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C60 composite top electrode for efficient and translucent perovskite solar cells
Solar Energy 196 (2020) 582–588
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A solution processed Ag-nanowires/C60 composite top electrode for efficient and translucent perovskite solar cells Nguyen Ha Khoaa, Yuki Tanakaa,b, Wei Peng Goha, Changyun Jianga, a b
T
⁎
Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, 138634, Singapore Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
A R T I C LE I N FO
A B S T R A C T
Keywords: Semi-transparent Perovskite solar cells Ag nanowires (AgNWs) Solution process
All solution-processed semi-transparent perovskite solar cells (PSCs) with high efficiency were fabricated with Ag nanowire (AgNW) top electrode directly coated on the organic electron transport layer (ETL). A unique AgNW/ETL interface morphology was attained by the fabrication of the AgNW-C60 composite electrode through a sequential spin-coating method. The C60 filler spin-coated on the AgNW network engendered a high contact area and favourable energy level alignment at the interface between the ETL and AgNW. As a result, devices based on AgNW-C60 electrode far outperformed devices with pristine AgNW electrodes. By adjusting the thickness of the AgNW film, the AgNW-C60 devices attained a PCE of 11.02%. The incorporation of C60 into the AgNW electrode also led to remarkable stability improvement. The low-temperature solution-processed and semi-transparent AgNW-C60 top electrode is also applicable to other PSC structures with various interfacial layers, thus showing a promising path towards fully-printable, flexible and translucent photovoltaic devices.
1. Introduction Perovskite solar cells (PSCs) have made impressive progress in just a few years with power conversion efficiencies (PCE) over 22% reported (Jung et al., 2019; NREL, 2019; Rong et al., 2018). Besides the high and fast-rising efficiency, PSCs have a much simpler production process in comparison to other photovoltaic technologies (silicon especially). Most of the functional layers of a PSC including the perovskite, electron transport layer (ETL) and hole transport layer (HTL) layer can be fabricated through solution processes at a moderate temperature, and the processes are readily scalable in a roll-to-roll printing/coating method. Usually, the top electrode is fabricated by vacuum thermal deposition of a thin metal film. To make the production cost more competitive, a low-temperature, all-solution-processed PSC is desirable. Thus, developing a solution-processed top electrode is becoming critical and urgently needed. Although there are several conductive pastes commercially available for printing electrodes, most of them cannot be used to fabricate the top electrode on the perovskite or organic layer due to harmful components such as binders, surfactants and some incompatible solvents. The binders and surfactants in the paste will leave residues in the electrode film processed at low temperature, which constitute severe charge injection barriers at the perovskite/electrode interface. Silver nanowire (AgNW) is a promising material for solutionprocessable top electrodes in organic/perovskite (Guo et al., 2015; K. ⁎
Han et al., 2018; Lee et al., 2016) and crystalline Si (Aurang et al., 2017; Jarrett et al., 2015) solar cells because of its low temperature processability, high electrical conductivity, mechanically flexibility, and high transparency. The optical transparency of AgNW top electrodes also enables the fabrication of high performance translucent PSCs (Pang et al., 2017; Yang et al., 2016) and perovskite/Si tandem solar cells (Bailie et al., 2015; Sahli et al., 2018; Werner et al., 2018). The fundamental challenge in developing a PSC with AgNW top electrode is posed by the solvent compatibility of the perovskite and ETL layers. In previous works, spray coating (Guo et al., 2015; K. Han et al., 2018; Yang et al., 2016) and transfer laminating (Bailie et al., 2015) were typically employed to mitigate the damage caused by the solvent carrier of AgNWs to the perovskite/ETL layer. In the spray coating process, the solvent dries within µs of reaching the substrate, thus the possibility of damage to the pre-coated active layers is significantly reduced, especially when a buffer layer is deposited before the AgNW deposition to provide further protection to the perovskite layer (Dai et al., 2016). In the transfer-lamination process, solvent contact with the active layer is eliminated. This process, however, is intricate and thus considerably less popular. Micro patterning techniques, such as etching, can produce highly transparent AgNW films which can be used as bottom electrodes of solar cell devices (Wu et al., 2016). However, they are typically unsuitable for the fabrication of top electrodes for PSCs since the processes
Corresponding author. E-mail address: [email protected] (C. Jiang).
https://doi.org/10.1016/j.solener.2019.12.038 Received 16 October 2019; Received in revised form 6 December 2019; Accepted 14 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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involved will damage the functional layers. Another issue for solution-processed AgNW top electrodes is the poor contact between the AgNWs and the planar ETL/HTL film, which arises from poor physical contact or unfavourable surfactant or binder trapped between the silver and ETL/HTL. Applying pressure on the AgNW layer was found to be effective in improving the contact between the AgNW network and the substrate as well as the contact among the network itself (Bailie et al., 2015; Yang et al., 2016). However, this method is not well-suited for a AgNW top electrode coated on the active layers because it involves a significant risk of puncturing the ultrathin perovskite active layers and causing a short circuit as a result. In this study, a semi-transparent and conductive AgNW electrode was deposited on the perovskite-ETL layer by a simple spin-coating process. To improve the contact and charge injection from the planar ETL to AgNWs, C60 as a filler is spin-coated on the AgNW network to engender a high contact area and favourable energy level alignment between the AgNWs and the ETL. Devices with AgNW-C60 electrodes far outperformed those with pristine AgNW electrodes. A PCE of 11.02% was achieved from a PSC with a thick AgNW-C60 electrode, which was comparable to that (PCE = 10.84%) of the device with thermally evaporated silver electrodes. The stability of AgNW-C60 based devices was also drastically improved through the incorporation of C60 in the electrode. The low-temperature solution-processed AgNWC60 composite electrode is a promising solution for the development of flexible and translucent PSCs or tandem solar cells.
Fig. 1. Device structure of planar perovskite solar cells with AgNW electrodes (left), and the energy level diagram of PC60BM, C60 and AgNW (right).
2.4. Preparation of other solutions The PCBM solution was prepared by dissolving PC60BM in DCB (23 mg/ml). The PCBM solution was then filtered through a 0.45 µm PTFE filter. The 10 mg/ml C60 solution in DCB was similarly prepared. To facilitate the dissolution, the solutions were heated at 60 °C for 2 h. Perovskite precursor solution was prepared by dissolving 264.5 mg of PbI2, 94.0 mg of FAI, 37.2 mg of PbBr2 and 11.3 mg of MABr in 400 µl of DMF and 100 µl of DMSO (Ye et al., 2018). The solution was heated up to 60 °C before deposition. 2.5. Device fabrication
2. Experimental
The schematic structure of the fabricated planar perovskite solar cells is shown in Fig. 1. The NiOx (HTL), perovskite, PC60BM (ETL) layers were sequentially spin-coated on cleaned ITO/glass substrates. ITO glass substrates were cleaned by sonication in detergent (2% Hellmanex® III solution), DI water, acetone and IPA. After drying, the substrates were UV ozone treated for 10 min at 100 °C right before deposition. NiOx ink was spin-coated on the ITO substrates at 3000 rpm for 45 s; the substrates were then dried on a hot plate at 100 °C for 10 min. Then, perovskite film of FA0.81MA0.15Pb(I0.836Br0.15)3 was deposited by spin-coating the perovskite precursor solution with antisolvent technique in a glovebox (Ye et al., 2018). The spin coating was done at 2000 rpm for 10 s and then at 6000 rpm for 30 s. CB (100 µl) was dropped on the substrate ~10 s into the second stage of spincoating. Upon finishing, the substrate was promptly transferred on a hot plate at 100 °C and kept for 50 min. The PCBM solution (23 mg/ml in DCB) was spin-coated at 1300 rpm for 40 s to form an ETL on top of the perovskite layer. The substrate was again annealed at 100 °C for 5 min and cooled down before the electrode deposition.
