Chen Weijie

The progress regarding development of solar cells and energy storage devices on paper substrates, where one or more of the main material layers are deposited via solution processing or printing, is reviewed. Paper is not only environmentally friendly and recyclable, but also thin, flexible, lightweight, biocompatible, inexpensive, and appealing as a substrate in the field of flexible printed electronics.

Abstract

Paper is a flexible material, commonly used for information storage, writing, packaging, or specialized purposes. It also has strong appeal as a substrate in the field of flexible printed electronics. Many applications, including safety, merchandising, smart labels/packing, and chemical/biomedical sensors, require an energy source to power operation. Here, progress regarding development of photovoltaic and energy storage devices on cellulosic substrates, where one or more of the main material layers are deposited via solution processing or printing, is reviewed. Paper can be used simply as the flexible substrate or, exploiting its porous fiber‐like nature, as an active film by infiltration or copreparation with electronic materials. Solar cells with efficiencies of up to 9% on opaque substrates and 13% on transparent substrates are demonstrated. Recent developments in paper‐based supercapacitors and batteries are also reviewed with maximum achieved capacity of 1350 mF cm⁻² and 2000 mAh g⁻¹, respectively. Analyzing the literature, it becomes apparent that more work needs to be carried out in continuing to improve peak performance, but especially stability and the application of printing techniques, even roll‐to‐roll processing, over large areas. Paper is not only environmentally friendly and recyclable, but also thin, flexible, lightweight, biocompatible, and inexpensive.

A self‐powered photodetector based on a gradient O‐doped CdS nanorod array and perovskite is presented. Compared to the pristine CdS/perovskite, the optimal gradient O‐doped CdS/perovskite shows increased on–off ratio, faster response speed, and higher detectivity. The improvement is attributed to the fact that gradient band bending promotes the photogenerated carrier transfer and hinders the recombination at the interface.

Abstract

Self‐powered photodetectors are highly desired to meet the great demand in applications of sensing, communication, and imaging. Manipulating the carrier separation and recombination is critical to achieve high performance. In this paper, a self‐powered photodetector based on the integrated gradient O‐doped CdS nanorod array and perovskite is presented. Through optimizing the degree of continuous built‐in band bending in the gradient‐O CdS, the photodetector demonstrates a remarkable detectivity of 2.1 × 10¹³ Jones. Under the self‐powered voltage mode, the responsivity can be as high as 0.48 A W⁻¹, and the rise and decay time are 0.54/2.21 ms. The comprehensive performance is comparable and even better than reported perovskite and other types of self‐powered photodetectors. The improved mechanism reveals that the gradient band bending promotes the photogenerated carrier transfer and hinders the recombination at the interface.

A facial and efficient method is demonstrated to simultaneously improve the efficiency and stability of the perovskite solar cells (PSCs) based on TiO₂ nanorod arrays by inserting uniform CdS shells. After optimizing the CdS shells, the PSCs achieve power conversion efficiency up to 17.71%, with current density of 22.36 mA cm⁻², V _oc of 1.05 V, and FF of 0.75.

Abstract

Both demands of stability and efficiency constitute the gargantuan barriers of perovskite solar cells (PSCs), hampering the academic communities and industrial production. Herein, it is demonstrated that, after depositing optimized CdS shell layers on TiO₂ nanorod arrays (NAs) by a simple and rapid chemical bath method at room temperature, the efficiency and stability of PSCs are simultaneously enhanced. The PSCs based on TiO₂/CdS core–shell NAs achieve higher power conversion efficiency up to 17.71%, compared to 15.93% of the pristine TiO₂ NAs‐based cells. Especially, the stability of the PSCs is prominently improved after optimized CdS layer modification without encapsulation. The significant enhancement of both efficiency and stability are mainly ascribed to that the type‐II structure of TiO₂/CdS coaxial nanorods can suppress recombination, and the oxygen vacancies on TiO₂ surfaces are not directly contacted with perovskite layers. The surface modification on TiO₂ NAs opens up an alternative approach toward improving the performance and stability of PSCs.

