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Efficient planar perovskite solar cells with large fill factor and excellent stability
Journal of Power Sources 297 (2015) 53e58
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Efficient planar perovskite solar cells with large fill factor and excellent stability Xichang Bao a, Yujin Wang b, Qianqian Zhu c, Ning Wang a, **, Dangqiang Zhu a, Junyi Wang a, Ailing Yang b, ***, Renqiang Yang a, d, * a
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China Department of Physics, Ocean University of China, Qingdao 266100, China College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China d State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
High-quality CH3NH3PbI3 perovskite films with a mirror-like surface were prepared. The fill factor of the perovskite solar cells approached 80.52%. The high-quality of the perovskite films greatly improves cell stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 21 July 2015 Accepted 26 July 2015 Available online xxx
The quality and stability of perovskite films are critical for solar cells using this technology. Here, we fabricate a high-quality CH3NH3PbI3 perovskite film by introducing small amounts of N-methyl-2pyrrolidone into a mixture of g-butyrolactone and dimethylsulphoxide. The perovskite film consists of compact grains (average sizes of ~57 nm) with unclear boundaries which show a mirror-like surface with the root mean square roughness of only 2.39 nm. The power conversion efficiency of 11.77% is obtained with the fill factor as high as 80.52% under one sun illumination (100 mW cm 2). Furthermore, the stability of the perovskite solar cells is greatly improved by forming compact perovskite films with excellent surfaces. The results indicate that high-quality perovskite films with compact grains are a promising choice for high-stable, efficient perovskite solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Planar perovskite solar cells High-quality films Fill factor Stability Degradation mechanism
1. Introduction
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (N. Wang), [email protected] (A. Yang), [email protected] (R. Yang). http://dx.doi.org/10.1016/j.jpowsour.2015.07.081 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Solution processed low-cost, efficient photovoltaic devices have been examined for several years for renewable solar to electrical energy. Since 2009, organic/inorganic hybrid perovskite methylammonium lead iodide (CH3NH3PbI3) material has received considerable attention on account of its perfect intrinsic properties for photovoltaic applications, such as an appropriate band gap
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(~1.55 eV), high absorption coefficient, long hole-electron diffusion length, and excellent carrier transport [1e15]. As far as efficiency is concerned, a highest power conversion efficiency (PCE) of over 15% has been achieved in both mesoporous structure and planar heterojunction perovskite solar cells [6e11]. The quality of perovskite films is extremely important for such cells. To date, several approaches, such as sequential deposition of inorganic and organic precursor [6], co-evaporation of the precursors [7], single step solution method [8e10], and vapor assistant solution process have been used to form perovskite films [16]. Among these methods, single step solution method is the simplest to fabricate low-cost solar cell devices. And this method can be compatible with low-cost printing or roll-to-roll manufacturing. However, it is very difficult to form high-quality and continuous perovskite films making use of spin coating the directly mixed lead iodine (PbI2) and methylammonium halide (CH3NH3I) due to the rapid crystallization of CH3NH3PbI3 [17,18]. Recently, several modified solution methods have been developed to improve the quality of perovskite films [8,9,19e22]. For instance, by using a mixture of g-butyrolactone (GBL) and dimethylsulphoxide (DMSO), Seok et al. fabricated uniform, dense perovskite layers via a CH3NH3IePbI2eDMSO intermediate phase and the performance of the corresponding devices was greatly improved [8]. Kim et al. effectively controlled the perovskite film by introducing small amounts of N-cyclohexyl-2-pyrrolidone as a morphology controller into N,N-dimethylformamide (DMF) [19]. However, the perovskite films still showed poor surface coverage, imperfect interfaces and structural defects due to aggregation of perovskite crystalline phases, which required further quality improvement. As far as structure is concerned, mesoporous or compact TiO2 layers are the substrates on which the perovskite films are grown in most state-of-the-art perovskite devices. However, a mesoporous or condensed TiO2 layer often requires high-temperature treatment (>450 C), which is not compatible with flexible, low-cost devices [23e28]. Recently, to solve this problem, perovskite solar cells without TiO2 layers have been prepared by several groups [8e10,29,30]. In particular, the low temperature processed perovskite solar cells using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as a hole transport layer and the fullerene-derivative phenyl-C61-butyric acid methyl ester (PCBM) as the electron transport layer have attracted much attention [29,30]. These interface layers are compatible with low-cost printable or roll-to-roll manufacturing in the future, and they can avoid hysteresis under different scanning conditions. As far as commercialization is concerned, the “golden triangle” for a solar technology involves cost, efficiency, and stability. The cost of perovskite solar cells is extremely low and their PCEs have been improved. Nevertheless, their stabilities are still a problem [31]. The main reason is that perovskite films are easily degraded into other chemical species in the presence of oxygen and moisture [32]. Therefore, it is highly desirable to investigate the degradation mechanism and develop high-quality perovskite films with enhanced stability. In this study, high-quality CH3NH3PbI3 perovskite films were prepared by using a mixture solution of GBL and DMSO with small amounts of N-methyl-2-pyrrolidone (NMP). The introducing of NMP can further inhibit grain growth, improving the surface morphology of perovskite films. The scanning electron microscope (SEM) results show that there are no clear grain boundaries and cracks. The films have mirror-like surface with a root mean square (RMS) value of only 2.39 nm. A perovskite solar cell with PEDOT:PSS as the hole transport layer and PCBM as the electron transport layer shows an improved PCE of 11.77% with large fill factor (FF) of 80.52%. In particular, the high-quality perovskite films can greatly improve the device stability. The reasons for this are discussed.
