Chen Weijie

Publication date: 15 June 2022

Source: Joule, Volume 6, Issue 6

Author(s): Yakun He, Ning Li, Thomas Heumüller, Jonas Wortmann, Benedict Hanisch, Anna Aubele, Sebastian Lucas, Guitao Feng, Xudong Jiang, Weiwei Li, Peter Bäuerle, Christoph J. Brabec

Monolayer graphene (MLG) is transferred crack free between a NiO _x and MAPbI₃ interface to serve as a template for van der Waals epitaxial growth of perovskite films with high crystallization for the first time. Additionally, the MLG can shield the defects of NiO _x to enhance the hole transport performance. And finally, the photovoltaic performance of inverted perovskite solar cells based on MLG is improved.

Inverted planar perovskite solar cells (PSCs) have intrigued great promise in negative hysteresis, simple fabrication process, and flexible substrate implementation, in which the shared hole transport materiel is NiO _x . However, the low-temperature solution processing of the NiO _x film is usually accompanied by defect formation, which deteriorates the perovskite quality and device performance. Meanwhile, the energy-level offset between the NiO _x and perovskite films is relatively large, limiting the interfacial charge transport. To suppress those setbacks, a defect-free monolayer graphene sheet is transferred onto the NiO _x film surface as a template for van der Waals epitaxial growth of perovskite films for the first time, leading to enhancing crystallinity with a large grain size of perovskite layer, 0.20 eV energy level offset drop, and accelerating charge transfer for the devices. Finally, the power conversion efficiency of 19.21% without hysteresis is achieved, exceeding 18.35% of the control device.

Herein, Ti₃C₂F_x quantum dots are prepared and selected as interfacial passivators to enhance the performance of CsPbI₃ perovskite solar cells (PSCs), which can tune the energy level, passivate defects, and form barrier layers for CsPbI₃ film. Consequently, CsPbI₃ PSC with an efficiency of 20.44% is obtained, which retains 93% of its initial efficiency after 600 h aging in ambient air without encapsulation.

Abstract

CsPbI₃ inorganic perovskites have attracted significant attention due to their desirable bandgap for tandem solar cells and excellent thermal stability. However, CsPbI₃ perovskite solar cells (PSCs) still exhibit low efficiency and high energy loss due to nonradiative recombination. Herein, functionalized Ti₃C₂F_x quantum dots (QDs) are prepared and selected as interface passivators to enhance the performance of CsPbI₃ PSCs. The systematic experimental results reveal that Ti₃C₂F_x QDs serve as effective passivators mainly in three aspects: 1) p-type Ti₃C₂F_x QDs can tune the energy level of perovskite films and provide an efficient pathway for hole transfer; 2) Ti₃C₂F_x QDs can effectively passivate defects and reduce interfacial nonradiative recombination, and 3) Ti₃C₂F_x QDs form a barrier layer to prevent water invasion and improve the stability of CsPbI₃ PSCs. Consequently, the champion CsPbI₃ PSC with Ti₃C₂F_x QDs treatment exhibits an excellent efficiency of 20.44% with a high open-circuit voltage of 1.22 V. Meanwhile, the corresponding device without encapsulation retained 93% of its initial efficiency after 600 h of storage in ambient air.

All the reported perovskite modules have a combination of different deposition methods for the perovskites and the charge-transport layers, which limits high-throughput module production. A combination of any amine molecules and 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) added in phenyl-C₆₁-butyric acid methyl ester (PCBM) allows the electrically conductive PCBM layers to conformally cover the perovskites and achieve high-efficiency PSCs and modules with all-bladed perovskite and charge-transport layers.

