All-perovskite tandem solar cells represent a significant advancement in next-generation photovoltaics toward higher power conversion efficiencies than single junction cells. A critical component of a monolithic tandem solar cell is the interconnecting layer, which facilitates the integration of the wide bandgap and low bandgap subcells. Conventional designs in all-perovskite tandem cells are based on an ultrathin metal recombination layer, typically Au, alongside a poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) hole transporting layer, which introduce optical and recombination losses, and instabilities. Here, we present a new interconnecting layer based on a graphene-oxide recombination layer, which facilitates the replacement of PEDOT:PSS with the preferred self-assembled monolayer [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz). This device architecture results in significantly reduced optical and nonradiative losses, leading to champion device efficiency of 23.4% compared to 19.7% with the conventional layers, along with improvements in stability. This work solves a critical challenge in all-perovskite tandem cell device design.

Thermal co-evaporation of halide perovskites is a solution-free, conformal, scalable, and controllable deposition technique with great potential for commercial applications, particularly in multi-junction solar cells.

Abstract As the demand for photovoltaic technologies continues to grow, the quest for efficient and sustainable transparent conducting electrodes (TCEs) rapidly rises. Traditional solutions, such as indium tin oxide (ITO), face challenges related to indium scarcity and environmental impact. To tackle these issues, a novel metal mesh rear TCE consisting of gold micro‐meshes is developed as ITO replacement in perovskite solar cells (PSCs). This study reveals that optimized Au meshes can guarantee 75% of the extracted photocurrent compared to reference devices with ITO and a promising power conversion efficiency (PCE) of 8.65%. By utilizing hybrid mesh structures with a 10‐nm ITO layer, the PCE further improves to 12.1%, with the extracted current exceeding 80% of the reference. Metal meshes can even serve to replace the opaque metal contact of PSCs, amplifying their functionality and efficiency through bifacial and multi‐junction applications. Here, aerosol jet‐printed silver meshes serve as front electrodes, combined with either 5–10 nm of Au, achieving efficient semi‐transparent devices (PCE 16.8%), or with 5–10 nm of ITO, providing enhanced bifacial properties while maintaining competitive efficiency. Overall, this work highlights remarkable features of metal meshes, making them promising alternatives to commonly used TCEs in optoelectronic applications.

Microscopy provides a proxy for assessing the operation of perovskite solar cells, yet most works in the literature have focused on bare perovskite thin films, missing charge transport and recombination losses present in full devices. Here we demonstrate a multimodal operando microscopy toolkit to measure and spatially correlate nanoscale charge transport losses, recombination losses and chemical composition. By applying this toolkit to the same scan areas of state-of-the-art, alloyed perovskite cells before and after extended operation, we show that devices with the highest macroscopic performance have the lowest initial performance spatial heterogeneity-a crucial link that is missed in conventional microscopy. We show that engineering stable interfaces is critical to achieving robust devices. Once the interfaces are stabilized, we show that compositional engineering to homogenize charge extraction and to minimize variations in local power conversion efficiency is critical to improve performance and stability. We find that in our device space, perovskites can tolerate spatial disorder in chemistry, but not charge extraction. Conventional microscopy is unable to correlate nanoscale properties of metal halide perovskites with the performance of solar cells. Frohna et al. present a multimodal operando microscopy toolkit as a diagnostic tool to address the issue.

This work explores electrochemical impedance spectroscopy to study recombination and ionic processes in all-perovskite tandem solar cells. We exploit selective excitation of each subcell to enhance or suppress the impedance signal from each subcell, allowing study of individual tandem subcells. We use this selective excitation methodology to show that the recombination resistance and ionic time constants of the wide gap subcell are increased with passivation. Furthermore, we investigate subcell-dependent degradation during maximum power point tracking and find an increase in recombination resistance and a decrease in capacitance for both subcells. Complementary optical and external quantum efficiency measurements indicate that the main driver for performance loss is the reduced capacity of the recombination layer to facilitate recombination due to the formation of a charge extraction barrier. This methodology highlights electrochemical impedance spectroscopy as a powerful tool to provide critical feedback to unlock the full potential of perovskite tandem solar cells.

