Three efficiency benefits from thin film plasmonic solar cells
In a race to reduce the cost per Watt of solar generated power, there is generally a tradeoff between high efficiency and low cost. By going to thinner devices, less material can be used; however, clever light management designs must be utilized to avoid the loss in current caused by reduced absorption in a thin active layer. Here we discuss such design schemes incorporating either dielectric or metallic structures to approach the bulk absorption limit in optically thin layers. As a specific example, a plasmonic back grating can result in absorption of 80% of the incident above bandgap light in a GaAs layer of only 200 nm. If the reduction in current upon thinning of the cell is limited, an improvement in the open circuit voltage can be obtained through a reduction of the bulk recombination current. Under the condition that the open circuit voltage increases more rapidly than the short circuit current decreases, thinner layers will produce more efficient cells. Finally, the incorporation of metallic scatterers can potentially improve the fill factor by reducing the sheet resistance of a top surface-passivating layer. We show experiments that suggest that the sheet resistance decreases for a metal particle decorated GaAs structure, which can be modeled using a simple circuit diagram. By combining all three effects, we consider the possibility of high efficiency solar cells that are an order of magnitude thinner than their bulk counterparts and consider their role for future photovoltaic device architectures.