P-Type and n-Type Doping for Polymer Semiconductor
Authors: Sakiyama Shin , Mizutani Naoki , Fujita Katsuhiko 
Published: February 2020

Nondoped organic semiconductors are usually used in organic devices such as organic light-emitting diodes (OLEDs) and solar cells, although inorganic semiconductor devices are realized by doping. Carrier doping has been attempted with dispersion of the donor or acceptor molecules in some organic semiconductors.  The conductivity of the organic semiconductor can be significantly improved. The doping efficiency is defined as the ratio of increased carrier density to dopant density. The doping efficiency in small-molecule semiconductors fabricated by a coevaporation method has been increased recently. The OLEDs and solar cells with the p-i-n structure consisting of small molecules have been realized by layer-by-layer deposition of p-type doped and n-type doped semiconductors. On the other hand, p-i-n doping in polymer semiconductors has not been realized yet, because there are some problems with the use of n-type molecular dopants. The representative dopants used in coevaporation such as Cs2CO3, FeCl3, and CsF are poorly soluble in organic solvents, which can dissolve polymer semiconductors, and aggregate easily in the polymer matrix during spin coating even when they are dispersed in a polymer solution. Therefore, the doping efficiency in polymer semiconductors has been very low, with 3% as the highest.We have developed a polymer-thin-film preparation method, evaporative spray deposition using an ultradilute solution (ESDUS), which enables the preparation of polymer semiconductor films using a highly diluted solution at 1 ppm.  In a previous study, on the basis of using this method of preparing polymer semiconductor films using an ultradilute solution, we have tried to prepare n-type doped polymer semiconductor poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) thin films with Cs2CO3, which is an effective n-type dopant with low solubility in an organic solvent. As a result, the conductivity of the device was significantly improved with a high doping efficiency of 60%. This result is caused by ESDUS, which suppresses aggregation of dopant molecules. Furthermore, we have fabricated pn junctions by layer-by-layer deposition of an n-doped polymer semiconductor film on a p-doped polymer poly(3-hexylthiophene) (P3HT) film. 

In the present study, we attempted p-type and n-type doping of a polymer semiconductor, poly[2-methoxy-5-(2’-methyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV LUMO, 3.1 eV, HOMO, 5.2 eV). It is one of the most widely used conductive polymers and a bipolar carrier-transporting semiconductor. FeCl3 (work function, 5.52 eV) as a p-type dopant and Cs2CO3(work function, 2.96 eV) as an n-type dopant were chosen for p-type and n-type doping in MEH-PPV, respectively.

MEH-PPV was dissolved in THF at 10 ppm. After the optimization of deposition conditions such as temperature, carrier gas flow rate, and solution feed flow rate, a homogeneous and continuous film of 20 nm thickness was obtained.

Each dopant molecule was dissolved in dehydrate ethanol at a concentration of 3 mg/ml and the solution was kept in a glove box with stirring overnight at 50 ℃. The dopant solutions were diluted with THF and added to a THF solution of MEH-PPV. The polymer concentration was kept constant at 10 ppm and the dopant concentration was varied at 0.02, 0.2, and 2 wt% against the polymer (0.02 wt% corresponds to FeCl3:0.096 mol% and Cs2CO3:0.047 mol% against the monomer of an MEH-PPV unit). The MEH-PPV solution containing each dopant was supplied to the ESDUS and formed into thin films.

Electron-only-device (EOD), Al/MEH-PPV (100 nm)/Ca, and hole-only-device (HOD), ITO/MEH-PPV (100 nm)/Au, were fabricated by vacuum deposition of the top electrode after ESDUS preparation of the MEH-PPV layer with the desired dopant concentration. Current-voltage (I-V) characteristics were measured using a Keithley 238 source meter without breaking vacuum after top electrode deposition. The hole and electron densities of the nondoped MEH-PPV and hole and electron mobilities were calculated from the I-V curves.

The Fermi levels of the MEH-PPV films were estimated using a surface potential meter (Kelvin probe, Riken Keiki, FAC-1) in nitrogen without exposing the samples to the ambient atmosphere after ESDUS film deposition. The nondoped and doped MEH-PPV films of 100 nm thickness were fabricated on Al and the Fermi level was estimated with respect to the Au standard (5.03 eV).

The current-voltage (I-V) characteristics of hall-only-devices (HOD) with varying FeCl3 (p-dopant) concentration and electron-only-devices (EOD) with varying Cs2CO3 (n-dopant) concentration clearly indicate that the current density increases significantly more than 1000 times as the doping concentration gets higher (Fig. 1). The carrier density increases with dopant concentration linearly and the doping efficiency is as high as 15% at low dopant concentration. The wide dispersion of the dopants in the ultradilute solution leads to high doping efficiency and large charge separation with the host polymer.

Figure 1