Organic solar cell (OSC) has been an active research field over the past decades due to the intrinsic advantages, Small molecule vacuum deposition is regarded as a promising fabrication method for the realization of multi-junction tandem solar cells (TSCs). One of the challenging topics is the development of novel photoactive electron donor materials that absorb in near infrared (NIR) region (λ > 780 nm). This kind of molecules is extremely rare compared to those whose absorption bands are in the visible light region (400<λ<780 nm), but they can make significant contribution in TSCs by completing the absorption. Taking advantage of the flexible chemical modification and tunable photophysical properties of boron dipyrromethene (BODIPY), we have been investigating high performance OSCs based on NIR BODIPY derivatives as electron donors.
Three aza-BODIPYs are synthesized via a modified route using phthalonitrile and organolithium reagents. Heterocyclic aromatic substituents (N-methylpyrrole, N-methylindole and 2-trimethylsilyl thiophene) with different electronic properties are introduced to tune the absorption positions. Moreover, the synthetic methods of aza-BODIPY derivatives with BF(CN) or B(CN)2 moiety are demonstrated, and three aza-BODIPY derivatives with BF(CN) moiety are prepared. The absorption bands of all the dyes are broadened and red-shifted from solution to solid state, and the maxima are all over 830 nm in thin film.
These aza-BODIPYs are used as electron donors in vacuum-processed bulk heterojunction (BHJ) OSCs with C60 as the acceptor. The optimal device presents a short-circuit current (jsc) of 9.0 mA cm-2, a moderate open-circuit voltage (Voc) of 0.61 V and a fill factor (FF) of 53%, giving a PCE of 3.0%. It is noteworthy that its external quantum efficiency (EQE) spectra covers from 650 to 950 nm, peaking around 850 nm, and the PCE value is reasonable for such long wavelength absorbing donor material.
A BODIPY with intense and long wavelength absorption can be achieved by an extension of the π-system and the introduction of an electron withdrawing group on the meso-C. Furan-fused BODIPYs with a CF3 group on the meso-C are synthesized, and their planar molecular structures are demonstrated by the single crystal X-ray diffraction measurements. Their intense sharp absorption bands in solution are greatly broadened in solid state, covering a wide region from 500 to 950 nm.
The best device gives a PCE of 6.1% with a high jsc of 13.3 mA cm-2. Moreover, its EQE spectrum spans a wide region from 550 to 900 nm, making it a suitable NIR donor for a TSC in cooperation with a “green” absorber. The serial TSC has complementary absorption peaking at 550 and 800 nm. Due to the matching photocurrent generated by the both sub-cells, a jsc of 9.9 mA cm-2 is achieved. With a high Voc of 1.70 V and a reasonable FF of 59%, a high PCE of 10% is obtained, which is an excellent value for vacuum-deposited TSCs.