Ligang Yuan

In article number https://doi.org/10.1002/admi.2019007181900718, Kai Wang, Bin Hu, and co‐workers report spin‐polarized electronic transport through Ni/CH₃NH₃PbI_3−x Cl _x spinterfaces at room temperature. Both anisotropic magnetoresistance (AMR) and spin‐valve based magnetoresistance (MR) are highly decided by the formations of the spinterfaces. The spin accumulation and the change of magnetic moment are further studied by the capacitancefrequency (C‐f) and electron paramagnetic (EPR) spectroscopies.

The high‐performing single‐junction organic solar cell blend, PM6:Y6, is examined to obtain an in‐depth understanding of the voltage losses, and charge recombination and extraction dynamics. The devices exhibit remarkable extraction coupled with moderate recombination losses. This behavior can most likely be credited to a beneficial morphology as evidenced by atomically resolved ¹⁹F magic‐angle‐spinning solid‐state NMR analysis.

Abstract

The highly efficient single‐junction bulk‐heterojunction (BHJ) PM6:Y6 system can achieve high open‐circuit voltages (V _OC) while maintaining exceptional fill‐factor (FF) and short‐circuit current (J _SC) values. With a low energetic offset, the blend system is found to exhibit radiative and non‐radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non‐geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long‐range ordering, as well as donor:acceptor (D:A) inter‐ and intramolecular interactions at an atomic‐level resolution. This is achieved using photo‐conductive atomic force microscopy, grazing‐incidence wide‐angle X‐ray scattering, and solid‐state ¹⁹F magic‐angle‐spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure–property relationships of this high‐performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high J _SC and FF to be retained despite the low energetic offset.

Back contact between [6,6]‐phenyl‐C61‐butyric acid methyl ester and metal electrode cathode in planar perovskite solar cell is modified via molecule Isatin, resulting in multiple beneficial effects, e.g., enhanced charge mobility, lower charge transfer resistance, higher charge conductivity, reduced energy barrier, and protective layer effect.

Abstract

The cathode interface plays a critical role in achieving high‐performance fullerene/perovskite planar solar cells. Herein, the simple molecule Isatin and its derivatives are introduced at the back contact [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/Al as a cathode modification interlayer. It is revealed that the Isatin interlayers facilitate electron transport/extraction and suppress electron recombination, attributed to the formation of negative dipole potential steps and the passivation of the interfacial trap density. The average power conversion efficiencies of the resulting devices are significantly improved by 11% from 17.68% to 19.74%, with an enhancement in all device parameters including short‐circuit current, open‐circuit voltage, and fill factor. The hysteresis index is found to disappear. In addition, such interlayer enhances device stability under ambient conditions compared to the control devices due to suppression of moisture‐induced degradation of the perovskite films. These findings provide a comprehensive understanding of the engineering of the back contact between PCBM and the metal electrode to improve efficiency and stability of perovskite solar cells.

Poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) is doped with 0.025 mol% molecular organic Lewis acid bis(pentafluorophenyl)zinc, which exhibits higher hole mobility and well‐matched energy. An enhanced highest power conversion efficiency of 17.49% is achieved for a perovskite solar cell based on doped P3HT without destroying its stability.

The molecular organic Lewis acid bis(pentafluorophenyl)zinc [Zn(C₆F₅)₂] is reported as an efficient p‐type dopant for poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), to be used as hole‐transporting material (HTM) in perovskite solar cells (PSCs) for the first time. To date, the most efficient PSCs use lithium bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and 4‐tert‐butylpyridine (tBP) as standard additives for HTMs. However, such dopants can induce deleterious effects on device stability. Herein, the effect of the concentration of Zn(C₆F₅)₂ in P3HT HTM on the performance of PSCs is investigated. The P3HT‐based PSCs using a low concentration of the dopant (0.025 mol%) in the HTM layer exhibit the best performance and the highest power conversion efficiency (PCE) of 17.49%, which is almost 3.5% higher than the achieved PCE for pristine P3HT‐based PSCs. The origin of the improved performance for PSCs is further investigated, by studying the conductivity and hole mobility of the thin films based on pristine and doped P3HT. Adding a small amount of Zn(C₆F₅)₂ to P3HT increases its thin‐film hole mobility and its hole extraction ability.