Abstract

Perovskite solar cells (PSCs) are promising to reduce the cost of photovoltaic system due to their low-cost raw materials and high-throughput solution process; however, fabrication of all the active layers in perovskite modules using a scalable solution process has not yet been demonstrated. Herein, the fabrication of highly efficient PSCs and modules in ambient conditions is reported, with all layers bladed except the metal electrode, by blading a 36 ± 9 nm-thick electron-transport layer (ETL) on perovskite films with a roughness of ≈80 nm. A combination of additives in phenyl-C₆₁-butyric acid methyl ester (PCBM) allows the PCBM to conformally cover the perovskites and still have a good electrical conductivity. Amine-functionalized molecules are added to enhance both the dispersity of PCBM and the affinity to perovskites. A PCBM dopant of 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) recovers the conductivity loss induced by the small amine molecules. PSCs (0.08 cm²) fabricated by the all-blading process reache an average efficiency of 22.4 ± 0.5% and a champion efficiency of 23.1% for perovskites with a bandgap of 1.51 eV, with much better stability compared to evaporated ETL PSCs. The all-bladed minimodule (25.03 cm²) shows an aperture efficiency of ≈19.3%, showing the good uniformity of the bladed ETLs.

This work, combined with experimental and theoretical results proves that the multifunctional modifier emtricitabine (FTC) has a positive effect on the power conversion efficiency and stability of perovskite solar cells (PSCs). At the same time, less lead leakage also indicates that the FTC has a bright application prospect in PSCs.

Though great achievements have been realized in perovskite solar cells (PSCs), there are still some thorny challenges that exist such as: 1) How to minimize the interfacial nonradiative recombination losses; 2) How to balance the power conversion efficiency (PCE) and environmental friendliness of the PSCs. Here, effective top-contacts-interface engineering is developed via using a new multi-active site Lewis base molecule named emtricitabine (FTC). Both, experimental and theoretical results confirm that a strong chemical interaction exists between FTC and Pb²⁺. After FTC treatment, the perovskite thin film has the lower density of defect than the control film, meanwhile, the interfacial hole extraction becomes better due to the more matched energy level. Upon the FTC passivation, the PCE of the PSCs is improved from 20.83% to 22.24%. Simultaneously, the humidity stability of the PSCs is improved after the FTC modification. Last but not least, the unpackaged target film showed less lead leakage than the control film.

Herein, a small molecule trifluoroacetamide (TFAA) with C═O, −NH₂, and F groups is incorporated into the perovskite precursor solution to alleviate the defect densities of the perovskite material from the source, obtaining a high-quality FA_0.85MA_0.15PbI₃ perovskite absorber and its assembled photovoltaic devices. The TFAA-modified perovskite solar cells yield a champion efficiency of 24.16% with outstanding atmospheric environment, thermal, and light stabilities.

The additive strategy is considered to be an effective scheme to purposefully passivate defect sites in perovskite materials. Herein, a small molecule trifluoroacetamide (TFAA) with C═O, −NH₂, and F groups is incorporated into the perovskite precursor solution to alleviate the defect densities of the perovskite material from the source, so as to obtain high-quality FA_0.85MA_0.15PbI₃ perovskite absorber and its assembled photovoltaic devices. Thanks to the interactions of Lewis acid of C═O and undercoordinated Pb²⁺, N—H and I⁻ via hydrogen bond, and F and FA⁺ fragments, the nonradiative recombination sites are effectively inhibited, simultaneously promoting the nucleation and grain growth of perovskite. The analysis results demonstrate that the introduction of TFAA additive greatly enhances the crystal quality of bulk perovskite absorber, ameliorates the surface morphology of perovskite film, and improves the extraction and transfer abilities of photogenerated carriers from perovskite absorber. The perovskite solar cells (PSCs) based on TFAA agent yield a champion power conversion efficiency of 24.16%, 8.3% better than that of the control device (22.31%). More importantly, the modified perovskite film has good harsh humidity stability and the unpackaged PSCs maintain outstanding photovoltaic performance in atmospheric environment, thermal, and light conditions.

A new passivator—benzoyl hydrazine (BH) with carboxyl, hydrazine and phenyl groups was developed to effectively passivate defects in CsPbI₃ through synergetic effects of these groups. Consequently, a highest efficiency of 20.47 % with a high open-circuit voltage and stability is achieved in a BH-CsPbI₃ solar cell, the highest efficiency among all pure CsPbI₃-based devices reported to date.

