High-performance metal-oxide-free perovskite solar cells based on organic electron transport layer and cathode

Accepted Manuscript High-performance metal-oxide-free perovskite solar cells based on organic electron transport layer and cathode Zhihai Liu, Xiaoyin Xie, Guanchen Liu, Eun-Cheol Lee PII:

S1566-1199(18)30547-0

DOI:

https://doi.org/10.1016/j.orgel.2018.10.032

Reference:

ORGELE 4948

To appear in:

Organic Electronics

Received Date: 26 June 2018 Revised Date:

18 October 2018

Accepted Date: 20 October 2018

Please cite this article as: Z. Liu, X. Xie, G. Liu, E.-C. Lee, High-performance metal-oxide-free perovskite solar cells based on organic electron transport layer and cathode, Organic Electronics (2018), doi: https://doi.org/10.1016/j.orgel.2018.10.032. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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High-performance metal-oxide-free perovskite solar cells based

Zhihai Liua, Xiaoyin Xieb, *, Guanchen Liub, and Eun-Cheol Leec,*

School of Opto-Electronic Information Science and Technology, Yantai University, Shandong 264005,

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a

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on organic electron transport layer and cathode

China b

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Department of Chemical Technology, Jilin Institute of Chemical Technology, Jilin 132022, China

Department of Nano-Physics, Gachon University, Gyeonggi 13120, Republic of Korea

*Corresponding authors at: Jilin Institute of Chemical Technology, Department of Chemical Technology, China. Tel.: +86 432 6218 5183; Fax: +86 432 6218 5153. E-mail address:

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[email protected] (X. Xie); and Gachon University, Department of Nano-Physics, Republic of Korea.

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Tel.: +82 31 750 8752; Fax: +82 31 750 8774. E-mail address: [email protected] (E.-C. Lee)

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ACCEPTED MANUSCRIPT Abstract We introduced phenyl-C61-butyric acid methyl ester (PCBM) as an electron transport layer to improve the performance of metal-oxide-free perovskite solar cells (PSCs) using high-conductivity poly(3,4-ethylenedioxylenethiophene):poly(styrene sulfonate) (PEDOT:PSS) as the cathode. The work

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function of the PEDOT:PSS was tuned from −5.08 to −4.05 eV by using polyethylenimine, improving the electron collection. Using PCBM improved the electron transport and suppressed the charge recombination of the PSCs. The power-conversion efficiency (PCE) of the rigid PSCs (on glass

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substrates) was significantly improved from 12.5% to 13.9%, and the open-circuit voltage, short-circuit current density, and fill factor were improved simultaneously. The long-term stability of the PSCs was

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also improved: the PCE degradation of the PSCs without encapsulation decreased from 18.4% to 13.0% after 114 h. Using a 37-nm PCBM layer, the flexible PSCs on polyethylene naphthalate substrates exhibited a high PCE of 11.4% with good bendability. Our results indicate that using PCBM as an electron transport layer in metal-oxide-free PSCs is a feasible method for the large-scale roll-to-roll

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production of PSCs.

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Keywords: PCBM, Electron transport layer, Metal-oxide-free, Perovskite solar cells

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ACCEPTED MANUSCRIPT 1. Introduction Since they were first introduced by Miyasaka et al. in 2009 [1], organic–inorganic hybrid perovskite materials have become increasingly popular for application in solar-cell fabrication [2–4]. As a new kind of solar absorber, perovskites have many superior photovoltaic properties, such as excellent optical

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absorption coefficients, long carrier diffusion lengths, and controllable direct bandgaps [1–6]. Recently, a high power-conversion efficiency (PCE) of 22.7% was reported for single perovskite solar cells (PSCs), indicating their great potential for the commercialization of PSCs [7].

sandwiched

between

TiO2

(electron

transport

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Standard PSCs usually employ F-doped tin oxide (FTO) as a cathode with perovskite absorbers material)

and

2,2’,7,7’-tetrakis-(N,N-di-p-

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methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD, hole transport material) layers [2, 4, 8]. However, the preparation of TiO2 layers usually requires very high temperatures (~500 °C) for sintering, whereas commercial production typically has low-temperature processing requirements [9–11]. Moreover, the rigid and fragile nature of metal-oxide materials (including FTO and TiO2) further limits

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the viability of PSCs in flexible applications [9–11]. Finding low-temperature-processable and flexible electrodes and electron transport materials to replace FTO and TiO2 has become an important research topic. As a promising approach, phenyl-C61-butyric acid methyl ester (PCBM) is widely used as an

