Electronic Materials Letters

, Volume 14, Issue 2, pp 155–160 | Cite as

Lead Acetate Based Hybrid Perovskite Through Hot Casting for Planar Heterojunction Solar Cells



Flawless coverage of a perovskite layer is essential in order to achieve realistic high-performance planar heterojunction solar cells. We present that high-quality perovskite layers can be efficiently formed by a novel hot casting route combined with MAI (CH3NH3I) and non-halide lead acetate (PbAc2) precursors under ambient atmosphere. Casting temperature is controlled to produce various perovskite microstructures and the resulted crystalline layers are found to be comprised of closely packed islands with a smooth surface structure. Lead acetate employed perovskite solar cells are fabricated using PEDOT:PSS and PCBM charge transporting layers, in pin type planar architecture. Especially, the outstanding open-circuit voltage demonstrates the high crystallinity and dense coverage of the produced perovskite layers by this facile route.

Graphical Abstract


CH3NH3PbI3 Lead acetate Hot casting Air processing Planar heterojunction solar cells 

1 Introduction

Perovskite-based hybrid solar cells have gained enormous attention as a promising candidate for next generation solar cells, with their continuously increasing power-conversion efficiency, which is comparable to conventional silicon solar cells [1, 2, 3, 4, 5, 6, 7, 8]. Particularly, recent studies have been mainly focused on simple planar structured solar cells, based on the perovskite absorber’s unique characteristics such as high carrier mobility and ambipolar transport, and also the processing development enabling full coverage perovskite films [9, 10]. Fabricating high-quality perovskite with a controlled morphology is indispensable for high-efficiency planar solar cells. Therefore various methods have been employed based on one-step or sequential approaches including two-step solution-based deposition, anti-solvent dripping, solvent-vapor annealing and vacuum-based deposition, etc. [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. However, most of these methods involve additional steps or equipments causing cost, time and reliability concerns. Hence, finding an efficient route producing full coverage and high quality perovskite films is needful. On the other hand, it has been reported that lead acetate (PbAc2) sources can result in faster perovskite formation with smoother surface roughness compared to conventional lead-halide counterparts, via the facile removal of by-product gas and thereby the acceleration of crystal growth [22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33]. Furthermore, a stringently controlled dry environment is generally required for high-quality perovskite layer synthesis since ambient moisture can lead to inactive PbI2 formation and poor coverage [34].

Here, we report an efficient perovskite-fabrication route employing hot casting in conjunction with MAI and non-halide PbAc2 precursors under ambient atmosphere. Fully covered, pinhole-free perovskite layers with high crystallinity were formed even without post-annealing. Besides, the relatively smooth surface roughness of a root-mean-square (RMS) of ~ 24 nm was obtained by the optimization of casting temperature, despite rapid crystallization. Inverted planar heterojunction solar cells were successfully fabricated, showing the best cell efficiency of 8.2%. Especially, the outstanding open-circuit voltage (Voc) of 0.98 V was confirmed, demonstrating the high crystallinity and dense coverage of the perovskite thin films fabricated through this route.

2 Experimental Procedure

Indium tin oxide (ITO) coated glass substrates were cleaned by ultrasonication sequentially in acetone, isopropanol, and deionized water, then exposed to ultraviolet-ozone to improve wettability. As a hole transporting material, poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was deposited by spin-coating at 4000 rpm for 40 s afterward heat-treated at 140 °C for 15 min in ambient air [35]. Perovskite precursors were prepared to a concentration of ~ 33 wt % by dissolving MAI and PbAc2 with a 3:1 molar ratio in anhydrous n,n-dimethylformamide (DMF), and filtrated with 0.45 μm polytetrafluoroethylene (PTFE) filters before spin casting. The PEDOT:PSS-coated substrates were preheated on a hot plate, and the top surface temperature was monitored and stabilized by the help of an IR-thermal gun. Following the fast transfer of the aforementioned substrates, the perovskite precursors were immediately spin-casted at 4000 rpm for 10 s, under ambient atmosphere. For electron-transporting layers, the solutions of phenyl-C61-butyric acid methyl ester (PCBM) with a concentration of 16 mg/ml in chlorobenzene were deposited at 1000 rpm for 60 s at room temperature in ambient air. As a buffer layer, 2,9-dimdimeth-4,7-diphenyl-1,10-phenanthroline (BCP) was dissolved in isopropanol and spin-coated at 4000 rpm for 30 s [36]. Finally, the devices were completed by thermal evaporation of 200 nm thick Ag electrodes.

