Non-perovskite Phase Research Articles

Potassium iodide reduces the stability of triple-cation perovskite solar cells.

The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices. Previous work on the use of potassium iodide (KI) in active layers to passivate defects in triple-cation mixed-halide perovskites has been shown to enhance their luminescence efficiency and reduce current–voltage hysteresis. However, the operational stability of KI passivated perovskite solar cells under ambient conditions remains largely unexplored. By investigating perovskite solar cell performance with SnO2 or TiO2 electron transport layers (ETL), we propose that defect passivation using KI is highly sensitive to the composition of the perovskite–ETL interface. We reconfirm findings from previous reports that KI preferentially interacts with bromide ions in mixed-halide perovskites, and – at concentrations >5 mol% in the precursor solution – modifies the primary absorber composition as well as leading to the phase segregation of an undesirable secondary non-perovskite phase (KBr) at high KI concentration. Importantly, by studying both material and device stability under continuous illumination and bias under ambient/high-humidity conditions, we show that this secondary phase becomes a favourable degradation product, and that devices incorporating KI have reduced stability.

Achieving Reproducible and High-Efficiency (>21%) Perovskite Solar Cells with a Presynthesized FAPbI3 Powder

The conventional precursor mixture based on highly pure and expensive PbI2 (purity >99.99%) is problematic in commercializing perovskite solar cells (PSCs) because of a large deviation in the batch-to-batch photovoltaic performance due to underlying nonstoichiometry and/or the nonperovskite phase. Here, we report on reproducibly efficient (>21%) PSCs based on the ambient-temperature-stable δ-phase FAPbI3 powder, synthesized by reacting PbI2 (either homemade or low-grade commercial product with purity <99%) with formamidinium iodide (FAI) at room temperature (∼93% yield). X-ray diffraction patterns confirm that annealing the spin-coated film leads to phase purity of cubic α FAPbI3. As compared to the conventional precursor mixture based on PbI2 (purity = 99.9985%), significantly reducing defects in the powder-based film is responsible for the stable and reproducible efficiency. In addition, the homogeneous light-intensity-dependent surface photocurrent contributes to the reproducibility of photovoltaic per...

A 0D/3D Heterostructured All-Inorganic Halide Perovskite Solar Cell with High Performance and Enhanced Phase Stability.

Although organic-inorganic hybrid perovskite solar cells (PVSCs) have achieved dramatic improvement in device efficiency, their long-term stability remains a major concern prior to commercialization. To address this issue, extensive research efforts are dedicated to exploiting all-inorganic PVSCs by using cesium (Cs)-based perovskite materials, such as α-CsPbI3 . However, the black-phase CsPbI3 (cubic α-CsPbI3 and orthorhombic γ-CsPbI3 phases) is not stable at room temperature, and it tends to convert to the nonperovskite δ-CsPbI3 phase. Here, a simple yet effective approach is described to prepare stable black-phase CsPbI3 by forming a heterostructure comprising 0D Cs4 PbI6 and γ-CsPbI3 through tuning the stoichiometry of the precursors between CsI and PbI. Such heterostructure is manifested to enable the realization of a stable all-inorganic PVSC with a high power conversion efficiency of 16.39%. This work provides a new perspective for developing high-performance and stable all-inorganic PVSCs.

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.

Effects of bromine substitution for iodine on structural stability and phase transition of CsPbI3

