Improving performance and reducing hysteresis in perovskite solar cells by using F8BT as electron transporting layer

Solar Energy Materials & Solar Cells 157 (2016) 79–84

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Improving performance and reducing hysteresis in perovskite solar cells by using F8BT as electron transporting layer Lixiao Zhao a,b, Xueyan Wang a, Xiaodong Li a, Wenjun Zhang a, Xiaohui Liu a, Yuejin Zhu b, Hai-Qiao Wang a,n, Junfeng Fang a,n a b

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China Faculty of Physics, Ningbo University, Ningbo 315211, China

art ic l e i nf o

a b s t r a c t

Article history: Received 12 January 2016 Received in revised form 7 April 2016 Accepted 11 May 2016

A conjugated polymer poly (9,9-dioctyfluorene-co-benzothiazole) (F8BT) was used as the electron transporting layer (ETL) to modify the cathode interface of inverted planar perovskite solar cell. Incorporation of the thin F8BT layer delivered greatly enhanced power conversion efficiency (PCE) of 13.9% compared to the PCE 7.9% of the device without F8BT, due to the improvements in various performance parameters like Voc, Jsc and FF. Moreover, great elimination of the hysteresis was demonstrated in F8BT based devices. These results suggest promising potential of polymeric materials in perovskite solar cells and possible approach to eliminate the hysteresis of perovskite solar cells with this class of interfacial materials. & 2016 Elsevier B.V. All rights reserved.

KEYWORDS: Perovskite solar cell F8BT Electron transporting layer Reduced hysteresis

1. Introduction The last five years have witnessed the swift surge of the perovskite solar cells [1–6]. Hybrid organic/inorganic perovskite materials with the formula CH3NH3PbX3 (X-halogen) have received a great deal of attention due to their good intrinsic properties for photovoltaic applications, such as appropriate band gap (
1.55 eV), high absorption coefficient, long hole-electron diffusion length, excellent carrier transport, low cost and ease of synthesis [7]. In 2009, perovskite materials were introduced into dyesensitized solar cells (DSSCs) as sensitizers for the first time, which showed a low power conversion efficiency (PCE) of 3.8% [1]. Until now, the PCE of perovskite solar cells has been achieved to 20.1% [8] accompanied by the rapid progresses made on each isolated materials and different architectures [6,9–12]. These fast progresses of perovskite solar cell have motivated the researchers in this field all over the world to focus their study on the scaffold, device architecture, formation of perovskite crystal and the interface engineering of this technique [4,6,13–15]. The interface engineering strategy by introducing proper interfacial materials to mainly optimize the interface properties between the perovskite layer and the charge-collecting electrode is also considered critically important to improve all the device parameters [4,6,9,16]. n

Corresponding authors. E-mail addresses: [email protected] (H.-Q. Wang), [email protected] (J. Fang). http://dx.doi.org/10.1016/j.solmat.2016.05.026 0927-0248/& 2016 Elsevier B.V. All rights reserved.

Besides classical metal oxide materials such as ZnO [16–18], TiO2 [19,20] etc., n-type organic materials have also been widely investigated in perovskite solar cells as electron extracting interfacial layers. Among them, fullerene derivatives like phenyl-C61butyric acid methyl ester (PC61BM) and Indene-C60 Bisadduct (ICBA) [21,22] or their combinations with other organic molecules [22,23] are the most reported. Compared to fullerene derivatives, conjugated polymers express structural variation and better film morphology. Conjugated polymers have been reported as electron interfacial layers in perovskite solar cells by Brabec [24] and Cao [11] groups in 2014, where the polymers were utilized in combination with PC61BM as the electron transporting layer (ETL) and improved device performances were achieved. In 2015, we demonstrated intrinsic polymeric ETLs in perovskite solar cells based on different polymers and achieved fair performance (PCE of 8.15%) compared to the reference device (PCE of 8.51%) [25]. All these results indicate excellent potential of polymeric application as ETL in perovskite solar cells especially when considering the advantages of abundant supply and easy tunable properties of polymers [25]. However, as far as we know, only a few conjugated polymers have been demonstrated to be effective ETLs in perovskite solar cells. Therefore, it is worth to explore and expand the family of polymeric electron transporting materials for achieving high electron extracting efficiency at the cathode. poly (9,9-dioctyfluorene-co-benzothiazole) (F8BT) is an efficient green-light emitter with photoluminescence efficiency up to 50–60% in solid films [26]. It has been first applied in polymer blend light emitting diodes (LEDs) as emission layer since 1990s,