2.1. Material All anhydrous solvents, including isopropanol alcohol (IPA), N,Ndimethylformamide (DMF), dimethyl sulfoxide (DMSO), chlorobenzene (CB), dichlorobenzene (DCB) were purchased from Sigma Aldrich. Nickel nitrate hexahydrate (99.999%) and bathocuproine (BCP) were also purchased from Sigma Aldrich. PC60BM (99.5%) was purchased from Solenne BV. Formamidinium iodine (FAI) and methylammonium bromide (MABr) were purchased from Dyesol. PbI2 and PbBr2 were purchased from Tokyo Chemical Industry Co., Ltd. AgNWs with average diameter/length (D/L) of 40 nm/45 µm in IPA were purchased from ACS Material (SKU: NWAG04I1). AgNWs with average D / L = 80 nm/81 µm were synthesized. Patterned indium tin oxide (ITO) glass substrates (10 Ω/□) were purchased from Xinyan Technology Ltd. 2.2. Synthesis of NiOx ink
Deposition of top electrodes
The preparation of NiOx nanoparticle ink is similar to a previously reported procedure (H. Zhang et al., 2016). Ni(NO3)2 (3.635 g) was dissolved in 2.5 ml of water. Ni(OH)2 was then precipitated with ~ 2.5 ml of 10 M NaOH solution. The light green precipitation was collected by centrifugation and washed thrice with deionized (DI) water. Afterwards, it was transferred to an oven at 80 °C and dried until the colour turned dark green. This solid was then transferred to an oven at 270 °C and kept for 2 h. The final product was a black powder of NiOx nanoparticles. The NiOx nanoparticles were then dispersed in DI water at 2.6 wt%. After sonicating for 2 h, the ink was filtered through a 0.45 µm PVDF filter. For best results, the ink was used at least one day after filtration.
(1) Spin-coating of AgNW electrodes On the PCBM layer, a AgNW top electrode was deposited by spincoating the AgNW dispersion (10 mg/ml) at 900 rpm for 30 s and dried at 90 °C for 30 s. The patterned area of the AgNW electrode was delimited by polyimide tape as mask. The active cell area was defined by the overlap of the patterned ITO and the patterned AgNW electrode, typically 0.09 cm2 for each cell. The electrode thickness can be increased by repeating the spincoating process. Devices with 1, 2 and 3-layered AgNW electrodes were fabricated. To deposit the PCBM or C60 filler, a 10 mg/ml solution of the respective filler was spin-coated on the AgNW layers at 1300 rpm for 45 s.
2.3. AgNW ink preparation
(2) Thermal evaporation of silver film electrodes
As-received AgNW dispersion was first purified by gentle centrifugation at 2000 rpm for 30 min. The AgNW precipitate was then redispersed in anhydrous IPA. Before using the AgNW dispersion, a sonication of not more than 30 s was performed.
For comparison, control devices with thermally evaporated Ag top electrode were also fabricated. BCP (0.5 mg/ml solution in anhydrous 583
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IPA) was first spin-coated on the PCBM ETL at 5000 rpm for 30 s. The devices were then masked, and silver was deposited under a vacuum condition of 5 × 10−6 mbar at about 0.1 Å/s. 2.6. Characterizations The surface morphologies of the AgNW films were investigated using a microscope (Olympus BX51), and a SEM (JEOL JSM-7001F) at 10 kV. Film transmittance spectra were recorded using a UV–visiblenear IR scanning spectrophotometer (UV-3600, Shimadzu). Film thicknesses were measured using a surface profiler (KLA-TENCOR P10). The work function of the electrode was measured using a photoelectron spectrometer (Riken Keiki, AC-2). The photoluminescence (PL) measurements were conducted using Shimadzu UV-2501 spectrofluorophotometer with an excitation wavelength of 520 nm. Current density-Voltage (J-V) characteristics for solar cell devices were measured in a nitrogen atmosphere with a solar simulator (SAN-EI Electric XES-301S 300 W Xe lamp JIS Class AAA) which was calibrated to 1000 W/m2 with a reference Si cell. For the stability test, the devices were stored in a nitrogen glovebox (H2O < 5 ppm, O2 < 10 ppm).
Fig. 2. Schematic drawing of the contact between the AgNW and the planar substrate of (a) the contact between a cylinder form nanowire and the planar ETL-1 surface, and b) suspended portion of AgNW, resulting in a gap between the wire and ETL. (c) and (d) show the filling of a second layer of ETL-2 (filler) in between the AgNW and the planar ETL-1.
3. Results and discussion
Han et al., 2018). Considering the cylindrical form of the AgNWs, the contact area between the nanowires and the planar ETL would be very small as shown in Fig. 2a. Additionally, the random overlapping of the AgNWs will result in a considerable portion of the nanowires being suspended and not in contact with the ETL as schematically shown in Fig. 2b. To improve the morphology of the contact between AgNWs and the planar ETL (hereby denoted ETL-1), a second ETL (ETL-2) was spincoated on the AgNW electrode as a filler layer, resulting in a nanocomposite electrode of AgNW and ETL-2. As shown in Fig. 2c and d, ETL-2 fills up the nano-scale voids between the AgNWs and the substrate, hence significantly increasing the contact area at the AgNW / ETL interface and decreasing the interfacial electrical resistance. For simplicity’s sake, PCBM was chosen as ETL-2 and spin-coated on the AgNW layer using a PCBM/CB solution in a lower concentration (10 mg/ml), forming a AgNW-PCBM nanocomposite electrode. Devices using AgNW-only and AgNW-PCBM electrodes were fabricated (with device structure of ITO/NiOx/perovskite/PCBM/AgNW or AgNWPCBM). Fig. 3a and b show the surface SEM images of the AgNW and AgNW-PCBM electrodes fabricated on two devices. As seen, the pure AgNW electrode (Fig. 3a) shows smooth surfaces on both the AgNWs and the bottom planar PCBM layer, while the AgNW-PCBM electrode (Fig. 3b) shows rougher surfaces on both the nanowires and the underlying PCBM layer. A closer contact between the AgNWs and the ETL is observed in Fig. 3b; some nanowires at the bottom can be seen partially embedded into the PCBM layer. It can be observed that the spincoating of the diluted PCBM solution not only resulted in the filling of the nano-scale voids between the substrate and the nanowires, but it also helped to form an ultrathin coating on the surface of each individual nanowire. It was often thought that the spin-coating of the second PCBM will dissolve and wash away the first planar PCBM layer. In this case, however, the AgNW network can act as a protective net to prevent the critical portion of PCBM under the AgNWs from being
As shown in Fig. 1, the planar perovskite solar cell has a structure of glass/ITO/NiOx/Perovskite/PCBM/AgNWs. All the active layers were sequentially deposited on the glass/ITO substrate by spin-coating. Asreceived AgNW dispersion contains traces of harmful solvents or surfactants which manifests in a colour change in the perovskite/PCBM surface upon contact. Thus, a solvent purification process, as described in the experimental section, is necessary. This procedure could enable a wider range of scalable fabrication methods such as drop casting, spray coating, Mayer-rod coating and various printing techniques. All the mentioned methods were reported to yield highly conductive and transparent AgNW films (Kwon et al., 2018). The AgNW size influences the film morphology and the ETL/AgNWs interface, which is crucial for realizing high performance translucent solar cells. Usually, larger sizes (diameter and length) of AgNWs yield lower sheet resistance. However, this creates larger voids in the film and hence, smaller contact area with the substrate (F. Han et al., 2018). AgNWs with diameters of 20–50 nm and aspect ratios 500–1000 are commonly used for perovskite solar cells fabrications (Bailie et al., 2015; Guo et al., 2015; K. Han et al., 2018; Xie et al., 2018). Here, high aspect ratio AgNWs with average size of D/L = 40 nm / 45 µm (aspect ratio ~1000) were used for devices fabrication. Bigger sizes of AgNWs with average D/L = 80 nm/81 µm were also tested for its characteristics as device electrodes, but it yielded much poorer device performance as compared to the AgNWs of D/L = 40 nm/45 µm (see Fig. S1 and Table S1 in the supporting information). Thus, AgNWs with an average D/L = 40 nm/45 µm were used for all the following studies. The PCEs of the devices with pristine AgNW layers coated on top of the ETL was low (< 6.18%, see Table 1). The voids among the AgNW network, and higher electrode sheet resistance (to be discussed later) might have contributed to the poor performance. However, the contact area between the AgNW electrode and the planar PCBM ETL is believed to be a major factor limiting the device performance (Dai et al., 2016; K. Table 1 Performance summary of PSCs with different AgNW top electrodes. AgNW Electrode
VOC(V)
Jsc (mA/cm2)
FF
PCE (%)
PCEmax (%)
Rs** Ω
Rsh Ω
AgNW AgNW-PCBM AgNW-C60
0.87 ± 0.02* 0.88 ± 0.01 0.92 ± 0.02
12.91 ± 0.54 14.17 ± 0.30 15.72 ± 0.55
0.50 ± 0.03 0.64 ± 0.02 0.60 ± 0.03
5.64 ± 0.43 7.90 ± 0.23 8.68 ± 0.35
6.18 8.09 9.17
550 110 130
~50000 ~80000 ~100000
* Data were taken from 6 to 8 sample devices. ** Rs and Rsh are calculated from the best device. 584
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Fig. 3. Surface SEM images of (a) 1-layered AgNW, (b) 1-layered AgNW-PCBM and (c) 3-layered AgNW electrodes fabricated on devices, and (d) a cross section image of a full device with structure of glass/ITO/NiOx/perovskite/PCBM/AgNW-C60.