DMSO‐molecule‐control is demonstrated to enhance the morphological and electronic qualities of Cs‐(FAPbI₃)_0.85(MAPbBr₃)_0.15 absorbers, thus increasing the efficiency (reaching 19.4%) and boosting the long‐term stability (retaining 90% of the initial efficiency after 50 days in ambient air) of 1 cm² sized planar perovskite solar cells.

While interfacial and grain‐boundary passivation presently attract enormous research interest for perovskite solar cells (PSCs), the improvement of Cs‐(FAPbI₃)_X(MAPbBr₃)_Y bulk quality still lacks systematical study, especially for constructing polycrystalline layers in planar configurations. Here, a DMSO‐molecule‐process for improving the quality of Cs‐(FAPbI₃)_0.85(MAPbBr₃)_0.15 is developed, where the molar ratio of precursors, the kind of anti‐solvents, and speed‐time profiles are found critical. The optimized treatment significantly enhanced the crystal orientation, grain size, surface roughness, photo‐response, carrier lifetime, and contact potential difference of absorbers. Cs‐(FAPbI₃)_0.85(MAPbBr₃)_0.15 absorbers also present superior charge transport, as well as reduced carrier recombination and decreased trap densities via DMSO‐molecule‐control, enabling performance improvement on both long‐term stability and photovoltaic parameters of 1 cm² PSCs. Champion planar cells demonstrated a power conversion efficiency (PCE) of 21.07% (0.159 cm²) and PCE of 19.4% (1 cm²) with negligible hysteresis. Moreover, 1 cm² devices retained 90% of initial PCE after aging 50 days in ambient air.

Gravure printing for flexible perovskite solar cells is presented. A perovskite layer is successfully printed based on both one‐ and two‐step processes. The all‐printed flexible perovskite solar cells fabricated by sequential gravure printing of hole‐transporting, perovskite, and electron‐transporting layers exhibit 17.2% champion efficiency. Remarkably, partly roll‐to‐roll processed, flexible perovskite solar cells show 9.7% champion efficiency.

Abstract

Recent advances in perovskite solar cells (PSCs) have resulted in greater than 23% efficiency with superior advantages such as flexibility and solution‐processability, allowing PSCs to be fabricated by a high‐throughput and low‐cost roll‐to‐roll (R2R) process. The development of scalable deposition processes is crucial to realize R2R production of flexible PSCs. Gravure printing is a promising candidate with the benefit of direct printing of the desired layer with arbitrary shape and size by using the R2R process. Here, flexible PSCs are fabricated by gravure printing. Printing inks and processing parameters are optimized to obtain smooth and uniform films. SnO₂ nanoparticles are uniformly printed by reducing surface tension. Perovskite layers are successfully formed by optimizing the printing parameters and subsequent antisolvent bathing. 2,2′,7,7′‐Tetrakis‐(N,N‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene is also successfully printed. The all‐gravure‐printed device exhibits 17.2% champion efficiency, with 15.5% maximum power point tracking efficiency for 1000 s. Gravure‐printed flexible PSCs based on a two‐step deposition of perovskite layer are also demonstrated. Furthermore, a R2R process based on the gravure printing is demonstrated. The champion efficiency of 9.7% is achieved for partly R2R‐processed PSCs based on a two‐step fabrication of the perovskite layer.

An air knife–assisted recrystallization method is developed to fabricate high‐quality perovskite film in air under daily variable weather. The presented method is a root fabrication based on a bath‐immersion process, offering useful insights for fabricating highly efficient ambient‐process perovskite solar cells toward real production and paves a way for dim‐light harvesting and recycling.

Abstract

The photovoltaic performance of perovskite solar cells is highly dependent on the control of morphology and crystallization of perovskite film, which usually requires a controlled atmosphere. Therefore, fully ambient fabrication is a desired technology for the development of perovskite solar cells toward real production. Here, an air‐knife assisted recrystallization method is reported, based on a simple bath‐immersion to prepare high‐quality perovskite absorbers. The resulted film shows a strong crystallinity with pure domains and low trap‐state density, which contribute to the device performance and stability. The proposed method can operate in a wide process window, such as variable relative humidity and bath‐immersion conditions, demonstrating a power conversion efficiency over 19% and 27% under 1 sun and 500–2000 lux dim‐light illumination respectively, which is among the highest performance of ambient‐process perovskite solar cells.

