Chen Weijie

A template‐assisted solution‐processed technique is developed to fabricate 3D nanowire‐coated macroporous titania thin films with outstanding optical and electrical properties. Perovskite solar cells based on newly prepared TiO₂ electron‐transporting layer deliver an impressive power conversion efficiency of up to 20.1% owing to enhanced light harvesting and facilitated charge collection.

Abstract

Microscopic design and morphological engineering of the semiconducting metal oxide as electron‐transporting layers (ETLs) is of vital importance for optical enhancement, photonic structuring, and charge collection optimization within optoelectronic devices. Herein, nanowire‐coated, branched macroporous titania (BMT) thin films are reported as a new type of ETL prepared by using silica spheres as a sacrificial template, followed by a sol–gel and subsequent alkaline‐assisted etching process. The BMT films feature 3D hierarchical structures and interconnected networks with tunable pore sizes, branch densities, and film thicknesses. The titania films are employed as ETLs in perovskite solar cells (PSCs), resulting in remarkable power conversion efficiencies (PCEs) of 20.1%; a noticeable 16% increase compared with titania nanowire (TNW) ETL‐based counterparts (PCE = 17.3%). The superior device performance of the BMT‐based PSCs can be attributed to the maximized light harvesting and charge collection capabilities. These beneficial properties are derived from the effective infiltration of the perovskite precursor into the titania macropores, efficient light confinement within the macropore structure, and the textured perovskite capping layer, as well as enhanced charge transport and reduced charge recombination through the BMT architecture. This work demonstrates a simple and effective approach for constructing branched macroporous metal‐oxide photoelectrodes toward high‐performance photovoltaic devices.

A very low‐cost and low‐temperature photovoltaic cell based on dopant‐free contact (transition metal oxide/n‐SiNWs/alkali metal salt) and Si nanowires arrays show a power conversion efficiency of 16.9%. Insights into the interaction between the self‐assembling interface passivation, interface polarities, and the performance of the device is demonstrated.

Abstract

Low‐cost and efficient interfacial layer construction with the required charge selectivity and compatibility is necessary for nanostructured solar cells, and the proper integration of the interfacial layer with the light‐trapping system is required to improve the power conversion efficiency of the cell. Herein, low‐cost Si nanowires‐based solar cells with tunneling heterojunctions are developed by the deposition of MoO _x and spin‐coating of Cs₂CO₃ as the carrier‐selective layers. The power conversion efficiency of 16.9% for a device of 4 cm² in area is achieved by Si nanowires solar cells by the self‐assembly of ultra‐thin SiO _x as the surface tunneling passivation layer. Self‐assembly is realized with an ultraviolet O₃ treatment process at room temperature. Quasi‐steady‐state photoconductance, microwave‐detected photoconductance decay, and constant current–voltage measurements are used to characterize the passivation quality and tunneling transportation properties of the ultra‐thin SiO _x layers. Interfacial charge recombination is suppressed and effective carrier tunneling properties are developed by the growth of ≈1.5 nm thick SiO _x layers on the surfaces of the Si nanowires. This proposed Si nanowires solar cell architecture featuring tunneling heterojunctions achieves high performance and may be suitable for fabricating industrialized Si nanowires‐based photovoltaic devices through a cost‐effective, simple, and low‐temperature process.

The primary factor limiting the efficiency of photocurrent generation is identified in polymer:nonfullerene acceptor (NFA) blends and its implications for materials design in the optimisation of organic solar cell performance is discussed. The magnitude of this geminate recombination loss pathway is found to be the key determinant of the efficiency of photocurrent generation in polymer:NFA blend solar cells.

Abstract

Herein, a meta‐analysis of the device performance and transient spectroscopic results are undertaken for various donor:acceptor blends, employing three different donor polymers and seven different acceptors including nonfullerene acceptors (NFAs). From this analysis, it is found that the primary determinant of device external quantum efficiency (EQE) is the energy offset driving interfacial charge separation, ΔE _CS. For devices employing the donor polymer PffBT4T blended with NFA and fullerene acceptors, an energy offset ΔE _CS = 0.30 eV is required to achieve near unity charge separation, which increases for blends with PBDTTT‐EFT and P3HT to 0.36 and ≈1.2 eV, respectively. For blends with PffBT4T and PBDTTT‐EFT, a 100 meV decrease in the LUMO of the acceptor is observed to result in an approximately twofold increase in EQE. Steady state and transient optical data determine that this energy offset requirement is not associated with the need to overcome the polymer exciton binding energy and thereby drive exciton separation, with all blends studied showing efficient exciton separation. Rather, the increase in EQE with larger energy offset is shown to result from suppression of geminate recombination losses. These results are discussed in terms of their implications for the design of donor/NFA interfaces in organic solar cells, and strategies to achieve further advances in device performance.

