3.1.

Spectroscopic Ellipsometry

$\mathrm{\Psi }$ and $\mathrm{\Delta }$ were measured with a variable angle spectroscopic ellipsometer (VASE^®, J.A. Woollam Co., Inc.), equipped with a rotating analyzer and two detectors. A rotating retarder plate (${\mathrm{MgF}}_2$ Berek waveplate) in the beam path behind the fixed polarizer allows for a precise determination of $\mathrm{\Delta }$.³⁹ Datasets were recorded between 250 and 1,700 nm (1,000 nm for ${\mathrm{PC}}_{71}\mathrm{BM}$). Transmission data of the samples were collected at normal incidence and by moving the VASE detector arm into the optical path behind the sample. Thus, the transmission spectra were determined on the very same spot as the ellipsometric data, allowing for their incorporation into the analysis. The datasets were analyzed with the software WVASE32^® (J. A. Woollam, version 3.774).

3.2.

Transmission Spectroscopy

Transmission spectra of diluted solutions and of solid films spin cast onto quartz-glass substrates were recorded with a spectrophotometer (Lambda 1050, Perkin Elmer), enabling a lower noise level than the measurements carried out with the ellipsometer.

3.3.

Materials and Sample Preparation

Soda-lime glass substrates were cleaned with acetone and isopropyl alcohol and then dried in a nitrogen stream. All materials were used as received and were deposited under inert conditions in a nitrogen glovebox (${\mathrm{O}}_2$ and ${\mathrm{H}}_2\mathrm{O}<1\mathrm{ppm}$).

${\mathrm{PC}}_{71}\mathrm{BM}$ (Solenne, $>99\%$) was dissolved in DCB (anhydrous, 99%, Sigma-Aldrich^®) at a concentration of $60\mathrm{mg}/\text{m}\text{L}$, stirred overnight at 80°C, and spin cast on substrates at room temperature with different spin coater settings to achieve thicknesses between 85 and 120 nm.

PSBTBT and PCPDTBT were both dissolved in DCB with a concentration of $13\mathrm{mg}/\text{mL}$, stirred overnight at 70 or 90°C, respectively, and were deposited from hot solution on hot soda-lime glass substrates. Different spin coater settings allowed for tuning the layer thicknesses between 40 and 60 nm for PSBTBT or 15 and 30 nm for PCPDTBT.

The blends $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ ($1:2$, layer thicknesses between 90 and 115 nm) and $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ ($1:3.4$, layer thicknesses between 90 and 130 nm) with a polymer concentration of $13\mathrm{mg}/\text{mL}$ in DCB were spin coated following the same conditions used for neat films. The blend ratios were chosen according to optimized process protocols.^27,29

SE data of the neat polymer films and the blend films were recorded between 250 and 1,700 nm at different angles of incidence $\alpha$ between 30 and 80 deg in 5 deg steps. Datasets were analyzed over the entire spectral range. For reasons of clarity, we only show a reduced dataset in the figures, i.e., we skip data in the infrared beyond 1,000 nm, restrict to some representative angles of incidence, and only show data for one sample of the multisample analysis.

Thin-film thicknesses were cross-checked with a stylus profiler (DektakXT, Bruker AXS GmbH) and compared with ellipsometry results.

4.1.

Glass Substrate

Soda-lime glass is used as substrate for all thin-films.³⁷ Therefore, we first set up an optical model for the glass substrate. As the substrate is $\sim 1\mathrm{mm}$ thick, unwanted backside reflections are expected. They can either be suppressed by roughening the substrate’s backside or be accounted for in the analysis software. Here we use the latter approach as this allows measuring the transmission spectra on the very same spot after the ellipsometry characterization.

The glass substrate is described with a Cauchy dispersion relation⁴⁰ with an Urbach-tail⁴¹ and a surface roughness layer of $\sim 1.3\mathrm{nm}$.

The derived complex refractive indices $(n,k)$ of the soda-lime glass substrates are depicted in Fig. 1 versus wavelength. In the following, we keep the parameters of the model constant and treat the surface roughness layer as an intermixed layer of the subsequently applied thin-film and the glass substrate.

