Abstract:
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_0.5I_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_0.5I_0.5)₃ films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br_0.5I_0.5)₃ is considered to be an important step in inducing halide mobility in photoirradiated films. 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.

Dr. Junsang Cho and Dr. Manjeet Chhetri's time at Notre Dame has sadly come to an end, but they are off new opportunities! Jun will be starting a faculty position at Duksung Women's University in Seoul, and Manjeet will be doing a postdoc at Clemson. We wish them all the luck in their science adventures!

Abstract:
Halide ion mobility in metal halide perovskites plays an important role in dictating the overall device performance and long-term stability of perovskite solar cells. Alloying with chloride (Cl), which is known to stabilize the perovskite solar cells, has now been found to suppress the photoinduced halide ion segregation in mixed halide (Br/I) perovskites. By varying the chloride concentration of 1–10% (as part of the halide composition), we have probed both photoinduced segregation and dark recovery kinetics at different temperatures. When we increased the concentration of Cl from 0 to 5%, we observed a decrease in the rate constant of segregation by a factor of ∼5 and a decrease in the fraction of halide segregation from 45 to 20%. The activation energy for photoinduced halide segregation increases (∼4 kJ/mol) upon the introduction of chloride into the mixed halide film, reflecting an increased energetic barrier for halide ion migration.

Abstract:
Visible light irradiation of a 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 a concurrent shift in the band edge to lower wavelengths. We have now employed mixed halide perovskites to uncover the influence of A-site cation [methylammonium (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 greater than 50%, the films become stable as the iodide expulsion is largely suppressed. The role of the A-site cation in dictating the mobility of halide ions is discussed.

Abstract:
Two-dimensional (2D) lead halide perovskites represent an emerging class of materials given their tunable optoelectronic properties and long-term stability in perovskite solar cells. In order to assess the halide ion mobility, we have tracked the changes in the bromide and iodide composition in physically paired 2D lead halide perovskite films of different layer numbers (n = 10–1). These low-dimensional perovskites suppressed halide ion migration as a result of their intercalated spacer ligands and their strong van der Waals interactions. The rate constants for halide exchange of low dimensionality perovskites follow the Arrhenius relationship with thermal activation energy ranging from 58 kJ/mol (n = 10) to 72 kJ/mol (n = 1). The suppression of halide ion mobility (and diffusion coefficient) with modulating perovskite layer number (n) provides further insight into the role of 2D perovskites in improving the performance of photovoltaic devices.

Abstract for Preethi's presentation: Visible light irradiation of the mixed halide perovskite film in contact with a solvent in which the film is otherwise stable leads to selective expulsion of iodide from the film with a concurrent shift in the band edge to lower wavelengths. Expulsion of iodide into solution can be tracked by the formation of I₃^- species in the solvent, allowing for calculation of a quantum yield for iodide expulsion process. In the absence of solvent contact the mixed halide perovskite films undergo photoinduced segregation. The rate of iodide expulsion in solvent is strongly dependent on the rate of photoinduced segregation.

Abstract for Jeff's presentation: In metal halide perovskite solar cells, electron transport layers (ETLs) such as TiO₂ dictate the overall photovoltaic performance. However, the same electron capture property of ETL indirectly impacts halide ion mobility as evident from the TiO₂-assisted halide ion segregation in mixed halide perovskite (MHP) films under pulsed laser excitation (387 nm, 500 Hz). This segregation is only observed when deposited on an ETL such as TiO₂ but not on insulating ZrO₂ substrate. Injection of electrons from excited MHP into the ETL (ket = 10¹¹ s^–1) followed by scavenging of electrons by O₂ causes hole accumulation in the MHP film. Localization of holes on the iodide site in the MHP induces instability causing iodide from the lattice to move away toward grain boundaries. Suppression of segregation occurs when holes are extracted by a hole transport layer (spiro-OMeTAD) deposited on the MHP, thus avoiding hole build-up. These results provide further insight into the role of holes in the phase segregation of MHPs and hole mobility in perovskite solar cells.

