Ligang Yuan

The current status of both low‐bandgap mixed Sn‐Pb perovskite solar cells and all‐perovskite tandem solar cells are summarized in this review. Possible strategies for further improving the performance and stability of the devices based on mixed Sn‐Pb perovskites are also discussed.

Abstract

Efficient organic–inorganic metal halide perovskite absorbers have gained tremendous research interest in the past decade due to their super optoelectronic properties and defect tolerance. Lead (Pb) halide perovskites enable highly efficient perovskite solar cells (PSCs) with a record power conversion efficiency (PCE) of over 23%. However, the energy bandgaps of Pb halide perovskites are larger than the optimal bandgap for single junction solar cells, governed by the Shockley–Queisser (SQ) radiative limit. Mixed tin (Sn)‐Pb halide perovskites have drawn significant attention, since their bandgap can be tuned to below 1.2 eV, which opens a door for fabricating all‐perovskite tandem solar cells that can break the SQ radiative limit. This review summarizes the development of low‐bandgap mixed Sn‐Pb PSCs and their applications in all‐perovskite tandem solar cells. Its aim is to facilitate the development of new approaches to achieve high efficiency low‐bandgap single‐junction mixed Sn‐Pb PSCs and all‐perovskite tandem solar cells.

Elusive β‐siteisomers are now the main products! PCBM‐like [70]methanofullerenes have been prepared from sulfur ylides in both site‐isomeric forms depending on the solvent polarity. Their use as template agent in perovskite solar cells afford high efficiency values (PCE∼20 %) depending on the site‐isomer.

Abstract

We report on the site‐selective synthesis of PCBM‐like [70]fullerene site‐isomers, where the elusive β‐site‐isomers are, for the first time, the major product in a (cyclo)addition chemical reaction involving [70]fullerene. The reaction involves an straightforward cyclopropanation of [70]fullerene from sulfonium salts, affording a mixture of α and β site‐isomers in good yields. Amazingly, the preference for the α‐ or β‐site‐isomer can be efficiently controlled by means of the solvent polarity! DFT theoretical calculations (DMF and toluene) nicely predict that, although the formation of the α‐adduct is, as expected, thermodynamically favored, the selectivity of the process is determined by the energy difference of the respective transition states. Furthermore, the employ of α or/and β site‐isomers, as pure materials or as a mixture of them, used as templating agent, has been evaluated in perovskite solar cells. The positive influence of the [70]fullerenes by passivating the voids/pin‐holes and/or deep slits, is reflected in highly efficient and stable bulk heterojunction perovskite solar cells, whose performance (around 20 %) is slightly but consistently depending on the isomeric fullerene composition. These experimental findings pave the way to investigate a new reactivity on C₇₀ and to explore the properties of the less‐known β‐derivatives.

The components with soft nature in the metal halide perovskite absorber usually generate lead (Pb)⁰ and iodine (I)⁰ defects during device fabrication and operation. These defects serve as not only recombination centers to deteriorate device efficiency but also degradation initiators to hamper device lifetimes. We show that the europium ion pair Eu³⁺-Eu²⁺ acts as the "redox shuttle" that selectively oxidized Pb⁰ and reduced I⁰ defects simultaneously in a cyclical transition. The resultant device achieves a power conversion efficiency (PCE) of 21.52% (certified 20.52%) with substantially improved long-term durability. The devices retained 92% and 89% of the peak PCE under 1-sun continuous illumination or heating at 85°C for 1500 hours and 91% of the original stable PCE after maximum power point tracking for 500 hours, respectively.

In contrast to conjugated donaor–acceptor (D–A) alternating copolymers, incorporating a third component, either D′‐ or A′‐unit, to their D–A type polymer backbones can improve their light absorption, and tune energy levels and interchain packing synergistically. Moreover, the well‐controlled stoichiometry for these components in terpolymers also provides further access to fine‐tune these factors, thus resulting in high photovoltaic performance in polymer solar cells.

Abstract

The development of conjugated alternating donor–acceptor (D–A) copolymers with various electron‐rich and electron‐deficient units in polymer backbones has boosted the power conversion efficiency (PCE) over 17% for polymer solar cells (PSCs) over the past two decades. However, further enhancements in PCEs for PSCs are still imperative to compensate their imperfect stability for fulfilling practical applications. Meanwhile development of these alternating D–A copolymers is highly demanding in creative design and syntheses of novel D and/or A monomers. In this regard, when being possible to adopt an existing monomer unit as a third component from its libraries, either a D′ unit or an A′ moiety, to the parent D–A type polymer backbones to afford conjugated D–A terpolymers, it will give a facile and cost‐effective method to improve their light absorption and tune energy levels and also interchain packing synergistically. Moreover, the rationally controlled stoichiometry for these components in such terpolymers also provides access for further fine‐tuning these factors, thus resulting in high‐performance PSCs. Herein, based on their unique features, the recent progress of conjugated D–A terpolymers for efficient PSCs is reviewed and it is discussed how these factors influence their photovoltaic performance, for providing useful guidelines to design new terpolymers toward high‐efficiency PSCs.

