Single-Step Formation of a Low Work Function Cathode Interlayer and n-type Bulk Doping from Semiconducting...

Area of Science:

• Organic electronics
• Materials science
• Interface engineering

Background:

• Polyethylenimine (PEI) is used as a thin interlayer in organic electronics to minimize Ohmic losses.
• PEI interlayers can form spontaneously via vertical phase separation in blended polymer solutions.
• The distinction between interfacial and bulk effects of PEI in organic semiconductors requires clarification.

Purpose of the Study:

• To investigate the interfacial and bulk effects of PEI in blended thin films with P(NDI2OD-T2) semiconductor.
• To determine the impact of PEI interlayers on electron injection barriers and contact resistance.
• To assess the extent of bulk doping and its influence on conductivity and carrier mobility.

Main Methods:

• Spin-coating of blended solutions of PEI and P(NDI2OD-T2) on indium tin oxide (ITO).
• Analysis of vertical phase separation using film morphology.
• Measurement of electron injection barriers, contact resistance, optical absorption, and electron paramagnetic resonance (EPR).

Main Results:

• Vertical phase separation leads to PEI accumulation at the film's top and bottom, sandwiching the semiconductor.
• PEI interlayers reduce the electron injection barrier, decreasing contact resistance by five orders of magnitude.
• EPR confirms low-level doping and increased carrier density, enhancing conductivity by four orders of magnitude.

Conclusions:

• Both interfacial and bulk effects of PEI are significant in blended semiconductor polymer/PEI films.
• The facile one-step procedure for PEI interlayers requires careful consideration due to simultaneous bulk modification.
• Unnoticed doping effects in the bulk semiconductor polymer can occur, necessitating thorough examination.