Performance Enhancement of Inverted Perovskite Solar Cells Based

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Surfaces, Interfaces, and Applications

Performance Enhancement of Inverted Perovskite Solar Cells Based on Smooth and Compact PC61BM:SnO2 Electron Transport Layers Yao Wang, Chenghao Duan, Jiangsheng Li, Wei Han, Min Zhao, Lili Yao, Yuanyuan Wang, Chao Yan, and Tonggang Jiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Applied Materials & Interfaces

Performance Enhancement of Inverted Perovskite Solar Cells Based on Smooth and Compact PC61BM:SnO2 Electron Transport Layers Yao Wang†,‡, Chenghao Duan†, Jiangsheng Li†, Wei Han†, Min Zhao†, Lili Yao†, Yuanyuan Wang†, Chao Yan‡, Tonggang Jiu*,†

†

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of

Sciences, Qingdao, Shandong 266101, P. R. China. ‡

College of Materials Science and Engineering, Jiangsu University of Science and

Technology, Zhenjiang, Jiangsu 212003, P. R. China. Email: [email protected]

Abstract: In this work, PC61BM:SnO2 electron transport layers (ETLs) were applied in inverted CH3NH3PbI3 perovskite solar cells and a high power conversion efficiency of 19.7% could be obtained. It increased by 49.0% in comparison with the device based on PC61BM-only ETL (13.2%). SnO2 nanocrystals with excellent dispersibility were employed here to fill the pin-holes and cover the valleys of PC61BM layer, forming smooth and compact PC61BM:SnO2 layers. Simultaneously, the electron traps caused by deep-level native defects of SnO2 were reduced by PC61BM proved by the space charge limited current analysis. Thus, PC61BM:SnO2 ETLs can inhibit both of

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the defects in PC61BM and SnO2 layers which contribute to the electron transport improvement and reduce the recombination loss. Moreover, the device stability based on the bilayer was significantly improved in comparison with the PC61BM-only device and the performance of 85% could be maintained after one month. Keywords: SnO2, Perovskite solar cell, Inverted structure, Electron traps, Device stability.

INTRODUCTION Organic-inorganic hybrid lead halide perovskite solar cells (PVSCs) have attracted great attention due to the superior photovoltaic properties of the halide perovskite materials such as high optical absorption coefficients, carrier diffusion length in micron scale, bipolar charge transport and long charge lifetime. In the past few years, great success has been achieved in this field where not only the PCE has been enhanced from 3.8% to 22.1% in last few year1-3 but also the device stability has been greatly guaranteed.4 These efforts push the low-cost solar energy conversion material to walk forth into the dawn of large-scale practical applications. The ambipolar semiconducting characteristic of perovskite materials endows the device constructions with conventional mesoscopic structure and planar structure which can be divided into n-i-p or p-i-n layouts.4,5 The PVSCs with record efficiency have been realized by adopting the electron transfer layers of metal oxide obtained through high-heat treatment in traditional n-i-p architecture. However, there is no doubt that the process of high-heat treatment increases the cost and confines certain photovoltaic


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applications such as the flexible solar cell and invert architecture. The PVSCs with p-i-n architecture are able to be easily realized by solution method under low temperature. PC61BM is the most commonly used material as electron transport layer (ETL) in inverted p-i-n PVSCs.6 However, due to its poor coverage on perovskite layer, modification needs to be carried out. Usually hole blocking materials such as bathocuproine (BCP) and LiF inserted between top electrodes and ETLs are employed to remedy the disadvantages of PC61BM film aiming to achieve higher device performance.7-11 However, due to the mismatching of the lowest unoccupied molecular orbital (LUMO) levels between BCP and PC61BM, it may result in a negative effect on electron transport.12 Moreover, Kuang et al. applied graphdiyne13-16 to modify PC61BM and improved the film conductivity and interfacial coverage of ETL.17 Jia et al. employed acene to modified PC61BM and a PCE of 17.07% could be achieved.18 Lin et al. reported that Indene-C60 bisadduct was employed to replace PC61BM for its higher LUMO level and a high PCE of 18.5% could be obtained.19 Gu et al. selected small organic molecule to replace PC61BM which achieved an optimal PCE of 18.2%.20 Additionally in most case, the improved methods of PC61BM or new substitutes are still under exploration in order to obtain better performance. In this study, SnO2 nanocrystals (NCs) were prepared to modify PC61BM layer aiming at obtaining smooth and compact PC61BM:SnO2 ETLs with the device structure of ITO/P3CT-K/MAPbI3/PC61BM:SnO2/Al. Surprisingly, the device showed an optimal PCE of 19.7% which increased by 49.0% in comparison with the PC61BM-only device (13.2%). By means of simple spin coating method, SnO2



nanocrystals with excellent dispersibility were employed to fill the pin-holes and cover the valleys on the PC61BM layer, forming smooth and compact PC61BM:SnO2 ETLs. Several measurements were performed to investigate the device behavior. Meanwhile, the stability of the device was tested and the outcome of maintaining 85% of the initial performance after one month was observed.

