High-performance Planar Perovskite Solar Cells Research Articles

Perovskite Crystallization Regulation via Antimonene Quantum Sheets for Highly Efficient and Stable Solar Cells.

The two-step deposition method offers significant advantages in the production of high-performance planar perovskite solar cells (PSCs). Nevertheless, there are still numerous challenges in regulating perovskite crystallization during the two-step process. In this work, two-dimensional (2D) material antimonene quantum sheets (AMQSs) as an additive are introduced to regulate the crystallization process of perovskite. As a result, perovskite films with high crystalline quality and vertical growth orientation are obtained by AMQSs providing heterogeneous nucleation sites with the penetration of a mixture solution of AMQSs and FAI into the PbI2 layer. Also, the influence mechanism of AMQSs on the crystallization of perovskite film is analyzed in details. At the same time, due to the chemical interaction between antimonene and the uncoordinated Pb2+, the defects in the perovskite are efficiently passivated. In addition, the energy level at the perovskite/SnO2 interface becomes more matched, leading to improved charge transport and extraction with the incorporation of AMQSs. Benefiting from the versatile AMQSs, the power conversion efficiency (PCE) of PSCs made by PbI2 + FAI:AMQSs is improved from 20.65 to 22.31% with the vastly enhanced Jsc and Voc. The ambient and operational stability of the unencapsulated PSCs fabricated using the PbI2 + FAI:AMQSs method were significantly improved, retaining 80% of the original PCE after being stored in a dark environment at a relative humidity of 30-40% for 18 days and 83% of the original PCE following continuous AM 1.5G illumination for 200 h.

A functionalized phosphorous tetrabenzotriazacorrole dye as a novel passivating agent of high-performance planar perovskite solar cells

Surface passivation is a crucial strategy to manage the film characteristics of solution-processed perovskite absorber layers, which greatly impact the photovoltaic properties of perovskite solar cells (PSCs). However, designing and synthesizing suitable passivating agents with required functional groups and solution-processability is challenging due to their complex molecular structure. In this study, we demonstrate a facile passivation technique using a functionalized phosphorous tetrabenzotriazacorrole dye (PTBC) for p-i-n PSCs. The tetrabenzotriazacorrole dye has amines and ether linkages that can closely interact with the undercoordinated Pb2+ with halogen vacancies through Lewis acid-base interactions, leading to a pinhole-free perovskite absorber layer with reduced trap density. Moreover, the central phosphorus of PTBC is functionalized with dimethoxy groups, which can provide a more hydrophobic surface in the perovskite absorber layer. Consequently, high-quality perovskite films (CsFAPbI3) prepared with PTBC exhibit improved optical, electrical, and morphological properties, as well as superior carrier transportation/recombination kinetics, boosting the power conversion efficiency of PSCs from 17.64 % to 20.08 %, with over 75 % of initial efficiency preserved after 1000 h of aging under ambient conditions. These results highlight PTBC as an effective surface passivation agent for perovskite absorber layers and emphasize the importance of suitable passivating agents in optimizing PSC performance.

Inhibited Aggregation of Lithium Salt in Spiro-OMeTAD for Perovskite Solar Cells

High-efficiency and stable hole transport materials (HTMs) play an essential role in high-performance planar perovskite solar cells (PSCs). 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-fluorene (Spiro-OMeTAD) is often used as HTMs in perovskite solar cells because of its excellent characteristics, such as energy level matching with perovskite, good film-forming ability, and high solubility. However, the accumulation and hydrolysis of the common additive Li-TFSI in Spiro-OMeTAD can cause voids/pinholes in the hole transport layer (HTL), which reduces the efficiency of the PSCs. In order to improve the functional characteristics of HTMs, in this work, we first used CsI as a dopant to modify the HTL and reduce the voids in the HTL. A small amount of CsI is introduced into Spiro-OMeTAD together with Li-TFSI and 4-tert-butylpyridine (TBP). It is found that CsI and TBP formed a complex, which prevented the rapid evaporation of TBP and eliminated some cracks in Spiro-OMeTAD. Moreover, the uniformly dispersed TBP inhibits the agglomeration of Li-TFSI in Spiro-OMeTAD, so that the effective oxidation reaction between Spiro-OMeTAD and air produces Spiro-OMeTAD+ in the oxidation state, thereby increasing the conductivity and adjusting the HTL energy. Correspondingly, the PCE of the planar PSC of the CsI-modified Spiro-OMeTAD is up to 13.31%. In contrast, the PSC without CsI modification showed a poor PCE of 10.01%. More importantly, the PSC of Spiro-OMeTAD treated with CsI has negligible hysteresis and excellent long-term stability. Our work provides a low-cost, simple, and effective method for improving the performance of hole transport materials and perovskite solar cells.

