Targeted nano-scale light trapping for the next generation of solar cells and photodetectors




To maximise the efficiency of a solar cell, it is necessary for the ‘active’ semiconductor layer to absorb as much of the solar spectrum as possible, at photon energies with sufficient energy to generate free charge carriers in the semiconductor.   Optimising the performance of a photodetector requires a different strategy as the wavelengths of light we are interested in are determined by the application.  For example: we may want to target the UVA and B band, to monitor the amount of dangerous radiation that people are exposed to for skin cancer prevention; or we may be interested in the near infrared for night imaging applications. 

Light trapping can be defined as increasing the path length, and hence the absorption of light in a film, and can lead to significant enhancements in device performance for photodetector and solar cells.  In particular, nano-photonic schemes – wavelength scale photonic structures resulting in diffraction, excitation of optical resonances, and enhanced optical fields – are increasingly interesting as optoelectronic devices get smaller and thinner [1].  In particular, diffraction gratings can be designed to provide light trapping at particular target wavelengths.

In this project we will design diffractive gratings to provide light trapping for a silicon-on-insulator (SOI) photodiodes [2].  We will target a variety of wavelength bands to demonstrate the usefulness of this concept for a range of applications, including UV and NIR detection, and broadband solar harvesting for solar cells.  The project will be based on a diffractive grating calculation program (GDCalc), which is an implementation of rigorous coupled wave analysis in Matlab.  


  1. Understand the concept of guided modes and calculate dispersion relations with existing Matlab code for a multilayer, SOI structure.
  2. Combine the concepts of diffraction and wave guiding to understand resonant coupling to guided modes in a multi-layered structure for light trapping applications
  3. Understand and use RCW theory (via GDCalc) to calculate diffraction efficiencies and the resulting absorption in SOI structures.
  4. Design light trapping diffractive gratings using GDCalc and demonstrate absorption enhancement in in SOI structures at target wavelengths.


This is a Matlab based modelling project, suitable for students with an interest in nanophotonics, solar cells, optical devices and optoelectronics. It would be particularly good experience for anyone interested in a research career, or in R&D in the area of photonics, semiconductor devices or solar cells. Background knowledge of diffraction and wave guiding/guided modes would be useful: a willingness to learn about this in detail is essential. Familiarity with Matlab is useful. The ability to think logically and creatively solve problems will be required.

Prerequisite: some background in optical physics

Suitable for: 6-12 unit courses for R&D students: ENGN 2707/12, 3706/12, 4706/12

Background Literature

  1. S. Mokkapati and K. R. Catchpole, "Nanophotonic light trapping in solar cells," J. Appl. Phys. 112, 0–19 (2012).
  2. Bozzola et al., "Photonic light-trapping versus Lambertian limits in thin film silicon solar cells with 1D and 2D periodic patterns", Optica Express, 1, 20, S2 / A224, 2012
  3. A. Afzalian and D. Flandre, "Physical modeling and design of thin-film SOI lateral PIN photodiodes," IEEE Trans. Electron Devices 52, 1116–1122 (2005).
  4. GDCalc documentation


  1. Engage with a novel research problem
  2. Develop a deeper understanding of nanophotonics and light trapping
  3. Gain hands on experience in optical modelling and matlab
  4. Get a chance to independently and creatively solve problems
  5. Undertake independent research and build up your analytical skills
  6. Project manage


Photonics, diffraction gratings, optical devices, photodetectors

Updated:  10 August 2021/Responsible Officer:  Dean, CECS/Page Contact:  CECS Marketing