The Role of Hole Transport in Hybrid Inorganic/Organic Silicon/Poly(3,4-ethylenedioxy-thiophene):Poly(styrenesulfonate) Heterojunction Solar Cells

In this paper, the fundamental advantage of highly conductive transparent polymers as hole transport layers in hybrid solar cells is demonstrated. The substantial efficiency improvement of hybrid n-type silicon (n-Si)/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells by adding organic solvents to the polymer dispersion is investigated, and a model that explains reasons and mechanisms for that improvement is given. Open-circuit voltages of 600 mV were measured, which are comparable to conventional diffused silicon pn-junction wafer cells. It is shown by means of X-ray photoelectron spectroscopy that the PEDOT versus PSS ratio plays an important role for charge carrier transport in the PEDOT:PSS layer as well as for charge carrier separation at the n-Si/PEDOT:PSS interface. A shell of insulating PSS segregates at the surface of PEDOT:PSS grains and represents a considerable barrier for charge carrier transport and charge carrier separation, influencing the conductivity of the polymer film and the open-circuit voltage of a processed solar cell, respectively. It could be demonstrated that a mixing of the PEDOT:PSS polymer blend with the organic solvent dimethylsulfoxide reduces the PSS insulator segregation at the surface of PEDOT:PSS grains and improves the performance of hybrid n-Si/PEDOT:PSS solar cells.