As an exciton is excited by the absorption of a photon in P3HT and subsequently the exciton is only split at the titania/P3HT interface over a built-in gradient in the elec-trochemical potential, a successful backfilling of P3HT into mesoporous titania films is of great importance for the performance of the final device. The experimental details
regarding backfilling are described in section 4.2.4. The P3HT-backfilled titania film, named as active layer, is the core part of hybrid solar cells which is responsible for the charge carrier generation and transport.
8.2.1 Active layer morphology
To obtain a more crystalline P3HT film, the backfilled sample is post-annealed at 120 ◦C for 10 min under nitrogen atmosphere.
Figure 8.12: a) SEM image of the P3HT back-filled hierarchical titania films measured at a tilt angle of 54 ◦ and b) corresponding cross-section SEM image.
Figure 8.12a shows the surface morphology of the hierarchical titania films (with meso-pores and artificial superstructures) backfilled with P3HT. To yield a better view, the sample stage is tilted with respect to the electron beam. After backfilling with P3HT, the mesoporous titania film is covered with P3HT and the artificial superstructures are pre-served instead of being filled completely. A similar observation is reported by Ding et al., who forged nanodome-superstructures onto mesoporous titania films and backfilled then with spiro-OMeTAD as a hole transport material [293]. It was shown that the preserved nanodomes after backfilling give a significant enhanced light-harvesting capability in the active layer. Therefore, the preservation of NIL-induced patterns is of importance. If the P3HT capping layer is too thin, the top electrode might contact with titania photoanodes directly, leading to shunting paths. If the capping layer is too thick, the pitch-like super-structures might be filled with P3HT completely, which results in less light harvesting in the active layer. Later on, gold electrodes need to be deposited on top of the active layer to finalize solar cells. To investigate the superstructure conditions after gold deposition, SEM is used to probe the sample. The sample stage is tilted as well. Figure 8.13 shows that the NIL-induced pattern on the active layer are still preserved after gold deposition.
The thickness of the active layer is revealed by cross-section SEM images (figure 8.12b).
A very thin layer of about 170 nm is observed. Moreover, the cross-section image shows a
Figure 8.13: SEM image of the superstructured active layer with gold contact on top. The sample is measured at a tilt angle of 54 ◦.
good infiltration of P3HT into the mesoporous titania films. As SEM measurements only provide the surface information, the inner morphology of the active layer is investigated with GISAXS measurements. The 2D GISAXS data of the superstructured active layer is displayed in figure 8.14a. As seen from the images, an arc locates at the high qz re-gion, which is identified as the (100) Bragg reflection of P3HT crystals. To characterize P3HT infiltration, a horizontal line cut is performed at the position of the P3HT Yoneda peak instead of the titania Yoneda peak. The cut along with the corresponding fit are presented in figure 8.14b. From data modeling, characteristic radii of (4.6 ±0.2) nm and (9.5±0.4) nm with center-to-center distances of (24.6±2.9) nm and (47.1±8.3) nm are determined for P3HT domains. From the GISAXS measurements on the nano-imprinted titania film after polymer extraction, the pore size is computed to be (19.0 ± 3.3) nm, which agrees well with the large-sized P3HT domains, suggesting a fairly good infiltration of P3HT into large-sized mesopores. In contrast, the small P3HT domains are slightly smaller than the small-sized pores, elucidating that P3HT infiltrates the small-sized meso-pores not perfectly. Moreover, the distances between P3HT domains can be calculated via equation 5.1 as well, to be (15.4 ± 2.7) nm and (28.1 ± 7.9) nm. Both values are commensurate with the small- and large-sized titania domains after polymer extraction, implying that the P3HT backfilling process does not influence the pre-existing titania nanostructures.
8.2.2 Optical properties of the active layer
To evaluate the enhancement of light harvesting in the superstructured active layer, the light absorption is investigated at various angles of light incidence. To make a comparison, the angular-dependent light absorption in the original active layer is measured as well.
