shuhui

Publication date: 21 August 2019

Source: Joule, Volume 3, Issue 8

Author(s): Chenchen Yang, Dianyi Liu, Matthew Bates, Miles C. Barr, Richard R. Lunt

Graphical Abstract

A new azeotropic promoted approach is proposed to successfully synthesize In doped CuCrO₂ under low temperatures in a short time. This In doped CuCrO₂ HTL has thermal stability up to 200 °C, and exhibits improved optical transmission and carrier mobility, which is beneficial for achieving high performance perovskite solar cells.

Abstract

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.

For developing low‐cost and high‐efficiency planar perovskite solar cells (PSCs), a straightforward low‐temperature chemical bath deposition process is developed to prepare a Co‐doped TiO₂ (Co‐TiO₂) electron transport layer (ETL); the optoelectrical properties of the TiO₂ ETL are significantly improved by Co‐doping. Finally, the efficiency of the PSCs is increased from 17.40% for TiO₂ to 19.10% for the Co‐TiO₂ ETL.

Planar hybrid perovskite solar cells (PSCs) attract great attention due to their obvious advantages of low‐temperature processing with a high power conversion efficiency (PCE) up to 23.32%. Here, Co‐doped TiO₂ (Co‐TiO₂) deposited by a straightforward low‐temperature chemical bath deposition (CBD) method is explored. Using Co‐TiO₂ as an electron transport layer (ETL) for the planar PSCs, the effects of doping on TiO₂ morphology, electronic properties, and solar cell performance are investigated. The PCE increases to 19.10% when the Co doping concentration is optimized at 5 mol%, an increase of 17.40% compared with that using the pristine TiO₂. Meanwhile, the notorious J–V hysteresis is suppressed to a greater extent. Considering that the low‐temperature CBD is comparable with continuous roll‐to‐roll processing, it makes the process and the Co‐TiO₂ ETL potential candidates for low‐cost commercialization.

The theme of this review is the progress of microcavity (MC) in organic solar cells (OSCs) in recent years. The principle of MC is described in detail. In addition, the application of MC in other photo‐electronic conversion devices is also briefly introduced. Finally, the summary and prospect of microcavity organic solar cells (MCOSCs) are given.

In recent decades, organic solar cells (OSCs) have drawn increasing interest due to their unique properties such as low cost, solution‐processing, flexibility, semitransparency, and nontoxicity. Due to some shortcomings of limited optical absorption in organic semiconductors as well as low carrier mobility and short exciton diffusion length, light‐trapping technologies such as surface plasmon resonance, photonic crystals, and microcavities (MCs) have been widely developed to improve device performance. Among these methods, the MC effect is liable to form and has unneglectable influences on the device efficiency. However, few reports systematically summarize the development of MC‐based OSCs. Herein, the principle of the MC effect is introduced first, and subsequently, the application and the development of MCs in single and multi‐junction OSCs are described in detail. Furthermore, in addition to the traditional MCs‐enhanced light absorption, other applications based on the MC structure in OSCs and other photo‐electronic conversion devices are also represented. Finally, the problems that need to be solved and the development directions of MC‐based OSCs in the future are outlined. It is believed that this review can provide new thinking for achieving high‐performance OSCs with optical means.

Micrometer‐thick stable CsPbBr₃ perovskite films are obtained through a facile vacuum drying process. Green emission with a brightness as high as 200 cd m⁻² is achieved from blue light with a back luminance of 1000 cd m⁻², which decays by only ≈2% when the films are tested after 18 days of exposure to ambient environment.

