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Enhancing the open circuit voltage of PEDOT:PSS-PC61BM based inverted planar mixed halide perovskite solar cells from 0.93 to 1.05 V by simply oxidizing PC61BM
Organic Electronics 59 (2018) 260–265
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Enhancing the open circuit voltage of PEDOT:PSS-PC61BM based inverted planar mixed halide perovskite solar cells from 0.93 to 1.05 V by simply oxidizing PC61BM
T
Yanqing Yaoa,b, Gang Wanga,b, Liping Liaoa,b, Debei Liua,b, Guangdong Zhoua,b, Cunyun Xua,b, Xiude Yanga,b, Rong Wuc, Qunliang Songa,b,∗ a b c
Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing, 400715, PR China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, Chongqing, 400715, PR China Key Laboratory of Solid-state Physics and Devices, School of Physical Science and Technology, Xinjiang University, Urumqi, 830046, China
A B S T R A C T
In this paper we propose a simple method of oxidization of PC61BM, which is commonly used in perovskite solar cells as the electron transport layer, to enhance the open circuit voltage (Voc) of PEDOT:PSS-PC61BM based inverted planar mixed halide perovskite solar cells. By simply oxidizing PC61BM, the Voc was enhanced from 0.93 to 1.05 V. The power conversion efficiency thus was increased from 14.22% to 16.45%. This improvement is attributed to the lower charge carrier recombination rate and faster charge transfer at the interface after oxidation PC61BM layer, as revealed by time resolved photoluminescence, transient photovoltage and transient photocurrent measurements. Ultra-violet photoelectron spectroscopy measurements indicate about 0.12 V shift of the work function of PC61BM after oxidation, supporting the above conclusion.
1. Introduction In a few years, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has risen from 3.8% to 22.7% [1,2]. Due to their high performance and low cost of solution process, PSCs have been considered to be great potential for commercialization [3–5]. The structure of PSCs can be mesoporous or planar with p–i–n or n–i–p layouts [6,7]. Among these structures, conventional mesoporous TiO2-based PSCs is one of the most widely used. This structure keeps the highest record PCE for PSCs so far. However, the fabrication of mesoporous TiO2 layer requires high sintering temperature (about 500 °C). Huge energy is consumed and in this process and it is incompatible for wearable electronic devices on flexible substrates [8]. Thus, in order to avoid this drawback, the latter one, another type of attractive structure of inverted planar PSCs has been introduced and exhibits comparable device performance with the mesoporous counterparts [9,10]. Due to its lowtemperature process, the potential of large-scale manufacturing with high speed as well as the simplified device architecture and compatibility with flexible devices, it is particularly attractive for potential commercialization [11–15]. But, from the related literatures [14,16–25], it is found that the typical inverted mixed halide PSCs (CH3NH3PbI3-xClx) with poly(3,4-
∗
ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the hole transport layer (HTL) and phenyl-C61-butyric acid methyl ester (PC61BM) as electron transport layer (ETL) often exhibit relatively low open circuit voltages (Voc), which is crucial to the PCE of PSCs. Xue and Liu pointed out in their articles [7,8] that Voc of typical PSCs with PEDOT:PSS as HTL and PC61BM as ETL is less than 1 V. For the PEDOT:PSS HTL side, the work function of it is about 5.0–5.2 eV [14,17,26], which is poorly matched with the valence band (VB) of CH3NH3PbI3-xClx (5.4 eV). In addition, PEDOT:PSS shows poor electron-blocking capability and relative low electrical conductivity [8,27–29]. These properties result in small Voc in inverted planar PSCs [30,31]. So there were many works [8,18,23,26,27,32–39] on modifying the PEDOT:PSS or synthesizing new HTLs to replace it. Liu et al. [8] enhanced electrical conductivity of PEDOT:PSS by F4-TCNQ doping, leading to an increase of Voc from 0.94 to 1.02 V. Non-wetting HTLs were employed by Cheng Bi et al. to replace PEDOT:PSS, showing an increased Voc [27]. As for PC61BM ETL, despite its high electron mobility and effective trap passivation property for inverted planar PSCs [40–42], there is still much potential to further enhance Voc by modifying PC61BM or using alternative new ETLs [17,24]. For instance, C60 [43], ICBA [44,45], and other n-type polymers [32] were applied in inverted planar PSCs as ETL. For example, Chien-Hung Chiang et al.
Corresponding author. Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing, 400715, PR China. E-mail addresses: [email protected], [email protected] (Q. Song).
https://doi.org/10.1016/j.orgel.2018.05.017 Received 2 April 2018; Received in revised form 4 May 2018; Accepted 14 May 2018 1566-1199/ © 2018 Elsevier B.V. All rights reserved.
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afterward in reverse from −1.2–1.2 V. The scan rate was 6.7 V/s. External quantum efficiency (EQE) was measured in the glove box with a Xenon arc lamp (Newport, 350 W, 66902), a monochromatic instrument (Newport, 74125) and a lock-in amplifier (Newport, 70104). The illumination beam was converged a small spot using a preconvex lens, the size on the sample about 2.0 × 2.0 mm2 and measurements were performed in a wavelength range from 300 to 800 nm with 5 nm step. The EQE was measured without background illumination or bias voltage. For the steady state photoluminescence (PL) and time-resolved photo-luminescence (TRPL) measurements used fluorescence spectroscopy (EDINBURGH INSTRUMENTS, FS 5). The sample was packaged in the glove box after fabrication. For TRPL, the 485 ± 10 nm laser was used as excitation light source. The laser hit the sample through the quartz side. For PL, we used a Xenon arc lamp as excitation light source, excited at 660 nm. After debugging, any parameter was not changed during the testing process. The light incident direction is from the quartz side. In transient photovoltage (TPV) and transient photocurrent (TPC) test, the cells were connected directly in series with a DSO (Agilent DSO-X02A) and the input impedance of the DSO is switched to 50 Ω (TPC test) or 106 Ω (TPV test). Then we used the 532 nm pulsed laser as the excitation light source. During the testing, the cell was illuminated under a white light bias. All tests were performed in the glove box. UPS (ESCALAB 250Xi) measurements were performed in an ultrahigh vacuum system at base pressures 10−7 Pa. He-UPS measurements with an excitation energy of 21.22 eV were conducted to determine the ionization potentials, and the energy resolution is ∼50 meV. The work function was determined by fitting a Boltzmann sigmoid function to the secondary electron cutoff.
