13.2% efficiency Si nanowire/PEDOT:PSS hybrid solar cell using a transfer-imprinted Au mesh electrode

In recent years, inorganic/organic hybrid solar cell concept has received growing attention for alternative energy solution because of the potential for facile and low-cost fabrication and high efficiency. Here, we report highly efficient hybrid solar cells based on silicon nanowires (SiNWs) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) using transfer-imprinted metal mesh front electrodes. Such a structure increases the optical absorption and shortens the carrier transport distance, thus, it greatly increases the charge carrier collection efficiency. Compared with hybrid cells formed using indium tin oxide (ITO) electrodes, we find an increase in power conversion efficiency from 5.95% to 13.2%, which is attributed to improvements in both the electrical and optical properties of the Au mesh electrode. Our fabrication strategy for metal mesh electrode is suitable for the large-scale fabrication of flexible transparent electrodes, paving the way towards low-cost, high-efficiency, flexible solar cells.


Chemical stability
Transparent electrodes should be able to withstand chemical attack in order to ensure durable device performance. For this reason, the chemical stabilities of Au mesh electrode and ITO electrode were evaluated by directly measuring sheet resistance after the samples were spincoated with acidic PEDOT:PSS (CLEVIOS PH1000) solution. The PEDOT:PSS solution has a potential of hydrogen (pH) value of 1.52.5. And, it has been well-known that metal-based transparent electrodes can be etched when it is exposed to PEDOT:PSS solution. Figure S2 shows the changes in the sheet resistance of the Au meshes and ITO during chemical stability test. The change in resistance is expressed as R/R 0 , where R 0 is the primary resistance and R is the measured resistance after chemical stability test. The R/R 0 value of the ITO electrode sample remarkably increased after 4 days. However, transfer-imprinted Au mesh electrode exhibited nearly constant R/R 0 value throughout the chemical stability test for 20 days. It is well-known that Au resists attacks by acid solution. Actually, the morphology of the Au mesh has not changed after this test (not shown in this article). These experimental results clearly indicate that Au mesh electrode could maintain the solar cell performances from the chemical attack.

Thermal stability
Highly robust transparent electrodes should be able to withstand high-temperature in the postprocessing stages in order to ensure superior device performance. For this reason, the thermal stabilities of the Au mesh electrode and ITO electrode were evaluated by directly measuring sheet resistance when the all transparent electrode samples were placed on a high-temperature (200, 250, 300, and 350°C) hot-plate for 60 min. In this work, Cu and Ag mesh electrodes were 3 also prepared using the same method for comparison. Figure S3ac shows the changes in the sheet resistance of the all transparent electrode samples during this test. The inset of the Figs.
S3ac shows the FESEM image of the Cu, Ag, and Au mesh electrodes on glass substrate after thermal stability test. Ag shown in Fig. S3a, the Cu mesh electrode showed stable electrical performance at 200°C for 60 min. However, sheet resistance increased significantly when it was heated to 250°C. In order to investigate the cause of the performance degradation, FESEM and energy dispersive spectroscopy (EDS) analyses of Cu mesh electrode were performed before and after the thermal stability test. As shown in the inset of the Fig. S3a, the Cu mesh showed a practically negligible change in its morphology after annealing at 250°C. However, as shown in the Fig. S3d, EDS analysis confirmed the thermal oxidation of the Cu mesh electrode after thermal stability test. The Ag mesh electrode showed stable electrical performance at 200 and 250°C for 60 min, as shown in the Fig. S3b. However, its sheet resistance increased rapidly when it was heated to 300°C. This is due to the morphology change of the Ag mesh after annealing at 300°C. Actually, as shown in inset of the Fig. S3b, annealing at 300°C for 20 min caused the Ag mesh to break up. The ITO glass showed stable electrical performance at 300°C for 60 min. However, as shown in Fig. S3c, the sheet resistance of the ITO glass slightly increased from 15.2 to 45.4 Ω/sq, even after exposure at 350°C for 60 min. In contrast, the Au mesh electrode showed stable electrical performance at high temperature (350°C), reflecting its excellent thermal stability, as shown in the Fig. S3c. As shown in the inset of Fig. S3c, the morphology of the Au mesh has not changed after annealing at 350°C for 60 min. This observation may be due to the higher thermal stability (higher melting point and low reactivity with air) of Au. This experimental result clearly indicates that thermal stability of the Au mesh electrode is higher than that of ITO electrode. 4