RESULTS AND DISCUSSION

Figure 1. (a) Illustration of the PVSCs with a device architecture of ITO/P3CT-K/MAPbI3/PC61BM:SnO2/Al. (b) The corresponding energy level diagram of the studied PVSCs. Top-sectional SEM images of (c) perovskite active layer growth on P3CT-K layer, (d) PC61BM film growth on P3CT-K/MAPbI3 layers and (e)


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PCBM:SnO2 bilayer growth on P3CT-K/MAPbI3 layers. TEM images of as-prepared SnO2 with (f) low- and (g) high-resolution respectively. (h) The three-dimensional structure illustration of PC61BM:SnO2 electron transport layers.

To examine the property of P61CBM:SnO2 ETLs in inverted PVSCs, the device with the configuration of ITO/P3CT-K/MAPbI3/PC61BM:SnO2/Al was designed as shown in Figure 1a. Poly[3-(4-carboxylbutyl) thiophene-K (P3CT-K) is used here as hole transport material. Bathocuproine, which used to be an exciton- or hole-blocking layer deposited between ETLs and top electrodes in the inverted p-i-n PVSCs, was abandoned in our process of device fabrication.10,19 The highest occupied molecular orbital (HOMO) level of SnO2 is lower than that of BCP,21 indicating that SnO2 demonstrates a better capability of blocking hole. Meanwhile its stepped conduction bands with MAPbI3 and PC61BM can facilitate electronic transport and reduce the accumulation of electrons at the interface as presented in Figure 1b.22,23 The surface morphology of the deposited perovskite active layer (MAPbI3) on top of ITO/P3CT-K is shown in Figure 1c.24 PC61BM films prepared by solution method have a strong tendency towards forming aggregates during the film drying process.25 When PC61BM was deposited on the perovskite active layer, many pinholes emerged in the PC61BM layer (Figure 1d and Figure S1), resulting in poor morphology and possible direct contact between MAPbI3 and top electrode. The coating of SnO2 dispersion on top of PC61BM layer made the morphology of whole film much better than before as shown in Figure 1e. SnO2, prepared by hydrothermal reaction, whose



nanocrystal size is about 8 nm on average (Figure 1f), can be dispersed very well in trifluoroethanol (Figure S2). X-ray powder diffraction (XRD) characteristic of the as-prepared SnO2 NCs are shown in Figure S3, indicating the great crystallinity of the NCs and the consistency of the result of high-resolution transmission electron microscope (TEM) with the lattice plane of (110) in SnO2 NCs (Figure 1g). Therefore, the clarified dispersion dripped on the PC61BM layer could fill the pin-holes and simultaneously cover the valleys on the PC61BM layer to obtain more smooth and compact film. As shown in Figure S4, the surface roughness of the PC61BM layer is 6.12nm and it can be reduced to 2.63nm after PC61BM:SnO2 bilayer formed. The resulting three-dimensional structure of PC61BM layer after modified by SnO2 is visually depicted in Figure 1h. In addition, the cross-sectional SEM image of the device based on the PC61BM:SnO2 bilayer is presented in Figure S5 to estimate the thicknesses of PC61BM layer and SnO2 layer, which are ~40nm and ~15 nm respectively. X-ray photoelectron spectroscopy (XPS) was carried out to explore the state of Sn and O in the SnO2 NCs. In Figure 2a, the spectra of Sn 3d can be deconvoluted into different peaks. The peaks of 486.8 and 495.2 eV are in agreement with Sn 3d3/2 and Sn 3d5/2 defined as rutile structure.26 The peaks which shift to lower energies indicate the existence of Sn atoms with lower valence state and oxygen vacancies in the SnO2 NCs which can lead to deep-level native defects and produce electron traps.27,28 The spectra of O 1s (Figure 2b) presents three peaks at 529.5, 531.1 and 532.8 eV which are attributed to metal-oxygen bonds, OH-bonds and absorbed water, respectively.


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Figure S6 shows the Sn 3d and O 1s XPS peaks of PC61BM:SnO2 bilayer which confirms that the electron traps also exist in the bilayer. To explore the trap density and the carrier motility of the electron transport layers, the space charge limited current

(SCLC)

technique

was

adopted

with

the

electron-only

devices

(ITO/Ag/PC61BM:SnO2 or PC61BM/Ag). For the PC61BM layer, the electron mobility increases remarkably after illuminating under a simulated sunlight compared with the condition in the dark (Figure 2c). PC61BM with a band gap of 2.0 eV (Figure 1b) prompts that it can adsorb a certain amount of sunlight in low wavelength range



Figure 2. XPS spectra of (a) Sn 3d and (b) O 1s. (b) log(J)-log(V) plots for the (c) PC61BM layer and (d) PCBM:SnO2 bilayer using the space-charge-limited current model in dark or under illumination with device structure of ITO/Ag/ETLs/Ag. Photocarrier dynamics in perovskite films. (e) Steady state and (f) transient PL spectra of perovskite film deposited by PC61BM layer and PC61BM:SnO2 bilayer. under illumination,29 and the photons in possession of greater energy than the band gap of PC61BM may generate charge carriers. Whereas, for the PC61BM:SnO2 bilayer, the change of electron mobility is very small after illuminating under a simulated


