No abstract

We review recent progress towards increasing solar cell efficiencies beyond the Shockley-Queisser efficiency limit. Four main approaches are highlighted: multi-junction cells, intermediate-band cells, hot carrier cells and spectrum conversion. Multi-junction cells use multiple solar cells that selectively absorb different regions of the solar spectrum. Intermediateband cells use one junction with multiple bandgaps to increase efficiencies. Hot-carrier cells convert the excess energy of abovebandgap photons into electrical energy. Spectrum conversion solar cells convert the incoming polychromatic sunlight into a narrower distribution of photons suited to the bandgap of the solar cell.The AM 1.5 solar spectrum along with the bandgaps of some semiconductors. Third generation photovoltaics strive to maximize the efficiency of converting polychromatic radiation into electricity.

A method is proposed and tested which allows for the accurate determination of the carrier collection efficiency and minority carrier diffusion length in Cu(In,Ga)Se2 solar cells using energy dependent electron beam induced current. Gallium composition gradients across the film thickness introduce quasielectric fields that are found to improve collection efficiency when they are located toward the rear of the sample. The quasielectric fields are also shown to reduce the influence of back surface recombination. The strengths and limitations of this technique are discussed and compared with external quantum efficiency measurements.

The surface contribution to the electrical transport properties of InN was directly measured and modulated by the electrolyte gated Hall effect. Undoped and Mg-doped films show different behaviors that can be effectively described by a multilayer model, taking into account the conduction contribution from both the surface and interface with the buffer layer. Gated photoluminescence experiments further show the surface accumulation layer enhances radiative electron-hole recombination in undoped InN.