Ligang Yuan

A set of machine learning models are used to predict the photovoltaic efficiency of organic donor–acceptor pairs based on the electronic and structural properties of both components. Using a data set of experimental observations on 262 donors and 76 acceptors for training, the models developed enable a full exploration of the space of combinations.

Abstract

In this work an instance of the general problem occurring when optimizing multicomponent materials is treated: can components be optimized separately or the optimization should occur simultaneously? This problem is investigated from a computational perspective in the domain of donor–acceptor pairs for organic photovoltaics, since most experimental research reports optimization of each component separately. A collection of organic donors and acceptors recently analyzed is used to train nonlinear machine learning models of different families to predict the power conversion efficiency of donor–acceptor pairs, considering computed electronic and structural parameters of both components. The trained models are then used to predict photovoltaic performance for donor–acceptor combinations for which experimental data are not available in the data set. Data structure, and the usefulness of the trained models are critically assessed by predicting some donor–acceptor pairs that recently appeared in the literature, and the best combinations are proposed as worth investigating experimentally.

Publication date: 20 November 2019

Source: Joule, Volume 3, Issue 11

Author(s): Enbing Bi, Wentao Tang, Han Chen, Yanbo Wang, Julien Barbaud, Tianhao Wu, Weiyu Kong, Peng Tu, Hong Zhu, Xiaoqin Zeng, Jinjin He, Shin-ichi Kan, Xudong Yang, Michael Grätzel, Liyuan Han

Context & Scale

Summary

Graphical Abstract

In article no. 1900224, Mohammad Khaja Nazeeruddin, Paul J. Dyson, Vytautas Getautis, and co‐workers present a novel hole transporting material, termed V1160, based on four N,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine‐type fragments, fused by a Tröger's base core. The material is synthetically robust and demonstrates a promising power conversion efficiency of over 18%. Moreover, V1160‐based devices exhibit improved performances in dopant‐free configurations and superior stability.

In article no. 1900119, Zong‐Xiang Xu and co‐workers synthesize isomer‐pure 2,9,16,24‐tetra‐n‐butyl‐Zn (II) phthalocyanine (RE‐ZnBu₄Pc) through ring expanding of symmetric tri‐n‐butyl‐substituted boron subphthalocyanine as a dopant‐free hole transporting material (HTM) in planar conventional perovskite solar cells, which offers higher efficiency, long‐term stability, and reproducibility than HTMs based on ZnBu₄Pc with an isomer mixture.

In article no. 1900078, Changlei Wang, Xiaofeng Li, Yanfa Yan, and co‐workers report that the introduction of block copolymer F127 could passivate grain boundaries and enhance the hydrophobicity of perovskite films simultaneously, resulting in highly efficient planar and flexible perovskite solar cells with good stability.

In article no. 1900199, Ana F. Nogueira and co‐workers modify perovskite surfaces with alkylammonium chloride, which increases the stability of the solar cells, making it last longer when exposed to environmental conditions. After the modification, 2D/3D structures are formed and their chemical structures are identified. This mixture makes the films more humidity tolerant.

This review provides a systematic overview of self‐powered integrated systems based on perovskite solar cells, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices are discussed to further the understanding of fundamental device physics. The current challenges and future perspective are provided.

Integrated smart portable devices (e.g., self‐powered devices) that utilize the environment‐friendly energy (e.g., solar energy) by means of photovoltaic technology (e.g., solar cell) are a popular concept in the current technological development trend. As a key component of integrated devices, photovoltaic devices acting as a bridge between solar energy and working devices play an important role in the whole system performance. The emergence of perovskite solar cells (PSCs) with high power conversion efficiencies (over 25%) allows for the possibility and appearance of many multifunctional self‐powered integrated devices. In this review, a systematic overview of self‐powered integrated devices based on PSCs that are reported so far is provided, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices and performance are also discussed to further the understanding of fundamental device physics. Finally, the current challenging issues and future perspective are provided to promote the development of self‐powered integrated devices based on PSCs in the near future.

Perovskite solar cells fabricated on aluminum‐doped zinc oxide (AZO)/quartz substrates are shown with a record efficiency of 15%, and their radiation hardness to 150 keV protons is presented. The cells show robust stability up to 10¹³ protons cm⁻², with degradation at 10¹⁴ and 10¹⁵ protons ccm⁻². Transient photovoltage measurements show an increase in minority carrier density and lifetime from 10¹² protons cm⁻².

