Chen Weijie

The non-fullerene acceptor ITIC-Th is compared with PCBM in semitransparent organic solar cells. Both blends show high transmissivity in the visible spectrum while maintaining high power conversion efficiencies between 7% and 9%. ITIC-Th devices show lower energy losses from nonradiative recombination compared to PCBM, leading to better performance. The origins of this difference are explained using a variety of techniques.

Organic solar cells that are transparent to visible light are highly desirable for applications such as window treatments or solar greenhouse panels. A key challenge is to simultaneously transmit most photons between 400 and 700 nm while retaining a high short-circuit current and power conversion efficiency (PCE). Here, organic bulk heterojunction (BHJ) solar cells consisting of a donor polymer (PM2) is reported and the non-fullerene acceptor ITIC-Th achieves a PCE of 9.3%, and the BHJ thin films exhibit an average visible transmittance over 40%. This value is achieved primarily due to a very high open-circuit voltage (V _OC) of 0.93 V, which represents a voltage loss of only 0.50 V relative to the material optical bandgap, E _opt. In PM2:PC₆₁BM devices, this voltage loss increases to 0.62 V (V _OC = 0.82 V). It is found that this difference in V _OC is due to higher nonradiative recombination in the fullerene-based solar cell, suggesting that non-fullerene acceptors may lead to better performance in semi-transparent devices. The optoelectronic properties associated with PM2:ITIC-Th and PM2:PC₆₁BM blends are further corroborated by different morphological features and local structures at the donor-acceptor interfaces characterized by atomic force microscopy, X-ray scattering, and solid-state NMR spectroscopy techniques.

Using a self-organized, discotic liquid crystal material (TP-2-PIE) as a third component it is possible to alter Flory–Huggins molecular interaction parameters in donors and enhance phase separations between the donor-dominated and the acceptor-dominated domains. This can tune the charge carrier mobilities and density of defect states in the active layer and improve device performance.

A donor–acceptor (D–A)-type discotic organic material (TP-2-PIE), having a self-organization ability, is selected to be the third component blending with poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-co-3-fluorothieno [3,4-b]-thiophene-2-carboxylate] (PTB7-Th): [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) in ternary bulk-heterojunction polymer solar cells. The complementary absorption and energy transfer between TP-2-PIE and PTB7-Th contributes to the enhancement in photocurrent generation, improving the short-circuit current. In addition, TP-2-PIE alters the molecular interaction leading to an enhanced phase separation, which promotes carrier transport with a good balance and minimizes the density of trap states simultaneously. In this way, the overall photovoltaic performances are markedly enhanced at an optimal condition of 10 wt% TP-2-PIE in donors with a 12.6% efficiency increase. Current understanding on the functionality of this third component in ternary solar cells may be able to guide future device development.

Publication date: 20 April 2022

Source: Joule, Volume 6, Issue 4

Author(s): Xiuhong Sun, Zhipeng Shao, Zhipeng Li, Dachang Liu, Caiyun Gao, Chen Chen, Bingqian Zhang, Lianzheng Hao, Qiangqiang Zhao, Yimeng Li, Xianzhao Wang, Yue Lu, Xiao Wang, Guanglei Cui, Shuping Pang

12.42% monolithic 25.42 cm² flexible organic solar cells (OSCs) are reported based on the nanogrid electrodes modified by amorphous indium tin oxide (ITO) modification layer. The modification of amorphous ITO also ensures excellent air shelf stability, as well as mechanical properties.

