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Publication date: October 2020

Source: Nano Energy, Volume 76

Author(s): Dong Woo Lee, Dong Geun Jeong, Jong Hun Kim, Hyun Soo Kim, Gonzalo Murillo, Gwan-Hyoung Lee, Hyun-Cheol Song, Jong Hoon Jung

Herein, an ink‐composition engineering approach for high‐performing flexible slot‐die‐coated indium tin oxide (ITO)‐free organic solar cells is presented. Optimized large‐area devices (0.88 cm²) show a power conversion efficiency of up to 10.21%, which is an efficiency retention factor of 0.86 when compared to optimized small‐area spin‐coated devices.

The potential for commercialization of organic solar cells (OSCs) has vastly increased in recent years as the device efficiency for small‐scale laboratory OSCs has continuously increased. There are, however, still multiple challenges that need to be addressed and overcome. Among them, upscaling of the device manufacturing techniques to be compatible with the potential attributes of low cost must be the pinnacle. Herein, a pathway for upscaling with an ink‐engineering approach toward in‐air optimization of large‐area OSCs is presented. Optimized flexible indium tin oxide (ITO)‐free OSCs based on a PTB7‐TH:IEICO‐4F:PC₇₁BM ternary blend show efficiencies up to 10.2% (device active area 0.88 cm²), which is the highest value reported to date (for in‐air slot‐die‐coated devices). This is achieved through ink modifications and optimizations as well as electrode and active layer compositional optimizations, leading to an impressive efficiency retention of 0.86 compared to the in‐literature optimized small‐scale devices.

A new ethanol‐soluble ionic fullerene derivative, C₆₀RT₆, is synthesized to be an additive of ground TiO₂ nanoparticles (NPs) for preparing a room‐temperature‐processed nanocomposite electron transporting layer (ETL) (R‐Fu/Lt‐TiO₂) to improve the photovoltaic performance of the corresponding planar regular perovskite solar cell (PSC). Rigid and flexible PSC–based R‐Fu/Lt‐TiO₂ achieves the power conversion efficiency of 20% and 16.2%, respectively.

Room‐temperature‐processed TiO₂ (R‐Lt‐TiO₂) electron transporting layers (ETLs) possess low conductivity and connectivity, resulting in poor photovoltaic performance. Herein, an ethanol (EtOH)‐soluble, highly conducting fullerene derivative, C₆₀RT₆, was used as an additive for Lt‐TiO₂ ETLs. Room‐temperature processed nanocomposite ETL (R‐Fu/Lt‐TiO₂) is prepared simply by spin coating a C₆₀RT₆ and G‐TiO₂ NPs (TiO₂ nanoparticle prepared by grinding the bulk TiO₂ powder) mixture. R‐Fu/Lt‐TiO₂ has better aligned with the frontier orbitals of the FA_xMA_1−xPbI₃, better continuity, conductivity, flatness, and higher surface hydrophilicity compared to Lt‐TiO₂ ETL. Perovskite films spin coated on R‐Fu/Lt‐TiO₂ ETLs also have slightly larger grains and thickness compared to those deposited on Lt‐TiO₂. Perovskite solar cells (PSCs) based on a R‐Fu/Lt‐TiO₂ ETL possess higher power conversion efficiency (PCE, up to 20% on glass substrate), less (negligible) current hysteresis, and better long‐term stability compared to those using R‐Lt‐TiO₂ as an ETL. The flexible PSC (used indium tin oxide/polyethylene terephthalate (ITO/PET) as a substrate) with a R‐Fu/Lt‐TiO₂ ETL achieves a PCE of 18.06% and retains 90% of the initial PCE after 500 bending cycles with a bending radius of 6 mm. The PCE of the flexible cell with a Lt‐TiO₂ ETL is only 8.2%, and loses 60% of the initial value after 500 bending cycles.