X-ray diffraction analyses (XRD, Ultima IV: RIGAKU) were used to ascertain the crystal structure of the produced perovskite layers. The morphology and surface topography were investigated by a field-emission scanning electron microscopy (FE-SEM, S-4300: HITACHI) and an atomic force microscopy (AFM, XE-150: Park System), respectively. The optical absorbance spectra were obtained by a UV–Vis spectrophotometer (UV-1601PC: Shimadzu). The photocurrent density–voltage (JV) characteristics of the devices were recorded from the reverse scan at a scan rate of 0.01 V/s by using a solar simulator (94021A: Newport). During the measurements, the solar cell devices were masked with an aperture area of 0.09 cm2 to avoid the edge effects.

3 Results and Discussion

Fully covered, pinhole-free perovskite layers are produced by a novel hot casting route, combined with non-halide PbAc2 precursors under ambient atmosphere. It is known that acetate sources can accelerate perovskite crystal formation due to the facile removal of MAAc by-product thereby resulting in high-quality, uniform and smooth layers compared to halide-based ones [22, 25]. Figure 1 represents the schematic illustration of the protocol, producing compact and high crystallinity perovskite layers even without post-annealing treatment under ambient air. First, PEDOT:PSS-coated substrates are preheated above the boiling point (~ 153 °C) of DMF solvent. Provided the fast transfer of the heated substrates to the spin coater, a mixture of MAI and PbAc2 (with a molar ratio of 3:1) is instantaneously dropped and spun cast under ambient atmosphere. As soon as the precursor mixture falls onto the substrates, the solutions transform into dark brown films indicating that evaporation of MAAc gas begins and direct crystallization occurs during the spinning. This accelerated crystal growth results in an efficient formation of well-defined highly crystalline perovskite layers without further heat treatment.
Fig. 1

Processing scheme for crystalline perovskite layers, through hot casting under ambient atmosphere. Substrates are preheated above the boiling point of DMF solvent, and then MAI and PbAc2 precursors are spin-coated on the substrates, leading to compact and high crystallinity perovskite layers without further annealing treatment

The produced perovskite layers’ crystal structure and composition were investigated according to casting temperatures, as shown in Fig. 2a. All the samples exhibit strong diffraction peaks corresponding to (110) and (002) planes, without any second phase. Furthermore, little change in texture is found within the casting temperature range, indicating the production of the highly crystalline layers that does not require subsequent heat treatment. The final composition resulted from the reactants can be extracted from the following balanced chemical equation [22]:
$$3{\text{MAI }} + {\text{ PbAc}}_{2} \to {\text{ MAPbI}}_{3} + \, 2{\text{MAAc}}.$$
Ideally, the reactants form the solid phases of MAPbI3 via the release of acetate-involved gas without other solid phases, which well explains the diffraction spectra of the final films by this route. The optical absorption spectra of the perovskite films are shown in Fig. 2b, and optical bandgaps were estimated from the Tauc plots with linear fits near the band edges. The values for all the samples were similar to ~ 1.55 eV, which is in agreement with previous reports, confirming the successful formation of MAPbI3.
Fig. 2

a X-ray diffraction patterns of the perovskite thin films. Casting temperature: 140 °C (bottom) to 200 °C (top) with the interval of 20 °C. b Optical absorption spectra (inset) and corresponding Tauc plots with linear fits near band edges

