Flexible solar cells based on foldable silicon wafers with blunted edges – Nature

Stress simulation

The solid mechanics module in COMSOL Multiphysics (v.5.6) was used to simulate the stress of a two-dimensional silicon wafer with the length and thickness set to 1 cm and 60 μm, respectively. The Young’s modulus, Poisson’s ratio and mass density of the wafer were 130 GPa, 0.26 and 2.33 g cm⁻³, respectively. The lower surface was textured with pyramids ranging from 5 μm to 8 μm in height and the initial angle between adjacent sharp pyramids was 71°. Three points around the midpoint of the top side of the wafer were fixed and bending forces of F_b = 1.2 mN were loaded on its two end points. The maximum von Mises stress was simulated as a function of the channel radius (R_p).

Atomistic simulation

Large-scale atomic/molecular massively parallel simulator (LAMMPS) package⁴⁷ was used to perform atomistic simulations of mode I loading on c-Si nanofilms with sharp and round channels between surface pyramids. The Tersoff potential⁴⁸ was used to describe the interatomic interaction between Si atoms. The simulated samples were 217.24 nm × 54.21 nm × 2.17 nm in size, containing approximately 1,150,000 Si atoms oriented along the [100], [010] and [001] directions with respect to the x, y and z axes, respectively. The R_p of the channels between the pyramids was increased from 0 to 15.81 nm. Periodic boundary conditions were imposed in the y and z directions of the simulation systems. Mode I loading was performed by uniformly stretching the simulation box with a strain rate of 5 × 10⁸ s^–1. Deformation processes coloured by the von Mises shear strain as well as stress−strain curves of the simulation samples with sharp and blunt notches were recorded as videos. Cracking of the blunted sample initiated at a higher loading strain of 17.3% compared with that of 9.3% for the untreated sample. Here, the simulation was qualitative because the R_p values were much smaller than those in the experimental conditions.

TEM characterization

An in situ bending test of a c-Si foil was conducted on an FEI Tecnai F30 TEM system using an electrical holder from PicoFemto. The c-Si foil was 6 μm × 12 μm × 70 nm in size, which was cut from the top surface of a wafer with sharp pyramids using a ThermoFisher Scios 2 FIB–SEM system, followed by deposition of a Pt film on the surface to protect the sharp pyramids. Then, the c-Si foil was welded onto a copper FIB holder with a diameter of 3 mm. A tungsten tip was used to contact the left side of the FIB c-Si foil; the movement of the foil was controlled by a piezo manipulator at a rate of about 0.01 nm s⁻¹, to apply a bending force on the edge of the c-Si foil with an estimated strain rate of 10⁻³ s⁻¹. For all bending processes, a 300-kV voltage with a weak electron beam was used in the TEM system to minimize the potential beam effects on the bending deformation. The real-time stress distribution was recorded by a charge-coupled device camera at a rate of 20 frames per second.

The fracture surfaces of two 60-μm wafers with sharp and round channels between pyramids were protected by a bilayer consisting of carbon and Pt films. In particular, a carbon film with a thickness of 100 nm was deposited by magnetron sputtering (ISC150 T Ion Sputter Coater) for nondestructive protection of the surface; then, the FIB-TEM foils were cut from the fracture surface using a ThermoFisher Scios 2 FIB–SEM system. STEM-HAADF observations were conducted at a depth of dozens of atoms from these fracture surfaces on an FEI Themis Z with a spherical aberration corrector for the illumination system.

Three-point bending test

Load–vertical displacement (F–D) curves of 4 cm × 2 cm × 140 μm textured c-Si wafers were obtained using a commercial Discovery DMA 850 instrument (Supplementary Fig. 15). The marginal regions of these textured wafers were blunted in 10 vol% HF:90 vol% HNO₃ solution for 0, 15 and 30 s.