[46] used PC71BM to replace PC61BM, then solvent annealed the PC71BM layer for 24 h. The Voc was enhanced from 0.93 to 1.05 V. Xue et al. [7] used dual interfacial modification to replace both PEDOT:PSS and PC61BM, then the high Voc of 1.13 V was achieved. All these efforts, no matter modifying or using new HTL/ETL, enhanced Voc but increased the cost and complexity of device fabrication. Thus, these efforts are only beneficial to fundamental understanding but against commercial requirements. In this study, we only exposed PC61BM ETL to ambient air (temperature ≈ 25 °C, humidity ≈ 25%) for 30 min without any other extra treatments. The Voc was improved significantly from 0.93 to 1.05 V, increasing the PCE from 14.22% to 16.45%. The mechanism for this Voc enhancement is also discussed. 2. Experimental section 2.1. Materials Lead iodide (PbI2), lead chloride (PbCl2), methyl ammonium iodide (MAI), PEDOT:PSS, PC61BM, 2,9-dimethyl-4,7-diphenyl-1,10Phenanthroline (BCP) were all purchased from Xi'an Polymer Light Technology Corp (China), while N,N-dimethylformide (DMF), chlorobenzene (CB), dimethyl sulfoxide (DMSO) and others were from Sigma–Aldrich. The mixed-halide perovskite precursor (CH3NH3PbI3xClx) was prepared according to the literature [47]. In brief, PbI2 (580.9 mg), PbCl2 (38.9 mg) and MAI (222.6 mg) were dissolved in DMF (900 μL) and DMSO (100 μL) at 1.26: 0.14: 1.4 M ratio and stirred overnight at room temperature. 20 mg PC61BM was dissolved in 1 mL chlorobenzene and stirred overnight at room temperature. 2.2. Perovskite solar cell preparation
3. Results and discussion
The structure of the PSCs is indium-tin oxide (ITO)/PEDOT:PSS/ CH3NH3PbI3-xClx/PC61BM/BCP/silver (Ag). The fabrication method was reported elsewhere [48,49]. First, the ITO substrate with a sheet resistance of ∼15 Ω·sq−1 was sequentially washed and sonicated in deionized water, ethanol, and acetone, then treated with oxygen plasma for 5 min. Then a single layer of PEDOT:PSS was spin-coating on ITO at 6000 rpm for 60 s and subsequently annealed at 120 °C for 20 min. The perovskite precursor solution was filtered with a PTFE syringe filter (Whatman, 0.45 mm) and spin-coated on PEDOT:PSS at 5000 rpm for 30 s. Then the perovskite coated film was heated at 85 °C for 25 min. The PC61BM layer was then deposited onto the perovskite film using spin-coating method with a speed of 5000 rpm for 30 s in the nitrogenfilled glove box. Most importantly, the PC61BM coated devices were transferred out of the nitrogen-filled glove box, exposed to ambient air (temperature ≈ 25 °C, humidity ≈ 25%) for 30 min. The reference devices were stored in the nitrogen-filled glove box for the same duration. Thus, the only difference is that the reference devices were not exposed to moisture and oxygen. Finally, 2 nm BCP and 120 nm Ag electrodes were sequentially deposited in high vacuum using a shadow mask. The cell area is 0.09 cm2, defined as the crossing area between the ITO and Ag electrode.
The J-V characteristic were measured under 1000 W m−2 air mass 1.5 global (AM 1.5G) illumination. The optimized performances of the devices are listed in Table 1. The reference device which was not exposed to ambient air (marked as PC61BM) exhibits a PCE of 14.22% with a Voc of 0.93 V, a Jsc of 20.66 mA cm−2, and an FF of 74%. In contrast, the device which was exposed to ambient air (marked as PC61BM-O) has a PCE of 16.45%, and the Voc, Jsc, FF are 1.05 V, 20.89 mA cm−2, 75% respectively. As shown in Fig. 1, the Voc increases significantly after exposing PC61BM-coated layer to ambient air for 30 min, though both Jsc and FF have slight improvements. The almost the same EQE results in Fig. 1b confirms the similar Jsc obtained from J-V measurements. Both PC61BM and PC61BM-O devices show very stable power output, as demonstrated in Fig. 1c of time-dependent power output of the two PSCs. The Voc of PC61BM-O device does increase a lot after exposure PC61BM to ambient air, as confirmed by the statistical data of the 220 devices shown in Fig. 1d. During the device fabrication, all other conditions are exactly the same except the exposure to ambient air after deposition of PC61BM layer for PC61BM-O device. Therefore, we believe that the increase of Voc is due to the effect of water or oxygen in the air. In order to verify which layer (PC61BM or perovskite layer) is affected by the water or oxygen and then contributes to the Voc increase, a set of comparative experiments have been conducted. Different from the PC61BM-O device,
2.3. Characterization Current density−voltage (J−V) measurements were conducted in the nitrogen-glove box using Newport 94043 A solar simulator with 100 mWcm−2 AM 1.5G simulated sunlight, verified by measuring the short-circuit current of a calibrated silicon solar cell. The silicon reference and the PSCs were temperature controlled to 25 °C during measurement. The active area of 0.09 cm2 was defined by the crossing area between the ITO and Al electrode, both 3 mm in width. J−V curves of the perovskite solar cells were scanned with a digital source meter (Keithley model 2400) in two consecutive sweeps without prebiasing of the device, first from 1.2 to −1.2 V, and immediately
Table 1 The device photovoltaic performance under 1000 W m−2 AM 1.5 G illumination.
