Efficient and Stable Inverted Perovskite Solar Cells Incorporating Secondary Amines

Interfacial Engineering of Nickel Oxide-Perovskite Interface with Amino Acid Complexed NiO to Improve Perovskite Solar Cell Performance

The interface between NiO and perovskite in inverted perovskite solar cells (PSCs) is a major factor that can limit device performance due to defects and inappropriate redox reactions, which cause nonradiative recombination and decrease in open-circuit voltage (VOC). In the present study, a novel approach is used for the first time, where an amino acid (glycine (Gly), alanine (Ala), and aminobutyric acid (ABA))-complexed NiO are used as interface modifiers to eliminate defect sites and hydroxyl groups from the surface of NiO. The Ala-complexed NiO suppresses interfacial non-radiative recombination, improves the perovskite layer quality and better energy band alignment with the perovskite, resulting in improved charge transfer and reduced recombination. The incorporation of the Ala-complexed NiO leads to a PCE of 20.27% with enhanced stability under the conditions of ambient air, light soaking, and heating to 85 °C, as it retains over 82%, 85%, and 61% of its initial PCE after 1000, 500, and 350 h, respectively. The low-temperature technique also leads to the fabrication of a NiO thin film that is suitable for flexible PSCs. The Ala-complexed NiO is fabricated on the flexible substrate and achieved 17.12% efficiency while retaining 71% of initial PCE after 5,000 bending.

Strategies for Improving Efficiency and Stability of Inverted Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted widespread research and commercialization attention because of their high power conversion efficiency (PCE) and low fabrication cost. The long-term stability of PSCs should satisfy industrial requirements for photovoltaic devices. Inverted PSCs with a p-i-n architecture exhibit considerable advantages because of their excellent stability and competitive efficiency. The continuously broken-through PCE of inverted PSCs shows huge application potential. This review summarizes the developments and outlines the characteristics of inverted PSCs including charge transport layers (CTLs), perovskite compositions, and interfacial regulation strategies. The latest effective CTLs, interfacial modification, and stability promotion strategies especially under light, thermal, and bias conditions are emphatically analyzed. Furthermore, the applications of the inverted structure in high-efficiency and stable tandem, flexible photovoltaic devices, and modules and their main obstacles are systematically introduced. Finally, the remaining challenges faced by inverted devices are discussed, and several directions for advancing inverted PSCs are proposed according to their development status and industrialization requirements.

Suppressing Redox Reactions at the Perovskite-Nickel Oxide Interface with Zinc Nitride to Improve the Performance of Perovskite Solar Cells

For p-i-n perovskite solar cells (PSCs), nickel oxide (NiOx) hole transport layers (HTLs) are the preferred interfacial layer due to their low cost, high mobility, high transmittance, and stability. However, the redox reaction between the Ni≥3+ and hydroxyl groups in the NiOx and perovskite layer leads to oxidized CH3NH3 + and reacts with PbI in the perovskite, resulting in a large number of non-radiative recombination sites. Among various transition metals, an ultra-thin zinc nitride (Zn3N2) layer on the NiOx surface is chosen to prevent these redox reactions and interfacial issues using a simple solution process at low temperatures. The redox reaction and non-radiative recombination at the interface of the perovskite and NiOx reduce chemically by using interface modifier Zn3N2 to reduce hydroxyl group and defects on the surface of NiOx. A thin layer of Zn3N2 at the NiOx/perovskite interface results in a high Ni3+/Ni2+ ratio and a significant work function (WF), which inhibits the redox reaction and provides a highly aligned energy level with perovskite crystal and rigorous trap-passivation ability. Consequently, Zn3N2-modified NiOx-based PSCs achieve a champion PCE of 21.61%, over the NiOx-based PSCs. After Zn3N2 modification, the PSC can improve stability under several conditions.

Key Roles of Interfaces in Inverted Metal-Halide Perovskite Solar Cells

Metal-halide perovskite solar cells (PSCs), an emerging technology for transforming solar energy into a clean source of electricity, have reached efficiency levels comparable to those of commercial silicon cells. Compared with other types of PSCs, inverted perovskite solar cells (IPSCs) have shown promise with regard to commercialization due to their facile fabrication and excellent optoelectronic properties. The interlayer interfaces play an important role in the performance of perovskite cells, not only affecting charge transfer and transport, but also acting as a barrier against oxygen and moisture permeation. Herein, we describe and summarize the last three years of studies that summarize the advantages of interface engineering-based advances for the commercialization of IPSCs. This review includes a brief introduction of the structure and working principle of IPSCs, and analyzes how interfaces affect the performance of IPSC devices from the perspective of photovoltaic performance and device lifetime. In addition, a comprehensive summary of various interface engineering approaches to solving these problems and challenges in IPSCs, including the use of interlayers, interface modification, defect passivation, and others, is summarized. Moreover, based upon current developments and breakthroughs, fundamental and engineering perspectives on future commercialization pathways are provided for the innovation and design of next-generation IPSCs.

Improved charge extraction in inverted perovskite solar cells with dual-site-binding ligands

Inverted (pin) perovskite solar cells (PSCs) afford improved operating stability in comparison to their nip counterparts but have lagged in power conversion efficiency (PCE). The energetic losses responsible for this PCE deficit in pin PSCs occur primarily at the interfaces between the perovskite and the charge-transport layers. Additive and surface treatments that use passivating ligands usually bind to a single active binding site: This dense packing of electrically resistive passivants perpendicular to the surface may limit the fill factor in pin PSCs. We identified ligands that bind two neighboring lead(II) ion (Pb2+) defect sites in a planar ligand orientation on the perovskite. We fabricated pin PSCs and report a certified quasi-steady state PCE of 26.15 and 24.74% for 0.05- and 1.04-square centimeter illuminated areas, respectively. The devices retain 95% of their initial PCE after 1200 hours of continuous 1 sun maximum power point operation at 65°C.