Abstract

All-inorganic CsPbI₃ perovskite presents preeminent chemical stability and a desirable band gap as the front absorber for perovskite/silicon tandem solar cells. Unfortunately, CsPbI₃ perovskite solar cells (PSCs) still show low efficiency due to high density of defects in solution-prepared CsPbI₃ films. Herein, three kinds of hydrazide derivatives (benzoyl hydrazine (BH), formohydrazide (FH) and benzamide (BA)) are designed to reduce the defect density and stabilize the phase of CsPbI₃. Calculation and characterization results corroborate that the carboxyl and hydrazine groups in BH form strong chemical bonds with Pb²⁺ ions, resulting in synergetic double coordination. In addition, the hydrazine group in the BH also forms a hydrogen bond with iodine to assist the coordination. Consequently, a high efficiency of 20.47 % is achieved, which is the highest PCE among all pure CsPbI₃-based PSCs reported to date. In addition, an unencapsulated device showed excellent stability in ambient air.

Nature Energy, Published online: 02 June 2022; doi:10.1038/s41560-022-01039-0

Printed carbon-electrode-based perovskite solar cells are developed by using the combination of poly(3-hexylthiophene nanoparticles and trioctylphosphine oxide. The poly(3-hexylthiophene-2,5-diyl) (P3HT) nanoparticles (NPs) dispersed in alcohol offer a cheap, effective, and stable hole transport layer that further retain the trioctylphosphine oxide (TOPO) passivation effect. The optimized perovskite solar cells exhibit promising device efficiency, excellent reproducibility, and device stability.

Rational design and engineering of top interface layers with combined properties of effective passivation, high thermal- and photo-stability are effective methods to advance the commercialization of perovskite photovoltaics. Here, an innovative anode interface combination is developed based on alcohol-dispersed poly(3-hexylthiophene-2,5-diyl) (P3HT) nanoparticles as the hole transport material and chlorobenzene-dissolved trioctylphosphine oxide (TOPO) as the passivation agent. It is shown that instead of the commonly used 2D passivation ligands, TOPO-passivated perovskite films exhibit greatly improved thermal stability. Furthermore, the passivation contributes to an enhanced carrier lifetime and reduced surface trap density, yielding an improvement in the quasi-fermi-level splitting of 57 meV. To maintain surface passivation during solution processing of further layers, it is necessary to develop a hole transport layer that can be processed from orthogonal solvents. P3HT nanoparticles formulated in alcoholic media fully meet this requirement, clearly benefitting from their high vertical conductivity and extremely low contact resistance with carbon electrodes. Based on this configuration, device efficiency of up to 18.4% is demonstrated for perovskite solar cells with fully solution-processed carbon electrodes, along with significantly improved device stability and reproducibility.

Herein, X-ray photoelectron spectroscopy on n–i–p and p–i–n architecture is used to study the interface at the perovskite and hole transporting layer. The band bending at the perovskite/hole transporting layer in both the architectures is found to be 0.5 eV. In n–i–p, the band bending is in the Spiro while in p–i–n is in the perovskite layer.

Interfaces between hybrid perovskite absorber and its adjacent charge-transporting layers are of high importance for solar cells performance. Understanding their chemical and electronic properties is a key step in designing efficient and stable perovskite solar cells. In this work, the tapered cross-section photoemission spectroscopy (TCS-PES) method is used to study the methylammonium lead iodide (CH₃NH₃PbI₃) (MAPI)-based solar cells in two configurations, that is, in an inverted p–i–n and in a classical n–i–p architecture. It is revealed in the results that the MAPI film deposited once on the n-type TiO₂ and once on the p-type NiO _x substrates is neither an intrinsic semiconductor nor adapts to the dopant nature of the substrate underneath, but it is heavily n-type doped on both substrates. In addition to that, the TCS-PES results identify that the band bending between the MAPI film and the hole transporting layer (HTL) layer depends on the perovskite solar cells architecture. In particular, a band bending on the HTL side in the n–i–p and at the MAPI in the p–i–n architecture is found. The flat band of NiO _x at the NiO _x /MAPI interface can be explained by the Fermi level pinning of the NiO _x at the interface.

