Chen Weijie

Publication date: 19 October 2022

Source: Joule, Volume 6, Issue 10

Author(s): Su Geun Ji, Ik Jae Park, Hogeun Chang, Jae Hyun Park, Geon Pyo Hong, Back Kyu Choi, Jun Ho Jang, Yeo Jin Choi, Hyun Woo Lim, You Jin Ahn, So Jeong Park, Ki Tae Nam, Taeghwan Hyeon, Jungwon Park, Dong Hoe Kim, Jin Young Kim

A novel interface layer of ABABr (4-(2-Aminoethyl) benzoic acid bromide) is developed to suppress the redox reaction at the interface between the NiO _x and different perovskites. An ABABr interface modification increases power conversion efficiency (PCE) by over 13% and the device maintains over 90% of its PCE after over 500 hours of continuous operation at maximum power point.

Abstract

The inorganic hole transport layer of nickel oxide (NiO _x ) has shown highly efficient, low-cost, and scalable in perovskite photovoltaics. However, redox reactions at the interface between NiO _x and perovskites limit their commercialization. In this study, ABABr (4-(2-Aminoethyl) benzoic acid bromide) between the NiO _x and different perovskite layers to address the issues has been introduced. How the ABABr interacts with NiO _x and perovskites is experimentally and theoretically investigated. These results show that the ABABr molecule chemically reacts with the NiO _x via electrostatic attraction on one side, whereas on the other side, it forms a strong hydrogen bond via the NH₃ ⁺ group with perovskites layers, thus directly diminishing the redox reaction between the NiO _x and perovskites layers and passivating the layer surfaces. Additionally, the ABABr interface modification leads to significant improvements in perovskite film morphology, crystallization, and band alignment. The perovskites solar cells (PSCs) based on an ABABr interface modification show power conversion efficiency (PCE) improvement by over 13% and maintain over 90% of its PCE after continuous operation at maximum power point for over 500 h. The work not only contributes to the development of novel interlayers for stable PSCs but also to the understanding of how to prevent interface redox reactions.

Sb₂S₃ is a kind of new light-absorbing material possessing high stability in ambient environment, high absorption coefficient in the visible range, and abundant elemental storage. Here, a surface modification scheme by lithium-doping is first introduced for Sb₂S₃ via a facile molten salt method. The device with the low-cost carbon electrode delivers a power conversion efficiency (PCE) of 6.16%, which is among the highest PCE reported for the full-inorganic Sb₂S₃ solar cells.

Abstract

Antimony sulfide (Sb₂S₃) is emerging as a promising light harvesting material owing to its brilliant photoelectric property. However, the performance of Sb₂S₃-based solar cells is partly limited by serious back contact interface recombination and hole transportation resistance. High-efficiency Sb₂S₃ devices typically use Spiro-OMeTAD and/or Au as back contact materials, but their stability and cost are a concern. In this sense, a surface modification scheme by lithium-doping is first introduced for Sb₂S₃ via a facile molten salt method. The ions in the molten state have high mobility and activity, enabling doping reactions to complete within a short time. The lithium-doped Sb₂S₃ thin film has a smooth and well-bonded surface, preferred (hk1) orientations, and an upshifted valence band maximum (VBM), which favors the hole extraction. Finally, a device using carbon as an electrode, which is more than a dozen times cheaper than gold, raises the short-circuit current density (J _SC) from 12.35 to 14.40 mA cm⁻², and the power conversion efficiency (PCE) from 4.47% to 6.16%. This is among the highest PCE reported for full-inorganic Sb₂S₃ solar cells, which demonstrates a facile interface modification technique via molten alkali salt to improve the performance of Sb₂S₃ solar cells.

Lead-free inorganic CsSnI_2.6Br_0.4 perovskite-based solar cells with a record efficiency of 11.2% are achieved via introducing dimethyl ketoxime (DMKO) as a multifunctional additive. The grain surface located DMKO not only mitigates Sn²⁺ oxidation but also passivates Sn(II)-related defects, forming an in situ encapsulation of the perovskite, which results in greatly enhanced ambient stability of the corresponding device.

