吴晓晓

Metallic lithium is applied to n-type crystalline silicon wafers and proven to be an excellent electron-selective, hole-blocking transport layer, yielding a low contact resistivity. By implementing a full-area electron-selective lithium contact, n-type c-Si solar cells with a fill factor of 81%, and efficiency of 19% are achieved.

Abstract

Separating photogenerated charge carriers by carrier-selective heterostructure contacts rather than by doped homojunctions is a promising pathway to approach the theoretical power conversion efficiency (PCE) limit of crystalline silicon (c-Si) solar cells. An electron-selective, hole-blocking lithium contact for c-Si solar cells is presented by simple thermal evaporation of air-stable Li₃N powder. It is found that this lithium contact introduces only a minimal Schottky-barrier height for electron transport at its interface with lightly doped n-type c-Si surfaces, resulting in a low contact resistivity of 12.8 mΩ cm². By implementing a full-area electron-selective lithium contact, an n-type c-Si solar cell with a PCE of 19% is achieved, representing a 4% absolute PCE improvement over reference devices with an aluminum contact. The choices of electron-selective contact materials for photovoltaic devices, using simple, scalable fabrication methods are extended.

This study reveals that Cu may act as a suitable electrode material for perovskite solar cells judging from both the in situ photoemission spectroscopy results and prototype device tests. The detailed insight obtained from the fundamental understanding can provide essential guidelines to advance the development of superior perovskite solar cells.

Abstract

In this study, the electronic properties and chemical stability of the Cu/CH₃NH₃PbI₃ interface are investigated in situ by a combination of X-ray photoelectron spectroscopy and synchrotron radiation photoemission spectroscopy (SRPES). The morphology of Cu deposited perovskite surface is monitored by scanning electron microscopy. The results show that the Cu/CH₃NH₃PbI₃ interface is very stable and no chemical reaction between Cu and the perovskite takes place. Moreover, a 0.45 eV interface dipole and a 0.15 eV upward band bending are obtained at the Cu/CH₃NH₃PbI₃ interface. Based on these fundamental findings, a prototype of Cu/CH₃NH₃PbI₃/NiO _x /indium tin oxide solar cell device is constructed to check the power conversion efficiency (PCE) and device stability. Although no electron transport material is used in this device, it still exhibits decent performance. The PCE of the device reaches up to 9.99% and remains almost unchanged over a long-time (49 d) storage in a N₂-filled glovebox. Through this study it is demonstrated that fundamental understanding of the interfacial structure of a perovskite solar cell is essential in pursuit of rational design of superior perovskite solar cells, and moreover, Cu is a promising electrode candidate for perovskite solar cells.

A new non-fullerene acceptor named BTP1O-4Cl-C12 which contains chlorinated end groups, extended inner side chains and asymmetric alkyl and alkoxy outer side chains is reported. These modifications help BTP1O-4Cl-C12-based devices achieve high efficiency of 17.1% and show its potential application in ternary organic solar cells.

Abstract

Chemical modifications of non-fullerene acceptors (NFAs) play vital roles in the development of high efficiency organic solar cells (OSCs). In this work, on the basis of the previously reported molecule named Y6-1O, chlorination and inner side-chain engineering are adopted to endow the corresponding devices with higher open-circuit voltage (V _OC) and short-circuit current density (J _SC) as well as good morphology for high fill factor (FF). As a result, the molecule named BTP1O-4Cl-C12 can help achieve a higher power conversion efficiency (PCE) of 17.1% than that of Y6-1O (16.1%). Furthermore, the following comparisons between BTP1O-4Cl-C12 and the two symmetric acceptors named BTP2O-4Cl-C12 and BTP-4Cl-C12 demonstrate the effect of asymmetric alkoxy substitution on the outer side chains, which not only achieves a balance between V _OC and J _SC, but also help obtain appropriate morphology for efficient charge dissociation and suppressed charge recombination. Therefore, the asymmetric BTP1O-4Cl-C12 can achieve a higher PCE compared to the symmetric BTP2O-4Cl-C12 and BTP-4Cl-C12. The work not only reports an excellent NFA for high-performance OSCs, but also puts forward a series of methods for consecutive chemical modifications on Y-series acceptors, which can be further applied to boost the PCE of OSCs to a higher level.

