Chen Weijie

A simple perovskite solar cell architecture, which is based on dopant‐free electron and hole conductors and carbon back contact deposited at room temperature, is demonstrated. The resulting architecture leads to the fabrication of cheap and highly efficient perovskite solar cells exhibiting unprecedented long‐term operational and UV stability thus hold immense potential for large‐scale deployment.

Abstract

Today's perovskite solar cells (PSCs) mostly use components, such as organic hole conductors or noble metal back contacts, that are very expensive or cause degradation of their photovoltaic performance. For future large‐scale deployment of PSCs, these components need to be replaced with cost‐effective and robust ones that maintain high efficiency while ascertaining long‐term operational stability. Here, a simple and low‐cost PSC architecture employing dopant‐free TiO₂ and CuSCN as the electron and hole conductor, respectively, is introduced while a graphitic carbon layer deposited at room temperature serves as the back electrical contact. The resulting PSCs show efficiencies exceeding 18% under standard AM 1.5 solar illumination and retain ≈95% of their initial efficiencies for >2000 h at the maximum power point under full‐sun illumination at 60 °C. In addition, the CuSCN/carbon‐based PSCs exhibit remarkable stability under ultraviolet irradiance for >1000 h while under similar conditions, the standard spiro‐MeOTAD/Au based devices degrade severely.

The formation of an inversion layer within n‐Si near the interface with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) based conductive thin films is evidenced. High power conversion efficiency in solar cells is correlated with a large contact‐induced band bending in Si, high polymer conductivity, and proper Si interfacial passivation.

Abstract

Heterojunctions formed by ultrathin conductive polymer [poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate)—PEDOT:PSS] films and n‐type crystalline silicon are investigated by photoelectron spectroscopy. Large shifts of Si 2p core levels upon PEDOT:PSS deposition provide evidence that a dopant‐free p–n junction, i.e., an inversion layer, is formed within Si. Among the investigated PEDOT:PSS formulations, the largest induced band bending within Si (0.71 eV) is found for PH1000 (high PEDOT content) combined with a wetting agent and the solvent additive dimethyl sulfoxide (DMSO). Without DMSO, the induced band bending is reduced, as is also the case with a PEDOT:PSS formulation with higher PSS content. The interfacial energy level alignment correlates well with the characteristics of PEDOT:PSS/n‐Si solar cells, where high polymer conductivity and sufficient Si‐passivation are also required to achieve high power conversion efficiency.

Here, an interface‐engineered perovskite 2D|3D‐heterojunction structure is employed to realize a multifunctional photonic device on‐chip, exhibiting power conversion efficiencies of up to 21.02% under AM1.5, and an external quantum efficiency for the light emitting diode up to 5.13%. This novel phenomenon is attributed to carrier transfer resulting in a high carrier density and enhanced carrier recombination at the 2D|3D interface.

Abstract

Although 2D|3D has shown potential for application in multifunctional devices, the principle of operation for multifunction devices (SOLAR Cell‐LED: SOLED) has not yet been revealed. However, most studies have reported that the devices have only one auspicious characteristic. Here in this study the SOLED devices are monitored and investigated in a 2D|3D heterostructure with a multidimensional perovskite. It is fond that a 2D|3D heterostructure with a multidimensional perovskite interface induces carrier transmission from the interface, increasing the density of electrons and holes, and increasing their recombination. An interface‐engineered perovskite 2D|3D‐heterojunction structure is employed to realize the multifunctional photonic device in on‐chip, exhibiting overall power conversion efficiencies of photovoltaics up to 21.02% under AM1.5, and external quantum efficiency of the light‐emitting diode up to 5.13%. This novel phenomenon is attributed to carrier transfer resulting in a high carrier density and enhanced carrier recombination at the 2D|3D interface.

Recently, sequential deposition of donor and acceptor layers has been demonstrated to be an alternative method to fabricate highly efficient bulk‐heterojunction organic solar cells. A simple “needle” model to simulate its morphology indicates a different morphological requirement which rationalizes the high exciton dissociation efficiency.

