Chen Weijie

The strong interaction between 4-(bis(9,9-dimethyl-9 H-flouren-2-yl)amino)-1-naphthoic acid (KTN) and iodide vacancies exposing Pb²⁺ reduces the nonradiative recombination and elongates the carrier lifetime, leading to an outstanding PCE approaching 23% with a notable increase in open-circuit voltage (V _OC) of 60 mV.

The photovoltaic performance of perovskite solar cells (PSCs) prepared by the low-temperature solution method has made rapid progress. However, the surface of the film is prone to defects that trap photogenerated charges, resulting in nonradiative recombination energy loss and limiting the open-circuit voltage and overall performance of the device. Interface passivation as an effective method can significantly reduce defects and inhibit nonradiative recombination. Herein, a simple method is introduced to passivate perovskite films by a carboxylated (–COOH) sensitizer that is applied in dye-sensitized solar cells (DSCs), 4-(bis(9,9-dimethyl-9H-flouren-2-yl)amino)-1-naphthoic acid (KTN) molecules. The research results show that the chemical interaction between KTN and iodide vacancies exposing Pb²⁺ can reduce the nonradiative recombination and elongate the carrier lifetime, which leads to an excellent power conversion efficiency (PCE) with 23% with an obvious increase in open-circuit voltage (V _OC) of 60 mV. Moreover, the defect passivation can significantly enhance the stability of corresponding PSC devices. The unencapsulated device with KTN passivation can readily maintain ≈90% of its initial efficiency value after 1400 h. These findings may provide a novel approach for interfacial defect passivation to further promote the performance and stability of PSCs.

Guanidine thiocyanate (GASCN) is selected to regulate quality of PEA_0.1FA_0.9SnI₃-based perovskite film. The GASCN additive can form Lewis adducts with uncoordinated Sn atoms, inhibit the oxidation of Sn²⁺, and passivate the trap states. Thus, the unsealed tin-based perovskite solar cells with GASCN additive at the optimal process demonstrate a champion efficiency of 10.06% with better stability.

Although tin-based perovskite solar cells (TPSCs) are regarded as one of the most potential candidates in the field of lead-free photovoltaics, the photoelectrical properties of TPSCs are limited because Sn²⁺ tends to be oxidized to be Sn⁴⁺ to produce tin vacancies, resulting in poor performance. Herein, guanidine thiocyanate (GASCN) is selected to regulate the quality of tin-based perovskite film to fabricate efficient TPSCs. The device structure is ITO/PEDOT:PSS/PEA_0.1FA_0.9SnI₃ with or without GASCN/PCBM/PEI/Ag. The effects of GASCN additive on the microstructure and photoelectrical properties of TPSCs are systematically investigated. The results show that the GASCN additive can form Lewis adducts with uncoordinated Sn atoms, inhibit the oxidation of Sn²⁺ to Sn⁴⁺, and passivate the trap states of perovskite films, resulting in the suppressed charge recombination and improved performance. Thus, the unsealed TPSCs with GASCN additive at the optimal process demonstrate a champion efficiency of 10.06% with better stability. Herein, an effective strategy to fabricate efficient and stable TPSCs is provided.

Dopant free hole transporting materials i.e., derivatives of dibenzothiophene and carbazole used in perovskite solar cells reaching power conversion efficiency of 20.90% which correlate with effects of 1) the hole transport, 2) the efficiency of the hole transfer from the perovskite phase to hole transporters, and 3) the charge collection at an electrode.

Replacement of hole-transporting materials (HTM) for additive-free perovskite solar cells (PSCs) is an urgent issue. In this work, three new derivatives of dibenzothiophene with methoxyphenyl, trimethoxyphenyl, carbazole moieties are synthesized as hole-transporting materials for PSCs. The hole density dynamics and hole transporting properties of synthesized dibenzothiophene derivatives are investigated by combination of the charge extraction by linearly increasing voltage (CELIV) and time-of-flight (TOF) techniques. The TOF hole mobility (μ _h) of one compound reaches the highest value of 4.2 × 10⁻³ cm² V⁻¹s⁻¹ at an electric field of 2.5 × 10⁵ V cm⁻¹, however additive-free layers in PSCs did not show the best performance. Instead, the PSC efficiency is determined by a trade-off between the hole-mobility properties and the “effective” hole recombination rate k _B ranging 0.5–40.3 ms⁻¹ as determined by means of the CELIV method. The best hole extraction properties are observed for a compound with μ _h of 9.45 × 10⁻⁴ cm² V⁻¹s⁻¹ and k _B of 11.8 ms⁻¹ which is coherent with its lowest energetic disorder σ of 78.2 meV. Having both appropriate hole density dynamics and hole-transporting properties, hole-transporting layer of that compound allows to reach PCE of 20.9% for additive-free PSC, which is among the state-of-art values for devices with undoped HTM.

