Chen Weijie

This article introduces a bottom layer modifier for ZnO-based methylammonium (MA)-free perovskite solar cells. Chlorine-containing phenanthroline derivative (Cl-phen) reduces the surface oxygen defects, improves the charge extraction efficiency, and moderately increases the hydrophobicity to achieve a large grain size. A high-performance solar cell with 21.15% is achieved. Its unencapsulated device maintains 91.5% of its stating PCE after 1500 h.

Formamidinium cesium (FACs) perovskite solar cells (PSCs) with the exclusion of methylammonium (MA) cations often have greatly improved device stability; however, their inferior performance compared with MA-based devices has impeded the real application. Among various device engineering strategies, bottom interfacial engineering is a promising method to simultaneously achieve the passivation of interfacial defects and the crystallization control of perovskite. Herein, a simple and effective bottom interfacial design is presented to improve the efficiency and stability of FACs PSCs by capping o-phenanthroline derivatives on the ZnO electron transporting layer (ETL). The most efficient modifier, 4,7-Dichloro-1,10-phenanthroline (Cl-phen), can improve the crystallinity of the perovskite film by chlorinated surface and passivate the defects of ZnO by reducing surface hydroxyl groups and oxygen vacancies. In addition, Cl-phen modified ZnO shows better energy alignment with FACs perovskite and increases the built-in electric field cascade by 80 mV. As a result, a champion device efficiency of 21.15% is obtained using ZnO/Cl-phen bilayer ETL. The stability has also been improved using ZnO/Cl-phen bilayer ETL, in which 91.5% of initial PCE is retained after 1500 h of storage at ambient environment (RH: 40–50%) without encapsulation.

The Sn-Pb alloyed perovskite films with stable α -phase and oxidation resistance are prepared by bulk doping (CsCl) and surface coordination (PbSO₄). The less oxidation of Sn²⁺ and enhanced stability are caused by reconfiguring perovskite crystallization and the formed dense water-insoluble hydrophobic surface. The ultimately fabricated perovskite solar cells deliver a champion power conversion efficiency of 10.39% and excellent stability.

Abstract

Novel all-inorganic Sn-Pb alloyed perovskites are developed aiming for low toxicity, low bandgap, and long-term stability. Among them, CsPb_{1− x} Sn _x I₂Br is predicted as an ideal perovskite with favorable band gap, but previously is demonstrated unable to convert to perovskite phase by thermal annealing. In this report, a series of CsPb_{1− x} Sn _x I₂Br perovskites with tunable bandgaps from 1.92 to 1.38 eV are successfully prepared for the first time via low annealing temperature (60 °C). Compared to the pure CsPbI₂Br, these Sn-Pb alloyed perovskites show superior stability. Furthermore, a novel α-phase-stabilization mechanism of the inorganic Sn-Pb alloyed perovskite by reconfiguring the perovskite crystallization process with chloride doping is provided. Simultaneously, a dense protection layer is formed by the coordination reaction between the surface lead dangling bonds and sulfate ion to retard the permeation of external oxygen and moisture, leading to less oxidation of Sn²⁺ in perovskite film. As a result, the fabricated all-inorganic Sn-Pb perovskite solar cells (PSCs) show a champion power conversion efficiency of 10.39% with improved phase stability and long-term ambient stability against light, heat, and humidity. This work provides a viable strategy in fabricating high-performance narrow-bandgap all-inorganic PSCs.

Annealing-free solution-processable NiO is developed and applied in inverted polymer solar cells based on non-fullerene system PTB7-Th:IEICO-4F. The inverted solar cells with annealing-free NiO exhibit equivalence efficiency and better stability without high-temperature annealing compared to the solar cells with the MoO _x hole transport layer fabricated by thermal evaporation.

Abstract

Nickel oxide (NiO) offers intrinsic p-type behavior and high thermal and chemical stability, making it promising as a hole transport layer (HTL) material in inverted organic solar cells. However, its use in this application has been rare because of a wettability problem caused by use of water as base solvent and high-temperature annealing requirements. In the present work, an annealing-free solution-processable method for NiO deposition is developed and applied in both conventional and inverted non-fullerene polymer solar cells. To overcome the wettability problem, the typical DI water solvent is replaced with a mixed solvent of DI water and isopropyl alcohol with a small amount of 2-butanol additive. This allows a NiO nanoparticle suspension (s-NiO) to be deposited on a hydrophobic active layer surface. An inverted non-fullerene solar cell based on a blend of p-type polymer PTB7-Th and non-fullerene acceptor IEICO-4F exhibits the high efficiency of 11.23% with an s-NiO HTL, comparable to the efficiency of an inverted solar cell with a MoO _x HTL deposited by thermal evaporation. Conventionally structured devices including this s-NiO layer show efficiency comparable to that of a conventional device with a PEDOT:PSS HTL.

