Chen Weijie

Tapered cross‐section photoelectron spectroscopy is introduced as a new method to analyze complete electronic devices in thermodynamic equilibrium and in operation. The power of the method is demonstrated by analyzing a state‐of‐the‐art mixed ion perovskite solar cell in the dark and under illumination.

Abstract

The purpose of this article is twofold. On the one hand the method of spacial resolved photoemission spectroscopy on small angle tapered cross‐sections (TCS) of complete devices is introduced to analyze simultaneously the chemical and electronic structure. On the other hand, a specific working principle of the analyzed cell type is revealed. Solar cells of 18% efficiency are prepared from a single precursor (FAPbI₃)_0.85(MAPbBr₃)_0.15 with excess of 15% PbI₂. It is shown that TCS‐phototoelectron spectroscopy allows to determine the chemical composition as well as the potential distribution across the full device in the dark and in operation. The energy converting contact is the hole extraction back contact. Interestingly the photopotential in the analyzed cell type is predominantly created within the hole extraction layer and not in the n‐doped perovskite absorber. With the addition of measured core level to valence band maximum positions of the respective layers, TCS line scans lead to the band diagram for the full device. In addition, depth variations of the chemical composition are found: the bromide concentration increases while the iodide concentration is reduced near and within the mesoporous TiO₂ layer.

Improved defect passivation of perovskite quantum dots (PQDs) is reported using glycine as a dual‐passivation ligand, which can simultaneously fill the A‐site (cesium) and iodine vacancies on the PQD surface. The enhanced photovoltaic performance is obtained in the glycine‐based PQD solar cells (PQDSCs) compared with that of the traditional Pb(NO₃)₂‐based PQDSCs, resulting from increased charge carrier extraction.

Abstract

Inorganic CsPbI₃ perovskite quantum dot (PQD) receives increasing attention for the application in the new generation solar cells, but the defects on the surface of PQDs significantly affect the photovoltaic performance and stability of solar cells. Herein, the amino acids are used as dual‐passivation ligands to passivate the surface defects of CsPbI₃ PQDs using a facile single‐step ligand exchange strategy. The PQD surface properties are investigated in depth by combining experimental studies and theoretical calculation approaches. The PQD solid films with amino acids as dual‐passivation ligands on the PQD surface are thoroughly characterized using extensive techniques, which reveal that the glycine ligand can significantly improve defect passivation of PQDs and therefore diminish charge carrier recombination in the PQD solid. The power conversion efficiency (PCE) of the glycine‐based PQD solar cell (PQDSC) is improved by 16.9% compared with that of the traditional PQDSC fabricated with Pb(NO₃)₂ treating the PQD surface, owning to improved charge carrier extraction. Theoretical calculations are carried out to comprehensively understand the thermodynamic feasibility and favorable charge density distribution on the PQD surface with a dual‐passivation ligand.

Incorporation of a small portion of a novel polymer donor named PBT(E)BTz with a deeper highest occupied molecular orbital level than that of the host materials is proven promising to construct highly efficient ternary polymer solar cells (PSCs). In addition to the role of a “solid additive” for ternary PSCs, PBT(E)BTz shows great potential to be a thermal and light stabilizer in ternary PSCs.

Abstract

Ternary strategies have attracted extensive attention due to their potential in improving power conversion efficiencies (PCEs) of single‐junction polymer solar cells (PSCs). In this work, a novel wide bandgap polymer donor (E _g ^opt ≈ 2.0 eV) named PBT(E)BTz with a deep highest occupied molecular orbital (HOMO) level (≈−5.73 eV) is designed and synthesized. PBT(E)BTz is first incorporated as the third component into the classic PBDB‐T‐SF:IT‐4F binary PSC system to fabricate efficient ternary PSCs. A higher PCE of 13.19% is achieved in the ternary PSCs with a 5% addition of PBT(E)BTz over binary PSCs (12.14%). Similarly, addition of PBT(E)BTz improves the PCE for PBDB‐T:IT‐M binary PSCs from 10.50% to 11.06%. The study shows that the improved PCE in ternary PSCs is mainly attributed to the suppressed charge carrier recombination and more balanced charge transport. The generality of PBT(E)BTz as a third component is further evidenced in another efficient binary PSC system—PBDB‐TF:BTP‐4Cl: an optimized PCE of 16.26% is realized in the ternary devices. This work shows that PBT(E)BTz possessing a deep HOMO level as an additional component is an effective ternary PSC construction strategy toward enhancing device performance. Furthermore, the ternary device with 5% PBT(E)BTz displays better thermal and light stability over binary devices.

Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Jianyu Yuan, Abhijit Hazarika, Qian Zhao, Xufeng Ling, Taylor Moot, Wanli Ma, Joseph M. Luther

Inorganic and organic electron transport layers (ETLs) have become a popular choice as selective contact materials for perovskite solar cells (PSCs). Herein, an overview of various inorganic and organic ETLs synthesis, properties, and their application in PSCs for different architectures, etc., to achieve high power conversion efficiency and functional stability is provided.

The presence of the electron transport layer (ETL) in perovskite solar cells (PSCs) is critical due to the requirement of enhancing the electron collection selectivity. ETLs are essential for achieving a high open‐circuit voltage (V _OC), high fill factor (FF), better transport of directional charges, better absorption of incoming light, and thermodynamically competent operation of photogenerated carrier populations. ETLs are sorted as organic, inorganic, or mixed, with different stability, cost effect, and directional charge transport ability. For instance, by using metal oxides as ETLs, power conversion efficiencies (PCEs) higher than 23% are reached for PSCs. Despite the advantages of metal oxide–ETLs and other organic or mixed ETLs, some questions still have to be addressed to achieve better PCEs, like how to passivate or eliminate the surface traps, how to upgrade the comprehension of the heterointerface, and optimization of morphology. Herein, different considerations of ETLs in different physical and environmental conditions, and different deposition methods used, are presented. Finally, the current studies and future challenges are analyzed in the domain of highly efficient PSCs with various ETLs.

A novel polymer additive polyaniline (PANI) is introduced to the CsPbI₂Br film of the carbon‐based all‐inorganic perovskite solar cell. The PANI effectively balances the electron and hole transport, passivates defects, and improves film quality, resulting in reduced E _loss and high V _oc of 1.33 V and power conversion efficiency (PCE) of 13.52%.

The energy loss of all‐inorganic metal halide perovskite solar cells is large, which reduces the open‐circuit voltage and photoelectron conversion efficiency of the device. Herein, it is found that the cathode electron transfer speed is much lower than the anode hole transfer speed in CsPbI₂Br perovskite solar cell with fluorine‐doped tin oxide (FTO) glass/SnO₂/CsPbI₂Br/carbon structure, which induces charge accumulation at the cathode and energy loss of the device accordingly. By introducing a new conductive polymer additive polyaniline (PANI) to the CsPbI₂Br film, the electron transfer speed at the cathode is enhanced, resulting in balanced charge transfer at both electrodes and reduced energy loss of the device. Ultraviolet photoelectron spectroscopy measurement reveals that the PANI pushes the conduction band minimum of CsPbI₂Br upward, leading to stronger driving force for electron extraction. Therefore, the nonradiative recombination at the SnO₂/CsPbI₂Br interface is greatly suppressed. In addition, PANI can also effectively passivate defects and promote the crystal quality of CsPbI₂Br, leading to reduced nonradiative recombination in perovskite materials. Accordingly, the optimized all‐inorganic CsPbI₂Br solar cell delivers a high V _oc of 1.33 V and power conversion efficiency (PCE) of 13.52%.

A low‐temperature seed‐assisted growth (SAG) method for high‐quality CsPbIBr₂ perovskite films through reducing the formation energy by introducing methylammonium halides is demonstrated. The device fabricated using optimized SAG‐based film yields a power conversion efficiency of 10.47% with a remarkable open circuit voltage (V _oc) of 1.21 V.

Abstract

All‐inorganic CsPbIBr₂ perovskite has recently received growing attention due to its balanced band gap and excellent environmental stability. However, the requirement of high‐temperature processing limits its application in flexible devices. Herein, a low‐temperature seed‐assisted growth (SAG) method for high‐quality CsPbIBr₂ perovskite films through reducing the crystallization temperature by introducing methylammonium halides (MAX, X = I, Br, Cl) is demonstrated. The mechanism is attributed to MA cation based perovskite seeds, which act as nuclei lowering the formation energy of CsPbIBr₂ during the annealing treatment. It is found that methylammonium bromide treated perovskite (Pvsk‐Br) film fabricated at low temperature (150 °C) shows micrometer‐sized grains and superior charge dynamic properties, delivering a device with an efficiency of 10.47%. Furthermore, an efficiency of 11.1% is achieved for a device based on high‐temperature (250 °C) processed Pvsk‐Br film via the SAG method, which presents the highest reported efficiency for inorganic CsPbIBr₂ solar cells thus far.