8.09% to 9.17%), with general increases in Jsc, Voc as well as FF. Hysteresis can also be observed in the JV measurement of the AgNWC60 devices but it is shown to be smaller than in the AgNW devices (see Supporting Information Fig. S2 and Table S2). The overall performance improvement can be ascribed to a favourable energy alignment at the C60/AgNW interface, high electron mobility of the C60 filler and its passivation of the surface defects. The obtained results concur with previous reports that C60 can improve device performance when used in conjunction with or to replace PCBM ETL (Liang et al., 2015; Namkoong et al., 2018). It must be highlighted that we also attempted to spin-coat a C60 layer directly on a planar PCBM film before the deposition of Ag electrode by vacuum thermal evaporation. All the resulting devices failed because the C60 spin-coating washed away the existing PCBM in the absence of the protective AgNW mesh. This again indicates the uniqueness of the solution-processable AgNW-C60 nanocomposite electrode. To further prove the effectiveness of the composite electrodes in extracting electrons from the perovskite layer, steady state PL was measured on samples with the structure of glass/perovskite/planar PCBM/AgNW-electrode, with incident light from the AgNW-electrode side. Electron transfer from the perovskite layer to the electrode will suppress the PL emission of the photoexcited perovskite. Fig. 4 shows the PL spectra of 3 samples with different electrodes of pristine AgNW, AgNW-PCBM and AgNW-C60, having peaks at 778 nm, 776 nm and 774 nm, respectively. The PL peaks are from the perovskite emission (Ye et al., 2017). As seen, pristine AgNW electrode produced the highest PL peak. The PL intensities were remarkably reduced when AgNWPCBM or AgNW-C60 were used as the electrodes, with the lowest intensity from the sample with AgNW-C60 electrode. Compared to the pristine AgNW sample, the lower PL intensity observed for the AgNWPCBM sample suggests an enhanced interface (contact) morphology facilitating electron flow from the perovskite. The lowest PL peak from the AgNW-C60 sample reaffirms the superior electron extraction capability of C60. It is noted that the PL quenching by the AgNW-C60 (or AgNW-PCBM) electrode interface is more efficient at long wavelength emission than short wavelength emission, hence, resulting in a blueshift of the peak after quenching, as shown in Fig. 4. Similar quenching and blue-shift effects were also reported by others (Namkoong et al.,
washed away. Notwithstanding, further precaution was exercised to reduce the risk of dissolving ETL-1. The second PCBM (ETL-2) layer was spin-coated using CB instead of DCB solvent, since the former dries faster and has lower solubility for PCBM (Wang and Hua, 2015). The statistical performance of the devices with pure AgNW and AgNW-PCBM composite electrodes are summarized in Table 1 (the first two rows) Table 1 shows that AgNW based devices without the filler have low PCEs (average 5.64%) due to poor Jsc (12.91 mA/cm2) and low FF (0.50). The very high series resistance (550 Ω) arose from the poor contact at the ETL/AgNW-electrode interface. In contrast, devices with AgNW-PCBM electrodes (with PCBM filler) possessed a much higher PCE (7.90%) due to increased Jsc (14.17 mA/cm2) and FF (0.64). Remarkably, the addition of the PCBM filler reduced the series resistance of the device to 110 Ω. The changes in the Jsc¸ FF and series resistance indicate that the PCBM ETL-2 filler has significantly improved the contact between AgNWs and PCBM-ETL. The PCBM between AgNW and perovskite facilitates electron extraction. However, the AgNW/PCBM interface is still not ideal for electron collection since a large energy gap of 0.7 eV exists between PCBM and AgNW (as shown in Fig. 1). The work function of AgNWs, as measured, is about 4.7 eV, higher than that of thermally evaporated silver films (4.4 eV) while the LUMO of PC60BM is commonly reported to be 4.0 eV (Docampo et al., 2013). A favourable energy alignment is necessary to form a good ohmic contact between the ETL and the electrode (Chen et al., 2016; Xie et al., 2018). C60, with a suitable LUMO level of 4.5 eV (Yan et al., 2015), is expected to improve electron extraction through a cascade energy level alignment cathode configuration, PCBM (4.0 eV)/C60 (4.5 eV)/AgNWs (4.7 eV). Furthermore, C60 electron mobility (1.6 cm2Vs−1) far exceeds that of PCBM (6.1 × 10−2cm2Vs−1) (Liang et al., 2015). Elsewhere, it was shown that C60 can effectively extract electrons from the perovskite layer (D. Liu et al., 2018). To further improve device performance, C60, instead of the PCBM ETL-2, was spin-coated on the AgNW layer to form a AgNW-C60 composite electrode. The performance of the AgNW-C60 based devices is presented in Table 1 (the last row). As shown in Table 1, the replacement of PCBM filler with C60 filler remarkably improved the device performance. The efficiency sees a relative improvement of 13% (from 585
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750 nm to 1350 nm and 0–20% in the visible light range (below 750 nm). The low transmittance in the visible light range is due to strong absorption of the thick perovskite layer (~600 nm, as shown in Fig. 3d). The transmittance, sheet resistance of the AgNW films and the corresponding devices’ performance are summarized in Table 2. As the thickness is increased from S1, S2 to S3, the transmittance peak of the AgNW films decreases from 65%, 55% to 40% (Fig. 5), the sheet resistance is reduced from 8.4, 8.2 to 4.1 Ω/square, and the devices average PCE increases from 8.51%, 9.03% to 9.44%, with the best PCE increasing from 10.09%, 10.32% to 11.02% (see Table 2). For comparison, devices with thermally evaporated silver electrode (Ag-TE) were fabricated, with the structure of /ITO/NiOx/Perovskite/ PCBM/BCP/Ag-TE; they are hereafter referred to as Ag-TE devices. The BCP cathode buffer layer was simply spin-coated on the planar PCBM layer. This BCP layer completes a cascade energy level alignment cathode configuration for a planar Ag-TE electrode (Chen et al., 2017), and prevents the harmful diffusion of evaporated Ag into the active layers (Philipp et al., 2015; Vogel et al., 2006). The performance of the Ag-TE devices is summarized in Table 2. The JV curves of the best AgTE device and the best S3-C60 device are presented in Fig. 6. As seen, the statistical performance of devices with spin-coated AgNW-C60 electrodes (S3-C60) is comparable to that of devices with thermally evaporated Ag thin film electrodes (Table 2). The best S3-C60 device even has a higher PCE (11.02%) than the best Ag-TE device (10.84%), due to the higher photocurrent obtained from the former device (see Fig. 6 and Table 2). However, all the AgNW devices exhibit a larger performance variation than the Ag-TE devices, as shown in Table 2. This is probably due to the random and inhomogeneous distribution of AgNWs. Realizing a uniform distribution of AgNWs in the film is one of the challenges for spin-coated AgNW films. Spray coating is a suitable method for fabrication of large-area and uniform AgNW films (K. Han et al., 2018). However, it remains to be investigated whether a spray coated C60 can realize a similar PCBM/AgNW-C60 interface morphology and nanowire surface coating as obtained in spincoated C60. In any case, further device performance improvements should be expected from a more uniform distribution of the AgNWs. Stability issue has been consistently reported for PSCs with AgNW electrodes. This distinctive instability is often attributed to the reactivity of the AgNWs with large exposed surface area (Guo et al., 2015; J. Zhang et al., 2016). Fig. 7 shows the normalized efficiency change with the storage time of three devices with top electrodes of S3, S3-C60 and S1-C60. As seen, the S3 device without filler rapidly degrades to 80% of its original efficiency after only 3 days of storage, while the S3C60 device shows significantly improved stability, virtually maintaining its original efficiency within 52 days of storage. In terms of storage stability, the S3-C60 device outperforms most reported AgNW based devices (J. Zhang et al., 2016; Liu et al., 2017), and is comparable to reported Ag-TE based devices (Teo et al., 2019; Wang et al., 2018; Zheng et al., 2018) (also see Tables S3 and S4 in Supporting Information). It is widely agreed that the degradation of the silver nanowire network is caused by its reaction with iodine compounds diffusing from the perovskite layer (Kato et al., 2015; Niu et al., 2015). Shielding the nanowire network from the iodine ions by using an additional oxide (Kim et al., 2016; Lee et al., 2018) or polymer (Jin et al., 2018) layer is an effective method to hamper the degradation. The improved stability in our AgNW-C60 based devices can be attributed to the shielding effect of the C60 layer (Fang et al., 2017; Liu et al., 2015; D. Liu et al., 2018). The compact molecular structure allows C60 to be densely packed in the filler layer, giving rise to its excellent protective capability. The C60 filler not only protects the nanowires, but also forms a hydrophobic cover on the devices which hinders the degradation of the perovskite layer by preventing harmful moisture ingression. Fig. 7 also shows that the S1-C60 device (with the thinner electrode) is less stable than the S3C60 device. This is reasonable because the thicker electrode, with denser AgNWs, can retain the critical conductivity of the AgNW
Fig. 4. PL spectra of samples with a structure of glass/perovskite/PCBM/electrode, where the electrodes are pristine AgNW, AgNW-PCBM and AgNW-C60.
