Infiltrated 2D/3D Heterojunction with Tunable Electric Field Landscape for Robust Inverted Perovskite Solar Cells with over 24% Efficiency.

In inverted perovskite solar cells, conventional planar 2D/3D perovskite heterojunctions typically exhibit a type-II band alignment, where the electric field tends to drive the electron motion in the opposite direction to the direction of electron transfer. Here, a 2D/3D gradient heterojunction is developed by allowing the 2D perovskite to infiltrate the 3D perovskite surface along the grain boundaries using the interaction between the organic cation of the 2D perovskite and the pseudohalogen thiocyanate ion (SCN-), which has the ability to diffuse downward. The infiltrated 2D perovskite not only fills the gaps of grain boundaries with improved structural stability, but it also reconstructs the original landscape of the electric field toward the n-doped surface to enable more rapid electron transfer and weaken the adverse type-II band alignment effect. Since 2D perovskite seals the GBs, the nonvolatile SCN- can accumulate at the top and bottom dual interfaces, releasing residual stress and significantly inhibiting nonradiative recombination. The device exhibits an excellent efficiency of 24.76% (certified 24.29%) and long-term stability that is >90% of the original PCE value after 800 h of heating at 85 °C or in high humidity (≈65%).

Organizing Uniform Phase Distribution in Methylammonium-Free 1.77 eV Wide-Bandgap Inverted Perovskite Solar Cells.

Disordered crystallization and poor phase stability of mixed halide perovskite films are still the main factors that compromise the performance of inverted wide bandgap (WBG; 1.77 eV) perovskite solar cells (PSCs). Great difficulties are evidenced due to the very different crystallization rates between I- and Br-based perovskite components through DMSO-alone assisted anti-solvent process. Here, a zwitterionic additive strategy is reported for finely regulating the crystal growth of Cs0.2 FA0.8 Pb(I0.6 Br0.4 )3 , thereby obtaining high-performance PSCs. The aminoethanesulfonic acid (AESA) is introduced to form hydrogen bonds and strong PbO bonds with perovskite precursors, realizing the complete coordination with both the organic (FAI) and inorganic (CsI, PbI2 , PbBr2 ) components, balancing their complexation effects, and realizing AESA-guided fast nucleation and retarded crystallization processes. This treatment substantially promotes homogeneous crystal growth of I- and Br-based perovskite components. Besides, this uniformly distributed AESA passivates the defects and inhibits the photo-induced halide segregation effectively. This strategy generates a record efficiency of 19.66%, with a Voc of 1.25 V and FF of 83.7% for an MA-free WBG p-i-n device at 1.77 eV. The unencapsulated devices display impressive humidity stability at 30 ± 5% RH for 1000 h and much improved continuous operation stability at MPP for 300 h.

Inhibiting Interfacial Diffusion in Heterojunction Perovskite Solar Cells by Replacing Low-Dimensional Perovskite with Uniformly Anchored Quaternized Polystyrene.

Surface heterojunction has been regarded as an effective method to improve the device efficiency of perovskite solar cells. Nevertheless, the durability of different heterojunction under thermal stress is rarely investigated and compared. In this work, benzylammonium chloride and benzyltrimethylammonium chloride are utilized to construct 3D/2D and 3D/1D heterojunctions, respectively. A quaternized polystyrene is synthesized to construct a three-dimensional perovskite/amorphous ionic polymer (3D/AIP) heterojunction. Due to the migration and volatility of organic cations, severe interfacial diffusion is found among 3D/2D and 3D/1D heterojunctions, in which the quaternary ammonium cations in the 1D structure are less volatile and mobile than the primary ammonium cations in the 2D structure. 3D/AIP heterojunction remains intact under thermal stress due to the strong ionic bond anchoring at the interface and the ultra-high molecular weight of AIP. Furthermore, the dipole layer formed by AIP can further reduce the voltage loss caused by nonradiative recombination at the interface by 0.088 V. Therefore, the devices based on the 3D/AIP heterojunction achieve a champion power conversion efficiency of 24.27% and maintain 90% of its initial efficiency after either thermal aging for 400 h or wet aging for 3000 h, showing a great promise for polymer/perovskite heterojunction towards real applications.

