Graphene/silicon (Gr/Si) Schottky junction solar cells have attracted much attention due to the ease and low cost of fabrication, along with the lucrative properties of high electron mobility, transparency and mechanical flexibility of graphene as a transparent conducting electrode.¹ Utilizing its inherent mechanical flexibility, graphene can be integrated with thin flexible crystalline Si substrates opening up a new regime of applications in flexible and wearable electronics. Reducing Si absorber thickness below 50 um offers advantages of reduced material cost, along with mechanical flexibility and light weight.² But Si at such thicknesses suffers from low photon absorption in the solar spectrum. To compensate for the low light absorption in such thin substrates, light management schemes become essential. Light trapping in Gr/Si solar cells is enabled by engineering the Si surface to form nanopillars, nanowires etc., which decreases the reflection loss and allows more light to couple in to the Si substrate. The structured Si absorber increases surface area and surface recombination, which is detrimental to the solar cell efficiency. Thus, it is imperative to use a light-trapping scheme devoid of Si structuring to enhance the photo-conversion efficiency.

We present an all-dielectric light-trapping scheme on planar Gr/Si Schottky junction solar cells with the use of bottom layer of titania spheres and top layer of silica spheres.³ An optimal Si thickness coupled with an optimized light-trapping scheme leads to efficient electron-photon harvesting. The photo-conversion efficiency of a 20 um thick nanosphere-decorated Gr/Si solar cell improves to 9%, which is 1.3x higher than the pristine cell’s PCE of 7%. FDTD simulations are performed for optimizing the diameter of nanoparticles in each of the layers. The ratio of size of nanoparticles in the top to bottom layer plays a crucial role in advanced light management. An optimized structure of silica spheres, having diameter larger than that of titania spheres, suppresses reflection over wide angles of incidence and increases absorption in active Si layer over AM1.5G solar spectrum. The non-absorbing dielectric spheres can be easily realized by the well-known Stober technique. Additionally, the photovoltaic characteristic of the laminated solar cell shows negligible change after several bending cycles having bend radius ranging from 5 mm to 10 mm. After continuous bending and straightening, the ultra-thin solar cell can retain its performance, revealing the excellent stability and flexibility of the device. Such simple, low-cost light trapping schemes are universal in nature, devoid of recombination losses and are potentially viable for any solar cell technology.

References

¹ B Li, et al. Adv. Mater. 22, 2743 (2010).

² K E Petersen, Proc. IEEE 70, 420 (1982).

³ S. Das, et al. Nano Energy 58, 47 (2019).