Methylammonium Lead Iodide Perovskite Films Research Articles

Defect passivation of organometal halide perovskite solar cells using low-cost green crystalline nanocellulose

Perovskite solar cells (PSCs) have made significant progress in recent years. However, the presence of defects in the photoactive absorption layer has become a limiting factor in the efficiency of PSCs. In this research, methylammonium lead iodide perovskite films with improved quality and reduced defect density were obtained with the addition of crystalline nanocellulose (CNC) biopolymer. CNC produced from biomass contains many hydroxyl groups that were able to passivate the defects in perovskite, increase grain size, and reduce recombination. The PSC made with CNC modification achieved an efficiency of 9.22 %, higher than similar PSC without the addition of CNC that produced an efficiency of 7.28 %. This work highlights the passivating impact of low-cost green cellulose for enhancing the electrical performance of PSC.

(Invited) Plasma Processes on Metal Halide Perovskite Interfaces for Photovoltaic Applications

Metal Halide Perovskite (MHP) semiconductors are currently standing out for their exceptional optoelectronic properties and, particularly, for their exploitation in photovoltaics. Their structure can be described by the formula , where A is usually an organic cation, such as methylammonium (MA) or formamidinium (FA), B is a metal cation and X is a halogen anion, typically I or Br.The exceptional properties of MHPs derive by their hybrid organic-inorganic nature, which also allows for low-cost fabrication processing. The raise of perovskite photovoltaics 1 followed progresses on three main research fronts: i) material deposition optimization, ii) material compositional tuning and iii) device interface engineering. The interfaces play a fundamental role in the device function affecting charge extraction, recombination processes and material/device overall stability. Therefore, to further improve the performances of these devices, many surface processes have been applied to solar cells interfaces, most of which include a solution-based methodology 2. The aim of these treatments is not only to improve solar cells efficiency in terms of carrier concentration and transport properties, but also to improve the device stability under working conditions, which is one of the main issues of these materials.Among the different surface treatments exploitable, the use of plasma represents a solvent-free and non-invasive promising strategy to boost MHP solar cells performances. Plasma-deposited coatings on perovskite, as fluorocarbon polymers, have shown to improve material resistance to humidity and photoluminescence properties 3.We have explored the effect of low-pressure plasmas fed with different gases, namely Ar, , and , on Metylammonium Lead Iodide surface4. An interesting improvement of perovskite photoluminescence and solar cell efficiency was observed for Ar and plasma treatments, ascribable both to the removal of organic components, proven to be beneficial to device performances 5, and to other chemical and morphological modifications depending on the gas used. Starting from these results, new plasma surface treatments, plasma-assisted deposition and encapsulation processes will be object of study of future research, to achieve a more complete understanding of the interfacial defects and charge carrier dynamics and to further minimize performance losses and instability issues. References NREL Best Research-Cell Efficiency Chart. https://www.nrel.gov/pv/cell-efficiency.html Han TH, Tan S, Xue J, Meng L, Lee JW, Yang Y. Interface and Defect Engineering for Metal Halide Perovskite Optoelectronic Devices. Advanced Materials. 2019;31(47). doi:10.1002/adma.201803515 Armenise V, Colella S, Milella A, Palumbo F, Fracassi F, Listorti A. Plasma-Deposited Fluorocarbon Coatings on Methylammonium Lead Iodide Perovskite Films. Energies (Basel). 2022;15(13):4512. doi:10.3390/en15134512 Andrea Listorti, Sara Covella, Alberto Perrotta, et al. A study on plasma-assisted modifications of Methylammonium Lead Iodide Perovskite surfaces for photovoltaic applications. Xiao X, Bao C, Fang Y, et al. Argon Plasma Treatment to Tune Perovskite Surface Composition for High Efficiency Solar Cells and Fast Photodetectors. Advanced Materials. 2018;30(9):1-7. doi:10.1002/adma.201705176 Figure 1

Acetylammonium chloride as an additive for crystallization control and defect passivation in MAPbI3 based perovskite solar cells

