Photovoltaic properties of bulk heterojunction solar cells with improved spectral coverage

Broadening the absorption of conjugated polymers by "click" functionalization with phthalocyanines

Conjugated copolymer derivatives of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT) containing 10% of alkyne functionalities in the side chains have been prepared using the sulfinyl precursor route and the Rieke method, respectively. With the aim of expanding the absorption range of these conjugated polymers for their use in bulk heterojunction (BHJ) polymer:fullerene solar cells, appropriate phthalocyanine (Pc) molecules have been covalently bound through a post-polymerization "click chemistry" reaction between the alkyne functionalities in the side chains of the copolymers and a Pc functionalized with an azide moiety. The resulting poly(p-phenylenevinylene)-Pc (PPV-Pc) material holds a 9 mol% content of Pcs, while the polythiophene-Pc material (PT-Pc) contains a 8 mol% of Pc-functionalization in the side chains. As expected, the presence of the Pc contributes to the extension of the absorption up to 700 nm. BHJ solar cells have been prepared using PPV-Pc and PT-Pc materials in combination with PCBM. Although the Pc absorption contributes to the generation of photocurrent, the overall power conversion efficiencies (PCE) obtained from these cells are lower than those obtained with BHJ P3HT:PCBM (1:1) and MDMO-PPV:PCBM (1:4) solar cells. A plausible explanation could be the moderate solubility of the PPV-Pc and PT-Pc materials that limits the processing into thin films.

Molecular aggregation state and photovoltaic properties of chlorophyll-doped conducting poly(3-hexylthiophene)/MCM-41 nanocomposites

Chlorophyll (Chl) was immobilized into a 1,4-butanediol-modified MCM-41 (BMCM-41) intercalated by poly(3-hexylthiophene) (P3HT) to form BMCM-41/P3HT/Chl nanocomposites having P3HT contents of 10, 30, 60, and 90 wt % from a solution-casting method. Wide-angle X-ray diffraction and transmission electron microscopy studies indicate that the pore structure of MCM-41 was retained after surface modification and a subsequent P3HT intercalation process. Scanning electron microscopy images showed that the BMCM-41 nanoparticles dispersed into the polymer matrix of BMCM-41/P3HT/Chl, and the sample with 10 wt % P3HT content gives the most homogeneous nanoparticle dispersion. Nitrogen adsorption-desorption results confirmed that the P3HT intercalation and Chl immobilization inside the BMCM-41 mesopore were successfully carried out. The pore volume and surface area of BMCM-41 decreased significantly when the amount of P3HT was increased from 10 to 90 wt %. The UV-vis study showed a blue shift of the pi-pi* transition band of P3HT in the spectra of BMCM-41/P3HT/Chl nanocomposites. The FT-IR study indicates an increase of the thiophene ring stretching and a decrease of the C horizontal lineO stretching when P3HT and Chl were inside the mesopore. The photovoltaic property of Chl-doped P3HT was improved significantly upon the addition of BMCM-41 nanoparticles, and BMCM-41/P3HT-10/Chl exhibits the highest incident photon-to-current conversion efficiency of 7.16%.

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.

Improvement of the light-harvesting efficiency in polymer/fullerene bulk heterojunction solar cells by interfacial dye modification

Enhancement of the light-harvesting efficiency in poly(3-hexylthiophene)/fullerene derivative (P3HT/PCBM) bulk heterojunction solar cells has been demonstrated by the introduction of near-infrared phthalocyanine molecules as the third component at the P3HT/PCBM interface. The introduction of silicon phthalocyanine derivative (SiPc) increased the short-circuit current density and hence improved the overall power conversion efficiency by 20%, compared to the P3HT/PCBM control device. For P3HT/PCBM/SiPc devices, two distinct external quantum efficiency (EQE) peaks were observed at wavelengths for the absorption bands of SiPc as well as P3HT before and after thermal annealing, suggesting that SiPc molecules are located at the P3HT/PCBM interface because of crystallization of the P3HT and PCBM domains. Furthermore, the EQE for the device increased even at wavelengths for the absorption band of P3HT by the introduction of SiPc molecules. This indicates that P3HT excitons can be dissociated into charge carriers more efficiently in the presence of SiPc molecules at the P3HT/PCBM interface by energy transfer from P3HT to SiPc molecules. These findings suggest that there are two origins for the increase in the photocurrent by the introduction of SiPc; SiPc molecules serve not only as a light-harvesting photosensitizer but also as an energy funnel for P3HT excitons at the P3HT/PCBM interface.