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High Efficiency Quantum Well Solar Cells.

The absorption of a p-i-n solar cell can be extended to lower photon energies by placing quantum wells into the intrisic region. Photogenerated carriers escape the quantum well via a thermally assisted tunneling process and contribute to the photocurrent of the solar cell. By using alternating compressive and tensile materials, a strain-balanced stack of quantum well and barrier layers can be grown, defect free, providing absorption-edge / lattice parameter combinations that are inaccessible using bulk materials. The high purity of the semiconductor material leads to radiatively dominated recombination enabling further efficiency gain by controlling the optical losses. When using compressively strained quantum well material, a small but fundamental efficiency advantage is realised over conventional bulk semiconductor solar cells on account of suppressed radiative emission from the light hole band. As a result, a strain-balanced GaAsP/InGaAs quantum well solar cell recently attained a power conversion efficiency of 28.3%.

To determine how the lipid environment affects membrane protein structure and function, strains of Escherichia coli were developed in which normal phospholipid composition can be altered or foreign lipids can be introduced. The properties of lactose permease (LacY) were investigated as a function of lipid environment. Assembly of LacY in membranes lacking phosphatidylethanolamine (PE) results in mis-orientation of the N-terminal six transmembrane domain (TM) bundle with loss of energy dependent uphill transport and retention of energy independent downhill transport. Post-assembly introduction of PE results in nearly complete regain of native orientation of TMs and restoration of uphill transport. Foreign lipids with no net charge can substitute for PE in supporting native LacY topology, but restoration of uphill transport is dependent on native topology and the proper folding of a solvent exposed domain. Increasing the positive charge density of the cytoplasmically exposed surface of LacY counters TM mis-orientation in the absence of neutral lipids demonstrating that charge interactions between these domains and the surface of the membrane bilayer are determinants of TM orientation. Therefore, membrane protein organization is determined during initial assembly through interactions between the protein and the lipid environment and is also dynamic and can change post-assembly dependent on changes in the lipid environment.