## 1 Introduction  
In the last few decades, a number of liquid-crystal (LC)-based devices have been demonstrated with functionalities such as phase shifters, filters, modulators, and polarizers, in the terahertz (THz) frequency range [1; 2; 3; 4]. Recently, we demonstrated an electrically tuned LC-based THz phase shifter with high transmittance and driving voltages compatible with complementary metal-oxide-semiconductor (CMOS) and thin-film transistor (TFT) technologies [5]. The key to the referred work [5] is nanostructured indium tin oxide (ITO) as a transparent conducting electrode (TCE). Only a few TCEs have been explored at THz frequencies in contrast with those in visible frequencies [6]. It should be noted that ITO film is not suitable as a THz-TCE layer because of its high absorption of THz waves [7]. To date, graphene, ITO nano-whisker (NW), and dimethylsulfoxide (DMSO)-doped poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate (PEDOT: PSS) thin layers have been successfully employed as TCE materials for THz device applications [5; 8; 9; 10; 11]. Nanostructured TCEs such as graphene and ITO nano-whiskers are relatively difficult to prepare. In contrast, PEDOT: PSS films can be easily fabricated by spin coating [12]. In a previous work, [PERSON] et al. [13] investigated THz phase imaging using an 800 \(\upmu\)m thick homogeneous LC device using pristine PEDOT: PSS electrode layers and achieved a 360\({}^{\circ}\) phase shift at 2.5 THz. More than 75 V of bias, however, was needed to achieve a 90\({}^{\circ}\) phase shift. The THz transmittance and conductivity of PEDOT: PSS thin film doped with either dimethyl sulfoxide (DMSO) or ethylene glycol (EG) have also been investigated. [PERSON] et al. [14] reported that PEDOT: PSS thin films doped with 10% EG and with thicknesses between 34 and 102 nm exhibited transmission between 62% and 83% in the frequency range of 0.3-1.2 THz. Moreover, [PERSON] et al. [15] reported that 6% DMSO-doped PEDOT: PSS thin film with thicknesses between 31 and 129 nm showed transmission between 30% and 72% in the frequency range of 0.5-2.5 THz. The THz transmittance decreased significantly with the increasing thickness of PEDOT: PSS layers doped with either EG or DMSO. In addition, the PEDOT: PSS thin layer displayed trade-off behaviors in conductivity, surface roughness, adhesion to the substrate, and lack of uniformity to increased surface to volume ratio during deposition by the spin-coating process [16]. Additional processes are needed to avoid the above-mentioned issues [17]. Recently, [PERSON] et al. [11] reported outstanding performances of homogeneously aligned LC-based THz phase shifters using DMSO-doped PEDOT: PSS as a TCE layer. The transmittance of this device, however, gradually decreased with frequency and became almost zero at 1.2 THz. This can be attributed to the fact that the transmittance of a homogenously aligned LC cell exhibits more explicit dependence on the wavelength [18]. In related work, [PERSON] et al. [19] fabricated a THz polarization converter using a twisted nematic (TN) LC cell structure with pristine PEDOT/PSS films as TCE. A 1 mm thick TN cell was used and a driving voltage of approximately 15 V was applied to decrease the transmittance of the device to zero. However, there is no discussion in the paper of the performance of TN-LC cell as a phase shifter. The TN-LC cell has additional advantages with a faster response time, a transmittance performance less dependent on wavelength, and a performance less dependent on the angle of incidence. Therefore, this structure has been widely used in displays [20]. Hence, it is worthwhile to study in detail the performance of TN-LC-based THz phase shifters using pristine PEDOT: PSS as a TCE layer. Furthermore, several new types of LC with high birefringence have recently been developed for THz applications, wherein the reduction of the cell thickness would be attractive.