Green-solvent-processed formamidinium-based perovskite solar cells with uniform grain growth and strengthened interfacial contact via a nanostructured tin oxide layer.

Green-solvent-processed perovskite solar cells (PSCs) have reached an efficiency of 20%, showing great promise in safe industrial production. However, the nucleation process in green-solvent-based deposition is rarely optimized, resulting in randomized crystallization and much lowered reported efficiencies. Herein, a nanostructured tin oxide nanorods (SnO2-NRs) substrate is utilized to prepare a high-quality formamidinium (FA)-based perovskite film processed from a green solvent of triethyl phosphate (TEP) with a low toxic antisolvent of dibutyl ether (DEE). Compared with SnO2 nanoparticles, the oriented SnO2-NRs can accelerate the formation of heterogeneous nucleation sites and retard the crystal growth process of the perovskite film, resulting in a high-quality perovskite film with uniform grain growth. Furthermore, a chlorine-terminated bifunctional supramolecule (Cl-BSM) is introduced to passivate the increasing interfacial defects due to the vast contact area in SnO2-NRs. Correspondingly, the substrate design of SnO2-NRs with Cl-BSM increases the power conversion efficiency (PCE) of green-solvent-processed PSCs to 22.42% with an open-circuit voltage improvement from 1.02 to 1.12 V, which can be attributed to the uniform grain growth and reduced carrier recombination at the SnO2-NRs/perovskite interface. More importantly, the photo and humidity stabilities of the unencapsulated device for up to 500 and 1000 hours are also achieved with negligible interfacial delamination after aging. This work provides a new perspective on the future industrial scale production of PSCs using environment-friendly solvents with compatible substrate design.

Direct In Situ Conversion of Lead Iodide to a Highly Oriented and Crystallized Perovskite Thin Film via Sequential Deposition for 23.48% Efficient and Stable Photovoltaic Devices.

In the sequential deposition method of perovskite films, the crystallinity and microstructure of PbI2 are often sacrificed to solve the problem of an incomplete reaction between organic halide and lead halide. As a result, the crystal orientation of the perovskite film prepared by the sequential deposition method is generally worse than that of the perovskite film prepared by a one-step antisolvent method. Here, we preplaced formamidine formate (FAFa) on the buried interface to regulate the formation mechanism from PbI2 to perovskite. As shown by the XPS measurement of the perovskite buried interface, the HCOO- anion of FAFa first partially replaces I- to coordinate with Pb2+. With the subsequent annealing process, some HCOO- anions were released and migrated upward, which promoted the recrystallization of PbI2, obtaining a PbI2 film with enhanced crystallinity and orientation. Additionally, the lift-off process proves that the HCOO- anions suppress the anion vacancy defects enriched at the buried interface and promote charge transport because the HCOO- anions are small enough to adapt to the iodide vacancy. Grazing incidence wide-angle X-ray scattering and X-ray diffraction measurements show that the in situ conversion mechanism is responsible for the PbI2-to-perovskite process, resulting in the highly oriented perovskite film without increasing the residual PbI2 content in the perovskite film. As a result, our strategies enabled a champion power conversion efficiency of 23.48% with improved storage stability and photostability. This work provides a new strategy to improve the crystallinity of sequential deposition perovskites without destabilizing the device due to more PbI2 residues.

Low-Temperature Phase-Transition for Compositional-Pure α-FAPbI3 Solar Cells with Low Residual-Stress and High Crystal-Orientation.

