Laser-assisted Nano-texturing for Flexible Ultrathin Crystalline Si Solar Cells

doi:10.21203/rs.3.rs-2566768/v1

Download PDF

Article

Laser-assisted Nano-texturing for Flexible Ultrathin Crystalline Si Solar Cells

https://doi.org/10.21203/rs.3.rs-2566768/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

Ultrathin crystalline Si wafers, which are more flexible than conventional ones, can apply to curved surfaces, enabling a wide range of applications such as building integrated photovoltaics, vehicle integrated photovoltaics and wearable devices. Thinner wafers require more effective light trapping; thus, surface texturing in microscale is a common approach to compensate for the reduced thickness by enhancing the light pathlength. Microscale textures, however, deteriorate the mechanical flexibility due to stress concentration in the valley of the micro-textures. In this study, we propose a laser-assisted nano-texturing process for enhanced flexibility of the ultrathin Si solar cells with a 50-µm thickness while maintaining light trapping performances. A nano-lens array was used to focus laser onto the Si wafers, inducing the formation of nanoparticle etch masks for nano-pyramid texturing in an alkaline solution. The origin of the enhanced flexibility of the nano-textured Si wafers was discussed by a micro-mechanics simulation study. We applied our nano-texturing technique to ultrathin Si based PERC cells and demonstrated the enhanced flexibility of the cells with a 26 mm critical bending radius. Also, we show that the nano-texture Si wafer provide a higher efficiency of 18.68%, whereas the micro-texture one exhibits 18.10%.

Physical sciences/Nanoscience and technology/Nanoscale devices/Electronic devices

Physical sciences/Materials science/Materials for energy and catalysis/Solar cells

nanosecond pulse laser

nano-pyramid texturing

ultrathin crystalline Si

flexible solar cells

mechanical bending test

Ultra-thin (UT) crystalline silicon (c-Si) solar cells with a thickness below 50 µm have recently attracted increased attention due to their properties of being thin, light, and bendable ^1,2. This makes them suitable for use in applications where a solar module weight is a significant factor like building-integrated photovoltaics (BIPV), vehicle- integrated photovoltaics (VIPV), and wearable electronics. However, as Si wafers become thinner and thinner, more effective light trapping as well as stronger surface passivation is demanded to maintain a high efficiency, which is one of challenging issues in the UT-Si based solar cells.

While the cost of silicon has shown some volatility in the past due to rapid demand growth and limited manufacturing capabilities ³, silicon wafers contribute an average of 14% to the total module production cost ⁴. Therefore, it is of economic interest to reduce the amount of silicon used in solar cells by reducing the Si wafer thickness ⁵. However, since crystalline silicon is an indirect semiconductor, it exhibits poor absorption for low-energy photons ⁶. To minimize optical absorption loss over a wide spectral range, the optical path length of light within silicon wafers must be increased by surface texturing.

Recently, many attempts have been made to manufacture Si solar cells with a UT-Si wafer having a thickness of less than 50 µm in order to realize a flexible solar cell by reducing the rigidity of the c-Si wafer ^7–12. UT-Si wafers without antireflection treatment exhibit extreme light absorption losses of more than 30% over the entire wavelength range because of the refractive index mismatch between air and c-Si. Moreover, UT c-Si exhibits a drastic absorption loss at wavelengths exceeding 900 nm due to its low absorption coefficient ¹. Because the sunlight in the long-wavelength region exceeding 900 nm constitutes about 18% of the solar radiation energy, the absorption of such long-wavelength light in the UT c-Si solar cells must be improved for achieving high efficiency. UT c-Si solar cells textured with nano- and micro-structures showed an outstanding efficiency of 18.9% ¹³. Although nano- and micro-structures can effectively increase the light absorption in thin c-Si solar cells, issues such as complicated fabrication processes and severe recombination of minority carriers persist. In terms of optical design, surface texturing using periodic nanostructured arrays like nanowires ¹⁴, nanocones ¹, nanopencils ¹⁵, and nanopyramids ^16,17 have been used for improved light trapping in UT-Si wafers with the potential to approach the Lambertian absorption limit ¹⁸. As for textures, small pyramidal textures in the range of sub-micrometer can improve photocurrent by more effectively increasing the overall path length of light propagation in silicon. In the case of UT-Si solar cells, it is difficult to produce micro-pyramid textures in the range of 3–10 µm while securing high production yield. In contrast, small pyramidal textures in the range of sub-micrometer are more compatible with UT-Si solar cell fabrication processes ¹⁹. In particular, the surface structures could function as crack initiation sites where the mechanical stress can concentrate. Such crack initiation sites would significantly degrade the flexibility of the UT-Si solar cells. Hence, the development of new light-trapping strategies is needed to enhance the light absorption in the entire wavelength region while maintaining the stable flexibility of the thin c-Si solar cells.

Most studies, however, have been performed on direct patterning of c-Si wafers using nanolithography such as e-beam lithography ^20–22, laser interference lithography ^23,24, and nanoimprint lithography ^25,26 to produce the aforementioned nanostructures. However, it is difficult to commercialize these technologies for large area solar cells due to production costs. There are many prior studies on the application of the nanostructures to the UT-Si wafers as a means to increase the absorption of UT-Si, but few studies have been reported on the effects of these structures on the mechanical strength of UT-Si solar cells.

In this work, we propose a novel nano-texturing process for c-Si wafers using a nanolens array and a ns pulse laser. The laser was focused through the nanolens on the UT-Si wafers of a 50 µm thickness and induces local heating to form silicon oxide that serves as nano etch masks for nano pyramid texturing. Nano textured UT-Si wafers have comparable light trapping performances to micro textured ones. Furthermore, the nano textured UT-Si wafers exhibited enhanced mechanical flexibility, which was revealed by a 4-point bending test. The mechanics simulation study was conducted to find an explanation for the enhanced flexibility. Finally, we successfully produced the flexible UT-Si based solar cells textured by the laser-assisted texturing technique developed in this study. The device performances and flexibility of the UT-Si solar cells were analyzed and discussed.