Next-generation low-cost semiconductors such as halide perovskites exhibit optoelectronic properties dominated by nanoscale variations in their structure, composition and photophysics. While microscopy provides a proxy for ultimate device function, past works have focused on neat thin-films on insulating substrates, missing crucial information about charge extraction losses and recombination losses introduced by transport layers. Here we use a multimodal operando microscopy toolkit to measure nanoscale current-voltage curves, recombination losses and chemical composition in an array of state-of-the-art perovskite solar cells before and after extended operational stress. We apply this toolkit to the same scan areas before and after extended operation to reveal that devices with the highest performance have the lowest initial performance spatial heterogeneity - a crucial link that is missed in conventional microscopy. We find that subtle compositional engineering of the perovskite has surprising effects on local disorder and resilience to operational stress. Minimising variations in local efficiency, rather than compositional disorder, is predictive of improved performance and stability. Modulating the interfaces with different contact layers or passivation treatments can increase initial performance but can also lead to dramatic nanoscale, interface-dominated degradation even in the presence of local performance homogeneity, inducing spatially varying transport, recombination, and electrical losses. These operando measurements of full devices act as screenable diagnostic tools, uniquely unveiling the microscopic mechanistic origins of device performance losses and degradation in an array of halide perovskite devices and treatments. This information in turn reveals guidelines for future improvements to both performance and stability.

Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfill ambitions for net-zero carbon dioxide equivalent (CO _2 eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TW _p in 2021 to 8.5 TW _p by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the PVs community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other.

Strain is an important property in halide perovskite semiconductors used for optoelectronic applications because of its ability to influence device efficiency and stability. However, descriptions of strain in these materials are generally limited to bulk averages of bare films, which miss important property-determining heterogeneities that occur on the nanoscale and at interfaces in multilayer device stacks. Here, we present three-dimensional nanoscale strain mapping using Bragg coherent diffraction imaging of individual grains in Cs(0.1)FA(0.9)Pb(I0.95Br0.05)(3) and Cs(0.15)FA(0.85)SnI(3) (FA = formamidinium) halide perovskite absorbers buried in full solar cell devices. We discover large local strains and striking intragrain and grain-to-grain strain heterogeneity, identifying distinct islands of tensile and compressive strain inside grains. Additionally, we directly image dislocations with surprising regularity in Cs(0.15)FA(0.85)SnI(3) grains and find evidence for dislocation-induced antiphase boundary formation. Our results shine a rare light on the nanoscale strains in these materials in their technologically relevant device setting.

The family of hybrid organic-inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, from light-emitting diodes to photovoltaics to X-ray detectors. Due to the inert nature of most organic molecules, the inorganic sublattice generally dominates the electronic structure and therefore the optoelectronic properties of perovskites. Here, we use optically and electronically active carbazole-based Cz-Ci molecules, where Ci indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3 to 5, as cations in the two-dimensional (2D) perovskite structure. By investigating the photophysics and charge transport characteristics of (Cz-Ci)2PbI4, we demonstrate a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, where photothermal deflection spectroscopy results remarkably reveal an organic-inorganic charge transfer state. Ultrafast transient absorption spectroscopy measurements demonstrate ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-Ci molecules, the efficiency of which increases by varying the chain length from i = 5 to i = 3. The charge transfer results in long-lived carriers (10-100 ns) and quenched emission, in stark contrast to the fast (sub-ns) and efficient radiative decay of bound excitons in the more conventional 2D perovskite (PEA)2PbI4, in which phenylethylammonium (PEA) acts as an inert spacer. Electrical charge transport measurements further support enhanced interlayer coupling, showing increased out-of-plane carrier mobility from i = 5 to i = 3. This study paves the way for the rational design of 2D perovskites with combined inorganic-organic electronic properties through the wide range of functionalities available in the world of organics.

Many loss processes in perovskite solar cells (PSCs) take place at interfaces. One effective mitigation strategy is introducing carbon derivatives at the interface of the charge transport layers with the halide perovskite absorber and/or with the contacts. In this work, single-walled carbon nanotubes are incorporated into the hole transporting material (HTM), either as composites or as a separate layer on top of the HTM. Both organic and inorganic hole transport materials are studied to validate this approach for different types of HTMs. It is shown that the CNTs facilitate charge transport from perovskite to the back contact, regardless of the HTM used. The use of CNTs results in an increase in a significant increase in short circuit current and power conversion efficiency. This study illustrates how SWCNTs can be utilised to improve the properties of a wide range of HTMs, which will pave the way for more efficient and PSCs.