Efficient nonfullerene organic solar cells (OSCs) are realized by combining a donor polymer, PffBT2T‐TT, and a small‐molecular acceptor, O‐IDTBR, which have identical bandgaps and close energy levels. Despite the small energy offsets for both hole and electron transfer, this system can still achieve efficient charge separation and a high efficiency of 10.4%.

Abstract

State‐of‐the‐art organic solar cells (OSCs) typically suffer from large voltage loss (V _loss) compared to their inorganic and perovskite counterparts. There are some successful attempts to reduce the V _loss by decreasing the energy offsets between the donor and acceptor materials, and the OSC community has demonstrated efficient systems with either small highest occupied molecular orbital (HOMO) offset or negligible lowest unoccupied molecular orbital (LUMO) offset between donors and acceptors. However, efficient OSCs based on a donor/acceptor system with both small HOMO and LUMO offsets have not been demonstrated simultaneously. In this work, an efficient nonfullerene OSC is reported based on a donor polymer named PffBT2T‐TT and a small‐molecular acceptor (O‐IDTBR), which have identical bandgaps and close energy levels. The Fourier‐transform photocurrent spectroscopy external quantum efficiency (FTPS‐EQE) spectrum of the blend overlaps with those of neat PffBT2T‐TT and O‐IDTBR, indicating the small driving forces for both hole and electron transfer. Meanwhile, the OSCs exhibit a high electroluminescence quantum efficiency (EQE_EL) of ≈1 × 10⁻⁴, which leads to a significantly minimized nonradiative V _loss of 0.24 V. Despite the small driving forces and a low V _loss, a maximum EQE of 67% and a high power conversion efficiency of 10.4% can still be achieved.

The influence of the surface effect of 2D layered perovskites before and after mechanical exfoliation is studied. The smooth 2D perovskite is less sensitive to ambient moisture and exhibits a considerably low dark current. This work reveals the strong dependence of the surface condition of 2D hybrid perovskite crystals on their moisture stability and optoelectronic properties.

Abstract

Despite the remarkable progress of optoelectronic devices based on hybrid perovskites, there are significant drawbacks, which have largely hindered their development as an alternative of silicon. For instance, hybrid perovskites are well‐known to suffer from moisture instability which leads to surface degradation. Nonetheless, the dependence of the surface effect on the moisture stability and optoelectronic properties of hybrid perovskites has not been fully investigated. In this work, the influence of the surface effect of 2D layered perovskites before and after mechanical exfoliation, representing rough and smooth surfaces of perovskite crystals, are studied. It is found that the smooth 2D perovskite is less sensitive to ambient moisture and exhibits a considerably low dark current, which outperforms the rough perovskites by 23.6 times in terms of photodetectivity. The superior moisture stability of the smooth perovskites over the rough perovskites is demonstrated. Additionally, ethanolamine is employed as an organic linker of the 2D layered perovskite, which further improves the moisture stability. This work reveals the strong dependence of the surface conditions of 2D hybrid perovskite crystals on their moisture stability and optoelectronic properties, which are of utmost importance to the design of practical optoelectronic devices based on hybrid perovskite crystals.

Publication date: November 2018

Source: Nano Energy, Volume 53

Author(s): Zhiwen Qiu, Ziqi Xu, Nengxu Li, Ning Zhou, Yihua Chen, Xingxing Wan, Jialiang Liu, Ning Li, Xiaotao Hao, Pengqing Bi, Qi Chen, Bingqiang Cao, Huanping Zhou

Abstract

Graphical abstract

Publication date: November 2018

Source: Nano Energy, Volume 53

Author(s): Cong Chen, Dali Liu, Yanjie Wu, Wenbo Bi, Xueke Sun, Xu Chen, Wei Liu, Lin Xu, Hongwei Song, Qilin Dai

Abstract

Graphical abstract

The dual interfacial modification mechanism represents an approach to achieve efficient PSCs with excellent stability, flexibility, large-area and well transmission.

Dopant‐free hole transporting materials for perovskite solar cells are reviewed. The efficiency growth of perovskite solar cells (PSCs) based on dopant‐free hole transporting materials (HTMs) is highlighted. Hydrophilic chemical dopants can accelerate the degradation of the perovskite layer. Dopant‐free HTMs play a significant role in both efficiency and stability of PSCs.

