Lattice Strain Regulation Enables High-Performance Formamidinium Perovskite Photovoltaics

Efficient perovskite solar modules enabled by a UV-stable and high-conductivity hole transport material

Ultraviolet (UV) radiation poses a substantial challenge to the stability of prevalent p-i-n (positive-intrinsic-negative) perovskite solar cells (PSCs), demanding more robust hole-transport layers (HTLs) due to light incident from the HTL side. Here, we unveil that commonly used self-assembled monolayer (SAM)-type HTLs suffer from poor UV stability that causes irreversible damage to hole extraction and impairs device stability. To address this issue, we develop a polymeric and UV-stable HTL named Poly-2PACz, which exhibits strong binding to substrates and exceptional UV resistance over SAM-type HTLs. The PSCs blade-coated under ambient conditions using Poly-2PACz HTL achieved a remarkable efficiency of 26.0% and outstanding UV stability. Our cells retain 80% of the initial PCE even after about 500 hours of high-intensity UV illumination [7.7 times higher than that of air mass 1.5 global (AM 1.5G) solar spectrum]. Furthermore, Poly-2PACz exhibits good wettability and high conductance, enabling the fabrication of blade-coated minimodules with an aperture efficiency of 22.2% and excellent uniformity.

Revealing Collaborative Effects of Binary Additives on Regulating Precursor Crystallization Toward Highly Efficient Perovskite Solar Cells

Fabricating high-quality perovskite layers is essential for achieving high-performance solar cells. Considering the significant advancements made in additive engineering for optimizing perovskite crystallization using single additive, exploring the collaborative effect of dual additives on precursor for perovskite crystallization may be an effective way for further advancing device performance. Herein, a binary additives strategy is proposed, where phenylmethylammonium iodide (PMAI) and [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) are introduced into the precursor. Compared with the precursor with no additives or a single additive (PMAI or 2PACz), the use of dual additives more effectively cleaves edge-shared Pb-I octahedra to expedite the transformation from PbI2 to PbI3 - complexes as prenucleation clusters and produces much larger colloidal particles with accelerated nucleation. Concurrently, the crystallization in both spin-coating and annealing processes is significantly retarded due to the stronger interaction between perovskite and binary additives. Benefiting from such rapid nucleation and slow crystallization, high-quality perovskite layer with larger-sized crystals and fewer defects is formed, resulting in mitigated microstrain, enhanced charge extraction, and suppressed nonradiative recombination. Consequently, the device derived from the use of dual additives could achieve an impressive efficiency of 26.05% (certified 25.49%) and retained 90% of its initial efficiency after 1200 h of maximum power point tracking.

Iodine Stabilization in Perovskite Lattice for Internal Stress Relief

Atomic iodine ionization in perovskite crystals leads to defect formation, lattice distortion, and the occurrence of localized micro-strain. These atomic-level chemical and mechanical effects significantly alter the electronic band landscape, profoundly affecting device performance. While iodine stabilization effects have traditionally been focused on stability, their impact on electrical properties, particularly the coupling effect with internal stress and lattice strain, remains underexplored. In this study, an iodine stabilization protocol using a parallel-π-stacked small molecule, [2,2]-paracyclophane (PCP) is implemented, which plays a beneficial role in relieving internal stress within the perovskite lattice, thereby improving the film's electrical properties. By leveraging this iodine stabilization strategy, internal stress in the perovskite film, resulting in a strain-free perovskite film and a corresponding device with an improved efficiency of 25.26% from 23.93% is successfully alleviated. The maximum power point tracking test of the perovskite device keeps 85% of its initial efficiency when illuminated under 1 sun for 1000 h, while the control device only maintains 57% of the initial efficiency under the same conditions. The good stability originates from the stable iodide ions in the perovskite lattice due to preventing iodide ions oxidation and perovskite degradation.