Grazing incidence small‐ and wide‐angle X‐ray scattering (GISAXS and GIWAXS) are extensively used for the characterization of film morphology of organic solar cells (OSCs). Herein, the use of these techniques to find the effect of chemistry of active layer materials and different pre‐ and postprocessing conditions on the film morphology of OSCs is discussed.

In recent years, a rapid evolution of organic solar cells (OSCs) has been achieved by virtue of structural design of active layer materials and optimization of film morphology. Along with other characterization techniques, grazing incidence small‐ and wide‐angle X‐ray scattering (GISAXS and GIWAXS) have played significant role in deeper understanding of film morphology. Herein, the importance of these techniques is explained with examples from various aspects of OSCs. Different pre‐ and post‐processing conditions such as solvent effect, solvent additive, solvent, and thermal annealing are studied in the framework of these techniques. Moreover, the impact of donor:acceptor ratio and molecular weight of semiconductor on microstructure is also explored. Finally, the effect of chemical structure of organic semiconductors (both polymers and small molecules) on the film morphology is discussed. These techniques provide valuable information about crystallinity, phase separation, and domain size of nanostructured film morphology, which helps to optimize the film morphology and enhances the performance of OSCs. The role of these techniques will become more important as the mystery of film morphology still has to be solved.

In article number https://doi.org/10.1002/adfm.2020026392002639, Liang Li, Hongqiang Wang, and co‐workers demonstrate a double‐barrier strategy that not only blocks the invasion of moisture but also employs the permeated moisture to increase the moisture durability of perovskite films, which results in an n–i–p perovskite solar cell with moisture stability over 115 days (relative humidity of 70%) and a champion efficiency up to 21.34%.

Soft routed benzimidazole clubbed phenoxazine‐based organic ionic plastic crystals with iodide and bromide anions successfully introduced as hole transporting materials in perovskite solar cells yield power conversion efficiencies exceeding 18%, which represents the best alternative to existing spiro‐OMeTAD due to high conductivity and hole mobility with a safer, stable, and efficient system.

Abstract

Organic ionic plastic crystals (OIPCs) are synthesized through a simple metal‐free, cost‐effective approach. The strategized synchronization of electron‐rich phenoxazine with benzimidazolium iodide (OIPC‐I) and bromide (OIPC‐Br) salts lead to enhanced hole mobility and conductivity of OIPCs which is suitable for an efficient alternative to conventional organic hole transporting materials (HTMs) for stable perovskite solar cells (PSCs). The fabricated PSCs with OIPC‐I as hole transporting layer yielded a power conversion efficiency of 15.0% and 18.1% without and with additive (Li salt) respectively, which are comparable with spiro‐OMeTAD based devices prepared under similar conditions. Furthermore, the PSCs with OIPCs show good stability compared to the spiro‐OMeTAD with or without additives. Here, first time benzimidazolium‐based OIPCs have been used as an alternative organic HTM for perovskite solar cells, which opens a window for the design of effective OIPCs for highly efficient PSCs with long‐term stability.

A well‐designed inorganic–organic double hole transporting layer (HTL) based on inorganic CuSCN and organic polymer dithiophene‐benzene is developed. A perovskite solar cell with this dopant‐free HTL exhibits a very high power conversion efficiency of 22.0% (certified: 21.7%) and significantly improved thermal, humidity, and light stabilities compared to 2,2′,7,7′‐tetrakis(N ,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (Spiro‐OMeTAD) HTL‐based devices.

