Author Correction: Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites
Author Correction: Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites, Published online: 14 February 2019; doi:10.1038/s41563-019-0316-5
Author Correction: Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites
A strategy to prepare efficient carbon‐based CsPbI2Br perovskite solar cells is explored by using Co3O4 nanomaterial as hole transport layer (HTM). It is found that the Co3O4 inorganic HTM effectively promotes photo‐generated charges separation and extraction, and suppress charge recombination at the CsPbI2Br/carbon electrode interface, leading to the enhanced performance.
Carbon‐based perovskite solar cells (PSCs) have gathered much attention due to their excellent thermal stability and low cost. However, the typically used hole‐conductor‐free PSCs based on carbon electrodes show the worst performance due to the serious charge recombination at the perovskite/carbon interface. In this work, the efficient and stable carbon‐based CsPbI2Br PSCs using Co3O4 as the hole transport material (HTM) are fabricated and their photoelectric properties are systematically investigated. It is found that the Co3O4 inorganic HTM effectively promotes photo‐generated charges separation and extraction, and suppresses charge recombination at the CsPbI2Br/carbon electrode interface, resulting in the improved photovoltaic performance. At the optimal Co3O4 concentration, the carbon‐based CsPbI2Br PSCs achieve the maximum efficiency of 11.21% with a negligible J–V hysteresis. This work provides a novel strategy to fabricate efficient and stable all‐inorganic PSCs.
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.
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.
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.
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.
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.
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.
Planar p–n homojunction perovskite solar cells with efficiency exceeding 21.3%
Planar p–n homojunction perovskite solar cells with efficiency exceeding 21.3%, Published online: 04 February 2019; doi:10.1038/s41560-018-0324-8
Carrier recombination limits the power conversion efficiency of perovskite solar cells. Here the authors construct a planar p–n homojunction perovskite solar cell to promote the oriented transport of carriers and reduce recombination, thus enabling power conversion efficiency of 21.3%.
Building blocks: Semi‐locked tetrathienylethene (TTE), featuring a hybrid planar and orthogonal molecular conformation, is introduced as the core for constructing state‐of‐the‐art hole‐transporting materials (HTMs). The resulting TTE achieves the best photovoltaic performance among dopant‐free HTM‐based planar n‐i‐p structured perovskite solar cells.
The construction of state‐of‐the‐art hole‐transporting materials (HTMs) is challenging regarding the appropriate molecular configuration for simultaneously achieving high morphology uniformity and charge mobility, especially because of the lack of appropriate building blocks. Herein a semi‐locked tetrathienylethene (TTE) serves as a promising building block for HTMs by fine‐tuning molecular planarity. Upon incorporation of four triphenylamine groups, the resulting TTE represents the first hybrid orthogonal and planar conformation, thus leading to the desirable electronic and morphological properties in perovskite solar cells (PSCs). Owing to its high hole mobility, deep lying HOMO level, and excellent thin film quality, the dopant‐free TTE‐based PSCs exhibit a very promising efficiency of over 20 % with long‐term stability, achieving to date the best performances among dopant‐free HTM‐based planar n‐i‐p structured PSCs.
An n‐doping of SnO2 is successfully realized through the use of the triphenylphosphine‐oxide molecule, where electrons are revealed to be transferred from the R3P+O− σ‐bond to the peripheral tin atoms and delocalized. That novel effect enlarges the built‐in‐field from 0.01 to 0.07 eV and declines the energy‐barrier from 0.55 to 0.39 eV at the SnO2–perovskite interface enabling a device conversion‐efficiency from 19.01% to 20.69%.
Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n‐type doping of oxide semiconductors. Herein, successful n‐type doping of SnO2 is demonstrated by a simple, air‐robust, and cost‐effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P+O−σ‐bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi‐photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO2, which enlarges the built‐in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO2/perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.
Methoxy‐functionalized triphenylamine‐imidazole derivatives, simultaneously working as hole transport materials and bifacial interface‐modifiers passivating defects in the perovskite and NiO x layers, are developed for high‐performance and stable perovskite solar cell. They are advantageous to improve charge‐extraction kinetics of devices and significantly enhance the stability of devices under constant UV illumination in air.
