MAPbI3 Layer Research Articles

Star-shaped molecule with planar triazine core and perylene diimide branches as an n-type additive for bulk-heterojunction perovskite solar cells

A novel star-shaped molecule, TPDI, featuring a triazine unit as the core and units of a perylene diimide derivative as its conjugated branches, has been synthesized for application as an n-type additive in methylammonium lead iodide (CH3NH3PbI3, MAPbI3)–based bulk-heterojunction perovskite solar cells (PVSCs). The architecture of the PVSCs was indium tin oxide/poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate)/MAPbI3:TPDI/[6,6]-phenyl-C61-butyric acid methyl ester/Ag. The star-shaped molecule TPDI was added into the MAPbI3 layer to minimize grain boundary defects and enhance the coverage and crystal grain sizes of the perovskite film. Upon the incorporation of a suitable amount of TPDI, the defects of the perovskite crystals were compensated for, and the absorbance and electron-transfer ability of the photoactive active layer were enhanced, thereby improving the photovoltaic performance of the PVSC. An open-circuit voltage of 0.83 V, a short-circuit current of 19.2 mA cm−2, a fill factor of 0.68, and a photoconversion efficiency of 10.84% were obtained for the TPDI-doped PVSC.

Analysis of light-induced degradation in inverted perovskite solar cells under short-circuited conditions

Abstract We present a study at both the device level and the interfacial analytic level on the light-induced degradation in the inverted CH3NH3PbI3 (MAPbI3) perovskite solar cells (PSCs) with an optimized structure, which is based on the Cu electrode and the charge transport layers of PTAA and Bphen. To accelerate the degradation process, the PSCs are exposed to concentrated sunlight of 2 suns with different illumination duration under the short-circuited condition. Compared to under the open-circuited condition, the PSCs presented here show a slightly more obvious decrease of power conversion efficiency (PCE) and a more distinct hysteresis with the time of light exposure. We infer that, under the short-circuited condition, the more dramatic migration of ions induced by photocurrents should result in greater ohmic dissipation and more trap states. But the weak degradation effects indicate that the devices still exhibit an unexpected photochemical stability against light exposure. Most importantly, there is a self-healing behavior to partly recover the degradation with the light exposure time of more than 8 h, which can be ascribed to the modification from the Cu electrode and the charge transport layers of PTAA and Bphen. Being wrapped tightly in these coverlayers, the possible volatile decomposition products are restrained inside of the MAPbI3 layers and converted back to MAPbI3 with a great probability.

Caffeine Improves the Performance and Thermal Stability of Perovskite Solar Cells

Summary To increase the commercial prospects of metal halide perovskite solar cells, there is a need for simple, cost-effective, and generalized approaches that mitigate their intrinsic thermal instability. Here we show that 1,3,7-trimethylxanthine, a commodity chemical with two conjugated carboxyl groups better known by its common name caffeine, improves the performance and thermal stability of perovskite solar cells based on both MAPbI3 and CsFAMAPbI3 active layers. The strong interaction between caffeine and Pb2+ ions serves as a “molecular lock” that increases the activation energy during film crystallization, delivering a perovskite film with preferred orientation, improved electronic properties, reduced ion migration, and greatly enhanced thermal stability. Planar n-i-p solar cells based on caffeine-incorporated pure MAPbI3 perovskites, which are notoriously unstable, exhibit a champion-stabilized efficiency of 19.8% and retain over 85% of their efficiency under continuous annealing at 85°C in nitrogen.

