(Invited) Microplasmas Technologies for Engineering of Silicon Based Quantum Dot Solar Cells

Abstract

Silicon nanocrystals (Si-ncs) are fascinating candidates for numerous applications such as photovoltaic cells. Solar cell technologies are highly dependent on silicon materials and Si-ncs with optimal energy band gap present potential for further development high efficiency solar cells. Especially, the tunability of the Si energy bandgap by precisely controlling the size of Si-ncs due to quantum confinement effects is expected to increase the efficiency of third generation all silicon tandem solar cells. Among the different semiconductor nanocrystals, Si-ncs have a special value as they are generally non-toxic and have been widely used on an industrial scale. A crucial aspect for Si-ncs integration into PV devices is represented by the control of the surface characteristics. Since quantum confinement effects are observed for small nanocrystal sizes (<10 nm in diameter) and at these dimensions the Si-nc surface chemistry plays a critical role for the overall optoelectronic properties and device performance. While the use of bulky and/or long molecular passivation is frequent and it is effective in stabilizing the Si-nc properties in colloids, it is highly detrimental for transport and device-related properties. In particular, photon absorption and subsequently exciton dissociation and carrier transport suffer from the separation between Si-ncs due to the bulky surfactants and considerably decrease the performance of devices. Recently, plasma-based approaches have been successfully applied to induce non-equilibrium chemical reactions on the surface of Si-ncs directly in colloids and to achieve surface-engineered Si-ncs without any bulky surfactants. Such approaches led to the enhancement of the Si-nc transport properties and its electronic interaction with a polymer matrix [1] as well as Si-nc long-term stability in water based solutions [2]. Another important issue is the doping of the Si-ncs. A number of issues especially regarding the large impurity formation energy by doping and transport are still in controversial debate. Doping of quantum confined Si-ncs offers unique opportunities to control the bandgap and the Fermi energy level. Particularly, boron-doped and phosphorus-doped quantum confined silicon nanocrystals (Si-ncs) are surface-engineered in colloidal solution by an atmospheric pressure radio frequency microplasma a will be underlined in this talk. In particular, we have performed the surface engineering of electrochemically etched p- and n-type Si-ncs where the no-change position of the dopant in Si-ncs is assured by room temperature processing [3]. We report on how the microplasma induced surface engineering can lead to drastically different outcomes due to different doping and how this impacts the corresponding optical and transport properties [3]. Next we report on Fermi level for doped Si-ncs. Our measurements demonstrate that surface engineering can tune the band energy levels and in particular the Fermi level. Indeed, we fabricate solar cells to test p-/n-type Si-ncs and induced surface chemistry to demonstrate the resulting effects. In particular it is noted that band alignment, dissociation efficiency and transport are all directly affected by the interplay of varying surface chemistry and doping levels. We demonstrate that surface chemistries induced on the Si-ncs strongly depend on the type of dopants and implications for third generation solar cells are discussed in details [4, 5]. As a final point, surfactant free microplasma surface-engineered Si-ncs can be integrated into the device architecture to be only optically active and provide a means of effective down-conversion of blue photons into red photons leading to 24% enhancement of the photocurrent under concentrated sunlight. We also show that the down-conversion effect under 1-sun is enhanced in the case of hybrid solar cells where microplasma engineered Si-ncs are also beneficial in the active absorption layer of the solar cells.