261
ETL
Voc[V]
Jsc [mA·cm−2]
FF[%]
PCE[%]
PC61BM PC61BM-O
0.93 1.05
20.66 20.89
74 75
14.22 16.45
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Fig. 1. (a) J–V characteristics, inset: the structure of the inverted planar PSCs. (b) EQE spectrum, and (c) Stability of PCE of the optimized PSCs measured at the maximum power point for devices with PC61BM ETL layer without (PC61BM) and with (PC61BM-O) oxidation. (d) Statistical Voc of the PC61BM and PC61BM-O PSCs. Table 2 Photovoltaic performances for PSCs based on PC61BM exposed to in different experimental atmosphere. atmosphere glove box humid nitrogen dry oxygen
average best average best average best
Voc [V]
Jsc [mA·cm−2]
FF [%]
PCE [%]
0.90 ± 0.04 0.93 0.89 ± 0.04 0.92 1.02 ± 0.03 1.03
19.35 ± 1.23 20.66 19.21 ± 1.07 20.36 19.58 ± 1.23 20.79
73 ± 2 74 73 ± 3 75 74 ± 3 76
13.61 ± 0.65 14.22 13.23 ± 0.89 14.05 15.73 ± 0.72 16.27
Fig. 2. (a) Steady state PL and (b) TRPL spectra of samples without PC61BM and with PC61BM (PC61BM for sample before ambient exposure and PC61BM-O for sample after ambient air exposure). The solid lines in (b) are bi-exponential fittings of the TRPL. 262
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Fig. 3. (a) TPV and (b) TPC measurements of PSCs with PC61BM ETL after 30 min oxidation (PC61BM-O) or without oxidation (PC61BM).
the literature report [26,34]. Compared with quartz/perovskite, the added PC61BM on perovskite film quenches more PL, and the most quenched PL is obtained for the ambient air exposed PC61BM (quartz/ perovskite/PC61BM-O). The PL results mean that PC61BM is a good electron transport material for electron extraction and the oxygen treated PC61BM is even better. The TRPL experiments were utilized to examine the charge dissociation and recombination process. The biexponential fitting I(t) = A1exp(−t/τ1)+A2exp(−t/τ2) of TRPL spectra are plotted in solid lines in Fig. 2b. The good fitting results mean that there are a fast (τ1) and a slow (τ2) decay processes in all the three samples. The quenching of the PL, transporting the photogenerated free carriers from the perovskite layer to the HTL or ETL, is believed to be the fast decay, and the slow decay process is the result of radiative recombination [24,51]. The fitted TRPL lifetimes of CH3NH3PbI3, CH3NH3PbI3/PC61BM, and CH3NH3PbI3/PC61BM-O layers are shown in the inset of Fig. 2b. For the pure perovskite film (quartz/perovskite), the fast decay lifetime (τ1) is 16.62 ns and the slow decay lifetime (τ2) is 60.54 ns while their weight fractions are 44% (A1) and 56% (A2), respectively, indicating that carrier recombination in perovskite bulk is the dominate process. The fitted parameters for the other two sample of quartz/perovskite/PC61BM, and quartz/perovskite/PC61BM-O are τ1 ∼ 4.66 ns (A1 ∼ 80%), τ2 ∼ 22.68 ns (A2 ∼ 20%) and τ1 ∼ 3.54 ns (A1 ∼ 87%), τ2 ∼ 19.22 ns (A2 ∼ 13%), respectively. All these data indicate that the photogenerated electrons are extracted and transferred from the perovskite film to ETL more effectively by adding PC61BM onto perovskite. The fastest decay of TRPL for the quartz/perovskite/PC61BM-O sample means the electron extraction and transfer ability can be further improved after ambient exposure. Another possible benefit for the oxidized PC61BM is the restrain of interface recombination by passivating the surface trap states. TPV [52] and TPC [40] are used to study the recombination process of solar cells. For TPV, the photovoltaic device is illuminated under a white light bias to achieve a steady-state equilibrium, then another weak pulsed laser is applied. The TPV is monitored to reflect the dynamic change of excess carriers generated by the pulsed laser. Due to the recombination of the excess carriers, the TPV signal decay to the steady-state equilibrium produced by the white light bias. As shown in Fig. 3a, the device with an oxidized PC61BM shows a slower photovoltage decay (2.98 ms) than that of the device with untreated PC61BM (0.97 ms). The longer lifetime of TPV signal indicates that the charge recombination rate in PC61BM-O device is lower. TPC measurement can be used to evaluate the charge transit time across the film after charge generation. As shown in Fig. 3b, PC61BM-O device has a much shorter charge transient time (0.22 μs) than the reference cell (0.51 μs). Thus by oxidizing PC61BM layer, the PC61BM oxidation increases lifetime as well as facilitates the charge transfer from the perovskite film to the
Fig. 4. Dark J–V curves of PSCs with PC61BM ETL after 30 min oxidation (PC61BM-O) or without oxidation (PC61BM). The inset is the logarithmic plot.