In article number 2100487, Ignacio Mínguez-Bacho and co-workers report the fabrication of highly ordered nanoporous transparent electrodes using nanosphere lithography and anodization. The self-assembly methods used result in perfect hexagonal order over domains of thousands of square micrometers. Solar cells made from them reach significantly improved performance compared to their disordered counterparts.

Herein, a novel ionic liquid solvent methylammonium acetate (MAAc) is introduced to prepare high-quality MAPbBr₃ perovskite films through one-step air-processing without antisolvent treatment. These films with excellent optical and moisture stability exhibit high efficiency and low threshold amplified spontaneous emission (ASE) under the irradiation of a nanosecond laser, which show great potential for stable perovskite lasers.

Abstract

The poor stability, in particular with respect to temperature, moisture, and light exposure, remains a ubiquitous impediment virtually for metal halide perovskite materials and devices in their future practical application. Herein, from the perspective of precursor solution chemistry, ionic liquid solvent methylammonium acetate (MAAc) is introduced to prepare high-quality MAPbBr₃ perovskite thin films in a one-step air-processing process without anti-solvent treatment. Due to formation of pinhole-free, uniform, and compact MAPbBr₃ perovskite film, excellent amplified spontaneous emission (ASE) with high emission efficiency and low threshold is obtained under nanosecond laser. Furthermore, the prepared MAPbBr₃ perovskite exhibits excellent two-photon induced ASE with a low threshold of 100 µJ cm⁻² under 800 nm femtosecond laser excitation. More importantly, in comparison with the traditional MAPbBr₃ films prepared with N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), the MAPbBr₃ film prepared with MAAc shows excellent optical stability: no signs of degradation under more than 2 h pulsed laser excitation, stable ASE emission spectra under the humidity of 95% and ASE spectra can be stimulated when films are kept in air for more than 6000 h without encapsulation.

Incorporation of thus far unreported 1D-Tpy₂Pb₃I₆ and 2D-TpyPb₃I₆ perovskites allows to form a multidimension coupled 1D-2D-3D perovskite, which is proved to effectively release the residual strain whilst improving the carriers transport ability. Accordingly, the as-fabricated 1D-2D-3D hybrid CsPbI₂Br perovskite solar cells demonstrate high power conversion efficiency whilst extraordinary stability within 1066 h in ambient atmosphere.

Abstract

Despite the rapid development of CsPbI _x Br_{3− x} (0 ≤ x ≤ 3) inorganic perovskite solar cells, associated with their superior thermal stability, their low moisture stability limits their commercial deployment. In this study, 1D-2D-3D multidimensional coupled perovskites are prepared by means of an in situ self-integration approach. This pioneering method allows incorporating thus far unreported 1D-Tpy₂Pb₃I₆ and 2D-TpyPb₃I₆ (Tpy; terpyridine) perovskites. Heterojunction perovskites demonstrate superior stability against water in comparison with control 3D CsPbI₂Br, which is related to the hydrophobicity of low-dimension (LD) perovskites. Remarkably, the spontaneous involvement of LD perovskites can adjust/reconstruct the interfacial structure. This modification allows releasing the residual strain, establishing effective charge transfer channels that increase the carrier transport ability. Accordingly, 1D-2D-3D hybrid CsPbI₂Br perovskite solar cells demonstrate a stabilized power conversion efficiency as high as 16.1%, which represents a very significant improvement, by a factor of 43%, with respect to control 3D CsPbI₂Br perovskite solar cell. Equally importantly, the multidimensional coupled perovskite solar cells exhibit extraordinary stability, well above 1000 h in ambient atmosphere.

Three cyanoacetate-containing donor-acceptor compounds (CA, CAMA, CAFA) are designed for perovskite modifications, combining surface passivation and secondary grain growth for synergistic effects. With proper selection of cation, the optimal CAMA contributes to lower energy barriers and fewer trap states, thus accounting for CAMA-treated PSCs with higher efficiency and better stability than the reference cells.