Suppressing Residual Lead Iodide and Defects in Sequential-Deposited Perovskite Solar Cell via Bidentate Potassium Dichloroacetate Ligand.

In sequential-deposited polycrystalline perovskite solar cells, the unreacted lead iodide due to incomplete conversion of lead iodide to perovskite phase, can contribute to ionic defects, such as residual lead ions (Pb2+ ). At present, passivation of interfacial and grain boundary defects has become an effective strategy to suppress charge recombination. Here, we introduced potassium acetate (KAc) and potassium dichloroacetate (KAcCl2 ) as additives in the sequential deposition of polycrystalline perovskite thin films and found that acetate ions (Ac- ) can effectively reduce the residual lead iodide. Compared with acetate (Ac), dichloroacetate (AcCl2 ) can form Pb-Cl and Pb-O bonding as "dual anchoring" bonds with residual Pb2+ , resulting in strong binding force and effective passivation of residual Pb2+ defects. Furthermore, K+ can enlarge grain size and restrain ion migration at the grain boundaries. Consequently, perovskite solar cells with KAcCl2 additive show power conversion efficiencies (PCE) from 19.67 % to 22.12 %, with the open-circuit voltage increasing from 1.06 V to 1.14 V. The unencapsulated device can maintain 82 % of the initial PCE under a humidity of 30±5 % for 1200 h. This work provides a new approach for the regulation of ionic defects and grain boundaries at the same time to develop high-performance planar perovskite solar cells.

High-Performance Planar Perovskite Solar Cells with a Reduced Energy Barrier and Enhanced Charge Extraction via a Na2WO4-Modified SnO2 Electron Transport Layer.

Tin oxide (SnO2) has been commonly used as an electron transport layer (ETL) in planar perovskite solar cells (p-PSCs) because it can be prepared by a low-temperature solution-processed method. However, the device performance has been restricted due to the limited electrical performance of SnO2 and its mismatched energy level alignment with the perovskite absorber. Considering these problems, sodium tungstate (Na2WO4) has been employed to modify the SnO2 ETL. The conduction band minimum of SnO2 increases and the defects at the ETL/perovskite interface decrease by the modification of the SnO2 ETL with Na2WO4, thus reducing the energy barrier between the ETL and perovskite. In addition, the electron extraction ability has been enhanced and the interface recombination between the ETL and perovskite has also been inhibited. As a result, the photovoltaic performance of p-PSCs based on the modified ETL has been improved, and a champion power conversion efficiency of 21.16% has been achieved compared with the control device of 17.30% with an open circuit voltage increased from 1.075 to 1.162 V.

Solution-processed Cu-doped SnO2 as an effective electron transporting layer for High-Performance planar perovskite solar cells

A high-quality electron transport layer (ETL) is indispensable for effective and stable planar perovskite solar cells (PSCs). Tin oxide (SnO2) is considered the most promising alternative to TiO2 as ETL due to its excellent optical and electrical properties. However, some of improvements need to be carried out for intrinsic SnO2, such as conductivity, energy loss, and lattice mismatch in the SnO2/perovskite interface. Herein, the Cu-doped SnO2 (Cu: SnO2) as ETL has been made through mixing the copper salt precursor solution with SnO2 colloidal solution in room-temperature solution-processed. The Cu incorporated into SnO2 increases the conductivity of ETL and uplifts the conduction band to obtain a more suitable band alignment in SnO2/perovskite interface for relieving the energy barrier in the charge transporting. Moreover, it further improves the quality of perovskite films with the enhanced crystallinity, enlarged grain size, which have an impact on light absorption and reduction of defect states for the perovskite layer. The power conversion efficiency (PCE) of the prepared planar PSCs with Cu:SnO2 ETL exceeds 21%, compared with the SnO2-based PSCs (19.63%), and the long-term stability was improved.