the perovskite film of the comparative device was exposed to ambient air for 30 min and then transferred back into nitrogen-filled glove box to spin coat PC61BM layer. The Voc of this comparative device is 0.92 V. The unchanged Voc compared with the reference device (0.93 V) indicates it is the PC61BM layer which is affected by the ambient air that contributes to the increased Voc. In order to further figure out the source (water, oxygen or water combined with oxygen) which affects the PC61BM layer, the process of atmospheric exposure was conducted under humid nitrogen (with a humidity of about 25%) and dry oxygen environment, respectively. The Voc of the device exposed in dry oxygen increases to about 1.02 V, similar to the device exposed in ambient air. However, the Voc of device exposed in humid nitrogen remains unchanged compared to the device fabricated in nitrogen-filled glove box. These results compared in Table 2 indicate that it is the oxygen not the moisture that benefits to the increased Voc. This conclusion is also supported by the work done by You-Jingbi et al. Almost the same Voc (0.86 and 0.85 V) they have obtained when annealed perovskite film in nitrogen-filled glove box and in a dry oxygen environment, respectively [50]. In order to understand the changes of PC61BM treated by O2, PL spectra and TRPL spectra measurements were employed to study the difference of interface between the perovskite and PC61BM layers. Fig. 2 show steady state PLs and TRPLs of quartz/perovskite, quartz/perovskite/PC61BM, quartz/perovskite/PC61BM-O, respectively. From Fig. 2a, PL peaks at about 776 ± 5 nm can be observed, consisting with 263
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Fig. 5. (a) Secondary cutoff region of the first derivative of UPS of PC61BM films on ITO. (b) Energy level diagrams of PC61BM before and after oxidation in air.
less dark current and then higher Voc. This study provides a simple and effective approach to enhance the Voc and then the performance of inverted planar mixed halide perovskite solar cells using PEDOT:PSS as HTL and PC61BM as ETL. Our study also gives a clue that previous modified PC61BM or using PC71BM to replace PC61BM might unintentionally introduce oxygen into the ETL during the annealing process, which can contribute to the increase of open circuit voltage.
PC61BM layer. To further explain the improved Voc, the J–V curves of the devices for PC61BM and PC61BM-O devices were measured in the dark. The smaller dark current density of PC61BM-O device, as shown in Fig. 4, indicates that the leakage current is suppressed after oxidizing PC61BM layer. According to equation (1) for Voc [53], the lower leakage current density the higher Voc would be. Therefore, PC61BM-O device with decreased leakage current has a higher Voc than that of reference cell.
Notes
nRT J nRT J ⎞ ln ⎛ sc ⎞ when (Jsc > > Jo) ⎞ ln ⎡ sc − 1⎤ ≈ ⎛ Voc = ⎛ ⎥ J F J 0⎠ ⎝ ⎠ ⎝ F ⎠ ⎢ ⎝ o ⎦ ⎣
The authors declare no competing financial interest.
(1)
where n is the ideal factor, R, T and F are the ideal gas constant, temperature and Faraday's constant, respectively. Jsc is the short circuit current and Jo is the dark current. Ultra-violet photoelectron spectroscopy (UPS) data were collected with a He lamp source which produces a resonance line He I (21.22 eV) by a cold cathode capillary discharge. UPS spectra of the secondary electron cutoff energy onset of PC61BM are shown in Fig. 5. The electron secondary cutoff energy increased from 16.17 eV with PC61BM to 16.29 eV with PC61BM-O for a total energy shift of about 0.12 eV. The corresponding work function value for PC61BM decreased from 5.05 eV to 4.93 eV [54,55]. Thus the effect of ambient exposure shifts the work function of ETL, consistent with the less dark current shown in Fig. 4 for the PC61BM-O device.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant 11274256 and 11774293), Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011), Wuhan University of Technology (2018-KF-14), Fundamental Research Funds for the Central Universities (XDJK2017A002). References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] https://www.nrel.gov/pv/assets/images/efficiency-chart.png. [3] X. Li, D. Bi, C. Yi, J.D. Decoppet, J. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Grätzel, A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells, Science 353 (2016) 58–62. [4] J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T. Jesper Jacobsson, M. Grätzel, A. Hagfeldt, The rapid evolution of highly efficient perovskite solar cells, Energy Environ. Sci. 10 (2017) 710–727. [5] X. Wang, M. Li, B. Zhang, H. Wang, Y. Zhao, B. Wang, Recent progress in organometal halide perovskite photodetectors, Org. Electron. 52 (2018) 172–183. [6] F. Azhar, S.M. Lukas, G.B. Germà, Rajan Jose, M.S. Ivan, Interfaces in perovskite solar cells, Adv. Energy Mater. 7 (2017) 1700623. [7] Q. Xue, Y. Bai, M. Liu, R. Xia, Z. Hu, Z. Chen, X.-F. Jiang, F. Huang, S. Yang, Y. Matsuo, H.-L. Yip, Y. Cao, Dual interfacial modifications enable high performance semitransparent perovskite solar cells with large open circuit voltage and fill factor, Adv. Energy Mater. 7 (2017) 1602333.
