(52e) Vapor Printing of Neutral Hole Transporting Polymer for Enhanced Efficiency and Stability of Organic Photovoltaics

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.