Abstract

Metal halide perovskite materials have attracted great attention owing to their fascinating optoelectronic characteristics and low cost fabrication via facile solution processing. One of the potential applications of these materials is to employ them as color‐conversion layers (CCLs) for visible blue light to achieve full‐color displays. However, obtaining thick perovskite films to realize complete color conversion is a key challenge. Here, the fabrication of micrometer‐level thick CsPbBr₃ perovskite films is presented through a facile vacuum drying approach. An efficient green photoconversion is realized in a 3.8 µm thick film from blue light @ 463 nm. For a back luminance of 1000 cd m⁻², the brightness of the resulting green emission can reach as high as 200 cd m⁻². Furthermore, only ≈2% of decay in brightness is observed when the films are tested after 18 days of exposure to ambient environment. In addition, a potential design is also proposed for full‐color displays with perovskite materials incorporated as CCLs.

The recent progress in double‐metallic lead‐free perovskite materials and devices is comprehensively reviewed. In particular, theory calculation, electronic structure, and fundamental properties of double perovskites are deliberated. The achievements and challenges in their application including solar cells, photon detectors, and laser devices, are summarized. In addition, the viewpoints for future research of this class of perovskites are also provided.

Lead halide perovskite (ABX₃) has attracted considerable attention due to its applicability as absorber layers in highly efficient photovoltaic cells. With regard to the lead toxicity, double‐metallic lead‐free perovskite, A₂B^IB^IIIX₆, in which the neighboring B⁺ and B³⁺ sites in the crystal microstructure are alternately occupied by monovalent‐metal and trivalent‐metal cations, is regarded to be a promising alternative to the widely used lead‐based perovskites. This review aims to summarize the recent advances in the new class of A₂B^IB^IIIX₆ double‐metallic lead‐free perovskites. In particular, the electronic structure, synthesis, property, and their applications in devices, for example, photovoltaics, photodetectors, and light emitting diodes, is carefully classified and presented. Notably, the theoretical calculations point out that there is much room toward potential applications for this new class of perovskite materials. The present review provides a holonomic conclusion and opens new perspectives toward realizing higher performance of A₂B^IB^IIIX₆‐based devices.

A lead‐bismuth (Pb‐Bi) binary metal based all‐inorganic perovskite film is successfully fabricated and applied as absorber layer to enhance the stability of perovskite solar cells (PSCs). High power conversion efficiency (PCE) of 11.9% is obtained for the all‐inorganic (PSC).The PCE only reduced by 10% under atmospheric humidity of 40% in 4 weeks.

Abstract

A lead‐bismuth (Pb‐Bi) binary metal based all‐inorganic perovskite film is successfully fabricated and applied as absorber layer to enhance the stability of perovskite solar cells (PSCs). Unlike the Pb‐only perovskite‐based device, the Pb‐Bi binary metal perovskite based one shows better tolerance to humidity and oxygen. High power conversion efficiency (PCE) of 11.9% is obtained for the all‐inorganic (PSC). Noticeably, the PCE only reduced by 10% under atmospheric humidity of 40% in four weeks. An electron‐only device also shows reduced trap states. The improved stability and PCE is ascribed to higher quality perovskite film with less trap states and smaller series resistance (R _s) in the device.

Me₄NBr is introduced to passivate the Sn–Pb based perovskite interface, leading to an improved efficiency of 13.97%, mainly due to the effective reduction of defects. By adopting the poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS)/poly(triarylamine) (PTAA) as the hole transport material (HTM), a Sn‐based perovskite solar cell with an efficiency of 14.56% is obtained. Furthermore, the Me₄NBr treated Sn–Pb perovskite cells also demonstrate a significant stability enhancement.

Abstract

Tin–lead (Sn–Pb) based hybrid perovskite solar cell is investigated as a potential solution to extend the light absorption spectrum range, and to reduce environmental hazard caused by lead in the perovskite materials. Nonetheless, due to the instability of tin, the Sn–Pb based perovskite solar cells suffer from more severe efficiency degradation when compared to the lead‐based perovskite solar cells, which restricts its further development. Here, a quaternary ammonium halide compound, Me₄NBr, is introduced to passivate the Sn–Pb based perovskite surface. The Me₄NBr effectively reduces the surface defects and enhances the open circuit voltage and fill factor of the Sn–Pb based perovskite solar cell. Moreover, the Me₄NBr treated Sn–Pb perovskite cells also demonstrate a significant stability enhancement when compared with the untreated Sn–Pb perovskite cells.