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electron transport material in inverted PSCs, which can be obtained via a solution process at low temperatures [12–14]. Recently, the use of PCBM in standard PSCs for replacing TiO2 has been

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reported, leading to high PCEs of 12.2%–15.3%, which are comparable to those of TiO2-based PSCs [15, 16]. In particular, the processing temperature for PSC fabrication in these studies was dramatically reduced from 500 to 100 °C, which is more suitable for commercialized large-scale production [15, 16]. Moreover, the use of PCBM-modified TiO2 (or ZnO) as the electron transport layer has improved the performance of PSCs, with the PCE greatly enhanced by 24%–42% [11, 17]. These results indicate the superior electron transport property of PCBM for PSCs compared with TiO2 [15–17]. On the other hand, various approaches have been developed to replace rigid and fragile FTO electrodes with flexible ones 3

ACCEPTED MANUSCRIPT to improve the flexibility of PSCs. For example, Ag nanowires (Ag NWs), a Ag NW/ZnO composite, an Al-doped ZnO/Ag NW/Al-doped ZnO composite, carbon nanotubes, and graphene have proven to be successful electrode materials for fabricating FTO-free PSCs with high flexibility [18–24]. As a result, PCEs of 0.8%–14.2% were achieved for metal-oxide-free PSCs. Although the PCEs are not as high as

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those of metal-oxide-based PSCs, the flexibility of the devices was improved, and the processing temperature was reduced [18–24], making the PSCs more suitable for commercialization. In addition, poly(3,4-ethylenedioxylenethiophene):poly(styrene sulfonate) (PEDOT:PSS) is a promising electrode

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material for solar cells because of its high conductivity, transparency, and flexibility [25, 26]. In our previous study, we demonstrated high PCEs of 12.4%–9.7% using rigid and flexible metal-oxide-free

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PSCs, in which polyethylenimine (PEI)-modified PEDOT:PSS layers were used as transparent cathodes [26]. However, in this type of PSC, the perovskite layer was directly deposited on the PEI-modified PEDOT:PSS electrode. There was no electron transport layer at the interface between the perovskite and the PEDOT:PSS cathode, which might be an important reason for the relatively low performance of the

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PSCs. Considering the superior electron transport performance of PCBM, it is very important to investigate PCBM as an electron transport layer for improving the performance of our PEDOT:PSSbased metal-oxide-free PSCs.

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In this study, we improved the performance of metal-oxide-free PSCs by using PCBM as the electron transport layer. The metal-oxide-free PSCs were fabricated using PEI-modified PEDOT:PSS as

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the cathode, which is more mechanically flexible than metal-oxides. We find that the hydrophobic nature of PCBM can improve the quality of the upper perovskite layers, enhancing the crystallinity and increasing the domain size. Moreover, the use of PCBM can result in more effective electron transport and suppress the charge recombination. By using 37-nm PCBM, the PCE of the PSCs (on glass substrates) was significantly improved from 12.5% to 13.9%, and the open-circuit voltage (Voc), shortcircuit current density (Jsc), and fill factor (FF) were enhanced simultaneously. Furthermore, the PCE degradation after long-term (144 h) storage was reduced from 18.4% to 13.0%, indicating enhanced 4

ACCEPTED MANUSCRIPT stability. The best PSC using PCBM as the electron transport layer exhibited a superior PCE of 14.3%, with a stable power output and negligible hysteresis. The flexible PSCs on polyethylene naphthalate (PEN) substrates exhibited a high PCE of 11.4%, with excellent bendability. Our results indicate that inserting PCBM as the electron transport layer in metal-oxide-free PSCs is a feasible method for the

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fabrication of high-performance PSCs and the large-scale roll-to-roll production of PSCs.