Efficient and photostable wide-bandgap (WBG) perovskites (≈1.8 eV) with only 25 mol% bromide are enabled by steric engineering via alloying dimethylammonium and chloride. The WBG single-junction cells, with a high efficiency of 17.7%, exhibit promising operational stability under 1-sun illumination (T ₉₀ of 1045 h). This strategy enables all-perovskite tandem with an impressive stabilized efficiency of 26.0%.

Abstract

Wide-bandgap (WBG, ≈1.8 eV) perovskite is a crucial component to pair with narrow-bandgap perovskite in low-cost monolithic all-perovskite tandem solar cells. However, the stability and efficiency of WBG perovskite solar cells (PSCs) are constrained by the light-induced halide segregation and by the large photovoltage deficit. Here, a steric engineering to obtain high-quality and photostable WBG perovskites (≈1.8 eV) suitable for all-perovskite tandems is reported. By alloying dimethylammonium and chloride into the mixed-cation mixed-halide perovskites, wide bandgaps are obtained with much lower bromide contents while the lattice strain and trap densities are simultaneously minimized. The WBG PSCs exhibit considerably improved performance and photostability, retaining >90% of their initial efficiencies after 1000 h of operation at maximum power point. With the triple-cation/triple-halide WBG perovskites enabled by steric engineering, a stabilized power conversion efficiency of 26.0% in all-perovskite tandem solar cells is further obtained. The strategy provides an avenue to fabricate efficient and stable WBG subcells for multijunction photovoltaic devices.

A new ternary system is developed by introducing an asymmetric electron acceptor BTP-2F2Cl into the PM1:L8-BO host system. The ternary system exhibits a record-high efficiency of 19.17% and demonstrates better long-term stability compared to the host system.

Abstract

The ternary strategy has been widely identified as an effective approach to obtain high-efficiency organic solar cells (OSCs). However, for most ternary OSCs, the nonradiative voltage loss lies between those of the two binary devices, which limits further efficiency improvements. Herein, an asymmetric guest acceptor BTP-2F2Cl is designed and incorporated into a PM1:L8-BO host blend. Compared with the L8-BO neat film, the L8-BO:BTP-2F2Cl blend film shows higher photoluminescence quantum yield and larger exciton diffusion length. Introducing BTP-2F2Cl into the host blend extends its absorption spectrum, improves the molecular packing of host materials, and suppresses the nonradiative charge recombination of the ternary OSCs. Consequently, the power conversion efficiency is improved up to 19.17% (certified value 18.7%), which represents the highest efficiency value reported for single-junction OSCs so far. The results show that improving the exciton behaviors is a promising approach to reducing the nonradiative voltage loss and realizing high-performance OSCs.

A series of acceptors PYSe-Clx (x = 0, 10, 20, and 30) is developed, in which the highest power conversion efficiency (PCE) of 14.21% is achieved for the PYSe-Cl20-based all-polymer solar cells (all-PSCs). With the addition of acceptor PTClo-Y, the ternary devices reach a PCE of 15.26%. Notable PCEs of 11.84% and 13.81% are achieved for both large-area (1.21 cm²) devices based on binary and ternary all-PSCs by blade-coating, respectively.

