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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Nanoscale Photoexcited Carrier Dynamics in Perovskites

The optoelectronic properties of lead halide perovskite thin films can be tuned through compositional variations and strain, but the associated nanocrystalline structure makes it difficult to untangle the link between composition, processing conditions, and ultimately material properties and degradation. Here, we study the effect of processing conditions and degradation on the local photoconductivity dynamics in [(CsPbI 3 ) 0.05 (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 ] and (FA 0.7 Cs 0.3 PbI 3 ) perovskite thin films using temporally and spectrally resolved microwave near-field microscopy with a temporal resolution as high as 5 ns and a spatial resolution better than 50 nm. For the latter FACs formulation, we find a clear effect of the process annealing temperature on film morphology, stability, and spatial photoconductivity distribution. After exposure of samples to ambient conditions and illumination, we find spectral evidence of halide segregation-induced degradation below the instrument resolution limit for the mixed halide formulation, while we find a clear spatially inhomogeneous increase in the carrier lifetime for the FACs formulation annealed at 180 degrees C.

14 SOLAR ENERGY↗
DOE OSTI2025

Phase segregation dynamics in mixed-halide perovskites revealed by plunge-freeze cryo-electron microscopy

Mixed-halide lead perovskites, with photoexcited charge-carrier properties suitable for high-efficiency photovoltaics, hold significant promise for high-efficiency tandem solar cells. However, phase segregation under illumination, where an iodide-rich phase forms carrier trap states, remains a barrier to application. This study employs plunge-freeze cryoelectron microscopy to visualize nanoscale phase segregation dynamics in CsPb(Br x I 1–x ) 3 films. By rapidly freezing the illuminated samples, we preserve transient photoexcited ion distributions for high-resolution structural and compositional analysis at the nanoscale. Cryogenic scanning transmission electron microscopy (STEM) techniques (electron energy loss spectroscopy [EELS] and 4D-STEM) captured the dynamics of photo-induced iodine migration from grain boundaries to centers, identified the buildup of anisotropic strain, and captured the heterogeneous evolution of this process within a single grain. These findings provide insight into microscopic phase segregation mechanisms and their dynamics, enhancing our understanding of mixed-halide perovskite photostability.

42 ENGINEERING↗
DOE OSTI2023

Sustainable materials acceleration platform reveals stable and efficient wide-bandgap metal halide perovskite alloys

The vast chemical space of emerging semiconductors, like metal halide perovskites, and their varied requirements for semiconductor applications have rendered trial-and-error environmentally unsustainable. Here, in this work, we demonstrate RoboMapper, a materials acceleration platform (MAP), that achieves 10-fold research acceleration by formulating and palletizing semiconductors on a chip, thereby allowing high-throughput (HT) measurements to generate quantitative structure-property relationships (QSPRs) considerably more efficiently and sustainably. We leverage the RoboMapper to construct QSPR maps for the mixed ion FA 1-y Cs y Pb(I 1-x Br x ) 3 halide perovskite in terms of structure, bandgap, and photostability with respect to its composition. We identify wide-bandgap alloys suitable for perovskite-Si hybrid tandem solar cells exhibiting a pure cubic perovskite phase with favorable defect chemistry while achieving superior stability at the target bandgap of ~1.7 eV. RoboMapper’s palletization strategy reduces environmental impacts of data generation in materials research by more than an order of magnitude, paving the way for sustainable data-driven materials research.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Triple-halide wide–band gap perovskites with suppressed phase segregation for efficient tandems

Wide–band gap metal halide perovskites are promising semiconductors to pair with silicon in tandem solar cells to pursue the goal of achieving power conversion efficiency (PCE) greater than 30% at low cost. However, wide–band gap perovskite solar cells have been fundamentally limited by photoinduced phase segregation and low open-circuit voltage. Here, we report efficient 1.67–electron volt wide–band gap perovskite top cells using triple-halide alloys (chlorine, bromine, iodine) to tailor the band gap and stabilize the semiconductor under illumination. We show a factor of 2 increase in photocarrier lifetime and charge-carrier mobility that resulted from enhancing the solubility of chlorine by replacing some of the iodine with bromine to shrink the lattice parameter. We observed a suppression of light-induced phase segregation in films even at 100-sun illumination intensity and less than 4% degradation in semitransparent top cells after 1000 hours of maximum power point (MPP) operation at 60°C. By integrating these top cells with silicon bottom cells, we achieved a PCE of 27% in two-terminal monolithic tandems with an area of 1 square centimeter.

