Strain Modulation for Light-Stable n-i-p Perovskite/Silicon Tandem Solar Cells

Transparent Recombination Layers Design and Rational Characterizations for Efficient Two-Terminal Perovskite-Based Tandem Solar Cells

Two-terminal (2T) perovskite-based tandem solar cells (TSCs) arouse burgeoning interest in breaking the Shockley-Queisser (S-Q) limit of single-junction solar cells by combining two subcells with different bandgaps. However, the highest certified efficiency of 2T perovskite-based TSCs (33.9%) lags behind the theoretical limit (42-43%). A vital challenge limiting the development of 2T perovskite-based TSCs is the transparent recombination layers/interconnecting layers (RLs) design between two subcells. To improve the performance of 2T perovskite-based TSCs, RLs simultaneously fulfill the optical loss, contact resistance, carrier mobility, stress management, and conformal coverage requirements. In this review, the definition, functions, and requirements of RLs in 2T perovskite-based TSCs are presented. The insightful characterization methods applicable to RLs, which are inspiring for further research on the RLs both in 2T perovskite-based two-junction and multi-junction TSCs, are also highlighted. Finally, the key factors that currently limit the performance enhancement of RLs and the future directions that should be continuously focused on are summarized.

Suppressing Halide Segregation via Pyridine-Derivative Isomers Enables Efficient 1.68 eV Bandgap Perovskite Solar Cells

Light-induced phase segregation is one of the main issues restricting the efficiency and stability of wide-bandgap perovskite solar cells (WBG PSCs). Small organic molecules with abundant functional groups can passivate various defects, and therefore suppress the ionic migration channels for phase segregation. Herein, a series of pyridine-derivative isomers containing amino and carboxyl are applied to modify the perovskite surface. The amino, carboxyl, and N-terminal of pyridine in all of these molecules can interact with undercoordinated Pb2+ through coordination bonds and suppress halide ions migration via hydrogen bonding. Among them, the 5-amino-3-pyridine carboxyl acid (APA-3) treated devices win the champion performance, enabling an efficiency of 22.35% (certified 22.17%) using the 1.68 eV perovskite, which represents one of the highest values for WBG-PSCs. This is believed to be due to the more symmetric spatial distribution of the three functional groups of APA-3, which provides a better passivation effect independent of the molecular arrangement orientation. Therefore, the APA-3 passivated perovskite shows the slightest halide segregation, the lowest defect density, and the least nonradiative recombination. Moreover, the APA-3 passivated device retains 90% of the initial efficiency after 985 h of operation at the maximum power point, representing the robust durability of WBG-PSCs under working conditions.

Suppressed Phase Segregation with Small A-Site and Large X-Site Incorporation for Photostable Wide-Bandgap Perovskite Solar Cells

Wide-bandgap (WBG) perovskite solar cells (PSCs) have been widely used as the top cell of tandem solar cells. However, photoinduced phase segregation and high open-circuit voltage loss pose significant obstacles to the development of WBG PSCs. Here, a two-step small-size A-site and large-size X-site incorporation strategy is reported to modulate the lattice distortion and improve the film quality of WBG formamidinium-methylammonium (FAMA) perovskite films for photostable PSCs based on two-step deposition method. First, CsI with content of 0-20% is introduced to tune the lattice distortion and film quality of FAMA perovskite with a bandgap of 1.70 eV. Then, 4% RbI is incorporated to further modulate the perovskite growth and lattice distortion, leading to the suppression of photoinduced phase segregation in the resultant RbCsFAMA quadruple cation perovskites. As a result, the 20%CsI/4%RbI-doped device obtains a promising efficiency of 20.6%, and the corresponding perovskite film shows good photothermal stability. Even without encapsulation, the device can maintain 92% of its initial efficiency after 1000 h of continuous operation under 1 sun equivalent white light-emitting diode illumination.

Recent Progress of Wide Bandgap Perovskites towards Two-Terminal Perovskite/Silicon Tandem Solar Cells

Perovskite/silicon tandem solar cells have garnered considerable interest due to their potential to surpass the Shockley-Queisser limit of single-junction Si solar cells. The rapidly advanced efficiencies of perovskite/silicon tandem solar cells benefit from the significant improvements in perovskite technology. Beginning with the evolution of wide bandgap perovskite cells towards two-terminal (2T) perovskite/silicon tandem solar cells, this work concentrates on component engineering, additives, and interface modification of wide bandgap perovskite cells. Furthermore, the advancements in 2T perovskite/silicon tandem solar cells are presented, and the influence of the central interconnect layer and the Si cell on the progression of the tandem solar cells is emphasized. Finally, we discuss the challenges and obstacles associated with 2T perovskite/silicon tandem solar cells, conducting a thorough analysis and providing a prospect for their future.

