Photon–Electron Conversion
The Exchange at the Heart of Every Optoelectronic Device
Our lab is named after a single physical process: photon–electron conversion. A solar cell, a camera sensor, a fibre-optic receiver and a medical X-ray detector all look very different, but underneath they perform the same exchange — a particle of light arrives, and mobile electric charge leaves. Run the same process backwards and you have an LED or a laser.
Understanding this exchange step by step is what lets us decide which material to use, where a device is losing performance, and what to change to make it better.
Four Steps From Light to Current
Converting a photon into usable current is not one event but a short chain of them. Each link has to succeed for the photon to count.
Figure 1: The photon-to-current chain. The overall efficiency is the product of the four step efficiencies, so a single weak link caps the whole device.
Because the steps multiply, a device that absorbs 95% of the light but separates only half of the pairs it makes is worse than one that is merely good at everything. Much of our engineering effort goes into identifying which step is the bottleneck in a given material system.
Where the Energy Goes
Even a perfectly built single-junction device discards a large share of the incoming sunlight, for two reasons that both follow directly from the bandgap.
Figure 2: The two intrinsic losses of a single-junction device. Photons below the bandgap are never absorbed; photons well above it are absorbed, but the surplus energy is shed as heat within picoseconds as the carrier relaxes to the band edge.
Lowering the bandgap harvests more photons but wastes more of each one; raising it does the reverse. That trade-off is the origin of the Shockley–Queisser limit, and it is why choosing — and tuning — the right material matters so much.
The Same Process, Run Backwards
Nothing in the chain is one-way. Drive current into the same structure and electrons and holes meet, pair up, and release a photon: that is an LED. The symmetry is quantitative as well as conceptual — a material that emits light efficiently is, by the same token, one with few pathways for wasting absorbed energy. We routinely use emission measurements to diagnose the quality of devices we are building in order to absorb.
How This Shapes Our Work
Each step in Figure 1 points to a different research question, and to the other pages in this section:
- Steps 1–2 are set by what the material is. See Novel Semiconductors.
- Steps 2–3 depend on how tightly the electron and hole are bound to one another, and on how the surrounding lattice responds. See Excitons and Polarons.
- Steps 3–4 are dominated by interfaces and contacts, which is where most of our device engineering happens. See Photodetectors.