Enhancement of photovoltaic power of single-walled carbon nanotube films by interface structures of different film thickness

In contrast to the electronic density of states (DOS) of graphite, single-walled carbon nanotubes (SWCNTs) possess spike-like DOS derived from one-dimensional van Hove singularity. Furthermore, SWCNT shows metallic or semiconducting DOS as a result of its chiral index. The optical transitions of SWCNTs arise between matching van Hove singularities in conduction and valence bands. In particular, semiconducting SWCNTs have two absorption peaks in the near-infrared light region, whereas metallic SWCNTs have one absorption peak in the visible light region [1]. Omari et al. illuminated infrared light on part of 90%-enriched semiconducting SWCNT thin films [2]. They reported that the photovoltage appeared only for asymmetric illuminations by the photothermovoltaic effect, which is explained on the basis of a photogenerated heat flow model, and the photo-thermoelectric power was attributed to the temperature gradient of both ends of the film. In general, the thermal conductivity of individual SWCNTs is high and ranges from 3000 to 5000 W/mK (at 25°C) due to ballistic phonon transport [3,4]. On the other hand, the thermal conductivity of randomly aggregated SWCNT films shows a low value of around 35 W/mK (at 25°C) [5].

The low thermal conductivity of randomly aggregated SWCNT films is due to phonons being scattered at the interface between nanotubes, where phonon transport is inhibited. In addition, theoretical considerations indicate that interface thermal resistance arises at the interface between nanotubes [6]. Therefore, numerous tube-tube junctions are due to dominant barriers to thermal transport in the SWCNT films. In contrast, pi-electrons are easily able to transport between nanotubes by pi-electron hopping [7]. Thus, the difference between pi-electron mobility and phonon mobility is considered to occur at the interface of the SWCNT film with different film thickness. Here, we produced SWCNT films with thin/thick interface structures in an effort to improve the photothermovoltaic effect by utilizing the low thermal and high electronic conductivities of SWCNT aggregates, and also investigated their photovoltages.

As-grown SWCNTs synthesized by an arc discharge method were purified using air oxidation and acid treatment, and the purified SWCNTs were annealed in a high vacuum (5.0 × 10^-5 Pa) at 1200 °C for 3 h [8]. We prepared two types of SWCNT films: one comprised thin/thick SWCNT films as shown in Fig. 1a, and the other comprised single-thickness SWCNT films. SWCNT films were prepared by spraying SWCNTs/ethanol dispersion on a glass slide with two Ag electrodes. The film thickness was controlled by measuring the transmittance of films at a wavelength of 700 nm. We set sample cells in a vacuum chamber and measured the photovoltage and temperature difference of both sides of the SWCNT films under solar light irradiation (yellow dotted square area in Fig. 1a) using an AM1.5 solar simulator (HAL-320, Asahi Spectra Co., Japan) with an irradiation power density of 100 mW/cm². The light was irradiated for 20 min.

The photovoltage and temperature difference between the electrodes of a single SWCNT film (%T = 48) with respect to the measurement time are shown in Fig. 1b. The photovoltage generated by light illumination increased with increasing temperature difference. Measurement of the thermoelectric power of this single SWCNT film revealed thermo-electromotive forces as shown in the curve of Fig. 1b, and the thermo-electromotive forces were proportional to the temperature difference. This experiment showed that the single SWCNT film possessed thermoelectric power, and the photovoltage is thought to be generated by the same mechanism as the thermoelectric power. We confirmed that the SWCNTs used in this study were constructed in approximately 10% metallic and 90% semiconducting SWCNTs from their UV-Vis-NIR spectra [9]. Electrons of all SWCNTs excited by photon absorption transport to metallic nanotubes. The presence of metallic nanotubes within a bundle will quench electronic excitation on adjacent semiconducting nanotubes. The light energy absorbed by light irradiation is converted into thermal energy (phonon). Therefore, thicker SWCNT films absorb more light and generate greater heat. Since the I-V curve of the single SWCNT film (%T = 48) is linear, the electric contact between the film and the Ag electrode represents an ohmic junction (inset of Fig. 1b). Judging by the identification of carriers from the polarity of the photovoltages and the temperature gradient, the carrier of the SWCNT film comprised holes in an air atmosphere and electrons in a vacuum. In the air atmospheric pressure environment, the photovoltage was positive. As air was gradually removed from the chamber, the photovoltage changed continuously from a positive state. When air was reintroduced into the chamber, the photovoltage again reversed polarity and once again became positive. This suggests that water molecules and oxygen molecules act as a dopant acceptor [10].

Figure 1c shows the photovoltages of a single film (%T = 8.4), a thin/thick film (%T = 8.4/3.3), and a thin/thick film (%T = 8.4/1.2) with respect to the measurement time. The photovoltages of these films were 0.77, 0.92, and 0.94 mV, respectively. In contrast, the photovoltage of a single film (%T = 0.64) was 0.79 mV, which was almost the same as that of the single film (%T = 8.4), even though the amount of photoabsorption of the single film (%T = 0.64) was higher than that of the thin/thick film (%T = 8.4/1.2). It is thought that the photovoltage was reduced because thermal diffusion occurs toward the upper left side of the single thick film, where the carriers also diffuse (Fig. 1d).

The photothermalvoltaic effect was improved using SWCNT films with thin/thick interface structures by utilizing the low thermal and high electronic conductivities of SWCNT aggregates. Thus, the photothermalvoltage property can be tuned by controlling the phonon mobility and pi-electron mobility at the interface between nanotubes with an aligned structure or a thickness-controlled structure. SWCNT aggregates controlled in a 3D structure will draw interest as optical absorbers through the use of their unique electric and optical characteristics.