2. Material and Methods

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2.1 PEDOT:PSS electrode fabrication

The PEDOT:PSS electrodes were fabricated by spin-coating pre-washed glass and PEN substrates

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(Ying Kou You Xuan Trade Co., Ltd., China) with a PEDOT:PSS (Clevios PH 1000, Germany) aqueous solution containing 5 wt% ethylene glycol (Aladdin, China), followed by annealing at 120 °C for 10 min in air. Subsequently, the samples were dipped into phosphoric acid (85 wt%, Sigma–Aldrich, USA) at 100 °C for 30 min. Then, the samples were washed three times using deionized water and

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baked at 120 °C for 20 min to fully remove the remaining solvent. 2.2 Device fabrication

PCBM and methylammonium iodide (MAI) were purchased from Nano-C (USA) and Xi’an

titanium

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Polymer Light Technology Corp. (China), respectively. Lead iodide (PbI2), spiro-OMeTAD, PEI, diisopropoxide

bis(acetylacetonate),

4-tert-butylpyridine

(tBP),

bis(trifluoromethane)

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sulfonamide lithium salt, chlorobenzene (CB), 2-methoxyethanol, ethanol, 1-butanol, isopropanol, and acetonitrile were all purchased from Sigma–Aldrich (USA). First, the PEI solution (0.4 wt% in 2methoxyethanol) was spin-coated onto the PEDOT:PSS electrodes at 5,000 rpm for 60 s. The films were then annealed at 100 °C for 15 min. Then, PCBM solutions (5, 8, 11, and 14 mg·mL-1 in CB) were spincoated onto PEDOT:PSS/PEI surfaces at 1,500 rpm for 50 s. Because PCBM is soluble in γbutyrolactone, N,N-dimethylformamide, and dimethyl sulfoxide solvents, a conventional one-step spincoating method for perovskite would destroy the underlying PCBM layer [11]. To protect the PCBM 5

ACCEPTED MANUSCRIPT layers, we used a two-step method for perovskite preparation, in accordance with previous studies [11, 16]. PbI2 layers approximately 400 nm thick were evaporated onto the PCBM layers using a thermal evaporator, followed by spin-coating with the MAI solution (10 mg in isopropanol). Then, the samples were annealed at 110 °C for 10 min in a glove box. The hole transport material (75 mg of spiro-

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OMeTAD, 9.4 mg of lithium salt, and 28 µL of TBP dissolved in 1 mL of CB) was spin-coated onto the perovskite films. Finally, a Au anode approximately 100 nm thick was thermally evaporated onto the spiro-OMeTAD layers under a vacuum of 10-4 Pa. The effective working area of the PSCs was 0.1 cm2,

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which was determined by a shadow mask. 2.3 Characterization

PEDOTPSS/PEI/PCBM

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The surface morphologies of the PEDOT:PSS, PEDOTPSS/PEI, and

layers were investigated via atomic force microscopy (AFM, Veeco, USA) in the tapping mode. Scanning electron microscopy (SEM, JEOL, Japan) images of the perovskite films and cross-sectional images of the PSCs were obtained at an acceleration voltage of 20 kV. The work function of

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PEDOT:PSS was measured using ultraviolet photoelectron spectroscopy (UPS, Kratos Analytical, UK). The He I (21.22 eV) radiation line from a discharge lamp was used, with an experimental resolution of 0.15 eV. Ultraviolet–visible transmission spectra were obtained using a Lambda 750 spectrometer

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(Perkin Elmer, USA). The photoluminescence (PL) spectra were measured using a spectrometer (FLS920, Edinburgh Instruments, UK). The X-ray diffraction (XRD) patterns were obtained using a

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Bruker D8 Advance XRD Instrument (Germany). The current density–voltage (J–V) characteristics of the PSCs were measured under an irradiation intensity of 10–100 mW cm-2 (0.1–1 sun). The incident photon-to-current efficiency (IPCE) was measured using a Solar Cell IPCE measurement system (Solar Cell Scan 100, Zolix, China).

3. Results and Discussion

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ACCEPTED MANUSCRIPT As shown in Fig. 1(a), the PEDOT:PSS layer had a smooth surface morphology with a low rootmean-square (RMS) roughness of 1.21 nm. The sheet resistance of the PEDOT:PSS layer was 68 Ω·sq-1, which was slightly higher than that (~15 Ω·sq-1) of the ITO electrode [26, 27]. As shown in Fig. 1(b), after coating with PEI, the surface became significantly rougher: the RMS roughness was drastically

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increased to 7.23 nm. This might be because of the island-like structures formed by PEI molecules on the PEDOT:PSS surface, as indicated in previous studies [26, 28]. The UPS spectra in Fig. S1 indicate that the work function of PEDOT:PSS was tuned from −5.08 to −4.05 eV by using PEI, which is similar

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to our previous work [26]. This is because of the formation of surface dipoles, which was induced by the amine groups in the PEI molecules (see inset of Fig. S1) [26, 28]. Fig. 1(c) shows that the surface

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became smoother again after being coated with PCBM, with a reduced RMS roughness of 2.52 nm, indicating full surface coverage of PCBM. The contact-angle measurements in Figs. 1(d)–(f) show that the surface became increasingly hydrophobic upon coating with PEI and PCBM. After the coating with PCBM, a large contact angle of 82° was obtained, suggesting the high hydrophobicity of the surface.

