Ligang Yuan

Lead‐free double perovskite Cs₂AgBiBr₆ is introduced to fabricate transparent photovoltaic (TPV) devices. An average visible transmittance up to 73% is produced from an indirect bandgap of 1.98 eV. Due to the optimized device parameters and narrower bandgap than other ultraviolet‐protective TPV absorbers, a record efficiency of 1.58% in the transparent solar cells with transmittance exceeding 70% is obtained.

Perovskite solar cells have attracted great research interest as a promising candidate for silicon solar cells. Plenty of work has been reported to use perovskites to semitransparent windows and transparent photovoltaic (TPV) devices to obtain multifunctional systems. However, the narrow bandgap and sharp absorption edge of the typical perovskites prevent them from achieving the highest transparency to satisfy the requirements of aesthetic and integration, and the poor stability and toxic Pb compositions hinder their practical application. Herein, lead‐free halide double perovskites with a wide bandgap and indirect bandgap characteristics is introduced to fabricate long‐term stable transparent photovoltaic devices exhibiting high visible transmittance (73%) and considerable energy conversion efficiency (1.56%). Through further theoretical calculation and evaluation, a new strategy using indirect bandgap material on TPV devices is proposed to combine the enhancement of these two parameters. This approach will be a significant compliment to near‐infrared‐absorbing solar cells to selectively harvest light in the invisible region to obtain highly performing multi‐junction smart windows on buildings, vehicles and mobile electronics, providing a new reasonable idea to realize TPVs with high efficiency and transparency simultaneously.

By utilizing excess formamidinium bromide (FABr) to passivate the surface defects and tune the energy band of nanocrystals (NCs), a pretty effective formamidinium lead bromide perovskite NCs' light‐emitting diode (LED) is demonstrated. The LED with the molar ratio FABr/PbBr₂ of 2.2:1 shows the record‐breaking maximum external quantum efficiency and current efficiency of 17.1% and 76.8 cd A⁻¹, respectively.

Abstract

Formamidinium lead bromide (FAPbBr₃) nanocrystals (NCs) demonstrate great potential in light‐emitting diode (LED) applications due to their pure green emission and excellent stability. However, the abundant defects at the surface of the NCs act as charge trapping centers and significantly increase the trap‐assisted nonradiative recombination channels, hampering the performance improvement of LEDs based on FAPbBr₃ NCs. Herein, a facile self‐passivation strategy of the surface defects is developed by introducing excess formamidinium bromide (FABr) during the colloidal synthesis of NCs, leading to much improved photoluminescence quantum yield (PLQY) of the obtained NCs. In addition, enhanced charge transport property is measured in the assembled films owing to the simultaneously declined insulating ligands at the surface of NCs. The molar ratio of FABr and PbBr₂ is rationally optimized during the synthesis of NCs and high‐efficient green‐emissive LEDs are fabricated with a champion current efficiency of 76.8 cd A⁻¹, corresponding to an external quantum efficiency of 17.1%, which is among the best‐performing green LEDs based on perovskite NCs so far.

Rubidium‐incorporated air‐stable Cs_{(1 −  x )}Rb_xPbI₂Br perovskite solar cells are fabricated through a surface passivation strategy. The resulting devices under optimal conditions yield an efficiency of over 15% with excellent long‐term thermal as well as light‐soaking stability in ambient atmosphere.

Inorganic CsPbI₂Br perovskite has gained growing attention due to its potential for improving device performance and stability. However, the notorious phase transition from the photoactive to photoinctive phase in ambient atmosphere hinders its further development. Herein, air‐stable rubidium (Rb)‐incorporated Cs_{(1 −  x )}Rb_xPbI₂Br perovskite with guanidinium bromide (GABr) post‐treatment is demonstrated. The incorporation of smaller monovalent Rb cation contributes to a contraction of the perovskite crystal, leading to an improvement in structure stability. In addition, GABr modification induces a 2D/3D heterostructure perovskite with high crystallinity, appropriate surface morphology, favorable electronic properties, and significantly reduced trap‐state density. Consequently, the fabricated perovskite solar cells deliver a power conversion efficiency (PCE) of 15.6%, which is much higher than the 12.9% reported for reference CsPbI₂Br‐based devices. Meanwhile, the significantly enhanced long‐term (88% of initial PCE after 60 days), thermal (76% of initial PCE after 30 days) as well as light soaking (90% of initial PCE after 300 min) stability in ambient atmosphere is demonstrated.

