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The desired α‐FAPbI₃ perovskite phase is stabilized by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium) overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇‐based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. It showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress.

Abstract

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α‐FAPbI₃ more suitable for solar‐cell applications than methylammonium lead iodide (MAPbI₃). However, its spontaneous conversion into the yellow non‐perovskite phase (δ‐FAPbI₃) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α‐FAPbI₃ perovskite phase by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.

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%.

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.

Interface energetics in 2D Ruddlesden–Popper perovskite solar cells are systematically investigated. The potential gradient across ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, suppressing the charge recombination and thus enhancing the open‐circuit voltage.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) are emerging as potential challengers to their 3D counterpart due to superior stability and competitive efficiency. However, the fundamental questions on energetics of the 2D RPPs are not well understood. Here, the energetics at (PEA)₂(MA) _{n −1}Pb _n I_{3 n +1}/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interfaces with varying n values of 1, 3, 5, 40, and ∞ are systematically investigated. It is found that n–n junctions form at the 2D RPP interfaces (n = 3, 5, and 40), instead of p–n junctions in the pure 2D and 3D scenarios (n = 1 and ∞). The potential gradient across phenethylammonium iodide ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, reducing charge recombination, which contributes to the smallest energy loss and the highest open‐circuit voltage (V _oc) in the perovskite solar cells (PSCs) based on the 2D RPP (n = 5)/PCBM. The mechanism is further verified by inserting a thin 2D RPP capping layer between pure 3D perovskite and PCBM in PSCs, causing the V _oc to evidently increase by 94 mV. Capacitance–voltage measurements with Mott–Schottky analysis demonstrate that such V _oc improvement is attributed to the enhanced potential at the interface.

In article number https://doi.org/10.1002/aenm.2019042341904234, Feng Liu and co‐workers report a detailed structure‐performance relationship to help understand the success of Y6 non‐fullerene acceptors. Through the analysis of the single crystal structure of Y6, it is found that Y6 forms a polymer‐like conjugated backbone through its banana‐shaped structure and π‐π interactions between molecules, and forms a 2D electron transport network under the ordered arrangement of the lattice.

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.

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).

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.

The slow photon of CsPbBr₃ inverse opal (IO) and the localized surface plasmon resonance of Au nanoparticles are coupled synergistically to enhance the performance of inorganic perovskite solar cells (PSCs). The synergetic effect leads to the enhanced light absorption and more efficient carriers transfer process. The PSC‐based Au‐CsPbBr₃ IO delivers a stabilized power conversion efficiency as high as 8.08%.

Abstract

Although all‐inorganic CsPbBr₃ are considered an ideal candidate for inorganic perovskite solar cells (PSCs) owing to their outstanding thermal‐ and moisture‐resistance, it still suffers from unfavorable charge transfer process and limited light harvesting ability. Herein, CsPbBr₃ inverse opal (IO) films coupled with Au nanoparticles (NPs) are rationally designed, and PSCs based on Au‐CsPbBr₃ IO achieve a stabilized photoelectric conversion efficiency up to 8.08%. By selectively tuning IO pore diameter, the slow photon region of CsPbBr₃ IO and localized surface plasmon resonance (SPR) region from Au NPs can be modulated to be overlapped to enhance the performance of inorganic CsPbBr₃ PSCs. The synergetic effect devotes to light utilization and charge transfer process, resulting in an enhanced light absorption capability and suppressed recombination rate of photogenerated electron–hole pairs. The introduction of Au not only triggers SPR effect, but also enhances efficient separation/injection of charge carriers owing to the Schottky barriers. Furthermore, it is revealed that simultaneous effect from SPR and IO photon effect are conducive to reduce exciton binding energy, enhancing exciton dissociation efficiency and leading to significant increase in free carrier density. This work provides a rational strategy for plasmonic metal/semiconductor composite light‐absorber for high‐performance inorganic PSCs.

Triplet materials are designed by introducing heavy atoms to enhance spin–orbit coupling or constructing donor and acceptor units with a twisted conformation to reduce ΔE _ST. However, the twisted materials have not been applied in solar cells due to weak absorption and low charge‐transport mobilities. Now two nonplanar acceptors with large π‐conjugated core were constructed that achieved over 15 % efficiency.

Abstract

Triplet acceptors have been developed to construct high‐performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T₁) are close to those of charge‐transfer states (³CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π‐conjugated core and D‐A structure, were confirmed to be triplet materials, leading to high‐performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D‐A structures result in large spin–orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T₁ is close to ³CT, facilitating the split of triplet exciton to free charges.

Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Zhenghui Luo, Ruijie Ma, Tao Liu, Jianwei Yu, Yiqun Xiao, Rui Sun, Guanshui Xie, Jun Yuan, Yuzhong Chen, Kai Chen, Gaoda Chai, Huiliang Sun, Jie Min, Jian Zhang, Yingping Zou, Chuluo Yang, Xinhui Lu, Feng Gao, He Yan

The incorporation of potassium can remarkably stabilize wide‐bandgap perovskites with a high Br content by the synergistic effect of the formation of 2D K₂PbI₄ at the grain boundaries and the interstitial occupancy in the perovskite lattices, which can effectively reduce the trap density and inhibit ion migration, thus suppressing the nonradiative recombination and photoinduced phase segregation.

