Embedded Metal Electrode for Organic–Inorganic Hybrid Nanowire

May 22, 2017 - Han-Don Um , Deokjae Choi, Ahreum Choi, Ji Hoon Seo, and Kwanyong Seo. Department of Energy Engineering, Ulsan National Institute of Sc...
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Embedded Metal Electrode for Organic− Inorganic Hybrid Nanowire Solar Cells Han-Don Um, Deokjae Choi, Ahreum Choi, Ji Hoon Seo, and Kwanyong Seo* Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea S Supporting Information *

ABSTRACT: We demonstrate here an embedded metal electrode for highly efficient organic−inorganic hybrid nanowire solar cells. The electrode proposed here is an effective alternative to the conventional bus and finger electrode which leads to a localized short circuit at a direct Si/metal contact and has a poor collection efficiency due to a nonoptimized electrode design. In our design, a Ag/SiO2 electrode is embedded into a Si substrate while being positioned between Si nanowire arrays underneath poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), facilitating suppressed recombination at the Si/Ag interface and notable improvements in the fabrication reproducibility. With an optimized microgrid electrode, our 1 cm2 hybrid solar cells exhibit a power conversion efficiency of up to 16.1% with an open-circuit voltage of 607 mV and a short circuit current density of 34.0 mA/cm2. This power conversion efficiency is more than twice as high as that of solar cells using a conventional electrode (8.0%). The microgrid electrode significantly minimizes the optical and electrical losses. This reproducibly yields a superior quantum efficiency of 99% at the main solar spectrum wavelength of 600 nm. In particular, our solar cells exhibit a significant increase in the fill factor of 78.3% compared to that of a conventional electrode (61.4%); this is because of the drastic reduction in the metal/ contact resistance of the 1 μm-thick Ag electrode. Hence, the use of our embedded microgrid electrode in the construction of an ideal carrier collection path presents an opportunity in the development of highly efficient organic−inorganic hybrid solar cells. KEYWORDS: hybrid solar cells, Si nanowire, metal electrode, microgrid, embedded electrode, PEDOT:PSS

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demonstrated superior light absorption close to the Lambertian limit by employing a nanocone−nanopillar dual structure.5 To improve the open-circuit voltage (VOC) of PEDOT:PSS/Si hybrid solar cells, a thin Si oxide3 or organic layer9,15,20 was introduced between PEDOT:PSS and Si nanostructures, resulting in the suppression of interface recombination. In addition, the PCEs of PEDOT:PSS/Si hybrid solar cells have been improved by using electron transport layers such as amorphous Si,19 titanium dioxide (TiO2),28 and cesium carbonate (Cs2CO3),21 resulting in the suppression of interface recombination and the formation of an ohmic contact at the rear surface. However, despite the tremendous efforts that have been devoted to optimizing the nanostructures, engineering the PEDOT:PSS/Si interface, and modifying the ohmic contact at the rear surface, the overall PCEs of hybrid solar cells have generally been lower than those of nanostructure-based Si solar cells with a conventional p−n junction. One major reason for the low PCE of hybrid solar cells is the nonoptimized front electrode that occurs due to the difficulty in

s a platform for cost-effective crystalline Si (c-Si) solar cells, an organic−inorganic hybrid structure has been proposed to replace the conventional p−n junction. The hybrid structure is composed of a transparent conducting polymer and c-Si.1−24 A Schottky junction at the conducting polymer/c-Si interface can easily be created using a simple spincoating process at room temperature, while a conventional Si p−n junction is formed via a high-temperature doping process.4,21,25 The Schottky junction allows the device to function as a solar cell despite the absence of a thermally formed p−n junction. This is because an energy barrier induced by the work function difference between the conducting polymer and c-Si can efficiently separate photogenerated carriers. Among the various conducting polymers, the conjugated polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely used due to its high transparency and conductivity.26,27 Considerable progress has been made in the improvement of the PCE of PEDOT:PSS/Si hybrid solar cells. In order to enhance the light-trapping efficiency, Si nanostructures such as nanopyramids,2,4,7 nanotubes,6 nanoholes,9 and nanowires1,8,12−14,18,20 have been applied, leading to an increase in the short circuit current density (JSC). Jian et al. © 2017 American Chemical Society

