91̽»¨

Abstract:

In mixed-halide perovskites, halide segregation results in rapid funnelling of charge carriers to the I-rich phase, increasing radiative recombination and slightly lowering their mobility, while sustaining effective charge-carrier extraction pathways. Mixed-halide perovskites offer ideal bandgaps for tandem solar cells, but they suffer from light-induced halide segregation, which compromises their operational stability. Here, we directly probe the impact of halide segregation on charge-carrier dynamics at the interface between a mixed-halide perovskite and charge transport layers by using a free-space synchronous multimodal spectroscopy approach, combining time-resolved microwave conductivity, time-resolved photoluminescence (PL) and steady-state PL. We present a method to distinguish directly between charge-carrier dynamics dominated by either majority or minority carriers, enabling us to isolate effects arising from charge-selective extraction from the perovskite to commonly used hole- or electron transport layers, i.e. poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and SnO 2 , respectively. We show that halide segregation creates iodide-rich phases that capture charge carriers within sub-nanoseconds, which slightly reduces their mobilities at microwave frequencies. We reveal that charge extraction from such iodide-rich domains is still surprisingly feasible, but competes with enhanced radiative recombination resulting from higher charge concentrations caused by funnelling into these minority phases. We demonstrate that together such effects reduce charge diffusion lengths and can account for the widely observed reduction in open-circuit voltages and short-circuit currents in solar cells under operational conditions. Our findings unravel the causes underpinning the adverse impact of halide segregation and provide guidelines to improve device performance.

Abstract:

Perovskite solar cells (PSCs) are emerging as a particularly promising technology to enhance the world’s renewable energy generation capacity. As PSCs are transitioning from research to industrial-scale production, there is an important opportunity to establish sustainable manufacturing pathways. Here, we present a closed-loop framework for the development of environmentally sustainable PSCs and highlight strategies to achieve this vision. First, we analyse the sourcing of raw materials and compare two established PSC fabrication techniques, vapour-phase deposition and solution processing, evaluating their respective advantages and limitations in terms of economic feasibility and environmental impact. Second, we examine solution processing methods, focusing on solvent system design for the preparation of high-quality perovskite films and on the use of non-hazardous or less-hazardous solvents. Third, we examine potential lead-release concerns during PSC operation and discuss approaches to minimize associated environmental risks. Fourth, we summarize effective recycling methods for main PSC components to 91̽»¨ a circular production model. Finally, we identify key challenges and outline future research directions to achieve fully sustainable, closed-loop PSC technologies.

Abstract:

The complex and varied relationship found in intermolecular interactions within the photo-active layers plays a decisive role in determining the photovoltaic energy conversion and overall device performance of organic solar cells (OSCs). Among different approaches, the ternary blend strategy serves as an effective technique to control the morphology within the active layer in OSCs. In this work, PM6:L8-BO is used as the main host system (binary) while the fullerene molecules PC61BM and PCBC6 are introduced to form ternary OSCs. The results highlight the important role of fullerenes in enhancing the performance of binary non-fullerene acceptor-based cells by suppressing trap-assisted recombination and optimizing the active layer morphology. The improved film phase microstructure, enabled by fullerene derivatives with higher lowest unoccupied molecular orbital (LUMO) energy levels in comparison to the host acceptor (L8-BO), facilitates more efficient charge collection and reduced non-radiative recombination. This results in an increase in the fill factor (FF) and open circuit voltage (Voc) in the ternary OSCs. Consequently, power conversion efficiencies (PCEs) of binary OSCs were increased from 17.28% to 18.10% and 18.38% for the PC61BM- and PCBC6-based ternary OSCs, respectively. Furthermore, the addition of the fullerene molecules in the active layer provided the devices with enhanced long-term photo and thermal stability. The ternary OSCs demonstrated degradation pathways distinct from those of binary cells (ISOS-L1-I and ISOS-D2-I protocols), as identified through in situ ultraviolet-visible (UV-Vis) absorption and Raman spectroscopy. Molecular dynamics (MD) simulations, for the first time, reveal the significant role of fullerene molecules as morphology regulators in non-fullerene acceptor (NFA)-based systems. Their presence ensures improved dispersion of blend components and promotes more uniform and isotropic thermal and mechanical behaviour. Finally, mini-modules with active areas of 3.8 cm2 were fabricated, achieving PCEs of 12.90%, 13.32%, and 13.70% for the binary and ternary cells using PC61BM-and PCBC6-based ternary cells, respectively. Our results demonstrate that regulation of the morphology of the photo-active layer in OSCs through fullerene incorporation reduces the non-radiative energy loss pathways, enabling high-efficiency, stable and scalable OSCs.