Abstract

Developing robust materials is very critical and faces a big challenge for high-performance large-area all-polymer solar cells (all-PSCs) by printing methods. Herein, the authors combine the advantages of the terpolymerization strategy with the non-conjugated backbone strategy to regulate the molecular aggregation rationally during the film-forming printing process, facilitating a facile printing process for large-area all-PSCs. A series of terpolymer acceptors PYSe-Clx (x = 0, 10, 20, and 30) is also developed, which can effectively fine-tune the morphology and photoelectric properties of the active layer. The PBDB-T: PYSe-Cl20-based all-PSC delivers a significantly improved power cconversion efficiency (PCE) than the one with PBDB-T: PYSe (14.21% vs 12.45%). By addition of a small amount of non-conjugated polymer acceptor PTClo-Y, the ternary all-PSC reaches a PCE of 15.26%. More importantly, the regulation of molecular aggregation enables a facile blade-coating process of the large-area device. A record PCE of 13.81% for large-area devices (1.21 cm²) is obtained, which is the highest value for large-area all-PSCs fabricated by blade-coating. The environmentally friendly solvent processed large-area device also obtains an excellent performance of 13.21%. This work provides a simple and effective molecular design strategy of robust materials for high-performance large-area all-PSCs by a printing process.

Herein, a novel histamine diiodate (HADI) is synthesized and incorporated into the SnO₂/perovskite interface to modulate its electronic properties. Experimental results and theoretical calculations demonstrate a bridge function of HADI. The HADI-SnO₂-based rigid/flexible perovskite solar cells (PSCs) achieve efficiencies of 24.79% and 22.44%, respectively, the highest reported values for rigid MA-free PSCs and flexible PSCs so far.

Abstract

Flexible perovskite solar cells (f-PSCs) have attracted great attention because of their unique advantages in lightweight and portable electronics applications. However, their efficiencies are far inferior to those of their rigid counterparts. Herein, a novel histamine diiodate (HADI) is designed based on theoretical study to modify the SnO₂/perovskite interface. Systematic experimental results reveal that the HADI serves effectively as a multifunctional agent mainly in three aspects: 1) surface modification to realign the SnO₂ conduction band upward to improve interfacial charge extraction; 2) passivating the buried perovskite surface, and 3) bridging between the SnO₂ and perovskite layers for effective charge transfer. Consequently, the rigid MA-free PSCs based on the HADI-SnO₂ electron transport layer (ETL) display not only a high champion power conversion efficiency (PCE) of 24.79% and open-circuit voltage (V _OC) of 1.20 V but also outstanding stability as demonstrated by the PSCs preserving 91% of their initial efficiencies after being exposed to ambient atmosphere for 1200 h without any encapsulation. Furthermore, the solution-processed HADI-SnO₂ ETL formed at low temperature (100 °C) is utilized in f-PSCs that achieve a PCE as high as 22.44%, the highest reported PCE for f-PSCs to date.

A strain modulation strategy to constrain phase segregation in a wide-bandgap perovskite absorber by reinforcing the energy barrier for ion migration is reported. With compressive strain, the single-junction devices yield one of smallest voltage deficits of 440 mV. Moreover, the resulting perovskite/silicon tandem solar cells achieve a champion efficiency of 26.95% with improved light stability at open-circuit.

Abstract

Perovskite/silicon tandem solar cells are promising to penetrate photovoltaic market. However, the wide-bandgap perovskite absorbers used in top-cell often suffer severe phase segregation under illumination, which restricts the operation lifetime of tandem solar cells. Here, a strain modulation strategy to fabricate light-stable perovskite/silicon tandem solar cells is reported. By employing adenosine triphosphate, the residual tensile strain in the wide-bandgap perovskite absorber is successfully converted to compressive strain, which mitigates light-induced ion migration and phase segregation. Based on the wide-bandgap perovskite with compressive strain, single-junction solar cells with the n–i–p layout yield a power conversion efficiency (PCE) of 20.53% with the smallest voltage deficits of 440 mV. These cells also maintain 83.60% of initial PCE after 2500 h operation at the maximum power point. Finally, these top cells are integrated with silicon bottom cells in a monolithic tandem device, which achieves a PCE of 26.95% and improved light stability at open-circuit.

Remarkably large organic photovoltaic modules with areas of 528.5 and 108 cm² that exhibit high power conversion efficiencies of 7.67% and 9.15%, respectively, are achieved on an indium-tin-oxide-free flexible transparent electrode. The sputtered ZnO and blade-coated ZnO nanoparticle bilayer for photovoltaic modules shows superior advantages, such as high uniformity, low-temperature processing capabilities, and large-area processability.