Nonfullerene‐based small‐molecule organic solar cells with a new record efficiency of 12.08% are achieved by first incorporation of near‐infrared absorbing molecules and by tuning the sequentially evolved crystalline morphology. The improved crystallinity of both donor and acceptor materials facilitates the formation of multilength scale morphologies, which further enhance charge mobility and extraction, and reduce the nongeminate recombination.

Abstract

In this paper, two near‐infrared absorbing molecules are successfully incorporated into nonfullerene‐based small‐molecule organic solar cells (NFSM‐OSCs) to achieve a very high power conversion efficiency (PCE) of 12.08%. This is achieved by tuning the sequentially evolved crystalline morphology through combined solvent additive and solvent vapor annealing, which mainly work on ZnP‐TBO and 6TIC, respectively. It not only helps improve the crystallinity of the ZnP‐TBO and 6TIC blend, but also forms multilength scale morphology to enhance charge mobility and charge extraction. Moreover, it simultaneously reduces the nongeminate recombination by effective charge delocalization. The resultant device performance shows remarkably enhanced fill factor and J _sc. These result in a very respectable PCE, which is the highest among all NFSM‐OSCs and all small‐molecule binary solar cells reported so far.

An n‐doping of SnO₂ is successfully realized through the use of the triphenylphosphine‐oxide molecule, where electrons are revealed to be transferred from the R₃P⁺O⁻ σ‐bond to the peripheral tin atoms and delocalized. That novel effect enlarges the built‐in‐field from 0.01 to 0.07 eV and declines the energy‐barrier from 0.55 to 0.39 eV at the SnO₂–perovskite interface enabling a device conversion‐efficiency from 19.01% to 20.69%.

Abstract

Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n‐type doping of oxide semiconductors. Herein, successful n‐type doping of SnO₂ is demonstrated by a simple, air‐robust, and cost‐effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P⁺O⁻σ‐bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi‐photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO₂, which enlarges the built‐in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO₂/perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.

In an inverted planar perovskite solar cell employing a hole transporting NiO thin film, photovoltaic performance is found to depend significantly on annealing atmosphere when preparing the NiO film. The best power conversion efficiency can be achieved from the NiO film annealed at the oxygen partial pressure of 30% in mixture of O₂ and N₂.

The effect of annealing atmosphere and importance of oxygen partial pressure upon annealing NiO film for achieving high efficiency inverted perovskite solar cells (PSCs) is reported. The solution‐process NiO films are deposited on an FTO (fluorine‐doped tin oxide) substrate and annealed at 400 °C under different atmospheres of air, O₂, N₂, and Ar. The devices using air‐ and O₂‐annealed NiO films show better photovoltaic performance than the N₂‐ and Ar‐annealed ones, mostly due to large difference in photocurrent density (J _sc) of ≈20 mA cm⁻² for air and O₂ vs ≈15 mA cm⁻² for N₂ and Ar. Oxygen‐excess condition leads to more p‐type characteristics along with better electrical and interfacial properties, leading to higher photovoltaic performance. When comparing air and O₂ condition, the air‐annealed NiO film shows slightly better power conversion efficiency (PCE) (15.68% for air vs. 14.93% for O₂), being indicative of importance of oxygen partial pressure. By carefully modifying oxygen content, the best photovoltaic performance is achieved from the NiO film annealed at the O₂/(O₂+N₂) ratio of 30%, delivering a PCE of 16.32%.

New‐type 2D/3D perovskites are designed by first introducing two hydrophobic ammonium salt cations with halogen functional groups into 3D perovskite. The 2D/3D perovskite devices exhibit optimal power conversion efficiency as high as 20.08% under 1 sun irradiation and superior stability when exposed to humidity, temperature, and continuous UV irradiation.

