Zidan Liu

Fill factor losses in nonfullerene‐acceptor‐based organic solar cells under illumination are caused by morphological traps due to diffusion limited aggregation of the nonfullerene acceptors in the mixed matrix. To achieve stable and high‐performance organic solar cells under illumination, it is essential to engineer the mixed regions from both thin‐film formation kinetics and materials intrinsic properties, e.g., materials compatibility and diffusion constant.

Abstract

As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long‐term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state‐of‐the‐art polymer donors with a commonly used nonfullerene acceptor (NFA), IT‐4F, are investigated. The active‐layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion‐limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene‐based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA‐based solar cells, which may be the key tactics to develop next‐generation OSCs with high efficiency as well as excellent stability.

A “welding” transparent flexible electrode, with respect to both the upper electrode and the underlying substrate, for fabricating high‐performance flexible OSCs is proposed, resulting in a record power conversion efficiency of single‐junction flexible organic solar cells (OSCs) with excellent mechanical properties.

Abstract

The power conversion efficiencies (PCEs) of flexible organic solar cells (OSCs) still lag behind those of rigid devices and their mechanical stability is unable to meet the needs of flexible electronics at present due to the lack of a high‐performance flexible transparent electrode (FTE). Here, a so‐called “welding” concept is proposed to design an FTE with tight binding of the upper electrode and the underlying substrate. The upper electrode consisting of solution‐processed Al‐doped ZnO (AZO) and silver nanowire (AgNW) network is well welded by utilizing the capillary force effect and secondary growth of AZO, leading to a reduction of the AgNWs junction site resistance. Meanwhile, the poly(ethylene terephthalate) is modified by embedding the AgNWs, which are then used to link with the AgNWs in the upper hybrid electrode, thus enhancing the adhesion of the electrode to the substrate. By this welding strategy, critical bottleneck issues relating to the FTEs in terms of optoelectronic and mechanical properties are comprehensively addressed. The single‐junction flexible OSCs based on this welded FTE show a high performance, achieving a record high PCE of 15.21%. In addition, the PCEs of the flexible OSCs are less influenced by the device area and display robust bending durability even under extreme test conditions.

Degradation of perovskite solar cells with excess PbI₂ is investigated. Excess PbI₂ in perovskite films undergoes photodecomposition (photolysis) under illumination, which produces lead and iodine and accelerates the degradation of PSCs.

Abstract

Excess/unreacted lead iodide (PbI₂) has been commonly used in perovskite films for the state‐of‐the‐art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI₂ has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI₂ crystals under continuous light illumination. Unreacted PbI₂ undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI₂ is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI₂ crystals in perovskite films is very important to improve device operational stability for diverse opto‐electronic applications in the future.

Fill factor losses in nonfullerene‐acceptor‐based organic solar cells under illumination are caused by morphological traps due to diffusion limited aggregation of nonfullerene acceptors in the mixed matrix. To achieve stable and high‐performance organic solar cells under illumination, it is essential to engineer the mixed regions from both thin film formation kinetics and materials intrinsic properties, e.g., materials compatibility and diffusion constant.

Abstract

As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long‐term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state‐of‐the‐art polymer donors with a commonly used nonfullerene acceptor (NFA), IT‐4F, are investigated. The active‐layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion‐limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene‐based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA‐based solar cells, which may be the key tactics to develop next‐generation OSCs with high efficiency as well as excellent stability.

A new S‐atom‐containing small molecule (TPE‐S) is introduced as a dopant‐free hole‐transporting layer in all‐inorganic and organic/inorganic hybrid perovskite solar cells (PVSCs) with a p–i–n inverted structure, leading to improved power conversion efficiencies of 15.4% and 21%, respectively. In addition, these devices also show enhanced photostability, with performance comparable to state‐of‐the‐art PVSCs based on the conventional n–i–p structure.

