Reproducibility Of Perovskite Solar Cells Research Articles

The Effect of Component Defects on the Performance of Perovskite Devices and the Low-Cost Preparation of High-Purity PbI2.

The efficiency and reproducibility of perovskite solar cells (PSCs) are significantly influenced by the purity of lead iodide (PbI2) in the raw materials used. Pb(OH)I has been identified as the primary impurity generated from PbI2 in water-based synthesis. Consequently, a comprehensive investigation into the impact of Pb(OH)I impurities on film and device performance is essential. In this study, PbI2, with varying stoichiometries, was synthesized to examine the effects of different Pb(OH)I levels on perovskite device performance. The characterization results revealed that even trace amounts of Pb(OH)I impede the formation of precursor prenucleation clusters. These impurities also increase the energy barrier of the α-phase and facilitate the transition of the intermediate phase to the δ-phase. These effects result in poor perovskite film morphology and sub-optimal photovoltaic device performance. To address these issues, a cost-effective method for preparing high-stoichiometry PbI2 was developed. The formation of Pb(OH)I was effectively inhibited through several strategies: adjusting solution pH and temperature, modifying material addition order, simplifying the precipitation-recrystallization process, and introducing H3PO2 as an additive. These modifications enabled the one-step synthesis of high-purity PbI2. PSCs prepared using this newly synthesized high-stoichiometry PbI2 demonstrated photovoltaic performance comparable to those fabricated with commercial PbI2 (purity ≥ 99.999%). Our novel method offers a cost-effective alternative for synthesizing high-stoichiometry PbI2, thereby providing a viable option for the production of high-performance PSCs.

Anti-solvent engineering to rapid purify PbI2 for efficient perovskite solar cells

The purity of PbI2 plays a critical role in determining the performance, stability, and reproducibility of perovskite solar cells (PSCs). However, the cost of high-purity PbI2 is much higher than that of low-purity, and even the expensive high-purity PbI2 suffers from batch variance issues. Although several purification methods have been developed, the straightforward and rapid purification of PbI2 remains a challenge. The solar cell efficiency achieved by previous direct purification of PbI2 is much lower than the state-of-the-art value. Therefore, people still doubt whether the low-purity PbI2 is eligible for the fabrication of efficient solar cells even after purification. Herein, we report an anti-solvent engineering strategy for the rapid and facile purification of low-purity PbI2, which allows us to efficiently eliminate both the insoluble and soluble impurities. Eventually, the formamidinium-cesium (FA0.95Cs0.05PbI3) PSCs made from directly purified PbI2 show much enhanced performance and reproducibility. A champion power conversion efficiency of 23.91% is realized, which is the highest value ever reported for PSCs made from purified low-purity PbI2. The in-depth PbI2 purification mechanism is further discussed from the perspective of solvent-coordination and solvent–solvent interactions by combining the donor number (DN) value of solvents and Hansen solution parameters. The results reveal that the weaker interaction between PbI2 and solvent, together with the stronger interaction between solvent and anti-solvent, is crucial for achieving successful purification. This study not only provides a simple and rapid method for the purification of PbI2 but also enhances our understanding of the selective crystallization of PbI2 and perovskite by elucidating the solvate and de-solvation processes of PbI2 within the perovskite precursor.

Crystal Growth Regulation of α‐FAPbI3 Perovskite Films for High‐Efficiency Solar Cells with Long‐Term Stability

AbstractThe two‐step sequentially deposition strategy has been widely used to produce high‐performance FAPbI3‐based solar cells. However, due to the rapid reaction between PbI2 and FAI, a dense perovskite film forms on top of the PbI2 layer immediately and blocks the FAI diffusion into the bottom of the PbI2 film for a complete reaction, which results in a low‐efficiency and limited reproducibility of perovskite solar cells (PSCs). Here, high‐quality α‐FAPbI3 perovskite films by crystal growth regulation with 4‐fluorobenzamide additives is fabricated. The additives can interact with FAI to suppress the fast reaction between the FAI and PbI2 and effectively passivate the under‐coordinated Pb2+ or I‐ defects. As a result, α‐FAPbI3 perovskite films with low trap density and large grain size are prepared. The modified PSCs present a high‐power conversion efficiency of 24.08%, maintaining 90% of their initial efficiency after 1400 h in high humidity. This study provides an efficient strategy of synergistic crystallization and passivation to form high‐quality α‐FAPbI3 films for high‐performance PSCs.