Abstract

Interfacial engineering methods have been developed to solve defect issues of perovskite solar cells (PSCs). However, traditional surface passivation has limited effects on eliminating defect-forming residuals, while secondary grain growth (SGG) is restricted by limited choices of additives and intrinsic properties of perovskites. Here, a pincer strategy of taking advantages of surface passivation and SGG is proposed to modify both exterior and interior of CH₃NH₃PbI₃ (MAPbI₃) perovskite, by employing cyanoacetate-containing donor-acceptor compounds (CA-D-A) including 2-cyano-3-(3,4,5-trimethoxyphenyl)acrylic acid (CA), methanaminium 2-cyano-3-(3,4,5-trimethoxyphenyl)acrylate (CAMA), and aminomethaniminium (Z)-2-cyano-3-(3,4,5-trimethoxyphenyl)acrylate (CAFA). In comparison to untreated perovskite, CA-D-A treated perovskites present better crystallinity because of SGG, lower trap densities due to the synergistic effect of surface passivation and SGG, and tuned energy levels induced by CA-D-A. Accordingly, the CA-D-A treated MAPbI₃-based PSCs exhibit higher open-circuit voltage and fill factor than the control PSC without any treatment, leading to improved power conversion efficiency (PCE) and enhanced device stability, especially the CAMA treated PSCs with an average PCE promoted from 17.77 (control PSCs) to 18.71%, and importantly an excellent PCE of 19.71% through further optimization. This work provides an effective strategy for developing highly efficient and stable PSCs with the assistance of both surface passivation and SGG.

In article number 2007011, Wei Ma and co-workers achieve simultaneous enhancement of efficiency, deformability and mechanical stability of organic solar cells by modulating the intermolecular interaction through incorporating a ductile third component. This strengthened intermolecular interaction enables the optimized ternary system with better morphology stability against force and heat.

A stable α-FAPbI₃ perovskite phase is achieved by employing benzylammonium iodide (BzI), which is elucidated by solid-state NMR spectroscopy and X-ray scattering measurements to obtain perovskite solar cells based on the FAPbI₃(BzI)_0.25 composition with power conversion efficiencies exceeding 20% accompanied by enhanced shelf-life and operational stability, maintaining 80% of the performance after one year at ambient conditions.

Abstract

There is an ongoing surge of interest in the use of formamidinium (FA) lead iodide perovskites in photovoltaics due to their exceptional optoelectronic properties. However, thermodynamic instability of the desired cubic perovskite (α-FAPbI₃) phase at ambient conditions leads to the formation of a yellow non-perovskite (δ-FAPbI₃) phase that compromises its utility. A stable α-FAPbI₃ perovskite phase is achieved by employing benzylammonium iodide (BzI) and the microscopic structure is elucidated by using solid-state NMR spectroscopy and X-ray scattering measurements. Perovskite solar cells based on the FAPbI₃(BzI)_0.25 composition achieve power conversion efficiencies exceeding 20%, which is accompanied by enhanced shelf-life and operational stability, maintaining 80% of the performance after one year at ambient conditions.

Ultrathin and ultra-lightweight organic solar cells (total thickness of less than 3 μm) with a stabilized power conversion efficiency of 15.5% and unprecedented power-per-weight of 32.07 W g⁻¹ at a weight of 4.83 g m⁻² are realized, which could be applied to almost any surface of wearable electronic devices, and can withstand the associated mechanical deformation.

Abstract

Ultraflexible and ultra-lightweight organic solar cells (OSCs) have attracted great attention in terms of power supply in wearable electronic systems. Here, ultrathin and ultra-lightweight OSCs, with a total thickness of less than 3 µm, with excellent mechanical properties in terms of their flexibility and ability to be stretched are demonstrated. A stabilized power conversion efficiency (PCE) of 15.5% and unprecedented power-per-weight of 32.07 W g⁻¹ at a weight of 4.83 g m⁻² is achieved, which represents one of the best-performing OSCs based on ultrathin foils substrate reported to date. The ternary strategy introduces the third component of amorphous conformation of the PC₇₁BM molecule, which can slightly reduce crystallization and aggregates without decreasing the electron mobility, thereby reducing rigidity and brittleness of the active layer. The increase in the ductility of the active layer significantly improves the mechanical flexibility of the device, resulting in over 90% retention in the PCE after 200 stretching–compression cycles. In addition, the ternary device exhibits excellent stability when stored in a N₂-filled glove box, resulting in the PCE retaining over 95% of its initial efficiency even after 1000 h. This ultraflexible and ultra-lightweight photovoltaic foils constitute a major step toward the integration of power supply into malleable electronic textiles.

