Side chain engineering of poly-thiophene and its impact on crystalline silicon based hybrid solar cells

Hybrid solar cells comprising inorganic and organic semiconductors enable the fabrication of very cost effective photovoltaic devices due to the solution based processing capability of organic compounds. Power conversion efficiencies of η > 10% have already been reported for planar hybrid solar cells based on Si/poly(3-hexylthiophene) (P3HT) or Si/poly(3,4-ethylene-dioxythiophene):polystyrene-sulfonate (PEDOT:PSS). The polymer films had a thickness of only 2–70 nm.^1,2 Hence, light is predominantly absorbed in the silicon. Compared with conventional semiconductor/metal Schottky-barrier devices, the improved performance is attributed to a reduced carrier recombination at the anode. This is ascribed to charge-carrier blocking of the organic semiconductor.¹ Furthermore, the presence of an organic semiconductor prevents the direct contact between the metal anode and the semiconductor. However, layer thickness and density have to be optimized to avoid metal permeation to the semiconductor. In addition, in these devices, Fermi-energy pinning is avoided that frequently limits the open circuit voltage, V_OC, in inorganic semiconductor/metal solar cells.^3,4

In the past, most research was focused on modifications of the polymer backbone, since most properties of the polymers are determined by the conjugated π-system.^5,6 In this paper, we show that changes in the peripheral side chains of polymers can result in superior properties. For this purpose, two derivatives of regioregular P3HT containing two and three ether groups in the side chains were investigated. The polymers poly(3-[3,6-dioxaheptyl]-thiophene) (P3DOT) and poly(3-[2,5,8-trioxanonyl] thiophene) (P3TOT) were synthesized by the GRIM method.⁷ They self-assemble into a microcrystalline lamellar structure with stacked backbones just like P3HT. Interestingly, the crystallinity of the polymers improves with increasing ether groups in the side chains, which is accompanied by a pronounced increase in the power conversion efficiency of hybrid solar cells.

All samples were prepared under ambient conditions on n-type c-Si with a (100) surface orientation. The wafers with a thickness of 330 μm and a resistivity of 5–10 Ω cm⁻¹ were cleaned using a conventional RCA process. The Si substrates were passivated with a thin oxide layer according to the following steps. First, the substrates were sonicated in acetone for 10 min and rinsed with high-purity water (18 MΩ cm). The residual native oxide was removed with dilute 5% hydrofluoric acid. Subsequently, the Si substrates were heated in high-purity water at a temperature of 80 °C for 10 min. Then the samples were rinsed with high-purity water and dried in a stream of nitrogen gas. The resulting oxide layers had a thickness of 1.3–1.5 nm and revealed an interface defect-density of D_it = 2 × 10¹²cm⁻² eV⁻¹, which was determined by surface photovoltage measurements.^8,9 To avoid recombination and to provide good selective collection of the photogenerated charge-carriers, an electron-selective back contact was used for the hybrid solar cells. Therefore, a phosphorous doped amorphous silicon layer with a thickness of 30 nm was deposited on the back side of the crystalline Si substrate.

The deposition of the organic layers of (poly-(3-hexylthiophene-2,5-diyl) (P3HT), poly-(3-[3,6-dioxaheptyl]-thiophene (P3DOT), and poly-(3-[2,5,8-trioxanyl]-thiophene] (P3TOT)) was carried out by spin coating at 2000 rpm for 30 s with a polymer concentration of 1 to 2 mg/ml solved in 1,2-dichlorbenzene. The resulting layers had a thickness of 20 nm for P3HT, 18 nm for P3DOT, and 12 nm for P3TOT. The films were dried in vacuum at 70 °C. As a final step, a circular 15 nm thick semi-transparent gold front contact was evaporated. The active cell area was defined by the front contact and amounted to 0.13 cm². Samples for UV/VIS measurements were spin coated on silica glass using the same parameters as for the devices.

The poly-thiophene based layers were characterized by UV/VIS and FT-IR measurements. Information on the crystallinity and molecular orientation were obtained from XRD measurements in Bragg-Brentano geometry. XRD and FT-IR measurements required thicker layers for sufficient signal intensity. XRD samples were spun under the same conditions as above using a higher concentration of the polymer in solution leading to a layer thickness of about 1 μm. The hybrid solar cells were characterized using a solar simulator under AM 1.5 G condition. An additional shadow mask prevented the underestimation of the active area.