2018; Shao et al., 2014). Light transmittance and electrical conductivity are two important properties of a AgNW top electrode, particularly when it is used to construct translucent solar cells. AgNW films have a random mesh structure, with the voids among the nanowires ranging from nanometer to micrometer in size, depending on the distribution density of the nanowires in the film. The device area beneath a large-sized void is a non-active area with negligible photocurrent generation because a significant proportion of carriers generated in this region cannot travel the large distance required to reach the AgNW network (Kumar, 2017). Generally, thinner AgNW films have higher transparency but also larger non-active area and higher sheet resistance, and thus result in poorer device performance. Thicker AgNW films, with smaller non-active area and lower sheet resistance, will result in devices with higher efficiency. PSCs of the same configuration (ITO/NiOx/perovskite/PCBM/ AgNW-C60) but with thicker AgNW films were studied. To increase the AgNW film thickness, multiple layers of AgNW were sequentially spincoated on the substrate. One-layered, two-layered and three-layered AgNW films are denoted S1, S2 and S3, respectively. The corresponding AgNW-C60 composite films are denoted S1-C60, S2-C60 and S3-C60, respectively. Multi-layered AgNW films show a denser distribution of nanowires, as shown in the SEM image in Fig. 3c. The transmittance of films of S1, S2, S3 and the S1-C60 device are presented in Fig. 5. The AgNW films show semi-transparency over a broad wavelength range and the transparency is observed to decrease with the film thickness. The highest transmittance of 65% is obtained from S1. The S1-C60 device shows transmittance of 30–45% in the wavelength range from
Fig. 5. Transmission spectra of AgNW films with different number of layers, S1 (1 layer), S2 (2 layers), S3 (3 layers), and a full device with S1-C60 electrode. 586
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Table 2 Performance summary of devices with top electrodes of varying AgNW thickness, and a thermally evaporated silver thin film (Ag-TE). Electrode
T* (peak)
Sheet resistivity (Ω/□)
Device performance VOC (V)
S1-C60 S2-C60 S3-C60 Ag-TE (75 nm)
65% 55% 40% 0%
8.4 8.2 4.1 < 0.5
0.95 0.95 0.97 0.99
± ± ± ±
**
0.04 0.03 0.03 0.02
Jsc (mA/cm2)
FF
17.33 16.33 16.35 15.75
0.52 0.58 0.59 0.62
± ± ± ±
0.23 0.69 0.77 0.82
PCE (%) ± ± ± ±
0.07 0.03 0.04 0.04
8.51 9.03 9.44 9.61
± ± ± ±
PCEmax (%) 0.98 0.53 0.80 0.56
10.09 10.32 11.02 10.84
* T is the Ag or AgNW film transmittance at 650 nm wavelength. ** Data were taken from 7 to 20 sample devices.
solar cells. The electrode was fabricated by a simple sequential spincoating method at low temperature (< 100 °C). The C60 filler spincoated on the AgNW network realized a high contact area and favourable cascade energy level alignment at the interface between the PCBM ETL and the AgNW electrode. As a result, devices with AgNWC60 electrodes outperformed devices with pristine AgNW electrodes. By adjusting the thickness of the AgNW film, the AgNW-C60 devices attained a PCE of 11.02%. Moreover, the AgNW-C60 devices also showed remarkable stability, showing virtually no decline in efficiency after 52 days of storage in a N2-filled glovebox, which can be ascribed to the C60 filler providing protection for the AgNW and perovskite layers. The efficiency and stability of the AgNW-C60 devices were comparable to conventional devices with thermally evaporated electrode. Hence, this low-temperature solution-processed and semi-transparent AgNW-C60 top electrode presents a promising path for developing fully-printable, flexible and translucent photovoltaic devices. Declaration of Competing Interest
Fig. 6. JV curves of the best S3-C60 device (red square) and Ag-TE device (black circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was financially supported by A*STAR SERC Grant (1425203141, A18A1b0045). The authors thank Hui HUANG from Singapore Institute of Manufacturing Technology, A*STAR, for providing partial AgNWs materials fabricated in their lab. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.12.038. References Aurang, P., Doganay, D., Bek, A., Turan, R., Unalan, H.E., 2017. Silver nanowire networks as transparent top electrodes for silicon solar cells. Sol. Energy. https://doi.org/10. 1016/j.solener.2016.11.021. Bailie, C.D., Christoforo, M.G., Mailoa, J.P., Bowring, A.R., Unger, E.L., Nguyen, W.H., Burschka, J., Pellet, N., Lee, J.Z., Grätzel, M., Noufi, R., Buonassisi, T., Salleo, A., McGehee, M.D., 2015. Semi-transparent perovskite solar cells for tandems with silicon and CIGS. Energy Environ. Sci. 8, 956–963. https://doi.org/10.1039/ C4EE03322A. Chen, C., Zhang, S., Wu, S., Zhang, W., Zhu, H., Xiong, Z., Zhang, Y., Chen, W., 2017. Effect of BCP buffer layer on eliminating charge accumulation for high performance of inverted perovskite solar cells. RSC Adv. 7, 35819–35826. https://doi.org/10. 1039/c7ra06365b. Chen, W., Zhu, Y., Yu, Y., Xu, L., Zhang, G., He, Z., 2016. Low cost and solution processed interfacial layer based on poly(2-ethyl-2-oxazoline) nanodots for inverted perovskite solar cells. Chem. Mater. 28, 4879–4883. https://doi.org/10.1021/acs.chemmater. 6b00964. Dai, X., Zhang, Y., Shen, H., Luo, Q., Zhao, X., Li, J., Lin, H., 2016. Working from both sides: composite metallic semitransparent top electrode for high performance perovskite solar cells. ACS Appl. Mater. Interfaces 8, 4523–4531. https://doi.org/10. 1021/acsami.5b10830. Docampo, P., Ball, J.M., Darwich, M., Eperon, G.E., Snaith, H.J., 2013. Efficient
Fig. 7. Normalized PCEs of devices with S3 AgNW (black circle), S3-C60 (red square) and S1-C60 (blue triangle) top electrodes over the storage period (storage condition: in nitrogen filled glovebox with H2O < 5 ppm, O2 < 10 ppm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
network for longer time even after some AgNWs are corroded. And, thicker electrode also has more C60 covering on the active layer, which can provide better protection to the perovskite.
