Versailles Project on Advanced Materials and Standards Interlaboratory Study on Measuring the Thickness and Chemistry of Nanoparticle Coatings Using XPS and LEIS

NiN-Passivated NiO Hole-Transport Layer Improves Halide Perovskite-Based Solar Cell

The interfaces between inorganic selective contacts and halide perovskites (HaPs) are possibly the greatest challenge for making stable and reproducible solar cells with these materials. NiO_x, an attractive hole-transport layer as it fits the electronic structure of HaPs, is highly stable and can be produced at a low cost. Furthermore, NiO_x can be fabricated via scalable and controlled physical deposition methods such as RF sputtering to facilitate the quest for scalable, solvent-free, vacuum-deposited HaP-based solar cells (PSCs). However, the interface between NiO_x and HaPs is still not well-controlled, which leads at times to a lack of stability and V_oc losses. Here, we use RF sputtering to fabricate NiO_x and then cover it with a Ni_yN layer without breaking vacuum. The Ni_yN layer protects NiO_x doubly during PSC production. Firstly, the Ni_yN layer protects NiO_x from Ni³⁺ species being reduced to Ni²⁺ by Ar plasma, thus maintaining NiO_x conductivity. Secondly, it passivates the interface between NiO_x and the HaPs, retaining PSC stability over time. This double effect improves PSC efficiency from an average of 16.5% with a 17.4% record cell to a 19% average with a 19.8% record cell and increases the device stability.

All-Inorganic Perovskite Solar Cells: Recent Advancements and Challenges

Organic–inorganic metal-halide-based hybrid perovskite solar cells (SCs) have attracted a great deal of attention from researchers around the globe with their certified power conversion efficiencies (PCEs) having now increased to 25.2%. Nevertheless, organic–inorganic hybrid halide perovskite SCs suffer the serious drawback of instability with respect to moisture and heat. However, all-inorganic perovskite SCs have emerged as promising candidates to tackle the thermal instability problem. Since the introduction of all-inorganic perovskite materials to the field of perovskite photovoltaics in 2014, a plethora of research articles has been published focusing on this research topic. The PCE of all-inorganic PSCs has climbed to a record 18.4% and research is underway to enhance this. In this review, I survey the gradual progress of all-inorganic perovskites, their material design, the fabrication of high-quality perovskite films, energetics, major challenges and schemes opening new horizons toward commercialization. Furthermore, techniques to stabilize cubically phased low-bandgap inorganic perovskites are highlighted, as this is an indispensable requirement for stable and highly efficient SCs. In addition, I explain the various energy loss mechanisms at the interface and in the bulk of perovskite and charge-selective layers, and recap previously published reports on the curtailment of charge-carrier recombination losses.

Charge transport materials for mesoscopic perovskite solar cells

An overview on recent advances in the fundamental understanding of how interfaces of mesoscopic perovskite solar cells (mp-PSCs) with different architectures, upon incorporating various charge transport layers, influence their performance.

Thermal Analysis of Metal-Organic Precursors for Functional Cu:ΝiOx Hole Transporting Layer in Inverted Perovskite Solar Cells: Role of Solution Combustion Chemistry in Cu:ΝiOx Thin Films Processing

Low temperature solution combustion synthesis emerges as a facile method for the synthesis of functional metal oxides thin films for electronic applications. We study the solution combustion synthesis process of Cu:NiO_x using different molar ratios (w/o, 0.1 and 1.5) of fuel acetylacetone (Acac) to oxidizer (Cu, Ni Nitrates) as a function of thermal annealing temperatures 150, 200, and 300 °C. The solution combustion synthesis process, in both thin films and bulk Cu:NiO_x, is investigated. Thermal analysis studies using TGA and DTA reveal that the Cu:NiO_x thin films show a more gradual mass loss while the bulk Cu:NiO_x exhibits a distinct combustion process. The thin films can crystallize to Cu:NiO_x at an annealing temperature of 300 °C, irrespective of the Acac/Oxidizer ratio, whereas lower annealing temperatures (150 and 200 °C) produce amorphous materials. A detail characterization study of solution combustion synthesized Cu:NiO_x, including XPS, UV-Vis, AFM, and Contact angle measurements, is presented. Finally, 50 nm Cu:NiO_x thin films are introduced as HTLs within the inverted perovskite solar cell device architecture. The Cu:NiO_x HTL annealed at 150 and 200 °C provided PVSCs with limited functionality, whereas efficient triple-cation Cs_0.04(MA_0.17FA_0.83)_0.96 Pb(I_0.83Br_0.17)₃-based PVSCs achieved for Cu:NiO_x HTLs for annealing temperature of 300 °C.