MoO3 doped PTAA for high-performance inverted perovskite solar cells

The hole transport layer (HTL) plays a key role in determining the performance of planar perovskite solar cells (PSCs). Poly[bis(4-phenyl)(2,4, 6-triMethylphenyl)aMine] (PTAA), a polymer with good stability, is one of the promising HTL candidates for PSCs. However, notable interfacial carrier recombination limits the performance of PSCs based on PTAA HTL, due to the large gap between the highest-occupied molecular orbital (HOMO) of PTAA and the valance band maximum (VBM) of MAPbI3 perovskite as well as the low intrinsic hole mobility of PTAA. Herein, PTAA was doped by 3 wt% of molybdenum oxide (MoO3) to move down its HOMO by 0.16 eV, leading to a better matching with the MAPbI3 perovskite. Meanwhile, the MoO3 dopant was demonstrated to improve the hole transport in PTAA and hole extraction at the PTAA/perovskite interface. Eventually, the fabricated inverted planar PSCs using PTAA HTL with 3 wt% MoO3 achieved the PCE of 20.06% as compared to that of 17.71% by using pristine PTAA HTL. The present work thus proposes a simple approach to utilize MoO3 as a dopant to substantially improve the molecular HTLs for high-performance PSCs and other perovskite-based optoelectronics.

Diluted-CdS Quantum Dot-Assisted SnO2 Electron Transport Layer with Excellent Conductivity and Suitable Band Alignment for High-Performance Planar Perovskite Solar Cells.

An electron transport layer (ETL) with excellent conductivity and suitable band alignment plays a key role in accelerating charge extraction and transfer for achieving highly efficient planar perovskite solar cells (PSCs). Herein, a novel diluted-cadmium sulfide quantum dot (CdS QD)-assisted SnO2 ETL has been developed with a low-temperature fabrication process. The slight addition of CdS QDs first enhances the crystallinity and flatness of SnO2 ETLs so that it provides a promising workstation to obtain high-quality perovskite absorption layers. It also amazingly increases the conductivity of the SnO2 ETL by an order of magnitude and regulates the energy level matching between the SnO2 ETL and perovskite. These outstanding properties greatly accelerate the charge extraction and transfer. Thus, the MAPbI3-based PSCs with such a diluted-CdSQD-assisted SnO2 ETL achieve a maximum power conversion efficiency of 20.78% and obtain a better stability of devices in air. These findings testify the importance and potential of semiconductor QD modification on ETLs, which may pave the way for developing such composite ETLs for further enhancing photovoltaic performance of planar PSCs.

Low-temperature processed tantalum/niobium co-doped TiO2 electron transport layer for high-performance planar perovskite solar cells

A low-temperature preparation process is significantly important for scalable and flexible devices. However, the serious interface defects between the normally used titanium dioxide (TiO2) electron transport layer (ETL) obtained via a low-temperature method and perovskite suppress the further improvement of perovskite solar cells (PSCs). Here, we develop a facile low-temperature chemical bath method to prepare a TiO2 ETL with tantalum (Ta) and niobium (Nb) co-doping. Systematic investigations indicate that Ta/Nb co-doping could increase the conduction band level of TiO2 and could decrease the trap-state density, boosting electron injection efficiency and reducing the charge recombination between the perovskite/ETL interface. A superior power conversion efficiency of 19.44% can be achieved by a planar PSC with a Ta/Nb co-doped TiO2 ETL, which is much higher than that of pristine TiO2 (17.60%). Our achievements in this work provide new insights on low-temperature fabrication of low-cost and highly efficient PSCs.

