Miguel Carrasco-Orozco

Towards 15% energy conversion efficiency: a systematic study of the solution-processed organic tandem solar cells based on commercially available materials

Owing to the lack of scalable high performance donor materials, studies on mass-produced organic photovoltaic (OPV) devices lag far behind that on lab-scale devices. In this work, we choose 6 already commercially available conjugated polymers and systematically investigate their potential in organic tandem solar cells. All the devices are processed under environmental conditions using doctor-blading, which is highly compatible with mass-production coating technologies. Power conversion efficiencies (PCE) of 6–7% are obtained for OPV devices based on different active layers. Optical simulations based on experimental data are performed for all realized tandem solar cells. An efficiency potential of ∼10% is estimated for these compounds in combination with phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor. In addition, we assume a hypothetical, optimized acceptor to understand the limitation of donors. It is suggested that a PCE of >14% is realistic for tandem solar cells based on these commercially available donor materials. Along with the demonstration of novel intermediate layers we believe that this systematic study provides valuable insight for those attempting to realize the high efficiency potential of tandem architectures.

Liquid‐Crystalline Perylene Diester Polymers with Tunable Charge‐Carrier Mobility

New classes of liquid-crystalline semiconductor polymers based on perylene diester benzimidazole and perylene diester imide mesogens are reported. Two highly soluble side-chain polymers, poly(perylene diester benzimidazole acrylate) (PPDB) and poly(perylene diester imide acrylate) (PPDI) are synthesized by nitroxide-mediated radical polymerization (NMRP). PPDB shows n-type semiconductor performance with electron mobilities of 3.2 × 10−4 cm2 V−1 s−1 obtained in a diode configuration by fitting the space-charge-limited currents (SCLC) according to the Mott–Gurney equation. Interestingly, PPDI performs preferentially as a p-type material with a hole mobility of 1.5 × 10−4 cm2 V−1 s−1, which is attributed to the less electron-deficient perylene core of PPDI compared to PPDB. Optical properties are investigated by UV-vis and fluorescence spectroscopy. The extended π-conjugation system due to the benzimidazole unit of PPDB leads to a considerably broader absorption in the visible region compared to PPDI. HOMO and LUMO levels of the polymers are also determined by cyclic voltammetry; the resulting energy band-gaps are 1.86 eV for PPDB and 2.16 eV for PPDI. Thermal behavior and liquid crystallinity are studied by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction measurements. The results indicate liquid-crystalline order of the polymers over a broad temperature range. These thermal, electrical, and optical properties make the perylene side-chain polymers attractive materials for organic photovoltaics.