Chen Weijie

Nature Materials, Published online: 01 July 2019; doi:10.1038/s41563-019-0416-2

Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Meng Zhang, Jueming Bing, Yongyoon Cho, Yong Li, Jianghui Zheng, Cho Fai Jonathan Lau, Martin A. Green, Shujuan Huang, Anita W.Y. Ho-Baillie

Graphical abstract

KI₃ complex as an additive stabilizes perovskite precursors as well as improving perovskite solar cell device performance.

The linking topology effect and the doping effect on the optical and electronic properties of a series of carbazole‐based hole‐transport materials (HTMs) with 2,7‐substitution and 3,6‐substitution are systematically investigated. The results clearly demonstrate that the 2,7‐substituted carbazole‐based HTMs display higher hole mobility and conductivity, thereby exhibiting better device performance in both solid‐state dye‐sensitized solar cells and perovskite solar cells.

Carbazole is a promising core for the molecular design of hole‐transport materials (HTMs) for solid‐state mesoscopic solar cells (ssMSCs), such as solid‐state dye‐sensitized solar cells (ssDSSCs) and perovskite solar cells (PSCs) due to its low cost and excellent optoelectronic properties of its derivatives. Although carbazole‐based HTMs are intensely investigated in ssMSCs and promising device performance is demonstrated, the fundamental understanding of the impact of linking topology on the properties of carbazole‐based HTMs is lacking. Herein, the effect of the linking topology on the optical and electronic properties of a series of carbazole‐based HTMs with 2,7‐substitution and 3,6‐substitution is systematically investigated. The results demonstrate that the 2,7‐substituted carbazole‐based HTMs display higher hole mobility and conductivity among this series of analogous molecules, thereby exhibiting better device performance. In addition, the conductivity of the HTMs is improved after light treatment, which explains the commonly observed light‐soaking phenomenon of ssMSCs in general. All these carbazole‐based HTMs are successfully applied in ssMSCs and one of the HTMs X50‐based devices yield a promising efficiency of 6.8% and 19.2% in ssDSSCs and PSCs, respectively. This study provides guidance for the molecular design of effective carbazole‐based HTMs for high‐performance ssMSCs and related electronic devices.

Three novel polytriphenylamine‐based polymers (H‐Z1, H‐Z2, and H‐Z3) are designed and applied as hole‐transport layers in all‐inorganic perovskite solar cells. Due to the gradual deepening of the highest occupied molecular orbital energy levels from H‐Z1, H‐Z2 to H‐Z3, the energy loss (E _loss) can be decreased from 0.69, 0.64, to 0.62 eV for H‐Z1, H‐Z2, and H‐Z3, respectively.

The energy loss (E _loss) control via interfacial engineering is a significant indispensible methodology to realize high‐performance all‐inorganic perovskite solar cells (PVSCs). Herein, three novel polytriphenylamine‐based polymer derivatives (H‐Z1, H‐Z2, and H‐Z3) are synthesized, and the energy levels of these polymers are tuned feasibly through introducing the electron‐withdrawing group of trifluoromethyl in the triphenylamine (TPA) unit. These very deep HOMO energy levels are very beneficial for improving the open‐circuit voltages (V_ocs) in PVSCs with the potentially decreased E _losss. Due to the gradual deepening of HOMO energy levels from H‐Z1, H‐Z2 to H‐Z3, the V_ocs are elevated from 1.23, 1.28 to 1.30 V, respectively, where the E _loss s are decreased from 0.69, 0.64, to 0.62 eV for H‐Z1, H‐Z2, and H‐Z3, respectively. Interestingly, both of the H‐Z1‐ and H‐Z2‐based devices show the highest PCEs, over 14%, in all‐inorganic PVSCs, which are effectively comparable to the results of reference device using Spiro‐OMeTAD as HTL. Thus, through the efficient atomic engineering and chemical modification in corresponding p‐typed polymers, excellent hole transport polymers are achieved for high‐performance and stable PVSCs with very low E _loss.

Biodegradable and biocompatible transparent conductive electrodes are fabricated from bamboo for flexible perovskite solar cells. After extensive mechanical tests, including bending and crumpling tests, they still exhibit excellent electrical performance and negligible decay. The bamboo‐based bioelectrode perovskite solar cell shows a record power conversion efficiency of 11.68%, maintaining over 70% of initial power conversion efficiency after the bending tests.

Abstract

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light‐weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo‐based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass‐based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.