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sunlight compared with the condition in the dark (Figure 2d), indicating that the charge carriers coming from PC61BM fill the electron traps in SnO2 firstly and then form a small amount of free electrons to enhance the conductivity of PC61BM:SnO2 ETLs.30 Hence, PC61BM:SnO2 ETLs can inhibit both of the defects in PC61BM and SnO2 layers. Steady photoluminescence (PL) was performed to estimate the charge transfer efficiency between the perovskite active layer and the ETLs. As shown in Figure 2e, all samples appear an emission peak at about 780 nm which originates from MAPbI3. For MAPbI3 film, the photoinduced carriers in the excited state can not be extracted by the extraction layers and result in radiative recombination. Obvious fluorescence quenching phenomenon is observed when PC61BM layer is deposited on the perovskite active layer. The reason is that a large number of photoinduced carriers can be extracted by PC61BM layer before the radiative recombination occur. More encouragingly, the fluorescence quenching can be further enhanced after applying SnO2 to form PC61BM:SnO2 ETLs because the stepped conduction bands between MAPbI3, PC61BM and SnO2 can provide the driving force for the extraction of photoinduced carriers, further reducing the probability of radiation recombination in the MAPbI3 film. In addition, the improvement of the interfacial contact between perovskite active layer, ETL and top electrode after PC61BM:SnO2 bilayer formed can also reduce the recombination. In order to further elucidate the effect of PC61BM:SnO2 ETLs on charge transfer dynamic, transient photoluminescence (TRPL) was conducted to explore the lifetime of the electron-hole pairs in MAPbI3. As shown



in Figure 2f, the curves were fitted by a biexponential function according to the computational formula: f(t) = A1exp(-t/τ1) + A2exp(-t/τ2).31,32 Here, t is the time, A1 and A2 represent the decay amplitude, τ1 is the decay time caused by the intrinsic excition relaxation and τ2 is the decay time caused by the non-geminate/free carrier recombination.33 Moreover, the average decay time can be calculated by the computational formula: τave = (A1τ12 + A2τ22 ) / (A1τ1 + A2τ2) and various parameters were listed in Table S1. The MAPbI3 film had a long average decay time of 67.027 ns because of long exciton diffusion length.34,35 When PC61BM layer or PC61BM:SnO2 layers were deposited on the MAPbI3 film, both of them demonstrate that the average decay time was significantly reduced. And the PC61BM:SnO2 layers have the lowest average decay time, indicating that the bilayer presents best electron extraction. The results of PL and TRPL indicate that the as-prepared SnO2 NCs have a positive effect on charge extraction from MAPbI3 active layer to ETLs and can inhibit extraction barrier at the interface between MAPbI3 perovskite active layer and electron extraction layer, thus reducing the recombination.36 Typical current-voltage (I-V) characteristics of the devices based on PC61BM ETL and PC61BM:SnO2 ETLs under illumination (AM 1.5, light intensity of 100mW/cm2) are shown in Figure 3a. The device based on PC61BM:SnO2 ETLs demonstrates better


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Figure 3. Current-voltage characteristics in the (a) illumination or (b) dark for the devices based on PC61BM:SnO2 ETLs and PC61BM-only ETL. (c) Plots of dV/dJ vs (Jsc-J)-1 and the linear fitting curves derived from Figure 3a. (d) Plots of ln(Jsc-J) against V+RsJ and the linear fitting curves derived from Figure 3a. (e) Nyquist plots for the devices based on PC61BM:SnO2 ETLs and PC61BM-only ETL. (The inserted picture is the equivalent circuit model.) (f) Enlarged image from the high frequency of Figure 3e.



device performances than the device based on PC61BM-only ETL, with the PCE of 19.7%, Voc of 1.12 V, Jsc of 23.15 mA/cm2 and fill factor (FF) of 76.0%. The dark current density of the devices is estimated in Figure 3b. The devices based on PC61BM:SnO2 ETLs present a small leakage current at low voltage and a larger current density at high voltage, indicating that the charge transport are improved.37 To further clarify the photovoltaic mechanism, the plots of -dV/dJ vs (Jsc - J)-1 and the linear fitting curves are presented in Figure 3c. The calculation originated from the illumination I-V curves according to the computational formula: -dV/dJ = AkBT/e(Jsc J)-1 + Rs. Here Rs is the series resistance, J is the current density flowing via the external load, A is the ideality factor, KB is the Boltzmann constant, T is the absolute temperature, e is elementary charge, and V is the DC bias voltage which is applied at the perovskite solar cell. An excellent linear relationship between –dV/dJ and (Jsc J)-1 indicates that both of the devices based on PC61BM ETL and PC61BM:SnO2 ETLs can be considered as well-behaved heterjunctions.38 The ideality factor and the series resistance are obtained from the slope and intercept of the linear fitting results. The values of the ideality factor based on both of PC61BM:SnO2 ETLs and PC61BM-only ETL are in the range between 1.3 and 2 which qualified with the well-behaved single heterojunction solar cells.39,40 In addition, The value of ideality factor for the device based on PC61BM:SnO2 ETLs is lower than the device based on PC61BM-only ETL, indicating that the carrier diffusion is easier and the recombination is reduced at the interface between MAPbI3 active layer and PC61BM:SnO2 ETLs. Moreover Rs


