Synthesis of Diketopyrrolopyrrole Containing Copolymers: A Study of Their Optical and Photovoltaic Properties

The diketopyrrolopyrrole-based copolymers PDPP-BBT and TDPP-BBT were synthesized and used as a donor for bulk heterojunction photovoltaic devices. The photophysical properties of these polymers showed absorption in the range 500−600 nm with a maximum peak around 563 nm, while TDPP-BBT showed broadband absorption in the range 620 − 800 nm with a peak around 656 nm. The power conversion efficiencies (PCE) of the polymer solar cells based on these copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were 0.68% (as cast PDPP-BBT:PCBM), 1.51% (annealed PDPP-BBT:PCBM), 1.57% (as cast TDPP-BBT:PCBM), and 2.78% (annealed TDPP-BBT:PCBM), under illumination of AM 1.5 (100 mW/cm2). The higher PCE for TDPP-BBT-based polymer solar cells has been attributed to the low band gap of this copolymer as compared to PDPP-BBT, which increases the numbers of photogenerated excitons and corresponding photocurrent of the device. These results indicate that PDPP-BBT and TDPP-BBT act as excellent electron donors for bulk heterojunction devices

Novel <i>p</i>-Phenylenevinylene Compounds Containing Thiophene or Anthracene Moieties and Cyano−Vinylene Bonds for Photovoltaic Applications

Two novel soluble compounds T and A that contain a central dihexyloxy-p-phenylenevinylene unit, intermediate moieties of thiophene or anthracene, respectively, and terminal cyano−vinylene nitrophenyls were synthesized and characterized. They showed moderate thermal stability and relatively low glass transition temperatures. These compounds displayed similar optical properties. Their absorption was broad and extended up to about 750 nm with the longer-wavelength maximum around 640 nm and an optical band gap of ∼1.70 eV. From the current−voltage characteristics of the devices using both compounds T and A, it was concluded that both compounds behave as p-type organic semiconductors with hole mobility on the order of 10−5 cm2/(V s). The power conversion efficiency (PCE) of the devices based on these compounds was 0.019% and 0.013% for compounds A and T, respectively. When compounds A and T were blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the PCE dramatically increased up to 1.66% and 1.36% for devices with A:PCBM and T:PCBM, respectively. The efficiencies of the devices were further enhanced upon thermal annealing up to 2.49% and 2.33% for devices based on A:PCBM and T:PCBM, respectively

Effect of the Incorporation of a Low-Band-Gap Small Molecule in a Conjugated Vinylene Copolymer: PCBM Blend for Organic Photovoltaic Devices

The effect of the incorporation of a low-band-gap small-molecule BTD-TNP on the photovoltaic properties of vinylene copolymer P:PCBM bulk heterojunction solar cells has been investigated. The introduction of this small molecule increases both the short-circuit photocurrent and the overall power conversion efficiency of the photovoltaic device. The incident photon-to-current efficiency (IPCE) of the device based on P:PCBM:BTD-TNP shows two distinct bands, which correspond to the absorption bands of P:PCBM and BTD-TNP. Furthermore, it was found that the IPCE of the device has also been enhanced even at the wavelengths corresponding to the absorption band of P:PCBM, when the thermally annealed blend was used in the device. This indicates that the excitons that are generated in copolymer P are dissociated into charge carriers more effectively in the presence of the BTD-TNP small molecule at the copolymer P:PCBM interface by energy transfer from P to the small molecule. Therefore, we conclude that the BTD-TNP small molecule acts as light-harvesting photosensitizer and also provides a path for the generated exciton in copolymer P toward the P:PCBM interface for efficient charge separation. The overall power conversion efficiency for the P:PCBM:BTD-TNP photovoltaic device is about 1.27%, which has been further enhanced up to 2.6%, when a thermally annealed blend layer is used