Chen Weijie

Publication date: August 2023

Source: Journal of Energy Chemistry, Volume 83

Author(s): Rongbo Wang, Meidouxue Han, Ya Wang, Juntao Zhao, Jiawei Zhang, Yi Ding, Ying Zhao, Xiaodan Zhang, Guofu Hou

Through introducing Nb₂O₅ into the back electrode interface (BEI) of a Cu₂ZnSn(S,Se)₄ solar cell, Nb (& O) diffusion from Nb₂O₅ to absorber bulk and BEI takes place during selenization. As a result, over 10% efficiency devices are achieved by simultaneously optimizing BEI and bulk based on synergistic passivation effects comprising the developed chemical passivation effect and field passivation effect.

As compared to the predecessor Cu(In,Ga)Se₂ device, the current efficiency of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is still much lower mainly due to the known carriers recombination issue within interface and absorber bulk. In contrast to the majority of researches concerning recombination issues that focus on either single absorber bulk or interface passivation strategy, this study is pioneering in constructing synergistic passivation effects (SPE) to address the bulk and interface recombination issue simultaneously. By introducing a novel niobium pentoxide passivation layer into the back electrode interface (BEI), it is identified that SPE can be constructed due to Nb (& O) diffusion from Nb₂O₅ layer to absorber bulk and BEI during high-temperature selenization. The chemical passivation effect is fulfilled via the intrinsic high resistance characteristic of Nb₂O₅ layer, and also through the NbO_x passivation aiming to absorber bulk benefited from Nb (& O) diffusion. Meanwhile, the occupations of Nb (& O) on the Mo (& Se) sites induce a conduction type inversion in MoSe₂ interfacial layer and create a preferable interface p⁺-Mo(Se,O)₂:Nb/CZTSSe, achieving an interfacial field passivation effect. Ultimately, the promoted absorber quality and improved charge carrier transportation from SPE contribute to the boost of device performance beyond 10% efficiency.

Herein, a steady-state method for the determination of spectral responsivity (SR) curves is presented. The solar cell is irradiated with multiple broadband spectra that all differ from each other, meanwhile measuring the short-circuit current. The relation between the incident spectral irradiance and the measured current gives an equation system which is solved mathematically and yields as solution the SR curve.

Herein, a method for the determination of the spectral responsivity (SR) and the short-circuit current under standard test conditions of photovoltaic devices (e.g., solar cells) is presented. This multi-spectrum SR method requires a spectrally tunable broadband light source irradiating the photovoltaic device with a large number of different spectra. For each spectrum, the light response of the device and the spectral irradiance in the measuring plane are measured. The spectral irradiances are integrated within predefined wavelength intervals and are incorporated together with the measured light response into an equation system which relates them to the (unknown) SR of the photovoltaic device. By solving the equation system, mathematically using regression algorithms, the SR is determined. Due to the usage of a broadband light source, the device operates at realistic injection conditions during measurements. The mathematical background of the multi-spectrum SR method is described and its applicability is demonstrated on three world-photovoltaic-scale-type solar cells and one large-area reference cell. Short-circuit currents from all SR curves are calculated using the tabulated AM1.5 G spectrum. In comparison to the SR reference data, the short-circuit currents from the multi-spectrum SR method deviate by less than 0.68%.

Two constitutional isomers, namely BTP-m-4Cl and BTP-o-4Cl, are synthesized with shortened synthesis steps and excellent yields. Both BTP-m-4Cl and BTP-o-4Cl work well with PM6, enabling highly efficient organic solar cells. More importantly, the figure of merit values of the isomers are among the best high-efficiency organic photovoltaic materials, revealing outstanding cost-effectiveness for future large-scale manufacture.

