Tailoring component incorporation for homogenized perovskite solar cells.

Deep-level traps at the buried interface of perovskite and energy mismatch problems between the perovskite layer and heterogeneous interfaces restrict the development of ideal homogenized films and efficient perovskite solar cells (PSCs) using the one-step spin-coating method. Here, we strategically employed sparingly soluble germanium iodide as a homogenized bulk in-situ reconstruction inducing material preferentially aggregated at the perovskite buried interface with gradient doping, markedly reducing deep-level traps and withstanding local lattice strain, while minimizing non-radiative recombination losses and enhancing the charge carrier lifetime over 9 µs. Furthermore, this gradient doping assisted in modifying the band diagram at the buried interface into a desirable flattened alignment, substantially mitigating the energy loss of charge carriers within perovskite films and improving the carrier extraction equilibrium. As a result, the optimized device achieved a champion power conversion efficiency of 25.24% with a fill factor of up to 84.65%, and the unencapsulated device also demonstrated excellent light stability and humidity stability. This work provides a straightforward and reliable homogenization strategy of perovskite components for obtaining efficient and stable PSCs.

Buried interface modification and light outcoupling strategy for efficient blue perovskite light-emitting diodes.

Perovskite light-emitting diodes (PeLEDs) exhibit remarkable potential in the field of displays and solid-state lighting. However, blue PeLEDs, a key element for practical applications, still lag behind their green and red counterparts, due to a combination of strong nonradiative recombination losses and unoptimized device structures. In this report, we propose a buried interface modification strategy to address these challenges by focusing on the bottom-hole transport layer (HTL) of the PeLEDs. On the one hand, a multifunctional molecule, aminoacetic acid hydrochloride (AACl), is introduced to modify the HTL/perovskite interface to regulate the perovskite crystallization. Experimental investigations and theoretical calculations demonstrate that AACl can effectively reduce the nonradiative recombination losses in bulk perovskites by suppressing the growth of low-n perovskite phases and also the losses at the bottom interface by passivating interfacial defects. On the other hand, a self-assembly nanomesh structure is ingeniously developed within the HTLs. This nanomesh structure is meticulously crafted through the blending of poly-(9,9-dioctyl-fluorene-co-N-(4-butyl phenyl) diphenylamine) and poly (n-vinyl carbazole), significantly enhancing the light outcoupling efficiency in PeLEDs. As a result, our blue PeLEDs achieve remarkable external quantum efficiencies, 20.4% at 487 nm and 12.5% at 470 nm, which are among the highest reported values. Our results offer valuable insights and effective methods for achieving high-performance blue PeLEDs.

Transparent Recombination Layers Design and Rational Characterizations for Efficient Two-Terminal Perovskite-Based Tandem Solar Cells.

Two-terminal (2T) perovskite-based tandem solar cells (TSCs) arouse burgeoning interest in breaking the Shockley-Queisser (S-Q) limit of single-junction solar cells by combining two subcells with different bandgaps. However, the highest certified efficiency of 2T perovskite-based TSCs (33.9%) lags behind the theoretical limit (42-43%). A vital challenge limiting the development of 2T perovskite-based TSCs is the transparent recombination layers/interconnecting layers (RLs) design between two subcells. To improve the performance of 2T perovskite-based TSCs, RLs simultaneously fulfill the optical loss, contact resistance, carrier mobility, stress management, and conformal coverage requirements. In this review, the definition, functions, and requirements of RLs in 2T perovskite-based TSCs are presented. The insightful characterization methods applicable to RLs, which are inspiring for further research on the RLs both in 2T perovskite-based two-junction and multi-junction TSCs, are also highlighted. Finally, the key factors that currently limit the performance enhancement of RLs and the future directions that should be continuously focused on are summarized.

Tailoring Perovskite Surface Potential and Chelation Advances Efficient Solar Cells.

