Chen Weijie

Ultra-stable all-inorganic lead-free Cs₄SnBr₆ perovskites were obtained by using SnF₂ as tin source. The doping of F ions can significantly suppress the oxidation of Sn²⁺ to Sn⁴⁺ in Cs₄SnBr₆ perovskites, thus achieving excellent stability against oxygen, moisture and light irradiation for 1200 h.

Abstract

Sn-based perovskites are the most promising alternative materials for Pb-based perovskites to address the toxicity problem of lead. However, the development of Sn^II-based perovskites has been hindered by their extreme instability. Here, we synthesized efficient and stable lead-free Cs₄SnBr₆ perovskite by using SnF₂ as tin source instead of easily oxidized SnBr₂. The SnF₂ configures a fluorine-rich environment, which can not only suppress the oxidation of Sn²⁺ in the synthesis, but also construct chemically stable Sn−F coordination to hinder the electron transfer from Sn²⁺ to oxygen within the long-term operation process. The SnF₂-derived Cs₄SnBr₆ perovskite shows a high photoluminescence quantum yield of 62.8 %, and excellent stability against oxygen, moisture, and light radiation for 1200 h, representing one of the most stable lead-free perovskites. The results pave a new pathway to enhance the optical properties and stability of lead-free perovskite for high-performance light emitters.

The stable graphite carbon nitride nanosheets (CNVx) with uniformly dispersed V₂O₅ nanoparticles are utilized as additives in the 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD)-based hole transport layer and the high performance and thermal stability of perovskite solar cells (PSCs) are obtained, which is attributed to the effective suppression in glass transition of spiro-OMeTAD and lithium ions migration in PSCs.

The undesired glass transition of 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD) and the lithium-ions migration are the intractable factors affecting the stability of perovskite solar cells (PSCs). Herein, the 2D graphite carbon nitride nanosheets (CNVx) with various loading masses of V₂O₅ are developed as additives for spiro-OMeTAD to achieve stable and efficient PSCs. The optimized CNVx possesses quantitative and controllable oxidation ability to spiro-OMeTAD under the inert atmosphere, yielding significantly improved conductivity and hole mobility of the modified hole transport layer (HTL) film. As a result, a power conversion efficiency (PCE) of 21.10% is obtained in the CNVx modified device with enhanced open-circuit voltage of 1.114 V and fill factor of 0.80. Furthermore, the glass transition temperature suppression in the CNVx-modified HTL film and effective Li⁺ ions capture significantly improve the stability of the unencapsulated device, which maintains 82% and 90% of the original PCE after aging at 85 °C in the N₂ atmosphere and storing in an ambient atmosphere with a relative humidity of 40% for 720 h, respectively. These results provide a deep understanding of PSCs stability enhancement with 2D carbon nanosheets for realizing high-performance PSCs.

Functionalized graphene oxide nanosheets (GNSs) are synthesized to modify the inorganic ZnO electron transport layer (ETL) in inverted organic photovoltaic devices (OPVs). The GNS-modification enables better energy-level alignment and facilitates charge transfer across the ZnO/BHJ interface. Moreover, it stabilizes the morphology of the ZnO ETL and enhances the thermodynamic nature of the photoactive layer. These combined advantages largely reduce the microstructure changes and the charge recombination in the BHJ layer under constant thermal/light stresses.

Abstract

As the power conversion efficiency (PCE) of organic photovoltaics (OPVs) approaches 19%, increasing research attention is being paid to enhancing the device's long-term stability. In this study, a robust interface engineering of graphene oxide nanosheets (GNS) is expounded on improving the thermal and photostability of non-fullerene bulk-heterojunction (NFA BHJ) OPVs to a practical level. Three distinct GNSs (GNS, N-doped GNS (N-GNS), and N,S-doped GNS (NS-GNS)) synthesized through a pyrolysis method are applied as the ZnO modifier in inverted OPVs. The results reveal that the GNS modification introduces passivation and dipole effects to enable better energy-level alignment and to facilitate charge transfer across the ZnO/BHJ interface. Besides, it optimizes the BHJ morphology of the photoactive layer, and the N,S doping of GNS further enhances the interaction with the photoactive components to enable a more idea BHJ morphology. Consequently, the NS-GNS device delivers enhanced performance from 14.5% (control device) to 16.5%. Moreover, the thermally/chemically stable GNS is shown to stabilize the morphology of the ZnO electron transport layer (ETL) and to endow the BHJ morphology of the photoactive layer grown atop with a more stable thermodynamic property. This largely reduces the microstructure changes and the associated charge recombination in the BHJ layer under constant thermal/light stresses. Finally, the NS-GNS device is demonstrated to exhibit an impressive T ₈₀ lifetime (time at which PCE of the device decays to 80% of the initial PCE) of 2712 h under a constant thermal condition at 65 °C in a glovebox and an outstanding photostability with a T ₈₀ lifetime of 2000 h under constant AM1.5G 1-sun illumination in an N₂-controlled environment.