Abstract

2D perovskites have attracted extensive attention due to their excellent stability compared with 3D perovskites. However, the intrinsic hydrophilicity of introduced alkylammonium salts effects the humidity stability of 2D/3D perovskites. Devices based on longer chain alkylammonium salts show improvement in hydrophobicity but lower efficiency due to the poorer charge transport among various layers. To solve this issue, two hydrophobic short‐chain alkylammonium salts with halogen functional groups (2‐chloroethylamine, CEA⁺ and 2‐bromoethylamine, BEA⁺) are introduced into (Cs_0.1FA_0.9)Pb(I_0.9Br_0.1)₃ 3D perovskites to form 2D/3D perovskite structure, which achieve high‐quality perovskite films with better crystallization and morphology. The optimal 2D/3D perovskite solar cells (PSCs) with 5% CEA⁺ display a power conversion efficiency (PCE) as high as 20.08% under 1 sun irradiation. Because of the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the optimal PSCs exhibit superior moisture resistance and retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. This work opens up a new direction for the design of new‐type 2D/3D PSCs with improved performance by employing proper alkylammonium salts with different functional groups.

Three polymers L24, L68, and L810 are developed as donor materials for organic solar cells. As the alkyl side chain of the fluorobenzotriazole (FTAZ) unit increases, the L810‐based device exhibits lower energy loss, better molecular face‐on orientation, and a higher absorption coefficient. Consequently, the power conversion efficiency is improved to 12.1%, which is one of the highest values for FTAZ‐based devices.

Abstract

The fluorobenzotriazole (FTAZ)‐based copolymer donors are promising candidates for nonfullerene polymer solar cells (PSCs), but suffer from relatively low photovoltaic performance due to their unsuitable energy levels and unfavorable morphology. Herein, three polymer donors, L24, L68, and L810, based on a chlorinated‐thienyl benzodithiophene (BDT‐2Cl) unit and FTAZ with different branched alkyl side chain, are synthesized. Incorporation of a chlorine (Cl) atom into the BDT unit is found to distinctly optimize the molecular planarity, energy levels, and improve the polymerization activity. Impressively, subtle side chain length of FTAZ realizes a dramatic improvement in all the device parameters, as revealed by the short‐current density (J _sc) improved from 7.41 to 20.76 mA cm⁻², fill‐factor from 36.3 to 73.5%, and even the open‐circuit voltage (V _oc) from 0.495 to 0.790 V. The best power conversion efficiency (PCE) of 12.1% is obtained from the L810‐based device, which is one of the highest values reported for FTAZ‐based PSCs so far. Notably, the corresponding external quantum efficiency curve keeps a very prominent value up to 80% from 500 to 800 nm. The notable performance is discovered from the reduced energy loss, improved molecular face‐on orientation, the down‐shifted energy levels, and optimized absorption coefficient regulated by side‐chain engineering.

Thermochromic lead‐free double perovskites that have potential applications in smart windows and temperature sensors are demonstrated. The anharmonic fluctuations and associated strong electron–phonon coupling, combined with the spin–orbit coupling effect, are responsible for the thermochromism. The findings on the structure modulation‐induced bandgap narrowing of Cs₂AgBiBr₆ provide new insights for the development of optoelectronic devices based on double perovskites.

Abstract

Lead‐free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature‐derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs₂AgBiBr₆ double perovskites is demonstrated. Along with the thermochromism, temperature‐dependent changes in the bond lengths of AgBr (R _AgBr) and BiBr (R _BiBr) are observed. The first‐principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the R _AgBr and R _BiBr at high temperatures. The synergy of anharmonic fluctuations and associated electron–phonon coupling, and the peculiar spin–orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs₂AgBiBr₆ shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead‐free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation‐induced bandgap narrowing of Cs₂AgBiBr₆ provide new insights for the further development of optoelectronic devices based on the lead‐free halide double perovskites.

Here a method to grow wafer‐size thin halide perovskite multiple crystals on aqueous solution surface is reported. The efficiency of lateral‐structure solar cells based on the single‐crystalline perovskite wafer reaches 5.9%.

Abstract

Solar‐grade single or multiple crystalline wafers are needed in large quantities in the solar cell industry, and are generally formed by a top‐down process from crystal ingots, which causes a significant waste of materials and energy during slicing, polishing, and other processing. Here, a bottom‐up technique that allows the growth of wafer‐size hybrid perovskite multiple crystals directly from aqueous solution is reported. Single‐crystalline hybrid perovskite wafers with centimeter size are grown at the top surface of a perovskite precursor solution. As well as saving raw materials, this method provides unprecedented advantages such as easily tunable thickness and rapid growth of the crystals. These crystalline wafers show high crystallinity, broader light absorption, and a long carrier recombination lifetime, comparable with those of bulk single crystals. Lateral‐structure perovskite solar cells made of these crystals demonstrate a record power conversion efficiency of 5.9%.