Received: April 3, 2017 Accepted: May 22, 2017 Published: May 22, 2017 6218

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ACS Nano directly forming a low-resistive and robust metal electrode onto the PEDOT:PSS layer. For large-scale solar cells, the specifically designed metal electrode is required to effectively collect photocarriers because long carrier diffusion through the PEDOT:PSS layer will lead to a resistive loss. In practice, the front electrode comprises a bus and fingers and is fabricated through the deposition of a thin Ag film using a shadow mask. However, the light absorption of the solar cells will be restricted due to a shading loss of 8%−10% induced by the nonoptimized bus/finger electrode. The low-resolution shadow mask method cannot form a microscale electrode that can efficiently collect carriers without shading loss. In addition, serious degradation of photovoltaic performance will occur due to an electrical short circuit because the sharp tips of Si nanostructures are frequently in direct contact with the metal electrode.13 However, there is no room for the optimization of the metal electrode in the device structures that have been proposed thus far. Highresolution metallization processes such as screen printing, electroplating, and lithography cannot be applied to hybrid solar cells because they require high temperature and wet processes; thus, they may deform the PEDOT:PSS layer. Therefore, a PEDOT:PSS/Si hybrid solar cell design should be developed for robust and reliable metal contact to prevent both optical and electrical losses. Here we demonstrate highly efficient (16.1%) PEDOT:PSS/ Si nanowire (SiNW) solar cells by designing an embedded electrode formed between the PEDOT:PSS layer and the Si substrate. A key feature of our high-efficiency hybrid solar cells is that the metal electrode is embedded into the Si substrate while being positioned between SiNW arrays underneath the PEDOT:PSS layer. By decoupling regions for light absorption and the robust electrode (planar Si), the embedded metal electrode overcomes an electrical short circuit induced by direct contact between the SiNWs and the metal electrode, leading to a dramatic improvement in the fabrication reproducibility of the PEDOT:PSS/SiNW hybrid solar cells. In addition, since the PEDOT:PSS layer is spin-coated in the final fabrication step after forming the metal electrode, our hybrid solar cells enable the utilization of conventional metallization processes such as lithography and electroplating to optimize the design of the front metal electrode. Thanks to the superior optical transmittance and electrical conductivity of an embedded Ag electrode with the microscale mesh patterns, the JSC and fill factor (FF) could be significantly improved compared to a bus/ finger electrode. As a result, our PEDOT:PSS/SiNW hybrid solar cells achieve a record PCE of up to 16.1% with a JSC of 34.0 mA/cm2, an open-circuit voltage (VOC) of 607 mV, and a FF of 78.3%. Compared to conventional hybrid solar cells with a bus/finger electrode (8.0%), our embedded electrode can effectively collect the photocarriers separated at the PEDOT:PSS/SiNW interface while preventing a short circuit at the direct metal/Si contact. Thus, the use of a metal contact in the construction of an ideal front electrode presents an opportunity in the development of highly efficient hybrid Si nanostructure solar cells.

Figure 1. Schematics of PEDOT:PSS/Si nanowire hybrid solar cells with (a) conventional and (b) embedded metal electrodes. (c) Current density−voltage curves and (d) efficiency distributions of hybrid solar cells with conventional (red) and embedded metal electrodes (blue). The inset of (c) shows optical images of hybrid solar cells with conventional (left) and embedded metal electrodes (right).

However, for the embedded electrode, the metal electrode is positioned between SiNW arrays underneath the PEDOT:PSS layer (Figures 1b and S1b). In addition, a thin SiO2 layer, acting as an electrical insulator, is inserted between the metal electrode and the Si substrate to prevent a short circuit due to the low Schottky barrier height (0.21 eV) of the Ag/Si contact. Figure 1c shows the current density−voltage (J−V) curves of the devices with conventional and embedded electrodes. As shown in optical images (inset of Figure 1c), both electrodes comprise a bus electrode with 10 fingers. The photovoltaic parameters of VOC, JSC, FF, and PCE are summarized in Table 1. By applying the embedded electrode, Table 1. Average Photovoltaic Parameters of PEDOT:PSS/ SiNW Hybrid Solar Cells with Conventional, Embedded, or Microgrid Electrodesa electrode design bus/fingers

conventional embedded

microgrid

embedded

VOC (mV)

