Modified Perovskite Films Research Articles

A-site assisted perovskite crystallization via ion-exchange MOFs for high efficient and stable perovskite solar cells

Here, a novel strategy is proposed targeting the volatility of A-site cations and the disordered arrangement of perovskite grains through employing Cs+ contained metal–organic frameworks In-aip(Cs) obtained by ion-exchange and crystalline transform. Interatomic forces between Cs-O atoms split the pore channels of the pristine In-aip, endowing In-aip(Cs) with multidimensional charge transport channels. In addition, the partially freed Cs+ in the interlayer compensates for the vacancy of A-site cations during the perovskite preparation process. The In-aip(Cs) modified perovskite films have a flat morphology, large grains and excellent optoelectronic properties. Benefiting from the high-quality perovskite films and faster charge extraction, the In-aip(Cs)-modified PSCs achieved a champion PCE of 23.03%, superior to the In-aip-modified (22.29%) and control device (21.13%). More importantly, the unencapsulated PSCs modified with In-aip(Cs) exhibited outstanding humidity and thermal stability. Over a period of almost 1000 h, the unencapsulated In-aip(Cs)-modified device retained 85% of its initial PCE after storing in a glove box at 85 °C, and retained 87% of the primary PCE upon storage in ambient condition at 25 °C under a humidity of 40%.

Multidentate anchoring strategy for synergistically modulating crystallization and stability towards efficient perovskite solar cells

CsFAMA-based perovskite solar cells (PSCs) have become promising photovoltaic materials due to their excellent photovoltaic properties. However, the inherent instability of perovskite severely hinders its commercialization, since the typical ionic nature of perovskite can cause the ionic migration and volatilization of organic components. Herein, allantoin molecules with multiple alternating carbonyl (C = O) and amino (–NH2 or –NH–) groups were introduced as a multidentate additive to synergistically anchor the perovskite components and passivate the defects, resulting in non-radiative recombination and ion migration being not susceptible to occur. Due to the favorable attraction ability of allantoin to the perovskite precursor monomers and the formation of hydrogen bonds between FAI/MAI and allantoin, the crystal quality of perovskite was significantly improved and the bulk/interface defects were effectively eliminated. Specifically, the interaction between allantoin and FA+/MA+ can suppress their deprotonations and prevent the volatilization of organic cations from the initial precursor solutions, drastically reducing the wastage of precursor materials. Consequently, the modified devices demonstrated a prominent elevation of performance with a higher efficiency of 21.49 %, which was better than that of control (19.67 %). Moreover, the modified perovskite films also demonstrated prominent stability under the continuous illumination and moisture aging owing to the reduction of trap defects. The allantoin additives also greatly enhanced the hydrophobicity of perovskite, and the modified device maintained 90 % of its initial efficiency for 1680 h at 10–20 % relative humidity without encapsulation.

Chemical Chelation-Assisted Highly Oriented Perovskite Solar Cells with Reduced Non-radiative Loss

Shallow- and/or deep-level defects on perovskite surfaces and at grain boundaries serving as recombination centers can negatively affect photovoltaic device performance and stability, thus they need to be minimized. In this study, we modified perovskite films with furan-2,5-dicarboxylic acid (FDCA) to modulate perovskite crystallization and passivate multiple intrinsic shallow- and deep-level defects using an antisolvent method. Characterizations and density functional theory simulation were performed to investigate the coordination and hydrogen bonding interactions between FDCA and perovskite that led to oriented grains and reduced defects. The interactions between FDCA and perovskite reduced non-radiative recombination and improved charge transport. The FDCA-based solar cells exhibited a superior power conversion efficiency of over 24% with improved operation and storage stabilities. This passivation strategy reveals the mechanism behind the improvement of device performance.

In-Depth Insight into the Effect of Hydrophilic-Hydrophobic Group Designing in Amidinium Salts for Perovskite Precursor Solution on Their Photovoltaic Performance.