2. Experimental section 2.1. Materials CH3NH3I was synthesized by reacting 24 mL of methylamine (33wt% in ethanol) and 10 mL of hydroiodic acid (57 wt% in water, Aladdin reagent, Shanghai, China), and 100 mL ethanol in a 250 mL round bottom flask under argon at 0 C for 2 h with stirring. After reaction, the white precipitate of CH3NH3I was recovered by rotary evaporation at 50 C and then dissolved in ethanol followed by sedimentation in diethyl ether by stirring the solution for 30 min. This step was repeated three times and the white CH3NH3I powder was finally collected and dried at 50 C in a vacuum oven overnight. PbI2 was also purchased from Aladdin reagent. The PEDOT:PSS aqueous solution (Clevios PVP Al4083) and PCBM was purchased from H. C. Starck (Leverkusen, Germany) and American Dye Sour, Quebec, Canada). The above materials were ces, Inc. (Baie-d'Urfe used as received. ITO glass substrates with a sheet resistance of 15 U/sq were obtained from Shenzhen Display (Shenzhen, China). 2.2. Solar cell fabrication ITO glass was cleaned in an ultrasonic bath with detergent, ultrapure water, acetone, and isopropyl alcohol for 20 min, respectively. After being treated in an oxygen plasma for 6 min, a thin layer of PEDOT:PSS (30 nm) was spin-coated onto the ITO glass with a speed of 4000 rpm and baked at 150 C for 20 min. To form the stoichiometric CH3NH3PbI3 precursor solution, the CH3NH3I and PbI2 (1 M) were dissolved in a mixture of anhydrous GBL: DMSO (7:3), with 5 v/v% NMP. Solutions were heated at 70 C overnight to encourage dissolution of solid material, cooled to room temperature, and then filtered with a 0.22 mm PTFE filter before use. Then, the precursor solution was spin-coated onto the PEDOT:PSS modified ITO glass at 4000 rpm for 50 s. During the spin coating, toluene was used to wash the surface to form high-quality surface coverage, as reported by Seok. After being thermal treated at 90 C for 20 min in a glovebox, a thin layer of PCBM (50 nm) was spincoated onto the surface of perovskite layer in a 15 mg mL 1 chlorobenzene solution at a speed of 1500 rpm. The devices were completed after thermal deposition of 10 nm calcium and 100 nm aluminum as the cathode at a pressure of 4 10 4 Pa. The detailed device configuration and the SEM cross-section image of the optimized perovskite solar cells are given in Fig. S1 (Supporting information). The device area was 10 mm2 for each cell defined by shadow mask. 2.3. Measurements The X-ray diffraction (XRD) pattern was obtained on a Bruker D8 ADVANCE (Billerica, MA). The absorption spectra of the films on ITO glass were observed by a scanning spectrophotometer (Varian Cary 50 UV/vis, Palo Alto, CA) in the range of 250e800 nm. Surface morphological characterizations of the films were characterized by a tapping-mode atomic force microscope (AFM, Agilent 5400, Keysight Technologies, Santa Rosa, CA) and scanning electron microscopy (SEM, Hitachi S-4800). The thicknesses of the films were measured by Veeco Dektak 150 surface profiler. Current densityevoltage (JeV) characteristics of the devices were measured with a Keithley 2420 source measurement unit (Cleveland, OH)under the illumination of AM 1.5G, 100 mW cm 2 with a Newport solar simulator (Irvine, CA). Light intensity was calibrated with a standard silicon solar cell. Stability of the PSCs was explored, and the unencapsulated devices were stored in glovebox and periodically tested. The JeV curves were measured by reverse scan (from bias 1.0 V to 0.1 V) and the step voltage was fixed at 10 mV.