Indoor photovoltaics are a promising technology in the field of self‐powered electronic devices for the Internet of Things. In article number https://doi.org/10.1002/aenm.2019019801901980, Shien‐Ping Feng and co‐workers report an I/Br/Cl triple‐anion perovskite material tailored for indoor light harvesting. The fitted spectrum, suppressed trap states and restrained halide segregation enable a record‐high efficiency and long‐term stability.

CsF is adopted to modify the PbI₂ seed for highly crystallized Cs‐doped perovskite film with very long carrier lifetime, and very high light, thermal and humidity stabilities. As a result, the planar perovskite solar cells based on the Cs‐doped film also show very good stability with negligible hysteresis, and display PCEs of over 21%.

Abstract

Adding a small amount of CsI into mixed cation‐halide perovskite film via a one‐step method has been demonstrated as an excellent strategy for high‐performance perovskite solar cells (PSCs). However, the one‐step method generally relies on an antisolvent washing process, which is hard to control and not suitable for fabricating large‐area devices. Here, CsF is employed and Cs is incorporated into perovskite film via a two‐step method. It is revealed that CsF can effectively diffuse into the PbI₂ seed film, and drastically enhances perovskite crystallization, leading to high‐quality Cs‐doped perovskite film with a very long photoluminescence carrier lifetime (1413 ns), remarkable light stability, thermal stability, and humidity stability. The fabricated PSCs show power conversion efficiency (PCE) of over 21%, and they are highly thermally stable: in the aging test at 60 °C for 300 h, 96% of the original PCE remains. The CsF incorporation process provides a new avenue for stable high‐performance PSCs.

In article number https://doi.org/10.1002/aenm.2019013411901341, Hongqiang Wang and co‐workers report that a droplet of laser generated anti‐colloidal‐solution which contains ligand‐free nanocrystals in a desired anti‐solvent can effectively boost the charge transfer of the photogenerated carriers in perovskite solar cells, resulting in a power conversion efficiency of 21.41%, as well as a pronounced stability over 5000 h in relative humidity of 40%.

Nonfullerene acceptors‐based terpolymer, SMD2, is designed and synthesized to continuously fabricate high‐performance organic solar cell (OSC) modules, and multifunctional hole transport layers are developed, and applied to flexible modules via an all‐solution process. the flexible OSC modules fabricated in an industrial production line have a PCE of 5.25% (P _max = 419.6 mW) on an area of 80 cm².

Abstract

To ensure laboratory‐to‐industry transfer of next‐generation energy harvesting organic solar cells (OSCs), it is necessary to develop flexible OSC modules that can be produced on a continuous roll‐to‐roll basis and to apply an all‐solution process. In this study, nonfullerene acceptors (NFAs)‐based donor polymer, SMD2, is newly designed and synthesized to continuously fabricate high‐performance flexible OSC modules. Also, multifunctional hole transport layers (HTLs), WO₃/HTL solar bilayer HTLs, are developed and applied via an all‐solution process called “ProcessOne” into inverted structure. SMD2, the donor terpolymer, has a deep highest occupied molecular orbital (HOMO) level and can achieve a power conversion efficiency (PCE) of 11.3% with NFAs without any pre‐/post‐treatment because of its optimal balance between crystallinity and miscibility. Furthermore, the integration of multifunctional HTLs enables the recovery of the drop in open circuit voltage (V _OC) caused by a mismatch in energy levels between the deep HOMO level of the NFAs‐based bulk‐heterojunction layer and the solution‐processed HTLs. Also, the photostability under ultraviolet‐exposure necessary for “ProcessOne” is greatly improved because of the integration of multifunctional HTLs. Consequently, because of the synergistic effects of these approaches, the flexible OSC modules fabricated in an industrial production line have a PCE of 5.25% (P _max = 419.6 mW) on an active area of 80 cm².

In article number https://doi.org/10.1002/aenm.2019017191901719, Jooho Moon and co‐workers demonstrate a cold antisolvent bathing method for fabricating highly efficient large‐area perovskite solar cells. This temperature tuned antisolvent bathing adjusts the nucleation and growth of the perovskite materials, leading to large grain‐sized perovskite films with preferred crystal orientation and fewer crystal defects. Reduced recombination and facilitated extraction of photogenerated charge carriers are observed, exhibiting champion efficiency of 18.50%.