Mechanical stability is essential for thin polymer functional films and their use in multilayer devices. However, many polymer films are prone to instability when in contact with a simple water drop. In article number 2009039, Hamid Kellay and co-workers show that the swelling of the films in contact with a liquid governs this instability and its phase diagram. Understanding the origin of the instability allows to control the quality of the films.

A novel method for subcell selective analysis of perovskite/Si tandem solar cells using a bichromatic light emitting diode (LED) light source is presented. Based on programmable LED biasing, light intensity-dependent measurements for each subcell can be conducted and subcell parameters extracted. Using a one-diode model for each subcell, their J−V characteristics can be reconstructed, providing important information about tandem operation.

In monolithic tandem solar cells, current−voltage (J−V) characteristics of subcells provide invaluable information about their quality and tandem operation. However, accessing the subcell J−Vs is challenging and requires sophisticated spectral methods. Herein, a customized, bichromatic light emitting diode setup (BCLED) for in-depth analysis of tandem solar cells, suitable for subcell operation analysis, and long-term stability testing is presented. For this, two spectrally independent LED arrays are used to selectively bias the two subcells. The power of the developed setup is demonstrated by successfully disentangling the tandem J−V curve into subcell J−V curves. The method is based on a one-diode model for each subcell and is validated by electrical simulations. Afterward, it is used on a fabricated 27.6% efficient perovskite/silicon tandem device, resulting in great agreement with the measured J−V curve. Therefore, the BCLED setup is a versatile tool, suitable for subcell characteristics and long-term stability analysis of tandem solar cells.

Hydrophobic organic ammonium halides (Cl, Br, and I) are used for the modification of inorganic CsPb(I_0.75Br_0.25)₃ perovskite solar cells. Benefiting from their passivation effects and hydrophobic long alkyl chain, the modified devices exhibit enhanced efficiency and stability. Among them, the hexadecyltrimethylammonium chloride (CTAC)-modified device shows the best performance with a power conversion efficiency (PCE) of 18.05%.

Inorganic cesium lead halide perovskite solar cells are promising candidates for next-generation photovoltaic applications. However, their phase instability and relatively low efficiency hinder their commercialization. Herein, hydrophobic organic ammonium halides (Cl, Br, and I) are rationally used for the modification of inorganic CsPb(I_0.75Br_0.25)₃ perovskite solar cells. Benefiting from their passivation effects and hydrophobic long alkyl chain, the modified devices exhibit enhanced efficiency and stability. Among them, the hexadecyltrimethylammonium chloride (CTAC)-modified device shows the best performance with a power conversion efficiency (PCE) of 18.05%. Furthermore, a gradient triple anion inorganic perovskite CsPb(I_0.75Br_0.25)_3−x Cl _x layer is formed in situ during the CTAC modification, which demonstrates better phase stability than CsPb(I_0.75Br_0.25)₃. As a result, the modified device also shows excellent stability, maintaining 94% of the initial efficiency after 35 days in N₂ atmosphere.

A series of 2D metal halide perovskite (MHP) nanosheets are synthesized and used as photocatalysts for CO₂ reduction without any organic sacrificial agents They display significantly improved stability and activity in comparsion with traditional MHP nanocrystals (NCs) in water-contained reaction system.

Metal halide perovskite (MHP) nanocrystals (NCs) have attained ever-increasing attention in the field of artificial photosynthesis, due to their desirable strong light-harvesting capacity and long lifetime of photogenerated carriers. However, the well-known inherent instability and inferior activity of MHP NCs limit their application foreground in photocatalysis. Herein, a series of 2D MHP nanosheets have been synthesized based on a facile approach at room temperature. These 2D materials are used as photocatalysts for the photocatalysis of CO₂ reduction without any organic sacrificial agents, which display significantly improved stability compared to traditional MHP NCs in water-contained reaction system. More importantly, benefitting from the large proportion of low-coordinated metal atoms and short carrier diffusion distance, these MHP nanosheets exhibit a remarkably increased performance for photocatalytic CO₂ reduction. The judicious modulation of halide component endows the MHP nanosheets with a highest electron consumption rate of 87.8 μmol g⁻¹ h⁻¹ among the reported pure MHP catalysts for photocatalytic CO₂ conversion, which is over seven times higher than that of traditional MHP NCs.