The small organic molecule (2‐(1,10‐phenanthrolin‐3‐yl)naphth‐6‐yl)diphenylphosphine oxide is explored as cathode interfacial material to reduce the extraction barrier between phenyl‐C61‐butyric acid methyl ester and Ag. With the better contact quality thanks to this molecule, both opaque and semitransparent p‐i‐n perovskite solar cell achieve improved performance and stability.

Abstract

Metal halide perovskite solar cells (PSCs) in the inverted planar p‐i‐n configuration often employ phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) as electron transport layer, onto which Ag is deposited as outer electrode. However, the energy offset between PC₆₁BM and Ag imposes an energy barrier for electron extraction. In this work, to improve the contact quality of this stack, a small organic molecule (2‐(1,10‐phenanthrolin‐3‐yl)naphth‐6‐yl)diphenylphosphine oxide (DPO) as a cathode interfacial material (CIM), inserted between PC₆₁BM and Ag, is introduced. In devices with the indium tin oxide (ITO)/NiO _x /methylammonium lead iodide (MAPbI₃)/PC₆₁BM/CIM/Ag configuration, it is found that this results in fill factor (FF) and short‐circuit current density values (J _SC) that are up to ≈34% and ≈1 mA cm⁻² higher, respectively, compared to DPO‐free devices. Inserting additional thin ZnO nanoparticle layers further improves the contact quality, leading to a power conversion efficiency of 18.2%. Semitransparent PSCs, utilizing DPO as an interlayer buffer layer are also realised. Resultant devices exhibit improved performance compared to DPO‐free devices. This proves that DPO withstands the sputtering of ITO, and may thus find application in perovskite‐based tandem devices. It is concluded that DPO acts as an excellent cathode modifier, opening new device‐engineering opportunities for p‐i‐n PSCs, especially in their semitransparent implementation.

Perovskite solar cells (PVSCs) with discrete SnO₂ nanoparticle modification layers are constructed via spin coating the SnO₂ dispersions on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The discrete SnO₂ nanoparticle film let holes pass and block electrons to diffuse toward PEDOT:PSS, which enhances the extraction efficiency, leading to an increase in a power conversion efficiency of p‐i‐n‐type PVSCs.

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most widely used hole transport materials for perovskite solar cells (PVSCs) with a p‐i‐n structure. However, the solar cells based on PEDOT:PSS show a low photoconversion efficiency due to the poor crystallinity of a perovskite film on it. Besides, the acidity of PEDOT:PSS performance critically influences the long‐term stability of PVSCs. Herein, a layer of the discrete SnO₂ nanoparticle film is deposited on the surface of PEDOT:PSS to modify the surface of the PEDOT:PSS film. This discrete SnO₂ nanoparticle film acts as the buffer layer between the PEDOT:PSS and MAPbI₃, which not only improves the crystallization of the quality of the perovskite film, but also provides a selective‐carrier pathway to enhance the extraction of holes and to block the diffusion of electrons. The SnO₂ modified devices show a power conversion efficiency of 18.04%, with a great improvement compared with the 12.24% efficiency of PEDOT:PSS only devices. This work demonstrates that it is possible to enhance the performance of PVSCs via n‐type nanoparticle modification of hole transport layer and provides a new guidance for PVSCs interface modification engineering.

Publication date: 18 September 2019

Source: Joule, Volume 3, Issue 9

Author(s): Minjin Kim, Gi-Hwan Kim, Tae Kyung Lee, In Woo Choi, Hye Won Choi, Yimhyun Jo, Yung Jin Yoon, Jae Won Kim, Jiyun Lee, Daihong Huh, Heon Lee, Sang Kyu Kwak, Jin Young Kim, Dong Suk Kim

Context & Scale

Summary

Graphical Abstract

A type of perovskite bifunctional device (PBD) with high photovoltaic (PV) and electroluminescence (EL) performance is developed. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is performed to turn the n‐type surface of the perovskite into p‐type and also correct the misalignment to form a well‐defined n–i–p heterojunction.