Modifying perovskite surface using various organic ammonium halide cations has proven to be an effective approach for enhancing the overall performance of perovskite solar cells. Nevertheless, the impact of the structural symmetry of these ammonium halide cations on perovskite interface termination has remained uncertain. Here, this work investigates the influence of symmetry on the performance of the devices, using molecules based on symmetrical bis(2-chloroethyl)ammonium cation (B(CE)A+) and asymmetrical 2-chloroethylammonium cation (CEA+) as interface layers between the perovskite and hole transport layer. These results reveal that the symmetrical B(CE)A+ cations lead to a more homogeneous surface potential and more comprehensive chelation with uncoordinated Pb2+ compared to the asymmetrical cations, resulting in a more favorable energy band alignment and strengthened defect healing. This strategy, leveraging the spatial geometrical symmetry of the interface cations, promotes hole carrier extraction between functional layers and reduces nonradiative recombination on the perovskite surface. Consequently, perovskite solar cells processed with the symmetrical B(CE)A+ cations achieve a power conversion efficiency (PCE) of 25.60% and retain ≈91% of their initial PCE after 500 h of maximum power point operation. This work highlights the significant benefits of utilizing structurally symmetrical cations in promoting the performance and stability of perovskite solar cells.

Spacer Conformation Induced Multiple Hydrogen Bonds in 2D Perovskite toward Highly Efficient Optoelectronic Devices.

Two-dimensional (2D) Dion-Jacobson (DJ) perovskites typically outperform Ruddlesden-Popper (RP) analogs in terms of photodetection (PD). However, the mechanism behind this enhanced performance remains elusive. Theoretical calculations for elucidating interlayer spacer conformation-induced multiple hydrogen bonds in 2D perovskite are presented, along with the synthesis of DPAPbBr4 (DPB) single crystals (SCs) and their PD properties under X-ray/ultraviolet (UV) excitation. The high-quality DPB SC enhances PD with exceptional photoresponse attributes, including a high on/off ratio (4.89 × 104), high responsivity (2.44 A W⁻1), along with large dynamic linear range (154 dB) and low detection limit (7.1 nW cm⁻2), which are currently the best results among 2D perovskite SC detectors, respectively. Importantly, high-resolution images are obtained under UV illumination with weak light levels. The SC X-ray detector exhibits a high sensitivity of 663 µC Gyair⁻1 cm-2 at 10 V and a detection limit of 1.44 µGyair s⁻1. This study explores 2D DJ perovskites for efficient and innovative optoelectronic applications.

Mixed tin-lead perovskites with balanced crystallization and oxidation barrier for all-perovskite tandem solar cells.

Mixed tin-lead perovskite solar cells have driven a lot of passion for research because of their vital role in all-perovskite tandem solar cells, which hold the potential for achieving higher efficiencies compared to single-junction counterparts. However, the pronounced disparity in crystallization processes between tin-based perovskites and lead-based perovskites, coupled with the easy Sn2+ oxidation, has long been a dominant factor contributing to high defect densities. In this study, we propose a multidimensional strategy to achieve efficient tin-lead perovskite solar cells by employing a functional N-(carboxypheny)guanidine hydrochloride molecule. The tailored N-(carboxypheny)guanidine hydrochloride molecule plays a pivotal role in manipulating the crystallization and grain growth of tin-lead perovskites, while also serving as a preservative to effectively inhibit Sn2+ oxidation, owing to the strong binding between N-(carboxypheny)guanidine hydrochloride and tin (II) iodide and the elevated energy barriers for oxidation. Consequently, single-junction tin-lead cells exhibit a stabilized power conversion efficiency of 23.11% and can maintain 97.45% of their initial value even after 3500 h of shelf storage in an inert atmosphere without encapsulation. We further integrate tin-lead perovskites into two-terminal monolithic all-perovskite tandem cells, delivering a certified efficiency of 27.35%.

Intermediate Phase Suppression with Long Chain Diammonium Alkane for High Performance Wide-Bandgap and Tandem Perovskite Solar Cells.

Wide bandgap (WBG) perovskite can construct tandem cells with narrow bandgap solar cells by adjusting the band gap to overcome the Shockley-Queisser limitation of single junction perovskite solar cells (PSCs). However, WBG perovskites still suffer from severe nonradiative carrier recombination and large open-circuit voltage loss. Here, this work uses an in situ photoluminescence (PL) measurement to monitor the intermediate phase evolution and crystallization process via blade coating. This work reports a strategy to fabricate efficient and stable WBG perovskite solar cells through doping a long carbon chain molecule octane-1,8-diamine dihydroiodide (ODADI). It is found that ODADI doping not only suppresses intermediate phases but also promote the crystallization of perovskite and passivate defects in blade coated 1.67 eV WBG FA0.7Cs0.25MA0.05Pb(I0.8Br0.2)3 perovskite films. As a result, the champion single junction inverted PSCs deliver the efficiencies of 22.06% and 19.63% for the active area of 0.07 and 1.02 cm2, respectively, which are the highest power conversion efficiencies (PCEs) in WBG PSCs by blade coating. The unencapsulated device demonstrates excellent stability in air, which maintains its initial efficiency at the maximum power points under constant AM 1.5G illumination in open air for nearly 500 h. The resulting semitransparent WBG device delivers a high PCE of 20.06%, and the 4-terminal all-perovskite tandem device delivers a PCE of 28.35%.