14 SOLAR ENERGY↗
DOE OSTI2021

Exploring Responses of Contact Kelvin Probe Force Microscopy in Triple-Cation Double-Halide Perovskites

Hybrid organic–inorganic perovskite (HOIP) solar cells have undergone unprecedentedly rapid improvement in power conversion efficiencies during the last decade, but controversy remains regarding the role of local phenomena and heterogeneity in grain by grain. These challenges in turn necessitate understanding the fundamental mechanisms operating at individual grains and exploring how these mechanisms give rise to the collective behaviors of the films. Although numerous efforts have targeted local phenomena in these materials using classical scanning probe microscopy modalities, the measurements remain difficult to interpret due to intrinsic coupling between possible ferroelectric, ion migration, bulk and surface electrochemical phenomena, and the effects of light on these behaviors. Here, we explore the electromechanical responses in (Cs 0.05 FA 0.85 MA 0.15 )Pb(I 0.85 Br 0.15 ) 3 perovskite thin film in the dark and under illumination using band excitation-piezoresponse force microscopy (BE-PFM). To identify and separate ferroic, electrostatic, and electrochemical responses, we utilize contact-Kelvin probe force microscopy (cKPFM). The cKPFM responses show weak variability with morphological features in the dark, while such variability significantly increases under illumination. Deconvolution of the cKPFM responses via principal component analysis (PCA) shows heterogeneity in grain to grain possibly due to variation in migration and segregation of halide ions in mixed perovskites at specific grains. Further, we propose an electrostatic mechanism based on the physics of semiconductors, which can directly link the observed cKPFM behaviors with the ionic and electronic density of states (DOS) and their time dynamics. Here, these findings suggest that cKPFM is a powerful probe of local ionic behaviors on the nanoscale, both in HOIPs and other materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Multifunctional Conjugated Ligand Engineering for Stable and Efficient Perovskite Solar Cells

Surface passivation is an effective way to boost the efficiency and stability of perovskite solar cells (PSCs). However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination without imposing energy barriers to charge extraction. Here, a novel multifunctional semiconducting organic ammonium cationic interface modifier inserted between the light-harvesting perovskite film and the hole-transporting layer is reported. It is shown that the conjugated cations can directly extract holes from perovskite efficiently, and simultaneously reduce interface non-radiative recombination. Together with improved energy level alignment and the stabilized interface in the device, a triple-cation mixed-halide medium-bandgap PSC with an excellent power conversion efficiency of 22.06% (improved from 19.94%) and suppressed ion migration and halide phase segregation, which lead to a long-term operational stability, is demonstrated. Finally, this strategy provides a new practical method of interface engineering in PSCs toward improved efficiency and stability.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Stable and Efficient Methylammonium–, Cesium–, and Bromide–Free Perovskite Solar Cells by In–Situ Interlayer Formation

The vast majority of high–performance perovskite solar cells (PSCs) are based on multi–cation mixed–anion compositions incorporating methylammonium (MA) and bromide (Br). Nevertheless, the thermal instability of MA and the tendency of mixed halide compositions to phase segregate limit the long–term stability of PSCs. However, reports of MA–free and/or Br–free compositions are rare in the community since their performance is generally inferior. Here, a strategy is presented to achieve highly efficient and stable PSCs that are altogether cesium (Cs)–free, MA–free and Br–free. An antisolvent quenching process is used to in–situ deposit a polymeric interlayer to promote the growth of phase–pure formamidinium lead tri–iodide perovskite crystals with reduced defect density and to assist in photo–excited charge extraction. Here, the PSCs developed are among the best–performing reported for such compositions. Moreover, the PSCs show superior stability under continuous exposure to both illumination and 85 °C heat.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Halide Ion Migration in Perovskite Nanocrystals and Nanostructures

The optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability. Halide mobility in perovskite films is an important factor influencing solar cell performance. One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites. The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum. Interestingly, mixed halide (e.g., MAPb(Br 0.5 I 0.5 ) 3 ) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination. Upon halting illumination, segregated phases mix to restore original mixed halide compositions. Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities. Long-term irradiation of MAPb(Br 0.5 I 0.5 ) 3 films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br 0.5 I 0.5 ) 3 is considered to be an important step in inducing halide mobility in photoirradiated films. Finally, this Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation.

