PbI3 Perovskite Research Articles

Understanding how chlorine additive in a dynamic sequential process affects FA0.3MA0.7PbI3 perovskite film growth for solar cell application

In this work, for the first time, we identify the effect of adding chlorine (Cl) in the dynamic sequential (no stoppage between the sequential steps and the precursor for the second-step is dispensed while the substrate is in motion) solution processing of formamidinium methylammonium lead iodide FA0.3MA0.7PbI3 perovskite solar cells by comparing (i) the roles of two different Cl sources (MACl and FACl) and (ii) the time at which the Cl sources were added: (a) into PbI2-DMSO in the first deposition step or (b) into MAI-FAI in the second deposition step. It is found that Cl in general (regardless of the type of source and at which step it is added) improves the perovskite film with better coverage, better crystallinity, and higher photoluminescence response and longer carrier lifetime due to reduced trap densities. Characterizations for investigating film formation mechanisms reveal that the amount of Cl remaining in the final film is most crucial in determining the film quality. This in turn is controlled by when Cl is added. When Cl is added in the second step, the higher Cl content which is highly diffusive initiates the Dimethyl sulfoxide (DMSO) complex formation and perovskite formation too rapidly producing films with excess PbI2, which are more disorder and have lower crystallinity. FACl exacerbates this while MACl reduces the detrimental effect by slowing down perovskite formation. Therefore, for dynamic sequential processing, it is recommended that MACl is used as the preferred source over FACl and Cl should be added in the 1st step of the process allowing sufficient Cl to leave the film in the 2nd step of the sequential process. The resulting film has better film quality producing a champion device with a 2.8% absolute efficiency increase over the best device fabricated without Cl additives. This work has provided great insights into the film formation mechanism by sequential processing with additive engineering.

In situ Near-Ambient Pressure X-ray Photoelectron Spectroscopy Reveals the Influence of Photon Flux and Water on the Stability of Halide Perovskite.

For several years, scientists have been trying to understand the mechanisms that reduce the long‐term stability of perovskite solar cells. In this work, we examined the effect of water and photon flux on the stability of CH3NH3PbI3 perovskite films and solar cells using in situ near‐ambient pressure X‐ray photoelectron spectroscopy (NAP‐XPS), field emission scanning electron microscopy (FESEM), and current density–voltage (J–V) characterization. The used amount of water vapor (up to 1 mbar) had a negligible impact on the perovskite film. The higher the photon flux, the more prominent were the changes in the NAP‐XPS and FESEM data; also, a faster decline in power conversion efficiency (PCE) and a more substantial hysteresis in the J‐V characteristics were observed. Based on our results, it can be concluded that the PCE decrease originates from the creation of Frenkel pair defects in the perovskite film under illumination. The stronger the illumination, the higher the number of Frenkel defects, leading to a faster PCE decline and more substantial hysteresis in the J‐V sweeps.

Why choosing the right partner is important: stabilization of ternary CsyGUAxFA(1-y-x)PbI3 perovskites

Why choosing the right partner is important: stabilization of ternary CsyGUAxFA(1-y-x)PbI3 perovskites

Multifunctional Self-Combustion Additives Strategy to Fabricate Highly Responsive Hybrid Perovskite Photodetectors.

To resolve the inherent trade-off issue between responsivity and detectivity in FA0.9Cs0.1PbI3 perovskite photodetectors, this paper proposes a novel strategy using multifunctional self-combustion additives (urea and ammonium nitrate). During the early stages of crystallization, urea allows for the formation of a strong Lewis complex-derived low-dimensional intermediate phase; this suppresses the formation of perovskite nuclei, while ammonium ions assist the preferred grain growth along the [110] direction. During the high-temperature annealing steps, a self-combusting exothermic reaction occurs between urea as a fuel and NH4NO3 as an oxidizer, through which a locally supplied heat facilitates the removal of residual urea and byproducts. These multifunctional roles of self-combustible additives facilitate the production of high-quality, enlarged grain-structured perovskite films with improved optoelectronic properties, as confirmed by various analyses, including impedance spectroscopy and intensity-modulated photocurrent spectroscopy. The resulting FA0.9Cs0.1PbI3-based photodiode-type photodetectors exhibit outstanding performance, such as a high responsivity (0.762 A W-1) and specific detectivity (over 5.08 × 1013 Jones) at a very low external reverse bias (-0.5 V). Our findings clearly suggest that the multifunctional self-combustion additives strategy could help realize the full potential of FA1-xCsxPbI3 as a photodiode-type photodetector.

