Markus Koppe

For bulk-heterojunction photovoltaic cells fabricated from conjugated polymers and a fullerene derivative, the relation between the open-circuit voltage (Voc) and the oxidation potential for different conjugated polymers is studied. A linear relation between Voc and the oxidation potential is found (see figure). Based on this relation, the energy-conversion efficiency of a bulk-heterojunction solar cell is derived as a function of the bandgap and the energy levels of the conjugated polymer.

Abstract The spectroscopic response of a poly(3‐hexylthiophene)/[6,6]‐phenyl‐C 61 ‐butyric acid methyl ester (P3HT/PCBM)‐based bulk heterojunction solar cell is extended into the near infrared region (NIR) of the spectrum by adding the low bandgap polymer poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4 H ‐cyclopenta[2,1‐ b ;3,4‐ b ´]‐dithiophene)‐ alt ‐4,7‐(2,1,3‐benzothiadiazole)] [PCPDTBT] to the blend. The dominant mechanism behind the enhanced photosensitivity of the ternary blend is found to be a two‐step process: first, an ultrafast and efficient photoinduced charge transfer generates positive charges on P3HT and PCPDTBT and a negative charge on PCBM. In a second step, the positive charge on PCPDTBT is transferred to P3HT. Thus, P3HT serves two purposes. On the one hand it is involved in the generation of charge carriers by the photoinduced electron transfer to PCBM, and, on the other hand, it forms the charge transport matrix for the positive carriers transferred from PCPDTBT. Other mechanisms, such as energy transfer or photoinduced charge transfer directly between the two polymers, are found to be absent or negligible.

Replacing a carbon atom with silicon (see figure) on the main chain of a conjugated polythiophene gives a polysilole with higher crystallinity, improved charge transport, reduced bimolecular recombination, and reduced formation of charge transfer complexes when blended with a fullerene derivative. Optimized bulk heterojunction solar cells using this blend give certified efficiencies of 5.2% under AM1.5 illumination.

Polymer/fullerene bulk heterojunctions have recently generated a lot of scientific interest due to their potential in low-cost photovoltaic applications. In this paper we detail the synthesis and characterization of two new low-band-gap polythiophenes, poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-4,7-bis(2-thienyl)-2,1,3-benzothiadiazole-5′,5′′-diyl] (PCPDTTBTT) and poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,3-dioctylquinoxaline-5,8-diyl] (PCPDTQ), for use in these applications. The PCPDTQ polymer did not produce efficient solar cells. A high power efficiency of 2.1% under one sun was found for a PCPDTTBTT/fullerene mixture. The high efficiency was achieved by alteration of the morphology using a solvent additive. Analysis of atomic force microscopy phase images shows that material phases with distinct mixing ratios are formed and altered with the addition of the solvent additive.