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where performances such as efficiency and maximum brightness were considerably improved [27–29]. After that it was widely utilized in field effect transistors [30–32] and photovoltaics [33– 35] to improve the device performances due to its high electron mobility (μe
10  3 cm2 V  1 s  1) and good compatibility. In 2014, Gill et al. incorporated a pure F8BT layer in flexible perovskite solar cells as the ETL and by which a modest PCE of 7.05% was achieved [36]. In this work, we reported the utilization of polymer F8BT in perovskite solar cells as an ETL combined with a fullerene (C60) layer. We fabricated the perovskite solar cells with the planar configuration: ITO/ poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/CH3NH3PbI3  xClx/F8BT/C60/Al. Promising performance with PCE of 13.9%, which almost double the performance of the recently reported device [36], was achieved for the obtained device due to the improved parameters. And even more excellent PCE of 15.9% was accomplished by using the mixture blend PCBM:F8BT for substitution of F8BT layer. Moreover, largely depressed hysteresis was recorded for the perovskite solar cells with appropriate F8BT modification, compared to the devices without F8BT.

2. Material and methods 2.1. Preparation of materials Methylammonium iodide (CH3NH3I) was synthesized by reacting 24 mL of methylamine (33 wt% in absolute ethanol, Sigma) and 10 mL of hydroiodic acid (57 wt% in water, Aldrich) in a 250 mL round-bottom flask at 0 °C for 2 h with stirring [37]. The precipitate was recovered by putting the solution on a rotary evaporator and carefully removing the solvents at 50 °C. The yellowish raw product CH3NH3I was re-dissolved in 80 mL absolute ethanol and precipitated with the addition of 300 mL diethyl ether. After removing the solvents with argon pressure, the step was repeated twice. At last, the product was collected and dried at 60 °C in a vacuum oven for 24 h. To prepare CH3NH3PbI3  xClx, the synthesized CH3NH3I (0.4198 g) and PbCl2 (0.2447 g, Aldrich) were mixed in 1 mL DMF at 60 °C for 12 h with a concentration of 40 wt% under stirring. 2.2. Device fabrication Patterned ITO glass with a sheet resistance of 15 Ω sq  1 was cleaned by ultrasonic in detergent, deionized water, acetone, isopropanol sequentially and then treated with UV-ozone for 20 min. A 30 nm thickness of poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS, Clevios PVP AI 4083) layer was formed on ITO substrates by spin coating an aqueous dispersion at 4000 rpm for 60 s followed by annealing at 140 °C for 20 min. The PEDOT:PSS-coated substrates were transferred into a N2 glovebox.

The CH3NH3PbI3 xClx precursor solution was spin-cast at 2000 rpm for 45 s and heated at 95 °C for around 2 h. F8BT solution in chlorobenzene was then spin-coated onto CH3NH3PbI3  xClx layer at 2000 rpm for 60 s. Finally, fullerene (C60, 50 nm) and aluminum (Al, 100 nm) were deposited on F8BT layer through a shadow mask under vacuum (ca. 10  6 Pa). A shadow mask was used during thermal evaporation to define the active area of 0.09 cm2. 2.3. Device characterization The thickness of F8BT was determined by Veeco profiler (Dektak 150). Device fabrication was carried out in a nitrogen atmosphere dry glovebox. The phase composition of the asprepared samples was analyzed using a powder X-ray diffractometer (Bruker AXS D8 Advance, Germany) equipped with Cu Kα radiation (λ ¼0.154 nm). The J-V characteristics were recorded with a Keithley 2400 source meter under irradiation of simulated AM 1.5 G solar spectrum. The incident photon to electron conversion efficiency (IPCE) spectra were measured by a Newport quantum efficiency measurement system (ORIEL IQE 200TM) with a xenon lamp and a lock-in amplifier. The light intensity was calibrated with a standard single-crystal Si/Ge photovoltaic cell. Optical absorption spectra were recorded by using a PerkinElmer Lambda950 UV–vis–NIR spectrophotometer. The scanning electron microscopy (SEM) images were performed by an S-4800 scanning electron microscope operated at an acceleration voltage of 4 kV.

3. Results and discussions The perovskite solar cells were fabricated with planar heterojunction configuration of glass/ITO/PEDOT:PSS/CH3NH3PbI3 xClx/ F8BT/C60/Al, as shown in Fig. 1(a). Fig. 1(b) presents the chemical structure of F8BT. The perovskite CH3NH3PbI3 xClx is employed as the light absorber and PEDOT:PSS as hole transporting layer (HTL). F8BT and C60 are incorporated together as ETL to modify the cathode. Similar configuration without F8BT is adopted to fabricate the reference device. In this work, the perovskite crystal film was prepared with one-step solution process. Fig. S1 shows the corresponding X-ray diffraction (XRD) of the as-prepared CH3NH3PbI3 xClx film on ITO/ PEDT:PSS substrate. The diffraction peaks at 14.25°, 28.51°, 32.04° and 43.37° can be assigned to the diffraction plane (110), (220), (310) and (330) of CH3NH3PbI3 xClx crystal respectively, which is consistent with the previously reported perovskite crystal structure and indicates a crystal structure of halide perovskite with high crystallinity [2,38]. Differently from those reported works where the polymer interfacial layers have always been contacting with the electrode, here the F8BT is incorporated directly contacting with the perovskite active layer so as to achieve the better energy level alignment. And between the F8BT and the Al electrode, a fullerene

Fig. 1. (a) Device structure of a F8BT-based perovskite solar cell. (b) The chemical structure of F8BT. (c) Energy level alignment of the materials used in our devices.

