Effects of Triphenylborane Addition to Decaphenylcyclopentasilane Thin Films

Takeo Oku1*, Naoki Hibi1, Atsushi Suzuki1, Tsuyoshi Akiyama1, Masahiro Yamada2, Sakiko Fukunishi3 and Kazufumi Kohno3

1Department of Materials Science, The University of Shiga Prefecture, Shiga 522-8533,

Japan 2Osaka Gas Co., Ltd., Osaka 554-0051, Japan 3Osaka Gas Chemicals Co., Ltd., Osaka 554-0051, Japan

E-mail: oku@mat.usp.ac.jp

(Received July 18, 2014)

Organic thin film solar cells are potential ‘next generation’ solar cells. Many p-type

semiconductors have been used in organic solar cells, but there have been far fewer reports

involving n-type organic semiconductors. Developing new n-type organic semiconductors is

therefore desirable. Decaphenylpentasilane (DPPS) thin films were spin-coated from solutions

containing boron (B), and the effects of B addition on film microstructures and electronic

properties were investigated. Microstructures of DPPS thin films were investigated by X-ray

diffraction, and DPPS thin films doped with B (DPPS(B)) showed the reduction of crystallinity

upon annealing at 300 °C, while DPPS thin films exhibited crystalline structures. DPPS(B) thin

films exhibited decreased electrical resistances upon the B doping and annealing. The desorption

of phenyl and methyl groups from the DPPS(B) thin films was observed by Raman scattering

measurements.

1. Introduction

Polysilanes are a p-type semiconductor, and have been applied as electronic conductive materials

and photovoltaic systems [1-11]. Polysilanes are organic polymers containing main chain Si-Si

bonds and side chain organic substituents, which are known as σ-conjugate polymers, and their

hole mobility is ~10–4 cm2 V–1 s–1 [1]. Although the polysilanes could be applied as a p-type

semiconductor on organic thin film solar cells for these characteristics, few studies on polysilane

solar cells have been reported. Doped polysilanes as an n-type semiconductor have been developed

and applied for organic solar cells [11,12].

The purpose of the present work is to prepare decaphenylcyclopentasilane thin films using

spin-coating mixture solutions of decaphenylpentasilane (DPPS) with triphenyl borane, and to

evaluate the effect of boron (B) and annealing temperatures on their electronic properties and

microstructures. It is expected that amorphous silicon doped with boron would function as a p-type

semiconductor. Spin-coating is a low-cost method, and is essential for mass production of solar

cells [13]. Optical property and desorption of phenyl group were investigated from

ultraviolet-visible (UV-vis) absorption and Raman scattering spectra. Microstructure analysis was

carried out by X-ray diffraction (XRD) analysis. The mechanism of microstructural change is

discussed on the basis of the experimental results.

2. Experimental

Figure 1 shows molecular structures of DPPS and triphenyl borane, which was used for boron

doping to DPPS. Indium tin oxide glass plates were cleaned by an ultrasonic bath with acetone and

JJAP Conf. Proc. (2015) 011404©2015 The Japan Society of Applied Physics

3Proc. Int. Conf. and Summer School on Advanced Silicide Technology 2014

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methanol, and then were dried with nitrogen gas. The triphenylborane and DPPS were mixed in 1

mL tetrahydrofuran. The mixed solution was spin-coated at 1000 rpm, and then annealed at

100~300 °C. Gold contacts were evaporated as a top electrode by vacuum deposition to measure

the electrical resistance of the film. The current density–voltage characteristics (Hokuto Denko

HSV-110) of the devices were measured to investigate the electrical resistances of the films. The

microstructures of the DPPS thin films were investigated by XRD analysis (Philips X’Pert-MPD

System) with CuKα radiation at 40 kV operating voltage and 40 mA operating current. Raman

scattering spectra were recorded with a laser Raman spectrometer (Jasco NRS-5100). The Raman

scattering spectra and optical images of the thin films after annealing were observed using an

excitation laser with a wavelength at 532 nm. Optical properties were investigated by UV-vis

absorption spectroscopy (Jasco, V-670). The molecular structures of the DPPS monomers were

optimized by ab-initio calculation based on the density functional theory using Gaussian 09. The

active modes in Raman scattering spectra were calculated by DFT/B3LYP/6-31G in the frequency

mode.

Fig. 1. Structures of (a) DPPS and (b) triphenylborane.

3. Results and Discussion

UV-vis absorption spectra of DPPS thin films before and after annealing are shown in Fig. 2. All

three exhibited obvious absorptions at 300–350 nm, which corresponds to excited states of π→π*

transition. The wavelength of the absorption edge increased upon B doping and annealing at 300 °C,

which suggests that shifted photoabsorption arose from the desorption of phenyl groups, from side

chains in DPPS, and/or an increase of the length of Si-Si chains.