4. Conclusion In summary, by simply exposing in ambient air for 30 min to oxidize the PC61BM layer, the Voc of inverted planar mixed halide perovskite solar cell based on using PEDOT:PSS as HTL and PC61BM as ETL can be enhanced significantly, from 0.93 to 1.05 V. The PCE thus is increased from 14.22% to 16.45%. This improvement is ascribed to the lower charge carrier recombination rate and faster charge transfer at the interface after oxidation PC61BM layer, as revealed by our TRPL, TPV and TPC measurements. Our UPS measurements indicate that the oxidation process can shift the work function of PC61BM about 0.12 V, leading to 264
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Enhancing the open circuit voltage of PEDOT:PSS-PC61BM based inverted planar mixed halide perovskite solar cells from 0.93 to 1.05 V by simply oxidizing PC61BM
T
Yanqing Yaoa,b, Gang Wanga,b, Liping Liaoa,b, Debei Liua,b, Guangdong Zhoua,b, Cunyun Xua,b, Xiude Yanga,b, Rong Wuc, Qunliang Songa,b,∗ a b c
Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing, 400715, PR China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, Chongqing, 400715, PR China Key Laboratory of Solid-state Physics and Devices, School of Physical Science and Technology, Xinjiang University, Urumqi, 830046, China
A B S T R A C T
In this paper we propose a simple method of oxidization of PC61BM, which is commonly used in perovskite solar cells as the electron transport layer, to enhance the open circuit voltage (Voc) of PEDOT:PSS-PC61BM based inverted planar mixed halide perovskite solar cells. By simply oxidizing PC61BM, the Voc was enhanced from 0.93 to 1.05 V. The power conversion efficiency thus was increased from 14.22% to 16.45%. This improvement is attributed to the lower charge carrier recombination rate and faster charge transfer at the interface after oxidation PC61BM layer, as revealed by time resolved photoluminescence, transient photovoltage and transient photocurrent measurements. Ultra-violet photoelectron spectroscopy measurements indicate about 0.12 V shift of the work function of PC61BM after oxidation, supporting the above conclusion.
1. Introduction In a few years, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has risen from 3.8% to 22.7% [1,2]. Due to their high performance and low cost of solution process, PSCs have been considered to be great potential for commercialization [3–5]. The structure of PSCs can be mesoporous or planar with p–i–n or n–i–p layouts [6,7]. Among these structures, conventional mesoporous TiO2-based PSCs is one of the most widely used. This structure keeps the highest record PCE for PSCs so far. However, the fabrication of mesoporous TiO2 layer requires high sintering temperature (about 500 °C). Huge energy is consumed and in this process and it is incompatible for wearable electronic devices on flexible substrates [8]. Thus, in order to avoid this drawback, the latter one, another type of attractive structure of inverted planar PSCs has been introduced and exhibits comparable device performance with the mesoporous counterparts [9,10]. Due to its lowtemperature process, the potential of large-scale manufacturing with high speed as well as the simplified device architecture and compatibility with flexible devices, it is particularly attractive for potential commercialization [11–15]. But, from the related literatures [14,16–25], it is found that the typical inverted mixed halide PSCs (CH3NH3PbI3-xClx) with poly(3,4-
∗
ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the hole transport layer (HTL) and phenyl-C61-butyric acid methyl ester (PC61BM) as electron transport layer (ETL) often exhibit relatively low open circuit voltages (Voc), which is crucial to the PCE of PSCs. Xue and Liu pointed out in their articles [7,8] that Voc of typical PSCs with PEDOT:PSS as HTL and PC61BM as ETL is less than 1 V. For the PEDOT:PSS HTL side, the work function of it is about 5.0–5.2 eV [14,17,26], which is poorly matched with the valence band (VB) of CH3NH3PbI3-xClx (5.4 eV). In addition, PEDOT:PSS shows poor electron-blocking capability and relative low electrical conductivity [8,27–29]. These properties result in small Voc in inverted planar PSCs [30,31]. So there were many works [8,18,23,26,27,32–39] on modifying the PEDOT:PSS or synthesizing new HTLs to replace it. Liu et al. [8] enhanced electrical conductivity of PEDOT:PSS by F4-TCNQ doping, leading to an increase of Voc from 0.94 to 1.02 V. Non-wetting HTLs were employed by Cheng Bi et al. to replace PEDOT:PSS, showing an increased Voc [27]. As for PC61BM ETL, despite its high electron mobility and effective trap passivation property for inverted planar PSCs [40–42], there is still much potential to further enhance Voc by modifying PC61BM or using alternative new ETLs [17,24]. For instance, C60 [43], ICBA [44,45], and other n-type polymers [32] were applied in inverted planar PSCs as ETL. For example, Chien-Hung Chiang et al.
Corresponding author. Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing, 400715, PR China. E-mail addresses: [email protected], [email protected] (Q. Song).
https://doi.org/10.1016/j.orgel.2018.05.017 Received 2 April 2018; Received in revised form 4 May 2018; Accepted 14 May 2018 1566-1199/ © 2018 Elsevier B.V. All rights reserved.
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afterward in reverse from −1.2–1.2 V. The scan rate was 6.7 V/s. External quantum efficiency (EQE) was measured in the glove box with a Xenon arc lamp (Newport, 350 W, 66902), a monochromatic instrument (Newport, 74125) and a lock-in amplifier (Newport, 70104). The illumination beam was converged a small spot using a preconvex lens, the size on the sample about 2.0 × 2.0 mm2 and measurements were performed in a wavelength range from 300 to 800 nm with 5 nm step. The EQE was measured without background illumination or bias voltage. For the steady state photoluminescence (PL) and time-resolved photo-luminescence (TRPL) measurements used fluorescence spectroscopy (EDINBURGH INSTRUMENTS, FS 5). The sample was packaged in the glove box after fabrication. For TRPL, the 485 ± 10 nm laser was used as excitation light source. The laser hit the sample through the quartz side. For PL, we used a Xenon arc lamp as excitation light source, excited at 660 nm. After debugging, any parameter was not changed during the testing process. The light incident direction is from the quartz side. In transient photovoltage (TPV) and transient photocurrent (TPC) test, the cells were connected directly in series with a DSO (Agilent DSO-X02A) and the input impedance of the DSO is switched to 50 Ω (TPC test) or 106 Ω (TPV test). Then we used the 532 nm pulsed laser as the excitation light source. During the testing, the cell was illuminated under a white light bias. All tests were performed in the glove box. UPS (ESCALAB 250Xi) measurements were performed in an ultrahigh vacuum system at base pressures 10−7 Pa. He-UPS measurements with an excitation energy of 21.22 eV were conducted to determine the ionization potentials, and the energy resolution is ∼50 meV. The work function was determined by fitting a Boltzmann sigmoid function to the secondary electron cutoff.
