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Nanostructured tin oxide (SnO2) is a very promising electron transport layer (ETL) for perovskite solar cells (PSCs) that allows low-temperature processing in the planar n–i–p architecture. However, minimizing current–voltage (J–V) hysteresis and optimizing charge extraction for PSCs in this architecture remains a challenge. In response to this, we study and optimize different types of single- and bilayer SnO2 ETLs. Detailed characterization of the optoelectronic properties reveals that a bilayer ETL composed of lithium (Li)-doped compact SnO2 (c(Li)-SnO2) at the bottom and potassium-capped SnO2 nanoparticle layers (NP-SnO2) at the top enhances the electron extraction and charge transport properties of PSCs and reduces the degree of ion migration. This results in an improved PCE and a strongly reduced J–V hysteresis for PSCs with a bilayer c(Li)-NP-SnO2 ETL as compared to reference PSCs with a single-layer or undoped bilayer ETL. The champion PSC with c(Li)-NP-SnO2 ETL shows a high stabilized PCE of up to 18.5% compared to 15.7%, 12.5% and 16.3% for PSCs with c-SnO2, c(Li)-SnO2 and c-NP-SnO2 as ETL, respectively.

The library of two-dimensional (2D) materials has been enriched over recent years with novel crystal architectures endowed with diverse exciting functionalities. Bulk perovskites, including metal-halide and oxide systems, provide access to a myriad of properties through molecular engineering. Their tunable electronic structure offers remarkable features from long carrier-diffusion lengths and high absorption coefficients in metal-halide perovskites to high-temperature superconductivity, magnetoresistance and ferroelectricity in oxide perovskites. Emboldened by the 2D materials research, perovskites down to the monolayer limit have recently emerged. Like other 2D species, perovskites with reduced dimensionality are expected to exhibit new physics and to herald next-generation multifunctional devices. In this Review, we critically assess the preliminary studies on the synthetic routes and inherent properties of monolayer perovskite materials. We also discuss how to exploit them for widespread applications and provide an outlook on the challenges and opportunities that lie ahead for this enticing class of 2D materials. Metal-halide and oxide perovskites are a rich playground for fundamental studies and applications. This Review focuses on the opportunities opened by reducing the dimensionality of these materials to two-dimensional monolayers.

Long-term operational stability remains a major challenge in the commercialization of perovskite solar cells (PSCs). While the major thrust of recent research effort has been on understanding the degradation mechanism, many new approaches are being explored to improve the performance stability of PSCs. In this issue are four Letters that discuss new strategies to achieve stabilization of PSCs. Editorial Advisory Board members highlight these new advances herein.

Lead halide based perovskite semiconductors self-assemble with distinct organic cations in natural multi-quantum-well structures. The emerging electronic properties of these two-dimensional (2D) materials can be controlled by the combination of the halide content and choice of chromophore in the organic layer. Understanding the photophysics of the perovskite semiconductor materials is critical for the optimization of stable and efficient optoelectronic devices. We use femtosecond transient absorption spectroscopy (fs-TAS) to study the mechanism of energy transfer between the organic and inorganic layers in a series of three lead-based mixed-halide perovskites such as benzylammonium (BA), 1-naphthylmethylammonium (NMA), and 1-pyrenemethylammonium (PMA) cations in 2D-lead-based perovskite thin films under similar experimental conditions. After optical excitation of the 2D-confined exciton in the lead halide layer, ultrafast energy transfer is observed to organic singlet and triplet states of the incorporated chromophores. This is explained by an effective Dexter energy transfer, which operates via a correlated electron exchange between the donating 2D-confined exciton and the accepting chromophore under spin conservation.

In the last decade, the power conversion efficiency (PCE) of solution-processed perovskite solar cells (PSCs) in the lab-scale has reached an incredible level of 25.5%. Generally, PSCs are composed of a stack consisting of a perovskite thin-film sandwiched between an electron transporting layer (ETL) and a hole transporting layer (HTL). Although the quality of the ETL and HTL interfaces with the perovskite thin-film is important, the quality of the perovskite thin-film is also critical to achieving high-performance PSCs. Low-temperature deposition of organic–inorganic perovskite thin-films by simple solution processes is one of the significant advantages of PSCs compared to other well-developed semiconductors for manufacturing solar cells. However, growing highly uniform and crystalline solution-processed perovskite thin-films is very challenging due to multiple phenomena during film formation, including solvent evaporation, wetting effects, inhomogeneous film stress and uncontrolled nucleation and growth. Therefore, understanding the different stages of perovskite crystallization is critical for achieving high-quality films and realizing higher PCEs. On the other hand, switching to large-scale solar modules leads to a substantial loss in performance, decreasing the chance of commercialization of this technology. Therefore, developing large-scale deposition techniques for reliable perovskite crystallization is very vital for scaling up PSCs. So far, several solution-processed methods such as anti-solvent and two-step processes have been developed for lab-scale perovskite thin-films deposition. However, these methods are not applicable for large-scale perovskite deposition. This review explores various scalable solution-processed perovskite deposition techniques. Moreover, different solvent quenching techniques as the most critical step of large-scale perovskite crystallization are discussed to provide a comprehensive view for achieving high-quality perovskite thin-films with large areas. Finally, the existing challenges and opportunities to push forward the commercialization of PSCs are discussed.

