Structure-induced resonant tail-state regime absorption in polymer: fullerene bulk-heterojunction solar cells

In this work, we present resonant tail-state regime absorption enhanced organic photovoltaics. We combine periodically structured ${\mathrm{TiO}}_2$ bottom electrodes with P3HT-PCBM bulk-heterojunction solar cells in an inverted device configuration. The wavelength-scale patterns are transferred to the electron-selective bottom electrodes via direct laser interference patterning, a fast method compatible with roll-to-roll processing. Spectroscopic and optoelectronic device measurements suggest polarization-dependent absorption enhancement along with photocurrent generation unambiguously originating from the population of tail states. We discuss the effects underlying these absorption patterns with the help of electromagnetic simulations using the discontinuous Galerkin time domain method. For this, we focus on the total absorption spectra along with spatially resolved power loss densities. Our simulations stress the tunability of the absorption resonances towards arbitrary wavelength regions.

Figure 1

The figure above shows an illustrative workflow description of inverted P3HT:PCBM BHJ solar cells featuring periodically structured ${\mathrm{TiO}}_2$ bottom electrodes. The employed materials along with the respective layer thickness are provided in (a). As shown in (b), direct laser interference patterning (DLIP) is employed to transfer a two-beam interference pattern to ${\mathrm{TiO}}_2$. After depositing an approximately 15-nm-thick capping layer (c) on structured as well as on flat reference devices, all remaining layers are deposited on top [cf. (a) and (d)].

Figure 2

The figure above shows in (a) and (b) the surface profile of two different ${\mathrm{TiO}}_2$ gratings created by DLIP. The measured absorption enhancements for the two solar cells built from these two different patterns with respect to the flat surface are shown in (c). In (d), the measured relative EQEs for two different solar cells with the same grating are shown. The two distinct absorption peaks in the red and IR in (c) that were obtained by careful design of the solar cell's structure lead to a significant enhancement of the EQE (d). Sensitive EQE measurement (e) quantitatively focusing on structure-induced tail-state photocurrent enhancement (black arrow). The measurements in (c)–(e) were done with unpolarized light.

Figure 3

Experimental and theoretical results for the absorption ($470\text{-nm}$ structure) for two different polarizations. (a) represents a sketch of the geometry. (b) focuses on the two experimental absorption peaks in the tail-state absorption region, whereas (c) and (d) present the complete numerical absorption, including the layer-resolved absorptions (colored areas). These layer-resolved absorptions add up to the total absorption (black line). (e) and (f) show numerical results for the enhancement in absorption of the total structure and P3HT:PCBM with respect to a flat reference, respectively.

Figure 4

Computations of the periodicity- and polarization-dependent absorption. Note the different numbers of resonances and the characteristic dependence of the different absorption peaks for different periodicities.

Figure 5

Spatial dependence of the power loss density ${\overline{p}}_{\text{loss}}(\mathbf{r},\omega )$ and time averaged Poynting vector (white arrows) at frequencies corresponding to the absorption peaks at $\lambda >700$ nm from the simulations. (f) sketches the geometry, for details we refer to Ref. [31]. (a)–(e) only show the portion of the structure highlighted in blue in (f). The active layer of the structure is P3HT-PCBM. Note that ${\overline{p}}_{\text{loss}}(\mathbf{r},\omega )$ in the photoactive P3HT-PCBM layer critically depends on the employed absorption coefficient.