shuhui

Protective coating: A tin‐based perovskite solar cell with significantly improved stability to oxidation was prepared by introducing hydroxybenzene sulfonic acid or a salt thereof as an antioxidant additive into the perovskite precursor solution. The resulting perovskite grains are encapsulated by a SnCl₂–additive complex layer.

Abstract

Tin‐based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin‐based PSCs because Sn²⁺ in perovskites can be easily oxidized to Sn⁴⁺ upon air exposure. Here we report the fabrication of air‐stable FASnI₃ solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl₂. The interaction between the sulfonate group and the Sn²⁺ ion enables the in situ encapsulation of the perovskite grains with a SnCl₂–additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin‐based PSCs.

Two new copper (II) phthalocyanine (CuPc) derivatives, namely CuPc‐Bu and CuPc‐OBu, are designed by molecular engineering of the non‐peripheral substituents of the Pc rings, and are further explored as dopant‐free hole‐transporting materials (HTMs) in perovskite solar cells (PSCs). The PSCs based on pristine CuPc‐OBu as HTMs afford a maximum power conversion efficiency of 17.6%, which is considerably higher than that of the devices with CuPc‐Bu (14.3%).

Abstract

Copper (II) phthalocyanines (CuPcs) have attracted growing interest as promising hole‐transporting materials (HTMs) in perovskite solar cells (PSCs) due to their low‐cost and excellent stability. However, the most efficient PSCs using CuPc‐based HTMs reported thus far still rely on hygroscopic p‐type dopants, which notoriously deteriorate device stability. Herein, two new CuPc derivatives are designed, namely CuPc‐Bu and CuPc‐OBu, by molecular engineering of the non‐peripheral substituents of the Pc rings, and applied as dopant‐free HTMs in PSCs. Remarkably, a small structural change from butyl groups to butoxy groups in the substituents of the Pc rings significantly influences the molecular ordering and effectively improves the hole mobility and solar cell performance. As a consequence, PSCs based on dopant‐free CuPc‐OBu as HTMs deliver an impressive power conversion efficiency (PCE) of up to 17.6% under one sun illumination, which is considerably higher than that of devices with CuPc‐Bu (14.3%). Moreover, PSCs containing dopant‐free CuPc‐OBu HTMs show a markedly improved ambient stability when stored without encapsulation under ambient conditions with a relative humidity of 85% compared to devices containing doped Spiro‐OMeTAD. This work thus provides a fundamental strategy for the future design of cost‐effective and stable HTMs for PSCs and other optoelectronic devices.

In article no. 1800217, Mingzhen Liu and co‐workers fabricate a Cs₂Ag‐BiBr₆ double perovskite film that is potentially desirable for lead‐free solar cell applications. The films exhibit high quality in terms of large compact grains, high uniformity, and long‐term stability.

Isomeric acceptors NBDTP‐F_out and NBDTP‐F_in, are synthesized for nonfullerene all‐small‐molecule solar cells (NF‐SMSCs). When blended with the molecular donor BDT3TR‐SF, NBDTP‐F_out achieves a high power conversion efficiency of 11.2% while NBDTP‐F_in shows almost no photovoltaic response. Detailed analyses indicate that the isomery‐dependent miscibility governs the performance, which provides an insight of molecular design for efficient NF‐SMSCs.

Abstract

Nonfullerene polymer solar cells develop quickly. However, nonfullerene small‐molecule solar cells (NF‐SMSCs) still show relatively inferior performance, attributing to the lack of comprehensive understanding of the structure–performance relationship. To address this issue, two isomeric small‐molecule acceptors, NBDTP‐F_out and NBDTP‐F_in, with varied oxygen position in the benzodi(thienopyran) (BDTP) core are designed and synthesized. When blended with molecular donor BDT3TR‐SF, devices based on the two isomeric acceptors show disparate photovoltaic performance. Fabricated with an eco‐friendly processing solvent (tetrahydrofuran), the BDT3TR‐SF:NBDTP‐F_out blend delivers a high power conversion efficiency of 11.2%, ranked to the top values reported to date, while the BDT3TR‐SF:NBDTP‐F_in blend almost shows no photovoltaic response (0.02%). With detailed investigations on inherent optoelectronic processes as well as morphological evolution, this performance disparity is correlated to the interfacial tension of the two combinations and concludes that proper interfacial tension is a key factor for effective phase separation, optimal blend morphology, and superior performance, which can be achieved by the “isomerization” design on molecular acceptors. This work reveals the importance of modulating the materials miscibility by interfacial‐tension‐oriented molecular design, which provides a general guideline toward efficient NF‐SMSCs.