Negative capacitance at the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, where the origin is still under discussion. Here we use Impedance Spectroscopy to analyse these responses in methylammonium lead bromide cells treated with lithium cation at the electron selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the Surface Polarization Model, we obtain a time constant associated to the kinetics of the interaction of ions/vacancies with the surface, {\tau}kin, in the range of 10^0 - 10^2 s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding to a flattening of the cyclic voltammetry. As consequence of these results we find that that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values., Comment: 17 pages, 4 figures

Passivation and interlayer engineering are important approaches to increase the efficiency and stability of perovskite solar cells. Thin insulating dielectric films at the interface between the perovskite and the charge carrier transport layers have been suggested to passivate surface defects. Here, we analyze the effect of depositing poly(methyl methacrylate) (PMMA) from a very low-concentration solution. Spatial- and time-resolved photoluminescence and atomic force microscopy analyses of samples with diverse morphologies demonstrate the preferential deposition of PMMA in topographic depressions of the perovskite layer, such as grain and domain boundaries. This treatment results in an increase in the fill factor of more than 4% and an absolute efficiency boost exceeding 1%, with a maximum efficiency of 20.4%. Based on these results, we propose a physical isolation mechanism rather than a chemical passivation of perovskite defects, which explains not only the data of this study but also most results found in earlier works. This work was partially funded by a Swiss Government Excellence Scholarship (2017.1080) and by the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie project PerSisTanCe with grant agreement No. 841005 (E.O.M.). It also received financial support from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is cofinanced by the Participating States and by the European Union’s Horizon 2020 research and innovation programme. E.O.M., R.D.O., P.F., and U.S. acknowledge financial support by the Adolphe Merkle Foundation. E.O.M.would like to thank Dr. Silver Hamill Turren-Cruz from Helmholtz-Zentrum Berlin for fruitful discussions and Dr. Jovana Milic from the University of Fribourg and Brian Carlsen from the Laboratory of Photomolecular Science (EPFL) for facilitating and carrying out stability measurements.

The stability of perovskite solar cells is a key issue for industrial development. One reason for this is the volatile organic methylammonium (MA) cation, which is prone to degas under elevated temperatures from the perovskite. At the same time, small amounts of MA are used for practically all highest performing solar cells. These compositions have also shown relatively promising stabilities. This raises the question of MA stability with respect to different, application-dependent stability requirements. Interestingly, MA stability was mainly studied on thin films that differ from full devices or with architectures which are also prone to degrade. Therefore, the degradation behavior on complete MA containing devices with a relatively stable architecture is required to quantify the long-term stability of MA. This enables to determine at which timescales MA is unstable and which role it can play in future compositions. If MA is indeed unstable at much longer timescales than previously recorded, it also indicates that more severe degradation pathways are currently underappreciated. Here, “weakest link” MAPbI3 solar cells show stability where devices retained 100% of their initial efficiency over 1000 h of aging under constant illumination and maximum power point tracking at 20 °C. At elevated temperatures of 50 and 65 °C, the devices retained 100% and 90% of their initial efficiency after 500 h of illumination, respectively. Impressively, at 95 °C the MAPbI3 device retained 85% of its initial efficiency after 500 h under constant illumination, which is some of the best stability data reported to date for MA. Thus, MA-containing devices require further studying. Nevertheless to achieve the necessary industrial lifetimes of more than 25 years, the complete removal of MA is a sensible precaution to systematically avoid any long-term risk factors.

Rubidium and Cesium cations (Rb + and Cs + ) incorporation recently emerged as a viable strategy to enhance perovskite solar cells (PSCs) efficiency. However, a clear understanding of the impact of these cations on the structure-function relationship in relation to the device performance is severely lacking. Here, we systematically investigate the influence of Rb + and Cs + on the carrier dynamics using transient optical spectroscopy and correlate with solar cell performance. Unlike Rb + , Cs + integrates well with methylammonium (MA + ) and formamidinium (FA + ) yielding increased perovskite grain size, longer charge carrier lifetimes and improved power conversion efficiency (PCE). Concomitant incorporation of Cs + /Rb + cooperatively retards radiative recombination by ~60% in the quaternary-cation based perovskite system (RbCsMAFA) compared to the dual-cation MAFA samples. By suppressing the defect density, PCEs around 20% are obtained along with more balanced charge carrier diffusion length and comparable photoluminescence quantum yield in quaternary-cation perovskites. While the synergistic addition of Rb + and Cs + is attractive for controlling defects and recombination losses in efficient solar cells development, sole incorporation of Rb + is still an engineering challenge. Importantly, our study explicates the underlying mechanisms behind the synergistic combination of cations to minimize the charge carrier losses and achieve high efficiency perovskite solar cells. Accepted version

