One of the promising technologies for third generation solar cells is represented by devices that use nanocrystals (quantum dots). In particular, solar cells with nanocrystals that exhibit new physical phenomena such as carrier multiplication (CM), present great potential for efficiency improvement and have attracted vast attention in the photovoltaic scientific community. The principle of exploiting the benefits of CM has been shown by integrating PbSe or PbS nanocrystals in quantum dot solar cells.

Nevertheless, the full assessment of the life-cycle, environmental aspects of PbSe/PbS nanocrystals is required. Indeed, other materials can instantaneously offer better opportunities that can be in principle more economically viable and with lower environmental impact. Since silicon-based technology is mature and dominates the solar cell market, a third generation photovoltaic technology based on elemental silicon (Si) nanocrystals represents a sensible approach.

While Si nanocrystals offer a range of opportunities that still need to be explored, CM effects are in this case triggered only for relatively higher photon energies. Therefore, alloying Si with another element, which decreases the band gap, might offer the possibility of activating CM at lower photons energies. It has to be noted that nanocrystals made of Si and germanium (Ge) have been already produced, however the Si-Ge system is not expected to show the benefits of a direct-bandgap semiconductor.

On the other hand, the silicon-tin (Si1-xSnx) system is an interesting candidate as an optically active material where the concentration of Sn can be effectively used to extend the range of achievable bandgaps below the energy gap of silicon (1.15 eV) down to 0.45 eV; furthermore, a transition from an indirect semiconductor behavior to a direct one is expected to occur with increasing Sn concentration in the nanocrystals. However, the Si-Sn system presents some synthetic challenges due to thermodynamic instability.

Very recently we have demonstrated the synthesis of semiconducting SiSn nanocrystals via a highly non-equilibrium spatially confined short-pulsed laser process. In this contribution, the potential of using spatially-confined plasma to induce the growth of SiSn nanocrystals via kinetic pathways will be discussed in details. Our investigations suggest that alloying between Si and Sn can occur at relatively high Sn concentrations (<50%) resulting in the synthesis of semiconducting SiSn nanocrysatls with quantum confinement effects. Due to the successful alloying, quantum confinement and band gap narrowing is observed in the red shift of the room temperature photoluminescence (PL) maximum with respect to elemental Si nanocrystals. Furthermore we will show that the integration of surfactant-free surface-engineered SiSn nanocrystals into thin films allows for adequate absorption and carrier transport as required for their implementation in successful quantum dot solar cell technology.

Vladimir Svrcek

AIST

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