Abstract

With the continuous development of organic semiconductor materials and on-going improvement of device technology, the power conversion efficiencies (PCEs) of organic solar cells (OSCs) have surpassed the threshold of 19%. Now, the low production cost of organic photovoltaic materials and devices have become an imperative demand for its practical application and future commercialization. Herein, the feasibility of simplified synthesis for cost-effective small-molecule acceptors via end-cap isomeric engineering is demonstrated, and two constitutional isomers, BTP-m-4Cl and BTP-o-4Cl, are synthesized and compared in parallel. These two non-fullerene acceptors (NFAs) have very similar optoelectronic properties but nonuniform morphological and crystallographic characteristics. Consequently, the OSCs composed of PM6:BTP-m-4Cl realize PCE of 17.2%, higher than that of the OSCs with PM6:BTP-o-4Cl (≈16%). When ternary OSCs are fabricated with PM6:BTP-m-4Cl:BTP-o-4Cl, the averaged PCE value reaches 17.95%, presenting outstanding photovoltaic performance. Most excitingly, the figure of merit (FOM) values of PM6:BTP-m-4Cl, PM6:BTP-o-4Cl, and PM6:BTP-m-4Cl:BTP-o-4Cl based devices are 0.190, 0.178, and 0.202 respectively. The FOM values of these systems are all among the top ones of the current high-efficiency OSC systems, revealing high cost-effectiveness of the two NFAs. This work provides a general but accessible strategy to minimize the efficiency-cost gap and promises the economic prospects of OSCs.

An electrostatic potential modulation (EPM) strategy is developed to enhance the interaction between the passivators and defects in FAPbI₃ film. Stable perovskite solar cells (PSCs) with a champion power conversion efficiency (PCE) of 24.67% are obtained based on the passivation of 1-Phenylbiguanide hydrochloride (PBGCl) with an enlarged electrostatic potential.

Abstract

The perovskite layer contains a large number of charged defects that seriously impair the efficiency and stability of perovskite solar cells (PSCs), thus it is essential to develop an effective passivation strategy to heal them. Based on theoretical calculations, it is found that enhancing the electrostatic potential of passivators can improve passivation effect and adsorption energy between charged defects and passivators. Herein, an electrostatic potential modulation (EPM) strategy is developed to design passivators for highly efficient and stable PSCs. With the EPM strategy, 1-phenylethylbiguanide (PEBG) and 1-phenylbiguanide (PBG) are designed. It is found that the charge distribution and electrostatic potential of phenyl- and phenylethyl- substituent on the biguanide are significantly enhanced. The N atom directly bonding to the phenyl group shows larger positive charge than that bonding to the phenylethyl group. The modulated electrostatic potential makes PBG bind stronger with the defects on perovskite surface. Based on the effective passivation of EPM, a champion efficiency of 24.67% is realized and the device retain 91.5% of its initial PCE after ≈1300 h. The promising EPM strategy, which provides a principle of passivator design and allows passivation to be controllable, may advance further optimization and application of perovskite solar cells toward commercialization.

Immersion doping methodology is presented as a method to improve the electrical properties of 2D Ruddlesden–Popper perovskites. Bulk inclusion of molecular dopants within the organic spacer layer is demonstrated with this approach with a judicial choice of solvent. This doping strategy of immersing the perovskite film in dopant solution increases the electrical current up to ≈60 times with maintaining clean film surface.

Abstract

Organic metal-halide perovskites (OHPs) have recently attracted much attention as next-generation semiconducting materials due to their outstanding opto-electrical properties. However, OHPs currently suffer from the lack of efficient doping methods, while the traditional method of atomistic doping having clear limitations in the achievable doping range. While doping with molecular dopants, has been suggested as a solution to this problem, the action of these dopants is typically restricted to perovskite surfaces, therefore significantly reducing their doping potential. In this study, successful bulk inclusion of “magic blue”, a molecular dopant, into 2D Ruddlesden–Popper perovskites is reported. This doping strategy of immersing the perovskite film in dopant solution increases the electrical current up to ≈60 times while maintaining clean film surface. A full mechanistic picture of such immersion doping is provided, in which the solvent molecule facilitates bulk diffusion of dopant molecule inside the organic spacer layer. Physical criteria for judicious choice of solvents in immersion doping are developed based on readily available solvent properties. The immersion doping method developed in this study that enables bulk molecular doping in OHPs will provide a strategic doping methodology for controlling electrical properties of OHPs for electronic and optoelectronic devices.

Ultrathin perovskite solar cells are successfully prepared by using stress-compensated bilayer indium tin oxide (ITO) to alleviate substrate deformation during ITO film growth. Cell efficiencies reach 18.2% under 1 sun, with a power-to-weight ratio of 24 W g⁻¹. Cell performance remains high under indoor lighting conditions. Using low-energy laser scribing, a functional, freestanding ultrathin 2.3 cm² 3-cell module was also realized.