Scintillators detect ionising radiation by converting energy deposited in them to a proportional number of photons. They are omnipresent in large-scale technical applications around us. Here, we report excellent scintillation properties of perovskites at low temperatures providing the potential for a new generation of cryogenic scintillators. One intriguing option would be replacing current medical scintillation detectors with cryogenic perovskites that could achieve higher imaging resolutions, for example for diagnosing early-stage brain cancer.

Herein, we describe the synthesis of a hole transporting material based on a partially planarised quart-p-phenylene core incorporating tetraketal and diphenylamine substituents that show optimal energy levels and solubility for perovskite solar cell applications. The triple-cation perovskite devices incorporating such quart-p-phenylene derivatives show power-conversion efficiencies, short circuit currents, open circuit voltages, and fill factors that are comparable to those of spiro-OMeTAD.

Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V-OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the V-OC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V-OC of the device. Importantly, the V-OC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the V-OC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the V-OC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.

Funder: Xunta de Galicia; Id: http://dx.doi.org/10.13039/501100010801, Lead‐halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next‐generation, efficient light‐emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge carriers into light and vice versa. However, despite the defect‐tolerance of LHPs, defects at the surface of colloidal NCs and grain boundaries in thin films play a critical role in charge‐carrier transport and nonradiative recombination, which lowers the PLQYs, device efficiency, and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes are critical for achieving advances in performance. This Review presents the current understanding of defects in halide perovskites and their influence on the optical and charge‐carrier transport properties. Passivation strategies toward improving the efficiencies of perovskite‐based LEDs and solar cells are also discussed.

Understanding and controlling the crystallization of organic-inorganic perovskite materials is important for their function in optoelectronic applications. This control is particularly delicate in scalable single-step thermal annealing methods. In this work, the crystallization mechanisms of flash infrared-annealed perovskite films, grown on substrates with lithographically patterned Au nucleation seeds, are investigated. The patterning enables the in situ observation to study the crystallization kinetics and the precise control of the perovskite nucleation and domain growth, while retaining the characteristic polycrystalline micromorphology with larger crystallites at the boundaries of the crystal domains, as shown by electron backscattering diffraction. Time-resolved photoluminescence measurements reveal longer charge carrier lifetimes in regions with large crystallites on the domain boundaries, relative to the domain interior. By increasing the nucleation site density, the proportion of larger crystallites is increased. This study shows that the combination of rapid thermal annealing with nucleation control is a promising approach to improve perovskite crystallinity and thereby ultimately the performance of optoelectronic devices. This work was partially supported by the Swiss National Science Foundation (SNSF) through grants numbers 153990 and 186453. A.G., E.B.-U., E.O.-M., and U.S. acknowledge the financial support by the Adolphe Merkle Foundation. The work of L.A.M. and B.E. is part of the Dutch Research Council (NWO) and was performed at the research institute, AMOLF. The work of L.A.M. was supported by NWO Vidi grant 016.Vidi.179.005. M.O. acknowledges financial support partially from the Swiss State Secretary for Education, Research, and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR program is co-financed by the participating states and by the European Union’s Horizon 2020 research and innovation program. E.O.-M. acknowledges support from the Marie Skłodowska Curie fellowship, H2020 grant agreement no. 841005.

Tin halide perovskites attract incremental attention to deliver lead free perovskite solar cells. Nevertheless, disordered crystal growth and low defect formation energy, related to Sn II oxidation to Sn IV , limit the efficiency and stability of solar cells. Engineering the processing from perovskite precursor solution preparation to film crystallization is crucial to tackle these issues and enable the full photovoltaic potential of tin halide perovskites. Herein, the ionic liquid n butylammonium acetate BAAc is used to tune the tin coordination with specific O Sn chelating bonds and N amp; 63743;H X hydrogen bonds. The coordination between BAAc and tin enables modulation of the crystallization of the perovskite in a thin film. The resulting BAAc containing perovskite films are more compact and have a preferential crystal orientation. Moreover, a lower amount of Sn IV and related chemical defects are found for the BAAc containing perovskites. Tin halide perovskite solar cells processed with BAAc show a power conversion efficiency of over 10 . This value is retained after storing the devices for over 1000 h in nitrogen. This work paves the way toward a more controlled tin based perovskite crystallization for stable and efficient lead free perovskite photovoltaics

