Residual PbI2 Research Articles

Zwitterion‐Functionalized SnO2 Substrate Induced Sequential Deposition of Black‐Phase FAPbI3 with Rearranged PbI2 Residue

Black-phase formamidinium lead iodide (FAPbI3 ) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black-phase crystallization and to eliminate the lead iodide (PbI2 ) residue, most sequential deposition methods of FAPbI3 -based perovskite will introduce external ions like methylammonium (MA+ ), cesium (Cs+ ), and bromide (Br- ) ions to the perovskite structure. Here a zwitterion-functionalized tin(IV) oxide (SnO2 ) is introduced as the electron-transport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI3 . The SnO2 ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI2 in the perovskite layer, which reduces the side effect of the residual PbI2 . Besides, the FSA functionalization also modifies SnO2 ETL to suppress deep-level defects at the perovskite/SnO2 interface. As a result, the FSA-FAPbI3 -based perovskite solar cells (PSCs) exhibit an excellent power conversion efficiency of up to 24.1% with 1000 h long-term operational stability. These findings provide a new interface engineering strategy on the sequential fabrication of black-phase FAPbI3 PSCs with improved optoelectronic performance.

Mandelic Acid as an Interfacial Modifier for High Performance NiOx‐based Inverted Perovskite Solar Cells

AbstractNickel oxide (NiOx) inverted perovskite solar cells have been witnessed as promising solar cells because of the low cast, easy fabrication and negligible hysteresis. However, the inferior interface feature of the link NiOx and perovskite layer often causes poor device property and repeatability. Here, Mandelic acid (MAD) is introduced to form a modified layer, which could improve the poor interface properties between perovskite layer and NiOx layer. The contact angel images revels MAD could increase the hydrophobicity and facilitate the growth of perovskite crystal. The X‐ray diffraction pattern and Fourier transform infrared spectroscopy have confirmed that the MAD small molecule could interact with residual PbI2 crystal, leading to better interface contact and high crystallinity. This is further demonstrated by the scanning electron microscopy test of NiOx/MAD/MAPbI3 film that average grain size is increased from 201.85 nm to 315.17 nm. The space‐charge limited current measurement proves the introduction of MAD molecule that result in the decreased trap state density from 3.58×1015 cm−3 to 2.721015 cm−3. As a consequence, The power conversion efficiency is significantly improved from 14.9% to 18.42% of MAPbI3‐based devices that retained nearly 80% initial PCE after 50 days in N2 without encapsulation.

Enhanced Hole Mobility and Decreased Ion Migration Originated from Interface Engineering for High Quality PSCs with Average FF beyond 80.

Perovskite solar cells (PSCs) have made significant progress in power conversion efficiency (PCE) by optimizing deposition method, composition, interface, etc. Although the two-step method demonstrates the advantage of being easy to operate, too much residual PbI2 not only forms defect centers, but affects the perovskite crystallization by arising more grain boundaries (GBs) due to the easy-to-crystallize nature of PbI2 . And GBs in polycrystalline perovskite usually provide main channel for ion migration, leading to accumulation of charges at the interface to form a barrier, thus reducing carrier mobility and resulting in degradation of perovskite devices. Here, an organic molecule N-(4-acetylphenyl)maleimide (N-APMI) is used to modify interface between perovskite and hole transport layer. X-ray photoelectron spectroscopy, scanning electron microscope, and nuclear magnetic resonance results show that ketone group (CO) in N-APMI forms a strong coordination with Pb2+ , which effectively reduces the residual amount of PbI2 nanoparticles on the perovskite surface, giving rise to improved crystallization of perovskite. Temperature-dependent current response demonstrates that ion migration is effectively suppressed, and hole mobility validly increases from 10.74 to 19.48 cm2 V-1 s-1 , leading to a champion fill factor (FF) of 82.5% (PCE 21.96%), and the maximum PCE of the device improves from 20.09% to 23.03%.