Abstract

Printed metal nanogrid electrode exhibits superior characteristics for use in flexible organic solar cells (OSCs). However, the high surface roughness and inhomogeneity between grid and blank region is adverse for performance improvement. In this work, a thin amorphous indium tin oxide (ITO) film (α-ITO) is introduced to fill the blank and to improve the charge transporting. The introduction of α-ITO significantly improves the comprehensive properties of metal grid electrode, which exhibits excellent bending resistance and long-term stability under double 85 condition (under 85 °C and 85% relative humidity) for 200 h. Both experimental and simulation results reveal α-ITO with a sheet resistance of 20 000 Ω □⁻¹ is sufficient to improve the charge transporting within the adjacent grids, leading to a remarkable efficiency of 16.54% for 1 cm² flexible devices. With area increased to 4.00, 9.00, and 25.42 cm², the devices still display a performance of 16.22%, 14.69%, and 12.42%, respectively, showing less efficiency loss during upscaling. And the 25.42 cm² monolithic flexible device exhibits a certificated efficiency of 12.03%. Moreover, the device shows significantly improved air stability relative to conventional high-conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate-modified device. All these make the α-ITO-modified Ag/Cu electrode promise to achieve high-efficient and long-term stable large-area flexible OSCs.

Publication date: 16 March 2022

Source: Joule, Volume 6, Issue 3

Author(s): Xiyue Yuan, Yunli Zhao, Dongsheng Xie, Langheng Pan, Xinyuan Liu, Chunhui Duan, Fei Huang, Yong Cao

Two sides of the same coin: the yellow δ-phase grows over time at the expense of the black γ-phase, following an Avrami like pathway after an incubating delay time. This kinetic study versus temperature has a predictive valence and allows identifying the beneficial role of Eu in stabilizing the black γ-phase in the range 30–100 °C.

The black γ-phase of CsPbI₃ with its bandgap at ≈1.75 eV can enable the take-off of tandem solar cells as soon as intrinsic instability to the yellow δ-phase is solved. Here, a black γ-phase is formed at low temperature (80–90 °C) by incorporating Eu through EuCl₃ or EuI₂. It is forced to become yellow by thermal heating under nitrogen. It is demonstrated how spectroscopic ellipsometry and the related critical points analysis provide a new diagnostic tool to monitor the transformation through the bandgap footprint. As two sides of the same coin, the consumption of the black γ-phase and the growth of the yellow δ-phase are addressed and modeled in the range of 60–100 °C using the Avrami's theory. This approach allows extracting the activation energies of the phase transformation that are 0.95 eV versus 1.15 eV using EuI₂ and EuCl₃, respectively. In situ transmission electron microscopy analyses combined with fast marching simulations highlight that the phase transformation occurs through constant nucleation and growth. Europium has indeed the capability to tackle those processes and to extend the durability of the black γ-phase at 30 °C in N₂ to ≈250 days, hugely above the one observed without Eu (≈hours).

Light causes ions to migrate two different directions in organic/inorganic lead halide: horizontal and vertical. A method combining cathodoluminescence and statistics is developed that quantifies the different types of ion migrations for both halides and cations in different types of perovskites. A possible mechanism for different modes of ion migration is discussed.

Abstract

Studying the compositional instability of mixed ion perovskites under light illumination is important to understand the mechanisms underlying their efficiency and stability. However, current techniques are limited in resolution and are unable to deconvolute minor ion migration phenomena. Here, a method that enables ion migration to be studied allowing different segregation mechanisms to be elucidated is described. Statistical analysis is applied to cathodoluminescence data to generate compositional distribution histograms. Using these histograms, two different ion migration phenomena, horizontal ion migration (HIM) and vertical ion migration (VIM), are identified in different perovskite films. It is found that most passivating agents inhibit HIM, but not VIM. However, VIM can be reduced by deposition of imidazolium iodide on the perovskite surface. This method can be used to study perovskite-based devices efficiency and stability by providing molecular level mechanistic understanding of passivation approaches leading to performance improvement of perovskite solar cells via rational design.

A high-performance indium-tin-oxide-free organic photovoltaic (OPV) with superior flexibility and upscaling capacity is demonstrated. The OPV device adopts a top-illuminated structure optimized by an anode buffer layer and charge collecting grid, and delivers a record efficiency of 15.56%. Moreover, the flexible OPV exhibits no degradation after 100 000 bending cycles with a radius of 4 mm, which represents the best result for flexible OPVs.