[46] used PC71BM to replace PC61BM, then solvent annealed the PC71BM layer for 24 h. The Voc was enhanced from 0.93 to 1.05 V. Xue et al. [7] used dual interfacial modification to replace both PEDOT:PSS and PC61BM, then the high Voc of 1.13 V was achieved. All these efforts, no matter modifying or using new HTL/ETL, enhanced Voc but increased the cost and complexity of device fabrication. Thus, these efforts are only beneficial to fundamental understanding but against commercial requirements. In this study, we only exposed PC61BM ETL to ambient air (temperature ≈ 25 °C, humidity ≈ 25%) for 30 min without any other extra treatments. The Voc was improved significantly from 0.93 to 1.05 V, increasing the PCE from 14.22% to 16.45%. The mechanism for this Voc enhancement is also discussed. 2. Experimental section 2.1. Materials Lead iodide (PbI2), lead chloride (PbCl2), methyl ammonium iodide (MAI), PEDOT:PSS, PC61BM, 2,9-dimethyl-4,7-diphenyl-1,10Phenanthroline (BCP) were all purchased from Xi'an Polymer Light Technology Corp (China), while N,N-dimethylformide (DMF), chlorobenzene (CB), dimethyl sulfoxide (DMSO) and others were from Sigma–Aldrich. The mixed-halide perovskite precursor (CH3NH3PbI3xClx) was prepared according to the literature [47]. In brief, PbI2 (580.9 mg), PbCl2 (38.9 mg) and MAI (222.6 mg) were dissolved in DMF (900 μL) and DMSO (100 μL) at 1.26: 0.14: 1.4 M ratio and stirred overnight at room temperature. 20 mg PC61BM was dissolved in 1 mL chlorobenzene and stirred overnight at room temperature. 2.2. Perovskite solar cell preparation
3. Results and discussion
The structure of the PSCs is indium-tin oxide (ITO)/PEDOT:PSS/ CH3NH3PbI3-xClx/PC61BM/BCP/silver (Ag). The fabrication method was reported elsewhere [48,49]. First, the ITO substrate with a sheet resistance of ∼15 Ω·sq−1 was sequentially washed and sonicated in deionized water, ethanol, and acetone, then treated with oxygen plasma for 5 min. Then a single layer of PEDOT:PSS was spin-coating on ITO at 6000 rpm for 60 s and subsequently annealed at 120 °C for 20 min. The perovskite precursor solution was filtered with a PTFE syringe filter (Whatman, 0.45 mm) and spin-coated on PEDOT:PSS at 5000 rpm for 30 s. Then the perovskite coated film was heated at 85 °C for 25 min. The PC61BM layer was then deposited onto the perovskite film using spin-coating method with a speed of 5000 rpm for 30 s in the nitrogenfilled glove box. Most importantly, the PC61BM coated devices were transferred out of the nitrogen-filled glove box, exposed to ambient air (temperature ≈ 25 °C, humidity ≈ 25%) for 30 min. The reference devices were stored in the nitrogen-filled glove box for the same duration. Thus, the only difference is that the reference devices were not exposed to moisture and oxygen. Finally, 2 nm BCP and 120 nm Ag electrodes were sequentially deposited in high vacuum using a shadow mask. The cell area is 0.09 cm2, defined as the crossing area between the ITO and Ag electrode.
The J-V characteristic were measured under 1000 W m−2 air mass 1.5 global (AM 1.5G) illumination. The optimized performances of the devices are listed in Table 1. The reference device which was not exposed to ambient air (marked as PC61BM) exhibits a PCE of 14.22% with a Voc of 0.93 V, a Jsc of 20.66 mA cm−2, and an FF of 74%. In contrast, the device which was exposed to ambient air (marked as PC61BM-O) has a PCE of 16.45%, and the Voc, Jsc, FF are 1.05 V, 20.89 mA cm−2, 75% respectively. As shown in Fig. 1, the Voc increases significantly after exposing PC61BM-coated layer to ambient air for 30 min, though both Jsc and FF have slight improvements. The almost the same EQE results in Fig. 1b confirms the similar Jsc obtained from J-V measurements. Both PC61BM and PC61BM-O devices show very stable power output, as demonstrated in Fig. 1c of time-dependent power output of the two PSCs. The Voc of PC61BM-O device does increase a lot after exposure PC61BM to ambient air, as confirmed by the statistical data of the 220 devices shown in Fig. 1d. During the device fabrication, all other conditions are exactly the same except the exposure to ambient air after deposition of PC61BM layer for PC61BM-O device. Therefore, we believe that the increase of Voc is due to the effect of water or oxygen in the air. In order to verify which layer (PC61BM or perovskite layer) is affected by the water or oxygen and then contributes to the Voc increase, a set of comparative experiments have been conducted. Different from the PC61BM-O device,
2.3. Characterization Current density−voltage (J−V) measurements were conducted in the nitrogen-glove box using Newport 94043 A solar simulator with 100 mWcm−2 AM 1.5G simulated sunlight, verified by measuring the short-circuit current of a calibrated silicon solar cell. The silicon reference and the PSCs were temperature controlled to 25 °C during measurement. The active area of 0.09 cm2 was defined by the crossing area between the ITO and Al electrode, both 3 mm in width. J−V curves of the perovskite solar cells were scanned with a digital source meter (Keithley model 2400) in two consecutive sweeps without prebiasing of the device, first from 1.2 to −1.2 V, and immediately
Table 1 The device photovoltaic performance under 1000 W m−2 AM 1.5 G illumination.