Abstract

Designing new hole‐transporting materials (HTMs) with desired chemical, electrical, and electronic properties is critical to realize efficient and stable inverted perovskite solar cells (PVSCs) with a p–i–n structure. Herein, the synthesis of a novel 3D small molecule named TPE‐S and its application as an HTM in PVSCs are shown. The all‐inorganic inverted PVSCs made using TPE‐S, processed without any dopant or post‐treatment, are highly efficient and stable. Compared to control devices based on the commonly used HTM, PEDOT:PSS, devices based on TPE‐S exhibit improved optoelectronic properties, more favorable interfacial energetics, and reduced recombination due to an improved trap passivation effect. As a result, the all‐inorganic CsPbI₂Br PVSCs based on TPE‐S demonstrate a remarkable efficiency of 15.4% along with excellent stability, which is the one of the highest reported values for inverted all‐inorganic PVSCs. Meanwhile, the TPE‐S layer can also be generally used to improve the performance of organic/inorganic hybrid inverted PVSCs, which show an outstanding power conversation efficiency of 21.0%, approaching the highest reported efficiency for inverted PVSCs. This work highlights the great potential of TPE‐S as a simple and general dopant‐free HTM for different types of high‐performance PVSCs.

In article number https://doi.org/10.1002/adma.2019070631907063, Himani Arora, Enrique Canovas, Artur Erbe, and co‐workers demonstrate broadband photodetectors based on a novel π–d conjugated Fe₃(THT)₂(NH)₃ 2D metal–organic framework (MOF), operative in the UV‐to‐NIR range. Due to the small IR bandgap of the MOF, the photodetectors are best operated at cryogenic temperatures by suppressing the thermally activated charge‐carrier population. Thus, a proof‐of‐concept MOF photodetector is reported, revealing MOFs as promising candidates for optoelectronics.

A new S‐atom‐containing small molecule (TPE‐S) is introduced as a dopant‐free hole‐transporting layer in all‐inorganic and organic/inorganic hybrid perovskite solar cells (PVSCs) with a p–i–n inverted structure, leading to improved power conversion efficiencies of 15.4% and 21%, respectively. In addition, these devices also show enhanced photostability, with performance comparable to state‐of‐the‐art PVSCs based on the conventional n–i–p structure.

Abstract

Designing new hole‐transporting materials (HTMs) with desired chemical, electrical, and electronic properties is critical to realize efficient and stable inverted perovskite solar cells (PVSCs) with a p–i–n structure. Herein, the synthesis of a novel 3D small molecule named TPE‐S and its application as an HTM in PVSCs are shown. The all‐inorganic inverted PVSCs made using TPE‐S, processed without any dopant or post‐treatment, are highly efficient and stable. Compared to control devices based on the commonly used HTM, PEDOT:PSS, devices based on TPE‐S exhibit improved optoelectronic properties, more favorable interfacial energetics, and reduced recombination due to an improved trap passivation effect. As a result, the all‐inorganic CsPbI₂Br PVSCs based on TPE‐S demonstrate a remarkable efficiency of 15.4% along with excellent stability, which is the one of the highest reported values for inverted all‐inorganic PVSCs. Meanwhile, the TPE‐S layer can also be generally used to improve the performance of organic/inorganic hybrid inverted PVSCs, which show an outstanding power conversation efficiency of 21.0%, approaching the highest reported efficiency for inverted PVSCs. This work highlights the great potential of TPE‐S as a simple and general dopant‐free HTM for different types of high‐performance PVSCs.

A thin‐film anatase TiO₂ substrate for single‐atom (SA) transmission electron microscopy (TEM) characterization is prepared. Control of the surface density of the SA traps results in tunable SA decoration on the anatase surface and superior photocatalytic H₂ generation compared to classic noble‐metal‐nanoparticle‐decorated TiO₂.