Perovskite solar cells with ZnO exhibit greatly improved stability when the methylammonium cation is excluded. The interfacial acid‐base reactions between methylammonium and ZnO are probed and the degradation kinetics are modulated by the acidity of the organic cation. Solar cells on ZnO films provide improved open circuit voltage, lower series resistance, and lower processing temperatures than those on SnO₂.

Abstract

Perovskite solar cells have achieved the highest power conversion efficiencies on metal oxide n‐type layers, including SnO₂ and TiO₂. Despite ZnO having superior optoelectronic properties to these metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI₃, ZnO is largely overlooked due to a chemical instability when in contact with metal halide perovskites, which leads to rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether all metal halide perovskites exhibit this instability with ZnO are yet to be undertaken. Experimental methods to elucidate the degradation mechanisms at ZnO–MAPbI₃ interfaces are developed. By substituting MA with formamidinium (FA) and cesium (Cs), the stability of the perovskite–ZnO interface is greatly enhanced and it is found that stability compares favorably with SnO₂‐based devices after high‐intensity UV irradiation and 85 °C thermal stressing. For devices comprising FA‐ and Cs‐based metal halide perovskite absorber layers on ZnO, a 21.1% scanned power conversion efficiency and 18% steady‐state power output are achieved. This work demonstrates that ZnO appears to be as feasible an n‐type charge extraction layer as SnO₂, with many foreseeable advantages, provided that MA cations are avoided.

Publication date: 21 August 2019

Source: Joule, Volume 3, Issue 8

Author(s): Chenchen Yang, Dianyi Liu, Matthew Bates, Miles C. Barr, Richard R. Lunt

Graphical Abstract

Solution‐processable Ga₂O₃ nanocrystals are developed as a novel electron‐transporting layer for high‐performance perovskite solar cells. The smooth film of Ga₂O₃ nanocrystals offers a better interface with perovskite and improves charge transport efficiency. The Ga₂O₃‐based device shows negligible hysteresis unlike the TiO₂‐based analogue.

The electron‐transporting layer (ETL) plays a very important role in perovskite solar cells (PSCs). The traditional TiO₂ ETL exhibits drawbacks such as complex preparation process and low stability. Devices incorporating TiO₂ as the ETL also show large hysteresis that limits their performance. Herein, Ga₂O₃ nanocrystals (NCs), prepared by a solution process, are applied as an ETL in n‐i‐p planar structured PSCs. The Ga₂O₃‐based devices exhibit negligible hysteresis and achieve higher performance than the TiO₂‐based devices. Due to better energy level matching and smoother surface morphology, films of Ga₂O₃ NCs make good interfacial contact with the perovskite top layer, improving the charge transport efficiency. The perovskite layer also exhibits high crystallinity. Unlike TiO₂, which is commonly prepared by high‐temperature sintering or solution hydrolysis, films of Ga₂O₃ NCs can be prepared by solution spin‐coating at a low temperature. This greatly reduces the complexity of fabrication and improves device performance.

The alcohol vapor post‐annealing treatment on Sb₂S₃ films is demonstrated as an effective method for high‐performance Sb₂S₃ planar heterojunction solar cells. Due to the higher polarity of methanol compared with that of ethanol and isopropanol, the meth‐annealed Sb₂S₃ devices show a power conversion efficiency of 5.27%, with an increase in 31% compared with the control device.

Solution‐processed Sb₂S₃ planar heterojunction solar cells have shown great progress in power conversion efficiency (PCE) in recent years. However, a conventional solution process yields Sb₂S₃ films with a small grain size. Herein, an alcohol vapor post‐annealing strategy is reported that uses alcohol vapors to facilitate grain growth of Sb₂S₃ films during annealing, achieving films with a larger grain size and better crystallinity. A series of alcohols with different polarities are used for the vapor post‐annealing. Methanol with the highest polarity provides films with the largest grain size. As a result, the Sb₂S₃ films prepared via vapor post‐annealing show enhanced light absorption, longer carrier lifetime, and less carrier recombination, which are verified by photoluminescence, transient photovoltage, suns‐short‐circuit photocurrent density, and suns‐open‐circuit voltage measurements. The Sb₂S₃ solar cells post‐annealed with methanol vapor exhibit a PCE of 5.27%, showing a drastic improvement of 31% compared with the non‐treated devices. The alcohol vapor post‐annealing approach presents a new avenue toward controlling the morphology and crystallinity of solution‐processed Sb₂S₃ films and achieving efficient Sb₂S₃ solar cells.

Sb₂Se₃ becomes an emerging photovoltaic absorber with high efficiency. However, its low photovoltage limits its overall performance. Herein, with a combined theoretical and experimental study, it is demonstrated that interface engineering via an oxygenated sputtered CdS window layer (CdS:O) is an effective approach to improve the open‐circuit voltage with a power conversion efficiency beyond 7% in CdS:O/Sb₂Se₃ thin‐film solar cells.