Cover Feature: A Self‐Formed Stable PbI2/NiOx Interface with Increased Ni3+ Centers for Perovskite Photovoltaics (Chem. Eur. J. 24/2022)

Ultraviolet ozone flushing (UVO) has been used to oxidize the NiOx hole-transport layer (yellow). The lattice bottom left shows the activation of Ni3+ (red balls) at the top of the NiOx film, which can enhance the electrical properties and hole extraction ability of NiOx. Then, by adopting sequential deposition of perovskites, residual PbI2 is automatically formed at the bottom interface (brown layer); this prevents the NiOx film from coming into direct contact with the perovskite layer, thus avoiding NiOx-related chemical reactions. More information can be found in the Research Article by S. Yang, Y. Hou, et al. (DOI: 10.1002/chem.202200202).

Low temperature-processed stable and high-efficiency carbon-based CsPbI2Br perovskite solar cells by additive strategy

All-inorganic perovskite solar cells require high-temperature preparation processes, which severely hinders their further commercial development. Inorganic perovskite materials have a high crystallization barrier, so the crystallinity is poor under low temperature conditions, resulting in poor device performance. Therefore, it is an important challenge to develop an excellent low-temperature preparation process. In this paper, the crystallization kinetics of the CsPbI2Br perovskite film under low temperature conditions was studied, and it was found that the perovskite was not completely crystallized during low temperature crystallization, and PbI2 remained. The PbI2 residue not only caused the appearance of lead defects but also formed insoluble CsBr due to the insufficient reaction of the raw materials. Through the interaction between the organic molecules 3-Amino-3-(3-pyridyl)propionic Acid (PA) and PbI2, the crystallization reaction process can be effectively improved. Eliminating the residual PbI2 also effectively inhibited the precipitation of CsBr, thereby significantly improved the crystallinity of CsPbI2Br. Efficiencies of 10.95% are achieved for low-temperature-processed CsPbI2Br planar-architecture FTO/SnO2/CsPbI2Br/carbon, and the obtained device also showed good stability in the air.

Methylammonium thiocyanate seeds assisted heterogeneous nucleation for achieving high-performance perovskite solar cells

Thiocyanate (SCN-) has been reported to manage crystallization of perovskite films and improve their optoelectronic properties in solar cells. However, the mechanism of SCN- in preparation of high-quality (MAFA)Pb(IBr)3 based light-absorbing layers has not been well investigated yet. In this work, we investigate the effects of SCN- on phase transition of (MAFA)Pb(IBr)3 perovskite films and the corresponding device performance by introducing methylammonium thiocyanate (MASCN) in a PbI2 precursor solution which is used to make perovskite by a two-step sequential deposition method. Characterization of the perovskite films has shown that a proper amount (1 mg/mL) of MASCN in PbI2 can produce PbI2-MASCN seeds which act as a nucleation center for assisting heterogeneous nucleation of the (MAFA)Pb(IBr)3. Meanwhile, MASCN helps to effectively control crystallization of the perovskite film by obtaining preferential crystal growth orientation. Furthermore, MASCN is beneficial to eliminate residual PbI2 and prepare perovskite films with low trap-state density. Consequently, the power conversion efficiency (PCE) of the perovskite solar cells increases from 18.36% to 20.4%. An enhanced device stability against moisture is also observed in the MASCN-treated device which exhibits 90% retention of the initial efficiency after exposed to the ambient air for 1780 h.