Fig. 1

Optical constants $n$ and $k$ of the soda-lime glass, which is used as substrate for all thin-films discussed in this study. The optical model includes a surface roughness layer of $\sim 1.3\hspace{0.17em}\text{nm}\hspace{0.17em}$.

4.2.

PC₇₁BM

Solar cells based on the polymers PCPDTBT and PSBTBT in combination with the electron acceptor ${\mathrm{PC}}_{71}\mathrm{BM}$ exhibit enhanced power conversion efficiencies. We first analyze ${\mathrm{PC}}_{71}\mathrm{BM}$. Therefore, the fullerene is dissolved in 1,2-dichlorobenzene (DCB) and then spin cast on the glass substrates as described in Sec. 3.3. A multisample SE analysis is performed on six samples with varying thicknesses. Figures 2(a) and 2(b) show the experimental data of a typical sample. Transmission spectra of ${\mathrm{PC}}_{71}\mathrm{BM}$ in DCB solution and of a solid ${\mathrm{PC}}_{71}\mathrm{BM}$ thin-film are recorded with a spectrophotometer, Fig. 2(c). From the absorption features present in all spectra, the starting values $E_{n,\text{input}}$ for the center energies of the Gaussian oscillators are derived, Table 1. Based on this estimation of $E_n$, an optical model is set up comprising nine Gaussian oscillators ${\mathrm{Gau}}_n$, a pole taking into account higher-energy transitions, and a real constant ${\epsilon }_{\mathrm{r},\infty }$. A stepwise fit ($\text{MSE}=11.14$), starting from the infrared and progressing toward higher energies, results in the oscillator parameters given in Table 1. The fitted center energies $E_n$ differ slightly from their respective starting values $E_{n,\text{input}}$. In this multisample analysis, all measured datasets are taken into account. If the fit is restricted to a single sample, lower MSEs can be achieved. As the multisample analysis only differs slightly from the results of the single-sample analysis and as we aim, here and in the following, at finding a widely applicable optical model, we utilize the multisample approach. The model-generated data ${\mathrm{\Psi }}^{\mathrm{mod}}$, ${\mathrm{\Delta }}^{\mathrm{mod}}$, and the optical constants are depicted in Figs. 2(a), 2(b), and 2(d), respectively. The good agreement with the optical constants in Ref. 42, supporting information, verifies the chosen approach. The good match between simulated and experimental transmission, which was measured on the same spot as the ellipsometry data, proves the fit quality, Fig. 3(a).

Fig. 2

(a) and (b) Spectroscopic ellipsometry data of an 87 nm [6,6]-phenyl C₇₁-butyric acid methyl ester (${\mathrm{PC}}_{71}\mathrm{BM}$) thin-film spin cast on a glass substrate for five angles of incidence. The colored solid lines correspond to the measured data; the dashed black lines correspond to the model-generated data. (c) Transmission spectra of ${\mathrm{PC}}_{71}\mathrm{BM}$ in diluted dichlorobenzene (DCB) solutions and of a solid film on fused silica measured with a spectrophotometer. The gray shaded area indicates the absorption of the cuvettes. The center energies of the absorption features ${\mathrm{Gau}}_1$ to ${\mathrm{Gau}}_9$ are used as starting values for the Gaussian oscillator center energies in the optical model. (d) Optical constants $n$ and $k$ are derived from a multisample analysis.

Table 1

From the transmission spectra in Fig. 1(c), the starting values En,input for the fit of the Gaussian oscillators’ center energies are derived. The optical model of [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) comprises nine Gaussian oscillators (Gaun with center energy En, amplitude An, and width Brn,33 a pole, and the real constant εr,∞=2.00  eV.

