Cite this Article
Li Long, Shi Cheng-Wu, Deng Xin-Lian, Wang Yan-Qing, Xiao Guan-Nan, Ni Ling-Ling. 420 nm thick CH 3 NH 3 PbI 3 − x Br x capping layers for efficient TiO2 nanorod array perovskite solar cells. Chinese Physics B, 2018, 27(1): 018804  
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420 nm thick CH 3 NH 3 PbI 3 − x Br x capping layers for efficient TiO2 nanorod array perovskite solar cells
Li Long, Shi Cheng-Wu †, Deng Xin-Lian, Wang Yan-Qing, Xiao Guan-Nan, Ni Ling-Ling
School of Chemistry and Chemical Engineering, Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, Hefei University of Technology, Hefei 230009, China

 

† Corresponding author. E-mail: shicw506@foxmail.com shicw506@hfut.edu.cn

Abstract

The rutile TiO2 nanorod arrays with 240 nm in length, 30 nm in diameter, and in areal density were prepared by the hydrothermal method to replace the typical 200–300 nm thick mesoporous TiO2 thin films in perovskite solar cells. The CH3NH3PbI3−xBrx capping layers with different thicknesses were obtained on the TiO2 nanorod arrays using different concentration complex precursor solutions in DMF and the photovoltaic performances of the corresponding solar cells were compared. The perovskite solar cells based on 240 nm long TiO2 nanorod arrays and 420 nm thick CH3NH3PbI3−xBrx capping layers showed the best photoelectric conversion efficiency (PCE) of 15.56% and the average PCE of 14.93±0.63% at the relative humidity of 50%–54% under the illumination of simulated AM 1.5 sunlight ( ).

PACS: 88.40.H-;88.40.hj;84.60.Jt
Keyword:rutile TiO2 nanorod array;CH3NH3PbI3-xBrx ;capping layer;perovskite solar cell
1. Introduction

Organometal halide perovskite solar cells (CH3NH3PbX 3, X =I, Br, Cl) have drawn tremendous attention due to their excellent photovoltaic performance such as long electron–hole diffusion length, strong optical absorption, tunable band gaps, and simple manufacturing process.[1–7] The thicknesses of the perovskite thin films strongly affect their absorbance and further influence the photovoltaic performance of the corresponding solar cells. Snaith[8] fabricated the perovskite solar cells with 200 nm thick CH3NH3PbI3−xClx capping layer and 260 nm thick mesoporous TiO2 thin film by a one-step solution method, and obtained the photoelectric conversion efficiency (PCE) of 8.44%. Park[9] prepared a ∼200 nm thick CH3NH3PbI3 capping layer using the 1.0 M PbI2 precursor solution by a two-step solution method and the corresponding perovskite solar cells with 220 nm thick mesoporous TiO2 thin film achieved the PCE of 12.5%. Seok[10] prepared a 200 nm thick perovskite capping layer using the 0.8 M CH3NH3PbI3 −xBrx precursor solution by a one-step solution method and the corresponding perovskite solar cells with 200 nm thick mesoporous TiO2 thin film exhibited the PCE of 15.9%. Mhaisalkar and Park[11] studied the influence of the rutile TiO2 nanorod array with lengths of 560 nm, 920 nm, and 1580 nm on the PCE of the corresponding perovskite solar cells and found that 560 nm long rutile TiO2 nanorod arrays gave the best PCE of 9.4%. Li, Xie, and Wang[12] assembled the perovskite solar cells with 200 nm long TiO2 nanorod array and 100 nm thick CH3NH3PbI3 capping layer using the 1.3 M CH3NH3PbI3 precursor solution, the corresponding perovskite solar cells achieved the best PCE of 18.22%. Although the perovskite solar cells with 200–300 nm thick mesoporous TiO2 thin films and 100–200 nm thick CH3NH3PbI3 capping layers have been widely investigated, the TiO2 nanorod array perovskite solar cells with high thickness CH3NH3PbI3 −xBrx capping layers have been rarely reported.

In this work, the rutile TiO2 nanorod arrays of 240 nm in length, 30 nm in diameter, and in areal density were prepared via hydrothermal method using an aqueous grown solution composed by 38 mW titanium isopropoxide and 6 M hydrochloric acid at for 75 min. 290 nm, 360 nm, and 420 nm thick CH3NH3PbI3−xBrx capping layers were obtained on the TiO2 nanorod arrays using 1.3 M, 1.5 M, and 1.7 M complex precursor solutions in DMF, respectivley. The influence of the CH3NH3PbI3 −xBrx capping layer thickness on the photovoltaic performance of the corresponding TiO2 nanorod array perovskite solar cells was compared.