GPA

The elastic strain distribution in the fractured c-Si wafers was mapped using GPA on the basis of the individual high-resolution STEM images. GPA, which was done on the basis of the formalism given in the literature¹⁷ and implemented in the Gatan Digital Micrograph as a plug-in, was used to calculate the in-plane components of the symmetric strain tensor ε_ij. Strain maps were plotted with respect to an internal reference lattice based on g1 = (200) and g2 = (020) using Lorentzian masks with a diameter of 0.5 nm⁻¹ (in reciprocal space). The maximum and minimum strains were set in the range of 5% to −5%.

SEM characterization

Top views, side views and fracture surfaces of c-Si wafers were observed using SEM (HITACHI, SU8020). Sharp channels between the pyramids of these wafers were blunted in 10 vol% HF:90 vol% HNO₃ solution for 0, 10, 20, 30, 40 and 90 s. The concentrations of HF and HNO₃ were 49% and 68%, respectively, diluted in water.

Ultraviolet–visible–infrared light characterization

The reflectivity of c-Si wafers from 300 to 1,200 nm was characterized using a UV−VIS−IR instrument (PerkinElmer Lambda 950).

Optical simulations

We used the electromagnetic wave module in COMSOL Multiphysics (v.5.6) to simulate the transmission, reflection and absorption spectra. A stack of a 10-nm a-Si:H layer and an 80-nm tungsten-doped indium oxide layer was coated onto a 60-μm silicon slab. This structure was surrounded by air. Three silicon slabs were simulated; their surfaces were planar, pyramidal (height: 5 μm; pyramid angle: 71°) and rounded (radius: 2 μm). For the nonplanar silicon slabs, the thickness was the average distance between their boundaries. The upper and lower boundaries of the simulation region were set as Floquet boundary conditions. The wavelength and incident angle of light were distributed from 300 to 1,200 nm and from 0° to 80°, respectively. The plane wave entered from one side of the slab. The refractive index of air was 1, whereas the refractive indices of the other materials were analysed using ellipsometry. The transmittance and reflectance, defined as the ratio of the energy of the transmitted wave and reflected wave to that of the incident wave, respectively, were obtained by integrating the Poynting vector. The absorbance of the whole structure (silicon layer) was the ratio of the dissipation energy in the whole structure (silicon layer) to the energy of the incident wave.

Ultrahigh-speed video camera characterization

High-speed imaging of the cracking process of a 60-μm c-Si wafer with sharp pyramids was studied using a Phantom V2511 ultrafast CMOS video camera. It recorded up to 100,000 frames per second through a Leica Z16 APO long-distance microscope. The resolution was approximately 17.5 µm per pixel.

Solar cell fabrication

Czochralski n-type c-Si wafers were purchased from Sichuan Yongxiang. Their thickness and electrical resistivity were 160 μm and 0.3−2.1 Ω cm, respectively. The saw damage was removed in a 20.0-vol% alkaline water solution at 80 °C and the duration was varied to obtain different wafer thicknesses. Then, the wafers were textured in a 2.1-vol% alkali water solution at 80 °C for 10 min to form microscale pyramids on the surfaces. To fabricate flexible solar cells, the approximately 2-mm-wide marginal region of these 60-μm textured wafers was blunted in 10 vol% HF:90 vol% HNO₃ solution for 90 s at room temperature. All wafers were cleaned using a standard RCA process to remove organics and metal ions. Next, they underwent cleaning in 2.0% hydrofluoric acid water solution for 3 min to etch the surface oxide. The creative thin c-Si technology developed previously has a great potential for flexible solar cells^49,50 because of sufficient utilization of the silicon material. Similar to the wet process, a dry method is also very efficient for improving the flexibility of the wafer (Supplementary Fig. 16). The marginal region of the wafer was blunted by a blending plasma (power, 120 W) of argon and fluorine ions for 30 min.