Abstract

Organic-free and lead-free CsSnI₃ perovskite solar cells (PSCs) have recently gained growing attention as a promising template to mitigate the thermal instability and lead toxicity of hybrid lead-based PSCs. However, the relatively low device efficiency due to the high content of Sn(II)-related defects hinders its further development. Herein, highly performed CsSnI _3−x Br _x compositional perovskite-based PSCs are achieved by using dimethyl ketoxime (C₃H₇NO, DMKO) as a multifunctional additive. As a commercially used deoxidant, DMKO can effectively neutralize the oxygen molecule and reduce Sn⁴⁺ back to Sn²⁺, enhancing the oxidation resistance of the film. Besides, the electron-rich oxime group (NOH) in DMKO tends to interact with Sn²⁺ ions with extremely low adsorption energy less than −15 eV and inhibits defect formation, resulting in films with low defect density. The corresponding PSCs deliver a considerable open-circuit voltage (V _oc) of 0.75 V with a record efficiency as high as 11.2%, which represents the highest reported efficiency for lead-free all-inorganic PSCs thus far. More importantly, the grain surface distributed DMKO provides an in situ encapsulation of the perovskite, which results in greatly enhanced ambient stability of the un-encapsulated devices.

The development of organic materials that magnetically order at room temperature remains a grand challenge central to integrating spin, magnetic, and quantum mechanical effects within emerging technologies. This work demonstrates that long-range π-correlations within a high-spin conjugated polymer provide a viable path toward spin-center generation, robust intermolecular exchange interactions, anisotropic spin ordering at elevated temperatures, and high chemical stability.

Abstract

The development of open-shell organic molecules that magnetically order at room temperature,which can be practically applied, remains a grand challenge in chemistry, physics, and materials science. Despite the exploration of vast chemical space, design paradigms for organic paramagnetic centers generally result in unpaired electron spins that are unstable or isotropic. Here, a high-spin conjugated polymer is demonstrated, which is composed of alternating cyclopentadithiophene and benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole heterocycles, in which macromolecular structure and topology coalesce to promote the spin center generation and intermolecular exchange coupling. Electron paramagnetic resonance (EPR) spectroscopy is consistent with spatially localized spins, while magnetic susceptibility measurements show clear anisotropic spin ordering and exchange interactions that persist at room temperature. The application of long-range π-correlations for spin center generation promotes remarkable stability. This work offers a fundamentally new approach to the implementation of this long-sought-after physical phenomenon within organic materials and the integration of manifold properties within emerging technologies.

Non-uniformly distributed tensile strain is likely to present in solution-processed perovskite thin films, which severely undermines the operational stability of resultant perovskite solar cells. A fundamental understanding and cautious control of residual strain within the perovskites is indispensable to derive stable perovskite materials with designed properties.

Abstract

Perovskite solar cells (PSCs) are rivaling most commercial photovoltaics in the aspect of efficiency and cost, while their intrinsic instability remains a major concern for their practical deployment. The presence of undesirable strain in PSCs during device fabrication and operation refers to the extension/narrowing of chemical bonds and expansion/shrinkage of lattice volume, which largely affects device stability due to promoted phase transition, chemical decomposition, and mechanical fragility. Pioneering investigations and remarkable achievements have revealed that strain control is indispensable in the design of stable PSCs. Herein, the evolution of strain in perovskite thin films and its effect on device performance is elucidated, and state-of-the-art strategies of strain modulation are systematically reviewed. A thorough understanding and cautious control of the strain-related phenomenon pave the pathway to derive perovskite materials with desired properties.

Two region-specific selenium-based non-fullerene acceptors flanking conjugated side chains are compared with their sulfur-based analogue. mPh4F-TS with selenium at the outer positions show the most rigid skeleton, red-shifted absorption and compact stacking. The PM6 : mPh4F-TS organic solar cells exhibit the lowest energetic disorders, the highest charge carrier mobility and the best photon response, affording a top-ranking efficiency of >18 %.

Abstract

Central π-core engineering of non-fullerene small molecule acceptors (NF-SMAs) is effective in boosting the performance of organic solar cells (OSCs). Especially, selenium (Se) functionalization of NF-SMAs is considered a promising strategy but the structure-performance relationship remains unclear. Here, we synthesize two isomeric alkylphenyl-substituted selenopheno[3,2-b]thiophene-based NF-SMAs named mPh4F-TS and mPh4F-ST with different substitution positions, and contrast them with the thieno[3,2-b]thiophene-based analogue, mPh4F-TT. When placing Se atoms at the outer positions of the π-core, mPh4F-TS shows the most red-shifted absorption and compact molecular stacking. The PM6 : mPh4F-TS devices exhibit excellent absorption, high charge carrier mobility, and reduced energy loss. Consequently, PM6 : mPh4F-TS achieves more balanced photovoltaic parameters and yields an efficiency of 18.05 %, which highlights that precisely manipulating selenium functionalization is a practicable way toward high-efficiency OSCs.

