Wangyikai

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Ming-Hua Li, Tian-Ge Sun, Jiang-Yang Shao, Yu-Duan Wang, Jin-Song Hu, Yu-Wu Zhong

Novel interface polarization induced field‐effect passivation based on amorphous transition metal oxide is developed for efficient and ambient‐air‐stable perovskite solar cells. Comprehensive insights into the interaction between the field‐effect passivation, interface polarities, and the performance of the device have been elucidated in detail.

Abstract

Organolead halide hybrid perovskite solar cells (PSCs) have become a shining star in the renewable devices field due to the sharp growth of power conversion efficiency; however, interfacial recombination and carrier‐extraction losses at heterointerfaces between the perovskite active layer and the carrier transport layers remain the two main obstacles to further improve the power conversion efficiency. Here, novel field‐effect passivation has been successfully induced to effectively suppress the interfacial recombination and improve interfacial charge transfer by incorporating interfacial polarization via inserting a high work function interlayer between perovskite and holes transport layer. The charge dynamics within the device and the mechanism of the field‐effect passivation are elucidated in detail. The unique interfacial dipoles reinforce the built‐in field and prevent the photogenerated charges from recombining, resulting in power conversion efficiency up to 21.7% with negligible hysteresis. Furthermore, the hydrophobic interlayer also suppresses the perovskite decomposition by preventing the moisture penetration, thereby improving the humidity stability of the PSCs (>91% of the initial power conversion efficiency (PCE) after 30 d in 65 ± 5% humidity). Finally, several promising research perspectives based on field‐effect passivation are also suggested for further conversion efficiency improvements and photovoltaic applications.

A newly conceived n‐type small molecule (Y‐Th2) is incorporated as an efficient additive in perovskite solar cells, achieving simultaneous improvements in device performance and stability. Y‐Th2 effectively passivates defects in perovskite crystals by Lewis acid–base interactions and intermolecular hydrogen bonds, obtaining high‐quality perovskite film. The inverted structure device exhibits a power conversion efficiency of 21.5% with notably enhanced operational stability.

Abstract

Significant efforts have been devoted to modulating the grain size and improving the film quality of perovskite in perovskite solar cells (PSCs). Adding materials to the perovskite is especially promising for high‐performance PSCs, because the additives effectively control the crystal structure. Although the additive engineering approach has substantially boosted the efficiency of PSCs, instability of the perovskite film has remained a primary bottleneck for the commercialization of PSCs. Herein, a newly conceived bithiophene‐based n‐type conjugated small molecule (Y‐Th2) is introduced to PSCs, which simultaneously enhances the performance and stability of the cell. The Y‐Th2 effectively passivates the defect states in perovskite through Lewis acid–base interactions, increasing the grain size and quality of the perovskite absorber. An inverted PSC containing the Y‐Th2 additive achieves a power conversion efficiency of 21.5%, versus 18.3% in the reference device. The operational stability is also considerably enhanced by the improved hydrophobicity and intermolecular hydrogen bonds in the perovskite.

An in‐situ formed polymeric interlayer enables enhanced photovoltaic performance of the methylammonium‐, cesium‐, and bromide‐free perovskite solar cells with superior photo‐ and thermal‐stability. The polymeric interlayer promotes growth of perovskite crystals with reduced defect density and improves the contact between the perovskite and hole transporting layers to assists in photo‐excited charge extraction.

Abstract

The vast majority of high‐performance perovskite solar cells (PSCs) are based on multi‐cation mixed‐anion compositions incorporating methylammonium (MA) and bromide (Br). Nevertheless, the thermal instability of MA and the tendency of mixed halide compositions to phase segregate limit the long‐term stability of PSCs. However, reports of MA‐free and/or Br‐free compositions are rare in the community since their performance is generally inferior. Here, a strategy is presented to achieve highly efficient and stable PSCs that are altogether cesium (Cs)‐free, MA‐free and Br‐free. An antisolvent quenching process is used to in‐situ deposit a polymeric interlayer to promote the growth of phase‐pure formamidinium lead tri‐iodide perovskite crystals with reduced defect density and to assist in photo‐excited charge extraction. The PSCs developed are among the best‐performing reported for such compositions. Moreover, the PSCs show superior stability under continuous exposure to both illumination and 85 °C heat.

