All-Solution-Processed Silver Nanowire Window Electrode-Based Flexible Perovskite Solar Cells Enabled with Amorphous Metal Oxide Protection
Eunsong Lee, Jihoon Ahn, Hyeok-Chan Kwon, Sunihl Ma, Kyungmi Kim, Seongcheol Yun, and Jooho Moon*
DOI: 10.1002/aenm.201702182
toward the development of solution pro-cessable alternative counterelectrodes, such as printable carbon, to replace ther-mally evaporated Au or Ag.[9–12] However, little attention has been paid toward the replacement of vacuum deposited trans-parent conducting electrodes (TCEs), which require not only high conductivity and high transparency but also high mechanical robustness for flexible devices. The currently used sputtered ceramic-based TCEs suffer from brittleness as well as high cost and process complexity.[13,14] Therefore, vacuum-free flexible TCEs are required to achieve the ultimate goal of all-solution-processed cost-effective and highly efficient flexible PVSCs.
Metal nanowire random networks, especially silver nanowire (AgNW) net-works, are attractive candidates for alter-native TCEs in various optoelectronic devices owing to their low sheet resist-ance, high transmittance, and solution processability.[15,16] Furthermore, the duc-tility of Ag leads to a flexible AgNW net-
work,[17] which makes it suitable for flexible PVSCs. However, there are several hurdles that need to be overcome before the application of AgNWs as TCEs. Most importantly, the chemical stability of AgNWs in the presence of halide perovskites needs to be ensured.[11,18] Especially, when it is employed as a bottom window electrode on which the perovskite precursor solution is subsequently deposited, the halogen ions in the perovskite absorber layer can damage the AgNW severely, resulting in the formation of a silver halide phase.[19] Furthermore, poor adhesion with substrates, rough surfaces, and voids between nanowires can possibly lead to a degradation in the electrical conduction and thermal stability.[20,21]
To prevent the loss of conductivity in AgNWs, attempts have been made to add a protection layer between the AgNW and the perovskite absorber layer. Han et al. reported an AgNW-based TCE together with a ≈100 nm thick pulsed laser-deposited pro-tection layer of ZnO:F; the resulting solar cell exhibited a PCE of 3.29%.[22] Im et al. demonstrated a protected AgNW composite electrode by embedding AgNWs in a glass fabric-reinforced plastic substrate followed by the vacuum deposition of indium tin oxide (ITO).[23] Although this method involved complex pro-cessing steps and a high-cost vacuum process, the fabricated
Silver nanowire (AgNW)-based transparent electrodes prepared via an all-solution-process are proposed as bottom electrodes in flexible perovskite solar cells (PVSCs). To enhance the chemical stability of AgNWs, a pinhole-free amorphous aluminum doped zinc oxide (a-AZO) protection layer is deposited on the AgNW network. Compared to its crystalline counterpart (c-AZO), a-AZO substantially improves the chemical stability of the AgNW network. For the first time, it is observed that inadequately protected AgNWs can evanesce via diffusion, whereas a-AZO secures the integrity of AgNWs. When an optimally thick a-AZO layer is used, the a-AZO/AgNW/AZO composite electrode exhibits a transmittance of 88.6% at 550 nm and a sheet resistance of 11.86 Ω sq−1, which is comparable to that of commercial fluorine doped tin oxide. The PVSCs fabricated with a configuration of Au/spiro-OMeTAD/CH3NH3PbI3/ZnO/AZO/AgNW/AZO on rigid and flexible substrates can achieve power conversion efficiencies (PCEs) of 13.93% and 11.23%, respectively. The PVSC with the a-AZO/AgNW/AZO composite elec-trode retains 94% of its initial PCE after 400 bending iterations with a bending radius of 12.5 mm. The results clearly demonstrate the potential of AgNWs as bottom electrodes in flexible PVSCs, which can facilitate the commercializa-tion and large-scale deployment of PVSCs.
E. Lee, Dr. J. Ahn, H.-C. Kwon, S. Ma, K. Kim, S. Yun, Prof. J. MoonDepartment of Materials Science and EngineeringYonsei UniversitySeoul 03722, Republic of KoreaE-mail: [email protected]
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201702182.