Abstract

Bulk heterojunction (BHJ) nonfullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq‐BHJ) have recently been shown to be highly efficient, environmentally friendly, and compatible with large area and roll‐to‐roll fabrication. However, the related photophysics at donor‐acceptor interface and the vertical heterogeneity of donor‐acceptor distribution, critical for exciton dissociation and device performance, have been largely unexplored. Herein, steady‐state and time‐resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlating with the luminescent efficiency of interfacial states and its nonradiative recombination, interfacial trap states are characterized to be about 40% more populated in the sq‐BHJ devices than the as‐cast BHJ (c‐BHJ), which probably limits the device voltage output. Cross‐sectional energy‐dispersive X‐ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly visualize the donor–acceptor vertical stratification with a precision of 1–2 nm. From the proposed “needle” model, the high exciton dissociation efficiency is rationalized. This study highlights the promise of sequential deposition to fabricate efficient solar cells, and points toward improving the voltage output and overall device performance via eliminating interfacial trap states.

Mixed A₁ cations are employed in layered two‐dimensional perovskites to investigate the interplay between alkylamine cations and unsaturated alkylamine cations with π‐electrons. It is revealed that alkylamine spacer cations are able to facilitate precursor assembly, which results in the orientated growth of perovskite crystals. Unsaturated alkylamine cations lead to reduced exciton binding energy, which improves the carrier pathway.

Abstract

Organic spacer cations in layered 2D (A₁)₂(A₂) _{n −1}B _n X_{3 n +1} (where A₁ is an organic cation acting as a spacer between the perovskite layers, A₂ is a monovalent cation, e.g., Cs⁺,CH₃NH₃ ⁺, CH(NH₂)₂ ⁺) perovskite materials improve the long‐term stability of the resulting solar cells, but hamper their power conversion efficiency due to poor carrier generation/transportation. Rational guidelines are thus required to enable the design of organic spacer cations. Herein, mixed A₁ cations are employed in layered 2D perovskites to investigate the interplay between alkylamine cations and unsaturated alkylamine cations. It is revealed that alkylamine spacer cations are able to facilitate precursor assembly, which results in the orientated growth of perovskite crystals. Unsaturated alkylamine cations further lead to reduced exciton binding energy, which improves carrier pathway in the 2D perovskites. By mixing both cations, substantially improved open circuit voltage is observed in the resultant photovoltaic cells with the efficiency of 15.46%, one of the highest one based on (A₁)₂(A₂)₃Pb₄I₁₃ layered 2D perovskites. The generality of the design principle is further extended to other cation combinations.

Publication date: 18 December 2019

Source: Joule, Volume 3, Issue 12

Author(s): Chenchen Yang, Dianyi Liu, Richard R. Lunt

Graphical Abstract

The acid treatment of TiO₂ weakens the bonding of octahedral chains in anatase TiO₂, rendering the formation of amorphous TiO₂ buffer layer on the surface of anatase TiO₂. This amorphous TiO₂ buffer layer contains rich oxygen vacancies, which increase the donor density of TiO₂.

Abstract

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high‐efficiency solar energy conversion. Now, a simple yet robust acid‐treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron‐transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂‐coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂‐based devices, the perovskite solar cells (PSCs) with acid‐treated TiO₂ ETL exhibit an enhanced short‐circuit current and power conversion efficiency.

Hybrid antiperovskites are expected to have the same potential in producing novel properties as hybrid perovskites. However, the known hybrid antiperovskites are very few. Now the design, synthesis, and characterization of seven ABX₃‐type hybrid antiperovskites is presented.