L. Zhao et al. / Solar Energy Materials & Solar Cells 157 (2016) 79–84

(C60) layer was introduced since its ability to reduce the current leakage [36]. Fig. 1(c) schematizes the relevant energy level alignment of the materials used in our devices according to the experiment data (Table S1) and references [4,21]. The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of F8BT are calculated according to the measured cyclic voltammetry curve and absorption of F8BT (Fig. S2) and the detailed data are listed in Table S1. The LUMO level of F8BT is determined to be  3.75 eV, which is the same as the conduction band (CB) (  3.75 eV) of CH3NH3PbI3  xClx. This could facilitate the electron transfer from C60 to Al electrode by raising the LUMO level of the C60 via energy level alignment by band bending and interface dipole effect [39,40] and thus benefit the electron extracting and the device performance. Meanwhile, the deep HOMO level (  5.91 eV) of F8BT can effectively block the hole transport. Thus the balanced HOMO/LUMO levels of F8BT would improve the electron extraction, hole blocking and simultaneously maintain a relatively high photovoltage. This is consistent with the obtained device performances. The perovskite solar cells incorporating optimal F8BT deliver remarkably improved PCE of 13.9% with a short circuit current (Jsc) of 22.40 mA cm  2, an open circuit voltage (Voc) of 0.971 V and a fill factor (FF) of 0.638 under irradiation of simulated AM 1.5 G (100 mW cm  2), compared to our reference device without F8BT (PCE 7.9%, Table 1) or the previously reported result (PCE 7.05%) [36]. To systematically investigate how the F8BT film thickness will influence the device performance, F8BT films with different thickness were deposited on perovskite layer to fabricate the devices by controlling the solution concentration. It turned out that the optimal PCE is obtained by a thin F8BT film with a thickness of about 10 nm deposited from a 2.5 mg/mL F8BT solution. Meanwhile, the thinner or thicker F8BT films delivered relatively lower device performance (Fig. 2(a)), probably due to the poor interface contact with insufficient coverage [39] of the thinner films and insufficient conductivity of the thicker films. Especially when the F8BT film is thicker than 40 nm, it can't provide a proper working device anymore (Fig. 2(a)). Detailed device parameters are shown in Table S2.

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Fig. 2(b) represents the external quantum efficiency (EQE) spectra of the obtained optimal device using F8BT. The EQE reaches a peak value ( 480%) at
550 nm, indicating that the device has a good charge collection efficiency and low charge recombination. It should be noted that, the integrated current density value from the EQE curve is slightly lower than the measured Jsc due to our device decay when measured in the atmosphere. In addition, similar series resistances are observed for both F8BT/C60 and reference (-/C60) devices while much increased shunt resistance is obtained for the F8BT/C60 device (Table 1). This improved Rsh suggests the eliminated trap states and depressed charge carrier recombination at the cathode interface due to the improved contact property, which should account for the enhanced performance of the F8BT device as well. The improved contact was also confirmed by the recorded high efficient photoluminescence (PL) quenching between perovskite and F8BT (Fig. S3). To further verify the presumption, morphology analysis was also carried out for the electrode interface with or without F8BT by SEM measurement. With F8BT underneath (Fig. 3, right column), improved interface with higher coverage, larger flat domains and less pin-holes was demonstrated for the C60 layer possibly due to the improved crystallinity [41], compared to the pure C60 interlayer without F8BT (Fig. 3, left column), suggesting less traps on the interface and improved contact to the Al electrode. Furthermore, the surface morphology of the perovskite layer with or without F8BT film on it was presented as well (Fig. S4). Different from the influence on C60 layer, no obvious morphology change on the perovskite layer was observed after the deposition of the thin F8BT film, which is evidenced again by the AFM images (Fig. S5). This excludes the contribution from the active layers for their observed different performance. To exclude the experiment accident errors and analyze the statistical distributions of the parameters for devices with/without F8BT ETL, the statistical data of the parameters are presented in the form of a standard box plot based on eight devices respectively, as shown in Fig. 4. The F8BT-based devices show relatively

Table 1. The parameters of optimal devices based on different ETL systems, with the structure of ITO/PEDOT:PSS/perovskite/ETL/Al. The data in bracket present the corresponding average value, which is obtained based on 8 individual devices. ETLs