Flexible perovskite solar cells deliver a 21.1% power conversion efficiency (certified 20.5%) with good bending resistance and long-term stability via creating a dual-functional 2D perovskite layer at the interface.

Abstract

Flexible perovskite solar cells (f-PSCs) have been attracting tremendous attention due to their potentially commercial prospects in flexible energy system and mobile energy system. Reducing the energy barriers and charge extraction losses at the interfaces between perovskite and charge transport layers is essential to improve both efficiency and stability of f-PSCs. Herein, 4-trifluoromethylphenylethylamine iodide (CF₃PEAI) is introduced to form a 2D perovskite at the interface between perovskite and hole transport layer (HTL). It is found that the 2D perovskite plays a dual-functional role in aligning energy band between perovskite and HTL and passivating the traps in the 3D perovskite, thus reducing energy loss and charge carrier recombination at the interface, facilitating the hole transfer from perovskite to the Spiro-OMeTAD. Consequently, the photovoltaic performance of f-PSCs is significantly improved, leading to a power conversion efficiency (PCE) of 21.1% and a certified PCE of 20.5%. Furthermore, the long-term stability of f-PSCs is greatly improved through the protection of 2D perovskite layer to the underlying 3D perovskite. This work provides an excellent strategy to produce efficient and stable f-PSCs, which will accelerate their potential applications.

Replacing one of the chlorinated terminal groups with norbornyl-modified one endows the reported asymmetric acceptors with not only enhanced solubility but also more favorable morphology and higher EQE_EL in the blend with PBDB-T. A power conversion efficiency of 16.82% with a J _sc over 26.5 mA cm⁻² and a ΔV _nr of 0.18 V are realized, representing the state-of-the-art in PBDB-T based organic solar cells.

Abstract

Three asymmetric non-fullerene acceptors (LL2, LL3, and LL4) are designed and synthesized with one norbornyl-modified 1,1-dicyanomethylene-3-indanone (CBIC) terminal group and one chlorinated 1,1-dicyanomethylene-3-indanone (IC-2Cl) terminal group. The three-dimensional shape-persistent CBIC terminal group can effectively enhance the solubility and tune the packing mode of acceptors. Compared with their symmetric counterparts (LL2-2Cl, LL3-2Cl, and LL4-2Cl) bearing two IC-2Cl terminals, the asymmetric acceptors show improved solubilities, giving rise to enhanced crystallinity and favored nanomorphology for charge transport in the blend films with PBDB-T. Asymmetric acceptors based organic solar cells (OSCs) also show much lower voltage loss due to their higher E _CT and EQE_EL values. Therefore, they exhibit 17−27% higher power conversion efficiency (PCE) than OSCs based on the corresponding symmetric acceptors. Among these six acceptors, LL3 with a central benzotriazole core shows the best PCE of 16.82% with an outstanding J _sc of 26.97 mA cm⁻² and a low nonradiative voltage loss (ΔV _nr) of 0.18 V, the best values for PBDB-T based OSCs. The J _sc and ΔV _nr also represent the best reported for asymmetric non-fullerene acceptors-based OSCs to date. The results demonstrate that the combination of the unique CBIC terminal group with the asymmetric strategy is a promising way to enhance the performance of OSCs.

A bifunctional SnO₂ colloid is developed using small molecular oxalate. The resultant SnO₂ films show “no annealing effect”, contributing to stabilized PCEs of 22.40% and 22.37% for high temperature process (HTP) SnO₂ planar and mesoporous PSCs, respectively. The high stability of HTP SnO₂ PSCs may ascribe to low oxygen vacancy and adsorbed water of HTP SnO₂.

