Chen Weijie

UV‐ozone treatments for different times (0, 10, 30, and 60 min) are examined on the 2D metallic Ti₃C₂T_x films to take advantage of the tunable optoelectronic properties of MXenes as electron transport layers in low‐temperature processed planar‐structured perovskite solar cells, resulting in augmentation of the power conversion efficiency (PCE) from 5.00% to the champion PCE of 17.17%.

Abstract

MXenes are a large and rapidly expanding family of 2D materials that, owing to their unique optoelectronic properties and tunable surface termination, find a wide range of applications including energy storage and energy conversion. In this work, Ti₃C₂T_x MXene nanosheets are applied as a novel type of electron transport layer (ETL) in low‐temperature processed planar‐structured perovskite solar cells (PSCs). Interestingly, simple UV‐ozone treatment of the metallic Ti₃C₂T_x that increases the surface TiO bonds without any change in its bulk properties such as high electron mobility improves its suitability as an ETL. Improved electron transfer and suppressed recombination at the ETL/perovskite interface results in augmentation of the power conversion efficiency (PCE) from 5.00% in the case of Ti₃C₂T_x without UV‐ozone treatment to the champion PCE of 17.17%, achieved using the Ti₃C₂T_x film after 30 min of UV‐ozone treatment. As the first report on the use of pure MXene layer as an ETL in PSCs, this work shows the great potential of MXenes to be used in PSCs and displays their promise for applications in photovoltaic technology in general.

An efficient control of the film quality and thickness distribution of alternating cations in the interlayer space of 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) quantum wells via incorporation of methylammonium chloride as an additive is demonstrated. The optimized device leads to more efficient charge transport and suppressed nonradiative charge recombination. Consequently, the optimized perovskite solar cell delivers an efficiency of 18.48%.

Abstract

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge‐transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low‐n and high‐n QWs and suppressed nonradiative charge recombination. Therefore, the additive‐treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open‐circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden–Popper, and Dion–Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.

Nature Materials, Published online: 16 September 2019; doi:10.1038/s41563-019-0480-7

Herein, amorphous‐Zn₂SnO₄ (am‐ZTO) is used to provide a large free energy difference (ΔG) to improve electron injection from perovskite to electron transport layers. In addition, the introduction of the am‐ZTO also leads to a dense physical contact between the am‐ZTO and the FTO substrate, leading to decreased leakage current. The optimized device exhibits a power conversion efficiency of 20.04%.

Charge extraction by electron transport layers (ETLs) plays a vital role in improving the performance of perovskite solar cells (PSCs). Here, PSCs with four different types of ETLs, such as SnO₂, amorphous‐Zn₂SnO₄ (am‐ZTO), am‐ZTO/SnO₂, and SnO₂/am‐ZTO, are successfully synthesized. The interface recombination behavior and the charge transport properties of the devices affected by four types of ETLs are systematically investigated. For dual am‐ZTO/SnO₂ ETLs, compact am‐ZTO ETL prepared by the pulsed laser deposition method provides a dense physical contact with FTO than the spin coating films, decreasing leakage current and improving charge collection at the interface of ETL/FTO. Moreover, dual am‐ZTO/SnO₂ ETLs lead to large free energy difference (ΔG), improving electron injection from perovskite to ETLs. One additional electron pathway from perovskite to am‐ZTO is formed, which can also improve electron injection efficiency. A power conversion efficiency of 20.04% and a stabilized efficiency of 19.17% are achieved for the device based on dual am‐ZTO/SnO₂ ETLs. Most importantly, the devices are fabricated at a low temperature of 150 °C, which offers a potential method for large‐scale production of PSCs, and paves the way for the development of flexible PSCs. It is believed that this work provides a strategy to design ETLs via controlling ΔG and interface contact to improve the performance of PSCs.

Recent progress in bandgap engineering strategies including the two main, widely used impurity and pressure as well as intermediate band, external electric field, and steric methods are reviewed comprehensively. Their underlying mechanism, achievements, and challenges are outlined. Additionally, future research directions are provided to realize direct and gap size continually tunable perovskites for further enhancing solar cell performance.

