Chen Weijie

Lead‐free double perovskite of Cs₂AgBiBr₆ was utilized as photocatalyst in a novel alcohol‐based photocatalytic system. Stable and highly efficient photocatlaytic degradation of dyes has been demonstrated.

Abstract

Composition engineering of halide perovskite allows the tunability of the band gap over a wide range so that photons can be effectively harvested, an aspect that is of critical importance for increasing the efficiency of photocatalysis under sunlight. However, the poor stability and the low photocatalytic activity of halide perovskites prevent use of these defect‐tolerant materials in wide applications involving photocatalysis. Here, an alcohol‐based photocatalytic system for dye degradation demonstrated high stability through the use of double perovskite of Cs₂AgBiBr₆. The reaction rate on Cs₂AgBiBr₆ is comparable to that on CdS, a model inorganic semiconductor photocatalyst. The fact of fast reaction between free radicals and dye molecules indicates the unique catalytic properties of the Cs₂AgBiBr₆ surface. Deposition of metal clusters onto Cs₂AgBiBr₆ effectively enhances the photocatalytic activity. Although the stability (five consecutive photocatalytic cycles without obvious decrease of efficiency) requires further improvements, the results indicate the significant potential of Cs₂AgBiBr₆‐based photocatalysis.

A 248 nm KrF excimer laser with high photon energy and low thermal effect is employed to perform a rapid surface modification on CH₃NH₃PbI₃ films for the first time. This approach can reduce the surface trap density of CH₃NH₃PbI₃ films effectively and improve the cell performance of perovskite solar cells obviously.

For perovskite solar cells (PSCs), the surface traps of perovskite films have great influence on the charge carrier behavior at the interface of perovskite and charge transport layers. In this investigation, a 248 nm KrF excimer laser with high photon energy and shallow penetration depth is introduced to perform surface modification on the CH₃NH₃PbI₃ film through irradiation for reducing its surface trap density for the first time. A whole excimer laser surface modification (ELSM) process can be completed in few seconds, and the actual interaction time of the excimer laser and perovskite film is only several hundred nanoseconds. After ELSM, the trap density of the CH₃NH₃PbI₃ film decreases from 1.61 × 10¹⁶ cm⁻³ to 5.81 × 10¹⁵ cm⁻³, and the nonradiative recombination is suppressed effectively. As a result, the open‐circuit voltage and the power conversion efficiency (PCE) of CH₃NH₃PbI₃‐based PSCs increase from 1082 ± 27 to 1117 ± 16 mV and from 16.69% ± 0.77% to 18.50% ± 0.65%, respectively, and the PCE of the champion device reaches 19.38%. In addition, the measured larger charge recombination resistance, slower open‐circuit photovoltage decay, and longer charge recombination lifetime confirm the effective suppression effect of ELSM on the charge carrier recombination in PSCs.

Thick‐film‐induced absorption compensation at short‐wavelength band is designed based on organic donors and acceptors with comparable bandgaps of 1.61 eV and suitable energy offsets for both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), giving high J _sc and V _oc close to perovskite solar cells. As a result, a suppressed trade‐off between J _sc and V _oc among OSCs is demonstrated.

Herein, a high‐mobility polymer (Si25) pairing a nonfullerene acceptor (O‐IDTBR) is introduced to construct active layers of organic solar cells (OSCs). The OSCs based on Si25 and O‐IDTBR with comparable bandgaps of 1.61 eV show high open‐circuit voltage (V _oc) of 1.03 V. Suitable energy level offsets between the donor and acceptor as well as sufficient photon absorbance by a 400 nm thick active layer afford a notable short‐circuit current (J _sc) of 21.11 mA cm⁻², indicating a significantly suppressed trade‐off between J _sc and V _oc among OSCs. In addition, notable high power conversion efficiency (PCE) between 10.2% and 11.54% can be achieved with thick blend films from 210 to 560 nm, a thickness range beneficial to pin‐hole free printing. The maximum PCE of 11.54% corresponds to a 400 nm thick blend film, which is a rare thickness for high‐efficiency nonfullerene‐based OSCs. The corresponding fill factors (FFs) are between 51.59% and 53.33%. The inferior FF is due to a very low electron–hole mobility ratio, offering space for future FF elevation. The results highlight the high V _oc and J _sc potentials for thick‐film nonfullerene OSCs based on a high hole mobility donor as well as looking forward to a high electron mobility nonfullerene acceptor.

