M. M. Mandoc

Effect of traps on the performance of bulk heterojunction organic solar cells

The effect of electron traps on the performance of bulk heterojunction organic solar cells made of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is investigated. By deliberately introducing 7,7,8,8-tetracyanoquinodimethane, which is a stronger electron accepting molecule than PCBM, the electron transport in the solar cells becomes trap limited. We demonstrate that the open circuit voltage and its light-intensity dependence are strongly affected by recombination of holes with trapped electrons. Depending on the amount of traps, their recombination strength, and the light intensity, the losses due to trap-assisted recombination can even dominate over the intrinsic bimolecular recombination.

Optimum charge carrier mobility in organic solar cells

In organic semiconductors the recombination mechanism is of the Langevin type, controlled by the mobility of the charge carriers. As a result, in organic solar cells the mobility simultaneously controls both the carrier extraction and the losses via carrier recombination. The authors demonstrate that the balance between carrier losses by extraction and by recombination leads to a distinct optimum in the carrier mobility with regard to the efficiency of organic solar cells. For low mobilities recombination losses limit the performance, whereas the efficient extraction at high mobilities leads to a reduction of the open-circuit voltage.

Origin of the efficiency improvement in all-polymer solar cells upon annealing

The origin of the enhanced efficiency upon annealing in solar cells based on blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] and poly{9,9-dioctylfluorene-2,7-diyl-alt-1,4-bis[2-(5-thienyl)-1-cyanovinyl]-2-methoxy-5-(3,7-dimethyl-octyloxy)benzene} is investigated. Current-voltage measurements on solar cells and single-carrier diodes reveal that the electron and hole transport is not improved for thermally treated devices. From the analysis of the photocurrent, the authors show that the dissociation efficiency of the bound electron-hole pairs increases upon annealing, due to an increase of the initial electron-hole separation distance.

Charge transport in MDMO-PPV:PCNEPV all-polymer solar cells

Charge transport properties are investigated of blends of poly [2-methoxy-5-(3′, 7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and poly-[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3′, 7′-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV). The hole transport in the MDMO-PPV donor phase of the 1:1 weight ratio blend is trap-free space-charge limited, with a mobility identical to the pristine polymer. The electron current in the PCNEPV acceptor phase is strongly reduced by traps that are exponentially distributed in energy. The current in MDMO-PPV:PCNEPV bulk heterojunction solar cells is therefore unbalanced and dominated by the holes in the MDMO-PPV phase.