The superior electrical conductivity and optical transparency of indium tin oxide (ITO) make it an ideal electrode material for use in optoelectronic devices such as solar cells. When ITO electrodes are fabricated on very thin plastic substrates, however, the internal stress of the ITO layer causes the substrate to deform, severely limiting the device's performance. Herein, it is shown that ITO bilayers composed of an amorphous base layer and a crystalline overlayer lead to deformation-free ITO electrodes. It is shown that an optimized bilayer structure is achieved when the internal stresses of the amorphous and crystalline layers approximately cancel. With this approach, mixed composition metal halide perovskite solar cells with ITO electrodes are successfully fabricated on 4 μm polyethylene naphthalate films. A power conversion efficiency (PCE) of 18.2% is obtained for the reference cell design, corresponding to a power-to-weight ratio of 24 W g⁻¹ before encapsulation. The devices retain 95% of the original PCE after 1000 bend cycles, while under simulated indoor lighting (white LED, 200 lux, 5000 K) the PCE reaches 28.3%. A 3-cell module with a designated area of 2.3 cm² is realized with a power output of 28.1 mW and an open-circuit voltage of 3.17 V.

Organic and perovskite optoelectronics have great potential as new-generation technology. They are solution-processable with tunable colors and reasonably high efficiency. They are lightweight and can be flexible. They are unstable and have low lifetimes. This review serves as a guide to characterization tools to probe the degradation mechanisms that lead to the low lifetime of organic and perovskites photovoltaics.

Organic solar cells (OSCs) and perovskite solar cells (PSCs) are promising due to their low cost and potential for renewable solar energy conversion. They are compatible with many substrates and varied deposition techniques, including solution processing. They can be coupled with other solar cell types in tandem and multijunction structures. Despite these great attributes and advancements in power conversion efficiencies over the years, they suffer from severe degradation, leading to low lifetime. In terms of research, their stability studies lag. One reason is the complexity of degradation studies and, sometimes, the lack of adequate tools to do an in-depth probe. Another reason is the lack of comprehensive literature on metrologies’ appropriateness for this kind of study. Although there are reviews on stability and improvement in the efficiency of devices, they focus either on the degradation mechanisms or efforts to use specific tools. There is little on comprehensive characterization tools for their degradation studies. Herein, the experimental tools and techniques researchers use in general to probe degradation in OSCs and PSCs are studied. This review is intended as a starting point and a go-to material for current and future researchers and (under-)graduate students interested in stability studies.

Derived from the IDTT and PTBTP backbones, an asymmetric molecular backbone PTBTT and PTBTT-based acceptors are designed and synthesized with greatly enlarged dipole moments. Both asymmetric PTBTT-4F/4Cl realize enhanced power conversion efficiencies (PCEs) over 14%. The PBDB-TF:PTBTT-4F devices achieve a maximum PCE of 14.49%, an open-circuit voltage of 0.88 V, a short-circuit current of 21.43 mA cm⁻², and fill factor of 76.73%.

Considerable progress on high-performance organic solar cells (OSCs) has been achieved in the past due to the rapid development of nonfullerene acceptors (NFAs). Typically, two kinds of methods have been employed to manipulate energy levels and aggregation of NFAs, i.e., molecular engineering on alkyl side chains and modification of the heterocyclic rings in the backbone. Herein, a novel asymmetric thiophene[3,2-b] pyrrole (TP)-based NFA with flipped molecular conformation, named as PTBTT-4F, is designed and synthesized. The introduction of the pyrrole ring in the novel NFA would not only afford extra reaction sites for side chain modification, but also induce substantial intramolecular charge transfer, thus leading to elevated energy levels of the NFA and thereby lower energy loss of the OSCs. When pairing with polymer donor PBDB-TF to fabricate OSCs, concurrent improvement in open-circuit voltage, short-circuit current (J _SC), and fill factor (FF) is realized, which delivers an outstanding power conversion efficiency (PCE) of 14.49%. Benefitting from effective molecular stacking and optimized phase separation induced by molecular conformation variation, asymmetric PTBTT-4F fabricated OSCs exhibit much higher J _SCs and FFs than the symmetrical PTBTP-4F devices.