Exploring Transport Behavior in Hybrid Perovskites Solar Cells via Machine Learning Analysis of Environmental-Dependent Impedance Spectroscopy

Hybrid organic-inorganic perovskites are one of the promising candidates for the next-generation semiconductors due to their superlative optoelectronic properties. However, one of the limiting factors for potential applications is their chemical and structural instability in different environments. Herein, the stability of (FAPbI₃ )_0.85 (MAPbBr₃ )_0.15 perovskite solar cell is explored in different atmospheres using impedance spectroscopy. An equivalent circuit model and distribution of relaxation times (DRTs) are used to effectively analyze impedance spectra. DRT is further analyzed via machine learning workflow based on the non-negative matrix factorization of reconstructed relaxation time spectra. This exploration provides the interplay of charge transport dynamics and recombination processes under environment stimuli and illumination. The results reveal that in the dark, oxygen atmosphere induces an increased hole concentration with less ionic character while ionic motion is dominant under ambient air. Under 1 Sun illumination, the environment-dependent impedance responses show a more striking effect compared with dark conditions. In this case, the increased transport resistance observed under oxygen atmosphere in equivalent circuit analysis arises due to interruption of photogenerated hole carriers. The results not only shed light on elucidating transport mechanisms of perovskite solar cells in different environments but also offer an effective interpretation of impedance responses.

Acute effects of CH3NH3PbI3 perovskite on Scenedesmus obliquus and Daphnia magana in aquatic environment

CH₃NH₃PbI₃ is one of the most widely studied and most promising photoelectric conversion materials for large-scale application. However, once it is discharged into the aquatic environment, it will release a variety of lethal substances to the aquatic organisms. Herein, two typical aquatic pollution indicators, Scenedesmus obliquus (a typical phytoplankton) and Daphnia magna (a typical zooplankton), were used to assess the acute effects of CH₃NH₃PbI₃ perovskite on aquatic organisms. The results showed that, when the initial CH₃NH₃PbI₃ perovskite level (CPL) was 40 mg L^-1 or higher, the growth of S. obliquus would be remarkably inhibited with significant decreases of chlorophyll content and protein content. And when the CPL was over 5 mg L^-1, the survival of D. magna would be notably threatened. Specifically, the 72 h EC-50 of CH₃NH₃PbI₃ perovskite to S. obliquus was calculated as 37.21 mg L^-1, and the 24 h LC-50 of this perovskite to D. magna adults and neonates were calculated as 37.53 mg L^-1 and 18.55 mg L^-1, respectively. Moreover, remarkably solution pH declination and large amounts of lead bio-accumulation was observed in the both acute experiments, which could be the main reasons causing the above acute effects. Considering the strong acute effects of these CH₃NH₃PbI₃ perovskite materials and their attractive application prospect, more attentions should be paid on their harmness to the environment.