In Fig. 1(a), XRD spectra of annealed polymer films on the Si/SiO_X substrates are shown. The data clearly indicate the existence of crystalline domains. For P3HT and P3DOT, the (100) reflection is observed at 2Θ ≈ 5.1°, which is due to a lamella structure.^10,11 It corresponds to a polymer chain spacing of 1.71 nm. The lamella structure is oriented parallel to the surface normal as depicted schematically in Fig. 1(b). A difference in the crystal structure related to the different length of the side chains for P3HT and P3DOT was not observed. The results obtained for P3HT and P3DOT are in good agreement with the data of Kline et al.¹² for high molecular weight regioregular P3HT. For P3TOT, the lamella spacing is slightly increased to 1.92 nm, which is caused by the longer trioxanonyl side chains compared with hexyl side chains.^13,14 The narrower (100) Bragg reflection of the P3TOT film points to a higher degree of crystallinity, compared with the other two polymers. According to the Scherrer formula,¹⁵ the average crystallite sizes of the polymers were determined to be 6 nm for P3HT and P3DOT, and 19 nm for P3TOT.

The UV-VIS absorption spectra of the three polymers spin-coated on the Si/SiO_X substrates are shown in Fig. 2. Prior to the measurement, the samples were annealed at 70 °C for 30 min. The spectra of P3HT and P3DOT films exhibit a well resolved vibronic structure and a bathochromic shift of the peak maximum of 0.5 eV to lower energy (see inset of Fig. 2), as expected for poly(3-alkylthiophenes).¹⁶ The absorption spectrum of the P3TOT film shows no vibronic structure and a less pronounced bathochromic shift.

While the absorption spectrum of P3TOT suggests little crystallinity, the XRD data reveal that the P3TOT film is crystalline with a higher structural order than the other polymer films [see Fig. 1(a)]. The variations in the polymer layer caused by addition of oxygen atoms in the side chains does not restrict to changes in the structure of the resulting layer as the contradiction in the results of XRD and UV/Vis measurements indicate. The intrinsic properties of the molecules are dominantly determined by the π-electron system. Thermochroism studies on poly(3-alkylthiophenes) identified the cause for the strong hypsochromic shift with increasing temperature to be an increasing twist between adjacent thiophene units.¹⁷ A similar effect comes into play for P3TOT. For this type of molecule, a preferential ordering of the side chains within the plane of the main polymer backbone is expected. The resulting lamellar structure has a high degree of planarity along a single polymer chain,¹⁸ resulting in the optical properties as can be observed for P3HT and P3DOT. However, P3TOT suffers from a steric hindrance of the side chains. The degree of planarity is lowered and results in a reduced degree of conjugation along the backbone. This has the following consequences. The bathochromic shift that is commonly observed when the polymer is cast from a solution into a thin film is reduced for P3TOT, and the vibronic structure vanishes [see Fig. 2].^17,19

The influence of the additional oxygen atoms in the side chains on the vibrational properties of the polymers were investigated using FT-IR spectroscopy. The symmetric stretching vibration of the thiophene ring is shown in Fig. 3(a). For P3HT, the symmetric stretching vibration occurs at ≈1509 cm⁻¹. When O atoms are introduced into the side chains, the vibrational mode shifts to 1511 and 1514 cm⁻¹ for P3DOT and P3TOT, respectively. The increase in frequency by about 1% indicates that the thiophene rings are subject to compressive strain. This is consistent with ab-initio density functional theory calculations that show a decrease of the C–C bond length in the thiophene ring by about 1% when 3 oxygen atoms are incorporated in the side chain.²⁰ 

The presence of O atoms in the side chains gives rise to vibrational modes. Fig. 3(b) shows the spectral region where the C–O–C stretching vibrations occur. It is important to note that P3HT does not show vibrational modes in this spectral region, which clearly establishes that these local vibrational modes are due to the presence of O atoms in the side chains. In P3DOT and P3TOT, two modes are observed at ν ≈ 1108 and 1135 cm⁻¹, which correspond to asymmetric stretching vibrations of alkyl ethers.²¹ With an increasing number of carbon atoms, the IR band shifts to lower frequencies. However, a precise assignment of the frequencies to the position of the oxygen atoms in the side chain is challenging. Based on the IR data of aliphatic ethers, it is likely that the mode at ν ≈ 1135 cm⁻¹ is related to the vibration of –CH₂CH₂–O–CH₃.²¹ 