Perovskite solar cells (PSCs) have gained increasing interest for space applications. However, before they can be deployed into space, their resistance to ionizing radiations, such as high‐energy protons, must be demonstrated. Herein, the effect of 150 keV protons on the performance of PSCs based on aluminum‐doped zinc oxide (AZO) transparent conducting oxide (TCO) is investigated. A record power conversion efficiency of 15% and 13.6% is obtained for cells based on AZO under AM1.5G and AM0 illumination, respectively. It is demonstrated that PSCs can withstand proton irradiation up to 10¹³ protons cm⁻² without significant loss in efficiency. From 10¹⁴ protons cm⁻², a decrease in short‐circuit current of PSCs is observed, which is consistent with interfacial degradation due to deterioration of the Spiro‐OMeTAD holes transport layer during proton irradiation. The structural and optical properties of perovskite remain intact up to high fluence levels. Although shallow trap states are induced by proton irradiation in perovskite bulk at low fluence levels, charges are released efficiently and are not detrimental to the cell's performance. This work highlights the potential of PSCs based on AZO TCO to be used for space applications and gives a deeper understanding of interfacial degradation due to proton irradiation.

Positive photoconductivity in CH₃NH₃PbBr₃ turns into negative photoconductivity after Bi doping. This transition is due to photogenerated DX‐like centers in Bi‐doped CH₃NH₃PbBr₃.

Abstract

Halide perovskites are known to be photoconductive for more than half a century, and their efficient photocarrier generation gives rise to positive photoconductivity (PPC). In this work, the discovery of negative photoconductivity (NPC) in hybrid perovskite CH₃NH₃PbBr₃ after Bi doping is reported. Transient photoconductivity measurements reveal a surprising bipolar behavior with a fast positive response followed by exponential negative photocurrent decay, resulting in an equilibrium photocurrent even below the dark level. The NPC effect in Bi‐doped CH₃NH₃PbBr₃ is among the largest ones reported so far for semiconductors. It is proposed that the transition to negative photoconductance is related to the presence of DX‐like centers in Bi‐doped halide perovskites, similar to doped III–V and chalcopyrite semiconductors. Such photogenerated DX‐like centers in the Bi‐doped CH₃NH₃PbBr₃ can trap mobile band electrons and enhance charge recombination, thus reducing the conductivity. This mechanism is consistent with the observations of crossover from PPC to NPC as functions of temperature, composition, and illumination. The results underscore the importance of defect engineering for tuning the optoelectronic properties of halide perovskites.

CsPbBr₃ perovskite quantum dots (QDs)‐loaded poly(methyl methacrylate) composite microspheres are easily prepared through in situ polymerization of methyl methacrylate in the presence of quantum dots in hexane. This method is very facile and the CsPbBr₃ perovskite QDs are evenly incorporated into the microspheres. Protected by the microsphere, the water and storage stability of CsPbBr₃ quantum dots are greatly improved.

Abstract

In this paper, a facile synthesis of water‐resistant CsPbBr₃ perovskite quantum dots (PQDs) loaded poly(methyl methacrylate) (PMMA) composite microspheres (CsPbBr₃@PMMA) is reported. The method is based on the precipitation polymerization of methyl methacrylate in hexane in the presence of CsPbBr₃ PQDs and stabilizer. The CsPbBr₃@PMMA microspheres show a tunable size and a narrow size distribution, with the CsPbBr₃ PQDs being uniformly dispersed in the PMMA microspheres. The effective incorporation of PQDs is attributed to the strong coordination interactions between Pb ions on the surface of PQDs and carbonyl groups (CO) from PMMA. Based on this mechanism, multicolor composite microspheres can be easily prepared through absorbing CsPbX₃ (X = Cl, Br, I) PQDs into blank PMMA microspheres. Protected by the PMMA microspheres, the imbedded CsPbBr₃ PQDs show improved water resistance and storage stability. Further, a wide‐color‐gamut (129%) white light‐emitting diode (LED) is demonstrated by combining the green‐emitting CsPbBr₃@PMMA composite microspheres and red‐emitting K₂SiF₆: Mn⁴⁺ with a blue LED, which enables to be used as backlights for liquid crystal displays.

A maximum external quantum efficiency of in situ fabricated CsPbBr₃ Nanoparticles (NPs) light‐emitting diode is demonstrated to be 17.4% by introducing sodium bromide to CsPbBr₃ NPs to passivate defect and promote the charge transfer ability.