Uniform and high-electronic-quality perovskite thin films are necessary for high efficiency perovskite solar cells (PSCs) and additives play a major role in improving the quality of the perovskite films. Here, we demonstrate acetylammonium chloride (AAC) as an additive to effectively control the morphology and crystal quality of the methylammonium lead iodide perovskite film. AAC incorporated PSCs have shown an improved power conversion efficiency (PCE) of 15.9% compared to 14.9% of the control device. Furthermore, AAC incorporated PSCs exhibit high operational stability by retaining 89% of the initial efficiency after 150 h of continuous operation while the control devices degrade to 51% of the initial PCE in just 45 h. The improved performance and stability of the AAC incorporated PSCs can be attributed to the (a) formation of uniform sized perovskite films of larger grains and (b) passivation of defects present in the grain boundaries and surfaces of the perovskite film. The defect density has reduced from 4.1 × 1016 to 1.97 × 1016 cm−3 on addition of AAC to the perovskite film. The reduction in defects-induced non-radiative recombination decay pathways, as further verified from impedance and capacitance–frequency measurements, has caused an enhancement in the open circuit voltage of the AAC incorporated devices and thus an improvement in the PCE and stability of the devices.

Achieving Efficient and Stable Perovskite Solar Cells in Ambient Air Through Non‐Halide Engineering

AbstractThe realization of highly efficient perovskite solar cells (PSCs) in ambient air is considered to be advantageous for low‐cost commercial manufacturing. However, it is fundamentally difficult to achieve comparable device performance to that obtained in an inert atmosphere, especially when the ambient humidity is high. Here, an effective precursor engineering that simultaneously employs non‐halide lead acetate and lead thiocyanate lead sources for fabricating high‐quality methylammonium lead iodide perovskite films in ambient air with enhanced moisture tolerance, is reported. The presence of Ac– and SCN– ions not only enables the facile formation of homogeneous and highly crystalized perovskite films, but also directs the uniform growth of the crystals along the (110) direction. Accordingly, a 20.55% efficiency is demonstrated, one of the best results for air‐processed MAPbI3 PSCs, which is also the highest value achieved with non‐halide lead sources. Furthermore, the unencapsulated device shows fivefold prolonged air stability (3600 h) compared to the conventional PbI2‐based PSC. Together with the use of non‐toxic antisolvent, this strategy is fully compatible with ambient air operation and thus of great potential for practical applications.

Single 3.3 fs multiple plate compression light source in ultrafast transient absorption spectroscopy

Ultrafast transient absorption spectroscopy is a powerful tool to reveal excited state dynamics in various materials. Conventionally, probe pulses are generated via bulk supercontinuum generation or (noncollinear) optical parametric amplifiers whilst pump pulses are generated separately using (noncollinear) optical parametric amplifiers. These systems are limited by either their spectral density, stability, spectral range, and/or temporal compressibility. Recently, a new intense broadband light source is being developed, the multi-plate compression, which promises to overcome these limitations. In this paper, we analyze the supercontinuum generated by a single Multiple Plate Compression system to set a benchmark for its use in the field of ultrafast pump-probe spectroscopy. We have compressed the supercontinuum to 3.3 fs using chirp mirrors alone, making it an excellent candidate for pump-probe experiments requiring high temporal resolution. Furthermore, the single light source can be used to generate both probe and pump pulses due to its high spectral density (>14.5 nJ/nm) between 490 and 890 nm. The intensity has an average shot-to-shot relative standard deviation of 4.6 % over 490 to 890 nm, calculated over 2,000 sequential shots. By using only 1,000 shot pairs, a Delta T/T noise level of 2.6times 10^{-4} RMS is achieved. Finally, as a proof of concept, the transient absorption spectrum of a methylammonium lead iodide perovskite film is taken, showing great signal to noise with only 1,000 shot pairs. These results show great potential for the employment of this technique in other spectroscopic techniques such as coherent multidimensional spectroscopy.

Co‐Evaporated Perovskite Light‐Emitting Transistor Operating at Room Temperature

AbstractSolution‐processed hybrid organic–inorganic perovskite light‐emitting transistors (PeLETs) suffer from low brightness and environmental instability induced by temperature‐activated trapping, ionic motion, and polarization effects, which so far have been hindering the realization of devices operating at room temperature. Here high quality thermally co‐evaporated methylammonium lead iodide perovskite films are employed to minimize ionic motion and significantly improve the electroluminescence characteristics of the perovskite active layer, enabling stable device operation up to ≈ 310 K. The demonstration of PeLETs operating at and beyond room‐temperature paves the way for practical applications in the rapidly evolving area of solid‐state lighting, active‐matrix displays, and visible light communications.

