Chen Weijie

Antisolvent engineering is employed to tune the crystal nucleation and grain growth of perovskite for achieving efficient perovskite solar cells. The engineering of perovskites treated with the green antisolvent MABr‐Eth, suppressing defects‐induced nonradiative recombination in perovskite solar cells, is developed. As expected, the device delivers over 21% power conversion efficiency and a better storage and light‐soaking stability.

Abstract

Organic–inorganic hybrid perovskites have attracted considerable attention due to their superior optoelectronic properties. Traditional one‐step solution‐processed perovskites often suffer from defects‐induced nonradiative recombination, which significantly hinders the improvement of device performance. Herein, treatment with green antisolvents for achieving high‐quality perovskite films is reported. Compared to defects‐filled ones, perovskite films by antisolvent treatment using methylamine bromide (MABr) in ethanol (MABr‐Eth) not only enhances the resultant perovskite crystallinity with large grain size, but also passivates the surface defects. In this case, the engineering of MABr‐Eth‐treated perovskites suppressing defects‐induced nonradiative recombination in perovskite solar cells (PSCs) is demonstrated. As a result, the fabricated inverted planar heterojunction device of ITO/PTAA/Cs_0.15FA_0.85PbI₃/PC₆₁BM/Phen‐NADPO/Ag exhibits the best power conversion efficiency of 21.53%. Furthermore, the corresponding PSCs possess a better storage and light‐soaking stability.

Molecular ordering of the highly crystalline nonfullerene acceptor C8‐IT‐4F is dependent on the film‐formation process and confinement induced by the preaggregated polymer donor PM7. An as‐cast polymer solar cell based on PM7:C8‐IT‐4F exhibits power conversion efficiency of up to 14.3% and high photostability with a stabilized fill factor due to molecular ordering and miscibility between the donor and the acceptor.

Molecular ordering and miscibility of donor and acceptor materials play critical roles in developing high‐performance as‐cast polymer solar cells (PSCs). In this work, a highly crystalline nonfullerene small molecular acceptor, namely, C8‐IT‐4F, based on alkylated indacenodithieno[3,2‐b]thiophene as the aromatic core and 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile moieties as end groups, is selected and synthesized. The π–π stacking distance in C8‐IT‐4F film can be tuned from 3.88 Å to a more compact (3.48 Å) state by a film‐formation process and the confinement induced by the preaggregated polymer donor PM7, leading to broadened absorption and fine phase separation in the blend film. The optimal morphology with a framework of preaggregated polymer donor, J‐type face‐on π–π stacked acceptor, and appropriate donor/acceptor miscibility facilitates charge generation and transport and reduces charge recombination. As a result, the best PSC based on the as‐cast PM7:C8‐IT‐4F blend film exhibits power conversion efficiency of 14.3%, with an open‐circuit voltage of 0.82 V, a short‐circuit current density of 22.7 mA cm⁻², a fill factor of 77.1%, and good photostability with a stabilized fill factor.

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): So Hyun Park, Sungmin Park, Seungjin Lee, Jiho Kim, Hyungju Ahn, Bumjoon J. Kim, Boknam Chae, Hae Jung Son

Publication date: December 2020

Source: Nano Energy, Volume 78

Author(s): Bowei Li, Yuren Xiang, K. D. G. Imalka Jayawardena, Deying Luo, Zhuo Wang, Xiaoyu Yang, John F. Watts, Steven Hinder, Muhammad T. Sajjad, Thomas Webb, Haitian Luo, Igor Marko, Hui Li, Stuart A.J. Thomson, Rui Zhu, Guosheng Shao, Stephen J. Sweeney, S. Ravi P. Silva, Wei Zhang

High‐temperature degradation of perovskite solar cells with spiro‐OMeTAD hole transport layer is investigated. The postdoping of the spiro‐OMeTAD layer by iodine released from an iodine‐containing perovskite layer at high temperature is discovered as one reason for the high‐temperature degradation. Using an iodine‐free perovskite absorber, thermally stable perovskite solar cells are demonstrated.