Suns-photoluminescence approach is used to spatially resolve open-circuit voltages and pseudo-fill factors of finished and partially finished perovskite solar cells at as-prepared and degraded states. This method is used to predict the cell performance from material selection until completion early. It also allows monitoring of the uniformity and repeatability of the cell fabrication process.

Early prediction of spatially resolved performance of perovskite solar cells (PSCs) is essential for process monitoring, control and fault diagnosis, and upscaling of this emerging technology. Herein, a fast, nonde structive, contactless imaging-based approach is developed to visualize the spatial distribution of possible light current density−voltage (pseudo-J−V) curves on finished and partly finished cells. This allows for the extraction of other critical spatially resolved properties including implied open-circuit voltage and pseudo-fill factor. The technique is applied to systematically investigate various degradation behaviors on PSCs including thermal stability, light soaking, and ambient air exposure. Finally, it is used to predict pseudo-J−V curves of various perovskite films with different compositions. These results demonstrate the significant value of this fast imaging technique for the research and development of PSCs ranging from material selection, process optimization, to degradation study.

The processing–property–performance correlation of a PPDT2FBT:PC₆₁BM blend system processed from a green solvent, ortho-xylene, is investigated in comparison with a traditional halogenated solvent, chlorobenzene. Green-solvent-processing devices show noteworthy higher power conversion efficiencies owing to the increased interfacial areas, favored orientation of PC₆₁BM in the vicinity of the PPDT2FBT aromatic core, faster extraction of charge carries, and suppressed recombination losses.

The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so-called green solvents during the fabrication processes. However, the effects of such green solvents on the molecular-level structures and optoelectronic properties lack in-depth characterization. Here, insights into the structure–processing–property correlation of a PPDT2FBT:PC₆₁BM bulk-heterojunction (BHJ) system processed from a green solvent, ortho-xylene (o-XY), is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The BHJ blends are characterized with various techniques probing at difference length scales, and an increased donor:acceptor (D:A) interfacial area as well as well-mixed features in the bulk morphologies of the active layer are observed for the o-XY processed BHJ blend. Furthermore, molecular-level differences in the D–A intermolecular interactions at the BHJ interfaces in o-XY and CB cast films are elucidated by 2-dimensional solid-state nuclear magnetic resonance (ssNMR) measurements and analysis. These results are consistent with the device properties, suggesting that the green-solvent-processed devices have longer charge carrier lifetimes and faster charge carrier extraction. The optimized PPDT2FBT:PC₆₁BM devices processed from o-XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short-circuit current density and fill factor.

This article reports two low-cost A-DA′D-A type non-fullerene acceptors (Y25,Y26) with pentacyclic fused backbone as the DA′D electron-deficient core and explains the reason why the PCE of the Y26-based device is higher than that of Y25 through the analysis of morphology and physicochemical properties. It would be a reference for the further design of non-fullerene acceptors with high performance.

Recent studies have almost focused on finding active layer materials with extended π-conjugation structures for high-performance organic solar cells (OSCs). However, with the extension of conjugate length, the synthesis difficulty and cost of materials will increase. Achieving high efficiency while reducing material costs is a prerequisite for the commercialization of OSCs. Herein, two low-cost A-DA′D-A-type (where A and D represent an electron-withdrawing unit and an electron-donating unit, respectively) nonfullerene acceptors (Y25,Y26) are synthesized with pentacyclic fused backbone as the DA′D electron-deficient core and 5,6-difluoro-3-(dicyandiamethyl) indigo as the end groups. Compared with classical Y series acceptors with heptacyclic backbone, although Y25 and Y26 own the reduced conjugated length, they still show moderate performance (11.65% and 13.34%), and the cost of synthesis is significantly reduced. Therefore, we provide a new molecular design idea for commercially efficient nonfullerene OSCs acceptors. We also find that adding alkyl chains to the β site of thiophenes is beneficial to obtaining the reduced energetic disorder, dominant molecular stacking, and desirable morphology, which can facilitate charge carrier transport and prompt higher short-circuit current density (J _sc) as well as fill factor.