Energy Barriers Restrict Charge Carrier Motion in MAPI Perovskite Films

AbstractUnderstanding of charge trapping processes in halide perovskites is vital to further improve performance of perovskite optoelectronic devices such as solar cells, photodetectors, and LEDs. In this work, transient photocurrent, time‐delayed collection field and transient fluorescence techniques along with numerical simulations are combined to address charge carrier trapping processes during their lateral motion in prototypical methylammonium lead iodide perovskite films formed on interdigitated electrodes. Carrier mobility decreases on hundreds of ns timescale, and its rate depends on the motion character—it is faster when charge carriers drift in the electric field and slower when the motion is caused by diffusion only. This difference becomes particularly evident at low temperatures. Based on the time‐delayed collection field data and carrier motion modelling results, it is demonstrated that the rapid mobility decay at low temperatures is mainly caused by the energy barriers, most likely formed at crystallite boundaries. Even though these barriers are surmountable at room temperature, they still play a major role in determining carrier mobility and diffusion rates. Suggested concept of the potential barriers moves beyond the conventional understanding of carrier mobility, diffusion, and recombination processes in hybrid perovskites.

Localized surface plasmon effects on the photophysics of perovskite thin films embedding metal nanoparticles

Herein we provide direct experimental evidence that proves that the photophysical properties of thin methylammonium lead iodide perovskite films are significantly enhanced by localized surface plasmon resonances (SPRs).

Long-range ballistic propagation of carriers in methylammonium lead iodide perovskite thin films

The performance of semiconductor devices is fundamentally governed by charge-carrier dynamics within the active materials1–6. Although advances have been made towards understanding these dynamics under steady-state conditions, the importance of non-equilibrium phenomena and their effect on device performances remains elusive7,8. In fact, the ballistic propagation of carriers is generally considered to not contribute to the mechanism of photovoltaics (PVs) and light-emitting diodes, as scattering rapidly disrupts such processes after carrier generation via photon absorption or electric injection9. Here we characterize the spatiotemporal dynamics of carriers immediately after photon absorption in methylammonium lead iodide perovskite films using femtosecond transient absorption microscopy (fs-TAM) with a 10 fs temporal resolution and 10 nm spatial precision. We found that non-equilibrium carriers propagate ballistically over 150 nm within 20 fs of photon absorption. Our results suggest that in a typical perovskite PV device operating under standard conditions, a large fraction of carriers can reach the charge collection layers ballistically. The ballistic transport distance appears to be limited by energetic disorder within the materials, probably due to disorder-induced scattering. This provides a direct route towards optimization of the ballistic transport distance via improvements in materials and by minimizing the energetic disorder. Our observations reveal an unexplored regime of carrier transport in perovskites, which could have important consequences for device performance. Charge-carrier dynamics are fundamental to the operation and performance of semiconductor devices. In methylammonium lead iodide perovskites, carriers in the non-equilibrium regime after excitation propagate ballistically over 150 nm within 20 fs.

Unusual Thermal Boundary Resistance in Halide Perovskites: A Way To Tune Ultralow Thermal Conductivity for Thermoelectrics.

Halide perovskites have emerged as promising candidates as the active material in photovoltaics and light-emitting diodes. They possess unusual bulk thermal transport properties that have been the focus of a number of studies, but there is much less understanding of thermal transport in thin films where a diverse range of structures and morphologies are accessible. Here, we report on the tuning of in-plane thermal conductivity in methylammonium lead iodide thin films by morphological control. Using 3-ω measurements, we find that the room temperature thermal conductivity of thermally evaporated methylammonium lead iodide perovskite films ranges from 0.31 to 0.59 W/(m K). We measure a discontinuity in thermal conductivity at the orthorhombic-tetragonal phase transition and explore this using density functional theory and attributing it to a collapse in the phonon group velocity along the c-axis of the tetragonal crystal. Moreover, we have quantified the thermal boundary resistance (Kapitza resistance) for thermally evaporated films, allowing us to estimate the Kapitza length, which is 36 ± 2 nm at room temperature and 15 ± 2 nm at 100 K. Curiously, the Kapitza resistance has a strong temperature dependence which we also explore using density functional theory, with these results suggesting an important role of methylammonium rotational modes in scattering phonons at the crystallite boundaries.

Controlling the Morphology of Organic-Inorganic Hybrid Perovskites through Dual Additive-Mediated Crystallization for Solar Cell Applications.