Figure 3 shows the SEM results of the perovskite layers deposited at different casting temperatures. From the plan-view images, complete coverage with island-shaped perovskite formation is clearly confirmed (Fig. 3a). The size of the islands is found to get larger with increasing casting temperature. The size distributions of the islands were extracted and analyzed from the SEM images by the Gaussian fits of the histograms and the average sizes were estimated to ~ 4.9, ~ 8.4, and ~ 11.6 μm for the casting temperatures of 170, 180, and 190 °C, respectively, (Fig. 3b). The formation of the large islands would be responsible for rapid growth from a few nucleation sites under the effect of thermal energy [37]. From the cross-sectional images, the distinct and densely grown morphology of the perovskite layers on the hole-transporting layers is clearly presented (Fig. 3c). The small internal grains composed the islands are clearly distinguished, and the average grain sizes exhibit the slight increase from ~ 80 to ~ 100 nm when the casting temperature increases from 170 to 190 °C. The average thickness also slightly increases from ~ 330 to ~ 380 nm with the casting temperature, due to the small difference in crystallization speed against the spillover of the solutions by spinning. Further increase of casting temperature to 200 °C caused a significant change in the average thickness to ~ 500 nm and the perovskite surface became very rough (The data are not shown here.).
Fig. 3

SEM analyses of the perovskite thin films. a Plan-view images with various casting temperatures. b Island-size distributions with Gaussian fits. c Cross-sectional SEM image. Casting temperature: 180 °C

Perovskite layers are desired to have flat surfaces in order to reduce subsequent charge-transporting layer thickness, hence leading to lower resistance and improved device performance. The AFM-analysis results of the perovskite films are presented in Fig. 4. The RMS-roughness values were calculated to ~ 30.3, ~ 24.9, and ~ 34.8 nm for 170, 180, and 190 °C, respectively, in the areas of 20 × 20 µm2. The line and height profiles also point out that the perovskite film produced at 180 °C has the smoothest surface.
Fig. 4

Surface-roughness analyses of the perovskite thin films. a AFM images with various casting temperatures. b Representative line profiles. c Height distributions around average heights

The fabricated inverted-type planar device architecture is shown in Fig. 5a. PEDOT:PSS, PCBM and BCP were used as hole-transporting, electron-transporting and buffer layers, respectively. Table 1 summarizes the photovoltaic performances of the devices produced with different perovskite-casting temperatures, and the representative JV curves are also plotted in Fig. 5b. The best efficiency of 8.2% is confirmed at 180 °C, which is due to the optimized perovskite microstructure such as surface roughness and crystallinity. Especially, the outstanding Voc of 0.98 V affirms the high crystallinity and dense coverage of the perovskite layers produced by this facile and efficient route. Despite of the process merit, the nonuniform perovskite thickness due to the island structure can cause severe recombination problems at the interface with the subsequent PCBM layer, which would be the main reason for the relatively low short-circuit current (Jsc) and fill factor (FF).
Fig. 5

a Device structure. b Representative JV curves with various casting temperatures

Table 1

Photovoltaic performances of the perovskite solar cell devices with different casting temperatures

Casting Temp. (°C)

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)
















4 Conclusions

We have fabricated the MAPbI3 perovskite films through a novel hot casting technique combined with MAI and non-halide PbAc2 sources, under ambient atmosphere. The X-ray diffraction and optical absorption analyses confirmed the formation of the high crystallinity MAPbI3 with a band gap of ~ 1.55 eV, even without post-annealing. The pinhole-free dense perovskite layers composed of the completely packed islands were verified, and the small internal grains inside the islands were also observed. Furthermore, the relatively smooth surface with the RMS roughness of ~ 24 nm was observed. The inverted type planar heterojunction perovskite solar cells were demonstrated using PEDOT: PSS and PCBM/BCP, showing the best efficiency of 8.2% with the high Voc of 0.98 V. As a future work, it is necessary to further develop perovskite-layer quality with optimizing device structure to improve the efficiency while maintaining the benefits of the synthetic route.



The present research was conducted by the research fund of Dankook University in 2015.


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Copyright information

© The Korean Institute of Metals and Materials 2018

Authors and Affiliations

  1. 1.Department of Materials Science and EngineeringDankook UniversityCheonanKorea
  2. 2.Department of PhysicsYildiz Technical UniversityIstanbulTurkey

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