Creating highly efficient carrier injection or collection contacts via soft-contact lamination of p-doped interlayers

Abstract

Molecular doping of organic semiconductors has long been investigated and is an effective method for increasing film conductivity and creating highly efficient contacts for electronic devices. It has been shown that inserting a (n- or p-) doped transport layer at the contacts could significantly improve the efficiency of small-molecule electronic devices, in which dopants can be precisely positioned via controlled vacuum co-deposition. However, precise interface confinement of dopants in a solution processed film is more challenging, due to limited choices of soluble molecular dopants and difficulties in stacking multilayers of polymer films.

We present here a novel approach, which consists in physically transferring onto the active layer of the device via soft-contact lamination (SCL) of an ultra-thin doped polymer film. This film, separately spin-coated from a solution containing a controlled amount of soluble dopant, creates a high-conductivity layer with controllable work function, that is also electronically and chemically compatible with the organic active layer. This approach affords unprecedented flexibility in the positioning of doped regions in organic devices.

In this work, p-doping of the host transport layer is achieved by co-solution of one of the following hole-transport polymers, i.e., poly(3-hexylthiophene-2,5-diyl) (P3HT), poly [(4,8-bis-(2 ethylhexyloxy)-benzo[1,2-b-4,5-b']dithiophene)-2,6-diylalt-(4-(2-ethylhexanoyl)-thieno[3,4 -b]thiophene)]-2,6-diyl (PBDTTT-C) or poly [N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (poly-TPD), and either the soluble oxidizing p-dopants molybdenum tris-[1-(methoxycarbonyl)- 2-(trifluoromethyl) ethane-1,2-dithiolene] (Mo(tfd-CO2Me)3) or molybdenum tris-[1-(trifl

uoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd-COCF3)3). The impact of p-doping

on electronic structure in the bulk is investigated via ultra-violet photoemission spectroscopy

(UPS) on doped and undoped polymer films, which shows a downward shift of Fermi level toward the highest occupied molecular orbital (HOMO) level by doping. Current-voltage (I - V ) measurements were also performed and the results show an enhancement of the conductivity in the host polymer layer by almost 6 orders of magnitude, up to 10􀀀E2 - 10E1 S/cm at room temperature. SCL of a thin doped polymer layer on the undoped polymer film is used to create spatially-conned doped regions, which serve as hole-injection contacts. This strategy is also successfully applied to create efficient hole-collecting contacts on solution-processed inverted

polymer solar cells.

The spatial stability of the dopants in polymer and polymer blend films is investigated via secondary ion mass spectrometry (SIMS) and I - V measurements. While the dopant is found to be very mobile in pure P3HT, it is far more stable in PBDTTT-C and poly-TPD. P3HT-fullerene and PBDTTT-C-fullerene bulk heterojunction (BHJ) solar cells with laminated doped films as hole-collection layers also show long term stability, consistent with the observation that dopants are stable at the interface with the BHJ. Our findings suggest a promising route to achieve spatially confined doping with long-term stability, leading to highly efficient hole collection /

injection contacts for all-solution processed polymer devices.