A study of the chemical and electronic properties of various layers across perovskite solar cell (PSC) stacks is challenging. Depth-profiling photoemission spectroscopy can be used to study the surface, interface, and bulk properties of different layers in PSCs, which influence the overall performance of these devices. Herein, sputter depth profiling (SDP) and tapered cross-sectional (TCS) photoelectron spectroscopies (PESs) are used to study highly efficient mixed halide PSCs. It is found that the most used SDP-PES technique degrades the organic and deforms the inorganic materials during sputtering of the PSCs while the TCS-PES method is less destructive and can determine the chemical and electronic properties of all layers precisely. The SDP-PES dissociates the chemical bonding in the spiro-MeOTAD and perovskite layer and reduces the TiO$_{2}$, which causes the chemical analysis to be unreliable. The TCS-PES revealed a band bending only at the spiro-MeOTAD/perovskite interface of about 0.7 eV. Both the TCS and SDP-PES show that the perovskite layer is inhomogeneous and has a higher amount of bromine at the perovskite/TiO$_{2}$ interface.

MIT and KRICT scientists recently reported the highest certified power conversion efficiency of 25.2% for perovskite solar cells. They highlighted that charge carrier management is essential to improve the photovoltaic parameters and overcome the theoretical efficiency limit.

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization. published

Perovskite solar cells (PSCs) have emerged recently as promising candidates for next generation photovoltaics and have reached power conversion efficiencies of 25.2%. Among the various methods to advance solar cell technologies, the implementation of nanoparticles with plasmonic effects is an alternative way for photon and charge carrier management. Surface plasmons at the interfaces or surfaces of sophisticated metal nanostructures are able to interact with electromagnetic radiation. The properties of surface plasmons can be tuned specifically by controlling the shape, size, and dielectric environment of the metal nanostructures. Thus, incorporating metallic nanostructures in solar cells is reported as a possible strategy to explore the enhancement of energy conversion efficiency mainly in semi‐transparent solar cells. One particularly interesting option is PSCs with plasmonic structures enable thinner photovoltaic absorber layers without compromising their thickness while maintaining a high light harvest. In this Review, the effects of plasmonic nanostructures in electron transport material, perovskite absorbers, the hole transport material, as well as enhancement of effective refractive index of the medium and the resulting solar cell performance are presented. Aside from providing general considerations and a review of plasmonic nanostructures, the current efforts to introduce these plasmonic structures into semi‐transparent solar cells are outlined., Implementation of nanoparticles (NPs) with plasmonic effects is an effective strategy for photon and charge dynamic management in perovskite solar cells (PSCs). The outstanding effects of plasmonic nanostructures such as Ag NPs decorated on TiO2 nanowires in electron transport materials as well as localized surface plasmon resonance of Au NPs in hole transport materials enhance the photovoltaic response of PSCs.

Tin based perovskites have drawn much attention for narrow bandgap and eco-friendliness. In our previous work, high quality Sn-Pb perovskite films with vertically aligned large grains and high crystallinity were achieved on a hydrophobic hole transport layer by a two-step processing method. However, it remains a challenge to achieve thick films which are required to enhance light absorption especially in near-infrared region, and thus the performance is limited by the low short-circuit current due to the thin films. Here, a 2D/3D structure was applied to obtain high quality thick Sn-Pb perovskite films and finally a PCE of 15.7% was obtained.

In 2009, a study on the use of halide perovskites as light sensi-tizers in liquid dye-sensitized solar cells went almost unnoticeduntil three years later, when an outstanding performance for all-solid perovskite solar cell was reported. Although these materialswere already well known in the literature, it was their use forphotovoltaic applications that boosted the interest in this familyof materials [...]

Perovskite solar cells (PSCs) have rapidly emerged as a potential competitive photovoltaic technology reaching high power conversion efficiencies (PCEs) from single digits to a certified 23.7% in just a few years. At this stage, the key issues are the further improvement of the PCE and long-term device stability. In this chapter, we summarize the recent developments in the quest to phase stabilize the perovskite material. Since the organic cations are volatile and sensitive to moisture, heat, light and thus a long-term risk factor for phase stability, our focus is on incorporation of inorganic cations such as cesium and rubidium to enhance the photo-active black phases by means of modifications in optoelectronic properties of perovskites. The challenges associated with PSC industrialization include the role of cation/anion in phase stability, phase segregation of the halides, toxicity, organic/inorganic ion mixing and determination of the best tuned-band gap composition for tandem applications.

Low toxicity and an ideal energy bandgap make two-dimensional (2D) Ruddlesden–Popper tin-based halide perovskites a promising photovoltaic material. However, the disordered crystal orientation and the oxidation of Sn2+ to Sn4+ still need to be addressed. Here, we demonstrate that the annealing of FASnI3 assisted by phenyl ethylammonium chloride enables the formation of more ordered 2D tin-based perovskite crystals oriented vertically. We use in situ synchrotron-based grazing incident X-ray diffraction to correlate the higher crystal orientation to the better device performance. We measured a maximum power conversion efficiency of more than 9%. Furthermore, we demonstrate that the phenyl ethylammonium chloride acts as a barrier layer at the surface of the crystals protecting the tin from the oxidation. Hence, this work paves the way for more efficient and stable lead-free perovskite solar cells.