High Efficiency (18.53%) of Flexible Perovskite Solar Cells via the Insertion of Potassium Chloride between SnO2 and CH3NH3PbI3 Layers

Flexible perovskite solar cells (PSCs) were ideal candidates for wearable devices due to the merits of flexibility, high efficiency, and being lightweight, and they could be fabricated in a continuous roll-to-roll production process to achieve large-area and low cost devices. Herein, the high efficiency (up to 18.53%) and fill factor (0.81) of flexible PSCs (ITO/SnO2/KCl/MAPbI3/spiro-OMeTAD/Ag) were achieved by low-pressure assisted solution processing under low temperature (⩽100 °C). The surface morphology and crystallinity of perovskite films were effectively promoted by the KCl modification and the defect density of perovskite films as well as the hysteresis of the corresponding devices was reduced accordingly. In addition, the stability and bendability of the KCl-modified flexible PSCs were improved simultaneously. To the best of our knowledge, both the efficiency and fill factor are the best among all flexible PSCs reported to date. Therefore, the insertion of KCl between SnO2 and MAPbI3 layers provi...

Hole transport materials doped to absorber film for improving the performance of the perovskite solar cells

In spite of the power conversion efficiency (PCE) of the PSCs has been rapidly improved, many changes, such as defects, poor morphology and expensive hole transporte materials (HTM), are still on the way to commercial application. In this paper, CuSCN was used as the HTM to substitute the popular Spiro-OMeTAD, because of its high hole transfer capability and much lower cost. A simple, planar construction device of FTO/c-TiO2/MAPbI3/CuSCN/Au has been prepared, yielding a best PCE of 10.41%. To improve the stability and performance of the device, 1 mol % CuSCN was innovatively introduced into the absorber layer. The grain size of the MAPbI3 layer has increased obviously and the defects in the device were also reduced. The fast charge transportation and extraction make the short-circuit currents increased from 18.39 mA/cm2 to 22.18 mA/cm2. The PCE of the CuSCN doped perovskite solar cells (PSCs) have increased 21.4% compared to the control device without added. In the meantime, the hysteresis and stability have also been improved effectively. The PCE of the CuSCN doped MAPbI3 device only dropped 28% from the initial efficiency after 500 h stored in an ambient air with temperature of 25°Cand humidity of 20%, whereas the device without CuSCN doped droped 48%. This result indicates that CuSCN has great potential as a doping to improve the morphology of perovskite films and enhance the performance of the PSCs.

Efficiency of MAPbI3-Based Planar Solar Cell Analyzed by Its Thickness-Dependent Exciton Formation, Morphology, and Crystallinity.

In spite of the impressive progresses regarding perovskite-type solar cells, a clear understanding about underlying mechanisms therein is still sparse, especially because of the absence of spatially resolved device characteristics which should be linked to exciton formation efficiency, morphology, and crystallinity being estimated as functions of positions within active layers. Here, the planar CH3NH3PbI3 (MAPbI3) perovskite solar cells (PeSCs) with ZnO as the electron-transporting layer (ETL) were fabricated. By varying the wide range of MAPbI3 active-layer thickness, we estimate their device parameters and external quantum efficiencies in addition to internal absorption spectra (Q) by means of the transfer matrix method. Furthermore, the spectrally and spatially resolved internal quantum efficiencies (IQEs) as a function of the active-layer thickness within PeSCs were calculated, and the relationship between IQE and device parameters extracted from the current-voltage ( J- V) behaviors was discussed. It was found that the PeSC with MAPbI3 film thickness around 303 nm has a relatively high IQE and PCE, indicating that there is more power loss of PeSCs when the thickness of the MAPbI3 layer is either less or more than about 300 nm. Furthermore, time-resolved photoluminescence together with the thickness-dependent morphology and crystallinity of the MAPbI3 film demonstrate that there are two power loss processes in the fabricated PeSCs: one at the ZnO/MAPbI3 interface and the other in bulk. The present research is beneficial for further understanding of the fundamental physics for the PeSCs based on the ZnO ETL.

Enhanced Efficiency of MAPbI₃ Perovskite Solar Cells with FAPbX₃ Perovskite Quantum Dots.

