Single-Step Formation of a Low Work Function Cathode Interlayer and n-type Bulk Doping from Semiconducting Polymer/Polyethylenimine Blend Solution

Abstract

The use of polyethylenimine (PEI) as a thin interlayer between cathodes and organic semiconductors in order to reduce interfacial Ohmic losses has become an important approach in organic electronics. It has also been shown that such interlayers can form spontaneously because of vertical phase separation when spin-coating a blended solution of PEI and the semiconductor. Furthermore, bulk doping of semiconducting polymers by PEI has been claimed. However, to our knowledge, a clear delineation of interfacial from bulk effects has not been published. Here, we report a study on thin films formed by spin-coating blended solutions of PEI and poly­{[N,N′-bis­(2-octyldodecyl)­naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P­(NDI2OD-T2)] on indium tin oxide. We observed the vertical phase separation in such films, where PEI accumulates at the bottom and the top, sandwiching the semiconductor layer. The PEI interlayer on ITO reduces the electron injection barrier to the minimum value determined by Fermi level pinning, which, in turn, reduces the contact resistance by 5 orders of magnitude. Although we find no evidence for doping-induced polarons in P­(NDI2OD-T2) upon mixing with PEI from optical absorption, more sensitive electron paramagnetic resonance measurements provide evidence for doping and an increased carrier density, at a very low level. This, in conjunction with an increased charge carrier mobility due to trap filling, results in an increase in the mixed polymer conductivity by 4 orders of magnitude relative to pure P­(NDI2OD-T2). Consequently, both interfacial and bulk effects occur with notable magnitude in thin films formed from blended semiconductor polymer/PEI solution. Thus, this facile one-step procedure to form PEI interlayers must be applied with attention, as modification of the bulk semiconductor polymer (here doping) may occur simultaneously and might go un-noticed if not examined carefully.

A high-conductivity n-type polymeric ink for printed electronics

“…When blended with PEI lin and annealed, the electrical conductivity increases to 0.10 ± 0.02 S cm −1 at 5 wt% PEI and saturates to 7.7 ± 0.5 S cm −1 for 50 wt% PEI content. Because of the high density of electron-donating secondary amine groups in PEI 39,40 , a 5 min treatment at 150°C is enough to reach 1 S cm −1 n-type conductivity, whereas the maximum conductivity of 8 S cm −1 is obtained after 2 h of annealing (Supplementary Fig. 12).…”

Disentangling Bulk and Interface Phenomena in a Molecularly Doped Polymer Semiconductor

“…Furthermore, ITO coated with polyethylenimine (PEI) also induced Fermi level pinning at the P(NDI2OD‐T 2 ) conduction band, and the resulting sample work function was also 4.0 eV. [ 30 ] However, the polymer PEI is not a one‐electron reductant as is OPR, and the reason for work function reduction was related to its intrinsic dipole moment instead of electron transfer. [ 40 ]…”

“…The values for the undoped P(NDI2OD-T 2 ) film are taken from literature. [30] www.advancedsciencenews.com www.advopticalmat.de…”

“…Since the conductance of the pristine P(NDI2OD‐T 2 ) film prepared from CB was too low to provide a reliable signal, a reference conductivity value of 2 × 10 −7 S cm –1 is taken from a previous P(NDI2OD‐T 2 ) study as upper limit. [ 30 ] Although using the same method as in that study, a variation in conductance can be due to the use of different solvents and processing conditions, as these parameters affect the polymer morphology and thus conductivity. [ 31–33 ] However, use of this reference value allows establishing an increase in conductivity by at least three orders of magnitude upon doping.…”

“…Figure 2.Conductivity and contact resistance of OPR-doped P(NDI2OD-T 2 ) thin films as a function of dopant concentration measured under inert nitrogen atmosphere. The values for the undoped P(NDI2OD-T 2 ) film are taken from literature [30]. …”