Given the increasing interest in searching for high-quality low-cost passivating contacts for c-Si solar cells, the fundamentals and development status of wide-bandgap metal-compound-based passivating contacts are reviewed and the challenges and potential solutions in developing highly transparent passivating contacts with excellent carrier selectivity are discussed. Based on in-depth data analysis and simulations, the improvement strategies for metal-compound-based passivating contacts design and device integration are pointed out.

Abstract

Advanced doped-silicon-layer-based passivating contacts have boosted the power conversion efficiency (PCE) of single-junction crystalline silicon (c-Si) solar cells to over 26%. However, the inevitable parasitic light absorption of the doped silicon layers impedes further PCE improvement. To this end, alternative passivating contacts based on wide-bandgap metal compounds (so-called dopant-free passivating contacts (DFPCs)) have attracted great attention, thanks to their potential merits in terms of parasitic absorption loss, ease-of-deposition, and cost. Intensive research activity has surrounded this topic with significant progress made in recent years. Various electron-selective and hole-selective contacts based on metal compounds have been successfully developed, and a champion PCE of 23.5% has been achieved for a c-Si solar cell with a MoO _x -based hole-selective contact. In this work, the fundamentals and development status of DFPCs are reviewed and the challenges and potential solutions for enhancing the carrier selectivity of DFPCs are discussed. Based on comprehensive and in-depth analysis and simulations, the improvement strategies and future prospects for DFPCs design and device implementation are pointed out. By tuning the carrier concentration of the metal compound and the work function of the capping transparent electrode, high PCEs over 26% can be achieved for c-Si solar cells with DFPCs.

It is observed that the amount of quenching of the photoluminescence in short-circuit conditions in perovskite solar cells is a good parameter to assess the efficiency and estimate the carrier extraction time of a photovoltaic device. Conversely, the use of open-circuit photoluminescence alone as an indicator of the solar cell efficiency can be a source of misleading conclusions.

The photoluminescence (PL) intensity is often used as an indicator of the performance of perovskite solar cells and indeed the PL technique is often used for the characterization of these devices and their constituent materials. Herein, a systematic approach is presented to the comparison of the conversion efficiency and the PL intensity of a cell in both open-circuit (OC) and short-circuit (SC) conditions and its application to multiple heterogeneous devices. It is shown that the quenching of the PL observed in SC conditions is a good parameter to assess the device efficiency. The authors explain the dependence of the PL quenching ratio between OC and SC on the cell efficiency with a simple model that is also able to estimate the carrier extraction time of a device.

The key mechanical properties of five all-polymer photovoltaic blend films are determined with a set of complementary characterization methods. The composition dependences of the film stretchability and elastic moduli of these all-polymer blends with different miscibility are well elucidated. Notably, the elastic moduli of these all-polymer blend films with various morphologies can be predicted by mechanical models for the first time.

Abstract

The rapid development of low bandgap polymer acceptors has promoted the efficiency up to ≈17% for all-polymer solar cells (all-PSCs). Nevertheless, the polymeric blend film, core to the photoelectric conversion of all-PSCs, has not been thoroughly understood in terms of the influence and regulatory factors of mechanical properties, which hinders the advances in flexible and wearable applications. Herein, a range of characterization methods is combined to investigate the mechanical properties, miscibility, and film microstructure of the blends based on several representative polymer donors (PTzBI-Si, PTVT-T, PM6 and PTQ10) and a benchmark polymer acceptor N2200, and to further reveal the miscibility-property relationships of the miscibility property. The results stress that fracture behaviors and elastic moduli of these blends with varied compositions show different changing trends, which are affected by molecular interactions and aggregated structure of the blends. The elastic moduli of the four all-polymer blends can be nicely predicted by different models that are deduced from macromolecular mechanics. Most crucially, the correlations between elastic modulus, morphology, and miscibility of all-polymer blends are elucidated for the first time. The derived relationships is validated with another high-efficiency blend and will be the key to the successful fabrication of mechanically robust and stretchable all-PSCs with high efficiency.