Addressing the Stability of Lead Halide Perovskites

The rapid rise in efficiencies of lead halide perovskite solar cells has hastened the case for commercialization. One of the key barriers, however, is stability of devices. In this issue of Joule, Zhou et al. studied the degradation mechanism of a FA-Cs mixed cation perovskite-based solar cell with experimental and modeling techniques to show that phase separation into FA-rich and Cs-rich compositions account for the observed degradation.

Cs4 PbI6 -Mediated Synthesis of Thermodynamically Stable FA0.15 Cs0.85 PbI3 Perovskite Solar Cells.

The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FAx Cs1 -x PbI3 has shown great promise owing to its improved thermal and moisture stability. However, the study of FAx Cs1 -x PbI3 is concentrated on formamidine (FA)-rich perovskite, whereas cesium (Cs)-rich FAx Cs1 -x PbI3 perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs4 PbI6 -mediated method is developed to synthesize Cs-rich FAx Cs1 -x PbI3 perovskites. It is demonstrated that Cs4 PbI6 intermediate phase has a low Cs cation diffusion barrier and therefore offers a fast ion exchange with the preformed FA-rich perovskite phase to finally form the Cs-rich FAx Cs1 -x PbI3 perovskite. The results indicate that ≈15% alloying with organic FA cations can sufficiently stabilize the perovskite phase with excellent phase and UV-irradiation stability. The FA0.15 Cs0.85 PbI3 perovskite solar cells achieve a champion power conversion efficiency of 17.5%, showing the great potential of Cs-based perovskites for efficient and stable solar cells.

Improvement of the stability of perovskite solar cells in terms of humidity/heat via compositional engineering

Compositional engineering is one of the ways of controlling the phase structure and improving the stability of perovskite solar cells. This study is the investigation of how adding cesium (Cs) to formamidium lead iodide (FAPbI3) perovskite affects the structure, morphology, performance and stability of hole-transport-free (HTF) mesoporous solar cells. It was shown that this addition leads to decrease in the lattice constant and the tolerance factor and causes perovskite band gaps to vary. The x-ray diffraction analysis also showed the removal of the non-photovoltaic perovskite phase and the conversion of the trigonal black phase to tetragonal structure. As 15% of Cs was added, the average efficiency of 7.41% and the highest efficiency of 11.78% were achieved for the HTF solar cells. Finally, FA0.85 Cs0.15PbI3 perovskite cells proved to be more stable than FAPbI3 reference cells at 150 °C and the humidity of 90%.

Light-Emitting Nanophotonic Designs Enabled by Ultrafast Laser Processing of Halide Perovskites.

Nanophotonics based on resonant nanostructures and metasurfaces made of halide perovskites have become a prospective direction for efficient light manipulation at the subwavelength scale in advanced photonic designs. One of the main challenges in this field is the lack of large-scale low-cost technique for subwavelength perovskite structures fabrication preserving highly efficient luminescence. Here, unique properties of halide perovskites addressed to their extremely low thermal conductivity (lower than that of silica glass) and high defect tolerance to apply projection femtosecond laser lithography for nanofabrication with precise spatial control in all three dimensions preserving the material luminescence efficiency are employed. Namely, with CH3 NH3 PbI3 perovskite highly ordered nanoholes and nanostripes of width as small as 250 nm, metasurfaces with periods less than 400 nm, and nanowire lasers as thin as 500 nm, corresponding to the state-of-the-art in multistage expensive lithographical methods are created. Remarkable performance of the developed approach allows to demonstrate a number of advanced optical applications, including morphology-controlled photoluminescence yield, structural coloring, optical- information encryption, and lasing.