Abstract:
Alloyed lead halide perovskites have taken a dominant role in the quest for third-generation solar cells. This is due to optimal light-harvesting properties, which can be tuned across the visible spectrum by mixing halide (X = Cl^–, Br^–, and I^–) anions and A⁺ cations (A⁺ = FA⁺, MA⁺, and Cs⁺). Durability issues related to ion movement within the perovskite lattice, however, impede large-scale commercialization. Uniformly mixed halide perovskites [e.g., APb(I_1–xBr_x)₃] reversibly segregate into narrow bandgap I-rich and wide bandgap Br-rich domains during continuous visible illumination. Subsequent I-rich domains reduce local open circuit voltages and decrease mixed halide perovskite solar cell power conversion efficiencies. In this review, we assess the known effects of halide segregation on the structural and optical properties of mixed halide materials, discuss ongoing research to suppress the phenomenon, and provide a mechanistic overview of its underlying origins.

Abstract:
In metal halide perovskite solar cells, electron transport layers (ETLs) such as TiO₂ dictate the overall photovoltaic performance. However, the same electron capture property of ETL indirectly impacts halide ion mobility as evident from the TiO₂-assisted halide ion segregation in mixed halide perovskite (MHP) films under pulsed laser excitation (387 nm, 500 Hz). This segregation is only observed when deposited on an ETL such as TiO₂ but not on insulating ZrO₂ substrate. Injection of electrons from excited MHP into the ETL (k_et = 10¹¹ s^–1) followed by scavenging of electrons by O₂ causes hole accumulation in the MHP film. Localization of holes on the iodide site in the MHP induces instability causing iodide from the lattice to move away toward grain boundaries. Suppression of segregation occurs when holes are extracted by a hole transport layer (spiro-OMeTAD) deposited on the MHP, thus avoiding hole build-up. These results provide further insight into the role of holes in the phase segregation of MHPs and hole mobility in perovskite solar cells.

Abstract:
We celebrate the contribution of female energy researchers who have published new advances from their laboratories in ACS Energy Letters. In order to inspire other scientists working in the field, we asked them to comment on their inspiration to engage in energy research, discuss an “aha” moment in research, and/or provide advice to newcomers in the field. These personal stories, collected from early career researchers to well-established senior scientists, span the successful career paths they have taken to become leaders in the community. It is our hope that these personal reflections can motivate many young researchers to tackle challenges in clean energy. In this two-part series, we compile papers published by women researchers in ACS Energy Letters along with their personal stories. This virtual issue is a compilation of one representative paper from each of these scientists. We would like to thank Stacey F. Bent, Sharon Hammes-Schiffer, Shelley D. Minteer, Eline M. Hutter, Aleksandra Vojvodic, Stephanie L. Wunder, Emily A. Carter, Emily A. Weiss, Esther S. Takeuchi, Jennifer M. Pringle, Anita W. Y. Ho-Baillie, R. Geetha Balakrishna, Annamaria Petrozza, Christy F. Landes, Hemamala I. Karunadasa, Laura M. Herz, and Lisa M. Utschig for their contributions to this virtual issue.

Abstract:
The strong binding between CsPbBr₃ nanocrystals and methyl viologen induces a long-lived charge-separated state following band gap excitation with important implications in photocatalytic processes. The unusually long-lived bleaching of the CsPbBr₃ excitonic peak in this case arises from the creation of a dipole with the hole residing in CsPbBr₃ and the electron in the surface-bound methyl viologen moiety.

Abstract:
The halide ion mobility in mixed halide perovskite exhibits two opposite trends in response to photo and thermal activation. While halides prefer to remain as Br-rich and I-rich domains under steady-state light irradiation of MAPbBr_1.5I_1.5 films, they prefer to remain in their stable mixed composition when kept in the dark. The activation energies as determined from the temperature-dependent rate constants are E_a,forward = 28.9 kJ mol^–1 for photoinduced segregation and E_a,reverse = 53.5 kJ mol^–1 for remixing of halides in the dark, respectively. The energy input from photoexcitation assists overcoming the dark (thermally activated) mixing to induce Br-rich and I-rich domains. This segregated state is maintained as long as the mixed halide film is irradiated continuously with visible light. The excitation intensity threshold above which segregation occurs follows a linear temperature dependence, such that phase separation occurs above I_exc = 30 μW/cm² white light at 23 °C. The threshold at 90 °C becomes higher with a minimum intensity requirement of 100 μW/cm² to induce segregation. The thermodynamic rationale behind this unusual halide mobility under photo and thermal excitation discussed here can aid in understanding the stability issues of perovskite solar cells.