Abstract

Developing indium-tin-oxide (ITO)-free flexible organic photovoltaics (OPVs) with upscaling capacity is of great significance for practical applications of OPVs. Unfortunately, the efficiencies of the corresponding devices lag far behind those of ITO-based rigid small-area counterparts. To address this issue, an advanced device configuration is designed and fabricated featuring a top-illuminated structure with ultrathin Ag as the transparent electrode. First, a conjugated polyelectrolyte layer, i.e., PCP-Li, is inserted to effectively connect the bottom Ag anode and the hole transport layer, achieving good photon to electron conversion. Second, charge collecting grids are deposited to suppress the increased resistance loss with the upscaling of the device area, realizing almost full retention of device efficiency from 0.06 to 1 cm². Third, the designed device delivers the best efficiency of 15.56% with the area of 1 cm² on polyimide substrate, representing as the record among the ITO-free, large-area, flexible OPVs. Interestingly, the device exhibits no degradation after 100 000 bending cycles with a radius of 4 mm, which is the best result for flexible OPVs. This work provides insight into device structure design and optimization for OPVs with high efficiency, low cost, superior flexibility, and upscaling capacity, indicating the potential for the future commercialization of OPVs.

High energy proton irradiation causes organic photovoltaic (OPV) cells degradation. The solubility of the active layer PTB7-Th:PC₇₁BM is substantially reduced, suggesting cross-linking can happen due to the irradiation. Raman spectroscopy reveals conformational changes on the PTB7-Th at high proton fluence. Interestingly, despite a drop in performance, the percentage drop is in fact lower than for III–V semiconductor-based PV.

Recent developments of solution-processed bulk-heterojunction organic photovoltaic (OPV) cells have demonstrated power conversion efficiencies (PCEs) as high as 18% for single-junction devices. Such a high PCE in addition to its desirable lightweight property and high mechanical flexibility can realize high specific power and small stowed volume, which are key considerations when choosing PV for space missions. To take one important step forward, their resilience to ionizing radiation should be well studied. Herein, the effect of proton irradiation at various fluences on the performance of benchmark OPV cells is explored under AM0 illumination. The remaining device performance is found to decrease with increasing proton fluence, which correlates to changes in electrical and chemical properties of the active layer. By redissolving the devices, the solubility of the active layer is found to decrease with increasing proton fluence, suggesting that the active materials are likely cross-linked. Additionally, Raman studies reveal conformational changes of the polymer leading to a higher degree of energetic disorder. Despite a drop in performance, the retaining percentage of the performance is indeed higher than the current market-dominating space PV technology—III–V semiconductor-based PV, demonstrating a high potential of the OPV cell as a candidate for space applications.

Change in fluorinated position of end groups leads to the nonfullerene acceptor with an enhanced charge transport and therefore a significantly increased power conversion efficiency, which suggests the important role of fluorinated end groups in affecting the photovoltaic performance of nonfullerene acceptors.

Intermolecular interaction of nonfullerene acceptors (NFAs) is essential for controlling their photovoltaic performance. Fluorinated substituents attached at the end groups of NFAs can significantly affect their frontier molecular orbitals and intermolecular interactions. Herein, four heteroheptacene-based NFAs (ML-1F, ML-2FO, ML-2FM, and ML-2FP) with different fluorinated end groups are designed, synthesized, and characterized. The impact of the end group on the crystal structures, optical, electrochemical, charge transport, and photovoltaic properties of these NFAs are systematically studied. In comparison with the acceptor with monofluorinated end groups, the acceptors with difluorinated end groups show reduced bandgaps and downshifted energy levels. Single-crystal results demonstrate that the intermolecular interactions including the π–π stacking distance and molecular aggregation behavior of these acceptors are also affected by the variation of end groups thereby leading to the acceptors with varied charge transport properties. In combination with the polymer donor of PM6, ML-2FM exhibits the highest power conversion efficiency (PCE) of 15.33% with a short-circuit current density of 23.73 mA cm⁻² and a fill factor of 0.734. However, ML-2FP displays an inferior PCE of 10.48% which is lower than that of ML-1F (11.41% PCE). The results show that the fluorine substituent number and position of end group are of vital importance in determining their photovoltaic performance.