Nature Energy, Published online: 19 September 2022; doi:10.1038/s41560-022-01096-5

The interfacial engineering using hole-selective self-assembled monolayers is vital to enhance power conversion efficiencies and stabilities of next generation photovoltaics.

Abstract

Inverted type perovskite solar cells (PSCs) have recently emerged as a major focus in academic and industrial photovoltaic research. Their multiple advantages over conventional PSCs include easy processing, hysteresis-free behavior, high stability, and compatibility for tandem applications. However, the maximum power conversion efficiency (PCE) of inverted PSCs still lags behind those of conventional PSCs because suitable charge-selective materials for inverted PSCs are limited. In this study, excellent hole-selective materials for inverted PSCs are introduced. A series of tricyclic aromatic rings containing O, S, or Se, respectively, as a core heteroatom, along with a phosphonic acid anchor, form a self-assembled monolayer (SAM) that directly contacts the perovskite absorber. The influence of heteroatoms in the aromatic structure on the molecular energetics and operating characteristics of the corresponding inverted PSCs is investigated using complementary experimental techniques as well as density functional theory (DFT) calculations. It is found that all of the SAMs formed an energetically well-aligned interface with the perovskite absorber. The interaction energy between the Se-containing SAM and perovskite absorber is the strongest among the series and it reduces the interfacial defect density, in turn leading to an extended charge carrier lifetime. As a result, PSCs incorporating the Se-containing SAM achieves a PCE of 22.73% and retains ≈96% of their initial efficiency after a maximum power point tracking test of 500 h without encapsulation under ambient conditions. All of the SAMs are then employed in organic solar cells (OSCs). Again, the Se-containing SAM-based OSCs demonstrates the highest PCE of 17.9% among the three molecular SAM-based OSCs. This study demonstrates the great potential for precisely engineered SAMs for use in high-performance solar cells.

A power conversion efficiency of 22.11%, a photoresponsibility of over 700 mA W⁻¹, a photodetectivity of 4.29 × 10¹⁴ cm Hz^1/2 W⁻¹, a linear dynamic range of 165 dB at room temperature, and dramatically boosted stability are demonstrated from the perovskite photovoltaics based on the Nd³⁺-substituted 2D:3D mixed perovskite composite thin films.

Abstract

2D perovskites are relatively stable but possess poor charge transport compared to 3D perovskites. To boost charge transport, novel 2D perovskites mixed with 3D perovskites are developed, where Pb²⁺ are partially substituted by the heterovalent neodymium cations (Nd³⁺) within both 2D and 3D perovskites (termed Nd³⁺-substituted 2D:3D mixed perovskites. Systematical studies reveal that the Nd³⁺-substituted 2D:3D mixed perovskites possess larger crystals, superior crystallinity, suppressed non-radiative charge recombination, and enhanced and balanced charge transport compared to the 2D:3D mixed perovskites. As a result, perovskite photovoltaics based on the Nd³⁺-substituted 2D:3D mixed perovskites exhibit a power conversion efficiency of 22.11%, a photoresponsibility of over 700 mA W⁻¹, a photodetectivity of 4.29 × 10¹⁴ cm Hz^1/2 W⁻¹, a linear dynamic range of 165 dB at room temperature, and dramatically boosted stability. These results demonstrate that, a facile way is developed to realize high-performance perovskite photovoltaics through partially heterovalent substituted Pb²⁺ by Nd³⁺ within 2D:3D mixed perovskites.

Publication date: 1 December 2022

Source: Nano Energy, Volume 103, Part A

Author(s): Simin Wu, Wanying Feng, Lingxian Meng, Zhe Zhang, Xiaodong Si, Yu Chen, Xiangjian Wan, Chenxi Li, Zhaoyang Yao, Yongsheng Chen

Publication date: 1 December 2022

Source: Nano Energy, Volume 103, Part A

Author(s): Hui Li, Zhongxiao Wang, Lian Wang, Bohong Chang, Zhen Liu, Lu Pan, Yutong Wu, Longwei Yin

Thiophenol ligands are introduced into the CsPbI₃ precursor, resulting in a high-quality perovskite film with low trap density and high carrier mobility, which facilitates the exciton separation and prolongs the charge lifetime, thus achieving a considerable power conversion efficiency of 20.1% for the all-inorganic perovskite solar cells.