Abstract

Substitution of A‐site and/or X‐site ions of ABX₃‐type perovskites with organic groups can give rise to hybrid perovskites, many of which display intriguing properties beyond their parent compounds. However, this method cannot be extended effectively to hybrid antiperovskites. Now, the design of hybrid antiperovskites under the guidance of the concept of Goldschmidt's tolerance factor is presented. Spherical anions were chosen for the A and B sites and spherical organic cations for the X site, and seven hybrid antiperovskites were obtained, including (F₃(H₂O) _x )(AlF₆)(H₂dabco)₃, ((Co(CN)₆)(H₂O)₅)(MF₆)(H₂dabco)₃ (M=Al³⁺, Cr³⁺, or In³⁺), (Co(CN)₆)(MF₆)(H₂pip)₃ (M=Al³⁺ or Cr³⁺), and (SbI₆)(AlF₆)(H₂dabco)₃. These new structures reveal that all ions at A, B, and X sites of inorganic antiperovskites can be replaced by molecular ions to form hybrid antiperovskites. This work will lead to the synthesis of a large family of hybrid antiperovskites.

Color, conduction, magnetism: Two new 2D hybrid perovskites exhibit both thermochromism and ferromagnetism. The (BED)₂CuCl₆ crystal shows a high thermochromic working temperature up to 443 K, with a dramatic temperature‐dependent conductivity change that spans six orders of magnitude.

Abstract

Two‐dimensional (2D) hybrid perovskites have shown many attractive properties associated with their soft lattices and multiple quantum well structure. Herein, we report the synthesis and characterization of two new multifunctional 2D hybrid perovskites, (PED)CuCl₄ and (BED)₂CuCl₆, which show reversible thermochromic behavior, dramatic temperature‐dependent conductivity change, and strong ferromagnetism. Upon temperature change, the (PED)CuCl₄ and (BED)₂CuCl₆ crystals exhibit a reversible color change between yellow and red‐brown. The associated structural changes were monitored by in situ temperature‐dependent powder X‐ray diffraction (PXRD). The (BED)₂CuCl₆ exhibits superior thermal stability, with a thermochromic working temperature up to 443 K. The conductivity of (BED)₂CuCl₆ changes over six orders of magnitude upon temperature change. The 2D perovskites exhibit ferromagnetic properties with Curie temperatures around 13 K.

Publication date: 15 January 2020

Source: Joule, Volume 4, Issue 1

Author(s): Luca Bertoluzzi, Caleb C. Boyd, Nicholas Rolston, Jixian Xu, Rohit Prasanna, Brian C. O’Regan, Michael D. McGehee

Two novel nonfullerene acceptors (NFAs) 4TFIC‐4F and 4TCIC‐4F are designed based on fluorene and carbazole. Compared with 4TFIC‐4F, 4TCIC‐4F exhibits higher lowest unoccupied molecular orbital (LUMO) level and narrower optical bandgap. Therefore, polymer solar cells based on PBDB‐T‐2Cl:4TCIC‐4F achieve a high‐power conversion efficiency of 13.02%, which is the highest value for the carbazole‐containing NFAs‐based devices.

It is important to tune the energy levels of nonfullerene acceptors (NFAs) to achieve more balanced open‐circuit voltage (V _oc) and short‐circuit current density (J _sc) to improve the device performance. Herein, two novel NFAs are designed via fusing fluorene or carbazole with two thieno[3,2‐b]thiophene and end capped with INIC‐2F, namely, 4TFIC‐4F and 4TCIC‐4F, respectively. The impact of the fluorene and carbazole unit on the PSC performance is systematically studied. Compared with 4TFIC‐4F, 4TCIC‐4F exhibits a higher lowest unoccupied molecular orbital (LUMO) energy level of −3.95 eV and a narrower optical bandgap of 1.51 eV owing to the stronger electron‐donating capacity of fused‐carbazole ring core. Consequently, the 4TCIC‐4F device achieves a high power conversion efficiencies (PCE) of 13.02% with a higher V _oc of 0.94 V and a larger J _sc of 18.98 mA cm⁻², whereas the 4TFIC‐4F device shows a PCE of 11.24%. The PCE of 13.02% is the highest value so far reported with the carbazole‐containing NFAs‐based PSCs. More importantly, the 4TCIC‐4F device shows good film thickness insensitive and long‐term thermal stability. The investigation demonstrates that the fused‐carbazole ring is a superior option to fused‐fluorene ring as electron‐donating core for designing high‐performance NFAs by improving V _oc and J _sc simultaneously.