A novel natural scaffold layer improving efficiency, stability and reproducibility of Perovskite solar cells

In this study, our hypothesis was to demonstrate the usability of a natural clay structure as scaffold layer in perovskite solar cells (PSCs). Sepiolite, which is a natural and environmentally friendly clay structure, has a very high active surface area and can easily be dispersed in solvents. In addition we predicted that crystallization could easily occur on their surfaces due to their surface chemistry. In the study, we firstly used a natural clay as scaffold layer in PSCs. It is observed that, efficiency, reproducibility and stability of PSCs have been significantly improved. Improvements in efficiency have been observed to be between 30 and 50% depending on the type of perovskite solvent used. In addition, the surface chemistry of the sepiolite resulted in better crystallization as well as stability. Due to its high-water adsorption capability, sepiolite makes the perovskite crystal more stable by trapping the residual water molecules as well as penetrated water molecules from environment. Consequently, we demonstrated that, a natural, low-cost and environmentally friendly clay may be an alternative material which may contribute to the commercialization of PSCs.

(Invited) Functional Materials and Sustainable Process for Commercialization of Perovskite Solar Cells

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 25% and further improvements are expected up to 27%. Now, the research on perovskite solar cells (PSCs) has been moving toward commercialization. To facilitate commercialization of these great solar cells, some technical issues such as long-term stability, large scale fabrication process, and Pb-related hazardous materials need to be solved. Also, flexible perovskite solar cell using plastic substrate can be used in niche applications such as portable electrical chargers, electronic textiles, and large-scale industrial roofing.This talk is dealing with our recent efforts to facilitate commercialization of perovskite solar cells. For examples, we introduce a recycling technology of perovskite solar cells, which covers the regeneration process of Pb contained perovskite layer as well as recycling process of Au electrodes and transparent conducting oxide glass. Also, simple fabrication technologies for realizing large scale perovskite module are introduced and recent effort for achieving high efficiency module is going to be presented. Precursor technology is of great importance for yielding high efficiency and reproducibility of PSC, which is one of topics in this talk.

A thioacetamide additive-based hybrid (MA0.5FA0.5)PbI3 perovskite solar cells crossing 21 % efficiency with excellent long term stability

An additive in hybrid perovskite is playing a vital role in the increment of power conversion efficiency (PCE), stability, and reproducibility of perovskite solar cells (PVSCs). Although, single-phase α-FAPbI3 perovskite has an ideal band gap but is continuously transforming to δ–FAPbI3, which is non-photoactive. Here, we controlled the methylammonium (MA) and formamidinium (FA) ratio in the (MAxFA1-x)PbI3 perovskite composition and tuned its morphology with the help of the thioacetamide (TAA) Lewis base additive. The optimum MA:FA ratio and fine-tuning of TAA additive result in a highly crystalline, large grain size and smooth surface of the (MA0.5FA0.5)PbI3 perovskite film. These highly uniform thin films with 850 nm grain size offered a superior interaction between the perovskite material and the electron transport layer (ETL) and a longer lifetime yielding a high PCE. The (MA0.5FA0.5)PbI3+1% TAA-based champion device exhibited the highest PCE of 21.29% for a small area (0.09 cm2) and 18.32% PCE for a large area (1 cm2). The TAA-assisted devices exhibited high stability with >85% retention over 500 h. These results suggest that the (MA0.5FA0.5)PbI3 along with the 1% TAA additive is a promising absorber layer that can produce >21% PCE.