To realize a high-efficiency perovskite solar cell (PSC), it is critical to optimize the morphology of the perovskite film for a uniform and smooth finish with large grain size during film formation. Using a chemical compound as an additive to the precursor solution has recently been established as a promising method to control the morphology of the perovskite film. In this study, we propose a new method to achieve an improved morphology of the methylammonium lead iodide perovskite film by simultaneous addition of dimethyl sulfoxide (DMSO) and methoxyammonium salt (MeO) (dual additives). We demonstrated that an appropriate amount of the MeO additive helps the precursors form a stable intermediated PbI2-DMSO adduct during film formation and enlarges the perovskite grains by retarding the kinetics of conversion of the adduct to the perovskite. Furthermore, we experimentally observed that the optical band gaps and crystal structures of perovskite films are reasonably unaffected by the MeO additive because MeO is almost eliminated during annealing. By optimizing the amount of MeO, we achieved improved device performances of the PSCs with a high power conversion efficiency of 19.71% that is ∼15% higher than that obtained for the control device (17.15%).

Surface polarity control in ZnO films deposited by pulsed laser deposition

We demonstrate a simple and inexpensive method of surface polarity control of ZnO grown by pulsed laser deposition (PLD). The polarity control is achieved in a straightforward way by changing the thickness of MgO buffer layer. The Zn- and O-polar ZnO films possess very distinct growth rate, electron concentration and mobility as well as different defect structures. These different structural and electronic properties result in significant differences in surface reactivity and device performance. For example, Pd Schottky diodes fabricated onto the O-polar ZnO film exhibit lower barrier height and ideality factor compared with the equivalent Zn-polar devices, while methylammonium lead iodide perovskite films are readily formed on O-terminated and rapidly decompose on Zn-terminated surfaces. This can be attributed to higher photocatalytic activity of Zn-terminated surface, as well as higher surface coverage of adsorbed hydroxyl groups. Consequently, our results indicate that polarity engineering to obtain favorable O-terminated surface can result in improved performance of ZnO-containing optoelectronic devices, while Zn-terminated surfaces could be of interest for photocatalytic and sensing applications.

Effect of Crystal Grain Orientation on the Rate of Ionic Transport in Perovskite Polycrystalline Thin Films.

In this work, we examine the effect of microstructure on ion-migration-induced photoluminescence (PL) quenching in methylammonium lead iodide perovskite films. Thin films were fabricated by two methods: spin-coating, which results in randomly oriented perovskite grains, and zone-casting, which results in aligned grains. As an external bias is applied to these films, migration of ions causes a quenching of the PL signal in the vicinity of the anode. The evolution of this PL-quenched zone is less uniform in the spin-coated devices than in the zone-cast ones, suggesting that the relative orientation of the crystal grains plays a significant role in the migration of ions within polycrystalline perovskite. We simulate this effect via a simple Ising model of ionic motion across grains in the perovskite thin film. The results of this simulation align closely with the observed experimental results, further solidifying the correlation between crystal grain orientation and the rate of ionic transport.

Direct Measurements of Carrier Transport in Polycrystalline Methylammonium Lead Iodide Perovskite Films with Transient Grating Spectroscopy.

Hybrid organic-inorganic halide perovskites have been proposed in many optoelectronic applications, but critical to their increasing functionality and utility is understanding and controlling carrier transport. Here, we use light-induced transient grating spectroscopy to probe directly carrier transport in polycrystalline methylammonium lead iodide perovskite thin films using a weakly perturbative and noncontact method. The data reveal intrinsic diffusion characteristics of the charge carriers in the material and agree well with a simulated model of charge transport in which grain boundaries act as barriers to carrier movement.

The evaluation of structure and optical properties of CH3NH3[LaxPb1-x]I3; (0 ≤ x ≤ 0.07) trihalide perovskite films prepared by solution growth approach