2.1. Fabrication process of nano-lens

The SNS solution was prepared with powders of 960 nm diameter spherical silica beads (Bang Laboratory), which were dispersed in a binary solution [N,N-dimethyl formamide (DMF, anhydrous 99.8%, Sigma-Aldrich): Ethylene glycol (EG, anhydrous 99.8%, Sigma-Aldrich) = 9:1, v/v] at a silica bead concentration of 400 mg/ml. The solutions were sonicated for 5 h to produce complete dispersion of the SNS. Test substrates consisted of Si wafers (1–5 Ω·cm, p-type, CZ, 525 µm thick) of a (100) crystal orientation of square pieces (30 mm × 30 mm) were washed in acetone, methanol, and ethanol with sonication for 15 min and were rinsed in deionized water for 10 min. In order to form a hydrophilic Si surface, they were subsequently cleaned in a piranha solution [H₂SO₄ (95%):H₂O₂ (30%) = 3:1] for 20 min followed by 10 min deionized water rinse The SNS solution of 150 µL was dropped and spun onto the clean Si substrates using a spin coater at 2000 rpm for 270 s. The SNS self-assembled monolayer in a hexagonal close-packed array were produced. Using this SNS monolayer as an etch mask, the RIE process is performed using SF₆ and O₂ gas. The master sample to be used for NIL was completed by a 5 min at a SF₆:O₂ partial pressure ratio of 8:2 RIE process.

Nano-Imprint Lithography (NIL) is the production of patterns by physically moving material into designated patterns. This is done using the master sample that was made earlier. The Si master sample is coated with fluorine, poured with PDMS, and baked at 90°C for 1 hour to form a pattern. Then, spin coat the HSQ resin on the PDMS mold and press it together with the glass substrate to fabricate a nano-lens. The fabricated nano-lens were subjected to additional UV treatment and heat treatment at 400°C for 1 hour were performed for curing and shape control. Please refer to Fig. 1 for the overall process.

2.2 Laser-assisted texturing

The fabricated nano-lens is placed on a Si wafer with a size of 30 mm × 30 mm in the air, and the entire area is irradiated using a diode-pumped Q-switched Nd:YVO₄ laser of 20 ns duration at a 532 nm wavelength. The sample stage was placed on an anti-vibrating table, and optimized laser fluence was 5.5 J/cm², and the number of scans on the wafer was performed 5 times. The current of the laser diode was 25 A, the laser frequency was 60 kHz, the scan speed was 200 mm/s, and the spacing between lines was 8 µm in the line scan mode. The samples after laser irradiation are immersed in 25 wt% HF solution for 30 seconds to remove the SiO₂ layer, and then etched for 90 seconds in a solution of 10 wt% KOH solution at 70°C and additive (SEA, Sunflower 100). After KOH etching, the Si substrates were rinsed in deionized water for a few minutes and rinsed in 25 wt% HF solution for a few seconds.

2.3 Four-point bending test

Non-, nano-, micro-textured samples were prepared for bending tests. 4-point bending test samples were fabricated by lase scribing with 3 mm and 13 mm of width and length, respectively. 4-point bending tests were performed using a universal testing machine (Instron 5948), and at a loading rate of 5 µm/s. Also, video was recorded during bending test, and critical bending radius was measured by image immediately before the fracture. All experiments were conducted under laboratory condition.

2.4 Finite element analysis (FEA) simulations

The FEA simulation was conducted to analyze the stress distribution and concentration according to surface texturing. Abaqus 6.14 and Abaqus CAE were used for FEA simulations and creation of input files, respectively. FEA simulations were performed for force control and linear elasticity condition. Two-dimensional deformable shell model was used, and the sketch of FEA simulation is identical to the typical SEM fracture images of textured samples (Fig. 8(a) and (b)). The mechanical properties of silicon were input using previous results ^27,28.

2.5 Cell fabrication and cycle bending test

For the fabrication of UT c-Si PERC cells, the (100) oriented p-type CZ Si wafers with a resistivity of 1 − 3 Ω·cm and a 50 µm thickness of square pieces (30 mm × 30 mm) were cleaned by RCA1 and RCA2 solutions. After proceeding to both sides of the laser-assisted texturing presented above, SiO₂ 200 nm is deposited as a diffusion barrier on the rear side by PECVD. A POCl₃ tube furnace was used to form a diffused emitter of a 100 Ω/□ sheet resistance. Phosphor-silicate glass (PSG) on the Si wafer surface and rear side diffusion barrier were removed by HF solution. For the rear passivation Al₂O₃ of a 20 nm thickness was deposited using atomic layer deposition (ALD), and a plasma-enhanced chemical vapor deposition (PECVD) SiN_x capping layer of an 80-nm thickness was used. Four neighboring cells of 10 mm × 10 mm were isolated by a SF₆ RIE etch process. While considering a passivation layer on the emitter instead of SiN_x, a 10 nm hydrogenated amorphous silicon (a-Si:H) layer used was selected. The a-Si:H layer could obtain excellent passivation performance. A local contact structure is formed by locally opening the a-Si:H layer to collect carriers without loss of series resistance. For that, a mask is made through photolithography, and the a-Si:H layer is opened using HNA solution. The Si sub-cells were finalized by deposition of a 2 µm thickness Al layer for the rear electrode using e-beam evaporation and the rear contact was formed through a laser fired contact (LFC) process. A diode-pumped Q-switched Nd:YVO₄ laser of 20 ns duration at a 532 nm wavelength was used for the LFC processing. The front electrode was formed of 800 nm thick Ag through photolithography and lift-off process. ZnS was deposited by 75nm thermal evaporation for SLARC, and ZnS 52 nm and MgF₂ 105 nm by thermal evaporation for DLARC. The solar cells were tested under a standard solar irradiation of AM 1.5 G at a 100 mW/cm² light intensity using a solar simulator (Oriel LSC-100), and the cell performance parameters were determined from the current–voltage characteristics. The external quantum efficiency (EQE) was also measured in a wavelength range from 350 nm to 1150 nm.