Micro/Nano Perovskite Materials for Advanced X-ray Detection and Imaging.

Halide perovskites have emerged as highly promising materials for ionizing radiation detection due to their exceptional characteristics, including a large mobility-lifetime product, strong stopping power, tunable band gap, and cost-effective crystal growth via solution processes. Semiconductor-type X-ray detectors employing various micro/nano perovskite materials have shown impressive progress in achieving heightened sensitivity and lower detection limits. Here, we present a comprehensive review of the applications of micro/nano perovskite materials for direct type X-ray detection, with a focus on the requirements for micro/nano crystal assembly and device properties in advanced X-ray detectors. We explore diverse processing techniques and optoelectronic considerations applied to perovskite X-ray detectors. Additionally, this review highlights the challenges and promising opportunities for perovskite X-ray detector arrays in real-world applications, potentially necessitating further research efforts.

Spontaneous crystallization of strongly confined CsSnxPb1-xI3 perovskite colloidal quantum dots at room temperature.

The scalable and low-cost room temperature (RT) synthesis for pure-iodine all-inorganic perovskite colloidal quantum dots (QDs) is a challenge due to the phase transition induced by thermal unequilibrium. Here, we introduce a direct RT strongly confined spontaneous crystallization strategy in a Cs-deficient reaction system without polar solvents for synthesizing stable pure-iodine all-inorganic tin-lead (Sn-Pb) alloyed perovskite colloidal QDs, which exhibit bright yellow luminescence. By tuning the ratio of Cs/Pb precursors, the size confinement effect and optical band gap of the resultant CsSnxPb1-xI3 perovskite QDs can be well controlled. This strongly confined RT approach is universal for wider bandgap bromine- and chlorine-based all-inorganic and iodine-based hybrid perovskite QDs. The alloyed CsSn0.09Pb0.91I3 QDs show superior yellow emission properties with prolonged carrier lifetime and significantly increased colloidal stability compared to the pristine CsPbI3 QDs, which is enabled by strong size confinement, Sn2+ passivation and enhanced formation energy. These findings provide a RT size-stabilized synthesis pathway to achieve high-performance pure-iodine all-inorganic Sn-Pb mixed perovskite colloidal QDs for optoelectronic applications.

Regulating Crystal Orientation via Ligand Anchoring Enables Efficient Wide-Bandgap Perovskite Solar Cells and Tandems.

Wide-bandgap (WBG) perovskite solar cells have attracted considerable interest for their potential applications in tandem solar cells. However, the predominant obstacles impeding their widespread adoption are substantial open-circuit voltage (VOC ) deficit and severe photo-induced halide segregation. To tackle these challenges, a crystal orientation regulation strategy by introducing dodecyl-benzene-sulfonic-acid as an additive in perovskite precursors is proposed. This method significantly promotes the desired crystal orientation, passivates defects, and mitigates photo-induced halide phase segregation in perovskite films, leading to substantially reduced nonradiative recombination, minimized VOC deficits, and enhanced operational stability of the devices. The resulting 1.66 eV bandgap methylamine-free perovskite solar cells achieve a remarkable power conversion efficiency (PCE) of 22.40% (certified at 21.97%), with the smallest VOC deficit recorded at 0.39 V. Furthermore, the fabricated semitransparent WBG devices exhibit a competitive PCE of 20.13%. Consequently, four-terminal tandem cells comprising WBG perovskite top cells and 1.25 eV bandgap perovskite bottom cells showcase an impressive PCE of 28.06% (stabilized 27.92%), demonstrating great potential for efficient multijunction tandem solar cell applications.

Large-n quasi-phase-pure two-dimensional halide perovskite: A toolbox from materials to devices.

