Silicon-nanowire hybrid solar cells fabricated with thin-film absorber
In recent years, great effort has been devoted to developing cost-effective silicon nanowires (SiNWs) on organic solar cells hybridized with silicon (Si) wafers.1–3 To date, these materials have demonstrated a promising power conversion efficiency (PCE) up to 10%.4–7 However, this excellent performance is substantially owed to the photocurrent generated in the costly, bulky Si wafer. For low-cost applications, alternative approaches that bypass bulky wafers—such as fabricating the SiNWs on thin-film Si—should be considered. Recently,8 we reported the development of SiNW/poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) hybrid solar cells using only a thin (2.2μm) layer of epitaxial Si absorber deposited on a heavily doped Si substrate.
Figure 1 shows the device architecture of the Si/PEDOT:PSS hybrid solar cells based on this thin film both with and without nanowire texturing. A 2.2μm-thick crystalline epitaxial Si thin film with a phosphorus doping concentration of ∼1.5×1016cm−3 was grown on top of a n++ arsenic-doped Si substrate using chemical vapor deposition. We then fabricated SiNW arrays on the epitaxial Si layer using electroless chemical etching. We formed the heterojunction by spin-coating a PEDOT:PSS solution on top of the SiNW arrays, which we followed by the deposition of rear and front electrodes.8 The n++ Si substrate has an extremely high doping concentration of ∼1×1020cm−3 and, therefore, the photocurrent contributed from it is negligible. Thus, the substrate and epitaxial layer enables us to mimic a Si thin film with an effective absorbing thickness of ∼2.2μm.
Figure 2 represents the cross-sectional view scanning electron micrograph of a 0.3μm SiNW array spin-coated with a PEDOT:PSS layer ∼125nm in thickness. The PEDOT:PSS forms a continuous canopy above the SiNW array and does not infiltrate the gaps among the SiNWs.8 Compared with the planar Si cell, the SiNW hybrid cells demonstrate an increased short-circuit current density (Jsc) from 12.5 to 13.6mA/cm2, PCE from 5.4% to 5.6%, and external quantum efficiency from 49.7% to 56.6%.8
The simple fabrication steps and relatively high efficiency of our cells show the viability of using thin-film Si for Si/PEDOT:PSS hybrid solar cells. We demonstrated the possibility of fabricating Si-organic hybrid cells on epitaxial Si, or even polycrystalline Si, thin films grown on low-cost substrates such as Si ribbons or glass. Thus, it suggests a new approach toward low-cost, efficient, and industrially viable photovoltaics using Si thin films and a fully solution-based process.
In the near future, the two major issues we need to address in this type of hybrid cell are severe recombination on SiNWs with defective surfaces and the instability of the organic material.
Rusli received his BS and MS in engineering from the National University of Singapore and his PhD in electrical engineering from the University of Cambridge, UK. He is now an associate professor.