Phenmethylammonium bromide (PMABr) applied as passivation agent for wide‐bandgap perovskite solar cells significantly improves the photovoltaic performance in 2‐terminal (2T) perovskite‐organic tandem solar cells. The novel structure of 2T perovskite‐organic tandem solar cells provide a promising strategy to explore the advanced application in photovoltaic‐driven water splitting system and also demonstrates the advantages of perovskite‐organic configuration on flexible tandem solar cells.

Abstract

Perovskite‐organic tandem solar cells are attracting more attention due to their potential for highly efficient and flexible photovoltaic device. In this work, efficient perovskite‐organic monolithic tandem solar cells integrating the wide bandgap perovskite (1.74 eV) and low bandgap organic active PBDB‐T:SN6IC‐4F (1.30 eV) layer, which serve as the top and bottom subcell, respectively, are developed. The resulting perovskite‐organic tandem solar cells with passivated wide‐bandgap perovskite show a remarkable power conversion efficiency (PCE) of 15.13%, with an open‐circuit voltage (V _oc) of 1.85 V, a short‐circuit photocurrent (J _sc) of 11.52 mA cm⁻², and a fill factor (FF) of 70.98%. Thanks to the advantages of low temperature fabrication processes and the flexibility properties of the device, a flexible tandem solar cell which obtain a PCE of 13.61%, with V _oc of 1.80 V, J _sc of 11.07 mA cm⁻², and FF of 68.31% is fabricated. Moreover, to demonstrate the achieved high V _oc in the tandem solar cells for potential applications, a photovoltaic (PV)‐driven electrolysis system combing the tandem solar cell and water splitting electrocatalysis is assembled. The integrated device demonstrates a solar‐to‐hydrogen efficiency of 12.30% and 11.21% for rigid, and flexible perovskite‐organic tandem solar cell based PV‐driven electrolysis systems, respectively.

In article https://doi.org/10.1002/smll.2019014661901466, Bo Chi, Gregory J. Wilson, and co‐workers investigate superior electronic properties of nanostructured tin (IV) oxide (SnO₂) as an ideal inorganic electron transport layer (ETL) in n–i–p perovskite solar cells. The bilayer ETL architecture attaining impressive power conversion efficiency (PCE) greater than 20% is depicted.

In article number https://doi.org/10.1002/adfm.2019059511905951, Jong H. Kim, Sang Hyuk Im, O‐Pil Kwon, and co‐workers report a series of electron transporting homochiral and heterochiral stereoisomers of naphthalene diimide crystalline materials and show simultaneous achievement of low‐temperature solution processability, high device performance, and long‐term temporal device stability.

Low‐temperature‐processed Zr/F co‐doped SnO₂ is an excellent successor of electron transport layers (ETLs) for high‐efficiency planar perovskite solar cells. Benefiting from an accurate energy level match and enhanced ETL conductivity, the photoelectric conversion efficiency, and hysteresis effect are obviously improved.

The energy band position and conductivity of electron transport layers (ETLs) are essential factors that restrict the efficiency of planar perovskite solar cells (p‐PSCs). Tin oxide (SnO₂) has become a primary material in ETLs due to its mild synthesis condition, but its low conduction band position and limited intrinsic carriers are disadvantageous in electron transport. To solve these problems, this work exquisitely designs a Zr/F co‐doped SnO₂ ETL. The doping of Zr can raise the conduction band of SnO₂, which reduces the energy barrier in electron extraction and inhibits the interface recombination between the ETL and perovskite. The open‐circuit voltage (V _OC) of p‐PSCs consequently increases. F⁻ doping belongs to n‐type doping. Thus, it equips SnO₂ with a large number of free electrons and improves the conductivity of the ETL and short‐circuit current (J _SC). The device based on Zr/F co‐doped ETL achieves a high efficiency of 19.19% and exhibits a reduced hysteresis effect, which is more satisfactory than that of a pristine device (17.35%). Overall, this research successfully adjusts the energy band match and boosts the conductivity of ETL via Zr/F co‐doping. The results provide an effective strategy for fabricating high‐efficiency p‐PSCs.