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Previous studies showed that a more hydrophobic surface is beneficial for the formation of the upper

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perovskite layer [29, 30], which will be discussed later.

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ACCEPTED MANUSCRIPT Fig. 1. AFM images of (a) bare PEDOT:PSS, (b) PEDOT:PSS covered by PEI, and (c) PEDOT:PSS/PEI covered by PCBM. Contact-angle measurements of (d) bare PEDOT:PSS, (e) PEDOT:PSS covered by PEI, and (f) PEDOT:PSS/PEI covered by PCBM.

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As shown in Figs. 2(a) and (b), the PSCs were fabricated on glass or PEN substrates with a structure of PEDOT:PSS/PCBM/perovskite/spiro-OMeTAD/Au. The cross-sectional SEM image (Fig. 1(a)) shows the dense layer-by-layer structure of the PSCs. The PEDOT:PSS electrode was found to be 80

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nm thick, while the perovskite and spiro-OMeTAD layers were approximately 400 and 210 nm thick, respectively, which were the typical thicknesses for the high-performance PSCs reported in a previous

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work [4–6, 24–26]. The energy-level alignment of the PSCs in Fig. 2(b) indicates that holes were transported by spiro-OMeTAD and collected by the An anode (to the left), while electrons were transported by PCBM and collected by the PEDOT:PSS cathode. The work function (−4.05 eV) of PEDOT:PSS—tuned by PEI—is very close to lowest unoccupied molecular orbital (LUMO) levels of

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PCBM (−3.80 eV) and perovskite (−3.75 eV), which is suitable for extracting electrons from perovskite

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and transferring them to the cathode [15, 26].

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Fig. 2. Cross-sectional SEM image (a) and energy-level alignment (b) of the layers in the PSCs.

as the electron transport layer.

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Forward scan J–V characteristics (c) and IPCE spectra (d) of the PSCs without and with 37-nm PCBM

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Table 1. Device parameters (average and best values for 12 individual devices in each group) for rigid PSCs without (control) and with PCBM layers (processed from 5, 8, 11, or 14-mg·mL-1 solutions).

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Device configuration (rigid) Without PCBM layer (control) With PCBM layer (5 mg·mL-1) With PCBM layer (8 mg·mL-1) With PCBM layer (11 mg·mL-1) With PCBM layer (14 mg·mL-1)

Voc (V) 0.99 ± 0.01 0.99 ± 0.01 1.00 ± 0.01 1.01 ± 0.01 1.00 ± 0.01

Jsc (mA cm-2) 17.7 ± 0.4 18.1 ± 0.3 18.5 ± 0.2 18.8 ± 0.3 18.4 ± 0.3

FF (%) 71.5 ± 1.3 72.1 ± 1.1 72.9 ± 1.1 73.3 ± 1.2 72.2 ± 0.9

Average PCE (%) 12.5 ± 0.4 12.9 ± 0.3 13.5 ± 0.2 13.9 ± 0.3 13.3 ± 0.3

Best PCE (%) 13.0 13.3 13.8 14.3 13.7

As shown in Table 1 and Fig. S2, the control PSCs without PCBM had an average PCE of 12.5%, which is similar to or higher than those of previously reported metal-oxide-free PSCs [18, 20, 21, 25, 9

ACCEPTED MANUSCRIPT 26]. When PCBM was used as the electron transport layer, the performance of the PSCs was gradually improved with the increase of the thickness of the PCBM layers, which was controlled by varying the concentration of the PCBM solutions. When an 11-mg·mL-1 solution was used for PCBM deposition, the highest PCE of 13.9% was obtained (Fig. 2(c)), which is comparable to that of metal-oxide-based

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PSCs [2–4, 15–17]. Additionally, the open-circuit voltage (Voc, from 0.99 to 1.01 V), short-circuit current density (Jsc, from 17.7 to 18.8 mA cm-2), and FF (from 71.5 to 73.3%) were improved simultaneously. As shown in the three-dimensional (3D) AFM height image of Fig. S3, the optimal

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thickness of the PCBM layer processed from the 11-mg·mL-1 solution was measured to be 37 nm. The spectra in Fig. 2(d) show an enhanced IPCE in a broad wavelength range of 400–750 nm for the case

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where 37-nm PCBM was used as the electron transport layer. The integrated Jsc values from the IPCE spectra were 17.3 and 18.3 mA cm-2 for PSCs without (control) and with a 37-nm PCBM layer, respectively. The difference between the measured (from J–V characteristics) Jsc and the calculated values (from IPCE spectra) was only 2.3%–2.7%, indicating the high reliability of our measurements.