Uridine as additive in antisolvent for improving performance and reproducibility of perovskite solar cells

Uridine as additive in antisolvent for improving performance and reproducibility of perovskite solar cells

NH4Br‐Assisted Two‐Step‐Processing of Guanidinium‐Rich Perovskite Films for Extremely Stable Carbon‐Based Perovskite Solar Cells in Ambient Air

The substitution of a portion of methylammonium (MA) for guanidinium (GA) has been verified to be able to enhance the stability of MA‐based devices. However, high‐dose guanidinium cation will introduce localized distortions to the perovskite lattice structure and destroy the microstructure of the perovskite films, impairing the stability and reproducibility of perovskite solar cells (PSCs) eventually. Herein, for the first time, the NH4Br‐assisted all‐atmospheric two‐step process is adopted to fabricate GA‐rich (20%) perovskite films. The NH4Br induces the formation of the intermediate phase NH4PbI3 and alleviates the disorder of the octahedron caused by the big GA. Consequently, the modified perovskite film shows increased tolerance for the roughness fluctuation and reduced risk of forming voids and pinholes. The fabricated compact GA‐rich perovskite films behave extremely well in photovoltaic performance when assembled as carbon‐based perovskite solar cells, delivering a high power conversion efficiency (PCE) of 16.19% and stability against moisture and sunlight. Especially, the unencapsulated devices in ambient air sustain 95.1%, 91.8%, and 95.7% of their initial PCEs after 2400 h of storage, after 1000 h of 65 °C heat environment, and after 800 h of sunlight illumination, respectively.

A combined molecular dynamics and experimental study of two-step process enabling low-temperature formation of phase-pure α-FAPbI3.

It is well established that the lack of understanding the crystallization process in a two-step sequential deposition has a direct impact on efficiency, stability, and reproducibility of perovskite solar cells. Here, we try to understand the solid-solid phase transition occurring during the two-step sequential deposition of methylammonium lead iodide and formamidinium lead iodide. Using metadynamics, x-ray diffraction, and Raman spectroscopy, we reveal the microscopic details of this process. We find that the formation of perovskite proceeds through intermediate structures and report polymorphs found for methylammonium lead iodide and formamidinium lead iodide. From simulations, we discover a possible crystallization pathway for the highly efficient metastable α phase of formamidinium lead iodide. Guided by these simulations, we perform experiments that result in the low-temperature crystallization of phase-pure α-formamidinium lead iodide.

Facile Physical Modifications of Polymer Hole Transporting Layers for Efficient and Reproducible Perovskite Solar Cells with Fill Factor Exceeding 80%

The hole transport materials that interact with the indium tin oxide (ITO) surface can be processed into monomolecular layers (MLs), which often exhibit different surface and electronic properties than their thin‐film counterparts. Herein, it is found that poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) films (R‐PTAA) can be easily processed into ML (M‐PTAA) due to the van der Waals interaction between ITO and PTAA. However, compared with R‐PTAA, the work function (WF) and conductivity of M‐PTAA are simultaneously reduced by the charge transfer at the ITO/PTAA interface. To address this issue, a modified monomolecular layer strategy (m‐MLS) is developed, where a small amount of 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) is introduced to enhance the interaction force between ITO and PTAA. PTAA treated by m‐MLS (F‐PTAA) has a hydrophilic physical surface, closely matching electronic energy level with the perovskite layer and smaller bulk resistance. As a result, the efficiency and reproducibility of perovskite solar cells (PSCs) are substantially improved. PSCs based on F‐PTAA demonstrated the highest power conversion efficiency (PCE) of 19.7% with a fill factor of over 80%. This study inspires the development of novel interface modification materials, and provides a simple and convenient direction for the fabrication of high‐performance and reproducible inverted PSCs with high fill factors.

