Si-quantum-dot heterojunction solar cells with 16.2% efficiency achieved by employing doped-graphene transparent conductive electrodes

To overcome small- and indirect-bandgap nature of crystalline bulk Si, a lot of efforts have been made to utilize Si quantum dots (SQDs) in optoelectronic devices. By controlling the size of Si quantum dots (SQDs), it is possible to vary the energy bandgap based on quantum confinement effect, which can maximize the power-conversion efficiency (PCE) of solar cells due to the energy harvesting in a broader spectral range. Here, we first employ graphene transparent conductive electrodes (TCEs) for SQDs-based solar cells, showing a maximum PCE of 16.2%, much larger than ever achieved in bulk-Si solar cells with graphene TCEs. In this work, the graphene TCEs are doped with two kinds of materials such as AuCl3 and Ag nanowires for efficient collection of the carriers photo-induced in SQDs. The encapsulation of the doped-graphene TCE with another graphene layer prevents the doping elements from being desorbed or oxidized, thereby making the PCE higher, its doping dependence more evident, and the long-term performance more stable. The observed unique solar cell characteristics prove to be dominated by the trade-off effects between doping-induced variations of diode quality, transmittance/sheet resistance of graphene, energy barrier at the graphene TCE/SQDs interface, and reflectance.