Dopant-Free Hole-Transport Materials with Germanium Compounds Bearing Pseudohalide and Chalcogenide Moieties for Perovskite Solar Cells

Hole-transport materials (HTMs) are key electronic components for the functioning of perovskite solar cells (PSCs) as they extract the photogenerated holes from the perovskite to be transported subsequently to the back electrode while minimizing the loss from electron recombination. Herein, we report the synthesis and characterization of novel germanium-based compounds with [{HC(CMeNAr)₂}GeNCS] (2), [{HC(CMeNAr)₂}Ge(S)NCS] (3), and [{HC(CMeNAr)₂}Ge(Se)NCS] (4) compositions, with Ar = 2,6-iPr₂C₆H₃ and the photovoltaic performance of 3 and 4 that is the same as for HTM in PSC. All compounds displayed excellent thermal properties and an appropriate alignment of energy levels for the perovskite with maximum optical absorption in the near-UV region. As revealed by space-charge limited-current (SCLC) measurements, compounds 3 and 4 have competing hole mobilities of about 1.37 × 10^-4 and 4.88 × 10^-4 cm² V^-1 s^-1, respectively. Upon assessing PSC devices using 3 and 4 with triple-cation perovskite absorber Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃, the power conversion efficiencies (PCEs) were about 13.03 and 9.23%, respectively, both without doping and additives, and were compared with benchmark HTM spiro-OMeTAD (2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene). Quantum chemical calculations with DFT showed that the optoelectronic properties are strongly influenced by the combined contributions of the germanium atom, the pseudohalide moiety (NCS^-), and chalcogenides (S^2- or Se^2-). Fine tuning the electronic properties of germanium is thus a good strategy for the targeted synthesis of potential conducting molecules in PSCs.

Influence of the cathode microstructure on the stability of inverted planar perovskite solar cells

One of the main challenges for perovskite solar cells (PSC) is their environmental stability, as oxygen and water induced aging may result in mobile decomposition compounds, which can enhance the recombination rate and react with charge carrier extraction layers or the contact metallization. In this contribution the importance of the microstructure of the contact metallization on the environmental cell stability is investigated. For this purpose, the storage stability of inverted planar methylammonium lead iodide (MAPI)-based perovskite solar cells without encapsulation is tested, using the metals aluminum (Al), silver (Ag), gold (Au) and nickel (Ni) as representative cathode materials. For this study, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis of the different electrodes as well as the perovskite is correlated with PSC device current–voltage (J–V) and impedance measurements. Our findings substantiate that the metal microstructure has a significant influence on the PSC aging properties. While a strong perovskite decomposition and iodide diffusion to the contacts were detected for devices using Al, Ag or Au cathodes with a polycrystalline microstructure, these effects were strongly reduced when Ni metallization was employed, where a nanocrystalline microstructure was exhibited under the chosen process conditions.

The microstructure of the metal contact has a significant influence on the PSC aging properties. In this contribution we show that perovskite decomposition and iodide diffusion can be suppressed by using nanocrystalline Ni metallization.

Roadmap on halide perovskite and related devices

Since the first report on solid-state perovskite solar cells (PSCs) with ∼10% power conversion efficiency (PCE) and 500 h-stability in 2012, tremendous effort has been being devoted to develop PSCs with higher PCE, longer stability and recycling hazardous lead waste. As a result, PCE over 23% was recorded in 2018 and stability over 10 000 h was reported. Beyond photovoltaics, lead halide perovskite materials demonstrated superb properties when they were applied to flat-panel x-ray detectors and non-volatile resistive switching memory. In this review, the progress of the lead halide perovskite in photovoltaics, x-ray imaging and memristors is investigated. Pb-based PSCs and non-Pb-based PSCs are compared, where technologies of non-Pb-based PSCs are not matured for commercialization. Pb-based PSCs were found to be highly suitable for both terrestrial and space photovoltaics. Higher sensitivity under low dose rate observed from the lead halide perovskite suggests a bright future for perovskite x-ray imaging systems. Moreover, high on/off ratio and low energy consumption observed in resistive switching enables perovskite to be a promising candidate for high density memristors.

Review on Recent Progress of All-Inorganic Metal Halide Perovskites and Solar Cells

All-inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic-inorganic hybrid counterparts. The power-conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high-quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead-free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low-bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge-carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.