X. Bao et al. / Journal of Power Sources 297 (2015) 53e58
3. Results and discussion 3.1. Film properties The structure of the perovskite film was investigated via XRD as shown in Fig. 1a. For comparison, XRD pattern of CH3NH3I and standard XRD of PbI2 were also given. From Fig. 1a, one can see that there are no peaks related to CH3NH3I and PbI2 in the synthesized perovskite layer, which indicate CH3NH3I and PbI2 reacted and transformed into CH3NH3PbI3 completely. Moreover, there are two strong peaks at 14.16 and 28.49 , corresponding to (110) and (220) planes, confirming the formation of an orthorhombic crystal structure [7]. The average grain size calculated by the Scherer formula is about 57.36 nm along the (110) peak [33]. According to previous reports [8,9,34,35], the grains of CH3NH3PbI3 are larger than 200 nm using the traditional single DMF or DMSO solution processes. Our result indicates that the grain sizes can be well controlled by the addition of NMP. Fig. 1b shows the UVevis absorption spectrum of the CH3NH3PbI3 film on the PEDOT:PSS modified ITO glass. The film absorbs a wide range of light from visible to the near-infrared, indicating the formation of CH3NH3 PbI3perovskite [2,18]. The inset of Fig. 1b is the picture of the prepared CH3NH3PbI3 perovskite film. The color of the film (left) is in agreement with the typical spectrum of CH3NH3PbI3 film coverage from visible to the near-infrared. Surface roughness and coverage are critical issues for planar
Fig. 1. (a) XRD patterns of PbI2, CH3NH3I, and the perovskite films. (b) UVevis absorption spectrum of the perovskite film, the inset is the photo from different angles.
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heterojunction perovskite solar cells. The film composed of small grains is smooth and its surface is similar to a mirror (right, inset of Fig. 1b, a pen shows a clear image in the film). Fig. 2a gives the result of AFM image and the RMS value is only 2.39 nm. Using the traditional preparative method, the RMS value is larger than 50 nm [36]. Through introducing small amounts of NMP into the mixture solution of GBL and DMSO, the film consists of compact grains with effectively controlled grain sizes. To the best of our knowledge, this is the lowest RMS value for solution-processed perovskite films. Fig. 2bed shows the SEM images of the perovskite film with different scale bars. From Fig. 2b, one can observe that the perovskite film has unclear grain boundaries. According to the previous report, the perovskite films have clear grain boundaries [8]. The compact films with unclear grain boundaries could be due to the introducing a small amount of NMP. After enlarging the scale bar, the results show that the perovskite film can be fully covered on the PEDOT:PSS layer without any cracks, pinholes and other serious aggregations (Fig. 2c, d). 3.2. Photovoltaic properties The enhanced perovskite film can be easily covered with a thin layer of PCBM and with formation of a good interfacial contact. Perovskite solar cells with a typical device architecture of ITO/ PEDOT:PSS/perovskite/PCBM/Ca/Al (Fig. S1a) were employed to evaluate the photovoltaic performance of the perovskite layer. Fig. 3a shows the photo- and dark-current density versus voltage curves of such a perovskite solar cell under revere scan with a step voltage 10 mV. The device exhibits an encouraging efficiency of 10.28%, with an open circuit voltage (VOC) of 0.867 V, an excellent FF of 80.83% and a moderate short circuit current (JSC) of 14.6 mA cm 2. As we know that the FFs of the perovskite devices are sensitive to the composition, coverage and thickness of the perovskite layers as well as those of the electron transport layers. The FF of 80.83% is one of the highest values in perovskite solar cells, which also indicates the quality of perovskite film is good and can form good interface contacts with the PEDOT:PSS layers and PCBM layers. Fig. 3b shows the dark current densityevoltage curve of the device. One can see that the dark current density is only 5.1 10 5 and 3.6 10 2 mA cm 2 under 0 V and with a large reverse bias of 1.5 V, respectively, which are much lower than the previous reported values [20]. The low dark current density is mainly ascribed to high-quality of perovskite film, which has less interfacial, structural and chemical defects, and thus reducing charge recombination. In addition, good surface morphology of the perovskite layer can be fully covered with a thin layer of PCBM, which can prevent leakage. These results show that high-quality perovskite film with good surface morphology can greatly improve device performance. The relatively lower PCE for above device is mainly due to low JSC. We know that JSC values of the high performance perovskite solar cells are higher than 17 mA cm 2 when the active layer is about 300 nm thick [7,37,38]. The thickness of the perovskite film is about 240 nm in the above cell. Thereafter, we further improved the device processing through increasing the thickness of the active layer to enhance JSC without sacrificing the large FF. When the thickness of the perovskite layer was increased to 290 nm (Fig. S1b), the performance of the perovskite solar cell improved. The PCE of such freshly-prepared perovskite solar cells approached 11.77%, with an enhanced JSC of 17.83 mA cm 2 and a large FF of 80.52% (Fig. 4). For comparison, a perovskite solar cell was prepared without NMP but with the same concentration and the same spin coating speed as given above [8]. Its PCE was 9.94% with a comparable FF of 74.10% (Fig. S2). The only difference of the two cells is whether a small amount of NMP was used in the preparing of the
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Fig. 2. (a) AFM image of the perovskite film, the scanning size is 10 mm 10 mm (bed) SEM images of the perovskite film with different bars.
Fig. 4. Current density versus voltage of the optimized perovskite solar cell.
perovskite films. As discussed in Fig. 2, the improved device performance was mainly due to the improved film quality by introducing a small amount of NMP.
3.3. Stability
Fig. 3. (a) Photo and dark current density versus voltage of the device. (b) Dark current density versus voltage of perovskite solar cell.