Three A₂‐A₁‐D‐A₁‐A₂ type small molecules are used as electron acceptors for fullerene‐free organic solar cells, and the cyano‐containing A₂ segments have a large effect on the open‐circuit voltage (V _OC) and power conversion efficiency (PCE). By utilizing the “same‐acceptor‐strategy” and chosing J71 as donor polymer, the dicyano containing molecule of BTA3 affords the highest PCE of 8.60% with a V _OC of 1.20 V.

To achieve efficient organic solar cells (OSCs), the design of promising non‐fullerene small molecular acceptors (SMAs) is crucially important and the relationship between the chemical structure and optoelectronic properties needs to be further investigated. Herein, an A₂‐A₁‐D‐A₁‐A₂ molecular skeleton is adopted to study the effect of end‐capped A₂ groups containing different numbers of cyano units, where D and A₁ are fixed as indacenodithiophene (IDT) and benzotriazole (BTA) units, respectively. Utilizing the “same‐acceptor‐strategy,” three BTA‐based SMAs, named as BTA701, BTA3, and BTA703, are paired with a BTA‐based p‐type polymer J71. The open‐circuit voltage (V _OC) gradually decreases with the enhancement of electron‐accepting ability of terminal A₂ units, from 1.32 V (BTA701) to 1.20 V (BTA3) and to 0.85 V (BTA703). The device J71:BTA3 eventually shows the best power conversion efficiency (PCE) of 8.60% with a V _OC up to 1.2 V because of the complementary light absorption, high and balanced hole and electron mobility, suitable phase separation, and crystallinity. This study indicates that appropriate cyano‐containing units in BTA‐based SMAs can effectively modulate the absorption, energy levels, charge mobility and surface free energy, which can provide valuable insights to the further design of SMAs. In addition, these results prove that the “same‐acceptor‐strategy” is simple and effective to realize a V _OC as high as 1.2V.

Additive‐free all‐polymer solar cells (all‐PSCs) devices based on TTFQx‐T1:N2200 are fabricated and the active layer (TTFQx‐T1:N2200) processed with THF and CHCl₃. The optimized device processed with THF exhibited better photovoltaic properties than that processed with CHCl₃. This work afforded a feasible strategy for constructing high‐performance all‐PSCs via a simple eco‐friendly processing method.

In this work, a medium bandgap quinoxaline (Qx) based polymer, named TTFQx‐T1, and a narrow bandgap n‐type polymer, named N2200, are employed to fabricate all‐polymer solar cells (all‐PSCs), which exhibited good light absorption for high short circuit current density (J _sc), well‐matched molecular energy level for good charge separation and high open circuit voltage (V _oc). Chlorinated solvents are harmful to both the environment and human beings; therefore, it is important to develop environmentally friendly solvents. Considering this, the green solvent tetrahydrofuran (THF) was employed to process all‐PSCs. The blend films based on TTFQx‐T1:N2200 processed with THF and thermal annealing (TA) exhibited better phase separation and preferential face‐on orientation toward the substrate, which benefited the exciton dissociation and charge carrier mobilities for higher FF and J _sc. The optimized device based on TTFQx‐T1:N2200 delivered an efficient power conversion efficiency of 8.63%, which is the highest value for all‐PSCs from Qx based polymers.

2D Dion–Jacobson perovskites have better carrier charge transport because of the closer interlayer distance. Solar cells based on Dion–Jacobson perovskites having mixed organic cations and using solvent‐engineering methods and hydriodic acid additive achieve higher efficiencies with high fill factors. Most importantly, the Dion–Jacobson perovskite solar cells exhibit better environmental stability compared with butylammonium‐based perovskites and 3D analogs.

Abstract

Hybrid halide 2D perovskites deserve special attention because they exhibit superior environmental stability compared with their 3D analogs. The closer interlayer distance discovered in 2D Dion–Jacobson (DJ) type of halide perovskites relative to 2D Ruddlesden–Popper (RP) perovskites implies better carrier charge transport and superior performance in solar cells. Here, the structure and properties of 2D DJ perovskites employing 3‐(aminomethyl)piperidinium (3AMP²⁺) as the spacing cation and a mixture of methylammonium (MA⁺) and formamidinium (FA⁺) cations in the perovskite cages are presented. Using single‐crystal X‐ray crystallography, it is found that the mixed‐cation (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ perovskite has a narrower bandgap, less distorted inorganic framework, and larger PbIPb angles than the single‐cation (3AMP)(MA)₃Pb₄I₁₃. Furthermore, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ films made by a solvent‐engineering method with a small amount of hydriodic acid have a much better film morphology and crystalline quality and more preferred perpendicular orientation. As a result, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃‐based solar cells exhibit a champion power conversion efficiency of 12.04% with a high fill factor of 81.04% and a 50% average efficiency improvement compared to the pristine (3AMP)(MA)₃Pb₄I₁₃ cells. Most importantly, the 2D DJ 3AMP‐based perovskite films and devices show better air and light stability than the 2D RP butylammonium‐based perovskites and their 3D analogs.