Internal Encapsulation for Lead Halide Perovskite Films for Efficient and Very Stable Solar Cells

AbstractExternal encapsulation technique as a straightforward craft process has been adopted to prevent the infiltration of moisture and oxygen, thereby improving environmental stabilities of lead halide perovskite solar cells (PSCs). However, irreversible light‐induced degradation originating from various vacancies and ion diffusion or migration inside the device cannot be efficiently solved by external encapsulation. Herein, an internal encapsulation strategy by introducing NbCl5 at the buried tin oxide/perovskite interface and spin‐casting n‐butylammonium bromide on top of perovskite is developed to comprehensively passivate the vacancies and hence block the channels for ion diffusion or migration. The internal encapsulation strategy results in better homogeneous electron transport layer and effective vacancy passivation at the buried interface and simultaneously generates a more homogeneous, better crystallized perovskite in the vertical direction with significantly reduced residual PbI2. Furthermore, fewer oxygen vacancies and formation of ultrathin Nb2O5 lead to a better interfacial energy level alignment for electron transfer. As a result, power conversion efficiency (PCE) of the resulting PSCs is as high as 24.01%. More importantly, the device demonstrates an excellent stability, retaining 88% of its initial PCE at its maximum power point tracking measurement (under 100 mW cm–2 white light illumination at ≈55 °C temperature, in N2 atmosphere) after 1000 h.

Wide Bandgap Interface Layer Induced Stabilized Perovskite/Silicon Tandem Solar Cells with Stability over Ten Thousand Hours

AbstractThe perovskite/silicon tandem solar cell (PK/c‐Si TSC) is a reasonable choice that can break through the efficiency limitations of silicon cells. Here, the p‐i‐n perovskite solar cell is conformally grown by the evaporation–solution combination technique on fully‐textured silicon heterojunction cells to realize two‐terminal PK/c‐Si TSCs. Due to the adverse effect of the residual PbI2 at the bottom of the perovskite bulk on device performance, a thermal‐evaporated CsBr thin layer is introduced between the perovskite layer and the hole transport layer to construct a gradient perovskite absorber for optimized energy level alignment, so as to improve the open‐circuit voltage and fill factor of the device. Finally, the PK/c‐Si tandem cell achieves an efficiency of 27.48% and is stable in nitrogen over 10 000 h.

Multifunctional 2D perovskite capping layer using cyclohexylmethylammonium bromide for highly efficient and stable perovskite solar cells

Constructing three-dimensional (3D)/two-dimensional (2D) hybrid perovskite heterojunction has been proved to be a promising approach to improve the power conversion efficiency (PCE) and environmental stability of perovskite solar cells (PSCs). Here, an organic ammonium salt, cyclohexylmethylammonium bromide (CMABr), is deposited atop the primary 3D perovskite film to form ultrathin 2D perovskite with residual PbI2 between the light-absorbing layer and the hole transport layer (HTL). The in-situ wide-band-gap 2D perovskite layer could play a multifunctional role in passivating the surface defects and protecting the underlying 3D perovskite film from humidity, resulting in the significantly suppressed non-radiative recombination process and superior moisture stability. As a result, a maximum PCE of 21.80% was obtained for the 3D/2D PSC as compared to that (19.62%) of pure 3D device. Moreover, the unencapsulated devices with 2D perovskite capping layer exhibited excellent long-term durability and mitigated hysteresis in contrast with the control device. Our study confirms that building 3D/2D perovskite heterojunctions by the utilization of organic ammonium salts is an effective way to develop highly efficient and stable PSCs.

Efficient p-n Heterojunction Perovskite Solar Cell without a Redundant Electron Transport Layer and Interface Engineering.

We report an electron transport layer-free perovskite solar cell (PSC) without interface engineering, whose key is a p-n heterojunction consisting of ITO/n-type FA0.9Cs0.1PbI3-xClx/p-type spiro-MeOTAD/Ag. The naturally matched energy levels between FA0.9Cs0.1PbI3-xClx and ITO make interface engineering unnecessary. The FA0.9Cs0.1PbI3-xClx film has a wide antisolvent processing window, favoring large-area production. The self-seeding growth method regulates the energy levels and work function of the FA0.9Cs0.1PbI3-xClx film via modulating the shape and distribution of residual PbI2. Interface band bending is confirmed by double-side photoluminescence (PL) measurement. Reduced PL is introduced for unambiguous charge transfer analysis without interference caused by different absorbance. Accordingly, enhanced ITO/perovskite interface energy level alignment and device performance (efficiency of 17.48% and Voc of 1.02 V) are demonstrated. Such simplified but efficient PSCs featuring a wide antisolvent processing window have great potential in practical applications.