Abstract

Single‐atom (SA) catalysis is a novel frontline in the catalysis field due to the often drastically enhanced specific activity and selectivity of many catalytic reactions. Here, an atomic‐scale defect engineering approach to form and control traps for platinum SA sites as co‐catalyst for photocatalytic H₂ generation is described. Thin sputtered TiO₂ layers are used as a model photocatalyst, and compared to the more frequently used (001) anatase sheets. To form stable SA platinum, the TiO₂ layers are reduced in Ar/H₂ under different conditions (leading to different but defined Ti³⁺‐O_v surface defects), followed by immersion in a dilute hexachloroplatinic acid solution. HAADF‐STEM results show that only on the thin‐film substrate can the density of SA sites be successfully controlled by the degree of reduction by annealing. An optimized SA‐Pt decoration can enhance the normalized photocatalytic activity of a TiO₂ sputtered sample by 150 times in comparison to a conventional platinum‐nanoparticle‐decorated TiO₂ surface. HAADF‐STEM, XPS, and EPR investigation jointly confirm the atomic nature of the decorated Pt on TiO₂. Importantly, the density of the relevant surface exposed defect centers—thus the density of Pt‐SA sites, which play the key role in photocatalytic activity—can be precisely optimized.

A novel bifunctional (anti)solvent system is developed for regulating the perovskite crystallization procedure. It can perform not only as an antisolvent at the spin‐coating step to rapidly generate crystal seeds, but also as a solvent for ripening the precursors to large crystal grains during the thermal‐annealing process. Therefore, it can significantly enhance the efficiency, stability, and reproducibility of perovskite solar cells.

Abstract

The preparation of high‐quality perovskite films is important for achieving high‐performance perovskite solar cells (PSCs). The effective balance between solvent and antisolvent is an essential factor for regulating high‐quality perovskite film during the spin‐coating and thermal‐annealing steps. In this work, a greener, nonhalogenated, nontoxic bifunctional (anti)solvent, methyl benzoate (MB), is developed not only as an antisolvent to rapidly generate crystal seeds at the perovskite spin‐coating step, but also as a digestive‐ripening solvent for the perovskite precursors, which can prevent the loss of organic components during the thermal‐annealing stage and effectively suppress the formation of miscellaneous lead halide phases. As a result, this novel bifunctional (anti)solvent is employed in planar n–i–p PSCs for engineering high‐quality perovskite layers and thus achieving a power conversion efficiency up to 22.37% with negligible hysteresis and >1300 h stability. Moreover, due to the high boiling point and low‐volatility characteristic of MB, high‐performance PSCs are achieved reproducibly at different operating temperatures (22–34 °C). Therefore, this developed bifunctional solvent system can provide a promising platform toward globally upscaling and commercializing PSCs in all seasons and regions.

All‐solution‐processed and printable nonfullerene organic solar cells are fabricated. All layers from the substrate to the top electrode are solution‐processed. Hydrogen molybdenum bronze is introduced to solve the charge extraction issue and the wetting issue of the top electrode (PEDOT:PSS) on a hydrophobic active layer. Efficiency over 10% (>1 cm²) is obtained for the all‐solution‐processed nonfullerene solar cells.

Abstract

All‐solution‐processed organic solar cells (from the bottom substrate to the top electrode) are highly desirable for low‐cost and ubiquitous applications. However, it is still challenging to fabricate efficient all‐solution‐processed organic solar cells with a high‐performance nonfullerene (NF) active layer. Issues of charge extraction and wetting are persistent at the interface between the nonfullerene active layer and the printable top electrode (PEDOT:PSS). In this work, efficient all‐solution‐processed NF organic solar cells (from the bottom substrate to the top electrode) are reported via the adoption of a layer of hydrogen molybdenum bronze (H_XMoO₃) between the active layer and the PEDOT:PSS. The dual functions of H_XMoO₃ include: 1) its deep Fermi level of −5.44 eV can effectively extract holes from the active layer; and 2) the wetting issues of the PEDOT:PSS on the hydrophobic surface of the NF active layer can be solved. Importantly, fine control of the H_XMoO₃ composition during the synthesis is critical in obtaining processing orthogonality between H_XMoO₃ and the PEDOT:PSS. Flexible all‐solution‐processed NF organic solar cells with power conversion efficiencies of 11.9% and 10.3% are obtained for solar cells with an area of 0.04 and 1 cm², respectively.