JSC (mA/cm2)

FF (%)

PCE (%)

320 (560) 553 (573) 593 (607)

10.3 (23.1) 23.0 (24.5) 32.3 (34.0)

29 (61.4) 58 (65.0) 76.2 (78.3)

4.2 (8.0) 8.6 (8.9) 15.8 (16.1)

a

Each entry is the average value, obtained from measurements on 32 devices. Values in the brackets are obtained from the best device.

the JSC value increases from 23.1 to 24.5 mA/cm2, although shading loss induced by light reflection from the metal electrode should be identical at 12% since the same bus/finger electrode design is used. Furthermore, the FF value (65.0%) of the embedded electrode cell is higher than that of the conventional electrode cell (61.4%). The performance improvement of the embedded electrode can be clearly observed from the increase in the average PCE value as obtained from 30 devices. Figure 1d presents a histogram of PCE values for the conventional and embedded electrode cells. The average PCE value of the embedded electrode cells (8.6%) is significantly

RESULTS AND DISCUSSION A main concept underlying this work is to reverse the position of the PEDOT:PSS and the metal electrode. Figure 1a,b shows schematics of the hybrid solar cells with conventional and embedded electrodes, respectively. Hybrid solar cells with a conventional electrode comprises three different layers: SiNWs, PEDOT:PSS layer, and a metal electrode (Figures 1a and S1a). 6219

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reverse bias. These results indicate that the device operation of the conventional electrode cell is more affected by the recombination behavior compared to the embedded electrode. To quantitatively estimate the recombination behavior of the conventional electrode cells, we simulated a double diode model and compared it with the experimental illumination J−V curve of the conventional electrode cell (Figure 2b). As shown in the equivalent circuit (Figure S3), an additional diode and resistance are connected to an ideal diode device containing the main junction with shunt and series resistances in parallel. For the double diode, the additional diode and resistance represent the regions with localized Schottky shunting or high recombination.30 When the saturation current density of the additional diode increases from 1 × 10−7 to 1 × 10−4 A/cm2, both JSC and FF values extracted from the calculated J−V curves decrease. Higher current density at the additional diode indicates increased current flow through the localized Schottky shunting of the PEDOT:PSS/Si diode, leading to decreased current flow through the external load and degradation of the FF value. For the reverse saturation current density of 2 × 10−5 A/cm2, the experimental J−V curve of the conventional electrode cell is well matched with the J−V curve calculated from the double diode model, proving that the performance degradation of the conventional electrode cell is mainly due to the severe recombination and localized Schottky shunting. Figure 2c shows a cross-sectional scanning electron microscopy (SEM) image of the conventional electrode. Even though the PEDOT:PSS layer was spin-coated onto SiNWs prior to the formation of the metal electrode, direct contact between the metal electrode and a SiNW is observed in the SEM image. This result can be explained by the fact that the thickness of the PEDOT:PSS layer (∼40 nm) is not enough to fully cover the sharp tips of SiNWs. As evidence of localized Schottky shunting, conductive atomic force microscopy (AFM) was used to analyze PEDOT:PSS-coated SiNWs. For the reverse bias condition, we applied a negative voltage of −1 V to the conductive probe tip. In the ideal case, the electrons cannot overcome the high barrier height of PEDOT:PSS/Si under the reverse bias, resulting in no current flow from PEDOT:PSS to SiNWs (Figure 2d). As shown in Figure 2f, however, a relatively high current is observed at several localized areas, whereas other regions of PEDOT:PSS-coated SiNWs show no current flow. In addition, the regions in which current flows are measured show high surface roughness (Figure 2e). According to SEM and AFM analyses, the conventional electrode exhibits undesired contact between the Ag electrode and the SiNWs, diminishing the diode property of the PEDOT:PSS/SiNW junction through the direct Ag electrode-Si NW contact. In contrast, the metal layer of the embedded electrode is designed onto the planar Si between SiNW arrays as shown in Figure 1b, resulting in the formation of a robust and continuous metal electrode. In order to prevent the short circuit of the metal electrode, a thin SiO2 layer is deposited onto the Si substrate via e-beam evaporation, followed by the deposition of the Ag film. Due to the electrically insulating layer (SiO2), no leakage current was observed at the metal/Si interface (Figure S4). These indicate that our embedded electrode can effectively collect the photocarriers while preventing electrical short circuits. Furthermore, we successfully demonstrated the ability to apply conventional semiconductor processes such as lithography, dry etching, and lift-off in hybrid solar cell fabrication.

