\section{Results}
\begin{figure}[h]
\centering
  \includegraphics[width=\columnwidth]{Fig2.pdf}
  \caption{Donor-polymers P3HT and MEH-PPV. Employed acceptors were C$_{\textup{70}}$ fullerene derivates and PC$_{\textup{60}}$BM  as reference.}
  \label{fgr:struc}
\end{figure}

P3HT (Rieke Metals), MEH-PPV (Sigma-Aldrich), PC$_{\textup{60}}$BM and PC$_{\textup{70}}$BM (Solenne B.V.) were used as purchased. Details about the synthesis of the fullerene dimers and C$_{\textup{70}}$-DPM-OE are reported elsewhere \cite{delgado2009}.
Samples were processed from chlorobenzene solutions (10~mg/ml) with a 1:1 weight ratio for blends. Thin films of $\sim$100~nm were spincast onto saphire substrates for use in PIA, PL and absorption. For ESR 200~$\mu l$ solution were vacuum-dried inside an ESR sample tube. All samples were annealed afterwards for 10~min at 130~$^{\circ}$ C.

Figure~\ref{fgr:absPL} shows that PC$_{\textup{70}}$BM has an increased absorption in the range of 400--700~nm compared to PC$_{\textup{60}}$BM, which is favorable in view of the AM~1.5 solar flux. Absorption was measured with a Perkin-Elmer Lambda 900 spectrometer under ambient conditions. At low temperatures (12~K) these samples emitted a weak, yet detectable PL when excited by a 532~nm laser. Intensities of the PL spectra were corrected for the spectral response of the spectrometer. The spectra of both fullerene derivatives showed well resolved vibronic structures and the PL of PC$_{\textup{70}}$BM was blue-shifted by $\sim$40~meV (17~nm) matching literature values~\cite{sibley1992}.



In figure~\ref{fgr:ESR} (left) the light-induced ESR spectra (normalized) of P3HT blended with the different fullerene derivatives are shown. Illumination was provided by a 532~nm laser and the samples were cooled to 100~K. All spectra inhibit the typical derivative-like absorption line at g=2.002, originating from positive polarons on the polymer chain. In P3HT:PC$_{\textup{60}}$BM blends, the radical anion of C$_{\textup{60}}$ can be detected at g=2.000. For blends consisting of P3HT and C$_{\textup{70}}$-derivatives no such signal can be identified directly, as the C$_{\textup{70}}$ radical anion line is strongly superimposed with the polymer polaron signal. Instead all C$_{\textup{70}}$-containing blends disclose a shoulder at g$\geq$2.005, being absent in the spectrum of P3HT:PC$_{\textup{60}}$BM. In figure~\ref{fgr:ESR} (right), the deconvolution of the experimental spectra into their contributions of the positive polymer polaron, the C$_{\textup{70}}$ (top) and  C$_{\textup{60}}$ (bottom) radical anion are shown. The deconvolution method has been described elsewhere \cite{poluektov2010}. We were able to simulate the spectra of the fullerene-anions (using Simfonia) with the following g-tensor components $(g_{xx} / g_{yy} / g_{zz})$: C$_{60}^{-}= (2.00058 / 2.00045 / 1.99845)$ and C$_{70}^{-}= (2.00592 / 2.00277 / 2.00211)$. A closer look at the spectrum of P3HT:C$_{\textup{60}}$--C$_{\textup{70}}$-dimer (e) revealed, that contributions of both fullerene anions are present, hinting that the charge transfer process is not dominated by one fullerene.

\begin{figure}[h]
  \centering
  \includegraphics[width=\columnwidth]{Fig3.pdf}
  \caption{~\textbf{Left side:} ESR spectra at 100~K of P3HT blended with different fullerenes: C$_{\textup{70}}$--C$_{\textup{70}}$-Dimer (a), C$_{\textup{70}}$-DPM-OE2 (b), C$_{\textup{70}}$-DPM-OE (c), PC$_{\textup{70}}$BM (d), C$_{\textup{60}}$--C$_{\textup{70}}$-Dimer (e), PC$_{\textup{60}}$BM (f). All C$_{\textup{70}}$-composites show a shoulder for g$\geq$2.005 (indicated by the red rectangle) which is assigned to the C$_{\textup{70}}$ anion. \textbf{Right side:} Deconvolution of the experimental spectra (black), the positive polaron P3HT$^{+}$ (dashed red), the fullerene anions (blue) and their simulated spectra (green) for composites (d) and (f).}
  \label{fgr:ESR}
\end{figure}
 