Self-passivated hybrid perovskite films for improved photovoltaic performance of solar cells

Defect passivation management of perovskite films is of great importance in improving the performance of perovskite solar cells. Various in situ or post-passivation strategies were adopted to modify the optoelectric properties of perovskite films. However, these modifications increased the fabrication complexity and cost of the devices. Here, we developed a facile self-passivation strategy to effectively decrease charge trap states and suppress the non-radiative recombination loss in perovskite films. By simply aging the precursor solution, we found that the fabricated perovskite films possess residual PbI2 phase that could decrease trap density, enhance the radiative recombination intensity, and facilitate charge transfer and collection. Therefore, the solar cells with this self-passivated perovskite film generated an increased efficiency of 21.1% compared to that of 19.9% for the control devices from fresh precursor solution.

Residual PbI2 Beneficial in the Bulk or at the Interface? An Investigation Study in Sputtered NiOx Hole-Transport-Layer-Based Perovskite Solar Cells

Investigation of the perovskite (MAPbI3; MA+ = CH3NH3+) crystallization, location of residual lead iodide (PbI2), and its influence on the optoelectronic and photovoltaic properties is important to resolve the low-efficiency issue in radio frequency (RF) sputtered nickel oxide (sp-NiOx) based perovskite solar cells (PSCs). Despite obtaining the perovskite layer from a solution of the stoichiometric ratio of precursor materials using a one-step method (OSM) for spin-coating, residual PbI2 grains are observed (from the top surface scanning electron microscopy (SEM) image) in the bulk of perovskite (deposited on sp-NiOx). To investigate the dependence of residual PbI2 on the preparation methods, we prepared the perovskite (on sp-NiOx) by a two-step method (TSM) in which methylammonium iodide (MAI) solution is spin-coated on top of the deposited PbI2 layer. The cross-sectional SEM image of the TSM-based perovskite device, confirms the presence of the residual PbI2 layer (20–50 nm) at the interface of perovskite/NiOx. Although a higher X-ray diffraction (XRD) peak intensity of residual PbI2 was observed, the higher crystallinity of the perovskite film prepared by TSM was confirmed based on the evaluation of the photoluminescence (PL) peaks and the PL lifetime. In comparison with the devices prepared by OSM, the higher photovoltaic performance of PSCs is observed for devices fabricated using TSM. An investigation on the relation of the device performance with the location of residual PbI2 is presented in this work and can be insightful to further improve the performance of sp-NiOx-based PSCs.

Composition Engineering of All‐Inorganic Perovskite Film for Efficient and Operationally Stable Solar Cells

AbstractCesium‐based inorganic perovskites have recently attracted great research focus due to their excellent optoelectronic properties and thermal stability. However, the operational instability of all‐inorganic perovskites is still a main hindrance for the commercialization. Herein, a facile approach is reported to simultaneously enhance both the efficiency and long‐term stability for all‐inorganic CsPbI2.5Br0.5 perovskite solar cells via inducing excess lead iodide (PbI2) into the precursors. Comprehensive film and device characterizations are conducted to study the influences of excess PbI2 on the crystal quality, passivation effect, charge dynamics, and photovoltaic performance. It is found that excess PbI2 improves the crystallization process, producing high‐quality CsPbI2.5Br0.5 films with enlarged grain sizes, enhanced crystal orientation, and unchanged phase composition. The residual PbI2 at the grain boundaries also provides a passivation effect, which improves the optoelectronic properties and charge collection property in optimized devices, leading to a power conversion efficiency up to 17.1% with a high open‐circuit voltage of 1.25 V. More importantly, a remarkable long‐term operational stability is also achieved for the optimized CsPbI2.5Br0.5 solar cells, with less than 24% degradation drop at the maximum power point under continuous illumination for 420 h.

