Charge transport in organic solar cells

Philippa, Bronson (2014) Charge transport in organic solar cells. PhD thesis, James Cook University.

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Abstract

The process of charge transport is fundamental to the operation of all electronic devices. In organic photovoltaics, high efficiencies can only be achieved if charge transport is able to extract charge carriers from the active layer with minimal recombination losses. This work presents new insights into the measurement of charge transport, the underlying physics, as well as new approaches for modelling. Numeric simulation software using a drift-diffusion-recombination model is developed and applied to organic photovoltaic devices. Specifically, this model is used to design and interpret charge transport experiments that are applicable to operational organic solar cells.

Charge carrier mobility is studied using photogenerated charge extraction by linearly increasing voltage (photo-CELIV) and the novel technique of resistance-dependent photovoltage (RPV). These experiments demonstrate the absence of "hot carrier" relaxation effects on the timescales of charge transport in several organic photovoltaic polymer:fullerene blends. This is surprising because it has previously been argued that such relaxation is the cause of the deterimental dispersive transport that affects many organic semiconductor devices. It is argued instead that dispersive transport arises from the loss of carriers to trap states. Next, the techniques are extended to recombination measurements, where the recombination coefficient in a benchmark polymer:fullerene system is found to depend upon the polymer's molecular weight.

Modelling of the steady-state photocurrent produced by a solar cell demonstrates the conditions under which non-geminate recombination may be avoided, and presents a design rule for avoiding non-geminate recombination. Experimental measurements on devices of varying thickness support the conclusion that the space-charge limited current is a fundamental threshold for high-efficiency photocurrent extraction.

Finally, fractional kinetics and generalised diffusion equations are explored. We show that the Poisson summation theorem permits the analytic solution of a fractional diffusion equation to be collapsed into closed form. Subsequently, these techniques are applied to a new type of kinetic model that is capable of unifying normal and dispersive transport within a single framework.

Item ID: 40726
Item Type: Thesis (PhD)
Keywords: charge extraction; charge transport; diffusion equations; fractional kinetics; high intensity RPV (HI- RPV); intensity dependent photocurrents (IPC); linearly increasing voltage; organic solar cells; photovoltaic cells; photovoltaic power generation; Poisson summation formula; polymers; resistance-dependent photovoltage (RPV); solar cells; solar-photovoltaic energy; superconductivity
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Philippa, B.W., White, R.D., and Robson, R.E. (2011) Analytic solution of the fractional advection-diffusion equation for the time-of-flight experiment in a finite geometry. Physical Review E, E84 (4). pp. 1-9.

Philippa, Bronson, Stolterfoht, Martin, White, Ronald D., Velusamy, Marrapan, Burn, Paul L., Meredith, Paul, and Pivrikas, Almantas (2014) Molecular weight dependent bimolecular recombination in organic solar cells. Journal of Chemical Physics, 141.

Philippa, Bronson, Stolterfoht, Martin, Burn, Paul L., Juška, Gytis, Meredith, Paul, White, Ronald D., and Pivrikas, Almantas (2014) The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells. Scientific Reports, 4.

Philippa, Bronson, Robson, R.E., and White, R.D. (2014) Generalized phase-space kinetic and diffusion equations for classical and dispersive transport. New Journal of Physics, 16.

Philippa, Bronson, Vijila, Chellappan, White, Ronald D., Sonar, Prashant, Burn, Paul L., Meredith, Paul, and Pivrikas, Almantas (2015) Time-independent charge carrier mobility in a model polymer: fullerene organic solar cell. Organic Electronics, 16. pp. 205-211.

Date Deposited: 14 Oct 2015 02:08
FoR Codes: 02 PHYSICAL SCIENCES > 0204 Condensed Matter Physics > 020404 Electronic and Magnetic Properties of Condensed Matter; Superconductivity @ 50%
02 PHYSICAL SCIENCES > 0204 Condensed Matter Physics > 020405 Soft Condensed Matter @ 50%
SEO Codes: 85 ENERGY > 8505 Renewable Energy > 850504 Solar-Photovoltaic Energy @ 50%
97 EXPANDING KNOWLEDGE > 970102 Expanding Knowledge in the Physical Sciences @ 50%
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