Highly efficient perovskite light‐emitting diodes with external quantum efficiency over 15.2% are achieved through control of surface termination. Excess bulky organoammonium halide, the choice of which is found to be important, is used to suppress grain growth. Also, a methylammonium iodide excess is shown to aid surface termination and passivation.

Abstract

Hybrid organic–inorganic metal halide perovskites are particularly promising for light‐emitting diodes (LEDs) due to their attractive optoelectronic properties such as wavelength tunability, narrow emission linewidth, defect tolerance, and high charge carrier mobility. However, the undercoordinated Pb and halide at the perovskite nanocrystal (NC) surface causes traps and nonradiative recombination. In this work, the external quantum efficiency of iodide‐based perovskite LEDs is boosted to greater than 15%, with an emission wavelength at 750 nm, by engineering the perovskite NC surface stoichiometry and chemical structure of bulky organoammonium ligands. To the stoichiometric precursor solution for the 3D bulk perovskite, 20% molar ratio of methylammonium iodide is added in addition to 20% excess bulky organoammonium iodide to ensure that the NC surface is organoammonium terminated as the crystal size is decreased to 5–10 nm. This combination ensures minimal undercoordinated Pb and halide on the surface, avoids 2D phases, and acts to provide nanosized perovskite grains which allow for smooth and pinhole‐free films. As a result of time‐resolved photoluminescence (PL) and PL quantum yield measurements, it is possible to demonstrate that this surface modification increases the radiative recombination rate while reducing the nonradiative rate.

A novel wide‐bandgap nonfullerene acceptor TfIF‐4FIC is synthesized. PBDB‐T‐2F:TfIF‐4FIC‐based organic solar cell acquires a power conversion efficiency (PCE) of 13.1%, a high open‐circuit voltage of 0.98 V, which is the best performed device with bandgap larger than 1.60 eV. When using PBDB‐T‐2F:TfIF‐4FIC as front cell and PTB7‐Th:PCDTBT:IEICO‐4F as back cell to construct tandem device, PCE of 15% is achieved.

Abstract

A tandem organic solar cell (OSC) is a valid structure to widen the photon response range and suppress the transmission loss and thermalization loss. In the past few years, the development of low‐bandgap materials with broad absorption in long‐wavelength region for back subcells has attracted considerable attention. However, wide‐bandgap materials for front cells that have both high short‐circuit current density (J _SC) and open‐circuit voltage (V _OC) are scarce. In this work, a new fluorine‐substituted wide‐bandgap small molecule nonfullerene acceptor TfIF‐4FIC is reported, which has an optical bandgap of 1.61 eV. When PBDB‐T‐2F is selected as the donor, the device offers an extremely high V _OC of 0.98 V, a high J _SC of 17.6 mA cm⁻², and a power conversion efficiency of 13.1%. This is the best performing acceptor with such a wide bandgap. More importantly, the energy loss in this combination is 0.63 eV. These properties ensure that PBDB‐T‐2F:TfIF‐4FIC is an ideal candidate for the fabrication of tandem OSCs. When PBDB‐T‐2F:TfIF‐4FIC and PTB7‐Th:PCDTBT:IEICO‐4F are used as the front cell and the back cell to construct tandem solar cells, a PCE of 15% is obtained, which is one of best results reported to date in the field of organic solar cells.

Indirect tail states formation by thermal-induced polar fluctuations in halide perovskites

Indirect tail states formation by thermal-induced polar fluctuations in halide perovskites, Published online: 29 January 2019; doi:10.1038/s41467-019-08326-7

Solution‐cast lead iodide films can exhibit different metastable solvated states in the presence of DMF. Using in situ diagnostics, it is shown that conversion of PbI₂ to MAPbI₃ from its crystalline solvated state can occur spontaneously at room temperature and lead to high‐quality perovskite films with reduced trap state density and a high power conversion efficiency.