4. Conclusions A unique AgNW-C60 composite film with high conductivity and semi-transparency was developed as the top electrode of perovskite 587
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A solution processed Ag-nanowires/C60 composite top electrode for efficient and translucent perovskite solar cells Nguyen Ha Khoaa, Yuki Tanakaa,b, Wei Peng Goha, Changyun Jianga, a b
T
⁎
Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, 138634, Singapore Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
A R T I C LE I N FO
A B S T R A C T
Keywords: Semi-transparent Perovskite solar cells Ag nanowires (AgNWs) Solution process
All solution-processed semi-transparent perovskite solar cells (PSCs) with high efficiency were fabricated with Ag nanowire (AgNW) top electrode directly coated on the organic electron transport layer (ETL). A unique AgNW/ETL interface morphology was attained by the fabrication of the AgNW-C60 composite electrode through a sequential spin-coating method. The C60 filler spin-coated on the AgNW network engendered a high contact area and favourable energy level alignment at the interface between the ETL and AgNW. As a result, devices based on AgNW-C60 electrode far outperformed devices with pristine AgNW electrodes. By adjusting the thickness of the AgNW film, the AgNW-C60 devices attained a PCE of 11.02%. The incorporation of C60 into the AgNW electrode also led to remarkable stability improvement. The low-temperature solution-processed and semi-transparent AgNW-C60 top electrode is also applicable to other PSC structures with various interfacial layers, thus showing a promising path towards fully-printable, flexible and translucent photovoltaic devices.
1. Introduction Perovskite solar cells (PSCs) have made impressive progress in just a few years with power conversion efficiencies (PCE) over 22% reported (Jung et al., 2019; NREL, 2019; Rong et al., 2018). Besides the high and fast-rising efficiency, PSCs have a much simpler production process in comparison to other photovoltaic technologies (silicon especially). Most of the functional layers of a PSC including the perovskite, electron transport layer (ETL) and hole transport layer (HTL) layer can be fabricated through solution processes at a moderate temperature, and the processes are readily scalable in a roll-to-roll printing/coating method. Usually, the top electrode is fabricated by vacuum thermal deposition of a thin metal film. To make the production cost more competitive, a low-temperature, all-solution-processed PSC is desirable. Thus, developing a solution-processed top electrode is becoming critical and urgently needed. Although there are several conductive pastes commercially available for printing electrodes, most of them cannot be used to fabricate the top electrode on the perovskite or organic layer due to harmful components such as binders, surfactants and some incompatible solvents. The binders and surfactants in the paste will leave residues in the electrode film processed at low temperature, which constitute severe charge injection barriers at the perovskite/electrode interface. Silver nanowire (AgNW) is a promising material for solutionprocessable top electrodes in organic/perovskite (Guo et al., 2015; K. ⁎
Han et al., 2018; Lee et al., 2016) and crystalline Si (Aurang et al., 2017; Jarrett et al., 2015) solar cells because of its low temperature processability, high electrical conductivity, mechanically flexibility, and high transparency. The optical transparency of AgNW top electrodes also enables the fabrication of high performance translucent PSCs (Pang et al., 2017; Yang et al., 2016) and perovskite/Si tandem solar cells (Bailie et al., 2015; Sahli et al., 2018; Werner et al., 2018). The fundamental challenge in developing a PSC with AgNW top electrode is posed by the solvent compatibility of the perovskite and ETL layers. In previous works, spray coating (Guo et al., 2015; K. Han et al., 2018; Yang et al., 2016) and transfer laminating (Bailie et al., 2015) were typically employed to mitigate the damage caused by the solvent carrier of AgNWs to the perovskite/ETL layer. In the spray coating process, the solvent dries within µs of reaching the substrate, thus the possibility of damage to the pre-coated active layers is significantly reduced, especially when a buffer layer is deposited before the AgNW deposition to provide further protection to the perovskite layer (Dai et al., 2016). In the transfer-lamination process, solvent contact with the active layer is eliminated. This process, however, is intricate and thus considerably less popular. Micro patterning techniques, such as etching, can produce highly transparent AgNW films which can be used as bottom electrodes of solar cell devices (Wu et al., 2016). However, they are typically unsuitable for the fabrication of top electrodes for PSCs since the processes
Corresponding author. E-mail address: [email protected] (C. Jiang).
https://doi.org/10.1016/j.solener.2019.12.038 Received 16 October 2019; Received in revised form 6 December 2019; Accepted 14 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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involved will damage the functional layers. Another issue for solution-processed AgNW top electrodes is the poor contact between the AgNWs and the planar ETL/HTL film, which arises from poor physical contact or unfavourable surfactant or binder trapped between the silver and ETL/HTL. Applying pressure on the AgNW layer was found to be effective in improving the contact between the AgNW network and the substrate as well as the contact among the network itself (Bailie et al., 2015; Yang et al., 2016). However, this method is not well-suited for a AgNW top electrode coated on the active layers because it involves a significant risk of puncturing the ultrathin perovskite active layers and causing a short circuit as a result. In this study, a semi-transparent and conductive AgNW electrode was deposited on the perovskite-ETL layer by a simple spin-coating process. To improve the contact and charge injection from the planar ETL to AgNWs, C60 as a filler is spin-coated on the AgNW network to engender a high contact area and favourable energy level alignment between the AgNWs and the ETL. Devices with AgNW-C60 electrodes far outperformed those with pristine AgNW electrodes. A PCE of 11.02% was achieved from a PSC with a thick AgNW-C60 electrode, which was comparable to that (PCE = 10.84%) of the device with thermally evaporated silver electrodes. The stability of AgNW-C60 based devices was also drastically improved through the incorporation of C60 in the electrode. The low-temperature solution-processed AgNWC60 composite electrode is a promising solution for the development of flexible and translucent PSCs or tandem solar cells.
Fig. 1. Device structure of planar perovskite solar cells with AgNW electrodes (left), and the energy level diagram of PC60BM, C60 and AgNW (right).
2.4. Preparation of other solutions The PCBM solution was prepared by dissolving PC60BM in DCB (23 mg/ml). The PCBM solution was then filtered through a 0.45 µm PTFE filter. The 10 mg/ml C60 solution in DCB was similarly prepared. To facilitate the dissolution, the solutions were heated at 60 °C for 2 h. Perovskite precursor solution was prepared by dissolving 264.5 mg of PbI2, 94.0 mg of FAI, 37.2 mg of PbBr2 and 11.3 mg of MABr in 400 µl of DMF and 100 µl of DMSO (Ye et al., 2018). The solution was heated up to 60 °C before deposition. 2.5. Device fabrication
2. Experimental
The schematic structure of the fabricated planar perovskite solar cells is shown in Fig. 1. The NiOx (HTL), perovskite, PC60BM (ETL) layers were sequentially spin-coated on cleaned ITO/glass substrates. ITO glass substrates were cleaned by sonication in detergent (2% Hellmanex® III solution), DI water, acetone and IPA. After drying, the substrates were UV ozone treated for 10 min at 100 °C right before deposition. NiOx ink was spin-coated on the ITO substrates at 3000 rpm for 45 s; the substrates were then dried on a hot plate at 100 °C for 10 min. Then, perovskite film of FA0.81MA0.15Pb(I0.836Br0.15)3 was deposited by spin-coating the perovskite precursor solution with antisolvent technique in a glovebox (Ye et al., 2018). The spin coating was done at 2000 rpm for 10 s and then at 6000 rpm for 30 s. CB (100 µl) was dropped on the substrate ~10 s into the second stage of spincoating. Upon finishing, the substrate was promptly transferred on a hot plate at 100 °C and kept for 50 min. The PCBM solution (23 mg/ml in DCB) was spin-coated at 1300 rpm for 40 s to form an ETL on top of the perovskite layer. The substrate was again annealed at 100 °C for 5 min and cooled down before the electrode deposition.