Abstract

To realize the commercialization of nonfullerene acceptor-based efficient organic solar cells, a low-cost and indium-tin-oxide-free (ITO-free) flexible electrode-based module that can be produced by roll-to-roll production should be developed. However, the low surface energy of ITO-free electrodes hinders the formation of a uniform charge transport layer through solution processing; this is the main cause of the efficiency drop. Herein, the realization of a highly uniform ZnO bilayer on an ultrathin Ag film-based transparent electrode that is suitable for flexible substrates and large-area organic photovoltaic (OPV) module fabrication is demonstrated. Based on the sputtered ZnO and blade-coated ZnO nanoparticle-based electron transport bilayer, large OPV module areas of 528.5 and 108 cm² with high efficiencies of 7.67% and 9.15%, respectively, are achieved.

Synthesized triethylene glycol-substituted fullerenes are miscible with perovskite precursors, enabling favorable vertical gradients of fullerene concentration within perovskite thin-films. This leads to grain boundary passivation by the fullerene species and enhanced electron collection by the fullerene overcoat in normal-type perovskite solar cells. Ultimately a high efficiency of 23.34% is produced, which is also certified by a national research institute.

Abstract

Fullerene-based n-type charge-collecting materials have emerged as a solution for high-performance perovskite solar cells. However, their application to perovskite solar cells is limited in the device architecture and only a small amount of fullerene additives have been introduced to the device system, because of the immiscibility of the fullerene species with polar solvents. To overcome this, triethylene glycol monomethyl ether chain-attached fullerene derivatives are synthesized and applied to normal-type perovskite solar cells. The newly synthesized fullerenes exhibit excellent solubility in polar solvents. A novel approach to introducing miscible fullerenes into perovskite devices and inducing a favorable vertical gradient is proposed. Forming an overcoat on an electron-transporting layer and waiting for a few minutes, the fullerene derivatives progressively permeate into the fullerene-doped perovskite active film. By fabricating perovskite solar cells combining direct mixing, overcoating, and waiting techniques, a remarkably high device efficiency of 23.34% is achieved. The high performance is attributed to the fullerene additives with a vertical gradient passivating the perovskite defect sites effectively and the overcoat enhancing the charge transfer. The device performance is certified by a national laboratory, which is the highest efficiency among the fullerene additives-used perovskite solar cells.

A 2D perovskite passivation layer is introduced as an electron blocking layer at the 3D perovskite/carbon interface in hole transporting layer (HTL) free carbon electrode-based perovskite solar cells. The substantial reduction of recombination losses due to the electron-blocking 2D perovskite results in a high power conversion efficiency for HTL-free devices (18.5%) with an improved stability.

Abstract

Interface engineering and passivating contacts are key enablers to reach the highest efficiencies in photovoltaic devices. While printed carbon–graphite back electrodes for hole-transporting material (HTM)-free perovskite solar cells (PSCs) are appealing for fast commercialization of PSCs due to low processing costs and extraordinary stability, this device architecture so far suffers from severe performance losses at the back electrode interface. Herein, a 2D perovskite passivation layer as an electron blocking layer (EBL) at this interface to substantially reduce interfacial recombination losses is introduced. The formation of the 2D perovskite EBL is confirmed through X-ray diffraction, photoemission spectroscopy, and an advanced spectrally resolved photoluminescence microscopy mapping technique. Reduced losses that lead to an enhanced fill factor and V _OC are quantified by electrochemical impedance spectroscopy and J _SC–V _OC measurements. This enables reaching one of the highest reported efficiencies of 18.5% for HTM-free PSCs using 2D perovskite as an EBL with a significantly improved device stability.

In situ electron microscopy, X-Ray-assisted and optoelectronic characterization techniques as the tools presenting the dynamic degradation changes of perovskite materials and devices’ microstructure, crystal structure, chemical composition, morphology, and photoelectric properties under different stress factors are effective to reveal the degradation mechanisms of perovskite solar cell and promote their commercial application.