This article reviews various dopant‐free hole transporting materials (HTMs) used in perovskite solar cells (PSCs) in three main categories including inorganic, polymeric, and small molecule HTMs. PSCs have undergone rapid progress, achieving power conversion efficiencies (PCEs) above 22%. With their low production cost and high efficiencies, PSCs are considered promising next‐generation solar cell technology. In all developed architectures for PSCs, including planar and mesoscopic with conventional and inverted structures, HTMs play a significant role in determining the photovoltaic performance of PSCs. Using p‐type dopants, however, is considered a common strategy to increase the hole conductivity of HTM, which is usually compensated by a more complicated fabrication procedure, higher production costs, and lower stability of PSC. Although several reviews on HTMs have been published, progress on dopant free HTMs needs to be reviewed and analyzed. Here, a review covering most of the published reports on dopant‐free HTMs is presented, and the device structure and fabrication method, HTM layer deposition techniques, and the efficiency and the stability of PSCs are addressed during discussions in each main category. Finally, an outlook on stability and PCE growth in PSCs based on dopant‐free HTMs is presented.

A novel organic compound 1‐(ammonium acetyl) pyrene is successfully introduced for preparing the 2D/3D heterostructured MAPbI₃ perovskite. Because of the functional organic pyrene group with high humidity resistance and strong absorption in the ultraviolet region, the 2D/3D perovskite showed notable stability, comparable photovoltaic performance in humid air atmosphere and ultraviolet irradiation.

Abstract

Despite the eminent performance of the organometallic halide perovskite solar cells (PSCs), the poor stability for humidity and ultraviolet irradiation is still major problem for the commercialization of PSCs. Herein, a novel functional organic compound 1‐(ammonium acetyl)pyrene is successfully introduced for preparing the 2D/3D heterostructured MAPbI₃ perovskite. Because of the functional organic pyrene group with high humidity resistance and strong absorption in the ultraviolet region, the 2D/3D perovskite film shows notable stability with no degradation in ≈60% relative humidity after even six months and exhibits a high ultraviolet irradiation stability which keeps nearly no degradation after 1 h in the UV Ozone treatment. Planar PSCs are fabricated in the ≈60% relative humidity air outside glovebox. The champion efficiency of (PEY₂PbI₄)_0.02MAPbI₃ perovskite solar cells is 14.7% with nearly no hysteresis which is equal performance of 3D MAPbI₃ devices (15.0%). This work presents a new direction for enhancing the solar cells' performance and stability by incorporating a functional organic aromatic compound into the perovskite layer.

Fully‐solution‐processed regular architecture perovskite solar cells utilizing a low‐temperature‐printed carbon electrode are demonstrated. The carbon electrode is printed from toluene ink with the active layer architecture ITO/self‐assembled monolayer/MAPI/hole transport materials/Ta‐WO _x /carbon, which shows dramati‐cally increased stability and reasonable performance.

Abstract

Thin‐film solar cells based on hybrid organo‐halide lead perovskites achieve over 22% power conversion efficiency (PCE). A photovoltaic technology at such high performance is no longer limited by efficiency. Instead, lifetime and reliability become the decisive criteria for commercialization. This requires a standardized and scalable architecture which does fulfill all requirements for larger area solution processing. One of the most highly demanded technologies is a low temperature and printable conductive ink to substitute evaporated metal electrodes for the top contact. Importantly, that electrode technology must have higher environmental stability than, for instance, an evaporated silver (Ag) electrode. Herein, planar and entirely low‐temperature‐processed perovskite devices with a printed carbon top electrode are demonstrated. The carbon electrode shows superior photostability compared to reference devices with an evaporated Ag top electrode. As hole transport material, poly (3′hexyl thiophene) (P3HT) and copper(I) thiocyanate (CuSCN), two cost‐effective and commercially available p‐type semiconductors are identified to effectively replace the costlier 2,2′,7,7′‐Tetrakis‐(N,N‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐MeOTAD). While methylammonium lead iodide (MAPbI₃)‐based perovskite solar cells (PSCs) with an evaporated Ag electrode degrade within 100 h under simulated sunlight (AM 1.5), fully solution‐processed PSCs with printed carbon electrodes preserve more than 80% of their initial PCE after 1000 h of constant illumination.

Lanthanide ions are doped into CsPbBr₃ films to modulate crystal lattice for high‐performance all‐inorganic perovskite solar cells. Arising from the improved grain size and carrier lifetime, the solar cell achieves a champion power conversion efficiency of 10.14%, an ultrahigh V _oc of 1.594 V, and excellent stability.