Despite their excellent environmental stability, low defect density, and high carrier mobility, large-n quasi-two-dimensional halide perovskites (quasi-2DHPs) feature a limited application scope because of the formation of self-assembled multiple quantum wells (QWs) due to the similar thermal stabilities of large-n phases. However, large-n quasi-phase-pure 2DHPs (quasi-PP-2DHPs) can solve this problem perfectly. This review discusses the structures, formation mechanisms, and photoelectronic and physical properties of quasi-PP-2DHPs, summarises the corresponding single crystals, thin films, and heterojunction preparation methods, and presents the related advances. Moreover, we focus on applications of large-n quasi-PP-2DHPs in solar cells, photodetectors, lasers, light-emitting diodes, and field-effect transistors, discuss the challenges and prospects of these emerging photoelectronic materials, and review the potential technological developments in this area.

Surface Microstructure Engineering in MAPbBr3 Microsheets for Performance-Enhanced Photodetectors.

Metal halide-perovskite-based photodetectors have recently emerged as a class of promising optoelectronic devices in various fields. Meanwhile, nano/microstructuring perovskite-based photodetectors are a facile integration with complementary metal-oxide semiconductors for miniaturized imaging systems. However, there are still challenges to be overcome in reducing the losses caused by light reflection on the surface of microstructural perovskites. In this work, surface microstructure engineering is employed in MAPbBr3 microsheets for reducing light reflection and improving light absorption, resulting in high-performance perovskite photodetectors. MAPbBr3 microsheets, which possess different surface morphologies of flat, upright hemisphere arrays and inverted hemisphere arrays (IHAs), are fabricated by a simple microstructure template-assisted space confinement process. The light absorption capacity of IHA MAPbBr3 is significantly higher than that of the other two structures. Hence, IHA photodetectors with excellent figures of merit, including low dark current, decent responsivity, and fast speed, are achieved. Furthermore, the noise of the IHA photodetectors is only ∼10-13 A/Hz, which results in the superior sensitivity for weak light detection with a specific detectivity up to 1011 Jones. Our results demonstrate that surface engineering is a simple, low-cost, yet effective approach to improve the performance of nano-/micro-optoelectronic devices.

Aspartate all-in-one doping strategy enables efficient all-perovskite tandems.

All-perovskite tandem solar cells hold great promise in surpassing the Shockley-Queisser limit for single-junction solar cells1-3. However, the practical use of these cells is currently hampered by the subpar performance and stability issues associated with mixed tin-lead (Sn-Pb) narrow-bandgap perovskite subcells in all-perovskite tandems4-7. In this study, we focus on the narrow-bandgap subcells and develop an all-in-one doping strategy for them. We introduce aspartate hydrochloride (AspCl) into both the bottom poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) and bulk perovskite layers, followed by another AspCl posttreatment. We show that a single AspCl additive can effectively passivate defects, reduce Sn4+ impurities and shift the Fermi energy level. Additionally, the strong molecular bonding of AspCl-Sn/Pb iodide and AspCl-AspCl can strengthen the structure and thereby improve the stability of Sn-Pb perovskites. Ultimately, the implementation of AspCl doping in Sn-Pb perovskite solar cells yielded power conversion efficiencies of 22.46% for single-junction cells and 27.84% (27.62% stabilized and 27.34% certified) for tandems with 95% retention after being stored in an N2-filled glovebox for 2,000 h. These results suggest that all-in-one AspCl doping is a favourable strategy for enhancing the efficiency and stability of single-junction Sn-Pb perovskite solar cells and their tandems.

Bifunctional Interface Passivation via Copper Acetylacetonate for Efficient and Stable Perovskite Solar Cells.

Manipulating interface defects can minimize interfacial nonradiative recombination, thus increasing the stability and performance of perovskite solar cells (PSCs). Here, copper acetylacetonate [Cu(acac)2] as a passivator is used to treat the interface between Spiro-OMeTAD and perovskite. Owing to the strong chelation, the uncoordinated Pb2+ could react with -C═O/-COH functional groups, firmly anchoring acetylacetonate at this interface or the grain boundaries (GBs) of perovskite films to construct multiple ligand bridges, accompanied by the p-type copper iodide formation with copper substituting lead. Simultaneously, Cu+-Cu2+ pairs transfer electrons from Pb0 to I0, suppressing deep level defects of Pb0 and I0 near the perovskite interface. These can be beneficial to hole-transferring. Moreover, the Schiff base complexes with hydrophobicity, from the reaction of acetylacetonate with perovskite, can lead to tightly packed adjacent perovskite surfaces and self-seal the GBs of the perovskite, inhibiting moisture diffusion for long-term stability. Consequently, the Cu(acac)2-based PSC has achieved more than 24% champion efficiency while retaining ca. 92% of the initial power conversion efficiency after 1680 h of storage.

