Research Article | Open Access
Gang Lu, Fengqin He, Shangzheng Pang, Haifeng Yang, Dazheng Chen, Jingjing Chang, Zhenhua Lin, Jincheng Zhang, Chunfu Zhang, "A PCBM-Modified TiO2 Blocking Layer towards Efficient Perovskite Solar Cells", International Journal of Photoenergy, vol. 2017, Article ID 2562968, 9 pages, 2017. https://doi.org/10.1155/2017/2562968
A PCBM-Modified TiO2 Blocking Layer towards Efficient Perovskite Solar Cells
Organometal halide perovskite as a promising absorber in solar cells has caught tremendous attention in the past few years. Herein, a hydrophobic PCBM layer was introduced into planar heterojunction perovskite solar cells to form the TiO2/PCBM double blocking layer. The bilayer structure assures the formation of a dense, smooth, and pinhole-free blocking layer. And the PCBM layer can also be in favor of larger grain size perovskite film and further passivate the TiO2 blocking layer. Scanning electron microscope, atomic force microscope, and X-ray diffraction (XRD) results suggest the formation of a uniform and compact CH3NH3PbI3−xClx perovskite layer on the PCBM-modified TiO2 blocking layer. The solar cell has reached a PCE up to 16.37%, which is greatly larger than the control device with a PCE of 10.81%.
Solar cells as an economic approach to utilize solar energy have been developed for over six decades, and the most pivotal part of a solar cell is the absorber [1–6]. Recently, one material named organometal halide perovskite has caught tremendous attention in solar cell society [7–10]. Solar cells employing this kind of material have reached stunning power conversion efficiencies (PCE) of over 20% in only 6 years . The organometal halide perovskite in the form of ABX3 (A = CH3NH3+; B = Pb2+ or Sn2+; and X = Cl−, Br−, or I−) is a kind of promising material to absorb sunlight owning to its suitable band structure and convenient deposition process. For example, a 1.55 eV direct bandgap of CH3NH3PbI3 is approaching the optimal bandgap for a single junction solar cell under the prediction of Shockley–Queisser limit . Moreover, the solution process of organometal halide perovskite enables the use of large-scale low-cost deposition method which will further lower the cost of organometal halide perovskite solar cells [13–15].
Due to the obvious advantages, great research interests have been attracted in organometal halide perovskite solar cells in recent years [16–21]. For the record efficiency of perovskite solar cells, a mesoporous n-i-p structure was employed where n, i, and p indicate the majority carrier type in each layer of a perovskite solar cell. Compared with the mesoporous structure device, a planar heterojunction solar cell may be in more favor due to its simple device structure. The mesoporous TiO2 high temperature (>450°C) sintering process will inevitably increase the cost of perovskite solar cells. However, for a planar n-i-p-structured organometal halide perovskite solar cell, the power conversion efficiency (PCE) still lags far behind that of mesoporous solar cells even though experiment has proved that mesoporous TiO2 is not necessary for the electron transportation . Achieving a high PCE in a planar n-i-p-structured organometal halide perovskite solar cell is not only beneficial to lower the cost of organometal halide perovskite solar cell but is also beneficial for understanding the interface properties between organometal halide perovskite solar cell and hole blocking layer.
By omitting the mesoporous TiO2, the planar heterojunction perovskite solar cell based on the TiO2 blocking layer has attracted much research interest. In order to achieve a better device performance, the TiO2 blocking layer should be smooth, dense, and pinhole-free. However, when the TiO2 is thin, pinholes usually form and when the TiO2 is thick, a large series resistance is introduced. Both of them will decrease the device performance. Herein, we report a solution-processed TiO2/PCBM double blocking layer to improve the device performance. It is also shown that the double blocking layer can improve the quality of CH3NH3PbI3−xClx perovskite thin films. Scanning electron microscope (SEM) and atomic force microscope (AFM) photographs suggest that a dense and uniform TiO2/PCBM blocking layer is obtained. X-ray diffraction (XRD) spectra suggest that the introduction of the PCBM layer and the following annealing process does not influence the underlying TiO2. The as-fabricated planar heterojunction organometal halide perovskite solar cell has achieved a PCE of 16.37% with a VOC of 1.05 V, a JSC of 21.96 mA/cm2, and a FF of 0.71, which is greatly larger than the control device with a PCE of 10.81%.