Replacing the acceptors 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 (ITIC) and another fuse ring acceptor with withdrawing units of 1,1‐dicyanomethylene‐3‐indanone and hexyl side chains (IDIC) with rhodanine‐benzothiadiazole‐coupled indacenodithiophene with branched 2‐ethylhexyl side chains (EH‐IDT) in nonfullerene organic solar cells can not only enhance power conversion efficiency but also extend device longevity under light. Good miscibility between the donor and the acceptor is found to be a key factor in stabilizing the film morphology of the active layer and contributes to an excellent photostability.

Nonfullerene organic solar cells (OSCs) have achieved an impressive power conversion efficiency (PCE) over the past few years, showing a great potential for real applications. However, the study on the photostability and degradation mechanism of nonfullerene OSCs is far behind than that of fullerene‐based solar cells, which is crucial for the commercial applications of the technology. Herein, an efficient and stable nonfullerene OSC based on PCE10:rhodanine‐benzothiadiazole‐coupled indacenodithiophene with branched 2‐ethylhexyl side chains (EH‐IDT) is fabricated from environmentally benign solvent. The PCE10:EH‐IDT solar cell shows a high PCE of 9.17% and a long operational lifetime (T ₈₀) of 2132 h, compared with other two OSCs based 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 (ITIC) and another fuse ring acceptor with withdrawing units of 1,1‐dicyanomethylene‐3‐indanone and hexyl side chains (IDIC) nonfullerene acceptors, with tested lifetimes of only 221 and 558 h, respectively. As indicated by the Flory–Huggins interaction parameters, ITIC and IDIC have poor miscibility with PCE10, which leads to morphology degradation, suppressed charge generation, increased trap states, and charge recombination in the photoaging test, which accounts for the significant loss of short‐circuit current density and fill factor during operation. The improved miscibility of the donor and the acceptor results in a more stable morphology, and the PCE10:EH‐IDT solar cells thus achieve an outstanding overall performance that combines high efficiency and superior photostability and paves the way for the potential practical applications of OSCs.

All‐polymer solar cells (all‐PSCs) fabricated via slot‐die printing are obtained. In situ grazing incidence wide‐angle X‐ray scattering reveals the multiple crystallization kinetics during film drying. Printing with 1,8‐diiodooctane leads to the formation of a multi‐length‐scale phase separation and eventually improves the solar cell efficiency up to 9.10%, which is the highest efficiency for printed all‐PSCs.

Herein, high‐performance printed all‐polymer solar cells (all‐PSCs) based on a bulk‐heterojunction (BHJ) blend film are demonstrated using PTzBI as the donor and N2200 as the acceptor. A slot‐die process is used to prepare the BHJ blend, which is a cost‐effective, high‐throughput approach to achieve large‐area photovoltaic devices. The real‐time crystallization of polymers in the film drying process is investigated by in situ grazing incidence wide‐angle X‐ray scattering characterization. Printing is found to significantly improve the crystallinity of the polymer blend in comparison with spin coating. Moreover, printing with 1,8‐diiodooctane as the solvent additive enhances the polymer aggregation and crystallization during solvent evaporation, eventually leading to multi‐length‐scale phase separation, with PTzBI‐rich domains in‐between the N2200 crystalline fibers. This unique morphology achieved by printing fabrication results in an impressively high power conversion efficiency of 9.10%, which is the highest efficiency reported for printed all‐PSCs. These findings provide important guidelines for controlling film drying dynamics for processing all‐PSCs.

In this work, organic ternary solar cells based on a model system comprising the polymers PDCBT and PTB7‐Th and PC₇₀BM are presented as electron accepting units. The photophysics of this blend is governed by a fast energy transfer process from PDCBT to PTB7‐Th allowed by a favorable molecular affinity between PDCBT and PTB7‐Th.