Solar Cells
1. Introduction
Organic–inorganic hybrid perovskites have been spotlighted as absorber layers of solar cells because of their unique proper-ties, such as long charge diffusion length, appropriate bandgap for photovoltaic devices, and high absorption coefficient.[1–3] Recently, a high power conversion efficiency (PCE) of 22.1% has been reported for perovskite solar cells (PVSCs); this is the highest efficiency achieved since the first report of PVSCs in 2009.[4,5] Furthermore, the solution processability and flexibility of perovskite absorbers suggests the possibility of roll-to-roll processes, which can easily be commercialized.[6–8] Meanwhile, a significant number of research efforts have been directed
PVSCs exhibited a high PCE of 14.15% on a flexible substrate. By contrast, Lu et al. introduced solution-processed graphene oxide as a buffer layer for AgNW and achieved a PCE of 7.92% on a flexible substrate.[24] Previously, our group demonstrated the feasibility of using AgNWs as all-solution-processed TCEs in which the AgNWs are sandwiched between combustion sol–gel derived conductive oxides of ITO and ZnO, resulting in a PCE of 8.44% on a rigid substrate.[19] However, it is highly desirable to replace indium in the substrate, which is quite scarce and expensive. Accordingly, in this study, we demon-strate that solution-processed indium-free amorphous Al-doped zinc oxide (a-AZO) can serve as a protective layer for AgNW. A pinhole-free a-AZO layer can form and sufficiently protect the underlying AgNWs, whereas combustion sol–gel derived crystalline AZO (c-AZO) with porous texture cannot protect AgNWs from chemical damage. Interestingly, if the AgNW net-work is not protected sufficiently, AgNW diffuses away to react with the halide compounds. Moreover, this amorphous layer could resolve the issues of thermal instability and poor sub-strate adhesion in addition to decreasing the surface roughness of AgNWs. After optimizing the thickness of the a-AZO layer on AgNWs, indium-free vacuum-free all-solution-processed transparent electrodes exhibit optoelectrical properties compa-rable to those of conventional fluorine doped tin oxide (FTO)
or ITO electrodes. When these AgNWs are utilized as window electrode in PVSCs, high PCEs of 13.93% (on a rigid substrate) and 11.23% (on a flexible substrate) could be achieved, which are comparable to the PCEs of traditional transparent electrode based PVSCs (15.72% on FTO glass and 13.56% on an ITO/polymer electrode). Notably, the PVSCs fabricated with flexible a-AZO/AgNW/AZO-based electrodes can endure 400 bending iterations with a bending radius of 12.5 mm, while maintaining 94% of their initial PCE.
2. Results and Discussion
The chemical instability of AgNWs with respect to halide-based perovskite precursors is the most critical issue, which if not addressed, will impede the application of AgNWs as a bottom window electrode in PVSCs. In order to resolve this problem, composite electrodes were fabricated with a sandwich configura-tion of AZO/AgNW/AZO (where the order of denotation from left to right corresponds with the sequence of layers from the top to bottom), as depicted in Figure 1a. For the bottom AZO, a combustion sol–gel solution, which can dramatically reduce the annealing temperature for crystallization, was used to deposit c-AZO. c-AZO exhibits much higher conductivity than
Adv. Energy Mater. 2018, 8, 1702182
Figure 1. a) Schematic representation of the c-AZO/AgNW/AZO and a-AZO/AgNW/AZO composite electrodes, b) XRD patterns of a combustion sol–gel derived crystalline AZO film and a conventional sol–gel derived amorphous AZO film. The peaks are indexed to hexagonal ZnO (JCPDS #36-1451). SEM images of c) AgNW/AZO, d) c-AZO/AgNW/AZO, and e) a-AZO/AgNW/AZO composite electrodes.
its amorphous counterpart after annealing.[25] In the combus-tion sol–gel solution, an exothermic reaction between the fuel and the oxidant contained in the solution provides internal thermal energy, thus enabling the film to crystallize (i.e., c-AZO) even at a low external thermal heating of 190 °C.[25] Such low-temperature processability enables us to use thermally sensitive plastic substrates, most of which undergo deformation above 200 °C. Subsequently, the AgNW dispersion was spin-coated on the c-AZO thin film, resulting in an AgNW network (Figure 1c). The areal density of the AgNW was determined based on image analysis and the covered area ratio was opti-mized at 28% to achieve the maximum efficiency (Figure S1, Supporting Information). For the top AZO layer, either c-AZO or a-AZO was used, as shown in the scheme depicted in Figure 1a; c-AZO was deposited with a combustion sol–gel solution in a manner similar to the deposition of the bottom AZO layer; however, it should be noted that the a-AZO layer was derived from a conventional sol–gel precursor solution. The deposition methods are described in detail in the Experimental Section.