Amidinium salts have been utilized in perovskite precursor solutions as additives to improve the quality of perovskite films. The design of hydrophilic or hydrophobic groups in amidinium salts is of great importance to photovoltaic device performance and stability in particular. Here we report a contrast study of a guanidinium iodide (GUI) additive with a hydrophilic NH2 group, and a N,1-diiodoformamidine (DIFA) additive with a hydrophobic C-I group, to investigate the group effect. The addition of GUI or DIFA was beneficial to achieve high quality perovskite film and superior photovoltaic device performance. Compared with GUI, the addition of the DIFA in a perovskite precursor solution enhanced the crystal quality, reduced the defect density, and protected the water penetration into perovskite film. The perovskite solar cell (PSC) devices showed the best power conversion efficiency (PCE) of 21.19% for those modified with DIFA, as compared to 18.85% for the control, and 20.85% for those modified with GUI. In benefit to the hydrophobic C-I group, the DIFA-modified perovskite films and PSC exhibited the best light stability, thermal stability, and humidity stability in comparison to the control films and GUI-modified films. Overall, the introduction of a hydrophobic group in the amidinium salts additive was demonstrated to be an efficient approach to achieve high quality and stable perovskite film and PSC devices.

Ultra-high moisture stability perovskite films, soaking in water over 360 min

Perovskite solar cells (PSCs) have attracted wide publicity via their excellent photoelectric properties. Nevertheless, the relative low formation energy and soft lattice of perovskite make it vulnerable to external environmental factors, especially the moisture. Inevitably, water molecules would remain in PSCs modules during practical production. Trace amounts of free water could not only destroy perovskite film, but also trigger metal electrode oxidation forming “snail tracks”. Most previous reports simply block perovskite film contact with water, ignoring the migration damage of free water in PSCs system. Herein, Sorbitan Monolaurate (Span 20), with multiple hydroxyl, ether, carbonyl groups and long alkyl chain, was adopted to post-treat the surface of perovskite. The hydrophilic groups can effectively absorb the slight free water in PSCs system, while the hydrophobic group would protect perovskite film from erosion. Shockingly, the modified perovskite films are hydrophilic, and present outstanding water stability, soaking in water over 360 min. The perovskite films morphology, crystallinity, photoelectric properties and decomposition processes are also improved by Span 20 passivation. In addition, the Span 20/IPA post-treatment has a large concentration operation window (5–100 mg/mL). The optimum power conversion efficiency (PCE) of devices increased to 22.13%, and preserved over 90% of the initial PCE during 29 d air aging (humidity: 60 ∼ 80 RH). This work overturns the popular concept of perovskite water stability, and provides a novel sight to improve the long-term stability of perovskite.

Lead fixation by spider web-like porphyrin polymer for stable and clean perovskite solar cells

Inhabiting the escape and migration of lead ions of perovskite films is crucial for the stable and clean perovskite solar cells (PSCs). The spider web is strong and sticky to catch the prey. Inspired by that, we employed an in-situ polymerized Zinc porphyrin (ZnP) with carbonyl group to fix lead into perovskite lattice to encapsulate the perovskite films. The carbonyl group can coordinate with lead ions in the modified perovskite films to inhibit the lead leakage when soaked into water, which could also reduce the trap density and suppressed non-radiative recombination within the modified film demonstrated by the space-charge-limited current, photoluminescence intensity and the electronic impedance spectroscopy measurements. Thus, the stabilities of the treated PSCs against heating and lighting improved obviously, and the optimal Power conversion efficiency (PCE) of PSCs also increased to 20.53% with the passivation of ZnP. This finding has provided an effective method to fix lead ions in the perovskite lattice.

Two‐Dimensional Metal–Organic Frameworks‐Based Grain Termination Strategy Enables High‐Efficiency Perovskite Photovoltaics with Enhanced Moisture and Thermal Stability

AbstractPerovskite degradation induced by surface defects and imperfect grain boundaries of films seriously damages the performance of perovskite solar cells (PSCs). Meanwhile, conventional organic molecules cannot maintain the long‐time passivation effects under the stimulation of external environmental factors. Here, efficient and stable grain passivation in perovskite films is realized by preparing formic acid‐functionalized 2D metal–organic frameworks (MOFs) as the terminated agent. Through robust interactions between exposed active sites and PbI2, the 2D MOFs tightly caps the surface of PbI2‐terminated perovskite grains to stabilize the perovskite phases and aids the adhesion of adjacent grains. The MOFs mainly distributed at the grain boundaries of the perovskite film is directly observed at the microscopic scale. The modified perovskite films have regular morphology, lower defect density, and superior optoelectronic properties. Benefiting from the suppressed charge recombination and faster charge extraction, a power conversion efficiency of 21.28% is achieved for the best‐performing PSC device. The unencapsulated PSCs with the MOFs modification maintain 88% and 81% of their initial efficiency after 750 h heating at 85 °C under N2 atmosphere and more than 1000 h storage in ambient environment (25 °C, RH ≈ 40%), respectively.