Herein, a comprehensive review on the recent progress of pure FAPbI₃-based perovskite solar cells (PSCs) is presented. From the development in both efficiency and stability, improvement on the FAPbI₃ film quality including morphology control, defect passivation, dimensional regulation, and strain engineering as well as optimization of the device structure and interface layers promotes the commercialization process of pure FAPbI₃-based PSCs.

Organic–inorganic hybrid perovskite solar cells (PSCs) have attracted great attentions due to their rapid increase of power conversion efficiency (PCE). Although the highest PCE of PSCs (25.7%) has been achieved via using formamidinium lead iodide (FAPbI₃) with a suitable bandgap, there is still a lack of systematic analysis on FAPbI₃-based PSCs toward high stability and high efficiency. Herein, the progress in FAPbI₃ films and achievements in their high-efficiency and long-term stability PSCs are comprehensively reviewed. First, the progress from the aspects of morphology, defect, dimension, and strain for FAPbI₃ film optimization is summarized and then the development of FAPbI₃ PSCs in both efficiency and stability is discussed. Then, the methods to improve the FAPbI₃ film quality by morphology control, defect passivation, dimensional regulation, and strain engineering, as well as strategies to optimize the device structure and interface layers, which are critical to promote device stability and efficiency, are evaluated. Finally, the outlook and strategies for realizing commercialized FAPbI₃ PSCs with high efficiency and long lifetime are discussed.

The effect of charge transport material on the overall optical performance of the device is examined by finite element method. Density functional theory calculates the microstructure of the charge transport layer material and comprehends the energy level matching of the device. Combining micro- and macroscales facilitates the development of high-efficiency perovskite solar cells.

CsPbBr₃ film possesses high stability and easy manufacturing characteristics, rendering it attractive for applications in perovskite solar cells (PSCs). However, optical loss and energy level matching of different material layers are still the major factors, limiting the performance of PSCs. Herein, the finite element method (FEM) and density functional theory (DFT) calculations composed of simulation interaction technology are used to study the effects of different electron transport layer and hole transport layer materials on the optical performance of different PSC configurations. The effect of the charge transport layer (CTL) material and CsPbBr₃ on the energy level matching and charge transport of the device is explained. The FEM simulation results show that inorganic CTL materials produce less parasitic absorption than the organic materials. The DFT calculation results give the microscopic design rules of the CTL material. In addition, PSCs with a light-trapping structure are designed, which effectively suppressed surface reflection. Finally, through the screening of CTL materials and the design of advanced light-trapping structures, the photocurrent of PSCs is increased by 69.8% (from 5.30 to 9.00 mA cm⁻²). This work provides a novel model for the screening of CTL materials for inorganic PSCs.

Low-temperature-processed perovskite solar cells have wide applicability in flexible and two-terminal tandem devices. In this review, recent breakthroughs in low-temperature-processed stable perovskite device architectures are summarized. A wide range of topics starting with degradation processes, stability-enhanced material developments, general trends, and process upscaling of notable low-temperature-processed materials are summarized.

Abstract

The impending commercialization of perovskite solar cells (PSCs) is plodding despite the booming power conversion efficiencies and high stabilities. Most high-performance, stable PSCs are often processed partially with high-temperature processes, increasing the cost of production and energy payback time. Low-temperature-processed PSCs are crucial as they cut down the expenses lowering the barriers to industrial use. In addition, low-temperature-processed methods have a wide range of applicability in flexible devices and for tandem applications with other photovoltaic technologies where the temperature budget is limited. Therefore, making stable PSCs under ambient conditions as well as providing low-cost fabrication techniques is highly desirable. Here, a detailed review is presented on the development of the low-temperature process strategies for fabricating highly stable PSCs and perovskite solar modules. The effectiveness of low-temperature processing in various classes of materials is also discussed. First, the authors introduce some major degradation processes in PSCs. Then, the developments and evolving strategies of notable materials using low-temperature processing routes and a correlation with stability are summarized. A few general trends which are related to stability are also discussed. Overall, this review contributes to a better understanding of the status of low-temperature-processed cells and modules.