Methoxy‐functionalized triphenylamine‐imidazole derivatives that can simultaneously work as hole transport materials (HTMs) and interface‐modifiers are designed for high‐performance and stable perovskite solar cells (PSCs). Satisfying the fundamental electrical and optical properties as HTMs of p‐i‐n planar PSCs, their energy levels can be further tuned by the number of methoxy units for better alignment with those of perovskite, leading to efficient hole extraction. Moreover, when they are introduced between perovskite photoabsorber and low‐temperature solution‐processed NiO x interlayer, widely featured as an inorganic HTM but known to be vulnerable to interfacial defect generation and poor contact formation with perovskite, nitrogen and oxygen atoms in those organic molecules are found to work as Lewis bases that can passivate undercoordinated ion‐induced defects in the perovskite and NiO x layers inducing carrier recombination, and the improved interfaces are also beneficial to enhance the crystallinity of perovskite. The formation of Lewis adducts is directly observed by IR, Raman, and X‐ray photoelectron spectroscopy, and improved charge extraction and reduced recombination kinetics are confirmed by time‐resolved photoluminescence and transient photovoltage experiments. Moreover, UV‐blocking ability of the organic HTMs, the ameliorated interfacial property, and the improved crystallinity of perovskite significantly enhance the stability of PSCs under constant UV illumination in air without encapsulation.
The latest breakthroughs in interface and defect engineering as applied to metal halide perovskite solar cells and light‐emitting diodes (LEDs) are reviewed in order to shed light on their necessity and importance in tuning the optoelectronic properties of devices in an attempt to realize the best‐performing solar cells and LEDs.
Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.
An in situ back‐contact passivation strategy is adopted to optimize the photovoltaic performance of n–i–p planar perovskite solar cells. Devices with a flat‐band alignment between the perovskite and polymer passivation layer achieve a high photovoltage of 1.15 V and fill factor of 83% with 1.53 eV bandgap perovskite, leading to a stabilized power conversion efficiency of 21.6%.
Organic–inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge–transport layers. Here, in situ back‐contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits—in contrast with previously studied insulator‐based passivants—the use of a relatively thick passivating layer. It is shown that a flat‐band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.
The optical absorption in monolithic perovskite/silicon tandem solar cells with flat Si front‐side is improved. The successful tailoring and incorporation of a nanocrystalline silicon oxide composite interlayer with tuneable refractive index is demonstrated on device by experiments and optical simulations. Improved short‐circuit current density (38.7 mA cm−2) combined with excellent contact properties lead to a cell with a certified stabilized conversion efficiency of 25.2%.
Perovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single‐junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, results in significant reflection losses at the internal junction between the cells in monolithic (two‐terminal) devices. Therefore, light management is crucial to improve photocurrent absorption in the Si bottom cell. Here it is shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate can be significantly reduced by using an optical interlayer consisting of nanocrystalline silicon oxide. It is demonstrated that 110 nm thick interlayers with a refractive index of 2.6 (at 800 nm) result in 1.4 mA cm−² current gain in the silicon bottom cell. Under AM1.5G irradiation, the champion 1 cm2 perovskite/silicon monolithic tandem cell exhibits a top cell + bottom cell total current density of 38.7 mA cm−2 and a certified stabilized power conversion efficiency of 25.2%.
Tetrafluoroborate (BF4 −) anion can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite solar cells. These advantages result in an improved power conversion efficiency of 20.16% from 17.55% due to enhanced open‐circuit voltage (V OC) and fill factor.
