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Nature Communications, Published online: 11 August 2020; doi:10.1038/s41467-020-17868-0

In article number https://doi.org/10.1002/aenm.2020007682000768, Atanu Jana, Jin Young Kim, Kwang S. Kim and co‐workers review the current and existing challenges facing perovskite stability. This Review focuses on materials, power conversion efficiencies, fundamental degradation mechanisms, and solar cell design for enhancing their environmental stability. Finally, existing challenges and a road map for future directions of perovskite stability are provided.

Emerging all‐inorganic CsPbI₃ perovskite quantum dots offer colloidal synthesis and processing using industrially friendly solvents. This is desirable for solar‐cell applications and will catalyze new structures for optoelectronic devices. In article number https://doi.org/10.1002/adma.2020004492000449, Jianyu Yuan, Wanli Ma, and co‐workers report the development of a new surface ligand management strategy, which improves on the state‐of‐the‐art efficiency of CsPbI₃ quantum‐dot solar cells.

Water is added into the precursor solution to assist crystal growths of quasi‐2D perovskite films featuring ordered phase distribution and favored crystal orientation. A champion efficiency of 18.04% is realized in (BA)₂(MA_0.8FA_0.15Cs_0.05)₄Pb₅I₁₆‐based quasi‐2D perovskite solar cells.

Abstract

Organic–inorganic hybrid quasi‐2D perovskites have shown excellent stability for perovskite solar cells (PSCs), while the poor charge transport in quasi‐2D perovskites significantly undermines their power conversion efficiency (PCE). Here, studies on water‐controlled crystal growth of quasi‐2D perovskites are presented to achieve high‐efficiency solar cells. It is demonstrated that the (BA)₂MA₄Pb₅I₁₆‐based PSCs (n = 5) processed with water‐containing precursors display an increased short‐circuit current density (J _sc) of 19.01 mA cm⁻² and PCE over 15%. The enhanced performance is attributed to synergetic growths of the 3D and 2D phase components aided by the formed hydration (MAI∙H₂O), leading to modulations on the crystal orientation and phase distribution of various n‐value components, which facilitate interphase charge transfer and charge sweepout throughout the device. The water‐assisted crystallization is further applied to triple cation‐based (BA)₂(MA_0.8FA_0.15Cs_0.05)₄Pb₅I₁₆ quasi‐2D perovskites, which generate a remarkable PCE of 18.04%. Despite the presence of water in the precursors, the devices exhibit a satisfactory thermal stability with the PCE degradation <15% under continuous thermal aging at 60 °C for over 500 h.

Publication date: December 2020

Source: Nano Energy, Volume 78

Author(s): Yanan Wang, Yajie Dong, Qi Liu, Xia Guo, Maojie Zhang, Yongfang Li

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

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.

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.

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.

An ultra‐flexible and transparent biomass‐derived conductive substrate is fabricated from polylactic acid. It exhibits high mechanical durability even when subjected to 15 000 bending cycles. Perovskite solar cells based on the biomass electrodes show good mechanical stability, retaining over 86% of its original power conversion efficiency after bending 1500 times at a curvature radius of 5 mm.

Biomass substrates are urgently needed to develop green electronics. Herein, an ultra‐flexible and transparent biomass‐derived conductive substrate is originated from nature polylactic acid (PLA) with silver nanowires (AgNWs) modified by PH1000. The composite electrode exhibits low sheet resistance of 25 Ω sq⁻¹, high transmittance (over 82% in the region of 400–800 nm), and excellent mechanical durability. After bending tests of 15 000 times at a curvature radius of 3 and 5 mm, the sheet resistances of the composite electrodes only increase to 89 and 51 Ω sq⁻¹, respectively. Biomass electrode–based flexible perovskite solar cells are demonstrated with a champion power conversion efficiency (PCE) of 11.44% and high bending tolerance with preserving over 86% of the initial PCE after 1500 bending cycles at a curvature radius of 5 mm. The biomass electrode exhibits great potential for the development of green flexible devices.

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.

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