Metal halide perovskites are attractive for highly efficient solar cells. As most perovskites suffer large or indirect bandgap compared with the ideal bandgap range for single‐junction solar cells, bandgap engineering has received tremendous attention in terms of tailoring perovskite band structure, which plays a key role in light harvesting and conversion. In this Review, various reported bandgap engineering strategies are summarized. The recently widely used two main strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap engineering are also investigated. Moreover, future research directions are outlined to guide the further investigation.

Using the oxygen‐containing functional group (—COOH and/or —C—OH)‐decorated multiwalled carbon nanotubes as the electrode, the power conversion efficiency of perovskite solar cells shows an improvement after long‐term storage. The reason is confirmed to be the self‐reconstruction ability of perovskite material and the interface reconstruction for its morphology and charge transfer resistance showing significant improvement.

Perovskite solar cells (PSCs) have attracted a lot of interest because of their high efficiency and low cost. However, in commercial applications, standard PSCs suffer from low stability of the cell components, including the hole transportation material (HTM). Owing to their characteristics of high chemical stability, hydrophobicity, and high conductivity, carbon nanotubes (CNTs) can be an alternative electrode to use to form HTM‐free PSCs. Enhancing the interaction with perovskite is vital not only for photovoltaic performance but also for the stability of CNT‐based PSCs. Herein, oxygen‐containing functional groups are introduced into CNTs via acid treatment to enhance the chemical interactions with perovskite. The self‐recrystallization ability of the perovskite material is discovered; its morphology shows significant improvement after long‐term storage. Results show that acid oxidization of CNTs enable the self‐recrystallization characteristics of MAPbI₃‐induced interfacial improvement, such that even with a dispersed initial photovoltaic performance, through storage in an ambient medium with relative humidity of 20–50%, the PSCs possess better interface contact, which results in lower charge transfer resistance, higher photovoltaic performance, and stability. As a result, PSCs with an initial power conversion efficiency range of 3.21–7.89% finally converge to within the range of 9.54–12.14% after long‐term storage.

Mesoporous boron‐doped TiO₂ (B‐TiO₂) is demonstrated as an improved electron transport layer (ETL) for perovskite solar cells for the reduction of hysteresis. The incorporation of boron dopant in TiO₂ ETL not only reduces the hysteresis but also improves device performance. Consequently, a methylammonium lead iodide photovoltaic device based on B‐TiO₂ ETL achieves a promising efficiency of 20.51% with negligible hysteresis.

Abstract

Perovskite solar cells (PSCs) have witnessed astonishing improvement in power conversion efficiency (PCE), more recently, with advances in long‐term stability and scalable fabrication. However, the presence of an anomalous hysteresis behavior in the current density–voltage characteristic of these devices remains a key obstacle on the road to commercialization. Herein, sol–gel‐processed mesoporous boron‐doped TiO₂ (B‐TiO₂) is demonstrated as an improved electron transport layer (ETL) for PSCs for the reduction of hysteresis. The incorporation of boron dopant in TiO₂ ETL not only reduces the hysteresis behavior but also improves PCE of the perovskite device. The simultaneous improvements are mainly ascribed to the following two reasons. First, the substitution of under‐coordinated titanium atom by boron species effectively passivates oxygen vacancy defects in the TiO₂ ETL, leading to increased electron mobility and conductivity, thereby greatly facilitating electron transport. Second, the boron dopant upshifts the conduction band edge of TiO₂, resulting in more efficient electron extraction with suppressed charge recombination. Consequently, a methylammonium lead iodide (MAPbI₃) photovoltaic device based on B‐TiO₂ ETL achieves a higher efficiency of 20.51% than the 19.06% of the pure TiO₂ ETL based device, and the hysteresis is reduced from 0.13% to 0.01% with the B‐TiO₂ based device showing negligible hysteresis behavior.

In this work, a “perovskite” template‐assisted structure is developed to fabricate high‐quality α‐FAPbI₃. The δ‐FAPbI₃ phases are avoided. Defects are substantially reduced with an excellent light harvesting. A power conversion efficiency of 21.24% (the highest efficiency reported for pure α‐FAPbI₃) is achieved. It also realizes a great stability in 800 h thermal ageing and 500 h light soaking.