Composition engineering is a particularly simple and effective approach especially using mixed cations and halide anions to optimize the morphology, crystallinity, and light absorption of perovskite films. However, there are very few reports on the use of anion substitutions to develop uniform and highly crystalline perovskite films with large grain size and reduced defects. Here, the first report of employing tetrafluoroborate (BF4 −) anion substitutions to improve the properties of (FA = formamidinium, MA = methylammonium (FAPbI3)0.83(MAPbBr3)0.17) perovskite films is demonstrated. The BF4 − can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite films and derived solar cells. These advantages benefit the performance of perovskite solar cells (PVSCs), resulting in an improved power conversion efficiency (PCE) of 20.16% from 17.55% due to enhanced open‐circuit voltage (V OC) and fill factor. This is the highest PCE for BF4 − anion substituted lead halide PVSCs reported to date. This work provides insight for further exploration of anion substitutions in perovskites to enhance the performance of PVSCs and other optoelectronic devices.
High‐performance inverted lead‐free perovskite solar cells (PVSCs) with enhanced UV stability are demonstrated via grain boundaries modification by PTN‐Br. The gradient band alignment of FASnI3 films with a PEDOT:PSS hole‐transport layer ensures excellent hole transportation and higher open‐circuit voltage. This study provides a strategy to develop high‐performance tin‐based PVSCs based on balanced charge transportation and reduced trap states.
High electronic quality perovskite films with a balanced charge transportation is critical for satisfying high‐performance for perovskite solar cells (PVSCs). However, the inferior band alignment of tin‐based perovskite films with an adjacent hole‐transport layer (HTL) leads to a poor hole transportation and collection. In this work, the semiconducting molecule poly[tetraphenylethene 3,3′‐(((2,2‐diphenylethene‐1,1‐diyl)bis(4,1‐phenylene))bis(oxy))bis(N,N‐dimethylpropan‐1‐amine)tetraphenylethene] (PTN‐Br) is introduced into a lead‐free perovskite precursor to form a bulk heterojunction film. In addition, the PTN‐Br molecule with the suitable highest occupied molecular orbital energy level (−5.41 eV) can fill into the grain boundaries of the perovskite film, serving as a hole‐transport medium between grains. The gradient band alignment of the perovskite film with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL ensures excellent hole transportation and higher open‐circuit voltage. In addition, the π‐conjugated polymer PTN‐Br can passivate trap states within the perovskite film due to the formation of Lewis adducts between uncoordinated Sn atoms and the dimethylamino of PTN‐Br. Consequently, a champion efficiency of 7.94% is achieved with significant enhancements in the open‐circuit voltage and fill factor. Furthermore, the PTN‐Br incorporated device shows better ultra violet (UV) stability because of the UV barrier and passivating effect of PTN‐Br, retaining about 66% of its initial efficiency after 5 h of continuous UV light irradiation.
Publication date: April 2019
Source: Nano Energy, Volume 58
Author(s): Xiaodong Li, Wenxiao Zhang, Wenjun Zhang, Hai-Qiao Wang, Junfeng Fang
Spontaneous grain polymerization strategy is proposed to fabricate efficient and stable perovskite solar cells (PSCs) through the incorporation of polymerizable additive ethyl 2-cyanoacrylate (E2CA). E2CA lies in and chemically anchors to grain boundaries (GBs) owing to –CN and -C=O groups’ coordination with PbI2, thus passivating the defects at GBs and leading to high devices efficiency of 21.03%. Importantly, E2CA in perovskite films will spontaneously polymerize to a hydrophobic polymer at GBs when exposed in moisture air, thus blocking GBs channel for moisture penetration and enhancing the moisture-resisting properties of perovskite films. As a result, PSCs with E2CA exhibit superior stability in moisture air (relative humidity: 40–60%), retaining ~90% of the maximum efficiency after aging over 1000 h. Even under high temperature (85 °C) in moisture air, non-encapsulated MAPbI3-E2CA devices still show good stability despite the burn-in degradation, retaining over 90% of the post burn-in efficiency after aging 200 h.
Spontaneous grain polymerization strategy is proposed to fabricate efficient and stable perovskite solar cells through the incorporation of ethyl 2-cyanoacrylate (E2CA) additive. E2CA chemically anchors to grain boundaries and then spontaneously polymerizes to hydrophobic polymer when exposed in air, thus passivating defects and blocking moisture penetration. The resulted devices exhibit high efficiency of 21.03% with good air and thermal stability.