GABr doping in ideal‐bandgap (≈1.34 eV) Sn–Pb binary perovskite films can efficiently reduce the defect density caused by Sn²⁺ oxidation in the perovskite and reduce the V _OC deficit. As a result, the best PCE of 20.63% with a record small V _OC deficit of 0.33 V is achieved in Sn–Pb binary 1.35 eV PSCs.

Abstract

1.5–1.6 eV bandgap Pb‐based perovskite solar cells (PSCs) with 30–31% theoretical efficiency limit by the Shockley–Queisser model achieve 21–24% power conversion efficiencies (PCEs). However, the best PCEs of reported ideal‐bandgap (1.3–1.4 eV) Sn–Pb PSCs with a higher 33% theoretical efficiency limit are <18%, mainly because of their large open‐circuit voltage (V _oc) deficits (>0.4 V). Herein, it is found that the addition of guanidinium bromide (GABr) can significantly improve the structural and photoelectric characteristics of ideal‐bandgap (≈1.34 eV) Sn–Pb perovskite films. GABr introduced in the perovskite films can efficiently reduce the high defect density caused by Sn²⁺ oxidation in the perovskite, which is favorable for facilitating hole transport, decreasing charge‐carrier recombination, and reducing the V _oc deficit. Therefore, the best PCE of 20.63% with a certificated efficiency of 19.8% is achieved in 1.35 eV PSCs, along with a record small V _oc deficit of 0.33 V, which is the highest PCE among all values reported to date for ideal‐bandgap Sn–Pb PSCs. Moreover, the GABr‐modified PSCs exhibit significantly improved environmental and thermal stability. This work represents a noteworthy step toward the fabrication of efficient and stable ideal‐bandgap PSCs.

A “welding” transparent flexible electrode, with respect to both the upper electrode and the underlying substrate, for fabricating high‐performance flexible OSCs is proposed, resulting in a record power conversion efficiency of single‐junction flexible organic solar cells (OSCs) with excellent mechanical properties.

Abstract

The power conversion efficiencies (PCEs) of flexible organic solar cells (OSCs) still lag behind those of rigid devices and their mechanical stability is unable to meet the needs of flexible electronics at present due to the lack of a high‐performance flexible transparent electrode (FTE). Here, a so‐called “welding” concept is proposed to design an FTE with tight binding of the upper electrode and the underlying substrate. The upper electrode consisting of solution‐processed Al‐doped ZnO (AZO) and silver nanowire (AgNW) network is well welded by utilizing the capillary force effect and secondary growth of AZO, leading to a reduction of the AgNWs junction site resistance. Meanwhile, the poly(ethylene terephthalate) is modified by embedding the AgNWs, which are then used to link with the AgNWs in the upper hybrid electrode, thus enhancing the adhesion of the electrode to the substrate. By this welding strategy, critical bottleneck issues relating to the FTEs in terms of optoelectronic and mechanical properties are comprehensively addressed. The single‐junction flexible OSCs based on this welded FTE show a high performance, achieving a record high PCE of 15.21%. In addition, the PCEs of the flexible OSCs are less influenced by the device area and display robust bending durability even under extreme test conditions.

Degradation of perovskite solar cells with excess PbI₂ is investigated. Excess PbI₂ in perovskite films undergoes photodecomposition (photolysis) under illumination, which produces lead and iodine and accelerates the degradation of PSCs.

Abstract

Excess/unreacted lead iodide (PbI₂) has been commonly used in perovskite films for the state‐of‐the‐art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI₂ has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI₂ crystals under continuous light illumination. Unreacted PbI₂ undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI₂ is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI₂ crystals in perovskite films is very important to improve device operational stability for diverse opto‐electronic applications in the future.

By depositing a sub‐nanometer‐thick Fe layer on STO substrates at room temperature, a 2DHG with the highest hole mobility ever reported for oxides is realized at the STO interface. The carrier type can be switched from p‐type (2DHG) to n‐type (2DEG) by controlling the Fe thickness. These findings provide a pathway to extremely low‐cost and high‐speed oxide electronics.