higher than that of the conventional electrode cells (4.2%). Furthermore, the embedded electrode cells exhibit similar VOC, JSC, and FF values with high reproducibility, whereas extremely low VOC and FF values are observed for the conventional electrode cells (see Figure S2). This result indicates that our embedded electrode has better reproducibility than the conventional electrode for PEDOT:PSS/SiNW hybrid solar cells. To elucidate a significant improvement in the manufacturing yield (100%) and the average PCE value (8.6%) of the solar cells using the embedded electrode, dark J−V curves of hybrid solar cells with conventional and embedded electrodes were measured as shown in Figure 2a. Based on an analysis of the

Figure 2. (a) Semilog scale current density−voltage curves of hybrid solar cells with conventional (red) and embedded metal electrodes (blue). The inset shows linear scale current density− voltage curves of hybrid solar cells. (b) Numerical calculation of double diodes with saturation current densities (JH) ranging from 1 × 10−4 to 1 × 10−7 A/cm2 (dashed lines). A double diode with a JH of 1 × 10−7 A/cm2 (blue dashed line) was matched with the experimental result of a conventional electrode (blue circles and solid line). (c) Cross-sectional SEM image of a hybrid solar cell with a conventional electrode indicating that a Si nanowire is in direct contact with the front metal electrode. (d) Band diagram schematic of PEDOT:PSS/Si diode under reverse bias. (e) AFM height mode and (f) conductive mapping images of PEDOT:PSS/ Si nanowires. The regions marked by yellow circles in panel (f) indicate the localized short circuit at the direct contact between the SiNWs and the metal.

curve slope at the forward bias, the diode ideality factor of the embedded electrode (1.8) is significantly lower than that of the conventional electrode (2.9), indicating that the diode behavior of the embedded electrode is closer to that of an ideal Schottky diode (ideality factor of 1) with thermionic charge transport across the barrier.29 In addition, the reverse saturation current density of the conventional electrode is relatively higher than that of the embedded electrode in dark J−V curves at the 6220

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width of 2 μm and a spacing of 200 μm, resulting in a superior transmittance of 94% (Figure S5). A 1 μm-thick Ag film was uniformly formed between the PEDOT:PSS and the Si substrate, as shown in Figure 3c, leading to a drastic decrease in the sheet resistance (3.2 Ω/sq) of the electroplated Ag microgrid when compared to an evaporated thin Ag film (32.4 Ω/sq). A PEDOT:PSS/SiNW hybrid solar cell with an embedded microgrid electrode is shown in the inset of Figure 4a, revealing

Although we fundamentally prevent localized short circuit between the metal and SiNWs, the PCE values of the hybrid solar cells are still lower than 10% because of the poor values of JSC (24.5 mA/cm2) and FF (65%). This is mainly due to severe shading loss (12%) originating from the bus/finger electrode. In order to improve the PCE of hybrid solar cells, the design of the metal electrode should be optimized by narrowing the distance between electrodes to collect holes from the relatively low conductive PEDOT:PSS layer to prevent electrical loss while minimizing the area of the metal electrode to reduce the shading loss. In previous reports, our group demonstrated a mesh-patterned microscale grid electrode as a highly transparent and conductive metal electrode for c-Si photovoltaic applications such as heterojunction solar cells31 and microwire solar cells.32,33 Thanks to the narrow distance between electrodes as well as the small fraction of the electrode area, the microgrid electrode significantly improved the JSC and FF values for c-Si solar cells. Here, we applied the microgrid design to the embedded electrode of PEDOT:PSS/SiNW hybrid solar cells. Figure 3a shows a schematic of the embedded microgrid