An N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (CSCO), is prepared for the first time as an electron transport layer for n‐i‐p planar perovskite solar cells, which leads to one of the highest power conversion efficiencies of 22.21%, and to remarkable stability, retaining over 83% of its initial power conversion efficiency without encapsulation after 130 days of storage in ambient conditions.

Abstract

The electron transport layer (ETL) has an important influence on the power conversion efficiency (PCE) and stability of n‐i‐p planar perovskite solar cells (PSCs). This paper presents an N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (abbreviated as CSCO) that is synthesized and prepared for the first time as an ETL for n‐i‐p planar PSCs, which leads to a high PCE of 22.21% after KCl treatment, one of the highest PCEs of n‐i‐p planar PSCs to date. Further analysis reveals that the high PCE is attributed to the excellent conductivity of CSCO because of its more delocalized electron cloud distribution due to its unique −O=C−O− group, and to the defect passivation of the Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ (denoted as CsFAMA) perovskite through the interaction between the O (Sn) atoms of CSCO and the Pb (halogen) atoms of CsFAMA at CSCO/CsFAMA interface, while the traditional ETL materials such as SnO₂ film lack this function. In addition to the high PCE, the optimal PSCs using CSCO as ETL show remarkable stability, retaining over 83% of its initial PCE without encapsulation after 130 days of storage in ambient conditions (≈25 °C at ≈40% humidity), much better than the traditional SnO₂‐based n‐i‐p PSCs.

A zwitterionic conjugated polyelectrolyte presents high hole mobility, compatible covalence level, and the ability for passivating surface defects of the perovskite film. The formation of a weak double‐layer capacitance, which is not strong enough to induce the migration of MA⁺ ions, contributes to low carrier transport resistance and interfacial charge accumulation, leading to high efficiency and stability.

Achieving rapid extraction and equivalent transport of charge carriers is an effective way to improve the performance of perovskite solar cells (PSCs). Herein, a thiophene‐based zwitterionic conjugated polyelectrolyte (poly(5‐amino‐5‐carboxy‐3‐oxapentyl)‐2,5‐thiophene [POWT]) is introduced into PSCs as a hole‐transporting and interfacial material. The polyelectrolyte has a high hole mobility of 5.74 × 10⁻³ cm² V⁻¹ s⁻¹ (similar to that of poly(triarylamine) [PTAA]) and compatible covalence level relative to the perovskite. Terminated with a zwitterionic pair of a‐amino acid, POWT layer builds up a weak double‐layer capacitance at the interface, which is not strong enough to induce the migration of MA⁺ ions in the perovskite layer. Deep electrical study on the PSC with the structure of indium tin oxide (ITO)/POWT/FA_0.2MA_0.8PbI_2.9Br_0.1/C₆₀/bathocuproine (BCP)/Ag discloses that the device has low carrier transfer resistance, low leakage current density, and minor interfacial charge accumulation. The open‐circuit voltage and the short‐circuit current density are much improved, and the power conversion efficiency (PCE) is up to 17.5%. With a‐amino acid zwitterions, POWT passivates the surface charge defects and grain boundaries of the perovskite film. The PSC presents negligible hysteresis and high stability. After 56 days, the unencapsulated PSC still remains at 85% of the original efficiency.

A new fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is utilized to passivate the surface of CsPbI₂Br perovskite for solar cells with enhanced efficiency as well as improved stability.