The cycle bending test was carried out using a specially designed bending test jig with a bending radius of 30 mm. UT-PERC cells with planar surface, micro-pyramid surface and nano-pyramid textured surface were tested with four samples of each cell structure. All samples were subjected to a total of 1,000 bending tests, and solar cell parameters were measured using a solar simulator after every 200 bending cycles. All the results of the cycle bending test can be seen in Figure S7.

3.1 Laser-assisted nanotexturing process

The overall nanopyramid texturing process helped with a ns pulse laser is illustrated in Fig. 1. The texturing process consists of three steps: (1) nanolens fabrication, (2) laser processing, (3) pyramid texturing in an alkaline solution as illustrated. As a first step, the HSQ nanolens is made of a convex optical structure and prepared through sequential fabrication processes of silica nanosphere (SNS) lithography and nanoimprint lithography (NIL). As a second step, the nanolens was placed on the c-Si wafer, irradiated with a ns pulse laser to form nanoparticle etch masks on the surface of silicon wafers, and finally, a typical KOH etching process was conducted to form nano-pyramidal structures on the c-Si wafers.

3.2 Nano-lens fabrication results

The nano-lens used for the laser-assisted texturing process was obtained by a NIL process after making a Si master sample through SNS lithography. As shown in Fig. 2(a), silica spheres were coated on the surface of the Si wafer for SNS lithography, and the Si nanostructures for the master sample were produced through the reactive ion etching (RIE) process. SF₆ and O₂ gases were used for RIE, and the shape of the nanostructures was adjusted by varying the partial pressure ratio of O₂ gas ²⁹. Since the nanolens array is fabricated by replicating the Si master sample, the shape of the master sample needs to be optimized for the effective light focusing. As shown in Fig. 2 (b), the surface of the silicon wafer subjected to the RIE process for 5 minutes under the condition of 10% O₂ partial pressure was rough; thus, it was excluded from the master sample. The 20% O₂ partial pressure ratio condition was selected as the most suitable condition because the surface of the master sample was smooth, and the lens shape was not too narrow as shwon in Fig. 2 (c). The condition of 30% O₂ partial pressure ratio was not either suitable as a master sample because the lens shape was too narrow as shown in Fig. 2 (d).

This master sample was coated with poly-dimethyl siloxane (PDMS) to make a pattern, and a nano-lens was manufactured through HSQ resin and NIL process.³⁰ In order to further modify the shape of the nano-lens, additional UV and heat treatments at 400°C for 1 hour were performed. There was a change in the shape of the nano-lens due to the UV treatment process, while the shape of the HSQ film without UV treatment easily collapsed as shown in Fig. 3. (a) and (b). As the UV treatment time increased to 30 minutes and 1 hour, the shape of the nano-lens was well maintained after the HSQ curing as seen in Fig. 3 (c) and (d). After 1 hour of the UV treatment and 1 hour of the heat treatment at 400°C, the shape of the nanolens as shown in Fig. 3 (d) showed nearly the same as that of the Si master sample, and thus this condition was used for the nanolens fabrication. The completed HSQ nanolens has a hexagonal arrangement of flat hexagonal heads with about 800 nm, as shown in Fig. 3(d). The height of each lens column is about 1 µm.

3.3 Laser-assisted texturing process

The fabricated nano-lens, serving as a focusing lens, was placed on a Si wafer with a size of 30 mm × 30 mm, and the entire area is irradiated using a scanning pulse laser. A 90 nm-thick SiO₂ layer was deposited on the Si wafers by plasma-enhanced chemical vapor deposition (PECVD). The role of this SiO₂ layer reduces optical reflection from the Si surface and increases the optical absorption in the Si wafers, as can be seen from the FDTD simulation result in Figure S1.³¹ The laser-irradiated Si wafers were dipped in the HF solution to removes the SiO₂ layer, and was textured through the KOH solution, which is generally used for Si pyramid texturing. Refer to the overall laser-assisted texturing process as illustrated in Fig. 1. The surface of the Si wafers after the laser irradiation was seen by the electron microscopy and shown in Fig. 4(a)-(f). The most important part of the laser-assisted texturing process is that the laser beam is focused through the nano-lens on the surface of SiO₂/Si to create unique nanoparticles on the Si surface by inducing local heating on the Si wafer surface. The appearance of these nanoparticles can be confirmed through scanning electron microscope (SEM) images in Fig. 4(a). Most nanoparticles remained even after HF treatment as seen in Fig. 4(b). The average size of the spherical nanoparticles is seen to be ~ 400 nm as confirmed in the SEM image of Fig. 4(c). These nanoparticles function as etch masks when Si wafers are textured in a KOH solution, making pyramid nucleation fast and dense in the beginning of the texture reaction. As can be seen in Fig. 4(d), by this mechanism, a Si surface with uniform and dense nano-pyramid can be obtained even with a noticeably short etching time of 90 seconds. A small etch loss along with a shorter etch time will be more advantageous for 50 µm-thick UT c-Si wafers. The magnified SEM images of the textured samples in plan and cross-sectional view show the produced pyramids are all sub micrometer scale, as shown in Figure (e) and (f). The origin of the nanoparticles induced by laser was analyzed by X-ray photoelectron spectroscopy (XPS). For comparative study, two Si wafers were deposited with 100 nm-thick SiO₂ thin films. One wafer was subjected to the laser process while the other remained intact. Afterwards, both wafers were treated in HF solution for 2 minutes to remove the SiO₂ thin film, and XPS spectra were taken. The XPS spectra in Fig. 4(g)-(h) revealed that a [Si]/[O] ratio and a Si-O bond peak increase after laser irradiation, supporting the nanoparticles are made of SiO₂ and generated by a laser passed through the nano-lens. The detailed values of the characteristic peaks in the XPS spectra are summarized in Table 1. To investigate the effect of the nanolens for laser focusing, the same laser processing experiment without nanolens was conducted on SiO₂ coated Si wafers. We observed that the formation of the SiO₂ nanoparticles was greatly suppressed, and as a result, the pyramid textures were non-uniformly formed and the texturing could not be completed at a given process time of 90 s. The SEM images of the Si wafers for the laser-assisted texturing cases with and without nanolens are presented in Figure S2 for reference with different texturing process time.