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Further increasing the thickness of the PCBM layer led to performance degradation, with the PCE decreasing to 13.3%. This might be caused by the high resistance and low transmittance of the thicker PCBM layers (see Fig. S4) [15]. As shown in Fig. 3(a), the best sample in the group exhibited a high

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PCE of 14.3%, with negligible hysteresis (PCE = 14.6%, reverse scan), indicating fewer defect-induced carrier traps of the PSC [13, 26]. The steady-state current density and PCE with respect to time for the

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best PSC are shown in Fig. 3(b). The current density stabilized at 18.2 mA cm-2 for 200 s, yielding a stabilized PCE of 13.8%, which indicates a stable output of the PSC [31, 32]. We also compared the long-term stability of the PSCs without and with PCBM under 1-sun irradiation (temperature of 25 °C, relative humidity of 30%), in accordance with previous studies [33, 34]. As shown in Fig. S5, after 3,600 s, the degradation of the PSC using PCBM was only 12.2%, which is significantly lower than that (27.8%) of the control PSC. Moreover, we tested the long-term stability of the PSCs with and without 37-nm PCBM by storing the PSCs in ambient conditions. Specifically, we stored the unencapsulated 10

ACCEPTED MANUSCRIPT PSCs in a dark environment before and after measuring their J–V characteristics. As shown in Fig. S6(a), the PSC using a 37-nm PCBM layer still had a PCE of 12.5% after 144 h, indicating a degradation of 13.0% (original PCE = 13.9%). In comparison, the control PSC without a PCBM layer showed faster performance degradation, with a PCE of 10.2% after the same storage time (original PCE = 12.5%).

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Thus, the long-term stability of the PSCs was improved by 29% by using PCBM as the electron transport layer. This can be explained by the protection of the PSCs from moisture with the use of PCBM. As shown in Figs. 1(e) and (f), PCBM is far more hydrophobic than PEDOT:PSS/PEI and thus

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could be more effective for blocking moisture from air [35–37].

Fig. 3. (a) J–V characteristics under forward and reverse scans for the best PSC using 37-nm PCBM as

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the electron transport layer. (b) Current density and PCE with respect to time for the best PSC under a

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forward bias of 0.76 V.

To identify the cause of the improvement in the performance of the J–V characteristics, we characterized the quality of the perovskite layers using SEM and XRD. As shown in Fig. 4(a), the perovskite layer deposited on PEDOT:PSS/PEI showed a rough surface morphology with small perovskite grains (average diameter = 230 nm). In comparison, the perovskite layer on PEDOT:PSS/PEI/PCBM showed a more compact morphology, with significantly larger perovskite 11

ACCEPTED MANUSCRIPT grains (average diameter = 350 nm). According to the XRD patterns in Fig. S6(b), both the perovskite layers with and without PCBM layers showed clear characteristic peaks for CH3NH3PbI3 tetragonal perovskite. However, the (110) and (220) peaks at 14.1° and 28.4° of the perovskite on PEDOT:PSS/PEI/PCBM are significantly higher and sharper than those of the perovskite on bare

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PEDOT:PSS/PEI, indicating that the crystallinity of the perovskite layer improved [29, 30, 38]. Previous studies demonstrated that a hydrophobic surface is beneficial for improving the perovskite quality because it can suppress the heterogeneous nucleation of perovskite at the initial stage and

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improve the boundary mobility of the perovskite grains [29, 30]. Less dense nuclei and higher boundary mobility are beneficial for perovskite growth during thermal annealing, resulting in a larger grain size

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and enhanced crystallinity [29, 30]. In our case, the surface of the PCBM is far more hydrophobic than that of PEI, as indicated by the contact-angle measurements (see Figs. 1(e) and (f)), which indicate a good water resistance property. The use of a PCBM layer did not significantly alter the thickness of the perovskite layers. As shown in Fig. S7(a), the thickness of the perovskite layer on PEDOT:PSS/PEI was

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shown in Figs. 2(a) and S6(b).