To analyze the crystallinity of c-AZO and a-AZO, X-ray dif-fraction (XRD) analysis was carried out (Figure 1b). Note that the sample films do not contain bottom c-AZO and AgNW; this condition was maintained to eliminate any interference. The XRD pattern of c-AZO obtained from the combustion sol–gel solution exhibits diffraction peaks attributed to ZnO. It is worth noting that due to the low doping concentration of Al (2 at%), no significant peak shift could be observed. On the other hand, the film from the conventional sol–gel solution shows only a hill-like hump under 40°, which is indicative of an amorphous phase. Therefore, it can be concluded that a-AZO was formed from the conventional sol–gel solution. Scanning electron microscopy (SEM) images reveal the morphological differences between the two AZO films (Figure S2, Supporting Informa-tion). c-AZO presents a crystalline grain structure with a rela-tively rough surface finish and occasional pinholes, whereas a-AZO exhibits a very smooth and compact surface structure without any grain boundaries. The morphological features of the composite electrodes (c-AZO/AgNW/AZO and a-AZO/AgNW/AZO) were also monitored by SEM (Figure 1d,e). It is apparent that both c-AZO and a-AZO upper layers uniformly cover AgNWs in which AgNW random network structure well maintains with a slight bulge formation at the junction due to the annealing at 190 °C (Figure 1c–e and Figure S3, Supporting Information). However, similar to the observations in single-layered c-AZO and a-AZO, c-AZO/AgNW/AZO reveals a rough surface with notable porosity but a-AZO/AgNW/AZO presents a smooth surface texture.
The transmittance spectra demonstrate that AgNW/AZO possesses optical properties comparable to those of a commer-cial FTO electrode in terms of the average visible transmittance (AVT, 86.7% for AgNW/AZO and 85.9% for FTO) defined in the 400–800 nm range (Figure 2a). When the top AZO layer is included in the electrode structure, the resultant composite electrodes show lower transparency than AgNW/AZO. How-ever, between the two sandwiched composite electrodes, the c-AZO/AgNW/AZO electrode exhibits an inferior AVT of 80.6% as compared to a-AZO/AgNW/AZO (84.6%). This could be explained by the fact that the single-layered c-AZO film has a lower optical transparency than the a-AZO film of similar
thickness (Figures S4 and S5, Supporting Information). This discrepancy can be attributed to the differences in the sur-face roughness and refractive indices; a crystalline granular film of c-AZO can effectively reflect incoming light due to its slightly higher refractive index.[26,27] The higher surface rough-ness, resulting from small c-AZO particles, contributes to light scattering and decreased transparency. On the other hand, the smooth a-AZO film exhibits good optical transmittance. In terms of electrical properties, I–V curves of a-AZO/AgNW/AZO and c-AZO/AgNW/AZO electrodes are shown in Figure S6a (Supporting Information). For the measurement, two gold thin films at the interval of 15 µm were thermally evaporated as shown in Figure S6b (Supporting Information). The com-posite electrode with c-AZO upper layer shows lower resistance because of higher conductivity of crystalline AZO compared to its amorphous counterpart.
To evaluate the stability of the AgNW-based composite elec-trodes against chemical attack by the halide ions contained in the perovskite precursor solution, resistance variations were monitored with a two-probe ohmmeter as functions of the aging time after depositing the perovskite film (Figure 2b). All the samples were stored at ambient environmental condi-tions (at room temperature and relative humidity of about 30%) without encapsulation. In the case of the AgNW/AZO electrode without the top AZO layer, an increase in the resist-ance is observed immediately after the deposition of the perov-skite film. In contrast, the c-AZO buffer layer seems to protect AgNW from initial chemical damage. However, after few days of aging in the ambient air, the resistance increases sharply due to the reaction between the iodine ions and AgNW even in the presence of the c-AZO buffer layer. Meanwhile, the a-AZO/AgNW/AZO composite electrode exhibits only a slight increase in the resistance even after being stored under the same conditions for 20 d. This distinct protection capability can be explained by the difference in the surface morphologies of a-AZO and c-AZO. The compact and pinhole-free surface struc-ture associated with a-AZO blocks contact between the iodine ions and AgNW efficiently, while the pinholes and/or grain boundaries in the c-AZO/AgNW/AZO electrode provide regions in which the chemical reaction occurs. Several studies inves-tigated the degradation mechanisms of the metallic electrodes used in PVSCs. In these studies, either an Ag or Au thin film or nanowire electrode was used as the top counterelectrode. One of the mechanisms is that the perovskite phase at the grain boundary is first decomposed into PbI2 and methylammonium iodide.[28–31] The I− and MA+ ions start to diffuse toward the metallic electrode through the charge transport layer. A second mechanism is based on the interdiffusion of both iodine com-pounds and the metallic element under illumination and/or thermal activations.[32–34] In both scenarios, metal iodide forma-tion is induced, which degrades the electrical conductivity of the electrode.
