Quantum well laser Energy electrons holes A quantum well laser is an improved LED. Electrons and holes are kept together inside the semiconductor at the center, which has a smaller gap. That makes it easier for electrons to find holes. It also creates quantized energy levels with a highly-concentrated density of states. Alferov Kroemer Nobel Prize in Physics 2000

Solar cell A photon from the Sun generates an electron-hole pair in a semiconductor. The electron is pulled to the front, the hole to the back of the solar cell, thereby creating a battery.

Energy diagram of a solar cell The electron and hole are pulled apart by the electric field between the p- and n-doped regions. It is critical not to lose electrons and holes on their way out. Crystalline semiconductors are good at that, but expensive.

100x100 square miles of solar cells would satisfy the electricity needs of the US. Goal High end Low end Here is the catch: x 0.4 TW = = 0.4 Trillion $ Crabtree and Lewis, Physics Today 60, March 2007, p. 37

Inexpensive thin film solar cells Use nanoparticles, molecules, polymers Less material, low temperature processing Print solar cells like newspaper, roll-to-roll Nanoparticle ink on metal foil (Nanosolar)

Shockley-Queisser limit for a single junction Photons below the band gap are not absorbed. Lose excess photon energy beyond the band gap. Converted by a crystalline silicon solar cell 1/3 This has been part of ALS Strategic Plan approved by SAC. The white paper is under preparation. 6

Use nanoparticles and molecules for tunability Dye-sensitized solar cell (Grätzel cell) Porphyrin dye: Metal atom in a cage of 4 nitrogens 7

Energy levels to play with in a solar cell A large energy drop between adjacent levels facilitates carrier separation (more current), but reduces the voltage.

Lose half the voltage on the donor side. Energy levels in a Grätzel cell Good example for tradeoff. Small voltage drop for ZnO, but electrons linger. Need to fill the hole fast, costs voltage. Lose half the voltage on the donor side. 9

Bottlenecks for Grätzel Cells  ½ Red , IR not absorbed  ½ Level mismatch on the donor side  ½ Fill factor: ( IV )max  ( Imax  Vmax ) ___________________________________  ⅛ (now 11%) Solutions: Several dyes + tandem cell, Solid state donor (electrolyte corrosive, needs Pt), EE