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decreased from 5.95 Ωcm2 to 2.14 Ωcm2 indicating that higher fill factor could be achieved based on the PC61BM:SnO2 ETLs.41 To further investigate the recombination properties of the as-prepared perovskite solar cells, the saturated recombination current denity (J0) was calculated by the plots of ln(Jsc - J) vs V+RsJ originating from the illumination I-V curves as shown in Figure 3d.39 It can be seen that the value of J0 is tremendously decreased from 3.58×10-7 mA cm2 to 0.414×10-7 mA cm2. In addition, Voc can be estimated by [AKBTln(Jsc/J0)]/e and increased by 66 mV for the device based on PC61BM:SnO2 ETLs in comparison with the PC61BM-only device which is almost consistent with the result of experiment.42 Electrochemical impedance spectroscopy (EIS) is often employed to study charge transport and the charge recombination in PVSCs. The Nyquist plots of the as-prepared devices based on PC61BM ETL and PC61BM:SnO2 ETLs are shown in Figure 3e. The inserted picture in Figure 3e is the equivalent circuit model of the devices. Charge transfer resistance (Rtr) and charge recombination resistance (Rrec), which can form parallel circuit with the corresponding capacitor respectively, are mainly caused by the interfaces between perovskite and ETL.43 The curve in the high frequency of Figure 3e is exhibited in Figure 3f. The semicircle in high frequency is associated with Rtr and the semicircle in low frequency accounts for Rrec. Compared with the device based on PC61BM ETL, a smaller radius of the semicircle in high frequency as well as a bigger radius in low frequency can be found in the device based on PC61BM:SnO2 ETLs, showing better charge transfer and lower charge recombination at the interface of perovskite/ETLs (PCBM:SnO2).44 Superior charge



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transfer is beneficial to avoid charge accumulation at the interface, thus reducing charge extraction barrier, which is in agreement with the above discussions.

Figure 4. (a) The best device performance under reverse scan and forward scan; (b) External quantum efficiency of the best device and integrated short-circuit current density; (c) Histograms for PCE and (d) The stability test with the structure of ITO/P3CT-K/MAPbI3/PC61BM:SnO2/Al.

Figure 4a and Figure S7 show the I-V curves of the PVSCs under the conditions of forward scan and reverse scan. The hysteresis index (HI) was applied to assess the degree

of

hysteresis

with

the

computational

formula

of

HI=[JF(Voc/2)-JR(Voc/2)]/[JF(Voc/2].45,46 Here, JF(Voc/2) and JR(Voc/2) represent the


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photocurrents at Voc/2 bias for the forward scan and the reverse scan respectively. HI trending to 0 indicates a decrease in hysteresis. Hence, the values of HI for the device based on PC61BM ETL and PC61BM:SnO2 ETLs are 0.26 and 0.005. Notably, the device based on PC61BM:SnO2 ETLs presents very small hysteresis. The lower hysteresis is attributed to the improved charge extraction barrier at the interface between MAPbI3 perovskite active layer and electron extraction layer.47,48 The stepped conduction bands between MAPbI3, PC61BM and SnO2 provide the driving force for the extraction of carriers and make the PC61BM:SnO2 ETLs extract photoinduced carriers from MAPbI3 perovskite active layer more easily than PC61BM-only ETL. Hence, the hysteresis phenomenon of the perovskite solar cells is minimal. The remaining hysteresis may be originated from the perovskite itself such as the migration of ionic defects.49 To confirm the reliability of the device parameters, the stabilized photocurrent was measured under a constant bias of 0.90V at maximum power point, as shown in Figure S8. The steady-state PCE was obtained with the value of 19.5%. Figure 4b exhibits the corresponding external quantum efficiency spectrum (EQE). The value of Jsc obtained by the optimal I-V curves is well matched with the integrated Jsc obtained from EQE (the error is about 4.5%), indicating that high Jsc in the device is reliable. Also, the EQE which is over 85% across 350-750 nm on average illuminates that the fabricated device possesses high photo-to-electron conversion, further confirming the reliability of the PVSCs.50 Moreover, the solar cell efficiency histogram of the PVSCs based on PC61BM ETL and PC61BM:SnO2 ETLs is illustrated in Figure 4c and Figure S9. The average results of the photovoltaic



parameters based on 40 devices on each condition are listed in Table S2. For the devices based on PC61BM:SnO2 ETLs, an average PCE of 19.0% can be achieved and over 90% of the fabricated devices can yield a PCE of higher than 18.7%. These results confirm the promise of such inverted planar PVSCs as well as its outstanding reproducibility. Figure 4d shows the stability of the fabricated devices in terms of storage time in insert atmosphere, indicating that our architecture of the PVSCs can retain about 85% of its original performance after one month, which is improved obviously in comparison with the PC61BM-only device (Figure S7), due to more perfect encapsulation effect of more compact PC61BM:SnO2 ETLs.