Development on inverted perovskite solar cells: A review

Recently, inverted perovskite solar cells (IPSCs) have received note-worthy consideration in the photovoltaic domain because of its dependable operating stability, minimal hysteresis, and low-temperature manufacture technique in the quest to satisfy global energy demand through renewable means. In a decade transition, perovskite solar cells in general have exceeded 25 % efficiency as a result of superior perovskite nanocrystalline films obtained via low temperature synthesis methods along with good interface and electrode materials management. This review paper presents detail processes of refining the stability and power conversion efficiencies in IPSCs. The latest development in the power conversion efficiency, including structural configurations, prospect of tandem solar cells, mixed cations and halides, films' fabrication methods, charge transport material alterations, effects of contact electrode materials, additive and interface engineering materials used in IPSCs are extensively discussed. Additionally, insights on the state of the art and IPSCs' continued development towards commercialization are provided.

Post-device Dimethylamine Treatment Enables Stable and Efficient Perovskite Solar Cells

The incorporation of organic ligands via post-device treatment is an effective strategy to improve the stability of perovskite solar cells (PSCs). Although the active area is protected by metal electrode under post-treatment, the aggression of post-treatment ligands into active area cannot be avoided thoroughly. Unfortunately, the size of long-chain amines is too large, and the three-dimensional (3D) perovskite cannot maintain its 3D perovskite structure once the cation substitution occurs during the post-treatment. Despite that the low-dimensional (LD) perovskites are beneficial to stability, long-chain amines are harmful to carrier transport in PSCs. Here, we introduce dimethylamine (DMA), a slightly oversized cation that can be doped into 3D perovskite structure, for post-device treatment to improve the efficiency and stability of PSCs. After exposure to DMA gas, the inactive area of Cs/FA/MA mixed cation perovskite device that is not covered by metal electrode is converted into LD perovskite, passivating the defects of 3D perovskite in the active region, suppressing non-radiation recombination and ion migration. As a result, we achieved a power conversion efficiency (PCE) of 22.29 % with negligible hysteresis and better stability after DMA post-treatment, which is much higher than that (20.40 %) of the control device.

Ligand Engineering Enables Efficient Pure Red Tin-Based Perovskite Light-Emitting Diodes

With increasing ecological and environmental concerns, tin (Sn)-based perovskite light-emitting diodes (PeLEDs) are competitive candidates for future displays because of their environmental friendliness, excellent photoelectric properties, and low-cost solution-processed fabrication. Nonetheless, their electroluminescence (EL) performance still lags behind that of lead (Pb)-based PeLEDs due to the fast crystallization rate of Sn-based perovskite films and undesired oxidation from Sn2+ to Sn4+ , leading to poor film morphology and coverage, as well as high density defects. Here, we propose a ligand engineering strategy to construct high-quality phenethylammonium tin iodide (PEA2 SnI4 ) perovskite films by using L-glutathione reduced (GSH) as surface ligands toward efficient pure red PEA2 SnI4 -based PeLEDs. We show that the hydrogen-bond and coordinate interactions between GSH and PEA2 SnI4 effectively reduce the crystallization rate of the perovskites and suppress the oxidation of Sn2+ and formation of defects. The improved pure red perovskite films not only show excellent uniformity, density, and coverage but also exhibit enhanced optical properties and stability. Finally, state-of-the-art pure red PeLEDs achieve a record external quantum efficiency of 9.32 % in the field of PEA2 SnI4 -based devices. This work demonstrates that ligand engineering represents a feasible route to enhance the EL performance of Sn-based PeLEDs.

Anion optimization for bifunctional surface passivation in perovskite solar cells

Pseudo-halide (PH) anion engineering has emerged as a surface passivation strategy of interest for perovskite-based optoelectronics; but until now, PH anions have led to insufficient defect passivation and thus to undesired deep impurity states. The size of the chemical space of PH anions (>106 molecules) has so far limited attempts to explore the full family of candidate molecules. We created a machine learning workflow to speed up the discovery process using full-density functional theory calculations for training the model. The physics-informed machine learning model allowed us to pinpoint promising molecules with a head group that prevents lattice distortion and anti-site defect formation, and a tail group optimized for strong attachment to the surface. We identified 15 potential bifunctional PH anions with the ability to passivate both donors and acceptors, and through experimentation, discovered that sodium thioglycolate was the most effective passivant. This strategy resulted in a power-conversion efficiency of 24.56% with a high open-circuit voltage of 1.19 volts (24.04% National Renewable Energy Lab-certified quasi-steady-state) in inverted perovskite solar cells. Encapsulated devices maintained 96% of their initial power-conversion energy during 900 hours of one-sun operation at the maximum power point.