First-principles density functional theory (DFT) and the variable-cell nudged-elastic-band (VC-NEB) method were used to investigate the roles of bromine substitution for iodine in perovskite to non-perovskite phase transition of CsPbI3. CsPb(I1-xBrx)3 mixed halide alloys with x = 0, 1/6, 1/3, and 1 have been inspected to understand the effects of this bromine substitution. From the VC-NEB simulation results, the phase transition mechanism has been elucidated. The perovskite cubic to non-perovskite orthorhombic phase transition of CsPbI3 could be described based on the following processes: the cubic symmetry breaking, which leads to monoclinic intermediate states; distortion and deformation of octahedral cages in response to the cubic symmetry breaking, which differentiate the length of PbI bond in PbI6 octahedra; loss of 3D corner-sharing octahedral connectivity due to some PbI bond becomes too long; and the formation of 1D edge-sharing octahedral chain, which is the type of octahedral connectivity for the non-perovskite yellow phase. From the results of mixed halide alloys, using bromine to replace iodine was found to increase the perovskite to non-perovskite phase's energy barrier. Thus, the bromine substitution has been clarified to be useful to prevent phase transition to the yellow phase. Surprisingly, the non-perovskite to perovskite phase's energy barrier of CsPbI2Br is anomalously low with large deviation from the linear trend. This indicates that the non-perovskite to perovskite phase's transformation is significantly easier to occur for CsPbI2Br. Therefore, the results from this study’ calculation suggest that bromine substitution not only prevents the formation of non-perovskite yellow phase but also exhibits capability to transform the yellow phase back to the desirable black phase.

Thermal unequilibrium of strained black CsPbI3 thin films.

The high-temperature, all-inorganic CsPbI3 perovskite black phase is metastable relative to its yellow, nonperovskite phase at room temperature. Because only the black phase is optically active, this represents an impediment for the use of CsPbI3 in optoelectronic devices. We report the use of substrate clamping and biaxial strain to render black-phase CsPbI3 thin films stable at room temperature. We used synchrotron-based, grazing incidence, wide-angle x-ray scattering to track the introduction of crystal distortions and strain-driven texture formation within black CsPbI3 thin films when they were cooled after annealing at 330°C. The thermal stability of black CsPbI3 thin films is vastly improved by the strained interface, a response verified by ab initio thermodynamic modeling.

Quantitative phase analysis on Cs- and Rb-doped FAPbI3 and corresponding solar cell efficiency simulations

Stability and efficiency are two key indicators to evaluate the development prospect of perovskite materials in solar cells (SCs). The partial substitution of Cs or Rb for Formamidinium cation (FA) is proven to significantly improve the stability of FAPbI3. Phase compositions of Cs- and Rb-doped FAPbI3 play a crucial role in determining the stability, which still lack a deep understanding. Besides, bandgap variations upon adding Cs or Rb content also affect the SC efficiency, awaiting a detailed discussion. This work quantitatively analyzed the phase compositions of CsxFA1−xPbI3 and RbyFA1−yPbI3. Moreover, efficiencies of the CsxFA1−xPbI3 and RbyFA1−yPbI3 SCs were simulated using experimental data. The synchrotron X-ray diffraction patterns reveal that the Cs0.2FA0.8PbI3 sample contains the largest amount of the perovskite phase with Pm-3m symmetry, the non-perovskite phase with P63mc symmetry of FAPbI3 is effectively suppressed and therefore the structural stability is significantly improved. However, efficiency of the Cs0.2FA0.8PbI3 SC is slightly lower than that of the Cs0.15FA0.85PbI3 SC, ascribed to its enlarged bandgap and consequent narrowed light absorption waveband. For the RbyFA1−yPbI3 samples, the Rb0.05FA0.95PbI3 sample shows the best structural stability, and the corresponding SC achieves the highest efficiency.

Preparation of Tortuous 3D γ‐CsPbI3 Films at Low Temperature by CaI2 as Dopant for Highly Efficient Perovskite Solar Cells

AbstractInorganic cubic CsPbI3 perovskite (α‐CsPbI3) has been widely explored for perovskite solar cells (PSCs) due to its thermal stability and suitable bandgap of 1.73 eV. However, α‐CsPbI3 usually requires high synthesis temperatures (&gt;320 °C). Additionally, it usually undergoes phase transition to the nonperovskite structure phase (β‐CsPbI3), which results in poor photoelectric performance in devices. In this study, it is first found that the tortuous 3D CsPbI3 phase (γ‐CsPbI3) can be prepared and used for PSCs by solution process without any additive at low temperature (60 °C). The γ‐CsPbI3 exhibits suitable bandgap of 1.75 eV and favorable photoelectric properties. However, γ‐CsPbI3 is a metastable phase and easily transforms into β‐CsPbI3 in ambient moisture. In order to improve the stability of γ‐CsPbI3, calcium ions (Ca2+) with a relatively small radius of 100 pm are used to partially substitute lead ions (119 pm). This research proves that Ca2+ can effectively improve the stability of the γ‐CsPbI3 at room temperature. By optimizing the doping concentration of Ca2+ (CsPb1−xCaxI3, x is from 0% to 2%), the Ca2+‐doped γ‐CsPbI3 PSCs achieve a hysteresis‐free J–V curve and a maximum power conversion efficiency (PCE) of 9.20%.