Abstract

Most of the high performance in perovskite solar cells (PSCs) have only been achieved with two organic hole transporting materials: 2,2′,7,7′‐tetrakis(N ,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (Spiro‐OMeTAD) and poly(triarylamine) (PTAA), but their high cost and low stability caused by the hygroscopic dopant greatly hinder the commercialization of PSCs. One effective alternative to address this problem is to utilize inexpensive inorganic hole transporting layer (i‐HTL), but obtaining high efficiency via i‐HTLs has remained a challenge. Herein, a well‐designed inorganic–organic double HTL is constructed by introducing an ultrathin polymer layer dithiophene‐benzene (DTB) between CuSCN and Au contact. This strategy not only enhances the hole extraction efficiency through the formation of cascaded energy levels, but also prevents the degradation of CuSCN caused by the reaction between CuSCN and Au electrode. Furthermore, the CuSCN layer also promotes the formation of a pinhole‐free and compact DTB over layer in the CuSCN/DTB structure. Consequently, the PSCs fabricated with this CuSCN/DTB layer achieves the power conversion efficiency of 22.0% (certified: 21.7%), which is among the top efficiencies for PSCs based on dopant‐free HTLs. Moreover, the fabricated PSCs exhibit high light stability under more than 1000 h of light illumination and excellent environmental stability at high temperature (85 °C) or high relative humidity (>60% RH).

The recent progress in flexible and stretchable organic solar cells (OSCs) is discussed. For flexible OSCs, the features of the commonly used flexible transparent electrodes and the relevant performance are selectively summarized and discussed. For stretchable OSCs, both the nonintrinsic and intrinsic processing methods are presented and discussed.

Abstract

Flexible and stretchable organic solar cells (OSCs) have attracted enormous attention due to their potential applications in wearable and portable devices. To achieve flexibility and stretchability, many efforts have been made with regard to mechanically robust electrodes, interface layers, and photoactive semiconductors. This has greatly improved the performance of the devices. State‐of‐the‐art flexible and stretchable OSCs have achieved a power conversion efficiency of 15.21% (16.55% for tandem flexible devices) and 13%, respectively. Here, the recent progress of flexible and stretchable OSCs in terms of their components and processing methods are summarized and discussed. The future challenges and perspectives for flexible and stretchable OSCs are also presented.

Solid‐state ¹⁹F magic angle spinning nuclear magnetic microscopy and elemental mapping are introduced to probe the structures of ternary and quaternary blends. The presence of the individual guest paths minimizes the influence on charge generation and transport of the host system, allowing cooperation of the parallel‐like subcells, producing impressive 17.2% efficiency via a quaternary strategy.

Abstract

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.

Rapid degradation of ion migration, measurement source‐induced damage, phase transition, and separation of perovskite materials hinder accurate evaluation by conventional characterization tools. Recent advanced characterization tools, such as cryogenic temperature assisted measurement, in situ observation, and multidimensional imaging/mapping are presented here that enable the correct diagnose perovskite properties.

Abstract

In the last 10 years, organic–inorganic hybrid perovskite solar cells have achieved unprecedented advances, to the point where they now exhibit extremely high efficiency. However, long‐term stability and areal scalability limitations impede the commercial application of perovskite materials, and appropriate diagnosistic tools have become necessary to evaluate perovskite materials. Characterization of perovskite materials is regularly misinterpretated, due to unique intrinsic and extrinsic factors: degradation from the measurement source, ion migration, phase transition, and separation. Herein, studies on perovskites are reviewed that have used advanced characterization tools to overcome characterization challenges. Cryogenic temperature assisted measurements mitigate degradation or phase transitions induced by the measurement source. In situ measurements can track the variation of perovskite materials depending on external stimuli. Spatial material properties are able to be evaluated by the use of multidimensional mapping techniques. An overview of these advanced characterization tools that can overcome the challenges associated with established tools provides the opportunity for further understanding perovskite materials and solving the remaining challenges on the road to commercialization.

The technology of pulsed laser irradiation in liquid from a solid target to liquid is pioneered, yielding liquid ternary supranano‐(<10 nm) alloys with a unique core–shell structure. The decoration of such supranano‐alloys as an electron mediator at grain boundaries promotes the electron extraction and transfer of the hybrid perovskite film of a perovskite solar cell and drives the efficiency up to 22.03%.

