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Perovskite solar cells (PSCs) represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of the relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path toward commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and the progress made toward addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field, which covers a broad range of topics related to PSC commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and move forward toward addressing the outstanding challenges in this fast-developing field.

Surface passivation with organic halide salts is a powerful strategy to enhance the performance of perovskite solar cells. However, the inevitable formed in-plane favored two-dimensional perovskite layers with low carrier mobility and high binding energy inhibit the interfacial charge transfer within the device. Herein, a bulky fluorinated phenmethylammonium salt is designed and synthesized to passivate the perovskite film without forming 2D perovskites. A strong interaction which is induced by an electron donation from passivation agent to perovskite not only reduces the defects at the top surface of the perovskite, but also suppresses the recombination reaction at the buried surface due to a permeation of the organic halide salt. Moreover, the results of time resolved photoluminescence and confocal microscopy images suggest that the interfacial charge transfer speed and uniformity are enhanced. As a result, the efficiency of a small-area device increases from 20.7 ± 0.9% to 22.8 ± 0.4% (aperture: 0.16 cm2). Moreover, a stabilized efficiency of 18.0% (aperture: 10.0 cm2) is achieved for larger-area modules with 6-series connected sub-cells. Equally important, the non-encapsulated modules show significantly improved stability at ambient conditions (ISOS-D-1). These significant improvements provided by a simple and reproducible procedure can be readily adopted in other types of devices.

Electron-transport-layer-free (ETL-free) perovskite solar cells (PSCs) show great promise for commercialization due to their simple design and ease of fabrication. However, the interface between the transparent conductive oxides such as indium-doped tin oxide (ITO) and the perovskite is not optimal due to differences in their work functions, surface defects, and wettability of the substrates. Surface modification of ITO through self-assembled monolayers (SAMs) to get ITO/SAM charge selective layers has shown great improvement in device performance in recent years, but little emphasis has been put on the stability of these devices. Here, we address this gap by introducing a series of newly synthesized naphthalene-imide derivatives which self-assemble at the interface between ITO and the perovskite interface and study their impact on the thermal stability of triple-cation PSCs. The chemical and thermal stabilities of the naphthalene-imide SAMs help improve the thermal stability of the devices, reaching T80 lifetimes exceeding 800 h for devices containing a pyridine-functionalized naphthalene diimide carboxylic acid at 85 °C in air. In addition, all SAMs improve the stabilized power output of the devices with respect to ITO-only reference devices. Drift-diffusion simulations reveal the strong influence of the ITO work function on the efficiency in ETL-free devices, and a work function reduction of 0.2 eV could improve efficiencies by over 30%. The functional diversity of naphthalene imides coupled with the ease of SAM deposition opens a pathway for stable, high-performing PSCs based on electron selective monolayers.

Electron-transport-layer-free (ETL-free) perovskite solar cells (PSCs) show great promise for commercialization due to their simple design and ease of fabrication. However, the interface between the transparent conductive oxides such as indium-doped tin oxide (ITO) and the perovskite is not optimal due to differences in their work functions, surface defects, and wettability of the substrates. Surface modification of ITO through self-assembled monolayers (SAMs) to get ITO/SAM charge selective layers has shown great improvement in device performance in recent years, but little emphasis has been put on the stability of these devices. Here, we address this gap by introducing a series of newly synthesized naphthalene-imide derivatives which self-assemble at the interface between ITO and the perovskite interface and study their impact on the thermal stability of triple-cation PSCs. The chemical and thermal stabilities of the naphthalene-imide SAMs help improve the thermal stability of the devices, reaching T80 lifetimes exceeding 800 h for devices containing a pyridine-functionalized naphthalene diimide carboxylic acid at 85 °C in air. In addition, all SAMs improve the stabilized power output of the devices with respect to ITO-only reference devices. Drift-diffusion simulations reveal the strong influence of the ITO work function on the efficiency in ETL-free devices, and a work function reduction of 0.2 eV could improve efficiencies by over 30%. The functional diversity of naphthalene imides coupled with the ease of SAM deposition opens a pathway for stable, high-performing PSCs based on electron selective monolayers.

Perovskite solar cells (PSCs) represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of the relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path toward commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and the progress made toward addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field, which covers a broad range of topics related to PSC commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and move forward toward addressing the outstanding challenges in this fast-developing field.