Stable, Efficient Near-Infrared Light-Emitting Diodes Enabled by α/δ Phase Modulation

Although formamidinium lead iodide (FAPbI3) perovskite shows excellent prospects in the field of optoelectronics, it is limited by the conversion of the black perovskite phase (α-FAPbI3) to the yellow non-perovskite phase (δ-FAPbI3). Here, we first introduce diamine cations [EDBE, 2,2-(ethylenedioxy)bis(ethylamine)] into FAPbI3 for stable and efficient near-infrared perovskite light-emitting diodes (NIR PeLEDs). By rationally regulating the α/δ phase, we have successfully prepared a NIR PeLED with an external quantum efficiency (EQE) of ≤11.40%. In addition, the perovskite film has excellent stability and does not undergo any significant change after being in air for 270 days. These results show that the introduction of EDBE reduces the formation energy of α-FAPbI3 perovskite and the hydrogen bonds formed between adjacent EDBE molecules increase the stability of the perovskite devices. This work could help guide the phase transition mechanism of FAPbI3 and further improve the performance of LEDs.

Potassium- and Rubidium-Passivated Alloyed Perovskite Films: Optoelectronic Properties and Moisture Stability.

Halide perovskites passivated with potassium or rubidium show superior photovoltaic device performance compared to unpassivated samples. However, it is unclear which passivation route is more effective for film stability. Here, we directly compare the optoelectronic properties and stability of thin films when passivating triple-cation perovskite films with potassium or rubidium species. The optoelectronic and chemical studies reveal that the alloyed perovskites are tolerant toward higher loadings of potassium than rubidium. Whereas potassium complexes with bromide from the perovskite precursor solution to form thin surface passivation layers, rubidium additives favor the formation of phase-segregated micron-sized rubidium halide crystals. This tolerance to higher loadings of potassium allows us to achieve superior luminescent properties with potassium passivation. We also find that exposure to a humid atmosphere drives phase segregation and grain coalescence for all compositions, with the rubidium-passivated sample showing the highest sensitivity to nonperovskite phase formation. Our work highlights the benefits but also the limitations of these passivation approaches in maximizing both optoelectronic properties and the stability of perovskite films.

Room Temperature Processing of Inorganic Perovskite Films to Enable Flexible Solar Cells

SummaryInorganic lead halide perovskite materials have attracted great attention recently due to their potential for greater thermal stability compared with hybrid organic perovskites. However, the high processing temperature to convert from the non-perovskite phase to the cubic perovskite phase in many of these systems has limited their application in flexible optoelectronic devices. Here, we report a room temperature processed inorganic perovskite solar cell (PSC) based on CsPbI2Br as the light harvesting layer. By combining this composition with key precursor solvents, we show that inorganic perovskite films can be prepared by the vacuum-assist method under room temperature conditions in air. Unencapsulated devices achieved power conversion efficiency up to 8.67% when measured under 1-sun irradiation. Exploiting this room temperature process, flexible inorganic PSCs based on an inorganic metal halide perovskite material are demonstrated.

Black and Stable: A Path to All-Inorganic Halide Perovskite Solar Cells

All-inorganic cesium lead halides are promising candidates for improving the stability of perovskite solar cells against decomposition in an ambient environment. Yet they are harder to process from solution and prefer crystallizing in yellow, non-perovskite phase. In this issue of Joule, Jiang et. al. demonstrate a new set of precursors to deposit thick Cs-based perovskite films and stabilize their black polymorph for 40 days without significant degradation.

