P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

Each year we are bombarded with B.Sc. and Ph.D. applications from students that want to improve the world. They have learned that their future depends on changing the type of fuel we use and that solar energy is our future. The hope and energy of these young people will transform future energy technologies, but it will not happen quickly. Organic photovoltaic devices are easy to sketch, but the materials, processing steps, and ways of measuring the properties of the materials are very complicated. It is not trivial to make a systematic measurement that will change the way other research groups think or practice. In approaching this chapter, we thought about what a new researcher would need to know about organic photovoltaic devices and materials in order to have a good start in the subject. Then, we simplified that to focus on what a new researcher would need to know about poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester blends (P3HT: PCBM) to make research progress with these materials. This chapter is by no means authoritative or a compendium of all things on P3HT:PCBM. We have selected to explain how the sample fabrication techniques lead to control of morphology and structural features and how these morphological features have specific optical and electronic consequences for organic photovoltaic device applications

Novel polymer electrolytes based on amorphous poly(ether-ester)s containing 1,4,7-trioxanonyl main chain units. Ionic conductivity versus polymer chain mobility

Melt condensation of 1,5-bis(9-hydroxy-1,4,7-trioxanonyl)naphthalene (2) with bis-acid chlorides, adipoyl chloride (3a), terephthaloyl chloride (3b), and 3,6,9,12-tetraoxatetradecane bis-acid chloride (3c), respectively, gives amorphous linear poly(ether-ester)s 1a-c, which contain 1,4,7-trioxanonyl (triethylene glycol) units at regular intervals in their main chain. Solid polymer electrolytes were prepared by mixing THF solutions of either LiClO4 with la-e or NaClO4 with 1b. The polymer electrolytes containing LiClO4 are fully amorphous, whereas in the case of NaClO4 and Na+/1b ratios larger than 0.125, crystalline NaClO4 is present. Despite the fact that the 1,4,7-trioxanonyl moieties in 1a-c are shorter than the minimum required for complete solvation of Li+ and Na+, dielectric relaxation spectroscopy shows that the solid polymer electrolytes Li+/1a, Li+/1b, and Li+/1c possess ionic conductivities of sigma = 3.2 x 10(-5), 1.9 x 10(-6), and even 1 x 10(-4) S cm(-1), respectively, at 368 K. A Vogel-Tammann-Fulcher (VTF) analysis of the ionic conductivity sigma and the relaxation time of the alpha-relaxation revealed a strong relationship between sigma and the relaxation behavior of the chain segments. By means of a fine structure analysis of the activation energy, the dielectric alpha-process around the glass transition was closely studied in the absence and presence of dissolved LiClO4 (1a-c) or NaClO4 (1b). From the highest apparent activation energy the T-g was determined and found to agree very well with values from DSC. In addition, the fractional free volume at T-g was quantified. It increases with increasing amount of dissolved salt; this becomes in particular clear from the fine structure analysis. Dielectric spectroscopy at T < T-g showed the presence of three secondary relaxations (gamma, beta(1), beta(2)), of which beta(1) and beta(2) strongly overlap. Two of them are assigned to local relaxations involving either free (gamma) or coordinated (beta(2)) EO sequences, resulting in a decrease or increase of the relaxation strength with salt concentration, respectively. Molecular modeling supports the idea that the beta(2) process arises from a chemical relaxation by the temporary breaking up and remaking of at least one O-Li+ coordination bond within the tetrahedral polymer-cation complex. The third (beta(1)) relaxation is in particular active in weakly complexed samples exposed to ambient humidity, suggesting a local motion involving the ester moieties.status: publishe

Effect of Polymer Crystallinity in P3HT:PCBM Solar Cells on Band Gap Trap States and Apparent Recombination Order

The non-geminate recombination of charge carriers in polymer-fullerene solar cells has been modeled in the last few years with a trap-assisted recombination model, which states that the apparent recombination order depends on the concentration of trapped charges tailing into the band gap. Higher concentrations of trapped charges lead to higher apparent recombination orders. In this work, the mass fraction f of highly crystalline nanofibrillar P3HT to the total P3HT content in P3HT:PCBM solar cells is consistently varied, controlling the temperature of a nanofibers-P3HT casting dispersion. A systematic study of the apparent recombination order, measured with a transient photovoltage technique, as a function of f is presented. A correlation is shown between the apparent recombination order, the P3HT crystallinity, and the trap concentration in the band gap measured with an admittance spectroscopy technique