Surface modification is demonstrated as an efficient strategy to enhance the efficiency and stability of perovskite solar cells (PVSCs). Fluorinated organic ammonium salts featuring a strong hydrophobic nature are seldom used as passivation agents for the surface modification of CsPbI₂Br perovskites. Herein, a fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is incorporated into the surface of CsPbI₂Br perovskite for the first time. After the CFPEAI modification, the defects of CsPbI₂Br perovskite are significantly passivated with reduced trap densities. The best‐performance PVSC with CFPEAI modification shows an excellent power conversion efficiency (PCE) of 16.07% with a high fill factor (FF) of 84.65%, a short‐circuit current density (J _SC) of 15.45 mA cm⁻², and an open‐circuit voltage (V _OC) of 1.23 V. In contrast, the control PVSCs without the surface modification exhibit a lower PCE of 14.50% with a FF of 80.56%, a J _SC of 15.05 mA cm⁻², and a V _OC of 1.20 V. With CFPEAI passivation, the CsPbI₂Br perovskite film exhibits enhanced hydrophobicity, thereby leading to improved stability for the corresponding PVSC in comparison with the control PVSC without any surface modification.

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).

Three novel donor–π‐bridge–acceptor (D–π–A)‐type small organic molecules are designed and synthesized as dopant‐free hole transport materials for perovskite solar cells. Combination of triazatruxene donor, terthiophene π‐bridge, and dicyanovinylene N‐ethyl rhodanine electron‐accepting unit as CI‐B3 creates well‐ordered edge‐on aggregated π–π stacking. Solar cell performance and long‐term stability are significantly improved.

Three donor–π‐bridge–acceptor (D–π–A)‐type organic small molecules coded CI‐B1, CI‐B2, and CI‐B3 are designed, synthesized, and used as dopant‐free hole transporting materials (HTMs) for perovskite solar cells (PSCs). The strong electron‐donating triazatruxene central core (D), terthiophene conjugated arms (π), and three different strong electron‐accepting units (A) provide high intramolecular charge transfer nature and eliminate the need of dopants during the fabrication of PSCs. HTMs are investigated to understand the effect of terminal functional groups on the PSC performance. Interestingly, due to the change of end‐capping, three different organizations of self‐assembly with π–π stacking are observed in the solid thin films. Dopant‐free CI‐B1, CI‐B2, CI‐B3, and spiro‐OMeTAD with dopants are used with triple cation perovskite composition Cs_0.1(MA_0.15FA_0.85)_0.9Pb(I_0.85Br_0.15)₃ (MA: CH₃NH₃ ⁺, FA: NHCHNH₃ ⁺) in n‐i‐p architecture. The cells prepared with CI‐B3 not only exhibits a comparable power conversion efficiency (PCE) of 17.54% to the state‐of‐art of spiro‐OMeTAD with dopants (18.02%), but also demonstrates improved long‐term stability, maintaining 88% of its original PCE after 1000 h of illumination. The superior photovoltaic performance, synthetic simplicity, dopant‐free nature, high durability, and edge‐on molecular orientation of CI‐B3 show its great promise as a HTM candidate for efficient and stable PSCs.

A synthetic polyhalide ligand (2‐picolyl)amine triiodide as a molecular glue is used to passivate halide vacancies at grain boundaries directionally and throughout grain bulk of perovskites. The inverted perovskite solar cells after passivation are allowed to be more efficient, and are profoundly stabilized in both ambient air and light‐soaking circumstances.

The fundamental instability of hybrid perovskite solar cells originates from the considerable halide vacancies. Furthermore, the local roles of halide vacancies between grain boundaries and grain bulk generally conflict, thus inhibiting complete passivation. To overcome this obstacle, a rational polyhalide ligand, di‐(2‐picolyl)amine triiodide, is designed as a molecular “glue” to achieve comprehensive passivation. Unlike a monohalide ligand, this ligand has multiple iodide ions and a quasiplanar tridentate chelation capability, contributing to directional passivation along the grain boundaries and overall passivation throughout the grain bulk. Using this molecular glue passivation, the best inverted solar cell yields an efficiency of 20.02%. Moreover, the relative stability of this cell in ambient air (≈40% humidity, 800 h aging) and under light‐soaking conditions (500 h aging) is profoundly enhanced by 33.33% and 22.26%, respectively. Herein, important insights into the design of passivating molecules to achieve low‐defect perovskites toward the development of multifunctional devices are provided.