14 SOLAR ENERGY↗
DOE OSTI2020

Iodine (I) Expulsion at Photoirradiated Mixed Halide Perovskite Interface. Should I Stay or Should I Go?

Visible light irradiation of the mixed halide perovskite film in contact with a solvent (dichloromethane, DCM) in which the film otherwise is stable, leads to selective expulsion of iodide (I) from the film with concurrent shift in the band edge to lower wavelengths. Here we have now employed mixed halide perovskites to uncover the influence of A-site cation (Methyl ammonium, MA and Cesium, Cs) on the mobility of iodide ions under photoirradiation. In the absence of solvent contact, the mixed halide perovskite films undergo photoinduced segregation with a rate constant that decreases with increasing Cs content. Interestingly, the iodide expulsion rate in DCM is strongly dependent on the rate of photoinduced segregation. At Cs atomic concentrations higher than 50%, the films become stable as the iodide expulsion is largely suppressed. The role of A-site cation in dictating the mobility of halide ions is discussed.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2020

A Review and Perspective on Cathodoluminescence Analysis of Halide Perovskites

Halide perovskite solar cells have achieved a certified efficiency of 25.2%, surpassing CdTe and CuInGaSe 2 , which have long been regarded as the most-efficient thin-film photovoltaic materials. As this exciting class of materials continues to mature, researchers will require characterization techniques capable of exposing the interplay among structure, chemistry, and optoelectronic properties to inform processing strategies and increase both device efficiencies and long-term stability. Cathodoluminescence microscopy is an ideal technique to provide such information due to the high spatial resolution and robust optical information acquired. Here, the current body of work related to cathodoluminescence analysis of halide perovskite materials for optoelectronic applications is surveyed. This review demonstrates how cathodoluminescence can monitor degradation due to environmental stressors, phase segregation resulting from material processing, and other halide perovskite-centric material issues. A persistent concern associated with e-beam-based analysis of halide perovskites is what effect the electron beam has on the material properties being probed. Addressing this, a detailed discussion is provided on the origin of the cathodoluminescence signal and a review of studies focused on revealing changes in the properties of halide perovskites resulting from e-beam excitation. Finally, a perspective on future opportunities to expand the role of cathodoluminescence analysis for halide perovskites is provided.

14 SOLAR ENERGY↗
DOE OSTI2022

Excitation Intensity- and Size-Dependent Halide Photosegregation in CsPb(I 0.5 Br 0.5 ) 3 Perovskite Nanocrystals

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. In this work, we quantitatively compare experimental, CsPb(I 0.5 Br 0.5 ) 3 nanocrystal (NC) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate local photogenerated carrier densities. We observe that mixed-anion perovskite NCs exhibit significant robustness to photosegregation, with photosegregation propensity decreasing with decreasing NC size. Observed size- and excitation intensity-dependent photosegregation data agree with model predicted size- and excitation intensity-dependent terminal halide stoichiometries. Established correspondence between experiment and theory, in turn, suggests that mixed-halide perovskite photostabilities can be predicted a priori using local gradients of (empirical) Vegard’s law expressions of composition-dependent band gaps.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Photoinduced iodide repulsion and halides-demixing in layered perovskites

Mixing halides in metal halide perovskites (MHPs) is an effective approach for adjusting the MHPs bandgap for applications in tandem solar cells. However, mixed-halide (MH-) MHPs undergo light-induced phase segregation (LIPS) under continuous illumination. Therefore, understanding the mechanism of LIPS is necessary for developing stable MH-MHPs. Here, we investigated LIPS in layered (L) MHPs and discovered a critical role of spacer cations in LIPS. Through probing chemical changes of LIPS, we unveil light-induced iodide repulsion and the formation of Br-rich-phase in illuminated regions during LIPS. This discovery also gives insight into the LIPS process in three-dimensional (3D) MHPs. By further investigating LIPS in 3D MHPs, we reveal that LIPS induces not only the formation of Br-rich and I-rich domains but also an overall change of halide distribution along the film thickness direction. Furthermore, LIPS is more significant in bulk due to a larger population of photogenerated charge carriers. Overall, this study reveals the chemical mechanism of LIPS in MHPs and its potential effect on device performance, offering insight into understanding the LIPS mechanism and improving the stability of MHPs.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Phase Segregation and Sequential Expulsion of Iodide and Bromide in Photoirradiated Ruddlesden–Popper 2D Perovskite Films