Organic–inorganic halide perovskites are promising as the light absorber of solar cells because of their efficient solar power conversion. An issue frequently occurring in perovskite solar cells (PSCs) with a hole transport layer of N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD) is a quick performance degradation at high temperature. Herein, it is discovered that postdoping of the spiro‐OMeTAD layer by iodine released from the perovskite layer is one possible mechanism for the high‐temperature PSC degradation. Iodine doping leads to the highest occupied molecular orbital level of the spiro‐OMeTAD layer becoming deeper and, therefore, induces the formation of an energy barrier for hole extraction from the perovskite layer. It is demonstrated that it is possible to suppress the high‐temperature degradation by using an iodine‐blocking layer or an iodine‐free perovskite in PSCs. These findings will guide the way for the realization of thermally stable perovskite optoelectronic devices in the future.

Saddle‐shaped small molecules α, β‐COTh‐Ph‐OMeTAD and β, β‐COTh‐Ph‐OMeTAD are synthesized and systemically characterized as dopant‐free hole‐transporting material (HTM) in inverted perovskite solar cells (i‐PSCs). High power conversion efficiencies (PCEs) (17.59% and 18.53%) and stable‐enhanced PSCs devices are achieved, and more than 80% of the maximum PCE is retained after storing in glove box for 150 days.

Two saddle‐shaped hole‐transporting materials (HTMs), α, β‐COTh‐Ph‐OMeTAD and β, β‐COTh‐Ph‐OMeTAD are designed with a strategy of flexible core with tunable conformation (FCTC) and applied in inverted planar perovskite solar cells (PSCs) as dopant‐free HTMs. As a result, the device based on α, β‐COTh‐Ph‐OMeTAD demonstrates a high power conversion efficiency (PCE) of 17.59% with J _sc = 21.32 mA cm⁻², V _oc = 1.02 V, and FF = 80.75%, and the one based on β, β‐COTh‐Ph‐OMeTAD yields a higher PCE of 18.53% with J _sc = 22.68 mA cm⁻², V _oc = 1.04 V, and FF = 78.48%. Moreover, the green‐solvent‐processed PSCs are also fabricated by dissolving the HTMs in ethyl acetate. Without any encapsulation, the devices based on both HTMs retain 80% of their initial PCEs after storage for 150 days in a glove box, and 60% of their initial PCEs after storing for 300 h in ambient air with 40% relative humidity. All these results demonstrate that the materials α, β‐COTh‐Ph‐OMeTAD and β, β‐COTh‐Ph‐OMeTAD based on FCTC strategy are promising HTMs for highly efficient and stable PSCs.

Molecular engineering of bridging atoms creates functional nonfullerene acceptors (NFAs) that not only afford decent photovoltaic performance but also ameliorate the fabrication process. DTSiC‐4Cl exhibits fine power conversion efficiency of 14.46% in binary bulk‐heterojunction organic solar cells (BHJ‐OSCs) and 15.04% in ternary BHJ‐OSCs without additives, manifesting great potential for both academic and industrial applications.

Herein, two novel nonfullerene acceptors (NFAs), DTCC‐4Cl and DTSiC‐4Cl, are synthesized by end‐capping dithienocyclopentacarbazole (DTCC) and dithieno‐silolocarbazole (DTSiC) cores with chlorinated IC (2Cl‐IC) units, respectively. With the better‐known advantage of having the extraordinary σ*–π* conjugation of silole unit embedded in the DTSiC core, DTSiC‐4Cl manifests upshifted lowest unoccupied molecular orbital (LUMO), blue‐shifted absorption, and increased π–π interaction in comparison with DTCC‐4Cl. Furthermore, to elucidate the effect of bridging atoms on the photovoltaic performance, T1 is selected as the polymer donor to be blended with DTCC‐4Cl and DTSiC‐4Cl. T1:DTCC‐4Cl‐based devices exhibit a fine power conversion efficiency (PCE) of 14.43% and T1:DTSiC‐4Cl‐based devices exhibit a comparable PCE of 14.46%. Interestingly, the T1:DTSiC‐4Cl‐based devices demonstrate an additive‐free feature, which is worthy of further applications. From the perspective of constructing high‐performance ternary devices, DTCC‐4Cl is expected to possess excellent compatibility with DTSiC‐4Cl owing to its structural similarity. As anticipated, the ternary T1:DTSiC‐4Cl:DTCC‐4Cl‐based device outperforms the binary T1:DTCC‐4Cl and T1:DTSiC‐4Cl‐based devices, affording a decent PCE of 15.04% with a V _OC of 0.97 V, a J _SC of 20.80 mA cm⁻², and an FF of 74.55% without any additive.

