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Title: Bulk Heterojunction Organic Solar Cells Based on Crosslinked Polymer Donor Networks
Authors: LIU BO
Keywords: organic photovoltaic, organic solar cell, bulk heterojunction, crosslink, network
Issue Date: 21-Aug-2012
Citation: LIU BO (2012-08-21). Bulk Heterojunction Organic Solar Cells Based on Crosslinked Polymer Donor Networks. ScholarBank@NUS Repository.
Abstract: The power conversion efficiency (PCE) of organic photovoltaic cells depends crucially on the morphology of their donor?acceptor heterostructure amongst other factors. While tremendous progress has been made to develop new donor and acceptor materials that better cover the solar spectrum, their heterostructure is still formed by a rather primitive process of spontaneous demixing. This is rather sensitive to processing conditions and hence difficult to realise over the large areas needed for manufacturing. In this thesis, it is demonstrated that the ideal interpenetrating heterostructure where the donor and acceptor phases are intimately mixed at the ten-nanometer length scale but contiguous over the device thickness can be readily created by acceptor doping into a lightly-crosslinked polymer donor network. The resultant nanotemplated network is markedly insensitive to processing conditions and resilient to phase coarsening. It also shows surprisingly the excellent local molecular order required for efficient carrier transport. A general 20% improvement in PCE for the prototypical regioregular poly(3-hexylthiophene) (P3HT): phenyl-C61-butyrate methyl ester (PCBM) donor?acceptor system to reach 4.2% has been found using this method over the usual spincast biblend devices. Since the donor?acceptor morphology is now predetermined by the crosslinking density independent of the P3HT: PCBM ratio, it is possible to critically test the standard optical?electrical model for P3HT: PCBM, and refine the parameters using data obtained in this work. To improve model reliability, we have moreover directly measured the built-in potential Vbi of these cells using electromodulated absorption spectroscopy to be 0.75 V, with negative polaron levels of P3HT and PCBM at 3.2 and 3.5 eV respectively. The open-circuit voltage deficit is thus only 0.1?0.15 V, which we have determined to arise here largely from majority carrier injection at the ohmic contacts. Excellent agreement between the model and experimental current-voltage characteristics were obtained over a wide thickness range using a single global parameter set. Analysis of the results further suggests: (a) the electron?hole recombination rate constant is 2?3 orders of magnitude lower than the Langevin constant, as other authors have reported; and (b) the interface mobile carrier density is 1?2 orders of magnitude lower than the actual delta-doped carrier density in the organic semiconductor at the contacts. The latter suggests significant energetic spread of the carriers. Using the refined parameter set, we have systematically examined the transport and optical-structure optimization landscapes of organic solar cells in general. We established: (i) the importance of high carrier mobilities, and of mobility mismatch to enhance photocarrier collection from an asymmetric exciton-generation profile, and (ii) the existence of a remarkably simple ?p / nPAL scaling law, where ?p is the absorption center wavelength and nPAL is the refractive index, that determines the optimal absorption thickness of the photoactive layer. These results reveal new device insights and lay down a clear path for the systematic optimization of organic solar cells.
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