We describe a method to enhance power conversion efficiency (PCE) of MAPbI3 perovskite solar cell by inserting a FAPbX3 perovskite quantum dots (QD-FAPbX3) layer. The MAPbI3 and QD-FAPbX3 layers were prepared using a simple, rapid spin-coating method in a nitrogen-filled glove box. The solar cell structure consists of ITO/PEDOT:PSS/MAPbI3/QD-FAPbX3/C60/Ag, where PEDOT:PSS, MAPbI3, QD-FAPbX3, and C60 were used as the hole transport layer, light-absorbing layer, absorption enhance layer, and electron transport layer, respectively. The MAPbI3/QD-FAPbX3 solar cells exhibit a PCE of 7.59%, an open circuit voltage (Voc) of 0.9 V, a short-circuit current density (Jsc) of 17.4 mA/cm2, and a fill factor (FF) of 48.6%, respectively.

Initial photochemical stability in perovskite solar cells based on the Cu electrode and the appropriate charge transport layers

This paper presents a study on the initial light-induced degradation in inverted CH3NH3PbI3 (MAPbI3) perovskite solar cells (PSCs) with a structure of indium tin oxide (ITO)/poly (bis(4-phenyl) (2,4,6-trimethylphenyl) amine) (PTAA)/MAPbI3/ [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/4,7-diphenyl-1, 10-phenanthroline (Bphen)/Cu under anoxic conditions. The pristine champion devices fabricated by our optimized techniques can achieve a power conversion efficiency (PCE) of ∼18% with a negligible hysteresis. Accelerated by exposure to concentrated sunlight of 200 mW/cm2, the short-term degradation tests show that the degradation effects of light exposure on the lattice and electronic structures of the MAPbI3 active layer are almost negligible and the photoelectric parameters of devices demonstrate superior photochemical stability against illumination, which can be ascribed to the introductions of the Cu electrode and the proper charge transport layers of PTAA and Bphen. As the MAPbI3 layers are integrated in devices and wrapped in these coverlayers, the volatile decomposition products originated from the photochemical reactions in MAPbI3 active layer should be restrained inside of the active layer and finally regenerated MAPbI3. Moreover, the Cu electrode strongly retards the photochemical reactions by restraining the diffusion of I− ions and further enhances photochemical stability of the devices.

Influence of drying temperature on morphology of MAPbI3 thin films and the performance of solar cells

Photoelectric conversion efficiency of organic-inorganic perovskite solar cells has been rapidly raised and attracted great attention in recent years. The quality of perovskite films is vital for the performance of devices. We used the anti-solvent method to prepare CH3NH3PbI3 thin films by spin coating and dried them at various temperature to transform adduct MAI·PbI2·DMSO into CH3NH3PbI3. We researched in detail on the relationship between surface morphology of MAPbI3 thin films fabricated by the anti-solvent method and various drying temperature. We found that surface roughness and grain size of CH3NH3PbI3 films together increased with increasing drying temperature. The larger grain size could efficiently reduce crystal boundaries which is advantageous for the suppression of photo-induced charge carrier recombination resulting in increase of FF. However, increase of surface roughness resulted in larger contact area at interface which might produce more tarp states and poorer wettability of HTM solution leading in decrease of Jsc. Surface morphology of MAPbI3 layer on the performance of solar cell devices is also an important research issue. By optimizing the drying temperature to 60 °C, the highest efficiency of 14.4% was achieved for the CH3NH3PbI3-based solar cell devices.

Precursor effects on methylamine gas-induced CH3NH3PbI3 films for stable carbon-based perovskite solar cells

Carbon-based perovskite solar cells (PSCs) without hole transport material have attracted much attention due to their high stability. However, their power conversion efficiency (PCE) is still low because of the lack of suitable perovskite deposition methods. In this work, a facile solid-gas reaction method was explored to prepare high quality perovskite and the effects of precursor composition (PbI2, PbI2 + HI, HPbI3 and PbI2 + NH4I) on the CH3NH2 (MA) gas-induced perovskite were systematically investigated. The lack of H+/I− and/or HI suppressed the conversion of PbI2 precursor to MAPbI3, while rapid and complete conversion was achieved for (PbI2 + HI), HPbI3 and (PbI2 + NH4I) precursors. Compared with HPbI3 precursor, the presence of H2O molecules in (PbI2 + HI) precursor lowered the perovskite crystallinity, while the release of NH3 from (PbI2 + NH4I) precursor slower the interaction of MA molecules with MAPbI3 for achieving the highest quality MAPbI3 layer. The application of an anti-solvent treatment during precursor deposition further improved the quality of HPbI3 and (PbI2 + NH4I)-processed perovskite layers, which as a result, significantly promoted the photovoltaic performance of C-PSCs. Both devices exhibited considerably high stability, especially for the (PbI2 + NH4I)-processed C-PSCs, who did not show any PCE degradation after storage in air atmosphere at room temperature for 120 days.