2. Materials and Methods
If not otherwise stated, all the materials are used as received. The materials include MAI (99.8%, Dyesol), FK-102 Cobalt salt (99.8%, Dyesol), PbI2 (99.999%, ultra dry, Alfa), PbCl2 (99.999%, Sigma-Aldrich), dimethyl sulphoxide (99.8%, anhydrous, Sigma-Aldrich), chlorobenzene (99.8%, anhydrous, Sigma-Aldrich), Li-TFSI (99.8%, Sigma-Aldrich), Toluene (99.8%, anhydrous, Sigma-Aldrich), γ-butyrolactone (99.8%, anhydrous, Aladdin), titanium diisopropoxide bis(acetylacetonate) (75 wt. % in isopropanol, Sigma-Aldrich), and 1-butanol (99.8%,Sigma-Aldrich).
2.2. Device Fabrication
Fluorine doped tin oxide (FTO, Zhuhai Kaivo Optoelectronic Technology Co. Ltd., 14 ohm/sq) glass was cleaned via sequentially in 5% Decon-90 solution, deionized water, acetone, and ethanol under ultrasonication for 15 min, respectively, and then treated with O2 plasma for 15 min. A compact TiO2 layer on the FTO glass was prepared by spin coating a 0.15 M titanium diisopropoxide bis(acetylacetonate) solution in 1-butanol at 2000, 4000, and 6000 rpm for 30 s, respectively, dried at 125°C for 5 min, and then repeated twice with 0.3 M of titanium diisopropoxide bis(acetylacetonate) solution, and finally, the as-prepared thin film was sintered at 500°C for 15 min. After that, the resultant TiO2 film was immersed into a 40 mM TiCl4 aqueous solution at 70°C for 30 min, washed with deionized water, and then heated at 500°C for 15 min. A 10 mg/mL PCBM solution in chlorobenzene was spin coated onto the TiO2 film at 2000, 4000, and 6000 rpm for 45 s, respectively, and the resulted thin films were annealed at 100°C for 5 min. A CH3NH3PbI3−xClx perovskite photoactive layer was deposited by dynamically spin coating a solution containing 0.14 M (39 mg) PbCl2, 1.26 M (581 mg) PbI2, and 1.3 M (209 mg) MAI in 1 mL cosolvent of DMSO:GBL (3 : 7 vol. ratio). It should be noted that the dynamically spin coating is used here so that the underlying PCBM can be avoided to be damaged when the perovskite precursor is deposited. The perovskite precursor solution was preheated at 80°C before spin coating. The spin coating was programmed to run at 1000 rpm for 40 s and then 4000 rpm for 60 s. At 20 s after the start of spin coating process, 240 μL hot CH3NH3PbI3−xClx solutions was injected onto the TiO2/PCBM blocking layer; at 60 s after the start of spin coating process, 500 μL anhydrous toluene was injected onto the spinning film to quench it. The transparent as-cast films were subsequently annealed at 100°C for 12 min. Then a spiro-MeOTAD solution containing 90 mg spiro-MeOTAD, 1 mL chlorobenzene, 45 μL LiTFSI/acetonitrile (170 mg/mL) solution, 75 μL tris (2-(1H-pyrazol-1-yl)-4-tertbutylpyridine) cobalt(III) bis(trifluoromethylsulphonyl) imide acetonitrile solution (100 mg/mL), and 10 μL tBP was spin coated on the substrates. Finally, a 100 nm Ag film was thermally evaporated as the anode.
The morphology measurement of the perovskite layers were measured by SEM (Quanta x50 FEG) and AFM (Bruker Dimension Icon). XRD test was conducted on Bruker D8 advanced, and the samples were prepared as the same process of device fabrication. The UV-visible absorption spectra of TiO2, TiO2/PCBM, and CH3NH3PbI3−xClx were recorded with UV-visible spectrophotometer (Perkin-Elmer Lambda 950) using the precleaned FTO glass as the blank signal. Photovoltaic performances were measured by using Keithley 2400 source meter under simulated sunlight from XES-70S1 solar simulator matching the AM 1.5G standard. The system was calibrated against a NREL-certified reference solar cell. All the measurements of the solar cells were performed with the active area of 0.07 cm2 under ambient atmosphere at room temperature without encapsulation.
3. Results and Discussion
The planar heterojunction perovskite solar cell in this study is structured as FTO/TiO2 (with or without PCBM)/CH3NH3PbI3−xClx/spiro-MeOTAD/Ag as shown in Figure 1, where the TiO2 and spiro-MeOTAD act as hole and electron blocking layers, respectively. Figure 2(a) shows the photocurrent-voltage (J-V) curves of TiO2 hole blocking layer with different spin coating speed. Table 1 summarizes the device performance of these perovskite solar cells. When the TiO2 spin coating speed is 2000 rpm, the device achieves a PCE of 7.90% with JSC of 17.09 mA/cm2, VOC of 0.81 V, and FF of 0.57. In increasing the spin coating speed from 2000 rpm to 4000 rpm, the PCE increases from 7.90 to 10.81%. However, when the spin coating speed of TiO2 is further increased to 6000 rpm, a decrease in PCE along and a poor reproductivity are observed.