Abstract

SnO₂ compact layer (c-SnO₂) frequently suffers from degradation in high temperature processes (HTP) such as crack, worse interfacial contact, and electrical properties, that is, annealing effect. To solve this problem, a kind of bifunctional SnO₂ colloid is developed by using small molecular oxalate whose organic components can be removed clearly at a low temperature process (LTP). The c-SnO₂ and SnO₂ mesoporous layer (m-SnO₂) derived from the fresh and aged sols with the same colloid show no annealing effect, decreasing oxygen vacancy, and adsorbing water on increasing annealing temperature. The champion devices of LTP and HTP SnO₂ planar perovskite solar cells (PSCs) achieve, respectively, stabilized photoelectric conversion efficiencies (PCEs) of 20.74% and 20.70%. In contrast, the performance of champion devices of their mesoporous counterparts is significantly improved, showing nearly hysteresis free character with stabilized PCEs of 22.40% and 22.37%, respectively. The inclusion of m-SnO₂ plays a role of an energy bridge, improving electrons collection efficiency, which is supported by photoluminescence and transient photoluminescence characterizations. HTP SnO₂ mesoporous PSCs can preserve 97.6% and 80% of their initial PCEs after aging for 25 weeks and 8-h irradiated/16-h dark cycle within 104 h. The high stability of HTP SnO₂ PSCs may ascribe to low oxygen vacancy and adsorbed water of HTP SnO₂.

Four emerging perovskite structures that have origins in pre-existing technologies such as silicon and chalcogenide-based photovoltaic cells are reviewed. The implementation of the new architectures holds great promise in reducing transport losses, light management, and stability. To guide further research into these areas key design criteria are identified and key characterization methods discussed.

Abstract

Over the past decade, perovskite solar cells (PSCs) have quickly established themselves as a promising technology boasting both high efficiency and low processing costs. The rapid development and success of PSCs is a product of substantial research effort addressing compositional engineering, thin film fabrication, surface passivation, and interfacial treatments. Recently, engineering of device architecture has entered a renaissance with the emergence of several new bulk and graded heterojunction structures. These structures promote a lateral approach to the development of single-junction PSCs affording new opportunities in light management, defect passivation, carrier extraction, and long-term stability. Following a short overview of the historic evolution of PSC architectures, a detailed discussion of the promising progress of the recently reported perovskite bulk heterojunction and graded heterojunction approaches are offered. To enable better understanding of these novel architectures, a range of approaches to characterizing the architectures are presented. Finally, an outlook and perspective are provided offering insights into the future development of PSC architecture engineering.

A high performance broadband photodetector is demonstrated with an inorganic perovskite CsPbBr₃/GeSn heterojunction. The detection range can be covered from 450 to 2200 nm, and the responsivity of the heterojunction device is 4.92 times larger than that of a GeSn one under illuminated light of 532 nm, and also the device is showing good stability.

Abstract

Photodetectors with broadband response spectrum have attracted great interest in many application areas such as imaging, gas sensing, and night vision. Here, a high performance broadband photodetector is demonstrated with inorganic perovskite CsPbBr₃/GeSn heterojunction, detection range can be covered from 450 to 2200 nm. The responsivity of heterojunction device can achieve as high as 129 mA W⁻¹ under illuminated light of 532 nm, which is 4.92 times larger than that of a GeSn based device. As the CsPbBr₃ can also act as anti-reflective coating for infrared wavelength, the infrared band responsivity at wavelength of 2200 nm can also be raised by 1.42 times. In addition, the device with all inorganic components is showed good stability, while keeping in the dry environment, the device can sustain its 90% original after 550 h storage. These results show the inorganic perovskite/GeSn heterojunction device is of great potential in broadband photodetection with high responsivity.

Asymmetric isomerization from BP6T-4F to ABP6T-4F not only lowers the exciton bonding energy but also optimizes the crystallization performance, achieving a pronounced isomer effect with 9.4% power conversion efficiency enhancement. Moreover, ternary devices are also fabricated, considering good compatibility between ABP6T-4F and CH1007, to deliver a power conversion efficiency over 17%.