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approximately 380 nm, which is similar to that of the perovskite layer on PEDOT:PSS/PEI/PCBM, as

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Fig. 4. SEM images of (a) the perovskite layer on the PEDOT:PSS/PEI substrate and (b) the perovskite

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layer on the PEDOT:PSS/PEI/PCBM substrate. (c) PL spectra of bare perovskite (on glass), perovskite on PEDOT:PSS/PEI, and perovskite on PEDOT:PSS/PEI/PCBM. (d) TRPL spectra of perovskite layers

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on PEDOT:PSS/PEI and PEDOT:PSS/PEI/PCBM.

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To evaluate the charge-extraction effect of the PCBM, we measured the PL and time-resolved PL (TRPL) spectra of bare perovskite (on glass), perovskite on PEDOT:PSS/PEI, and perovskite on PEDOT:PSS/PEI/PCBM. As shown in Fig. 4(c), the bare perovskite layer exhibited a strong PL signal at a wavelength of 778 nm, which is similar to the results of previous studies using CH3NH3PbI3 perovskite [26, 38]. The use of PEDOT:PSS/PEI and PEDOT:PSS/PEI/PCBM as underlayers both significantly quenched the PL signals. However, the PL intensity of the perovskite layer using PCBM was 32% lower than that of the perovskite without PCBM, indicating that the charge-extraction property 13

ACCEPTED MANUSCRIPT improved [39]. Moreover, the PL position was slightly blue-shifted (from 778 to 773 nm) by using PCBM, which is attributed to the enlarged grain size and enhanced crystallinity of the perovskite [29]. The TRPL spectra in Fig. 4(d) show a significantly reduced PL lifetime of the perovskite with the use of PCBM. By fitting bi-exponential curves to the data, the PL lifetime was dramatically decreased from

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112 to 79 ns, indicating that the charge extraction from the perovskite was enhanced using PCBM [26, 39]. Because PCBM is an N-type electron transport material, the PL and TRPL results further demonstrated the improved electron transport property, which was consistent with the enhanced Jsc and

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FF in the PSCs [13, 39]. As shown in Fig. 2(a), the LUMO level of PCBM (−3.80 eV) is slightly lower than that of perovskite (−3.75 eV) but higher than the work function (−4.05 eV) of the PEI-modified

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PEDOT:PSS electrode. Thus, using PCBM could facilitate more efficient transport of electrons from the

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perovskite layer and to the cathode, which explains the PL and TRPL results [13, 39].

Fig. 5. Light-intensity dependence of (a) Jsc and (b) Voc for PSCs with and without 37-nm PCBM as the electron transport layer.

We also analyzed the charge-recombination properties of the PSCs by measuring the current density and voltage dependence on the incident light intensity (from 0.1 to 1 sun). Previous studies indicated that Jsc has a power-law dependence on the light intensity (Plight), Jsc = Plightα [39, 40]. When α = 0.75, a 14

ACCEPTED MANUSCRIPT device is space-charge-limited because of the interfacial barrier or carrier imbalance. When α = 1, no space charge exists for the related device [40−42]. As shown in Fig. 5(a), the calculated α from the dependence of Jsc on the light intensity for the PSCs without (control) and with the 37-nm PCBM layer are 0.91 and 0.99, respectively. This indicates that there was suppressed bimolecular recombination in

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the PSCs when PCBM was used [40−42]. As shown in Fig. 2(b), the PCBM had a deep highest occupied molecular orbital level (−6.1 eV), which could effectively block holes from the perovskite layer [13, 15]. Thus, the use of PCBM can reduce the charge-recombination probability at the interface

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[13, 15]. On the other hand, according to the dependence of Voc on the light intensity in Fig. 5(b), the PSCs without a PCBM layer showed a slope of 1.69 kT/q, where k is the Boltzmann constant, T is the

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absolute temperature (approximately 300 K), and q is the elementary charge. For the PSCs with a 37-nm PCBM, the slope was significantly decreased to 1.23 kT/q, indicating that there was a reduced trapassisted recombination of the PSCs [40−43]. This can be explained by the improved quality of the perovskite film using PCBM, as shown in Figs. 4 and S5(b). A high-quality perovskite layer with good

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crystallinity has few defects, reducing the number of carrier traps in the PSCs [29, 30, 38]. The slightly improved Voc can be described by the Shockley equation [16, 44−46]:

nkT J ln( sc ) , q J0

(1)

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Voc ≈

where J0 is the reverse saturation current density, and n is the ideality factor. The decreased charge

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recombination from the light-intensity dependence analysis indicates that J0 was reduced. Thus, the suppressed charge recombination (resulting in a decreased J0) and enhanced Jsc are the reasons for the improvement in Voc (from 0.99 to 1.01 V).

