Juan Dong, Jiangjian Shi, Yanhong Luo, Dongmei Li, Qingbo Meng

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 bysuspending 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 obtainthe Mg-doped ZnO nanorods,a certain amount of magnesium nitrate was added into the precursor growth solution. After stirring for20min, three different Mg(NO3)2doped 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 nanorodsvia 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 substratewas 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 minwhich 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-thicknesswas 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 dopedwith 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/cm2at 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/cm2at 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.