Transition of δ-phase formamidinium lead triiodide (δ-FAPbI3 ) to pure α-phase FAPbI3 (α-FAPbI3 ) typically requires high processing temperature (150 °C), which often results in unavoidable residual stress. Besides, using methylammonium chloride (MACl) as additive in fabrication will cause MA residue in the film, compromising the compositional purity. Here, a stress-released and compositional-pure α-FAPbI3 thin-film is fabricated using 3-chloropropylammonium chloride (Cl-PACl) by two-step annealing. The 2D template of n = 2 can preferentially form in perovskite with the introduction of Cl-PACl at a temperature as low as 80 °C. Such a 2D template can guide the free components to form ordered α-FAPbI3 and promote the transition of the formed δ-FAPbI3 to α-FAPbI3 by reducing the phase transition energy. As a result, the obtained perovskite films via low-temperature phase-transition have a high degree of crystal orientation and reduced residual stress. More importantly, most of the Cl-PACl is volatilized during the subsequent high-temperature annealing process accompanied by the disintegration of the 2D templates. The residual trace of Cl-PA+ is mainly concentrated at the grain boundary near the perovskite surface layer, stabilizing α-FAPbI3 and passivating defects. Perovskite solar cell based on pure α-FAPbI3 achieves a power conversion efficiency of 23.03% with excellent phase stability and photo-stability.

Bottom-Up Templated and Oriented Crystallization for Inverted Triple-Cation Perovskite Solar Cells with Stabilized Nickel-Oxide Interface.

Inverted-structure perovskite solar cells (PSCs) are known for their superior device stability. However, based on nickel-oxide (NiOx ) substrate, disordered crystallization and bottom interface instability of perovskite film are still the main factors that compromise the power conversion efficiency (PCE) of PSCs. Here, 2D perovskite of thiomorpholine 1,1-dioxide lead iodide (Td2 PbI4 ) is introduced as a template to prepare 3D perovskite thin film with high crystal orientation and large grain size via a bottom-up growth method. By adding TdCl to the precursor solution, pre-crystallized 2D Td2 PbI4 seeds can accumulate at the bottom interface, lowering the barrier of nucleation, and templating the growth of 3D perovskite films with improved (100) orientation and reduced defects during crystallization. In addition, 2D Td2 PbI4 at the bottom interface also hinders the interfacial redox reaction and reduces the hole extraction barrier on the buried interface. Based on this, the Td-0.5 PSC achieves a PCE of 22.09% and an open-circuit voltage of 1.16 V. Moreover, Td-0.5 PSCs show extremely high stability, which retains 84% of its initial PCE after 500 h of continuous illumination under maximum power point operating conditions in N2 atmosphere. This work paves the way for performance improvement of inverted PSCs on NiOx substrate.

Resolving Mixed Intermediate Phases in Methylammonium-Free Sn-Pb Alloyed Perovskites for High-Performance Solar Cells.

The complete elimination of methylammonium (MA) cations in Sn-Pb composites can extend their light and thermal stabilities. Unfortunately, MA-free Sn-Pb alloyed perovskite thin films suffer from wrinkled surfaces and poor crystallization, due to the coexistence of mixed intermediate phases. Here, we report an additive strategy for finely regulating the impurities in the intermediate phase of Cs0.25FA0.75Pb0.6Sn0.4I3 and, thereby, obtaining high-performance solar cells. We introduced d-homoserine lactone hydrochloride (D-HLH) to form hydrogen bonds and strong Pb-O/Sn-O bonds with perovskite precursors, thereby weakening the incomplete complexation effect between polar aprotic solvents (e.g., DMSO) and organic (FAI) or inorganic (CsI, PbI2, and SnI2) components, and balancing their nucleation processes. This treatment completely transformed mixed intermediate phases into pure preformed perovskite nuclei prior to thermal annealing. Besides, this D-HLH substantially inhibited the oxidation of Sn2+ species. This strategy generated a record efficiency of 21.61%, with a Voc of 0.88 V for an MA-free Sn-Pb device, and an efficiency of 23.82% for its tandem device. The unencapsulated devices displayed impressive thermal stability at 85 °C for 300 h and much improved continuous operation stability at MPP for 120 h.

Ultralow Set Voltage and Enhanced Switching Reliability for Resistive Random-Access Memory Enabled by an Electrodeposited Nanocone Array.