Abstract

Herein, asymmetric isomer effects are systematically explored by designing and synthesizing two benzo[c][1,2,5]thiadiazole (BT)-fused nonacyclic electron acceptors. By changing from BP6T-4F to asymmetric ABP6T-4F, significantly enhanced dielectric constant and inhibited excessive molecular aggregation and unfavorable edge-on orientation could be achieved. The reduced exciton binding energy also facilitates a more efficient dissociation process in PM6:ABP6T-4F compared to PM6:BP6T-4F with the same energy offset. Moreover, the weaker crystallization behavior enables a significantly enhanced miscibility between PM6 and ABP6T-4F than that between PM6 and BP6T-4F, which leads to an optimized micromorphology with smooth surface, suitable domain size, and ordered π–π stacking. Organic solar cells (OSCs) based on PM6:ABP6T-4F achieve a 15.8% power conversion efficiency (PCE), which is remarkably higher than that of PM6:BP6T-4F-based OSCs (6.4%). Furthermore, ternary devices are also fabricated considering good compatibility between ABP6T-4F and CH1007 to deliver a PCE over 17%. This study reveals the effectiveness and great potential of asymmetric isomerization strategy in regulating molecular properties, which will provide guidance for the future design of non-fullerene acceptors.

Perovskite solar cells (PSCs) are fabricated using a novel organic cation (TTMAI) treatment on a 3D perovskite, which enables higher power conversion efficiency (PCE) and improves stability. The PCE enhancement is explained by the drift-diffusion modeling. In addition, TTMAI-treated 3D perovskite-based semitransparent PSCs are also realized, and a notable increase in PCE and stability is obtained.

Abstract

Perovskite surface treatment with additives has been reported to improve charge extraction, stability, and/or surface passivation. In this study, treatment of a 3D perovskite ((FAPbI₃)_{1− x} (MAPbBr₃) _x ) layer with a thienothiophene-based organic cation (TTMAI), synthesized in this work, is investigated. Detailed analyses reveal that a 2D (n = 1) or quasi-2D layer does not form on the PbI₂-rich surface 3D perovskite. TTMAI-treated 3D perovskite solar cells (PSCs) fabricated in this study show improved fill factors, providing an increase in their power conversion efficiencies (PCEs) from 17% to over 20%. It is demonstrated that the enhancement is due to better hole extraction by drift-diffusion simulations. Furthermore, thanks to the hydrophobic nature of the TTMAI, PSC maintains 82% of its initial PCE under 15% humidity for over 380 h (the reference retains 38%). Additionally, semitransparent cells are demonstrated reaching 17.9% PCE with treated 3D perovskite, which is one of the highest reported efficiencies for double cationic 3D perovskites. Moreover, the semitransparent 3D PSC (TTMAI-treated) maintains 87% of its initial efficiency for six weeks (>1000 h) when kept in the dark at room temperature. These results clearly show that this study fills a critical void in perovskite research where highly efficient and stable semitransparent perovskite solar cells are scarce.

A universal morphology optimization method is developed by applying anthracene as a solid additive to improve the photovoltaic performance of polymer solar cells. Anthracene can restrict the over-aggregation of nonfullerene acceptors during the film-forming process, and then facilitate bicontinuous phase separation during the kinetic process of its removal in the blend under thermal annealing.

Abstract

Currently, morphology optimization methods for the fused-ring nonfullerene acceptor-based polymer solar cells (PSCs) empirically follow the treatments originally developed in fullerene-based systems, being unable to meet the diverse molecular structures and strong crystallinity of the nonfullerene acceptors. Herein, a new and universal morphology controlling method is developed by applying volatilizable anthracene as solid additive. The strong crystallinity of anthracene offers the possibility to restrict the over aggregation of fused-ring nonfullerene acceptor in the process of film formation. During the kinetic process of anthracene removal in the blend under thermal annealing, donor can imbed into the remaining space of anthracene in the acceptor matrix to form well-developed nanoscale phase separation with bi-continuous interpenetrating networks. Consequently, the treatment of anthracene additive enables the power conversion efficiency (PCE) of PM6:Y6-based devices to 17.02%, which is a significant improvement with regard to the PCE of 15.60% for the reference device using conventional treatments. Moreover, this morphology controlling method exhibits general application in various active layer systems to achieve better photovoltaic performance. Particularly, a remarkable PCE of 17.51% is achieved in the ternary PTQ10:Y6:PC₇₁BM-based PSCs processed by anthracene additive. The morphology optimization strategy established in this work can offer unprecedented opportunities to build state-of-the-art PSCs.