Effect of PbI2 passivation to grain boundary of perovskite film with cation and anion co-mixing on performance of photovoltaic devices

Perovskite components have tremendous influence on the film quality and device performance. There are obvious impurity defects, holes, etc. in the films of conventional perovskite solar cells (PSCs). In this work, the perovskite component was tuned to [HC(NH2)2]0.3(CH3NH3)0.7PbI2.4Cl0.3Br0.3 (FA0.3MA0.7PbI2.4Cl0.3Br0.3) via cation and anion co-mixing, it was observed that the residual PbI2 at perovskite film had a good passivation effect on the grain boundaries and further ameliorated the quality of the film. The photoelectric conversion efficiency (PCE) of low-cost carbon electrode-based photovoltaic devices we prepared increased from 8.58% to 12.40%. Our work will also be more beneficial to the commercialization of PSCs and the preparation of other optoelectronic devices.

Efficient Nanorod Array Perovskite Solar Cells: A Suitable Structure for High Strontium Substitution in Nature.

Organic-inorganic hybrid perovskite solar cells (PSCs) have become a research hotspot because of their excellent power conversion efficiency (PCE), but the presence of toxic lead (Pb) in perovskite films has significantly limited their commercial applications. In this study, using a TiO2 nanorod array (TNRA) as the electron transport layer, strontium chloride (SrCl2) was chosen to fabricate lead-less PSCs in air (relative humidity, RH = 50%) by a simple two-step spin-coating method. The influence of introduced strontium (Sr) on the perovskite films and cell properties was systematically investigated by various characterization methods. With an increasing Sr substitution amount from 0 to 15 mol %, the formed perovskite films with a compact structureand large crystalline size essentially remained invariable, while the amount of residual PbI2 was reduced, which is beneficial for the cell performance. An optimal PCE of 16.08% (average PCE = 15.37%) was obtained for the 5 mol % Sr-substituted PSCs because of the enhanced charge extraction from the perovskite films to the TNRAs and the suppressed charge recombination in the PSCs. Both the humidity and thermal stability of the Sr-substituted PSCs were improved. More importantly, the 15 mol % Sr-substituted PSCs still exhibited a PCE of 15.09% in air (RH = 50%), maintaining 99% of the cell efficiency of the pristine (0 mol % Sr) PSC (15.27%), suggesting that the structure of TNRAs is suitable for the synthesis of high-performance Sr-substituted lead-less PSCs.

Acetic Acid Assisted Crystallization Strategy for High Efficiency and Long-Term Stable Perovskite Solar Cell.

Improving the quality of perovskite poly‐crystalline film is essential for the performance of associated solar cells approaching their theoretical limit efficiency. Pinholes, unwanted defects, and nonperovskite phase can be easily generated during film formation, hampering device performance and stability. Here, a simple method is introduced to prepare perovskite film with excellent optoelectronic property by using acetic acid (Ac) as an antisolvent to control perovskite crystallization. Results from a variety of characterizations suggest that the small amount of Ac not only reduces the perovskite film roughness and residual PbI2 but also generates a passivation effect from the electron‐rich carbonyl group (C=O) in Ac. The best devices produce a PCE of 22.0% for Cs0.05FA0.80MA0.15Pb(I0.85Br0.15)3 and 23.0% for Cs0.05FA0.90MA0.05Pb(I0.95Br0.05)3 on 0.159 cm2 with negligible hysteresis. This further improves device stability producing a cell that maintained 96% of its initial efficiency after 2400 h storage in ambient environment (with controlled relative humidity (RH) <30%) without any encapsulation.

