Women in Renewable Energy

The demand for sustainable and renewable energy is increasing due to the depletion of fossil fuels and the global climate crisis. Thus, the need for affordable, clean, safe energy conversion and storage systems that are sustainable is vital and requires new approaches and disruptive technologies to contribute to global energy production. This inspires researchers and engineers from multidisciplinary research areas to discover novel materials for energy conversion and storage. Moreover, it is paramount to unveil their structure–property–performance relationship and fundamental processes of energy materials and devices.

This special issue on Women in Renewable Energy encompasses 3 reviews and 15 research articles on the most updated research progress in renewable energy conversion and storage areas conducted by leading female researchers globally. It covers a broad range of innovative functional materials discovery, synthetic methodologies, structure–property relationships, and their applications in organic solar cells, perovskite solar cells, rechargeable batteries, and capacitors.

For energy conversion, organic and perovskite solar cells (PSCs) are emerging photovoltaic (PV) technology that has advanced due to their tunable optoelectronic properties. These technologies are solution-processable and can be fabricated using cost-effective commercially available coating techniques representing a substantial benefit for lowering manufacturing costs. This emerging PV technology minimizes the levelized cost of electricity and energy payback time as compared to the silicon PV technology.^[2] So far, halide PSCs have achieved a certified record power conversion efficiency^[1] (PCE) of 26.7% for single-junction solar cells, while for single-junction organic solar cells (OSCs) reached 19.2%. In both types of solar cells, the exploration of new absorber and charge transport materials, new device designs, fabrication techniques is particularly relevant to improving the efficiency and stability of not only single-junction but also multi-junction tandem solar cells.

In high-efficiency tandem solar cells, wide-bandgap perovskites as the top cell are desirable, which requires a large amount of Br concentration in the composition of the perovskite to expand the bandgap; however, the photo instability and heterogeneous halide distribution are brought on by a rise in Br concentration. Nogueira et al. report that the addition of MACl destabilizes the intermediate phases and favors the formation of wide-bandgap perovskites with high crystallinity and homogenizes the distribution of halides, thus improving the performance of PSCs (2307104). Similarly, for low-bandgap perovskite, Petrozza et al. explore that alloying formamidinium lead iodide (FAPbI₃) perovskite absorber with small amount of Br⁻ anion increases the radiative recombination yield and semiconductor photostability, which is beneficial for photonics applications (2308545).

Further to substitute the toxic lead (Pb), low-dimensional and wide-bandgap perovskite-inspired materials such as halide elpasolite, vacancy-ordered double perovskite, pnictogen-based metal halide, Ag-Bi-I families of materials, their device performance within and outside of photovoltaics are reviewed by Paola et al., along with their defect-driven optical and charge-carrier transport features (2307441). In the same essence, Leppert, Hutter et al. report that large and indirect bandgap of Cs₂AgBiBr₆ perovskite limits efficient light harvesting, which can be manipulated from indirect to direct transition by replacing >50% of Bi³⁺ with Fe³⁺ via mechanochemical synthesis, and a remarkable tunable absorption onset between 2.1 and ≈1 eV can be observed due to lowering of the conduction band upon the introduction of Fe³⁺, by density functional theory calculations (2306106).

Optimizing the interface between active materials and electron transport (ETL) or hole transport layers (HTL) is well recognized as being important in the field of thin-film photovoltaics. Recombination losses need to be minimized, and overall performance is to be improved. Reduced Shockley–Read–Hall and interface recombination can greatly increase the open-circuit voltage (V_OC) and fill factor (FF), bringing solar cells closer to their theoretical Shockley–Queisser limit. In a review, Kazim et al. showed how the introduction of MXenes can be beneficial in both solar cells and batteries. In solar cells, it improves the FF and V_oc, as MXenes can control the charge-transporting interfaces and the surfaces of perovskite grains, while in batteries, MXenes have been used as electrode materials (2315694). Similarly, Loi et al. discover a self-assembled bilayer, consisting of a covalent monolayer (Br-2PACz) and a noncovalent wetting layer (4CzNH3I) as the HTL in Pb/Sn-based p-i-n solar cells to improve the performance and stability (2306571).

Besides, exploring new charge transporting and absorber layer, to reduce the levelized cost of electricity and energy payback time, recycling expensive TCO glasses from end-of-life perovskite modules is another important aspect for commercialization. Grancini et al. report a sustainable recycling procedure to recover electron-transporting material and recycling of substrates to fabricate solar cells and to achieve similar PCE (2306040).