Layer-edge device of two-dimensional hybrid perovskites

Layer-edge device of two-dimensional hybrid perovskites, Published online: 05 December 2018; doi:10.1038/s41467-018-07656-2

Ternary organic solar cells with improved power conversion efficiency (PCE) and ambient stability are developed by combining a nonfullerene acceptor and a fullerene acceptor. Such a ternary system is insensitive to the content of the third component, and PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of the binary reference device.

Abstract

The ternary structure that combines fullerene and nonfullerene acceptors in a photoactive layer is demonstrated as an effective approach for boosting the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, highly efficient ternary OSCs comprising a wide‐bandgap polymer donor (PBT1‐C), a narrow‐bandgap nonfullerene acceptor (IT‐2F), and a typical fullerene derivative (PC₇₁BM) are reported. It is found that the addition of PC₇₁BM into the PBT1‐C:IT‐2F blend not only increases the device efficiency up to 12.2%, but also improves the ambient stability of the OSCs. Detailed investigations indicate that the improvement in photovoltaic performance benefits from synergistic effects of increased photon‐harvesting, enhanced charge separation and transport, suppressed trap‐assisted recombination, and optimized film morphology. Moreover, it is noticed that such a ternary system exhibits excellent tolerance to the PC₇₁BM component, for which PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of PBT1‐C:IT‐2F binary reference device.

The rapid development in large‐area organic solar cells (OSCs) is reviewed. Materials requirements, modular designs, and printing methods for large‐area OSCs are discussed. By combining thick‐film material systems with efficient modular designs, and then by employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

Abstract

The printing of large‐area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small‐area devices is approaching 15%, whereas the PCE for large‐area devices has also surpassed 10% in a single cell with an area of ≈1 cm². Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large‐area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large‐area fabrication, including knife coating, slot‐die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick‐film material systems with efficient modular designs exhibiting low‐efficiency losses and employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

In article number 1802323, Chih‐Ping Chen and co‐workers demonstrate hydrophilic carbon nanodots efficient additives in perovskite solar cells (PSC). The p‐i‐n PSC device incorporating these additives demonstrated a power conversion efficiency of 20.2% and exhibited excellent air‐stability, maintaining high PCEs (25 °C and a humidity of 40%) for over 500 h.

The optical properties of 2D halide perovskite (C₆H₅C₂H₄NH₃)₂PbBr₄ are significantly modified by lattice compression. A pressure‐induced new broadband emission with a large Stokes shift is observed in 2D Pb‐Br perovskite by improving lattice distortion to increase exciton–phonon coupling, promoting its current and future optoelectronic device applications through improved materials‐by‐design.

Abstract

2D Ruddlesden–Popper halide perovskites, which incorporate hydrophobic organic interlayers to considerably improve environmental stability and optical properties diversity, have attracted substantial research attention for optoelectronic applications. The burgeoning broad emission arising from exciton self‐trapping of 2D perovskites shows a strong dependence on a deformable structure. Here, the pressure‐induced broadband emission of layered (001) Pb‐Br perovskite with a large Stokes shift in the visible region is observed by finely improving lattice distortion to increase exciton–phonon coupling under hydrostatic pressure. Band gap narrows ≈0.5 eV under modest pressure, mainly due to the large compressibility of the orientational organic layer, confirming that the bulky organic cations notably influence the structure and, in turn, the various properties of materials. Sequential amorphization of the organic and inorganic layer is confirmed by high pressure Raman and X‐ray diffraction measurements, suggesting the particularity of layered crystal structures. The mechanism constructed here offers a new route for tuning the optical properties of 2D perovskites.

Trivalent cations (Sb³⁺ and In³⁺) can be spontaneously distributed with a gradient in organometal halide perovskites from homogeneous precursors, because of their difference in ionic size and electrostatic interaction between the dopants and the host atoms. This phenomenon can facilitate charge separation and collection of photoelectrons, leading to excellent photovoltaic performance.