Abstract

Strontium titanate (SrTiO₃ or STO) is important for oxide‐based electronics as it serves as a standard substrate for a wide range of high‐temperature superconducting cuprates, colossal magnetoresistive manganites, and multiferroics. Moreover, in its heterostructures with different materials, STO exhibits a broad spectrum of important physics such as superconductivity, magnetism, the quantum Hall effect, giant thermoelectric effect, and colossal ionic conductivity, most of which emerge in a two‐dimensional (2D) electron gas (2DEG) formed at an STO interface. However, little is known about its counterpart system, a 2D hole gas (2DHG) at the STO interface. Here, a simple way of realizing a 2DHG with an ultrahigh mobility of 24 000 cm² V⁻¹ s⁻¹ is demonstrated using an interface between STO and a thin amorphous FeO _y layer, made by depositing a sub‐nanometer‐thick Fe layer on an STO substrate at room temperature. This mobility is the highest among those reported for holes in oxides. The carrier type can be switched from p‐type (2DHG) to n‐type (2DEG) by controlling the Fe thickness. This unprecedented method of forming a 2DHG at an STO interface provides a pathway to unexplored hole‐related physics in this system and enables extremely low‐cost and high‐speed oxide electronics.

A flexible and self‐powered photodetector based on a nested inverse opal (NIO) perovskite structure is presented. The NIO device shows excellent photoresponse performance and mechanical durability. The remarkable improvement is attributed to the NIO‐structure‐enabled enhanced light harvesting, fast carrier transport, high perovskite crystallinity, and efficient stress release.

Abstract

Flexible and self‐powered perovskite photodetectors have attracted tremendous research interests due to their applications in wearable and portable devices. However, the conventional planar structured photodetectors are always accompanied with limited device performance and undesired mechanical stability. Herein, a nested inverse opal (NIO) structured perovskite photodetector via a facile template‐assisted spin‐coating method is reported. The coupling effect of enhanced light capture, increased carrier transport, and improved perovskite film quality enables NIO device to exhibit superior photoresponse performance. The NIO photodetector exhibits a high responsivity of 473 mA W⁻¹ and detectivity up to 1.35 × 10¹³ Jones at 720 nm without external bias. The NIO structure can efficiently release mechanical stress during the bending process and the photocurrent has no degradation even after 500 cycles of bending. Moreover, the unencapsulated NIO device can operate for over 16 d under ambient conditions, presenting a significantly enhanced environmental stability compared to the planar device. This work demonstrates that deliberate structural design is an effective avenue for constructing self‐powered, flexible, and stable optoelectronic devices.

Fill factor losses in nonfullerene‐acceptor‐based organic solar cells under illumination are caused by morphological traps due to diffusion limited aggregation of the nonfullerene acceptors in the mixed matrix. To achieve stable and high‐performance organic solar cells under illumination, it is essential to engineer the mixed regions from both thin‐film formation kinetics and materials intrinsic properties, e.g., materials compatibility and diffusion constant.

Abstract

As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long‐term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state‐of‐the‐art polymer donors with a commonly used nonfullerene acceptor (NFA), IT‐4F, are investigated. The active‐layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion‐limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene‐based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA‐based solar cells, which may be the key tactics to develop next‐generation OSCs with high efficiency as well as excellent stability.

A polymerization‐assisted grain growth strategy in the sequential deposition method of perovskite thin films is demonstrated by triggering a polymerization process during PbI₂ film annealing. This strategy effectively passivates undercoordinated lead ions, reduces defect density, and boosts power conversion efficiency up to 23.0%, together with a prolonged lifetime.