Silicon/nickel oxide core/shell nanospheres as a hole transport layer for high efficiency and light-stable perovskite solar cells

Metal halide perovskite solar cells (PSCs) possess huge potential due to their high power conversion efficiency. However, instability is still a key factor limiting their applications. Therefore, we have found a feasible strategy to improve the light stability of PSCs. Specifically, a core-shell material with a silicon nanosphere core and a nickel oxide nanosheet shell serves as the hole transport layer in our PSCs. Due to the selective absorption of ultraviolet light by the silicon nanoparticles, the ultraviolet light content of the natural light that reaches the perovskite layer is reduced. Compared with a control device (without Si), the PSCs with the silicon/nickel oxide hole transport layer possessed a higher current density of 22.09 mA cm^-2 and a higher power conversion efficiency of 18.54%, with both values increased by 2.7% and 6.1%, respectively. More importantly, the PSCs based on a silicon/nickel oxide hole transport layer maintains 85% of its initial power conversion efficiency value after 700 hours of natural light exposure. These results indicate that the silicon/nickel oxide hole transport layer is an important functional component of the PSCs, which improves the photovoltaic performance and reduces ultraviolet light-induced photodegradation, thereby improving the device stability.

Graphene-Like Conjugated Molecule as Hole-Selective Contact for Operationally Stable Inverted Perovskite Solar Cells and Modules

Further enhancing the operational lifetime of inverted-structure perovskite solar cells (PSCs) is crucial for their commercialization, and the design of hole-selective contacts at the illumination side plays a key role in operational stability. In this work, the self-anchoring benzo[rst]pentaphene (SA-BPP) is developed as a new type of hole-selective contact toward long-term operationally stable inverted PSCs. The SA-BPP molecule with a graphene-like conjugated structure shows a higher photostability and mobility than that of the frequently-used triphenylamine and carbazole-based hole-selective molecules. Besides, the anchoring groups of SA-BPP promote the formation of a large-scale uniform hole contact on ITO substrate and efficiently passivate the perovskite absorbers. Benefiting from these merits, the champion efficiencies of 22.03% for the small-sized cells and 17.08% for 5 × 5 cm² solar modules on an aperture area of 22.4 cm² are achieved based on this SA-BPP contact. Also, the SA-BPP-based device exhibits promising operational stability, with an efficiency retention of 87.4% after 2000 h continuous operation at the maximum power point under simulated 1-sun illumination, which indicates an estimated T₈₀ lifetime of 3175 h. This novel design concept of hole-selective contacts provides a promising strategy for further improving the PSC stability.

Bridging the Buried Interface with Piperazine Dihydriodide Layer for High Performance Inverted Solar Cells

Given that it is closely related to perovskite crystallization and interfacial trap densities, buried interfacial engineering is crucial for creating effective and stable perovskite solar cells. Compared with the in-depth studies on the defect at the top perovskite interface, exploring the defect of the buried side of perovskite film is relatively complicated and scanty owing to the non-exposed feature. Herein, the degradation process is probed from the buried side of perovskite films with continuous illumination and its effects on morphology and photoelectronic characteristics with a facile lift-off method. Additionally, a buffer layer of Piperazine Dihydriodide (PDI₂ ) is inserted into the imbedded bottom interface. The PDI₂ buffer layer is able to lubricate the mismatched thermal expansion between perovskite and substrate, resulting in the release of lattice strain and thus a void-free buried interface. With the PDI₂ buffer layer, the degradation originates from the growing voids and increasing non-radiative recombination at the imbedded bottom interfaces are suppressed effectively, leading to prolonged operation lifetime of the perovskite solar cells. As a result, the power conversion efficiency of an optimized p-i-n inverted photovoltaic device reaches 23.47% (with certified 23.42%) and the unencapsulated devices maintain 90.27% of initial efficiency after 800 h continuous light soaking.

DFT Studies on a Metal Oxide@Graphene-Decorated D-π1-π2-A Novel Multi-Junction Light-Harvesting System for Efficient Dye-Sensitized Solar Cell Applications

Graphene nanocomposites have emerged as potential photoanode materials for increased performance of the dye-sensitized solar cells (DSSCs) via charge transfer. Various metal-oxide-decorated graphene nanocomposites have widespread applications in energy devices, such as solar cells, fuel cells, batteries, sensors, electrocatalysis, and photocatalysis. However, the possible role of these composites in DSSC applications has largely remained unexplored. Herein, we studied a Sb₂O₃-decorated graphene-D-π₁-π₂-A sensitized TiO₂ nanocomposite (dye-(TiO₂)₉/Sb₂O₃@GO) as a model multi-junction light-harvesting system and examined the impact of various π-bridges on the optical and photovoltaic properties of the push-pull dye system employed in this light-harvesting system. We have shown that by changing the spacer unit, the light sensitivity of nanocomposites can be varied from visible to near-infrared wavelengths. Furthermore, with the integration of metal-oxide-decorated graphene nanocomposites on D-π₁-π₂-A systems and D-π-A systems, composite photoelectrodes displayed better optical and photovoltaic characteristics with an enhanced absorption spectrum in the wavelength range of 800-1000 nm. The performance of the D-π₁-π₂-A system has been evaluated in terms of various photovoltaic parameters such as the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, excited-state oxidation potential (E _dye ^*), free energy of electron injection (G _inject), total reorganization energy (λ_total), and open-circuit voltage (V _oc). This work throws light on the current trends and the future opportunities in graphene-metal oxide nanocomposite-based DSSCs for better harvesting of the solar spectrum and better performance of solar devices.