Table 1

XPS element analysis of the Si wafers without and with laser processing: binding energy, FWHM of Si 2p, O1s peaks, and O/Si atomic ratio.
Sample	Element	Binding Energy (eV)	FWHM (eV)	[O]/[Si]
Bare Si wafer	Si 2p	99.36	1.39	0.009
Bare Si wafer	O 1s	532.14	2.65	0.009
Laser-processed	Si 2p	99.34	1.44	0.156
Laser-processed	O 1s	532.26	2.6	0.156

3.4 Light trapping performance of textured UT-Si

The size distribution of the pyramids in the nanotextured wafers was analyzed by image processing and shown in Fig. 5(a). The average size of the nanopyramids was 692 nm. For comparison, the Si wafers were also textured by a conventional KOH texturing process and in the same manner, the size distribution of the pyramids was analyzed and shown in Fig. 5(b). The average size of the pyramids was 5.76 µm, which is a typical pyramid size produced in a standard KOH texturing process. Effective light trapping in 50 µm-thick UT c-Si wafers is more demanded compared with the case of a conventional solar grade wafers with a 200 µm thickness.^10,32 In Fig. 5(c), the total reflectance of 50 µm-thick UT c-Si wafers with a planar surface, a typical micro-pyramid textured surface, and a nano-pyramid textured surface by a laser-assisted texturing process are compared. It can be seen that the total reflectance of the nano-pyramid textured Si wafer is lower at a short wavelength of 400 nm or less, but slightly higher at a long wavelength of 800 nm or more compared with the micro-pyramid textured Si wafer. The solar weighted reflectances for the micro-pyramid and nano-pyramid textured sample were calculated and were 13.09% and 13.81%, respectively. The results show that the reflectance loss in the nano-pyramid textured sample is comparable to the nano-pyramid one. Additionally, the total reflectance of a sample deposited with single layer anti-reflection coatings (SLARC) of 75 nm ZnS on a nano-pyramid textured Si wafer and a double layer (DL) ARC sample combining 105 nm MgF₂ and 52 nm ZnS were also measured and shown in Fig. 5(d).³³ The calculated weighted reflectance of the SLARC and DLARC structures were 6.72% and 4.82%, respectively. If the uniformity of the laser-assisted texturing process is improved in the future, the overall light trapping performance of the nano-pyramid texture can be improved.

3.5 Mechanical flexibility of UT c-Si wafers

In order to analyze the surface texturing effects on flexural strength, a 4-point bending test was conducted using single-side textured Si wafers. Because of the catastrophic failure of the Si wafers, more than 30 4-point bending tests were conducted for each sample. 4-point bending tests were performed using a bending jig with distances between the support and load spans of 8 mm and 4 mm. The schematic of our 4-point testing setup is illustrated in Fig. 6(a). The bending samples were fabricated with width and length of 3 mm and 13 mm by laser scribing after surface texturing process. Also, textured surface is located on support span to apply tensile deformation and analyze the surface texturing effect on flexural strength. Through 4-point bending tests, flexural strength can be calculated using Eq. (1) ^34,35:

$${\sigma }_{f}= \frac{3PL}{4w{t}^{2}}$$

where P, L, w, and t are the fracture force, distance between support spans, width of sample, and sample thickness. Figure 6(b) shows the Weibull analysis based on results of 4-point bending tests. Weibull analysis is a method for brittle materials such as silicon to analyze the stress distribution. Through Weibull analysis, characteristic strength (${\sigma }_{0}$) and Weibull modulus (m) can be obtained for each texturing condition.

$$P\left({V}_{0}\right)=exp[-({\frac{\sigma -{\sigma }_{u}}{{\sigma }_{0}})}^{m}]$$

In Eq. (2), P(${V}_{0}$), σ and ${\sigma }_{u}$ are the survival probability of the material, applied stress and threshold stress, respectively. Threshold stress is equal to 0 when the material has brittle fracture. Therefore, Eq. (2) can be revised to Eqs. (3) and (4):

$$P\left({V}_{0}\right)=exp[-({\frac{\sigma }{{\sigma }_{0}})}^{m}]$$

$$lnln\left[\frac{1}{P\left({V}_{0}\right)}\right]=m(ln\sigma -ln{\sigma }_{0})$$