Fig. 3

Measured (red lines) and simulated transmission (black dashed lines) of (a) ${\mathrm{PC}}_{71}\mathrm{BM}$, (b) poly{[4,4’-bis(2-ethylhexyl)dithieno(3,2-b;2’,3’-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT), (c) poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), (d) $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$, and (e) $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$. The measured transmission spectra were determined at the same spot as the ellipsometry data by rotating the ellipsometer’s detector straight behind the sample. The noise levels are higher than in the spectrophotometer measurements that were described in Sec. 3.2 and that are depicted in Figs. 2(c), 5(c), and 7(c). The simulated data in Figs. 3(b)–3(e) rely on anisotropic models.

All thin-film thicknesses $t_{\text{Stylus}}$ as determined by profilometry are well within the 90% confidence limit of the thickness $t_{\mathrm{SE}}$ as determined by SE.

4.3.

Polymers

Both polymers, PCPDTBT and PSBTBT, exhibit strong structural similarities. They differ only in the 5-position, where PCPDTBT comprises a carbon atom, whereas PSBTBT features a silicon atom, Fig. 4(a). It was shown that the silicon substitution in PSBTBT reduces the steric hindrance from the bulky alkyl groups and improves molecular packing.⁴³ The main absorption features of both polymers are still comparable though.

Fig. 4

(a) Chemical structure of PSBTBT and PCPDTBT and definition of the polymer-chain reference frame (a, b, and c). The $c$ axis is oriented along the polymer backbone. (b) The optical constants $n_{xy}$ and $k_{xy}$ describe light-polymer interaction for polymers with their $c$ axes being aligned parallel to the substrate surface, while the $z$-component describes vertically oriented polymers. ($x$, $y$, and $z$) is the substrate reference frame. Schemes of the morphology of (c) neat PSBTBT, (d) neat PCPDTBT, (e) $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$, and (f) $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ showing different degrees of aggregate alignment relative to the surface. Black dots represent ${\mathrm{PC}}_{71}\mathrm{BM}$.

The measured datasets $\mathrm{\Psi }$ and $\mathrm{\Delta }$ for a PSBTBT thin-film spin cast on a glass substrate are shown in Figs. 5(a) and 5(b), respectively. The isotropic optical model is constructed in analogy to the procedure described for ${\mathrm{PC}}_{71}\mathrm{BM}$. The transmission spectra of PSBTBT in DCB solution and of a solid PSBTBT thin-film are recorded, Fig. 5(c). The analysis of the absorption features leads to the starting values $E_{n,\text{input}}$ for the center energies $E_n$ of the Gaussian oscillators. Then, all oscillator parameters, a pole for the high-energy transitions and ${\epsilon }_{\mathrm{r},\infty }$ are fitted. The respective values are summarized in Table 2. Based on this model, the isotropic optical constants are derived, Fig. 6(a).

Fig. 5

(a) and (b) Spectroscopic ellipsometry data $\mathrm{\Psi }$ and $\mathrm{\Delta }$ of a typical neat PSBTBT thin-film spin cast on a glass substrate. The colored solid lines correspond to the measured data; the dashed black lines correspond to data generated from a uniaxial optical model. (c) Transmission spectra of PSBTBT in diluted DCB solutions and of a solid film on fused silica. The starting values for the Gaussian oscillator center energies $E_{n,\text{input}}$ are derived from the center energies of the absorption features, Table 2. (d) The optical constants comprise an in-plane (index $xy$) and out-of-plane component (index $z$) and are derived from a multisample analysis with an overall mean squared error $(\text{MSE})=6.37$.

Table 2

Isotropic, parameterized description of poly{[4,4’-bis(2-ethylhexyl)dithieno(3,2-b;2’,3’-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT). The starting values En,input are derived from the transmission spectra in Fig. 5(c). The center energies En change only slightly upon performing the fit. The parameters An and Brn are determined defining the Gaussian oscillators Gaun. The offset was found to be εr,∞=2.46  eV.

Fig. 6

Optical constants of (a) PSBTBT and (b) PCPDTBT within an isotropic description. We note the different scaling of the $y$ axes.