Rational Selection of the Lewis Base Molecules Targeted for Lead-Based Defects of Perovskite Solar Cells: The Synergetic Co-passivation of Carbonyl and Carboxyl Groups

Defect passivation through Lewis acid-base chemistry has recently attracted significant interest because of its proven ability to improve the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). However, tedious trial-and-error procedures are commonly used for the selection of Lewis molecules due to their abundant variety. Herein, two typical Lewis base molecules, the M molecule containing only carbonyl groups and the 3M molecule containing both carbonyl and carboxyl groups, are proposed to passivate the Pb-based defects and mitigate their negative impacts on PSC performance. The results indicated that much stronger coordination bonds can be formed between the 3M molecule and uncoordinated Pb²⁺ than with the M molecule. Because of the benefit from the synergetic co-passivation effect of carbonyl and carboxyl groups, an impressive maximum PCE of 24.07% was achieved via 3M modification. More importantly, the modified devices demonstrated remarkably improved operational stability.

Regulating surface potential maximizes voltage in all-perovskite tandems

The open-circuit voltage (V_OC) deficit in perovskite solar cells is greater in wide-bandgap (over 1.7 eV) cells than in perovskites of roughly 1.5 eV (refs. ^1,2). Quasi-Fermi-level-splitting measurements show V_OC-limiting recombination at the electron-transport-layer contact^3-5. This, we find, stems from inhomogeneous surface potential and poor perovskite-electron transport layer energetic alignment. Common monoammonium surface treatments fail to address this; as an alternative, we introduce diammonium molecules to modify perovskite surface states and achieve a more uniform spatial distribution of surface potential. Using 1,3-propane diammonium, quasi-Fermi-level splitting increases by 90 meV, enabling 1.79 eV perovskite solar cells with a certified 1.33 V V_OC and over 19% power conversion efficiency (PCE). Incorporating this layer into a monolithic all-perovskite tandem, we report a record V_OC of 2.19 V (89% of the detailed balance V_OC limit) and over 27% PCE (26.3% certified quasi-steady state). These tandems retained more than 86% of their initial PCE after 500 h of operation.

Continuous Modification of Perovskite Film by a Eu Complex to Fabricate the Thermal and UV-Light-Stable Solar Cells

Perovskite solar cells (PSCs) with simple and low-cost processability have shown promising photovoltaic performances. However, internal defects, external UV light, and heat sensitivity are principal obstacles on their way toward commercialization. Herein, we prepare an Eu complex and directly dope it into the perovskite precursor as a UV filter to decrease the photodegradation of PSCs. The formation of hydrogen bonds between the organic cation of perovskite and the -CF₃ in the Eu complex could restrain the escape of organic cations under heating. The Eu complex acts as a redox shuttle to reduce metallic lead (Pb⁰) and iodine (I⁰) defects when the PSCs have a long-time operation. Additionally, the ligand-containing aromatic rings could reduce the trace amount of I⁰ existing as electronic defects in perovskites and together with the long alkyl chain retard the moisture immersion into the PSCs. The best efficiency of PSCs modified by the Eu complex improves up to 20.9%. The excellent thermal stability and UV-light resistance are also realized. This strategy provides a method to design a passivator which continuously modifies the imperfections and inhibits the chemical chain reactions in perovskite film, thereby enhancing the performance and stability of PSCs.

Dimethylammonium Cation-Induced 1D/3D Heterostructure for Efficient and Stable Perovskite Solar Cells

Mixed-dimensional perovskite engineering has been demonstrated as a simple and useful approach to achieving highly efficient and more-durable perovskite solar cells (PSCs), which have attracted increasing research interests worldwide. In this work, 1D/3D mixed-dimensional perovskite has been successfully obtained by introducing DMAI via a two-step deposition method. The additive DMA⁺ can facilitate the crystalline growth and form 1D DMAPbI₃ at grain boundaries of 3D perovskite, leading to improved morphology, longer charge carrier lifetime, and remarkably reduced bulk trap density for perovskite films. Meanwhile, the presence of low-dimension perovskite is able to prevent the intrusion of moisture, resulting in enhanced long-term stability. As a result, the PSCs incorporated with 1D DMAPbI₃ exhibited a first-class power conversion efficiency (PCE) of 21.43% and maintained 85% of their initial efficiency after storage under ambient conditions with ~45% RH for 1000 h.

Interplay of Kinetic and Thermodynamic Reaction Control Explains Incorporation of Dimethylammonium Iodide into CsPbI3

CsPbI3 is a promising material for optoelectronics owing to its thermal robustness and favorable bandgap. However, its fabrication is challenging because its photoactive phase is thermodynamically unstable at room temperature. Adding dimethylammonium (DMA) alleviates this instability and is currently understood to result in the formation of DMA x Cs1-x PbI3 perovskite solid solutions. Here, we use NMR of the 133Cs and 13C local structural probes to show that these solid solutions are not thermodynamically stable, and their synthesis under thermodynamic control leads to a segregated mixture of yellow one-dimensional DMAPbI3 phase and δ-CsPbI3. We show that mixed-cation DMA x Cs1-x PbI3 perovskite phases only form when they are kinetically trapped by rapid antisolvent-induced crystallization. We explore the energetics of DMA incorporation into CsPbI3 using first-principles calculations and molecular dynamics simulations and find that this process is energetically unfavorable. Our results provide a complete atomic-level picture of the mechanism of DMA-induced stabilization of the black perovskite phase of CsPbI3 and shed new light on this deceptively simple material.