Charge-Carrier Dynamics of Evaporated Bismuth-Based Chalcogenide Thin Films Probed with Time-Resolved Microwave Conductivity.

Chalcogenide-based semiconductors are emerging as a set of highly promising candidates for optoelectronic devices, owing to their low toxicity, cost-effectiveness, exceptional stability, and tunable optoelectronic properties. Nonetheless, the limited understanding of charge recombination mechanisms and trap states of these materials is impeding their further development. To fill this gap, we conducted a comprehensive study of bismuth-based chalcogenide thin films and systematically investigated the influence of post-treatments via time-resolved microwave conductivity and temperature-dependent photoluminescence. The key finding in this work is that post-treatment with Bi could effectively enhance the crystallinity and charge-carrier mobility. However, the carrier density also increased significantly after the Bi treatment. On the contrary, post-treatment of evaporated Bi2S3 thin films with sulfur could effectively increase the carrier lifetime and mobility by passivating the trap states on the grain boundaries, which is also consistent with the enhanced radiative recombination efficiency.

A generic lanthanum doping strategy enabling efficient lead halide perovskite luminescence for backlights.

Metal halide perovskites films have attracted great interest because of their solution-proceed fabrication and potential applications for next-generation displays. However, their non-ideal photoluminescence quantum efficiency (PLQE) and stability still does not meet the requirements of displays. Here, we adopt lanthanum ion (La3+) doping strategy to enhance its luminescence performance and the possibility of taking it to commercialization. In addition to the entry of lanthanum ions into the lattice to greatly improve the crystal quality, the excess La3+ gives rise to the formation of newly developed formamidinium cesium lanthanum bromine, (FA, Cs)2LaBr5, which provides additional energy transport pathways, therefore concentrating more energy onto the perovskite host. Consequently, a near-unity PLQE of 99.5% is realized in standardized green-emission cesium (Cs)/formamidinium (FA) mixed FA0.7Cs0.3PbBr3 perovskite films. The introduction of La3+ can also lead to markedly stability with nearly 1000 days' shelf storage half-lifetime and 400 hours' light-irradiation T90 lifetime as well as much-enhanced color purity. Moreover, the films with La3+ doping enable full-visible spectral enhancement and high-performance white light emission and trichromatic luminescence with 98.9% coverage of the color gamut area required for the Rec. 2020 standard, which suggest that perovskite films have great application potential for backlights in liquid crystal display technologies.

A Multifunctional Additive Strategy Enables Efficient Pure-Blue Perovskite Light-Emitting Diodes.

Lead halide perovskites have shown exceptional performance in light-emitting devices (PeLEDs), particularly in producing significant electroluminescence in sky-blue to near-infrared wavelengths. However, PeLEDs emitting pure-blue light at 465-475 nm are still not satisfactory. Herein, efficient and stable pure-blue PeLEDs are reported by controlling phase distribution, passivation of defects, as well as surface modifications using multifunctional phenylethylammonium trifluoroacetate (PEATFA) in reduced-dimensional p-F-PEA2 Csn-1 Pbn (Br0.55 Cl0.45 )3n+1 polycrystalline perovskite films. Compared with 4-fluorophenylethylammonium (p-F-PEA+ ) in the pristine films, phenylethylammonium (PEA+ ) has lower adsorption energy while interacting with perovskites, resulting in large-n low-dimensional perovskites, which can greatly facilitate charge transport within the low-dimensional perovskite films. The interaction between the CO group in trifluoroacetate (TFA- ) and perovskites significantly reduces defects in the perovskite films. Additionally, the electron-giving CF3 group in TFA- uplifts surface potential in the films, resulting in smooth electronic injection in devices. The multifunctional additive strategy leads to elevated radiative recombination and efficient carrier transport in the films and devices. As a result, the devices exhibit a maximum external quantum efficiency (EQE) of 11.87% at 468 nm with stable spectral output, the highest reported to date for pure-blue PeLEDs. Thus, this study extends the way for high-efficiency pure-blue LED with perovskite polycrystal films.