Single-walled carbon nanotubes (SWCNTs) have been deployed in perovskite solar cells (PSCs) via a simple transfer route, achieving power conversion efficiencies of 19% and 18% on rigid and flexible substrates, respectively. Moreover, unique features of the SWCNT network, including the high environmental and chemical stability, outstanding mechanical robustness, have greatly extended the lifetime and durability of SWCNT-based PSCs.

Abstract

The unprecedented advancement in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has rendered them a promising game-changer in photovoltaics. However, unsatisfactory environmental stability and high manufacturing cost of window electrodes are bottlenecks impeding their commercialization. Here, a strategy is introduced to address these bottlenecks by replacing the costly indium tin oxide (ITO) window electrodes via a simple transfer technique with single-walled carbon nanotubes (SWCNTs) films, which are made of earth-abundant elements with superior chemical and environmental stability. The resultant devices exhibit PCEs of ≈19% on rigid substrates, which is the highest value reported to date for ITO-free PSCs. The facile approach for SWCNTs also enables application in flexible PSCs (f-PSCs), delivering a PCE of ≈18% with superior mechanical robustness over their ITO-based counterparts due to the excellent mechanical properties of SWCNTs. The SWCNT-based PSCs also deliver satisfactory performances on large-area (1 cm² active area in this work). Furthermore, these SWCNT-based PSCs can retain over 80% of original PCEs after exposure to air over 700 h while ITO-based devices only sustain ≈60% of initial PCEs. This work paves a promising way to accelerate the commercialization of ITO-free PSCs with reduced material cost and prolonged lifetimes.

Publication date: October 2021

Source: Nano Energy, Volume 88

Author(s): Devika Mudusu, Koteeswara Reddy Nandanapalli, Geon Dae Moon, Sungwon Lee

Fluoroethylamine passivates defects in perovskite films as a result of F distribution throughout the bulk and even at the surface. The nonradiative recombination in perovskite films using derivatives of this compound is suppressed, the carrier-lifetime is prolonged, and the film–air interface offers greater hydrophobicity. The corresponding solar cells deliver high efficiency of up to 23.40%. The unencapsulated device shows good environmental stability, maintaining 87% of its initial efficiency after exposure to the ambient environment for 1200 h.

Abstract

Defects in perovskite layers usually cause nonradiative recombination, impairing device performance and stability. Here, fluoroethylamine (FC₂H₄NH₃, FEA) is integrated into the perovskite film to passivate defects. By engineering of different amounts of fluorine in the molecule, it is found that when 2-fluoroethylamine (1FEA), in which one F bonds to the first carbon atom at the end of the molecule's structure, is used, the F atoms appear to be distributed throughout the bulk to the very surface. When 2,2-difluoroethylamine (2FEA) and 2, 2, 2-trifluoroethylamine (3FEA) are used, F is prone to distribution in the bulk of the perovskite film, while there appears to be no detectable F content on the surface. With the FEA passivation, the nonradiative recombination is suppressed, the carrier-lifetime is improved to 840.01 ns, and the film-air interface offers greater hydrophobicity, especially in the case of 1FEA, where because it is distributed throughout the film thickness, it passivates more defects and delivers the highest efficiency, as much as 23.40%. The device with 3FEA shows the best environmental stability; specifically, the bare cell without any encapsulation maintains 87% of its initial efficiency after exposure to the ambient environment for 1200 h.

Charge dynamics and energy loss (E _loss) in ternary organic solar cell (OSCs) with an impressive fill factor (FF) of 80.88% are thoroughly investigated by transient characterization techniques, which is expect to aid development of high-FF and low-E _loss ternary OSCs.

Abstract

Ternary architecture is a promising strategy to enhance power conversion efficiencies (PCEs) of organic solar cells (OSCs). However, among all the photovoltaic parameters that govern the final PCEs, the fill factor (FF) for ternary OSCs is generally below 78%, limiting solar cells’ performance. Here, charge dynamics in the ternary cells PM6:DRTB-T-C4:Y6 with a FF of 80.88% and a PCE of 17.05% are thoroughly investigated by a series of transient characterization technologies, including transient absorption spectroscopy, transient photovoltage, and transient photocurrent measurements. The impressive FF results from effective exciton dissociation, enhanced charge transport and suppressed recombination in ternary cells. Moreover, the correlation between the measured FF and the charge recombination-extraction competition is quantitatively analyzed by using a circuit model. The ternary cells also show small energy loss (E _loss). The findings here provide insight into achieving high-FF and low-E _loss ternary OSCs.

