Quantitative measurements of photobleaching effects in organic semiconductors

(supervisor: Chris Finlayson)

Nature of project: experimental, instrumental

Available to full-time physicists or joint students.

Project description and methodology

The quantitative measurement of light emission from semiconductors is a critical issue when characterizing new materials for applications as wide as LEDs, lasers and solar cells. A key parameter is the quantum efficiency or "quantum yield" of samples, with may be defined in simple terms as the ratio of photons emitted (i.e. photo-luminescence) to those absorbed (i.e. photo-excitation).[1]

In many practical situations, where such fluorophores are subject to incident excitation light, a process known as "photobleaching" may take place, which results in an adverse degradation of light emission over time. This is a significant problem in many state-of-the-art semiconductor materials, including organic/polymeric semiconductors.[2]

In this experimental project, the student will characterise these effects using an "integrating sphere"[3,4]; an instrument which allows luminescent emission to be collected over all solid angles.

A successful project will develop beyond the above in one/some of the following directions:
As an extension, students might study the quantitative effects of photobleaching of samples as a function of laser excitation, exposure time and the environment (i.e. in air vs. inert atmosphere).

When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.

Additional scope or challenge if taken as a Year-4 project: There is scope to develop this project at Y4 level. Changes in the instrumental design to allow the incorporation and study of LEDs is one such avenue; extending the study to (photovoltaic) blend systems of two materials provides another.

Initial literature for students:

  1. http://en.wikipedia.org/wiki/Quantum_yield (and links therein)
  2. http://www.nature.com/nnano/focus/organics/index.html (and links therein)
  3. J. Tomes & C.E. Finlayson, Eur. J. Phys., vol.37, 055501 (2016)
  4. N.C. Greenham et al, Chem. Phys. Lett. vol.241 , p89 (1995)

Novelty, degree of difficulty and amount of assistance required

The scientific content involved in this project is highly suitable for the average Y3 physics student.

Project milestones and deliverables (including timescale)

milestoneto be completed by
Experimental design outlineChristmas
Rig/Instrument constructionend of February
Calibration and testing completemid-March
Completion of experimental resultsEaster

Students taking this project will have to submit a full risk assessment form