Low-Temperature synthesis of FeOOH Quantum Dots as Promising Electron-Transporting Layers for High-Performance Planar Perovskite Solar Cells

The development of a low-temperature processed electron-transporting layers (ETL) is highly desirable for high-performance planar perovskite solar cells (PSCs) toward large-scale and flexible devices, yet remains a great challenge. Herein, a low-temperature route was applied to synthesize FeOOH quantum dots (QDs) as promsing ETL for PSCs. A systematic survey of the structure, morphology and band structure was carried out, and the device performance further illustrates that the device based on FeOOH QDs ETL achieves a high efficiency of 17.3% with negligible hysteresis. Moreover, a stable current output at 0.8 V in 400 s was observed in the champion device. As a result, the low-temperature preparation of FeOOH QDs ETL and the corresponding excellent performance of PSCs present great commercial potential for future applications.

Fluorine substitution position effects on spiro(fluorene-9,9′-xanthene) cored hole transporting materials for high-performance planar perovskite solar cells

Fluorine substitution in molecular design has become an effective strategy for improving the overall performance of organic photovoltaics. In this study, three low-cost small molecules of spiro-linked hole transporting materials (SFX-o-2F, SFX-m-2F, and SFX-p-2F) endowed with two-armed triphenylamine moieties were synthesized via tuning of the fluorine substitution position, and they were employed for use in highly efficient perovskite solar cells (PSCs). Despite the fluorine substitution position playing a negligible role in the optical and electrochemical properties of the resulting small molecules, the photovoltaic performance thereof was observed to vary significantly. The planar n-i-p PSCs based on SFX-m-2F demonstrated superior performance (18.86%) when compared to that of the corresponding SFX-o-2F (9.7%) and SFX-p-2F (16.33%) under 100 mW cm−2 AM1.5G solar illumination, which is competitive with the performance of the benchmark spiro-OMeTAD-based device (18.98%). Moreover, the SFX-m-2F-based PSCs were observed to be more stable than the spiro-OMeTAD-based devices under ambient conditions. The improved performance of SFX-m-2F is primarily associated with improved morphology, more efficient hole transport, and extraction characteristics at the perovskite/HTM interface. This work demonstrated the application of fluorination engineering to the tuning of material film morphology and charge transfer properties, showing the promising potential of fluorinated SM-HTMs for the construction of low-cost, high-efficiency PSCs.

The dual interfacial modification of 2D g-C3N4 for high-efficiency and stable planar perovskite solar cells.

Carrier recombination and charge loss at the interfaces of perovskite layers have a significant influence on high-performance planar perovskite solar cells (PSCs). We employed two-dimensional graphitic carbon nitride (g-C3N4), which is a heat-resistant n-type semiconductor, to modify the electron-transport layer/perovskite and perovskite/hole-transport layer interfaces, respectively. g-C3N4 could passivate the surface trap states of the methylammonium lead iodide light absorber through the formation of a Lewis adduct between N and the under-coordinated Pb, and it could also remarkably reduce the grain boundaries between perovskite crystal particles. A maximum power conversion efficiency (PCE) of 19.67% (Voc = 1.14 V, Jsc = 21.45 mA cm−2, FF = 0.807) could be obtained from planar PSCs with long-term stability using dual-positioned g-C3N4. Therefore, we consider that ultrathin semiconductor films with a Lewis base nature are suitable as dual-functional transport materials for devices. This work provides new guidance for dual-interfacial modification to improve the PCE and stability of devices.

Red‐Carbon‐Quantum‐Dot‐Doped SnO2 Composite with Enhanced Electron Mobility for Efficient and Stable Perovskite Solar Cells