Device stability is another important factor for practical application of solar cells. The unencapsulated device was stored in glovebox for 20 days and tested periodically to investigate the stability. Fig. 5 shows the normalized photovoltaic characteristics as a function of time. The detailed device performance parameters are
X. Bao et al. / Journal of Power Sources 297 (2015) 53e58
Fig. 5. Degradation of the perovskite solar cell stored in glovebox. Normalized PCE, FF, VOC, and JSC versus time.
given in Table 1. One can observe that the device exhibits excellent stability. The VOC was almost constant during the whole experiment. It showed a slight decrease of 0.012 V over 20 days. Both the FF and JSC decay slightly with time. From the first day to the last day of observations, there was only a 3.8% and 7.7% decrease in FF and JSC. The device efficiency decreases from 10.28% to 9.02%, a loss of 12.3% over 20 days. While preparing the manuscript, JeV curve of the optimized CH3NH3PbI3 perovskite solar cell was measured again after 20 days of storage (Fig. 4). The PCE was 10.24% with a JSC of 17.06 mA cm 2 and a FF of 75.15%, showing good stability. Because of the device structure, its performance degradation may be a result of changes in the two transport layers (Ca/Al film and PEDOT:PSS). The Ca/Al, as highly reactive material, will tend to react with slight amount of oxygen and moisture and result in the degradation of the devices [39,40]. The ITO/PEDOT:PSS interface is not stable due to the etching of ITO by the strongly acidic nature of PEDOT:PSS. The degraded interface of ITO/PEDOT:PSS can act as a defect site and lead to deterioration of device performance, especially in regard to its long-term stability [41,42]. Furthermore, the stability of the perovskite films should be considered. According to previous work, it can be easily degraded in the presence of oxygen and moisture [43]. The XRD pattern of a control perovskite film was measured after 20 days of storage in a glovebox, as shown in Fig. 1a. One can see that the peaks match well with those of a fresh film, suggesting that there are no changes in the crystallographic structure of the perovskite film over time. For comparison, the XRD pattern of perovskite films stored for 20 days in air was also obtained as shown in Fig. S3. Most of the peaks match well with the fresh perovskite film except for one weak peak
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at 12.71, which may be identified with PbI2. These results show that only a small amount of perovskite is decomposed in air over 20 days. During this time, the color of the perovskite film remains the same (Fig. S4), so the film appears to be very stable under these conditions. However, after storage in air for 20 days, a scratch in the film shows a slight yellow coloration, which means that degradation mainly occurs at the grain boundaries. In order to verify this deduction, a perovskite film was also prepared by spin coating the direct mixed CH3NH3I and PbI2 in DMF solution with the same concentration and spin speed. The film shows poor coverage (Fig. S5) and low photovoltaic performance (Fig. S2). The device efficiency decreases quickly even in glovebox. The film decomposed completely when stored for only 3 days in air because of its grain boundaries exposed to air (Fig. S4, only showing the presence of PbI2). Our results demonstrate that after adding NMP, the stability of the perovskite film can be enhanced by reducing grain sizes and improving film quality with unclear compact grain boundaries. High-quality perovskite films can not only improve device efficiency, but also enhance device stability. For further improvement of the stability, perovskite solar cells which do not use the unstable low work function metal Ca and strongly acidic PEDOT:PSS are being developed for practical applications. 4. Conclusion By introducing small amounts of NMP into the solution of GBL and DMSO, high-quality CH3NH3PbI3 perovskite films were prepared. The grains have been effectively controlled with the average sizes of ~57 nm. The perovskite film consists of compact grains with undefined boundaries, which show a mirror-like surface with the RMS value of only 2.39 nm. The PCE of 11.77% is obtained with large FF of 80.52% based on the structure of ITO/PEDOT:PSS/perovskite/ PCBM/Ca/Al under one sun illumination (100 mW cm 2). The FF of 80.52% is one of the highest values in perovskite solar cells. The results show that the degradation mainly occurred at the grain boundaries. The stability of the perovskite film can be greatly improved by formation of compact films with well-defined surfaces. Such high-quality films are very stable, and point the way to the development of perovskite solar cells for practical applications. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2014CB643501, 2010DFA52310), the National Natural Science Foundation of China (61107090, 51173199, 41172110, 21274161 and 21204097), Qingdao Municipal Science and Technology Program (11-2-4-22-hz, 14-2-4-28-jch), and Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (IIMDKFJJ-14-08). Appendix A. Supporting information
Table 1 Detail device performance parameters of the perovskite solar cell.
Supporting information related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.07.081.