Structural Mechanisms of Quasi-2D Perovskites for Next-Generation Photovoltaics

Quasi-two-dimensional (2D) perovskite embodies characteristics of both three-dimensional (3D) and 2D perovskites, achieving the superior external environment stability structure of 2D perovskites alongside the high efficiency of 3D perovskites. This effect is realized through critical structural modifications in device fabrication. Typically, perovskites have an octahedral structure, generally ABX3, where an organic ammonium cation (A') participates in forming the perovskite structure, with A'(n) (n = 1 or 2) sandwiched between A(n-1)B(n)X(3n+1) perovskite layers. Depending on whether A' is a monovalent or divalent cation, 2D perovskites are classified into Ruddlesden-Popper perovskite or Dion-Jacobson perovskite, each generating different structures. Although each structure achieves similar effects, they incorporate distinct mechanisms in their formation. And according to these different structures, various properties appear, and additive and optimizing methods to increase the efficiency of 3D perovskites also exist in 2D perovskites. In this review, scientific understanding and engineering perspectives of the quasi-2D perovskite is investigated, and the optimal structure quasi-2D and the device optimization is also discussed to provide the insight in the field.

Regulating Strain and Suppressing Defect States Through Polymer Ionization and Chemical Chelation for Efficient Inverted Flexible Perovskite Solar Cells

Flexible perovskite solar cells (FPSCs) have great promise for applications in wearable technology and space photovoltaics. However, the unpredictable crystallization of perovskite on flexible substrates results in significantly lower efficiency and mechanical durability than industry standards. A strategy is investigated employing the polymer electrolyte poly(allylamine hydrochloride) (PAH) to regulate crystallization and passivate defect states in perovskite films on flexible substrates. The weakly acidic precursor allows PAH to undergo partial ionization, leading to the protonation of some ─NH3 + groups into ─NH3 +. Simulations and experimental results indicate that multifunctional PAH forms strong chemical interactions with precursors of perovskite materials including Formamidine Hydroiodide (FAI) and Lead Iodide (PbI2), facilitating homogenous nucleation and growth of crystals of perovskite films. High-resolution transmission electron microscopy (HR-TEM) reveals that PAH strongly anchors to grain boundaries (GBs), consistent with findings from photo-induced force microscopy-based infrared spectroscopy (PiFM-IR). PAH improves the uniformity distribution of Young's modulus between the grains and GBs, facilitating stress relief in the perovskite films. Therefore, the champion efficiency of PAH-modified FPSCs reaches 24.19% and exhibits strong bending durability, retaining 87.9% of their initial efficiency after 2500 bending cycles (r = 5 mm), demonstrating their practical application potential in outdoor wearable electronic products.

Solvent-Engineering-Assisted Ligand Exchange Strategy for High-Efficiency AgBiS2 Quantum Dot Solar Cells

As the initial synthesized colloidal quantum dots (CQDs) are generally capped with insulating ligands, ligand exchange strategies are essential in the fabrication of CQD films for solar cells, which can regulate the surface chemical states of CQDs to make them more suitable for thin-film optoelectronic devices. However, uncontrollable surface adsorption of water molecules during the ligand exchange process introduces new defect sites, thereby impairing the resultant device performance, which attracts more efforts devoted to it but remains a puzzle. Here, we develop a solvent-engineering-assisted ligand exchange strategy to revamp the surface adsorption, improve the exchange efficiency, and modulate the surface chemistry for the environmentally friendly lead-free silver bismuth disulfide (AgBiS2) CQDs. The optimized AgBiS2 CQD solar cells deliver an outstanding champion power conversion efficiency (PCE) of up to 8.95 % and improved long-term stability. Our strategy is less environment-dependent and can produce solar cells with negligible performance variance for several batches across several months. Our work demonstrates the critical role of solvents for ligand exchange in the surface chemistry of CQDs and the realization of high-performance photovoltaic devices in a highly reproducible manner.

Ligand-Tuned AgBiS2 Planar Heterojunctions Enable Efficient Ultrathin Solar Cells

AgBiS2 quantum dots (ABS QDs) have emerged as highly promising candidates for photovoltaic applications due to their strong sunlight absorption, nontoxicity, and elemental availability. Nevertheless, the efficiencies of ABS solar cells currently fall far short of their thermodynamic limits due in large part to sluggish charge transport characteristics in nanocrystal-derived films. In this study, we overcome this limitation by tuning the surfaces of ABS semiconductor QDs via a solvent-induced ligand exchange (SILE) strategy and provide key insights into the role of surface composition on both n- and p-type charge transfer doping, as well as long-range charge transport. Using this approach, the electronic properties of ABS films were systematically modulated, thereby enabling the design of planar p-n heterojunctions featuring favorable band alignment for solar cell applications. Carrier transport and separation are significantly enhanced by the built-in electric fields generated within the ultrathin (30 nm) ABS heterojunction absorber layers, resulting in a notable solar-cell power conversion efficiency of 7.43%. Overall, this study presents a systematic and straightforward strategy to tune not only the surfaces of ABS, but also the electronic properties of solid-state films, thereby enabling junction engineering for the development of advanced semiconductor structures tailored for photovoltaic applications.

