shuhui

A platinum(II) complexation strategy is developed to regulate the crystallinity of a newly designed s‐tetrazine‐containing wide‐bandgap copolymer donor PSFTZ, and optimize the morphology of the PSFTZ:Y6 active blend film, which boosts successfully the power conversion efficiency of the resulting nonfullerene polymer solar cells (NF‐PSCs) from 13.03% to 16.35%. 16.35% is the new record for single‐junction NA‐PSCs at present.

Abstract

A new strategy of platinum(II) complexation is developed to regulate the crystallinity and molecular packing of polynitrogen heterocyclic polymers, optimize the morphology of the active blends, and improve the efficiency of the resulting nonfullerene polymer solar cells (NF‐PSCs). The newly designed s‐tetrazine (s‐TZ)‐containing copolymer of PSFTZ (4,8‐bis(5‐((2‐butyloctyl)thio)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3,6‐bis(4‐octylthiophen‐2‐yl)‐1,2,4,5‐tetrazine) has a strong aggregation property, which results in serious phase separation and large domains when blending with Y6 ((2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile)), and produces a power‐conversion efficiency (PCE) of 13.03%. By adding small amount of Pt(Ph)₂(DMSO)₂ (Ph, phenyl and DMSO, dimethyl sulfoxide), platinum(II) complexation would occur between Pt(Ph)₂(DMSO)₂ and PSFTZ. The bulky benzene ring on the platinum(II) complex increases the steric hindrance along the polymer main chain, inhibits the polymer aggregation strength, regulates the phase separation, optimizes the morphology, and thus improves the efficiency to 16.35% in the resulting devices. 16.35% is the highest efficiency for single‐junction PSCs reported so far.

By soldering adjacent perovskite grains with the synergistic effects of formamidinium and chloride ions, the trap state density and non‐radiative recombination rate of perovskite films are remarkably reduced. Perovskite solar cells, using unannealed nickel oxide as the hole transport layer, could achieve a power conversion efficiency of 19.6% with open‐circuit voltages as high as 1.10 V.

Abstract

Grain boundaries (GBs) are one of the major sources of defects in a polycrystalline perovskite solar cell and can greatly increase the rate of charge carrier recombination. In the push to optimize the efficiency of perovskite solar cells, it is therefore extremely important to maximize the grain size and minimize the number of GBs. In the present work, the number of GBs is effectively reduced by introducing a suitable number of formamidinium and chloride ions into the methylammonium lead iodide (MAPbI₃) absorber layer. Inverted perovskite solar cells, using NiO _x nanocrystals as the low‐temperature‐fabricated hole transport layer, are prepared; the champion cell has an efficiency of 19.6%. This work demonstrates a simple method of minimizing the number of grain boundaries, which is critical to the future development of this technology.

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.

Nonfullerene n‐type organic semiconductors possess unique advantages over inorganic semiconductors and/or fullerene derivatives in perovskite solar cells. This research news article summarizes and discusses the recent development of the multifunctional nonfullerene n‐type organic semiconductors used in perovskite solar cells.

Abstract

Compared to inorganic semiconductors and/or fullerene derivatives, nonfullerene n‐type organic semiconductors present some advantages, such as low‐temperature processing, flexibility, and molecule structure diversity, and have been widely used in perovskite solar cells (PSCs). In this research news article, the recent advances in nonfullerene n‐type organic semiconductors which function as electron‐transporting, interface‐modifying, additive, and light‐harvesting materials in PSCs are summarized. The remaining challenges and promising future directions of nonfullerene‐based PSCs are also discussed.

A novel strategy is reported where control over the surface‐adsorbed water on a transparent conducting oxide substrate is used to mediate the in situ nanocrystalline regrowth of a SnO₂ electron transport layer (ETL) at near room temperature. The new ETL is key to achieving a high power conversion efficiency of 20.5% and 17.5% in rigid and flexible perovskite solar cells, respectively.