Protonation of polyethylenimine ethoxylated (PEIE) can effectively passivate the chemical reaction between the PEIE and a nonfullerene (NF) active layer. As a result, the PEIE can work very efficiently as a low‐work‐function interface for NF solar cells. These flexible solar cells exhibit power conversion efficiency up to 12.5% with a room‐temperature‐processed PEIE interface.

Abstract

Nonfullerene (NF) organic solar cells (OSCs) have been attracting significant attention in the past several years. It is still challenging to achieve high‐performance flexible NF OSCs. NF acceptors are chemically reactive and tend to react with the low‐temperature‐processed low‐work‐function (low‐WF) interfacial layers, such as polyethylenimine ethoxylated (PEIE), which leads to the “S” shape in the current‐density characteristics of the cells. In this work, the chemical interaction between the NF active layer and the polymer interfacial layer of PEIE is deactivated by increasing its protonation. The PEIE processed from aqueous solution shows more protonated N⁺ than that processed from isopropyl alcohol solution, observed from X‐ray photoelectron spectroscopy. NF solar cells (active layer: PCE‐10:IEICO‐4F) with the protonated PEIE interfacial layer show an efficiency of 13.2%, which is higher than the reference cells with a ZnO interlayer (12.6%). More importantly, the protonated PEIE interfacial layer processed from aqueous solution does not require a further thermal annealing treatment (only processing at room temperature). The room‐temperature processing and effective WF reduction enable the demonstration of high‐performance (12.5%) flexible NF OSCs.

Within two steps, an all CH activation strategy is developed to synthesize diketopyrrolopyrrole–benzothiadiazole–diketopyrrolopyrrole (DBD) copolymers for printed complementary logic. Printed PDBD‐Se organic field‐effect transistors (OFETs) demonstrate hole/electron mobility up to 6.70 and 4.30 cm² V⁻¹ s⁻¹ as plastic OFETs and find their use in organic complementary circuits, such as inverters and NAND gates.

Abstract

High mobility ambipolar conjugated polymers are seriously absent regardless their great potential for flexible and printed plastic devices and circuits. Here, ambipolar polymers with ultrahigh balanced hole and electron mobility are developed via a two‐step CH activation strategy. Diketopyrrolopyrrole‐benzothiadiazole‐diketopyrrolopyrrole (DBD) and its copolymers with thiophene/selenophene units (short as PDBD‐T and PDBD‐Se) are used as examples. PDBD‐Se exhibits highly efficient ambipolar transport with hole and electron mobility up to 8.90 and 7.71 cm² V⁻¹ s⁻¹ in flexible organic field‐effect transistors, presenting a milestone for ambipolar copolymer screening. Based on this performance metrics and good solubility, PDBD‐Se is investigated as inkjet‐printable semiconductor ink for organic complementary logic circuits. Under ambient processing, maximum hole and electron mobilities reach 6.70 and 4.30 cm² V⁻¹ s⁻¹, respectively. Printed complementary inverter and NAND gates with transition voltages near V _DD/2 are fabricated, providing an easy‐handling, general material for printed electronics and logic.

A synergistic effect is proposed by employing a dielectric mirror and a ternary strategy to precisely tune the color perception as well as semitransparent organic solar cell (ST‐OSC) performance. It results in the highest efficiency reported for neutral‐color ST‐OSCs to date.

Abstract

Neutral‐colored semitransparent organic solar cells (ST‐OSCs) have attracted considerable attention owing to their unique application in no‐visual‐obstacle building‐integrated photovoltaics. Toward this promising potential application, a synergistic effect is first proposed by employing a dielectric mirror and ternary photoactive layer with near‐infrared absorption to tune the color perception as well as ST‐OSC performance precisely. As a result, a neutral‐color ST‐OSC with high average transmittance of over 21% is successfully constructed, and a remarkable color‐rendering index approaching 100 and high power conversion efficiency (PCE) of 9.37% are simultaneously achieved. To the best of our knowledge, this is the highest PCE reported for neutral‐color ST‐OSCs to date. Importantly, this synergistic effect is demonstrated to be a universal strategy that is not only suitable for various photoactive layer systems, but can also be implanted in flexible substrate. The resulting neutral‐color flexible ST‐OSCs also show a promising PCE of 8.76%.