The rapid development of Internet of Things mobile terminals has accelerated the market's demand for portable mobile power supplies and flexible wearable devices. Here, an embedded metal-mesh transparent conductive electrode (TCE) is prepared on poly(ethylene terephthalate) (PET) using a novel selective electrodeposition process combined with inverted film-processing methods. This embedded nickel (Ni)-mesh flexible TCE shows excellent photoelectric performance (sheet resistance of ≈0.2-0.5 Ω sq-1 at high transmittance of ≈85-87%) and mechanical durability. The PET/Ni-mesh/polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS PH1000) hybrid electrode is used as a transparent electrode for perovskite solar cells (PSCs), which exhibit excellent electric properties and remarkable environmental and mechanical stability. A power conversion efficiency of 17.3% is obtained, which is the highest efficiency for a PSC based on flexible transparent metal electrodes to date. For perovskite crystals that require harsh growth conditions, their mechanical stability and environmental stability on flexible transparent embedded metal substrates are studied and improved. The resulting flexible device retains 76% of the original efficiency after 2000 bending cycles. The results of this work provide a step improvement in flexible PSCs.

State-of-the-art cesium-containing multiple-cation perovskites are generally synthesized from stock solutions of perovskite precursors and CsI in DMSO and DMF. However, compositional diversity of multi-component perovskites is significantly hampered due to the poor solubility of other cesium halides in these solvents. Here, we show how insoluble CsCl, as a new source of cesium cation, can be integrated into a multiple-cation perovskite material by a one-step method involving grinding of the precursors. The resulting polycrystalline powder is fully soluble in a DMSO/DMF mixture and allows formation of perovskite thin films. 133Cs solid-state MAS NMR data indicate that the cesium cation is almost fully (90%) incorporated into the 3D perovskite lattice, while the remaining 10% forms a cesium-rich mixed-halide secondary phase. The planar heterojunction device fabricated using this original mechanoperovskite yielded a power conversion efficiency of 19.12% and an open circuit voltage of 1.16 V. Moreover, we show that the introduction of CsCl improves both interfacial and bulk photovoltaic metrics. Our one-step approach provides an efficient general method for incorporating poorly soluble salts into multi-component perovskite crystal lattices.

Organic–inorganic perovskite solar cells (PSCs) have gained considerable attention owing to their impressive photovoltaic properties and simple device manufacturing. In general, PSC employs a perovskite absorber material sandwiched between an electron and hole selective transport layer optimized with respect to optimal band alignment, efficient charge collection, and low interfacial recombination. The interfaces between the perovskite absorber and respective selective contacts play a crucial role in determining photovoltaic performance and stability of PSCs. However, a fundamental understanding is lacking, and there is poor understanding in controlling the physical processes at the interfaces. Herein, we investigate the interfacial characteristics of PSCs with both planar and mesoporous architecture that provide a deeper insight into the charge recombination and accumulation mechanism and the origin of open-circuit voltage (Voc). The effect of electron- and hole-selective contacts in the final cell perfor...

In perovskite solar cells (PSCs), the most commonly used hole transport material (HTM) is spiro-OMeTAD, which is typically doped by metalorganic complexes, for example, based on Co, to improve charge transport properties and thereby enhance the photovoltaic performance of the device. In this study, we report a new hemicage-structured iron complex, 1,3,5-tris(5'-methyl-2,2'-bipyridin-5-yl)ethylbenzene Fe(III)-tris(bis(trifluoromethylsulfonyl)imide), as a p-type dopant for spiro-OMeTAD. The formal redox potential of this compound was measured as 1.29 V vs. the standard hydrogen electrode, which is slightly (20 mV) more positive than that of the commercial cobalt dopant FK209. Photoelectron spectroscopy measurements confirm that the iron complex acts as an efficient p-dopant, as evidenced in an increase of the spiro-OMeTAD work function. When fabricating planar PSCs with the HTM spiro-OMeTAD doped by 5 mol % of the iron complex, a power conversion efficiency of 19.5 % (AM 1.5G, 100 mW cm-2 ) is achieved, compared to 19.3 % for reference devices with FK209. Open circuit voltages exceeding 1.2 V at 1 sun and reaching 1.27 V at 3 suns indicate that recombination at the perovskite/HTM interface is low when employing this iron complex. This work contributes to recent endeavors to reduce recombination losses in perovskite solar cells.