These polymers were used to fabricate Si/polymer heterojunction solar cells. For spin-coating, low polymer concentrations and high rotational velocities were chosen to obtain homogeneous films with a thickness of 10–20 nm. In Fig. 4, the dark current voltage characteristics of hybrid solar cells with P3HT, P3DOT, and P3TOT are plotted. For comparison, the j–U curve of a metal-insulator-semiconductor (MIS) structure consisting of Au–SiO_X–c-Si is shown. The MIS and the P3HT based devices exhibit similar diode characteristics with saturation currents of about j₀ = 1.6 × 10⁻³ and 2.4 × 10⁻³mA/cm², respectively. According to the transmission electron microscopy data, the similarity is due to intermixing of Au and P3HT during the metal evaporation. On the other hand, when P3DOT and P3TOT are used to fabricate hybrid solar cells, the saturation current-density decreases by about 2 orders of magnitude (see Fig. 4). For these devices, intermixing of Au and the polymer layers was not observed. It is likely that this is directly related to an improvement of the structural quality of the polymers due to the presence of the O atoms in the side chains (see Fig. 1). Similar j–U curves were obtained when a MoO₃ diffusion barrier was deposited between the polymer and the Au layer. This clearly shows that side chain engineering can be used to improve device performance without the need for additional layers (i.e., diffusion barrier).

The hybrid solar cells were further characterized by measuring their j–U curves under AM 1.5G illumination. The device characteristics are plotted in Fig. 5. The performance of the MIS device is in good agreement with results from literature, approaching the theoretical limit for a gold n-type silicon diode with typical open circuit voltages below 0.3 V.^22,23 This value is mainly caused by the thermionic emission current flow of the electrons from the metal to semiconductor, limiting the open circuit voltage of Schottky devices compared with p–n junctions. A possibility to overcome this problem is the introduction of an interlayer, which further reduces the current flow across the barrier.²⁴ A basic precondition for the organic layers to perform this task is to prevent the direct contact of the metal with the semiconductor. However, the similarity of the MIS device to the hybrid solar cell with a P3HT interlayer indicates that these devices suffer from the same problem. As noted earlier, side chain engineering overcomes this drawback. Adding oxygen atoms in the side chains results in an increase in the open circuit voltage from 0.3 V to 0.46 V and to 0.5 V for P3HT, P3DOT, and P3TOT, respectively. This is consistent with the observed change of the saturation current-density (see Fig. 4), since V_OC = nkT/e × ln(j_SC/j₀). Here, j_SC denotes the short circuit current, e is the electric charge, n is the ideality factor of the diode, k is the Boltzmann constant, and T is the temperature. The solar cells fabricated with side chain engineered polymers exhibit a pronounced increase of the power conversion efficiency (see inset in Fig. 5). The devices prepared with P3DOT appear to be affected by shunting through pinholes, which leads to a reduced efficiency. On the other hand, hybrid solar cells prepared with P3TOT reach efficiencies close to η = 10%. The device characteristic in Fig. 5 indicates that the performance of the hybrid solar cell is limited by a series resistance. Most likely, this is due to poor charge-transport properties in the polymer layer.

In summary, we have shown that changes of the peripheral side chain of regioregular P3HT influence structural, electrical, and optical properties of the polymers and hybrid solar cells. The ether groups in the side chains of P3DOT and P3TOT resulted in better adhesion and, therefore, more homogeneous and electronically enhanced polymer/SiO_x interfaces. With increasing number of ether groups, the crystallinity is improved. For P3HT, the symmetric stretching vibration occurs at ≈1509 cm⁻¹, which shifts to 1511 and 1514 cm⁻¹ for P3DOT and P3TOT, respectively. This indicates that the thiophene rings are exposed to compressive strain. P3DOT and P3TOT exhibit two modes due to stretching vibrations of alkyl ethers at ν ≈ 1108 and 1135 cm⁻¹. The efficiency of planar hybrid solar cells made from Si/SiO_X/polymer structures depends on the amount of ether groups in the polymer side chains. The highest efficiency of η = 9.6% was achieved for P3TOT, which has three ether groups in the polymer side chains.

The authors are indebted to C. Klimm for providing SEM micrographs. K. Jacob and E. Conrad are acknowledged for the silicon wafer preparation. Financial support was provided by the Helmholtz-Energy-Alliance “Hybrid Photovoltaics.”