A multifunctional pseudohalogen salt additive (BDPF₆) is incorporated into the perovskite precursor solution. The BDPF₆ additive promotes the crystallization of perovskite films and reduces defect densities, achieving high power conversion efficiency (PCE) of 22.68%. Additionally, it performs with significantly improved long-term stability.

The accumulation of defects and ion migration at the surfaces and grain boundaries of perovskite impedes the improvement of performance and stability in perovskite solar cells. Therefore, developing strategies to reduce trap-assisted nonradiative recombination and suppress ion migration in perovskite films is urgently needed. Herein, a multifunctional pseudohalogen salt additive, (benzotriazol-1-yloxy) dipyrrolidinocarbenium hexafluorophosphate (denoted as BDPF₆), composed of a benzotriazole derivative cation and hexafluorophosphate anion, is incorporated into the perovskite precursor solution. The anion vacancies of perovskite films are filled by PF6−$\text{PF}_{6}^{-}$, whereas the cation and anion in BDPF₆ form ionic and coordination bonds with the perovskites. The BDPF₆ additive promotes the crystallization of perovskite films with large grain size, reducing defect densities, prolonging carrier lifetimes, and inhibiting ion migration. Thus, the power conversion efficiency (PCE) of the BDPF₆-modified device remarkably improves from 20.36% to 22.68%. The unencapsulated BDPF₆-modified device maintains 97% of its initial PCE after 1,400 h of exposure to an ambient environment with a relative humidity of 10–20%, whereas the control device maintains only 85% of its initial PCE. Similarly, the BDPF₆-modified device maintains 78% of its original PCE after aging at 60 °C for 1,400 h, whereas the control device only maintains 55%.

Recent research progress on the strategies to fabricate phase-pure α-FAPbI₃ perovskite and the advances in their photovoltaic application is reviewed. The fundamental challenges of preparing efficient α-FAPbI₃ perovskite solar cells (PSCs) and some perspectives on the further development of high-quality phase-pure α-FAPbI₃ for reliable PSCs are discussed.

Formamidinium lead triiodide (FAPbI₃) with an optimal bandgap of 1.48 eV and superior thermal stability is regarded as one of the most promising perovskite-based materials for the application in efficient single-junction solar cells. However, the metastable properties of FAPbI₃ due to phase transition from photoactive α phase into an undesired nonperovskite δ phase in ambient conditions become the major factors limiting its further development. Challenges remain in stabilizing α phase structure and preparing phase-pure α-FAPbI₃ films for high-efficiency and stable perovskite solar cells (PSCs) devices. Herein, the recent research progress on the strategies is reviewed to fabricate phase-pure α-FAPbI₃ perovskite and the advances in their photovoltaic application. The physical parameters affecting the phase instability of intrinsic FAPbI₃ are first discussed, followed by various methodologies for regulating phase transition behavior, such as additive engineering, solvent optimization, dimensionality engineering, and fabrication techniques. Finally, the fundamental challenges of preparing efficient α-FAPbI₃ PSCs are discussed, and the perspectives on the further development of high-quality phase-pure α-FAPbI₃ for reliable PSCs are proposed.

PbI₂ precursor films are deposited using a sputtering process and treated with iodination, thermal annealing, and dimethyl sulfoxide to improve their properties. Perovskite solar cells are fabricated using direct contact conversion and methylamine vapor annealing, achieving 12.2% efficiency with potential for further improvement. Uniform perovskite films are deposited on large-area textured silicon, enabling application in perovskite/silicon tandem solar cells.

Conformal deposition of perovskite films on textured silicon surfaces using a dry process is crucial for producing high-performance perovskite/silicon tandem solar cells. Herein, a radio frequency magnetron sputtering process is used with a PbI₂ target to deposit precursor films. Iodination, thermal annealing, and dimethyl sulfoxide treatment are employed as posttreatment processes to improve the stoichiometry, crystallinity, and surface morphology of the PbI₂ precursor. The precursor films are converted into perovskite using direct contact conversion process, and the interfacial and bulk properties are enhanced by methylamine vapor annealing to fabricate perovskite solar cells with a power conversion efficiency of 12.2%. Also, 18.3% efficiency is confirmed at a wider voltage sweep range, which suggests that further efficiency improvement is possible by removing defects inside the perovskite. Finally, uniform perovskite films are conformally deposited on a 25 cm² textured silicon surface. With such high-efficiency potential and conformality, the method of sputtering PbI₂ can open a new way to fabricate perovskite/silicon tandem solar cells.