To understand the degradation mechanism in our sandwich-type composite electrode, X-ray photoelectron spectroscopy (XPS) depth profile analysis was performed (Figure 3). Both a-AZO/AgNW/AZO and c-AZO/AgNW/AZO samples were subjected to annealing at 100 °C for 20 h in ambient air after the deposition of perovskite films. This particular annealing temperature was chosen to accelerate degradation, after taking
into consideration the rise in temperature during the actual operation of the solar cell under illumination. Further, an additional 50 nm thick ZnO layer was deposited on top of the two different composite electrodes to exactly simulate a PVSC, where ZnO plays the role of an electron transport layer (ETL). The surface morphology of the ZnO layer, which was derived from the combustion sol–gel method, exhibits a grainy struc-ture and resembles that of c-AZO (Figure S7, Supporting Infor-mation). In the XPS contour plots, the x-axis corresponds to the etching time related to the depth of the multilayer structure, a sequential stack of perovskite/ZnO/c- or a-AZO/AgNW/AZO. In the c-AZO protected composite electrode (Figure 3a), Ag is unexpectedly found in the top regime of the cell stack. Con-sidering the position of other elements, such as Zn, Pb, and I, which indicate the presence of ZnO and the perovskite layer, it is presumed that Ag migrates upward. In contrast, in the case of the a-AZO protected composite electrode, Ag remains inside the Zn-distributed regime, implying that it is immobilized. The cross-sectional structure and elemental distributions of the perovskite/ZnO/AZO/AgNW/AZO structure (Figure 4) agree well with the XPS observations. In the case of the c-AZO pro-tected electrode, Ag is mainly located in the top region where
the perovskite phase is supposed to exist, as indicated by the presence of iodide. The migration of Ag can be manifested by the empty spaces inside the Zn region, where the AgNWs had been located. In contrast, the AgNW domains are present within the Zn regime, without any dislocation or migration, when the a-AZO-based composite electrode was used.
In order to explain this phenomenon, it is worth mentioning that the migration of AgNWs does not arise from prolonged thermal annealing at 100 °C but from the chemical reaction-driven process. Figure S8 (Supporting Information) shows the XPS pattern of the c-AZO/AgNW/AZO sample stored for 2 weeks in the ambient conditions without annealing. Once again, Ag can be found outside the Zn region, analogous to the annealed sample; this implies that Ag can diffuse even at ambient conditions. In addition, the XRD results imply that the c-AZO/AgNW/AZO electrode can maintain its initial state in the absence of the perovskite layer even after annealing at 100 °C. On the other hand, the presence of iodine compounds like PbI2 and CH3NH3PbI3 in the vicinity of AgNW results in the forma-tion of the silver iodide (AgI) phase (Figure S9, Supporting Infor-mation). These facts indicate that the chemical reaction between AgNW and iodine species is the primary event and suggest that
Adv. Energy Mater. 2018, 8, 1702182
Figure 2. a) Transmission spectra of AgNW/AZO, AZO/AgNW/AZO composite electrodes and an FTO electrode. b) Resistance variation in each com-posite electrode after the deposition of perovskite film, as a function of the aging time. The right-side plot presents a magnified view of the left-side plot. The samples were stored at ambient condition of 25 °C and a relative humidity of 30%. The value at 0 d refers to the relative resistance after the deposition of perovskite film on the TCEs followed by annealing at 100 °C, whereas the value before 0 d refers to the initial resistance of the electrodes prior to the deposition of the perovskite film.
Figure 3. Elemental contour plots of the Ag, I, Zn, and Pb XPS signals for the a) CH3NH3PbI3/ZnO/c-AZO/AgNW/AZO and b) CH3NH3PbI3/ZnO/a-AZO/AgNW/AZO structures. Both measurements were followed by annealing at 100 °C for 20 h after the deposition of the perovskite layer.
the formation of AgI is strongly related to the diffusion and van-ishing of AgNW. Consequently, we can propose the protection/degradation mechanisms associated with the a-/c-AZO layers, as depicted in Scheme 1. In the case of c-AZO, iodine species can pass through the permeable pinholes and react with AgNW to form AgI. It is hypothesized that Ag readily diffuses out of
AgNW through the pinholes and/or grain boundaries for further chemical reaction with the iodine species, resulting in an upward diffusion of Ag during the aging process. On the other hand, the pinhole-free and dense a-AZO can act as a robust iodine-blocking layer that can efficiently protect AgNW from chemical damage, and thus it preserves the structural integrity of AgNW.
