This thesis concerns the use of time-resolved optical techniques to study the efficiency-limiting processes in high open-circuit voltage solar cells. Solution-processed large-area organic solar cells offer the potential for inexpensive and scalable production, but suffer from low efficiency. Currently the quantum efficiency in organic solar cells (the number of electrons extracted per incident photon) is only high when a significant amount of the energy from an absorbed photon is lost during the formation of the photo-induced charge-transfer state. This limits the voltage obtainable from state-of-the-art organic solar cells, and thus their overall efficiency. We investigate organic solar cells that retain more of the photon energy in the charge-transfer state, and hence allow for higher operating voltages to be reached. However, in these systems quantum efficiency is low and severely limits their overall power conversion efficiency. This work identifies that the dominant loss mechanism in such a system is terminal recombination into a triplet exciton. This mechanism for recombination will be energetically accessible in all high open-circuit voltage solar cells made from organic materials, and therefore must be considered in the design and optimisation of these devices. Due to the efficiency of this recombination mechanism, concurrent high densities of charges, singlet excitons, and triplet excitons are observed. This allows the interactions between these excited states to be determined. We measure the annihilation rates salient to optoelectronic device operation, allowing for detailed modelling of second-order kinetic effects to be undertaken. Finally, we investigate a polymer:small molecule solar cell and observe how, in this case, charge-transfer is aided by an initial transient of fast charge motion. However, after this faster motion subsides, the charges recapture one another and recombine quickly. This bimolecular recombination explains the observed efficiency of the polymer:small molecule solar cell, and also the dependence of this efficiency on incident light intensity.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:604270 |
| Date | January 2009 |
| Creators | Howard, I. A. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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