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 p–i–n 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.
KeywordsCH3NH3PbI3 Lead acetate Hot casting Air processing Planar heterojunction solar cells
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 .
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 . 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 . 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 (J–V) 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
Photovoltaic performances of the perovskite solar cell devices with different casting temperatures
Casting Temp. (°C)
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.
- 8.Heo, J.H., Im, S.H., Noh, J.H., Mandal, T.N., Lim, C.S., Chang, J.A., Lee, Y.H., Kim, H.J., Sarkar, A., Nazeeruddin, M.K., Grätzel, M., Seok, S.I.: Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors. Nat. Photonics 7, 486 (2013)CrossRefGoogle Scholar
- 22.Zhang, W., Saliba, M., Moore, D.T., Pathak, S.K., Hörantner, M.T., Stergiopoulos, T., Stranks, S.D., Eperon, G.E., Alexander-Webber, J.A., Abate, A., Sadhanala, A., Yao, S., Chen, Y., Friend, R.H., Estroff, L.A., Wiesner, U., Snaith, H.J.: Ultrasmooth organic-inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells. Nat. Commun. 6, 6142 (2015)CrossRefGoogle Scholar
- 26.Fu, Y., Meng, F., Rowley, M.B., Thompson, B.J., Shearer, M.J., Ma, D., Hamers, R.J., Wright, J.C., Jin, S.: Solution growth of single crystal methylammonium lead halide perovskite nanostructures for optoelectronic and photovoltaic applications. J. Am. Chem. Soc. 137, 5810 (2015)CrossRefGoogle Scholar
- 28.Zhang, W., Pathak, S., Sakai, N., Stergiopoulos, T., Nayak, P.K., Noel, N.K., Haghighirad, A.A., Burlakov, V.M., deQuilettes, D.W., Sadhanala, A., Li, W., Wang, L., Ginger, D.S., Friend, R.H., Snaith, H.J.: Enhanced optoelectronic quality of perovskite thin films with hypophosphorous acid for planar heterojunction solar cells. Nat. Commun. 6, 10030 (2015)CrossRefGoogle Scholar
- 31.Zhao, L., Luo, D., Wu, J., Hu, Q., Zhang, W., Chen, K., Liu, T., Liu, Y., Zhang, Y., Liu, F., Russell, T.P., Snaith, H.J., Zhu, R., Gong, Q.: High-performance inverted planar heterojunction perovskite solar cells based on lead acetate precursor with efficiency exceeding 18%. Adv. Funct. Mater. 26, 3508 (2016)CrossRefGoogle Scholar