In the past decade, organic–inorganic hybrid perovskite solar cells (PSCs) have made unprecedented progress and recently achieved high efficiency of over 25%, comparable with commercial silicon solar cells. However, PSCs still face poor long-term stability hindering their commercial application. Because PSCs undergo severe degradation under environmental stress factors, such as moisture, heat, light, and electrical bias. Thus, exploring and evaluating the degradation pathways of perovskites and the degradation mechanisms of PSCs is quite essential. In situ diagnostic techniques can track the real-time changes of structure, morphology, and optoelectronic properties of the materials in the device during the degradation process. Herein, the progress on in situ characterization for understanding the degradation in PSCs is reviewed, including advanced characterization techniques in the aspects of electron microscopy, X-Ray, and optoelectronic spectroscopy. Besides, in situ characterization tracking the degradation process of perovskite material films from typical methylamine (MA) perovskite to formamidinium (FA)–cesium (Cs) mixed-cation perovskite and PSCs dependent on external factors is also discussed. This overview can provide a further understanding of the stability of PSCs and solve the problems on their road to commercialization. Finally, the future perspectives of in situ characterization for understanding the degradation of PSCs are provided at the end of this review.

Cobalt modulation of SnO₂ electron transporting layer (ETL) significantly improves the performance and reduces the hysteresis of flexible perovskite solar cells (PSCs). Cobalt nitrate improves the crystallinity of SnO₂ nanoparticles, reduces surface defects of ETL and facilitates perovskite film growth, leads to hysteresis-less flexible PSCs with efficiencies over 20% in regular structure.

Flexible perovskite solar cells (PSCs) have great potential for portable electronics, however, suffer from large hysteresis in regular structure. Insufficient charge extraction in commonly used tin dioxide (SnO₂) electron transporting layer (ETL) is regarded as one possible origin of hysteresis due to the low crystallinity and energy level mismatching. Here, the hysteresis of flexible PSCs is suppressed by synthesizing cobalt-modified SnO₂ ETLs, which improve electron extraction capability due to the high carrier mobility and well-aligned energy levels. Moreover, cobalt modification passivates the defects on the ETL surface, facilitates sequential perovskite film growth, and inhibits carrier recombination. As a result, flexible PSCs with efficiencies exceeding 20% are obtained with significantly reduced hysteresis and enhanced illumination stability. Comprehensive optoelectronic simulations are conducted to unveil the deep mechanisms of eliminated hysteresis. The proposed work provides an efficient and facile strategy for the fabrication of high-performance flexible PSCs upon future commercialization.

An environmental-friendly PBAT polymer is adopted to implant the perovskite film with an anti-solvent method, which can passivate the uncoordinated Pb²⁺ and neutral iodine defects of perovskite material and markedly improve device efficiency and operational stability. More importantly, the polymer network can prevent nearly 98% of Pb²⁺ from leaking by directly immersing the polymer-coated perovskite film in water.

Abstract

Although perovskite solar cells (PSCs) are on the road to industrialization, the operational stability under high efficiency still needs to be improved, and the water solubility of lead ions (Pb²⁺) will cause environmental pollution problems. Herein, it is successfully implanted an environment-friendly (biodegradability) poly(butylene adipate-coterephthalate) polymer (PBAT) into the perovskite film, which can passivate the uncoordinated Pb²⁺ and neutral iodine defects of the perovskite material because of the adequate carbonyl groups and benzene rings in PBAT polymer, thereby regulating the crystallization of perovskite film with lower trap density, inhibiting the nonradiative recombination and improving charge carrier transport. As a result, the polymer-incorporated inverted PSCs achieve optimal conversion efficiencies of 22.07% (0.1 cm²) and 20.31% (1 cm²). Meanwhile, the incorporated device, after being encapsulated, exhibits a prominent improvement in operational stability of high-efficiency device under maximum power point tracking and continuous one sunlight illumination, maintaining the initial efficiency of 80% for 3249 h. More importantly, the polymer network can protect Pb²⁺ from being dissolved by water and prevent nearly 98% of Pb²⁺ from leaking by directly immersing the polymer-coated perovskite film in water. Environmental-friendly molecules provide new hope for solving lead poisoning and improving device operational stability under high efficiency.

