Improvement in current density of nano- and micro-structured Si solar cells by cost-effective elastomeric stamp process

In recent years, nano- and micro-structures have been investigated for cost-effective solar cell applications with better performance. Nano-structures exhibit excellent light absorption performance owing to their strong light trapping effect;^1,2 however, they provide poor step coverage of surfaces.³ In contrast, micro-structures provide better step coverage but poor light absorption.⁴ Therefore, high performance solar cells can be realized by combining the advantages of nano- and micro-structures.

There are several methods of fabricating micro-structures on silicon surface, such as deep reactive ion etching,⁵ catalytic vapor–liquid–solid chemical vapor deposition,⁶ micro-contact printing,⁷ electron beam lithography,⁸ and X-ray lithography.⁹ However, these processes have limitations owing to expensive equipment and materials, such as photo masks, contact aligners, and tracking systems for cost-effective solar cells, and the usage of toxic chemicals as developer and photoresist. As a result, the elastomeric stamp process^10,11 with polydimethylsiloxane (PDMS), which is a soft lithography technique, has been introduced as a cost effective and practical method compared to conventional lithography. In addition, the stamp of a mother pattern can be reused with good reproducibility of micro-structures on silicon surface. The metal-assisted chemical etching (MACE) process has been considered as a simple and cost-effective tool for fabricating nano-structures.^12–14 As mentioned above, a combined nano- and micro-structure (CNMS) can improve the performance of solar cells through mutual supplementation.

In this study, a CNMS was periodically arrayed using the elastomeric stamp and MACE processes on the surface of solar graded silicon. Then, the improvement in the performance of silicon solar cells was investigated.

As shown in Fig. 1, CNMS Si was prepared on a 200-μm thick p-type (100) monocrystalline silicon wafer. Micro-patterned PDMS was prepared as an elastomeric mother pattern with a diameter of 6 μm, a height of 2 μm, and a spacing of 2 μm. Then, the Si wafer was cut into a size of 12 × 12 mm², cleaned in a mixed solution of NH₄OH:H₂O₂:H₂O (1:1:5) at 80 °C for 10 min, and immersed in diluted hydrofluoric acid (DHF) for 1 min. UV resin (mr-UV Cur06/micro resist technology GmbH, Germany) was coated on the wafer at 3000 rpm for 1 min using a spin-coating method. The elastomeric mother stamp was contacted onto the resin-coated silicon wafer. Then, the resin-coated wafer transferred from mother stamp was heated at 100 °C for 20 s and cured using a Xe lamp (365 nm) for 90 s. The surface of the transferred resin with micro-structures was treated through O₂-plasma-based reactive ion etching (RIE-1C/Samco, Japan). For the formation of nano-structures, Ag nanoparticles were dispersed on the micro-pattern using a mixed solution of deionized water, HF (4.8 M), and AgNO₃ (0.05 M) for 90 s. MACE was performed in a mixed solution of deionized water, HF (4.8 M), and H₂O₂ (0.5 M) for 2 min, 4 min, and 6 min. This resulted in the formation of micro-structures and nano-structures with an axial height of 2.8–3.8 μm and 380 nm–780 nm, respectively. Residual Ag nanoparticles and UV resin were removed using nitric acid solution and O₂ plasma treatment.

Phosphorous oxychloride (POCl₃) gas was supplied to a furnace at 860 °C. DHF (10%) was used to remove phosphosilicate glass. A silicon nitride (SiN_x) layer with a thickness of 80 nm was deposited through plasma enhanced chemical vapor deposition (SNTEK, Korea). Ag and Al pastes were screen printed, dried, and co fired at 915 °C for the front and rear electrodes.

The CNMS was investigated through field emission scanning electron microscopy (SEM, JSM-6701F/Jeol, Japan). The optical reflectance and external quantum efficiency of CNMS Si were observed using UV-visible spectroscopy (V-670/Jasco, Japan) and incident photon-to-current efficiency (CEP-25BX/Jasco, Japan). The conversion efficiency of CNMS solar cells was measured by employing a solar simulator (WXS-105H/Wacom, Japan) under the standard testing condition (1 sun, AM 1.5G, 25 °C).

Figure 2 shows the SEM images and etching rate of CNMS Si in a range of 2–6 min of etching time. The micro-structures are formed regularly, as shown in Fig. 2(a), and the nano-structures are uniformly distributed on the micro-structures; UV resin is effective as an etching mask. As etching time increased from 2 min to 6 min, the average height of the micro-structures increased from 2.8 to 3.8 μm and etching rate decreased from 1.4 to 0.63 μm/min. Etching rate decreased owing to the sidewall reaction effect.¹⁵ Participated transport ions should react at the bottom of the silicon wafer surface during the MACE process. However, they react with the sidewall of the CNMS because of increase in the aspect ratio; this results in decrease in etching rate. The radius of the micro-structures was approximately 4.5 μm and less than that of the mother pattern (6 μm). The average height of the nano-structures increased from 350 to 785 nm with etching time, and the structures exhibited poor uniformity in formation. The optical reflectance of CNMS Si after the MACE process and that of planar Si are shown in Fig. 3(a). The reflectance of planar Si is 29.92%, which is too high to fabricate solar cells. As the duration of the MACE process increases to 6 min, the reflectance of CNMS samples decreases considerably to 4.53% in a wavelength range of 400–1100 nm. This is attributed to the improvement in the light trapping effect with increase in the aspect ratio of the micro- and nano-structures.^1,2 Figure 3(b) shows external quantum efficiency (EQE) of planar silicon solar cells and CNMS Si. EQE is defined as the percentage of electrons collected per incident photon. Higher EQE values are observed for CNMS Si in the longer wavelength range because of lower reflectance and increase in light absorption through multiple scattering in the nano-structures.¹⁶ The J–V curves (J_sc: Short circuit current density, V_oc: Open circuit voltage) of CNMS Si and planar Si solar cells are shown in Figure 4. The V_oc for all samples is similar (∼0.6 V), and the J_sc of the CNMS is 1.5 times that of the planar Si structure. Other detailed cell parameters are shown in Table I. As the duration of MACE increases from 2 min to 6 min, the value of J_sc increases from 31.13 mA/cm² to 34.72 mA/cm². The improvement in optical properties because of the light trapping effect contributes toward the increase in current density and EQE. However, the fill factor (FF) decreased considerably from 82.3% to 64%. The higher surface area and number of defect sites in the CNMS^16,17 resulted in a rapid carrier-recombination velocity at the surface. In addition, this was attributed to nonconformal coverage of the n⁺ emitter and electrode through the crosslinking of the nano-structures. As seen in Fig. 5, series resistance (R_ss) increases and shunt resistance (R_sh) decreases during the MACE process. The highest conversion efficiency of CNMS Si solar cells was 13.65%; the efficiency of the CNMS was 25% higher than that of the planar Si structure.

CNMS Si solar cells were fabricated using the elastomeric PDMS stamp process, followed by the MACE method. We prepared periodic micro- and nano-structures without using expensive photolithography. As the duration of MACE increased to 6 min, reflectance decreased to 4.53% and current density improved from 22.35 to 34.72 mA/cm². The highest conversion efficiency of CNMS Si solar cells was 13.65% after 4 min of the MACE process. The surface passivation of the CNMS provided a higher FF through reduction of surface recombination. Therefore, the elastomeric PDMS stamp process could be a good alternative to conventional expensive photolithography.

This work was supported by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Knowledge Economy (20163030014070) and financially supported by the Project of the Korea Institute of Industrial Technology (KITECH-JG 170018).