2. Experimental section
2.1. Preparation of the CH3NH3PbI3 −xBrx capping layers

complex precursor solutions with 1.3 M, 1.5 M, 1.7 M in DMF[13] were dropped on TiO2 nanorod arrays and spin-coated at 3000 rpm for 20 s. Immediately, , 0.465 M methylammonium halide mixture of CH3NH3I/CH3NH3Br=85/15 (molar ratio) in isopropanol[13] was dropped. After waiting for 20 s, the spin-coater was started to remove the residual methylammonium halide mixture solution and the parameters of the spin-coating procedure were 5000 rpm and 30 s. Subsequently, these CH3NH3PbI3−xBrx capping layers were heated on a preheated hot plate in the glove box with the relative humidity of 10%–15% and the annealing temperature and time were and 30 min, respectively. Moreover, the spin-coater was immediately started (after waiting for 0 s) and these CH3NH3PbI3−xBrx capping layers were obtained to analyze the complete conversion of and the content of residual PbI2 for comparisons.

2.2. Preparation of the rutile TiO2 nanorod arrays and fabrication of perovskite solar cells

The rutile TiO2 nanorod arrays were prepared by hydrothermal method similar to that in our previous reports.[14,15] The aqueous grown solution was composed of 38 mW titanium isopropoxide and 6 M hydrochloric acid. The growth temperature and time were and 75 min, respectivley. The rutile TiO2 nanorod arrays were annealed at 300 °C for 30 min in air prior to use.[11] The preparation of the 60 nm thick TiO2 compact layer, spiro-OMeTAD layer, 60 nm thick gold electrode and the fabrication of perovskite solar cells were the same as in our previous reports.[13,14]

2.3. Characterization

The characterization with field emission scanning electron microscopy (FE-SEM, Sirion200, FEI), x-ray diffraction (XRD, D/MAX2500V, Rigaku, Japan), ultraviolet–visible spectroscopy (UV–Vis, U-3900H, Hitachi, Japan), ultraviolet–visible–near infrared spectroscopy (UV–Vis–NIR, U-3900H, Hitachi, Japan), electrochemical impedance spectroscopy (EIS, electrochemical workstation, Shanghai), and energy dispersive x-ray spectroscopy (EDS, JSM-6490LV, Japan) were identical to that in our previous reports.[13–15] The photovoltaic performance of the perovskite solar cells was measured by a solar simulator (Oriel, Newport, USA) with a source meter (Keithley 2420) controlled by Testpoint software under the illumination of simulated AM 1.5 sunlight ( . The irradiation intensity was calibrated with the standard crystalline silicon solar cell (Oriel, Newport, USA). The black and opaque film with the square aperture (3 mm×3 mm) adhered on FTO and the active area of 0.09 cm2 were defined and the PCE measurements were carried out in atmosphere air ambient with the relative humidity of 50%–54%.

3. Results and discussion
3.1. Microstructure, crystal phase, and optical absorption of the TiO2 nanorod arrays

Figure 1 shows the microstructure, XRD pattern, and UV–Vis spectrum of the TiO2 nanorod array on 60 nm thick TiO2 compact layer and SnO2:F transparent conductive glass (FTO). From Figs. 1(a) and 1(b), it is found that the TiO2 nanorod arrays have the length of 240 nm, the diameter of 30 nm, and the areal density of . The 240 nm length is similar to the typical 200–300 nm thickness of the mesoporous TiO2 thin films in perovskite solar cells.[16,17] Compared with the planar perovskite solar cell without mesoporous TiO2 thin films, the introduction of the TiO2 nanorod array can improve the charge separation in the interface of perovskite/TiO2.[14] As shown in Fig. 1(c), in addition to the strong diffraction peaks of FTO, the weak peaks appearing at and 62.8° match well with the spacings of (101) and (002) planes of the tetragonal rutile phase, and the (101) plane is the preferred orientation of the TiO2 nanorod arrays.[15] And the intensities of the diffraction peaks of TiO2 increase with the increase of the hydrothermal growth time.[18] As shown in Fig. 1(d), the absorption onset of the TiO2 nanorod arrays is 353 nm and nearly no obvious absorbance can be observed from 400 nm to 800 nm.