Nature Energy, Published online: 14 April 2022; doi:10.1038/s41560-022-00997-9

Publication date: 20 April 2022

Source: Joule, Volume 6, Issue 4

Author(s): Jiwei Liang, Xuzhi Hu, Chen Wang, Chao Liang, Cong Chen, Meng Xiao, Jiashuai Li, Chen Tao, Guichuan Xing, Rui Yu, Weijun Ke, Guojia Fang

Superhalogen is a promising candidate for defect passivation and stabilization of metal-halide perovskites due to its higher electronegativity and electron affinity than halides. This perspective gives an overview of studies regarding the use of superhalogen to develop efficient and stable perovskite solar cells and concludes with an outlook of further research directions.

Metal-halide perovskites are optoelectronic materials applied to solar cells as a light absorber due to their excellent optoelectronic properties. The power conversion efficiency of perovskite solar cells (PSCs) reaches 25.7% certified, which stands in comparison with Si solar cells. Importantly, compositional engineering of perovskites has been one of the keys to the breakthrough. However, the presence of defects within perovskites is a matter of importance as it can cause nonradiative recombination of charge carriers. In addition, defect migration can degrade the photovoltaic performance and stability of PSCs. Previous studies have commonly addressed that iodide-related defects such as interstitial iodide and iodide vacancy are problematic due to their low formation energy. Thus, halide engineering is imperative to mitigate the defect-related dynamics and improve the materials quality of perovskites. In this sense, superhalogen is a promising candidate for defect passivation and stabilization of perovskites based on its higher electronegativity and electron affinity than halides, which are beneficial to the formation of a more robust interaction with adjacent elements in perovskites. This perspective gives an overview of studies regarding the use of superhalogen to develop efficient and stable PSCs and concludes with an outlook of further research directions.

A two-step orthogonal solvent method that enables straightforward fabrication of multilayer perovskite films is applied to construct the perovskite/carbon heterojunction for carbon-based perovskite solar cells (PSCs). Such a bulk heterojunction (BHJ) improves the interface contact and facilitates the hole transport between perovskite and the carbon electrode, significantly boosting the power conversion efficiency (PCE) to 16.4% with certification.

An efficient perovskite junction is critical for the functioning of perovskite solar cells (PSCs). However, carbon-based perovskite solar cells (C-PSCs) have been plagued by the paucity of ways to construct an efficient junction between perovskite and carbon, staggering around an efficiency much lower than other state-of-the-art PSCs. Herein, a perovskite/carbon bulk heterojunction (BHJ) for C-PSCs is innovated and systematically studied. First, a two-step orthogonal solvent method is developed to deposit a series of high-quality perovskite films directly on the preformed perovskite film, allowing to manipulate compositions, band alignment, and charge transfer of the perovskite junction in a low-cost and straightforward fashion. Second, by adopting this method to a porous carbon electrode as originally motivated, fabrication of perovskite/carbon BHJ with perovskite crystals by seamlessly filling in the porous carbon film is successfully done, thus providing a high contact area of perovskite/carbon heterojunction. Such a BHJ accelerates hole collection of the carbon electrode from the perovskite layer, thus significantly boosting the performance of C-PSCs with MAPbI₃ as the active layer from 12% to over 16% with certification. The device is shown to be stable with no obvious degradation after 1700 h of continuous light soaking near the maximum power point.

Crystal structural collapse in perovskite films governs the Pb leakage after exposure to water as indicated by Noyes-Whitney Model simulation. It is effectively retarded by constructing a chemically stable Dion-Jacobson two-dimensional (2D) perovskite (DOE)PbI_4−x Cl _x at the surface of the film.

Abstract

Perovskite solar cells (PSCs) have become a promising candidate for the next-generation photovoltaic technologies. As an essential element for high-efficiency PSCs however, the heavy metal Pb is soluble in water, causing a serious threat to the environment and human health. Due to the weak ionic bonding in three-dimensional (3D) perovskites, drastic structure decomposition occurs when immersing the perovskite film in water, which accelerates the Pb leakage. By introducing the chemically stable Dion-Jacobson (DJ) 2D perovskite at the 3D perovskite surface, the film dissolution is significantly slowed down, which retards lead leakage. As a result, the Pb contamination is dramatically reduced under various extreme conditions. In addition, the PSCs device deliver a power conversion efficiency (PCE) of 23.6 % and retain over 95 % of their initial PCE after the maximum power point tracking for over 1100 h.