Wide‐bandgap perovskites have great potential to enable high‐efficiency tandem photovoltaics by combining with the well‐established low‐bandgap absorbers. However, such wide‐bandgap perovskites are often necessarily constructed with a high Br content, and thus faced with issues of phase segregation–induced photoinstability and high defect density, severely hindering their photovoltaic performance. Herein, a remarkable boost of the stability and efficiency of wide‐bandgap perovskite solar cells (PSCs) is demonstrated by simply incorporating potassium ions. Experiments have shown the interstitial occupancy of potassium ions in the perovskite lattice and the formation of 2D K₂PbI₄ at the grain boundaries, both can reduce the trap density and inhibit ion migration, and thus suppress nonradiative recombination and photoinduced phase segregation. The average power conversion efficiency (PCE) of photovoltaic devices based on the perovskite with 40% Br is improved from 15.28% to 17.94%, among which the champion efficiency is 18.38% with an optimal 15% KI incorporation. Importantly, the champion open‐circuit voltage (V _oc) remains unchanged (≈1.25 V) even when the bandgap reduces from 1.80 to 1.75 eV due to KI doping, effectively reducing the V _oc deficit. In addition, the unencapsulated cells can sustain 94% of the initial PCE after 2000 h of storage in ambient atmosphere, affirming their outstanding stability.

A novel non‐conjugated polymer based on the polyethylene backbone, PVCz‐OMeTPA, with suitable energy levels, good hole mobility, as well as excellent film‐forming ability is developed as an efficient dopant‐free hole‐transporting material (HTMs) for inverted quasi‐2D perovskite solar cells (PSCs). Quasi‐2D PSCs using the dopant‐free PVCz‐OMeTPA as HTM exhibit an excellent power conversion efficiency of 17.22% and long‐term environmental stability.

Quasi‐2D perovskites with excellent stability have been recognized as an alternative to 3D counterparts for perovskite solar cells (PSCs). Although the power conversion efficiency (PCE) of quasi‐2D PSCs has increased over 18% by the compositional controlling and solvent engineering of perovskites, fewer studies have been conducted to exploit charge transport layers and investigate their interface relationships with quasi‐2D perovskites. To achieve high efficiency and good long‐term stability for quasi‐2D PSCs, hole‐transporting materials (HTMs) with matched energy levels and good chemical compatibility with quasi‐2D perovskites are explored and investigated. Herein, a novel non‐conjugated polymer based on polyethylene backbone, poly[3,6‐(4,4′‐dimethoxytriphenylamino)‐9‐vinyl‐9H‐carbazole] (PVCz‐OMeTPA), is easily synthesized and investigated as a promising dopant‐free HTM for quasi‐2D PSCs. Due to its more suitable energy levels, good hole mobility, as well as excellent film‐forming ability to assist the formation of high‐quality quasi‐2D perovskite films, the optimized p–i–n structured quasi‐2D PSCs based on PVCz‐OMeTPA exhibit the best PCE of 17.22%. The unencapsulated quasi‐2D PSCs based on PVCz‐OMeTPA maintain 82% of the initial efficiency after 1400 h under a relative humidity of ≈40% and sustain over 81% of the original efficiency after aging for 600 h upon 70 °C of continuous annealing.

High‐efficiency monolithic silicon‐based tandem solar cells require an adapted bandgap of the top cell. The perovskite composition FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ has a theoretically optimal bandgap of 1.68 eV. Implementation in p–i–n tandem devices gives highest certified efficiency of 25.1%, whereas a substantial efficiency increase is observed over time. By eliminating remaining interfacial and reflection losses, >30% efficiency is feasible.

Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p–i–n tandem configuration. A methylammonium‐free FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ perovskite with high Cs content is investigated for improved stability. A 10% molarity increase to 1.1 m of the perovskite precursor solution results in ≈75 nm thicker absorber layers and 0.7 mA cm⁻² higher short‐circuit current density. With the optimized absorber, tandem devices reach a high fill factor of 80% and up to 25.1% certified efficiency. The unencapsulated tandem device shows an efficiency improvement of 2.3% (absolute) over 5 months, showing the robustness of the absorber against degradation. Moreover, a photoluminescence quantum yield analysis reveals that with adapted charge transport materials and surface passivation, along with improved antireflection measures, the high bandgap perovskite absorber has the potential for 30% tandem efficiency in the near future.

A straightforward approach is developed to decouple the degradation effects occurring at the interfaces between the lead halide absorber with a hole‐transport and electron‐transport layers in perovskite solar cells. The impact of the hole‐transport layer is shown to depend on its composition: materials such as nickel oxide aggressively interact with the perovskite, whereas organic polytriarylamine provides a stable interface.