Abstract

Inorganic perovskite CsPbI₃ has exhibited promising performance in single-junction solar cells, but the grain boundaries (GBs) in its film cause the formation of the defects with deep energy levels (such as iodide vacancy (V _I)) and impede the transport of carriers, worsening the efficiency and stability of the solar cells. Here, a CsPbI₃ precursor is devised with thiophenol series ligands (TP-ligands) containing both SH and π-conjugated molecules. The strong interaction between the SH group (Lewis base) and PbI₂ (Lewis acid) suppresses the formation of PbI₄ ²⁻ in perovskite solution, thus suppressing the formation of V _I in the film accordingly. The terminal groups, such as -F and -NH₂, are employed to achieve an appropriate evaporation speed of the ligands and prevent the oxidation of the thiophenol group, then a high-quality perovskite film with low trap density is obtained. In addition, the functional group π-conjugated molecules provide additional carrier transmission channels at the GBs to increase carrier mobility, which facilitates the exciton separation and prolongs the charge lifetime. The CsPbI₃ solar cell shows a considerable power conversion efficiency of 20.1% with an open-current voltage of 1.18 V and a high fill factor of 83.5% and excellent working stability.

Through machine learning, the influences of five factors, including grain size, defect density, bandgap, fluorescence lifetime, and surface roughness, on the efficiency and stability of perovskite solar cells have been revealed. The surface roughness and grain size are most influential to the long-term stability. A mathematical model is given to predict efficiency based on fluorescence lifetime and bandgap.

Abstract

Understanding the key factor driving the efficiency and stability of semiconductor devices is vital. To date, the key factor influencing the long-term stability of perovskite solar cells (PSCs) remains unknown because of the many influencing factors. In this work, through machine learning, the influences of five factors, including grain size, defect density, bandgap, fluorescence lifetime, and surface roughness, on the efficiency and stability of PSCs have been revealed. It is found that the bandgap has the greatest influence on the efficiency, and the surface roughness and grain size are most influential to the long-term stability. A mathematical model is given to predict efficiency based on fluorescence lifetime and bandgap. Guided by the model, four groups of experiments are conducted to confirm the machine-learning predictions and a PSC with 23.4% efficiency and excellent long-term stability is obtained, as manifested by retention of 97.6% of the initial efficiency after 3288 h aging in the ambient environment, the best stability under these conditions. This work shows that machine learning is an effective means to enrich semiconductor physical models.

A quaternary strategy is used to achieve desirable carrier behaviors and optimized multiphase morphology; thus, the device shows an outstanding power conversion efficiency (PCE) of 19.32% (certified 19.35%). Furthermore, the device with ≈300 nm-thick film shows a high efficiency of 17.55%, and the large-area devices (1.05 and 72.25 cm²) deliver encouraging PCEs of 18.25% and 12.20%, which are among the highest values reported so far.

Abstract

With the continuous breakthrough of the efficiency of organic photovoltaics (OPVs), their practical applications are on the agenda. However, the thickness tolerance and upscaling in recently reported high-efficiency devices remains challenging. In this work, the multiphase morphology and desired carrier behaviors are realized by utilizing a quaternary strategy. Notably, the exciton separation, carrier mobility, and carrier lifetime are enhanced significantly, the carrier recombination and the energy loss (E _loss) are reduced, thus beneficial for a higher short-circuit density (J _SC), fill factor (FF), and open-circuit voltage (V _OC) of the quaternary system. Moreover, the intermixing-phase size is optimized, which is favorable for constructing the thick-film and large-area devices. Finally, the device with a 110 nm-thick active layer shows an outstanding power conversion efficiency (PCE) of 19.32% (certified 19.35%). Furthermore, the large-area (1.05 and 72.25 cm²) devices with 110 nm thickness present PCEs of 18.25% and 12.20%, and the device with a 305 nm-thick film (0.0473 cm²) delivers a PCE of 17.55%, which are among the highest values reported. The work demonstrates the potential of the quaternary strategy for large-area and thick-film OPVs and promotes the practical application of OPVs in the future.

A new acceptor of FTCCBr and frequently-used ITCC is studied, due to their similar optical and electrochemistry properties. Under a light intensity of 1000 lux, the PB2:ITCC-based device with a small electrostatic potential (ESP) offset gives a power conversion efficiency (PCE) of 25.4%, while the PB2:FTCCBr-based device with a large ESP offset achieves a record PCE of 30.2%.