Abstract

Ternary blends with broad spectral absorption have the potential to increase charge generation in organic solar cells but feature additional complexity due to limited intermixing and electronic mismatch. Here, a model system comprising the polymers poly[5,5‐bis(2‐butyloctyl)‐(2,2‐bithiophene)‐4,4‐dicarboxylate‐alt‐5,5‐2,2‐bithiophene] (PDCBT) and PTB7‐Th and PC₇₀BM as an electron accepting unit is presented. The power conversion efficiency (PCE) of the ternary system clearly surpasses the performance of either of the binary systems. The photophysics is governed by a fast energy transfer process from PDCBT to PTB7‐Th, followed by electron transfer at the PTB7‐Th:fullerene interface. The morphological motif in the ternary blend is characterized by polymer fibers. Based on a combination of photophysical analysis, GIWAXS measurements and calculation of the intermolecular parameter, the latter indicating a very favorable molecular affinity between PDCBT and PTB7‐Th, it is proposed that an efficient charge generation mechanism is possible because PTB7‐Th predominantly orients around PDCBT filaments, allowing energy to be effectively relayed from PDCBT to PTB7‐Th. Fullerene can be replaced by a nonfullerene acceptor without sacrifices in charge generation, achieving a PCE above 11%. These results support the idea that thermodynamic mixing and energetics of the polymer–polymer interface are critical design parameter for realizing highly efficient ternary solar cells with variable electron acceptors.

High‐performance and stable perovskite solar cells with a power conversion efficiency exceeding 20% are achieved based on a low‐temperature solution‐processed ZnO electron transport layer. Dual passivation layers with TiO _x and phenyl‐C61‐butyric acid methyl ester (PCBM) enable the devices to exhibit good stability under an ambient air condition.

ZnO is considered as a potential electron transport layer (ETL) in solar cells due to its high charge carrier mobility and low‐temperature processability. However, it is challenging to obtain highly efficient and stable perovskite solar cells (PSCs) using low‐temperature ZnO ETL due to the poor chemical compatibility between ZnO and perovskite films. Hence, proper surface passivation strategies of ZnO ETL are developed to enhance the ZnO/perovskite interface stability for highly efficient PSCs. In this study, low‐temperature TiO _x post‐treatment is performed to passivate the ZnO surface, suppress the interface charge recombination, and stabilize the ZnO/perovskite interface. The fullerene treatment is further applied to enhance the electron transfer from perovskite to ZnO and reduce the hysteresis behavior. Finally, high‐performance PSCs with an efficiency of over 20% and improved stability are achieved.

Surface passivation of perovskite film for efficient solar cells

Surface passivation of perovskite film for efficient solar cells, Published online: 01 April 2019; doi:10.1038/s41566-019-0398-2

Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene)

Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene), Published online: 27 March 2019; doi:10.1038/s41586-019-1036-3

A 1D closely packed and high aspect ratio metal oxide nanowire‐based thin film is demonstrated to achieve favorable bifacial contact junction engineering for large‐area perovskite solar cells with efficiency exceeding 21% owing to the facilitated electron extraction, effective hole blocking, and suppressed charge recombination.

Abstract

Ordered 1D metal oxide structure is desirable in thin film solar cells owing to its excellent charge collection capability. However, the electron transfer in 1D electron transporting layer (ETL)‐based devices is still limited to a submicrometer‐long pathway that is vertical to the substrate. Here, an innovative closely packed rutile TiO₂ nanowire (CRTNW) network parallel to the facet of fluorine‐doped tin oxide (FTO) substrate is reported, which can serve as a 1D nanoscale electron transport pathway for efficient perovskite solar cells (PSCs). The PSC constructed using newly prepared CRTNW ETL achieves an impressive power conversion efficiency of 21.10%, which can be attributed to the facilitated electron extraction induced by the favorable junctions formed at FTO/ETL and ETL/perovskite interfaces and also the suppressed charge recombination originating from improved perovskite morphology with large grains, flat surface, and good surface coverage. The bifacial contact junctions engineering also enables large‐area device fabrication. The PSC with 1 cm² aperture yields an efficiency of 19.50% under one sun illumination. This work highlights the significance of controlling the orientation and packing density of the ordered 1D oxide nanostructured thin films for highly efficient optoelectronic devices in a large‐scale manner.

The [7]helicenes with stable partial open‐shell biradical ground states are demonstrated as effective surface modifiers of the inorganic NiO _x hole‐transporting layer in p–i–n perovskite solar cells. Their nonpolar feature improves the crystallinity of the perovskite films grown on them. Meanwhile, their biradical character provides a certain defect passivation function to facilitate charge transfer/extraction across the perovskite interface.