Abstract

All‐inorganic perovskite has attracted much attention because of the higher stability. Many organic additives such as alkyl chain ammonium and polymers are usually introduced into perovskite to improve their performance. However, the long chain ammonium cations in perovskite may restrain the carrier transfer ability and ultimately deteriorate the performance of light‐emitting diodes (LEDs). In this work, the CsPbBr₃ nanoparticles (NPs) are in situ fabricated by the synergistic effect of poly(ethylene oxide) and phenethylammonium bromide (PEABr). Particularly, sodium bromide (NaBr) with better conductivity is successfully introduced into CsPbBr₃ NPs to substitute PEA partially, ultimately to passivate the defect and promote the carrier transfer ability. Besides, the addition of NaBr results in a better promotion for electron mobility than for hole mobility leading to a more balanced charge transport in devices. It enables NaBr based CsPbBr₃ NPs green LEDs to exhibit a maximum external quantum efficiency (EQE_max) of 17.4%, which presents obvious enhancement compared to the LEDs without NaBr (EQE_max = 12%). Further, NaBr based CsPbBr₃ NPs LEDs with a large area of 108 mm² still show a high maximum EQE of 10.2%. Above all, this work provides a feasible way of adding metal additive in perovskite films to improve the performance of perovskite LEDs.

Anomalous experimental evidences of 2D perovskites from different aspects are presented, which deviate from the general carrier dynamics with irreversible trapping but agree well with the energy reservoir model. A transient energy reservoir mechanism outcompeting nonradiative loss is established in the 2D perovskites, providing a new clue to understand the lauded defect tolerance of perovskites.

Abstract

2D Ruddlesden−Popper type perovskites have attracted enormous attention due to their natural multiquantum‐well structure. However, there is still mystery regarding the behavior of photocarriers, especially the exciton fine structure behind the excellent optoelectronic performance. The coexistence of two strikingly different decay components in time‐resolved photoluminescence is inconsistent with the high internal quantum yield (QY_IN = ≈0.7) in the conventional model for radiative and nonradiative recombinations (QY_IN = τ_nr/(τ_nr + τ_r) = 17%). Here it is revealed that there is a special transient energy reservoir outcompeting nonradiative loss in 2D Ruddlesden−Popper type perovskites. Upon optical excitation, the bright excitons rapidly relax into the low‐lying energy reservoir before nonradiative recombination occurs. Interestingly, the energy in the reservoir is not lost. The carriers in this energy reservoir can spontaneously transfer back to the bright states and can still effectively contribute to the photovoltaic and photonic properties of the perovskites. This investigation provides a novel insight into the mechanism for the lauded defect tolerance of 2D perovskites by highly efficient energy storage via a transient reservoir.

Record‐efficiency near‐infrared fluorescent organic light‐emitting diodes (OLEDs) are demonstrated using energy transfer from a thermally activated delayed fluorescence sensitizer to a cyanine pyrrolopyrrole derivative. The OLEDs show a maximum external quantum efficiency of 5.4% and spectrally narrow IR emission centered at λ = 790 nm. The cohost energetics are found to play an important role in determining the device efficiency.

Abstract

Through various triplet‐harvesting approaches, fluorescent organic light‐emitting diodes (OLEDs) that emit in the visible spectrum can now be fabricated with efficiencies rivaling those of their phosphorescent counterparts. However, achieving high efficiencies in the near‐infrared (NIR) is considerably more challenging. This is in part due to the low quantum yield of most fluorescent NIR emitters and inefficient triplet exciton harvesting in such devices. Here, fluorescent NIR OLEDs with an external quantum efficiency of 5.4% and a peak emission wavelength of 790 nm are demonstrated. The OLEDs are fabricated by combining a deep‐red host that undergoes thermally assisted delayed fluorescence with a near‐infrared cyanine dye that emits with high efficiency. The devices show nearly pure NIR emission with a NIR cut‐on wavelength of 749 nm and >90% emitted power at wavelengths above 750 nm. It is also shown that the host polarity strongly affects the device performance.

Using the single‐component Eu³⁺–Tb³⁺‐containing metallopolymer Poly(NVK‐co‐2‐co‐7) with the straightforward high‐quality white‐lights as the emitting layer, its reliable WPLED with stepwise alignments of both HOMO and LUMO levels affords the record‐renewed electroluminescent performance among previous organo‐Ln³⁺‐based WOLEDs/WPLEDs.

Abstract

Despite the excellent physical properties of single‐component Eu³⁺–Tb³⁺‐containing metallopolymers, the development of their flexible white polymer light‐emitting diodes (WPLEDs) for portable full‐color flat displays remains a formidable challenge. Herein, the WPLEDs from a metallopolymer Poly(NVK‐co‐2‐co‐7) are reported, in which [Eu(DBM)₃(4‐vp‐PBI)] (2) and [Tb(tba‐PMP)₃(4‐vp‐PBI)] (7) with different localized circumstances are grafted into poly(N‐vinyl‐carbarzole) (PVK). In this design, both Dexter and Förster energy transfers occur, which endow a photoluminescent quantum yield up to 22.3% of the straightforward high‐quality white‐lights. Contributing from the stepwise alignment of frontier molecular orbitals of Poly(NVK‐co‐2‐co‐7) as the emitting layer in combination with CBP‐ and BCP‐assisted carrier‐transports, a reliable WPLED with the record‐renewed electroluminescent performance (L ^Max = 388.0 cd m⁻², η_c ^Max = 31.1 cd A⁻¹, η_p ^Max = 15.0 lm W⁻¹, η_EQE ^Max = 18.1%, and weak efficiency‐roll‐off) among previous organo‐Ln³⁺‐based white organic light‐emitting diodes/WPLEDs is achieved. This finding renders a single‐component Eu³⁺–Tb³⁺‐containing metallopolymers a potential new platform to cost‐effective flexible WPLEDs for practical applications.