Highly crystalline CH3NH3[LaxPb1-x]I3 (0 ≤ x ≤ 0.07) films are developed on quartz substrates using solution growth approach. The influence of lanthanum ions on the structural, morphological and optical properties of the La-doped methylammonium lead iodide perovskite films were examined by the X-ray diffraction, scanning electron microscope, Raman spectroscopy and UV-vis spectrophotometery. The films exhibited polycrystalline orthorhombic perovskite crystal structure, suggesting the solubility of the La ions in the CH3NH3PbI3 perovskite crystals. The orthorhombic distortion factor disclosed the incorporation of 3 mol% of La ions in the CH3NH3PbI3 perovskite crystal promote the symmetry of these crystal. The Raman spectra revealed three transverse optical phonon modes and three longitudinal optical phonons modes. The decrease of the ratio of the 96/110 bands and the peak position shift towards low frequency, implying the decrease of the concentrations of the PbI2 into the orthorhombic perovskite crystal and these films become more polarizable. Among all films, the CH3NH3[La0.03Pb0.07]I3 film has shown high optical absorption, low transmittance and very low reflectance. The increase of the amount of La ions from 0 to 3 mol% resulted in a decrease of the band gap from 1.51 eV to 1.44 eV, then an increase to 1.47 eV when the concentration reached to 7 mol%. The CH3NH3[La0.03Pb0.07]I3 film showed very low dissipation factor and high optical conductivity, implying the opportunity to use this composition for the development of photovoltaics and photodiodes applications.

Surface Fluorination of ALD TiO2 Electron Transport Layer for Efficient Planar Perovskite Solar Cells

AbstractPerovskite solar cells (PSCs) are emerging among the photovoltaic (PV) technologies due to their high power conversion efficiency (PCE) in combination with potentially low cost manufacturing processing. In this contribution, the fabrication of efficient planar n‐i‐p PSCs by the modification of the electron transport layer (ETL) adopted as n‐type contact is demonstrated. Specifically, a fluorine‐based plasma treatment prior to perovskite deposition leads to surface fluorination of the TiO2 ETL. The presence of fluorine on the TiO2 surface drastically improves the adhesion between the ALD layer and the methylammonium lead iodide perovskite film, and leads to a more favourable energy band alignment, accompanied by a faster electron carrier extraction at the interface. As consequence of surface fluorination, we observe a significant reduction in the current density‐voltage curve hysteresis with respect to the ALD based reference sample, as well as a remarkable improvement in power conversion efficiency from 4% up to a stable 14.8%.

Photoconductive noise microscopy revealing quantitative effect of localized electronic traps on the perovskite-based solar cell performance

We developed a “photoconductive noise microscopy” method to directly image electronic charge traps distributed on a methylammonium lead iodide perovskite film in a solar cell device. The method enabled quantitative imaging of trap densities along with local photocurrents on the solar cell film. By analyzing the imaging data, we could reveal quantitative correlations between the trap distribution and local photocurrents. The results show that the spatial density of the charge traps has a power-law relationship with the short-circuit currents during a solar cell operation as well as localized photocurrents under a sample bias, indicating that a charge trap distribution in a perovskite film can be a major factor determining the performance of the perovskite-based solar cells.

Hindered Amine Light Stabilizers Increase the Stability of Methylammonium Lead Iodide Perovskite Against Light and Oxygen.

Methylammonium lead iodide perovskite (MAPI) is a promising material for highly efficient photovoltaic devices. However, it suffers from photooxidation, which imposes strict requirements for its protection from oxygen during processing and operation. A hindered amine light stabilizer (HALS) has been found to exert a stabilization effect on methylammonium iodide (MAI) and MAPI against photooxidation. The HALS prevents the degradation of MAI by inhibiting the oxidation of iodide to iodine. Chemical modification of HALS allows its incorporation in MAPI films, which extends the resistivity of MAPI against photodegradation in ambient air from a couple of hours to several days, while causing no significant changes in key properties, such as optical absorption and charge transport. These results represent an important advance in the stabilization of MAPI against decomposition and demonstrate for the first time that antioxidants improve the stability of MAPI.

Perovskite solar cells: Shedding light on film crystallization.

A study on the formation of methylammonium lead iodide perovskite films reveals that light illumination influences the crystallization kinetics, therefore affecting the final photovoltaic performance of these materials.

Grain-Size-Limited Mobility in Methylammonium Lead Iodide Perovskite Thin Films

We report a systematic study of the gigahertz-frequency charge carrier mobility found in methylammonium lead iodide perovskite films as a function of average grain size using time-resolved microwave conductivity and a single processing chemistry. Our measurements are in good agreement with the Kubo formula for the AC mobility of charges confined within finite grains, suggesting (1) that the surface grains imaged via scanning electron microscopy are representative of the true electronic domain size and not substantially subdivided by twinning or other defects not visible by microscopy and (2) that the time scale of diffusive transport across grain boundaries is much slower than the period of the microwave field in this measurement (∼100 ps). The intrinsic (infinite grain size) minimum mobility extracted form the model is 29 ± 6 cm2 V–1 s–1 at the probe frequency (8.9 GHz).