PCPDTBT is investigated accordingly. The respective isotropic datasets are shown in Fig. 7, Table 3, and Fig. 6(b). The spectral shape of the complex indices of refraction is comparable with the isotropic data presented by Guerrero et al.⁴² As they used PCPDTBT from a different supplier, the amplitudes are different though.

Fig. 7

(a) and (b) Spectroscopic ellipsometry data $\mathrm{\Psi }$ and $\mathrm{\Delta }$ of a typical neat PCPDTBT thin-film spin cast on a glass substrate. The solid lines correspond to the measured data; the dashed black lines correspond to the model-generated data within a uniaxial description. (c) Transmission spectra of PCPDTBT in dilute DCB solutions and of a film on fused silica. The center energies $E_{n,\text{input}}$ of the absorption features ${\mathrm{Gau}}_n$ are given in Table 3. (d) Anisotropic optical constants derived from a multisample analysis with an overall $\text{MSE}=7.51$.

Table 3

Poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole] (PCPDTBT). Starting values En,input as derived from Fig. 4(c). The parameters En, An, and Brn determine the optical model. εr,∞=2.28  eV. Gau7 is outside the ellipsometer’s measurement range; the pole function covers its contribution.

Both polymer models are derived independently. However, the derived optical models utilize very similar sets of Gaussian oscillators, which reflect their strong structural similarity. A comparison of the isotropic optical constants further shows that the overall optical density of PSBTBT is somewhat higher than the optical density of PCPDTBT. This can be either attributed to a higher intrinsic absorption or to a denser packing of the polymer chains.⁴³

For both polymers, we assign the transitions ${\mathrm{Gau}}_1$ and ${\mathrm{Gau}}_2$ to the 0–0 and 0–1 absorption peaks, while ${\mathrm{Gau}}_3$ comprises higher vibronic progressions and an amorphous background. The nature of ${\mathrm{Gau}}_4$ remains unclear, but may be attributed to transitions involving more localized states.⁴⁴

A more detailed model has to take into account the anisotropy of the films. OSC polymers often exhibit elongated structures and, hence, form mostly flat two-dimensional molecules.^45–48 Therefore, we probe the thin-films for anisotropy, assuming no preferential order in the in-plane direction as indicated by the invariance of $\mathrm{\Psi }$ under in-plane sample rotation.^32,49,50 This assumption allows us to utilize a uniaxial optical description, only distinguishing between the $xy$- and the $z$-contribution. [The xy-component is sometimes referred to as $\parallel$ (in-plane) and z as $\perp$ (out-of-plane).] For this orientation, the off-diagonal elements of the Jones matrix vanish.³²

In the uniaxial configuration, the optical constants $n_{xy}$ and $k_{xy}$ are determined by the $xy$-layer, and the $z$-component by the $z$-layer. The $xy$-optical constants describe light-matter interaction for light under normal incidence with its electrical field vector $E$ parallel to the sample surface. For light impinging at grazing incidence, $E$ is almost normal to the sample surface and the interaction with the polymer is described with $n_z$ and $k_z$.

For the anisotropic model, the isotropic oscillator parameters serve as input parameters for both the $xy$- and $z$-layer, consequently doubling the number of fit parameters. Since doubling of the fit parameters does hamper finding a unique solution, we simplify our model by assuming that the same optical transitions will be observed in both directions ($xy$ and $z$), changing their relative amplitude only. Accordingly, the oscillator amplitudes $A_{n,xy}$ and $A_{n,z}$ are defined as fit parameters.