A natural product betulin-based insulating polymer is used as filler in various organic solar cells (OSCs). Donor–acceptor–insulator ternary OSCs are developed with improved open-circuit voltage (V _oc). Herein, the variety of filler materials in OSCs to biomass is broadened, and the filler strategy is made a feasible and promising strategy toward highly efficient, eco, and low-cost OSCs.

Introduction of filler materials into organic solar cells (OSCs) are a promising strategy to improve device performance and thermal/mechanical stability. However, the complex interactions between the state-of-the-art OSC materials and filler require careful selection of filler materials and OSC fabrication to achieve lower cost and improved performance. In this work, the introduction of a natural product betulin-based insulating polymer as filler in various OSCs is investigated. Donor–acceptor–insulator ternary OSCs are developed with improved open-circuit voltage (V _oc) due to decreased trap-assisted recombination. Furthermore, filler-induced vertical phase separation due to mismatched surface energy can strongly affect charge collection at the bottom interface and limit the filler ratio. A quasi-bilayer strategy is used in all-polymer systems to circumvent this problem. Herein, the variety of filler materials in OSCs to biomass is broadened, and the filler strategy is made a feasible and promising strategy toward highly efficient, eco, and low-cost OSCs.

The review focuses on the benefits of downconversion (DC) materials in improving the perovskite solar cells’ (PSCs) performance and photostability. It contains a conclusive analysis of the reported two categories called the lanthanides and nonlanthanides-based DC materials for PSCs. It also suggests that the DC materials-based PSCs devices can work well under ultraviolet-rich environments such as space.

Perovskite solar cells (PSCs) have made game-changing progress in the last decade and reached a power conversion efficiency (PCE) of up to 25%. Furthermore, the development of material chemistry, structure design, active layer composition, process engineering, etc. has contributed to improving PSCs’ stability. The significant PCE losses experienced by PSCs are related to spectral mismatch between the incident solar spectra and absorption range of the active layer, which thereby limits the PCE. Besides PSCs’ performance, the photoinduced degradation is also a major concern. Recently, lanthanide (rare-earth) and nonlanthanide-based downconversion (DC) materials have been introduced to resolve these spectral mismatch losses as well as reduce the photoinduced degradation. The DC materials improve the photovoltaic performance by converting ultraviolet (UV) light to visible, also providing UV shielding, and thus contribute to increasing the efficiency as well as stability of PSCs. Moreover, the Shockley–Queisser efficiency limit of the solar cell can be crossed with the help of DC materials. In this review, the importance, processing, and the reported DC materials for PSCs are thoroughly discussed. Furthermore, the development of DC materials and their impact on PSCs’ performance and stability, along with their future perspectives, are focused.

The ball-milled MoS₂ nanosheets were used as the passivation layer between hole transport layer and perovskite. The Mo²⁺–Mo⁴⁺ ion pair could promote electron transfer from Pb^° to I^°, suppressing the deep defects at this interface near perovskite side and passivating the surface (or grain boundaries) defects through a Mo—I or Pb—O bond. The corresponding perovskite solar cell achieves a champion efficiency of 22.39%.