Abstract

All‐inorganic cesium lead bromide (CsPbBr₃) perovskite solar cells have attracted enormous attention owing to their outstanding stability in comparison with organic–inorganic hybrid devices. The greatest weakness for inorganic CsPbBr₃ solar cells is their lower power conversion efficiencies, mainly arising from inferior light‐absorbance range and serious charge recombination at interfaces or within perovskite films. To address this issue, the lattice doping of lanthanide ions (Ln³⁺ = La³⁺, Ce³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Ho³⁺, Er³⁺, Yb³⁺, and Lu³⁺) into CsPbBr₃ films for all‐inorganic solar cells free of hole‐transporting materials and precious metal electrodes is presented. Arising from the enlarged grain size and prolonged carrier lifetimes upon incorporating Ln³⁺ ions into perovskite lattice, the performances of these inorganic CsPbBr₃ solar cell devices are significantly enhanced, achieving a champion efficiency as high as 10.14% and an ultrahigh open‐circuit voltage of 1.594 V under one sun illumination. Meanwhile, the nearly unchanged efficiency upon persistent attack by 80% RH in air atmosphere over 110 d and enhanced thermal stability at 80 °C over 60 d provide new opportunities of promoting commercialization of all‐inorganic CsPbBr₃ perovskite solar cells.

A new group of aniline‐based enamine hole‐transporting materials is synthesized, characterized, and tested in perovskite solar cells, yielding a champion power conversion efficiency over 20%. The investigated materials are obtained via one‐step synthesis procedure, without the use of a transition metal catalyst, from a very common and inexpensive precursor—aniline.

Abstract

Metal‐halide perovskites offer great potential to realize low‐cost and flexible next‐generation solar cells. Low‐temperature‐processed organic hole‐transporting layers play an important role in advancing device efficiencies and stabilities. Inexpensive and stable hole‐transporting materials (HTMs) are highly desirable toward the scaling up of perovskite solar cells (PSCs). Here, a new group of aniline‐based enamine HTMs obtained via a one‐step synthesis procedure is reported, without using a transition metal catalyst, from very common and inexpensive aniline precursors. This results in a material cost reduction to less than 1/5 of that for the archetypal spiro‐OMeTAD. PSCs using an enamine V1091 HTM exhibit a champion power conversion efficiency of over 20%. Importantly, the unsealed devices with V1091 retain 96% of their original efficiency after storage in ambient air, with a relative humidity of 45% for over 800 h, while the devices fabricated using spiro‐OMeTAD dropped down to 42% of their original efficiency after aging. Additionally, these materials can be processed via both solution and vacuum processes, which is believed to open up new possibilities for interlayers used in large‐area all perovskite tandem cells, as well as many other optoelectronic device applications.

A low‐temperature and substrate‐independent growth method is demonstrated to grow millimeter‐level inorganic perovskite monocrystalline thin films. These films present good optical and electrical properties comparable to bulk ones. What is more, they exhibit excellent long‐term stability toward humidity and thermal treatment. The as‐grown CsPbBr₃ monocrystalline films are fabricated into photodetectors with high photodetecting performance.

Abstract

Stability is a key problem that hinders the practical application of lead halide perovskite. Therefore, all‐inorganic perovskite CsPbX₃ monocrystalline films are urgently needed to fabricate photoelectric devices. Herein, a low‐temperature and substrate‐independent growth method is demonstrated to grow millimeter‐level inorganic perovskite monocrystalline thin films. These films present good optical and electrical properties comparable to bulk ones. What is more, they exhibit excellent long‐term stability toward humidity and thermal treatment. The as‐grown CsPbBr₃ monocrystalline films are then fabricated into photodetectors with high photodetection performance. These results demonstrate that the CsPbBr₃ monocrystalline films have potential in fabricating high‐performance optoelectronic devices.

A sequential‐vapor‐deposition method is exploited to successfully fabricate Cs₂AgBiBr₆ double perovskite films that are desirable for lead‐free solar cell applications. The films exhibit high quality in terms of large compact grains, high uniformity, and long‐term stability. The planar structure solar cells based on the vapor deposited films show a power conversion efficiency of 1.37%.

Lead‐free double perovskites have been demonstrated as promising alternatives to solve the toxicity and stability issues in conventional lead trihalide perovskites. However, different solubility of components in the precursors hinders fabrication of double perovskite films with commonly used solution procedures. Here, for the first time, the authors successfully prepared double perovskite Cs₂AgBiBr₆ thin films throughout a sequential‐vapor‐deposition procedure. The obtained thin films with pure double perovskite phase show large grain sizes, uniform, and smooth surface properties. In addition, the high‐quality vapor‐deposited Cs₂AgBiBr₆ films exhibit a photoluminescence (PL) lifetime of 117 ns, indicative of significant potential in photovoltaic applications. The resulting solar cells with planar device structure show an optimized power conversion efficiency of 1.37%, which can be maintained at 90% after 240 h of storage under ambient condition. Our results demonstrate the feasibility of employing vapor deposition technique to fabricate high‐quality double perovskite thin films, which paves the way for further development of various optoelectronic devices based on these promising lead‐free semiconductors.

Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring

Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring, Published online: 24 September 2018; doi:10.1038/s41467-018-06425-5