By increasing the spin coating speed of TiO2 from 2000 rpm to 4000 rpm, the VOC increased from 0.81 V to 0.98 V with the JSC increased from 17.09 mA/cm2 to 19.07 mA/cm2 and the fill factor does not change much. This indicates that a thicker TiO2 blocking layer caused a large series resistance and this series resistance leads to the VOC and JSC losses. While further increasing the spin coating speed does not improve the device performance, at a TiO2 spin coating speed of 6000 rpm, the corresponding devices show a lower FF which may correspond to the increase of current leakage. It is worth to notice that optimizing the spin coating speed of TiO2 does not yield a FF over 70% which can be easily obtained in a mesoporous n-i-p-structured perovskite solar cell. Here, PCBM is introduced to modify the TiO2 layer and PCBM in this structure is expected to perform threefold functions: firstly, PCBM can reduce the energy mismatch between TiO2 and the perovskite film and facilitate the electron transport from the perovskite film to the TiO2 layer; secondly, PCBM can improve the compactness of the electron transporting layer and avoid the direct contact between perovskite film and the FTO electrode; thirdly, PCBM is expected to have a passivation effect in the devices, which has been proved by recent studies [19, 20]. The photocurrent-voltage (J-V) curves with PCBM-modified TiO2 layer are shown in Figure 2(b).
From Figure 2(b), it could be seen that the insertion of PCBM is mainly in favor of the increase of FF and VOC. As it is known, the VOC of a solar cell corresponds to the splitting of quasi-fermi level and the insertion of PCBM could reduce the trap state density which can further increase the splitting . Hence compared with TiO2 only device, PCBM-modified device has reached a VOC of 1.05 V and the FF enhancement may owe the suitable band structure of PCBM which may reduce the energy offset between perovskite and TiO2. With the help of PCBM layer, the device achieved a PCE of 16.37% with a JSC of 21.96 mA/cm2, VOC of 1.05 V, and FF of 0.71 when the PCBM spin coating speed is 6000 rpm. When the PCBM layer is thick, the series resistance is large. Then with the increasing PCBM spin coating speed, the device PCE is improved. However, a too high PCBM spin coating speed will make it difficult to form a continuous and full coverage PCBM layer as shown in the formation of TiO2 layer. In fact, a PCBM spin coating speed more than 6000 rpm will lead to the decreased device performance and the 6000 rpm for PCBM solution is optimal.
It should be noted that all the devices above are fabricated by the dynamic coating method when depositing the perovskite precursor on top of the underlying PCBM layer. It was found that the coating method could greatly affect the final device performance. Figure 1 shows the performance difference between the two devices fabricated by the different coating methods. For both the devices, the PCBM layer is formed by spin coating a 10 mg/mL PCBM solution in chlorobenzene onto the TiO2 film at 6000 rpm for 45 s. The difference between them is that one device is fabricated by coating the following perovskite precursor with the dynamic method (labeled as “dynamic coating”) and another one is fabricated by coating the following perovskite precursor with the static method (labeled as “static coating”). The corresponding device parameters are summarized in Table 2. For the device with the static coating method, it achieved a PCE of 12.3% with a JSC of 19.10 mA/cm2, VOC of 1.04 V, and FF of 0.62, which is greatly inferior to the device with the dynamic coating method. It could be clearly obtained that the dynamic coating method could greatly improve the device performance. We believe that the reason is that the dynamically spin coating could avoid the serious damage of the underlying PCBM when the perovskite precursor is deposited. The PCBM layer plays an important role to improve the device performance. Xing et al.  showed that there are high density hole-deficient traps located at the TiO2 surface, which leads to an electron transfer potential barrier between CH3NH3PbI3 and TiO2. This potential barrier is absent between the perovskite layer and the PCBM layer, and then the PCBM layer has a faster charge extraction compared to TiO2 [24, 25]. In order to ensure the function of PCBM, the PCBM layer should be avoided from the damage when the perovskite precursor is deposited. This is why the device with the dynamic coating method has a better device performance.
