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 (λmax ~ 850 nm) with a large bandgap
polymer (λmax ~ 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.
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 (ΦAg ~4.4 eV) to 5.5 eV (ΔΦ ~ 1.1 eV) for 1H,1H,2H,2H-perfluorinated alkanethiols. Hexadecanethiol on the other hand shifted ΦAg 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 ΦAu 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 ΦAg 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.
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