Abstract

Producing high efficiency solar cells without high‐temperature processing or use of additives still remains a challenge with the two‐step process. Here, the solution processing of MAPbI₃ from PbI₂ films in N,N‐dimethylformamide (DMF) is investigated. In‐situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) measurements reveal a sol–gel process involving three PbI₂‐DMF solvate complexes—disordered (P₀) and ordered (P₁, P₂)—prior to PbI₂ formation. When the appropriate solvated state of PbI₂ is exposed to MAI (methylammonium Iodide), it can lead to rapid and complete room temperature conversion into MAPbI₃ with higher quality films and improved solar cell performance. Complementary in‐situ optical reflectance, absorbance, and quartz crystal microbalance with dissipation (QCM‐D) measurements show that dry PbI₂ can take up only one third of the MAI taken up by the solvated‐crystalline P₂ phase of PbI₂, requiring additional annealing and yet still underperforming. The perovskite solar cells fabricated from the ordered P₂ precursor show higher power conversion efficiency (PCE) and reproducibility than devices fabricated from other cases. The average PCE of the solar cells is greatly improved from 13.2(±0.53)% (from annealed PbI₂) to 15.7(±0.35)% (from P₂) reaching up to 16.2%. This work demonstrates the importance of controlling the solvation of PbI₂ as an effective strategy for the growth of high‐quality perovskite films and their application in high efficiency and reproducible solar cells.

A high‐efficiency parallel‐like ternary organic photovoltaic device is developed through synergetic effects among a wide‐bandgap donor polymer, a narrow‐bandgap nonfullerene acceptor, and fullerene acceptors. Morphological optimization of the ternary devices via the incorporation of fullerenes yields simultaneous enhancement of the charge generation and extraction. An efficiency of 12.1% at an energy loss of 0.61 eV is realized.

Abstract

Ternary organic photovoltaic (OPV) devices with multiple light‐absorbing active materials have emerged as an efficient strategy for realizing further improvements in the power conversion efficiency (PCE) without building complex multijunction structures. However, the third component often acts as recombination centers and, hence, the optimization of ternary blend morphology poses a major challenge to improving the PCE of these devices. In this work, the performance of OPVs is enhanced through the morphological modification of nonfullerene acceptor (NFA)‐containing binary active layers. This modification is achieved by incorporating fullerenes into the layers. The uniformly dispersed fullerenes are sufficiently continuous and successfully mediate the ordering of NFA without charge or energy transfer. Owing to the simultaneous improvement in the charge generation and extraction, the PCE (12.1%) of these parallel‐linked ternary devices is considerably higher than those of the corresponding binary devices (9.95% and 7.78%). Moreover, the additional energy loss of the ternary device is minimized, compared with that of the NFA‐based binary device, due to the judicious control of the effective donor:acceptor composition of the ternary blends.

Zwitterions have emerged as a novel class of materials for organic/perovskite solar cells, light‐emitting devices, and lithium ion batteries. These materials have demonstrated tremendous performance in the enabling of highly efficient devices. The mechanism of action, structure–property relationships, and more device applications of zwitterionic materials must be explored to exploit the potential of these materials to the full.

Abstract

Zwitterions, a class of materials that contain covalently bonded cations and anions, have been extensively studied in the past decades owing to their special features, such as excellent solubility in polar solvents, for solution processing and dipole formation for the transfer of carriers and ions. Recently, zwitterions have been developed as electrode modifiers for organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light‐emitting devices (OLEDs), as well as electrolyte additives for lithium ion batteries (LIBs). With the rapid advances of zwitterionic materials, high‐performance devices have been constructed with enhanced efficiencies by introducing them as interface layers and electrolyte additives. In this review, recent progress in OSCs, PVSCs, OLEDs, and LIBs by using zwitterions is highlighted. The authors also elaborate the role of various zwitterionic materials as interfacial layers and additives for highly efficient OSCs, PVSCs, OLEDs, and LIBs. This article presents an overview of device performance of zwitterionic materials. The structure–property relationship is also discussed. Finally, the prospects of zwitterion materials are also addressed.