Abstract

Currently, photovoltaic/electroluminescent (PV/EL) perovskite bifunctional devices (PBDs) exhibit poor performance due to defects and interfacial misalignment of the energy band. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is introduced to reduce energy loss, through adding corrosion‐free 3,3′‐(2,7‐dibromo‐9H‐fluorene‐9,9‐diyl) bis(n,n‐dimethylpropan‐1‐amine) (FN‐Br) into a HTL free of lithium salt. This strategy can turn the n‐type surface of perovskite into p‐type and thus correct the misalignment to form a well‐defined N–I–P heterojunction. The tailored PBD achieves a high PV efficiency of up to 21.54% (certified 20.24%) and 4.3% EL external quantum efficiency. Free of destructive additives, the unencapsulated devices maintain >92% of their initial PV performance for 500 h at maximum power point under standard air mass 1.5G illumination. This strategy can serve as a general guideline to enhance PV and EL performance of perovskite devices while ensuring excellent stability.

Solution‐processed double‐walled carbon nanotubes function as transparent electrodes in inverted‐type planar heterojunction perovskite solar cells. Double‐walled carbon nanotubes exhibit high optical conductivity and solubility. Good energy level alignment and morphology of the electrodes leads to an operating power conversion efficiency of 17.2%, which is the highest among the carbon nanotube electrode‐based perovskite solar cells.

Abstract

Double‐walled carbon nanotubes are between single‐walled carbon nanotubes and multiwalled carbon nanotubes. They are comparable to single‐walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. Exploiting such advantages, solution‐processed transparent electrodes are demonstrated using double‐walled carbon nanotubes and their application to perovskite solar cells is also demonstrated. Perovskite solar cells which harvest clean solar power have attracted a lot of attention as a next‐generation renewable energy source. However, their eco‐friendliness, cost, and flexibility are limited by the use of transparent oxide conductors, which are inflexible, difficult to fabricate, and made up of expensive rare metals. Solution‐processed double‐walled carbon nanotubes can replace conventional transparent electrodes to resolve such issues. Perovskite solar cells using the double‐walled carbon nanotube transparent electrodes produce an operating power conversion efficiency of 17.2% without hysteresis. As the first solution‐processed electrode‐based perovskite solar cells, this work will pave the pathway to the large‐size, low‐cost, and eco‐friendly solar devices.

This work reports an efficient post‐treatment method for CsPbI₃ perovskite quantum dots (QDs) using cesium cations, which can passivate the CsPbI₃ surface and improve the electron coupling of QDs. Finally, the best CsPbI₃ QD solar cell with an impressive efficiency of 14.10% is achieved by cesium acetate (CsAc) and exhibits improved stability against moisture.

Abstract

Surface manipulation of quantum dots (QDs) has been extensively reported to be crucial to their performance when applied into optoelectronic devices, especially for photovoltaic devices. In this work, an efficient surface passivation method for emerging CsPbI₃ perovskite QDs using a variety of inorganic cesium salts (cesium acetate (CsAc), cesium idodide (CsI), cesium carbonate (Cs₂CO₃), and cesium nitrate (CsNO₃)) is reported. The Cs‐salts post‐treatment can not only fill the vacancy at the CsPbI₃ perovskite surface but also improve electron coupling between CsPbI₃ QDs. As a result, the free carrier lifetime, diffusion length, and mobility of QD film are simultaneously improved, which are beneficial for fabricating high‐quality conductive QD films for efficient solar cell devices. After optimizing the post‐treatment process, the short‐circuit current density and fill factor are significantly enhanced, delivering an impressive efficiency of 14.10% for CsPbI₃ QD solar cells. In addition, the Cs‐salt‐treated CsPbI₃ QD devices exhibit improved stability against moisture due to the improved surface environment of these QDs. These findings will provide insight into the design of high‐performance and low‐trap‐states perovskite QD films with desirable optoelectronic properties.