Store days
PCE (%)
FF (%)
VOC (V)
JSC (mA/cm2)
0 1 2 3 6 8 10 13 15 20
10.28 9.93 9.62 9.71 9.49 9.35 9.14 9.07 9.05 9.02
80.83 78.48 79.41 78.03 79.5 78.96 75.73 76.84 77.24 77.79
0.867 0.859 0.847 0.856 0.855 0.842 0.859 0.851 0.85 0.855
14.67 14.72 14.31 14.54 13.96 14.07 13.91 13.88 13.78 13.55
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Efficient planar perovskite solar cells with large fill factor and excellent stability Xichang Bao a, Yujin Wang b, Qianqian Zhu c, Ning Wang a, **, Dangqiang Zhu a, Junyi Wang a, Ailing Yang b, ***, Renqiang Yang a, d, * a
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China Department of Physics, Ocean University of China, Qingdao 266100, China College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China d State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
High-quality CH3NH3PbI3 perovskite films with a mirror-like surface were prepared. The fill factor of the perovskite solar cells approached 80.52%. The high-quality of the perovskite films greatly improves cell stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 21 July 2015 Accepted 26 July 2015 Available online xxx
The quality and stability of perovskite films are critical for solar cells using this technology. Here, we fabricate a high-quality CH3NH3PbI3 perovskite film by introducing small amounts of N-methyl-2pyrrolidone into a mixture of g-butyrolactone and dimethylsulphoxide. The perovskite film consists of compact grains (average sizes of ~57 nm) with unclear boundaries which show a mirror-like surface with the root mean square roughness of only 2.39 nm. The power conversion efficiency of 11.77% is obtained with the fill factor as high as 80.52% under one sun illumination (100 mW cm 2). Furthermore, the stability of the perovskite solar cells is greatly improved by forming compact perovskite films with excellent surfaces. The results indicate that high-quality perovskite films with compact grains are a promising choice for high-stable, efficient perovskite solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Planar perovskite solar cells High-quality films Fill factor Stability Degradation mechanism
1. Introduction
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (N. Wang), [email protected] (A. Yang), [email protected] (R. Yang). http://dx.doi.org/10.1016/j.jpowsour.2015.07.081 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Solution processed low-cost, efficient photovoltaic devices have been examined for several years for renewable solar to electrical energy. Since 2009, organic/inorganic hybrid perovskite methylammonium lead iodide (CH3NH3PbI3) material has received considerable attention on account of its perfect intrinsic properties for photovoltaic applications, such as an appropriate band gap
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(~1.55 eV), high absorption coefficient, long hole-electron diffusion length, and excellent carrier transport [1e15]. As far as efficiency is concerned, a highest power conversion efficiency (PCE) of over 15% has been achieved in both mesoporous structure and planar heterojunction perovskite solar cells [6e11]. The quality of perovskite films is extremely important for such cells. To date, several approaches, such as sequential deposition of inorganic and organic precursor [6], co-evaporation of the precursors [7], single step solution method [8e10], and vapor assistant solution process have been used to form perovskite films [16]. Among these methods, single step solution method is the simplest to fabricate low-cost solar cell devices. And this method can be compatible with low-cost printing or roll-to-roll manufacturing. However, it is very difficult to form high-quality and continuous perovskite films making use of spin coating the directly mixed lead iodine (PbI2) and methylammonium halide (CH3NH3I) due to the rapid crystallization of CH3NH3PbI3 [17,18]. Recently, several modified solution methods have been developed to improve the quality of perovskite films [8,9,19e22]. For instance, by using a mixture of g-butyrolactone (GBL) and dimethylsulphoxide (DMSO), Seok et al. fabricated uniform, dense perovskite layers via a CH3NH3IePbI2eDMSO intermediate phase and the performance of the corresponding devices was greatly improved [8]. Kim et al. effectively controlled the perovskite film by introducing small amounts of N-cyclohexyl-2-pyrrolidone as a morphology controller into N,N-dimethylformamide (DMF) [19]. However, the perovskite films still showed poor surface coverage, imperfect interfaces and structural defects due to aggregation of perovskite crystalline phases, which required further quality improvement. As far as structure is concerned, mesoporous or compact TiO2 layers are the substrates on which the perovskite films are grown in most state-of-the-art perovskite devices. However, a mesoporous or condensed TiO2 layer often requires high-temperature treatment (>450 C), which is not compatible with flexible, low-cost devices [23e28]. Recently, to solve this problem, perovskite solar cells without TiO2 layers have been prepared by several groups [8e10,29,30]. In particular, the low temperature processed perovskite solar cells using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as a hole transport layer and the fullerene-derivative phenyl-C61-butyric acid methyl ester (PCBM) as the electron transport layer have attracted much attention [29,30]. These interface layers are compatible with low-cost printable or roll-to-roll manufacturing in the future, and they can avoid hysteresis under different scanning conditions. As far as commercialization is concerned, the “golden triangle” for a solar technology involves cost, efficiency, and stability. The cost of perovskite solar cells is extremely low and their PCEs have been improved. Nevertheless, their stabilities are still a problem [31]. The main reason is that perovskite films are easily degraded into other chemical species in the presence of oxygen and moisture [32]. Therefore, it is highly desirable to investigate the degradation mechanism and develop high-quality perovskite films with enhanced stability. In this study, high-quality CH3NH3PbI3 perovskite films were prepared by using a mixture solution of GBL and DMSO with small amounts of N-methyl-2-pyrrolidone (NMP). The introducing of NMP can further inhibit grain growth, improving the surface morphology of perovskite films. The scanning electron microscope (SEM) results show that there are no clear grain boundaries and cracks. The films have mirror-like surface with a root mean square (RMS) value of only 2.39 nm. A perovskite solar cell with PEDOT:PSS as the hole transport layer and PCBM as the electron transport layer shows an improved PCE of 11.77% with large fill factor (FF) of 80.52%. In particular, the high-quality perovskite films can greatly improve the device stability. The reasons for this are discussed.