The characteristic strength and Weibull modulus can be obtained by Weibull distribution graph of Fig. 6(b) and Eq. (4) ^36,37. The characteristic strength corresponds to a survival probability of 37%, and Weibull modulus means distribution of the strength. Because Weibull modulus is obtained by slope of graph in Fig. 6(a), a lower value means a larger stress distribution. As a result of Weibull analysis, the characteristic strengths of non-, nano-, and micro-textured Si wafers were 186.0 MPa, 103.8 MPa, and 91.0 MPa, respectively. The surface texture acts as a stress concentrator, and the strength of textured Si wafers is lowered. Based on previous research ^35,38, in case of a single notch, stress concentration is independent of notch angle when the notch angle is less than 90°, and notch depth and tip radius affects stress concentration. When the nano- and micro-texture are compared, tip radii of nano- and micro-texture are 29 nm and 28 nm, respectively. Also, the maximum notch depths of nano- and micro-textures are 1 µm and 10 µm, respectively. Although the tip radii of nano-texture and micro-texture are similar, the notch depth of micro-texture is much deeper than that of nano-texture. Therefore, the characteristic strength of nano-textured Si samples is greater than that of micro-textured Si samples. Weibull moduli of non-, nano-, micro-textured samples are 3.58, 4.77, and 5.27, respectively. In other words, the stress distribution of non-textured Si samples is wider than that of textured Si wafers. It means that a flexural strength of non-textured Si samples is dominated by the highest stress concentrator that can be included stochastically. In case of textured samples, surface textures act as a highest stress concentrator, and they dominate in flexural strength. Also, Weibull modulus of micro-textured Si samples is higher than that of nano-textured samples. It means that a larger stress concentration occurs in the micro pyramidal texture. Figure 6(c-e) show the images immediately before the fracture of three samples. Through these images, we obtained the critical bending radii. The critical bending radii were 27.2 (± 6.6) mm for non-textured samples, 45.2 (± 6.6) mm for nano-textured samples, and 53.9 (± 12.5) mm for micro-textured samples. The critical bending radius of nano-textured samples is 19.2% smaller than that of micro textured samples. All the parameters extracted from the 4-point bending tests are summarized in Table 2.

Table 2

Weibull modulus(m), critical bending radius (R_crit), characteristic strengths (σ₀) of UT-Si wafers by Weibull distribution analysis.
Parameters	Planar	Nano-texture	Micro-texture
m	3.58	4.77	5.27
${\sigma }_{0}$ (MPa)	186.0	103.8	91.0
${R}_{crit.}$ (mm)	27.2 (± 6.6)	45.2 (± 6.55)	53.9 (± 12.5)

3.6 Flexible UT-PERC solar cells: device performances, flexibility

PERC cells were fabricated with 50 µm-thick UT c-Si wafers with a planar texture, a typical micro-pyramid texture, and a nano-pyramid texture to compare their performances. The current-voltage characteristics measured under a standard solar irradiation of AM 1.5G with a 100 mW/cm2 light intensity are presented in Fig. 7(a). The external quantum efficiency (EQE) was also measured and shown in Fig. 7(b). It can be divided into a SLARC structure and a DLARC structure, and the overall performances of the UT-PERC cell are shown in Table 3. In both SLARC and DLARC structures, the UT-PERC cell with nano-pyramid textures showed the best efficiency. The average efficiency of the SLARC nano-pyramid UT-PERC cell was 18.25%, which was 0.15% higher in absolute efficiency than that of the micro-pyramid UT-PERC cell, 18.10%. The average efficiency of DLARC nano-pyramid UT-PERC cells was 18.56%, which was 0.16% higher in absolute efficiency than that of micro-pyramid UT-PERC cells, 18.40%. Apart from the ARC structure, it was confirmed that the difference between nano-pyramid and micro-pyramid showed a similar efficiency difference of about 0.15%. The champion device that showed the highest efficiency was a DLARC nano-pyramid UT-PERC cell with an efficiency of 18.68%. A closer look at the characteristics parameters of the nano-pyramid and micro-pyramid UT-PERC cells shows that there is no significant difference in V_oc, the cells of the micro-pyramid structure are higher in J_sc, and the cells of the nano-pyramid structure are superior in FF. These results are thought to be due to the emitter formation properties of the nanostructures. The emitter profile formed in the nano-pyramid has a higher surface density and deeper junction depth than the conventional textured, which might be a reason for the loss in the lower current density and superior FF of the nano-pyramid UT-PERC cell. The J-V curve and EQE results in Fig. 7(a) and (b) show only the results of the champion device among the DLARC PERC cells with different surface condition.

Table 3

Performance parameters of the UT-Si wafer based solar cells with various textures and ARCs. The champion cell performance parameters in each cell are shown in parentheses.
ARC	Texture	V_oc(mV)	J_sc(mA/cm²)	FF(%)	Eff(%)
SLARC	Planar	619.00 ± 2.40 (622)	33.84 ± 0.34 (34.29)	76.18 ± 0.30 (76.35)	15.96 ± 0.25 (16.28)
	Micro	620.80 ± 2.53 (622)	38.01 ± 0.28 (38.25)	76.84 ± 0.20 (76.88)	18.10 ± 0.11 (18.31)
	Nano	622.00 ± 2.80 (628)	37.66 ± 0.20 (37.65)	78.00 ± 0.22 (77.91)	18.25 ± 0.10 (18.33)
DLARC	Planar	618.20 ± 1.79 (621)	35.14 ± 0.13 (35.18)	76.13 ± 0.10 (76.12)	16.54 ± 0.10 (16.63)
	Micro	620.33 ± 1.86 (623)	38.54 ± 0.12 (38.56)	76.96 ± 0.08 (77.12)	18.40 ± 0.10 (18.53)
	Nano	620.14 ± 2.03 (624)	38.34 ± 0.10 (38.32)	78.08 ± 0.14 (78.13)	18.56 ± 0.17 (18.68)