Abstract

Creating colloids of liquid metal with tailored dimensions has been of technical significance in nano‐electronics while a challenge remains for generating supranano (<10 nm) liquid metal to unravel the mystery of their unconventional functionalities. Present study pioneers the technology of pulsed laser irradiation in liquid from a solid target to liquid, and yields liquid ternary nano‐alloys that are laborious to obtain via wet‐chemistry synthesis. Herein, the significant role of the supranano liquid metal on mediating the electrons at the grain boundaries of perovskite films, which are of significance to influence the carriers recombination and hysteresis in perovskite solar cells, is revealed. Such embedding of supranano liquid metal in perovskite films leads to a cesium‐based ternary perovskite solar cell with stabilized power output of 21.32% at maximum power point tracing. This study can pave a new way of synthesizing multinary supranano alloys for advanced optoelectronic applications.

Ambipolar black phosphorene (BP) nanosheets with tailored thicknesses concurrently enhance carrier extraction at both the electron‐transport layer/perovskite and hole‐transport layer/perovskite interfaces for high‐efficiency perovskite solar cells, demonstrating the appealing implementation of BP as a dual‐functional carrier‐transport material for a diversity of optoelectronic devices, including solar cells, photodetectors, sensors, light‐emitting diodes, etc.

Abstract

2D black phosphorene (BP) carries a stellar set of physical properties such as conveniently tunable bandgap and extremely high ambipolar carrier mobility for optoelectronic devices. Herein, the judicious design and positioning of BP with tailored thickness as dual‐functional nanomaterials to concurrently enhance carrier extraction at both electron transport layer/perovskite and perovskite/hole transport layer interfaces for high‐efficiency and stable perovskite solar cells is reported. The synergy of favorable band energy alignment and concerted cascade interfacial carrier extraction, rendered by concurrent positioning of BP, delivered a progressively enhanced power conversion efficiency of 19.83% from 16.95% (BP‐free). Investigation into interfacial engineering further reveals enhanced light absorption and reduced trap density for improved photovoltaic performance with BP incorporation. This work demonstrates the appealing characteristic of rational implementation of BP as dual‐functional transport material for a diversity of optoelectronic devices, including photodetectors, sensors, light‐emitting diodes, etc.

In article number https://doi.org/10.1002/adma.2020024952002495, Yang Li, Jianrong Qiu, Gang Han, and co‐workers report highly thermotolerant metal halide perovskite solids that maintain outstanding quantum efficiency after high‐temperature heating. These perovskite solids enable the construction of a white‐light‐emitting diode that exhibits superior working life under high working current.

Contact resistance and intrinsic gain are important performance indicators of organic field‐effect transistors. Staggered‐structure devices based on highly crystallized monolayer organic semiconductor provide outstanding contact, carrier mobility, and intrinsic gain. Organic 2D materials can potentially play an important role in next‐generation flexible electronics.

Abstract

The contact resistance limits the downscaling and operating range of organic field‐effect transistors (OFETs). Access resistance through multilayers of molecules and the nonideal metal/semiconductor interface are two major bottlenecks preventing the lowering of the contact resistance. In this work, monolayer (1L) organic crystals and nondestructive electrodes are utilized to overcome the abovementioned challenges. High intrinsic mobility of 12.5 cm² V⁻¹ s⁻¹ and Ohmic contact resistance of 40 Ω cm are achieved. Unlike the thermionic emission in common Schottky contacts, the carriers are predominantly injected by field emission. The 1L‐OFETs can operate linearly from V _DS = −1 V to V _DS as small as −0.1 mV. Thanks to the good pinch‐off behavior brought by the monolayer semiconductor, the 1L‐OFETs show high intrinsic gain at the saturation regime. At a high bias load, a maximum current density of 4.2 µA µm⁻¹ is achieved by the only molecular layer as the active channel, with a current saturation effect being observed. In addition to the low contact resistance and high‐resolution lithography, it is suggested that the thermal management of high‐mobility OFETs will be the next major challenge in achieving high‐speed densely integrated flexible electronics.