Thermal Shock Fabrication of Ion-Stabilized Perovskite and Solar Cells

A highly crystalline tempered-glass-like perovskite grain structure with compressed surface lattice realized by a thermal-shocking fabrication is shown. The strained perovskite grain structure is stabilized by Cl^- -reinforcing surface lattice and shows enhanced bonding energy and ionic activation temperature, which contributes to hysteresis-free operation of perovskite solar cells (PSCs) at much higher temperature up to 363 K in thermal-shocking-processed MAPbCl_x I_{3- x} (T-MPI). The PSCs can be fabricated by a high-speed fully air process without post-annealing based on the scalable bar-coating technique. Both high efficiency and stability are achieved in T-MPI PSC with a power conversion efficiency (PCE) up to 22.99% and long-term operational stability with T₈₀ lifetime exceeding 4000 h.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel

CdS quantum dots (QDs), synthesized by a sol-gel method, exhibit significantly Stokes shifted bright photoluminescence (PL), predominantly from the trap states. Surprisingly, the PL decay at the emission maximum is single-exponential. This is an unusual observation for as-prepared QDs and indicates a narrow distribution in the nature of trap states. A closer look reveals an additional fast component for the decays at shorter emission wavelengths, presumably due to the band edge emission, which remains elusive in the steady-state spectra. Indeed, a significantly narrower and blue-shifted emission band is observed in the decay-associated spectra. The contribution of this component to the steady-state PL intensity is shown to be overwhelmed by that of the significantly stronger trap emission. Exciton dynamics in the quantum dots is elucidated using transient absorption spectra, in which the stimulated emission is observed even at low pump power.

How Do A-Site Cations Regulate Trap States at Defective Surfaces of Lead Iodide Perovskites?

The defect properties at surfaces or grain boundaries of metal halide perovskites are largely unexplored due to the complexity of surface structures stirred by the rotational A-site cations with varied dipole moments. Using a combination of density functional theory (DFT) and time-dependent DFT methods, we study the nature of iodine vacancies at the surfaces of lead iodide perovskites (APbI₃) with A-site cations including methylammonium (MA = CH₃NH₃⁺), formamidinium, and cesium. It is found that the light-induced charge distributions are sensitively dependent on MA orientation at the MAI-terminated surfaces with vacancies at the apical position while the electronic excitation is marginally affected by A-site species at both the AI- and PbI-terminated surfaces with vacancies at the equatorial site. Such variations of electronic excitation are rationalized by analyzing the electrostatic interactions between the A-site cations and charged defects as well as the projected p orbitals of Pb atoms at the bottom of the conduction band.

Bridging the Interfacial Contact for Improved Stability and Efficiency of Inverted Perovskite Solar Cells

Inverted perovskite solar cells (PSCs) have received widespread attention due to their facile fabrication and wide applications. However, their power conversion efficiency (PCE) is reported lower than that of regular PSCs because of the undesirable interfacial contact between perovskite and the hydrophobic hole transport layer (HTL). Here, an interface regulation strategy is proposed to overcome this limitation. A small molecule ([2-(9H-carbazol-9-yl) ethyl] phosphonic acid, abbreviated as 2P), composed of carbazole and phosphonic acid groups, is inserted between perovskite and HTL. Morphological characterization and theoretical calculation reveal that perovskite bonds stronger on 2P-modified HTL than on pristine HTL. The improved interfacial contact facilitates hole extraction and retards degradation. Upon the incorporation of 2P, inverted PSCs deliver a high PCE of over 22% with superior stability, keeping 84.6% of initial efficiency after 7200 h storage under an ambient atmosphere with a relative humidity of ≈30-40%. This strategy provides a simple and efficient way to boost the performance of inverted PSCs.

Manipulating Ion Migration and Interfacial Carrier Dynamics via Amino Acid Treatment in Planar Perovskite Solar Cells

Instability caused by the migrating ions is one of the major obstacles toward the large-scale application of metal halide perovskite optoelectronics. Inactivating mobile ions/defects via chemical passivation, e.g., amino acid treatment, is a widely accepted approach to solve that problem. To investigate the detailed interplay, L-phenylalanine (PAA), a typical amino acid, is used to modify the SnO₂/MAPbI₃ interface. The champion device with PAA treatment maintains 80% of its initial power conversion efficiency (PCE) when stored after 528 h in an ambient condition with the relative humidity exceeding 70%. By employing a wide-field photoluminescence imaging microscope to visualize the ion movement and calculate ionic mobility quantitatively, we propose a model for enhanced stability in perspective of suppressed ion migration. Besides, we reveal that the PAA dipole layer facilitates charge transfer at the interface, enhancing the PCE of devices. Our work may provide an in-depth understanding toward high-efficiency and stable perovskite optoelectronic devices.

Large Photomultiplication by Charge-Self-Trapping for High-Response Quantum Dot Infrared Photodetectors

PbS colloidal quantum dots (CQDs) are emerging as promising candidates for next-generation, low-cost, and high-performance infrared photodetectors. Recently, photomultiplication has been explored to improve the detectivity of CQD infrared photodetectors by doping charge-trapping material into a matrix. However, this relies on remote doping that could influence carrier transfer giving rise to limited photomultiplication. Herein, a charge-self-trapped ZnO layer is prepared by a surface reaction between acid and ZnO. Photogenerated electrons trapped by oxygen vacancy defects at the ZnO surface generate a strong interfacial electrical field and induce large photomultiplication at extremely low bias. A PbS CQD infrared photodiode based on this structure shows a response (R) of 77.0 A·W^-1 and specific detectivity of 1.5 × 10¹¹ Jones at 1550 nm under a -0.3 V bias. This self-trapped ZnO layer can be applied to other photodetectors such as perovskite-based devices.