In a cluster plasma-enhanced chemical vapour deposition system (VHF-PECVD, IE Sunflower, OAK-DU-5; ULVAC CME-400), 5 nm i-a-Si:H and 15 nm p-a-Si:H, and 4 nm i-a-Si:H and 6 nm n-a-Si:H were deposited on the back and front sides of the wafers, respectively, in which the process temperatures were 200 ± 5 °C. The i-a-Si:H layers had a bilayer architecture; the first layer was grown using pure SiH₄, whereas the second layer was grown using diluted SiH₄ in H₂ with a flow ratio of 1:10. A 15-s H₂ plasma was used to improve the passivation quality at the interface of i-a-Si:H and n-c-Si. The power density, chamber pressure and gas flow ratio during deposition of the n-a-Si:H layer were 33 mW cm⁻², 80 Pa and [PH₃]:[SiH₄]:[H₂] = 1.5:100:1,000, respectively. The p-a-Si:H layer also had a bilayer architecture, for which the deposition power density, chamber pressure and gas flow ratio were 20/20 mW cm⁻², 80/80 Pa and [B₂H₆]:[SiH₄]:[H₂] = 1:100:100/2:100:400, respectively. Tungsten-doped indium oxide was deposited by reactive plasma deposition at 150 °C and the target was 1.0% tungsten dissolved in an indium oxide target. Electrode busbars and fingers were screen-printed on the surfaces of the devices using a low-temperature silver paste, followed by two-step annealing at 150 °C for 5 min and 185 °C for 30 min. On the sides of the certified SHJ solar cells that were exposed to sunlight, fine busbars were screen-printed and a 110-nm MgF₂ layer was deposited by electron-beam evaporation to improve the light-harvesting efficiency.

Solar cell (module) characterization

The current–voltage characteristics of SHJ solar cells (modules) were tested with a solar simulator (Halm IV, ceitsPV-CTL2) and the light intensity was calibrated using a National Renewable Energy Laboratory reference cell. A 60-μm flexible cell was independently tested by the National Institute of Metrology in China. To compare the current density, a 140-μm brittle cell was independently tested by ISFH CalTeC in Germany. All devices were tested under a standard illumination of 100 mW cm⁻² at 25 °C.

Bending cycle stability

The edge of a 60-μm flexible SHJ solar cell was folded to touch the opposite edge; this bending was maintained for more than 10 s. The bending speed was approximately 1,000 mm min⁻¹. The J_sc, V_oc, FF and PCE of this cell were tested with a solar simulator during 1,000 bending cycles under standard illumination of 100 mW cm⁻² at 25 °C. The bending test was conducted in directions vertical and parallel to the direction of the busbars. We also monitored the sheet resistance of the 80-nm tungsten-doped indium oxide layer on a 60-μm quasiplanar c-Si substrate during 500 side-to-side bending cycles (Supplementary Fig. 17).

Vibration experiment

A 1,260 mm × 860 mm flexible SHJ module was installed on a large vibration platform, in which the module was supported by metal holders with a height of approximately 3 cm. The module vibrated in the z direction, expressed as Z(t) = Z₀sin(2πt/T), where the vibration amplitude Z₀ = 5 mm and the vibration period T = 200 ms. Electroluminescence images and the power of this flexible module before and after 18,000 vibration periods were obtained.

Free-falling test

We made a 5.4-kg, 520 mm × 520 mm module using our flexible SHJ solar cells, which was subjected 15 times to continuous free-falling from a height of approximately 500 mm. Its power was recorded before and after the free-falling cycles.

Thermal cycling stability

Thermal cycling was conducted between −70 °C and 85 °C for 120 h. In each cycle, the module was maintained at −70 °C for 1 h and at 85 °C for 1 h.

Violent storm stability

Flexible SHJ solar cells were encapsulated in a large (>10,000 cm²) module, which was attached to a large soft gasbag inflated with air to support this flexible module. The pressure inside the gasbag was 94.7−830 Pa higher than the atmospheric pressure. A powerful fan was used to blow air on the module at a wind speed of 30 m s⁻¹ to model a violent storm (Beaufort number 11: 28.5−32.6 m s⁻¹). The power and electroluminescence images of this module before and after continuous impact by this air flow for 20 min were obtained.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.