Figure 4. Current density−voltage curve of a hybrid solar cell with an embedded microgrid electrode. The inset shows an optical image of the hybrid solar cell. (b) Reflectance spectra of PEDOT:PSS/Si nanowires without a metal electrode (green) and with various metal electrodes (black: conventional bus/finger electrode, red: embedded bus/finger electrode, blue: embedded microgrid electrode). (c) EQE and (d) IEQ of hybrid solar cells with a conventional bus/finger (black), embedded bus/finger (red), and embedded microgrid electrodes (blue).

the metal electrode to be uniformly formed in large area of 1 cm × 1 cm. The embedded microgrid electrode not only increases the PCE from 9.2 to 16.1% but also improves the FF and JSC values (Figure 4b and Table 1) compared to an embedded electrode with a bus and fingers. Note that the FF value of the microgrid electrode (78.3%) is notably improved compared to that of a bus/finger electrode (65.0%) which requires a relatively longer carrier diffusion length (up to half the distance between fingers, ≈400 μm) through the low conductive PEDOT:PSS layer. This significant improvement in the FF value can be explained by the fact that the contact resistance (Rc) value of the microgrid electrode (spacing of 200 μm) is reduced by as much as four times compared to a bus/ finger electrode (spacing of 800 μm) as

Figure 3. (a) Schematic of the fabrication process of an embedded metal electrode. (b, c) Cross-sectional SEM images of an embedded microgrid electrode with an electroplated thick Ag film.

electrode for PEDOT:PSS/SiNW hybrid solar cells. The SiNWs were fabricated via a metal-assisted chemical etching method on the Si substrate. The microgrid was designed so as to surround the SiNW arrays, thereby ensuring that photogenerated holes in the SiNWs were effectively collected through the metal electrode due to the short diffusion length, which is up to half of the grid spacing (≈100 μm). In addition, the Si region between SiNW arrays was deeply etched to produce a space in which the thick metal electrode could be formed. A thin SiO2 layer was deposited using an e-beam evaporator, followed by the deposition of a 100 nm-thick Ag film. In order to improve the electrical conductivity of the microgrid electrode, additional metal electroplating was used to form a thick Ag film. Compared to physical vapor deposition, electroplating is a cost-effective method for producing a metal film with a high aspect ratio. The PEDOT:PSS layer was finally coated onto the SiNWs with an embedded microgrid electrode. As shown in Figure 3b, the Ag microgrid electrode has a line

Rc =

R shρc L

⎛ W coth⎜⎜ ⎝2

R sh ρc

⎞ ⎟a s ⎟ 2 ⎠

where Rsh, L, ρc, W, a, and s represent the sheet resistance, the grid length, the contact resistivity, the grid width, the unit cell length, and the grid spacing, respectively. In addition, the metal resistance (Rm) value of the microgrid electrode is also reduced by electroplating a thick Ag film. This improved FF can therefore be attributed to a decrease in the sum of Rc and Rm by using the microgrid electrode. 6221