Figure 8.14: a) 2D GISAXS data of the active layer with artificial superstructures. b) The black curve represents the horizontal line cut obtained from the 2D GISAXS data. The grey line indicates the fit to the data.
For UV/Vis measurements, the active layers are fabricated on glass substrates and the light is shone through the glass substrate to mimic the real situation of illumination for solar cells. In order to eliminate the influence of the glass surface reflection, the UV-Vis absorption spectra of the pure glass is deducted from the spectra of glass-substrate/active-layer sample at each angle of incidence. The corrected UV/Vis absorption spectra of the active layers at various angles of incidence are shown in figure 8.15. For both active layers (without and with superstructures), the main absorption is in the wavelength region of 400 - 600 nm, which is assigned to the absorption characteristics of P3HT [123]. Moreover, all spectra have a similar shape and the peak positions stay nearly unchanged, indicating that the NIL pattern does not lead to remarkable changes in the optical properties.
Under standard conditions (at 0◦ light incident angle), the maximum absorbance of both active layers at 525 nm is approximately 0.54. The absorption resemblance implies that the light absorption at 0◦ incidence is not affected by the artificial superstructures, which is due to the fact that the incident light is perpendicular to the pitch-like super-structures in the hierarchical titania film and thereby no obvious scattering occurs. In addition, both active layers show a similar thickness, (154±9) nm for the original sample and (157 ± 19) nm for the non-imprinted active layer. The thickness is measured with height profilometry, as described in section 3.1.1. With increasing incidence angles, the
Figure 8.15: Angular dependent UV/Vis absorption spectra. The data are recorded every 5◦: a) Original active layer and b) superstructured active layer.
active layers behave quite differently with respect to the degree of light absorption. Only a slight increase of light absorption is formed in the original active layer whereas a signif-icant enhancement of light absorption is present in the nano-imprinted active layer. For the original active layer, the increase arises from a geometrically elongated pathway at nonzero angles. The light path with a certain incident angle is schematically illustrated in figure 8.16. In the present study, the refractive index of titania and P3HT is 2.2 and 2.37,
Figure 8.16:Schematic illustration of the light path in the active layer. Black arrows indicate the light path, X and L indicate the light traveling distances at nanozero and zero incidence angle, respectively. θ1 and θ2 represent incidence angle and refraction angle, respectively.
respectively. The porosity of mesoporous titania films is about 0.62, which is determined by white light interferometry. The refractive index of the active layer can be calculated
to be 2.30 via a so-called Bruggeman effective medium approximation which is described in literature [311]. The relation between θ1 and θ2 can be interpreted by Snell’s law:
n1sinθ1 =n2sinθ2 (8.1)
For example, the light incident angle is assumed to be 45 ◦, by knowing n1 = 1 for the air and n2 = 2.30 for the active layer, the θ2 can be computed to be about 18 ◦. Thus, X = 1.051 L is obtained. From theoretical calculation, the light pathway at 45 ◦ incident angle is 5.1 % longer than that at zero incident angle. The UV/Vis measurements show that the absorbance at 45 ◦ incident angle is 5.8% higher as compared to 0 ◦ incident angle. The increased absorbance agrees well with the calculated elongated light pathway, confirming that the changes of absorption in the original active layer are caused by the distance changes of the light pathway.
For the nano-imprinted active layer, the absorbance at 45 ◦ incident angle is about 0.66 at 525 nm, i.e. almost 20 % enhancement as compared to 0 ◦ absorbance. Besides the longer light travel path (about 6 %), the rest of 14 % enhancement of the light absorption originates from scattering in the active layer superstructures. Moreover, it is noticed that the first prominent increase in light absorbance occurs at 35 ◦ incident angle (figure 8.15b), indicating this angle is a threshold for large amount of light scattering in this specific film with special superimposed structural order.