\begin{figure}[h]
  \centering
  \includegraphics[width=\columnwidth]{Fig4.pdf}
  \caption{PIA spectra at 15--30~K. While the structure of the spectra is similar, the peak at 0.92~eV shows up only in blends with C$_{\textup{70}}$-derivates.  \textbf{Inset}: Subtraction (red) of the normalized spectra of P3HT:PC$_{\textup{60}}$BM from P3HT:PC$_{\textup{70}}$BM together with a lorentzian fit of the lineshape (dashed black).}
  \label{fgr:PIA}
\end{figure}

Further, we studied the photophysical properties by photoinduced absorption (PIA). The excitation by a 532~nm laser leads to singlet excitons, which are separated at the polymer:fullerene interface yielding positive and negative charge carriers in the polymer and the fullerene phase, respectively. The resulting change in transmission of a white light continuum probe beam was measured ($-\Delta T / T$). The transmitted light was focussed into a monochromator and the detection was provided by a silicon photodiode (550--1030~nm) and a liquid nitrogen cooled InSb-detector (1030--5500~nm). In most polymer:fullerene blends, transitions ascribed to polaronic states (radical cations) are located at 0.3~eV and 1.2~eV. For P3HT or PPV blended with PC$_{\textup{60}}$BM these spectral signatures are well documented \cite{osterbacka2000,montanari2002}. 

In the PIA spectra of all polymer:C$_{\textup{70}}$ blends, similar polaronic absorption bands could be detected, indicating efficient charge separation. Additionally we found an absorption at 0.92~eV (figure~\ref{fgr:PIA}), which is absent in the reference P3HT:PC$_{\textup{60}}$BM spectrum and therefore related to the C$_{\textup{70}}$. As the examined fullerene derivatives have different side chains, we can exclude electrons being localized on here. 
Charge transfer states as possible candidates for a new subgap absorption could be ruled out by replacing the host polymer P3HT with MEH-PPV, which has different HOMO-LUMO energies \cite{holt2005,oku2008}.
Furthermore the relation of the 0.92~eV peak to C$_{\textup{70}}$-derivatives is also consistent with the different temperature dependence of the PIA peaks in the blend of MEH-PPV:PC$_{\textup{70}}$BM (data not shown). The polaron peaks at 0.3~eV and 1.2~eV show both an identical decrease in signal intensity when increasing the temperature from 30~K to 300~K, while the 0.92~eV peak decreases steeper and vanishes at 300~K. 
We were able to separate the contribution of the underlying peak by subtracting the normalized spectrum of P3HT:PC$_{\textup{60}}$BM from P3HT:PC$_{\textup{70}}$BM . This is shown together with a Lorentzian fit of the spectrum in the inset in figure~\ref{fgr:PIA} (E$_{0}=$~0.92~eV / 1350~nm and FWHM~$=$~0.11~eV).

To summarize, we have studied a variety of solid films of C$_{\textup{70}}$-fullerene derivates and dimers blended with two different conjugated polymers, with the emphasis on the photogenerated radical anion. We identified spectral features originating from C$_{\textup{70}}$ in PIA and ESR spectroscopy, namely an additional subgap PIA peak at 0.92~eV and an ESR-shoulder at g$\geq$2.005. By comparing results from different blends we can infer that these features are indeed signatures of the C$_{\textup{70}}$ radical anion and not originating from the side chains of the fullerenes or charge transfer states at polymer:C$_{\textup{70}}$-fullerene interfaces. As anticipated, ESR signatures related to the C$_{\textup{70}}$ radical anion were found and the corresponding spectra could be simulated with the g-values from our previous findings \cite{poluektov2010}. The obtained signatures are of importance for studying the charge transfer reactions in C$_{\textup{70}}$-based bulk-heterojunctions.



\begin{acknowledgments}
The work at the University of W\"urzburg was supported by the German Research Foundation, DFG, within the SPP 1355 ``Elementary processes in organic photovoltaics'' (DY18/6-1 and -2). The MICINN of Spain (project CT2008-00795/BQU, R\&C program, and Consolider-Ingenio 2010C-07-25200) and the CAM (project P-PPQ-000225-0505) are also acknowledged. J. L. D. thanks MICINN of Spain for a Ramon y Cajal contract cofinanced by the EU social funds.
\end{acknowledgments}

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