Interface Regulation by an Ultrathin Wide-Bandgap Halide for Stable and Efficient Inverted Perovskite Solar Cells

The nonradiative recombination between hole transport layers (HTLs) and perovskites generally leads to obvious energy losses. The trap states at the HTL/perovskite interface directly influence the improvement of the power conversion efficiency (PCE) and stability. Interface regulation is a simple and commonly used method to decrease nonradiative recombination in inverted perovskite solar cells (PSCs). Here, a wide-bandgap halide was used to regulate the PTAA/MAPbI₃ interface, in which n-hexyltrimethylammonium bromide (HTAB) was used to modify the upper surface of poly[bis(4-phenyl)-(2,4,6-trimethylphenyl)amine] (PTAA). Upon introduction of the HTAB layer, the contact between PTAA and MAPbI₃ is strengthened, the defect state density in PSCs is reduced, the MAPbI₃ crystallinity is improved, and the nonradiative recombination loss is suppressed. The device with HTAB delivers the highest PCE of 21.01% with negligible hysteresis, which is significantly higher than that of the control device (17.71%), and it maintains approximately 87% of its initial PCE for 1000 h without encapsulation in air with a relative humidity of 25 ± 5%. This work reveals an effective way of using a wide-bandgap halide to regulate the PTAA/MAPbI₃ interface to simultaneously promote the PCE and stability of PSCs.

Enhanced Perovskite Solar Cell Performance via 2-Amino-5-iodobenzoic Acid Passivation

The intrinsic stability issues of the perovskite materials threaten the efficiency and stability of the devices, and stability has become the main obstacle to industrial applications. Herein, the efficient and facile passivation strategy by 2-amino-5-iodobenzoic acid (AIBA) is proposed. The impact of AIBA on the properties of the perovskite films and device performance is systemically studied. The results show that the trap states are eliminated without affecting the crystal properties of the perovskite grains, leading to the enhanced performance and stability of the perovskite solar cells (PSCs). A high power conversion efficiency (PCE) of 20.23% and lower hysteresis index (HI) of 1.49‰ are achieved, which represent one of the most excellent PCE and HI values for the inverted PSCs based on MAPbI₃/[6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM) planar heterojunction structure. Moreover, the UV stability of the perovskite films and the thermal and moisture stability of the devices are also enhanced by the AIBA passivation. The PCE of the device with AIBA can maintain about 83.41% for 600 h (40 RH %) and 64.06% for 100 h (55-70 RH %) of its initial PCE value without any encapsulation, while the control device can maintain only about 72.91 and 45.59% of its initial PCE. Density functional theory calculations are performed to study the origins of enhanced performance. Interestingly, the results show that the surface states induced by AIBA can facilitate the photoexcited charge transfer dynamics and reduce the electron-hole recombination loss. The passivation method developed in this work provides an efficient way to enhance the stability and performance of inverted PSCs.

Mixed Dimethylammonium/Methylammonium Lead Halide Perovskite Crystals for Improved Structural Stability and Enhanced Photodetection

The solvent acidolysis crystallization technique is utilized to grow mixed dimethylammonium/methylammonium lead tribromide (DMA/MAPbBr₃ ) crystals reaching the highest dimethylammonium incorporation of 44% while maintaining the 3D cubic perovskite phase. These mixed perovskite crystals show suppression of the orthorhombic phase and a lower tetragonal-to-cubic phase-transition temperature compared to MAPbBr₃ . A distinct behavior is observed in the temperature-dependent photoluminescence properties of MAPbBr₃ and mixed DMA/MAPbBr₃ crystals due to the different organic cation dynamics governing the phase transition(s). Furthermore, lateral photodetectors based on these crystals show that, at room temperature, the mixed crystals possess higher detectivity compared to MAPbBr₃ crystals caused by structural compression and reduced surface trap density. Remarkably, the mixed-crystal devices exhibit large enhancement in their detectivity below the phase-transition temperature (at 200 K), while for the MAPbBr₃ devices only insignificant changes are observed. The high detectivity of the mixed crystals makes them attractive for visible-light communication and for space applications. The results highlight the importance of the synthetic technique for compositional engineering of halide perovskites that governs their structural and optoelectronic properties.

Imidazole cores for the construction of dopant-free asymmetric hole-transporting materials for efficient inverted perovskite solar cells

An asymmetric molecule endowed with an extended electron donor generates a PCE over 16% in n–i–p structured inverted PSCs.

Enhanced Activation Energy Released by Coordination of Bifunctional Lewis Base d-Tryptophan for Highly Efficient and Stable Perovskite Solar Cells

Perovskite defect passivation with molecule doping shows great potential in boosting the efficiency and stability of perovskite solar cells (PSCs). Herein, an efficient and low-cost bifunctional Lewis base additive d-tryptophan is introduced to control the crystallization and growth of perovskite grains and passivation defects. It is found that the additive doped in the solution precursors could retard crystal growth by increasing activation energy, resulting in improved crystallization of large grains with reduced grain boundaries, as well as inhibiting ion migration and PbI₂ aggregation. As a result, the PSCs incorporated with d-tryptophan additives achieve an improved power conversion efficiency from 18.18 to 21.55%. Moreover, the d-tryptophan passivation agent improves the device stability, which retains 86.85% of its initial efficiency under ambient conditions at room temperature after 500 h. This work provides Lewis base small-molecule d-tryptophan for efficient defect passivation of the grain boundaries toward efficient and stable PSCs.