Real‐Time In Situ Observation of Microstructural Change in Organometal Halide Perovskite Induced by Thermal Degradation

AbstractOrganometal halide perovskites have demonstrated remarkable achievements in solar cell applications and have attracted tremendous attention as next‐generation photovoltaic materials. Regardless of the unprecedented success, the degradation of the perovskite has caused the performance of the perovskite solar cells to be unreliable and prevented their commercialization. However, the detailed degradation mechanism of the perovskite has yet to be elucidated. In this study, the entire procedure of the thermally induced degradation of methylammonium lead iodide (MAPbI3) is reported using real‐time in situ transmission electron microscopy. The in situ investigation directly illustrates the detailed process of precipitating trigonal PbI2 grains during thermal degradation and confirms that trigonal PbI2 is precipitated from the amorphized MAPbI3 layer via intermediate states. The intermediate states and their stackings enable the generation of 3D linear‐empty spaces that can be utilized as passages by elements during the decomposition and intercalation of the perovskite. This report will provide critical clues for the commercialization of the perovskite‐based solar cells and for further investigation of the synthesis of the perovskite, which is not fully understood.

Influence of interfacial Sn-doping on band alignment of ZnO-nanorods/MAPbI3 interface: The density functional calculation

This work used density functional theory to investigate both electronic and atomic structures at the interface between MAPbI3 and Sn-doped ZnO-nanorods. In the model, we considered two possible surfaces of ZnO-nanorod in forming interfaces with MAPbI3 layers, i.e. polar (0001) and non-polar (1010) surfaces. From the results, the undoped ZnO(1010)/MAPbI3 interface presents type-II band alignment, whereas the undoped ZnO(0001)/MAPbI3 presents type-I band alignment. The partial density of state analysis reveals that the trap state only occurs in ZnO(0001)/MAPbI3. However, when Sn atoms substitute the Zn atoms at the interface, the band alignment was found to change from type-II to type-I in Sn-doped ZnO(1010)/MAPbI3, and the band gap of ZnO was found reduced. However, for the Sn-doped ZnO(0001)/MAPbI3, the band alignment is still type-I and the band gap is almost unchanged. This means that how the band structure of Sn-doped ZnO/MAPbI3 realigns depends on how ZnO terminates at the interface (i.e. polar or non-polar), when Sn-doping is introduced. In addition, this band alignment modification of Sn-doped ZnO/ MAPbI3 was found to be originated from the contribution of Sn 5s-orbital at the band edge, which adjusts the band structure of ZnO at the interface. These therefore suggest that using Sn-doped ZnO nanorods as electron transporting layers may be useful for band engineering. Specifically, the band alignment and band offsets can be tuned with controlling of Sn concentration.

Influence of organic cations on intrinsic properties of lead iodide perovskite solar cells

