Optical anisotropy in PEDOT:PSS and its effect on the photovoltaic performance of crystalline Si/PEDOT:PSS Schottky solar cells Hajime SHIRAI, Keiji UENO, and Ryo ISHIKAWA Saitama University (Japan) Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) has attracted considerable attention in the field of conducting polymers because of its high electrical conductivity, transparency, structural stability, and other desirable properties. Several attempts have been made to improve conductivity in PEDOT:PSS by adding polar solvents such as water, methanol (MeOH), dimethyl sulfonate (DMSO), graphene oxide (GO), ZnO nanoparticles, and ethylene glycol (EG). These solvents modified the fine structure of the PEDOT:PSS random coil. Among these polar solvents, the addition of EG to PEDOT:PSS enhanced the electrical conductivity. This enhancement was attributed to the phase separation of hydrophilic conductive PEDOT and hydrophobic insulating PSS, which enhanced hole mobility. In the present study, PEDOT:PSS exhibited a strong uniaxial optical anisotropy, i.e., the ordinary optical constant, index of refraction no, and extinction coefficient ko indicated metallic behavior, while the extraordinary optical constant nex and its extinction coefficient kex exhibited a more dielectric behavior. Recently, efficient crystalline Si (c-Si)/PEDOT:PSS solar cells based on a c-Si/PEDOT:PSS heterojunction structure with a power conversion efficiency η of 10-11% have been reported. The effect of uniaxial optical anisotropy on the photovoltaic performance of c-Si/PEDOT:PSS solar cells, however, remains an open question. In the present paper, we demonstrate the effect of uniaxial optical anisotropy on the carrier transport parameter and photovoltaic performance of c-Si/PEDOT:PSS solar cells using MeOH and EG as a cosolvent. Spectroscopic ellipsometry revealed that the extraordinary index of refraction increased as film thickness was decreased by adding cosolvent. In contrast, the ordinary optical constant indicated a metallic property and was nearly independent of the MeOH/EG concentration ratio. On the other hand, the conductivity showed a maximum at a wt% ratio of (PEDOT:PSS):(MeOH):(EG) of 1:1:0.1. The c-Si heterojunction with cosolvent-modified PEDOT:PSS that demonstrated the maximum conductivity exhibited a relatively high power conversion efficiency of 11.23%. These findings suggest that the increased extraordinary index of refraction contributes to the enhanced hole mobility of PEDOT:PSS, resulting in improved photovoltaic performance. Also, the effect of GO, molybdenum trioxide MoO3, and ferroelectric P(VDF-TrFE) additions into conductive PEDOT:PSS and the insertion at the c-Si/PEDOT:PSS interface on the photovoltaic performance of c-Si/PEDOT:PSS hybrid solar cell will be discussed. Figure 1 shows the chemical structure of PEDOT:PSS and EG, and a schematic of the c-Si/PEDOT:PSS solar cell used in this study. Highly conductive 5 wt% DMSO-added PEDOT:PSS was then spin-coated using MeOH and EG as a cosolvent with different MeOH/EG weight percent ratios on RCA-cleaned c-Si wafers at 1000 rpm for 60 s, followed by thermal annealing at 140˚C for 30 FIG. 1 Chemical structures of PEDOT:PSS and min. Finally, the top electrode (5 mm×5 mm in ethylene inner size, 0.1 mm in width) was formed using c-Si/PEDOT:PSS heterojunction solar cells. glycol (EG). Schematic of silver paste and a pattern printer. Electronic, structural, and morphological characterizations of the PEDOT:PSS films were analyzed using spectroscopic ellipsometry (SE: Jobin Yvon UVISEL), atomic force nanotechnology, X-ray microscopy (AFM: SII SPA-300/SPI-3800), and photoelectron spectroscopy (XPS: ULVAC-PH1). Figure 2(a) shows measured index of refraction <n> and extinction coefficient <k> spectra of MeOH-modified, ~100-nm-thick and pristine, cosolvent-modified PEDOT:PSS films on c-Si at a probe light FIG. 2 (a) Measured <n> and <k> spectra of incidence angle of θi=70º. No marked c-Si/PEDOT:PSS films: pristine, MeOH-modified, and differences in the <n> and <k> spectra were MeOH/EG-modified PEDOT:PSS films on c-Si at a θi observed, suggesting no in-plane optical of 70º. The best fitted results are included as black and anisotropy, but the measured spectra did dotted lines. (b) Optical model used for spectral depend on θi. The measured <n> and <k> analysis. (c) no and ko spectral components and (d) nex spectra were analyzed using a Tauc-Lorentz and kex spectral components for the corresponding (TL) oscillator and a Drude model that took cosolvent-modified PEDOT:PSS films. the optical anisotropy into consideration. The optical model used in this study is shown in Fig. 2(b), which consists of a single layer with uniaxial optical anisotropy. The best fitted (n0, k0) and (nex, kex) spectra for the bulk component in pristine, MeOH-modified, and cosolvent-modified PEDOT:PSS films are also shown in Figs. 2(b) and 2(c), respectively. The no and ko spectra showed metallic