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Scheme 1. Schematic illustration of the degradation/protection processes of a) c-AZO/AgNW/AZO and b) a-AZO/AgNW/AZO composite electrodes when CH3NH3PbI3 films are formed.
Figure 4. Cross-sectional STEM image and elemental maps of Ag, I, and Zn of the a) CH3NH3PbI3/ZnO/c-AZO/AgNW/AZO and b) CH3NH3PbI3/ZnO/a-AZO/AgNW/AZO structures. Both measurements were followed by annealing at 100 °C for 20 h after the deposition of the perovskite layer (scale bar = 100 nm).
It is also worth noting that the protection ability of a-AZO is verified when the annealing temperature is lowered to 150 °C that is suitable to flexible substrates (Figure S10, Supporting Information).
To fabricate PVSCs based on all-solution-processed AZO/AgNW/AZO composite electrodes, a combustion sol–gel ZnO layer, a CH3NH3PbI3 layer, and a 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9-spirobifluorene (spiro-OMeTAD) layer were deposited as the ETL, absorber layer, and the hole transport layer (HTM), respectively; this operation was fol-lowed by the thermal evaporation of the gold counterelectrode, as depicted in Figure 5a,b. The ZnO layer was deposited from the combustion sol–gel solution because crystalline ZnO is more suitable as an ETL and exhibits better carrier mobility than its amorphous counterpart.[35] Generally, ZnO ETL is known to induce the decomposition of the perovskite layer when in direct contact because of residual acetate ligands and hydroxyl group on its surface.[36] However, it should be noted that the perovskite was not decomposed when placed on our combustion sol–gel derived ZnO layer. This phenomenon can be attributed to the different surface properties of the ZnO films. Combustion sol–gel solution involves nitrate precursor, while conventional sol–gel solution typically uses acetate pre-cursors. Moreover, the combustion reaction can supply local additional heat energy, enabling removal of unwanted hydroxyl groups. Comparison of the stability of the CH3NH3PbI3 layer on various ZnO films is shown in Figure S11 (Supporting Information). Observation of color change clearly confirms that the combustion sol–gel derived ZnO ensures the stability of CH3NH3PbI3 layer. The current density–voltage (J–V) curves of the selected cells are plotted in Figure 5c and the perfor-mance parameters and deviations in each device are displayed
in Figure S12 (Supporting Information) and summarized in Table 1. The PVSC with the AgNW/AZO electrode exhibits a low PCE, resulting in a very low JSC and fill factor. This is pro bably a result of the degradation of AgNW during the deposition of the perovskite film, resulting in a low conductivity and a high series resistance, as shown in Figure 2b. On the other hand, the PVSC with the a-AZO/AgNW/AZO composite electrode exhibits much better performance than the AgNW/AZO elec-trode. This implies that a-AZO acts as an efficient buffer layer; this observation is in sync with the resistance measurement, XPS, and transmission electron microscopy (TEM) results. However, it shows a relatively low performance as compared to the PVSCs with a commercial FTO electrode. The slight drop in the VOC can be attributed to the surface roughness of the AgNW-based electrode, which gives rise to recombination sites near the interface. The difference in the JSC of these two cells, i.e., 19.9 mA cm−2 for the FTO-based cell and 18.0 mA cm−2
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Figure 5. a) Schematic illustration of the perovskite solar cell structure on an AZO/AgNW/AZO composite electrode, b) cross-sectional SEM image of the device, and c) photovoltaic performances of the perovskite solar cells on AgNW/AZO/glass, c-AZO/AgNW/AZO/glass, a-AZO/AgNW/AZO/glass, and FTO/glass electrodes under standard 1 sun AM 1.5 G illumination.
Table 1. Photovoltaic performances of the PVSCs and average values of the device parameters with each electrode.
for the a-AZO/AgNW/AZO electrode-based device, presumably results from the lower transparency and higher sheet resist-ance of the AgNW composite electrode as compared to FTO. The lower fill factor in the composite electrode cell is ascribed to poor charge transport from the ETL through the poorly conductive a-AZO to the AgNW, as defined by the high series resistance. Meanwhile, the c-AZO/AgNW/AZO electrode-based PVSCs exhibit inferior cell performances as compared to the a-AZO/AgNW/AZO electrode-based device, possibly due to transmittance loss and electrode degradation, as discussed ear-lier. The device performance deviation between these types of cells confirms that a-AZO is suitable as a blocking layer against undesirable chemical reactions between Ag and iodine species, thus enabling indium-free all-solution-processed transparent window electrodes for PVSCs for the first time.