Anion Confinement for Homogeneous Mixed Halide Perovskite Film Growth by Electrospray

Wide-bandgap perovskites are promising absorbers for state-of-the-art tandem solar cells to feasibly surpass Shockley-Queisser limit with low cost. However, the commonly used mixed halide perovskites suffer from poor stability; particularly, photoinduced phase segregation. Electrospray deposition is developed to bridge the gap of growth rate between iodide and bromide components during film growth by spatially confining the anion diffusion and eliminating the kinetic difference, which universally improves the initial homogeneity of perovskite films regardless of device architectures. It thus promotes the efficiency and stability of corresponding solar cells based on wide-bandgap (1.68 eV) absorbers. Remarkable power conversion efficiencies (PCEs) of 21.44% and 20.77% are achieved in 0.08 cm2 and 1.0 cm2 devices, respectively. In addition, these devices maintain 90% of their initial PCE after 1550 h of stabilized power output (SPO) tracking upon one sun irradiation (LED) at room temperature.

Extrinsic Interstitial Ions in Metal Halide Perovskites: A Review

Perovskite solar cells have rapidly developed as a promising technology for the next generation of low-cost photovoltaics, receiving enormous attention from researchers and industries. Compared to traditional semiconducting materials, metal halide perovskite exhibits outstanding tolerance to extrinsic ions. At a certain range of doping concentration, the interstitial occupancy of extrinsic ions provides appealing benefits to the perovskite films, contributing to higher performance and stability of the devices. This review summarizes the research progress of interstitial ions for metal halide perovskite, providing insights into the mechanism and identification of interstitial doping of extrinsic ions, covering the benefits of interstitial ions in regulating crystal growth, inhibiting ion migration, and reducing defect density. Finally, based on the latest progress and findings, further topics and directions of research on interstitial ions in metal halide perovskite are proposed to advance the understanding of interstitial ions in perovskite and promote the development of perovskite photovoltaic technology.

Tuning Film Stresses for Open-Air Processing of Stable Metal Halide Perovskites

Challenges to upscaling metal halide perovskites (MHPs) include mechanical film stresses that accelerate degradation, dominate at the module scale, and can lead to delamination or fracture. In this work, we demonstrate open-air blade coating of single-step coated perovskite as a scalable method to control residual film stress after processing and introduce beneficial compression in the thin film with the use of polymer additives such as gellan gum and corn starch. The optoelectronic properties of MHP films with compression are improved with higher photoluminescence yields. MHP film stability is significantly improved under compression, under humidity, heat, and thermal cycling. By measuring the evolution of film stresses, we demonstrate for the first time that stress relaxation occurs in MHP films with tensile stress that correlates with film degradation. This discovery of a new mechanism underpinning MHP degradation shows that film stress can be used as a parameter to screen MHP devices and modules for quality control before deployment as a design for reliability criterion.

Dual-Interface Modulation with Covalent Organic Framework Enables Efficient and Durable Perovskite Solar Cells

Dual-interface modulation including buried interface as well as the top surface has recently been proven to be crucial for obtaining high photovoltaic performance in lead halide perovskite solar cells (PSCs). Herein, for the first time, the strategy of using functional covalent organic frameworks (COFs), namely HS-COFs for dual-interface modulation, is reported to further understand its intrinsic mechanisms in optimizing the bottom and top surfaces. Specifically, the buried HS-COFs layer can enhance the resistance against ultraviolet radiation, and more importantly, release the tensile strain, which is beneficial for enhancing device stability and improving the order of perovskite crystal growth. Furthermore, the detailed characterization results reveal that the HS-COFs on the top surface can effectively passivate the surface defects and suppress non-radiation recombination, as well as optimize the crystallization and growth of the perovskite film. Benefiting from the synergistic effects, the dual-interface modified devices deliver champion efficiencies of 24.26% and 21.30% for 0.0725 cm2 and 1 cm2 -sized devices, respectively. Moreover, they retain 88% and 84% of their initial efficiencies after aging for 2000 h under the ambient conditions (25 °C, relative humidity: 35-45%) and a nitrogen atmosphere with heating at 65 °C, respectively.

Mitigating Ion Migration with an Ultrathin Self-Assembled Ionic Insulating Layer Affords Efficient and Stable Wide-Bandgap Inverted Perovskite Solar Cells

Wide-bandgap perovskite solar cells (PSCs) are attracting increasing attention because they play an irreplaceable role in tandem solar cells. Nevertheless, wide-bandgap PSCs suffer large open-circuit voltage (VOC ) loss and instability due to photoinduced halide segregation, significantly limiting their application. Herein, a bile salt (sodium glycochenodeoxycholate, GCDC, a natural product), is used to construct an ultrathin self-assembled ionic insulating layer firmly coating the perovskite film, which suppresses halide phase separation, reduces VOC loss, and improves device stability. As a result, 1.68 eV wide-bandgap devices with an inverted structure deliver a VOC of 1.20 V with an efficiency of 20.38%. The unencapsulated GCDC-treated devices are considerably more stable than the control devices, retaining 92% of their initial efficiency after 1392 h storage under ambient conditions and retaining 93% after heating at 65 °C for 1128 h in an N2 atmosphere. This strategy of mitigating ion migration via anchoring a nonconductive layer provides a simple approach to achieving efficient and stable wide-bandgap PSCs.