Crystallization tailoring (F⁻ doping) of perovskite and construction of multilayer cascade charge transport layers (NiO _x /Zn:CuGaO₂ and TiO₂/PC₆₁BM/ZnO) for inverted CsPbI₂Br solar cells are collaboratively presented, resulting in excellent device efficiency (over 15%) with improved stability. The present strategy can be extended to hybrid wide‐bandgap perovskite solar cells.

It is imperative to improve the quality of light absorber and reduce the charge‐carrier recombination for efficient perovskite solar cells (PSCs). Herein, a synergistic regulation strategy that combines the tailoring of crystallinity and construction of multilayer cascade charge transport layers (CTLs) for inverted CsPbI₂Br solar cells is presented. The film quality of CsPbI₂Br is well tuned via F⁻ doping. In addition, gradient energy alignment between perovskite and CTLs, i.e., NiO _x /Zn:CuGaO₂/perovskite and perovskite/TiO₂/PC₆₁BM/ZnO, favors the charge transfer and depresses carrier recombination. Noticeably, the TiO₂ interlayer with deep valence band maximum effectively blocks the hole back‐transfer from perovskite to PC₆₁BM. These unique characteristics of the novel structured CsPbI₂Br device give a champion power conversion efficiency (PCE) of 15.10% along with good thermal and operational stability. Moreover, the graded CTLs can be expanded to methylammonium‐free hybrid perovskite device (E _g = ≈1.76 eV) by delivering a PCE of 18.12%, showing great promise in tandem solar cells for use as top cell.

The M13 bacteriophage functions as an effective perovskite growth template and a passivator in perovskite solar cells. This is owing to its filamentous and uniform dimension, as well as the amino acids on its surface. These effects enhance when the M13 viruses are denatured at high temperature. The efficiency increases from 17.8% to 20.1% upon addition of the denatured viruses.

Abstract

The M13 bacteriophage, a nature‐inspired environmentally friendly biomaterial, is used as a perovskite crystal growth template and a grain boundary passivator in perovskite solar cells. The amino groups and carboxyl groups of amino acids on the M13 bacteriophage surface function as Lewis bases, interacting with the perovskite materials. The M13 bacteriophage‐added perovskite films show a larger grain size and reduced trap‐sites compared with the reference perovskite films. In addition, the existence of the M13 bacteriophage induces light scattering effect, which enhances the light absorption particularly in the long‐wavelength region around 825 nm. Both the passivation effect of the M13 bacteriophage coordinating to the perovskite defect sites and the light scattering effect intensify when the M13 virus‐added perovskite precursor solution is heated at 90 °C prior to the film formation. Heating the solution denatures the M13 bacteriophage by breaking their inter‐ and intra‐molecular bondings. The denatured M13 bacteriophage‐added perovskite solar cells exhibit an efficiency of 20.1% while the reference devices give an efficiency of 17.8%. The great improvement in efficiency comes from all of the three photovoltaic parameters, namely short‐circuit current, open‐circuit voltage, and fill factor, which correspond to the perovskite grain size, trap‐site passivation, and charge transport, respectively.

Publication date: December 2020

Source: Nano Energy, Volume 78

Author(s): Zijia Li, Jaehong Park, Hansol Park, Jongmin Lee, Yeongkwon Kang, Tae Kyu Ahn, Bong-Gi Kim, Hui Joon Park

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): You-Hyun Seo, Jun Hee Kim, Do-Hyung Kim, Hee-Suk Chung, Seok-In Na

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): Qiyao Guo, Jihuai Wu, Yuqian Yang, Xuping Liu, Weihai Sun, Yuelin Wei, Zhang Lan, Jianming Lin, Miaoliang Huang, Hongwei Chen, Yunfang Huang

The mechanism for binary solvent additives to enhance photovoltaic device performance via advanced technology characterizations is unveiled. The binary additives improve polymer order, maintain high crystallinity, and obtain the preferable morphology of photoactive films. As a result, new binary additives result in an enhanced short circuit current and fill factor, and the device performance is improved from 9.11% to 10.64%.

