shuhui

Publication date: 20 February 2019

Source: Joule, Volume 3, Issue 2

Author(s): Jie Ding, Qiwei Han, Qian-Qing Ge, Ding-Jiang Xue, Jing-Yuan Ma, Bo-Ya Zhao, Yao-Xuan Chen, Jie Liu, David B. Mitzi, Jin-Song Hu

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Publication date: February 2019

Source: Nano Energy, Volume 56

Author(s): Hang Guo, Junqing Zhao, Qingshun Dong, Liduo Wang, Xueyan Ren, Song Liu, Chi Zhang, Guifang Dong

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Publication date: February 2019

Source: Nano Energy, Volume 56

Author(s): Xingyue Liu, Xianhua Tan, Zhiyong Liu, Haibo Ye, Bo Sun, Tielin Shi, Zirong Tang, Guanglan Liao

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Boosting the efficiency of carbon-based planar CsPbBr₃ perovskite solar cells by a modified multistep spin-coating technique and interface engineering is demonstrated. A champion power conversion efficiency (PCE) of 8.79% is obtained for the as-prepared PSCs, which is the highest efficiency for planar CsPbBr₃ PSCs reported yet.

Lead‐free 2D perovskites: Using symmetrical imidazolium‐based cations, 2D tin perovskites with suitable band gaps and improved stability for solar cell applications could be obtained. Hole‐transport material (HTM)‐free devices show encouraging power conversion efficiencies measured under 1 sun illumination in ambient conditions.

Abstract

Organic‐inorganic hybrid perovskites have attracted great attention over the last few years as potential light‐harvesting materials for efficient and cost‐effective solar cells. However, the use of lead iodide in state‐of‐the‐art perovskite devices may demonstrate an obstacle for future commercialization due to toxicity of lead. Herein we report on the synthesis and characterization of low dimensional tin‐based perovskites. We found that the use of symmetrical imidazolium‐based cations such as benzimidazolium (Bn) and benzodiimidazolium (Bdi) allow the formation of 2D perovskites with relatively narrow band gaps compared to traditional ‐NH₃ ⁺ amino groups, with optical band gap values of 1.81 eV and 1.79 eV for Bn₂SnI₄ and BdiSnI₄ respectively. Furthermore, we demonstrate that the optical properties in this class of perovskites can be tuned by formation of a quasi 2D perovskite with the formula Bn₂FASn₂I₇. Additionally, we investigate the change in band gap in the mixed Sn/Pb solid solution Bn₂Sn _x Pb _x−1I₄. Devices fabricated with Bn₂SnI₄ show promising efficiencies of around 2.3 %.

Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): Qian Kang, Long Ye, Bowei Xu, Cunbin An, Samuel J. Stuard, Shaoqing Zhang, Huifeng Yao, Harald Ade, Jianhui Hou

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In article no. 1800173, Baomin Xu and Chun Cheng describe an effective strategy for introducing a polymer‐assisted small molecule layer as a hole transport layer to develop highly efficient inverted perovskite solar cells, in which the champion device exhibits a power conversion efficiency of >21%. A much smoother surface for the mixed hole transport layer and largely reduced charge recombination is obtained, and the energy level alignment is more matched with that of the perovskite.

Cellulose paper, as one of the four great inventions of China, possesses admirable properties like biocompatibility, biodegradability, and low cost and is a promising alternative to conventional substrates for perovskite solar cells (PSCs). In article no. 1800175, Bingqiang Cao, Jinghua Yu, and co‐workers fabricate a flexible holetransport‐ material‐free PSC with a power conversion efficiency of 9.05% on the cellulose paper, for the first time, which could be utilized in wearable electronics.

This research provides insightful investigation into the optical and electrical properties of alkali metals doped Sb2S3 films. The results show that Cs‐doping leads to the most efficient improvement in carrier concentration and film formability. The device delivers a power conversion efficiency of 6.56%, which is the highest efficiency in planar heterojunction Sb2S3 solar cells.

Sb₂S₃ as a stable light harvesting material has received increasing attention for solar cell applications. To improve the device efficiency, much effort has been put into the materials synthesis, interfacial engineering, and device structure design. Here, it is demonstrated that doping of alkali metal (Li, Na, K, Rb, Cs) ions essentially affects the morphological, crystal, optical and electrical properties of Sb₂S₃ films. The structural and compositional analysis shows that the doping is in form of alkali metal‐sulfur chemical bond at both surface and grain boundaries of Sb₂S₃ films. Compared with the pristine Sb₂S₃, we observe that Li and Na doping are not able to improve the device efficiency, while K, Rb, and Cs notably increase energy conversion efficiency. The most effective enhancement is found in Cs‐doped Sb₂S₃, where the carrier concentration, crystallinity, and film formability show remarkable improvement. The device based on the Cs‐Sb₂S₃ film delivers a power conversion efficiency of 6.56%, which is the highest efficiency in planar heterojunction Sb₂S₃ solar cells, regardless of whether the films are fabricated by a solution process or thermal deposition. This research identifies that doping of heavy alkali metals is an effective approach to improve the performance of Sb₂S₃ solar cells.