Structural and mechanical properties
Strong adhesion between metal-based transparent electrode and substrate is essential for practical use. For this reason, the fabricated various metal meshes on glass were subjected to a repeated adhesive tape test to evaluate the adhesion between metal mesh film and substrate. The adhesion tape test was done as follows: An adhesive tape (810, 3M) was firmly pressed onto the surface of the transparent electrodes and slowly peeled off. For comparison, a sample of ITO glass was also tested. Figure S4 shows the changes in sheet resistance of all transparent electrode samples during the test. As shown in Fig. S4, the sheet resistance of the ITO glass is almost the same as before the adhesion test. Similarly, the sheet resistance of the metal mesh electrodes remained unchanged after 10 cycles. This suggests that the adhesion between the metal mesh film and substrate was very strong even without any additional treatments.
The mechanical flexibility of transparent electrodes is very important to apply in flexible optoelectronic devices. For this reason, the Au mesh on polyethylene terephthalate (PET) film and ITO on PET film were rolled around a bending radius of 5 mm, and then unrolled at a speed of 1.0 cycle/s, after which the sheet resistance of each transparent electrode was compared to its initial value. Figure S5a shows the changes in sheet resistance of the Au mesh and ITO samples during bending-fatigue test. As shown in Fig. S5a, the ITO on PET film appeared to have the resistance increased sharply after only a few cycles. This is because ITO film on flexible substrate is brittle and easily cracks. Inset of Fig. S5a shows a microscope image of the ITO on PET film after this test. As shown in the inset of Fig. S5a, the ITO on PET film shows many cracks on its surface after mechanical flexibility test. In contrast, the Au mesh on PET film 5 showed stable electrical performance after 10,000 bending cycles. This indicates that the Au mesh on PET film possessed high mechanical flexibility. Figure S5b shows an image of a red light-emitting diode by using an electrically connected flexible and transparent Au mesh electrode.

Long-term stability
The long-term stability of transparent electrode is of prime importance for its practical use in optoelectronic devices. To evaluate the long-term stability of all transparent electrode samples to oxidation, metal mesh electrodes and ITO electrode were exposed to high humidity conditions (85~90% relative humidity, RH) at 80~85°C for 30 days. In this work, Cu and Ag mesh electrodes were also prepared using the same method for comparison. Figure S6 shows the changes in sheet resistance of all samples during the long-term stability test. As shown in Fig. S6, the sheet resistance of Cu and Ag mesh samples increased significantly during this test.
Especially, the change in sheet resistance of Cu mesh is more rapid than that of Ag mesh. The sheet resistance of the Cu mesh could no longer be measured after only 7 days. This observation may be due to the higher oxidation rate of Cu. It is well-known that Cu is more easily oxidized than Ag when they are exposed to same oxidative conditions. In contrast, the ITO electrode showed very stable electrical performance during long-term stability test. Au mesh electrode also showed stable electrical performance for 30 days, because Au does not readily react with the air and water. This suggests that the performance of Au mesh electrode could maintain for a longer time. These experimental results clearly confirm that the long-term stability of the Au mesh was comparable to that of ITO electrode.

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In this study, we performed a comparative analysis of the durability of various metal (Cu, Ag, and Au) meshes, which were fabricated by using simple transfer imprinting method. As a result, we propose a highly robust and flexible Au mesh for alternative transparent electrode of ITO.
The Au mesh electrode exhibited excellent optical and electrical properties. In addition, the Au mesh electrode exhibited the excellent long-term stability to oxidation as well as the chemical stability and strong adhesion, comparable to commercial ITO electrode. Surprisingly, the thermal stability and mechanical flexibility of the Au mesh electrode was higher than that of ITO electrode. From these experimental results, we strongly believe that our Au mesh electrode will be broadly applicable to high-performance optoelectronic devices.             Cu mesh Ag mesh Au mesh ITO glass