This work unveils periodic module rejuvenation as a new strategy to bring online the next-generation of perovskite tandem photovoltaic technologies earlier using experimental investigation and life cycle assessment modeling as an integrated tool.

All-perovskite tandemsolar cells beckon as lower costalternativesto conventional single-junction cells. Solution processing has enabledrapid optimization of perovskite solar technologies, but new depositionroutes will enable modularity and scalability, facilitating technologyadoption. Here, we utilize 4-source vacuum deposition to deposit FA(0.7)Cs(0.3)Pb-(I x Br1-x )(3) perovskite, wherethe bandgap is changed through fine control over the halide content.We show how using MeO-2PACz as a hole-transporting material and passivatingthe perovskite with ethylenediammonium diiodide reduces nonradiativelosses, resulting in efficiencies of 17.8% in solar cells based onvacuum-deposited perovskites with a bandgap of 1.76 eV. By similarlypassivating a narrow-bandgap FA(0.75)Cs(0.25)Pb(0.5)Sn(0.5)I(3) perovskite and combining itwith a subcell of evaporated FA(0.7)Cs(0.3)Pb-(I0.64Br0.36)(3), we report a 2-terminalall-perovskite tandem solar cell with champion open circuit voltageand efficiency of 2.06 V and 24.1%, respectively. This dry depositionmethod enables high reproducibility, opening avenues for modular,scalable multijunction devices even in complex architectures.

Despite the rapid rise in the performance of a variety of perovskite optoelectronic devices with vertical charge transport, the effects of ion migration remain a common and longstanding Achilles heel limiting the long-term operational stability of lead halide perovskite devices. However, there is still limited understanding of the impact of tin (Sn) substitution on the ion dynamics of lead (Pb) halide perovskites. Here, we employ scan-rate-dependent current-voltage measurements on Pb and mixed Pb-Sn perovskite solar cells to show that short circuit current losses at lower scan rates, which can be traced to the presence of mobile ions, are present in both kinds of perovskites. To understand the kinetics of ion migration, we carry out scan-rate-dependent hysteresis analyses and temperature-dependent impedance spectroscopy measurements, which demonstrate suppressed ion migration in Pb-Sn devices compared to their Pb-only analogues. By linking these experimental observations to first-principles calculations on mixed Pb-Sn perovskites, we reveal the key role played by Sn vacancies in increasing the iodide ion migration barrier due to local structural distortions. These results highlight the beneficial effect of Sn substitution in mitigating undesirable ion migration in halide perovskites, with potential implications for future device development.

Investigation of the inherent field-driven charge transport behaviour of 3D lead halide perovskites has largely remained a challenging task, owing primarily to undesirable ionic migration effects near room temperature. In addition, the presence of methylammonium in many high performing 3D perovskite compositions introduces additional instabilities, which limit reliable room temperature optoelectronic device operation. Here, we address both these challenges and demonstrate that field-effect transistors (FETs) based on methylammonium-free, mixed-metal (Pb/Sn) perovskite compositions, that are widely studied for solar cell and light-emitting diode applications, do not suffer from ion migration effects as their pure Pb counterparts and reliably exhibit hysteresis free p-type transport with high mobility reaching 5.4 cm^2/Vs, ON/OFF ratio approaching 10^6, and normalized channel conductance of 3 S/m. The reduced ion migration is also manifested in an activated temperature dependence of the field-effect mobility with low activation energy, which reflects a significant density of shallow electronic defects. We visualize the suppressed in-plane ionic migration in Sn-containing perovskites compared to their pure-Pb counterparts using photoluminescence microscopy under bias and demonstrate promising voltage and current-stress device operational stabilities. Our work establishes FETs as an excellent platform for providing fundamental insights into the doping, defect and charge transport physics of mixed-metal halide perovskite semiconductors to advance their applications in optoelectronic devices.