Publication date: April 2019
Source: Nano Energy, Volume 58
Author(s): Guangbao Wu, Jiyu Zhou, Jianqi Zhang, Rui Meng, Boxin Wang, Baoda Xue, Xuanye Leng, Dongyang Zhang, Xuning Zhang, Shiqing Bi, Qian Zhou, Zhixiang Wei, Huiqiong Zhou, Yuan Zhang
Control over the crystallization in quantum well Ruddlesden-Popper phase halide perovskites is vitally important for the photovoltaic performance. Through managing the molecular stacking in 2-dimentioanl BA2MA3Pb4I13 (n = 4) perovskites based on PTAA hole transporting layers, we achieve enhanced vertical crystal orientations in BA2MA3Pb4I13 polycrystalline films, leading to a champion power conversion efficiency (PCE) of 14.3% (n ≤ 4) with negligible hysteresis in PTAA based p-i-n perovskite solar cells. The enhanced PCE is ascribed to the suppression on change recombination associated with an expedited charge extraction, revealed by transient opto-electrical analyses. Benefitted from the enhanced molecular arrangement revealed by GIWAXS, efficient charge generation at low temperatures (T) is enabled, leading to a negative T-dependence of efficiency in the hot-cast device, showing a peak PCE of 15.0% at 210 K. This trend is likely correlated to the reduced potential barriers in the quantum wells with which the detrapping of photo-carriers is facilitated at smaller thermal energy. Contrastingly, the solar cells with more randomly oriented crystals are found to suffer more from these unfavorable barriers, resulting in decreased PCEs with lowered T. Our findings highlight the opportunity through crystallization management coupled with interface engineering to achieve high efficiency and stable 2D perovskite solar cells within a wide T-window.
The management on the orientational crystallization in 2-dimensional BA2MA3Pb4I13 perovskite solar cells based on the polymeric PTAA hole transporting layer leads to boosted power conversion efficiency (PCE) to 14.28% with a small hysteresis at room temperature. The device also exhibits a negative temperature dependence of PCE reaching 15% at 210 K with supreme stability.
Publication date: April 2019
Source: Nano Energy, Volume 58
Author(s): Ankur Solanki, Pankaj Yadav, Silver-Hamill Turren-Cruz, Swee Sien Lim, Michael Saliba, Tze Chien Sum
Rubidium and Cesium cations (Rb+ and Cs+) incorporation recently emerged as a viable strategy to enhance perovskite solar cells (PSCs) efficiency. However, a clear understanding of the impact of these cations on the structure-function relationship in relation to the device performance is severely lacking. Here, we systematically investigate the influence of Rb+ and Cs+ on the carrier dynamics using transient optical spectroscopy and correlate with solar cell performance. Unlike Rb+, Cs+ integrates well with methylammonium (MA+) and formamidinium (FA+) yielding increased perovskite grain size, longer charge carrier lifetimes and improved power conversion efficiency (PCE). Concomitant incorporation of Cs+/Rb+ cooperatively retards radiative recombination by ~60% in the quaternary-cation based perovskite system (RbCsMAFA) compared to the dual-cation MAFA samples. By suppressing the defect density, PCEs around 20% are obtained along with more balanced charge carrier diffusion length and comparable photoluminescence quantum yield in quaternary-cation perovskites. While the synergistic addition of Rb+ and Cs+ is attractive for controlling defects and recombination losses in efficient solar cells development, sole incorporation of Rb+ is still an engineering challenge. Importantly, our study explicates the underlying mechanisms behind the synergistic combination of cations to minimize the charge carrier losses and achieve high efficiency perovskite solar cells.
In article number 1801743, Xugang Guo and co‐workers develop two phthalimide‐based polymers featuring a D‐A1‐D‐A2 backbone motif. Eliminating benzodithiophene leads to polymers with substantial mobility. Nonfullerene polymer solar cells utilizing these high‐mobility polymers achieve a remarkable power conversion efficiency >13%. The results demonstrate that phthalimides are excellent building blocks for enabling polymer semiconductors with outstanding solar cell performances and benzodithiophenes are not necessary for constructing such polymers.
