Near Infrared Boron Dipyrromethene as Donor Materials for Vacuum-processed Organic Solar Cells

Organic solar cell (OSC) has been an active research field over the past decades, due to their intrinsic advantages, such as low consumption of materials and energy, the applicability on flexible substrates and the degradability of the organic components. Compared with the solution processing technology using polymers as electron donor materials, small molecule vacuum deposition is regarded as a promising fabrication method, avoiding the use of toxic aromatic solvents and guaranteeing constant batch-to-batch performance. Moreover, it is much easier to realize multi-junction tandem solar cells (TSCs) by thermal deposition, and the leading power conversion efficiency (PCE) of 13.2% was achieved using three different absorbers by vacuum deposition (“Heliatek sets new organic photovoltaic world record efficiency of 13.2%” 2016). In this dissertation, novel electron donor materials are synthesized based on the molecular skeleton of a famous chromophore, boron dipyrromethene (BODIPY), and chemical modifications are carried out to tune the intense absorption bands of these dyes to near infrared (NIR, λ>750,nm) region. Efficient small molecule NIR absorbers are highly required for TSCs, because they can construct a complementary absorption over the visible and NIR spectral region in cooperation with a wide bandgap material. Three β-fused aza-BODIPY molecules with heterocyclic substituents on α-positions are prepared using organolithium reagents and phthalonitrile as the starting materials. The organolithium reagents, namely N-methylpyrrole, N-methylindole and 2-trimethylsilylthiophene, are used instead of commonly used Grignard reagents. Moreover, three corresponding aza-BODIPY derivatives are obtained by replacing one fluorine atom in the BF_{2} moiety by a cyano group. UV-vis absorption spectra reveal that all these materials are strong NIR absorbers, and their abortion in solid state cover a wide range from 600 to 1000,nm. OSCs with these aza-BODIPY donors give a best PCE of 3.0%, which is a reasonable value for the NIR devices with the maximum and the onset of the EQE spectrum around 850 and 950,nm respectively. A series of furan-fused BODIPYs with a electron withdrawing CF_{3} group on the meso-C are synthesized, and the photophysical/electrochemical properties can be tuned easily by the electronic properties of the substituents on the peripheral aromatic rings. The most promising candidate gives a high PCE of 6.1% in a single junction OSC with a J_{sc} of 13.3,mA/cm^{2}, a V_{oc} of 0.73,V, and a FF of 62.7%. A serial connected TSC is fabricated using this BODIPY as the low bandgap donor and a “green” donor, and its EQE spectrum covers a wide range from 400 to 900,nm. The PCE reaches 9.9% with a J_{sc} of 9.9,mA/cm^{2}, a V_{oc} of 1.70,V, and a FF of 59.0%. Based on the general structure of furan-fused BODIPY, alkyl or fluorinated alkyl substituents with larger volume is introduced on either peripheral aromatic rings or the meso-C. The variations that caused by these substituents on the photophysical and electrochemical properties are negligible. The investigations on the OSCs demonstrate that the introduction of these alkyl chain substituents have positive influence on the PCE values, which benefit mainly from the increased photocurrent. However, there is no positive relationship between the device performance and the volume of the alkyl chain substituents. BODIPY molecules have been demonstrated as efficient and promising NIR electron donor materials for vacuum-deposited OSCs. Taking advantages of facile molecular modification, oustanding photophysical behaviors and tunable electrochemical properties, this series of dyes are also intereting for other semiconductor devices