Photoconductive charge transfer complexes made of p-type polymers and n-type PCBM are employed as charge transport as well as power conversion layers to improve the efficiency and stability of inverted perovskite solar cells.

Abstract

Charge transport layers (CTLs) are critical for achieving high power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, the p-type bulk heterojunction (p-BHJ, i.e., PCBM doped PTAA) and n-type BHJ (n-BHJ, i.e., PBDTTT-C-T doped PCBM) charge transfer complexes are employed as hole and electron transport layers, respectively, to fabricate inverted PSCs. The photo-induced charge transfer between p-type and n-type organic semiconductors in the BHJ layers provides extra photoconductivity for enhanced charge transport and quasi-Fermi level splitting, hence enhancing the fill factor and open-circuit voltage of PSCs. The p-BHJ layer helps to improve the crystallinity and light absorption of perovskite, whilst the n-BHJ layer provides extra light absorption and charge generation to boost the short-circuit current. The combination of p-BHJ and n-BHJ CTLs in Cs_0.05(FA_0.92MA_0.08)_0.95Pb (I_0.92Br_0.08)₃ based inverted PSCs synergistically enhances the PCE from 18.3% to 22.6% with superior operational and thermal stabilities, and showing a negligible dependence on the thickness of these BHJ CTLs. Density functional theory simulations show that the formation energy of BHJ complex is critical in determining the doping effect and the ultimate performance enhancement of PSCs.

A holistic solution is proposed that is generally applicable for perovskite solar cells with either n–i–p or p–i–n structures to improve their heat transfer by introducing hexagonal boron nitride inside of absorber and the radiator fin outside of device. The optimized device retains 93% of its initial efficiency after 2451 h of maximum power point tracking.

Abstract

Special attention should be devoted to the thermal stability of hybrid perovskite solar cells (PSCs), because they are often operated at elevated temperatures. However, effective strategies are lacking for manipulation of heat flow in PSCs to improve their thermal stability. Here, a holistic solution is reported for the rapid removal of dissipated heat within the absorber by introducing hexagonal boron nitride (h-BN) inside and radiator fin outside of the device. This strategy significantly improves the thermal conductivity of perovskite and speeds up the heat transfer of device, which effectively reduces the cell temperature under illumination of simulated AM 1.5G standard spectrum by ≈6.5 °C. Regardless of device configurations, the corresponding PSCs exhibit prolonged lifetimes aged at different temperatures, continuously operated under white light-emitting diode (LED) lamp or full-spectrum illumination. Of particular note, the optimized h-BN/Cu device with n–i–p structure keeps 88% and 93% of its initial PCE after 1776 h of 85 °C thermal aging and 2451 h of maximum power point (MPP) tracking, respectively, and the device with p–i–n structure maintains 96% and 92% of its original PCE after 1704 h of 85 °C thermal aging and 2164 h of MPP tracking.

Narrow-gap indium arsenide colloidal quantum dot (CQD) solids are employed as an electron transporting layer in nonfullerene-based organic photovoltaics and lead to high-efficiency, air- and photo-stable devices under 1 sun illumination. The best-performing InAs CQD-based device shows a power conversion efficiency of 15.1% while retaining original efficiency of over 80% under continuous 1 sun illumination over 1000 min in ambient air.

Abstract

The past decade has seen a dramatic surge in the power conversion efficiency (PCE) of next-generation solution-processed thin-film solar cells rapidly closing the gap in PCE of commercially-available photovoltaic (PV) cells. Yet the operational stability of such new PVs leaves a lot to be desired. Specifically, chemical reaction with absorbers via high-energy photons transmitted through the typically-adapted metal oxide electron transporting layers (ETLs), and photocatalytic degradation at interfaces are considered detrimental to the device performance. Herein, the authors introduce a device architecture using the narrow-gap, Indium Arsenide colloidal quantum dots (CQDs) with discrete electronic states as an ETL in high-efficiency solution-processed PVs. High-performing PM6:Y6 organic PVs (OPVs) achieve a PCE of 15.1%. More importantly, as the operating stability of the device is significantly improved, retaining above 80% of the original PCE over 1000 min under continuous illumination, a Newport-certified PCE of 13.1% is reported for nonencapsulated OPVs measured under ambient air. Based on operando studies as well as optical simulations, it suggested that the InAs CQD ETLs with discrete energy states effectively cut-off high-energy photons while selectively collecting electrons from the absorber. The findings of this works enable high-efficiency solution-processed PVs with enhanced durability under operating conditions.

