Chen Weijie

A monoatomic layer of Al₂O₃ and oxygen plasma are used to increase the hole concentration and induce ordered columnar growth of Sb₂Se₃ thin film deposited by a RTE method. A solar cell with a new structure of FTO/CdS/P‐Sb₂Se₃/P+‐Sb₂Se₃/Al₂O₃/Au is fabricated and its efficiency is increased to 6.7%.

Antimony selenide (Sb₂Se₃) thin films are attractive light‐absorbing materials for low‐cost and highly efficient thin‐film solar cells. Optimizing columnar growth of the grains and the proper hole concentration will be very helpful for improving the efficiency of Sb₂Se₃ thin‐film solar cells. In this paper, a monoatomic layer of Al₂O₃ prepared by the atomic layer deposition (ALD) method is used to increase the hole concentration of the Sb₂Se₃ film. The increase in the hole concentration is mainly due to the suppression of n‐type defects, such as V _se, or the increase in p‐type defects, such as V _sb. In addition, a simple and environmentally friendly oxygen plasma method is used to modify the CdS film to induce ordered columnar growth of the Sb₂Se₃ film deposited by the rapid thermal evaporation (RTE) method. Finally, an oxygen plasma treatment on CdS and a monoatomic Al₂O₃ layer covering the Sb₂Se₃ was combined to fabricate a solar cell with a new structure, FTO/CdS/P‐Sb₂Se₃/P⁺‐Sb₂Se₃/Al₂O₃/Au, and its efficiency was increased from 2.48% to 6.7%. These simple, nontoxic, and industrially applicable methods provide potential avenues for preparing low‐cost and highly efficient solar cells.

An n‐doped organic‐inorganic hybrid perovskite film is realized directly incorporating AgI into the CH₃NH₃PbI₃ precursor solution. AgI doping resulted in a remarkable increase in the electron mobility by the aligned orientation of MA⁺ ions owing to the Ag⁺‐influenced distribution of the electron cloud density. Consequently, a stable perovskite solar cell with a power conversion efficiency of 20.02% is obtained.

The disparity of hole and electron behavior is a ubiquitous issue in methylammonium lead halide perovskites. The carrier mobility imbalance, which will result in a built‐in electric field thus increase the device resistance, is regarded as one of main limiting factors for the further improvement of device performance in perovskite solar cells (PSCs). Here, we realized an n‐doped organic‐inorganic hybrid perovskite by directly incorporating AgI into the CH₃NH₃PbI₃ precursor solution, to fabricate high‐performance PSCs. AgI doping resulted in a balanced charge transporting owing to a remarkable increase in the electron mobility, which was attributed to the aligned orientation of MA⁺ ions owing to the Ag⁺‐influenced distribution of electron cloud density. Meanwhile, AgI could act as an additive to control the perovskite crystallization with improved crystallinity and film morphology. Consequently, a maximum power conversion efficiency of over 20% is achieved. The finding in this work provides a direction to fabricate high‐performance PSCs by controlling the charge balance via intensive doping technique.

Isomeric acceptors NBDTP‐F_out and NBDTP‐F_in, are synthesized for nonfullerene all‐small‐molecule solar cells (NF‐SMSCs). When blended with the molecular donor BDT3TR‐SF, NBDTP‐F_out achieves a high power conversion efficiency of 11.2% while NBDTP‐F_in shows almost no photovoltaic response. Detailed analyses indicate that the isomery‐dependent miscibility governs the performance, which provides an insight of molecular design for efficient NF‐SMSCs.

Abstract

Nonfullerene polymer solar cells develop quickly. However, nonfullerene small‐molecule solar cells (NF‐SMSCs) still show relatively inferior performance, attributing to the lack of comprehensive understanding of the structure–performance relationship. To address this issue, two isomeric small‐molecule acceptors, NBDTP‐F_out and NBDTP‐F_in, with varied oxygen position in the benzodi(thienopyran) (BDTP) core are designed and synthesized. When blended with molecular donor BDT3TR‐SF, devices based on the two isomeric acceptors show disparate photovoltaic performance. Fabricated with an eco‐friendly processing solvent (tetrahydrofuran), the BDT3TR‐SF:NBDTP‐F_out blend delivers a high power conversion efficiency of 11.2%, ranked to the top values reported to date, while the BDT3TR‐SF:NBDTP‐F_in blend almost shows no photovoltaic response (0.02%). With detailed investigations on inherent optoelectronic processes as well as morphological evolution, this performance disparity is correlated to the interfacial tension of the two combinations and concludes that proper interfacial tension is a key factor for effective phase separation, optimal blend morphology, and superior performance, which can be achieved by the “isomerization” design on molecular acceptors. This work reveals the importance of modulating the materials miscibility by interfacial‐tension‐oriented molecular design, which provides a general guideline toward efficient NF‐SMSCs.