Passivation of the Buried Interface via Preferential Crystallization of 2D Perovskite on Metal Oxide Transport Layers

The open-circuit voltage (V_oc ) of perovskite solar cells is limited by non-radiative recombination at perovskite/carrier transport layer (CTL) interfaces. 2D perovskite post-treatments offer a means to passivate the top interface; whereas, accessing and passivating the buried interface underneath the perovskite film requires new material synthesis strategies. It is posited that perovskite ink containing species that bind strongly to substrates can spontaneously form a passivating layer with the bottom CTL. The concept using organic spacer cations with rich NH₂ groups is implemented, where readily available hydrogens have large binding affinity to under-coordinated oxygens on the metal oxide substrate surface, inducing preferential crystallization of a thin 2D layer at the buried interface. The passivation effect of this 2D layer is examined using steady-state and time-resolved photoluminescence spectroscopy: the 2D interlayer suppresses non-radiative recombination at the buried perovskite/CTL interface, leading to a 72% reduction in surface recombination velocity. This strategy enables a 65 mV increase in V_oc for NiO_x based p-i-n devices, and a 100 mV increase in V_oc for SnO₂ -based n-i-p devices. Inverted solar cells with 20.1% power conversion efficiency (PCE) for 1.70 eV and 22.9% PCE for 1.55 eV bandgap perovskites are demonstrated.

Mobile ions determine the luminescence yield of perovskite light-emitting diodes under pulsed operation

The external quantum efficiency of perovskite light-emitting diodes (PeLEDs) has advanced quickly during the past few years. However, under pulsed operation, an operation mode which is important for display and visible light communication, the performance of PeLEDs changes a lot and requires in-depth understanding to facilitate these applications. Here, we report the response of PeLEDs under pulsed operation in the range of 10 Hz to 20 kHz. Beyond transient effects in the low frequencies, we find that for higher frequencies (>500 Hz) the transient electroluminescence intensity depends strongly on the duty cycle. This feature is much more pronounced and of different origin than that in conventional LEDs. We rationalise our experimental observations using a mathematical model and assign these features to the effect of mobile ionic charges in the perovskite. Our work also provides important implications for the operation of PeLEDs under the steady state, where accumulation of mobile ions at the interfaces could be beneficial for high electroluminescence yields but harmful for the long-term stability.

The Main Progress of Perovskite Solar Cells in 2020-2021

Perovskite solar cells (PSCs) emerging as a promising photovoltaic technology with high efficiency and low manufacturing cost have attracted the attention from all over the world. Both the efficiency and stability of PSCs have increased steadily in recent years, and the research on reducing lead leakage and developing eco-friendly lead-free perovskites pushes forward the commercialization of PSCs step by step. This review summarizes the main progress of PSCs in 2020 and 2021 from the aspects of efficiency, stability, perovskite-based tandem devices, and lead-free PSCs. Moreover, a brief discussion on the development of PSC modules and its challenges toward practical application is provided.

Efficient and stable inverted perovskite solar cells with very high fill factors via incorporation of star-shaped polymer

Stabilizing high-efficiency perovskite solar cells (PSCs) at operating conditions remains an unresolved issue hampering its large-scale commercial deployment. Here, we report a star-shaped polymer to improve charge transport and inhibit ion migration at the perovskite interface. The incorporation of multiple chemical anchor sites in the star-shaped polymer branches strongly controls the crystallization of perovskite film with lower trap density and higher carrier mobility and thus inhibits the nonradiative recombination and reduces the charge-transport loss. Consequently, the modified inverted PSCs show an optimal power conversion efficiency of 22.1% and a very high fill factor (FF) of 0.862, corresponding to 95.4% of the Shockley-Queisser limited FF (0.904) of PSCs with a 1.59-eV bandgap. The modified devices exhibit excellent long-term operational and thermal stability at the maximum power point for 1000 hours at 45°C under continuous one-sun illumination without any significant loss of efficiency.

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

Engineering the Optical Emission and Robustness of Metal-Halide Layered Perovskites through Ligand Accommodation

The unique combination of organic and inorganic layers in 2D layered perovskites offers promise for the design of a variety of materials for mechatronics, flexoelectrics, energy conversion, and lighting. However, the potential tailoring of their properties through the organic building blocks is not yet well understood. Here, different classes of organoammonium molecules are exploited to engineer the optical emission and robustness of a new set of Ruddlesden-Popper metal-halide layered perovskites. It is shown that the type of molecule regulates the number of hydrogen bonds that it forms with the edge-sharing [PbBr6 ]4- octahedra layers, leading to strong differences in the material emission and tunability of the color coordinates, from deep-blue to pure-white. Also, the emission intensity strongly depends on the length of the molecules, thereby providing an additional parameter to optimize their emission efficiency. The combined experimental and computational study provides a detailed understanding of the impact of lattice distortions, compositional defects, and the anisotropic crystal structure on the emission of such layered materials. It is foreseen that this rational design can be extended to other types of organic linkers, providing a yet unexplored path to tailor the optical and mechanical properties of these materials and to unlock new functionalities.