CONCLUSIONS In summary, we introduced PC61BM:SnO2 ETLs into the fabrication of inverted p-i-n perovskite solar cells. The SnO2 NCs could fill the pin-holes and cover the valleys observed by SEM on the PC61BM layers, thus forming smooth and compact ETLs. Moreover, electron traps resulting from the oxygen vacancies in SnO2 NCs as certified by XPS were successfully filled by the charge carriers stemming from PC61BM under illumination. The performance improvement of electron transport and recombination contributes to an optimal PCE of 19.7%, increased by 49% in comparison with the PC61BM-only devices (13.2%) and the optimal performance could maintain 85% after one month. These desirable properties provide a new direction for the further enhancement of the performance of inverted perovskite solar cells.

EXPERIMENTAL SECTION


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Preparation of SnO2 NCs. SnO2 NCs were prepared by hydrothermal reaction according to previous report with a little modification.51 0.350 g of SnCl4·5H2O was dissolved in the mixed solvent of water and ethanol (v/v=1:1). 2.16 mL of aqueouste tramethylammonium hydroxide (TMAH) was added into the solution mentioned above. The solution was finally transferred to an autoclave in 200 oC for 12 h. The as-prepared products were collected by adding acetone and drying under vacuum conditions. Devices Fabrication. ITO was washed by water, acetone and isopropanol. At air environment, 2 mg/mL of aqueous poly[3-(4-carboxylbutyl) thiophene-K (P3CT-K) was deposited on ITO by simple spin-coating method at 3000 rpm for 60 s , then thermal treating for 15 min at 140 oC. The as-prepared ITO/P3CT-K substrate was transferred to the glove box with nitrogen atmosphere in order to complete the following operation. The precursor solution of MAPbI3 active layer was spin-coated on ITO/P3CT-K at 5000rpm/30s, then thermal treating at 60 and 80 oC. PC61BM was deposited on MAPbI3 active layer at 2000 rpm for 30 s by use of 20 mg/mL PC61BM chlorobenzene solution, then SnO2 was employed to modify PC61BM layer by spin-coating 10mg/mL of SnO2 trifluoroethanol dispersion at 2500 rpm for 60 s on it. Finally, 100 nm Ag electrode was deposited via thermal evaporation through a shadow mask to form 0.06 cm2 devices at a pressure of 9×10-8 Torr.

Supporting Information SEM image of the pin-hole on PC61BM layer, photograph of SnO2 NCs dispersion,



XRD of SnO2 NCs, AFM images of PC61BM layer and PC61BM:SnO2 layers on perovskite film, cross-sectional SEM image of the device based on PC61BM:SnO2 ETLs, Sn 3d and O 1s XPS peaks of PC61BM:SnO2 film, current-voltage characteristics under reverse scan and forward scan, stabilized photocurrent density and PCE at the maximum power point, histograms of PCE and stability tests for the devices based on PC61BM ETL and PC61BM:SnO2 ETLs, values of decay amplitude constants and decay time, summary of photovoltaic parameters.

Acknowledgements The project was supported by National Natural Science Foundation of China (51672288) and Youth Innovation Promotion Association of Chinese Academy of Sciences. This study was also supported by the Major Basic Research Program of Shandong Natural Science Foundation (ZR2017ZB0313).

Reference (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (3) Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S.


Page 18 of 27

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S.;

Seo,

J.;

Kim,

E.

K.;

Noh,

J.

H.

Iodide

Management

in

Formamidinium-Lead-Halide-Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376-1379. (4) Aitola, K.; Domanski, K.; Correa-Baena, J. P.; Sveinbjörnson, K.; Saliba, M.; Abate, A.; Grätzel, M.; Kauppinen, E.; Johansson, E. M. J.; Tress, W.; Hagfeldt, A.; Boschloo, G. High Temperature-Stable Perovskite Solar Cell Based on Low-Cost Carbon Nanotube Hole Contact. Adv. Mater. 2017, 29, 1606398. (5) Tan, H.; Jain, A.; Voznyy, O.; Lan, X. Z. F.Pelayo, G. D. A.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M. J.; Zhang, B.; Zhao, Y. C.; Fan, F. J.; Li, P. C.; Quan, L. N.; Zhao, Y. B.; Lu, Z. H.; Yang, Z. Y.; Hoogland, S.; Sargent, E. H. Efficient and Stable Solution-Processed Planar Perovskite Solar Cells via Contact Passivation. Science 2017, 355, 722-726. (6) Li, J.; Jiu, T.; Duan, C.; Wang, Y.; Zhang, H.; Jian, H.; Zhao, Y.; Wang, N.; Huang, C.; Li. Y. Improved Electron Transport in MAPbI3 Perovskite Solar Cells Based on Dual Doping Graphdiyne. Nano Energy 2018, 46, 331-337. (7) Chen, S. S.; Yang, S. W.; Sun, H.; Zhang, L.; Peng, J. J.; Liang, Z. Q.; Wang, Z. S. Enhanced Interfacial Electron Transfer of Inverted Perovskite Solar Cells by Introduction of Cose Into the Electron-Transporting-Layer. J. Power Sources 2017, 353, 123-130. (8) Lee, K. R.; Yu, J.; Yu, H.; Yun, J.; Lee, J.; Jang, J. Enhanced Efficiency and Air-Stability of NiOX-Based Perovskite Solar Cells via PCBM Electron Transport Layer Modification with Triton X-100. Nanoscale 2017, 9, 16249-16255.