Reduced-Dimensional α-CsPbX3 Perovskites for Efficient and Stable Photovoltaics

Summary Inorganic CsPbX3 perovskites have gained great attention owing to their excellent thermal stability and carrier transport properties. However, the power conversion efficiency (PCE) of solution-processed CsPbX3 perovskite solar cells is still far inferior to that of their hybrid analogues. Insufficient film thickness and undesirable phase transition are the two major obstacles limiting their device performance. Here, we show that by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). We further introduced a judicious amount of phenylethylammonium iodide (PEAI) into the system to induce reduced-dimensional perovskite formation. Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed undesirable phase transition and thus reduced the film's trap density. Following this approach, we reported a state-of-the-art PCE to date, 12.4%, for reduced-dimensional α-CsPbI3 perovskite photovoltaics with greatly improved performance longevity.

Restoration of perovskite phase in the top layer of thin BTO film by plasma treatment and annealing

We report a simple method to restore the perovskite phase in the top surface/sub-surface region of a thin film (~100 nm) of barium titanate (BTO) fabricated by pulsed laser deposition on a platinized silicon surface and thus enhance its dielectric and ferroelectric properties. Phase evolution, surface morphology with local chemical composition of as-grown BTO films have been studied as a function of laser fluence. Investigations using x-ray diffraction, grazing-angle incidence x-ray diffraction and depth resolved x-ray photoelectron spectroscopy show that even after achieving a good phase formation there can be a presence of non-perovskite TiO2 phase at the surface and subsurface in such films that degrades its dielectric and ferroelectric response. The restoration of the degraded top layer was done by a combination of low energy Ar plasma treatment followed by an annealing process that enhances Ba content.

Thermochromic halide perovskite solar cells.

Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excellent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings, automobiles, information displays, and potentially many other technologies.

The stability of perovskite solar cells

Solar energy as a widespread clean energy is considered to be the key to solving future energy problems, solar cell technology is also evolving, of which perovskite solar cells because of its rapid growth efficiency gains wide attention. The great breakthroughs have been observed in organic-inorganic halide perovskite solar cells. However, the long-term stability due to decomposition and the toxicity of heavy metal lead have not been perfectly solved. Solving the stability problem will be the key to the future application of perovskite solar cells. This review focuses the factors affecting the stability of perovskite solar cells from the perspective of perovskite solar cell structure, including perovskite materials, hole transport materials, electron transport materials, and electrodes. Through the analysis we found that for perovskite materials, grain size and defect density affect the stability of perovskite. The grain with large size will inhibit the defect state, improve the stability of the device. Moisture, oxygen and light will promote the decomposition process of perovskite, even in the case of isolated air. The thermal stability of perovskite is also poor and will be self-decomposition at a certain temperature. While perovskite will also have the problem of crystal structure instability, the perovskite may changes to the non-perovskite phase, which will result in a sharp attenuation of efficiency. The dopant in the hole transport material may cause destruction and decomposition of the perovskite structure. The titanium dioxide commonly used in electron transport materials can lead to the decomposition of perovskite and the reduction of electron transport efficiency due to oxygen vacancy and strong electron extraction. Silver and aluminum commonly used in electrode will be associated with the perovskite layer formation of halide, which may reduce the conductivity of the electrode and destroy the perovskite structure. In order to solve the above problems, we propose that new preparation should be developed to get thin films with large grain size. The researchers also should find new perovskite material insensitive to moisture and oxygen, appropriate package to isolate the air from the water and oxygen. The inorganic perovskite materials with enhancing thermal stability of the perovskite film should be considered to improve the stability of perovskite solar cells. Developing organic hole transport materials that do not require additives or inorganic hole transport materials can reduce the possibility of perovskite crystals being destroyed by hole transport materials. Avoid direct contact between perovskite and titanium dioxide or use other electron transport materials instead of titanium dioxide to prevent the decomposition of perovskite. Furthermore, we recommend mesoscopic framework based on all inorganic metal oxides for organic-inorganic perovskite based photoelectric conversion devices.