Resistive random-access memory (RRAM) has been extensively investigated for 20 years due to its excellent advantages, including scalability, switching speed, compatibility with the complementary metal oxide semiconductor process, and neuromorphic computing application. However, the issue of memristor reliability for cycle to cycle and device to device resulting from the random ion drift and diffusion in solid-state thin films is still a great challenge for commercialization. Therefore, controlling the internal ionic process to improve the memristor performance and reliability is a primary and urgent task. Here, a Ni nanocone array prepared by an electrodeposition method is integrated with an Ag/Al2O3/Pt resistive switching device. The nanocone-array-based memristor yields superior switching performance, including an ultralow set voltage (-0.37 V), a concentrated voltage/resistance distribution (CV 14.8%/32.7%), robust endurance (>105 cycles), and multilevel storage capability. A finite element analysis, transmission electron microscope observation, and current mapping test indicate that the local enhancement of the electric field confines the ionic migration process and yields a predictable formation and dissolution process of the conductive filament. The nanocone-array-based RRAM device provides a new and feasible method to control the conductive filament growth reliably, which paves the way for memristor development.

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.

Surface-Anchored Acetylcholine Regulates Band-Edge States and Suppresses Ion Migration in a 21%-Efficient Quadruple-Cation Perovskite Solar Cell.

Although incorporating multiple halogen (bromine) anions and alkali (rubidium) cations can improve the open-circuit voltage (Voc ) of perovskite solar cells (PSCs), severe voltage loss and poor stability have remained pivotal limitations to their further commercialization. In this study, acetylcholine (ACh+ ) is anchored to the surface of a quadruple-cation perovskite to provide additional electron states near the valence band maximum of the perovskite surface, thereby enhancing the band alignment and minimizing the Voc loss significantly. Moreover, the quaternary ammonium and carbonyl units of ACh+ passivate the antisite and vacancy defects of the organic/inorganic hybrid perovskite. Because of strong interactions between ACh+ and the perovskite, the formation of lead clusters and the migration of halogen anions in the perovskite film are suppressed. As a result, the device prepared with ACh+ post-treatment delivers a power conversion efficiency (PCE) (21.56%) and a value of Voc (1.21 V) that are much higher than those of the pristine device, along with a twofold decrease in the hysteresis index. After storage for 720 h in humid air, the device subjected to ACh+ treatment maintained 70% of its initial PCE. Thus, post-treatment with ACh+ appears to be a useful strategy for preparing efficient and stable PSCs.

Lead-free bright blue light-emitting cesium halide nanocrystals by zinc doping.

Cesium lead halide perovskite nanocrystals (NCs) have attracted extensive attention for photoelectric device application due to their excellent optoelectronic properties. However, the toxicity of lead has hindered their commercialization. Consequently, lead free cesium metal halide NCs have been developed, but these materials suffer from low photoluminescence quantum yield (PLQY) and poor stability. Here, a new class of lead-free non-perovskite blue-emitting cesium bromine (CsBr) and cesium iodine (CsI) halide NCs are realized by zinc doping. High PLQYs of 79.05% and 78.95% are achieved by CsBr:Zn and CsI:Zn NCs, respectively, attributed to the improved local structural order and reduced strain between the lattices of the NCs after storing under ambient conditions for 20 to 30 days. Moreover, zinc doped cesium halide NCs show excellent air stability for at least 50 days. Our results for zinc doped cesium halide NCs have shown a new avenue to fabricate lead-free halide NCs for blue lighting and display applications.

Silver Metallic Cyclodextrin-Core Star mPEG.

A novel silver metallic ß-cyclodextrin (ß-CD)-core star methoxypolyethylene glycol (mPEG) (SM-CD-SPEG) is presented. For this, a well-defined water-soluble ß-CD-core star mPEG with aldehyde groups (CD-star(ald)PEG, III) is first designed and synthesized via the copper-catalyzed azide-alkyne click reaction. Then, silver nanoparticles are formed within III through the oxidation-reduction reaction between the aldehyde groups and silver ions. The oxidation-reduction reaction is characterized by means of 1 H NMR spectra. And the preparation conditions of SM-CD-SPEG are optimized. The resulting SM-CD-SPEG aqueous solutions exhibit good stability and a unimolecular micelle morphology. Significantly, it is able to obtain solid SM-CD-SPEG samples with molecular water-solubility. This is convenient for storage and further application of SM-CD-SPEG. The SM-CD-SPEG samples also show good catalytic performance toward the reduction reaction of methylene blue (MB) by sodium borohydride (NaBH4 ).