A strategy is demonstrated for efficacious regulation of perovskite crystallinity using glycolic acid (GA) against nonvolatile thioglycolic acid (TGA) following dimethyl sulfoxide sublimation, resulting in enhanced device performance. A champion power conversion efficiency as high as 21.32% is achieved for the GA‐based device, which is almost 13% or 20% higher than those of the control device or TGA‐based device.

Abstract

A strategy for efficaciously regulating perovskite crystallinity is proposed by using a volatile solid glycolic acid (HOCH₂COOH, GA) in an FA_0.85MA_0.15PbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite precursor solution that is different from the common additive approach. Accompanied with the first dimethyl sulfoxide sublimation process, the subsequent sublimation of GA before 150 °C in the FA_0.85MA_0.15PbI₃ perovskite film can artfully regulate the perovskite crystallinity without any residual after annealing. The improved film formation upon GA modification induced by the strong interaction between GA and Pb²⁺ delivers a champion power conversion efficiency (PCE) as high as 21.32%. In order to investigate the role of volatility in perovskite solar cells (PSCs), nonvolatile thioglycolic acid (HSCH₂COOH, TGA) with a similar structure to GA is utilized as an additive reference. Large perovskite grains are obtained by TGA modification but with obvious pinholes, which directly leads to an increased defect density accompanied by a decline in PCE. Encouragingly, the champion PCE achieved for GA‐based PSC device (21.32%) is almost 13% or 20% higher than those of the control device or TGA‐based device. In addition, GA‐modified PSCs exhibit the best stability in light‐, thermal‐, and humidity‐based tests due to the improved film formation.

A method is introduced to experimentally measure the efficiency potential of any neat perovskite film on glass with/without attached transport layers using intensity‐dependent photoluminescence measurements. This approach allows decoupling efficiency losses due to insufficient charge transport, bulk, interface, and surface recombination. These findings also shine light on the ideality factor in perovskite solar cells and thereby fill factor limitations.

Abstract

Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1‐sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non‐radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open‐circuit voltage and the internal quasi‐Fermi level splitting (QFLS), the transport resistance‐free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity‐dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non‐radiative fill factor and open‐circuit voltage loss. It is found that potassium‐passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and employed in perowskite solar cells, reaching efficiencies of 19.4 %. Although all these HTMs show very similar chemical and physical properties, they provide different carrier recombination kinetics.

Abstract

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and successfully employed in triple‐cation mixed‐halide PSCs, reaching efficiencies of 19.4 %. The efficiencies, comparable to those obtained using spiro‐OMeTAD, point them out as promising candidates for easily attainable and cost‐effective alternatives for PSCs, given their facile synthesis from commercially available materials. Interestingly, although all these HTMs show similar chemical and physical properties, they provide different carrier recombination kinetics. Our results demonstrate that is feasible through the molecular design of the HTM to minimize carrier losses and, thus, increase the solar cell efficiencies.

A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with engineered bandgap (1.65 eV ≤ E _g ≤ 1.85 eV) which results in improved stabilized PCEs and open‐circuit voltages for opaque and semitransparent perovskite solar cells. Four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells with stabilized PCEs of up to 25.7% and 25.0%, respectively, are demonstrated.

Abstract

Wide‐bandgap perovskite solar cells (PSCs) with optimal bandgap (E _g) and high power conversion efficiency (PCE) are key to high‐performance perovskite‐based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with engineered bandgap (1.65 eV ≤ E _g ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open‐circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four‐terminal tandem configuration with respect to variations in the perovskite bandgap for two state‐of‐the‐art bottom solar cells is experimentally validated.