Post‐treating the mesoporous TiO₂/ZrO₂/carbon triple layer by alkali metal sulfonate compounds enables a significantly enhanced photovoltage for hole‐conductor‐free printable mesoscopic perovskite solar cells. The devices demonstrate high operational stability, retaining 91.7% of their initial efficiency after 1000 h continuous operation at the maximum power point under 1 sun illumination.

Triple‐mesoscopic perovskite solar cells (PSCs) based on TiO₂/ZrO₂/carbon architecture have attracted much attention due to their excellent long‐term stability and screen‐printing technique‐based fabrication process. However, the relatively low open‐circuit voltage (V _OC) limits the further improvement of power conversion efficiency (PCE) for triple‐mesoscopic PSCs. Herein, 2‐phenyl‐5‐benzimidazole sulfonate‐Na to post‐treat the triple‐mesoscopic structured scaffold is introduced. The conduction band of the mesoporous TiO₂ layer (electron transport layer [ETL]) is significantly shifted up from −4.22 to −4.11 eV, which favors the electron transfer from the perovskite absorber to the ETL. At the same time, the recombination at the interface of ETL/perovskite is effectively suppressed. Correspondingly, the V _OC and fill factor (FF) of the devices are enhanced without sacrificing the photocurrent density (J _SC). With optimal post‐treatment conditions, the champion device delivers a V _OC of 1.02 V and an FF of 0.70 with J _SC of 23.06 mA cm⁻², showing an overall PCE of 16.51%. After 1000 h continuous operation at the maximum power point under AM1.5G 1 sun illumination, the devices can maintain 91.7% of the initial efficiency. This simple procedure and significant photovoltage enhancement render this method promising for fabricating efficient PSCs based on mesoporous charge transport layers.

Intensity‐dependent absolute photoluminescence studies on perovskite neat materials and partial cell stacks highlight how interface recombination can account for ideality factors between 1 and 2, commonly observed in perovskite devices. The findings are rationalized via a recombination model which details how interface recombination can lead to ideality factors of unity, in this case, not representative of a better device.

Abstract

The measurement of the ideality factor (n _id) is a popular tool to infer the dominant recombination type in perovskite solar cells (PSC). However, the true meaning of its values is often misinterpreted in complex multilayered devices such as PSC. In this work, the effects of bulk and interface recombination on the n _id are investigated experimentally and theoretically. By coupling intensity‐dependent quasi‐Fermi level splitting measurements with drift diffusion simulations of complete devices and partial cell stacks, it is shown that interfacial recombination leads to a lower n _id compared to Shockley–Read–Hall (SRH) recombination in the bulk. As such, the strongest recombination channel determines the n _id of the complete cell. An analytical approach is used to rationalize that n _id values between 1 and 2 can originate exclusively from a single recombination process. By expanding the study over a wide range of the interfacial energy offsets and interfacial recombination velocities, it is shown that an ideality factor of nearly 1 is usually indicative of strong first‐order non‐radiative interface recombination and that it correlates with a lower device performance. It is only when interface recombination is largely suppressed and bulk SRH recombination dominates that a small n _id is again desirable.

The different aggregation forms of hole‐transporting materials (HTMs) affect intermolecular charge transfer and hole transporting in achieving highly efficient dopant‐free perovskite solar cells. The combination of twisted periphery groups with planar core units shows an efficient approach to regulate the state of molecular aggregation after a systematical investigation of 6,12‐dihydroindeno[1,2‐b]fluorine (IDF)‐HTMs with the same IDF core and the different periphery groups.

Abstract

Although several hole‐transporting materials (HTMs) have been designed to obtain perovskite solar cells (PSCs) devices with high performance, the dopant‐free HTMs for efficient and stable PSCs remain rare. Herein, a rigid planar 6,12‐dihydroindeno[1,2‐b]fluorine (IDF) core with different numbers of bulky periphery groups to construct dopant‐free HTMs of IDF‐SFXPh, IDF‐DiDPA, and IDF‐TeDPA is modified. Thanks to the contributions of the planar IDF core and the twisted SFX periphery groups, the dopant‐free IDF‐SFXPh‐based PSCs device achieves a device performance of 17.6%, comparable to the doped 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based device (17.6%), with much enhanced device stability under glovebox and ambient conditions.