Perovskite oxides with high piezoelectricity and low bandgap are realized by an efficient strategy. For example, the Ni²⁺ mediated (1‐x)Na_0.5Bi_0.5TiO₃‐xBa(Ti_0.5Ni_0.5)O_3–δ with morphotropic phase boundary composition, shows enhanced piezoelectricity and exhibits three gap states with lowest sub‐bandgap of 0.9 eV that is responsible for visible/near‐infrared absorption. The designed ferroelectric semiconductor is very promising for multifunctional applications.

Abstract

Defect‐engineered perovskite oxides that exhibit ferroelectric and photovoltaic properties are promising multifunctional materials. Though introducing gap states by transition metal doping on the perovskite B‐site can obtain low bandgap (i.e., 1.1–3.8 eV), the electrically leaky perovskite oxides generally lose piezoelectricity mainly due to oxygen vacancies. Therefore, the development of highly piezoelectric ferroelectric semiconductor remains challenging. Here, inspired by point‐defect‐mediated large piezoelectricity in ferroelectrics especially at the morphotropic phase boundary (MPB) region, an efficient strategy is proposed by judiciously introducing the gap states at the MPB where defect‐induced local polar heterogeneities are thermodynamically coupled with the host polarization to simultaneously achieve high piezoelectricity and low bandgap. A concrete example, Ni²⁺‐mediated (1–x)Na_0.5Bi_0.5TiO₃‐xBa(Ti_0.5Ni_0.5)O_3–δ (x = 0.02–0.08) composition is presented, which can show excellent piezoelectricity and unprecedented visible/near‐infrared light absorption with a lowest ever bandgap ≈0.9 eV at room temperature. In particular, the MPB composition x = 0.05 shows the best ferroelectricity/piezoelectricity (d ₃₃ = 151 pC N^–1, Pr = 31.2 μC cm^–2) and a largely enhanced photocurrent density approximately two orders of magnitude higher compared with classic ferroelectric (Pb,La)(Zr,Ti)O₃. This research provides a new paradigm for designing highly piezoelectric and visible/near‐infrared photoresponsive perovskite oxides for solar energy conversion, near‐infrared detection, and other multifunctional applications.

A new method to control current–voltage hysteresis of planar structured FA_0.83Cs_0.17PbI₃ perovskite solar cells (PSCs) is presented while enhancing device efficiency through tailoring the crystal structure of the perovskite compound with a guanidinium cation (Gu⁺)‐dopant. New insights into the correlation of the dynamics of device hysteresis with the interfacial charge recombination and accumulation in the PSCs are revealed.

Abstract

Current–voltage hysteresis of perovskite solar cells (PSCs) has raised the concern of accurate performance measurement in practice. Although various theories have been proposed to elucidate this phenomenon, the origin of hysteresis is still an open question. Herein, the use of guanidinium cation (Gu⁺)‐dopant is demonstrated to tailor the crystal structure of mixed‐cation formamidinium‐cesium lead triiodide (FA_0.83Cs_0.17PbI₃) perovskite, resulting in an improved energy conversion efficiency and tunable current–voltage hysteresis characteristic in planar solar cells. Particularly, when the concentration of Gu‐dopant for the perovskite film increases, the normal hysteresis initially observed in the pristine PSC is first suppressed with 2%‐Gu‐dopant, then changed to inverted hysteresis with a higher Gu‐dopant. The hysteresis tunability behavior is attributed to the interplay of charge/ion accumulation and recombination at interfaces in the PSC. Furthermore, compared to the cell without Gu⁺‐dopant, the optimal content of 2% Gu⁺‐dopant also increases the device efficiency by 14%, reaching over 17% under one sun illumination.

F4‐TCNQ is applied to manipulate the morphological, electrical, and photovoltaic properties of nonfullerene solar cells. Adding a trace amount of F4‐TCNQ yields a higher current density and fill factor, in comparison to the reference device. The combined techniques evidence that the addition of F4‐TCNQ increases charge lifetime, charge mobility, and mean‐square composition variation.

Abstract

Fluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells.

Publication date: 21 November 2018

Source: Joule, Volume 2, Issue 11

Author(s): Yanfa Yan

The absence of a charge transfer (CT) state is found in the (6,5) single‐walled carbon nanotube:PC₇₀BM system and a detailed analysis of the open‐circuit voltage (V _OC) is reported. The analysis reveals that the lack of the CT state enables very small radiative as well as nonradiative V _OC losses for an organic cell, despite the ultranarrow bandgap of this system.