Lattice Strain Regulation and Halogen Vacancies Passivation Enable High-Performance Formamidine-Based Perovskite Solar Cells

Formamidinium lead iodide (FAPbI3) perovskite has lately surfaced as the preferred contender for highly proficient and robust perovskite solar cells (PSCs), owing to its favorable bandgap and superior thermal stability. Nevertheless, volatilization and migration of iodide ions (I-) result in non-radiating recombination centers, and the presence of large formamidine (FA) cations tends to cause lattice strain, thereby reducing the power conversion efficiency (PCE) and stability of PSCs. To solve these problems, the lead formate (PbFa) is added into the perovskite solution, which effectively mitigates the halogen vacancy and provides tensile strain outside the perovskite lattice, thereby enhancing its properties. The strong coordination between the C═O of HCOO- and Pb-I backbones effectively immobilizes anions, significantly increases the energy barrier for anion vacancy formation and migration, and reduces the risk of lead ion (Pb2+) leakage, thereby improving the operation and environmental safety of the device. Consequently, the champion PCE of devices with Ag electrodes can be increased from 22.15% to 24.32%. The unencapsulated PSCs can still maintain 90% of the original PCE even be stored in an N2 atmosphere for 1440 h. Moreover, the target devices have significantly improved performance in terms of light exposure, heat, or humidity.

Improvement of Photovoltaic Performance of Perovskite Solar Cells by Synergistic Modulation of SnO2 and Perovskite via Interfacial Modification

In the past decade, perovskite solar cell (PSC) photoelectric conversion efficiency has advanced significantly, and tin dioxide (SnO2) has been extensively used as the electron transport layer (ETL). Due to its high electron mobility, strong chemical stability, energy level matching with perovskite, and easy low-temperature fabrication, SnO2 is one of the most effective ETL materials. However, the SnO2 material as an ETL has its limitations. For example, SnO2 films prepared by low-temperature spin-coating contain a large number of oxygen vacancies, resulting in energy loss and high open-circuit voltage (VOC) loss. In addition, the crystal quality of perovskites is closely related to the substrate, and the disordered crystal orientation will lead to ion migration, resulting in a large number of uncoordinated Pb2+ defects. Therefore, interface optimization is essential to improve the efficiency and stability of the PSC. In this work, 2-(5-chloro-2-benzotriazolyl)-6-tert-butyl-p-cresol (CBTBC) was introduced for ETL modification. On the one hand, the hydroxyl group of CBTBC forms a Lewis mixture with the Sn atom, which reduces the oxygen vacancy defect and prevents nonradiative recombination. On the other hand, the SnO2/CBTBC interface can effectively improve the crystal orientation of perovskite by influencing the crystallization kinetics of perovskite, and the nitrogen element in CBTBC can effectively passivate the uncoordinated Pb2+ defects at the SnO2/perovskite interface. Finally, the prevailing PCE of PSC (1.68 eV) modified by CBTBC was 20.34% (VOC = 1.214 V, JSC = 20.49 mA/cm2, FF = 82.49%).

Multi-Functional Spirobifluorene Phosphonate Based Exciplex Interface Enables Voc Reaching 95% of Theoretical Limit for Perovskite Solar Cells

Metal halide perovskite solar cells (PSCs) show significant advancements in power conversion efficiency (PCE). However, the open-circuit voltage (VOC) of PSCs is limited by interfacial factors such as defect-induced recombination, energy band mismatch, and non-intimate interface contact. Here, an exciplex interface is first developed based on the strategically designed and synthesized two spirobifluorene phosphonate molecules to mitigate VOC loss in PSCs. The exciplex interface constructed by the intimate contact between the multi-functional molecules and hole transport layer takes the roles to promote the hole extraction by donor-acceptor interaction, passivate coordination-unsaturated Pb2+ defects by equipped phosphonate groups, and optimize the energy level alignment. As a result, a record VOC of 1.26 V with a perovskite bandgap of 1.61 eV is achieved, representing over 95% of theoretical limit. This advancement leads to an increase in PCE from 21.29% to 24.12% and improved stability. The exciplex interface paves the way for addressing the long-standing challenge of VOC loss and promotes the wider application of PSCs.