In article number https://doi.org/10.1002/aenm.2019018051901805, Doojin Vak, Seok‐In Na and co‐workers report a highly efficient single‐junction ternary polymer solar cell (PSC) based on PTB7‐Th, PC₇₁BM, and COi8DFIC using slot‐die coating. This approach is readily translated into large‐area module and roll‐to‐roll processed PSCs, which produce the highest power conversion efficiency among the printing‐based PSCs.

A dopant‐free polymeric hole transport material (HTM) is synthesized to fabricate perovskite solar cells. The carbonyl groups can passivate defects of under‐coordinated Pb atoms that exist in the surface of perovskite films. A PBT1‐C based device shows a power conversion efficiency of 19.06% with a fill factor of 81.22%, which is the highest value among the dopant‐free polymeric HTMs.

Abstract

Although perovskite solar cells (PVSCs) have achieved rapid progress in the past few years, most of the high‐performance device results are based on the doped small molecule hole‐transporting material (HTM), spiro‐OMeTAD, which affects their long‐term stability. In addition, some defects from under‐coordinated Pb atoms on the surface of perovskite films can also result in nonradiative recombination to affect device performance. To alleviate these problems, a dopant‐free HTM based on a donor‐acceptor polymer, PBT1‐C, synthesized from the copolymerization between the benzodithiophene and 1,3‐bis(4‐(2‐ethylhexyl)thiophen‐2‐yl)‐5,7‐bis(2‐alkyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione units is introduced. PBT1‐C not only possesses excellent hole mobility, but is also able to passivate the surface traps of the perovskite films. The derived PVSC shows a high power conversion efficiency of 19.06% with a very high fill factor of 81.22%, which is the highest reported for dopant‐free polymeric HTMs. The results from photoluminescence and trap density of states measurements validate that PBT1‐C can effectively passivate both surface and grain boundary traps of the perovskite.

Mechanochemistry has recently reemerged as a promising green‐chemistry approach for the development of lead halide perovskites and related materials. Simple grinding of halide precursors yields phase‐pure multinary metal halides with excellent optoelectronic properties within minutes. Recent examples of their implementation in high‐efficiency devices are described, alongside promising single‐source vacuum deposition methods for fully solvent‐free processing.

Abstract

Mechanochemical synthesis has recently emerged as a promising route for the synthesis of functional lead halide perovskites as well as other (lead‐free) metal halides. Mechanochemical synthesis presents several advantages with regards to more commonly used solution‐based processes such as an inherent lower toxicity by avoiding organic solvents and a finer control over stoichiometry of the final products. The ease of implementation, either through the use of a simple mortar and pestle or with an electrically powered ball‐mill, and low amount of side products make mechanochemical synthesis appealing for upscaling the production of halide perovskites. Due to the defect tolerance of lead halide perovskites, they are ideally suited to be prepared by this solvent‐free method. However, the implementation of these semiconductors in high‐efficiency optoelectronic devices requires the transformation of synthesized powder into smooth thin films where still some hurdles remain to be cleared.

Machine learning (ML) is used to predict the material bandgap and perovskite solar cell device performances. The findings from the ML model matches well with the trend in the solar cell theory derived from the “Shockley and Queisser limit.” Other findings, which are beneficial for the fabrication of high‐performing perovskite solar cells are also discussed.

Abstract

Perovskite solar cells (PSCs) have recently received considerable attention due to the high energy conversion efficiency achieved within a few years of their inception. However, a machine learning (ML) approach to guide the development of high‐performing PSCs is still lacking. In this paper ML is used to optimize material composition, develop design strategies, and predict the performance of PSCs. The ML models are developed using 333 data points selected from about 2000 peer reviewed publications. These models guide the design of new perovskite materials and the development of high‐performing solar cells. Based on ML guidance, new perovskite compositions are experimentally synthesized to test the practicability of the model. The ML model also shows its ability to predict underlying physical phenomena as well as the performance of PSCs. The PSC model matches well with the theoretical prediction by the Shockley and Queisser limit, which is almost impossible for a human to find from an ensemble of data points. Moreover, strategies for developing high‐performing PSCs with different bandgaps are also derived from the model. These findings show that ML is very promising not only for predicting the performance, but also for providing a deeper understanding of the physical phenomena associated with the PSCs.

Recent progress on additive engineering during perovskite film formation is reported according to the following common categories: Lewis acid, Lewis base, ammonium salts, low‐dimensional perovskites, and ionic liquid. Then, various additive‐assisted strategies for interface optimization are compared. Finally, an outlook on the research trends with respect to additive engineering in perovskite solar cell development is provided.