Self‐crystallized multifunctional 2D perovskite (M2P) is formed on top of a 3D perovskite light absorber. The M2P layer performs as a hole‐transfer facilitator and a surface‐trap passivator in perovskite solar cells (PSCs). PSCs using the developed 3D/2D perovskites achieve a power conversion efficiency of 20.79% with highly improved long‐term stability compared to devices without M2P.

Abstract

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.

With respect to common single‐interlayer approach, a unique co‐interlayer ligand strategy is demonstrated to realize quasi‐2D perovskite light‐emitting diodes with a high external quantum efficiency of 15.1%. This is enabled by a facile and controllable one‐step spin‐coating method without additional antisolvent treatments.

Abstract

With respect to three‐dimensional (3D) perovskites, quasi‐two‐dimensional (quasi‐2D) perovskites have unique advantages in light‐emitting devices (LEDs), such as strong exciton binding energy and good phase stability. Interlayer ligand engineering is a key issue to endow them with these properties. Rational design principles for interlayer materials and their processing techniques remain open to investigation. A co‐interlayer engineering strategy is developed to give efficient quasi‐2D perovskites by employing phenylbutylammonium bromide (PBABr) and propylammonium bromide (PABr) as the ligand materials. Preparation of these co‐interlayer quasi‐2D perovskite films is simple and highly controllable without using antisolvent treatment. Crystallization and morphology are readily manipulated by tuning the ratio of co‐interlayer components. Various optical techniques, including steady and ultrafast transient absorption and photoluminescence spectroscopies, are used to investigate their excitonic properties. Photoluminescence quantum yield (PLQY) of the perovskite film is dramatically improved to 89% due to the combined optimization of exciton binding energy and suppression of trap state formation. Accordingly, a high current efficiency of 66.1 cd A⁻¹ and an external quantum efficiency of 15.1% are achieved for green co‐interlayer quasi‐2D perovskite LEDs without using any light out‐coupling techniques, indicating that co‐interlayer engineering is a simple and effective approach to develop high‐performance perovskite electroluminescence devices.

Electroabsorption spectroscopy is employed to monitor the optical signatures of functional layers in an operating perovskite‐based light‐emitting diode (PeLED). The spectroscopic and device modeling results reveal that the degradation of the PeLED may initiate from the top surface of the perovskite layer. Surface treatment of the perovskite by phenethylammonium iodide is shown to greatly improve PeLED stability.

Abstract

The past few years have seen a significant improvement in the efficiency of organometal halide‐perovskite‐based light‐emitting diodes (PeLEDs). However, poor operation stability of the devices still hinders the commercialization of this technology for practical applications. Despite extensive studies on the degradation mechanisms of perovskite thin films, it remains unclear where and how degradation occurs in a PeLED. Electroabsorption (EA) spectroscopy is applied to study the degradation process of PeLEDs during operation and directly evaluates the stability of each functional layer (i.e., charge transporting layers and light‐emitting layer) by monitoring their unique optical signatures. The EA measurements unambiguously reveal that the degradation of the PeLEDs occurs predominantly in the perovskite layer. With finite‐element method‐based device modeling, it is further revealed that the degradation may initiate from the interface between the perovskite and hole transporting layers and that vacancy, antisite, or interstitial defects can further accelerate this degradation. Inspired by these observations, a surface‐treatment step is introduced to passivate the perovskite surface with phenethylammonium iodide. The passivation leads to a drastic enhancement of the PeLED stability, with the operation lifetime increased from 1.5 to 11.3 h under a current density of 100 mA cm⁻².

A 2D/3D layered heterostructure with 2D perovskites self‐assembled atop 3D MAPbI₃ via a one‐step printing process is reported. The 2D perovskite capping layer significantly suppresses nonradiative recombination of the devices, leading to a remarkably high open‐circuit voltage of 1.2 V. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of 2D perovskites.