2-Furfurylammonium is first developed as a new spacer cation for 2D perovskites. The best-performing perovskite solar cell with a power conversion efficiency (PCE) of 15.66% was obtained by optimizing the crystallization and orientation of the perovskite film via an additive-assisted technique. This perovskite film can also be fabricated in ambient air and delivered a PCE of 15.24%.

2D perovskites have recently emerged as promising materials for solar cells due to their appealing ambient stability and structural diversity. The organic spacer cation plays an important role in the stability and properties of the 2D perovskites. Herein, a new organic ammonium cation, 2-furfurylammonium (FuMA⁺), is developed as the spacer cation for 2D Ruddlesden–Popper perovskites. Thin films of (FuMA)₂(MA)₄Pb₅I₁₆ with increased crystal size and enhanced vertical orientation are obtained via an additive-assisted film forming technique, which dramatically improves the efficiency of 2D perovskite solar cells from 4.90% to 15.66%. Moreover, the perovskite films based on FuMA⁺ can be prepared in ambient air, and the devices based on the perovskite films prepared in ambient air with 30% relative humidity can perform a power conversion efficiency (PCE) of 15.24%.

The Sn-vapor is provided during the synthesis of Cu₂ZnSn(S,Se)₄ (CZTSSe) to inhibit the decomposition. As expected, the density of acceptor defects and the interface recombination are significantly decreased. This advanced strategy improves the performance of CZTSSe solar cells and enables a 12.03% efficiency. This work provides a new route to enhancing the photovoltaic performance of CZTSSe solar cells.

Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, which are emerging as promising photovoltaic devices, are currently suffering serious issues of large open-circuit voltage deficit and low fill factor. Decomposition of CZTSSe is one of many factors limiting the efficiency improvement of CZTSSe solar cells, which can lead to the deviation of the chemical environment during the synthesis of CZTSSe. Herein, the Sn-vapor is provided during the synthesis of CZTSSe to inhibit the decomposition, and the effect of decomposition on the intrinsic defects and interface recombination are systematically investigated. The high-quality CZTSSe without the secondary phase and with the low Zn_Sn and Cu_Zn acceptor defects density is obtained. By inhibiting the decomposition, the recombination activation energy at depletion region is improved and the interface defect density is dramatically decreased, indicating the interface recombination is effectively reduced. Consequently, the performance of CZTSSe thin film solar cells, especially the open-circuit voltage and fill factor, has been significantly improved. Finally, based on the excellent CZTSSe film, a photovoltaic device with 12.03% efficiency (active area efficiency is 12.96%) is prepared. These encouraging results provide a new route to controlling the defects and interface recombination of CZTSSe solar cells.

The nonionic surfactant polymer (P123) serves to suppress the aggregation of SnO₂ nanoparticles for a uniform SnO₂ electron transport layer (ETL). The size distribution of SnO₂ colloid resides in the nanoscale of 10 nm, providing an ideal template for ultrathin and compact SnO₂ ETL deposition. As a result, the devices based on P123-SnO₂ ETL demonstrate a champion efficiency with enhanced open-circuit voltage (V _OC) of 1.162 V, also exhibiting good reproducibility and long-term stability.