Assessment of Reproducibility, Hysteresis, and Stability Relations in Perovskite Solar Cells Using Machine Learning

Herein, the hysteresis and reproducibility of perovskite solar cells and their relations with power conversion efficiency and long‐term stability are analyzed using machine learning tools. A hysteresis dataset containing 387 cells from 194 articles in the literature is constructed and analyzed using association rule mining, while the reproducibility is analyzed using the pooled variance of 24 142 cells from 438 articles. It is found that mixed cation perovskites, two‐step spin coating or multiple spin coating in one step, dimethylformamide + dimethyl sulfoxide as precursor solution, poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine as hole transport layer (HTL), lithium bis(trifluoromethanesulfonyl)imide + 4‐tert‐butylpyridine + tris(2‐(1H‐pyrazol‐1‐yl)‐4‐tert‐butylpyridine) cobalt(III) as HTL dopant, and carbon as back contact are found to be beneficial for both low hysteresis and high reproducibility in regular (n–i–p) cells. In addition to the perovskite material and deposition techniques mentioned earlier, the toluene as antisolvent, bathocuproine as electron transport layer interlayer, and Ag as back contact are found to have positive impacts in inverted (p–i–n) cells. It is also found that those factors are also highly relevant for power conversion efficiency and stability, clearly relating these four most commonly discussed performance measures for perovskite cells.

Tetrahydrofuran as an Oxygen Donor Additive to Enhance Stability and Reproducibility of Perovskite Solar Cells Fabricated in High Relative Humidity (50%) Atmosphere

In sequential deposition method of lead‐halide perovskite material, the PbI2 layer morphology and remnant PbI2 play an important role in enhancing the power conversion efficiency (PCE) of the perovskite solar cell. However, humidity levels affect the PbI2 and perovskite film morphology, resulting in defect sites and recombination centers on the surface and within the bulk of the material, thus impeding the overall device performance and stability. To address this, incorporation of tetrahydrofuran (THF) additive in PbI2–dimethylformamide (DMF) precursor solution is reported, to improve the quality of PbI2 thin films and to prevent the water interaction directly with PbI2 under high humidity environments. The O‐donor THF interacts with PbI2, resulting in a homogeneous, dense, and pinhole‐free layer as compared with the PbI2 layer without additive. The perovskite layer so obtained from the pinhole‐free PbI2 layer is compact, resulting in a significant reduction of defects/traps. The device is fabricated with modified perovskite in ≈50% humidity atmosphere, resulting in 15% efficiency with high reproducibility. Moreover, the THF‐modified non‐encapsulated perovskite device retains 80% PCE after exposure to 50% relative humidity for 20 days. A strategy to fabricate perovskite solar cells, with reproducible efficiency in high humidity atmosphere viable for large‐scale production, is demonstrated.

Optimization of anti-solvent engineering toward high performance perovskite solar cells

Abstract

Direct formed tri-iodide ions stabilizing colloidal precursor solution and promoting the reproducibility of perovskite solar cells by solution process

Solution process is the most widely used method to prepare perovskite films for efficient solar cells due to its facile fabrication and low-cost. However, the aged perovskite precursor solutions usually suffer from degradation, which impedes the reproducibility of the solution. In this paper, we demonstrate the direct formed tri-iodide ions stabilizing colloidal precursor solution for efficient halide perovskite solar cells. The added I2 molecules dissolved in perovskite precursor sustain the dynamic balance between I− ions and I3− ions by the reversible reaction: I− + I2 ⇌ I3−, which effectively stabilizes the whole I− ion concentration in perovskite solution and prevents the colloidal particles from aggregation. The resulting perovskite films with better crystallization have less bulk defects that act as carrier recombination centers. As a result, the devices using the aged solution with the additional tri-iodide ions show the less efficiency drop from ∼19% to ∼16% with the increased aging time up to 15 days, compared with the devices using the as-prepared precursor solution degrading from ∼18% to ∼6%. Our results represent an important advance in the stabilization of perovskite colloidal precursor and promote the reproducibility of perovskite solar cells by solution process.

