Correlation of open-circuit voltage and energy levels in zinc-phthalocyanine: C${}_{60}$ bulk heterojunction solar cells with varied mixing ratio

The maximum open-circuit voltage $V_{\mathrm{OC}}$ of bulk-heterojunction solar cells is limited by the effective HOMO(donor)-LUMO(acceptor) gap of the photoactive absorber blend. We investigate blend layers comprising zinc-phthalocyanine (ZnPc) and the buckminster fullerene ${\mathrm{C}}_{60}$ with ultraviolet, x-ray, and inverse photoelectron spectroscopy. By varying the volume mixing ratio ZnPc:${\mathrm{C}}_{60}$ from 6:1 to 1:6, we observe a linear increase of the HOMO(ZnPc)-LUMO(${\mathrm{C}}_{60}$) gap by 0.25 eV. The trend in this gap correlates with the change in the charge transfer energy measured by Fourier-transform photocurrent spectroscopy as well as with the observed open-circuit voltage of solar cells containing ZnPc:${\mathrm{C}}_{60}$ as the photoactive absorber layer. Furthermore, the morphology of different ZnPc:${\mathrm{C}}_{60}$ blend layers is investigated by grazing-incidence x-ray diffraction. As physical origins for the changed energy levels, a suppressed crystallization of the ${\mathrm{C}}_{60}$ phase in the presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are suggested.

Figure 1

Open-circuit voltage $V_{\mathrm{OC}}$ of pii-BHJ solar cells as a function of the ZnPc:${\mathrm{C}}_{60}$ mixing ratio. The device structure is shown in the inset of the graph and the chemical structures of the employed molecules above.

Figure 2

High binding energy cutoff (HBEC) and highest occupied molecular orbital (HOMO) region of the UPS spectra of 15 nm ZnPc:${\mathrm{C}}_{60}$ (1:6, 1:1, and 6:1 volume mixing ratios) layers on gold. For determining the HOMO onsets of both materials within the blend layer, the mixed layer spectrum is fitted by subspectra (dashed-dotted: ${\mathrm{C}}_{60}$, dashed: ZnPc) originally obtained from separate intrinsic reference samples. For the different mixing ratios the ${\mathrm{C}}_{60}$ and ZnPc subspectra are adapted in intensity. The obtained HOMO onsets are indicated by circles. The dotted gray lines represent assumed Touggard-type backgrounds.

Figure 3

(a) ZnPc and ${\mathrm{C}}_{60}$ HOMO onset values within mixed thin films determined by fitting UPS spectra (Fig. 2) as functions of the mixing concentration. (b) Corresponding HOMO(${\mathrm{C}}_{60}$)-HOMO(ZnPc) difference in comparison to $V_{\mathrm{OC}}$ of respective BHJ solar cells. (c) Work function and (d) resulting individual IEs of ZnPc and ${\mathrm{C}}_{60}$ determined by the corresponding HOMO onsets.

Figure 4

Absorption spectra of 30 nm ZnPc:${\mathrm{C}}_{60}$ blends with different mixing ratios evaporated on quartz glass.

Figure 5

IPES and UPS spectra of pure ZnPc and ${\mathrm{C}}_{60}$ films as well as of 1:1.4 and 2.2:1 blends on gold. Mixed-layer spectra are fitted by subspectra (dashed-dotted: ${\mathrm{C}}_{60}$, dashed: ZnPc) originally obtained from the intrinsic reference samples. The HOMO (LUMO) onsets of both materials are indicated by colored circles (diamonds). The dotted gray lines represent assumed Touggard-type backgrounds.

Figure 6

Open-circuit voltage $V_{\mathrm{OC}}$ and charge-transfer energy $E_{\mathrm{CT}}$ of ZnPc:${\mathrm{C}}_{60}$ pii-BHJ solar cells (inset) plotted versus the ${\mathrm{C}}_{60}$ concentration (mixing ratio varied from 5:1 to 1:5). The $E_{\mathrm{CT}}$ values are obtained by fitting the corresponding FTPS spectra (cf. Fig. 7).

Figure 7

FTPS spectra of ZnPc:${\mathrm{C}}_{60}$ pii-BHJ solar cells varying the mixing ratio from 1:5 to 5:1 (solid lines; device structure shown in the inset of Fig. 6). The red lines indicate the fit curves obtained by fitting the FTPS spectra with Eq. (2). The dashed line refers to a similar pii-FHJ solar cell containing only 50 nm ZnPc as photoactive layer.

Figure 8

Grazing-incidence x-ray diffraction (GIXRD) spectra of ZnPc:${\mathrm{C}}_{60}$ blends with varied mixing ratio thermally evaporated on glass at room temperature. The dotted lines indicate pure ZnPc (gray) or ${\mathrm{C}}_{60}$ (black) diffraction patterns.

Figure 9

Level alignment of incrementally evaporated ZnPc:${\mathrm{C}}_{60}$ blends (1:6, 0.6/1.5/3/10 nm) on either (a) Au/MeO-TPD:F6-TCNNQ (10 wt.%, 5 nm)/ZnPc (5 nm) or (b) Au/ZnPc (8 nm).