Interface defects can generate serious nonradiative recombination in perovskite solar cells (PSCs) and need to be restrained for further optimization of device performance. Herein, common and easily available 2D MoS₂ nanosheets prepared by a ball-milled method are demonstrated. The obtained MoS₂ has bigger specific surface area and narrower pore distribution, inducing more water to be trapped on the surface or in pores of MoS₂. With annealing, the locally absorbed water can be desorbed, and much more Mo atoms at the outermost surface layers could bond with oxygen to repair the uncoordinated Mo at edge sites for more ordered Mo—S—Mo bonds, which can weaken the catalytic activity of MoS₂ to stabilize the heterojunction interface of hole transport layer (HTL)/perovskite. When it is used as the passivation layer between HTL and perovskite, the Mo²⁺–Mo⁴⁺ ion pair can promote electron transfer from Pb^° to I^°, suppressing the deep defects at this interface near the perovskite side. Meanwhile, MoS₂ can passivate the surface (or grain boundaries) defects through the bond of Mo—I or Pb—O. The resultant PSCs give a champion efficiency of 22.39% with MoS₂ treatment, thus enabling the decorated devices with excellent long-term stability.

Energy yield (EY) estimation for various solar cell architectures using a combination of illumination, optical, and electrical models. With respect to a monofacial silicon solar cell, the EY of monofacial perovskite/silicon tandem solar cells increases by 42% and 43% for 2- and 4-terminal configurations, and with bifacial operation by 55% and 61%, respectively.

The power conversion efficiency of conventional silicon solar cells approaches its theoretical limit. Bifacial operation and the perovskite/silicon tandem device architecture are promising approaches for increasing the energy yield of photovoltaic modules. Here, an energy yield calculation tool for (bifacial) perovskite/silicon tandem solar cells is presented. It uses a chain of models for irradiance, optical absorption, and temperature-dependent electrical performance. Each step is validated with irradiance and performance data from a rooftop installation with mono- and bifacial silicon solar cells in Jerusalem, Israel. Selecting the data for two days (one in summer, one in winter) and considering the high-reflective ground of this particular installation (albedo 60%) a 20% increased energy yield for a bifacial module with respect to a monofacial module is modeled. This result matches well with experimental data. When “upgrading” the silicon solar cell to a perovskite/silicon tandem solar cell, the case study predicts up to 40% additional energy yield. Combining the concepts of bifacial solar operation and perovskite/silicon tandem solar cells results in up to 60% increased energy with a high albedo ground, and is therefore a promising approach to further decrease the levelized cost of electricity for photovoltaic electricity generation.

Herein, the impact of the Cs cation and halide anions on the microstructure, crystal structure, structural defects, optoelectronic properties, and photovoltaic parameters of FAPbI₃ perovskite solar cells with the use of CsX additives (X = I, Br, and Cl) is systematically studied.

The key role of Cs cation and X halide anions (X = I, Br, Cl) on the microstructure, crystal structure, structural defects, optoelectronic properties, and photovoltaic parameters of FAPbI₃-based perovskite solar cells is investigated. The CsCl–FAPbI₃ perovskite film shows the highest photoluminescence (PL) intensity, longest PL lifetime, and highest power conversion efficiency compared with the CsI–FAPbI₃ and CsBr–FAPbI₃ perovskite films. The morphology and crystallography of { 111 }_c nanotwins and stacking faults of perovskite films are studied using transmission electron microscopy and selected-area electron diffraction. The microstructure, crystallography, and atomic structure model of intersecting twin boundaries are presented. Finally, the degradation pathways and the mechanism behind the formation of FAPbI₃-based perovskites under ambient conditions are systematically studied. The grain boundaries of the perovskite films are nonuniformly damaged, resulting in many black nanoparticles after 4 weeks. Electron diffraction analyses of the black nanoparticles confirm the hexagonal PbI₂ phase formation in all CsX–FAPbI₃ perovskite samples after 4 weeks of aging.

An interface modifier (IM) that interacts not only with perovskite but also with hole transporting material is introduced for efficient and stable perovskite solar cells. The best solar cell employing rational IM exhibits a power conversion efficiency of 23.6% with a 2D/3D perovskite structure. Thermal and operational stabilities of interface-optimized devices are demonstrated.