Abstract

Electron transport layer (ETL) is a functional layer of great significance for boosting the power conversion efficiency (PCE) of perovskite solar cells (PSCs). To date, it is still a challenge to simultaneously reduce the surface defects and improve the crystallinity in ETLs during their low‐temperature processing. Here, a novel strategy for the mediation of in situ regrowth of SnO₂ nanocrystal ETLs is reported: introduction of controlled trace amounts of surface absorbed water on the fluorinated tin oxide (FTO) or indium–tin oxide (ITO) surfaces of the substrates using ultraviolet ozone (UVO) pretreatment. The optimum amount of adsorbed water plays a key role in balancing the hydrolysis–condensation reactions during the structural evolution of SnO₂ thin films. This new approach results in a full‐coverage SnO₂ ETL with a desirable morphology and crystallinity for superior optical and electrical properties, as compared to the control SnO₂ ETL without the UVO pretreatment. Finally, the rigid and flexible PSC devices based on the new SnO₂ ETLs yield high PCEs of up to 20.5% and 17.5%, respectively.

Highly‐efficient, low‐cost, solution‐processed perovskite solar cells, exhibiting remarkable environmental stability, are reported. The fabrication strategy relies on the rational design of the molecular structure of arylamine‐substituted copper(II) phthalocyanine (CuPc) derivatives, which are used as dopant‐free hole‐transport materials. The resulting devices reach a power conversion efficiency of 19.7% and display enhanced long‐term stability with respect to standard (doped) materials.

Abstract

A power conversion efficiency (PCE) as high as 19.7% is achieved using a novel, low‐cost, dopant‐free hole transport material (HTM) in mixed‐ion solution‐processed perovskite solar cells (PSCs). Following a rational molecular design strategy, arylamine‐substituted copper(II) phthalocyanine (CuPc) derivatives are selected as HTMs, reaching the highest PCE ever reported for PSCs employing dopant‐free HTMs. The intrinsic thermal and chemical properties of dopant‐free CuPcs result in PSCs with a long‐term stability outperforming that of the benchmark doped 2,2′,7,7′‐Tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐Spirobifluorene (Spiro‐OMeTAD)‐based devices. The combination of molecular modeling, synthesis, and full experimental characterization sheds light on the nanostructure and molecular aggregation of arylamine‐substituted CuPc compounds, providing a link between molecular structure and device properties. These results reveal the potential of engineering CuPc derivatives as dopant‐free HTMs to fabricate cost‐effective and highly efficient PSCs with long‐term stability, and pave the way to their commercial‐scale manufacturing. More generally, this case demonstrates how an integrated approach based on rational design and computational modeling can guide and anticipate the synthesis of new classes of materials to achieve specific functions in complex device structures.

The technological potential of emerging perovskite solar cells is determined by the stability of the power generated. Alkali salts used as interface modifiers positively affect device performance and stability and mitigate current–voltage hysteresis. Devices modified with potassium nitrate deliver a power conversion efficiency of 19.2% attributed to an improved charge carrier extraction and suppression of transient capacitive effects in device operation.

After demonstration of a 23% power conversion efficiency, a high operational stability is the next most important scientific and technological challenge in perovskite solar cells (PSCs). A potential failure mechanism is tied to a bias‐induced ion migration, which causes current–voltage hysteresis and a decay in the device performance over time. Herein, alkali salts are shown to mitigate hysteresis and stabilize device performance in n‐i‐p hybrid planar PSCs. Different alkali salts of potassium chloride, iodide, and nitrate as well as sodium chloride and iodide are deposited from aqueous solution onto the n‐type contact, based on SnO₂, prior to deposition of the perovskite absorber Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. Introduction of potassium‐based alkali salts suppresses the current–voltage hysteresis and stabilizes the operational device stability at the maximum power point. This is attributed to the suppression of hole trapping at the n‐type selective transport layer (SnO₂)/perovskite interface observed by surface photovoltage spectroscopy, which is interpreted to reduce interfacial recombination and improve charge carrier extraction. The best and most stable performance of 19% is achieved using potassium nitrate as the interface modifier. Devices with higher and more stable performance exhibit substantially lower current transients, analyzed during maximum power point tracking.