An efficient electron transport layer (ETL) plays a key role in promoting carrier separation and electron extraction in planar perovskite solar cells (PSCs). An effective composite ETL is fabricated using carboxylic-acid- and hydroxyl-rich red-carbon quantum dots (RCQs) to dope low-temperature solution-processed SnO2 , which dramatically increases its electron mobility by ≈20 times from 9.32 × 10-4 to 1.73 × 10-2 cm2 V-1 s-1 . The mobility achieved is one of the highest reported electron mobilities for modified SnO2 . Fabricated planar PSCs based on this novel SnO2 ETL demonstrate an outstanding improvement in efficiency from 19.15% for PSCs without RCQs up to 22.77% and have enhanced long-term stability against humidity, preserving over 95% of the initial efficiency after 1000 h under 40-60% humidity at 25 °C. These significant achievements are solely attributed to the excellent electron mobility of the novel ETL, which is also proven to help the passivation of traps/defects at the ETL/perovskite interface and to promote the formation of highly crystallized perovskite, with an enhanced phase purity and uniformity over a large area. These results demonstrate that inexpensive RCQs are simple but excellent additives for producing efficient ETLs in stable high-performance PSCs as well as other perovskite-based optoelectronics.

Low-temperature growth of uniform ultrathin TiO2 blocking layer for efficient perovskite solar cell

Abstract Although chemical bath of TiCl4 treatment has been used to serve as a compact layer for planar perovskite solar cells (PSCs), uneven TiO2 particle sizes and chlorine residual would decrease the device performance. In this study, we modify the TiCl4 treatment process through well-controlled the kinetics grown of TiO2 nanoparticle, leading to a uniform and ultrathin TiO2 blocking layer. A dense and pinholeless TiO2 blocking layer is crucial for high-performance PSCs because of its effective charge extraction and elimination of carrier shunting path. The as-fabricated devices using modified process of TiO2 blocking layer exhibit the best power conversion efficiency (PCE) of 17.89% with an average PCE of 17.5%. Scanning electron microscopy and cyclic voltammetry analysis of TiO2 blocking layer reveal that this process improved surface uniformity and the carrier blocking capability.

Ethyl acetate green antisolvent process for high-performance planar low-temperature SnO2-based perovskite solar cells made in ambient air

One-step antisolvent deposition has been considered as one of the most feasible methods to obtain high-performance perovskite solar cells (PSCs). However, most of the reported high-performance PSCs are based on the toxic anti-solvents, which is a major issue for the potential commercialization of PSCs. SnO2 has been successfully used as an efficient electron transport layer (ETL) material in PSCs, but the preparation of low-temperature processed crystallization SnO2 ETLs is still a challenge. In this work, ethyl acetate (EA) as a green antisolvent is introduced into the perovskite crystallization process, resulting in uniform and compact perovskite films with large grain size, reduced grain boundaries, and low defect density. Low-temperature (100 °C) processed SnO2 ETL provides good interface contact between ETL and perovskite layer, facilitating photoelectron extraction and transport. As a result, a champion power conversion efficiency (PCE) of 17.83% has been achieved. More importantly, unencapsulated PSC retains 84.80% of its original PCE value after storage in atmosphere for 80 days (>1900 h). Apart from great air stability, the final devices also show excellent thermal (100 °C) stability. It is particularly noteworthy that all the preparation and measurement processes were performed under ambient conditions. These findings present a green path towards manufacturing efficient and stable air-processed PSCs.

High-Performance Planar Perovskite Solar Cells with Negligible Hysteresis Using 2,2,2-Trifluoroethanol-Incorporated SnO2

SummaryAn efficient electron transport layer (ETL) between the perovskite absorber and the cathode plays a crucial role in obtaining high-performance planar perovskite solar cells (PSCs). Here, we incorporate 2,2,2-trifluoroethanol (TFE) in the commonly used tin oxide (SnO2) ETL, and it successfully improves the power conversation efficiency (PCE) and suppresses the hysteresis of the PSCs: the PCE is increased from 19.17% to 20.92%, and the hysteresis is largely reduced to be almost negligible. The origin of the enhancement is due to the improved electron mobility and optimized work function of the ETL, together with the reduced traps in the perovskite film. In addition, O2 plasma is employed to treat the surface of the TFE-incorporated SnO2 film, and the PCE is further increased to 21.68%. The concept here of incorporating organic small molecules in the ETL provides a strategy for enhancing the performance of the planar PSCs.