Antimony chalcogenide Sb₂Se₃ is an emerging photovoltaic absorber due to its appropriate bandgap (≈1.1 eV), high absorption coefficient (>10⁵ cm⁻¹), suitable p‐type conductivity, low toxicity, earth abundance, and excellent stability. However, the stringent growth condition and low photovoltage limit its power conversion efficiency (PCE). Herein, via a combined theoretical and experimental study, interface engineering via an oxygenated cadmium sulfide (CdS) window layer (CdS:O) is found to be an effective approach to improve the device performance of CdS:O/Sb₂Se₃ solar cells. The sputtered oxygenated CdS:O window layer can be used to replace conventional chemical‐bath‐deposited CdS window layer in the Sb₂Se₃ devices. The best PCE of 7.01% is demonstrated in the superstrate configuration of fluorine‐doped SnO₂/CdS:O/Sb₂Se₃/graphite with a high open‐circuit voltage of 0.432 V, where Sb₂Se₃ is fabricated using the close space sublimation technique. The interfacial diffusion between Sb₂Se₃ and sputtered CdS:O is significantly suppressed by introducing oxygen at the interface, which prevents Cd diffusion and the formation of Cd interstitials. Combined device physics characterizations and theoretical calculations reveal that oxygen in the CdS:O/Sb₂Se₃ interface can increase depletion region, built‐in voltage, and reduce interfacial recombination. These findings provide the guidance to optimize quasi‐one‐dimensional non‐cubic earth‐abundant chalcogenide photovoltaic devices through interface engineering.

Perovskite solar cells are modified by dropping alkylammonium solutions over CH₃NH₃PbI₃ films and lead to an increase in the stability after exposure to humidity. In the presence of the alkylammonium chains, the bulk perovskite is converted to a 2D/3D structure that helps the device to retain its performance for longer.

Hybrid organic and inorganic perovskite solar cells lack long‐term stability, and this negatively impacts the widespread application of this emerging and promising photovoltaic technology. Herein, aiming to increase the stability of perovskite films based on CH₃NH₃PbI₃ and to deeply understand the formation of 2D structures, solutions of alkylammonium chlorides containing 8, 10, and 12 carbons are introduced during the spin‐coating on the surface of 3D perovskite films leading to the in situ formation of 2D structures. It is possible to identify the chemical formulae of some 2D structures formed by X‐ray diffraction and UV–vis analysis of the modified films. Interestingly, the increase in the stability of the CH₃NH₃PbI₃ films due to the formation of a 2D + 3D perovskite network is only possible in planar TiO₂ substrates. The increase in stability of the CH₃NH₃PbI₃ films follows the surfactant molecule order: octylammonium (8C) > decylammonium (1 °C) > dodecylammonium (12C) chlorides > standard. An increase of 17.6% in the lifetime of the devices assembled with octylammonium‐modified perovskite film is observed compared with that of the standard device, which is directly linked to the improvement of the charge carrier lifetimes obtained from time‐correlated single photon counting measurements.

A new azeotropic promoted approach is proposed to successfully synthesize In doped CuCrO₂ under low temperatures in a short time. This In doped CuCrO₂ HTL has thermal stability up to 200 °C, and exhibits improved optical transmission and carrier mobility, which is beneficial for achieving high performance perovskite solar cells.

Abstract

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.

For developing low‐cost and high‐efficiency planar perovskite solar cells (PSCs), a straightforward low‐temperature chemical bath deposition process is developed to prepare a Co‐doped TiO₂ (Co‐TiO₂) electron transport layer (ETL); the optoelectrical properties of the TiO₂ ETL are significantly improved by Co‐doping. Finally, the efficiency of the PSCs is increased from 17.40% for TiO₂ to 19.10% for the Co‐TiO₂ ETL.

Planar hybrid perovskite solar cells (PSCs) attract great attention due to their obvious advantages of low‐temperature processing with a high power conversion efficiency (PCE) up to 23.32%. Here, Co‐doped TiO₂ (Co‐TiO₂) deposited by a straightforward low‐temperature chemical bath deposition (CBD) method is explored. Using Co‐TiO₂ as an electron transport layer (ETL) for the planar PSCs, the effects of doping on TiO₂ morphology, electronic properties, and solar cell performance are investigated. The PCE increases to 19.10% when the Co doping concentration is optimized at 5 mol%, an increase of 17.40% compared with that using the pristine TiO₂. Meanwhile, the notorious J–V hysteresis is suppressed to a greater extent. Considering that the low‐temperature CBD is comparable with continuous roll‐to‐roll processing, it makes the process and the Co‐TiO₂ ETL potential candidates for low‐cost commercialization.