Nature Energy, Published online: 24 June 2021; doi:10.1038/s41560-021-00848-z

Polyaromatic passivator 4-hydroxybiphenyl substituted naphthalene-1,8-dicarboximide provides chemical passivation (protonic/Lewis-base groups system) and energetic passivation (creating benign midgap states) effects. The Lewis-base/polyaromatic conjugation/protonic system reduces defects efficiently and avoids superoxide anions in perovskite solar cells.

Abstract

As game-changers in the photovoltaic community, perovskite solar cells are making unprecedented progress while still facing grand challenges such as improving lifetime without impairing efficiency. Herein, two structurally alike polyaromatic molecules based on naphthalene-1,8-dicarboximide (NMI) and perylene-3,4-dicarboximide (PMI) with different molecular dipoles are applied to tackle this issue. Contrasting the electronically pull–pull cyanide-substituted PMI (9CN-PMI) with only Lewis-base groups, the push–pull 4-hydroxybiphenyl-substituted NMI (4OH-NMI) with both protonic and Lewis-base groups can provide better chemical passivation for both shallow- and deep-level defects. Moreover, combined theoretical and experimental studies show that the 4OH-NMI can bind more firmly with perovskite and the polyaromatic backbones create benign midgap states in the excited perovskite to suppress the damage by superoxide anions (energetic passivation). The polar and protonic nature of 4OH-NMI facilitates band alignment and regulates the viscosity of the precursor solution for thicker perovskite films with better morphology. Consequently, the 4OH-NMI-passivated perovskite films exhibit reduced grain boundaries and nearly three-times lower defect density, boosting the device efficiency to 23.7%. A more effective design of the passivator for perovskites with multi-passivation mechanisms is provided in this study.

Effective piezo-phototronic enhancement of flexible photodetectors based on ferroelectric EA₄Pb₃Br₁₀ single-crystalline thin-films is demonstrated. Under external strains, the responsivity of the photodetectors can be enhanced by as much as 284%. These findings shed light on the piezo-phototronic effect for optimizing the performance of perovskite-based optoelectronic devices, and offer a promising avenue to broaden functionalities of hybrid perovskites.

Abstract

2D hybrid perovskites are very attractive for optoelectronic applications because of their numerous exceptional properties. The emerging 2D perovskite ferroelectrics, in which are the coupling of spontaneous polarization and piezoelectric effects, as well as photoexcitation and semiconductor behaviors, have great appeal in the field of piezo-phototronics that enable to effectively improve the performance of optoelectronic devices via modulating the electro-optical processes. However, current studies on 2D perovskite ferroelectrics focus on bulk ceramics that cannot endure irregular mechanical deformation and limit their application in flexible optoelectronics and piezo-phototronics. Herein, we synthesize ferroelectric EA₄Pb₃Br₁₀ single-crystalline thin-films (SCFs) for integration into flexible photodetectors. The in-plane multiaxial ferroelectricity is evident within the EA₄Pb₃Br₁₀ SCFs through systematic characterizations. Flexible photodetectors based on EA₄Pb₃Br₁₀ SCFs are achieved with an impressive photodetection performance. More importantly, optoelectronic EA₄Pb₃Br₁₀ SCFs incorporated with in-plane ferroelectric polarization and effective piezoelectric coefficient show great promise for the observation of piezo-phototronic effect, which is capable of greatly enhancing the photodetector performance. Under external strains, the responsivity of the flexible photodetectors can be modulated by piezo-phototronic effect with a remarkable enhancement up to 284%. Our findings shed light on the piezo-phototronic devices and offer a promising avenue to broaden functionalities of hybrid perovskite ferroelectrics.

Two narrow-bandgap block copolymers PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6 are designed and synthesized for single-component polymer solar cells, and a record-high efficiency of 8.64% is obtained. Moreover, these block copolymers exhibit relatively small energy loss and improved storage stability under both ambient condition and continued 80 °C thermal stresses for over 1000 h.