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): Junsheng Luo, Jianxing Xia, Hua Yang, Chunlin Sun, Ning Li, Haseeb Ashraf Malik, Hongyu Shu, Zhongquan Wan, Haoli Zhang, Christoph J. Brabec, Chunyang Jia

Publication date: 19 August 2020

Source: Joule, Volume 4, Issue 8

Author(s): Caleb C. Boyd, R. Clayton Shallcross, Taylor Moot, Ross Kerner, Luca Bertoluzzi, Arthur Onno, Shalinee Kavadiya, Cullen Chosy, Eli J. Wolf, Jérémie Werner, James A. Raiford, Camila de Paula, Axel F. Palmstrom, Zhengshan J. Yu, Joseph J. Berry, Stacey F. Bent, Zachary C. Holman, Joseph M. Luther, Erin L. Ratcliff, Neal R. Armstrong

Publication date: 19 August 2020

Source: Joule, Volume 4, Issue 8

Author(s): Ening Gu, Xiaofeng Tang, Stefan Langner, Patrick Duchstein, Yicheng Zhao, Ievgen Levchuk, Violetta Kalancha, Tobias Stubhan, Jens Hauch, Hans Joachim Egelhaaf, Dirk Zahn, Andres Osvet, Christoph J. Brabec

A low‐temperature crystallization strategy of CsPbIBr₂ perovskite solar cells is reported. The additive n‐butylammonium iodide (BAI) is incorporated into the perovskite precursor to improve crystallinity, optimize morphology, and passivate defects at 160 °C. As a result, a high‐level PCE of 10.78% with a high open‐circuit voltage (V _OC) of 1.25 V is achieved.

Inorganic cesium lead halide perovskite solar cells (PSCs) have been widely explored due to their outstanding thermal stability and photovoltaic performance. However, the application and development of CsPbIBr₂‐based PSCs is still hindered by major challenges such as high fabrication temperature and large voltage loss. To address these difficulties, additive engineering is conducted using n‐butylammonium iodide (BAI). It is found that it not only improves the crystallization and morphology of perovskite layers but also substantially decreases the annealing temperature. In addition, the BAI incorporation decreases trap state density and restrains nonradiative recombination. As such, a high power conversion efficiency (PCE) of 10.78% is achieved, 21% higher compared with that of the control sample (8.88%). It should be noted that this is particularly high for the CsPbIBr₂ PSCs fabricated at low temperatures (<200 °C) that are required for flexible devices based on polymeric substrates.

An “S‐shaped, hook‐like” naphthalene diimide derivate, NDI‐BN, is adopted as a cathode interface layer in inverted perovskite solar cells and good power conversion efficiency of 21.32% with enhanced stability is achieved. The relationship between the molecular packing motif of the organic interface layer and the interfacial degradation mechanism is explored.

Abstract

Ion migration induced interfacial degradation is a detrimental factor for the stability of perovskite solar cells (PSCs) and hence requires special attention to address this issue for the development of efficient PSCs with improved stability. Here, an “S‐shaped, hook‐like” organic small molecule, naphthalene diimide derivative (NDI‐BN), is employed as a cathode interface layer (CIL) to tailor the [6,6]‐phenylC61‐butyric acid methylester (PCBM)/Ag interface in inverted PSCs. By realizing enhanced electron extraction capability via the incorporation of NDI‐BN, a peak power conversion efficiency of 21.32% is achieved. Capacitance–voltage measurements and X‐ray photoelectron spectroscopy analysis confirmed an obvious role of this new organic CIL in successfully blocking ionic diffusion pathways toward the Ag cathode, thereby preventing interfacial degradation and improving device stability. The molecular packing motif of NDI‐BN further unveils its densely packed structure with π–π stacking force which has the ability to effectually hinder ion migration. Furthermore, theoretical calculations reveal that intercalation of decomposed perovskite species into the NDI clusters is considerably more difficult compared with the PCBM counterparts. This substantial contrast between NDI‐BN and PCBM molecules in terms of their structures and packing fashion determines the different tendencies of ion migration and unveils the superior potential of NDI‐BN in curtailing interfacial degradation.