Abstract A comprehensive study of the intrinsic properties including internal absorption, charge recombination and internal quantum efficiency of lead iodide perovskite solar cells (PeSCs) based on CH3NH3PbI3 (MAPbI3) and HC(NH2)2PbI3 (FAPbI3) perovskites is presented. The optical simulation of PeSCs shows that FAPbI3 layer exhibits a wider internal absorption spectra than MAPbI3 layer due to its lower bandgap determined by ultraviolet photoemission spectroscopic (UPS) and band structure calculation. The photo-induced charge extraction by linearly increasing voltage (photo-CELIV) investigation and photocurrent density (Jph) versus effective voltage (Veff) measurement indicate that FAPbI3 PeSCs possess lower charge recombination and higher charge collection efficiency because of the superior transport property of FAPbI3 perovskite, leading to higher internal quantum efficiency (IQE) than MAPbI3 PeSCs. In addition, by comparing the calculated short-circuit current (JSC) by Spectroscopic Limited Maximum Efficiency (SLME) mathematical models, the predicted JSC by transfer matrix optical model (TMM) and the experimental JSC from current density versus voltage (J-V) characteristics, we find that the light losses from the absorption of non-perovskite layers and the reflection at the incident interface of device, and the charge recombination in perovskite layer are the main factors limiting the photocurrent of PeSCs based on these two perovskites.

Insights about the Absence of Rb Cation from the 3D Perovskite Lattice: Effect on the Structural, Morphological, and Photophysical Properties and Photovoltaic Performance.

Efficiencies >20% are obtained from the perovskite solar cells (PSCs) employing Cs+ and Rb+ based perovskite compositions; therefore, it is important to understand the effect of these inorganic cations specifically Rb+ on the properties of perovskite structures. Here the influence of Cs+ and Rb+ is elucidated on the structural, morphological, and photophysical properties of perovskite structures and the photovoltaic performances of resulting PSCs. Structural, photoluminescence (PL), and external quantum efficiency studies establish the incorporation of Cs+ (x < 10%) but amply rule out the possibility of Rb-incorporation into the MAPbI3 (MA = CH3 NH3+ ) lattice. Moreover, morphological studies and time-resolved PL show that both Cs+ and Rb+ detrimentally affect the surface coverage of MAPbI3 layers and charge-carrier dynamics, respectively, by influencing nucleation density and by inducing nonradiative recombination. In addition, differential scanning calorimetry shows that the transition from orthorhombic to tetragonal phase occurring around 160 K requires more thermal energy for the Cs-containing MAPbI3 systems compared to the pristine MAPbI3 . Investigation including mixed halide (I/Br) and mixed cation A-cation based compositions further confirms the absence of Rb+ from the 3D-perovskite lattice. The fundamental insights gained through this work will be of great significance to further understand highly promising perovskite compositions.

Influence of Perovskite Grain Size and TiO2 Surface States to the Performance of Perovskite Solar Cell