A simple, spin‐coating‐free, and directly scalable drop‐cast method is developed to prepare 2D‐perovskite films at arbitrary shapes. The precursor solutions can self‐assemble into highly oriented 2D‐perovskite films on a preheated substrate, producing perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9%, the highest PCE to date for a solar cell with 2D‐perovskite layers fabricated by a nonspin‐coating method.

Abstract

2D organic–inorganic hybrid Ruddlesden–Popper perovskites have emerged recently as candidates for the light‐absorbing layer in solar cell technology due largely to their impressive operational stability compared with their 3D‐perovskite counterparts. The methods reported to date for the preparation of efficient 2D perovksite layers for solar cells involve a nonscalable spin‐coating step. In this work, a facile, spin‐coating‐free, directly scalable drop‐cast method is reported for depositing precursor solutions that self‐assemble into highly oriented, uniform 2D‐perovskite films in air, yielding perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9% (certified PCE of 14.33% ± 0.34 at 0.078 cm²). This is the highest PCE to date for a solar cell with 2D‐perovskite layers fabricated by nonspin‐coating method. The PCEs of the cells display no evidence of degradation after storage in a nitrogen glovebox for more than 5 months. 2D‐perovskite layer deposition using a slot‐die process is also investigated for the first time. Perovskite solar cells fabricated using batch slot‐die coating on a glass substrate or R2R slot‐die coating on a flexible substrate produced PCEs of 12.5% and 8.0%, respectively.

Spectral engineering and ternary blend approaches were employed to demonstrate an efficient semitransparent polymer solar cell tailored for greenhouse application. The semitransparent device transmits mainly blue and red lights for photosynthesis, and shows a high efficiency of 7.75% with a crop growth factor of 24.8%. Optimal sunlight harvesting in photovoltaics and photosynthesis will be beneficial for future greenhouse application.

Abstract

In this study, a wavelength selective semitransparent polymer solar cell (ST‐PSC) with a proper transmission spectrum for plant growth is proposed for greenhouse applications. A ternary strategy combining a wide bandgap polymer donor with a near‐infrared absorbing nonfullerene acceptor and a high electron mobility fullerene acceptor is introduced to achieve PSCs with power conversion efficiency (PCE) over 10%. The addition of PC₇₁BM into J52:IEICO‐4F binary blend contributes to the suppressed trap‐assisted recombination, enhanced charge extraction, and improved open‐circuit voltage simultaneously. ST‐PSC based on the J52:IEICO‐4F:PC₇₁BM ternary blend shows an optimized performance with PCE of 7.75% and a defined crop growth factor of 24.8%. Such high‐performance ST‐PSC is achieved by carefully engineering the absorption spectrum of the light harvesting materials. As a result, the transmission spectra of the semitransparent devices are well‐matched with the absorption spectra of the photoreceptors, such as chlorophylls, in green plants, which provides adequate lighting conditions for photosynthesis and plant growth, and therefore making it a competitive candidate for photovoltaic greenhouse applications.

To finely tune blend miscibility, a novel chemical tool of steric engineering is proposed. It renders a high PCE over 12% for the polymer with middle steric structure, due to a more balanced blend miscibility without sacrificing charge‐carrier transport. Therefore the steric effect‐induced miscibility (SEIM) as a novel chemical strategy exhibits very simple and promising potential in optimizing morphology.

Abstract

Morphology and miscibility control are still a great challenge in polymer solar cells. Despite physical tools being applied, chemical strategies are still limited and complex. To finely tune blend miscibility to obtain optimized morphology, chemical steric engineering is proposed to systemically investigate its effects on optical and electronic properties, especially on a balance between crystallinity and miscibility. By changing the alkylthiol side chain orientation different steric effects are realized in three different polymers. Surprisingly, the photovoltaic device of the polymerPTBB‐m with middle steric structure affords a better power conversion efficiency, over 12%, compared to those of the polymers PTBB‐o and PTBB‐p with large or small steric structures, which could be attributed to a more balanced blend miscibility without sacrificing charge‐carrier transport. Space charge‐limited current, atomic force microscopy, grazing incidence wide angle X‐ray scattering, and resonant soft X‐ray scattering measurements show that the steric engineering of alkylthiol side chains can have significant impacts on polymer aggregation properties, blend miscibility, and photovoltaic performances. More important, the control of miscibility via the simple chemical approach has preliminarily proved its great potential and will pave a new avenue for optimizing the blend morphology.