behavior, whereas dielectric characteristics were more dominant for the nex and kex spectra of all samples. The no and ko spectra components showing metallic character were nearly independent of MoO3 concentration. On the other hand, change in the nex and kex spectra components were prominent. In particular, the ko spectra in the infrared region of 0.8-1.0 eV corresponding to free carrier absorption [Fig. 2(c)] implies that the free carrier concentration N was markedly increased by cosolvent addition.16,17 In contrast, nex was significantly increased by adding cosolvent, with no significant changes in kex. Furthermore, the standard optical transmittance/reflectance spectra of PEDOT:PSS films were nearly independent of EG concentration. These findings suggest that MeOH solvent mixing modifies the fine structure of the PEDOT:PSS random coil, resulting in a change in the uniaxial optical anisotropy.18-20 Furthermore, the addition of EG to conductive PEDOT:PSS decreased the film thickness and improved film uniformity. The viscosity for pristine, MeOH-modified, and MeOH/EG cosolvent-modified films were 16.1, 13.8, and 13.2 mPa·s, respectively, suggesting an increased ionic mobility of PEDOT:PSS in the solution. These findings imply that cosolvent mixing enhanced the flexibility of FIG. 3 J-V curves of c-Si heterojunction solar cells with pristine, MeOH-modified, and MeOH/EG cosolvent-modified buffer layers (a) in the dark and (b) under illumination of a simulated AM1.5G light. (c) EQE spectra of the corresponding c-Si heterojunction solar cells. PEDOT-PSS polymer chains and promoted densification of the PEDOT:PSS network. Therefore, there exists an appropriate MeOH:EG concentration ratio that minimizes the sheet resistance. The carrier transport parameters, i.e., the free carrier concentration N and carrier mobility μ were determined from plasma frequency ωp(=Ne2/ε0m*)0.5 and μ=e/m*Γd relations, where Γd is a damping factor determined by the dielectric constant ε(E)= ε∞-[Ad/(E2+Γd2)]. Here, μ represents the average hole mobility within an average sized grain. ε∞ and Ad are the dielectric constant ε in the high-frequency region and a constant, respectively. Here, an effective mass m* of 0.82m0 was used, where m0 is the free-electron mass. ωp, N, and μ were 1.41 eV, 1.18×1021 cm-3, and 1.23 cm2/Vs for the pristine film, 1.38 eV, 1.13×1021 cm-3, and 2.14 cm2/Vs for the MeOH modified film, and 1.44 eV, 1.23×1021 cm-3, and 3.92 cm2/Vs for the MeOH/EG cosolvent film, respectively. These μ values determined from the SE spectra agreed well with those based on conductivity (σ=Neμ) using the results shown in Fig. 3. Therefore, the SE analysis supports the optical constant as well as the carrier transport parameter. Figure 3 shows J-V characteristics observed in the dark and under AM1.5G illumination of 100 mW/cm2 simulated solar light, as well as external quantum efficiency (EQE) spectra from c-Si heterojunction solar cells based on pristine, MeOH-modified, and MeOH/EG cosolvent-modified PEDOT:PSS. The dark current density Jd was significantly suppressed by MeOH mixing [Fig. 4(a)]. Therefore, the reduction in the forward and reverse dark currents originated from the MeOH solvent. Furthermore, the reduction of dark current is clear evidence of the suppression of electron recombination. These results imply that the carrier transport property can be improved by using MeOH as a solvent without creating additional defects at the junction interface. The devices with a pristine PEDOT:PSS layer exhibited an average η of 6.38%, whereas an MeOH/EG-modified device demonstrated an improved η of 11.23% with a short-circuit current density Jsc of 28.79 mA/cm2, a open-circuit voltage Voc of 0.548 V, and an FF of 0.711 [Fig. 3(b)]. Obviously, the improvements in device performance can be attributed to the use of the MeOH/EG cosolvent. If the improvement of photovoltaic performance is governed by the changes in N, the EQE near 1000-1200 nm corresponding to free carrier absorption should be more prominent, because an increase in free carrier absorption would suppress the EQE value. However, the EQE was enhanced over the entire wavelength region by the addition of MeOH/EG cosolvent [Fig. 3(c)]. As a result, the photovoltaic performance was improved for the PEDOT:PSS layer showing higher electrical conductivity [Fig. 3]. Thus, the improved photovoltaic performance of c-Si/PEDOT:PSS heterojunction solar cells originated from an increase of μ rather than of N. We demonstrated the effect of uniaxial optical anisotropy on the photovoltaic performance of c-Si/organic heterojunction solar cells using an MeOH/EG cosolvent. nex increased by mixing cosolvent, while film thickness decreased, promoting the densification of PEDOT:PSS network. As a consequence, μ increased. The corresponding c-Si/PEDOT:PSS heterojunction solar cell exhibited a relatively efficient η of 11.23%, with a Jsc of 28.79 mA/cm2, a Voc of 0.548 V, and an FF of 0.71.