Two-dimensional (2D) Ruddlesden–Popper mixed-halide perovskite films, BA 2 PbBr 2 I 2 , undergo phase segregation when excited with visible light to generate bromide- and iodide-rich regions, as marked by absorption and emission changes. Upon stopping illumination, the process reverses, allowing original film compositions to be restored. However, if films are in contact with dichloromethane, light irradiation causes the sequential expulsion of iodide and bromide and introduces irreversible changes to the 2D films. The sequential disappearance of I– and Br– from pristine films (BA 2 Pb 2 Br 4 and BA 2 Pb 2 I 4 ) under photoirradiation, as observed from variances in expulsion rates, reflects differences in halide ion mobilities in these films. The photoinstability of 2D films raises questions about their use in stabilizing bulk, three-dimensional (3D) perovskite solar cells through 3D/2D interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Photophysical Properties and Phase Behavior of Ultrawide Photovoltaic Bandgap Cesium–Lead-Based Triple Halide Perovskites

Metal halide perovskite films in the top cell of triple-junction tandems require bandgaps around 2.0 eV to achieve current matching, assuming that the middle absorbing layer is the commonly used FAPbI 3 composition and the bottom cell has a bandgap around 1.1 eV. Unfortunately, mixed organic/inorganic metal halide perovskites that have the necessary Br content to reach a bandgap of 2.0 eV segregate into iodine-rich and bromine-rich phases under illumination, limiting their obtainable voltage. Previous reports have shown improved photostability using either Cs-based inorganic compositions or Cl incorporation on the X-site. Here, we investigate the inorganic triple halide compositional space CsPb­(I 1–x–y Br y Cl x ) 3 where bandgaps near 2.0 eV are expected based on the knowledge that CsPbI2Br has a bandgap of 1.90 eV. Incorporation of Cl occurs readily for x ≤ 0.07–0.10 within perovskites with a Br content of 0.3 ≤ y ≤ 0.42. When x >0.1, X-ray diffraction and photoluminescence (PL) measurements indicate that multiple compositional phases form. We hypothesize that the variable sizes of the three halide ions are not supported within the rigid Cs lattice, resulting in the formation of multiple compositional phases. The photoluminescence quantum yield of the single-phase compositional space–CsPb­(I 1–x–y Br y Cl x ) 3 where x ≤ 0.07was typically 0.001–0.004%, most likely as a result of a high defect density, including mobile iodine species. PL light-soaking measurements of many perovskite compositions with bandgaps in the range of 1.89–2.05 eV demonstrate that phase segregation occurs when initial bandgaps are above 1.95 eV regardless of halide content: indicating further iodide oxidation and corresponding migration under illumination. The conclusion is that further compositional or additive engineering is necessary for the development of inorganic triple halide compositions that accomplish the elusive goal of fabricating high-quality and photostable 2.0 eV films for use in multijunction tandems.

Electrical conductivity↗
DOE OSTI2023

Enhanced Photostability of “Hollow” Mixed Halide Wide-Bandgap Perovskite Films

We investigated the role of hollow perovskite architectures in enhancing the photostability of mixed halide wide-bandgap perovskites. We focused on mitigating photoluminescence (PL) peak shifts caused by phase segregation when exposed to light. By analyzing the optical and structural properties of mixed bromide/iodide hollow perovskite thin films, we observed that the incorporation of hollow structures reduced the ionic conductivity in the films, leading to improved photostability compared to non-hollow perovskite samples. The mixed halide hollow perovskite thin films exhibited increased the bandgap. High-power laser irradiation was used to induce phase segregation, and changes in the PL emission spectra were measured as a function of irradiation time. The mixed halide hollow perovskite thin films exhibited reduced PL peak shifts compared to the control samples. The inclusion of enI 2 (en = ethylenediamine) resulted in a reduction in the overall ionic conductivity of the films and a lower trap density. Hollow perovskite films incorporated in solar cells indicated that while the initial efficiency of the solar cells decreased with increasing enI 2 concentration, the open-circuit voltage value increased, potentially due to the slight enhancement of the band gap. Furthermore, the findings highlight the potential of hollow perovskite architectures in enhancing the photostability of mixed halide perovskites

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