Solvent Engineering Using a Volatile Solid for Highly Efficient and Stable Perovskite Solar Cells.

A strategy for efficaciously regulating perovskite crystallinity is proposed by using a volatile solid glycolic acid (HOCH2COOH, GA) in an FA0.85MA0.15PbI3 (FA: HC(NH2)2; MA: CH3NH3) perovskite precursor solution that is different from the common additive approach. Accompanied with the first dimethyl sulfoxide sublimation process, the subsequent sublimation of GA before 150 °C in the FA0.85MA0.15PbI3 perovskite film can artfully regulate the perovskite crystallinity without any residual after annealing. The improved film formation upon GA modification induced by the strong interaction between GA and Pb2+ delivers a champion power conversion efficiency (PCE) as high as 21.32%. In order to investigate the role of volatility in perovskite solar cells (PSCs), nonvolatile thioglycolic acid (HSCH2COOH, TGA) with a similar structure to GA is utilized as an additive reference. Large perovskite grains are obtained by TGA modification but with obvious pinholes, which directly leads to an increased defect density accompanied by a decline in PCE. Encouragingly, the champion PCE achieved for GA‐based PSC device (21.32%) is almost 13% or 20% higher than those of the control device or TGA‐based device. In addition, GA‐modified PSCs exhibit the best stability in light‐, thermal‐, and humidity‐based tests due to the improved film formation.

A Three-Dimensional Lead Halide Perovskite-Related Ferroelectric

Three-dimensional (3D) organic-inorganic lead halides represented by [CH3NH3]PbI3 perovskite have attracted great interest for their diverse functional properties and promising optoelectronic applications. However, 3D lead halides are still very rare and their ferroelectricity remains controversial. Here, we report an unprecedented 3D lead halide perovskite-related ferroelectric [2-trimethylammonioethylammonium]Pb2Cl6 ([TMAEA]Pb2Cl6), which contains a 3D lead chloride framework of corner- and edge-sharing PbCl6 octahedral, with the [TMAEA]+ cations occupying the voids of the framework. [TMAEA]Pb2Cl6 shows a ferroelectric-to-paraelectric phase transition with the Curie temperature as high as 412 K, a typical ferroelectric hysteresis loop at 293 K with a spontaneous polarization of 1 μC/cm2, and a clear ferroelectric domain switching. To the best of our knowledge, [TMAEA]Pb2Cl6 is the first 3D lead halide showing such an excellent ferroelectricity. Additionally, it also exhibits semiconducting property with a direct bandgap of 3.43 eV. This finding enriches the family of 3D hybrid lead halides, and inspires the exploration of 3D lead halide ferroelectrics.

Triphenylamine-based hole transporting materials with thiophene-derived bridges for perovskite solar cells

A triphenylamine-based compound containing benzoyl[1,2-b:4,5-b’]dithiophene (BDT) derivative (BTPA-2) has been synthesized and employed in FA0.85MA0.15PbI3 perovskite solar cells (PSCs) as the hole transporting material (HTM). The reference BTPA-1 containing thiophene bridge was also prepared for a comparative study. Compared to the thiophene unit in BTPA-1, the hexyloxy substituted BDT unit in BTPA-2 made the HOMO energy level lower. The more conjugated system with the BDT unit can improve the conductivity and hole mobility. The BTPA-1 and BTPA-2 based FA0.85MA0.15PbI3 PSCs showed the best power conversion efficiencies (PCE) of 12.76 % and 13.97 %, respectively. Larger Voc and higher Jsc for the BTPA-2 based PSC were attributed to its reduced interfacial recombination and more conjugated system compared to the BTPA-1 based PSCs. More hydrophobic property of BTAP-2 made its PSC exhibit better environmental stability than BTPA-1.