The current status and recent advances in perovskite‐based tandem solar cells, including perovskite–silicon, perovskite–perovskite, and perovskite–copper indium gallium selenide (CIGS) integrations, are comprehensively reviewed. Different configurations, key issues regarding the photoelectric properties, and material design are discussed. The critical role of perovskite bandgap optimization, interface engineering, and recombination layers are analyzed to outline the roadmaps for future investigations.

Metal halide perovskite‐based solar cells have achieved rapidly increasing efficiencies of up to 23.7%. However, it is still far away from the Shockley–Quiesser limit of 33.16%. Tandem solar cells, consisting of two subcells with complementary absorption, are suggested as an alternative to beat this limit due to the fact that a maximum efficiency of 42% can be reached using two subcells with bandgaps of 1.9 eV/1.0 eV, opening up a great potential to develop perovskite‐based tandem solar cells. In this review, the current status of and recent advances in perovskite‐based tandem solar cells are highlighted, including perovskite–silicon, perovskite–perovskite, and perovskite–copper indium gallium selenide (CIGS) integrations. Different configurations, key issues regarding the photoelectric properties, present efficiency limitations, and material design are discussed. The critical role of perovskite bandgap optimization, interface engineering, and recombination layers are also analyzed to outline the roadmaps for future investigation. The current challenging issues and future perspectives are also provided. It is hoped that the findings will provide new perspectives for perovskite‐based tandem solar cells with an unprecedented performance and the opportunity for commercialization.

Publication date: 17 July 2019

Source: Joule, Volume 3, Issue 7

Author(s): Dong Hoe Kim, Christopher P. Muzzillo, Jinhui Tong, Axel F. Palmstrom, Bryon W. Larson, Chungseok Choi, Steven P. Harvey, Stephen Glynn, James B. Whitaker, Fei Zhang, Zhen Li, Haipeng Lu, Maikel F.A.M. van Hest, Joseph J. Berry, Lorelle M. Mansfield, Yu Huang, Yanfa Yan, Kai Zhu

Context & Scale

Summary

Graphical Abstract

A novel strategy is reported where control over the surface‐adsorbed water on a transparent conducting oxide substrate is used to mediate the in situ nanocrystalline regrowth of a SnO₂ electron transport layer (ETL) at near room temperature. The new ETL is key to achieving a high power conversion efficiency of 20.5% and 17.5% in rigid and flexible perovskite solar cells, respectively.

Abstract

Electron transport layer (ETL) is a functional layer of great significance for boosting the power conversion efficiency (PCE) of perovskite solar cells (PSCs). To date, it is still a challenge to simultaneously reduce the surface defects and improve the crystallinity in ETLs during their low‐temperature processing. Here, a novel strategy for the mediation of in situ regrowth of SnO₂ nanocrystal ETLs is reported: introduction of controlled trace amounts of surface absorbed water on the fluorinated tin oxide (FTO) or indium–tin oxide (ITO) surfaces of the substrates using ultraviolet ozone (UVO) pretreatment. The optimum amount of adsorbed water plays a key role in balancing the hydrolysis–condensation reactions during the structural evolution of SnO₂ thin films. This new approach results in a full‐coverage SnO₂ ETL with a desirable morphology and crystallinity for superior optical and electrical properties, as compared to the control SnO₂ ETL without the UVO pretreatment. Finally, the rigid and flexible PSC devices based on the new SnO₂ ETLs yield high PCEs of up to 20.5% and 17.5%, respectively.

An ingenious surface chlorination treatment method is used to passivate the interface defects of perovskite/zinc oxide (ZnO), which effectively reduces the interface charge recombination loss and improves the poor interface chemical characteristics. Thus, the fabricated zinc oxide–chlorine (ZnO–Cl)‐based device achieves an enhanced efficiency and suppressed hysteresis, as well as strengthened stability in perovskite solar cells.