2.1. Material All anhydrous solvents, including isopropanol alcohol (IPA), N,Ndimethylformamide (DMF), dimethyl sulfoxide (DMSO), chlorobenzene (CB), dichlorobenzene (DCB) were purchased from Sigma Aldrich. Nickel nitrate hexahydrate (99.999%) and bathocuproine (BCP) were also purchased from Sigma Aldrich. PC60BM (99.5%) was purchased from Solenne BV. Formamidinium iodine (FAI) and methylammonium bromide (MABr) were purchased from Dyesol. PbI2 and PbBr2 were purchased from Tokyo Chemical Industry Co., Ltd. AgNWs with average diameter/length (D/L) of 40 nm/45 µm in IPA were purchased from ACS Material (SKU: NWAG04I1). AgNWs with average D / L = 80 nm/81 µm were synthesized. Patterned indium tin oxide (ITO) glass substrates (10 Ω/□) were purchased from Xinyan Technology Ltd. 2.2. Synthesis of NiOx ink
Deposition of top electrodes
The preparation of NiOx nanoparticle ink is similar to a previously reported procedure (H. Zhang et al., 2016). Ni(NO3)2 (3.635 g) was dissolved in 2.5 ml of water. Ni(OH)2 was then precipitated with ~ 2.5 ml of 10 M NaOH solution. The light green precipitation was collected by centrifugation and washed thrice with deionized (DI) water. Afterwards, it was transferred to an oven at 80 °C and dried until the colour turned dark green. This solid was then transferred to an oven at 270 °C and kept for 2 h. The final product was a black powder of NiOx nanoparticles. The NiOx nanoparticles were then dispersed in DI water at 2.6 wt%. After sonicating for 2 h, the ink was filtered through a 0.45 µm PVDF filter. For best results, the ink was used at least one day after filtration.
(1) Spin-coating of AgNW electrodes On the PCBM layer, a AgNW top electrode was deposited by spincoating the AgNW dispersion (10 mg/ml) at 900 rpm for 30 s and dried at 90 °C for 30 s. The patterned area of the AgNW electrode was delimited by polyimide tape as mask. The active cell area was defined by the overlap of the patterned ITO and the patterned AgNW electrode, typically 0.09 cm2 for each cell. The electrode thickness can be increased by repeating the spincoating process. Devices with 1, 2 and 3-layered AgNW electrodes were fabricated. To deposit the PCBM or C60 filler, a 10 mg/ml solution of the respective filler was spin-coated on the AgNW layers at 1300 rpm for 45 s.
2.3. AgNW ink preparation
(2) Thermal evaporation of silver film electrodes
As-received AgNW dispersion was first purified by gentle centrifugation at 2000 rpm for 30 min. The AgNW precipitate was then redispersed in anhydrous IPA. Before using the AgNW dispersion, a sonication of not more than 30 s was performed.
For comparison, control devices with thermally evaporated Ag top electrode were also fabricated. BCP (0.5 mg/ml solution in anhydrous 583
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IPA) was first spin-coated on the PCBM ETL at 5000 rpm for 30 s. The devices were then masked, and silver was deposited under a vacuum condition of 5 × 10−6 mbar at about 0.1 Å/s. 2.6. Characterizations The surface morphologies of the AgNW films were investigated using a microscope (Olympus BX51), and a SEM (JEOL JSM-7001F) at 10 kV. Film transmittance spectra were recorded using a UV–visiblenear IR scanning spectrophotometer (UV-3600, Shimadzu). Film thicknesses were measured using a surface profiler (KLA-TENCOR P10). The work function of the electrode was measured using a photoelectron spectrometer (Riken Keiki, AC-2). The photoluminescence (PL) measurements were conducted using Shimadzu UV-2501 spectrofluorophotometer with an excitation wavelength of 520 nm. Current density-Voltage (J-V) characteristics for solar cell devices were measured in a nitrogen atmosphere with a solar simulator (SAN-EI Electric XES-301S 300 W Xe lamp JIS Class AAA) which was calibrated to 1000 W/m2 with a reference Si cell. For the stability test, the devices were stored in a nitrogen glovebox (H2O < 5 ppm, O2 < 10 ppm).
Fig. 2. Schematic drawing of the contact between the AgNW and the planar substrate of (a) the contact between a cylinder form nanowire and the planar ETL-1 surface, and b) suspended portion of AgNW, resulting in a gap between the wire and ETL. (c) and (d) show the filling of a second layer of ETL-2 (filler) in between the AgNW and the planar ETL-1.
3. Results and discussion
Han et al., 2018). Considering the cylindrical form of the AgNWs, the contact area between the nanowires and the planar ETL would be very small as shown in Fig. 2a. Additionally, the random overlapping of the AgNWs will result in a considerable portion of the nanowires being suspended and not in contact with the ETL as schematically shown in Fig. 2b. To improve the morphology of the contact between AgNWs and the planar ETL (hereby denoted ETL-1), a second ETL (ETL-2) was spincoated on the AgNW electrode as a filler layer, resulting in a nanocomposite electrode of AgNW and ETL-2. As shown in Fig. 2c and d, ETL-2 fills up the nano-scale voids between the AgNWs and the substrate, hence significantly increasing the contact area at the AgNW / ETL interface and decreasing the interfacial electrical resistance. For simplicity’s sake, PCBM was chosen as ETL-2 and spin-coated on the AgNW layer using a PCBM/CB solution in a lower concentration (10 mg/ml), forming a AgNW-PCBM nanocomposite electrode. Devices using AgNW-only and AgNW-PCBM electrodes were fabricated (with device structure of ITO/NiOx/perovskite/PCBM/AgNW or AgNWPCBM). Fig. 3a and b show the surface SEM images of the AgNW and AgNW-PCBM electrodes fabricated on two devices. As seen, the pure AgNW electrode (Fig. 3a) shows smooth surfaces on both the AgNWs and the bottom planar PCBM layer, while the AgNW-PCBM electrode (Fig. 3b) shows rougher surfaces on both the nanowires and the underlying PCBM layer. A closer contact between the AgNWs and the ETL is observed in Fig. 3b; some nanowires at the bottom can be seen partially embedded into the PCBM layer. It can be observed that the spincoating of the diluted PCBM solution not only resulted in the filling of the nano-scale voids between the substrate and the nanowires, but it also helped to form an ultrathin coating on the surface of each individual nanowire. It was often thought that the spin-coating of the second PCBM will dissolve and wash away the first planar PCBM layer. In this case, however, the AgNW network can act as a protective net to prevent the critical portion of PCBM under the AgNWs from being
As shown in Fig. 1, the planar perovskite solar cell has a structure of glass/ITO/NiOx/Perovskite/PCBM/AgNWs. All the active layers were sequentially deposited on the glass/ITO substrate by spin-coating. Asreceived AgNW dispersion contains traces of harmful solvents or surfactants which manifests in a colour change in the perovskite/PCBM surface upon contact. Thus, a solvent purification process, as described in the experimental section, is necessary. This procedure could enable a wider range of scalable fabrication methods such as drop casting, spray coating, Mayer-rod coating and various printing techniques. All the mentioned methods were reported to yield highly conductive and transparent AgNW films (Kwon et al., 2018). The AgNW size influences the film morphology and the ETL/AgNWs interface, which is crucial for realizing high performance translucent solar cells. Usually, larger sizes (diameter and length) of AgNWs yield lower sheet resistance. However, this creates larger voids in the film and hence, smaller contact area with the substrate (F. Han et al., 2018). AgNWs with diameters of 20–50 nm and aspect ratios 500–1000 are commonly used for perovskite solar cells fabrications (Bailie et al., 2015; Guo et al., 2015; K. Han et al., 2018; Xie et al., 2018). Here, high aspect ratio AgNWs with average size of D/L = 40 nm / 45 µm (aspect ratio ~1000) were used for devices fabrication. Bigger sizes of AgNWs with average D/L = 80 nm/81 µm were also tested for its characteristics as device electrodes, but it yielded much poorer device performance as compared to the AgNWs of D/L = 40 nm/45 µm (see Fig. S1 and Table S1 in the supporting information). Thus, AgNWs with an average D/L = 40 nm/45 µm were used for all the following studies. The PCEs of the devices with pristine AgNW layers coated on top of the ETL was low (< 6.18%, see Table 1). The voids among the AgNW network, and higher electrode sheet resistance (to be discussed later) might have contributed to the poor performance. However, the contact area between the AgNW electrode and the planar PCBM ETL is believed to be a major factor limiting the device performance (Dai et al., 2016; K. Table 1 Performance summary of PSCs with different AgNW top electrodes. AgNW Electrode
VOC(V)
Jsc (mA/cm2)
FF
PCE (%)
PCEmax (%)
Rs** Ω
Rsh Ω
AgNW AgNW-PCBM AgNW-C60
0.87 ± 0.02* 0.88 ± 0.01 0.92 ± 0.02
12.91 ± 0.54 14.17 ± 0.30 15.72 ± 0.55
0.50 ± 0.03 0.64 ± 0.02 0.60 ± 0.03
5.64 ± 0.43 7.90 ± 0.23 8.68 ± 0.35
6.18 8.09 9.17
550 110 130
~50000 ~80000 ~100000
* Data were taken from 6 to 8 sample devices. ** Rs and Rsh are calculated from the best device. 584
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Fig. 3. Surface SEM images of (a) 1-layered AgNW, (b) 1-layered AgNW-PCBM and (c) 3-layered AgNW electrodes fabricated on devices, and (d) a cross section image of a full device with structure of glass/ITO/NiOx/perovskite/PCBM/AgNW-C60.