Introduction Currently, perovskite solar cells (PSCs) have received great curiosity from solar cell field, due to rapid improvement in their photoelectric conversion efficiencies (PCEs) from 9% to over 22% [1]. PSCs can be constructed with either invert or regular structure. In invert type, organic materials are employed as the hole transporting layer (HTL) and electron transporting layer (ETL). The PSCs with invert structure has reached PCE 20.15% [2]. The PSCs with regular structure typically employs both mesoporous and planar inorganic material of TiO2 to serve as ETL because the affinity of perovskite layers with TiO2 is generally lower than that with organic ETL. When considering the long stability, inorganic material would be better than organic material as ETL. The current best performing PSCs with mesoporous TiO2 as ETL perform PCE >22% because mesoporous TiO2 helps to support perovskite layer acting as a scaffold and make better connection with perovskite layer[1]. On the other hand, PSCs with planar TiO2 also shows PCE about 20.2%, which is lower than with mesoporous TiO2 but provides a more straightforward platform for fundamental investigations [3]. However, in planar structure, the quality of each layers is an important factor that affect overall cell performance, especially ETL [4]. However, a mechanism depending on ETL on cell performances should be investigated in detail. For ETL, various deposition methods are explored, such as spin coating, atomic layer deposition and sputtering, especially spray-coating has a great potential for the large-scale production due to the low cost and capability to obtain thin films in large areas for industrial application. The polycrystalline has grain boundaries. When having polycrystalline Si with more grain boundaries, Si solar cell shows lower performance because grain boundaries act as recombination centers for electrons and holes and become scattering centers of free carriers [5]. CuInSe2 (CIS) solar cell showed PCE about 20% without special grain boundaries passivation [6]. Similar to CIS solar cell, perovskite grain boundaries seem to have small effect on photoelectronic properties because even high efficient PSCs have smaller grain size less than 1 um. Therefore, the relationship between grain size and performance of PSCs have not been clarified clearly. Here, Kim et al. showed planar PSCs of FTO/compact TiO2/MAPbI3/spiro-OMeTAD/Au. They obtained dense MAPbI3 layers with different grain sizes ranging from ~100 to ~500 nm. With increasing grain size of perovskites, all the photovoltaic parameters were improved. As a result, the best performance was observed for the largest perovskite grain size (~500 nm) with PCE of 19.4% [7]. In this paper, we investigated the relationship between PSC performance and grain size distribution on the top of planar TiO2 by spray pyrolysis technique to construct more defined structure. SEM image from top view and cross section were observed and used to identify the grain size distribution. The surface roughness of TiO2 measured by AFM used for investigation of the relationship with perovskite grain size. These findings indicated that wide variations in the device efficiency also influence by perovskite grain size distribution in planar heterojunction structure. Experimental An around 80-100 nm thick compact-TiO2 layer was deposited on ITO by spray pyrolysis at 350, 450 and 550°C to investigate the of spray temperature (Ts) effect on compact-TiO2 surface state. Also, ethanol, propanol and 1-butanol were used as a precursor solvent with different boiling temperature (Tb) to find their effects on compact-TiO2 surface state. Next, samples were annealed at different temperatures (400, 450, 500 or 600°C) for 1 hr to study an effect of annealing temperature (Ta) as shown in Table 1. The device structure consists of; ITO/compact-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au with active area 0.5 cm2. Results and discussion In this work, planar PSCs were assembled with different ETL fabrication condition for comparison. The uniform TiO2 film without pin hole were successfully deposited by using 1-butanol as a solvent. However, the wide range variations of PCE values were observed even at the same fabrication condition. So, grain size distribution was measured and the results indicated that higher device efficiencies at the same ETL fabrication condition resulted from larger perovskite grain size distribution. The obtained dense MAPbI3 layers showed different grain sizes ranging from ~100 to ~300 nm. In addition, the relationship between roughness of TiO2 and perovskite grain size showed that the larger grain sizes were from lower roughness of TiO2 film. Therefore, higher PCEs were strongly achieved from larger perovskite grain size on planar TiO2. Figure 1

Fully doctor-bladed planar heterojunction perovskite solar cells under ambient condition

Abstract It is highly desirable to develop large-scale, low-cost fabrication techniques to process perovskite solar cells (PSCs) under ambient condition for accelerating their potential commercialization. Herein, efficient planar heterojunction (PHJ) PSCs with a simple structure of ITO/NiOx/CH3NH3PbI3 (MAPbI3)/PC61BM/Ag is fabricated via fully doctor blading under ambient condition with the humidity of ∼40%, in which the NiOx layer, MAPbI3 layer, and PC61BM layer are deposited via doctor blading subsequently. The high-quality perovskite CH3NH3PbI3 films are fabricated by one-step doctor-bladed deposition using modified precursors with the co-solvents of dimethylsulphoxide (DMSO) and dimethylformamide (DMF) along with the suitably excess MAI. The well control on solvent recipe retards the crystallization time, leading to the formation of homogeneous and uniform perovskite film with large-size domains. Furthermore, the excess MAI in the modified precursor ink is helpful to form large-size perovskite grains and high crystallinity. With the optimization, the fully doctor-bladed PHJ-PSCs under ambient condition with the power conversion efficiency (PCE) up to 10.92% is achieved, yielding an average PCE of 10.16%. The research suggests that solvent engineering is a good route for fabricating high-quality perovskite films via one-step, doctor-blading deposition, and it is feasible to achieve efficient PHJ-PSCs through fully doctor-balding technique under ambient condition.