Defect states on the zinc oxide (ZnO) surface cause severe interfacial charge recombination and perovskite decomposition during device operation, which inevitably leads to efficiency loss and poor device stability, making the usage of ZnO in perovskite solar cells (PSCs) problematic. Herein, a simple and effective method of inorganic chlorination treatment is used to passivate the surface defects of the ZnO electron transport layer. It is shown that chlorine (Cl) effectively fills the oxygen vacancy defects of ZnO, suppressing charge recombination and facilitating charge transport at the perovskite/ZnO interface. Therefore, the resulting CH₃NH₃PbI₃‐based device achieves an enhanced power conversion efficiency with suppressed hysteresis. Meanwhile, the chlorination of the ZnO surface protects the perovskite layer from decomposition, thus improving device stability. Herein, an ingenious method is developed to further improve the device performance of ZnO‐based PSCs and useful guidance is provided for the development of other perovskite optoelectronics, especially those with ZnO as the charge transport layer.

Nature Photonics, Published online: 27 May 2019; doi:10.1038/s41566-019-0435-1

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.

Highly‐efficient, low‐cost, solution‐processed perovskite solar cells, exhibiting remarkable environmental stability, are reported. The fabrication strategy relies on the rational design of the molecular structure of arylamine‐substituted copper(II) phthalocyanine (CuPc) derivatives, which are used as dopant‐free hole‐transport materials. The resulting devices reach a power conversion efficiency of 19.7% and display enhanced long‐term stability with respect to standard (doped) materials.

Abstract

A power conversion efficiency (PCE) as high as 19.7% is achieved using a novel, low‐cost, dopant‐free hole transport material (HTM) in mixed‐ion solution‐processed perovskite solar cells (PSCs). Following a rational molecular design strategy, arylamine‐substituted copper(II) phthalocyanine (CuPc) derivatives are selected as HTMs, reaching the highest PCE ever reported for PSCs employing dopant‐free HTMs. The intrinsic thermal and chemical properties of dopant‐free CuPcs result in PSCs with a long‐term stability outperforming that of the benchmark doped 2,2′,7,7′‐Tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐Spirobifluorene (Spiro‐OMeTAD)‐based devices. The combination of molecular modeling, synthesis, and full experimental characterization sheds light on the nanostructure and molecular aggregation of arylamine‐substituted CuPc compounds, providing a link between molecular structure and device properties. These results reveal the potential of engineering CuPc derivatives as dopant‐free HTMs to fabricate cost‐effective and highly efficient PSCs with long‐term stability, and pave the way to their commercial‐scale manufacturing. More generally, this case demonstrates how an integrated approach based on rational design and computational modeling can guide and anticipate the synthesis of new classes of materials to achieve specific functions in complex device structures.

Crystalline DRCN5T is used to optimize the performance of thick‐film ternary organic solar cells by forming obvious interpenetrating network morphology with decreased π‐π stacking and enhanced domain purity. More importantly, DRCN5T can precisely modulate vertical distribution of the active layer due to contrasting miscibility with PTB7‐Th and PC₇₀BM, which drives the enrichment of PTB7‐Th on the active layer surface.

Abstract

Blending multidonor or multiacceptor organic materials as ternary devices has been recognized as an efficient and potential method to improve the power conversion efficiency of bulk heterojunction devices or single‐junction components in tandem design. In this work, a highly crystalline molecule, DRCN5T, is involved into a PTB7‐Th:PC₇₀BM system to fabricate large‐area organic solar cells (OSCs) whose blend film thickness is up to 270 nm, achieving an impressive performance of 11.1%. The significant improvement of OSCs after adding DRCN5T is due to the formation of an interconnected fibrous network with decreased π–π stacking and enhanced domain purity, in addition to the optimized vertical distribution of PTB7‐Th and PC₇₀BM, producing more effective charge separation, transport, and collection. The optimized morphology and performance are actually determined by the miscibility in different components, which can be quantitatively described by the Flory–Huggins interaction parameter of −0.80 and 2.94 in DRCN5T:PTB7‐Th and DRCN5T:PC₇₀BM blends, respectively. The findings in this work can potentially guide the selection of an appropriate third additive for high‐performance OSCs for the sake of large‐area printing and roll‐to‐roll fabrication from the view of miscibility.