Strain-Engineering of Mesoporous Cs3 Bi2 Br9 /BiVO4 S-Scheme Heterojunction for Efficient CO2 Photoreduction

Slow charge kinetics and unfavorable CO₂ adsorption/activation strongly inhibit CO₂ photoreduction. In this study, a strain-engineered Cs₃ Bi₂ Br₉ /hierarchically porous BiVO₄ (s-CBB/HP-BVO) heterojunction with improved charge separation and tailored CO₂ adsorption/activation capability is developed. Density functional theory calculations suggest that the presence of tensile strain in Cs₃ Bi₂ Br₉ can significantly downshift the p-band center of the active Bi atoms, which enhances the adsorption/activation of inert CO₂ . Meanwhile, in situ irradiation X-ray photoelectron spectroscopy and electron spin resonance confirm that efficient charge transfer occurs in s-CBB/HP-BVO following an S-scheme with built-in electric field acceleration. Therefore, the well-designed s-CBB/HP-BVO heterojunction exhibits a boosted photocatalytic activity, with a total electron consumption rate of 70.63 µmol g^-1 h^-1 , and 79.66% selectivity of CO production. Additionally, in situ diffuse reflectance infrared Fourier transform spectroscopy reveals that CO₂ photoreduction undergoes a formaldehyde-mediated reaction process. This work provides insight into strain engineering to improve the photocatalytic performance of halide perovskite.

Efficient Semi-Transparent Wide-Bandgap Perovskite Solar Cells Enabled by Pure-Chloride 2D-Perovskite Passivation

Wide-bandgap (WBG) perovskite solar cells suffer from severe non-radiative recombination and exhibit relatively large open-circuit voltage (VOC) deficits, limiting their photovoltaic performance. Here, we address these issues by in-situ forming a well-defined 2D perovskite (PMA)2PbCl4 (phenmethylammonium is referred to as PMA) passivation layer on top of the WBG active layer. The 2D layer with highly pure dimensionality and halide components is realized by intentionally tailoring the side-chain substituent at the aryl ring of the post-treatment reagent. First-principle calculation and single-crystal X-ray diffraction results reveal that weak intermolecular interactions between bulky PMA cations and relatively low cation-halide hydrogen bonding strength are crucial in forming the well-defined 2D phase. The (PMA)2PbCl4 forms improved type-I energy level alignment with the WBG perovskite, reducing the electron recombination at the perovskite/hole-transport-layer interface. Applying this strategy in fabricating semi-transparent WBG perovskite solar cells (indium tin oxide as the back electrode), the VOC deficits can be reduced to 0.49 V, comparable with the reported state-of-the-art WBG perovskite solar cells using metal electrodes. Consequently, we obtain hysteresis-free 18.60%-efficient WBG perovskite solar cells with a high VOC of 1.23 V.

Recent Advances in Wide-Bandgap Organic-Inorganic Halide Perovskite Solar Cells and Tandem Application

Perovskite-based tandem solar cells have attracted increasing interest because of its great potential to surpass the Shockley-Queisser limit set for single-junction solar cells. In the tandem architectures, the wide-bandgap (WBG) perovskites act as the front absorber to offer higher open-circuit voltage (VOC) for reduced thermalization losses. Taking advantage of tunable bandgap of the perovskite materials, the WBG perovskites can be easily obtained by substituting halide iodine with bromine, and substituting organic ions FA and MA with Cs. To date, the most concerned issues for the WBG perovskite solar cells (PSCs) are huge VOC deficit and severe photo-induced phase separation. Reducing VOC loss and improving photostability of the WBG PSCs are crucial for further efficiency breakthrough. Recently, scientists have made great efforts to overcome these key issues with tremendous progresses. In this review, we first summarize the recent progress of WBG perovskites from the aspects of compositions, additives, charge transport layers, interfaces and preparation methods. The key factors affecting efficiency and stability are then carefully discussed, which would provide decent guidance to develop highly efficient and stable WBG PSCs for tandem application.

Strain Release and Defect Passivation in Formamidinium-Dominated Perovskite via a Novel in-Plane Thermal Gradient Assisted Crystallization Strategy

It is essential to release annealing induced strain during the crystallization process to realize efficient and stable perovskite solar cells (PSCs), which does not seem achievable using the conventional annealing process. Here we report a novel and facile thermal gradient assisted crystallization strategy by simply introducing a slant angle between the preheated hot plate and the substrate. A distinct crystallization sequence resulted along the in-plane direction pointing from the hot side to the cool side, which effectively reduced the crystallization rate, controlled the perovskite grain growth, and released the in-plane tensile strain. Moreover, this strategy enabled uniform strain distribution in the vertical direction and assisted in reducing the defects and aligning the energy bands. The corresponding device demonstrated champion power conversion efficiencies (PCEs) of 23.70% and 21.04% on the rigid and flexible substrates, respectively. These highly stable rigid devices retained 97% of the initial PCE after 1097 h of storage and more than 80% of the initial PCE after 1000 h of continuous operation at the maximum power point. This novel strategy opens a simple and effective avenue to improve the quality of perovskite films and photovoltaic devices via strain modulation and defect passivation.