Credit: TSINGHUA UNIV./ © 2010 WILEY-VCH

Graphene — a single layer of carbon atoms arranged in a honeycomb lattice — is already attracting interest for its various applications in electronics and optoelectronics, including its use as a transistor, photodetector and saturable absorber of light. It now seems that silicon photovoltaics may also benefit from this interesting material by exploiting it both as a semitransparent electrode and an antireflection coating.

Xinming Li and colleagues from Tsinghua University and Peking University in China have now reported a graphene-on-silicon solar cell with promising, albeit unoptimized, characteristics (Adv. Mater. doi: 10.1002/adma.200904383; 2010). In particular, their cell has an efficiency of ∼1.65%, an open-circuit voltage of ∼0.48 V, a short-circuit current density of ∼6.5 mA and a fill factor of ∼56% when measured under the well-known 'air mass 1.5 global illumination' conditions.

In essence, the team constructed a Schottky junction solar cell by depositing a graphene sheet (GS, pictured) onto an n-silicon wafer. First, the n-silicon wafer, covered with a 300-nm-thick SiO2 layer, was patterned by photolithography and wet-etching of oxide to obtain a square window with a junction area of 0.1 cm2. The backside electrical contact was formed from Ti/Pd/Ag deposited on the lower surface of the n-silicon wafer. The frontside contact consisted of small regions of Au sputtered onto the SiO2, on top of which a graphene layer of thickness 10–100 nm was deposited, creating a conformal coating with the Au layer and the underlying n-silicon wafer.

The researchers explain that in addition to its role as a semitransparent upper electrode and antireflection layer, graphene's intrinsic electric field properties aid electron–hole separation and hole transport. The built-in field helps guide photogenerated holes and electrons towards the graphene sheet and n-silicon, respectively.

Although the efficiency of their graphene-on-silicon solar cell is currently quite low, the researchers envisage that it can be improved by optimizing the conductivity and transparency of the graphene films used, and by using surface passivation of silicon to improve the graphene/n-silicon interface.

“This concept demonstrates a new class of photovoltaic device that has the merits of low cost and easy fabrication, leading to a new trend in the development of carbon-based solar cells,” say the researchers.