(9) Kim, G. H.; Jang, H.; Yoon, Y. J.; Jeong, J.; Park, S. Y.; Walker, B.; Jeon, I. Y.; Jo, Y.; Yoon, H.; Kim, M.; Baek, J. B.; Kim, D. S.; Kim, J. Y. Fluorine Functionalized Graphene Nano Platelets for Highly Stable Inverted Perovskite Solar Cells. Nano Lett. 2017, 17, 6385-6390. (10) Matsumoto, F.; Vorpahl, S. M.; Banks, J. Q.; Sengupta, E.; Ginger, D. S. Photodecomposition and Morphology Evolution of Organometal Halide Perovskite Solar Cells. J. Phys. Chem. C 2015, 119, 20810-20816. (11) Ye, S. Y.; Rao, H. X.; Zhao, Z. R.; Zhang, L. J.; Bao, H. L.; Sun, W. H.; Li, Y. L.; Gu, F. D.; Wang, J. Q.; Liu, Z. W.; Bian, Z. Q.; Huang, C. H. A Breakthrough Efficiency of 19.9% Obtained in Inverted Perovskite Solar Cells by Using an Efficient Trap State Passivator Cu (Thiourea) I. J. Am. Chem. Soc. 2017, 139, 7504-7512. (12) Jeng, J. Y.; Chiang, Y. F.; Lee, M. H.; Peng, S. R.; Guo, T. F.; Chen, P.; Wen, T. C. CH3NH3PbI3 Perovskite/Fullerene Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2013, 25, 3727-3732. (13) Li, Y. Design and Self-assembly of Advanced Functional Molecular Material-From Low Dimension to Multi-dimension. Sci. Sin. Chim. 2017, 47, 1045-1056. (14) Huang, C. -S.; Li, Y. -L. Structure of 2D Graphdiyne and its Application in Energy Fields. Acta Phys. -Chim. Sin. 2016, 32, 1314-1329. (15) Chen, Y.; Liu, H.; Li, Y. Progress and Prospect of two Dimensional Carbon Graphdiyne. Chin. Sci. Bull. 2016, 61, 2901-2912. (16) Li, Y. J.; Li, Y. L. Two Dimensional Polymers-progress of Full Carbon


Page 20 of 27

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphyne. Acta Polymerica Sinica 2015, 2, 147-165. (17) Kuang, C.; Tang, G.; Jiu, T.; Yang, H.; Liu, H.; Li, B.; Luo, W.; Li, X.; Zhang, W.; Lu, F.; Fang, J.; Li, Y. Highly Efficient Electron Transport Obtained by Doping PCBM with Graphdiyne in Planar-Heterojunction Perovskite Solar Cells. Nano Lett. 2015, 15, 2756-2762. (18) Jia, Z. Y.; Jiu, T. G.; Li, Y. J.; Li, Y. L. New Method for the Synthesis of a Highly-Conjugated Acene Material and Its Application in Perovskite Solar Cells. Mater. Chem. Front. 2017, 1, 2261-2264. (19) Lin, Y. Z.; Chen, B.; Zhao, F. W.; Zheng, X. P.; Deng, Y. H.; Shao, Y. C.; Fang, Y. J.; Bai, Y.; Wang, C. R.; Huang, J. S. Matching Charge Extraction Contact for Wide-Bandgap Perovskite Solar Cells. Adv. Mater. 2017, 29, 1700607. (20) Gu, P. Y.; Wang, N.; Wang, C. Y.; Zhou, Y. C.; Long, G. K.; Tian, M. M.; Chen, W. Q.; Sun, X. W.; Kanatzidis, M. G.; Zhang, Q. C. Pushing up the Efficiency of Planar Perovskite Solar Cells to 18.2% with Organic Small Molecules as the Electron Transport Layer. J. Mater. Chem. A 2017, 5, 7339-7344. (21) Zhu, Z. L.; Zheng, X. L.; Bai, Y.; Zhang, T.; Wang, Z. L.; Xiao, S.; Yang, S. H. Mesoporous SnO2 single crystals as an effective electron collector for perovskite solar cells. Phys. Chem. Chem. Phys. 2015, 17, 18265-18268. (22) Zhu, Z. L.; Bai, Y.; Zhang, T.; Liu, Z. K.; Long, X.; Wei, Z. H.; Wang, Z. L.; Zhang, L. X.; Wang, J. N.; Yan, F.; Yang, S. H. High-Performance Hole-Extraction Layer of Sol-Gel-Processed NiO Nanocrystals for Inverted Planar Perovskite Solar Cells. Angew. Chem. 2014, 126, 12779-12783.