Publication date: 21 August 2019

Source: Joule, Volume 3, Issue 8

Author(s): Jaehoon Chung, Seong Sik Shin, Geunjin Kim, Nam Joong Jeon, Tae-Youl Yang, Jun Hong Noh, Jangwon Seo

Context & Scale

Summary

Graphical Abstract

The incorporation of π‐extended phosphoniumfluorene electrolytes as hole‐blocking layers in planar perovskite solar cells results in a significant enhancement in both the fill factor and the open‐circuit voltage of the devices. The latter can be enhanced by up to 120 mV as compared to the commonly used bathocuproine hole blocking layer.

Abstract

Four π‐extended phosphoniumfluorene electrolytes (π‐PFEs) are introduced as hole‐blocking layers (HBL) in inverted architecture planar perovskite solar cells with the structure of ITO/PEDOT:PSS/MAPbI₃/PCBM/HBL/Ag. The deep‐lying highest occupied molecular orbital energy level of the π‐PFEs effectively blocks holes, decreasing contact recombination. It is demonstrated that the incorporation of π‐PFEs introduces a dipole moment at the PCBM/Ag interface, resulting in significant enhancement of the built‐in potential of the device. This enhancement results in an increase in the open‐circuit voltage of the device by up to 120 mV, when compared to the commonly used bathocuproine HBL. The results are confirmed both experimentally and by numerical simulation. This work demonstrates that interfacial engineering of the transport layer/contact interface by small molecule electrolytes is a promising route to suppress nonradiative recombination in perovskite devices and compensates for a nonideal energetic alignment at the hole‐transport layer/perovskite interface.

The role of cation and halide mixing is revealed using in situ X‐ray scattering measurements during spin‐coating. Modulating the cation/halide composition directly impacts the lifetime of the sol–gel precursor film and its easy and reproducible conversion to the perovskite phase to yield solar cells with 20% power conversion efficiency.

Abstract

Perovskite solar cells increasingly feature mixed‐halide mixed‐cation compounds (FA_{1− x − y} MA _x Cs _y PbI_{3− z} Br_z) as photovoltaic absorbers, as they enable easier processing and improved stability. Here, the underlying reasons for ease of processing are revealed. It is found that halide and cation engineering leads to a systematic widening of the anti‐solvent processing window for the fabrication of high‐quality films and efficient solar cells. This window widens from seconds, in the case of single cation/halide systems (e.g., MAPbI₃, FAPbI₃, and FAPbBr₃), to several minutes for mixed systems. In situ X‐ray diffraction studies reveal that the processing window is closely related to the crystallization of the disordered sol–gel and to the number of crystalline byproducts; the processing window therefore depends directly on the precise cation/halide composition. Moreover, anti‐solvent dripping is shown to promote the desired perovskite phase with careful formulation. The processing window of perovskite solar cells, as defined by the latest time the anti‐solvent drip yields efficient solar cells, broadened with the increasing complexity of cation/halide content. This behavior is ascribed to kinetic stabilization of sol–gel state through cation/halide engineering. This provides guidelines for designing new formulations, aimed at formation of the perovskite phase, ultimately resulting in high‐efficiency perovskite solar cells produced with ease and with high reproducibility.

The recent progress in lead‐free tin (Sn)‐based perovskite solar cells (PSCs) is reviewed. After briefing the structural and optoelectronic properties of Sn‐based perovskites, the film deposition methods and the strategies toward high performance in Sn‐based PSCs are then summarized. The challenges and prospective opportunities in this field are also discussed.