Colloidal copper chalcogenide CuSeS/CdS core/shell quantum dots (QDs) with engineered optoelectronic properties and type‐II band structure are developed, showing strong visible absorption, near‐infrared emission (≈915 nm), and ultralong lifetime (≈9.5 μs). These core/shell QDs are used to fabricate solar‐driven photoelectrochemical cell, exhibiting a saturated photocurrent density of ≈4 mA cm⁻² with preeminent durability.

Colloidal alloyed copper chalcogenide Cu_2 − x Se_1 − y S _y (CuSeS) quantum dots (QDs) are promising building blocks for solar energy applications because of their unique size/composition‐tunable optical bandgap, which is well matched to the sunlight spectrum. Nevertheless, poor charge separation/transfer and low photostability induced by their intrinsically abundant surface defects/traps tremendously hinder the realization of high‐performance solar energy conversion systems such as solar‐driven photoelectrochemical (PEC) devices. Herein, the synthesis of copper chalcogenide CuSeS core QDs with effective CdS shell passivation is presented. These core/shell QDs exhibit optimized optoelectronic properties with a particularly ultralong lifetime, indicating the formation of type‐II band structure for efficient spatial charge separation, which is further probed using ultrafast transient absorption (TA) spectroscopy. To demonstrate the feasibility of solar energy conversion, solar‐driven PEC cells using such core/shell QDs as light‐converters are fabricated, yielding an impressive saturated photocurrent density of ≈4 mA cm⁻² with preeminent durability under 1 sun illumination. This finding highlights a potential technique to engineer the optoelectronic properties of colloidal alloyed copper chalcogenide nanomaterials for high efficiency and stable solar energy conversion devices.

A new small molecule (SM)‐acceptor, POIT‐IC4F, is developed. Due to the synergistic effects of side‐chain engineering and fluorination on the SM‐acceptor to simultaneously broaden spectral response and minimize voltage loss, the annealing‐free organic solar cells achieve a high device efficiency of 13.8%.

Herein, three small molecule (SM)‐acceptors (POIT‐IC, POIT‐IC2F, and POIT‐IC4F) are developed by combining the side‐chain engineering located on the sp³‐hybridized carbon atoms of the fused‐ring core and the fluorination of end groups. From ITIC to POIT‐IC, POIT‐IC2F, and then to POIT‐IC4F, the SM‐acceptors show gradually broadened absorption spectra, increased maximum extinction coefficient, crystallinity, and electron mobilities due to the synergistic effects of side‐chain engineering and fluorination. Compared with nonfluorinated ITIC and POIT‐IC, as fluorination broadens the molecular spectra, POIT‐IC2F and POIT‐IC4F with alkoxyphenyl side chains show less decreased LUMO levels than IT‐IC2F and IT‐IC4F with alkylphenyl side chains, which are conducive to both higher V _oc and J _sc for organic solar cells (OSCs). Combined with polymer donor PM6, the POIT‐IC4F‐based OSCs achieve a device efficiency of up to 13.8% with a high V _oc of 0.91 V and J _sc of 20.9 mA cm⁻², which are significantly higher than that of the control OSCs based on ITIC (8.9%), POIT‐IC (10.1%), or IT‐IC4F (12.2%). An efficiency of 13.8% is one of the highest PCEs reported for the annealing‐free OSCs. Our results show that the synergistic effects of side‐chain engineering and fluorination on SM‐acceptor can simultaneously broaden spectral response and minimize voltage loss of OSCs and ultimately achieve high device efficiency.

The theme of this review is the progress of microcavity (MC) in organic solar cells (OSCs) in recent years. The principle of MC is described in detail. In addition, the application of MC in other photo‐electronic conversion devices is also briefly introduced. Finally, the summary and prospect of microcavity organic solar cells (MCOSCs) are given.

In recent decades, organic solar cells (OSCs) have drawn increasing interest due to their unique properties such as low cost, solution‐processing, flexibility, semitransparency, and nontoxicity. Due to some shortcomings of limited optical absorption in organic semiconductors as well as low carrier mobility and short exciton diffusion length, light‐trapping technologies such as surface plasmon resonance, photonic crystals, and microcavities (MCs) have been widely developed to improve device performance. Among these methods, the MC effect is liable to form and has unneglectable influences on the device efficiency. However, few reports systematically summarize the development of MC‐based OSCs. Herein, the principle of the MC effect is introduced first, and subsequently, the application and the development of MCs in single and multi‐junction OSCs are described in detail. Furthermore, in addition to the traditional MCs‐enhanced light absorption, other applications based on the MC structure in OSCs and other photo‐electronic conversion devices are also represented. Finally, the problems that need to be solved and the development directions of MC‐based OSCs in the future are outlined. It is believed that this review can provide new thinking for achieving high‐performance OSCs with optical means.

