilization efficiency (i.e., the product of PCE and AVT) of 4.0. This is amongst the highest values for semitransparent devices processed from non-halogenated solvents.
The optimization of these devices is based on thorough characterization complemented by optical and electrical simulations which shall be discussed in the talk.

Due to a number of desirable attributes, including tailorable optical properties and scalable, low-embodied energy fabrication techniques, next-generation photovoltaics based on organic semiconductors and perovskites show great potential for a variety of niche applications. Indoor photovoltaics (IPVs) are one such application; in the coming decades, IPVs promise to reduce the carbon footprint associated with networked Internet-of-Things devices by recycling low-intensity, artificial light (generated by, e.g., LEDs and fluorescent lamps) to power them.^[1] Due to the relative infancy of the field, however, the performance limits of organic semiconductors and perovskites in indoor applications are poorly understood, and the most suitable photo-active materials are yet to be identified. In the work presented here, we therefore step beyond the conventional Shockley-Queisser model to present a thermodynamic limit of IPV performance that accounts for the effects of intrinsic material characteristics, including sub-gap absorption, energetic disorder, and non-radiative open-circuit voltage loss.^[2-4] Following this, we present a methodology that utilizes a device’s photovoltaic external quantum efficiency spectrum and its open-circuit voltage under one-Sun conditions to predict its performance under illumination by any spectrum, at any intensity.^[5] We apply this methodology to current state-of-the-art photovoltaics (including inorganics, organics, and perovskites) to predict which could perform best under typical indoor light sources.

Despite recent developments in organic photovoltaic (OPV) devices at lab scales (< 1 cm²), commercially viable OPV devices suffer performance losses at scale (> 10 cm²).^[1,2] This is largely due to the large series resistance exhibited by transparent conducting electrodes (TCEs), even when using current state-of-the-art transparent conductive oxides (TCOs) such as indium tin oxide. To reduce the series resistance of TCEs, one solution is to use the TCO in conjunction with a metallic grid to form gridded-TCEs (g-TCEs),^[3–5] which essentially divides a cell into an array of smaller sub-cells.^[6] Although TCO materials exhibit a trade-off between average visible transmittance (AVT) and conductivity, the use of TCOs with metallic grids allows the electro-optical properties of high-performance TCEs to be optimised. In this work, the scaling-related performance barrier is presented for OPVs. The performance impact of critical grid geometry is studied with varying light intensity for different OPV applications. At high irradiances, smaller grid critical dimensions (CD) result in improved OPV device performance and can be combined with TCOs with high AVT. At low irradiances, the requirement for a highly conductive TCE reduces, because of the reduced photocurrent, allowing coarser (greater CD) grid structures to be utilised. As grid structures with high topographies result in challenging surface morphologies for scalable printing techniques, a method to recess metallic grids into substrates is demonstrated through exemplary planar (± 30 nm) 5 cm² g-TCEs, demonstrated with silver nanowires and aluminium-doped zinc oxide (AZO). The AZO g-TCEs were shown to provide a sheet resistance of 0.5 Ω/□, with AVT greater than 77 %.  These results show that high-performance g-TCE structures are a highly versatile approach for utilising OPV in various applications.

Organic solar cells (OSCs) have garnered significant interest for potential commercial uses because of their light weight, mechanical flexibility, semitransparency, and large-area manufacturing properties. In the past few years, the efficiency of organic solar cells (OSCs) has greatly improved owing to the emergence of Y-series non-fullerene acceptors (Y-NFAs) and the advancements in polymer donors. Given the rapid progress in efficiency, it becomes crucial to prioritize the examination of the stability of photovoltaic materials. These materials play an important role in determining the lifetime of OSCs under real-world conditions, ensuring they meet the necessary criteria for future commercialization. However, the connection between the molecular structure and the outdoor stability of their devices remains elusive. A comprehensive operational outdoor study paired with photo- and thermo-induced degradation has yet to be documented.