Interface engineering of C60/ fluorine doped tin oxide on the photovoltaic performance of perovskite solar cells using the physical vapor deposition technique

Fullerene (C60) has been demonstrated, using vapor deposition, to be a good electron transport layer (ETL) and different thicknesses have been utilized in n-i-p configuration perovskite solar cells. However, an underlying reason for the variation in thicknesses, which hinders the reproducibility of perovskite solar cells employing a C60 ETL, has not been well-examined. This study reveals that the surface roughness of the conducting glass, such as fluorine doped tin oxide (FTO), affects the photovoltaic performance while optimizing the thickness of the vacuum deposited compact C60 ETL. A low-thickness C60 ETL retains a surface roughness and Ohmic behavior similar to bare FTO due to physical defects at the C60/FTO interface. Increasing the thickness further reduces the defects at the C60/FTO interface and facilitates an enhanced electron extraction from a vacuum co-deposited methyl-ammonium lead iodide perovskite light absorber. As a result, a perovskite solar cell with a homogenously covered C60 ETL on an FTO substrate delivers a power conversion efficiency of 14.63%. These comprehensive characterizations support the finding that suppression of defects at the C60/FTO interface results in an improved photovoltaic performance. This work demonstrates that the surface roughness of FTO needs to be considered for a decisive compact ETL for enhanced photovoltaic performance and reproducibility.

Unveiling the Role of tBP-LiTFSI Complexes in Perovskite Solar Cells.

Lead halide-based perovskite materials have been applied as an intrinsic layer for next-generation photovoltaic devices. However, the stability and performance reproducibility of perovskite solar cells (PSCs) needs to be further improved to match that of silicon photovoltaic devices before they can be commercialized. One of the major bottlenecks that hinders the improvement of device stability/reproducibility is the additives in the hole-transport layer, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4- tert-butylpyridine (tBP). Despite the positive effects of these hole-transport layer additives, LiTFSI is hygroscopic and can adsorb moisture to accelerate the perovskite decomposition. On the other hand, tBP, the only liquid component in PSCs, which evaporates easily, is corrosive to perovskite materials. Since 2012, the empirical molar ratio 6:1 tBP:LiTFSI has been wildly applied in PSCs without further concerns. In this study, the formation of tBP-LiTFSI complexes at various molar ratios has been discovered and investigated thoroughly. These complexes in PSCs can alleviate the negative effects (decomposition and corrosion) of individual components tBP and LiTFSI while maintaining their positive effects on perovskite materials. Consequently, a minor change in tBP:LiTFSI ratio results in huge influences on the stability of perovskite. Due to the existence of uncomplexed tBP in the 6:1 tBP:LiTFSI mixture, this empirical tBP-LiTFSI molar ratio has been demonstrated not as the ideal ratio in PSCs. Instead, the 4:1 tBP:LiTFSI mixture, in which all components are complexed, shows all positive effects of the hole-transport layer components with dramatically reduced negative effects. It minimizes the hygroscopicity of LiTFSI, while lowering the evaporation speed and corrosive effect of tBP. As a result, the PSCs fabricated with this tBP:LiTFSI ratio have the highest average device efficiency and obviously decreased efficiency variation with enhanced device stability, which is proposed as the golden ratio in PSCs. Our understanding of interactions between hole-transport layer additives and perovskite on a molecular level shows the pathway to further improve the PSCs' stability and performance reproducibility to make them a step closer to large-scale manufacturing.

Gas-solid reaction based over one-micrometer thick stable perovskite films for efficient solar cells and modules

Besides high efficiency, the stability and reproducibility of perovskite solar cells (PSCs) are also key for their commercialization. Herein, we report a simple perovskite formation method to fabricate perovskite films with thickness over 1 μm in ambient condition on the basis of the fast gas−solid reaction of chlorine-incorporated hydrogen lead triiodide and methylamine gas. The resultant thick and smooth chlorine-incorporated perovskite films exhibit full coverage, improved crystallinity, low surface roughness and low thickness variation. The resultant PSCs achieve an average power conversion efficiency of 19.1 ± 0.4% with good reproducibility. Meanwhile, this method enables an active area efficiency of 15.3% for 5 cm × 5 cm solar modules. The un-encapsulated PSCs exhibit an excellent T80 lifetime exceeding 1600 h under continuous operation conditions in dry nitrogen environment.