Investigation of sol-gel and nanoparticle-based NiOx hole transporting layer for high-performance planar perovskite solar cells

We conduct a comprehensive study on and comparison of sol-gel and nanoparticles (NPs)-based nickel oxide hole-transporting layer (HTL) for high-performance planar perovskite solar cells (PSCs). The characteristics and film properties of sol-gel and NPs were systemically investigated using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and photoluminescence (PL), and its effect on device-performance was also examined using J-V characteristics, quantum-efficiency, and the VOC dependence of the light intensity. Through this comparison of two types of HTL and their device-performances, these studies can provide sufficient and robust information for nickel oxide-based PSCs, and furthermore, the overall results and discussions can be useful for high-performance PSCs.

Facile synthesis of composite tin oxide nanostructures for high-performance planar perovskite solar cells

Metal oxide carrier transporting layers have been investigated widely in organic/inorganic lead halide perovskite solar cells (PSCs). Tin oxide (SnO2) is a promising alternative to the titanium dioxide commonly used in the electron transporting layer (ETL), due to its tunable carrier concentration, high electron mobility, amenability to low-temperature annealing processing, and large energy bandgap. In this study, a facile method was developed for the preparation of a room-temperature-processed SnO2 electron transporting material that provided a high-quality ETL, leading to PSCs displaying high power conversion efficiency (PCE) and stability. A novel physical ball milling method was first employed to prepare chemically pure ground SnO2 nanoparticles (G-SnO2), and a sol–gel process was used to prepare a compact SnO2 (C-SnO2) layer. The effects of various types of ETLs (C-SnO2, G-SnO2, composite G-SnO2/C-SnO2) on the performance of the PSCs are investigated. The composite SnO2 nanostructure formed a robust ETL having efficient carrier transport properties; accordingly, carrier recombination between the ETL and mixed perovskite was inhibited. PSCs incorporating C-SnO2, G-SnO2, and G-SnO2/C-SnO2 as ETLs provided PCEs of 16.46, 17.92, and 21.09%, respectively. In addition to their high efficiency, the devices featuring the composite SnO2 (G-SnO2/C-SnO2) nanostructures possessed excellent long-term stability—they maintained 89% (with encapsulation) and 83% (without encapsulation) of their initial PCEs after 105 days (>2500 h) and 60 days (>1400 h), respectively, when stored under dry ambient air (20 ± 5 RH %).

Mixed-steam annealing treatment for perovskite films to improve solar cells performance

To improve the photovoltaic performance of the perovskite solar cells, it is necessary to reduce the density of surface defect for perovskite film with smooth surface. In this paper, we demonstrate a DMSO/CB mixed vapor annealing process to fabricate high-performance planar perovskite solar cells. The mixed vapor annealing treatment can significantly enhance the crystallization of perovskite film, leading to less crystal surface defects, effective charge-separation and the electron transport rate at the perovskite interface. Finally, the efficiency and reproducibility of the cells has been greatly improved. The power conversion efficiency (PCE) improved from 16.6% to 18.4% under AM 1.5G 100 mW·cm−2 irradiation. We anticipate that the mixed vapor annealing treatment will become a promising crystallization method for the fabrication of high performance PSCs in the future commercialization.

Low temperature solution-derived TiO2-SnO2 bilayered electron transport layer for high performance perovskite solar cells

Planar lead halide perovskite solar cells have shown a promising application in the field of printable solar cells. However, high-performance planar perovskite solar cells typically need a high-temperature process to achieve crystallized titanium oxide films as the electron transport layers, which hinders their application in flexible plastic substrates. Here, we demonstrate a bilayered TiO2-SnO2 film as an excellent substitute for electron transport layer using a low temperature liquid phase method. The bilayered TiO2-SnO2 film exhibits efficient electron extraction and hole blocking ability even at a low processing temperature of 150 °C. The as-obtained solar cells exhibit a champion power conversion efficiency of 18.85% (Voc = 1.100 V, Jsc = 22.52 mA cm−2 and FF = 0.761) under one sun illumination, which is much higher than the devices based on individual SnO2 or TiO2 electron transport layers. The higher electron extraction driving force at the SnO2/perovskite interface and the stronger hole blocking ability due to the defect-free physical contact at the TiO2/FTO interface are suggested to be the main reasons for the improved device performance.