In this study, we examine the stability of various Y-NFAs when paired with a common polymer donor, and vice versa. For Y-NFAs, we establish a connection between the molecular structure, specifically the end-group and side-chain, and their photostability. Through the combination of density functional theory (DFT) calculations on the energy barrier of photoisomerization and the analysis of device photostability, we have discovered that suppressing light-induced vinyl rotation can significantly improve the lifetime of the devices. Shifting our focus to polymer donors, we delve into the significance of various building blocks in determining the device lifetime. In order to evaluate the potential occurrence of side-chain breakage, we introduce a molecular descriptor for predicting the photostability of polymer donors and validate its effectiveness by testing it on 28 different polymeric materials. Under illumination, the chemical changes occurring in both Y-NFAs and polymer donors result in a noticeable increase in trap-assisted recombination, leading to a degradation in device performance when exposed to outdoor conditions. Furthermore, we conduct a systematic comparison of the photostability, thermostability, and outdoor stability of devices based on state-of-the-art materials. This comprehensive analysis enables us to gain a thorough understanding of how the photoactive layers impact the long-term performance, particularly in the hot and sunny climate of Saudi Arabia. Our findings provide valuable insights into the design and synthesis of photoactive materials, with the goal of attaining high efficiency and long-term stability in OSCs for outdoor applications.

Low power high voltage sources can be used to drive many interesting devices, for example, dielectric elastomer actuators (DEAs)[1], electroaerodynamic propulsion (EAD thruster)[2], micro-electromechanical systems (MEMS)[3], among others. These devices show great potential for smart life applications in the future. However, one common bottleneck is power supply, which usually comes from cables or converters via a battery. The cables limit working space, while batteries restrict working time. To overcome these drawbacks, some groups developed triboelectric nanogenerators to drive DEAs[4]. However, this type of energy supply is based on motion and the performance relies on vibration frequency which is confined for real applications. Developing a solid state, high efficiency and environmentally compatible high voltage source is therefore timely. Harvesting light energy from environment by photovoltaics (solar cells) is a common way to realize energy autonomy. One single junction solar cell usually can generate a voltage of 0.5-1.5 V, depending on the active material and the illumination intensity. To achieve high voltage output, interconnecting single solar cells in series to a module is an efficient way. A lot has been done to meet the need of MEMS in the range from dozens to hundreds of Volts[5,6,7]. But this is not high enough for DEAs and EAD thrusters that usually need thousands of Volts. To the best of our knowledge, no mini solar module with an open circuit voltage (V_OC) higher than 1000 V has been reported thus far. Here we report a mini organic solar module with 1640 individual solar cells interconnected in series on 3.6 × 3.7 cm² realized by laser patterning. We used two different active materials, namely PV-X plus and PM6:GS-ISO. Under 100 k lux warm white LED illumination, the PV-X plus gives a V_OC of 1362 V, with a short circuit current (I_SC) of 97.2 µA, fill factor (FF) of 0.67 and a maximum power (P_max) of 89.3 mW, while the PM6:GS-ISO gives a V_OC of 1640 V, I_SC of 25.2 µA, FF of 0.59 and P_max of 24.4 mW. Noteworthy, the PV-X plus device can still provide a V_OC of 1141 V under an illuminance as low as 1000 lux. This means that the solar modules can work excellently under normal indoor environment even without any additional illumination sources. As a power source, stability is also important for real applications. We kept the devices near the V_OC point to test the stability of the organic solar modules under 100 k lux illumination. The performance of PV-X plus shows almost no degradation at all within 30 min, while the PM6:GS-ISO degrades slightly, but still keeps 89.6% of the initial value.  To proof the usefulness of the high voltage modules, we combined several modules to get a higher voltage output and could this way successfully drive different types of DEAs. In addition, we also powered a DEA thruster. In a hanging mode, the DEA thruster moved about 20 mm in 0.5 s. The thrust force is measured as 136 µN. Future work on the solar modules will include the use of flexible substrates and enhancing V_OC values by further minimizing the individual cell size and thus the number of series interconnected cells hence providing more potential for integration with DEAs and EAD thrusters.