Improved performance and reproducibility of perovskite solar cells by jointly tuning the hole transport layer and the perovskite layer deposition

Solution processed organometal trihalide materials spur tremendous attention due to their unprecedented performance in photovoltaic applications. However, submicron thick perovskite films are prone to morphological defects in the form of cracks, pinholes and porosity; the traits originated from their solution phase processing and subsequent crystallization. Moreover, pinholes and cracks in the thin film of spincoated Spiro-OMeTAD hole transport layer reduce the performance reliability by forming micro shorts and weaken the defense against moisture ingress to the perovskite layer. For the large scale processing of perovskite solar cell from the economically prudent solution phase processing, morphological shortcomings of both perovskite and hole transport layers need an urgent address. By selecting non-conventional lead precursor (lead acetate) and implementing anti-solvent treatment during film deposition, we able to form pinhole free and compact perovskite film. Crack free hole conducting layer is obtained by blending Spiro-OMeTAD with a conducting polymer without compromising in the solar cell performance. A detail investigation of the charge transport and charge extraction properties of the developed hole transport layers have been carried out. The developed CH3NH3PbI3 based perovskite solar cells show improved repeatability and performance.

Enhanced crystallinity of CH3NH3PbI3 by the pre-coordination of PbI2-DMSO powders for highly reproducible and efficient planar heterojunction perovskite solar cells.

Solution processable CH3NH3PbI3 has received considerable attention for highly-efficient perovskite solar cells. However, the different solubility of PbI2 and CH3NH3I is problematic, initiating active solvent engineering research using dimethyl sulfoxide (DMSO). Here we investigated the pre-coordination of PbI2–DMSO powders for planar heterojunction perovskite solar cells fabricated by a low-temperature process (≤100 °C). Pre-coordination was carried out by simple mechanical mixing using a mortar and pestle. The composition of PbI2–DMSOx (x = 0, 1, or 2) in the powder mixture was investigated by gradually increasing mechanical mixing time, and a dominant composition of PbI2–DMSO1 was obtained after a 10 min mixing process. The pre-coordinated PbI2–DMSO powders were then blended with CH3NH3I in DMF to make the CH3NH3PbI3 film by toluene-assisted spin-coating and heat treatment. Compared with the one-step blending of CH3NH3I, PbI2, and DMSO in DMF, the pre-coordination method resulted in better dissolution of PbI2, larger grain size, and pinhole-free morphology. Consequently, absorption, fluorescence, carrier lifetime, and charge extraction were enhanced. The average open-circuit voltage (1.046 V), short-circuit current (22.9 mA cm−2), fill factor (73.5%), and power conversion efficiency (17.6%) were increased by 2–12% with decreased standard deviations (13–50%), compared with the one-step blending method. The best efficiency was 18.2%. The simple mechanical pre-coordination of PbI2–DMSO powders was very effective in enhancing the crystallinity of CH3NH3PbI3 and photovoltaic performance.

Al2 O3 Underlayer Prepared by Atomic Layer Deposition for Efficient Perovskite Solar Cells.

Perovskite solar cells, as an emergent technology for solar energy conversion, have attracted much attention in the solar cell community by demonstrating impressive enhancement in power conversion efficiencies. However, the high temperature and manually processed TiO2 underlayer prepared by spray pyrolysis significantly limit the large-scale application and device reproducibility of perovskite solar cells. In this study, lowtemperature atomic layer deposition (ALD) is used to prepare a compact Al2 O3 underlayer for perovskite solar cells. The thickness of the Al2 O3 layer can be controlled well by adjusting the deposition cycles during the ALD process. An optimal Al2 O3 layer effectively blocks electron recombination at the perovskite/fluorine-doped tin oxide interface and sufficiently transports electrons through tunneling. Perovskite solar cells fabricated with an Al2 O3 layer demonstrated a highest efficiency of 16.2 % for the sample with 50 ALD cycles (ca. 5 nm), which is a significant improvement over underlayer-free PSCs, which have a maximum efficiency of 11.0 %. Detailed characterization confirms that the thickness of the Al2 O3 underlayer significantly influences the charge transfer resistance and electron recombination processes in the devices. Furthermore, this work shows the feasibility of using a high band-gap semiconductor such as Al2 O3 as the underlayer in perovskite solar cells and opens up pathways to use ALD Al2 O3 underlayers for flexible solar cells.