The fitted datasets $\mathrm{\Psi }$ and $\mathrm{\Delta }$ for PSBTBT are shown in Figs. 5(a) and 5(b) and the derived anisotropic optical constants in Fig. 5(d). The MSE improves from 7.23 to 6.37 upon utilizing an anisotropic model. The respective datasets for PCPDTBT are shown in Fig. 7. For PCPDTBT, the MSE improves from 7.77 to 7.51. Evidence of the fit quality is provided in Figs. 3(b) and 3(c), where we compare the experimental transmission data that were measured on the same spot as the ellipsometry data, with the simulated transmission based on the ellipsometric model. Both neat polymers exhibit pronounced uniaxial anisotropy. Since the polymer $\pi \to {\pi }^{*}$ transitions, here modeled with ${\mathrm{Gau}}_1$—${\mathrm{Gau}}_3$, are excited by an electrical field that is polarized parallel to the polymer backbone, i.e., the polymer $c$ axis as defined in Fig. 4(a), a detailed analysis of the anisotropic optical constants can further reveal morphological details.^51,52,53 The optical constants $n_{xy}$ and $k_{xy}$ describe light-polymer interaction for polymers with their $c$ axes aligned parallel to the substrate surface, while the z-component describes upright polymers with their $c$ axes oriented in $z$-direction, Fig. 4(b).

The extinction coefficient of PSBTBT splits up in an $xy$- and a $z$-component between $\sim 540$ and 800 nm, Fig. 5(d). Toward lower wavelengths, no splitting is observed, indicating a different nature of the involved transitions. According to similar observations in the literature, we attribute the shoulder around 761 nm (${\mathrm{Gau}}_1$) to polymer aggregation.⁴³ As the contribution of the $xy$-component is more pronounced than the contribution of the $z$-component, we conclude that aggregates of polymers with a $c$ axis aligned in-plane prevail, Fig. 4(c).

For PCPDTBT, the $xy$- and $z$-extinction coefficients also split up and show anisotropic behavior in the low-wavelength regime, Fig. 7(d). The low-energy shoulder, which is visible in the $k_{xy}$ data, does not appear in the $k_z$ data. In analogy to PSBTBT, the shoulder in $k_{xy}$ can be attributed to aggregates of polymers with $c$ axes aligned parallel to the $xy$-plane, whereas the interpretation of $k_z$ leads to ambiguous results. This can either be attributed to nonaggregated standing polymers or an isotropic amorphous background, Fig. 4(d).

4.4.

Blends

The measured spectroscopic ellipsometry data $\mathrm{\Psi }$ and $\mathrm{\Delta }$ of a $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ and a $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ thin-film are depicted in Figs. 8(a) and 8(c) and in Figs. 8(b) and 8(d), respectively. The optical properties of the polymer:fullerene blends are modeled within an effective medium approximation (EMA). Therefore, the optical constants of polymer and fullerene constituents are superimposed. As the analysis of neat polymers in Sec. 4.3 revealed a preferred orientation of the PSBTBT and PCPDTBT $c$ axes parallel to the substrate, we choose uniaxial models for the blends as well. The oscillator parameters in Tables 1, 2, and 3 are used as starting parameters.

Fig. 8

Spectroscopic ellipsometry data $\mathrm{\Psi }$ and $\mathrm{\Delta }$ of [(a) and (c)] a typical $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ ($1:2$) thin-film and [(b) and (d)] a typical $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ ($1:3.4$) thin-film. The solid lines correspond to the measured data; the dashed black lines correspond to the model-generated data within a uniaxial description. Anisotropic optical constants of (e) $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ and (f) $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ are derived from a multisample analysis with an overall $\text{MSE}=3.90$ and $\text{MSE}=4.64$, respectively.

After determining the layer thicknesses in the transparent spectral regime, the volume ratio of polymer to ${\mathrm{PC}}_{71}\mathrm{BM}$ is fitted. The resulting ratios are listed in Table 4 showing excellent agreement with volume ratios being calculated on the basis of material density as determined by helium pycnometry in the literature.^54,55 This result shows that SE can be an alternative and contactless metrology approach to determine material density ratios in blend films. If the material density of one component is already known, the absolute material density of the other component can be determined.