Abstract

Intrinsically, detrimental defects accumulating at the surface and grain boundaries limit both the performance and stability of perovskite solar cells. Small molecules and bulkier polymers with functional groups are utilized to passivate these ionic defects but usually suffer from volatility and precipitation issues, respectively. Here, starting from the addition of small monomers in the PbI₂ precursor, a polymerization‐assisted grain growth strategy is introduced in the sequential deposition method. With a polymerization process triggered during the PbI₂ film annealing, the bulkier polymers formed will be adhered to the grain boundaries, retaining the previously established interactions with PbI₂. After perovskite formation, the polymers anchored on the boundaries can effectively passivate undercoordinated lead ions and reduce the defect density. As a result, a champion power conversion efficiency (PCE) of 23.0% is obtained, together with a prolonged lifetime where 85.7% and 91.8% of the initial PCE remain after 504 h continuous illumination and 2208 h shelf storage, respectively.

Near‐infrared (NIR) perovskite light‐emitting diodes (PeLEDs) with comparable operation stability to NIR organic LEDs are demonstrated by incorporation of binary alkali cations. Surface‐distributed Rb⁺ and bulk‐distributed Cs⁺ effectively reduce nonradiative recombination and block I⁻ migration pathways, leading to a record external quantum efficiency of 15.84% in alkali‐cation‐incorporated FAPbI₃‐based PeLEDs and a half‐lifetime over 3600 min.

Abstract

The poor stability of perovskite light‐emitting diodes (PeLEDs) is a key bottleneck that hinders commercialization of this technology. Here, the degradation process of formamidinium lead iodide (FAPbI₃)‐based PeLEDs is carefully investigated and the device stability is improved through binary‐alkalication incorporation. Using time‐of‐flight secondary‐ion mass spectrometry, it is found that the degradation of FAPbI₃‐based PeLEDs during operation is directly associated with ion migration, and incorporation of binary alkali cations, i.e., Cs⁺ and Rb⁺, in FAPbI₃ can suppress ion migration and significantly enhance the lifetime of PeLEDs. Combining experimental and theoretical approaches, it is further revealed that Cs⁺ and Rb⁺ ions stabilize the perovskite films by locating at different lattice positions, with Cs⁺ ions present relatively uniformly throughout the bulk perovskite, while Rb⁺ ions are found preferentially on the surface and grain boundaries. Further chemical bonding analysis shows that both Cs⁺ and Rb⁺ ions raise the net atomic charge of the surrounding I anions, leading to stronger Coulomb interactions between the cations and the inorganic framework. As a result, the Cs⁺–Rb⁺‐incorporated PeLEDs exhibit an external quantum efficiency of 15.84%, the highest among alkali cation‐incorporated FAPbI₃ devices. More importantly, the PeLEDs show significantly enhanced operation stability, achieving a half‐lifetime over 3600 min.

In article number https://doi.org/10.1002/adma.2019080871908087, Rüdiger Berger, Hyunsik Yoon, Kookheon Char, and co‐workers demonstrate the versatility of nanoconfinement on various semiconducting polymers for enhancing vertical charge transport. Both homogeneous and blended semiconducting polymers show more than two orders of magnitude higher charge mobility along the out‐of‐plane direction due to the enhanced crystallinity and compatibility of the polymers through the polymer assembly under nanoconfined geometries.

A nonresonant and broadband hybrid metamaterial is obtained by confining and separating Ni nanoparticles in natural cellulose fibers. The hybrid metamaterial delivers high solar‐to‐water efficiencies from 47.9% to 65.8% for the solar purification of seawater and sewage. The low cost and high scalability of the metamaterial makes it promising for large‐scale solar energy harvesting.

Abstract

Sophisticated metastructures are usually required to broaden the inherently narrowband plasmonic absorption of light for applications such as solar desalination, photodetection, and thermoelectrics. Here, nonresonant nickel nanoparticles (diameters < 20 nm) are embedded into cellulose microfibers via a nanoconfinement effect, producing an intrinsically broadband metamaterial with 97.1% solar‐weighted absorption. Interband transitions rather than plasmonic resonance dominate the optical absorption throughout the solar spectrum due to a high density of electronic states near the Fermi level of nickel. Field solar purification of sewage and seawater based on the metamaterial demonstrates high solar‐to‐water efficiencies of 47.9–65.8%. More importantly, the solution‐processed metamaterial is mass‐producible (1.8 × 0.3 m²), low‐cost, flexible, and durable (even effective after 7 h boiling in water), which are critical to the commercialization of portable solar‐desalination and domestic‐water‐purification devices. This work also broadens material choices beyond plasmonic metals for the light absorption in photothermal and photocatalytic applications.