Abstract

Two narrow-bandgap block conjugated polymers with a (D1–A1)–(D2–A2) backbone architecture, namely PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6, are designed and synthesized for single-component organic solar cells (SCOSCs). Both polymers contain same donor polymer, PBDB-T, but different polymerized nonfullerene molecule acceptors. Compared to all previously reported materials for SCOSCs, PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6 exhibit narrower bandgap for better light harvesting. When incorporated into SCOSCs, the short-circuit current density (J _sc) is significantly improved to over 15 mA cm⁻², together with a record-high power conversion efficiency (PCE) of 8.64%. Moreover, these block copolymers exhibit low energy loss due to high charge transfer (CT) states (E _ct) plus small non-radiative loss (0.26 eV), and improved stability under both ambient condition and continuous 80 °C thermal stresses for over 1000 h. Determination of the charge carrier dynamics and film morphology in these SCOSCs reveals increased carrier recombination, relative to binary bulk-heterojunction devices, which is mainly due to reduced ordering of both donor and acceptor fragments. The close structural relationship between block polymers and their binary counterparts also provides an excellent framework to explore further molecular features that impact the photovoltaic performance and boost the state-of-the-art efficiency of SCOSCs.

Molecular hole-transporting materials with different anchoring groups are synthesized. The anchoring groups with a stronger bonding strength enable greatly enhanced compactness of self-assembly monolayer, which benefits hole-extraction and electron-blocking in complete devices. When applied in inverted perovskite solar cells, 1 cm² devices show a promising power conversion efficiency of over 20% with high stability.

Abstract

Anchoring-based self-assembly (ASA) has emerged as a material-saving and highly scalable strategy to fabricate charge-transporting monolayers for perovskite solar cells (PSCs). However, the interfacial hole-extraction and electron-blocking performances are highly dependent on the compactness of the ASA monolayers, which has been largely ignored though it is very crucial to the efficiency and stability of PSCs. Here, strategically designed hole-transporting molecules with different anchoring groups are incorporated to investigate the effect of bonding strength on monolayer quality and correlate these with the performance of p-i-n structured PSCs. It is unraveled that the anchoring groups with a stronger bonding strength are advantageous for improving the assembly rate, density, and compactness of ASA monolayer, thus enhancing charge collection and suppressing interfacial recombination. The prototypical PSCs based on optimal ASA monolayer achieve a high power conversion efficiency (PCE) of 21.43% (0.09 cm²). More encouragingly, when enlarging the device area by tenfold, a comparable PCE of 20.09% (1.0 cm²) can be obtained, suggesting that the ASA strategy is practically useful for scaling-up. The robust anchoring of the ASA monolayer also enhances devices stability, retaining 90% of initial PCE after three months. This study provides important insights into the ASA charge-transporting monolayers for efficient and stable PSCs.

This study develops a universal bottom interface modification method with diverse 2D spacers, which significantly enhance the device performance of inverted perovskite solar cells from 20.7% to 21.6%. The lift-off method is used to directly study the change of optoelectronic properties at the bottom interface and unveils the formation of 2D/3D heterojunction as the general mechanism underlying the device performance enhancement.

Abstract

Although the 2D spacer modification is widely studied in perovskite solar cells (PVSCs), the energy level alignment between the 2D/3D interfaces makes it unfavorable for top surface passivation in the inverted p-i-n device structure. To address this issue, the effect of bottom interface modification is studied with three representative 2D spacers, i.e., the Ruddlesden-Popper 2D spacer, Dion-Jacobson 2D spacer, and strong passivation 2D spacer, in inverted p-i-n PVSCs. After optimization, the PVSCs with these 2D spacer modifications universally exhibit the best efficiencies of ≈21.6%, which constitutes dramatic improvement compared to the control device (20.7%). By lifting off the perovskite layer, the optoelectronic properties of the bottom surface are studied, and the mechanism underlying the improved device performance is unveiled to be uniformly originated from the formation of 2D/3D heterojunction, where the cascade valence band facilitates the hole collection and electron back scattering field suppresses the charge recombination at the anode interface. Besides, the unencapsulated device retains 90% of initial efficiency after 30 days of storage in ambient air with a relative humidity of 30 ± 5%, indicating excellent stability against moisture and oxygen. This study provides insight into the bottom interface modification with diverse 2D spacers for high-performance p-i-n structured PVSC devices.

Nature Photonics, Published online: 21 June 2021; doi:10.1038/s41566-021-00830-x