Photoinduced Dynamics of Charge Carriers in Metal Halide Perovskites from an Atomistic Perspective

Perovskite solar cells have attracted intense attention over the past decade because of their low cost, abundant raw materials, and rapidly growing power conversion efficiency (PCE). However, nonradiative charge carrier losses still constitute a major factor limiting the PCE to well below the Shockley-Queisser limit. This Perspective summarizes recent atomistic quantum dynamics studies on the photoinduced excited-state processes in metal halide perovskites (MHPs), including both hybrid organic-inorganic and all-inorganic MHPs and three- and two-dimensional MHPs. The simulations, performed using a combination of time-domain ab initio density functional theory and nonadiabatic molecular dynamics, allow emphasis on various intrinsic and extrinsic features, such as components, structure, dimensionality and interface engineering, control and exposure to various environmental factors, defects, surfaces, and their passivation. The detailed atomistic simulations advance our understanding of electron-vibrational dynamics in MHPs and provide valuable guidelines for enhancing the performance of perovskite solar cells.

Enhanced Perovskite Solar Cell Efficiency Via the Electric-Field-Induced Approach

The stability issue hinders the commercialization of the perovskite solar cells (PSCs), which is widely recognized. The efficiency generally decreases over time during the working condition. Here, we report an efficiency enhancement phenomenon of PSCs in the stability test at the maximum power point, which is speculated to be related to the electric-field-induced ion migration. The defect density and efficiency-related parameters were traced in situ by admittance spectroscopy and transient photovoltage when the cell works under bias voltage. The performance enhancement was revealed to be attributed to the reduction of the cell defects owing to ion migration. An efficiency of 22.3% can be achieved after the bias voltage was kept for 8 h. These findings suggest that ion migration is a double-edged sword that affects the electrical stability of PSCs, which presents a potential approach to improve the device's stability by appropriately controlling the defect states.

Grain Growth of MAPbI3 via Diethylammonium Bromide Induced Grain Mergence

Various additives are used to improve the film morphology, crystal quality, and grain size for the sake of enhancing the performance of three-dimensional perovskite solar cells. Although significant enhancement in the performance of devices has been made due to the introduction of additives, an in-depth understanding of the additive-related crystallization kinetics and the growth mechanism is still lacking. Here, the grain growth mechanism of diethylammonium bromide (DABr)-doped MAPbI₃ is investigated using in situ dynamic microscopy techniques. The results reveal that the alkyl chains of DABr restrain the growth of grains of MAPbI₃ during spin-coating, and DABr-induced grain mergence during the annealing process, achieving large grains on the micrometer scale. Meanwhile, the crystallization of MAPbI₃ with DABr is significantly improved and the number of defects is reduced. The solar cell with optimized DABr doping MAPbI₃ as the active layer presents a higher power conversion efficiency (PCE) of 19.58% with a fill factor of 79.81%.

High-performance inverted perovskite solar cells using 4-diaminomethylbenzoic as a passivant

Grain boundary (GB) and interface passivation of perovskite films impacts the efficiency and stability of perovskite solar cells (PSCs) dramatically. Here, an efficient passivation strategy using 4-diaminomethylbenzoic (4-DA) followed by thermal annealing treatment is proposed to improve the performance of PSCs. We have systemically investigated the impact of 4-DA on the physical properties of the perovskite layer and corresponding performance of the inverted PSCs. The results show that the contact between crystalline grains is improved, and high quality MAPbI₃ films are successfully prepared, which result in the elimination of trap states and enhanced performance of the devices. The highest power conversion efficiency (PCE) of 20.58% is achieved in this work. Meanwhile, the devices show enhanced stability and the average PCE values almost remained the same after 168 hours of storage without any encapsulation. The passivation method developed in this work shows a novel strategy toward the fabrication of inverted PSCs with high efficiency and high stability.

Interfacial and structural modifications in perovskite solar cells

The rapid and continuous progress made in perovskite solar cell (PSC) technology has drawn considerable attention from the photovoltaic research community, and the application of perovskites in other electronic devices (such as photodetectors, light-emitting diodes, and batteries) has become imminent. Because of the diversity in device configurations, optimization of film deposition, and exploration of material systems, the power conversion efficiency (PCE) of PSCs has been certified to be as high as 25.2%, making this type of solar cells the fastest advancing technology until now. As demonstrated by researchers worldwide, controlling the morphology and defects in perovskite films is essential for attaining high-performance PSCs. In this regard, interface engineering has proven to be a very efficient way to address these issues, obtaining better charge collection efficiency, and reducing recombination losses. In this review, the interfacial modification between perovskite films and charge-transport layers (CTLs) as well as CTLs and electrodes of PSCs has been widely summarized. Grain boundary (GB) engineering and stress engineering are also included since they are closely related to the improvement in device performance and stability.

Steric Impediment of Ion Migration Contributes to Improved Operational Stability of Perovskite Solar Cells

The operational instability of perovskite solar cells (PSCs) is known to mainly originate from the migration of ionic species (or charged defects) under a potential gradient. Compositional engineering of the "A" site cation of the ABX₃ perovskite structure has been shown to be an effective route to improve the stability of PSCs. Here, the effect of size-mismatch-induced lattice distortions on the ion migration energetics and operational stability of PSCs is investigated. It is observed that the size mismatch of the mixed "A" site composition films and devices leads to a steric effect to impede the migration pathways of ions to increase the activation energy of ion migration, which is demonstrated through multiple theoretical and experimental evidence. Consequently, the mixed composition devices exhibit significantly improved thermal stability under continuous heating at 85 °C and operational stability under continuous 1 sun illumination, with an extrapolated lifetime of 2011 h, compared to the 222 h of the reference device.