Half metals have promising applications in advanced spintronic devices. A new half metal LaCu₃Fe₂Re₂O₁₂ with a record-high Curie temperature (T _C) in perovskite oxides, combining with a wide spin-up energy gap (E _g) and a large magnetization (M _N), is synthesized. The current LaCu₃Fe₂Re₂O₁₂ thus shows the best synthetic performance (T _C × E _g × M _N) among all half-metallic oxides discovered so far.

Abstract

Half metals, in which one spin channel is conducting while the other is insulating with an energy gap, are theoretically considered to comprise 100% spin-polarized conducting electrons, and thus have promising applications in high-efficiency magnetic sensors, computer memory, magnetic recording, and so on. However, for practical applications, a high Curie temperature combined with a wide spin energy gap and large magnetization is required. Realizing such a high-performance combination is a key challenge. Herein, a novel A- and B-site ordered quadruple perovskite oxide LaCu₃Fe₂Re₂O₁₂ with the charge format of Cu²⁺/Fe³⁺/Re^4.5+ is reported. The strong Cu²⁺(↑)Fe³⁺(↑)Re^4.5+(↓) spin interactions lead to a ferrimagnetic Curie temperature as high as 710 K, which is the reported record in perovskite-type half metals thus far. The saturated magnetic moment determined at 300 K is 7.0 μ_B f.u.⁻¹ and further increases to 8.0 μ_B f.u.⁻¹ at 2 K. First-principles calculations reveal a half-metallic nature with a spin-down conducting band while a spin-up insulating band with a large energy gap up to 2.27 eV. The currently unprecedented realization of record Curie temperature coupling with the wide energy gap and large moment in LaCu₃Fe₂Re₂O₁₂ opens a way for potential applications in advanced spintronic devices at/above room temperature.

Aiming for hybrid perovskite solar cells, a novel strategy is developed that using ethylenediamine diiodide and phenethylammonium iodide as co-modifiers to selectively targeting anchor with lead–tin binary perovskite materials, resulting in comprehensively healing the dual-sourced defects. As a result, a champion efficiency of 22.51% is achieved, which is the record efficiency among the ideal-bandgap perovskite solar cells.

Abstract

Mixed lead–tin perovskite solar cells (LTPSCs) with an ideal bandgap are demonstrated as a promising candidate to reach higher power conversion efficiency (PCE) than their Pb-counterparts. Herein, a Br-free mixed lead–tin perovskite material, FA_0.8MA_0.2Pb_0.8Sn_0.2I₃, with a bandgap of 1.33 eV, as a perovskite absorber, is selected. Through density functional theory calculations and optoelectronic techniques, it is demonstrated that both Pb- and Sn-related A-site vacancies are pushed into deeper energetic depth, causing severe nonradiative recombination. Hence, a selective targeting anchor strategy that employs phenethylammonium iodide and ethylenediamine diiodide as co-modifiers to selectively anchor with Pb- and Sn-related active sites and passivate bimetallic traps, respectively, is established. Furthermore, the selectivity of the molecular oriented anchor passivation is demonstrated through energetic depth specificity of Pb- and Sn-related traps. As a result, a substantially enhanced open-circuit voltage (V _OC) from 0.79 to 0.90 V for the LTPSCs is achieved, yielding a champion PCE of 22.51%, which is the highest PCE among the reported ideal-bandgap PSCs. The V _OC loss is reduced to 0.43 V.

TEASCN (2-thiopheneethylamine thiocyanate) was synthesized to construct a bilayer structure on a Sn-Pb perovskite surface, which can passivate perovskite and ensure effective carrier transfer, enabling the device to reach a certified efficiency of 21.1 %. The mechanism for the growth of the uniform bilayer structure is revealed by simulation based on density functional theory.