Abstract

A gradient heterosturcture is one of the basic methods to control the charge flow in perovskite solar cells (PSCs). However, a classical route for gradient heterosturctures is based on the diffusion technique, in which the guest ions gradually diffuse into the films from a concentrated source of dopants. The gradient heterosturcture is only accessible to the top side, and may be time consuming and costly. Here, the “intolerant” n‐type heteroatoms (Sb³⁺, In³⁺) with mismatched cation sizes and charge states can spontaneously enrich two sides of perovskite thin films. The dopants at specific sides can be extracted by a typical hole‐transport layer. Theoretical calculations and experimental observations both indicate that the optimized charge management can be attributed to the tailored band structure and interfacial electronic hybridization, which promote charge separation and collection. The strategy enables the fabrication of PSCs with a spontaneous graded heterojunction showing high efficiency. A champion device based on Sb³⁺ doped film shows a stabilized power‐conversion efficiency of 21.04% with a high fill factor of 0.84 and small hysteresis.

Ferromagnetic insulators, innately rare in nature, are important components for realizing dissipationless quantum electronic/spintronic devices and solid‐state quantum computing. Room‐temperature ferromagnetic insulating films are successfully synthesized by deliberate control of the cation ratio and ordering in oxide double perovskites Sr₂Fe_{1+ x} Re_{1− x} O₆, which can be used for future development of advanced quantum devices working at ambient temperature.

Abstract

Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, FMIs are innately rare to find in nature as ferromagnetism generally accompanies metallicity. Here, novel room‐temperature FMI films that are epitaxially synthesized by deliberate control of the ratio between two B‐site cations in the double perovskite Sr₂Fe_{1+ x} Re_{1‐ x} O₆ (−0.2 ≤ x ≤ 0.2) are reported. In contrast to the known FM metallic phase in stoichiometric Sr₂FeReO₆, an FMI state with a high Curie temperature (T _c ≈ 400 K) and a large saturation magnetization (M _S ≈ 1.8 µ_B f.u.⁻¹) is found in highly cation‐ordered Fe‐rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe³⁺Fe³⁺ and Fe³⁺Re⁶⁺ bonding states, which originate from the relatively excess Fe ions owing to the deficiency in Re ions. The emerging FMI state created by controlling cations in the oxide double perovskites opens the door to developing novel oxide quantum materials and spintronic devices.

A novel device structure of double‐side‐passivated perovskite solar cells is devised through intentionally distributing PbI₂ to both front/rear‐side surfaces and grain boundaries of perovskite film and a stabilized efficiency of 22% is achieved. Double‐side‐passivation effectively boosts the limits of open circuit voltage toward a record potential loss of 0.38 V for 1.53 eV‐bandgap perovskites.

An ideal crystal quality in the grain interior of perovskite polycrystalline films is well recognized; therefore, understanding interfacial impact and the ways to limit interfacial recombination is critical to fabricating highly efficient solar cells. In perovskite solar cells, PbI₂ has been used to passivate defects at grain boundaries, yet a systematic PbI₂ passivation engineering to boost the high‐performance perovskite solar cells has not been fully explored. Here, a novel device structure comprised of double‐side‐passivated perovskite solar cells (DSPC) is devised through intentionally distributing PbI₂ to both the front/rear‐side surfaces and grain boundaries of the formamidinium‐lead‐iodide‐based (FAPbI₃‐based) perovskite film. The minority carrier lifetime in double‐side‐passivated perovskite is extended to 1.1 μs with single‐exponential decay using time‐resolved photoluminescence. This result indicates a generic passivation effect of PbI₂ on perovskite interfaces, resembling SiO₂ passivation in silicon solar cells. Correspondingly, the best photovoltaic device with TiO₂‐based planar structure presents a stabilized efficiency of 22%. Moreover, DSPC effectively boosts the limits of open circuit voltages toward a record potential loss of 0.38 V for 1.53 eV‐bandgap perovskites. The architecture of double‐side‐passivated perovskite opens up new opportunities to exceed the efficiency of state‐of‐the‐art perovskite solar cells.

Melamine hydroiodide (MLAI) has been successfully introduced for preparing hetero structured MAPbI₃ perovskite for enhancing the photovoltaic performance and stability of perovskite solar cells in a robust humid ambient atmosphere (35 °C, 60–70% relative humidity). The power conversation efficiency of perovskite solar cells based on MLAI functionalized perovskite is 25% higher than that of pristine MAPbI₃ perovskite with nearly no hysteresis and high stability.