261
ETL
Voc[V]
Jsc [mA·cm−2]
FF[%]
PCE[%]
PC61BM PC61BM-O
0.93 1.05
20.66 20.89
74 75
14.22 16.45
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Fig. 1. (a) J–V characteristics, inset: the structure of the inverted planar PSCs. (b) EQE spectrum, and (c) Stability of PCE of the optimized PSCs measured at the maximum power point for devices with PC61BM ETL layer without (PC61BM) and with (PC61BM-O) oxidation. (d) Statistical Voc of the PC61BM and PC61BM-O PSCs. Table 2 Photovoltaic performances for PSCs based on PC61BM exposed to in different experimental atmosphere. atmosphere glove box humid nitrogen dry oxygen
average best average best average best
Voc [V]
Jsc [mA·cm−2]
FF [%]
PCE [%]
0.90 ± 0.04 0.93 0.89 ± 0.04 0.92 1.02 ± 0.03 1.03
19.35 ± 1.23 20.66 19.21 ± 1.07 20.36 19.58 ± 1.23 20.79
73 ± 2 74 73 ± 3 75 74 ± 3 76
13.61 ± 0.65 14.22 13.23 ± 0.89 14.05 15.73 ± 0.72 16.27
Fig. 2. (a) Steady state PL and (b) TRPL spectra of samples without PC61BM and with PC61BM (PC61BM for sample before ambient exposure and PC61BM-O for sample after ambient air exposure). The solid lines in (b) are bi-exponential fittings of the TRPL. 262
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Fig. 3. (a) TPV and (b) TPC measurements of PSCs with PC61BM ETL after 30 min oxidation (PC61BM-O) or without oxidation (PC61BM).
the literature report [26,34]. Compared with quartz/perovskite, the added PC61BM on perovskite film quenches more PL, and the most quenched PL is obtained for the ambient air exposed PC61BM (quartz/ perovskite/PC61BM-O). The PL results mean that PC61BM is a good electron transport material for electron extraction and the oxygen treated PC61BM is even better. The TRPL experiments were utilized to examine the charge dissociation and recombination process. The biexponential fitting I(t) = A1exp(−t/τ1)+A2exp(−t/τ2) of TRPL spectra are plotted in solid lines in Fig. 2b. The good fitting results mean that there are a fast (τ1) and a slow (τ2) decay processes in all the three samples. The quenching of the PL, transporting the photogenerated free carriers from the perovskite layer to the HTL or ETL, is believed to be the fast decay, and the slow decay process is the result of radiative recombination [24,51]. The fitted TRPL lifetimes of CH3NH3PbI3, CH3NH3PbI3/PC61BM, and CH3NH3PbI3/PC61BM-O layers are shown in the inset of Fig. 2b. For the pure perovskite film (quartz/perovskite), the fast decay lifetime (τ1) is 16.62 ns and the slow decay lifetime (τ2) is 60.54 ns while their weight fractions are 44% (A1) and 56% (A2), respectively, indicating that carrier recombination in perovskite bulk is the dominate process. The fitted parameters for the other two sample of quartz/perovskite/PC61BM, and quartz/perovskite/PC61BM-O are τ1 ∼ 4.66 ns (A1 ∼ 80%), τ2 ∼ 22.68 ns (A2 ∼ 20%) and τ1 ∼ 3.54 ns (A1 ∼ 87%), τ2 ∼ 19.22 ns (A2 ∼ 13%), respectively. All these data indicate that the photogenerated electrons are extracted and transferred from the perovskite film to ETL more effectively by adding PC61BM onto perovskite. The fastest decay of TRPL for the quartz/perovskite/PC61BM-O sample means the electron extraction and transfer ability can be further improved after ambient exposure. Another possible benefit for the oxidized PC61BM is the restrain of interface recombination by passivating the surface trap states. TPV [52] and TPC [40] are used to study the recombination process of solar cells. For TPV, the photovoltaic device is illuminated under a white light bias to achieve a steady-state equilibrium, then another weak pulsed laser is applied. The TPV is monitored to reflect the dynamic change of excess carriers generated by the pulsed laser. Due to the recombination of the excess carriers, the TPV signal decay to the steady-state equilibrium produced by the white light bias. As shown in Fig. 3a, the device with an oxidized PC61BM shows a slower photovoltage decay (2.98 ms) than that of the device with untreated PC61BM (0.97 ms). The longer lifetime of TPV signal indicates that the charge recombination rate in PC61BM-O device is lower. TPC measurement can be used to evaluate the charge transit time across the film after charge generation. As shown in Fig. 3b, PC61BM-O device has a much shorter charge transient time (0.22 μs) than the reference cell (0.51 μs). Thus by oxidizing PC61BM layer, the PC61BM oxidation increases lifetime as well as facilitates the charge transfer from the perovskite film to the
Fig. 4. Dark J–V curves of PSCs with PC61BM ETL after 30 min oxidation (PC61BM-O) or without oxidation (PC61BM). The inset is the logarithmic plot.