Recently, inorganic nanoparticles have been studied as hole transporting materials (HTM) in perovskite solar cells (PSCs). Inorganic HTMs are believed to provide better stability compared to conventional organic HTMs, such as spiro-OMeTAD. One of the challenges of nanoparticle HTMs is the high density of defects at the interface with the perovskite film, which reduces the device's fill factor. In this study, we use N, N '-di (naphthalene-1-yl)-N, N ' diphenyl benzidine (NPB) small molecules to passivate the surface of perovskite films in carbon-based PSCs with CuInS2 nanoparticles as the HTM. By applying NPB, trap density decreases from 1.35 x 10(16) cm(-3) to 5.09 x 10(15) cm(-3). NPB interface passivation results in a power conversion efficiency (PCE) of 16.11% compared to the control device with PCE of 14.92% in carbon-based perovskite solar cells. Moreover, improved contact angle was obtained for NPB surface-treated film (61.9 degrees) compared to the reference film (40.6 degrees), providing better protection against moisture induced degradation. The surface-treated devices maintained 92% of their efficiency after 4000 h of storage at ambient condition, while the control device showed a rapid and notable degradation, losing >58% of the initial efficiency under the same condition.

Photoinduced spin-charge interconversion in semiconductors with spin-orbit coupling could provide a route to optically addressable spintronics without the use of external magnetic fields. However, in structurally disordered polycrystalline semiconductors, which are being widely explored for device applications, the presence and role of spin-associated charge currents remains unclear. Here, using femtosecond circular-polarization-resolved pump-probe microscopy on polycrystalline halide perovskite thin films, we observe the photoinduced ultrafast formation of spin domains on the micrometre scale formed through lateral spin currents. Micrometre-scale variations in the intensity of optical second-harmonic generation and vertical piezoresponse suggest that the spin-domain formation is driven by the presence of strong local inversion symmetry breaking via structural disorder. We propose that this leads to spatially varying Rashba-like spin textures that drive spin-momentum-locked currents, leading to local spin accumulation. Ultrafast spin-domain formation in polycrystalline halide perovskite films provides an optically addressable platform for nanoscale spin-device physics.

Lead halide perovskite solar cells have made enormous progress over the past decade, and record power conversion efficiencies are now exceeding those of established silicon technologies. In addition, lead halide perovskites are finding applications in other optoelectronic applications, such as LEDs and sensors. However, the toxicity of lead (Pb) is a formidable hurdle towards commercializing lead halide perovskites. A host of Pb-free perovskite materials are being developed as less toxic alternatives. These materials replace Pb with other metals such as tin, bismuth, and germanium. In this chapter we discuss the material and optoelectronic properties of Pb-free perovskite materials and their performance in solar cells, LEDs, memristors, and various detectors. Despite promising properties, many of these materials still need big advances in terms of stability and performance. Key to achieving properties on par with Pb-based perovskites will be a continued research effort into controlling the deposition of these materials and an intricate understanding of material properties, which will allow targeted improvement strategies.

Halide perovskites have shown promise to advance the field of light detection in next-generation photodetectors, offering performance and functionality beyond what is currently possible with traditional inorganic semiconductors. Despite a relatively high density of defects in perovskite thin films, long carrier diffusion lengths and lifetimes suggest that many defects are benign. However, perovskite photodetectors show detection behavior that varies with time, creating inconsistent device performance and difficulties in accurate characterization. Here, we link the changing behavior to mobile defects that migrate through perovskites, leading to detector currents that drift on the time scale of seconds. These effects not only complicate reproducible device performance but also introduce characterization challenges. We demonstrate that such transient phenomena generate measurement artifacts that mean the value of specific detectivity measured can vary by up to 2 orders of magnitude even in the same device. The presence of defects can lead to photoconductive gain in photodetectors, and we show batch-to-batch processing variations in perovskite devices gives varying degrees of charge carrier injection and photocurrent amplification under low light intensities. We utilize the passivating effect of aging to reduce the impact of defects, minimizing current drifts and eliminating the gain. This work highlights the potential issues arising from mobile defects, which lead to inconsistent photodetector operation, and identifies the potential for defects to tune photodetection behavior in perovskite photodetectors.

Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfil ambitions for net-zero carbon dioxide equivalent (CO2eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TWp in 2021 to 8.5 TWp by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the photovoltaics community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other.