8.09% to 9.17%), with general increases in Jsc, Voc as well as FF. Hysteresis can also be observed in the JV measurement of the AgNWC60 devices but it is shown to be smaller than in the AgNW devices (see Supporting Information Fig. S2 and Table S2). The overall performance improvement can be ascribed to a favourable energy alignment at the C60/AgNW interface, high electron mobility of the C60 filler and its passivation of the surface defects. The obtained results concur with previous reports that C60 can improve device performance when used in conjunction with or to replace PCBM ETL (Liang et al., 2015; Namkoong et al., 2018). It must be highlighted that we also attempted to spin-coat a C60 layer directly on a planar PCBM film before the deposition of Ag electrode by vacuum thermal evaporation. All the resulting devices failed because the C60 spin-coating washed away the existing PCBM in the absence of the protective AgNW mesh. This again indicates the uniqueness of the solution-processable AgNW-C60 nanocomposite electrode. To further prove the effectiveness of the composite electrodes in extracting electrons from the perovskite layer, steady state PL was measured on samples with the structure of glass/perovskite/planar PCBM/AgNW-electrode, with incident light from the AgNW-electrode side. Electron transfer from the perovskite layer to the electrode will suppress the PL emission of the photoexcited perovskite. Fig. 4 shows the PL spectra of 3 samples with different electrodes of pristine AgNW, AgNW-PCBM and AgNW-C60, having peaks at 778 nm, 776 nm and 774 nm, respectively. The PL peaks are from the perovskite emission (Ye et al., 2017). As seen, pristine AgNW electrode produced the highest PL peak. The PL intensities were remarkably reduced when AgNWPCBM or AgNW-C60 were used as the electrodes, with the lowest intensity from the sample with AgNW-C60 electrode. Compared to the pristine AgNW sample, the lower PL intensity observed for the AgNWPCBM sample suggests an enhanced interface (contact) morphology facilitating electron flow from the perovskite. The lowest PL peak from the AgNW-C60 sample reaffirms the superior electron extraction capability of C60. It is noted that the PL quenching by the AgNW-C60 (or AgNW-PCBM) electrode interface is more efficient at long wavelength emission than short wavelength emission, hence, resulting in a blueshift of the peak after quenching, as shown in Fig. 4. Similar quenching and blue-shift effects were also reported by others (Namkoong et al.,
washed away. Notwithstanding, further precaution was exercised to reduce the risk of dissolving ETL-1. The second PCBM (ETL-2) layer was spin-coated using CB instead of DCB solvent, since the former dries faster and has lower solubility for PCBM (Wang and Hua, 2015). The statistical performance of the devices with pure AgNW and AgNW-PCBM composite electrodes are summarized in Table 1 (the first two rows) Table 1 shows that AgNW based devices without the filler have low PCEs (average 5.64%) due to poor Jsc (12.91 mA/cm2) and low FF (0.50). The very high series resistance (550 Ω) arose from the poor contact at the ETL/AgNW-electrode interface. In contrast, devices with AgNW-PCBM electrodes (with PCBM filler) possessed a much higher PCE (7.90%) due to increased Jsc (14.17 mA/cm2) and FF (0.64). Remarkably, the addition of the PCBM filler reduced the series resistance of the device to 110 Ω. The changes in the Jsc¸ FF and series resistance indicate that the PCBM ETL-2 filler has significantly improved the contact between AgNWs and PCBM-ETL. The PCBM between AgNW and perovskite facilitates electron extraction. However, the AgNW/PCBM interface is still not ideal for electron collection since a large energy gap of 0.7 eV exists between PCBM and AgNW (as shown in Fig. 1). The work function of AgNWs, as measured, is about 4.7 eV, higher than that of thermally evaporated silver films (4.4 eV) while the LUMO of PC60BM is commonly reported to be 4.0 eV (Docampo et al., 2013). A favourable energy alignment is necessary to form a good ohmic contact between the ETL and the electrode (Chen et al., 2016; Xie et al., 2018). C60, with a suitable LUMO level of 4.5 eV (Yan et al., 2015), is expected to improve electron extraction through a cascade energy level alignment cathode configuration, PCBM (4.0 eV)/C60 (4.5 eV)/AgNWs (4.7 eV). Furthermore, C60 electron mobility (1.6 cm2Vs−1) far exceeds that of PCBM (6.1 × 10−2cm2Vs−1) (Liang et al., 2015). Elsewhere, it was shown that C60 can effectively extract electrons from the perovskite layer (D. Liu et al., 2018). To further improve device performance, C60, instead of the PCBM ETL-2, was spin-coated on the AgNW layer to form a AgNW-C60 composite electrode. The performance of the AgNW-C60 based devices is presented in Table 1 (the last row). As shown in Table 1, the replacement of PCBM filler with C60 filler remarkably improved the device performance. The efficiency sees a relative improvement of 13% (from 585
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750 nm to 1350 nm and 0–20% in the visible light range (below 750 nm). The low transmittance in the visible light range is due to strong absorption of the thick perovskite layer (~600 nm, as shown in Fig. 3d). The transmittance, sheet resistance of the AgNW films and the corresponding devices’ performance are summarized in Table 2. As the thickness is increased from S1, S2 to S3, the transmittance peak of the AgNW films decreases from 65%, 55% to 40% (Fig. 5), the sheet resistance is reduced from 8.4, 8.2 to 4.1 Ω/square, and the devices average PCE increases from 8.51%, 9.03% to 9.44%, with the best PCE increasing from 10.09%, 10.32% to 11.02% (see Table 2). For comparison, devices with thermally evaporated silver electrode (Ag-TE) were fabricated, with the structure of /ITO/NiOx/Perovskite/ PCBM/BCP/Ag-TE; they are hereafter referred to as Ag-TE devices. The BCP cathode buffer layer was simply spin-coated on the planar PCBM layer. This BCP layer completes a cascade energy level alignment cathode configuration for a planar Ag-TE electrode (Chen et al., 2017), and prevents the harmful diffusion of evaporated Ag into the active layers (Philipp et al., 2015; Vogel et al., 2006). The performance of the Ag-TE devices is summarized in Table 2. The JV curves of the best AgTE device and the best S3-C60 device are presented in Fig. 6. As seen, the statistical performance of devices with spin-coated AgNW-C60 electrodes (S3-C60) is comparable to that of devices with thermally evaporated Ag thin film electrodes (Table 2). The best S3-C60 device even has a higher PCE (11.02%) than the best Ag-TE device (10.84%), due to the higher photocurrent obtained from the former device (see Fig. 6 and Table 2). However, all the AgNW devices exhibit a larger performance variation than the Ag-TE devices, as shown in Table 2. This is probably due to the random and inhomogeneous distribution of AgNWs. Realizing a uniform distribution of AgNWs in the film is one of the challenges for spin-coated AgNW films. Spray coating is a suitable method for fabrication of large-area and uniform AgNW films (K. Han et al., 2018). However, it remains to be investigated whether a spray coated C60 can realize a similar PCBM/AgNW-C60 interface morphology and nanowire surface coating as obtained in spincoated C60. In any case, further device performance improvements should be expected from a more uniform distribution of the AgNWs. Stability issue has been consistently reported for PSCs with AgNW electrodes. This distinctive instability is often attributed to the reactivity of the AgNWs with large exposed surface area (Guo et al., 2015; J. Zhang et al., 2016). Fig. 7 shows the normalized efficiency change with the storage time of three devices with top electrodes of S3, S3-C60 and S1-C60. As seen, the S3 device without filler rapidly degrades to 80% of its original efficiency after only 3 days of storage, while the S3C60 device shows significantly improved stability, virtually maintaining its original efficiency within 52 days of storage. In terms of storage stability, the S3-C60 device outperforms most reported AgNW based devices (J. Zhang et al., 2016; Liu et al., 2017), and is comparable to reported Ag-TE based devices (Teo et al., 2019; Wang et al., 2018; Zheng et al., 2018) (also see Tables S3 and S4 in Supporting Information). It is widely agreed that the degradation of the silver nanowire network is caused by its reaction with iodine compounds diffusing from the perovskite layer (Kato et al., 2015; Niu et al., 2015). Shielding the nanowire network from the iodine ions by using an additional oxide (Kim et al., 2016; Lee et al., 2018) or polymer (Jin et al., 2018) layer is an effective method to hamper the degradation. The improved stability in our AgNW-C60 based devices can be attributed to the shielding effect of the C60 layer (Fang et al., 2017; Liu et al., 2015; D. Liu et al., 2018). The compact molecular structure allows C60 to be densely packed in the filler layer, giving rise to its excellent protective capability. The C60 filler not only protects the nanowires, but also forms a hydrophobic cover on the devices which hinders the degradation of the perovskite layer by preventing harmful moisture ingression. Fig. 7 also shows that the S1-C60 device (with the thinner electrode) is less stable than the S3C60 device. This is reasonable because the thicker electrode, with denser AgNWs, can retain the critical conductivity of the AgNW
Fig. 4. PL spectra of samples with a structure of glass/perovskite/PCBM/electrode, where the electrodes are pristine AgNW, AgNW-PCBM and AgNW-C60.