Methylammonium lead iodide (MAPbI₃) exhibits exceptional photovoltaic performance, but there remains substantial controversy over the existence and impact of ferroelectricity on the photovoltaic response. We confirm ferroelectricity in MAPbI₃ single crystals and demonstrate mediation of the electronic response by ferroelectric domain engineering. The ferroelectric response sharply declines above 57°C, consistent with the tetragonal-to-cubic phase transition. Concurrent band excitation piezoresponse force microscopy–contact Kelvin probe force microscopy shows that the measured response is not dominated by spurious electrostatic interactions. Large signal poling (>16 V/cm) orients the permanent polarization into large domains, which show stabilization over weeks. X-ray photoemission spectroscopy results indicate a shift of 400 meV in the binding energy of the iodine core level peaks upon poling, which is reflected in the carrier concentration results from scanning microwave impedance microscopy. The ability to control the ferroelectric response provides routes to increase device stability and photovoltaic performance through domain engineering.

A thin layer of polyethylene oxide (PEO) is introduced to modify the energy level alignment at the interface between an FA_0.8Cs_0.2PbI_2.64Br_0.36 perovskite and a carbon electrode. The PEO‐modified perovskite cell shows 22% increase in power conversion efficiency and enhanced stability keeping 77% of the initial value after being aged for 192 h under the conditions of 85 °C and 85% humidity without encapsulation.

Abstract

Perovskite solar cells (PSCs) have attracted great attention in the past few years due to their rapid increase in efficiency and low‐cost fabrication. However, instability against thermal stress and humidity is a big issue hindering their commercialization and practical applications. Here, by combining thermally stable formamidinium–cesium‐based perovskite and a moisture‐resistant carbon electrode, successful fabrication of stable PSCs is reported, which maintain on average 77% of the initial value after being aged for 192 h under conditions of 85 °C and 85% relative humidity (the “double 85” aging condition) without encapsulation. However, the mismatch of energy levels at the interface between the perovskite and the carbon electrode limits charge collection and leads to poor device performance. To address this issue, a thin‐layer of poly(ethylene oxide) (PEO) is introduced to achieve improved interfacial energy level alignment, which is verified by ultraviolet photoemission spectroscopy measurements. Indeed as a result, power conversion efficiency increases from 12.2% to 14.9% after suitable energy level modification by intentionally introducing a thin layer of PEO at the perovskite/carbon interface.

The approaches and the consequences of lead replacement in lead halide perovskite solar cells are summarized. The theoretical understanding of the electronic, optical, and defect properties of lead and lead‐free halide perovskites and perovskite derivatives is reviewed, explaining why all reported lead‐free perovskite solar cells underperform compared to lead halide perovskite solar cells.

Abstract

Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb‐free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb‐free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature. The theoretical understanding explains well why Pb halide perovskites exhibit superior photovoltaic properties, but Pb‐free perovskites and perovskite derivatives do not.

A novel thermal radiated hot‐cast method (THCM) is specially designed to fabricate highly efficient perovskite solar cells (PSCs) in ambient air. THCM creates constant temperature and ultralow relative humidity fields for perovskite growth, and is a universal protocol suitable for both inverted and regular PSCs, of which the champion power conversion efficiency of 17.2 and 19.1% is achieved, respectively.

Perovskite solar cells (PSCs) have developed rapidly in the past few years. However, highly efficient PSCs prepared in ambient air have remained intractable, since the crystallization and film morphology of perovskite are highly sensitive to moisture. Here, a thermal radiated hot‐cast method (THCM) to prepare high quality perovskite films in ambient air is introduced. The proposed THCM not only eliminates the temperature gradient across the perovskite film, but also forms a significantly reduced and constant relative humidity field at the local space above the substrate (ca. 6%); these conditions result in a smooth, compact, oriented perovskite film with largely reduced grain boundaries. THCM is a universal protocol, based on the application to the devices with both inverted and regular architectures, and it enables improved J–V performance with largely reduced hysteresis. The champion power conversion efficiencies of 17.2% for inverted and 19.1% for regular devices are achieved by THCM. These are comparable to the top efficiencies of fully air‐processed PSCs, demonstrating that THCM is a promising protocol for commercialization of PSCs in the near future.