Multifunctional Trifluoroborate Additive for Simultaneous Carrier Dynamics Governance and Defects Passivation to Boost Efficiency and Stability of Inverted Perovskite Solar Cells

The main obstacles to promoting the commercialization of perovskite solar cells (PSCs) include their record power conversion efficiency (PCE), which still remains below the Shockley-Queisser limit, and poor long-term stability, attributable to crystallographic defects in perovskite films and open-circuit voltage (Voc) loss in devices. In this study, potassium (4-tert-butoxycarbonylpiperazin-1-yl) methyl trifluoroborate (PTFBK) was employed as a multifunctional additive to target and modulate bulk perovskite defects and carrier dynamics of PSCs. Apart from simultaneously passivating anionic and cationic defects, PTFBK could also optimize the energy-level alignment of devices and weaken the interaction between carriers and longitudinal optical phonons, resulting in a carrier lifetime of greater than 3 μs. Furthermore, it inhibited non-radiative recombination and improved the crystallization capacity in the target perovskite film. Hence, the target rigid and flexible p-i-n PSCs yielded champion PCEs of 24.99 % and 23.48 %, respectively. More importantly, due to hydrogen bonding between formamidinium and fluorine, the target devices exhibited remarkable thermal, humidity, and operational tracking at maximum power point stabilities. The reduced Young's modulus and residual stress in the perovskite layer also provided excellent bending stability for flexible target devices.

Seed-Mediated Growth for High-Efficiency Perovskite Solar Cells: The Important Role of Seed Surface

Additive engineering has emerged as one of the most promising strategies to improve the performance of perovskite solar cells (PSCs). Among additives, perovskite nanocrystals (NCs) have a similar chemical composition and matched lattice structure with the perovskite matrix, which can effectively enhance the efficiency and stability of PSCs. However, relevant studies remain limited, and most of them focus on bromide-involved perovskite NCs, which may undergo dissolution and ion exchange within the FAPbI3 host, potentially resulting in an enlarged band gap. In this work, we employ butylamine-capped CsPbI3 NCs (BPNCs) as additives in PSCs, which can be well maintained and serve as seeds for regulating the crystallization and growth of perovskite films. The resultant perovskite film exhibits larger domain sizes and fewer grain boundaries without compromising the band gap. Moreover, BPNCs can alleviate lattice strain and reduce defect densities within the active layer. The PSCs incorporating BPNCs show a champion power conversion efficiency (PCE) of up to 25.41 %, well over both Control of 22.09 % and oleic acid/oleylamine capped CsPbI3 NC (PNC)-based devices of 23.11 %. This work illustrates the key role of nanosized seed surfaces in achieving high-performance photovoltaic devices.

Phase-Pure α-FAPbI3 Perovskite Solar Cells via Activating Lead-Iodine Frameworks

Narrow bandgap cubic formamidine perovskite (α-FAPbI3 ) is widely studied for its potential to achieve record-breaking efficiency. However, its high preparation difficulty caused by lattice instability is criticized. A popular strategy for stabilizing the α-FAPbI3 lattice is to replace intrinsic FA+ or I- with smaller ions of MA+ , Cs+ , Rb+ , and Br- , whereas this generally leads to broadened optical bandgap and phase separation. Studies show that ions substitution-free phase-pure α-FAPbI3 can achieve intrinsic phase stability. However, the challenging preparation of high-quality films has hindered its further development. Here, a facile synthesis of high-quality MA+ , Cs+ , Rb+ , and Br- -free phase-pure α-FAPbI3 perovskite film by a new solution modification strategy is reported. This enables the activation of lead-iodine (Pb─I) frameworks by forming the coated Pb⋯O network, thus simultaneously promoting spontaneous homogeneous nucleation and rapid phase transition from δ to α phase. As a result, the efficient and stable phase-pure α-FAPbI3 PSC is obtained through a one-step method without antisolvent treatment, with a record efficiency of 23.15% and excellent long-term operating stability for 500 h under continuous light stress.