A method is introduced to experimentally measure the efficiency potential of any neat perovskite film on glass with/without attached transport layers using intensity‐dependent photoluminescence measurements. This approach allows decoupling efficiency losses due to insufficient charge transport, bulk, interface, and surface recombination. These findings also shine light on the ideality factor in perovskite solar cells and thereby fill factor limitations.

Abstract

Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1‐sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non‐radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open‐circuit voltage and the internal quasi‐Fermi level splitting (QFLS), the transport resistance‐free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity‐dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non‐radiative fill factor and open‐circuit voltage loss. It is found that potassium‐passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.

A facile strategy for a vapor‐phase patterning inorganic perovskite single crystal array is demonstrated to fabricate high‐performance micro‐optoelectronic devices. This strategy allows growth of high‐quality inorganic perovskite single crystals with controllable size and location. It provides new opportunities to explore the inorganic perovskite single‐crystal array for various integrated optoelectronics applications.

Abstract

Inorganic perovskite single crystals have emerged as promising vapor‐phase processable structures for optoelectronic devices. However, because of material lattice mismatch and uncontrolled nucleation, vapor‐phase methods have been restricted to random distribution of single crystals that are difficult to perform for integrated device arrays. Herein, an effective strategy to control the vapor‐phase growth of high‐quality cesium lead bromide perovskite (CsPbBr₃) microplate arrays with uniform morphology as well as controlled location and size is reported. By introducing perovskite seeds on substrates, intractable lattice mismatches and random nucleation barriers are surpassed, and the epitaxial growth of perovskite crystals is accurately controlled. It is further demonstrated that CsPbBr₃ microplate arrays can be monolithically integrated on substrates for the fabrication of high‐performance lasers and photodetectors. This strategy provides a facile approach to fabricate high‐quality CsPbBr₃ microplates with controllable size and location, which offers new opportunities for the scalable production of integrated optoelectronic devices.

A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared through defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, but with intrinsic defect concentration largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere. By completely eliminating the labile and expensive components in traditional perovskite solar cells (PSCs), these all‐inorganic PSCs exhibit high photovoltaic performances.

Abstract

The emergence of cesium lead iodide (CsPbI₃) perovskite solar cells (PSCs) has generated enormous interest in the photovoltaic research community. However, in general they exhibit low power conversion efficiencies (PCEs) because of the existence of defects. A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared by defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, while the intrinsic defect concentration is largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere and thus a glovebox is not required. By completely eliminating the labile and expensive components in traditional PSCs, the all‐inorganic PSCs based on CsPbI₃:Br:InI₃ and carbon electrode exhibit PCE and open‐circuit voltage as high as 12.04% and 1.20 V, respectively. More importantly, they demonstrate excellent stability in air for more than two months, while those based on CsPbI₃ can survive only a few days in air. The progress reported represents a major leap for all‐inorganic PSCs and paves the way for their further exploration in order to achieve higher performance.

A new S‐atom‐containing small molecule (TPE‐S) is introduced as a dopant‐free hole‐transporting layer in all‐inorganic and organic/inorganic hybrid perovskite solar cells (PVSCs) with a p–i–n inverted structure, leading to improved power conversion efficiencies of 15.4% and 21%, respectively. In addition, these devices also show enhanced photostability, with performance comparable to state‐of‐the‐art PVSCs based on the conventional n–i–p structure.