Fig. 1. TiO2 nanorod array: (a) surface SEM image, (b) cross-sectional SEM image, (c) XRD pattern, (d) UV–Vis absorption spectrum.
3.2. Crystal phase, morphology, and optical absorption of the CH3NH3PbI3−xBrx capping layers

Figure 2 shows the XRD patterns of the CH3NH3PbI3−xBrx capping layers using 1.3 M, 1.5 M, and 1.7 M complex precursor solutions after waiting for 0 s and 20 s. The CH3NH3PbI3 −xBrx capping layers at 0 s all exhibit the weak diffraction peaks of PbI2,[19] while the diffraction peaks of PbI2 do not appear at 20 s and the diffraction peaks of CH3NH3PbI3−xBrx appear at 2θ = 14.15°, 20.07°, 23.54°, 24.60°, 28.50°, 31.97°, 40.79°, and 43.33°.[13,14] The result demonstrates that does not completely convert to CH3NH3PbI3−xBrx at 0 s and the pure phase CH3NH3PbI3−xBrx capping layers can be obtained at 20 s.

Fig. 2. (color online) XRD patterns of the CH3NH3PbI3−xBrx capping layers using (a) 1.3 M, (b) 1.5 M, (c) 1.7 M complex precursor solutions after waiting for 0 s and 20 s.

Figure 3 displays the cross-sectional and surface SEM images of the CH3NH3PbI3−xBrx capping layers using 1.3 M, 1.5 M, and 1.7 M complex precursor solutions after waiting for 20 s. From Figs. 3(a)–3(c), the thicknesses of the CH3NH3PbI3 −xBrx capping layers are 290 nm of 1.3 M, 360 nm of 1.5 M, and 420 nm of 1.7 M and increase with the concentration. And the total thicknesses of the CH3NH3PbI3−xBrx absorbing layers reach 530 nm of 1.3 M, 600 nm of 1.5 M, and 660 nm of 1.7 M. From Figs. 3(d)–3(f), the CH3NH3PbI3−xBrx capping layers with the thicknesses of 360 nm and 420 nm have more uniform grain sizes, less grain boundaries, and more smooth surfaces than that of 290 nm, and the grain size of perovskite is 500–800 nm in the 420 nm thick CH3NH3PbI3−xBrx capping layer. The EDS analysis reveals that the chemical composition of the 420 nm thick CH3NH3PbI3−xBrx capping layer is the atomic ratio of Pb: I: Br = 1: 3.18: 0.24. Combined with the results of XRD, SEM, and EDS, the Br-doped CH3NH3PbI3−xBrx capping layers with high thickness of 420 nm are successfully obtained. It is worthwhile to note that the concentration of the complex precursor saturated solution is ∼1.7 M, the CH3NH3PbI3 −xBrx capping layers with thicknesses lager than 420 nm are not prepared in this paper.

Fig. 3. Cross-sectional and surface SEM images of the CH3NH3PbI3−xBrx capping layers using (a), (d) 1.3 M, (b), (e) 1.5 M, and (c), (f) 1.7 M complex precursor solutions.

Figure 4 exhibits the UV–Vis–NIR absorption spectra of the CH3NH3PbI3−xBrx capping layers using 1.3 M, 1.5 M, and 1.7 M complex precursor solutions after waiting for 20 s. The absorption onsets of the CH3NH3PbI3−xBrx capping layers are all 780 nm and the absorbance in the range of 500–800 nm increases with the increase of the complex precursor solution concentration and the CH3NH3PbI3−xBrx capping layer thickness, which is accordance with the result of SEM in Fig. 3.

Fig. 4. (color online) UV–Vis–NIR absorption spectra of CH3NH3PbI3−xBrx capping layers using (i) 1.3 M, (ii) 1.5 M, and (iii) 1.7 M complex precursor solutions.
3.3. Photovoltaic performance of the TiO2 nanorod array CH3NH3PbI3−xBrx solar cells

Table 1 lists the photovoltaic performance parameters of the TiO2 nanorod array perovskite solar cells with the different thicknesses of CH3NH3PbI3−xBrx capping layers and the photocurrent–photovoltage characteristics, and the incident photon-to-electron conversion efficiency (IPCE) spectra of the corresponding champion solar cells are shown in Fig. 5. With the increase of the CH3NH3PbI3−xBrx capping layer thickness from 290 nm to 360 nm and 420 nm, the short-circuit photocurrent density ( and open-circuit voltage ( increase from and 0.81±0.03 V of 290 nm, to and 0.91±0.03 V of 360 nm, and 0.98±0.04 V of 420 nm. The increase of should be related to the thickness and absorbance increases of the CH3NH3PbI3 −xBrx capping layers. From the IPCE spectra in Fig. 5(b), the integrated of the corresponding champion solar cells (1.3 M, 1.5 M, 1.7 M) are 20.42 mA cm−2, 20.43 mA cm−2, and 21.33 mA cm−2, which are in close agreement with the measured values. When the thickness of the CH3NH3PbI3−xBrx capping layers is increased, the quantity of conduction band electrons injecting from CH3NH3PbI3−xBrx to TiO2 increases under the illumination and the level of the TiO2 conduction band up-shifts.