Solution processing from nanoparticle dispersions allows the use of eco-friendly processing agents for the deposition of organic semiconductor thin-films for photovoltaic and other optoelectronic applications. Omitting surfactants, which are commonly used to stabilize dispersions, is essential to not jeopardize the solar cell performance. So far, solar cells could only be fabricated from surfactant-free P3HT dispersions which show some intrinsic self-stabilization. In this work, the self-stabilization of P3HT nanoparticle dispersions is demystified, and electrostatic effects are identified as the origin of self-stabilization. By application of this gained knowledge, novel surfactant-free nanoparticle dispersions from other, high-performance organic semiconductors are synthesized by nanoprecipitation. Electrical doping via oxidation of the polymer by iodine promotes the electrostatic repulsion of the nanoparticles and hence the colloidal stability of the respective dispersions. For the first time, the corresponding solar cells achieved power conversion efficiencies of up to 10.6%, demonstrating the general feasibility of this alternate, all-eco-friendly processing route.

For the consolidation of organic photovoltaics (OPV), it is crucial to create market pull through the identification and target of strategic niches, where this technology can exploit its fundamental differentiators.[1] For instance, materials engineering has enabled wavelength-selective harvesting with transparent OPV for power-generating windows[2] and building-integrated photovoltaics.[3] Therein, a simultaneous high efficiency and high transparency are needed. While the community has made relevant developments to maximize the optoelectronic properties of OPV devices, little attention has been paid to their structural properties. High-volume manufacturing technologies such as plastic thermoforming and injection moulding can help expand the opportunities, the capabilities, and the seamless integration of OPV.

In this work we demonstrate, for the first time, the feasibility of fabricating OPV cells and modules embedded into structural plastic parts through injection molding. This process yields lightweight OPV devices with enhanced device robustness and durability, thanks to the hermetical and conformable encapsulation resulting from the plastic injection. We discuss the interplay between the plastic processing conditions and the OPV device performance and stability, as well as highlight relevant optomechanical and physico-chemical material properties, including recyclable thermoplastic polymeric materials that might facilitate material reuse. Finally, we also show how plastic processing can be used to fabricate low-cost, three‑level hierarchically organized micro/nanometric surface textures that provide additional functionalities, such as light management or self-cleaning. [4]

The active layer of Organic Solar Cells typically comprises a mixture of donor and acceptor molecules, forming a so-called bulk-heterojunction (BHJ), whose optimal nanostructure is highly material specific. The nanostructure of a BHJ blend comprises both mixed regions where the donor and acceptor molecules are in intimate contact as well as relatively pure domains of either blend component, which facilitate exciton dissociation and charge extraction, respectively. The poor intrinsic stability arises because the optimal nanostructure of a best performing BHJ tends to be far away from thermodynamic equilibrium¹. As a result, the initial BHJ nanostructure can evolve with time through short- and/or long-range diffusion of either of the blend components resulting in a decrease in device performance^2,3.

Here, we explore whether the use of acceptor mixtures comprising more than two components can substantially increase the active layer's thermal stability. The use of acceptor mixtures with more than two components is motivated by our recent observation that blending of up to eight perylene derivatives can lead to mixtures with an unprecedented ability to form a molecular glass, driven by the formation of a high-entropy ordered liquid composed of perylene aggregates⁴. In the current work, up to five Y-series (ITIC-derivative) acceptors are mixed, in analogy to bulk metallic glasses, which tend to comprise up to five elements^5,6,7. The combination of several acceptors has a minimal effect on their electronic disorder and blending with the widely used donor polymer PM6 results in hexanary blends with best device efficiencies of 17.6 %. The hexanary blends display a high degree of thermal stability, independent of the film thickness (up to 390 nm), resulting in an unaltered photovoltaic performance upon annealing at 130 ^oC for 23 days (552 hours) in the dark and under inert conditions.

Hybrid halide perovskite has established its credibility as high performance thin film photovoltaic technology. In only one-decade, the hybrid organic-inorganic halide perovskite solar cell achieved to compete with all mature crystalline technologies, by reaching a certified 25.7 % power conversion efficiency (PCE) on cells and 17.9 % PCE on small modules.[1] Perovskite’s strength stem from their remarkable opto-electronic properties. However, the technology still requires significant considerations regarding stability. Rapid structural and electronic degradation can be engendered when exposed under various external stressors (temperature [2-3], humidity [4-6], light [7-8], electrical bias [9]). 