The atomic structure and electronic properties of Ruddlesden–Popper (RP) faults and grain boundaries (GBs) in CsPbBr₃ are revealed using electron microscopy and density functional theory calculations. The halide concentration at these planar defects is found to be crucial to their electronic properties. The GBs are predicted to repel electrons and attract holes, whereas the RP faults repel both.

Abstract

To evaluate the role of planar defects in lead‐halide perovskites—cheap, versatile semiconducting materials—it is critical to examine their structure, including defects, at the atomic scale and develop a detailed understanding of their impact on electronic properties. In this study, postsynthesis nanocrystal fusion, aberration‐corrected scanning transmission electron microscopy, and first‐principles calculations are combined to study the nature of different planar defects formed in CsPbBr₃ nanocrystals. Two types of prevalent planar defects from atomic resolution imaging are observed: previously unreported Br‐rich [001](210)∑5 grain boundaries (GBs) and Ruddlesden–Popper (RP) planar faults. The first‐principles calculations reveal that neither of these planar faults induce deep defect levels, but their Br‐deficient counterparts do. It is found that the ∑5 GB repels electrons and attracts holes, similar to an n–p–n junction, and the RP planar defects repel both electrons and holes, similar to a semiconductor–insulator–semiconductor junction. Finally, the potential applications of these findings and their implications to understand the planar defects in organic–inorganic lead‐halide perovskites that have led to solar cells with extremely high photoconversion efficiencies are discussed.

The rapid development in large‐area organic solar cells (OSCs) is reviewed. Materials requirements, modular designs, and printing methods for large‐area OSCs are discussed. By combining thick‐film material systems with efficient modular designs, and then by employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

Abstract

The printing of large‐area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small‐area devices is approaching 15%, whereas the PCE for large‐area devices has also surpassed 10% in a single cell with an area of ≈1 cm². Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large‐area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large‐area fabrication, including knife coating, slot‐die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick‐film material systems with efficient modular designs exhibiting low‐efficiency losses and employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.

Publication date: 20 February 2019

Source: Joule, Volume 3, Issue 2

Author(s): Kai Wang, Dong Yang, Congcong Wu, Joe Shapter, Shashank Priya

Publication date: 20 February 2019

Source: Joule, Volume 3, Issue 2

Author(s): Long Ye, Sunsun Li, Xiaoyu Liu, Shaoqing Zhang, Masoud Ghasemi, Yuan Xiong, Jianhui Hou, Harald Ade

Context & Scale

Summary

Graphical Abstract

A megasonic spray‐coating system is developed for continuous fabrication of highly uniform, and large‐grain MAPbI₃ films. The large‐area MAPbI₃ layer (56.25 cm²) is coated via a megasonic spray‐coating system and the fabricated solar cells achieve a maximum efficiency of 16.9% with an average efficiency of 16.4% for small active area cells and 14.2% for large active area (1 cm²) cells, respectively.

Abstract

A simple, low‐cost, large area, and continuous scalable coating method is proposed for the fabrication of hybrid organic–inorganic perovskite solar cells. A megasonic spray‐coating method utilizing a 1.7 MHz megasonic nebulizer that could fabricate reproducible large‐area planar efficient perovskite films is developed. The coating method fabricates uniform large‐area perovskite film with large‐sized grain since smaller and narrower sized mist droplets than those generated by existing ultrasonic spray methods could be generated by megasonic spraying. The volume flow rate of the CH₃NH₃PbI₃ precursor solution and the reaction temperature are controlled, to obtain a high quality perovskite active layer. The devices reach a maximum efficiency of 16.9%, with an average efficiency of 16.4% from 21 samples. The applicability of megasonic spray coating to the fabrication of large‐area solar cells (1 cm²), with a power conversion efficiency of 14.2%, is also demonstrated. This is a record high efficiency for large‐area perovskite solar cells fabricated by continuous spray coating.

Multichannel strategies involving modulation of the counterions, end‐capped substituents, and dimerization are established to regulate the concentrations of azaphenalene diradicals for the first time. The generated anion‐radicals substantially decrease the work functions of the cathode. The all‐solution‐processed bulk heterojunction organic solar cells fabricated with azaphenalene salts based cathode interfacial layers achieve a high power conversion efficiency over 10%.