The bending radius test with the fabricated UT-PERC cells was performed using the specially designed bending test jigs with different bending radius shown in Figure S3 to check if the result of the flexibility experiment conducted through the bare wafer has any difference in the actual cell stage. With each UT-PERC cell having a planar surface, a typical micro-pyramid textured surface, and a nano-pyramid textured surface could be used to identify a critical bending radius at which the cell fracture occurs. Interesting results from critical bending radius tests of UT-PERC cells with different surface structures can be seen in Fig. 7(c). A critical bending radius of planar UT-PERC cells were 26 mm, micro-pyramid UT-PERC cells were 28 mm and a critical bending radius of nano-pyramid UT-PERC cells were 26 mm, respectively. The nano-pyramid UT-PERC cells reduced the critical bending radius by 2 mm compared with micro-pyramid UT-PERC cells. The critical bending radius of nano-pyramid UT-PERC cells was found to be equivalent to that of planar UT-PERC cells, but it is necessary to look more closely at the 28 mm test results. When looking at the test results at a bending radius of 28 mm where the fracture started, planar samples had a breakage probability of 50%, while nano-pyramid samples had a breakage probability of 40%. The critical bending radius test results of these UT-PERC cells are consistent with the previous 4-point bending test results. The nano-pyramid samples clearly show higher mechanical flexibility than the micro-pyramid samples. The nano-pyramid samples do not show a significant difference compared to the planar samples but show slightly improved mechanical flexibility. In the additional cycle bending test, as shown in Fig. 7 (d), micro-pyramid UT-PERC cells showed a decrease in efficiency of 4.3% compared to the initial efficiency after 1,000 bending tests, whereas planar UT-PERC cells and nano-pyramid UT-PERC cells showed a decrease in efficiency of 3.0% and 2.0%, respectively. As shown in Fig. S7, this decrease in efficiency after the cycle bending test is mainly due to the decrease in FF and series resistance. It is thought that the series resistance increased as the electrode was damaged during the bending test.

4.1. Laser-assisted nano-texturing mechanism

As mentioned in the results part, the role of the nano-lens is to concentrate the light passing through the lens. It is necessary to find a process window that does not damage the nano-lens while obtaining a uniform and dense texture shape by optimizing the irradiated laser fluence condition. As shown in Figure S4, when the laser fluence is too low, even if a nano-lens is used, a uniformly textured surface cannot be obtained in a short time. Also, like the condition of 7.13 J/cm², if the laser fluence is too strong, the nano-lens is damaged and the shape of the texture surface also increases, so that the nano-pyramid is not formed. Through experiments, the critical power for laser-assisted nano-texturing using a nano-lens was determined to be 4.1 J/cm², and the optimal laser condition for the most uniform texturing was 5.5 J/cm². Looking at the results of Figure S4 along with the laser power condition, it was found that the density of initial pyramidal nucleation was related to the number of scans of the laser. When analyzing particles using Image-J, it can be seen that the number and density of pyramidal nuclei generated at the beginning of the texture increase as the number of laser scans increases 1, 3, and 5 times.

Recall that the laser-assisted texturing process causes the local heating on the Si wafer by a focused laser beam through the nano-lens, creating a unique spherical SiO₂ mask on the Si surface. The distribution of these unique spherical SiO₂ masks can be seen through the AFM image in Figure S6. Analyzing the distribution of these unique spherical SiO₂ masks, it can be confirmed that the spacing between the SiO₂ masks is about 775 nm, similarly to the spacing of our nano-lens. Therefore, as mentioned above, the unique spherical SiO₂ masks generated by the nano-lens, laser, and the 90 nm SiO₂ layer make the initial pyramidal nucleation fast and dense when the Si wafer is textured in KOH solution. It is a key factor that allows obtaining a uniform nano-pyramid surface in a short time of 90 seconds.

4.2. Finite element method (FEM) simulation and mechanical flexibility

The cross-sectional SEM images of the nano- and micro-textured Si wafers in the same scale are presented in Fig. 8(a) and (b). FEA simulations were conducted to analyze the stress concentration and distribution of micro-and nano-textured samples. As shown in Fig. 8(c) and (d), the maximum stress is concentrated on micro texture which has highest depth for micro-textured sample. On the other hand, in periodic nano-textured samples, the stress is distributed by around texture, and the stress concentration effect is less than that of micro-textured samples. Through FEA results, a stress concentration factor (SCF) was calculated using the following Eq. 3^5,37:

$$K= \frac{{\sigma }_{max}}{{\sigma }_{nom}}$$

where K is a stress concentration factor, and ${\sigma }_{max}$ is a maximum stress at the notch tip. ${\sigma }_{nom}$ is the nominal stress far from the texture. Stress concentration factors analyzed by 4-point bending tests were 1.79 for nano-textured samples and 2.05 for micro-textured samples. Calculated stress concentration factors based on the FEA results were 6.13 for nano-textured samples and 8.27 for micro-textured samples. Because the FEA simulations were performed using two-dimensional models, stress dispersion does not occur effectively. Therefore, the calculated SCF is higher than the measured SCF. Through bending results and FEA results, it is found that stress is dispersed effectively in nano textured Si wafers. In other words, the flexibility of nano-textured samples is better than that of micro-textured samples due to the stress dispersion effect.

In summary, we have developed a laser assisted nano-texturing process using a hydrogen silsesquioxane (HSQ) film as a nano lens to focus a pulse laser onto US-Si wafers. The focused laser induces silicon oxide nanoparticles on the UT-Si wafers, which serve as etch mask in alkaline solution for nanoscale pyramid texturing. The mechanical flexibility of the UT-Si wafers textured random pyramid structures was analyzed by a 4-pint bending test and compared with planar UT-Si wafers. We introduced the nanoscale pyramid textures on the UT-Si based cell fabrication in the passivated emitter rear cell (PERC) architecture. The PERC solar cells, which are currently a mainstream technology in the PV industry, were manufactured using 50 µm-thick UT c-Si wafers with different surface textures, and their performance was verified. Through this experiment, we lead to a conclusion that the mechanical flexibility of the US-Si wafers with the nano-pyramid textures is excellent compared to the planar and typical micro-pyramid textured wafers. In addition, the PERC flexible solar cells maintain their efficiency under 1,000 cyclic bending tests with a 3 mm bending radius. Therefore, we believe that the laser texturing technology using the nano lens we proposed can be a promising candidate for the manufacture of a thin flexible solar cell.

