Supporting Information
Controlling the Conduction Band Offset for Highly
Efficient ZnO Nanorods Based Perovskite Solar Cell
Juan Dong, Jiangjian Shi, Yanhong Luo, Dongmei Li, Qingbo Meng
Key Laboratory for Renewable Energy, Chinese Academy of Sciences; Beijing Key
Laboratory for New Energy Materials and Devices; Institute of Physics, Chinese Academy of
Sciences, Beijing 100190, P. R. China
Experimental Section
1. Materials:
PbI2 (99%) and N, N-dimethylformide (DMF, 99.7%) were purchased from Sigma-Aldrich
and Alfa Aesar, respectively. All the chemicals were directly used without further
purification. CH3NH3I was synthesized by the literature method. Substrates of the cells are
fluorine-doped tin oxide conducting glass (FTO) (Pilkington; thickness 2.2 mm, sheet
resistance 14 Ω/square). Before using, FTO glass was first washed with mild detergent,
rinsed with distilled water for several times and subsequently with ethanol in an ultrasonic
bath, finally dried under air stream.
2. Devices Fabrication
2.1. Mg-doped ZnO Nanorods Synthesis
ZnO nanorods were grown on the polycrystalline ZnO seed layer according to the
previous related works. 30 nm of the ZnO seed layer was first deposited on FTO substrate by
spin-coating zinc acetate dihydrate in methanol. The ZnO nanorods were synthesized by
suspending the seeded substrates facedown in a solution of zinc nitrate,
hexamethylenetetramine and ammonium hydroxide in deionized (DI) water at 90 °C for 1
hour. To obtain the Mg-doped ZnO nanorods, a certain amount of magnesium nitrate was
added into the precursor growth solution. After stirring for 20min, three different Mg(NO3)2
doped precursor growth solutions were prepared and the Mg/Zn (molar ratio) were 0%
(undoped ZnO nanorods); 5% and 10%. After a growth period, the substrates were
thoroughly rinsed with DI water, dried, and annealed at 450 ° C for 30 min.
2.2. Fabrication of the perovskite solar cell
The CH3NH3PbI3 layer was then deposited onto the ZnO nanorods via a two-step
deposition method in air. Firstly, the solution of 1.3 M PbI2 dissolved in DMF was
spin-coated onto ZnO nanorods layer at a speed of 3000 rpm for 30 s, and the substrate was
heated at 90 °C for 2 min to remove the residual DMF solvent. After cooling down to the
room temperature, the film was spin coated with the PbI2 solution again to increase the
amount of PbI2 and dried at 90 °C for another 10 min. Then the substrate was the immersed
in a 10 mg/mL solution of CH3NH3I in 2-propanol for 3 min which has already been heated
to 90 °C, and rinsed with 2-propanol thoroughly. Then, the film was heated at 90 °C for
another 40 min in air on a hotplate. Finally, hole-transport layer was formed by spin-coating
spiro-MeOTAD solution at 2500 rpm for 30 s. Au electrode of 80 nm-thickness was
deposited onto the prepared film by thermal evaporation at an atmospheric pressure of 10−7
Torr to complete the fabrication of the perovskite solar cells.
2.3. Characterizations
The current-voltage (I−V) characteristics were measured by an additional voltage from
the 2602 system source meter of Keithley together with a sunlight simulator (Oriel Solar
Simulator 91192, AM 1.5100 mW/cm2) calibrated with a standard silicon reference cell. The
solar cells were masked with a black aperture to define the active area of 0.1 cm2 and
measured in a lab-made light-tight sample holder. The morphologies of the films were
obtained with scanning electron microscopy (SEM, FEI, and XL30 S-FEG). X-ray
photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi
using 200 W monochromated Al Kα radiation. The 500 μm X-ray spot was used for XPS
analysis. The base pressure in the analysis chamber was about 3×10-10 mbar. Typically the
hydrocarbon C1s line at 284.8 eV from adventitious carbon is used for energy referencing.
Samples were analyzed on Thermo Scientific ESCALab 250Xi using ultraviolet
photoelectron spectroscopy (UPS). The gas discharge lamp was used for UPS, with helium
gas admitted and the HeI (21.22 eV) emission line employed. The helium pressure in the
analysis chamber during analysis was about 2×10-8 mbar. The data were acquired with -10V
bias. Photoluminescence (PL) were obtained on a PL Spectrometer (Edinburgh Instruments,
FLS 900), and excited with a picosecond pulsed diode laser (EPL-445, 0.8 μJ/cm2, 1 MHz) at
445 nm. The incident-photon-to current conversion efficiency (IPCE) was measured by DC
method using a lab-made IPCE setup illuminated. Impedance spectra (IS) for the cell were
measured on a ZAHNER IM6e electrochemical workstation in dark ranging from 0.1 to 105
Hz with a perturbation amplitude of 10 mV. Transient photovoltage were measured with a
pulsed Nd:YAG laser (Brio, 20 Hz) at 532 nm and a nanosecond resolved digital oscilloscope
(Tektronix DPO 7104).
Characteristics:
Figure S1. Box charts of (a) JSC, (b) VOC, (c) FF and (d) PCE for perovskite solar cells with
ZnO NRs doped with different Mg concentration: 0%, 5% and 10%.
After several repeated experiments, statistic results of the cell performance were shown
in Figure S1 as box charts. As seen in Figure S1(a), JSC decreased slightly when the Mg
doping concentration was 5% and 10%. VOC of the cell was obviously improved up to 1032
mV with Mg doping treatment, as shown in Figure S1(b). Impressively, the average FF in
FigureS1(c) was increased from 0.65 to 0.69 with increasing Mg doping concentration. Thus,
the average PCE of the cells has been improved from 13.6% to 15.1% after doping with 5%
Mg concentration, as shown in Figure S1(d). XRD spectra of ZnO nanorods doping with
different Mg concentration: 0%, 5% and 10% was shown in Figure S2.
For perovskite solar cells based on ZnO nanorods, hysteresis does really exists. The
hysteresis has been generally shown to be strongly dependent on voltage sweep rate, delay
time, light soaking, scanning directions of applied voltage, and preconditioning of the devices,
making it difficult to accurately evaluate the cell performance. Then, we obtain the steady
state power output of the perovskite solar cells doped with 0%, 5% and 10% Mg, as shown in
Figure S3. Photocurrent density as a function of time for an undoped ZnO based perovskite
solar cell held at a forward bias of 680 mV. The cell was placed in the dark prior to the start
of the measurement. The photocurrent stabilizes within seconds to approximately 19 mA/cm2,
yielding a stabilized power conversion efficiency of 12.9%, measured after 120 s. For a 5%
Mg doped ZnO based perovskite solar cell, the photocurrent stabilizes to approximately 18.5
mA/cm2 at a forward bias of 754 mV, obtaining a stabilized power conversion efficiency of
13.9%. Meanwhile, after doped with 10% Mg, the stabilized photocurrent of the perovskite
solar cells is 17.5 mA/cm2 at a forward bias of 776 mV, and a slightly lower stabilized power
conversion efficiency is obtained, about 13.5%.
Figure S2. XRD spectra of ZnO nanorods doping with different Mg concentration: 0%, 5%
and 10%.
Figure S3. Photocurrent density as a function of time for the Mg doped ZnO based
perovskite solar cell held at the forward bias in different doping concentrations.