Abstract

Interface modification of perovskite solar cells (PSCs) has been widely explored not only to achieve defect passivation but also to facilitate charge transport and stabilize the physical/electrical contact at device interfaces. In this study, [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (CEPA) is introduced as an interface modifier at the interface of perovskite and the hole transporting material (HTM) layer into n-i-p PSCs. CEPA reduces surface traps, manipulates the surface dipole for energy-level alignment, and induces molecular interaction at the interface of the CEPA-HTM for enhanced interfacial adhesion energy and good mechanical stability. The power conversion efficiency of interface-optimized PSC is 23.6% using a 2D/3D perovskite structure, representing the highest efficiency among poly(triarylamine) HTM-based devices. The encapsulated CEPA-treated PSCs maintain nearly 90% of their initial efficiency during a damp heat lasting for more than 1530 h and retain their initial efficiency during continuous operation under illumination.

Mixed lead-tin perovskites are crucial for achieving all-perovskite tandem devices for photovoltaics. This study shows that these materials degrade in air via deep-trap formation which deteriorates charge-carrier diffusing lengths, in contrast to the mechanism of shallow tin-vacancy formation and self-doping dominant in tin-only perovskites. Development of passivation strategies tuned specifically to mixed lead-tin perovskites is suggested to boost long-term performance.

Abstract

Owing to the bandgap-bowing effect, mixed lead-tin halide perovskites provide ideal bandgaps for the bottom subcell of all-perovskite tandem photovoltaic devices that offer fundamentally elevated power-conversion efficiencies. However, these materials suffer from degradation in ambient air, which worsens their optoelectronic properties and hinders their usability for photovoltaic applications. Such degradation pathways are not yet fully understood, especially for the perovskites in the middle of the APb_xSn_1-xI₃ solid solution line, which offer the narrowest bandgaps across the range. This study unravels the degradation mechanisms of APb_xSn_1-xI₃ perovskites, reporting clear differences between mixed lead-tin (x = 0.5) and tin-only (x = 0) perovskites. The dynamic optoelectronic properties, electronic structure, crystal structure, and decomposition products of the perovskite thin films are examined in situ during air exposure. Both perovskite compositions suffer from the formation of defects over the timescale of hours, as indicated by a significant reduction in their charge-carrier diffusion lengths. For tin-only perovskite, degradation predominantly causes the formation of energetically shallow tin vacancies and hole doping. However, for mixed lead-tin perovskite, deep trap states are formed that significantly accelerate charge-carrier recombination, yet leave mobilities relatively unaffected. These findings highlight the need for passivation strategies tailored specifically to mixed lead-tin iodide perovskites.

Based on an in-depth discussion of the guanidine halide (GuX) passivation mechanisms, a 2D/3D hybrid perovskite solar cells with a dual (bulk- and surface-) passivation approach achieves significantly enhanced efficiency (22.53%) compared with that of the control device (20.31%).

Abstract

In order to improve both performance and stability of perovskite solar cells, a design is provided by combining the advantages of high-efficiency 3D perovskite solar cells (PSCs) and long-term stability 2D PSCs. A 2D/3D hybrid perovskite film with a dual (bulk- and surface-) passivation approach is realized, based on in-depth discussion of the guanidine halide (GuX) passivation mechanisms. The approach can reduce the charge carrier losses in the bulk and prevent decomposition at the surface. Based on a combination of ab initio molecular dynamics simulations, Urbach energy, and photovoltage measurements, it is indicated that the engineered components of GuX salts are GuCl for bulk treatment and GuI surface treatment, respectively. The former can lower the nonradiative recombination, and the latter can prevent the halogen-out. Moreover, drive-level capacitance profiling is employed in the context of the 2D/3D perovskite structure for revealing and proving the passivation mechanism. The 2D/3D hybrid perovskites achieve long-term stability (efficiency degradation <10% after 30 days without encapsulation) and significant enhanced efficiency (22.53%) compared with the efficiency (20.31%) of the control device. This work represents a rational design strategy for bulk and surface passivation treatment in 2D/3D hybrid perovskites.