Chen Weijie

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

Abstract

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

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

Abstract

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

Nature Materials, Published online: 13 July 2020; doi:10.1038/s41563-020-0720-x

Publication date: 19 August 2020

Source: Joule, Volume 4, Issue 8

Author(s): Nengxu Li, Yanqi Luo, Zehua Chen, Xiuxiu Niu, Xiao Zhang, Jiuzhou Lu, Rishi Kumar, Junke Jiang, Huifen Liu, Xiao Guo, Barry Lai, Geert Brocks, Qi Chen, Shuxia Tao, David P. Fenning, Huanping Zhou

Solar Cell Fabrication

In article number 2000087, Gang Wu and co‐workers present a new additive‐assisted method, free of hot‐casting, to realize the fabrication of high‐quality two‐dimensional Dion–Jacobson perovskite films with preferred vertical orientation growth. This method avoids the hindrance of the insulating organic spacer cation layer to the charge transport and promotes the current along the vertical direction, resulting in improved power conversion efficiency of the photovoltaic device.

Boron sub‐(na)phthalocyanine chloride–based organic photovoltaic devices exhibit extremely low recombination loss (≈0.50 eV) while not penalizing charge separation (internal quantum efficiency  > 80%). Unique intrinsic properties of these materials and hybridization collectively result in low recombination loss and effective charge separation at the same time.

Hybridization between the charge transfer (CT) state of a donor–acceptor pair and lowest exciton state of the donor or the acceptor is reported to be effective for reducing recombination loss in organic photovoltaic (OPV) devices. Although this approach shows great success in maximizing open circuit voltage (V _oc), it is typically accompanied by low device performance. Here, “complete boron sub‐(na)phthalocyanine devices” with strong hybridization resulting in lower recombination loss (≈0.47 eV) while not penalizing charge separation dynamics (internal quantum efficiency (IQE) > 80% and fill factor (FF) > 70%) are reported. Interestingly, when boron sub‐(na)phthalocyanine is paired with any other active material used in this study (“partial boron sub‐(na)phthalocyanine device”), recombination losses are still consistently maintained at lower levels (<0.53 eV). These observations denote the capability of boron sub‐(na)phthalocyanine to result in lower recombination loss devices while pairing with other materials. Special intrinsic characteristics of these materials (high dielectric constant, sharp absorption edge, unusually high absorption coefficient) and hybridization collectively result in reduced recombination loss and efficient charge generation in these systems.

The expanded analogs of 3D halide perovskites reported herein can hold relatively large aromatic cations in the spacious inorganic sublattice. The band gap of these materials is set by the empty low‐lying orbitals of the organic molecules and the filled orbitals of the inorganic components. The molecular nature of the conduction band allows us to electronically dope the materials through simple redox chemistry.

Abstract

Replacing the Pb−X octahedral building unit of A^IPbX₃ perovskites (X=halide) with a pair of edge‐sharing Pb−X octahedra affords the expanded perovskite analogs: A^IIPb₂X₆. We report seven members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrazinium‐based cations with low‐lying π* orbitals that form the conduction band, substantially decreasing the band gap of the expanded lattice. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with A^II=pyridinium and ammonium, we can isolate the contributions of the pyrazinium‐based orbitals in the band gap transition of A^IIPb₂X₆. The organic‐molecule‐based conduction band and the inorganic‐ion‐based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.

Publication date: 19 August 2020

Source: Joule, Volume 4, Issue 8

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

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

Abstract

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

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

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

A sol–gel material optimized for screen printing is investigated. The liquid paste can be converted to borosilicate glass (BSG) by thermal annealing. The derived films show similar doping performance compared with chemical‐vapor‐deposited dopant sources. The printed layers are integrated in the fabrication of prototype back‐contact back‐junction (BCBJ) solar cells, demonstrating a high potential of the material in photovoltaics manufacturing.

Borosilicate glass films deposited by chemical vapor deposition are used as boron dopant sources in silicon solar‐cell manufacturing, to reduce the fabrication costs of, e.g., back‐contact back‐junction (BCBJ) solar cells. Herein, an alternative dopant source is investigated, which can replace such layers by a printing step. The necessary paste is synthesized by the sol–gel method and optimized for screen printing as demonstrated. The liquid paste can be converted to a glass by thermal annealing, as evaluated by spectroscopic investigations of the resulting thin films. The resulting layers are uniform and crack‐free with a thickness of around 150 nm on silicon surfaces. It is shown that such layers can act as boron dopant sources on silicon, sufficient for the fabrication of BCBJ solar cells, as demonstrated by prototype devices.

Cyano‐based π‐conjugated molecules composed of indacenodithieno[3,2‐b]thiophene (IDTT) and the cyano group are used to passivate defects at the surface and grain boundaries of metal–halide perovskite films. These molecules are self‐anchored at the grain boundaries due to their strong binding to undercoordinated Pb²⁺ and enhance the power conversion efficiencies up to 21.2%, with improved stability of the perosvkite solar cells.