The thickness of the top a-AZO layer would significantly affect the device performance and consequently it needs to be opti-mized. At a fixed solution concentration, the top a-AZO layer thickness was varied by varying the number of coating iterations from one (35 nm) to three (105 nm). The transmittance and sheet resistance of composite electrodes with three different thicknesses were measured (Figure S13a, Supporting Information) and the figure of merit (FoM, ΦTC) (Figure S13b, Supporting Information) was plotted using the Haacke equation (Equation (1))
TC
10
S
T
RΦ = (1)
The sheet resistance (RS) increases with an increase in the thickness of a-AZO, whereas the optical transmittance (T) shows a different tendency. The composite electrode with
a-AZO thickness of 70 nm (two times coating) exhibits the highest transparency. This can be explained by analyzing the cross-sectional SEM images (Figure S14, Supporting Infor-mation). When the top a-AZO layer is thin, the surface of the composite electrode is rough like the AgNW, whereas after the second coating, the AZO layer becomes thick enough to reduce the effects of the rough surface, resulting in a flat and smooth morphology. However, when the a-AZO is too thick, it leads to a loss in transparency. In terms of both optical transmittance and sheet resistance, the double-coated a-AZO/AgNW/AZO electrode exhibits the highest FoM value of 25.1 × 10−3 Ω−1 (T: 88.6% @ 550 nm, RS: 11.86 Ω sq−1). The optical/electrical prop-erties of this optimized composite electrode are comparable to those of commercial FTO (T: 86.4% @ 550 nm, RS: 7.50 Ω sq−1).
PVSCs based on the composite electrodes (with varying top a-AZO layer thickness) were also fabricated and their device performances are summarized in Table S1 (Supporting Infor-mation). As expected from the high FoM value, the a-AZO/AgNW/AZO composite electrode with a 70 nm thick a-AZO layer exhibits the best cell performance. We achieved a PCE of 13.93% for the AgNW-based composite electrode cell con-taining the optimally thick a-AZO layer and this performance is comparable to that of a cell based on a commercial FTO electrode, as shown in Figure 6a. Especially, VOC is noticeably increased across the flat surface of the composite electrode, as shown in Figure S14 (Supporting Information), which in turn gives rise to a lesser number of recombination sites at the inter-face. However, when the top a-AZO layer becomes too thick (≈105 nm), the cell performance deteriorates, accompanied by a degradation in all the device parameters. The J–V hysteresis characteristics of the cells based on the optimized a-AZO/
Figure 6. Photovoltaic performances of the champion cell with the optimized a-AZO/AgNW/AZO composite electrode. a) The current density–voltage characteristics, b) J–V hysteresis characteristics measured with a dwell time of 500 ms, c) external quantum efficiency, and d) stabilized current density and PCE of the champion cell.
AgNW/AZO electrode were also measured using a dwell time of 500 ms for both positive (JSC to VOC) and negative (VOC to JSC) scan directions (Figure 6b). A PCE of 13.53% was achieved in the negative scan, whereas the PCE value was 12.15% in the positive scan. The JSC value of the champion cell calculated from the external quantum efficiency (EQE) measurements was 18.15 mA cm−2 (Figure 6c), which corresponds well with the measured value of 18.5 mA cm−2 in the J–V curve. At the stabilized current density and PCE, a constant DC voltage of 0.846 V (which was the point of maximum power) and 1 sun illumination were applied for 300 s, and the change in current density was measured as a function of time (Figure 6d). Both J and PCE gradually increase to the saturation point within 100 s due to the light soaking effect; the stabilized PCE was found to be 12.42%. The overall PCEs are still around 15%, while the reported highest performance of PVSCs is 22.1% to date.[37] However, it should be noted that the fabricated PVSCs are opti-mized for low-temperature process. Moreover, the PCEs show rather high values when compared to the PVSCs with metal nanowire-based electrodes.[19,22–24] Further improvement in per-formance is expected with the optimization of absorber layer by changing its composition as mixed halide or cation, as well as with the optimization of charge transport layers.