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ACS Nano By applying the microgrid electrode, the JSC value increased significantly from 24.5 to 34.0 mA/cm2. While the bus/finger electrode occupies 12% of the front surface, the microgrid electrode covers an area of 3%, leading to a significant reduction in the shading loss of the incident light at the front electrode. The reflectance spectra of SiNWs with and without the electrodes were measured to clarify the difference in the surface reflection depending on the electrode structures (Figure 4b). The SiNWs fabricated by metal-assisted chemical etching show excellent antireflecting behavior with an extremely low reflectance of ∼1% at the broad range of the wavelength; this is because the SiNWs gradually reduce the mismatching of the refractive index between air and Si.34 With the microgrid electrode, the reflectance, averaged over a spectral range from 400 to 1000 nm, increases slightly from 1.3 to 3.4%. On the other hand, the bus/finger electrode shows a much higher reflectance of 14.8% regardless of the electrode structure (conventional or embedded electrodes). The reflectance spectra of various electrodes show similar profiles for the difference in the absolute reflectance values. The increase in the reflectance coincides with a difference in the metal electrode area, as shown in the inset of Figure 4b. According to the electrode area, the external quantum efficiency (EQE) spectra also show a similar tendency as the reflectance spectra (Figure 4c). Although the reflectance spectrum of the conventional electrode (bus/finger) is almost the same as that of the embedded electrode (bus/finger) because of the same electrode design and area, the EQE of embedded electrode (bus/finger) is larger than that of the conventional electrode (bus/finger). This result can be explained by that fact that localized short circuits are removed from the embedded SiO2/Ag electrode. However, for the microgrid electrode, the rate of increase in the JSC value (38.7%) is higher than that in the light absorption (13.6%). This result is mainly due to the effective carrier collection of the microgrid electrode. To evaluate the carrier collection efficiency, we analyzed the internal quantum efficiencies (IQEs) of the hybrid solar cells with different electrodes (Figure 4d). Thanks to the meshstructured microgrid, the maximum distance that the holes need to move along the PEDOT:PSS layer is approximately 100 μm (half of the spacing between microgrid patterns), which is relatively short in comparison with the bus/finger electrode (400 μm). Because the microgrid electrode diminishes the possibility of recombination of hole carriers due to the reduced required diffusion path length, the embedded microgrid significantly enhances the IQE values over a broad range of wavelengths. Note that a superior quantum efficiency of 99% is observed at a wavelength of 600 nm (i.e., the highest photon flux wavelength of the solar spectrum), indicating that the lossless carrier collection is demonstrated through the application of the embedded microgrid electrode. Regardless of the electrode design, IQE values tend to initially increase with the wavelength up to 600 nm, but then decrease at wavelengths in the near-infrared (NIR) region. This result is mainly due to the high recombination velocity at the rear contact. Since the holes are generated near the rear of the device due to the low absorption coefficient (≤1 × 102 /cm) of Si at the NIR region, the holes (i.e., minority carriers in n-Si) can suffer from recombination at the interface due to defects between the Si and metal. Even though we applied highly doped Si (n+) between moderately doped Si and a metal contact, the n+-Si layer still has issues such as Auger recombination and interfacial defects between Si and the

metal. This recombination at the rear surface can be prevented by using electron selective contact layers between the moderately doped Si and the metal contact. Since electron selective contact layers such as calcium (Ca),35 titanium dioxide (TiO2),36 and magnesium fluoride (MgF2)37 can reduce the barrier height at the Si/metal interface, VOC and JSC values in crystalline Si solar cells have been significantly improved. Therefore, further VOC and JSC improvements are possible through the insertion of electron selective contact layers between Si and the metal contact at the rear.

CONCLUSION In summary, we have demonstrated highly efficient (16.2%) PEDOT:PSS/SiNW hybrid solar cells by using an embedded microgrid electrode as the front contact. Through analyses using conductive AFM and a double diode model, we found that localized short circuit originated from the direct contact between SiNW tips and a metal electrode, leading to performance degradation and poor reproducibility for a conventional electrode. However, the embedded electrode proposed here was designed to decouple the planar area for the metal electrode from the SiNWs for light absorption, thus avoiding the localized short circuit and serious degradation of photovoltaic performance observed with a conventional electrode. In order to improve the PCE values of the hybrid solar cells, an embedded microgrid electrode was fabricated using conventional metallization processes such as photolithography, lift-off, and electroplating, resulting in the formation of a highly transparent and conductive metal electrode. The superior light transmittance and low metal/ contact resistances of the microgrid electrode improved the JSC value dramatically (from 24.5 to 34.0 mA/cm2), with an IQE of 99% for the main solar spectrum wavelength (600 nm), resulting in over a two times higher PCE (16.1%) compared to a conventional electrode (8.0%). This embedded electrode is therefore believed to be a very promising design for achieving reproducible and highly efficient organic−inorganic hybrid solar cells. EXPERIMENTAL SECTION Fabrication of Heavily Doped Si for Ohmic Contact. PEDOT:PSS/Si nanowire hybrid solar cells were fabricated from Czochralski (CZ) n-type Si wafers (resistivity of 1−3 Ω·cm, 350 μm thick). The heavily doped region (n+) was formed by phosphorus diffusion via the spin-on-dopant (P509, Filmtronics) method at the front of the solar cell for an ohmic contact between Si and back electrode. The diffusion doping was carried out in a tube furnace under a mixed ambient of 20% O2 and 80% N2 at 900 °C. The sheet resistance of the n+-Si region was 130−150 Ω/sq. For the bottom contact, 300 nm-thick Al film was deposited on the bottom of samples using a thermal evaporator. Fabrication of Si Nanowires via Metal-Assisted Chemical Etching. After removing phosphorus glass and SiOx layer, the Si nanowires were fabricated by the metal-assisted chemical etching (MACE) onto the front of the Si substrate. The rear surface of the Si substrate was covered by the polymer film to prevent the formation of Si nanowires. The Si substrate was dipped into the diluted HF solution to remove the native oxide. The Ag nanoparticles were first deposited onto a hydrogen-terminated Si substrate by the electroless deposition in a mixed solution of 4.8 M hydrofluoric acid (HF) and 0.005 M silver nitrate (AgNO3) for 60 s at the room temperature. After the deposition of the Ag nanoparticle, the substrate was immersed in an aqueous solution of 4.8 M HF and 0.6 M hydrogen peroxide (H2O2) for 30 s at the room temperature. The Si nanowires were then dipped in the diluted nitric acid (HNO3) for 10 min so as to remove the Ag 6222