Raman scattering spectra of DPPS thin films after annealing are shown in Fig. 3. Raman active

modes of the DPPS film were observed at ~1000, ~1600 and ~3000 cm–1. From calculation of

Raman spectra, peaks at ~1000, ~1600 and ~3000 cm–1 in the spectra of DPPS were attributed to

vibration modes of phenyl group–Si, phenyl ring itself and C-H bonds in phenyl group, respectively.

Strong vibrations of the Si crystal lattice at ca. 520 cm–1 [14] were not observed in the spectra of

the thin films. Adding triphenylborane to DPPS caused the peaks to disappear, which suggests

desorption of the phenyl group and enhancement of the decomposition of the DPPS to the

amorphous structure.

Si Si

Si

Si

Si B

(a) (b)

011404-2JJAP Conf. Proc. (2015) 0114043

Fig. 2. UV-vis absorption spectra of DPPS thin films with and without B doping.

Fig. 3. Raman spectra of DPPS thin films annealed with or without B doping.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

300 350 400 450 500

Ab

so

rban

ce (

a. u

.)

Wavelength (nm)

DPPS

DPPS(B) annealed

DPPS annealed

500 1000 1500 2000 2500 3000 3500

Inte

nsit

y (

a.

u.)

Raman shift (cm-1)

DPPS

DPPS(B) annealed

DPPS annealed

011404-3JJAP Conf. Proc. (2015) 0114043

Fig. 4. XRD patterns of DPPS thin films annealed at 300 °C with or without B doping.

Table I. Summary of XRD results for DPPS thin films.

Sample Annealing

temperature (°C)

Position (2θ) FWHM (2θ) d-spacing (nm)

DPPS – 8.389 0.048 1.053

DPPS 300 8.363 0.086 1.056

DPPS(B) 300 9.036 0.038 0.9780

Figure 4 shows XRD patterns of DPPS thin films before and after annealing at 300 °C.

Diffraction peaks of DPPS were observed in the range of 8~9° in XRD patterns. After annealing at

300 °C, the diffraction intensity was increased. On the other hands, the intensity of the diffraction

peak decreased by triphenylborane addition to DPPS thin film, which indicates the decomposition

of the DPPS structure, and formation of an amorphous structure from the crystalline structure by

boron doping. XRD results for DPPS thin films were summarized in Table I. The d-spacings of ~1.0 nm

might be distances of the DPPS plane layers.

5 10 15 20 25 30 35 40

Inte

ns

ity (

a. u

.)

2θ (degree)

DPPS

DPPS(B) annealed

DPPS annealed

011404-4JJAP Conf. Proc. (2015) 0114043

J-V characteristics of the DPPS thin films were measured in the dark, and electrical resistances

of the DPPS thin films were calculated. Film thickness was measured to be ~200 nm by an atomic

force microscope. Volume resistivities of as-prepared, annealed, and annealed with B doping DPPS

were measured to be 6.6×104, 5.6×104, and 5.5×104 Ω cm, respectively. The DPPS(B) thin films

exhibited decreased electrical resistances upon B doping and annealing.

Fig. 5. Calculated electronic structures of DPPS.

Fig. 6. Proposed changes in microstructure of DPPS(B) during annealing.

Absorption spectra of DPPS thin films in Fig. 2 indicated that the absorption edge shifted to longer waves

by the B doping and annealing at 300 °C, which suggested that the increase of the length of Si-Si chain

would decrease the energy gaps by increasing highest occupied molecular orbital (HOMO) energy

levels keeping the lowest unoccupied molecular orbital (LUMO) energy levels [15]. A calculated

result of HOMO and LUMO is shown in Fig. 5. The length of main Si-Si chain affects the

non-locality of σ electrons, which would affect the absorption spectra.

Figure 6 shows proposed mechanisms for B doping in DPPS. A Si ring opening reaction

occurred in DPPS. Si-Si bonds were broken, and numerous phenyl groups desorbed from DPPS

side chains during annealing at 300 °C, and B bound to DPPS side chains, and an amorphous

structure formed from the Si chain structure [16]. This type of DPPS thin films would be used for a

p-type semiconductor, which would be expected for the use of pn junction solar cells.

HOMOLUMO

Si Si

Si

Si

Si B

Si Si

Si

Si

Si

Si SiSi

SiSiB B

Si

Si

Si

Si

Si

300 ºC

B

011404-5JJAP Conf. Proc. (2015) 0114043

4. Conclusion

DPPS thin films were fabricated by spin-coating, and the effect of doping with B was investigated.

DPPS(B) thin films exhibited an amorphous structure after annealing at 300 °C, while DPPS thin

films were crystalline after annealing. DPPS thin films exhibited decreased electrical resistances

upon annealing and B doping. The B doping and annealing at 300 °C caused the desorption of

phenyl groups from DPPS(B) side chains.

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