Credit authorship contribution statement

Youngseok Lee: Methodology, Investigation, Data curation, Writing-original draft. Jeong-Hyun Woo: Methodology, Investigation, Writing-original draft. Keonhee Kim: Writing-review & editing. Kyeong Seok Lee: Investigation, Methodology.Yeonjoo Jeong: Investigation, Validation. Jaewook Kim: Investigation, Validation. Gyu Weon Hwang: Writing-review & editing. Doh-Kwon Lee: Funding acquisition, Writing-review & editing. Ju-Young Kim: Conceptualization, Methodology, Writing-review & editing. Inho Kim: Supervision, Funding acquisition, Writing-review & editing.

Declaration of Competing Interest

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.

Data availability

Data will be made available on request.

Acknowledgements

This work was partly supported by KIST (Korea Institute of Science and Technology) institutional program and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government (MOTIE) (20213091010010). I. Kim and D.-K. Lee acknowledged the financial support form the National Research Foundation of Korea (Grant No. 2019K1A3A1A61091348).

Wang, K. X., Yu, Z., Liu, V., Cui, Y. & Fan, S. Absorption Enhancement in Ultrathin Crystalline Silicon Solar Cells with Antireflection and Light-Trapping Nanocone Gratings. Nano Letters 12, 1616–1619 (2012). https://doi.org:10.1021/nl204550q
He, J. et al. Realization of 13.6% Efficiency on 20 µm Thick Si/Organic Hybrid Heterojunction Solar Cells via Advanced Nanotexturing and Surface Recombination Suppression. ACS Nano 9, 6522–6531 (2015). https://doi.org:10.1021/acsnano.5b02432
Bye, G. & Ceccaroli, B. Solar grade silicon: Technology status and industrial trends. Solar Energy Materials and Solar Cells 130, 634–646 (2014). https://doi.org:https://doi.org/10.1016/j.solmat.2014.06.019
Woodhouse, M. A., Smith, B., Ramdas, A. & Margolis, R. M. Crystalline Silicon Photovoltaic Module Manufacturing Costs and Sustainable Pricing: 1H 2018 Benchmark and Cost Reduction Road Map. (United States, 2019).
Louwen, A., van Sark, W., Schropp, R. & Faaij, A. A cost roadmap for silicon heterojunction solar cells. Solar Energy Materials and Solar Cells 147, 295–314 (2016). https://doi.org:https://doi.org/10.1016/j.solmat.2015.12.026
Green, M. A. Self-consistent optical parameters of intrinsic silicon at 300K including temperature coefficients. Solar Energy Materials and Solar Cells 92, 1305–1310 (2008). https://doi.org:https://doi.org/10.1016/j.solmat.2008.06.009
Hadibrata, W., Es, F., Yerci, S. & Turan, R. Ultrathin Si solar cell with nanostructured light trapping by metal assisted etching. Solar Energy Materials and Solar Cells 180, 247–252 (2018). https://doi.org:https://doi.org/10.1016/j.solmat.2017.06.029
Wang, S. et al. Large-Area Free-Standing Ultrathin Single-Crystal Silicon as Processable Materials. Nano Letters 13, 4393–4398 (2013). https://doi.org:10.1021/nl402230v
Zhou, S. et al. Wafer-Scale Integration of Inverted Nanopyramid Arrays for Advanced Light Trapping in Crystalline Silicon Thin Film Solar Cells. Nanoscale Research Letters 11, 194 (2016). https://doi.org:10.1186/s11671-016-1397-6
Yu, K. J. et al. Light Trapping in Ultrathin Monocrystalline Silicon Solar Cells. Advanced Energy Materials 3, 1401–1406 (2013). https://doi.org:https://doi.org/10.1002/aenm.201300542
Pagliaro, M., Ciriminna, R. & Palmisano, G. Flexible Solar Cells. ChemSusChem 1, 880–891 (2008). https://doi.org:https://doi.org/10.1002/cssc.200800127
Hwang, I. et al. Effective Photon Management of Non-Surface-Textured Flexible Thin Crystalline Silicon Solar Cells. Cell Reports Physical Science 1, 100242 (2020). https://doi.org:https://doi.org/10.1016/j.xcrp.2020.100242
Hwang, I., Um, H.-D., Kim, B.-S., Wober, M. & Seo, K. Flexible crystalline silicon radial junction photovoltaics with vertically aligned tapered microwires. Energy & Environmental Science 11, 641–647 (2018). https://doi.org:10.1039/C7EE03340K
Garnett, E. & Yang, P. Light Trapping in Silicon Nanowire Solar Cells. Nano Letters 10, 1082–1087 (2010). https://doi.org:10.1021/nl100161z
He, J. et al. Enhanced Electro-Optical Properties of Nanocone/Nanopillar Dual-Structured Arrays for Ultrathin Silicon/Organic Hybrid Solar Cell Applications. Advanced Energy Materials 6, 1501793 (2016). https://doi.org:https://doi.org/10.1002/aenm.201501793
Mavrokefalos, A., Han, S. E., Yerci, S., Branham, M. S. & Chen, G. Efficient Light Trapping in Inverted Nanopyramid Thin Crystalline Silicon Membranes for Solar Cell Applications. Nano Letters 12, 2792–2796 (2012). https://doi.org:10.1021/nl2045777
Lee, Y., Woo, Y., Lee, D.-K. & Kim, I. Fabrication of quasi-hexagonal Si nanostructures and its application for flexible crystalline ultrathin Si solar cells. Solar Energy 208, 957–965 (2020). https://doi.org:https://doi.org/10.1016/j.solener.2020.08.063
Han, S. E. & Chen, G. Toward the Lambertian Limit of Light Trapping in Thin Nanostructured Silicon Solar Cells. Nano Letters 10, 4692–4696 (2010). https://doi.org:10.1021/nl1029804