Abstract

Self‐doped cathode interfacial layers (CILs) are crucial to enable Ohmic‐like contact between the electrode and organic functional layers and thus profoundly promote the performances of organic optoelectronic devices. Herein, multifarious azaphenalene‐embedded organic salts with variable counterions, substituent groups, and repeating units are prepared, and their impacts on producing homologous diradicals are established. Electron paramagnetic resonance and X‐ray photoelectron spectroscopy studies reveal the existence of free radicals of these azaphenalene salts in the solid state. Density functional theory simulations indicate that the thermal energy of counterion‐induced proton transfer is crucial to produce diradicaloids, which can be manipulated in tailoring the azaphenalene backbones. Noticeably, the formed diradicaloids that are delocalized over the π‐conjugated systems will be beneficial to enhance the carrier density of the matrix and remarkably decrease the work functions of the Al electrode. The all‐solution‐processed bulk heterojunction organic solar cells are fabricated by employing them as CILs, which results in high power conversion efficiency of 10.24% in contrast to the 7.34% of the reference device without CILs.

Protective coating: A tin‐based perovskite solar cell with significantly improved stability to oxidation was prepared by introducing hydroxybenzene sulfonic acid or a salt thereof as an antioxidant additive into the perovskite precursor solution. The resulting perovskite grains are encapsulated by a SnCl₂–additive complex layer.

Abstract

Tin‐based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin‐based PSCs because Sn²⁺ in perovskites can be easily oxidized to Sn⁴⁺ upon air exposure. Here we report the fabrication of air‐stable FASnI₃ solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl₂. The interaction between the sulfonate group and the Sn²⁺ ion enables the in situ encapsulation of the perovskite grains with a SnCl₂–additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin‐based PSCs.

Large‐scale flexible photodetector arrays are fabricated based on patterned CH₃NH₃PbI_{3− x} Cl _x film. In addition, the device, with outstanding optoelectronic performance and excellent electrical stability, is applied to capture a real‐time light trajectory and detect a multipoint light distribution, indicating that it has widespread potential in photosensing and imaging for optical communication, imaging, and artificial electronic skin applications.

Abstract

The quest for novel deformable image sensors with outstanding optoelectronic properties and large‐scale integration becomes a great impetus to exploit more advanced flexible photodetector (PD) arrays. Here, 10 × 10 flexible PD arrays with a resolution of 63.5 dpi are demonstrated based on as‐prepared perovskite arrays for photosensing and imaging. Large‐scale growth controllable CH₃NH₃PbI_{3− x} Cl _x arrays are synthesized on a poly(ethylene terephthalate) substrate by using a two‐step sequential deposition method with the developed Al₂O₃‐assisted hydrophilic–hydrophobic surface treatment process. The flexible PD arrays with high detectivity (9.4 × 10¹¹ Jones), large on/off current ratio (up to 1.2 × 10³), and broad spectral response exhibit excellent electrical stability under large bending angle (θ = 150°) and superior folding endurance after hundreds of bending cycles. In addition, the device can execute the functions of capturing a real‐time light trajectory and detecting a multipoint light distribution, indicating that it has widespread potential in photosensing and imaging for optical communication, digital display, and artificial electronic skin applications.

Mixed lead–tin perovskites are shown to be efficient near‐infrared light emitters with tunable emission wavelengths from 850 to 950 nm. Devices based on MAPb_0.6Sn_0.4I₃ films with 4‐fluorobenzylammonium iodide additives reach external quantum efficiency of 5%. It is revealed that there is no phase separation during the operation of devices with various Pb:Sn ratios and different mixed iodide and bromide compositions.

Abstract

Near‐infrared (NIR) light‐emitting diodes (LEDs), with emission wavelengths between 800 and 950 nm, are useful for various applications, e.g., night‐vision devices, optical communication, and medical treatments. Yet, devices using thin film materials like organic semiconductors and lead based colloidal quantum dots face certain fundamental challenges that limit the improvement of external quantum efficiency (EQE), making the search of alternative NIR emitters important for the community. In this work, efficient NIR LEDs with tunable emission from 850 to 950 nm, using lead–tin (Pb‐Sn) halide perovskite as emitters are demonstrated. The best performing device exhibits an EQE of 5.0% with a peak emission wavelength of 917 nm, a turn‐on voltage of 1.65 V, and a radiance of 2.7 W Sr⁻¹ m⁻² when driven at 4.5 V. The emission spectra of mixed Pb‐Sn perovskites are tuned either by changing the Pb:Sn ratio or by incorporating bromide, and notably exhibit no phase separation during device operation. The work demonstrates that mixed Pb‐Sn perovskites are promising next generation NIR emitters.