The identification of recombination centers in perovskite solar cells is highly challenging. Here, we demonstrate the red and blue excitation wavelength resolved impedance response in state-of-the-art perovskite solar cells (PSCs) providing insights into charge recombination and ion accumulation. To get insight into the interfacial electronic characteristics, we fabricated PSCs with a planar architecture containing state-of-the-art triple-cation perovskite materials as absorber layers. The capacitance–frequency response under various blue and red illumination conditions were used to investigate interfacial charge accumulations and found that under high energy photons irradiation maximum charge or ion accumulations at interface (ETL/perovskite) take place. Ideality factor of PSCs was also calculated from the obtained value of high frequency resistance equaling 2.1 and 1.8 for devices measured under blue and red excitation wavelength, respectively. Our study illustrates that EIS measurements at different excitation wavelengths, reveals information about charge or ion accumulation and also provides a tool to diagnose PSCs in a more localized way.

Recently organic–inorganic perovskite solar cells (PSCs) have emerged as promising candidates for photovoltaics because of their relatively high efficiency and low processing costs. However, for possible commercialisation, long-term stability remains a key obstacle, especially when compared to silicon or GaAs. Thus, future research will significantly focus on stability. The most relevant industry standards for the stability of solar cells are issued by the International Electrotechnical Commission (IEC), summarized in the so-called IEC 61215 norm. The IEC 61215 is a series of very detailed, time-consuming and interconnected stress tests that provide accelerated aging conditions to extrapolate the potential long-term lifetime of a solar module. Established silicon, for example, passes the full IEC 61215. To gain the confidence of investors and customers, passing the full IEC 61215 is a necessary minimum requirement for the commercialization of perovskites. Interestingly, the IEC 61215 is not openly accessible which may be one reason why there are often references to outdated versions. To remedy this situation, we introduce and analyse the most current IEC 61215 stability standards for solar cells and to which degree perovskites have passed them. We then elaborate on the most pertinent challenges for the long-term stability of PSCs in the coming years. This includes less explored stability tests such as potential-induced degradation (IEC TS 62804-1) and ammonia corrosion (IEC 62716). From this, it is evident that currently underappreciated degradation modes such as mechanical stability, high applied voltages and reverse bias, where especially hot spots could become problematic, must be considered in the coming years when evaluating the long-term stability of PSCs.

The improvement of the quality of the hole transporting layer (HTL) plays a key role in the fabrication of highly efficient and stable perovskite solar cells (PSCs). Here, we used rubrene as a surface treatment agent on top of a spiro HTL. We found that rubrene can cover the pinholes of the spiro layer and provide an excellent contact layer for planar PSCs. Based on this modification, mobile gold ions from the metal electrode are prevented from diffusing through the HTL hindering the degradation of PSCs. The optimized device shows a maximum power conversion efficiency (PCE) of 19.87% and a 79% fill factor (FF), which are higher than the 17.98% PCE and 72% FF of the reference device. In addition, the device after modification demonstrates better stability after 100 h under continuous light illumination retaining 97% of its initial performance compared with 65% of the reference device.

As metal halide solar cells reach photoconversion efficiencies over 25%, one question that is often being asked is, “What is next?” Interestingly, there are plenty of new opportunities to explore in the perovskite world. The four articles selected for the Energy Spotlight in this issue are just a few examples to show the dynamic nature of metal halide perovskite research. New inroads are being made to explore new heterostructures and luminescent semiconductor nanoclusters, understand phase stability, and offer new insights into excited state processes. Such advances provide new avenues to further expand the scope of perovskite research, offering new areas to explore.

Perovskite solar cells (PSCs) have been introduced as an attractive photovoltaic technology over the past decade due to their low-cost processing, earth-abundant raw materials, and high power conversion efficiencies (PCEs) of up to 25.2%. However, the relatively high density of defects within the bulk, grain boundaries, and surface of polycrystalline perovskite films acts as recombination centers and facilitates ion migration, lowering the theoretical PCE ceiling, often leading to inferior device stability. Therefore, understanding the defect sources and developing passivation methods are key factors for reaching higher PCEs and stabilities in perovskite photovoltaics. Herein, various passivation methods, including bulk and surface treatment of perovskite films, are explored. In the bulk treatment, the passivating agents should be directly added to the perovskite precursor. However, in the surface treatment method, the surface of perovskite films can be treated by inducing passivating agents during the intermediate phase or after annealing steps, denoted here as in-film or surface posttreatment. In addition, different kinds of passivating agents are categorized based on their functional groups. Finally, the outline directions to minimize the defects in perovskite films are highlighted.