Temperature Dependent Charge Carrier Dynamics in Formamidinium Lead Iodide Perovskite.

The fundamental opto-electronic properties of organic–inorganic hybrid perovskites are strongly affected by their structural parameters. These parameters are particularly critical in formamidinium lead iodide (FAPbI3), in which its large structural disorder leads to a non-perovskite yellow phase that hinders its photovoltaic performance. A clear understanding of how the structural parameters affect the opto-electronic properties is currently lacking. We have studied the opto-electronic properties of FAPbI3 using microwave conductivity measurements. We find that the mobility of FAPbI3 increases at low temperature following a phonon scattering behavior. Unlike methylammonium lead iodide (MAPbI3), there are no abrupt changes after the low-temperature β/γ phase transition and the lifetime is remarkably long. This absence of abrupt changes can be understood in terms of the reduced rotational freedom and smaller dipole moment of the formamidinium, as compared to methylammonium.

Bismuth Incorporation Stabilized α-CsPbI3 for Fully Inorganic Perovskite Solar Cells

All-inorganic CsPbI3 perovskite is emerging to be an alternative light-harvesting material in solar cells owing to the enhanced stability and comparable photovoltaic performance compared to organic–inorganic hybrid perovskites. However, the desirable black phase α-CsPbI3 is not stable at room temperature and degrades rapidly to a nonperovskite yellow phase δ-CsPbI3. Herein, we introduce a compositional engineering approach via incorporating Bi3+ in CsPbI3 to stabilize the α-phase at room temperature. Fully inorganic solar cells based on the Bi-incorporated α-CsPb1–xBixI3 compounds demonstrate a high PCE of 13.21% at an optimal condition (incorporation of 4 mol % Bi3+) and maintain 68% of the initial PCE for 168 h under ambient conditions without encapsulation. This is the first attempt of partial substitution of the “B”-site of the perovskite to stabilize the α-CsPbI3, which paves the way for further developments of such perovskites and other optoelectronic devices.

The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead Mixed-Halide Perovskites Solar Cells

Metal halide perovskite solar cells that use the inorganic cation Cs have been shown to have better thermal stability than the organic cation containing counterparts, and CsPbI2Br has a more suitable (lower) band gap than CsPbIBr2 as a photovoltaic energy harvesting material. However, increase in iodine content reduces structural stability due to the preference toward the non-perovskite orthorhombic phase when the film is exposed to air. In this work, the effect of varying stoichiometry of CsPbI2Br perovskite on film quality such as the grain size, presence of impurities and nature of impurity grains, photoluminescence, morphology, and elemental distribution are studied. Details on how to vary the stoichiometry during the dual source thermal evaporation process are reported. It is found that the air stability of CsPbI2Br film correlates with the CsBr-to-PbI2 deposition rate ratio, in which the CsBr-rich CsPbI2Br is the most stable upon air exposure, while the stoichiometrically balanced CsPbI2Br perovskit...

Structural, optical, and electrical properties of phase-controlled cesium lead iodide nanowires

Cesium lead iodide (CsPbI3), in its black perovskite phase, has a suitable bandgap and high quantum efficiency for photovoltaic applications. However, CsPbI3 tends to crystalize into a yellow non-perovskite phase, which has poor optoelectronic properties, at room temperature. Therefore, controlling the phase transition in CsPbI3 is critical for practical application of this material. Here we report a systematic study of the phase transition of one-dimensional CsPbI3 nanowires and their corresponding structural, optical, and electrical properties. We show the formation of perovskite black phase CsPbI3 nanowires from the non-perovskite yellow phase through rapid thermal quenching. Post-transformed black phase CsPbI3 nanowires exhibit increased photoluminescence emission intensity with a shrinking of the bandgap from 2.78 to 1.76 eV. The perovskite nanowires were photoconductive and showed a fast photoresponse and excellent stability at room temperature. These promising optical and electrical properties make the perovskite CsPbI3 nanowires attractive for a variety of nanoscale optoelectronic devices.