Perovskite solar cells (PSCs) have achieved state‐of‐the‐art efficiency, approaching monocrystalline silicon solar cells due to the superior optoelectronic properties and intensive research efforts, fulfilling its forthcoming commercial use at affordable costs. Nevertheless, the toxicity of lead (Pb) is still one of the obstacles hindering future large‐scale production. Herein, the recent progress of emerging lead‐free tin (Sn)‐based PSCs is reviewed. First, the structural and photovoltaic‐related properties of Sn‐based perovskites are summarized. Following a brief introduction of film deposition methods, strategies recently adopted to obtain high performance are then discussed in detail. Finally, the current challenges and prospective opportunities are provided to help the further progression of Sn‐based PSCs.

Nature Energy, Published online: 17 June 2019; doi:10.1038/s41560-019-0400-8

Chlorine‐substituted graphdiyne (ClGD) is employed into electron transport layers of MAPbI₃‐based perovskite solar cells. It is experimentally and theoretically demonstrated that the interactions of derivated graphdiyne and PCBM stem from four types of noncovalent bonds, which contribute to the improved device performance. Perovskite solar cells based on the ClGD‐PCBM obtain an enhanced power conversion efficiency (PCE) of 20.34%.

Chlorine‐substituted graphdiyne (ClGD) is employed into electron transport layers (ETLs) of MAPbI₃‐based perovskite solar cells for the first time, forming a high‐quality film with superior film morphology and electrical conductivity as compared with pristine [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) film. Strikingly, a champion power conversion efficiency of 20.34% is achieved, showing a 19% enhancement compared with the counterparts (17.08%). Simultaneously, ClGD‐PCBM‐based devices show suppressed J–V hysteresis. It is experimentally and theoretically demonstrated that the interactions of derivated graphdiyne and PCBM stem from four types of noncovalent bonds, which contribute to the improved device performance. The results suggest that derivated graphdiyne‐based interfacial material is promising for the applications in solar cells and other photoelectric devices.

Metal oxides are used as charge transporting layers to effectively separate the photogenerated electrons and holes in perovskite solar cells (PSCs). The metal oxide layers require a wide bandgap, a good charge mobility, and a compatible band alignment with the perovskite layers. This review summarizes and correlates the preparation and performance of the various metal oxides used in PSCs.

Abstract

Currently, the efficiency of perovskite solar cells (PSCs) is ≈24%. For the fabrication of such high efficiency PSCs, it is necessary to use both electron and hole transport layers to effectively separate the charges generated by light absorption of the perovskite layer and selectively transfer the separated electrons and holes. In addition to the efficiency, the materials used for transporting charges must be resilient to light, heat, and moisture to ensure long‐term stability of PSCs; furthermore, low‐cost fabrication is required to form a charge transport layer at low temperatures by a solution process. For this purpose, metal oxides are best suited as charge transport materials for PSCs because of their advantages such as low cost, long‐term stability, and high efficiency. In this Review, the metal oxide electron and hole transport materials used in PSCs are reviewed and preparation of these materials is summarized. Finally, the challenges and future research direction for metal oxide‐based charge transport materials are described.

In this contribution, the detailed pathways for the sequential deposition of FAPbI₃ are investigated using in situ X‐ray techniques and the influence of additive ions on the crystallization and grain orientation of the resultant perovskite films is revealed; the optimal film preparation conditions are obtained through in situ experimental results.

Abstract

Metal halide perovskites have revolutionized the development of highly efficient, solution‐processable solar cells. Further advancements rely on improving perovskite film qualities through a better understanding of the underlying growth mechanism. Here, a systematic in situ grazing‐incidence X‐ray diffraction investigation is performed, facilitated by other techniques, on the sequential deposition of formamidinium lead iodide (FAPbI₃)‐based perovskite films. The active chemical reaction, composition distribution, phase transition, and crystal grain orientation are all visualized following the entire perovskite formation process. Furthermore, the influences of additive ions on the crystallization speed, grain orientation, and morphology of FAPbI₃‐based films, along with their photovoltaic performances, are fully evaluated and optimized, which leads to highly reproducible and efficient perovskite solar cells. The findings provide key insights into the perovskite growth mechanism and suggest the fabrication of high‐quality perovskite films for widespread optoelectronic applications.