Herein, spray deposition technology is developed to fabricate unique M13 bacteriophage-templated SnO₂ nanoparticle (M13-SnO₂) films as electron transport layer (ETL) for perovskite solar cells (PSCs). Spray-deposited M13-SnO₂ biohybrid ETLs exhibit mesoporous morphologies with a high optical transmittance of >85%. The PSC-based M13-SnO₂ ETLs and FAPbI₃-based perovskite show the highest power conversion efficiency > 22.08%.

In recent years, researchers have developed spray deposition technology to fabricate tin oxide electron transport layer (ETL) with the aim of manufacturing high-efficiency, large-area perovskite solar cell (PSC). However, the power conversion efficiency (PCE) of PSC based on sprayed SnO₂ ETL remains inferior to that of the spin-coated SnO₂ ETL. Herein, the combined use of spray deposition and genetically engineered M13 bacteriophages for the deposition of M13-SnO₂ biohybrid ETL over large-area (62.5 cm²) substrates is demonstrated. The spray-deposited M13-SnO₂ ETLs exhibit mesoporous morphologies with >85% transmittance in UV–vis region. Through the use of M13-SnO₂ ETL, the sequential-deposited PSCs achieve a maximum PCE of ≈22.1%. The improved performance of the PSC is attributable to the mesoporous morphology of M13-SnO₂ ETL that facilitates the growth of larger perovskite grains. The PSCs based on M13-SnO₂ ETLs also display highly consistent photovoltaic performance which manifests the excellent scalability of the spraying process. Furthermore, M13-SnO₂-based PSCs exhibit higher ambient stability compared to the SnO₂-based PSCs, showing that the use of M13 bacteriophage is incredibly beneficial to both the efficiency and stability of PSCs.

The surface post-treatment of FAPbI₃ perovskite film with multifunctional molecule 4-hydroxypicolinic acid (4HPA) shows excellent optoelectronic properties with improved crystallinity, pure α-phase FAPbI₃, and favorable energy band bending. The 4HPA post-treated PeSC achieves a champion power conversion efficiency of 23.28% in 0.12 cm² cells and 19.26% in 36 cm² modules, with excellent environmental and thermal stabilities.

Abstract

Perovskite solar cells (PeSCs) using FAPbI₃ perovskite films often exhibit unfavorable phase transitions and defect-induced nonradiative interfacial recombination, resulting in considerable energy loss and impairing the performance of PeSCs in terms of efficiency, stability, and hysteresis. In this work, a facile interface engineering strategy to control the surface structure and energy-level alignment of perovskite films by tailoring the interface between the FAPbI₃ film and hole-transporting layer using 4-hydroxypicolinic acid (4HPA) is reported. According to density functional theory studies, 4HPA has prominent electron delocalization distribution properties that enable it to anchor to the perovskite film surface and facilitate charge transfer at the interface. By enabling multiple bonding interactions with the perovskite layer, including hydrogen bonds, PbO, and PbN dative bonds, 4HPA passivation significantly reduces the trap density and efficiently suppresses nonradiative recombination. The obtained perovskite films exhibit superior optoelectronic properties with improved crystallinity, pure α-phase FAPbI₃, and favorable energy band bending. Following this strategy, 4HPA post-treatment PeSCs achieve a champion power conversion efficiency of 23.28% in 0.12 cm² cells and 19.26% in 36 cm² modules with excellent environmental and thermal stabilities.

Present work demonstrates an efficient strategy of the particle boundaries (PBs) embedding of multifunctional p-type semiconducting CdTe nanocrystals for inhibited carrier losses at PBs, which can serve as efficient PBs mediator for boosting the electrons mobility of TiO₂ ETL by maximally three orders of magnitude and consequently result in a new benchmark PCE over 25% in planar PSCs.