Abstract

As perovskite solar cells (PSCs) are highly efficient, demonstration of high‐performance printed devices becomes important. 2D/3D heterostructures have recently emerged as an attractive way to relieving the film inhomogeneity and instability in perovskite devices. In this work, a 2D/3D ensemble with 2D perovskites self‐assembled atop 3D methylammonium lead triiodide (MAPbI₃) via a one‐step printing process is shown. A clean and flat interface is observed in the 2D/3D bilayer heterostructure for the first time. The 2D perovskite capping layer significantly suppresses nonradiative charge recombination, resulting in a marked increase in open‐circuit voltage (V _OC) of the devices by up to 100 mV. An ultrahigh V _OC of 1.20 V is achieved for MAPbI₃ PSCs, corresponding to 91% of the Shockley–Queisser limit. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of the 2D perovskites. These results suggest a viable approach for scalable fabrication of highly efficient perovskite solar cells with enhanced environmental stability.

Ligands around inorganic perovskite nanocrystals (PNCs) play a critical role in improving the PNCs‐based solar cell performance. Herein, a facile hexane/ethyl acetate solvent treatment method to manage the ligand amount around PNCs is reported. Finally, a power conversion efficiency of 12.2%, which is the highest performance reported for mixed‐halide CsPbX₃ NCs solar cells, is achieved.

CsPbX₃ (X = Cl, Br, I) inorganic perovskite nanocrystals (PNCs) not only maintain the excellent optical and electronic properties of bulk material but also possess the features of nano‐materials, such as tunable bandgap and easily processable colloidal ink, and enable them to be suitable for incorporation into various electronic devices and compatible with printing techniques. In contrast to the traditional II‐VI and III‐V semiconductor nanocrystals, the unique defect‐tolerance effect makes the CsPbX₃ PNCs promising materials for optoelectronic applications. The ligands around the PNCs play a critical role in the optoelectronic devices performance. Herein, through a facile hexane/ethyl acetate (MeOAc) solvent treatment method to control the ligand amount around CsPbBrI₂ PNCs, the impact of ligand amount on the performance of solar cell is systematically demonstrated and the ligand amount is quantified precisely via the nuclear magnetic resonance internal standard method. Through controlling the ligand amount, the film quality, charge transfer, and transport properties are largely improved. In addition, a simple annealing process is applied to improve the interface properties by partial crystal fusion. As a consequence, the photovoltaic power conversion efficiency of 12.2% is achieved, which is the highest performance reported for mixed‐halide CsPbX₃ PNCs solar cells.

Nano‐TiO₂ is unprecedentedly used to load the commercial dye N719 forming N719@TiO₂ nanoparticles which promotes charge extraction and suppress shallow defects in the fully printable carbon‐based perovskite solar cells due to surface carboxyl groups comprising the ligands of the N719 dye. Accordingly, the optimal power conversion efficiency increases from 12.00% (control) to 13.95% (N719@TiO₂).

Shallow defects are one of the energy states that trap photoexcited electrons leading to charge recombination and limit the increase in the photocurrent of perovskite solar cells (PSCs). Due to the large perovskite thickness and uncontrollable crystallization processes, suppressing shallow defects, especially methylamine (MA) vacancies, has become a key challenge for fully printable PSCs. Herein, nano‐TiO₂ is unprecedentedly used to load the commercial dye N719, forming N719@TiO₂ nanoparticles, which crucially improves the passivation effect of MA vacancies on the surface of perovskite and charge extraction, by the unbounded carboxyl group of N719 as a shell on the surface of TiO₂. Meanwhile, the core TiO₂ serves as a centre to bind the dyes, assisting the perovskite crystallization and enhancing the passivation effect. It is found that the charge extraction increases to 1.8007 × 10⁻⁹ C for the devices based on N719@TiO₂ from 1.5507 × 10⁻⁹ C for the control group. Simultaneously, the short‐circuit current density (J _sc) is significantly enhanced to 23.58 mA cm⁻² in the device containing N719@TiO₂ over that of the control device (21.95 mA cm⁻²). This opens up a novel pathway to reduce shallow defects in PSCs via organic passivator with carboxyl anchoring group loaded on n‐type semiconductors (nano‐TiO₂).