Organic solar cells have attracted great attention due to their potential to enable lightweight, flexible, large-area, and cost-effective photovoltaic technology. In order to optimize their performance, a variety of hole transporting materials has been applied to them. Among the materials, poly(3,4-ethylenedioxy-thiophene):polystyrene sulfonate (PEDOT:PSS) is one of the most promising candidates due to deep work function (suitable HOMO level for organic donor materials), high transparency in the visible-light region, and earth-abundant element composition (e.g., C, H, O, and S) facilitating future cost-effective, large-scale manufacturing. However, the strong acidity from PSS corrodes organic donor and anode materials, and thus degrades the stability of organic photovoltaic devices with PEDOT:PSS.[33-35] Therefore, new neutral hole-transporting polymers are needed to upgrade the stability of organic photovoltaic devices.
In this sense, poly(3,4-dimethoxythiophene) (PDMT) can be a neutral alternative to PEDOT:PSS due to the same earth-abundant element composition, but typical PDMT does not have a high conductivity compared to PEDOT:PSS. We have modified PDMT via oxidant chemical vapor deposition (oCVD), which has a remarkably improved conductivity relative to the typical PDMT. Specifically, the nature of oxidative polymerization achieved in oCVD reactors generates doped PDMT with anion dopant ions (Cl–), which is advantageous for increasing conductivity. Consequently, oCVD-processed PDMT can have a high conductivity without acidity, and thus it becomes one of the most suitable candidates to replace PEDOT:PSS.
In this study, PDMT hole transporting layer (HTL) is successfully integrated into organic photovoltaic devices for the first time. Though PDMT is insoluble and infusible, and thus typically difficult to process, patterned thin films of this regioregular polymer were easily prepared using a vacuum-based vapor-printing technique (i.e., oCVD combined with in-situ shadow masking). Vapor-printed PDMT HTL functions better than spin-coated PEDOT:PSS HTL by enhancing short-circuit current and fill factor in DBP:C60 photovoltaic devices. The maximum power conversion efficiency (PCE) was 4.1% for employing vapor-printed PDMT HTL, and 3.5% for using spin-coated PEDOT:PSS HTL (See Figure 1). Furthermore, vapor-printed PDMT HTL demonstrates much longer-term stability in terms of PCE because PDMT is a neutral material, unlike acidic PEDOT:PSS (See Figure 2). The photovoltaic device with vapor-printed PDMT HTL maintained 83% of its optimum efficiency after 17 days in a N2-filled glove box, while one with spin-coated PEDOT:PSS HTL retained only 12% of its best efficiency under the same conditions. The advances of this work can also be applied to different-type solar cells and any other organic electronics because oCVD is a powerful platform technology, independent of material solubility and substrate properties.
Figure 1. J-V (Current density – voltage) curves for 62 nm PEDOT:PSS HTL and 75 nm doped PDMT HTL.
Figure 2. Long-term efficiency stability measurement for 62 nm PEDOT:PSS HTL and 75 nm doped PDMT HTL.