Abstract

Designing new hole‐transporting materials (HTMs) with desired chemical, electrical, and electronic properties is critical to realize efficient and stable inverted perovskite solar cells (PVSCs) with a p–i–n structure. Herein, the synthesis of a novel 3D small molecule named TPE‐S and its application as an HTM in PVSCs are shown. The all‐inorganic inverted PVSCs made using TPE‐S, processed without any dopant or post‐treatment, are highly efficient and stable. Compared to control devices based on the commonly used HTM, PEDOT:PSS, devices based on TPE‐S exhibit improved optoelectronic properties, more favorable interfacial energetics, and reduced recombination due to an improved trap passivation effect. As a result, the all‐inorganic CsPbI₂Br PVSCs based on TPE‐S demonstrate a remarkable efficiency of 15.4% along with excellent stability, which is the one of the highest reported values for inverted all‐inorganic PVSCs. Meanwhile, the TPE‐S layer can also be generally used to improve the performance of organic/inorganic hybrid inverted PVSCs, which show an outstanding power conversation efficiency of 21.0%, approaching the highest reported efficiency for inverted PVSCs. This work highlights the great potential of TPE‐S as a simple and general dopant‐free HTM for different types of high‐performance PVSCs.

Publication date: May 2020

Source: Solar Energy Materials and Solar Cells, Volume 208

Author(s): Shuang Ma, Xuepeng Liu, Yunzhao Wu, Ye Tao, Yong Ding, Molang Cai, Songyuan Dai, Xiaoyan Liu, Ahmed Alsaedi, Tasawar Hayat

A simple low‐temperature‐processed In₂O₃/SnO₂ bilayer electron‐transport layer (ETL) is used for fabricating efficient perovskite solar cells (PSCs). The bilayer ETL with appropriate energy alignment is beneficial for charge transfer, thus minimizing open‐circuit voltage (V _OC) loss. An optimized planar PSC with a power conversion efficiency (PCE) of 23.24% is obtained. In contrast, devices based on single SnO₂ only achieve efficiency of 21.42%.

Abstract

An electron‐transport layer (ETL) with appropriate energy alignment and enhanced charge transfer is critical for perovskite solar cells (PSCs). However, interfacial energy level mismatch limits the electrical performance of PSCs, particularly the open‐circuit voltage (V _OC). Herein, a simple low‐temperature‐processed In₂O₃/SnO₂ bilayer ETL is developed and used for fabricating a new PSC device. The presence of In₂O₃ results in uniform, compact, and low‐trap‐density perovskite films. Moreover, the conduction band of In₂O₃ is shallower than that of Sn‐doped In₂O₃ (ITO), enhancing the charge transfer from perovskite to ETL, thus minimizing V _OC loss at the perovskite and ETL interface. A planar PSC with a power conversion efficiency of 23.24% (certified efficiency of 22.54%) is obtained. A high V _OC of 1.17 V is achieved with the potential loss at only 0.36 V. In contrast, devices based on single SnO₂ layers achieve 21.42% efficiency with a V _OC of 1.13 V. In addition, the new device maintains 97.5% initial efficiency after 80 d in N₂ without encapsulation and retains 91% of its initial efficiency after 180 h under 1 sun continuous illumination. The results demonstrate and pave the way for the development of efficient photovoltaic devices.

In article number https://doi.org/10.1002/adma.2019056741905674, Tae‐Hee Han, Yang Yang, and co‐workers suggest an effective solution to mitigate the drawbacks of metal halide perovskites (MHPs) using strategically designed surface‐2D/bulk‐3D heterophased core–shell‐like MHP nanograins for long‐term‐stable light‐emitting diodes.

An efficient control of the film quality and thickness distribution of alternating cations in the interlayer space of 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) quantum wells via incorporation of methylammonium chloride as an additive is demonstrated. The optimized device leads to more efficient charge transport and suppressed nonradiative charge recombination. Consequently, the optimized perovskite solar cell delivers an efficiency of 18.48%.

Abstract

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge‐transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low‐n and high‐n QWs and suppressed nonradiative charge recombination. Therefore, the additive‐treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open‐circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden–Popper, and Dion–Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.