High performance perovskite solar cells using multiple hole transport layer and modulated FAxMA1−xPbI3 active layer

NiO layer and NiO/Ag/NiO triple layers were respectively deposited on indium-tin-oxide-coated glass substrates as the hole transport layers of MAPbI3 and FAxMA1−xPbI3 perovskite solar cells using a radio frequency magnetron sputtering system. Compared with the 60-nm-thick NiO hole transport layer with an average transmission of 66.8% between the wavelength range from 300 to 800 nm, the average transmission was improved to 70.4% for the NiO/Ag/NiO (30/7/30 nm) hole transport layer. Consequently, the power conversion efficiency of MAPbI3 perovskite solar cells using the NiO/Ag/NiO (30/7/30 nm) hole transport layer was improved from 11.02 to 11.68%. To broaden light absorption, various FAxMA1−xPbI3 active layers were obtained by blending various formamidine iodide (FAI) contents with methylammonium iodide (MAI). The power conversion efficiency of the FA0.1MA0.9PbI3 perovskite solar cells using 60-nm-thick NiO hole transport layer was improved to 12.30%. By integrating the optimal FA0.1MA0.9PbI3 active layer and the optimal NiO/Ag/NiO (30/7/30 nm) hole transport layer, the power conversion efficiency of the resulting perovskite solar cells was further improved to 12.67%.

Revealing the perovskite formation kinetics during chemical vapour deposition

High control chemical vapour deposition of (Cs,FA)PbI3 perovskite provides insights into CVD kinetics and perovskite phase evolution.

Why choosing the right partner is important: stabilization of ternary CsyGUAxFA(1-y-x)PbI3 perovskites.

Lead halide perovskites with mixtures of monovalent cations have attracted wide attention due to the possibility of preferentially stabilizing the perovskite phase with respect to photovoltaically less suitable competing phases. Here, we present a theoretical analysis and interpretation of the phase stability of binary (CH6N3)x[HC(NH2)2](1-x)PbI3 = GUAxFA(1-x)PbI3 and ternary CsyGUAxFA(1-y-x)PbI3 mixtures. We first estimate if such mixtures are stable and if they lead to a stabilization of the perovskite phase based on static Density Functional Theory (DFT) calculations. In order to investigate the finite temperature stability of the phases, we also employ first-principles molecular dynamics (MD) simulations. It turns out that in contrast to the FA+-rich case of FA/Cs mixtures, although mixing of FA/GUA is possible, it is not sufficient to stabilize the perovskite phase at room temperature. In contrast, stable ternary mixtures that contain 17% of Cs+ can be formed that lead to a preferential stabilization of the perovskite phase. In such a way, the enthalpic destabilization due to the introduction of a too large/too small cation that lies outside the Goldschmidt tolerance range can be (partially) compensated through the introduction of a third cation with complementary size. This allows to suggest a new design principle for the preparation of stable perovskite structures at room temperature with cations that lie outside the Goldschmidt range through mixtures with size-complementary cations in such a way that the effective average cation radius of the mixture lies within the stability range.

Chemical Composition and Phase Evolution in DMAI-Derived Inorganic Perovskite Solar Cells

Inorganic CsPbI3 is promising to enhance the thermal stability of perovskite solar cells. The dimethylamine iodide (DMAI) derived method is currently the most efficient way to achieve high efficiency, but the effect of DMAI has not been fully explained. Herein, the chemical composition and phase evolution of the mixed DMAI/CsPbI3 layer during thermal treatment has been studied. The results demonstrate that, with the common DMAI/CsI/PbI2 recipe in DMSO solvent, a mixed perovskite DMA0.15Cs0.85PbI3 is first formed through a solid reaction between DMAPbI3 and Cs4PbI6. Further thermal treatment will transform the mixed perovskite phase directly to γ-CsPbI3 and then spontaneously convert to δ-CsPbI3. It has been also demonstrated that the DMA0.15Cs0.85PbI3 phase is thermodynamically stable and shows a bandgap of 1.67 eV, which is narrower than 1.73 eV of γ-CsPbI3. The device efficiency of the mixed DMA0.15Cs0.85PbI3 perovskite is therefore highly improved in comparison with the pure inorganic γ-CsPbI3 perovskite.