A hidden ferromagnetic insulating state with a high Curie temperature is found by Ho Nyung Lee and co‐workers in epitaxial 3d–5d double perovskite films, as described in article number 1805389. Such an intriguing state is rare in nature but critical for realizing quantum electronic and computing devices without dissipating electric power. The front cover image illustrates the spin and orbital structures of Sr₂FeReO₆, composed of localized 3d orbitals (red) and spin–orbit‐coupled 5d orbitals (blue).

SnO₂‐based perovskite solar cells display a strong light‐soaking effect during the first minutes of operation. It is found that this is caused by light‐induced desorption of hydrogen from the SnO₂ lattice. The resulting lower trap density and free carrier concentration lead to a lower recombination rate and improved device performance.

Abstract

Lead halide perovskite solar cells that use SnO₂ as the electron‐transporting material are known to improve upon light soaking. Photoluminescence measurements and electrochemical impedance spectroscopy show that this improvement is due to reduced non‐radiative recombination and is accompanied by a reduction in the extrinsic electron concentration in SnO₂. This performance enhancement can also be achieved by exposing these devices to high vacuum at ambient temperature. This study postulates that the performance increase stems from desorption of hydrogen from oxygen vacancies in SnO₂. Furthermore, Ga‐doped SnO₂‐based devices exhibit a reduced light‐soaking effect and have fewer oxygen vacancies, as is shown by X‐ray photoelectron spectroscopy measurements. It is concluded that high extrinsic electron concentrations in SnO₂ are undesirable because of their role in non‐radiative recombination. The reduction in electron density when SnO₂ is incorporated into a perovskite diode is therefore advantageous for solar cell performance.

The employment of anions as spacers to synthesize a new 2D hybrid perovskite family is reported. Moreover, this new class of 2D Cs‐based halide perovskites exhibits novel morphology and strong photoluminescence, which is potentially feasible for optoelectronic applications.

Abstract

The low‐dimensional halide perovskites have received enormous attention due to their unique photovoltaic and optoelectronic performances. Periodic spacers are used to inhibit the growth of 3D perovskite and fabricate a 2D counterpart with layered structure, mostly based on organic/inorganic cations. Herein, by introducing organic anions (e.g., pentanedioic acid (PDA) and hexanedioic acid (HDA) simultaneously), leaf‐shaped (Cs₃Pb₂Br₅)₂(PDA–HDA) microplates with low‐dimensional structure are synthesized. They also exhibit significant photoluminescence (PL) centered at 540 nm with a narrow emission peak. The synthesis of single crystals of Pb(PDA) and Pb(HDA) allows to further clarify the crystal structure of (Cs₃Pb₂Br₅)₂(PDA–HDA) perovskite and its structural evolution mechanism. Moreover, the cooperative introduction of dicarboxylic acid pairs with appropriate lengths is thermodynamically favored for the low‐dimensional perovskite crystallization. The temperature‐dependent PL indicates a V‐shaped Stokes shift with elevated temperature that could be associated with the localization of excitons in the inorganic layers between organic dicarboxylic acid molecules. This work demonstrates low‐dimensional halide perovskite with anionic spacers, which also opens up a new approach to the growth of low‐dimensional organic–inorganic hybrid perovskite crystals.

Introducing functional groups to end‐groups of acceptor‐donor‐acceptor‐structured nonfullerene small‐molecule acceptors is a convenient way to tune their optoelectronic and morphological properties. This work combines molecular dynamics simulations and density functional theory calculations, and examines the molecular‐scale impact of methoxy substitution position in the end‐group of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno [2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene based acceptors on the intermolecular packing and electron‐transfer properties in neat films.

Abstract

Nonfullerene small‐molecule acceptors (SMAs) are considered as a key component of next‐generation organic photovoltaics. Introducing functional groups to the end‐groups of “acceptor‐donor‐acceptor”‐type SMAs is a facile and convenient way to tune their optoelectronic and morphological properties. Here, molecular dynamics simulations are combined with long‐range corrected density functional theory calculations to explore the molecular‐scale impact that the position of methoxy substitution in the end‐group has on the molecular packing and electron‐transfer properties in neat films. The focus here is on 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno [2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IT‐OM), where three end‐group methoxy substitution positions are evaluated. Changing the methoxy substitution position is found to influence, to different extents, the planarity of the end‐groups and thus the intermolecular packing density. The effect on the intermolecular electron‐transfer rates is also examined and leads to markedly different sizes of strongly interconnected clusters. Overall, these findings are fully consistent with the experimental evolution of electron mobility in the neat IT‐OM film as a function of methoxy substitution position.

The reduction and/or the fluorination of the TT‐units of the PBDTT‐TT polymers have crucial effects on the photostability of their polymer:fullerene solar cells. Reduction improves the photostability of the solar cells while fluorination destabilizes the photostability.