A sulfur/nitrogen‐enriched polyimide (E‐PI) with high glass transition temperature (>200 °C) is synthesized and introduced as an interlayer for inverted‐type polymer:nonfullerene solar cells. The 3 nm‐thick E‐PI interlayers result in the improved efficiency and stability of poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b″]dithiophene))‐alt‐(5,5‐(1″,3″‐di‐2‐thienyl‐5″,7″‐bis(2‐ethylhexyl)benzo[1″,2″‐c:4″,5″‐c″]dithiophene‐4,8‐dione))] (PBDB‐T):3,9‐bis(6‐methyl‐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) solar cells due to the increased work function (electron mobility) of zinc oxide electron‐collecting buffer layers.

Herein, it is reported that sulfur/nitrogen‐enriched polyimide can act as a stable interlayer for inverted‐type polymer:nonfullerene solar cells because it improves the power conversion efficiency (PCE) and stability of the devices. The sulfur/nitrogen‐enriched polyimide (E‐PI) interlayers are prepared on the zinc oxide layers via the thermal imidization of corresponding films of soluble precursor polymer, poly(N‐(2‐((carboxymethyl)(2‐((5‴‐methyl‐[2,2″:5″,2″:5″,2‴‐quaterthiophen]‐5‐yl)amino)‐2‐oxoethyl)amino)ethyl)‐N‐(2‐(methylamino)‐2‐oxoethyl)glycine acid), which is synthesized from ethylenediaminetetraacetic dianhydride and 5,5‴‐diamino‐2,2″:5″,2″:5″,2‴‐quaterthiophene. The E‐PI films exhibit high glass transition temperature (≈204 °C) and broad optical absorption up to ≈1000 nm (absorption edge). Results show that the average PCE of polymer:nonfullerene solar cells is increased from 10.86% to 11.6% at the E‐PI thickness of 3 nm. In particular, the stability of polymer:nonfullerene solar cells is clearly improved by inserting the 3 nm‐thick E‐PI interlayers.

Nickel oxide (NiO _x ) as the most commonly used hole transport layer in inverted perovskite solar cells is doped by nitrogen for the first time, affording an obvious enhancement of average power conversion efficiency from 15.28% to 17.02%. This is primarily due to increased electrical conductivity and lowered valence band energy of the NiO _x film after nitrogen doping.

Nickel oxide (NiO _x ) is commonly used as a hole transport layer (HTL) in inverted‐structure (p‐i‐n) planar perovskite solar cells (PSCs), playing a critical role in the device performance. However, a solution‐processed NiO _x HTL usually suffers from low electrical conductivity, consequently resulting in an inefficient interfacial charge transport. Herein, a facile method is developed to prepare nitrogen‐doped NiO _x (N:NiO _x ), which is applied as a novel HTL in inverted PSCs for the first time, achieving a decent improvement in average power conversion efficiency (PCE) from 15.28% to 17.02%. The effects of nitrogen doping on the electrical conductivity and the energy band structure of NiO _x as well as the quality of CH₃NH₃PbI₃ perovskite layer atop are studied by a series of characterizations, revealing that nitrogen doping leads to increased electrical conductivity and lowered valence band energy of the NiO _x film, which are beneficial to interfacial hole transport. In addition, the trap density of the CH₃NH₃PbI₃ perovskite film atop N:NiO _x layer is reduced, prohibiting unfavorable charge recombination.

Publication date: 21 August 2019

Source: Joule, Volume 3, Issue 8

Author(s): Xiaopeng Zheng, Joel Troughton, Nicola Gasparini, Yuanbao Lin, Mingyang Wei, Yi Hou, Jiakai Liu, Kepeng Song, Zhaolai Chen, Chen Yang, Bekir Turedi, Abdullah Y. Alsalloum, Jun Pan, Jie Chen, Ayan A. Zhumekenov, Thomas D. Anthopoulos, Yu Han, Derya Baran, Omar F. Mohammed, Edward H. Sargent

Context & Scale

Summary

Graphical Abstract