Abstract

The combination of comprehensive surface passivation and effective interface carriers transfer plays a critical role in high-performance perovskite solar cells. A 2D structure is an important approach for surface passivation of perovskite film, however, its large band gap could compromise carrier transfer. Herein, we synthesize a new molecule 2-thiopheneethylamine thiocyanate (TEASCN) for the construction of bilayer quasi-2D structure precisely on a tin-lead mixed perovskite surface. This bilayer structure can passivate the perovskite surface and ensure effective carriers transfer simultaneously. As a result, the open-circuit voltage (V _oc) of the device is increased without sacrificing short-circuit current density (J _sc), giving rise to a high certified efficiency from a credible third-party certification of narrow band gap perovskite solar cells. Furthermore, theoretical simulation indicates that the inclusion of TEASCN makes the bilayer structure thermodynamically more stable, which provides a strategy to tailor the number of layers of quasi-2D perovskite structures.

A solid n-PEDOT:POM powder with neutral pH is synthesized by utilizing polyoxometalate as an oxidizing reagent, which exhibits excellent water solubility, high chemical stability and superior hole collection ability. Organic solar cells (OSCs) with n-PEDOT:POM as a hole transport layer exhibit a photovoltaic efficiency of 17.62% with a fill factor of 0.80. In addition, the neutral pH of PEDOT:PSS effectively improves the long-term stability of OSCs.

Abstract

Although PEDOT:PSS is the most prominently used conducting polymer as hole transporting layer (HTL) material in organic solar cells (OSCs), the strong acidity of PEDOT:PSS has been proved to cause corrosion on electrodes, which is largely responsible for device instability. At present, the development of a non-corrosive and stable PEDOT with comparable performance to PEDOT:PSS remains a great challenge in the field. Herein, a solid n-PEDOT:POM powder with neutral pH is synthesized by utilizing polyoxometalate (POM) as an oxidizing reagent, which exhibits excellent water solubility, high chemical stability, and superior hole collection ability. Impressively, an amazing fill factor of 0.80 along with a photovoltaic efficiency of 17.62% is obtained in the OSC by using n-PEDOT:POM as the HTL, suggesting an exceptional hole collection ability for n-PEDOT:POM. In addition, the neutral pH of PEDOT:PSS effectively improves the long-term stability of OSCs. X-ray photoelectron spectroscopy results reveal that, compared to the electrode/PEDOT:PSS interface, the permeation of dissociative indium from the electrode to n-PEDOT:POM can be greatly retarded, which proves the non-corrosive effect of n-PEDOT:POM and its improvement of device stability. The successful preparation of a non-corrosive and stable PEDOT derivative without sacrificing its hole collection ability provides the most fruitful new insight into developing high-performance HTL materials.

Seven prediction models built by machine learning algorithms demonstrate incredible prediction performance. A series of Shapley Additive Explanations explanation strategies for power conversion efficiency (PCE) were proposed, which not only comprehensively analyzed the 13 main features affecting PCE, but also ranked the most influential feature as FA (formamidinium/NH₂CHNH₂ ⁺).

Characterizing the electrical parameters of perovskite solar cells (PSCs) usually requires a lot of time to fabricate complete devices. Here, machine learning (ML) is used to reduce the device fabrication process and predict the electrical performance of PSCs. Using ML algorithms and 814 valid data cleaned from 2735 peer-reviewed publications, ML prediction models are built for bandgap, conduction band minimum, valence band maximum of perovskites, and electrical parameters of PSCs. These prediction models have excellent accuracy, and the root mean square error of the prediction models for bandgap and power conversion efficiency (PCE) reaches 0.064 eV and 1.58%, respectively. Among the many factors that affect the performance of PSCs, those factors play a major role in the lack of comprehensive explanation. Through the prediction model of electrical parameters and Shapley Additive explanations theory, the factors affecting the PCE of PSCs are explained and analyzed. It can not only verify the objective physical laws from the perspective of ML, but also conclude that among the 13 features, the content of formamidinium/NH₂CHNH₂ ⁺ plays the most important role in improving the PCE of PSCs. These results show that ML has great application possibilities in the PSC field.