2. Experimental section 2.1. Materials CH3NH3I was synthesized by reacting 24 mL of methylamine (33wt% in ethanol) and 10 mL of hydroiodic acid (57 wt% in water, Aladdin reagent, Shanghai, China), and 100 mL ethanol in a 250 mL round bottom flask under argon at 0 C for 2 h with stirring. After reaction, the white precipitate of CH3NH3I was recovered by rotary evaporation at 50 C and then dissolved in ethanol followed by sedimentation in diethyl ether by stirring the solution for 30 min. This step was repeated three times and the white CH3NH3I powder was finally collected and dried at 50 C in a vacuum oven overnight. PbI2 was also purchased from Aladdin reagent. The PEDOT:PSS aqueous solution (Clevios PVP Al4083) and PCBM was purchased from H. C. Starck (Leverkusen, Germany) and American Dye Sour, Quebec, Canada). The above materials were ces, Inc. (Baie-d'Urfe used as received. ITO glass substrates with a sheet resistance of 15 U/sq were obtained from Shenzhen Display (Shenzhen, China). 2.2. Solar cell fabrication ITO glass was cleaned in an ultrasonic bath with detergent, ultrapure water, acetone, and isopropyl alcohol for 20 min, respectively. After being treated in an oxygen plasma for 6 min, a thin layer of PEDOT:PSS (30 nm) was spin-coated onto the ITO glass with a speed of 4000 rpm and baked at 150 C for 20 min. To form the stoichiometric CH3NH3PbI3 precursor solution, the CH3NH3I and PbI2 (1 M) were dissolved in a mixture of anhydrous GBL: DMSO (7:3), with 5 v/v% NMP. Solutions were heated at 70 C overnight to encourage dissolution of solid material, cooled to room temperature, and then filtered with a 0.22 mm PTFE filter before use. Then, the precursor solution was spin-coated onto the PEDOT:PSS modified ITO glass at 4000 rpm for 50 s. During the spin coating, toluene was used to wash the surface to form high-quality surface coverage, as reported by Seok. After being thermal treated at 90 C for 20 min in a glovebox, a thin layer of PCBM (50 nm) was spincoated onto the surface of perovskite layer in a 15 mg mL 1 chlorobenzene solution at a speed of 1500 rpm. The devices were completed after thermal deposition of 10 nm calcium and 100 nm aluminum as the cathode at a pressure of 4 10 4 Pa. The detailed device configuration and the SEM cross-section image of the optimized perovskite solar cells are given in Fig. S1 (Supporting information). The device area was 10 mm2 for each cell defined by shadow mask. 2.3. Measurements The X-ray diffraction (XRD) pattern was obtained on a Bruker D8 ADVANCE (Billerica, MA). The absorption spectra of the films on ITO glass were observed by a scanning spectrophotometer (Varian Cary 50 UV/vis, Palo Alto, CA) in the range of 250e800 nm. Surface morphological characterizations of the films were characterized by a tapping-mode atomic force microscope (AFM, Agilent 5400, Keysight Technologies, Santa Rosa, CA) and scanning electron microscopy (SEM, Hitachi S-4800). The thicknesses of the films were measured by Veeco Dektak 150 surface profiler. Current densityevoltage (JeV) characteristics of the devices were measured with a Keithley 2420 source measurement unit (Cleveland, OH)under the illumination of AM 1.5G, 100 mW cm 2 with a Newport solar simulator (Irvine, CA). Light intensity was calibrated with a standard silicon solar cell. Stability of the PSCs was explored, and the unencapsulated devices were stored in glovebox and periodically tested. The JeV curves were measured by reverse scan (from bias 1.0 V to 0.1 V) and the step voltage was fixed at 10 mV.