Abstract:
Cesium lead halide perovskite films with a systematic change in the halide composition of CsPbBr_3−xI_x, in which iodide concentration varies from x = 0 to x = 3, provide a built-in gradient band structure. Such a gradient structure allows for the integrated capture of visible photons and directs them to the energetically low-lying iodide rich region. Annealing gradient halide perovskite films at temperatures ranging from 50 °C to 90 °C causes the films to homogenize into mixed halide perovskites. The movement of halide ions during the homogenization process was elucidated using UV-Visible absorbance and X-ray photoelectron spectroscopy. The halide ion movement in CsPbBr_3−xI_x gradient films was tracked via absorbance changes in the visible region of the spectrum that enabled us to measure the temperature dependent rate constant and energy of activation (74.5 kJ/mol) of halide ion homogenization. Excited state processes of both gradient and homogenized films probed through transient absorption spectroscopy showed the direct flow of charge carriers and charge recombination in both films.

Abstract:
Interfacial charge transfer between a semiconductor nanocrystal and a molecular relay is an important step in nanomaterial photocatalysis. The ferrocene redox couple (Fc⁺/Fc⁰, E⁰ = −4.9 eV vs vacuum) has now been used as a model redox relay system to investigate photocatalytic properties of CsPbBr₃ perovskite nanocrystals. The photocatalytic reduction of ferrocenium (Fc⁺) to ferrocene (Fc⁰) with CsPbBr₃ nanocrystals was dictated by the surface interactions. Whereas a rapid quenching and subsequent recovery of CsPbBr₃ emission is seen at low Fc⁺ concentrations, the quenched emission was sustained at higher Fc⁺ concentrations. The photoinduced interfacial electron transfer between CsPbBr₃ and ferrocenium (Fc⁺) studied using transient absorption spectroscopy occurred with a rate constant of 1.64 × 10¹⁰ s^–1. Better understanding of interfacial processes using redox probes can lead to the improvement in photocatalytic performance of perovskite nanocrystals.

Undergraduate researcher James Drysdale gives a video presentation for his poster entitled "Halide Composition and Temperature Dependent Properties of Perovskite Solar Cells" for the Notre Dame Summer Undergraduate Research Symposium.

Abstract:
MAPbBr₃ and MAPbI₃ films cast onto glass slides and physically paired together undergo halide exchange to form mixed halide films. The change in halide composition in these two ∼130 nm thick films occurs concurrently with Br^– diffusing toward the MAPbI₃ film and I^– diffusing toward the MAPbBr₃ film. The diffusion of these halide species, which is tracked through changes in the absorption, offers a direct measurement of thermally activated halide diffusion in perovskite films. The increase in the rate constant of halide diffusion with increasing temperature (from 8.3 × 10^–6 s^–1 at 23 °C to 3.7 × 10^–4 s^–1 at 140 °C) follows an Arrhenius relationship with activation energy of 51 kJ/mol. The thermally activated halide exchange shows the challenges of employing layers of different metal halide perovskites in stable tandem solar cells.

Rebecca Scheidt is entering her fourth year as a graduate student advised by Prof. Prashant Kamat in the Department of Chemistry and Biochemistry. She presented "Interfacial Charge Transfer between Excited CsPbBr₃ Nanocrystals and TiO₂: Charge Injection versus Photodegradation,” at the ND Energy PD&GS Luncheon in May. In 2018, Scheidt received the Patrick and Jana Eilers Graduate Student Fellowship for Energy Related Research for her project looking at cesium lead bromide perovskite nanocrystals and how they operate in a photovoltaic device stack. Because these next-generation materials absorb light so well, these nanocrystal solar cells can be placed into thin film and flexible technologies. “What I specifically look at is how they interacted with different charge transfer materials in the solar cell and whether that caused degradation under certain conditions,” Scheidt said. “In terms of making them more stable long-term, and looking at how they break down or degrade, we aim to make them more efficient overall.”

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Abstract:
The control of grain size and surface properties is an important parameter in controlling the optoelectronic and photovoltaic properties of metal halide perovskites. When CsPbBr₃ nanocrystal (10 nm in diameter) films were annealed at 100–125 °C, they grow in size to produce 400 nm diameter crystallites while transforming into bulk perovskite films. Characteristic changes in the optical properties were noted when such transformation occurred from nanocrystals into bulk. By tracking absorbance and emission spectra and morphological changes of CsPbBr₃ films at different annealing times and temperature, we were able to establish the mechanism of particle growth. The presence of nanocrystals and larger crystals during the intermediate annealing steps and narrowing size distribution confirmed the Ostwald ripening mechanism for the crystal growth. The energy of activation of crystal growth as determined from the temperature dependent optical properties was estimated to be 75 kcal/mol.

