Received: 14 Jul 2018
Revised: 03 Aug 2018
Accepted: 06 Aug 2018
Published online: 08 Aug 2018
Mohammad Mahdi Tavakoli,1,2,* Daniel Prochowicz,3 Pankaj Yadav,4 Rouhollah Tavakoli,1 and Michael Saliba5
1Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran.
2Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA
3Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
4Department of Solar Energy, School of Technology, Pandit Deendayal Petroleum University, Gandhinagar-382 007, Gujarat, India
5Adolphe Merkle Institute, Chemins des Verdiers 4, CH-1700 Fribourg
Email: email@example.com (M. Tavakoli)
Zinc stannate nanorod array as an ETL in perovskite solar cell improves the charge extraction as well as efficiency and reduces hysteresis.
Careful engineering of the electron transfer layer (ETL) is a promising approach to improve the efficiency of perovskite solar cells (PSCs). In this study, we demonstrate the potential of using zinc stannate (Zn2SnO4, or ZSO from here) as ETL for the fabrication of highly efficient PSCs. ZSO was deposited on top of FTO glass as thin films and nanorod arrays using ultrasonic spray pyrolysis (USP) technique. Optical characterizations reveal that perovskite films deposited on such nanorod arrays of ZSO have a lower transmittance exhibiting better charge extraction properties compared to the planar ZSO thin films. The best ZSO-based device reached a power conversion efficiency (PCE) of 18.24% and a high current density of 23.8 mA/cm2. Moreover, these devices showed a lower hysteresis index in compared to the ZSO planar based devices.
Reaching high efficiency and high stability are among the most important challenges in the field of organic-inorganic perovskite solar cells (PSCs).1-5 The steady improvement is mainly reached using interface engineering, compositional engineering, crystal engineering and the utilization of passivation agents.6-10 This recently results in a PSC device with a certified PCE of 22.6%.11 Among these techniques, the improvement of charge extraction properties in electron transporting layer (ETL) and light absorption in the device are the most effective approaches for boosting the photovoltaic parameters of PSCs.12-17 In general, TiO2, ZnO, and SnO2 are commonly used as ETLs for the fabrication of PSCs.18-23 For example, Mahmood et al.24 demonstrated a PSC using N-doped ZnO nanorods. This resulted in a PCE of 16.1% and low hysteresis. Jiang et al.25 reported a PSC with an efficiency of 11.7% based on a TiO2 nanowire ETL. Besides these metal oxides, Zn2SO4 (ZSO) is another alternative ETL for the fabrication of efficient PSCs, which has been recently utilized in the form of thin films and quantum dots (QDs). ZSO shows interesting electrical properties and its band levels are well-matched with perovskite absorber.26-32 Bera et al.31 deposited a compact layer of ZSO and reported a PCE of 13.34%, while Shin et al.32 synthesized ZSO QDs at low temperature process resulting in a PCE of 15.3%.
In this work, we have successfully grown ZSO layers as thin films and in the form of nanorod arrays using an ultrasonic spray pyrolysis (USP) technique. We then employed them as ETLs for the fabrication of PSCs. The fabricated ZSO nanorods has a strong quenching effect in compared to ZSO based thin film indicating improved charge extraction. Moreover, PSCs based on ZSO nanorod photoelectrode exhibit a maximum PCE of 18.24% in compared to planar device with PCE of 17.04%, (mainly due to a higher current density).
Deposition of ZSO thin film and nanorod array: FTO glass (NSG-10) was selected as substrates for PSC devices. First, the substrates were etched by using zinc powder and HCl solution (2 M). Then, the substrates were cleaned carefully by 20 min sonication in Triton X100 (1 vol% in deionized water), deionized water, acetone, and ethanol, respectively. Ultrasonic spray pyrolysis was employed to fabricate ZSO planar film and nanorod array. In this regard, ZnCl2 and SnCl2⸱2H2O (molar ratio = 2:1) were dissolved in 25 mL isopropanol (IPA). The concentration of solution was fixed to 0.1 M for both planar film and nanorod array. After 1 h stirring, the solution was sprayed on substrates using ultrasonic system and flowing oxygen as a carrier gas. The diameter of nozzle was 5 cm with many small holes. The substrate temperatures for thin film and nanorod array were 420 °C and 380 °C, respectively. The distance between nozzle and substrate was optimized and fixed for both cases (4 cm). For nanorod growth, we have reduced the gas pressure as compared to thin film growth. Notably, for nanorod growth a seed layer of ZSO solution was spin-coated on the substrate, followed by annealing at 420 °C.
Device fabrication: After USP deposition of ZSO on the FTO glass and plasma cleaning for 2 min, MAPbI3 perovskite was deposited from a solution of PbI2 (1.2 M, TCI) and methylammonium iodide (1.15 M, dyesol) in a mixed solvent of DMF:DMSO (4:1). The spin coating process was performed at 1000 rpm for 10 s and 6000 rpm for 30 s. 200 µL of chlorobenzene was applied to film 10 seconds before end of spinning. Then, the film was annealed at 100 °C for 1 h. For HTL layer, a solution of spiro (52 mg/500µL CB) with 23 µL of 4-tert-butyl pyridine (TBP) and 12.34 µL of bis(trifluoromethylsulfonyl)imide lithium salt (Li-TFSI, Sigma-Aldrich) (520 mg/1 mL acetonitrile) was spin-coated at 4000 rpm for 20 s (with ramp rate of 2000/s). The device was completed finally by thermal evaporation of 100 nm-thick gold as a back contact.
The ideality factor was calculated from the following formula:40
Ln (I) = Ln (I0) + (q/nkT)V
where I, I0, q, n, k, T, and V are current, saturation current, electron charge, ideality factor, Boltzmann constant, temperature, and voltage, respectively.
Film characterization: The ZSO and perovskite morphologies were studied by scanning electron microscopy (SEM, ZEISS Merlin). Varian Cary 5 and Fluorolog 322 (Horiba Jobin Ybon Ltd) were employed to measure UV-vis and Steady-state photoluminescence (PL) spectra, respectively. X-ray diffraction (XRD) pattern was measured by Bruker D8 X-ray Diffractometer (USA) utilizing a Cu Kα radiation.
Device measurement: For J-V measurement a 450 W xenon lamp (Oriel, USA) and a digital source meter (Keithley model 2400, USA) were employed. All devices were measured using a black aperture with 0.1 cm2 area. During the measurement, the scan rate was 10 mVs-1. The intensity of lamp was 1000 W/m2 for device measurement. The device was scanned from 0 to 1.2 V and reverse with a step size of 0.005 V and a delay time of 200 ms at each point.