Abstract

Perovskite solar cells (PSCs) have reached a certified 25.2% efficiency in 2019 due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap. However, due to the nature of solution processing and rapid crystal growth of perovskite thin films, a variety of defects can form as a result of the precursor compositions and processing conditions. The use of additives can affect perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, as well as influence the interface tuning of structure and energetics. Here, recent progress in additive engineering during perovskite film formation is discussed according to the following common categories: Lewis acid (e.g., metal cations, fullerene derivatives), Lewis base based on the donor type (e.g., O‐donor, S‐donor, and N‐donor), ammonium salts, low‐dimensional perovskites, and ionic liquid. Various additive‐assisted strategies for interface optimization are then summarized; additives include modifiers to improve electron‐ and hole‐transport layers as well as those to modify perovskite surface properties. Finally, an outlook is provided on research trends with respect to additive engineering in PSC development.

In article number https://doi.org/10.1002/aenm.2019018631901863, Fang‐Chung Chen and co‐workers use calculations of the Shockley‐Queisser limits under indoor light sources to reveal an unusual bandgap (E _g) zone, in which the dependence of the power conversion efficiencies (PCEs) on the value of E _g exhibits different trends from that under solar irradiation. Therefore, an increase in the Eg of perovskite materials improves the PCEs of perovskite solar cells under indoor lighting conditions.

The polystyrene is introduced into perovskite solar cells as the buffer layer between the SnO₂ and perovskite, which can release the stress during the perovskite annealing. A large lattice, fewer defect, and low ion‐immigration‐energy perovskite can be obtained by releasing stress. Finally, 21.89% efficiency is obtained and the cell can maintain almost 97% of the initial efficiency after 5 days.

Abstract

The mixed halide perovskites have become famous for their outstanding photoelectric conversion efficiency among new‐generation solar cells. Unfortunately, for perovskites, little effort is focused on stress engineering, which should be emphasized for highly efficient solar cells like GaAs. Herein, polystyrene (PS) is introduced into the perovskite solar cells as the buffer layer between the SnO₂ and perovskite, which can release the residual stress in the perovskite during annealing because of its low glass transition temperature. The stress‐free perovskite has less recombination, larger lattices, and a lower ion migration tendency, which significantly improves the cell's efficiency and device stability. Furthermore, the so‐called inner‐encapsulated perovskite solar cells are fabricated with another PS capping layer on the top of perovskite. As high as a 21.89% photoelectric conversion efficiency (PCE) with a steady‐state PCE of 21.5% is achieved, suggesting that the stress‐free cell can retain almost 97% of its initial efficiency after 5 days of “day cycle” stability testing.

The electron extracting properties of the widely used electron transporting layer (ETL) material Phen‐NaDPO are remarkably enhanced via simple addition of the wide‐bandgap inorganic material tin (II) thiocyanate (Sn(SCN)₂). Use of this hybrid ETL system in organic and perovskite solar cells results in consistent efficiency improvements due to the reduced trap‐assisted recombination and efficient electron extraction.

Abstract

A simple approach that enables a consistent enhancement of the electron extracting properties of the widely used small‐molecule Phen‐NaDPO and its application in organic solar cells (OSCs) is reported. It is shown that addition of minute amounts of the inorganic molecule Sn(SCN)₂ into Phen‐NaDPO improves both the electron transport and its film‐forming properties. Use of Phen‐NaDPO:Sn(SCN)₂ blend as the electron transport layer (ETL) in binary PM6:IT‐4F OSCs leads to a remarkable increase in the cells' power conversion efficiency (PCE) from 12.6% (Phen‐NaDPO) to 13.5% (Phen‐NaDPO:Sn(SCN)₂). Combining the hybrid ETL with the best‐in‐class organic ternary PM6:Y6:PC₇₀BM systems results to a similarly remarkable PCE increase from 14.2% (Phen‐NaDPO) to 15.6% (Phen‐NaDPO:Sn(SCN)₂). The consistent PCE enhancement is attributed to reduced trap‐assisted carrier recombination at the bulk‐heterojunction/ETL interface due to the presence of new energy states formed upon chemical interaction of Phen‐NaDPO with Sn(SCN)₂. The versatility of this hybrid ETL is further demonstrated with its application in perovskite solar cells for which an increase in the PCE from 16.6% to 18.2% is also demonstrated.