Finally, the optimized a-AZO/AgNW/AZO composite elec-trode was utilized to fabricate flexible PVSCs on polyethersul-fone (PES) substrates. Mechanical bending tests were carried out on the commercial ITO and AgNW composite electrodes prior to full device fabrication and the sheet resistance was measured after every 100 cycles. As shown in Figure S15 (Sup-porting Information), the ITO electrode loses its conductivity and exhibits an increase of ≈20 times in its resistance after only 100 cycles with a bending radius of 12.5 mm because of its inherent brittleness. Meanwhile, the sheet resistance of the a-AZO/AgNW/AZO composite electrode does not exhibit any noticeable variation even after 300 cycles of bending with a bending radius of 5 mm, due to the high flexibility of AgNWs. In addition, direct comparison of mechanical durability between the two electrodes by SEM images before and after the bending tests was shown in Figure S16 (Supporting Information). Large occasional cracks were observed in commercial ITO electrode after 400 bending cycles at a bending radius of 5 mm, while no noticeable cracks appeared in the AgNW-based composite electrode. Flexible PVSCs based on the a-AZO/AgNW/AZO
electrode were fabricated with a configuration similar to that used in rigid PVSCs. The cell performances of the flexible device based on our AgNW composite electrode on PES and the reference cell based on an ITO/polyethylene naphthalate (PEN) electrode are shown in Figure 7a and summarized in Table 2. The device parameters of the rigid devices based on both types of electrodes are included. The a-AZO/AgNW/AZO composite electrode-based flexible PVSC achieved a PCE of 11.23%, which is comparable to that of commercial ITO-based flexible PVSCs (13.56%). A slight discrepancy in the PCEs of flexible and rigid PVSCs might arise owing to the uneven surface finish of plastic substrates and their hydrophobic properties, which may reduce the quality of the overlying spin-coated films. Mechan-ical bending tests were performed on the two types of flexible PVSCs and the variation in the normalized PCE is shown in Figure 7b as a function of the number of bending cycles. At a bending radius of 12.5 mm, flexible PVSCs based on AgNW composite electrodes maintain 94% of their initial PCE even after 400 bending cycles, while commercial ITO-based flexible cells retain only 42% of their initial PCE. This observation is consistent with the bending test results of the two electrodes. The slight decrease in the PCE of the AgNW composite elec-trode-based cell can be explained by the structural disintegra-tion of the brittle ZnO ETL under mechanical loading, which not only degenerates the electron transporting ability but also leads to the delamination of upper layers.
3. Conclusions
In summary, all-solution-processed and indium-free trans-parent window electrodes were fabricated with highly
Figure 7. a) Current density–voltage characteristics and b) mechanical bending test results of the perovskite solar cells on ITO/PEN and a-AZO/AgNW/AZO/PES electrodes (bending radius of 12.5 mm).
Table 2. Photovoltaic performances of the champion PVSCs with each electrode and substrate.
conductive and flexible AgNWs. AgNW was protected against chemical attack by introducing an a-AZO buffer layer, which prevents reaction between Ag and iodine species of the perov-skite absorber layer. From structural analysis, it was found that a-AZO has a more compact morphology than c-AZO; hence, a-AZO can efficiently protect AgNW. The a-AZO/AgNW/AZO composite electrode can maintain its conductivity after perov-skite deposition, annealing, and aging in ambient air. Notably, it was observed that when AgNW has a chance of reacting with iodine, it vanishes via diffusion and participates in the chem-ical reaction. At the optimum top a-AZO layer thickness, the a-AZO/AgNW/AZO electrode exhibits a transparency of 88.6% @ 550 nm and a sheet resistance of 11.86 Ω sq−1. The PVSC based on a-AZO/AgNW/AZO exhibits outstanding per-formance with a maximum PCE of 13.93%. Furthermore, when the composite electrode is deposited on flexible substrates, 94% of the initial PCE was retained even after 400 bending cycles. Therefore, our research suggests that AgNW-based composite systems are promising alternative bottom electrodes for PVSCs; our approach precludes the necessity for highly expensive indium element or high vacuum processes.
4. Experimental SectionPreparation of AZO Precursor Solutions: To prepare the AZO precursor
solutions, initially, aluminum nitrate nonahydrate (Al(NO3)·9H2O, 99.997%, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in Zn precursor solutions to obtain Al/(Zn + Al) ratio of 2 at%. To prepare the combustion sol–gel solution for crystalline AZO, zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%, Alfa Aesar, Ward Hill, MA, USA) and zinc acetylacetonate (Zn(C2O5H5)2, 99.995%, Sigma-Aldrich) were dissolved in 2-methoxyethanol (anhydrous, 99.8%, Sigma-Aldrich) at a molar ratio of 1:1. The total concentration of the metal precursors was 0.15 m. The solution was stirred at room temperature for 24 h. To prepare the conventional sol–gel solution for amorphous AZO, zinc acetate dihydrate (Zn(CH3COO)2·2H2O, ≥98%, Sigma-Aldrich) was dissolved in 2-methoxyethanol to prepare a 0.4 m solution. After vigorously stirring the solution at 60 °C for 30 min, ethanolamine (≥99%, Sigma-Aldrich) was added as a stabilizing agent. The solution was then stirred at 60 °C for 2 h followed by 24 h of aging at room temperature.