Abstract

Formamidinium (FA) lead halide (α‐FAPbI₃) perovskites are promising materials for photovoltaic applications because of their excellent light harvesting capability (absorption edge 840 nm) and long carrier diffusion length. However, it is extremely difficult to prepare a pure α‐FAPbI₃ phase because of its easy transformation into a nondesirable δ‐FAPbI₃ phase. In the present study, a “perovskite” template (MAPbI₃‐FAI‐PbI₂‐DMSO) structure is used to avoid and suppress the formation of δ‐FAPbI₃ phases. The perovskite structure is formed via postdeposition involving the treatment of colloidal MAI‐PbI₂‐DMSO film with FAI before annealing. In situ X‐ray diffraction in vacuum shows no detectable δ‐FAPbI₃ phase during the whole synthesis process when the sample is annealed from 100 to 180 °C. This method is found to reduce defects at grain boundaries and enhance the film quality as determined by means of photoluminescence mapping and Kelvin probe force microscopy. The perovskite solar cells (PSCs) fabricated by this method demonstrate a much‐enhanced short‐circuit current density (  J _sc) of 24.99 mA cm⁻² and a power conversion efficiency (PCE) of 21.24%, which is the highest efficiency reported for pure FAPbI₃, with great stability under 800 h of thermal ageing and 500 h of light soaking in nitrogen.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Jianxing Xia, Junsheng Luo, Hua Yang, Chunlin Sun, Zhongquan Wan, Haseeb Ashraf Malik, Haoli Zhang, Yu Shi, Chunyang Jia

Graphical abstract

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Linxiang Zeng, Zongao Chen, Shudi Qiu, Jinlong Hu, Chaohui Li, Xianhu Liu, Guangxing Liang, Christoph J. Brabec, Yaohua Mai, Fei Guo

Graphical abstract

All inorganic: The crystal structure, phase transition, band gap and optical properties of perovskite‐phase RbPbBr₃ were analyzed theoretically and experimentally. This new perovskite microsphere can serve as a gain medium and microcavity to achieve broadband (475–540 nm) single‐mode lasing with a high Q of about 2100.

Abstract

Rubidium lead halides (RbPbX₃), an important class of all‐inorganic metal halide perovskites, are attracting increasing attention for photovoltaic applications. However, limited by its lower Goldschmidt tolerance factor t≈0.78, all‐inorganic RbPbBr₃ has not been reported. Now, the crystal structure, X‐ray diffraction (XRD) pattern, and band structure of perovskite‐phase RbPbBr₃ has now been investigated. Perovskite‐phase RbPbBr₃ is unstable at room temperature and transforms to photoluminescence (PL)‐inactive non‐perovskite. The structural evolution and mechanism of the perovskite–non‐perovskite phase transition were clarified in RbPbBr₃. Experimentally, perovskite‐phase RbPbBr₃ was realized through a dual‐source chemical vapor deposition and annealing process. These perovskite‐phase microspheres showed strong PL emission at about 464 nm. This new perovskite can serve as a gain medium and microcavity to achieve broadband (475–540 nm) single‐mode lasing with a high Q of about 2100.

Nature Communications, Published online: 10 September 2019; doi:10.1038/s41467-019-12132-6

By rationally tuning the molecular interaction and energy level alignments of the donors and acceptors, when both donor and acceptor are fluorinated or both are not fluorinated, high‐performance organic photovoltaics can be realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, devices based on the PBDB‐T‐F/Y1‐4F blend and PBDB‐T‐F/Y6 exhibit power conversion efficiencies as high as 14.8% and 15.9%, respectively.

Abstract

The performance of organic photovoltaics (OPVs) has rapidly improved over the past years. Recent work in material design has primarily focused on developing near‐infrared nonfullerene acceptors with broadening absorption that pair with commercialized donor polymers; in the meanwhile, the influence of the morphology of the blend film and the energy level alignment on the efficiency of charge separation needs to be synthetically considered. Herein, the selection rule of the donor/acceptor blend is demonstrated by rationally considering the molecular interaction and energy level alignment, and highly efficient OPV devices using both‐fluorinated or both‐nonfluorinated donor/acceptor blends are realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, the devices based on the PBDB‐T‐F/Y1‐4F blend and PBDB‐T‐F/Y6 exhibit champion power conversion efficiencies as high as 14.8% and 15.9%, respectively.

Nature Materials, Published online: 09 September 2019; doi:10.1038/s41563-019-0478-1