An open‐shell donor–acceptor conjugated polymer enhances charge delocalization in the reduced state and is demonstrated as a highly stable anode in supercapacitors, with 90% capacitance retention after 2000 redox cycles. Asymmetric supercapacitors using this n‐dopable polymer operate with a wide 3 V potential window, with a best‐in‐class energy density and a long cycle life critical to energy storage and management.

Abstract

Supercapacitors have emerged as an important energy storage technology offering rapid power delivery, fast charging, and long cycle lifetimes. While extending the operational voltage is improving the overall energy and power densities, progress remains hindered by a lack of stable n‐type redox‐active materials. Here, a new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated polymer is demonstrated, which exhibits an open‐shell ground state, intrinsic electrical conductivity, and enhanced charge delocalization in the reduced state. These attributes afford very stable anodes with a coulombic efficiency of 99.6% and that retain 90% capacitance after 2000 charge–discharge cycles, exceeding other n‐dopable organic materials. Redox cycling processes are monitored in situ by optoelectronic measurements to separate chemical versus physical degradation mechanisms. Asymmetric supercapacitors fabricated using this polymer with p‐type PEDOT:PSS operate within a 3 V potential window, with a best‐in‐class energy density of 30.4 Wh kg⁻¹ at a 1 A g⁻¹ discharge rate, a power density of 14.4 kW kg⁻¹ at a 10 A g⁻¹ discharge rate, and a long cycle life critical to energy storage and management. This work demonstrates the application of a new class of stable and tunable redox‐active material for sustainable energy technologies.

In this Review, the important role of the interface in tin‐based perovskites and their PSCs device is demonstrated. The up‐to‐date studies on interface engineering of tin‐based PSCs are summarized. At last, a future perspective and remaining challenges in this field are given to provide some new thoughts on interface engineering for efficient tin‐based PSCs device.

Abstract

As a rising star of lead‐free perovskite solar cells (PCSs), tin‐based PSCs have drawn much attention and made promising progress during the past few years. Notably, interfaces in the tin‐based PSCs device have great impacts on performance enhancements. In this Review, the authors first demonstrate why the interface is especially crucial for tin‐based PSCs device. It is proposed that the engineering of i) interface between perovskite grains in the film and ii) interface within the PSCs device are of great significance on the improvement of device functionality and stability. Then, the up‐to‐date studies on interface engineering of tin‐based PSCs are reviewed, including the following strategies: i) passivation of trap states; ii) modification of interfacial layers; iii) construction of 2D/3D structure. At last, a future perspective and remaining challenges in this field are given, aiming to provide a comprehensive understanding of interfaces in tin‐based PSCs and give some new thoughts on interface engineering for efficient PSCs device.

A nonfullerene acceptor based active layer with high halogen contents is designed to fabricate efficient thick‐film organic solar cells. The conventional structure device using chlorinated acceptor F–2Cl and fluorinated donor PM6 exhibits a power conversion efficiency over 10% with an active layer thickness of 600 nm.

Abstract

Developing efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick‐film organic solar cells employing a nonfullerene acceptor F–2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achieved under optimized conditions. PCE of 9.03% is obtained for the inverted device with active layer thickness of 500 nm. It is worth noting that the conventional structure device still maintains the PCE of over 10% when the film thickness of the active layer is 600 nm, which is the highest value for the NF‐OSCs with such a large active layer thickness. It is found that the performance difference between the thick active layer films based conventional and inverted devices is attributed to their different vertical phase separation in the active layers.