Hybrid solar cells with potential to push through efficiency barrier

The university’s Cavendish Laboratory, in the Department of Physics, has developed the new hybrid cells, which absorb both red and blue light. Present-day solar cells lose most of the blue photon energy as heat.

The new cells are able to harness the blue light energy through using the organic semiconductor material pentacene. This is applied alongside an inorganic semiconductor material, lead sulfide.

The research was led by professors Neil Greenham and Sir Richard Friend, and was published in January, in the journal Nano Letters. PhD candidate Bruno Ehrler was the lead author on the paper and he told pv magazine that despite the very low efficiency achieved in the prototype cell, of only one percent, this is being improved on rapidly. He continued that in principal the paper proves the concept that a hybrid approach can break through existing theoretical barriers to photovoltaic efficiency gains.

Ehrler explained, "In conventional solar cells, because the sun has a very broad spectrum, you have to decide either you will absorb a lot of that spectrum or will you use as much energy as possible from each individual absorbed light particle. For that trade off you end up having a maximum conversion efficiency of 34 percent."

By contrast, because the Cambridge team is using more of the high-energy light, the blue light, in principal an efficiency of 44 percent can be achieved.

The team intends to progress their research by improving the hybrid cells set out in the most recent paper as, in Ehrler’s words, "there are a lot of knobs to turn to improve the efficiency."

The approach developed in Cambridge’s Cavendish Laboratory differs from conventional photovoltaic cells in a number of ways. The deposition technique is to print the cells, after dissolving the semiconductor materials, in a very thin layer. The coupling of the organic pentacene material with an inorganic material is also of significance, as potentially it opens the way for this hybrid approach to be applied to silicon or more conventional solar cells.

"This is first prototype, but in principal because silicon cells are already relatively close to that theoretical limit, it is definitely worth exploring ways to circumvent that," said Ehrler.

Another advantage is that the organic material, pentacene, is also in abundance, as it is carbon based. Ehrler pointed out however, that at this stage it remains expensive. The cost of the cells need not be high because of this though, because of the printed deposition technique, the semiconductor layer, and therefore materials needed, are minimal.

This stage of the Cambridge research is expected to continue for three to five years and a commercial application of the technique may be a considerable distance away. However, the potential for printable, hybrid photovoltaic cells is certainly great. "It’s a very active and interesting field and this is a valuable contribution to the field," concluded Ehrler.