Achieving Long-Term Operational Stability of Perovskite Solar Cells with a Stabilized Efficiency Exceeding 20% after 1000 h

Perovskite solar cells (PSCs) with mesoporous TiO2 (mp-TiO2) as the electron transport material attain power conversion efficiencies (PCEs) above 22%; however, their poor long-term stability is a critical issue that must be resolved for commercialization. Herein, it is demonstrated that the long-term operational stability of mp-TiO2 based PSCs with PCE over 20% is achieved by isolating devices from oxygen and humidity. This achievement attributes to systematic understanding of the critical role of oxygen in the degradation of PSCs. PSCs exhibit fast degradation under controlled oxygen atmosphere and illumination, which is accompanied by iodine migration into the hole transport material (HTM). A diffusion barrier at the HTM/perovskite interface or encapsulation on top of the devices improves the stability against oxygen under light soaking. Notably, a mp-TiO2 based PSC with a solid encapsulation retains 20% efficiency after 1000 h of 1 sun (AM1.5G including UV) illumination in ambient air.

A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells

Perovskite solar cells have achieved photo-conversion efficiencies greater than 20%, making them a promising candidate as an emerging solar cell technology. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degradation can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temperatures can be over 45 °C higher than ambient temperatures when operating under direct sunlight. Herein, recent advances in resolving thermal stability problems are highlighted through literature review. Moreover, the most recent and promising strategies for overcoming thermal degradation are also summarized.

The Interplay of Contact Layers: How the Electron Transport Layer Influences Interfacial Recombination and Hole Extraction in Perovskite Solar Cells

Charge-selective contact layers in perovskite solar cells influence the current density-voltage hysteresis, an effect related to ion migration in the perovskite. As such, fullerene-based electron transport layers (ETLs) suppress hysteresis by reducing the mobile ion concentration. However, the impact of different ETLs on the electronic properties of other constituent device layers remains unclear. In this Kelvin probe force microscopy study, we compared potential distributions of methylammonium lead iodide-based solar cells with two ETLs (planar TiO₂ and C60-functionalized self-assembled monolayer) with different hysteretic behavior. We found significant changes in the potential distribution of the organic hole transport layer spiroMeOTAD, suggesting the formation of a neutral spiroMeOTAD/iodide interface due to a reaction between iodide with p-doped spiroMeOTAD in the TiO₂ cell. Our results show that the ETL affects not only the mobile ion concentration and the recombination at the perovskite/ETL interface but also the resistance and capacitance of the spiroMeOTAD.

Molecule-Doped Nickel Oxide: Verified Charge Transfer and Planar Inverted Mixed Cation Perovskite Solar Cell

Both conductivity and mobility are essential to charge transfer by carrier transport layers (CTLs) in perovskite solar cells (PSCs). The defects derived from generally used ionic doping method lead to the degradation of carrier mobility and parasite recombinations. In this work, a novel molecular doping of NiO_x hole transport layer (HTL) is realized successfully by 2,2'-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ). Determined by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, the Fermi level (E_F ) of NiO_x HTLs is increased from -4.63 to -5.07 eV and valence band maximum (VBM)-E_F declines from 0.58 to 0.29 eV after F6TCNNQ doping. The energy level offset between the VBMs of NiO_x and perovskites declines from 0.18 to 0.04 eV. Combining with first-principle calculations, electrostatic force microscopy is applied for the first time to verify direct electron transfer from NiO_x to F6TCNNQ. The average power conversion efficiency of CsFAMA mixed cation PSCs is boosted by ≈8% depending on F6TCNNQ-doped NiOx HTLs. Strikingly, the champion cell conversion efficiency of CsFAMA mixed cations and MAPbI₃ -based devices gets to 20.86% and 19.75%, respectively. Different from passivation effect, the results offer an extremely promising molecular doping method for inorganic CTLs in PSCs. This methodology definitely paves a novel way to modulate the doping in hybrid electronics more than perovskite and organic solar cells.