Nicotinamide as Additive for Microcrystalline and Defect Passivated Perovskite Solar Cells with 21.7% Efficiency.

Passivation of electronic defects on the surface and at grain boundaries (GBs) of perovskite films has become one of the most effective tactics to suppress charge recombination in perovskite solar cells. It is demonstrated that trap states can be effectively passivated by Lewis acid or base functional groups. In this work, nicotinamide (NTM, commonly known as vitamin B3 or vitamin PP) serving as a Lewis base additive is introduced into the PbI2 and/or FAI: MABr: MACl precursor solution to obtain NTM modified perovskite films. It has been found that the NTM in the perovskite film can well passivate surface and GBs defects, control the film morphology and enhance the crystallinity via its interaction with a lone pair of electrons in nitrogen. In the presence of the NTM additive, we obtained enlarged perovskite crystal grain about 3.6 μm and a champion planar perovskite solar cell with efficiency of 21.72% and negligible hysteresis. Our findings provide an effective route for crystal growth and defect passivation to bring further increases on both efficiency and stability of perovskite solar cells.

Amphoteric imidazole doping induced large-grained perovskite with reduced defect density for high performance inverted solar cells

Intrinsic defect density in polycrystalline halide perovskite films are required to be low enough to suppress charge recombination loss for improvement in performance of perovskite solar cells (PeSCs). In this paper, we propose the use of amphoteric imidazole to achieve high crystalline quality of CH3NH3PbI3 perovskite absorption layer. The imidazole additive plays a synergistic role in controlling the perovskite crystal growth for large grain size and passivating the uncoordinated ions (e.g., Pb2+) defects, resulting in improved carrier transport/lifetime and suppressed non-radiative recombination. The champion power conversion efficiency (PCE) of PeSCs with imidazole is improved to 16.88%, from the control device with a PCE value of 14.65%. Besides, the stability of imidazole modified perovskite films is further improved by limiting ion immigration at grain boundaries against moisture and heat stresses. The findings pave an avenue for synergistically modulating crystallization and healing defect in perovskite to achieve efficient and stable solar cells.

Enhanced emission from CH3NH3PbBr3 perovskite films by graphene quantum dot modification

Organic-inorganic hybrid perovskites have emerged as promising emitters with the benefits of low cost and high color purity, but their low luminescence efficiency is a drawback for practical application on light emitting devices. Here we show that by incorporating proper amount of graphene quantum dots (GQDs) into perovskite precursor, dense CH3NH3PbBr3 films with reduced grain size and well passivated grain boundaries could be obtained. This gives rise to enhanced emission from GQD modified perovskite films. Our work thus provides a viable way to prepare highly luminescent perovskite films for optoelectronic applications.

Improving the performance of perovskite solar cells by surface passivation

Abstract NiO has a perfect-aligned energy level with CH3NH3PbI3 perovskite such that it serves as a hole transport layer (HTL), but NiO-based perovskite solar cells (PSCs) still suffer from low efficiency due to the poor interface contact between the perovskite layer and the NiO HTL, and haphazardly stacked perovskite grains. Herein, poly[(9,9­dioctylfluorenyl­2,7-diyl)-alt-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (PFBT) is introduced between the NiO and perovskite layers in the form of a polymer aggregate to enhance perovskite crystallinity and decrease the interface charge recombination between perovskite and NiO in PSCs, resulting in an improved performance. Moreover, PFBT modified perovskite films showed sharper, smoother, and more compact crystalline grains with fewer grain boundaries, leading to the decreased nonradiative recombination. This study offers a simple strategy to achieve highly efficient PSCs with the incorporation of polymer semiconductor aggregates to passivate the interface between the perovskite and NiO layers.