Fig. 5. (color online) (a) Photocurrent–photovoltage characteristics of the TiO2 nanorod array perovskite solar cells with (i) 290 nm, (ii) 360 nm, and (iii) 420 nm thick CH3NH3PbI3−xBrx capping layers and (b) the corresponding IPCE spectra.
Table 1.

The photovoltaic performance parameters of TiO2 nanorod array perovskite solar cells using 1.7 M, 1.5 M, and 1.3 M complex precursor solutions.

.

The grain boundary energy should be minimized and the charge transportation can be improved because the CH3NH3PbI3−xBrx capping layers with the thicknesses of 360 nm and 420 nm have more uniform grain sizes, less grain boundaries, and more smooth surface than that of 290 nm.[20] The of 360 nm and 420 nm is higher than that of 290 nm thick CH3NH3PbI3−xBrx capping layer. Therefore, the perovskite solar cell based on the combination of 420 nm thick CH3NH3PbI3−xBrx capping layers and 240 nm long rutile TiO2 nanorod arrays achieves the best PCE of 15.56% with of , of 0.99 V, and FF of 0.70 and the average PCE of with of , of V, and FF of 0.70±0.04 at the relative humidity of 50%–54% under the illumination of simulated AM 1.5 sunlight ( .

3.4. EIS analysis of the TiO2 nanorod array perovskite solar cells with different CH3NH3PbI3 −xBrx capping layer thicknesses

The effect of the CH3NH3PbI3−xBrx capping layer thickness on the photovoltaic performance of perovskite solar cells can be further investigated by analyzing the corresponding EIS.[21–25] Figure 6 shows the Nyquist plots of the perovskite solar cells with different CH3NH3PbI3−xBrx capping layer thicknesses, and the corresponding fitted data by an equivalent circuit ( CPE) are listed in Table 2. The serial resistance describes the resistance of the FTO and Au electrode, is the charge recombination resistance of the electrons in the TiO2 conduction band and the holes in the CH3NH3PbI3−xBrx thin film, and Y 0 and n represent the capacitance and the contact extent of the TiO2/CH3NH3PbI3−xBrx thin film interface. Therefore, the arc difference in the Nyquist plots (Fig. 6) should be derived from the thickness difference of the CH3NH3PbI3 −xBrx thin films. From Table 2, with the increase of the CH3NH3PbI3−xBrx capping layer thickness from 290 nm to 360 nm and 420 nm, and n both increase from 243.8 Ω, 0.87 to 634.2 Ω, 0.89 and 687.3 Ω and 0.98, respectively, while Y 0 decreases from to and . The increasing and n can be attributed to the more uniform grain sizes, less grain boundaries, and smoother surfaces with the increasing CH3NH3PbI3−xBrx capping layer thickness which suppress the charge recombination of the electrons in the TiO2 conduction band and the holes in the CH3NH3PbI3−xBrx thin film and are benefit to better charge transporting in the CH3NH3PbI3−xBrx thin film; this is consistent well with the SEM image results of the CH3NH3PbI3−xBrx capping layers. On the other hand, the increasing CH3NH3PbI3 −xBrx capping layer thickness also leads to a decrease of the capacitance which causes the decrease of Y 0. The results of and Y 0 are in accordance with the increasing and of the corresponding perovskite solar cells.

Fig. 6. (color online) Nyquist plots of the TiO2 nanorod array perovskite solar cells with 290 nm (a, a-fit), 360 nm (b, b-fit), and 420 nm (c, c-fit) thick CH3NH3PbI3−xBrx capping layers.
Table 2.

Parameters obtained by fitting the experimental spectra with the equivalent circuit ( CPE).

.
4. Conclusion

The rutile TiO2 nanorod arrays with 240 nm in length, 30 nm in diameter, and in areal density were prepared via a hydrothermal method using an aqueous grown solution consisted of 38 mW titanium isopropoxide and 6 M hydrochloric acid at for 75 min. 290 nm, 360 nm, and 420 nm thick CH3NH3PbI3−xBrx capping layers were successfully obtained on the TiO2 nanorod arrays using 1.3 M, 1.5 M, and 1.7 M complex precursor solutions in DMF and the corresponding CH3NH3PbI3−xBrx solar cells presented the best PCE of 11.80%, 13.89%, and 15.56% and the average PCE of 10.77±2.03%, 12.45±1.44%, and 14.93±0.63% at the relative humidity of 50%–54% under the illumination of simulated AM 1.5 sunlight ( ).

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