Abstract
Recently, with further intensified energy crisis and consciousness of environmental protection, solar energy has caught much attention due to its non-polluting and highly abundant properties. However, an efficiency-to-cost ratio of photovoltaic techniques is still too low to substitute for fossil energy. Therefore, developing a cost-effective or high-efficient solar device has aroused many interests and become a must in the field of energy harvesting. Here, a potential candidate that combines a silicon substrate with a polymer layer as a heterojunction solar cell will be elaborated. In this study, a Si nanostructure/poly(3,4-ethylenedioxythiophene): poly(stylenesulfonate) (PEDOT:PSS) heterojunction solar cell is proposed and examined. First, different lengths of Si nanowire (SiNW) structures formed by metal-assisted chemical etching (MaCE) were fabricated to discuss a trade-off between light absorption efficiency and amounts of e--h+ recombination centers at surface defects. A solar device with 200 nm SiNWs possesses relatively low reflectance and less trapping defects, resulting in the best performance among the designed lengths of the NWs. Nonetheless, PEDOT:PSS cannot fully infiltrate into 200-nm-length SiNWs. Moreover, without passivation of PEDOT:PSS, there appear lots of surface defects at the bottom region of SiNWs. Hence, a step of post-KOH dipping is executed after the MaCE process to widen the spacing among SiNWs. Several lengths of SiNWs were used as starting substrates and devices with 150-nm-long nanostructures owned the best performance for both the starting substrates with the 200- and 300-nm-long SiNWs that might stem from suppressed surface recombination and also reduced contact resistance, benefiting from a better coverage of PEDOT:PSS on the surface of SiNWs. Moreover, influences from PEDOT:PSS as a hole transport layer are also discussed. In principle, the mobility of PEDOT:PSS (~10-2 cm2V-1S-1 for pristine film) is much smaller than Si’s (~103 cm2V-1S-1); thus, there exists a recombination region at the interface between Si and PEDOT:PSS due to the unbalanced mobility between an electron and a hole. Hence, in order to escalate the mobility of PEDOT:PSS, secondary dopants such as dimethyl sulfoxide (DMSO) and graphene oxide (GO) were mixed into PEDOT:PSS and a solar device based on the modified PEDOT:PSS of 0.2 wt% GO addition provided the best mobility and thus the best efficiency. Such Si nanostructure/PEDOT:PSS heterojunction solar cell could be simply fabricated via low temperature wet etching and spin-coating methods that can dramatically reduce fabrication cost. After optimizing the Si nanostructures and the amount of the secondary dopants, power conversion efficiency above 13% can be achieved and is believed to be ready for applications of energy harvesting.