Abstract:
Owing to its high hole conductivity and ease of preparation, CuI was among the first inorganic hole-transporting materials that were introduced early on in metal halide perovskite solar cells, but its full potential as a semiconductor material is still to be realized. We have now performed ultrafast spectroelectrochemical experiments on ITO/CuI electrodes to show the effect of applied bias on the excited-state dynamics in CuI. Under operating conditions, the recombination of excitons is dependent on the applied bias, and it can be accelerated by decreasing the potential from +0.6 to −0.1 V vs Ag/AgCl. Prebiasing experiments show the persistent and reversible “memory” effect of electrochemical bias on charge carrier lifetimes. The excitation of CuI in a CuI/CsPbBr₃ film provides synergy between both CuI and CsPbBr in dictating the charge separation and recombination.

Read the latest paper from the Kamat Lab!

Transformation of Sintered CsPbI₃ Nanocrystals to Cubic CsPbI₃ and Gradient CsPbBr_xI_3-x through Halide Exchange

Abstract: All-inorganic cesium lead halide (CsPbX₃, X = Br^-, I^-) perovskites could potentially provide comparable photovoltaic performance with enhanced stability compared to organic-inorganic lead halide species. However, small-bandgap cubic CsPbI₃ has been difficult to study due to challenges forming CsPbI3 in the cubic phase. Here, a low-temperature procedure to form cubic CsPbI₃ has been developed through a halide exchange reaction using films of sintered CsPbBr₃ nanocrystals. The reaction was found to be strongly dependent upon temperature, featuring an Arrhenius relationship. Additionally, film thickness played a significant role in determining internal film structure at intermediate reaction times. Thin films (50 nm) showed only a small distribution of CsPbBr_xI_3-x species, while thicker films (350 nm) exhibited much broader distributions. Furthermore, internal film structure was ordered, featuring a compositional gradient within film. Transient absorption spectroscopy showed the influence of halide exchange on the excited state of the material. In thicker films, charge carriers were rapidly transferred to iodide-rich regions near the film surface within the first several picoseconds after excitation. This ultrafast vectorial charge-transfer process illustrates the potential of utilizing compositional gradients to direct charge flow in perovskite-based photovoltaics.

Read the latest paper from the Kamat Lab!

Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation

Abstract: Mixed halide lead perovskites (e.g., CH₃NH₃PbBr_xI_3-x) undergo phase segregation creating iodide-rich and bromide-rich domains when subjected to visible irradiation. This intriguing aspect of halide ion movement in mixed halide films is now being tracked through excited-state behavior using emission and transient absorption spectroscopy tools. These transient experiments have allowed us to establish the time scale with which such separation occurs under laser irradiation (405 nm, 25 mW/cm² to 1.7 W/cm²) as well as dark recovery. While the phase separation occurs with a rate constant of 0.1-0.3 s^-1, the recovery occurs over a time period of several minutes to an hour. The relative photoluminescence quantum yield observed for Br-rich regions (em. max 530 nm) is nearly 2 orders of magnitude lower than that of I-rich regions (em. max 760 nm) and arises from the fact that I-rich regions serve as sinks for photogenerated charge carriers. Understanding such cascading charge transfer to localized sites could further enable the design of gradient halide structures in mixed halide systems.

Read the latest paper from the Kamat Lab!

Intriguing Optoelectronic Properties of Metal Halide Perovskites

Abstract: A new chapter in the long and distinguished history of perovskites is being written with the breakthrough success of metal halide perovskites (MHPs) as solution-processed photovoltaic (PV) absorbers. The current surge in MHP research has largely arisen out of their rapid progress in PV devices; however, these materials are potentially suitable for a diverse array of optoelectronic applications. Like oxide perovskites, MHPs have ABX₃ stoichiometry, where A and B are cations and X is a halide anion. Here, the underlying physical and photophysical properties of inorganic (A = inorganic) and hybrid organic-inorganic (A = organic) MHPs are reviewed with an eye toward their potential application in emerging optoelectronic technologies. Significant attention is given to the prototypical compound methylammonium lead iodide (CH₃NH₃PbI₃) due to the preponderance of experimental and theoretical studies surrounding this material. We also discuss other salient MHP systems, including 2-dimensional compounds, where relevant. More specifically, this review is a critical account of the interrelation between MHP electronic structure, absorption, emission, carrier dynamics and transport, and other relevant photophysical processes that have propelled these materials to the forefront of modern optoelectronics research.