(23) Liang, P. W.; Chueh, C. C.; Williams, S. T.; Jen, A. K. Y. Roles of Fullerene-Based Interlayers in Enhancing the Performance of Organometal Perovskite Thin-Film Solar Cells. Adv. Energy Mater. 2015, 5, 1402321. (24) Chiang, C. H.; Nazeeruddin, M. K.; Grätzel, M.; Wu, C. G. The Synergistic Effect of H2O and DMF towards Stable and 20% Efficiency Inverted Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 808-817. (25) Huang, Y. C.; Liao, Y. C.; Li, S. S.; Wu, M. C.; Chen, C. W.; Su, W. F. Study of the Effect of Annealing Process on the Performance of P3HT/PCBM Photovoltatic Device Using Scanning-probe Microscopy. Sol. Energy Mat. Sol. C. 2009, 93, 888-892. (26) Wang, L. P.; Leconte, Y.; Feng, Z. X.; Wei, C.; Zhao, Y.; Ma, Q.; Xu, W. Q.; Bourrioux, S.; Azais, P.; Srinivasan, M.; Xu, Z. J. Novel Preparation of N-Doped SnO2 Nanoparticles via Laser-Assisted Pyrolysis: Demonstration of Exceptional Lithium Storage Properties. Adv. Mater. 2016, 29, 1603286. (27) Kitano, M.; Funatsu, K.; Matsuoka, M.; Ueshima, M.; Anpo, M. Preparation of Nitrogen-Substituted TiO2 Thin Film Photocatalysts by the Radio Frequency Magnetron Sputtering Deposition Method and Their Photocatalytic Reactivity under Visible Light Irradiation. J. Phys.Chem. B 2006, 110, 25266-25272. (28) Godinho, K. G.; Walsh, A.; Watson, G. W. Energetic and Electronic Structure Analysis of Intrinsic Defects in SnO2. J. Phys. Chem. C 2011, 113, 439-448. (29) Mirza, S.; Rahman, S.; Sarar, A.; Rayfield, G. Carbon Nanotubes for Optical Power Limitiing Application. Lecture Notes in Nanoscale Science & Technology 2010,


Page 22 of 27

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9, 101-129. (30) Xie, J. S. Huang, K.; Yu, X. G.; Yang, Z. R.; Xiao, K.; Qiang, Y. P.; Zhu, X. D.; Xu, L. B.; Wang, P.; Cui, C.; Yang, D. R. Enhanced Electronic Properties of SnO2 via Electron Transfer from Graphene Quantum Dots for Efficient Perovskite Solar Cells. Acs Nano 2017, 11, 9176-9182. (31) Zhao, D.; Yu,Y.; Wang, C.; Liao, W.; Shrestha, N.; Grice, C.; Cimaroli, A.; Guan, L.; Ellingson, R.; Zhu, K. Low-bandgap Mixed Tin-Lead Iodide Perovskite Absorbers with Long Carrier Lifetimes for All-perovskit Tandem Solar Cells. Nat. Energy 2017, 2, 17018. (32) Wang, S.; Bi, C.; Yuan, J.; Zhang, L.; Tian, L. Original Core-shell Structure of Cubic CsPbBr3@Amorphous CsPbBrX Perovskite Quantum Dots with a High Blue Photoluminescence Yield of over 80%. ACS Energy Lett. 2017, 3, 245-251. (33) Yang, D.; Yang, R.; Zhang, J.; Yang, Zhou.; Liu, S.; Li, C. High Efficiency Flexible Perovskite Solar Cells using Superior Low Temperature TiO2. Energy Environ. Sci. 2015, 8, 3208-3214. (34) You, J.; Hong, Z.; Yang, Y. M.; Chen, Q.; Cai, M.; Song, T. B.; Chen, C. C.; Lu, S.; Liu, Y.; Zhou, H.; Yang, Y. Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility. ACS Nano 2014, 8, 1674-1680. (35) Ryu, S.; Seo, J.; Shin, S. S.; Kim, Y. C.; Jeon, N. J.; Noh, J. H.; Seok, S. Fabrication of Metal-Oxide-Free CH3NH3PbI3 Perovskite Solar Cells Processed at Low Temperature. J. Mater. Chem. A 2015, 3, 3271-3275.