Fabrication of Composite Electrodes: The AZO/AgNW/AZO composite electrodes were prepared by a successive spin-coating process. Soda lime glasses used as the rigid substrates were cleaned with acetone, distilled water, and ethanol in an ultrasonic bath for 10 min with each solution and treated with O2 plasma for 10 min. PES was used as the flexible substrate after peeling off the protective film and exposing to O2 plasma (similar to the rigid substrate). The bottom AZO layer was deposited by spin coating the combustion sol–gel solution at 2000 rpm for 30 s. After sequentially drying at 150 °C for 2 min and 190 °C for 2 min, the same coating procedure was repeated once again, followed by annealing at 190 °C for 30 min. Later, AgNW dispersion ink (AgNW-25, Seashell Technology, CA, USA) was spin coated on the bottom c-AZO layer, followed by drying at 150 °C for 1 min and rinsing with distilled water to remove any organic contaminants and binders. Finally, the top AZO layer was deposited by spin coating either the combustion solution (for crystalline AZO) or the conventional solution (for amorphous AZO) at 2000 rpm for 40 s and annealed at 190 °C for 30 min. The thickness of the top AZO layer was controlled by controlling the concentration of the solution and number of coating cycles.
Optical, Structural, and Electrical Characterization: The optical transmittances of the transparent electrodes were measured with a UV–vis spectrophotometer (V-670, Jasco, Tokyo, Japan) and the soda lime glass substrate was used for the baseline measurement. The surfaces
and cross sections of the samples were analyzed by field-emission SEM (FE-SEM, JSM-7001F, JEOL Ltd, Tokyo, Japan) and TEM (Talos F200X, FEI, USA). The depth profile analysis was carried out using XPS (K-alpha, Thermo Fisher Scientific Inc., Waltham, MA, USA). The sheet resistances of the transparent electrodes were measured using a 4-point probe system (RS8, BEGA Technologies, Seoul, Korea). All the reported resistances are the averages of at least five measurements. The crystallinities of the films were determined using an HR-XRD instrument (Rigaku Smartlab, TX, USA).
Fabrication of Perovskite Solar Cells: To fabricate the electron transport layer, a 0.15 m ZnO combustion solution (it was prepared with the same recipe as the AZO combustion precursor solution except for the addition of aluminum nitrate nonahydrate.) was coated three times on the composite or commercial electrode substrates, followed by annealing at 190 °C for 30 min. To fabricate the perovskite absorber layer, a 53 wt% mixture of PbI2, CH3NH3I, and dimethyl sulfoxide (Sigma-Aldrich) (1:1:1 molar ratio) was dissolved in dimethylformamide (Sigma-Aldrich) and then spin coated on the ZnO-coated substrates at 4000 rpm for 25 s, followed by drying at 65 °C for 3 min and annealing at 100 °C for 7 min. The HTM precursor solution was prepared by dissolving spiro-OMeTAD (99.7%, Borun molecular, China, 72 mg) in chlorobenzene (1 mL) followed by the addition of 4-t-butylpyridine (28.8 µL) and Li salt solution (520 mg mL−1 lithium bis(trifluoromethylsulfonyl)imide in acetonitrile, 17.6 µL). The HTM precursor solution was spin coated on the perovskite layer at 3000 rpm for 30 s without any additional annealing process. Finally, a 70 nm thick gold electrode layer was thermally evaporated onto the spiro-OMeTAD film.
Characterization of Photovoltaic Properties: The photovoltaic performances of the perovskite solar cells were evaluated in terms of their J–V characteristics with a solar simulator (Sol3A Class AAA, Oriel Instruments, Stratford, CT, USA) and a Keithley 2400 source measurement unit (Keithley Instruments Inc., Cleveland, OH, USA) under air mass (AM) 1.5 and 1 sun (100 mW cm−2) conditions. The 1 sun intensity level was calibrated using a standard Si reference cell certified by the Newport Corporation (Irvine, CA, USA). The active area of the perovskite solar cells was 0.06 cm2, as determined by the aperture mask. Scanning was carried out over the −0.1 to 1.2 V range at a rate of 0.52 V s−1 with a dwell time of 50 ms at each point and at a rate of 0.048 V s−1 with a dwell time of 500 ms for the J–V hysteresis measurement. The EQE was obtained using a quantum efficiency measurement system (QEX10, PV Measurements, Inc.).
Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.
AcknowledgementsThis work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2012R1A3A2026417).
Conflict of InterestThe authors declare no conflict of interest.
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