Tan, X., Yan, W., Tu, Y. & Deng, C. Small pyramidal textured ultrathin crystalline silicon solar cells with double-layer passivation. Opt. Express 25, 14725–14731 (2017). https://doi.org:10.1364/OE.25.014725
Lu, Y. & Lal, A. High-Efficiency Ordered Silicon Nano-Conical-Frustum Array Solar Cells by Self-Powered Parallel Electron Lithography. Nano Letters 10, 4651–4656 (2010). https://doi.org:10.1021/nl102867a
Fischer, P. B. & Chou, S. Y. Sub-50 nm high aspect‐ratio silicon pillars, ridges, and trenches fabricated using ultrahigh resolution electron beam lithography and reactive ion etching. Applied Physics Letters 62, 1414–1416 (1993). https://doi.org:10.1063/1.108696
Chen, Y. Nanofabrication by electron beam lithography and its applications: A review. Microelectronic Engineering 135, 57–72 (2015). https://doi.org:https://doi.org/10.1016/j.mee.2015.02.042
Zhao, L. et al. Antireflection silicon structures with hydrophobic property fabricated by three-beam laser interference. Applied Surface Science 346, 574–579 (2015). https://doi.org:https://doi.org/10.1016/j.apsusc.2015.04.058
Lee, H.-S. et al. Enhanced efficiency of crystalline Si solar cells based on kerfless-thin wafers with nanohole arrays. Scientific Reports 8, 3504 (2018). https://doi.org:10.1038/s41598-018-21381-2
Mårtensson, T. et al. Nanowire Arrays Defined by Nanoimprint Lithography. Nano Letters 4, 699–702 (2004). https://doi.org:10.1021/nl035100s
Yu, Z., Gao, H., Wu, W., Ge, H. & Chou, S. Y. Fabrication of large area subwavelength antireflection structures on Si using trilayer resist nanoimprint lithography and liftoff. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena 21, 2874–2877 (2003). https://doi.org:10.1116/1.1619958
Goel, S., Kovalchenko, A., Stukowski, A. & Cross, G. Influence of microstructure on the cutting behaviour of silicon. Acta Materialia 105, 464–478 (2016).
Vodenitcharova, T., Borrero-López, O. & Hoffman, M. Mechanics prediction of the fracture pattern on scratching wafers of single crystal silicon. Acta materialia 60, 4448–4460 (2012).
Cheon, S.-E. et al. Fabrication of parabolic Si nanostructures by nanosphere lithography and its application for solar cells. Scientific Reports 7, 7336 (2017). https://doi.org:10.1038/s41598-017-07463-7
Lee, H.-S. et al. Design of mid-infrared filter array based on plasmonic metal nanodiscs array and its application to on-chip spectrometer. Scientific Reports 11, 12218 (2021). https://doi.org:10.1038/s41598-021-91762-7
Mohsin, A. S. M., Mobashera, M., Malik, A., Rubaiat, M. & Islam, M. Light trapping in thin-film solar cell to enhance the absorption efficiency using FDTD simulation. Journal of Optics 49, 523–532 (2020). https://doi.org:10.1007/s12596-020-00656-w
Andreani, L. C., Bozzola, A., Kowalczewski, P., Liscidini, M. & Redorici, L. Silicon solar cells: toward the efficiency limits. Advances in Physics: X 4, 1548305 (2019). https://doi.org:10.1080/23746149.2018.1548305
Diedenhofen, S. L. et al. Broadband and omnidirectional anti-reflection layer for III/V multi-junction solar cells. Solar Energy Materials and Solar Cells 101, 308–314 (2012). https://doi.org:https://doi.org/10.1016/j.solmat.2012.02.022
Lee, J. S. et al. The Flexural Strengths of Silicon Substrates with Various Surface Morphologies for Silicon Solar Cells. Korean Journal of Materials Research 19, 18–23 (2009).
Woo, J.-H. et al. Critical bending radius of thin single-crystalline silicon with dome and pyramid surface texturing. Scripta Materialia 140, 1–4 (2017).
Ćurković, L., Bakić, A., Kodvanj, J. & Haramina, T. Flexural strength of alumina ceramics: Weibull analysis. Transactions of FAMENA 34, 13–18 (2010).
Kashyap, K. et al. Elimination of strength degrading effects caused by surface microdefect: A prevention achieved by silicon nanotexturing to avoid catastrophic brittle fracture. Scientific reports 5, 1–13 (2015).
Pilkey, W. D., Pilkey, D. F. & Bi, Z. Peterson's stress concentration factors. (John Wiley & Sons, 1997).

(Not answered)

SupplementaryinformationNPGAsiaMatSubmission.docx

Download PDF

Version 1

posted

You are reading this latest preprint version

Laser-assisted Nano-texturing for Flexible Ultrathin Crystalline Si Solar Cells

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2.1. Fabrication process of nano-lens

2.2 Laser-assisted texturing

2.3 Four-point bending test

2.4 Finite element analysis (FEA) simulations

2.5 Cell fabrication and cycle bending test

3. Results

3.1 Laser-assisted nanotexturing process

3.2 Nano-lens fabrication results

3.3 Laser-assisted texturing process

3.4 Light trapping performance of textured UT-Si

3.5 Mechanical flexibility of UT c-Si wafers

3.6 Flexible UT-PERC solar cells: device performances, flexibility

4. Discussion

4.1. Laser-assisted nano-texturing mechanism

4.2. Finite element method (FEM) simulation and mechanical flexibility

5. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1