Abstract

Defects at the surface and grain boundaries of metal–halide perovskite films lead to performance losses of perovskite solar cells (PSCs). Here, organic cyano‐based π‐conjugated molecules composed of indacenodithieno[3,2‐b]thiophene (IDTT) are reported and it is found that their cyano group can effectively passivate such defects. To achieve a homogeneous distribution, these molecules are dissolved in the antisolvent, used to initiate the perovskite crystallization. It is found that these molecules are self‐anchored at the grain boundaries due to their strong binding to undercoordinated Pb²⁺. On a device level, this passivation scheme enhances the charge separation and transport at the grain boundaries due to the well‐matched energetic levels between the passivant and the perovskite. Consequently, these benefits contribute directly to the achievement of power conversion efficiencies as high as 21.2%, as well as the improved environmental and thermal stability of the PSCs. The surface treatment provides a new strategy to simultaneously passivate defects and enhance charge extraction/transport at the device interface by manipulating the anchoring groups of the molecules.

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

Abstract

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

1,3‐dimethyl‐2‐(thiophen‐2‐yl)‐2,3‐dihydro‐1H‐benzo[d]imidazole (DMBI‐2‐Th) and its iodine ionized molecule DMBI‐2‐Th‐I are developed to regulate the electronic states of bulk perovskite for efficient p–i–n pero‐SCs, leading to a significant improvement in electron trap density, electron concentration, ambipolar charge transporting property, and electronic extraction efficiency. Finally, a promising power conversion efficiency of 20.90% with excellent moisture stability is obtained.

Abstract

The power conversion efficiency (PCE) of planar p–i–n perovskite solar cells (pero‐SCs) is commonly lower than that of the n–i–p pero‐SCs, due to the severe nonradiative recombination stemming from the more p‐type perovskite with prevailing electron traps. Here, two n‐type organic molecules, DMBI‐2‐Th and DMBI‐2‐Th‐I, with hydrogen‐transfer properties for the doping of bulk perovskite aimed at regulating its electronic states are synthesized. The generated radicals in these n‐type dopants with high‐lying singly occupied molecular orbitals enable easy transfer of the thermally activated electrons to the MAPbI₃ perovskite for the realization of n‐doped perovskites. The n‐doping degree could be further enhanced by using the iodine ionized dopant DMBI‐2‐Th‐I. The doping effect could reduce the electron trap density, increase the electron concentration of the bulk perovskite, and simultaneously improve the surface electronic contact. When the DMBI‐2‐Th‐I‐doped perovskite is used in planar p–i–n pero‐SCs, the nonradiative recombination is significantly suppressed. As a result, the photovoltaic performance improved significantly, as evidenced by an excellent PCE of 20.90% and a robust ambient stability even under high relative humidity. To the best of the knowledge, this work represents the first example where organic n‐type dopants are used to tune the electronic states of a bulk perovskite film for efficient planar p–i–n pero‐SCs.

A formamidinium (FA)‐based quasi‐2D Ruddlesden–Popper (RP) perovskite, namely, (ThMA)₂(FA) _{n −1}Pb _n I_{3 n +1} (nominal n = 5), is successfully demonstrated with high photovoltaic performance by using an organic‐salt‐assisted crystal growth method. The optimized device exhibits a champion efficiency of 19.06%, which is a record for quasi‐2D RP perovskite solar cells with nominal n‐value lower than 6.

Abstract

Quasi‐2D Ruddlesden–Popper (RP) perovskite solar cells (PSCs) have drawn significant attention due to their appealing environmental stability compared to their 3D counterparts. However, the relatively low power conversion efficiency (PCE) greatly limits their applications. Here, high photovoltaic performance is demonstrated for quasi‐2D RP PSCs using 2‐thiophenemethylammonium as spacer with nominal n‐value of 5, which is based on the stoichiometry of the precursors. The incorporation of formamidinium (FA) in quasi‐2D RP perovskites reduces the bandgap and improves the light absorption ability, resulting in enlarged photocurrent and an increased PCE of 16.18%, which is higher than that of reported analogous methylammonium (MA)‐based quasi‐2D PSC (≈15%). A record high PCE of 19.06% is further demonstrated by using an organic salt, namely, 4‐(trifluoromethyl)benzylammonium iodide, assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the charge recombination losses. More importantly, the devices based on OACG‐processed quasi‐2D RP perovskites show remarkable environmental stability and thermal stability, for example, the PCE retaining ≈96% of its initial value after storage at 80 °C for 576 h, while only ≈37% of the original efficiency left for FAPbI₃‐based

3D PSCs.

Marrying inverse vulcanization and silane chemistry yields solution‐processable high sulfur content polysulfide‐alkoxysilanes, which could be cured via polycondensation. With this strategy, coated surfaces and particles as well as crosslinked materials can be obtained. This method extends the applicability and control of materials prepared via inverse vulcanization.

Abstract

Sulfur as a side product of natural gas and oil refining is an underused resource. Converting landfilled sulfur waste into materials merges the ecological imperative of resource efficiency with economic considerations. A strategy to convert sulfur into polymeric materials is the inverse vulcanization reaction of sulfur with alkenes. However, the materials formed are of limited applicability, because they need to be cured at high temperatures (>130 °C) for many hours. Herein, we report the reaction of elemental sulfur with styrylethyltrimethoxysilane. Marrying the inverse vulcanization and silane chemistry yielded high sulfur content polysilanes, which could be cured via room temperature polycondensation to obtain coated surfaces, particles, and crosslinked materials. The polycondensation was triggered by hydrolysis of poly(sulfur‐r‐styrylethyltrimethoxysilane) (poly(S_n‐r‐StyTMS) under mild conditions (HCl, pH 4). For the first time, an inverse vulcanization polymer could be conveniently coated and mildly cured via post‐polycondensation. Silica microparticles coated with the high sulfur content polymer could improve their Hg²⁺ ion remediation capability.