SnO₂ electron transport layer (ETL) plays a critical role in constructing a planar perovskite solar device. Improving SnO₂ ETL properties and understanding of interfacial energy loss are key factors to fabricate highly efficient and reproducible perovskite solar cells (PSCs). Herein, a nonionic surfactant, polyethylene oxide-polypropylene oxide-polyethylene oxide (P123), is introduced to suppress the aggregation of SnO₂ nanoparticles for a uniform SnO₂ ETL. The P123 polymer can maintain the SnO₂ colloidal size around 10 nm over 72 h at 35 °C and thus promote the dispersion of nanoparticles in SnO₂ precursor. By spin coating P123-doped SnO₂ (SnO₂-P) colloid, the compactness and uniformity of SnO₂ layer are improved significantly. Correspondingly, SnO₂-P-based devices demonstrate a champion efficiency with enhanced open-circuit voltage (V _OC) of 1.162 V. Due to the SnO₂/perovskite interface binding interaction, the devices gain a high long-term stability, retaining 85% of their initial performance after 1000 h storage in air. Equally important, the polymer-regulated ETL allows a competitive efficiency of 17.44% with active area of 1.00 cm², exhibiting much potential for large-scale solar devices. This ETL modification approach provides a new and simple route to improve the quality of SnO₂ colloid solution for fabricating efficient perovskite devices.

A cascade defect passivation strategy is proposed to doubly reduce perovskite defects by introducing free radical azo-initiators. Both the surface and the interior perovskite defects can be healed efficiently and thus the modified CH₃NH₃PbI₃ (MAPbI₃)-based inverted perovskite solar cells (PSCs) exhibit significantly enhanced efficiency with the power conversion efficiency (PCE) approaching 20% as well as high long-term stability.

Remarkable progress in perovskite solar cells (PSCs) has been made by virtue of surface and bulk modification for defect control of perovskite layers, however, the interior defects in the perovskite layer can be hardly passivated by means of conventional passivation strategies, which makes perovskites prone to decomposition with inferior device performance. Here, the authors propose a cascade defect passivation strategy to doubly reduce perovskite defects by introducing a series of azo radical initiators. The primary passivation process of the azo compounds is realized on the perovskite surfaces through strong coordination interactions between Pb²⁺ and carbonyl/cyano groups. The secondary passivation occurs after thermal treatment, and the decomposed products diffuse into the interior defects of the initial passivated perovskite films through grain boundaries to accomplish the cascade defect passivation. Excitingly, this passivation strategy inhibits unwanted defect-assisted recombination effectively, and thus CH₃NH₃PbI₃ (MAPbI₃)-based inverted PSCs exhibit a significant increase in power conversion efficiency (PCE) from 16.92% to 19.69%. Moreover, the dual-passivated PSCs show only 10% PCE loss after 3000 h in an inert atmosphere. Overall, the first proposed cascade defect passivation strategy is highly efficient in promoting defect healing of perovskites, demonstrating a new way to prepare high-quality perovskite films for high-performance PSCs.

Nature Materials, Published online: 05 May 2022; doi:10.1038/s41563-022-01244-y

Publication date: July 2022

Source: Nano Energy, Volume 98

Author(s): Jiakai Zhou, Xianglin Su, Qian Huang, Yuheng Zeng, Dian Ma, Wei Liu, Baojie Yan, Jichun Ye, Jie Yang, Xinyu Zhang, Hao Jin, Ying Zhao, Guofu Hou

Publication date: 18 May 2022

Source: Joule, Volume 6, Issue 5

Author(s): Kiwan Jeong, Junseop Byeon, Jihun Jang, Namyoung Ahn, Mansoo Choi

PTAA (poly[bis(4-phenyl) (2,4,6-trimethyl-phenyl)amine) is one of the most promising hole transport materials, widely used in perovskite-based solar cells. A progress review on PTAA for its application in various perovskite solar cells is provided, both in n–i–p and p–i–n structures. This work enables a better understanding of PTAA and approaches to unlock its tremendous potential.

Perovskite solar cells (PSCs) with a power conversion efficiency (PCE) overpassing 25% have proved to be the most promising competitor for the next-generation photovoltaic technology. Massive efforts are devoted to the improvement of the performance and stability of PSCs, whereas the hole transport layer (HTL) has attracted significant interest. Among diverse hole transport materials, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) is one of the most promising candidates due to its ease of fabrication, transparency to visible light, mechanical flexibility, conductivity, and stability. Over the past few years, there has been an increasing amount of research using PTAA as the HTL first in n–i–p and then in p–i–n PSCs with extended applications in flexible, large-area, and tandem devices. Herein, a progress review on PTAA for PSC applications is provided, which enables a better understanding of the advantages and disadvantages of PTAA, as well as the approaches to fully realizing its tremendous potential. The emerging and promising research directions for PTAA-based PSCs are discussed, shedding light on the practical applications of PTAA.