There is growing evidence that the stability of perovskite solar cells (PSCs) is strongly dependent on the interface chemistry between the absorber films and adjacent charge‐transport layers, whereas the exact mechanistic pathways remain poorly understood. Herein, a straightforward approach is presented for decoupling the degradation effects induced by the top fullerene‐based electron transport layer (ETL) and various bottom hole‐transport layer (HTL) materials assembled in p‐i‐n PSCs. It is shown that chemical interaction of MAPbI₃ absorber with ETL comprised of the fullerene derivative most aggressively affects the device operational stability. However, washing away the degraded fullerene derivative and depositing fresh ETL leads to restoration of the initial photovoltaic performance when bottom perovskite/HTL interface is not degraded. Following this approach, it is possible to compare the photostability of stacks with various HTLs. It is shown that poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and NiO_x induce significant degradation of the adjacent perovskite layer under light exposure, whereas poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) provides the most stable perovskite/HTL interface. A time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) analysis allows identification of chemical origins of the interactions between MAPbI₃ and HTLs. The proposed research methodology and the revealed degradation pathways should facilitate the development of efficient and stable PSCs.

Organic solar cells with unusual inverted structure (sequentially processed heterojunction) are fabricated by sequentially spin coating the acceptor layer FOIC1 after the donor layer PTB7‐Th, which yields better‐controlled vertical phase separation and improved efficiency compared with traditional bulk heterojunction devices.

A new fused‐ring electron acceptor FOIC1 is designed and synthesized. FOIC1 exhibits intense absorption in the range of 600–1000 nm, the highest occupied molecular orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO) energy levels of −5.39/−3.99 eV, and electron mobility of 1.8 × 10⁻³ cm² V⁻¹ s⁻¹. Organic solar cells based on sequentially processed heterojunction (SHJ) with an unusual inverted structure are fabricated. Through sequentially spin‐coating polymer donor PTB7‐Th as the bottom layer and acceptor FOIC1 as the top layer, a better vertical phase distribution is formed in this SHJ compared with that in traditional bulk heterojunction (BHJ). In the upper‐half part, a more balanced donor/acceptor distribution is beneficial for exciton dissociation. At the bottom interface, more FOIC1 accumulation is beneficial for exciton generation and charge transport. Overall, the SHJ cells exhibit power conversion efficiency as high as 12.0%, higher than that of the BHJ counterpart (11.0%).

Radio frequency magnetron sputtered SnO₂ is used as an electron transport layer (ETL) for PbS quantum dot solar cells with an efficiency of 8.4%. Further to modify the SnO₂ surface, a thin sol–gel ZnO layer is spin‐coated on top of SnO₂ forming a SnO₂–ZnO double ETL. The best device with double ETL achieves an efficiency over 10%.

Herein, for the first time, it is successfully demonstrated that radio frequency (RF) magnetron sputtered SnO₂ can be a qualified alternative electron transport layer (ETL) for a high‐efficiency PbS quantum dot (QD) solar cell. The highest performing device using such a SnO₂ ETL obtains an efficiency of 8.4%, which is comparable to the sol–gel ZnO‐based one (8.8%). The excellent performance mainly results from the improved current density, which is attributed to the superior properties of the SnO₂ ETL, such as high electron mobility and excellent optical transmittance. However, it is also found that the sputtered SnO₂‐based devices show smaller voltage and fill factor due to the unsatisfied surface morphology and energy level alignment. By combining a thin (around 10 nm) sol–gel ZnO film on top of a sputtered SnO₂ film to form the double ETL, the best efficiency of 10.1% is obtained, which is the highest efficiency using SnO₂ ETL in a PbS QD solar cell. The work not only provides a new avenue to improve the efficiency of PbS QD solar cells but also offers the possibility to use an industry compatible sputtering technique for PbS QD solar cells.

A new spiro derivative, SPS‐4F, is designed and synthesized as a nonfullerene electron transport material in perovskite solar cells. An efficiency of 20.31% and high device stability are simultaneously achieved in the resultant devices. This work opens up opportunities to obtain a new family of spiro‐based electron transport materials and paves a way for realizing high‐performance devices with low cost.

Abstract

Electron transport materials (ETMs) play a significant role in perovskite solar cells (PSCs). However, conventional solution processable organic ETMs are mainly restricted to fullerene derivatives and it is challenging to obtain nonfullerene ETMs with satisfactory properties. In this work, a new organic semiconductor SPS‐4F is synthesized by utilizing the classical spiro[fluorine‐9′9‐thioxanthene] unit to construct a π‐extended core. Although spiro is normally used in hole transport materials, the new spiro derivative SPS‐4F is successfully used as an ETM in inverted PSCs with power conversion efficiency over 20%. In addition, SPS‐4F can strongly coordinate with MAPbI₃ perovskite and lead to efficient surface trap passivation. The resultant PSCs exhibit excellent stability in air because of the hydrophobic property of SPS‐4F. This work opens up opportunities to obtain a new family of ETMs based on spiro and paves a way to the fabrication of high‐performance PSCs with low cost.