Chemical inhibition of reversible decomposition for efficient and super-stable perovskite solar cells

Despite the remarkable photovoltaic characteristics and printability of perovskite solar cells, their intrinsic instability has been the most serious drawback toward future commercialization. In this work, we have investigated the stability of perovskite films in terms of morphology, electronic properties and chemical compositions. Specifically, the chemical decomposition inhibition strategy was introduced in perovskite films through iodine bromide to modify the crystal defects, leading to PSCs with suppressed hysteresis effects, superior durability and attractive PCE of 21.5%. Femto-second transient absorption spectra and GIWAXS measurements provide deep insight into the reduced carrier recombination and indicate the improved crystallinity of the modified perovskite films. Furthermore, an efficient hole-transporting material, PDPP4T, without using any doping process is applied to achieve PSCs with enhanced open-circuit voltage and better repeatability. As a consequence, the modified PSCs could maintain 82% of their initial efficiency after 5000 h of storage in ambient conditions and 90% of their initial efficiency after 1000 h of light soaking process. An excellent water resistance up to 100 h of the PSCs is also obtained by encapsulation technology. Besides, after coating Ce3+-CsPbI3 nanocrystals as luminescent down-shifting layers on the front side of the PSCs, the PCE of the device was further improved to 22.16%.

The effect of phase purification on photovoltaic performance of perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have witnessed a rapid rising in power conversion efficiency (PCE) over the past few years; however, they still suffer from recombination loss via interface defects in perovskite films. In this study, we implement an efficient phase purification strategy by incorporating isopropyl alcohol (IPA) post-treatment of perovskite films that reduces defect states and improves charge transport. It is found that the DMSO-PbI2-MAI complex in the perovskite film is eliminated after IPA post-treatment. A suit of opto-electric characterizations demonstrates that the nonradiative recombination is greatly diminished, and charge extraction is effectively boosted in the modified perovskite films. The perovskite solar cells with phase-pure MAPbI3 achieve an impressively larger PCE of 18.78% than that of 17.1% for the control devices. Our work presents a facile and efficient path to performance improvement of PSCs.

Enhanced photovoltaic properties of perovskite solar cells by the addition of cellulose derivatives to MAPbI3 based photoactive layer

In this study, chlorodeoxyhydroxyethylcellulose (CDHC) was synthesized from hydroxyethylcellulose (HEC) through chlorination and then both HEC and CDHC were applied individually as additives within the methylammonium lead iodide (CH3NH3PbI3, MAPbI3) layers of perovskite solar cells (PVSCs). The architecture of the PVSCs was indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/MAPbI3:cellulose derivative/[6,6]-phenyl-C61-butyric acid methyl ester/Ag. The photovoltaic (PV) properties of the HEC- and CDHC-incorporated PVSCs were superior to those of the corresponding pristine PVSC prepared without an additive, a result of decreases in the number of grain boundary defects as well as increases in the crystal grain sizes, crystallinities, and absorption intensities of the modified perovskite films. Moreover, the polymer chains of CDHC, presenting chlorine atoms, were particularly beneficial for enhancing the crystal size and crystallinity of the MAPbI3 film, resulting in the highest absorbance and PV performance in this study being those of a CDHC-doped PVSC. Indeed, this CDHC-incorporated PVSC displayed a short-circuit current density of 17.73 mA cm−2, an open-circuit voltage of 0.96 V, a fill factor of 0.61, and a power conversion efficiency of 10.38%.

Surface Modification of Methylamine Lead Halide Perovskite with Aliphatic Amine Hydroiodide.

By spin-coating method, a thin layer of dodecylamine hydroiodide (DAHI) is introduced to the surface of perovskite CH3NH3PbI xCl3- x. This layer of DAHI successfully changes the surface of perovskite from hydrophilic to hydrophobic as revealed by the water contact angle measurement. Significantly enhanced fluorescence intensity and prolonged fluorescence lifetime are found for these modified films in comparison to those of unmodified perovskite films, suggesting that the number of structure defects is reduced dramatically. The compatibility between the perovskite and hole transfer layer (HTL) is also improved, which leads to more efficient hole collection from the perovskite layer by HTL as revealed by the fluorescence spectra, fluorescence decay dynamics, as well as the transient photocurrent measurements. Moreover, the perovskite solar cells (PSCs) fabricated from these modified perovskite films exhibit significantly improved humidity stability as well as promoted photoelectron conversion efficiency (PCE). The result of this research reveals for the first time that the layer of aliphatic amino hydroiodide is a multiple functions layer, which can not only improve the humidity stability but also promote the performance of PSCs by reducing the defect number and improve the compatibility between perovskite and HTL. Because the structure of aliphatic amines can be functionalized with myriad of other groups, this perovskite modification method should be very promising in promoting the performance of PSCs.