Polymer solar cells have a promising future for applications in our day to day usage of energy in small appliances and portable devices. However, their performance in terms of efficiency is limited by a number of factors, among which is their low open circuit voltage (V _oc). It is, therefore, understandable that much effort is channeled by the scientific community in improving the V _oc. One way to achieve this goal is the development of novel materials, for example, polymers, through chemical structure modification. Typical examples are addition (chlorination, fluorination, or sulfonylation) and/or reduction (from alkyl‐ester to ketone substituents) mechanisms. This paper reports on the study of the effect of these structural changes for V _oc enhancement on the performance of the polymers in polymer:fullerene solar cells. In particular, it looks at seven polymers of the polybenzodithiophene‐thienothiophene family, identifying the structural changes in the thienothiophene units or their moieties as a function of V _oc behavior in relation to their UV‐stability. The findings reveal that the fluorination of the TT‐units or having alkyl‐ester groups as substituents on the TT‐units is bad for photostability. However, when these alkyl‐ester groups are reduced into ketone substituents, the photostability behavior improves.

A strategy to prepare efficient carbon‐based CsPbI₂Br perovskite solar cells is explored by using Co₃O₄ nanomaterial as hole transport layer (HTM). It is found that the Co₃O₄ inorganic HTM effectively promotes photo‐generated charges separation and extraction, and suppress charge recombination at the CsPbI₂Br/carbon electrode interface, leading to the enhanced performance.

Carbon‐based perovskite solar cells (PSCs) have gathered much attention due to their excellent thermal stability and low cost. However, the typically used hole‐conductor‐free PSCs based on carbon electrodes show the worst performance due to the serious charge recombination at the perovskite/carbon interface. In this work, the efficient and stable carbon‐based CsPbI₂Br PSCs using Co₃O₄ as the hole transport material (HTM) are fabricated and their photoelectric properties are systematically investigated. It is found that the Co₃O₄ inorganic HTM effectively promotes photo‐generated charges separation and extraction, and suppresses charge recombination at the CsPbI₂Br/carbon electrode interface, resulting in the improved photovoltaic performance. At the optimal Co₃O₄ concentration, the carbon‐based CsPbI₂Br PSCs achieve the maximum efficiency of 11.21% with a negligible J–V hysteresis. This work provides a novel strategy to fabricate efficient and stable all‐inorganic PSCs.

Precise amount of DRCN5T incorporation into perovskite precursors could effectively passivate the defect states on the surface, which results in improved the life time and mobility of carriers. An impressive PCE of 20.60% is realized with lower hysteresis and high stability under ambient conditions (RH 50–60%).

The additive engineering to hybrid organic‐inorganic perovskite precursors is an effective technique toward highly efficient stable photovoltaic devices, however, there is still a deficiency in fundamental understanding on how these additives affect the perovskite film and device performance as well. Herein is introduced a small organic molecule, DRCN5T, into a double‐cation perovskite precursor and the function on device performance is systematically investigated. An appropriate amount of DRCN5T into the precursor can promote the crystallization of film with successful suppression of δ‐FAPbI₃ phase, reduce grain boundaries and adequately passivate the native defect sites. In addition, the incorporation of DRCN5T also regulates the energy level alignment of the perovskite to charge transport layer suitably. This leads to the promotion of charge transport, reduction in non‐radiative recombination, and boosts the efficiency to a value of 20.60% with greatly reduced hysteresis in the device. Moreover, the treatment by DRCN5T also significantly increases the stability of the devices in ambient environment. These findings open the gate to produce highly crystallized perovskite/organic‐molecule active layers toward commercialization of perovskite solar cells.

An effective strategy is developed to synthesize high‐nuclearity Cu₅₃ clusters that are surface‐capped by alkynyl and acetate ligands. These nanoclusters are unexpectedly soluble in ether, enabling the easy formation of high‐quality films. The cluster films are readily converted into high‐quality CuI thin films for applications as a hole transport layer in perovskite solar cells.

Abstract

An effective strategy is developed to synthesize high‐nuclearity Cu clusters, [Cu₅₃(RCOO)₁₀(C≡CtBu)₂₀Cl₂H₁₈]⁺ (Cu₅₃ ), which is the largest Cu^I/Cu⁰ cluster reported to date. Cu powder and Ph₂SiH₂ are employed as the reducing agents in the synthesis. As revealed by single‐crystal diffraction, Cu₅₃ is arranged as a four‐concentric‐shell Cu₃@Cu₁₀Cl₂@Cu₂₀@Cu₂₀ structure, possessing an atomic arrangement of concentric M₁₂ icosahedral and M₂₀ dodecahedral shells which popularly occurs in Au/Ag nanoclusters. Surprisingly, Cu₅₃ can be dissolved in diethyl ether and spin coated to form uniform nanoclusters film on organolead halide perovskite. The cluster film can subsequently be converted into high‐quality CuI film via in situ iodination at room temperature. The as‐fabricated CuI film is an excellent hole‐transport layer for fabricating highly stable CuI‐based perovskite solar cells (PSCs) with 14.3 % of efficiency.