Ambipolar black phosphorene (BP) nanosheets with tailored thicknesses concurrently enhance carrier extraction at both the electron‐transport layer/perovskite and hole‐transport layer/perovskite interfaces for high‐efficiency perovskite solar cells, demonstrating the appealing implementation of BP as a dual‐functional carrier‐transport material for a diversity of optoelectronic devices, including solar cells, photodetectors, sensors, light‐emitting diodes, etc.

Abstract

2D black phosphorene (BP) carries a stellar set of physical properties such as conveniently tunable bandgap and extremely high ambipolar carrier mobility for optoelectronic devices. Herein, the judicious design and positioning of BP with tailored thickness as dual‐functional nanomaterials to concurrently enhance carrier extraction at both electron transport layer/perovskite and perovskite/hole transport layer interfaces for high‐efficiency and stable perovskite solar cells is reported. The synergy of favorable band energy alignment and concerted cascade interfacial carrier extraction, rendered by concurrent positioning of BP, delivered a progressively enhanced power conversion efficiency of 19.83% from 16.95% (BP‐free). Investigation into interfacial engineering further reveals enhanced light absorption and reduced trap density for improved photovoltaic performance with BP incorporation. This work demonstrates the appealing characteristic of rational implementation of BP as dual‐functional transport material for a diversity of optoelectronic devices, including photodetectors, sensors, light‐emitting diodes, etc.

A well‐designed inorganic–organic double hole transporting layer (HTL) based on inorganic CuSCN and organic polymer dithiophene‐benzene is developed. A perovskite solar cell with this dopant‐free HTL exhibits a very high power conversion efficiency of 22.0% (certified: 21.7%) and significantly improved thermal, humidity, and light stabilities compared to 2,2′,7,7′‐tetrakis(N ,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (Spiro‐OMeTAD) HTL‐based devices.

Abstract

Most of the high performance in perovskite solar cells (PSCs) have only been achieved with two organic hole transporting materials: 2,2′,7,7′‐tetrakis(N ,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (Spiro‐OMeTAD) and poly(triarylamine) (PTAA), but their high cost and low stability caused by the hygroscopic dopant greatly hinder the commercialization of PSCs. One effective alternative to address this problem is to utilize inexpensive inorganic hole transporting layer (i‐HTL), but obtaining high efficiency via i‐HTLs has remained a challenge. Herein, a well‐designed inorganic–organic double HTL is constructed by introducing an ultrathin polymer layer dithiophene‐benzene (DTB) between CuSCN and Au contact. This strategy not only enhances the hole extraction efficiency through the formation of cascaded energy levels, but also prevents the degradation of CuSCN caused by the reaction between CuSCN and Au electrode. Furthermore, the CuSCN layer also promotes the formation of a pinhole‐free and compact DTB over layer in the CuSCN/DTB structure. Consequently, the PSCs fabricated with this CuSCN/DTB layer achieves the power conversion efficiency of 22.0% (certified: 21.7%), which is among the top efficiencies for PSCs based on dopant‐free HTLs. Moreover, the fabricated PSCs exhibit high light stability under more than 1000 h of light illumination and excellent environmental stability at high temperature (85 °C) or high relative humidity (>60% RH).

Soft routed benzimidazole clubbed phenoxazine‐based organic ionic plastic crystals with iodide and bromide anions successfully introduced as hole transporting materials in perovskite solar cells yield power conversion efficiencies exceeding 18%, which represents the best alternative to existing spiro‐OMeTAD due to high conductivity and hole mobility with a safer, stable, and efficient system.

Abstract

Organic ionic plastic crystals (OIPCs) are synthesized through a simple metal‐free, cost‐effective approach. The strategized synchronization of electron‐rich phenoxazine with benzimidazolium iodide (OIPC‐I) and bromide (OIPC‐Br) salts lead to enhanced hole mobility and conductivity of OIPCs which is suitable for an efficient alternative to conventional organic hole transporting materials (HTMs) for stable perovskite solar cells (PSCs). The fabricated PSCs with OIPC‐I as hole transporting layer yielded a power conversion efficiency of 15.0% and 18.1% without and with additive (Li salt) respectively, which are comparable with spiro‐OMeTAD based devices prepared under similar conditions. Furthermore, the PSCs with OIPCs show good stability compared to the spiro‐OMeTAD with or without additives. Here, first time benzimidazolium‐based OIPCs have been used as an alternative organic HTM for perovskite solar cells, which opens a window for the design of effective OIPCs for highly efficient PSCs with long‐term stability.