This work reports an efficient post‐treatment method for CsPbI₃ perovskite quantum dots (QDs) using cesium cations, which can passivate the CsPbI₃ surface and improve the electron coupling of QDs. Finally, the best CsPbI₃ QD solar cell with an impressive efficiency of 14.10% is achieved by cesium acetate (CsAc) and exhibits improved stability against moisture.

Abstract

Surface manipulation of quantum dots (QDs) has been extensively reported to be crucial to their performance when applied into optoelectronic devices, especially for photovoltaic devices. In this work, an efficient surface passivation method for emerging CsPbI₃ perovskite QDs using a variety of inorganic cesium salts (cesium acetate (CsAc), cesium idodide (CsI), cesium carbonate (Cs₂CO₃), and cesium nitrate (CsNO₃)) is reported. The Cs‐salts post‐treatment can not only fill the vacancy at the CsPbI₃ perovskite surface but also improve electron coupling between CsPbI₃ QDs. As a result, the free carrier lifetime, diffusion length, and mobility of QD film are simultaneously improved, which are beneficial for fabricating high‐quality conductive QD films for efficient solar cell devices. After optimizing the post‐treatment process, the short‐circuit current density and fill factor are significantly enhanced, delivering an impressive efficiency of 14.10% for CsPbI₃ QD solar cells. In addition, the Cs‐salt‐treated CsPbI₃ QD devices exhibit improved stability against moisture due to the improved surface environment of these QDs. These findings will provide insight into the design of high‐performance and low‐trap‐states perovskite QD films with desirable optoelectronic properties.

Solution‐processed double‐walled carbon nanotubes function as transparent electrodes in inverted‐type planar heterojunction perovskite solar cells. Double‐walled carbon nanotubes exhibit high optical conductivity and solubility. Good energy level alignment and morphology of the electrodes leads to an operating power conversion efficiency of 17.2%, which is the highest among the carbon nanotube electrode‐based perovskite solar cells.

Abstract

Double‐walled carbon nanotubes are between single‐walled carbon nanotubes and multiwalled carbon nanotubes. They are comparable to single‐walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. Exploiting such advantages, solution‐processed transparent electrodes are demonstrated using double‐walled carbon nanotubes and their application to perovskite solar cells is also demonstrated. Perovskite solar cells which harvest clean solar power have attracted a lot of attention as a next‐generation renewable energy source. However, their eco‐friendliness, cost, and flexibility are limited by the use of transparent oxide conductors, which are inflexible, difficult to fabricate, and made up of expensive rare metals. Solution‐processed double‐walled carbon nanotubes can replace conventional transparent electrodes to resolve such issues. Perovskite solar cells using the double‐walled carbon nanotube transparent electrodes produce an operating power conversion efficiency of 17.2% without hysteresis. As the first solution‐processed electrode‐based perovskite solar cells, this work will pave the pathway to the large‐size, low‐cost, and eco‐friendly solar devices.

A type of perovskite bifunctional device (PBD) with high photovoltaic (PV) and electroluminescence (EL) performance is developed. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is performed to turn the n‐type surface of the perovskite into p‐type and also correct the misalignment to form a well‐defined n–i–p heterojunction.

Abstract

Currently, photovoltaic/electroluminescent (PV/EL) perovskite bifunctional devices (PBDs) exhibit poor performance due to defects and interfacial misalignment of the energy band. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is introduced to reduce energy loss, through adding corrosion‐free 3,3′‐(2,7‐dibromo‐9H‐fluorene‐9,9‐diyl) bis(n,n‐dimethylpropan‐1‐amine) (FN‐Br) into a HTL free of lithium salt. This strategy can turn the n‐type surface of perovskite into p‐type and thus correct the misalignment to form a well‐defined N–I–P heterojunction. The tailored PBD achieves a high PV efficiency of up to 21.54% (certified 20.24%) and 4.3% EL external quantum efficiency. Free of destructive additives, the unencapsulated devices maintain >92% of their initial PV performance for 500 h at maximum power point under standard air mass 1.5G illumination. This strategy can serve as a general guideline to enhance PV and EL performance of perovskite devices while ensuring excellent stability.