Binary solvent additive engineering is an effective strategy to optimize photoactive films for high‐efficiency organic solar cells, however, the effect of single components on device performance and the combination principle of binary solvent additives remain unclear. Herein, synchrotron‐based grazing incident X‐ray diffraction, Derjaguin–Muller–Toporov modulus imaging, and plasmon energy shift imaging acquired by scanning transmission electron microscopy to investigate the effect of new binary solvent additive of 1,8‐diiodooctane (DIO) and less‐toxic and p‐anisaldehyde (AA) on device performance of solar cells based on poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3‴‐di(2‐octyldodecyl)2,2′;5′,2″;5″,2‴‐quaterthio‐phen‐5,5‴‐diyl)] (PffBT4T‐2OD) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) are used. It is found that AA mainly favors polymer order and high crystallinity of PffBT4T‐2OD. Differently, DIO mainly enables PC61BM diffusing into PffBT4T‐2OD polymer matrix, leading to enlarged donor–acceptor (D–A) interface. As expected, by combining AA and DIO, the composite film provides large D–A interface and more balanced charge carrier transport. Consequently, their beneficial synergistic effect results in enhanced short circuit current and fill factor, and thereby increased power conversion efficiency of 10.64%, improved by 16% with respect to the control device. Herein, a general mechanism of enhancing device performance via the combination of solvent additives with different contributions to photoactive film is unveiled.

Atom‐thick 2D WS₂ nanosheets’ electron‐transport layers (ETLs) facilitate enhanced coupling with the perovskite absorber layer, promoting a highly active interfacial charge transfer dynamic. The single‐crystalline nature of the WS₂ ETL also provides the facile transportation of photogenerated electrons to the electrode for a high‐performance and high‐stability perovskite solar cell.

The structure and the electronic properties of the electron‐transport layer (ETL) of perovskite solar cells (PSCs) govern the interfacial charge transfer and charge transportation to the electrode. The ETLs of two dimensions, that are atom thick, and have a planar structure that possesses special electronic properties, such as the surface collective motion of excitons or charge transfer–driven defect state relief, that is 2D transition metal dichalcogenide, allow a highly energetic carrier dynamic process for enhanced photovoltaic effect. Herein, it is discovered that planar, few‐atom‐thick 2H–WS₂ nanosheets' ETLs drive ultrafast charge transfer and transportation along the ETL during the photovoltaic process. Time‐resolved photoluminescence and electrochemical impedance spectroscopy analysis results indicate that the charge transfer from the perovskite to the ETL occurs as fast as 5.9 ns with charge transfer resistance as low as 25.6 Ω. This allows the PSC device to produce a power conversion efficiency of 18.21% with short‐circuit current density, open‐circuit voltage, and fill factor as high as 22.24 mA cm², 1.12 V, and 0.731, respectively. The PSC retains 96.87% of its performance when being aged in nitrogen atmosphere for 33 days. Atom‐thick planar WS₂ ETL nanosheets can be the basis for the development of high‐performance PSC devices.

Pb and Pb‐free perovskite absorbers are analyzed using a 1D simulator for n‐i‐p devices. SCAPS‐1D simulations suggest: 1) theoretically determined efficiency limit of Cs₂PtI₆ perovskites is comparable with (FA,MA,Cs)Pb(I,Br)₃, 2) FA₄GeSbCl₁₂ is a promising photoabsorber; and 3) for efficient photoconversion with Sn‐, Ge‐, Ti‐, or Ag‐based perovskite absorbers, reduction in defect density and increase in absorption coefficient is needed.

Optoelectronic properties of organic–inorganic halide perovskites are exceptional with solar cells showing efficiency comparable with conventional photovoltaic technologies. However, with issues of material stability and toxicity of Pb, it is important to understand if Pb can be replaced while maintaining the high power conversion efficiencies of (FA,MA,Cs)Pb(I,Br)₃. Herein, practical efficiency limits of Pb and Pb‐free perovskite absorbers are analyzed using a 1D simulator for n‐i‐p or p‐i‐n device structures. SCAPS‐1D baseline models for perovskite absorber materials with and without Pb are developed to numerically reproduce the experimental current density–voltage (JV) and external quantum efficiency (EQE) of champion devices from literature. From these baseline models, the efficiency limits are determined based on optimizing the interface band alignments, reduction in midgap defect density, increased absorption coefficient, and no parasitic losses. SCAPS‐1D simulations suggest that 1) theoretically determined efficiency limit of Cs₂PtI₆ perovskites is comparable with (FA,MA,Cs)Pb(I,Br)₃ perovskites, 2) FA₄GeSbCl₁₂ is a promising photoabsorber; and 3) for efficient photoconversion with Sn‐, Ge‐, Ti‐, or Ag‐based compounds, a reduction of defect density and increase in absorption coefficient is needed.