Limit of incorporating cesium cations into formamidinium-methylammonium based mixed halide perovskite solar cells

Incorporating formamidinium (FA) into methylammonium (MA) based perovskite has brought significant thermal and environmental stability including best device performance. It has been shown that addition of Cesium (Cs) makes perovskite robust in terms of thermodynamic stability as well. We explore the means of incorporating Cs into a base perovskite of mixed cation (FA/MA) and mixed halide (I/Br) that has a proven track record of high performance through an inter-diffusion approach. With this approach, it has been shown that perovskites form a smooth film without any residual PbI2 and exhibit higher absorbance. Though the residual PbI2 disappeared with the increase in added Cs, the film morphology became rough for Cs concentration higher than 15%. Addition of small amounts of PbCl2 allowed inclusion of more Cs content, which resulted in smooth film surface and further improved device performance.

Molecule occupancy by a n-butylamine treatment to facilitate the conversion of PbI2 to perovskite in sequential deposition.

Herein, we propose a new approach of molecule occupancy via a vapor treatment to facilitate the conversion of PbI2 to perovskite in sequential deposition. We have shown that the morphology of PbI2 and the subsequent crystallization of perovskite can be effectively tuned, thus leading to the elimination of residual PbI2 and promotion of perovskite growth.

Investigating the Growth of CH3NH3PbI3 Thin Films on RF‐Sputtered NiOx for Inverted Planar Perovskite Solar Cells: Effect of CH3NH3+ Halide Additives versus CH3NH3+ Halide Vapor Annealing

AbstractInvestigating the low efficiency issue of radio frequency‐sputtered nickel oxide (sp‐NiOx)‐based perovskite solar cells (PSCs) due to a limited understanding of the correlation between perovskite growth and sp‐NiOx on the optoelectronic properties and photovoltaic device performance is critical. Herein, the crystallization of methylammonium (MA) lead iodide (MAPbI3) thin film (obtained from stoichiometric precursor ratio) on sp‐NiOx is shown, resulting in appearance of residual PbI2 grains. This is in contrast to perovskite growth on solution‐processed NiOx. The amount of residual PbI2 is suppressed by 1) adding excess MACl/MAI additives and 2) annealing the perovskite film in MACl/MAI vapor atmosphere. Structural and morphological results reveal significant reduction in the amount of residual PbI2 and enhanced grain size for all the cases while photophysical measurements reveal mitigation of trap/defect sites (within the bulk and at the interfaces) only for MACl/MAI vapor annealing case. As a result, photovoltaic devices exhibit improved performance only for the vapor annealing case. These results elucidate the critical role of maintaining stoichiometric ratio in perovskite and its crystallization on sp‐NiOx by eliminating the associated defects (influenced by sp‐NiOx) in rendering improved performance, which can be insightful to further enhance the performance of PSCs.

Effect of Precursor Composition on Ion Migration in Hybrid Perovskite CH3NH3PbI3

We used thin-film transistors (TFTs) to understand the effect of precursor composition on ion migration in the hybrid perovskite CH3NH3PbI3(MAPbI3). For all the precursor compositions we investigated (i.e. MAI/PbI2 ratios of 0.5, 0.8, 1.0, and 1.2), the TFTs exhibited anomalous hysteresis in their current-voltage (I-V) characteristics, indicating mixed ionic-electronic conduction. The hysteresis in the TFTs with MAI/PbI2 = 0.5 was the largest, followed by that of the TFTs with ratios of 1.2, 1.0, and 0.8. Although residual PbI2 was the source of hysteresis in the PbI2-rich TFTs (MAI/PbI2 = 0.5), we conclude that it may enhance electronic conduction and minimize ion migration when present in small amounts (as in the case of MAI/PbI2 = 0.8). Ion vacancy defects in grain boundaries are pathways for ion migration and the PbI2 residues passivate these defects, there by minimizing the vacancy-mediated migration of ions in the MAPbI3.