2018; Shao et al., 2014). Light transmittance and electrical conductivity are two important properties of a AgNW top electrode, particularly when it is used to construct translucent solar cells. AgNW films have a random mesh structure, with the voids among the nanowires ranging from nanometer to micrometer in size, depending on the distribution density of the nanowires in the film. The device area beneath a large-sized void is a non-active area with negligible photocurrent generation because a significant proportion of carriers generated in this region cannot travel the large distance required to reach the AgNW network (Kumar, 2017). Generally, thinner AgNW films have higher transparency but also larger non-active area and higher sheet resistance, and thus result in poorer device performance. Thicker AgNW films, with smaller non-active area and lower sheet resistance, will result in devices with higher efficiency. PSCs of the same configuration (ITO/NiOx/perovskite/PCBM/ AgNW-C60) but with thicker AgNW films were studied. To increase the AgNW film thickness, multiple layers of AgNW were sequentially spincoated on the substrate. One-layered, two-layered and three-layered AgNW films are denoted S1, S2 and S3, respectively. The corresponding AgNW-C60 composite films are denoted S1-C60, S2-C60 and S3-C60, respectively. Multi-layered AgNW films show a denser distribution of nanowires, as shown in the SEM image in Fig. 3c. The transmittance of films of S1, S2, S3 and the S1-C60 device are presented in Fig. 5. The AgNW films show semi-transparency over a broad wavelength range and the transparency is observed to decrease with the film thickness. The highest transmittance of 65% is obtained from S1. The S1-C60 device shows transmittance of 30–45% in the wavelength range from
Fig. 5. Transmission spectra of AgNW films with different number of layers, S1 (1 layer), S2 (2 layers), S3 (3 layers), and a full device with S1-C60 electrode. 586
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Table 2 Performance summary of devices with top electrodes of varying AgNW thickness, and a thermally evaporated silver thin film (Ag-TE). Electrode
T* (peak)
Sheet resistivity (Ω/□)
Device performance VOC (V)
S1-C60 S2-C60 S3-C60 Ag-TE (75 nm)
65% 55% 40% 0%
8.4 8.2 4.1 < 0.5
0.95 0.95 0.97 0.99
± ± ± ±
**
0.04 0.03 0.03 0.02
Jsc (mA/cm2)
FF
17.33 16.33 16.35 15.75
0.52 0.58 0.59 0.62
± ± ± ±
0.23 0.69 0.77 0.82
PCE (%) ± ± ± ±
0.07 0.03 0.04 0.04
8.51 9.03 9.44 9.61
± ± ± ±
PCEmax (%) 0.98 0.53 0.80 0.56
10.09 10.32 11.02 10.84
* T is the Ag or AgNW film transmittance at 650 nm wavelength. ** Data were taken from 7 to 20 sample devices.
solar cells. The electrode was fabricated by a simple sequential spincoating method at low temperature (< 100 °C). The C60 filler spincoated on the AgNW network realized a high contact area and favourable cascade energy level alignment at the interface between the PCBM ETL and the AgNW electrode. As a result, devices with AgNWC60 electrodes outperformed devices with pristine AgNW electrodes. By adjusting the thickness of the AgNW film, the AgNW-C60 devices attained a PCE of 11.02%. Moreover, the AgNW-C60 devices also showed remarkable stability, showing virtually no decline in efficiency after 52 days of storage in a N2-filled glovebox, which can be ascribed to the C60 filler providing protection for the AgNW and perovskite layers. The efficiency and stability of the AgNW-C60 devices were comparable to conventional devices with thermally evaporated electrode. Hence, this low-temperature solution-processed and semi-transparent AgNW-C60 top electrode presents a promising path for developing fully-printable, flexible and translucent photovoltaic devices. Declaration of Competing Interest
Fig. 6. JV curves of the best S3-C60 device (red square) and Ag-TE device (black circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was financially supported by A*STAR SERC Grant (1425203141, A18A1b0045). The authors thank Hui HUANG from Singapore Institute of Manufacturing Technology, A*STAR, for providing partial AgNWs materials fabricated in their lab. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.12.038. References Aurang, P., Doganay, D., Bek, A., Turan, R., Unalan, H.E., 2017. Silver nanowire networks as transparent top electrodes for silicon solar cells. Sol. Energy. https://doi.org/10. 1016/j.solener.2016.11.021. Bailie, C.D., Christoforo, M.G., Mailoa, J.P., Bowring, A.R., Unger, E.L., Nguyen, W.H., Burschka, J., Pellet, N., Lee, J.Z., Grätzel, M., Noufi, R., Buonassisi, T., Salleo, A., McGehee, M.D., 2015. Semi-transparent perovskite solar cells for tandems with silicon and CIGS. Energy Environ. Sci. 8, 956–963. https://doi.org/10.1039/ C4EE03322A. Chen, C., Zhang, S., Wu, S., Zhang, W., Zhu, H., Xiong, Z., Zhang, Y., Chen, W., 2017. Effect of BCP buffer layer on eliminating charge accumulation for high performance of inverted perovskite solar cells. RSC Adv. 7, 35819–35826. https://doi.org/10. 1039/c7ra06365b. Chen, W., Zhu, Y., Yu, Y., Xu, L., Zhang, G., He, Z., 2016. Low cost and solution processed interfacial layer based on poly(2-ethyl-2-oxazoline) nanodots for inverted perovskite solar cells. Chem. Mater. 28, 4879–4883. https://doi.org/10.1021/acs.chemmater. 6b00964. Dai, X., Zhang, Y., Shen, H., Luo, Q., Zhao, X., Li, J., Lin, H., 2016. Working from both sides: composite metallic semitransparent top electrode for high performance perovskite solar cells. ACS Appl. Mater. Interfaces 8, 4523–4531. https://doi.org/10. 1021/acsami.5b10830. Docampo, P., Ball, J.M., Darwich, M., Eperon, G.E., Snaith, H.J., 2013. Efficient
Fig. 7. Normalized PCEs of devices with S3 AgNW (black circle), S3-C60 (red square) and S1-C60 (blue triangle) top electrodes over the storage period (storage condition: in nitrogen filled glovebox with H2O < 5 ppm, O2 < 10 ppm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
network for longer time even after some AgNWs are corroded. And, thicker electrode also has more C60 covering on the active layer, which can provide better protection to the perovskite.
4. Conclusions A unique AgNW-C60 composite film with high conductivity and semi-transparency was developed as the top electrode of perovskite 587
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