Vertical Organic–Inorganic Hybrid Perovskite Schottky Junction Transistors

AbstractWhile organolead halide perovskite materials have attracted considerable attention for high efficiency solar cells, they have shown little potential for use in field‐effect transistors (FETs) due to the limited current modulation at room temperature. Here, by developing a vertically gated methylammonium lead iodide (MAPbI3)/indium‐tin oxide (ITO) Schottky junction, a room temperature‐operable vertical MAPbI3 thin‐film transistor is reported. Due to the injection‐controlled gating mechanism as well as the vertical channel structure, a current modulation ratio up to 104 is achieved. Fabrication of a nanoporous ITO source electrode forming a highly rectifying Schottky junction with the MAPbI3 layer is the key for the large current modulation. It is discovered that the electron transporting layers on top of the MAPbI3 layer play an important role in achieving a large current rectification of 107 at the Schottky junction and hence large current on/off ratios of the resulting transistor.

First-Principles Study of Electron Injection and Defects at the TiO2/CH3NH3PbI3 Interface of Perovskite Solar Cells.

We investigated electron injection rates and vacancy defect properties by performing first-principles calculations on the interface of an anatase-TiO2(001) and a tetragonal CH3NH3PbI3(110) (MAPbI3(110)). We found that the coupling matrix element between the lowest unoccupied molecular orbital of MAPbI3 and the TiO2 conduction band (CB) minimum is negligibly small, the indication being that electron-injection times for low-energy excited states are quite long (more than several tens of picoseconds). We also found that higher-lying CB states coupled more strongly; injection was expected to take place on a femtosecond time scale. Furthermore, we found that vacancy defects in the TiO2 layer produced undesired defect levels that caused hole traps and recombination centers. Whereas most of the vacancy defects in the MAPbI3 layer produced no additional states in the MAPbI3 gap, a Pb vacancy (VPb) at the interface created an energy level below the MAPbI3 CB edge and had a lower energy of formation than the VPb defect in bulk because of the interaction with the TiO2 surface.

Electrochemical deposition of PbI2 for perovskite solar cells

In this work, PbI2 is coated on the TiO2 films via the cost-effective electrochemical deposition method and a MAPbI3 layer is formed by exposing the deposited PbI2 to CH3NH3I subsequently. The structure, composition and morphology of the deposits are characterized in detail by the measurements of X-ray diffraction, scanning electron microscopy, and energy dispersion spectroscopy, respectively. The results suggest that a high coverage and uniform perovskite layer has been fabricated by virtue of the electrochemical deposition method. The influence of the deposition time in the morphology of the obtained perovskite is investigated and the optical properties of the prepared PbI2 and CH3NH3PbI3 are studied as well. As a result, the assembled perovskite solar cells exhibited a maximum PCE of 6.62% with the corresponding photocurrent of 12.5 mA cm−2 and open-circuit voltage of 0.957 V at AM 1.5 solar light of 100 mW cm−2, thus demonstrating the feasible application of electrochemical deposition in perovskite solar cells.

Hybrid perovskite by mixing formamidinium and methylammonium lead iodides for high-performance planar solar cells with efficiency of 19.41%

Organic lead halide layer is crucial to a high-efficiency planar perovskite solar cell. In order to reduce defects and improve the crystallinity of the perovskite layer, we introduce formamidinium lead iodide (FAPbI3) as crystal seeds to modulate the growth of methylammonium lead iodide (MAPbI3) crystals and form a mixed-cation perovskite layer (MA1−xFAxPbI3) by a facial two-step spin-coating method. The field-emission scanning electron microscopy and X-ray diffraction pattern show the better crystallinity and larger grain for mixed-cation perovskite layer than the pristine MAPbI3 layer. Time-resolved photoluminescence decay measurements indicate that the mixed perovskite layer has less defect and longer lifetime for charge recombination than pristine MAPbI3 layer. Under optimal conditions, the planar perovskite solar cell based on MA0.5FA0.5PbI3 achieves a champion power conversion efficiency (PCE) of 19.41%, while the planar perovskite solar cell based on pristine MAPbI3 obtains a PCE of 15.95% under the same conditions.