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

PCBM/C60 -/C60 F8BT/C60 PCBM:F8BT/C60

0.959(0.961) 0.795(0.787) 0.971(0.967) 0.995(0.986)

22.34(22.09) 19.59(19.49) 22.40(21.71) 24.61(24.11)

66.6(63.1) 50.7(49.0) 63.8(63.3) 65.1(65.3)

14.3(13.6) 7.9(7.4) 13.9(13.1) 15.9(14.8)

5.4(7.2) 10.3(12.7) 10.0(9.6) 5.3(5.6)

356(328) 479(232) 566(573) 256(262)

Fig. 2. (a) Optimal device performance with different thickness of the electron transport material. (b) External quantum efficiency of the optimal device.

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Fig. 3. SEM images of perovskite/C60 film (a, c) and perovskite/F8BT/C60 film (b, d) on PEDOT:PSS substrate. Scale bars: top 10 mm, bottom 5 mm.

Fig. 4. Statistical data of key parameters of the devices with or without F8BT as a standard box plot shown in (a) Voc, (b) Jsc, (c) FF, and (d) PCE.

higher and overall smaller deviation of parameter values than the control devices, indicating good reproducibility of the well performed F8BT-based devices.

Finally, the J-V hysteresis of the devices with and without F8BT was tested as well, with different scanning directions and scan rates. With a scan rate of 0.21 V/s in different scan directions, very

L. Zhao et al. / Solar Energy Materials & Solar Cells 157 (2016) 79–84

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Fig. 5. J  V measurements of control and F8BT-based devices with different scan directions (a) and different scan rates of 0.21, 0.15 and 0.1 V/s (b).

F8BT (nm)

Scan direction

Voc (V) Jsc (mA/ cm2)

FF (%) PCE (%) PCE deviation (%)

0

SC-FB FB-SC

0.826 0.857

18.39 10.07

46.8 43.2

7.10 3.73

47.9


3

SC-FB FB-SC

0.810 0.769

20.79 20.60

59.7 61.2

10.05 9.57

4.8

control device or the previously reported result, mainly due to the balanced LUMO/HOMO levels, improved interface contact and charge transport at the electrode interface. And even higher PCE of 15.9% was achieved when mixture blend of PCBM:F8BT was used as substitution of the F8BT layer. Moreover, greatly elimination of hysteresis was confirmed for the thin F8BT film devices. This work demonstrates the promising potential of F8BT as interfacial modification layer in perovskite solar cells to improve the device performance and provide possible approach for reducing the hysteresis of this technique.


6

SC-FB FB-SC

0.920 0.898

21.18 20.89

61.1 59.5

11.91 11.14

6.5

Notes

10

SC-FB FB-SC

0.968 0.969

21.35 21.45

62.4 64.0

12.90 13.31

3.2

25

SC-FB FB-SC

0.898 0.892

14.43 12.66

55.0 49.3

7.14 5.58

21.9

SC-FB FB-SC

0.848 0.828

0.34 0.17

36.7 34.8

0.11 0.05

54.6

Table 2. The parameters and PCE deviation of the devices with different thickness of F8BT and constant thickness of F8BT-C60 (60 nm) when measured in different directions.

The authors declare no competing financial interest.

Acknowledgments

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slight hysteresis was recorded for the F8BT device in forward/ reverse scanning, while larger hysteresis was observed for the control device (Fig. 5(a)). And less hysteresis was demonstrated for the F8BT-based device as well in different scanning rates (0.21, 0.15 and 0.1 V/s) measurement, compared to the control devices. These results indicate efficient modification of F8BT on the interface of the device. In addition, the recorded hysteresis of the devices with different thicknesses of F8BT but constant thickness of F8BT-C60 (60 nm) is presented in Table 2. The devices incorporating a thin F8BT film show a relatively low hysteresis in the J-V curves scanned in different directions. While larger hysteresis and meanwhile largely decreased device performance was recorded for the thicker F8BT film based devices.

4. Conclusions In conclusion, we have demonstrated high efficient planar heterojunction perovskite solar cells by modifying the perovskite film with a polyfluorene derivative F8BT. Considerably higher power conversion efficiency of 13.9% with good reproducibility was demonstrated for the device with configuration of ITO/PEDOT: PSS/perovskite/F8BT/C60/Al, compared to the PCE of 7.9% of the

This research was supported by Natural Science Foundation of China (No. 61474125, 51502313), Zhejiang Provincial Natural Science Foundation of China (LR14E030002, LY16E02006), and Ningbo Science and Technology Bureau (2014B82010). The work was also supported by National Young Top-Notch Talent Program of China and Hundred Talent Program of Chinese Academy of Sciences.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2016.05.026.

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