High-Rubidium–Formamidinium-Ratio Perovskites for High-Performance Photodetection with Enhanced Stability

Formamidinium lead trihalide perovskites have emerged as promising photovoltaic materials owing to their superior absorption coefficient properties. However, one big challenge is the material phase stability and thermal stability at high temperature. In this work, a large quantity of rubidium (Rb) ions is incorporated into formamidinium (FA) perovskite thin films to improve the material phase stability and thermal stability. Photodiodes based on optimized FA0.7Rb0.3PbI3 perovskites deliver a high responsivity of 0.43 A W-1, a detectivity of >1012 Jones, a relatively large linear dynamic range of 125 dB, and an ultrafast response speed of approximately 300 ns. Moreover, these photodiodes present lower dark current and higher photocurrent after baking at high temperature. These results are very promising for photodetection at high operational temperature. In addition, the high-ratio rubidium-incorporated perovskite films may have great potential in fabricating other high-performance optoelectronic devices, i.e., light-emitting diodes and solar cells with excellent phase stability and high temperature thermostability.

Deconstruction-assisted perovskite formation for sequential solution processing of Cs0.15(MA0.7FA0.3)0.85PbI3 solar cells

Organic-inorganic hybrid lead halide perovskite has shown to be one of the best light-harvesting materials for solar cell in the last decade. However, there still is needed a deeper understanding of phase and film formation for better control of device fabrication. In this work, we visualise the formation mechanism of Cs0.15(MA0.7FA0.3)0.85PbI3 perovskite by the sequential spin-coating method and how changes in the dispensing timing and substrate motion affect the formation process and properties of the final film quality. In particular, this is the first time that we are able to visualise and identify the different stages of the film formation: i) “initial formation”; ii) “perovskite deconstruction” and iii) “perovskite re-crystallisation”. This particularly applies to films that are sequentially spin-coated and involve the use of dimethyl sulfoxide (DMSO) as the “deconstruction” is caused by the formation of intermediate-DMSO-complex. These findings are validated by FTIR and XRD measurements. Comparison among processes also suggests that motion causes an earlier onset of deconstruction, which will lead to a slower re-crystallisation resulting in better quality perovskite film with less non-perovskite phase. This can be achieved by motion dispensing and dynamic processing (where there is no stoppage between the two sequential steps). Reasons for the earlier onset of deconstruction are the higher kinetic energy supplied by the dynamic process. This work has provided more insights into the complex stages involved in perovskite conversion specific to sequential processing. The knowledge will aid future process optimisation for better device fabrication.