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Fig. 6. (a) J–V characteristic of the flexible metal-oxide-free PSC on a PEN substrate using PCBM as the electron transport layer. The inset shows the flexible PSC with an effective working area of 0.1 cm2.

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(b) Bending-test results for the flexible PSC using PCBM as the electron transport layer, in terms of the normalized PCEs, plotted with respect to the number of bending cycles.

In addition, we fabricated flexible PSCs using PCBM as the electron transport layer on PEN substrates, as shown in the inset of Fig. 6(a). According to the J–V characteristic in Fig. 6(a), the

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flexible PSCs had a PCE of 11.4%, with Voc, Jsc, and FF values of 1.00 V, 17.4 mA cm-2, and 65.5%, respectively. This PCE is 17% higher than that of the PSCs without PCBM presented in our previous

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work [26]. However, the difference between the PCEs of the rigid and flexible PSCs using PCBM as the electron transport layer (13.9% and 11.4%, respectively) is 2.5%, which is slightly better than that

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(2.7%) of the PSCs without PCBM in our previous study [26]. The reduction of the PCE was mainly due to the reductions in Jsc and FF, which were probably caused by the rougher PEN substrate and higher series resistance of the flexible PSCs [26, 47]. Fig. 6(b) shows the bending-test results for the flexible PSCs with a bending radius of ~3 mm. After 500 bending cycles, the PSC still had a PCE of 8.1%, corresponding to a degradation of 29%, which is similar to the results of our previous study [26]. The results of the bending test indicate that using PCBM does not play a significant role in improving the flexibility of the PSCs. The bending-induced performance degradation might be caused by the rigid 16

ACCEPTED MANUSCRIPT and fragile nature of the perovskite layer, as demonstrated in previous studies [47–49]. Thus, we conclude that using PCBM cannot prevent flaw formation in perovskite layers during bending tests.

4. Conclusions

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We inserted a PCBM electron transport layer into metal-oxide-free PSCs using PEI-modified PEDOT:PSS as a cathode. The SEM and XRD results indicated that the crystallization of the perovskite layer was improved with PCBM as the underlayer. PL and TRPL analysis revealed the enhanced

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charge-extraction property of the PSCs using PCBM. Light-intensity dependence measurements showed the suppressed charge recombination of the PSCs using PCBM. As a result, a high average PCE of

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13.9% was obtained for the rigid PSCs (on a glass substrate) using 37-nm PCBM as an electron transport layer, which was 11.2% higher than that of PSCs without PCBM. The best rigid PSC using PCBM as an electron transport layer showed a superior PCE of 14.3%, with a stable power output and negligible hysteresis. Moreover, the long-term stability of the PSCs was significantly improved, with

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reduced PCE degradation (from 18.4% to 13.0%) after 144 h, which was caused by the hydrophobic property of the PCBM. The PSCs using PCBM as an electron transport layer (on PEN substrates) exhibited an excellent PCE of 11.4%, along with good flexibility. Our results indicate that using PCBM

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free PSCs.

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as the electron transport layer is a simple and effective way to improve the performance of metal-oxide-

Acknowledgements

This work was supported by the Department of Science & Technology of Jilin Province (Developmental Project of Science and Technology of Jilin Province [funding number 20160414043GH]) and by the National Research

Foundation of Korea (grant numbers

2016R1A2B2015389 and 2017R1C1B5075448).

17

ACCEPTED MANUSCRIPT Appendix A. Supplementary Material

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Supplementary data associated with this article can be found in the online version, at xx

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ACCEPTED MANUSCRIPT ·High-performance metal-oxide-free PSC produced with PCBM as electron transport layer. ·High PCE of 13.9% was achieved for rigid PSCs on a glass substrate.

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·Long term stability of the PSCs was also improved upon using PCBM. ·Flexible PSCs on a PEN substrate exhibited a high PCE of 11.4% with good

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bendability.