Thin silicon foils produced by epoxy-induced spalling of silicon for high efficiency solar cells

Abstract

We report on the drastic improvement of the quality of thin silicon foils produced by epoxy-induced spalling. In the past, researchers have proposed to fabricate silicon foils by spalling silicon substrates with different stress-inducing materials to manufacture thin silicon solar cells. However, the reported values of effective minority carrier lifetime of the fabricated foils remained always limited to ∼100 μs or below. In this work, we investigate epoxy-induced exfoliated foils by electron spin resonance to analyze the limiting factors of the minority carrier lifetime. These measurements highlight the presence of disordered dangling bonds and dislocation-like defects generated by the exfoliation process. A solution to remove these defects compatible with the process flow to fabricate solar cells is proposed. After etching off less than 1 μm of material, the lifetime of the foil increases by more than a factor of 4.5, reaching a value of 461 μs. This corresponds to a lower limit of the diffusion length of more than 7 times the foil thickness. Regions with different lifetime correlate well with the roughness of the crack surface which suggests that the lifetime is now limited by the quality of the passivation of rough surfaces. The reported values of the minority carrier lifetime show a potential for high efficiency (>22%) thin silicon solar cells.

Controlled Cracking for Large-Area Thin Film Exfoliation: Working Principles, Status, and Prospects

“…Characterization of the fractured surface of the as-exfoliated Si via electron spin resonance (ESR) and photoluminescence (PL) imaging shows that the defects cause significant reduction in the minority carrier lifetime. 28 The defects at the fracture surface can be appropriately removed by etching away a few micrometers of Si from the fracture surface via a wet-etching process. 64 Another approach to fabricate cost-effective heterojunction solar cells is through the utilization of 2D materials.…”

“…To avoid the above-mentioned disadvantages, a thermosetting polymer based thin film can be used as a stressor layer. − The polymer layer can be easily deposited by drop-casting or spin coating on a target substrate. In addition, the large difference in thermal expansion coefficients of the polymer layer and the semiconductor allows a significant reduction in the annealing temperature to below 150 °C to induce sufficient thermal misfit stress for crack growth.…”

“…It is noted that the crack-assisted layer transfer method may induce defects on the newly formed fracture surface of the spalled Si and donor substrate. Characterization of the fractured surface of the as-exfoliated Si via electron spin resonance (ESR) and photoluminescence (PL) imaging shows that the defects cause significant reduction in the minority carrier lifetime . The defects at the fracture surface can be appropriately removed by etching away a few micrometers of Si from the fracture surface via a wet-etching process …”

Kerfless exfoliated thin crystalline Si wafers with Al metallization layers for solar cells

“…16 Martini et al reached minority carrier lifetimes of 461 ls on a 100 lm thick foil of 1 X cm n-type material. 17 Measuring quasi-steady-state photoconductance (QSSPC) 18 on freestanding exfoliated silicon films, Saha et al found no degradation of the bulk carrier lifetime compared to the parent substrate before lift-off and showed a solar cell produced from a 25 lm thick exfoliated SOM silicon layer with nickel rear contact and an efficiency of 14.9% on a small area. 19 These results are encouraging for using spalled layers as solar cell absorber materials.…”

“…17,21 Within this paper, we present exfoliated silicon foils that were produced using an evaporated Al film as stressor layer. The use of a metal shows a distinct advantage for photovoltaic applications: To reduce the wafering costs it is preferable not to remove the stressor layer after lift-off but to use it as a functional layer.…”