During the formation of films, such as a hole transporting layer (HTL) in normal optoelectronic devices, the pre-treatment of a transparent conductive substrate and the post-treatment of HTLs are always necessary in order to realize good surface energy matching and qualified properties. Take the most commonly used HTL material, for example water-dissolved poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS), it faces the problem of wetting a pristine substrate. As a result, the underlying substrate needs to be ultraviolet-ozone (UVO) or oxygen-plasma treated to realize a good surface energy match with a water-based PEDOT:PSS solution. Also, PEDOT:PSS needs thermal annealing after the film formation to remove the residual water. Otherwise, it will still re-absorb water if the samples are not used immediately.
Meanwhile, various kinds of organic donor materials have emerged in the past several years, and they typically have different highest occupied molecular orbital (HOMO) values. The most studied donor polymer, poly(3-hexylthiophene) (P3HT), has a HOMO energy level of around 5.00 eV. As another example, conjugated polymers based on the backbone of alternating benzo[1,2-b;4,5-b′]dithiophene (BDT) and thieno[3,4-b]thiophene (TT) building units have HOMO values that vary from 5.07 eV (PBDTTT-C) to 5.33 eV (PBDT-TS1). As the alignment of work function (WF) of HTL with the donor's HOMO is a very important issue to prevent energy loss at the interface, developing WF tunable films (e.g. acting as HTL) becomes desirable.
New methods and apparatus that assist in advancing technological needs and industrial applications are desirable.