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ACS Nano nanoparticles. In order to minimize the surface recombination, the Si nanowires were covered by the 1 nm-thick Al2O3 layer deposited using atomic layer deposition (Lucida D100, NCD) at 250 °C in which trimethylaluminum (TMA) and H2O were used. Fabrication of PEDOT:PSS/Si Nanowire Hybrid Solar Cells. For the hybrid solar cells with the conventional electrode, a PEDOT:PSS solution mixed with 5 wt % dimethyl sulfoxide (DMSO) and 0.1 wt % Triton X-100. (Sigma-Aldrich) was coated onto the Si nanowires at 2000 rpm for 40 s, followed by annealing samples at 125 °C for 10 min. For bus and finger Ag electrodes, a 100 nm-thick Ag film was deposited via thermal evaporation and shadow mask onto the PEDOT:PSS-coated Si nanowires. For the embedded electrodes, the Si nanowires were covered with photoresist (AZ4330E, AZ electronic materials) using a lithography process, followed by the reactive ion etch (RIE) of Si nanowires. A 20 nm-thick SiO2 and 100 nm-thick Ag film was deposited via e-beam and thermal evaporation, respectively. The photoresist was removed by dipping samples in acetone solution. In order to further improve the embedded microgrid electrode, electroplating was conducted to deposit a thick Ag film. After the deposition of thin seed layer (100 nm-thick Ag film), electroplating deposition was performed in an Ag plating solution (HT Silver 701, HanTechPMC) at 40 °C under the current density of 0.25 mA/cm2. The active area of the PEDOT:PSS/Si nanowire hybrid solar cells is 1 cm2. Characterization of PEDOT:PSS/Si Nanowire Hybrid Solar Cells. The surface morphologies and current mapping of PEDOT:PSS-coated Si nanowires were characterized by atomic force microscopy (AFM, Veeco microscope). The photovoltaic properties of our solar cells were investigated using a solar simulator (Class AAA, Oriel Sol3A, Newport) under AM 1.5G illumination. Incident flux was measured using a calibrated power meter and double-checked using a NREL-calibrated solar cell (PV Measurements, Inc.). EQE was measured using a Xe light source and a monochromator in the wavelengths range of 400−1100 nm. Optical reflection measurements were performed over wavelengths of 400−1100 nm using a UV−vis/ NIR spectrophotometer (Cary 5000, Agilent).

Ministry of Science, ICT & Future Planning (NRF2017R1A2B4002738, 2015R1D1A1A01059726).

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02322. SEM images of PEDOT:PSS/Si nanowires hybrid solar cells; distribution of photovoltaic parameters for conventional and embedded electrodes; equivalent circuit of PEDOT:PSS/Si nanowires hybrid solar cells; leakage current measurement of embedded electrode (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Han-Don Um: 0000-0002-2087-160X Kwanyong Seo: 0000-0002-8443-7933 Author Contributions

H.D.U. and K.S. conceived and designed the research. H.D.U., D.C., A.C., and J.H.S. performed the experiments and analyzed the data. H.D.U. and K.S. wrote the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was funded by the Ulsan National Institute of Science and Technology (Research Funds 1.170003.01). It was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the 6223

DOI: 10.1021/acsnano.7b02322 ACS Nano 2017, 11, 6218−6224

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DOI: 10.1021/acsnano.7b02322 ACS Nano 2017, 11, 6218−6224