the perovskite film of the comparative device was exposed to ambient air for 30 min and then transferred back into nitrogen-filled glove box to spin coat PC61BM layer. The Voc of this comparative device is 0.92 V. The unchanged Voc compared with the reference device (0.93 V) indicates it is the PC61BM layer which is affected by the ambient air that contributes to the increased Voc. In order to further figure out the source (water, oxygen or water combined with oxygen) which affects the PC61BM layer, the process of atmospheric exposure was conducted under humid nitrogen (with a humidity of about 25%) and dry oxygen environment, respectively. The Voc of the device exposed in dry oxygen increases to about 1.02 V, similar to the device exposed in ambient air. However, the Voc of device exposed in humid nitrogen remains unchanged compared to the device fabricated in nitrogen-filled glove box. These results compared in Table 2 indicate that it is the oxygen not the moisture that benefits to the increased Voc. This conclusion is also supported by the work done by You-Jingbi et al. Almost the same Voc (0.86 and 0.85 V) they have obtained when annealed perovskite film in nitrogen-filled glove box and in a dry oxygen environment, respectively [50]. In order to understand the changes of PC61BM treated by O2, PL spectra and TRPL spectra measurements were employed to study the difference of interface between the perovskite and PC61BM layers. Fig. 2 show steady state PLs and TRPLs of quartz/perovskite, quartz/perovskite/PC61BM, quartz/perovskite/PC61BM-O, respectively. From Fig. 2a, PL peaks at about 776 ± 5 nm can be observed, consisting with 263
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Fig. 5. (a) Secondary cutoff region of the first derivative of UPS of PC61BM films on ITO. (b) Energy level diagrams of PC61BM before and after oxidation in air.
less dark current and then higher Voc. This study provides a simple and effective approach to enhance the Voc and then the performance of inverted planar mixed halide perovskite solar cells using PEDOT:PSS as HTL and PC61BM as ETL. Our study also gives a clue that previous modified PC61BM or using PC71BM to replace PC61BM might unintentionally introduce oxygen into the ETL during the annealing process, which can contribute to the increase of open circuit voltage.
PC61BM layer. To further explain the improved Voc, the J–V curves of the devices for PC61BM and PC61BM-O devices were measured in the dark. The smaller dark current density of PC61BM-O device, as shown in Fig. 4, indicates that the leakage current is suppressed after oxidizing PC61BM layer. According to equation (1) for Voc [53], the lower leakage current density the higher Voc would be. Therefore, PC61BM-O device with decreased leakage current has a higher Voc than that of reference cell.
Notes
nRT J nRT J ⎞ ln ⎛ sc ⎞ when (Jsc > > Jo) ⎞ ln ⎡ sc − 1⎤ ≈ ⎛ Voc = ⎛ ⎥ J F J 0⎠ ⎝ ⎠ ⎝ F ⎠ ⎢ ⎝ o ⎦ ⎣
The authors declare no competing financial interest.
(1)
where n is the ideal factor, R, T and F are the ideal gas constant, temperature and Faraday's constant, respectively. Jsc is the short circuit current and Jo is the dark current. Ultra-violet photoelectron spectroscopy (UPS) data were collected with a He lamp source which produces a resonance line He I (21.22 eV) by a cold cathode capillary discharge. UPS spectra of the secondary electron cutoff energy onset of PC61BM are shown in Fig. 5. The electron secondary cutoff energy increased from 16.17 eV with PC61BM to 16.29 eV with PC61BM-O for a total energy shift of about 0.12 eV. The corresponding work function value for PC61BM decreased from 5.05 eV to 4.93 eV [54,55]. Thus the effect of ambient exposure shifts the work function of ETL, consistent with the less dark current shown in Fig. 4 for the PC61BM-O device.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant 11274256 and 11774293), Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011), Wuhan University of Technology (2018-KF-14), Fundamental Research Funds for the Central Universities (XDJK2017A002). References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] https://www.nrel.gov/pv/assets/images/efficiency-chart.png. [3] X. Li, D. Bi, C. Yi, J.D. Decoppet, J. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Grätzel, A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells, Science 353 (2016) 58–62. [4] J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T. Jesper Jacobsson, M. Grätzel, A. Hagfeldt, The rapid evolution of highly efficient perovskite solar cells, Energy Environ. Sci. 10 (2017) 710–727. [5] X. Wang, M. Li, B. Zhang, H. Wang, Y. Zhao, B. Wang, Recent progress in organometal halide perovskite photodetectors, Org. Electron. 52 (2018) 172–183. [6] F. Azhar, S.M. Lukas, G.B. Germà, Rajan Jose, M.S. Ivan, Interfaces in perovskite solar cells, Adv. Energy Mater. 7 (2017) 1700623. [7] Q. Xue, Y. Bai, M. Liu, R. Xia, Z. Hu, Z. Chen, X.-F. Jiang, F. Huang, S. Yang, Y. Matsuo, H.-L. Yip, Y. Cao, Dual interfacial modifications enable high performance semitransparent perovskite solar cells with large open circuit voltage and fill factor, Adv. Energy Mater. 7 (2017) 1602333.
4. Conclusion In summary, by simply exposing in ambient air for 30 min to oxidize the PC61BM layer, the Voc of inverted planar mixed halide perovskite solar cell based on using PEDOT:PSS as HTL and PC61BM as ETL can be enhanced significantly, from 0.93 to 1.05 V. The PCE thus is increased from 14.22% to 16.45%. This improvement is ascribed to the lower charge carrier recombination rate and faster charge transfer at the interface after oxidation PC61BM layer, as revealed by our TRPL, TPV and TPC measurements. Our UPS measurements indicate that the oxidation process can shift the work function of PC61BM about 0.12 V, leading to 264
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