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3. Results and discussion 3.1. Film properties The structure of the perovskite film was investigated via XRD as shown in Fig. 1a. For comparison, XRD pattern of CH3NH3I and standard XRD of PbI2 were also given. From Fig. 1a, one can see that there are no peaks related to CH3NH3I and PbI2 in the synthesized perovskite layer, which indicate CH3NH3I and PbI2 reacted and transformed into CH3NH3PbI3 completely. Moreover, there are two strong peaks at 14.16 and 28.49 , corresponding to (110) and (220) planes, confirming the formation of an orthorhombic crystal structure [7]. The average grain size calculated by the Scherer formula is about 57.36 nm along the (110) peak [33]. According to previous reports [8,9,34,35], the grains of CH3NH3PbI3 are larger than 200 nm using the traditional single DMF or DMSO solution processes. Our result indicates that the grain sizes can be well controlled by the addition of NMP. Fig. 1b shows the UVevis absorption spectrum of the CH3NH3PbI3 film on the PEDOT:PSS modified ITO glass. The film absorbs a wide range of light from visible to the near-infrared, indicating the formation of CH3NH3 PbI3perovskite [2,18]. The inset of Fig. 1b is the picture of the prepared CH3NH3PbI3 perovskite film. The color of the film (left) is in agreement with the typical spectrum of CH3NH3PbI3 film coverage from visible to the near-infrared. Surface roughness and coverage are critical issues for planar
Fig. 1. (a) XRD patterns of PbI2, CH3NH3I, and the perovskite films. (b) UVevis absorption spectrum of the perovskite film, the inset is the photo from different angles.
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heterojunction perovskite solar cells. The film composed of small grains is smooth and its surface is similar to a mirror (right, inset of Fig. 1b, a pen shows a clear image in the film). Fig. 2a gives the result of AFM image and the RMS value is only 2.39 nm. Using the traditional preparative method, the RMS value is larger than 50 nm [36]. Through introducing small amounts of NMP into the mixture solution of GBL and DMSO, the film consists of compact grains with effectively controlled grain sizes. To the best of our knowledge, this is the lowest RMS value for solution-processed perovskite films. Fig. 2bed shows the SEM images of the perovskite film with different scale bars. From Fig. 2b, one can observe that the perovskite film has unclear grain boundaries. According to the previous report, the perovskite films have clear grain boundaries [8]. The compact films with unclear grain boundaries could be due to the introducing a small amount of NMP. After enlarging the scale bar, the results show that the perovskite film can be fully covered on the PEDOT:PSS layer without any cracks, pinholes and other serious aggregations (Fig. 2c, d). 3.2. Photovoltaic properties The enhanced perovskite film can be easily covered with a thin layer of PCBM and with formation of a good interfacial contact. Perovskite solar cells with a typical device architecture of ITO/ PEDOT:PSS/perovskite/PCBM/Ca/Al (Fig. S1a) were employed to evaluate the photovoltaic performance of the perovskite layer. Fig. 3a shows the photo- and dark-current density versus voltage curves of such a perovskite solar cell under revere scan with a step voltage 10 mV. The device exhibits an encouraging efficiency of 10.28%, with an open circuit voltage (VOC) of 0.867 V, an excellent FF of 80.83% and a moderate short circuit current (JSC) of 14.6 mA cm 2. As we know that the FFs of the perovskite devices are sensitive to the composition, coverage and thickness of the perovskite layers as well as those of the electron transport layers. The FF of 80.83% is one of the highest values in perovskite solar cells, which also indicates the quality of perovskite film is good and can form good interface contacts with the PEDOT:PSS layers and PCBM layers. Fig. 3b shows the dark current densityevoltage curve of the device. One can see that the dark current density is only 5.1 10 5 and 3.6 10 2 mA cm 2 under 0 V and with a large reverse bias of 1.5 V, respectively, which are much lower than the previous reported values [20]. The low dark current density is mainly ascribed to high-quality of perovskite film, which has less interfacial, structural and chemical defects, and thus reducing charge recombination. In addition, good surface morphology of the perovskite layer can be fully covered with a thin layer of PCBM, which can prevent leakage. These results show that high-quality perovskite film with good surface morphology can greatly improve device performance. The relatively lower PCE for above device is mainly due to low JSC. We know that JSC values of the high performance perovskite solar cells are higher than 17 mA cm 2 when the active layer is about 300 nm thick [7,37,38]. The thickness of the perovskite film is about 240 nm in the above cell. Thereafter, we further improved the device processing through increasing the thickness of the active layer to enhance JSC without sacrificing the large FF. When the thickness of the perovskite layer was increased to 290 nm (Fig. S1b), the performance of the perovskite solar cell improved. The PCE of such freshly-prepared perovskite solar cells approached 11.77%, with an enhanced JSC of 17.83 mA cm 2 and a large FF of 80.52% (Fig. 4). For comparison, a perovskite solar cell was prepared without NMP but with the same concentration and the same spin coating speed as given above [8]. Its PCE was 9.94% with a comparable FF of 74.10% (Fig. S2). The only difference of the two cells is whether a small amount of NMP was used in the preparing of the
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Fig. 2. (a) AFM image of the perovskite film, the scanning size is 10 mm 10 mm (bed) SEM images of the perovskite film with different bars.
Fig. 4. Current density versus voltage of the optimized perovskite solar cell.
perovskite films. As discussed in Fig. 2, the improved device performance was mainly due to the improved film quality by introducing a small amount of NMP.
3.3. Stability
Fig. 3. (a) Photo and dark current density versus voltage of the device. (b) Dark current density versus voltage of perovskite solar cell.