Abstract

Electron transport layers (ETLs) with pronounced electron conducting capability are essential for high performance planar perovskite photovoltaics, with the great challenge being that the most widely used metal oxide ETLs unfortunately have intrinsically low carrier mobility. Herein is demonstrated that by simply addressing the carrier loss at particle boundaries of TiO₂ ETLs, through embedding in ETL p–n heterointerfaces, the electron mobility of the ETLs can be boosted by three orders of magnitude. Such embedding is encouragingly favorable for both inhibiting the formation of rutile phase TiO₂ in ETL, and initiating the growth of high-quality perovskite films with less defect states. By virtue of these merits, creation of formamidinium lead iodide perovskite solar cells (PSCs) with a champion efficiency of 25.05% is achieved, setting a new benchmark for planar PSCs employing TiO₂ ETLs. Unencapsulated PSCs deliver much-improved environmental stability, i.e., more than 80% of their initial efficiency after 9000 h of air storage under RH of 40%, and over 90% of their initial efficiency at maximum power point under continuous illumination for 500 h. Further work exploring other p-type nanocrystals for embedding warrants the proposed strategy as a universal alternative for addressing the low-carrier mobility of metal oxide based ETLs.

By regulating the electronic structure with ThFABr, an ultralong carrier lifetime exceeding 20 µs and carrier diffusion lengths longer than 6.5 µm is achieved in 2D/3D polycrystalline perovskite films. These excellent properties enable the ThFA-based devices yielding a champion efficiency of 24.69% and a high V _OC of 1.21 V, coupled with significantly improved operational stability.

Abstract

The carrier lifetime is one of the key parameters for perovskite solar cells (PSCs). However, it is still a great challenge to achieve long carrier lifetimes in perovskite films that are comparable with perovskite crystals owning to the large trap density resulting from the unavoidable defects in grain boundaries and surfaces. Here, by regulating the electronic structure with the developed 2-thiopheneformamidinium bromide (ThFABr) combined with the unique film structure of 2D perovskite layer caped 2D/3D polycrystalline perovskite film, an ultralong carrier lifetime exceeding 20 µs and carrier diffusion lengths longer than 6.5 µm are achieved. These excellent properties enable the ThFA-based devices to yield a champion efficiency of 24.69% with a minimum V _OC loss of 0.33 V. The unencapsulated device retains ≈95% of its initial efficiency after 1180 h by max power point (MPP) tracking under continuous light illumination. This work provides important implications for structured 2D/(2D/3D) perovskite films combined with unique FA-based spacers to achieve ultralong carrier lifetime for high-performance PSCs and other optoelectronic applications.

A synergistic modulation strategy of two-dimensional (2D) perovskite with alternating cations in the interlayer space (ACI) and multisite ligand 2-mercapto-1,3,4-thiadiazole (MTD) is proposed to fabricate high-quality methylammonium-free perovskite films. The significantly inhibited nonradiative recombination enables the realization of high-efficiency inverted devices with a fascinating power conversion efficiency (PCE) of 24.58%, which is one of the highest PCEs reported for inverted devices.

Abstract

The preparation of high-quality perovskite films is key to realize efficient and stable inverted perovskite solar cells. The trap-assisted nonradiative recombination at grain boundary (GB) and surface poses a serious challenge for fabricating high-quality perovskite films. Here, a synergistic modulation strategy of two-dimensional (2D) perovskite with alternating cations in the interlayer space (ACI) and multisite ligand 2-mercapto-1,3,4-thiadiazole (MTD) for fabricating high-quality methylammonium-free perovskite films is reported. The formation of ACI 2D perovskite promotes the nucleation of three-dimensional (3D) perovskites, suppresses the generation of yellow phase, and promotes the formation of black phase, leading to increased grain size and crystallinity. Due to the synergistic effect of ACI 2D perovskite and MTD, the defects at GBs and surface are healed simultaneously. The significantly inhibited nonradiative recombination enables realization of high-efficiency inverted devices with a fascinating power conversion efficiency (PCE) of 24.58%, which is one of the highest PCEs reported for inverted devices. The synergistically modified unsealed device demonstrates an excellent long-term ambient stability, retaining 90.5% of its initial PCE after 3000 h under a relative humidity of 30–40%. This work provides deep insights into minimizing nonradiative recombination losses through the rational synergistic engineering of GB and surface toward efficient and stable inverted devices.

Nature Energy, Published online: 10 May 2023; doi:10.1038/s41560-023-01261-4