Abstract

Current state‐of‐the‐art organic solar cells (OSCs) still suffer from high losses of open‐circuit voltage (V _OC). Conventional polymer:fullerene solar cells usually exhibit bandgap to V _OC losses greater than 0.8 V. Here a detailed investigation of V _OC is presented for solution‐processed OSCs based on (6,5) single‐walled carbon nanotube (SWCNT): [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester active layers. Considering the very small optical bandgap of only 1.22 eV of (6,5) SWCNTs, a high V _OC of 0.59 V leading to a low E _gap/q − V _OC = 0.63 V loss is observed. The low voltage losses are partly due to the lack of a measurable charge transfer state and partly due to the narrow absorption edge of SWCNTs. Consequently, V _OC losses attributed to a broadening of the band edge are very small, resulting in V _OC,SQ − V _OC,rad = 0.12 V. Interestingly, this loss is mainly caused by minor amounts of SWCNTs with smaller bandgaps as well as (6,5) SWCNT trions, all of which are experimentally well resolved employing Fourier transform photocurrent spectroscopy. In addition, the low losses due to band edge broadening, a very low voltage loss are also found due to nonradiative recombination, ΔV _OC,nonrad = 0.26 V, which is exceptional for fullerene‐based OSCs.

Four new polymers containing the novel asymmetrical backbone, thienobenzodithiophene, are synthesized and applied in high‐performance nonfullerene solar cells. The asymmetrical backbone can dramatically effect the polymer geometric configuration and modulate the polymer aggregation and crystallinity. This work reveals that the versatile asymmetric backbone is an excellent moiety to construct light‐harvesting copolymers and to modulate the microstructure for highly efficient PSCs.

Abstract

In this work, a new asymmetrical backbone thienobenzodithiophene (TBD) containing four aromatic rings is designed, and then four polymers PTBD‐BZ, PTBD‐BDD, PTBD‐FBT, and PTBD‐Tz are synthesized. The planar and high degree of π‐conjugation configuration can guarantee effective charge carrier transport and the distinct flanked dihedral angles between the TBD core and conjugated side chain can subtly regulate the molecular aggregation and crystallinity. The four polymer/3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]‐dithiophene (ITIC) blending films exhibit predominantly face‐on orientation. The photovoltaic devices based on wide bandgap polymers PTBD‐BZ and PTBD‐BDD achieve power conversion efficiencies (PCEs) as high as 12.02% and 11.39% without any post‐treatment. For the medium bandgap polymers PTBD‐FBT and PTBD‐Tz, the devices also show good PCEs of 10.18% and 11.02% with high V _OC of 0.94 and 1.02 V, respectively, which indicates simultaneously achieving a V _OC > 1 V and a high J _SC is feasible to further improve the PSCs' performance by modifying this new backbone. This work reveals that the versatile asymmetric backbone is an excellent moiety to construct light‐harvesting copolymers and to modulate the microstructure for highly efficient PSCs.

The intrinsic instability of metal halide perovskites is one of the main factors that limit the commercialization of perovskite solar cells. This review highlights the recent progress in the composition engineering of metal halide perovskites for improving the stability of perovskite solar cells. The strategy of using mixed‐ion hybrid perovskites, low‐dimensional hybrid perovskites, and all‐inorganic perovskites is discussed in detail.

Abstract

Metal halide perovskite solar cells (PSCs) have emerged as promising candidates for photovoltaic technology with their power conversion efficiencies over 23%. For prototypical organic–inorganic metal halide perovskites, their intrinsic instability poses significant challenges to the commercialization of PSCs. Recently, the scientific community has done tremendous work in composition engineering to develop more robust light‐absorbing layers, including mixed‐ion hybrid perovskites, low‐dimensional hybrid perovskites, and all‐inorganic perovskites. This review provides an overview of the impact of these perovskites on the efficiency and long‐term stability of PSCs.

The bulk and surface defects of perovskite films are suppressed by using SnO₂/TiO₂ double layer oxide, addition of methylammonium chloride (MACl) as a crystallization aid to the precursor solution, and surface passivation of perovskite films with iodine solution, due to the formation of high‐quality large‐grain perovskite films and retardation of radiationless carrier recombination.

Abstract

The presence of bulk and surface defects in perovskite light harvesting materials limits the overall efficiency of perovskite solar cells (PSCs). The formation of such defects is suppressed by adding methylammonium chloride (MACl) as a crystallization aid to the precursor solution to realize high‐quality, large‐grain triple A‐cation perovskite films and that are combined with judicious engineering of the perovskite interface with the electron and hole selective contact materials. A planar SnO₂/TiO₂ double layer oxide is introduced to ascertain fast electron extraction and the surface of the perovskite facing the hole conductor is treated with iodine dissolved in isopropanol to passivate surface trap states resulting in a retardation of radiationless carrier recombination. A maximum solar to electric power conversion efficiency (PCE) of 21.65% and open circuit photovoltage (V _oc) of ≈1.24 V with only ≈370 mV loss in potential with respect to the band gap are achieved, by applying these modifications. Additionally, the defect healing enhances the operational stability of the devices that retain 96%, 90%, and 85% of their initial PCE values after 500 h under continuously light illumination at 20, 50, and 65 °C, respectively, demonstrating one of the most stable planar PSCs reported so far.

In article number 1801892, Steve Albrecht, Vytautas Getautis and co‐workers demonstrate a novel promising concept for the formation of a hole selective monolayer in perovskite solar cells. A low temperature dopant‐free technique makes it suitable for different substrates.