(36) Zuo, L. J.; Chen, Q.; Marco, N. D.; Hsieh, Y. T.; Chen, H. J.; Sun, P. Y.; Chang, S. Y.; Zhao, H. X.; Dong, S. Q.; Yang, Y. Tailoring the Interfacial Chemical Interaction for High-Efficiency Perovskite Solar Cells. Nano Lett. 2017, 17, 269-275. (37) Chen, C.; Li, H.; Jin, J. J.; Chen, X.; Cheng, Y.; Zheng, Y.; Liu, D. L.; Xu, L.; Song, H. W.; Dai, Q. L. Long-Lasting Nanophosphors Applied to UV-Resistant and Energy Storage Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1700758. (38) Hegedus, S. S.; Shafarman, W. N. Thin-Film Solar Cells: Device Measurements and Analysis. Prog. Plotovolt: Res. Appl. 2004, 12, 155-176. (39) Shi, J. J.; Dong, J.; Lv, S. T.; Xu, Y. Z.; Zhu, L. F.; Xiao, J. Y.; Xu, X.; Wu, H. J.; Li, D. M.; Luo, Y. H.; Meng, Q. B. Hole-Conductor-Free Perovskite Organic Lead Iodide Heterojunction Thin-Film Solar Cells: High Efficiency and Junction Property. Appl. Phys. Lett. 2014, 104, 063901. (40) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed; Wiley: New York, 2006. (41) Bag, S.; Gunawan, O.; Gokmen, T.; Zhu, Y.; Todorov, K. T.; Mitzi, D. B. Low Band Gap Liquid-Processed CZTSe Solar Cell with 10.1% Efficiency. Energy Environ. Sci. 2012, 5, 7060-7065. (42) Zhang, H. Y.; Shi, J. J.; Xu, X.; Zhu, L. F.; Luo, Y. H.; Li, D. M.; Meng, Q. B. Mg-Doped TiO2 Boosts the Efficiency of Planar Perovskite Solar Cells to Exceed 19%. J. Mater. Chem. A 2016, 4, 15383-15389. (43) Park, M.; Kim, J. Y.; Son, H. J.; Lee, C. H.; Jang, S. S.; Min, J. K. Low-Temperature Solution-processed Li-Doped SnO2 as An Effective Electron


Page 24 of 27

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Transporting Layer for High-Performance Flexible and Wearable Perovskite Solar Cells. Nano Energy 2016, 26, 208-215. (44) Wang, N.; Zhao, K.; Ding, T.; Liu, W. B.; Ahmed, A. S.; Wang, Z. R.; Tian, M. M.; Sun, X. W.; Zhang, Q. C. Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer. Adv. Energy Mater. 2017, 7, 1700522. (45) Yu, H.; Lu, H.; Xie, F.; Zhou, S.; Zhao, N. Native Defect-Induced Hysteresis Behavior in Organolead Lodide Perovskite Solar Cells. Adv. Funct. Mater. 2016, 26, 1411-1419. (46) Liu, X.; Bu, T.; Li, J.; He, J.; Li, T.; Zhang, J.; Li, W.; Ku, Z.; Peng, Y.; Huang, F.; Cheng, Y. B.; Zhong, J. Stacking n-type Layer: Effective Route towards Stable, Efficent and Hysteresis-Free Planar Perovskite Solar Cells. Nano Energy 2018, 44, 34-42. (47) Tress, W.; Correa Baena, J. P.; Saliba, M.; Abate, A.; Graetzel, M.; Inverted Current–Voltage Hysteresis in Mixed Perovskite Solar Cells: Polarization, Energy Barriers, and Defect Recombination. Adv. Energy Mater. 2016, 6, 1600396. (48) Correa baena, J. P.; Steier, L.; Tress, W.; Saliba, M.; Neutzner, S.; Matsui, T.; Giordano, F.; Jacobsson, T. J.; Srimath Kandada, A. R.; Zakeeruddin, M.; Petrozza, A.; Abate, A.; Nazeeruddin, M. K.; Gratzel, M.; Hagfeldt, A. Highly Efficient Planar Perovskite Solar Cells through Band Alignment Engineering. Energy Environ. Sci. 2015, 8, 2928-2934. (49) Li, C.; Tscheuschner, S.; Paulus, F.; Hopkinson, P. E.; Kiessling, J.; Kohle, A.;



Vaynzof, Y.; Huettner, S. Iodine Migration and its Effect on Hysteresis in Perovskite Solar Cells. Adv. Mater. 2016, 28, 2446-2454. (50) Zhu, Z. L.; Bai, Y.; Liu, X.; Chueh, C. C.; Yang, S. H.; Jen, A. K. Y. Enhanced Efficiency and Stability of Inverted Perovskite Solar Cells Using Highly Crystalline SnO2 Nanocrystals as the Robust Electron-Transporting Layer. Adv. Mater. 2016, 28, 6478-6484. (51) Wang, X.; Han, X.; Xie, S.; Kuang, Q.; Jiang, Y.; Zhang, S.; Mu, X.; Chen, G.; Zheng, L. Controlled Synthesis and Enhanced Catalytic and Gas-Sensing Properties of Tin Dioxide Nanoparticles with Exposed High-Energy Facets. Chem. Eur. J. 2012, 18, 2283.

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Performance Enhancement of Inverted Perovskite Solar Cells Based

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