Conclusions of the Third-generation PV workshop for high efficiency through full spectrum utilization

As a result of the presentations during the workshop and the subsequent

discussion, the participants have agreed the following conclusions:

• Present solar cells are not likely to reach a cost that will allow penetration of the PV electricity market because, in their present form—with poor utilization of the solar spectrum—they make ineffective use of the solar resource that—although immense—comes with moderate densities.

• A number of options were presented that, in principle, may ensure better use of the solar resource.

• Of such options, multi-junction solar cells seem to be the one closer to practical exploitation. It was generally agreed that high concentration was needed to render them cost effective. Operation of the cells in high concentration has also been proven in many cases. Additional R&D including the use of nanotechnology (superlattices) might enable this promising line of research to reach its full potential.

• Alternative options based on better utilization of the low-energy photons could be achieved with intermediate-band or – level solar cells. In this option, two lower-energy photons are used to raise the free energy of the electrons to be delivered to the external circuit to a high level. Some theoretical work has been done recently on this topic and initial experimental work is based on nanotechnology, mainly quantum dot arrays. The concept, although apparently a long-term one, is worth exploration. Practical ideas to separate the up-converting function from solar cell fabrication, using optical coupling, have been presented in this workshop for the first time. Concentration seems to be one way of making the complex devices based on this principle cost effective.

• However, thin films based on this concept may also be a way to develop them in a low cost way, possibly with a poorer performance. Quantum dots based on a highly porous material grown in cheap ways or even organic semiconductors operating with the two-photon principle may become one of the most attractive ways to realize cost-effective high-efficiency solar cells.

• Using the high energy of the electrons excited by high-energy photons, before they thermalize with the lattice, may allow for higher efficiency devices. This can, in principle, be achieved by ionization of a second electron-hole pair or by extracting the electrons while they are still at a high temperature. Concepts for both solutions have been presented although they still seem to be far from demonstration. Arrays of quantum dots seem to be important in all these technologies and mastering this new field of applied science seems to be the key for possible exploitation of such concepts.

• Thermophotovoltaics and thermophotonics, which is extracting electricity by a solar cell illuminated by a heated emitter, i. e. a heated LED, presents several theoretical advantages. In principle, such devices could reach the Carnot efficiency if the cell and LED were ideal. Practical thermophotovoltaic devices operating with fuel-heated emitters already exist. Further research on this concept and ways of heating the emitter with solar energy are worth further study. This might become a medium-term option to compete with the other options studied.

• There is a general agreement that many of the solutions studied in this workshop may be cost effective only under highly concentrated sunlight. It was stressed that while many technological aspects of solar cells have advanced greatly, the optical concepts used for the concentrator are still based on concepts that were already known in the Ancient World. However, such devices were not the best that could be achieved and a need to exhaust the theoretical potential of concentrators was felt necessary to reach cost – effective solutions. New synthesis methods are available and further research on them seems justified to render cost-effective concentrators for novel PV cells.

• Novel technological aspects, some based on micromechanics, appear to be of utility for the purpose of this workshop. Direct bonding techniques were presented as an alternative to monolithic multi-junction cells in which some of the constraints of their fabrication as well as many of the drawbacks of conventional stacking disappear. This option is worth further exploration because it might lead to unexpected and interesting results.

[1] The potential drop across the inserted substructure is 0.25 V (or more) [46]. The measured Voc of the best MIND models is about 0.58 V, so the total barrier is 0.83 V.

[2] TEM images show a thin (10 nm) a-Si layer at this depth buried within the single-crystal Si. The CCL corresponds to the position of the upper a-Si/c-Si interface.

[3] efficient,

(2) suitable for mass production (repetitive and inexpensive),