One of the limitations of present organic solar cells is the relatively poor spectral overlap
of their absorption bands with the solar spectrum. Semiconducting polymers as poly(3-hexyl
thiophene) have a bandgap higher than 2.0 eV (600 nm), thereby limiting the maximum
possible absorption of the solar spectrum to about 30%. A way to overcome this limitation is a
tandem solar cell where two bulk heterojunction single cells are stacked in series, each with a
different bandgap. The combined absorption then covers a broader region of the solar
spectrum. So far, solution-processed tandem solar cells have not been realized due to
incompatibility of the solvents. We demonstrate a solution-processed polymer tandem cells by
stacking two single cells in series. The tandem cell consist of two bulk heterojunction subcells separated by a thin semitransparent electrode of gold. This middle electrode serves in
three different ways; as a charge recombination centre, as a protecting layer for first cell
during spin coating of the second cell, and as a semitransparent layer that creates optical
cavities, which allows tuning of the optical transmission through the first (bottom) cell to
optimize the optical absorption of the second (top) cell. To cover a broader region of the solar
spectrum we combined a small bandgap polymer (λ<sub>max</sub> ~ 850 nm) with a large bandgap
polymer (λ<sub>max</sub> ~ 550 nm). These sub cells are electronically coupled in series, which leads to
an open-circuit voltage that equals the sum of each sub cell. A high open-circuit voltage of 1.4
Volt is achieved. The current density of the tandem cell follows the current of the top cell,
which has a lower, limiting current. The tandem architecture and proper materials give us the
possibility to cover a very broad spectral range of the solar spectrum to make highly efficient
organic solar cells in the near future.
Bulk-heterojunction solar cells made from blend films of regioregular poly(3-hexylthiophene) (P3TH) and methanofullerene (PCBM) exhibit a ten-fold increase of the power conversion efficiency upon post production annealing. We have applied a numerical model to explain this dramatic enhancement of the efficiency. The mobilities of electrons and holes were selectively determined, and it was found that the hole mobility of P3HT in the blend increases by more than three orders of magnitude. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red-shift, improving the spectral overlap with the solar emission, which result in an increase of more than 60% in the generation rate of charge carriers. Our numerical model demonstrates that the unannealed devices suffer from the buildup of space-charge, as a consequence of the low hole mobility. Furthermore, at short-circuit the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90%, which explains the large quantum efficiencies measured in P3HT:PCBM blends.
We have measured the electron and hole mobility in blends of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C<sub>61</sub>-butyric acid methyl ester (PCBM) with varying MDMO-PPV/PCBM composition. It is shown that the electron mobility in the PCBM-rich phase gradually increases up to 80 wt.% PCBM, due to an increased number of percolated pathways from bottom to top electrode. In contrast to the expectations the hole mobility in the MDMO-PPV phase shows a similar behavior as a function of fullerene concentration; Starting at 40 wt.% with the value of pristine MDMO-PPV the hole mobility strongly increases and saturates beyond 67 wt.% at a value which is more than two order of magnitude higher. The large enhancement of the hole mobility and its saturation is related to recent findings on the film morphology of this material system.
Tuning the work functions of metals was demonstrated by chemically modifying the metal surface through the formation of chemisorbed self-assembled monolayers (SAMs) derived from 1H,1H,2H,2H-perfluorinated alkanethiols and hexadecanethiol. The ordering inherent in the SAMs creates an effective, molecular dipole at the metal/SAM interface, which increased the work function of Ag (Φ<sub>Ag</sub> ~4.4 eV) to 5.5 eV (ΔΦ ~ 1.1 eV) for 1H,1H,2H,2H-perfluorinated alkanethiols. Hexadecanethiol on the other hand shifted Φ<sub>Ag</sub> toward 3.8 eV (ΔΦ ~ 0.6 eV) and raised the energy barrier for hole injection. These SAMs on Au were less efficient. 1H,1H,2H,2H-perfluorodecanethiol raised ΦAu (4.9 eV) by 0.5 eV to 5.4 eV, whereas hexadecanethiol decreased Φ<sub>Au</sub> by only 0.1 eV. These chemically modified electrodes were applied in the fabrication of pLEDs and the hole conduction of MEH-PPV was investigated. An ohmic contact for hole injection between a silver electrode functionalized with the perfluorinated SAMs, and MEH-PPV with a HOMO of 5.2 eV was established. Conversely, a silver electrode modified with a SAM of hexadecanethiol lowered Φ<sub>Ag</sub> to 3.8 eV, creating an efficient energy barrier for hole injection. This method demonstrates a simple and attractive approach to modify and improve metal/organic contacts in organic electronic devices like LEDs and photovoltaic cells.