Abstract

Organic–inorganic hybrid perovskites have realized a high power conversion efficiency (PCE) in both n–i–p and p–i–n device configurations. However, since the p–i–n structure exempts the sophisticated processing of charge‐transporting layers, it seems to possess better potential for practical applications than the n–i–p one. Currently, the inorganic NiO _x is the most prevailing hole‐transporting layer (HTL) used in p–i–n perovskite solar cells. Nevertheless, defects might exist on its surface to influence the charge transfer/extraction across the interface with perovskite and to affect the quality of the perovskite film grown on it. Herein, two novel [7]helicenes with stable open‐shell singlet biradical ground states at room temperature are demonstrated as an effective surface modifier of the NiO _x HTL. Their nonpolar feature effectively promotes the crystallinity of the perovskite film grown on them; meanwhile, their unique partial biradical character seems to provide a certain degree of defect passivation function at the perovskite interface to facilitate interfacial charge transfer/extraction. As a result, both 1ab‐ and 1bb‐modifed devices yield a PCE of >18%, exceeding the value (15.6%) of the control device using a sole NiO _x HTL, and the maximum PCE can reach 19%. Detailed characterizations are carefully conducted to understand the underlying reasons behind such enhancement.

Phenylethylamine bromide is demonstrated to be capable of improving the ordering extent of alternatively arranged [AgX₆]⁵⁻ and [BiX₆]³⁻ octahedra in a Cs₂AgBiBr₆ single crystal, and thereby tuning the band gap and suppressing self‐trapped exciton formation, which consequently promotes its application in an X‐ray detector with faster current response and higher sensitivity, which largely outperforms the devices based on lower‐ordering single crystals.

Abstract

The double perovskite Cs₂AgBiBr₆ single crystal holds great potential for detecting applications because of its low minimum detectable dose rate and toxic‐free merit. Nevertheless, the disordered arrangement of Ag⁺/Bi³⁺ usually gives rise to unexpected structural distortion and thereafter heavily influences the photoelectric properties of the Cs₂AgBiBr₆ single crystal. Herein, phenylethylamine bromide is demonstrated to be capable of in situ regulation of the order–disorder phase transition in the Cs₂AgBiBr₆ single crystal. The improved ordering extent of alternatively arranged [AgX₆]⁵⁻ and [BiX₆]³⁻ octahedra is theoretically and experimentally proven to decrease the defect density and suppress self‐trapped exciton formation, and thereby tune the band gap and enhance the carrier mobility, which consequently promotes its application in an X‐ray detector. The performance of a corresponding detector based on PEA‐Cs₂AgBiBr₆ single crystal displays superior performances, e.g., longer carrier drift distance, higher photoconductive gain, and faster current response (13 vs 3190 µs). Prominently, the as‐fabricated PEA‐Cs₂AgBiBr₆ single‐crystal X‐ray detector has an extremely high sensitivity with a value of 288.8 µC Gy_air ⁻¹ cm⁻² under a bias of 50 V (22.7 V mm⁻¹), which largely outperforms those of their counterparts with lower ordering structure.

A controllable ultraviolet/ozone (UVO) treatment is employed to prepare a high‐quality electrochemically deposited NiO_x hole transport layer (HTL). Under optimal conditions of UVO treatment, the increased hole conductivity in the HTL reduces defects at the HTL/perovskite interface, and a narrowed offset of the valence band between the HTL and perovskite film are obtained, which results in high‐performance perovskite solar cells with an efficiency of 19.67%.

Nickel oxide (NiO_x) has exhibited great potential as a hole transport layer (HTL) for fabricating efficient and stable perovskite solar cells (PSCs). However, it has been greatly limited by its fabrication and manipulation process. In this work, a simple processing method on an ultrathin electrochemical mesoporous NiO_x film manipulated by controllable ultraviolet/ozone (UVO) treatmentis employed; the duration of UVO treatment on the NiO_x film significantly affects the photovoltaic properties of the PSCs. When the exposure duration increases, the wettability, electrical conductivity, nonstoichiometry, and valence band energy of the NiO_x film are improved with varying degrees. Besides, the perovskite grain size, recombination resistance at the perovskite/NiO_x interface, and build‐in potential of the device also increase, resulting in higher short‐circuit current density (J _SC) and open‐circuit voltage (V _OC). Combining these factors together, an optimal exposure time of UVO treatment on the NiO_x film has been achieved at 5 min, which results in a significantly high performance with an efficiency of 19.67%, large V _OC (>1.1 V), and J _SC (>23 mA cm⁻²). Furthermore, the experimental results are coincide well with simulation results on the different corresponding subjects. Hopefully, this work could facilitate material manipulation toward scalable, high efficiency, and stable solar cells.

In this work, the origin of the graded phase distribution in 2D perovskite films is revealed: DMSO suppresses the graded distribution effectively. An intermediate phase involved DMSO is found to be responsible for the less graded films. Solar cells with a less graded phase distribution shows an improved power conversion efficiency exceeding 14%.