With a power conversion efficiency (PCE) exceeding 22 %, perovskite solar cells (PSCs) have thrilled photovoltaic research. However, the interface behavior is still not understood and is a hot topic of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, which makes perovskite interface physics intriguing. Herein, through femtosecond transient absorption spectroscopy with spectral coverage extending into the crucial IR region, the ultrafast interface-specific processes at standard and newly molecularly engineered perovskite interfaces in state-of-the-art PSCs are interrogated. Ultrafast interfacial charge injection occurs with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the timescale for the first step involved in the complex charge-transfer process. This is also true for 20 % efficient devices measured under real operation, for which the femtosecond injection is followed by a slower picosecond component. These findings provide compelling evidence for the femtosecond interfacial charge-injection step and demonstrate a robust method for the straightforward identification of interfacial non-equilibrium processes on the ultrafast timescale. Fil: Grancini, Giulia. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Viola, Daniele. Politecnico di Milano; Italia Fil: Lee, Yonghui. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Saliba, Michael. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Paek, Sanghyun. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Cho, Kyung Taek. Ecole Polytechnique Federale de Lausanne; Suiza Fil: Orlandi, Simonetta. Consiglio Nazionale delle Ricerche; Italia Fil: Cavazzini, Marco. Consiglio Nazionale delle Ricerche; Italia Fil: Fungo, Fernando Gabriel. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina Fil: Hossain, Mohammad I.. Qatar Environment And Energy Research Institute; Qatar Fil: Belaidi, Abdelhak. Qatar Environment And Energy Research Institute; Qatar Fil: Tabet, Nouar. Qatar Environment And Energy Research Institute; Qatar Fil: Pozzi, Gianluca. Consiglio Nazionale delle Ricerche; Italia Fil: Cerullo, Giulio. Politecnico di Milano; Italia Fil: Nazeeruddin, Mohammad Khaja. Ecole Polytechnique Federale de Lausanne; Suiza

Thermal stability of perovskite solar cells is crucial for their practical use. In this report, we show good 74% stability retention after a thermal stress test at 85 °C for 350 h) with high efficiency (>18%) using Rb modified perovskite absorber layers. It is posited that the mechanism of the thermal stability improvement is due to the formation of RbPbl3 which suppresses Pbl2 growth. This facile way for improving thermal stability is promising for realizing solar cells with high efficiency and stability.

State-of-the-art halide perovskite solar cells employ semiconductor oxides as electron transport materials. Defects in these oxides, such as oxygen vacancies (Ovac), act as recombination centres and, in air and UV light, reduce the stability of the solar cell. Under the same conditions, the PbZrTiO3 ferroelectric oxide employs Ovac for the creation of defect-dipoles responsible for photo-carrier separation and current transport, evading device degradation. We report the application of PbZrTiO3 as the electron extraction material in triple cation halide perovskite solar cells. The application of a bias voltage (poling) up to 2 V, under UV light, is a critical step to induce charge transport in the ferroelectric oxide. Champion cells result in power conversion efficiencies of ∼11% after poling. Stability analysis, carried out at 1-sun AM 1.5 G, including UV light in air for unencapsulated devices, shows negligible degradation for hours. Our experiments indicate the effect of ferroelectricity, however alternative conducting mechanisms affected by the accumulation of charges or the migration of ions (or the combination of them) cannot be ruled out. Our results demonstrate, for the first time, the application of a ferroelectric oxide as an electron extraction material in efficient and stable PSCs. These findings are also a step forward in the development of next generation ferroelectric oxide-based electronic and optoelectronic devices., M. G. and S. M. Z. thank the European Union's Horizon 2020 programme, through a FET Open research and innovation action under grant agreement No. 687008 and the King Abdulaziz City for Science and Technology (KACST) for the financial support. H.-S. K. is grateful for the postdoctoral fellowship grant (NRF-2016R1A6A3A03012393). M. S. acknowledges support from the co-funded Marie Skłodowska Curie fellowship, H2020 grant agreement no. 665667. To the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295 and for the postdoctoral contract for H. X.; for the grant ENE2016-79282-C5-2-R and the OrgEnergy Excelence Network CTQ2016-81911-REDT. To the Agència de Gestió d'Ajuts Universitaris i de Recerca for the support to the consolidated Catalonia research group 2017 SGR-329 and the Xarxa de Referència en Materials Avançats per a l'Energia (Xarmae). To the Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) for the contract ENE2015-74275-JIN to A. P. To the CERCA Programme/Generalitat de Catalunya and to the European COST Action StableNextSol project MP1307.

The search for new classes of hole transporting materials (HTMs) is a very important task on the way towards commercialization of perovskite solar cells (PSCs). In this work, the synthesis and performance in PSCs of new enamine-based HTMs are presented. The synthesis scheme is very short, consisting of only one step, starting from commercially available aromatic amines. Moreover, the reaction does not require metal catalysts and is based on condensation chemistry with water as a by-product. It was shown that PSCs with such materials reach high power conversion efficiencies (PCE), with the highest PCE of 19% achieved for the V1021-based device. This work establishes enamines as a very promising class of materials for application in PSCs.