Abstract

The correlation between molecular structure and photovoltaic performance is lagging for constructing high-performance indoor organic photovoltaic (OPV) cells. Herein, this relationship is investigated in depth by employing two medium-bandgap nonfullerene acceptors (NFAs). The newly synthesized NFA of FTCCBr exhibits a similar bandgap and molecular energy level, but a much stronger dipole moment and larger average electrostatic potential (ESP) compared with ITCC. After blending with the polymer donor PB2, the PB2:ITCC and PB2:FTCCBr blends exhibit favorable bulk-heterojunction morphologies and the same driving force, but the PB2:FTCCBr blend exhibits a large ESP difference. In OPV cells, the PB2:ITCC-based device produces a power conversion efficiency (PCE) of 11.0%, whereas the PB2:FTCCBr-based device gives an excellent PCE of 14.8% with an open-circuit voltage (V _OC) of 1.05 V, which is the highest value among OPV cells with V _OC values above 1.0 V. When both acceptor-based devices work under a 1000 lux of 3000 K light-emitting diode, the PB2:ITCC-based 1 cm² device yields a good PCE of 25.4%; in contrast, the PB2:FTCCBr-based 1 cm² device outputs a record PCE of 30.2%. These results suggest that a large ESP offset in photovoltaic materials is important for achieving high-performance OPV cells.

Perovskite single crystals with thicknesses ranging from 20 to 300 µm can still exhibit efficient charge collection in the absence of an external voltage bias, resulting in high power conversion efficiency. The electron-diffusion length in these crystals is calculated to be longer than 0.4 mm. This work demonstrates that perovskite crystals can offer more than what is currently expected.

Abstract

Single-crystal halide perovskites exhibit photogenerated-carriers of high mobility and long lifetime, making them excellent candidates for applications demanding thick semiconductors, such as ionizing radiation detectors, nuclear batteries, and concentrated photovoltaics. However, charge collection depreciates with increasing thickness; therefore, tens to hundreds of volts of external bias is required to extract charges from a thick perovskite layer, leading to a considerable amount of dark current and fast degradation of perovskite absorbers. However, extending the carrier-diffusion length can mitigate many of the anticipated issues preventing the practical utilization of perovskites in the abovementioned applications. Here, single-crystal perovskite solar cells that are up to 400 times thicker than state-of-the-art perovskite polycrystalline films are fabricated, yet retain high charge-collection efficiency in the absence of an external bias. Cells with thicknesses of 110, 214, and 290 µm display power conversion efficiencies (PCEs) of 20.0, 18.4, and 14.7%, respectively. The remarkable persistence of high PCEs, despite the increase in thickness, is a result of a long electron-diffusion length in those cells, which was estimated, from the thickness-dependent short-circuit current, to be ≈0.45 mm under 1 sun illumination. These results pave the way for adapting perovskite devices to optoelectronic applications in which a thick active layer is essential.

A room-temperature molten salt dimethylamine acetate is developed as the solvent for precursor solutions, which also regulates the phase conversion process of the CsPbI₃ film. Consequently, 1.25 V of the open-circuit voltage and >21% power conversion efficiency are achieved, which is the record highest for CsPbI₃ perovskite solar cells reported so far.

Abstract

All-inorganic CsPbI₃ perovskite has emerged as an important photovoltaic material due to its high thermal stability and suitable bandgap for tandem devices. Currently, the cell performance of CsPbI₃ solar cells is mainly subject to a large open-circuit voltage (V _OC) deficit. Herein, a multifunctional room-temperature molten salt, dimethylamine acetate (DMAAc) is demonstrated, which not only directly acts as a solvent for precursor solutions, but also regulates the phase conversion process of the CsPbI₃ film for high-efficiency photovoltaics. DMAAc can stabilize the DMAPbI₃ structure and eliminate the Cs₄PbI₆ intermediate phase, which is easily spatially segregated. Meanwhile, a new homogeneous intermediate phase DMAPb(I,Ac)₃ is formed, which finally affords high-quality CsPbI₃ films. With this approach, the charge capture activity of defects in the CsPbI₃ film is significantly suppressed. Consequently, a V _OC of 1.25 V and >21% power conversion efficiency are achieved, which is the record highest reported thus far. This intermediate phase-regulation strategy is believed to be applicable to other perovskite material systems.