Device stability is another important factor for practical application of solar cells. The unencapsulated device was stored in glovebox for 20 days and tested periodically to investigate the stability. Fig. 5 shows the normalized photovoltaic characteristics as a function of time. The detailed device performance parameters are
X. Bao et al. / Journal of Power Sources 297 (2015) 53e58
Fig. 5. Degradation of the perovskite solar cell stored in glovebox. Normalized PCE, FF, VOC, and JSC versus time.
given in Table 1. One can observe that the device exhibits excellent stability. The VOC was almost constant during the whole experiment. It showed a slight decrease of 0.012 V over 20 days. Both the FF and JSC decay slightly with time. From the first day to the last day of observations, there was only a 3.8% and 7.7% decrease in FF and JSC. The device efficiency decreases from 10.28% to 9.02%, a loss of 12.3% over 20 days. While preparing the manuscript, JeV curve of the optimized CH3NH3PbI3 perovskite solar cell was measured again after 20 days of storage (Fig. 4). The PCE was 10.24% with a JSC of 17.06 mA cm 2 and a FF of 75.15%, showing good stability. Because of the device structure, its performance degradation may be a result of changes in the two transport layers (Ca/Al film and PEDOT:PSS). The Ca/Al, as highly reactive material, will tend to react with slight amount of oxygen and moisture and result in the degradation of the devices [39,40]. The ITO/PEDOT:PSS interface is not stable due to the etching of ITO by the strongly acidic nature of PEDOT:PSS. The degraded interface of ITO/PEDOT:PSS can act as a defect site and lead to deterioration of device performance, especially in regard to its long-term stability [41,42]. Furthermore, the stability of the perovskite films should be considered. According to previous work, it can be easily degraded in the presence of oxygen and moisture [43]. The XRD pattern of a control perovskite film was measured after 20 days of storage in a glovebox, as shown in Fig. 1a. One can see that the peaks match well with those of a fresh film, suggesting that there are no changes in the crystallographic structure of the perovskite film over time. For comparison, the XRD pattern of perovskite films stored for 20 days in air was also obtained as shown in Fig. S3. Most of the peaks match well with the fresh perovskite film except for one weak peak
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at 12.71, which may be identified with PbI2. These results show that only a small amount of perovskite is decomposed in air over 20 days. During this time, the color of the perovskite film remains the same (Fig. S4), so the film appears to be very stable under these conditions. However, after storage in air for 20 days, a scratch in the film shows a slight yellow coloration, which means that degradation mainly occurs at the grain boundaries. In order to verify this deduction, a perovskite film was also prepared by spin coating the direct mixed CH3NH3I and PbI2 in DMF solution with the same concentration and spin speed. The film shows poor coverage (Fig. S5) and low photovoltaic performance (Fig. S2). The device efficiency decreases quickly even in glovebox. The film decomposed completely when stored for only 3 days in air because of its grain boundaries exposed to air (Fig. S4, only showing the presence of PbI2). Our results demonstrate that after adding NMP, the stability of the perovskite film can be enhanced by reducing grain sizes and improving film quality with unclear compact grain boundaries. High-quality perovskite films can not only improve device efficiency, but also enhance device stability. For further improvement of the stability, perovskite solar cells which do not use the unstable low work function metal Ca and strongly acidic PEDOT:PSS are being developed for practical applications. 4. Conclusion By introducing small amounts of NMP into the solution of GBL and DMSO, high-quality CH3NH3PbI3 perovskite films were prepared. The grains have been effectively controlled with the average sizes of ~57 nm. The perovskite film consists of compact grains with undefined boundaries, which show a mirror-like surface with the RMS value of only 2.39 nm. The PCE of 11.77% is obtained with large FF of 80.52% based on the structure of ITO/PEDOT:PSS/perovskite/ PCBM/Ca/Al under one sun illumination (100 mW cm 2). The FF of 80.52% is one of the highest values in perovskite solar cells. The results show that the degradation mainly occurred at the grain boundaries. The stability of the perovskite film can be greatly improved by formation of compact films with well-defined surfaces. Such high-quality films are very stable, and point the way to the development of perovskite solar cells for practical applications. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2014CB643501, 2010DFA52310), the National Natural Science Foundation of China (61107090, 51173199, 41172110, 21274161 and 21204097), Qingdao Municipal Science and Technology Program (11-2-4-22-hz, 14-2-4-28-jch), and Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (IIMDKFJJ-14-08). Appendix A. Supporting information
Table 1 Detail device performance parameters of the perovskite solar cell.
Supporting information related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.07.081.
Store days
PCE (%)
FF (%)
VOC (V)
JSC (mA/cm2)
0 1 2 3 6 8 10 13 15 20
10.28 9.93 9.62 9.71 9.49 9.35 9.14 9.07 9.05 9.02
80.83 78.48 79.41 78.03 79.5 78.96 75.73 76.84 77.24 77.79
0.867 0.859 0.847 0.856 0.855 0.842 0.859 0.851 0.85 0.855
14.67 14.72 14.31 14.54 13.96 14.07 13.91 13.88 13.78 13.55
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