Despite the remarkable performance of organometallic halide perovskite solar cells (PSCs), their ultimate stability is still a major issue that inhibits the commercialization of this eminent technology. Herein, melamine hydroiodide (MLAI) is added to function methyl ammonium (CH3NH3⁺, MA⁺) lead iodide perovskite for fabricating structured perovskite with enhanced photovoltaic performance and stability in the harsh ambient atmosphere (35 °C, 60–70% relative humidity). Nearly no new phase formed even incorporated 25 mol.% MLAI induces the strain in the perovskite crystal structure. The MLAI‐structured perovskite film shows a denser and smoother surface than the pristine MAPbI₃ perovskite. Planar PSCs based on 2 mol.% MLAI‐functionalized perovskite show 17.2% power conversion efficiency with nearly no hysteresis which is much higher than pristine MAPbI₃ PSCs. Most importantly, the solar cell devices based on 2 mol.% MLAI‐functionalized perovskite still retain over 90% of the initial performance after being kept in ambient atmosphere for more than 560 h without encapsulation.

The energy level mismatch between a FA0.6MA0.4 Sn0.6Pb0.4I3 absorber and PEDOT:PSS based hole transporting layer are reduced and an improved V_OC over 50 mV and hence power conversion efficiencies of up to 15.85% are achieved.

Small bandgap Sn‐Pb mixed perovskite is generally regarded as the most promising structure to further enhance the power conversion efficiency of perovskite solar cells. However, the open circuit voltages (V _OCs) are usually lower than expected. In this work, by doping the generally used hole transporting layer (HTL) poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) with a perfluorinated ionomer (PFI), we can tune the work function of it from −5.02 to −5.19 eV. This reduces the energy level mismatch between the FA_0.6MA_0.4 Sn_0.6Pb_0.4I₃ (FAMA) absorber and HTL, giving rise to enhanced built‐in voltage and better carrier extraction. The V _OC improves to over 50 mV, up to 0.783 V, resulting in an improved champion power conversion efficiency (PCE) of 15.85%. Moreover, the devices based on modified HTLs show improved stability at maximum power point. These results demonstrate that energy level tuning of the HTL is a promising strategy for the improvement of the PCEs of Sn‐Pb mixed inverted PSCs, and the addition of PFI is an effective method to tune the work function of PEDOT:PSS.

Next‐generation solar cells consisting of organic materials are studied. To develop novel dyes for dye‐sensitized solar cells, the essential dye structures are explored to attain high efficiency. Additionally, the interfaces in the perovskite solar cells are characterized via electrochemical methods, and newly developed laser deposition methods for perovskite layers are discussed.

Abstract

Next‐generation organic solar cells such as dye‐sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are studied at the National Institute of Advanced Industrial Science and Technology (AIST), and their materials, electronic properties, and fabrication processes are investigated. To enhance the performance of DSSCs, the basic structure of an electron donor, π‐electron linker, and electron acceptor, i.e., D–π–A, is suggested. In addition, special organic dyes containing coumarin, carbazole, and triphenylamine electron donor groups are synthesized to find an effective dye structure that avoids charge recombination at electrode surfaces. Meanwhile, PSCs are manufactured using both a coating method and a laser deposition technique. The results of interfacial studies demonstrate that the level of the conduction band edge (CBE) of a compact TiO₂ layer is shifted after TiCl₄ treatment, which strongly affects the solar cell performance. Furthermore, a special laser deposition system is developed for the fabrication of the perovskite layers of PSCs, which facilitates the control over the deposition rate of methyl ammonium iodide used as their precursor.

Charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed tin–lead halide perovskite solar cells. It is found that bromine incorporation can passivate grain boundaries and lower the dark current density by two to three orders of magnitude. The champion cell shows an open‐circuit voltage deficit of 0.384 V and power conversion efficiency exceeding 19%.

Abstract

The unsatisfactory performance of low‐bandgap mixed tin (Sn)–lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all‐perovskite tandem solar cells. By analyzing dark‐current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed Sn–Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two–three orders of magnitude. By optimizing the Br concentration, low‐bandgap (1.272 eV) mixed Sn–Pb halide perovskite solar cells are fabricated with open‐circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The best‐performing device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low‐bandgap mixed Sn–Pb halide perovskite solar cells.