Recent progress of tin and mixed Pb–Sn halide perovskite solar cells is summarized, including an introduction of device structures, fabrication methods, strategies to improve both performance and stability, and an outlook of pure tin‐based halide, mixed Pb–Sn halide, and monolithic all‐perovskite tandem solar cells.

Abstract

Metal halide perovskites have recently attracted enormous attention for photovoltaic applications due to their superior optical and electrical properties. Lead (Pb) halide perovskites stand out among this material series, with a power conversion efficiency (PCE) over 25%. According to the Shockley–Queisser (SQ) limit, lead halide perovskites typically exhibit bandgaps that are not within the optimal range for single‐junction solar cells. Partial or complete replacement of lead with tin (Sn) is gaining increasing research interest, due to the promise of further narrowing the bandgaps. This enables ideal solar utilization for single‐junction solar cells as well as the construction of all‐perovskite tandem solar cells. In addition, the usage of Sn provides a path to the fabrication of lead‐free or Pb‐reduced perovskite solar cells (PSCs). Recent progress in addressing the challenges of fabricating efficient Sn halide and mixed lead–tin (Pb–Sn) halide PSCs is summarized herein. Mixed Pb–Sn halide perovskites hold promise not only for higher efficiency and more stable single‐junction solar cells but also for efficient all‐perovskite monolithic tandem solar cells.

Judicious incorporation of ambiopolar black phosphorene with tailored thickness to concurrently impart electron and hole extractions in perovskite solar cells is reported by Jinsong Huang, Zhiqun Lin, and co‐workers in article number https://doi.org/10.1002/adma.2020009992000999. This work underpins the potential implementation of black phosphorene as a dual‐functional transport material for a diversity of optoelectronic devices, including photodetectors, sensors, and light‐emitting diodes.

An ultrathin polyelectrolyte PEIE is Inserted underneath of SnO₂ layer to form a double‐layered PEIE/SnO₂ electron transport composite. Work function and surface energy of the SnO₂ top surface is lowered. This reduces energy level mismatch at the interface for a better electron transport and induces large grain sizes in perovskite layers. Photovoltaic performance of the device is substantially improved.

Abstract

A double‐layered poly(ethylenimine) ethoxylated (PEIE)/SnO₂ composite structure, the ultrathin PEIE in contact with an indium‐tin‐oxide electrode, and an SnO₂ layer interfaced with the perovskite, is developed as an electron‐transport layer (ETL) in the preparation of perovskite solar cells. The surface energy and the work function of the top SnO₂ side of the composite ETL can be finely adjusted by tuning the underneath polyelectrolyte PEIE layer. These control the nucleation process in the crystallization of the perovskite layer and reduce the energy level mismatch between the electron transport and perovskite layers. High performance perovskite solar cells having a certified power conversion efficiency of 21.3%, with negligible hysteresis are achieved.

A correlative study investigates the influence of the novel KI‐passivation treatment on halide perovskite solar cell materials. By comparing the local electrical and chemical properties using an array of high‐spatial resolution imaging techniques, this research shows the spatial distribution of excess passivating material and links the nanoscale properties with macroscopic device performance.

Abstract

Perovskite semiconductors are an exciting class of materials due to their promising performance outputs in photovoltaic devices. To boost their efficiency further, researchers introduce additives during sample synthesis, such as KI. However, it is not well understood how KI changes the material and, often, leaves precipitants. To fully resolve the role of KI, multiple microscopy techniques are applied and the electrical and chemical behavior of a Reference (untreated) and a KI‐treated perovskite are compared. Upon correlation between electrical and chemical nanoimaging techniques, it is discovered that these local properties are linked to the macroscopic voltage enhancement of the KI‐treated perovskite. The heterogeneity revealed in both the local electrical and chemical responses indicates that the additive partially migrates to the surface, yet surprisingly does not deteriorate the performance locally, rather, the voltage response homogeneously increases. The research presented within provides a diagnostic methodology, which connects the nanoscale electrical and chemical properties of materials, relevant to other perovskites, including multication and Pb‐free alternatives.