Under some exactly predesigned growth conditions identified by utilizing thousands of chemicals through a potential screening process, some intrinsic defects or defect complexes can spontaneously incorporate into the grain boundary (GB) cores, and effectively eliminate the harmful deep‐levels induced by the low‐energy GBs in lead‐free halide double perovskites (type‐I and type‐II).

Abstract

Halide double perovskites (HDPs) are promising lead‐free perovskites for various optoelectronic applications. However, the device performances of HDPs are far below the optimized values, which open a critical question regarding the origin of low performance in these HDPs. In this article, using first‐principles calculations, it is found that some types of grain boundaries (GBs) are easy to form in polycrystalline HDPs. Importantly, the existence of low‐energy Σ5(310) GBs can induce harmful deep‐level defect states within the bandgaps of type‐I (e.g., Cs₂AgInCl₆) and type‐II (e.g., Cs₂AgBiCl₆) HDPs, which may dramatically reduce the device performances. Interestingly, it is found that the formation of some intrinsic defects and defect complexes could effectively eliminate these deep‐levels in type‐II and type‐I HDPs, respectively. Under some exactly predesigned growth conditions identified by utilizing thousands of chemicals through a potential screening process, these defects or defect complexes can spontaneously incorporate into the GB cores, meanwhile the harmful deep‐level defects in the bulk can also be effectively eliminated. In addition, the self‐passivated GBs could generate band bending, which may be beneficial for charge separation. The understanding of GB formation as well as the self‐passivation mechanism in HDPs can provide a new viewpoint and guidance for designing polycrystalline perovskites with improved optoelectronic performance.

Amino‐functionalized TiO₂ nanoparticles are synthesized in situ by a facile onestep, low‐temperature, nonhydrolytic approach, and are applied as the electrontransport layer of regular‐structure planar heterojunction perovskite solar cells, offering a dramatic performance increase due to the passivation of the surface trap states of the perovskite film.

Abstract

Titanium oxide (TiO₂) has been commonly used as an electron transport layer (ETL) of regular‐structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO₂ layer. However, a high temperature annealing (over 450 °C) treatment is mandatory, which is incompatible with low‐cost fabrication and flexible devices. Herein, a facile one‐step, low‐temperature, nonhydrolytic approach to in situ synthesizing amino‐functionalized TiO₂ nanoparticles (abbreviated as NH₂‐TiO₂ NPs) is developed by chemical bonding of amino (‐NH₂) groups, via TiN bonds, onto the surface of TiO₂ NPs. NH₂‐TiO₂ NPs are then incorporated as an efficient ETL in n‐i‐p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs_0.05FA_0.83MA_0.12PbI_2.55Br_0.45 (abbreviated as CsFAMA) PHJ PSC devices based on NH₂‐TiO₂ ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO₂ ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current–voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.

Mesoscale‐structured materials offer broad applications owing to their high surface areas and tunable surface energy. A novel route to fabricate organic‐based mesoscale‐structured interfaces for perovskite solar cells using a roll‐to‐roll compatible process is presented. The efficient infiltration of organic porous structures based on assembled crystalline nanoparticles allows engineering perovskite solar cells with excellent efficiency, stability, and lateral homogeneity.

Abstract

Mesoscale‐structured materials offer broad opportunities in extremely diverse applications owing to their high surface areas, tunable surface energy, and large pore volume. These benefits may improve the performance of materials in terms of carrier density, charge transport, and stability. Although metal oxides–based mesoscale‐structured materials, such as TiO₂, predominantly hold the record efficiency in perovskite solar cells, high temperatures (above 400 °C) and limited materials choices still challenge the community. A novel route to fabricate organic‐based mesoscale‐structured interfaces (OMI) for perovskite solar cells using a low‐temperature and green solvent–based process is presented here. The efficient infiltration of organic porous structures based on crystalline nanoparticles allows engineering efficient “n‐i‐p” and “p‐i‐n” perovskite solar cells with enhanced thermal stability, good performance, and excellent lateral homogeneity. The results show that this method is universal for multiple organic electronic materials, which opens the door to transform a wide variety of organic‐based semiconductors into scalable n‐ or p‐type porous interfaces for diverse advanced applications.