The search for lead-free alternatives to lead-halide perovskite photovoltaic materials resulted in the discovery of copper(I)-silver(I)-bismuth(III) halides exhibiting promising properties for optoelectronic applications. The present work demonstrates a solution-based synthesis of uniform CuAgBiI thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI at x = 1, 2D CuAgBiI at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, CuAgBiI has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, a valence band level at -5.25 eV, improved carrier lifetime, and higher recombination resistance as compared to CuAgBiI. These differences are mirrored in the power conversion efficiencies of the CuAgBiI and CuAgBiI solar cells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The latter value is the highest reported for this class of materials owing to the favorable film morphology provided by the hot-casting method. Future performance improvements might emerge from the optimization of the CuAgBiI layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated CuAgBiI solar cells display storage stability over 240 days.

Metal-halide perovskite solar cells (PSCs) exhibit outstanding power conversion efficiencies (PCEs) when fabricated as mm-sized devices, but creation of high-performing large-area modules that are stable on a sufficiently long timescale still presents a significant challenge. Herein, the quality of large-area perovskite film is improved by using ionic liquid additives via forming a new Pb-N bonding between the ionic liquid and Pb2+. This new bond can be modulated by a critical screening of the anion structure of the ionic liquid. The selected ionic liquid effectively reduces the defects of the perovskite films and markedly elongate their carrier lifetimes. As a result, a champion PCE of 24.4% for small-area (0.148 cm2) devices and 20.4% for larger-area (10.0 cm2) modules under AM 1.5G irradiation is achieved. More importantly, the modified devices retain 90% of their peak PCE after aging for 1900 h at 65 ± 5 °C (ISOS-T-1) and 80% after continuous light soaking for 750 h. The non-encapsulated modules maintained 80% of their peak PCE after 1100 h of aging in the air with a relative humidity of 35 ± 5% and temperature of 25 ± 5 °C under dark (ISOS-D-1), showing great potential for future commercialization.

Back-contact electrodes for hybrid organic-inorganic perovskite solar cells (PSCs) eliminate the parasitic absorption losses caused by the transparent conductive electrodes that are inherent to conventional sandwich-architecture devices. However, the fabrication methods for these unconventional architectures rely heavily on expensive photolithography, which limits scalability. Herein, we present an alternative cost-effective microfabrication technique in which the conventional photolithography process is replaced by microsphere lithography in which a close-packed polystyrene microsphere monolayer acts as the patterning mask for the honeycomb-shaped electrodes. A comprehensive comparison between photolithography and microsphere lithography fabrication techniques was conducted. Using microsphere lithography, we achieve highly efficient devices having a stabilized power conversion efficiency (PCE) of 8.6%, twice the reported value using photolithography. Microsphere lithography also enabled the fabrication of the largest back-contact PSC to date, having an active area of 0.75 cm2 and a stabilized PCE of 2.44%.

While perovskite solar cell (PSC) efficiencies are soaring at a laboratory scale, these are most commonly achieved with evaporated gold electrodes, which would present a significant expense in large-scale production. This can be remedied through the use of significantly cheaper carbon electrodes that, in contrast to metals, also do not migrate through the device. To this end, the present work investigates simple-to-prepare aluminum-supported carbon electrodes derived from commercially available, inexpensive materials that can be applied onto various hole-transporting materials and enable photovoltaic performances on par with those provided by gold electrodes. Successful integration of the new carbon-based electrode into flexible devices produced by a roll-to-roll printing technology by both pressing and lamination is demonstrated. However, temperature cycling durability tests reveal that the use of carbon electrodes based on commercial pastes is hindered by incompatibility of adhesive additives with the key components of the PSCs under heating. Resolving this issue, tailor-made graphite electrodes devoid of damaging additives are introduced, which improve the PSC stability under temperature cycling test protocol to the level provided by benchmark gold electrodes. The study highlights current challenges in developing laminated carbon electrodes in PSCs and proposes strategies toward the resolution thereof.