Triarylamine-based polymers with different functional groups were synthetized as hole-transport materials (HTMs) for perovskite solar cells (PSCs). The novel materials enabled efficient PSCs without the use of chemical doping (or additives) to enhance charge transport. Devices employing poly(triarylamine) with methylphenylethenyl functional groups (V873) showed a power conversion efficiency of 12.3 %, whereas widely used additive-free poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) demonstrated 10.8 %. Notably, devices with V873 enabled stable PSCs under 1 sun illumination at maximum power point tracking for approximately 40 h at room temperature, and in the dark under elevated temperature (85 °C) for more than 140 h. This is in stark contrast to additive-containing devices, which degrade significantly within the same time frame. The results present remarkable progress towards stable PSC under real working conditions and industrial stress tests.

CH3NH3PbX3 (MAPbX3) perovskites have attracted considerable attention as absorber materials for solar light harvesting, reaching solar to power conversion efficiencies above 20%. In spite of the rapid evolution of the efficiencies, the understanding of basic properties of these semiconductors is still ongoing. One phenomenon with so far unclear origin is the so-called hysteresis in the current–voltage characteristics of these solar cells. Here we investigate the origin of this phenomenon with a combined experimental and computational approach. Experimentally the activation energy for the hysteretic process is determined and compared with the computational results. First-principles simulations show that the timescale for MA+ rotation excludes a MA-related ferroelectric effect as possible origin for the observed hysteresis. On the other hand, the computationally determined activation energies for halide ion (vacancy) migration are in excellent agreement with the experimentally determined values, suggesting that the migration of this species causes the observed hysteretic behaviour of these solar cells., The origin of hysteresis remains an open question in lead-halide perovskite solar cells. Here, Meloni et al. investigate the causes of hysteresis using an experimental and computational approach, finding that the observed hysteresis is due to halide ion-vacancy movement in the perovskite.

Tandem solar cells combining silicon and perovskite absorbers have the potential to outperform state-of-the-art high efficiency silicon single junction devices. However, the practical fabrication of monolithic silicon/perovskite tandem solar cells is challenging as material properties and processing requirements such as temperature restrict the device design. Here, we fabricate an 18% efficient monolithic tandem cell formed by a silicon heterojunction bottom-and a perovskite top-cell enabling a very high open circuit voltage of 1.78 V. The monolithic integration was realized via low temperature processing of the semitransparent perovskite sub-cell where an energetically aligned electron selective contact was fabricated by atomic layer deposition of tin oxide. The hole selective, transparent top contact was formed by a stack of the organic hole transport material spiro-OMeTAD, molybdenum oxide and sputtered indium tin oxide. The tandem cell design is currently limited by the photo-current generated in the silicon bottom cell that is reduced due to reflectance losses. Based on optical modelling and first experiments, we show that these losses can be significantly reduced by combining optical optimization of the device architecture including light trapping approaches.

Small-molecule hole transporting materials based on methoxydiphenylamine-substituted fluorene fragments were synthesized and incorporated into a perovskite solar cell, which displayed a power conversion efficiency of up to 19.96%, one of the highest conversion efficiencies reported. The investigated hole transporting materials were synthesized in two steps from commercially available and relatively inexpensive starting reagents, resulting in up to fivefold cost reduction of the final product compared with spiro-OMeTAD. Electro-optical and thermoanalytical measurements such as UV/Vis, thin-film conductivity, hole mobility, DSC, TGA, ionization potential and current voltage scans of the full perovskite solar cells have been carried out to characterize the new materials.

Lead halide perovskites have attracted considerable interest as photoabsorbers in PV-applications over the last few years. The most studied perovskite material achieving high photovoltaic performance has been methyl ammonium lead iodide, CH3NH3PbI3. Recently the highest solar cell efficiencies have, however, been achieved with mixed perovskites where iodide and methyl ammonium partially have been replaced by bromide and formamidinium. In this work, the mixed perovskites were explored in a systematic way by manufacturing devices where both iodide and methyl ammonium were gradually replaced by bromide and formamidinium. The absorption and the emission behavior as well as the crystallographic properties were explored for the perovskites in this compositional space. The band gaps as well as the crystallographic structures were extracted. Small changes in the composition of the perovskite were found to have a large impact on the properties of the materials and the device performance. In the investigated compositional space, cell efficiencies, for example, vary from a few percent up to 20.7%. From the perspective of applications, exchanging iodide with bromide is especially interesting as it allows tuning of the band gap from 1.5 to 2.3 eV. This is highly beneficial for tandem applications, and an empirical expression for the band gap as a function of composition was determined. Exchanging a small amount of iodide with bromide is found to be highly beneficial, whereas a larger amount of bromide in the perovskite was found to cause intense sub band gap photoemission with detrimental results for the device performance. This could be caused by the formation of a small amount of an iodide rich phase with a lower band gap, even though such a phase was not observed in diffraction experiments. This shows that stabilizing the mixed perovskites will be an important task in order to get the bromide rich perovskites, which has a higher band gap, to reach the same high performance obtained with the best compositions.