Effect of Cs-Incorporated NiO x on the Performance of Perovskite Solar Cells

The effect of Cs-incorporated NiO _x on perovskite solar cells with an inverted structure was investigated, where NiO _x and PCBM were used as selective contacts for holes and electrons, respectively. It was found that the generation of an Ni phase in an NiO _x layer was significantly suppressed by employing cesium. Furthermore, Cs-incorporated NiO _x enabled holes to be efficiently separated at the interface, showing the improved photoluminescent quenching and thus generating higher short-circuit current. The effect of Cs incorporation was also prominent in the inhibition of recombination. The recombination resistance of Cs-incorporated NiO _x was noticeably increased by more than three-fold near the maximum power point, leading to a higher fill factor of 0.78 and consequently a higher power conversion efficiency of 17.2% for the device employing Cs-incorporated NiO _x .

Lead-free perovskite solar cells using Sb and Bi-based A3B2X9 and A3BX6 crystals with normal and inverse cell structures

Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3+, Ag+ and Cu+). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb3+ and TiO2/Cu3BiI6, which should be addressed in the future study.

NiO x Hole Transport Layer for Perovskite Solar Cells with Improved Stability and Reproducibility

In this study, highly stable, low-temperature-processed planar lead halide perovskite (MAPbI_3-x Cl _x ) solar cells with NiO _x interfaces have been developed. Our solar cells maintain over 85% of the initial efficiency for more than 670 h, at the maximum power point tracking (MPPT) under 1 sun illumination (no UV-light filtering) at 30 °C, and over 73% of the initial efficiency for more than 1000 h, at the accelerating aging test (85 °C) under the same MPPT condition. Storing the encapsulated devices at 85 °C in dark over 1000 h revealed no performance degradation. The key factor for the prolonged lifetime of the devices was the sputter-deposited polycrystalline NiO _x hole transport layer (HTL). We observed that the properties of NiO _x are dependent on its composition. At a higher Ni³⁺/Ni²⁺ ratio, the conductivity of NiO _x is higher, but at the expense of optical transmittance. We obtained the highest power conversion efficiency of 15.2% at the optimized NiO _x condition. The sputtered NiO _x films were used to fabricate solar cells without annealing or any other treatments. The device stability enhanced significantly compared to that of the devices with PEDOT:PSS HTL. We clearly demonstrated that the illumination-induced degradation depends heavily on the nature of the HTL in the inverted perovskite solar cells (PVSCs). The sputtered NiO _x HTL can be a good candidate to solve stability problems in the lead halide PVSCs.

Effects of Small Polar Molecules (MA+ and H2O) on Degradation Processes of Perovskite Solar Cells

Degradation mechanisms of methylammonium lead halide perovskite solar cells (PSCs) have drawn much attention recently. Herein, the bulk and surface degradation processes of the perovskite were differentiated for the first time by employing combinational studies using electrochemical impedance spectroscopy (EIS), capacitance frequency (CF), and X-ray diffraction (XRD) studies with particular attention on the roles of small polar molecules (MA⁺ and H₂O). CF study shows that short-circuit current density of the PSCs is increased by H₂O at the beginning of the degradation process coupled with an increased surface capacitance. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSCs involves a lattice expansion process, which facilitates MA⁺ ion diffusion by creating more efficient channels. These results provide a better understanding of the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film.

Effect of Selective Contacts on the Thermal Stability of Perovskite Solar Cells

Thermal stability of CH₃NH₃PbI₃ (MAPbI₃)-based perovskite solar cells was investigated for normal structure including the mesoporous TiO₂ layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI₃ was found to be intrinsically stable from 85 °C to 120 °C in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 °C. Photoluminescence (PL) intensity and the time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI₃ into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 °C-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD, and MAPbI₃ is proved to be thermally stable. Spiro-MeOTAD with additives was crystallized at 85 °C due to a low glass transition temperature, and hole mobility was significantly deteriorated, which was responsible for the thermal instability. Thermal stability was significantly improved for the inverted structure with the NiO hole transporting layer, where the power conversion efficiency (PCE) was maintained at 74% of its initial PCE of 14.71% after the 80th thermal cycle (one cycle: heating at 85 °C for 2 h and cooling at 25 °C for 2 h). This work implies that the thermal stability of perovskite solar cells depends on selective contacts.