Ruddlesden‐Popper perovskites (2D perovskites) show tremendous potential in photovoltaic devices for their superior stability compared with their 3D counterparts, while their lower power conversion efficiencies severely hinder their further progress for practical application. Generally, the 2D perovskite films fabricated by spin‐coating present graded distribution of 2D phases, and the small‐n phases located at the film bottom will impede the interlayer charge transport. In this work, the origin of the graded phase distribution is explained, and the roles of DMSO in Ruddlesden‐Popper perovskites film crystallization are systematically investigated. DMSO was found to homogenize the film composition, and therefore suppress the proportion of the small‐n 2D phases. Through X‐ray diffraction and grazing‐incidence wide‐angle X‐ray scattering measurements, it has been revealed that an intermediate phase is responsible for the less graded films and rationalizes the effect of DMSO in film crystallization. More importantly, solar cells with an efficiency exceeding 14% are fabricated by this facile method, which deliver good light, thermal, and ambient stability without encapsulation. These findings significantly benefit the understanding of crystal growth in 2D perovskites film and demonstrate that 2D perovskites are promising candidates for high‐performance solar cells.

This paper presents aqueous, ultrasonically sprayed NiO_x hole transport layers (HTLs) with large‐area scalability and high photovoltaic performance in double cation perovskite solar cells, outperforming spin‐coated NiO_x from organic precursors and dramatically improving the fracture energy, a key metric of thermomechanical reliability. This robust and scalable HTL technology therefore has the potential to become a platform for scaling perovskite modules.

Abstract

Organometal halide perovskites have powerful intrinsic potential to drive next‐generation solar technology, but their insufficient thermomechanical reliability and unproven large‐area manufacturability limit competition with incumbent silicon photovoltaics. This work addresses these limitations by leveraging large‐area processing and robust inorganic hole transport layers (HTLs). Inverted perovskite solar cells utilizing NiO_x HTLs deposited by rapid aqueous spray‐coating that outperform spin‐coated NiO_x and lead to a 5× improvement in the fracture energy (G _c), a primary metric of thermomechanical stability, are presented. The morphology, chemical composition, and optoelectronic properties of the NiO_x films are characterized to understand and optimize compatibility with an archetypal double cation perovskite, Cs_.17FA_.83Pb(Br_.17I_.83)₃. Perovskite solar cells with sprayed NiO_x show higher photovoltaic performance, exhibiting up to 82% fill factor and 17.7% power conversion efficiency (PCE)—the highest PCE reported for inverted cell with scalable charge transport layers—as well as excellent stability under full illumination and after 4000 h aging in inert conditions at room temperature. By utilizing open‐air techniques and aqueous precursors, this combination of robust materials and low‐cost processing provides a platform for scaling perovskite modules with long‐term reliability.

A unique and versatile strategy is proposed to fine‐tune the persistent luminescence over the entire visible spectral region with narrow bandwidth and highly synchronized afterglow decay. It uses a CaAl₂O₄:Eu²⁺,Nd³⁺ (CAO) afterglow phosphor as the single energy storage source and all‐inorganic CsPbX₃ (X=Cl, Br, and I) perovskite quantum dots (PeQDs) as efficient light conversion materials.

Abstract

Applications of persistent luminescence phosphors as night or dark‐light vision materials in many technological fields have fueled up a growing demand for rational control over the emission profiles of the phosphors. This, however, remains a daunting challenge. Now a unique strategy is reported to fine‐tune the persistent luminescence by using all‐inorganic CsPbX₃ (X=Cl, Br, and I) perovskite quantum dots (PeQDs) as efficient light‐conversion materials. Full‐spectrum persistent luminescence with wavelengths covering the entire visible spectral region is achieved through tailoring of the PeQD band gap, in parallel with narrow bandwidth of PeQDs and highly synchronized afterglow decay owing to the single energy storage source. These findings break through the limitations of traditional afterglow phosphors, thereby opening up opportunities for persistent luminescence materials for applications such as a white‐emitting persistent light source and dark‐light multicolor displays.

Publication date: 11 April 2019

Source: Chem, Volume 5, Issue 4

Author(s): Lingfeng Chao, Yingdong Xia, Bixin Li, Guichuan Xing, Yonghua Chen, Wei Huang

The Bigger Picture

Summary

Graphical Abstract