In addition to the sustainable recycling of the PV materials, new fabrication techniques that are scalable for industrial processing are also crucial. Morales-Masis et al. explore the pulsed laser deposition (PLD) method using a single source of MA_1−xFA_xPbI₃ perovskite precursor materials. As a proof of concept, fabricated solar cells demonstrated a PCE improvement of up to 14% without passivation. PLD is a desirable technique due to its advantages as an all-dry process and conformal growth of thin films over textured surfaces (2300588).Likewise, thermal evaporation process was used to deposit ultra-thin (10 nm) perovskite films for the fabrication of semi-transparent perovskite cells (ST-PSCs), where homogenous, compact thin films of controlled thicknesses are desirable. Making a balance between average visible transmittance (AVT) and efficiency is a critical factor. Vaynzof et al. report the fabrication of ST-PSCs with an AVT of 54.26% and achieve a PCE of 3.6% using a thermal evaporation process (2307471).

On the contrary, the discovery of near-infrared absorbing small molecule acceptors has led to organic solar cells with 20% PCE. Nguyen et al. show the mechanistic insight and influence of the metal electrodes (Al and Ag) on the interfacial morphological and chemical stability of active layers in PM6:Y6 bulk heterojunction and their OPV performance using multiscale characterization approaches such as solid-state nuclear magnetic resonance spectroscopy (NMR), Electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (2308618). Energy-resolved impedance and intrinsic photoconduction measurements are used to explain why the dissociation of charge-transfer (CT) states in a PM6:Y6 solar cell is not a thermally activated process. Using these techniques, Köhler et al. report the density of states distributions of the relevant HOMO and LUMO states, and a Coulomb binding energy (E_b, _CT) of about 150 meV is obtained, which is 250 meV less than what would be predicted for two localized charges that are separated by 1 nm (2302520).

Improving the photo-oxidative stability of the organic PV materials is crucial for enhancing the durability and efficiency of PV devices. Turkovic et al. described the role of boron sub-phthalocyanines (SubPcs) to improve the photo-oxidative stability of organic components used in PV devices (2310222).

Semi-transparent organic solar cells (ST-OSCs) can be used as power-generating windows because they can absorb low-energy photons in the near-infrared spectrum to generate photocurrents while letting visible light pass through for human vision. Zou et al. reported the development of ST-OSCs with high light utilization efficiency of 4.02%, and color rendering index of 90.67%, via a synergistic material and device engineering strategy (2305017).

Kohler, Stingelin et al. demonstrated that the critical aggregation temperatures in solution, and during film formation, serve as valuable indicators for polymer assembly into anticipated aggregates, and the mechanistic understanding of polymer assembly for processing of the functional plastics and their device performance (2314729).

In the same way, for energy storage, in electrochemical devices (rechargeable lithium batteries and supercapacitors), the necessity to develop and design new materials & sustainable synthesis methods and characterize the structure, morphology and properties to implement in advanced prototypes is paramount. These devices are fundamental to promote the large-scale use of renewable energies and the electrification of transport, in addition to combating the climate change's crisis and reducing environmental pollution.

Huang et al. focus on developments in characterization techniques to reveal underlying mechanisms of Li-ion diffusion and degradation of the disordered rock-salt structured cathode material for Li-ion batteries and offer guidance for further research on this class of materials (2308165). Clément et al. reveal how planar defects (twin boundaries) and off-stoichiometry affected the Li intercalation characteristics of LiNiO₂ by the use of magnetometry, transmission electron microscopy, solid-state NMR, and synchrotron X-ray diffraction (2306168).

Tolbert et al. report the use of nanostructured MoS₂ materials as an additive with controlled size and disorder can cause suppression of Li-intercalation-induced phase transitions and synergistically enhance charge storage kinetic performance of pseudocapacitors (2304896).

Brunetti et al. report an innovative example of flexible, hybrid energy conversion and storage device. A flexible perovskite solar mini-module for energy conversion and ultra-flexible screen-printed interdigitated carbon supercapacitors on paper for energy storage are combined to create an all-flexible self-power hybrid photo-supercapacitor. Under 1 sun illumination, the integrated system exhibited 2.8% of overall efficiency with a wide-range bias window of 3.8 V, indicating considerable prospective for portable and versatile wearable applications (2313267).

The innovation in structural design, cost-effective synthetic procedures, and versatility in deposition and fabrication methodologies of energy materials have provided new opportunities to tailor their functionalities in energy conversion and storage systems. With this newfound knowledge and techniques presented, we should be able to rationally design and implement methodologies for fine-tuning the structural properties of materials and fabrication of energy conversion and storage devices for improving performance and stability.

The authors are delighted to serve as the guest editors of this special issue to celebrate Women in Renewable Energy and express their gratitude to all the authors for their excellent contributions and reviewers for providing timely feedback. Furthermore, the authors wish to thank the editorial team of the Advanced Functional Materials, especially Dr. Richard Murray for their help to make this special issue possible.