Carrier lifetime measurement at grain boundaries in thin-film solar cells

The total solar energy reaching the Earth in less than an hour is greater than the annual global energy demand. Solar cells provide valuable renewable energy by converting sunlight into electricity. Silicon currently dominates solar cell technology, although it is an intrinsically poor absorber of light. On the other hand thin-film solar cells utilise strongly absorbing materials so that the material volume is reduced to a thin layer a few micrometers in thickness, thereby lowering cost. Examples include CdTe and CIGS, the former being commercially produced with an annual output exceeding 1 Giga Watt.

A common feature of thin-film solar cells is the high density of crystal defects known as 'grain boundaries'. A grain boundary demarcates the region where two crystals (i.e. grains) of different orientation impinge on one another. They typically reduce the overall solar cell efficiency and need to be passivated for optimum device operation. Indeed in CdTe solar cells a 'chlorine activation' step is used post-deposition where the device is annealed in a chlorine-rich environment leading to a ten-fold improvement in efficiency. It is thought that chlorine segregates to the grain boundaries thereby passivating them, although the precise mechanism is unknown.

In this project we will use state-of-the-art measurements of carrier lifetimes at individual grain boundaries to explore the fundamental mechanism(s) behind chlorine activation. The carrier lifetime is an important parameter specifying the electrical activity of a given grain boundary and its effect on device operation. In order to carry out such measurements a high spatial resolution (few nanometres) must be combined with excellent temporal resolution (a few picoseconds) the technical demands for which have only recently been overcome. We will carry out the very first measurements of grain boundary carrier lifetimes in CdTe thin-film solar cells using the new Attolight scanning electron microscope recently installed at EPFL, Switzerland. This is the only scientific instrument of its kind in the world capable of carrying out such analyses. We will also explore the relative effectiveness of different chlorine activation routes (i.e. standard activation using solid CdCl2 and gas activation methods) for passivating grain boundaries. This has important long term commercial benefits such as the production of efficient CdTe thin-film solar cells as well as reducing the environmental impact of the chlorine activation process.

Silicon currently dominates (>80%) solar cell technology although it is a poor absorber of light. A significant fraction of the cost is therefore due to purifying and refining the silicon. This is a major obstacle to reducing the cost of solar energy to the $1 per peak Watt value where it becomes economically competitive with fossil fuels. On the other hand thin-film solar cells utilise efficient light absorbing materials such that the material volume is reduced to a few micrometer thick layer in which the material quality is not as critical as silicon. CdTe is one such example- it is already in commercial production with one company (First Solar) achieving an annual output in excess of 1 Giga Watt. 'Chlorine activation' is an important step in the CdTe device fabrication process where the efficiency is increased ten-fold from its as-deposited value. The standard method for chlorine activation, using solid CdCl2, is however environmentally unfriendly and hence gaseous routes are under investigation as alternatives. This project will examine the underlying mechanism for chlorine activation and will also compare the relative merits of CdCl2 vs. gas activation methods. The aim is to produce more efficient devices using smaller amounts of chlorine-based chemicals. The analyses will be carried out on 16.5% efficiency samples that are close to the 16.7% world record for CdTe (samples will be provided by Prof Ken Durose, Liverpool University). Insights from this project could therefore help surpass the current world record efficiency for CdTe.

Another important aspect is the development of high spatial resolution carrier lifetime measurement as a new state-of-the-art characterisation technique for solar cells. This has important applications in grain boundary electrical activity characterisation and carrier dynamics of nanowire-based solar cells. We will carry out the very first measurements in each of these cases. The new research methodology will be of use to groups working on thin-film solar cells and third generation nanowire-based solar cells.