Improving the performance of low-temperature planar perovskite solar cells by adding functional fullerene end-capped polyethylene glycol derivatives

Functional fullerene derivatives play an important role in improving the performance of perovskite solar cells (PSCs) by promoting charge transfer and passivating trap states in perovskite film. In this work, the planar PSCs with the structure of FTO/TiO2/modified CH3NH3PbI3/Spiro-OMeTAD/Ag are fabricated by one-step method. Fullerene end-capped polyethylene glycol derivative (PCBPEG) is synthesized by a simple process and added into the perovskite precursor solution to improve microstructure and photoelectric properties of PSCs. At the optimum concentrations of PCBPEG additives and the annealing time of perovskite film, PSCs with PCBPEG additives yield the average efficiency of over 17.3%, being much higher than 15.28% of the reference PSC. Moreover, the unencapsulated PSCs with PCBPEG additives prepared at the optimal process demonstrate the enhanced stability. Compared to the reference perovskite film, the modified perovskite films demonstrate larger grain size, improved electric properties at nanoscale level and reduced electron trap state density, which will contribute to the favorable photovoltaic performance. The improved performance of the modified PSCs is primarily attributed to the promoted carrier transfer and suppressed charge recombination. These results provide a facile and feasible method to fabricate functional fullerene for high performance PSCs.

Enhancing the efficiency of low-temperature planar perovskite solar cells by modifying the interface between perovskite and hole transport layer with polymers

In this work, planar perovskite solar cells (PSCs) based on CH3NH3PbI3 perovskite layer and low-temperature processed TiO2 have been fabricated. Polymers including poly(methylmethacrylate) (PMMA), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-pheny- lenevinylene] (MEH-PPV) and polyethylene glycol (PEG) in chlorobenzene solution have been selected to modify the interface between perovskite and hole transport layer (HTL), respectively. The concentrations of the three polymer solutions have been optimized. The effect of interfacial modification by different polymer solutions on the photoelectric properties of perovskite layer and the performance of PSCs has been systematically investigated. The microstructure and photoelectric properties of the modified perovskite films has been systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), conducting force microscopy (CFM) and Kelvin probe force microscopy (KPFM). The results reveal that the modified perovskite films with tetrahedral perovskite structure have lager grain size, lower roughness and better photoelectric properties compared with the reference sample. The electron trap state density (Dtrap), charge extraction, carrier transfer and recombination process in the PSCs have been investigated by current-voltage (I-V) characteristic curves, steady-state photoluminescence (PL), photo-voltage decay and electrochemical impedance spectroscopy (EIS). The results indicate that the polymeric interface modification at the optimum concentration can reduce the Dtrap, promote the charge transfer and suppress carrier recombination, resulting in the improved performance of PSCs. All of the modified PSCs at an optimum concentration exhibit the improved fill factor (FF) and open circuit voltage (Voc), thus the power conversion efficiency (PCE) is enhanced to over 17% from 15.49%.

Enhanced efficiency and stability of carbon based perovskite solar cells using terephthalic acid additive

We report an effective method to modulate the perovskite crystallization by incorporating terephthalic acid (TPA) additive into the perovskite precursor solutions. The addition of TPA additive simultaneously facilitate nucleation and modulate the kinetics of crystal growth from solutions, enabling preparation of smooth and compact perovskite morphology with improved crystallization, grain uniformity, loading of perovskite into the TiO2 mesopores and coverage area. Before deposition of perovskite films, an ultrathin wide bandgap dielectric MgO nanolayer is incorporated onto porous TiO2 to retard the carrier recombination at the interface in the devices. The mechanism of the interaction between TPA and Pb2+ on the film-forming process of perovskite crystal was also analyzed. Consequently, by optimizing the TPA concentration, we obtained highly efficient and stable low-temperature processed electron transport material (HTM)-free carbon based perovskite solar cells (PSCs). The performance of the PSCs is significantly improved with the mean efficiency increasing from 11.5% ± 0.67% for the control structure to 14.29% ± 0.37% for the TPA modified structure. Furthermore, the TPA modified perovskite films and devices present much better thermal, UV and ambient air stability than the pristine perovskite films and devices, and the corresponding PSCs with TPA modification show excellent stability with efficiency approaching 94% after being exposed to ambient air for 21 days without encapsulation.