In order to achieve a good performance, the TiO2 layer in the planar heterojunction perovskite solar cell should be compact and pinhole-free. If there are pinholes in the TiO2 layer, it is possible that the perovskite film can direct contact with the FTO electrode, which will lead to severe recombination at the FTO and perovskite film interface and result in a low short-circuit current density (JSC) and a low open-circuit voltage (VOC). It is possible to achieve a pinhole-free TiO2 layer by increasing the TiO2 film thickness. However, a too thick TiO2 blocking layer corresponds to a large series resistance and low fill factor as shown in the previous work . To investigate the influence of spin coating speed on the morphology of TiO2 thin film, SEM photographs were taken on TiO2 thin films prepared at 2000, 4000, and 6000 rpm, corresponding to Figures 3(a)–3(c), respectively (the SEM photographs in large scale in Supplementary Figure S1). Pinholes are observed on the TiO2 thin film which is prepared at a spin coating speed of 6000 rpm. And no pinholes are found on TiO2 thin films at the spin coating speed of 2000 and 4000 rpm.
Figures 4(a)–4(c) show the corresponding AFM images. The root mean square roughness (RMS) value of TiO2 thin films prepared at 2000, 4000, and 6000 rpm is 10.4 nm, 13.2 nm, and 18.4 nm, respectively. It could be seen that the film roughness is improved with the increase of the spin coating speed. When there are no pinholes, the roughness increasing is small. However, when the pinholes appear, the roughness increasing is severe. Thus, when the TiO2 thin film is prepared at a spin coating speed of 4000 rpm, the film is pinhole-free and at the same time, the thickness is low and relative smooth. This TiO2 thin film would be used for the following discussion.
By introducing PCBM into the blocking layer, the blocking layer is improved which will be in favor for the electron extraction as the electrical measurements shown in Figure 2(b). Figure 4 and Supplementary Figure S1 present the AFM and SEM photographs of PCBM/TiO2 blocking layer, the spin coating speed of TiO2 is fixed to 4000 rpm, and the spin coating speed of PCBM varied from 2000 rpm to 6000 rpm at a 2000 interval. As the Supplementary Figure S1 (d), (e), and (f) depicted, when PCBM was introduced, no pinholes are observed, and the spin coating speed could hardly affect the morphology of TiO2/PCBM thin films. As shown in the AFM photographs, the roughness of TiO2/PCBM thin films could be deduced. As Figures 4(d)–4(f) illustrated, the RMS value of TiO2/PCBM thin films prepared at 2000, 4000, and 6000 rpm is almost the same and in the range from 15 nm to 16 nm. By increasing the spin coating speed, negligible variances in RMS values are observed. It is shown that the RMS values are mainly determined by the underlying TiO2 layer.
To further investigate the morphology of CH3NH3PbI3−xClx organometal halide perovskite material deposited on TiO2/PCBM thin films, SEM and AFM photographs of CH3NH3PbI3−xClx are taken on TiO2/PCBM thin films, with TiO2 spin coating speed fixed at 4000 rpm and the PCBM spin coating speed varied from 2000 rpm to 6000 rpm at a 2000 interval. It could be found that dense and uniform CH3NH3PbI3−xClx organometal halide perovskite thin film is obtained on the TiO2/PCBM thin film. SEM photographs in Figures 3(d)–3(f) (the SEM photographs in large scale in Supplementary Figure S1) suggest that the grain sizes of the CH3NH3PbI3−xClx film are in a range of 450–550 nm, which is almost double the size of CH3NH3PbI3−xClx directly on the TiO2 layer as shown in our previous work . This is because that the PCBM layer is more hydrophobic than the TiO2 layer, and a hydrophobic PCBM layer can help increase the perovskite crystal size . This grain size value is slightly larger than the film thickness (about 300 nm from the XRD measurement and also could be seen from the Figure 5), which implies that it is great possible that the carrier could pass the perovskite film without encountering any grain boundaries. The granular particle between CH3NH3PbI3−xClx grain boundaries is PbI2 as in the previous report , which might passivate CH3NH3PbI3−xClx material. This result also coincidences with our following XRD analysis. Figures 4(g)–4(i) show the AFM photographs of CH3NH3PbI3−xClx organometal halide perovskite materials. With a constant spin coating speed of TiO2 at 4000 rpm, the spin coating speed and RMS roughness of PCBM are (Figure 4(g)) 2000 rpm with a 15.6 nm RMS roughness, (Figure 4(h)) 4000 rpm with a 13.8 nm RMS roughness, and (Figure 4(i)) 6000 rpm with a 12.2 nm RMS roughness. It suggests that increasing the spin coating speed results in a decrease of RMS roughness.