In article number 2007772, Xiaoyang Zhu, Hongbo Lan, and co-workers develop a maskless, templateless, and plating-free fabrication approach for high-performance flexible transparent electrodes with an embedded silver mesh by combining electric-field-driven microscale 3D printing and hybrid hot-embossing. The fabricated flexible, transparent electrode exhibits excellent photoelectric properties, remarkable mechanical stability, and environmental adaptability.

A self-sufficient micrometer-level vacuum growth chamber based on encapsulated MAPbBr₃ microcrystals is designed by Rui Chen, Zhipeng Wei, and co-workers, as described in article number 2100466. Perovskite materials with enhanced environmental, thermal, and optical stability through the reduction of deep-level trap states due to self-structural healing are observed. This greatly improves the lasing performance and service cycle.

This review summarizes the state-of-the-art electrical and optical characteristics of typical perovskite photodetectors. The electrical manipulations with advanced device structures and optical manipulations with artificial photonic nanostructures are detailed to improve light absorption, photoelectric conversion, and carrier transmission performance in perovskite photodetectors. This review aims to provide strategies to achieve high-performance photodetectors.

Abstract

Photodetectors built from conventional bulk materials such as silicon, III–V or II–VI compound semiconductors are one of the most ubiquitous types of technology in use today. The past decade has witnessed a dramatic increase in interest in emerging photodetectors based on perovskite materials driven by the growing demands for uncooled, low-cost, lightweight, and even flexible photodetection technology. Though perovskite has good electrical and optical properties, perovskite-based photodetectors always suffer from nonideal quantum efficiency and high-power consumption. Joint manipulation of electrons and photons in perovskite photodetectors is a promising strategy to improve detection efficiency. In this review, electrical and optical characteristics of typical types of perovskite photodetectors are first summarized. Electrical manipulations of electrons in perovskite photodetectors are discussed. Then, artificial photonic nanostructures for photon manipulations are detailed to improve light absorption efficiency. By reviewing the manipulation of electrons and photons in perovskite photodetectors, this review aims to provide strategies to achieve high-performance photodetectors.

The in situ optical spectra reveal a significantly prolonged crystallization window during the perovskite deposition via additive strategy. Finer thickness gradient by n values in the direction orthogonal to the substrate leads to more efficient charge transport between quantum wells and suppressed charge recombination in the additive-treated film. Finally, the power conversion efficiency of 14.4% is obtained.

Abstract

New structural type of 2D AA′ _{n −1}M _n X_{3 n +1} type halide perovskites stabilized by symmetric diammonium cations has attracted research attention recently due to the short interlayer distance and better charge-transport for high-performance solar cells (PSCs). However, the distribution control of quantum wells (QWs) and its influence on optoelectronic properties are largely underexplored. Here effective phase-alignment is reported through dynamical control of film formation to improve charge transfer between quantum wells (QWs) for 2D perovskite (BDA)(MA) _{n -1}Pb _n I_{3 n +1} (BDA = 1,4-butanediamine, 〈n〉 = 4) film. The in situ optical spectra reveal a significantly prolonged crystallization window during the perovskite deposition via additive strategy. It is found that finer thickness gradient by n values in the direction orthogonal to the substrate leads to more efficient charge transport between QWs and suppressed charge recombination in the additive-treated film. As a result, a power conversion efficiency of 14.4% is achieved, which is not only 21% higher than the control one without additive treatment, but also one of the high efficiencies of the low-n (n ≤ 4) AA′ _{n −1}M _n X_{3 n +1} PSCs. Furthermore, the bare device retains 92% of its initial PCE without any encapsulation after ambient exposure for 1200 h.

Nature Energy, Published online: 25 May 2021; doi:10.1038/s41560-021-00838-1

Nature, Published online: 26 May 2021; doi:10.1038/d41586-021-01331-1

Nature, Published online: 26 May 2021; doi:10.1038/s41586-021-03492-5