Table 4

The normalized volume ratios in the blends as derived from ellipsometry fittings show excellent agreement with the volume ratio calculated from the material densities. ρPC71BM=1.52  g/cm3, ρPSBTBT=1.17  g/cm3, and ρPCPDTBT=1.10  g/cm3 as determined by helium pycnometry in the group of Dadmun et al.54,55

In the next step, we investigate whether the polymer’s anisotropy is preserved upon blending with ${\mathrm{PC}}_{71}\mathrm{BM}$. Therefore, the polymer oscillator amplitudes are fitted leading to the anisotropic optical constants shown in Fig. 8(e) for $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ and in Fig. 8(f) for $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$. As indicated in Figs. 8(a) and 8(c) and in Figs. 8(b) and 8(d), both multisample fits describe the experimental data very well ($\text{MSE}=3.90$ and 4.64). Again, the comparison of the simulated and the experimental transmission in Figs. 3(d) and 3(e) confirms the good fit quality. Further, the anisotropic optical constants of $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ match the shape of the isotropic optical constants known in the literature.⁵⁶ When performing the fits, only the amplitudes of the oscillators ${\mathrm{Gau}}_1$ to ${\mathrm{Gau}}_3$ change considerably. In analogy to the analysis of the neat polymers, those oscillators are an indicator for the degree of polymer aggregation.

For $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$, a pronounced uniaxial anisotropy becomes visible in the optical constants, Fig. 8(e). Similar to neat PSBTBT, both the in-plane and the out-of-plane extinction coefficients exhibit shoulders around 760 nm, where the $xy$-contribution dominates the $z$-contribution. The dominance of the $xy$-contribution can be attributed to polymer aggregates with their $c$ axes aligned in-plane as illustrated in Fig. 4(e). This interpretation of the polymer orientation from SE experiments fits well to the time-resolved x-ray analysis that we published earlier, where we found that PSBTBT nucleation takes place in defined orientation at an interface, while randomly oriented aggregates form in the bulk during solvent evaporation.²⁹ As the degree of $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ anisotropy is similar to the anisotropy of neat PSBTBT, the polymer orientation remains mostly unaffected upon blending with ${\mathrm{PC}}_{71}\mathrm{BM}$.

For $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$, the optical constants in $xy$- and in $z$-directions differ slightly, Fig. 8(f). The PCPDTBT low-energy shoulder appears in both directions. This can be attributed to randomly oriented PCPDTBT aggregates as depicted in Fig. 4(f). In the blend model, the low-energy shoulder is more pronounced than in the model for neat PCPDTBT, indicating a higher degree of aggregation in the blend.

The higher degree of the $c$ axis in-plane alignment within the $\mathrm{PSBTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ blend as compared to the $\mathrm{PCPDTBT}:{\mathrm{PC}}_{71}\mathrm{BM}$ blend, probably induced by the substrate surface, may be explained by the lower ${\mathrm{PC}}_{71}\mathrm{BM}$ volume fraction, Table 4.

4.5.

Limitations

The EMA approach requires negligible cross-coupling of optical transitions between the blend’s components. For example, charge transfer (CT) absorption could cause such a new excitation channel. Since CT absorption in state-of-the-art OSCs is some magnitudes lower than the $\pi \to {\pi }^{*}$ transition,⁵⁷ we disregard CT absorption.

Both polymers that are investigated in this study exhibit a lower degree of crystallinity than other semicrystalline materials, such as P3HT.⁵⁸ Upon blending more crystalline materials, the degree of crystallinity may be reduced significantly. Therefore, in order to generate a suitable model for highly crystalline materials, care should be taken that the degree of crystallinity between the neat material and the blend is comparable. For example, for neat P3HT, we suggest deposition from a low-boiling point solvent, such as chloroform, thus accelerating the film drying time and, thereby, effectively suppressing crystallization of the neat polymer.

The determination of the anisotropic blend models benefits from the weak spectral overlap of the polymer and fullerene extinction coefficients, which allows separating both components and performing a precise fit. Fortunately, this applies to the vast majority of polymer:fullerene blends. Even more stable fit results can be achieved by taking into account advanced photophysical descriptions.⁵⁹ So far, such a detailed description has been elaborated for P3HT only.^60,61 A description of the complex absorption spectra of the low-band gap polymers is still lacking. Therefore, we suggest utilizing the phenomenological derived values of the oscillator center energies and robust multisample fits as discussed in this work.