After the above discussion on morphology, X-ray diffraction measurement is carried out to investigate the crystallization of the thin films. As shown in Figure 6, the peaks at 37.74° and 51.50° belongs to FTO and the peaks at 26.52° and 36.71° belongs to TiO2 suggesting an anatase crystal structure. As shown in Supplementary Figure S2, the TiO2 films prepared at 2000 rpm, 4000 rpm, and 6000 rpm show the similar XRD spectra and this means that the spin coating speed does not change the underlying layer. By analyzing the XRD spectra of TiO2 and TiO2/PCBM thin films, it is found that the introduction of PCBM at various spin coating speeds and the consequence annealing process does not influence the crystallization of TiO2 and CH3NH3PbI3−xClx as shown in Figure 6 and Supplementary Figure S3. For the TiO2/PCBM/CH3NH3PbI3−xClx XRD spectra, the weak peak at 12.72° corresponds to PbI2 and this coincides with the SEM photograph. Different from the spectra of TiO2 and TiO2/PCBM thin films, three strong peaks appeared at 14.17°, 28.51°, and 31.95°, which correspond to <110>, <220>, and <310> lattice planes of the tetragonal perovskite structure, respectively.
To further illustrate the optical characteristics of TiO2/PCBM thin films, UV-vis spectra are carried out. Figure 7(a) depicts the UV-vis spectra of TiO2 thin films, and Figure 7(b) depicts the UV-vis spectra TiO2/PCBM thin films. As it could be seen from Figure 7, increasing the spin coating speed of TiO2 leads to a decrease in transmittance. On the contrary, increasing the spin coating speed of PCBM leads to an increase in transmittance. The conflict lies in the morphology and optical absorption of TiO2 and PCBM. As AFM photographs demonstrate, when spin coated at 6000 rpm, pinholes are observed on TiO2 surface along with an increase in RMS roughness. In agreement with our previous report , this roughness causes the scattering of incident light hence leads to the decrease in transmittance. While PCBM is a commonly used material in organic solar cell, a thicker PCBM layer absorbs more incident light, which leads to the decrease in transmittance.
This decrease in PCE is attributed to the pinholes in TiO2 prepared at 6000 rpm spin coating speed that has been supported by the SEM photograph. In Figure S4, PL spectra of TiO2 thin film have suggested a peak at 518.4 nm corresponding to the thickness of TiO2 thin film. Since our exciton wavelength is 515 nm, which could not satisfy the excitation requirement of bulk TiO2, the peak corresponds to the surface trap of TiO2 thin film which may greatly influence the photovoltaic performance of solar cells [27, 28]. After spin coated a thin PCBM layer as shown in Figure 8, the PL peak at 518.4 nm could hardly be observed, which further proves the suggestion that the peak corresponds to the surface trap of TiO2 thin film and the PCBM could passivate the traps.
Herein, a versatile TiO2/PCBM double blocking layer was introduced into the planar heterojunction perovskite solar cell. The PCBM layer has faster charge extraction compared to TiO2. At the same time, by employing PCBM, the TiO2 thin film was efficiently passivated and the hydrophobic PCBM also yields a large grain size pinhole-free CH3NH3PbI3−xClx thin film. Utilizing the as-prepared thin film, the planar heterojunction organometal halide perovskite solar cell has shown improved performance compared to the control device, achieving a PCE of 16.37%, a VOC of 1.05 V, a JSC of 21.96 mA/cm2, and an FF of 0.71.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
This study was partly financially supported by the National Natural Science Foundation of China (61334002, 61106063, and 61534004) and the Fundamental Research Funds for the Central Universities (JB141106).
Supplementary 1. Figure S1: SEM photographs of TiO2 thin films prepared at (a) 2000 rpm, (b) 4000 rpm, and (c) 6000 rpm, respectively. SEM photographs of TiO2/PCBM thin films with TiO2 prepared at 4000 rpm and PCBM prepared at (d) 2000 rpm, (e) 4000 rpm, and (f) 6000 rpm, respectively. And SEM photographs of CH3NH3PbI3-xClx on TiO2/PCBM thin films, the PCBM spin-coating speed is (g) 2000 rpm, (h) 4000 rpm, and (i) 6000 rpm.
Supplementary 2. Figure S2: XRD spectra of TiO2 thin films prepared at (a) 2000 rpm, (b) 4000 rpm, and (c) 6000 rpm, respectively.
Supplementary 3. Figure S3: XRD spectra of TiO2 thin films prepared at (a) 2000 rpm, (b) 4000 rpm, and (c) 6000 rpm, respectively.
Supplementary 4. Figure S4: XRD spectra of TiO2 thin films prepared at (a) 2000 rpm, (b) 4000 rpm, and (c) 6000 rpm, respectively.
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