Homojunction bilayer electron transport layers (ETLs) are developed by stacking Sb‐doped SnO₂ (Sb‐SnO₂) and SnO₂ ETLs via a low‐temperature process. The perovskite solar cells with the Sb‐SnO₂/SnO₂ bilayer ETLs achieve the best power conversion efficiency of 20.73%. Due to Sb‐SnO₂/SnO₂, the bilayer ETL with a cascade energy arrangement enhances charge separation and reduces carrier recombination.

Energy alignment between electron transport layers (ETLs) and perovskite has a strong influence on the device performance of perovskite solar cells (PSCs). Two approaches are deployed to tune the energy level of ETLs: 1) doping ETLs with aliovalent metal cations and 2) constructing heterojunction bilayers with different materials. However, the abrupt interfaces in the heterojunction bilayers introduce undesirable carrier recombination. Herein, a homojunction bilayer ETL is developed by stacking Sb‐doped SnO₂ (Sb‐SnO₂) and SnO₂ ETLs via low‐temperature spin‐coating processes. The energy levels of ETLs are tuned by the incorporation of Sb and altering stacking orders. Bilayer ETL of Sb‐SnO₂/SnO₂ with cascade energy alignment promotes the best power conversion efficiency of 20.73%, surpassing single‐layer ETLs of SnO₂ (18.23%) and Sb‐SnO₂ (19.15%), whereas the SnO₂/Sb‐SnO₂ bilayer with barricade energy alignment receives the poorest device performance. The cascade bilayer ETL facilitates charge separation and suppresses carrier recombination in PSCs, which is verified by photoluminescence, conductivity, and impedance characterizations. The homojunction bilayer ETLs with adjustable energy levels open a new direction for interface engineering toward efficient PSCs.

The interface and bulk defects of perovskite solar cells are distinguished and quantified, and are for the first time traced in situ using an expanded admittance model. A fullerene derivative [6, 6]‐phenyl‐C61‐butyric acid (PCBA) is introduced into the TiO₂/perovskite interface to release the interface stress.

Abstract

The stability issue that is obstructing commercialization of the perovskite solar cell is widely recognized, and tremendous effort has been dedicated to solving this issue. However, beyond the apparent thermal and moisture stability, more intrinsic semiconductor mechanisms regarding defect behavior have yet to be explored and understood. Herein, defects are quantified; especially interface defects, within the cell to reveal their impact on device performance and especially stability. Both the bulk and interface defects are distinguished and traced in situ using an expanded admittance model when the cell degrades in its efficiency under illumination or voltage. The electric field‐induced interface, rather than bulk defects, is found to have a direct correlation to stability. Releasing the interface strain using a fullerene derivative is an effective way to suppress interface defect formation and improve stability. Overall, this work provides a quantitative approach to probing the semiconductor mechanism behind the stability issue, and the inherent correlation discovered here among the electric field, interface strain, interface defects, and cell stability has important implications for ongoing device stability engineering.

2D materials have shown great potential for use as photovoltaic materials owing to their outstanding properties. The application of a wide variety of emerging 2D materials for efficient, scalable, and stable perovskite solar cells is reviewed. Interface engineering, energy level alignment, film morphology control, instability issues, hysteresis phenomena, and other key factors are discussed.

Abstract

Perovskite solar cells (PSCs) are now at the forefront of the state‐of‐the‐art photovoltaic technologies due to their high efficiency and low fabrication costs. To further realize the potential of this fascinating class of solar cells, nanostructured functional materials have been playing important roles. 2D layered materials have attracted a great deal of interest due to their fascinating properties and unique structure. Recently, the exploration of a wide range of novel 2D materials for use in PSCs has seen considerable progress, but still a lot remains to be done in this field. In this progress report, the advancements that have recently been made in the application of these emerging 2D materials, beyond graphene, for PSCs are presented. Both the advantages and challenges of these 2D materials for PSCs are highlighted. Finally, important directions for the future advancements toward efficient, low‐cost, and stable PSCs are outlined.