To cope with the stability issue, it is mandatory to precisely understand the multiple degradation pathways of the perovskite. In situ or operando characterization techniques are central characterization tools in order to clarify the different degradation pathways. In this communication, we will be discussing the degradation of different perovskite composition on the basis of humidity or temperature-controlled in situ x-ray diffraction and corroborated with in situ electron spin resonance spectroscopy and in situ transmission electron microscopy. For example, one key finding which we will discuss is that α-FAPbI3 degradation is substantially accelerated when temperature is combined to illumination and when it is interfaced with the extraction layers, and, second the existence of a temperature gap region which takes place only under illumination involving an intermediate stage between the thermal-induced perovskite degradation and the formation of PbI2 by-product. [10]

The staggering rise in perovskite solar cells (PSCs) efficiency in past years has led to multi-fold advancements. They are promising candidate for aesthetically appealing, electrically and optically efficient building integrated photovoltaics, BIPVs. However, the losses in power conversion efficiency are limited due to the spectral mismatch between the incident solar spectra and absorption range of the active perovskite layer. Besides PSCs’ performance, the photoinduced degradation/instability is also a major concern. This talk is about three cross-dependent aspects: band-gap engineering vs “thinning” for semi-transparent perovskite solar cells, transparent front contact and its effect on PCE and, photon management strategy by down-converting (DC) fluorophores to enhance carrier generation.

The discussion further points out the advantages of perovskites for diffuse light irradiance. It is important for indoor light applications where the incident power is already too low for power generation. The talk will also emphasize on the true challenge in the realization of DC effect. The flux and wavelength dependent PCE study shows the real promise of DC effect in PSCs. Further research is imperative for in-depth mechanistic understanding of the semi-transparent PSCs operations.

Fully decarbonising our energy system will require increased implementation of renewable energy technologies such as photovoltaics, but also more efficient utilisation of the energy that is generated. This can be achieved through design of systems where the photovoltaic generation is integrated into the overall system to maximise efficiency of energy use, or by combining photovoltaic generation with other functions such as production of fuels, energy storage, heat management, clean water, or other uses of space. Optimising the integrated system requires knowledge of the optical and electrical response of the solar module in different conditions and control of those properties as well models of the whole system performance, resource availability and demand. The choice of PV technology may not be the most important factor. We have developed a flexible energy system modelling tool [1], that can be applied to such optimisations. We  will present examples of the integration of PV in different systems, and consider how system design and operation influence overall performance.

Impedance spectroscopy (IS) and current-voltage measurement (jV) form the most useful tandem-techniques to characterise in depth interfacial processes taking place in perovskite solar cells (PSCs). Among these processes, hysteresis is one of the most remarkable, due to its relationship with the instability of these devices. This phenomenon refers to a difference between forward scan (from short circuit to open circuit) and reverse scan (open circuit to short circuit) during jV measures. Normal hysteresis (NH) is considered when the reverse scan (RS) performs with better photovoltaic response. This type of behaviour has been associated with the lower-frequency capacitance in the IS spectra. However, the opposite effect is also usually found: forward scan (FS) with improved fill factor (FF) and photovoltage (V_oc), known as inverted hysteresis (IH). In addition, a funny feature known as loop or inductance in some impedance spectra, is also frequently reported for perovskite devices. The capacitance versus frequency of this feature, leads to a negative capacitance (NC). We have demonstrated experimentally how both phenomena, IH and NC, are governed by the same process. Using the surface polarization model (SPM), obtaining the time constant, τ_kin, associated. The kinetics obtained have been associated with surface interactions involving ions/vacancies at the ESL/perovskite interface. These interactions lead to a decrease in the overall recombination resistance, modifying the low frequency perovskite response and yielding a flattening of the cyclic voltammetry. We have proved these findings in different types of perovskites (bromide and iodide) and under different treatments such as cation addition, which limits NC and thus IH. The shared origin between inverted hysteresis (IH) and negative capacitance (NC) is crucial for understanding perovskite devices. In the context of perovskite solar cells (PSCs), detecting IH and NC helps to study recombination pathways that impact photovoltage losses. In the case of emerging devices like perovskite-based memristors, the presence of both IH and NC indicates the occurrence of ionic motion required for encoding information. This presentation will show how ionic motion can be reduced to minimize photovoltage losses in PSCs, and how, instead, can be favoured to obtain high-performance memristors with outstanding endurance, allowing to uncover the underlying mechanisms through the negative capacitance analysis.