Fluorine-containing low-energy-gap organic dyes with low voltage losses for organic solar cells

- Isomer selective synthesis was demonstrated.
- New chemical purification method can be used for almost all the D-A dyes due to low solubility.
- The new donor materials do not show significant decomposition below 300 °C under argon.
- Optical band gaps between 1.57 and 1.62 eV were determined from the onset of the thin film absorption spectra.
- The non-radiative voltage loss of the PRTF device is only 0.26 V, which is much lower than typically observed values for organic solar cells (>0.34 V).

Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum‍-deposited organic solar cells. These molecules comprise a donor‍-‍acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-‍donating and electron-‍withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-‍vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8%. Moreover, the voltage loss for the PRTF device due to the non-‍radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (>0.34 V).

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