Application of Atomic Layer Deposition for Fabrication of Solar Cells

Atomic layer deposition (ALD) is exceptional deposition technique because it allows for growth of high quality films on large are substrates at low deposition temperatures. In our contribution ALD was employed for preparation of transparent conducting electrodes and for encapsulation of fabricated solar cells. Al-doped ZnO films were used for transparent conducting electrodes. Deposition was carried out at temperatures between 150 and 250 °C on Si and quartz substrates. Diethyl zinc and trimethyl aluminum were used as precursors and water vapors as reactant. Al-doping was performed by inserting Al2O3 cycles in ZnO growth. Deposition of 1 Al2O3 layer per 7 deposited ZnO layers gave the best results. Resistivity of the Al-doped ZnO films depended on the deposition temperature and film thickness. Transition electron microscopy revealed typical columnar growth of the films with fine grained polycrystalline region close to the substrate. The films with the thickness above 100 nm prepared at 250 °C showed resistivity of 1 mΩcm. This corresponds to the sheet resistance of 40 Ω/square. Hall measurement revealed electron concentration of 4*1020 cm-3 and mobility of 15 cm2Vs-1 for the films with resistivity of 1 mΩcm. Work function of 4.4 eV was measured by Kelvin probe. Optical transmittance of the films was higher than 80% in the wavelength range between 400 and 900 nm. Al-doped ZnO transparent electrodes were used for fabrication of perovskite-type conventional planar solar cell. CH3NH3 PbI3 perovskite was used as absorbing layer, Spiro-OMeTAD as hole transport layer and Au/Ag as top contact layer. Properties of the solar cell with Al-doped ZnO electrode are compared with inverted planar solar cell using indium tin oxide as a transparent electrode. Inorganic-organic hybrid perovskite solar cells suffer from poor stability in ambient atmosphere. Oxygen and moisture are believed to be reason for the solar cell degradation. To protect the cells against ambient atmosphere influence ALD prepared Al2O3 films were employed for encapsulation. Trimethyl aluminum was used as precursor while either water vapors or ozone were applied as reactants. Deposition of the Al2O3 films took place at 50°C. Power conversion efficiency of the solar cell encapsulated by 24 nm Al2O3 thin film deposited using water vapors as reactant decreased to 60% while unprotected solar cell degraded down to 10% of its initial value after 20 days in ambient atmosphere. In our contribution we compare properties of perovskite solar cell encapsulated by ALD Al2O3 films prepared using water vapors or ozone as reactants.

Simultaneous Cesium and Acetate Coalloying Improves Efficiency and Stability of FA0.85MA0.15PbI3 Perovskite Solar Cell with an Efficiency of 21.95%

A simple coalloying strategy is applied to improve the efficiency and stability of FA0.85MA0.15PbI3 perovskite solar cells (PSCs) by using cesium acetate (CsAc) as an additive. It is found that the simultaneous incorporation of cation (Cs+) and anion (Ac−) into the FA0.85MA0.15PbI3 film is an effective approach to realize lattice contraction, grain size enlargement, photoelectric properties improvement, band structure modulation, and therefore the optimization of the efficiency and stability of PSCs. At optimal CsAc alloying, the FA0.85MA0.15PbI3 PSCs achieve a maximum power conversion efficiency (PCE) of 21.95% and an average of over 21%. In addition, the alloyed PSCs retain 97% of their initial PCE values after aging for 55 days in air without encapsulation.

Ultrasonic Spray-Coated Mixed Cation Perovskite Films and Solar Cells

A solar absorber composed of mixed cation perovskite films containing methylammonium (MA) and formamidinium (FA) lead triiodide (MA1–xFAxPbI3) was prepared by continuous ultrasonic spray-coating. This process combined features of substitutional modifications in perovskites and simple yet versatile processing to realize scalable fabrication. Herein, simultaneous growth from a precursor solution with controlled MA/FA stoichiometry in the inverted perovskite solar cells based on MA1–xFAxPbI3 was systematically investigated to link the thin film quality, optical properties, and crystal structures with photovoltaic performance. The result of the inherent advantages of increased photoabsorption, better film quality, and enhanced crystallinity, the perovskite solar cell composed of the MA0.75FA0.25PbI3 (x = 0.25) perovskite achieved an optimum power conversion efficiency (PCE) as high as 15.61% and an average PCE of 15.15 ± 0.45%. In particular, the spray-deposited MA0.75FA0.25PbI3 unencapsulated cells exhibited almost no hysteresis in the current density–voltage (J–V) curve and excellent long-term stability under ambient conditions (25 °C, 30–40% relative humidity). These results highlight the importance of choosing a suitable mixed cation perovskite composition and scalable spraying technique for high-throughput manufacturing of perovskite solar cells to prolong device lifetime and achieve good reproducibility.