Efficient Narrow-Bandgap Mixed Tin-Lead Perovskite Solar Cells via Natural Tin Oxide Doping.

Narrow-bandgap (NBG) mixed tin/lead-based (Sn-Pb) perovskite solar cells (PSCs) have attracted extensive attention for use in tandem solar cells. However, they are still plagued by serious carrier recombination due to inferior film properties resulting from the alloying of Sn with Pb elements, which leads to p-type self-doping behaviors. This work reports an effective tin oxide (SnOx ) doping strategy to produce high-quality Sn-Pb perovskite films for utilization in efficient single-junction and tandem PSCs. SnOx can be naturally oxidized from tin diiodide raw powders and successfully incorporated into Sn-Pb perovskite films. Consequently, Sn-Pb perovskite films doped with SnOx exhibit dramatically improved morphology, crystallization, absorption, and more interestingly, upward-shifted Fermi levels. The resulting narrow-bandgap Sn-Pb PSCs with natural SnOx doping have considerably reduced carrier recombination, therefore delivering a maximum power conversion efficiency (PCE) of 22.16% for single-junction cells and a remarkable PCE of 26.01% (with a steady-state efficiency of 25.33%) for two-terminal all-perovskite tandem cells. This work introduces a facile doping strategy for the manufacture of efficient single-junction narrow-bandgap PSCs and their tandem solar cells.

Antimony Potassium Tartrate Stabilizes Wide-Bandgap Perovskites for Inverted 4-T All-Perovskite Tandem Solar Cells with Efficiencies over 26.

Wide-bandgap (WBG) perovskites have been attracting much attention because of their immense potential as a front light-absorber for tandem solar cells. However, WBG perovskite solar cells (PSCs) generally exhibit undesired large open-circuit voltage (VOC) loss due to light-induced phase segregation and severe non-radiative recombination loss. Herein, antimony potassium tartrate (APTA) is added to perovskite precursor as a multifunctional additive that not only coordinates with unbonded lead but also inhibits the migration of halogen in perovskite, which results in suppressed non-radiative recombination, inhibited phase segregation and better band energy alignment. Therefore, a APTA auxiliary WBG PSC with a champion photoelectric conversion efficiency of 20.35% and less hysteresis is presented. They maintain 80% of their initial efficiencies under 100 mW cm-2 white light illumination in nitrogen after 1,000 h. Furthermore, by combining a semi-transparent WBG perovskite front cell with a narrow-bandgap tin-lead PSC, a perovskite/perovskite four-terminal tandem solar cell with an efficiency over 26% is achieved. Our work provides a feasible approach for the fabrication of efficient tandem solar cells.

Intermediate Phase Engineering with 2,2-Azodi(2-Methylbutyronitrile) for Efficient and Stable Perovskite Solar Cells.

Sequential deposition has been widely employed to modulate the crystallization of perovskite solar cells because it can avoid the formation of nucleation centers and even initial crystallization in the precursor solution. However, challenges remain in overcoming the incomplete and random transformation of PbI2 films with organic ammonium salts. Herein, a unique intermediate phase engineering strategy has been developed by simultaneously introducing 2,2-azodi(2-methylbutyronitrile) (AMBN) to both PbI2 and ammonium salt solutions to regulate perovskite crystallization. AMBN not only coordinates with PbI2 to form a favorably mesoporous PbI2 film due to the coordination between Pb2+ and the cyano group (C≡N), but also suppresses the vigorous activity of FA+ ions by interacting with FAI, leading to the full PbI2 transformation with the preferred orientation. Therefore, perovskites with favorable facet orientations are obtained, and the defects are largely suppressed owing to the passivation of uncoordinated Pb2+ and FA+ . As a result, a champion power conversion efficiency over 25% with a stabilized efficiency of 24.8% is achieved. Moreover, the device exhibits an improved operational stability, retaining 96% of initial power conversion efficiency under 1000 h continuous white-light illumination with an intensity of 100 mW cm-2 at ≈55 °C in N2 atmosphere.