Hole transporting materials (HTMs) play essential roles in facilitating hole extraction and suppressing recombination in lead halide perovskite solar cells (PSCs). High levels of p-doping in HTMs is necessary for achieving high device performance, attributed to an increased electrical conductivity. In this work, we provide evidences that the poor performance of PSCs with low levels of doping (i.e., 4 mol% spiro-OMeTAD+) in spiro-OMeTAD is mainly caused by the presence of a Schottky barrier at the perovskite/spiro-OMeTAD interface, hampering hole injection. Under continuous illumination at open-circuit condition, the barrier gradually diminishes, increasing the PSC power conversion efficiency by 70-fold after 7 h. This process is completely reversible, returning to the initial poor performance after dark storage. We attribute this improvement in performance to a gradual photodoping of spiro-OMeTAD, triggered by the transfer of photogenerated holes and mediated by the slow migration of halide anions from perovskite to compensate the newly formed spiro-OMeTAD+. In-situ parallel analyses with impedance spectroscopy (IS) and photoluminescence are employed to gain insights into the charge dynamics along with light soaking. We find that the Schottky barrier resistance overlays with the recombination signal at the high frequency arc of IS, having important implications for the IS data analysis for PSCs. The work elucidates a major mechanism causing the slow efficiency variations during light/dark cycling, commonly observed in PSCs, which complicates the determination of long-term stability.

As the performance of organic–inorganic halide perovskite solar cells approaches their practical limits, the use of back-contact architectures, which eliminate parasitic light absorption, provides an effective route toward higher device efficiencies. However, a poor understanding of the underlying device physics has limited further performance improvements. Here a mesoporous charge-transporting layer is introduced into quasi-interdigitated back-contact perovskite devices and the charge extraction behavior with an increased interfacial contact area is studied. The results show that the incorporation of a thin mesoporous titanium dioxide layer significantly shortens the charge-transfer lifetime and results in more efficient and balanced charge extraction dynamics. A high short-circuit current density of 21.3 mA cm–2 is achieved using a polycrystalline perovskite layer on a mesoscopic quasi-interdigitated back-contact electrode, a record for this type of device architecture.

Copper redox mediators have enabled open-circuit voltages (V-OC) of over 1.0 V in dye-sensitized solar cells (DSCs) and have helped to establish DSCs as the most promising solar cell technology in low-light conditions. The addition of additives such as 4-tert-butylpyridine (tBP) to these electrolytes has helped in achieving high solar cell performances. However, emerging evidence suggests that tBP coordinates to the Cu(II) species and limits the performance of these electrolytes. To date, the implications of this coordination are poorly understood. Here, the importance of Lewis base additives for the successful implementation of copper complexes as redox mediators in DSCs is demonstrated. Two redox couples, [Cu(dmp)(2)](+/2+)and [Cu(dpp)(2)](+/2+)(with dmp = 2,9-dimethyl-1,10-phenanthroline and dpp = 2,9-diphenyl-1,10-phenanthroline) in combination with three different Lewis bases, TFMP (4-(trifluoromethyl)pyridine), tBP, and NMBI (1-methyl-benzimidazole), are considered. Through single-crystal X-ray diffraction analysis, absorption, and(1)H-NMR spectroscopies, the coordination of Lewis bases to the Cu(II) centers are studied. This coordination efficiently suppresses recombination losses and is crucial for high performing solar cells. If, however, the coordination involves a ligand exchange, as is the case for [Cu(dpp)(2)](+/2+), the redox mediator regeneration at the counter electrode is significantly retarded and the solar cells show current limitations.

For the first time, we intentionally deposit an ultrathin layer of excess methylammonium iodide (MAI) on top of a methylammonium lead iodide (MAPI) perovskite film. Using photoelectron spectroscopy, we investigate the role of excess MAI at the interface between perovskite and spiro-MeOTAD hole-transport layer in standard structure perovskite solar cells (PSCs). We found that interfacial, favorable, energy-level tuning of the MAPI film can be achieved by controlling the amount of excess MAI on top of the MAPI film. Our XPS results reveal that MAI dissociates at low thicknesses (<16 nm) when deposited on MAPbI(3). It is not the MAI layer but the dissociated species that leads to the interfacial energy-level tuning. Optimized interface energetics were verified by solar cell device testing, leading to both an increase of 19% in average steady-state power conversion efficiency (PCE) and significantly improved reproducibility, which is represented by a much lower PCE standard deviation (from 15 +/- 2% to 17.2 +/- 0.4%).