CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC) and compared to CH3NH3PbI(3-x)Clx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, LD, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI(3-Clx ones, where LD values are comparable for both charge types, do not.

We present "design rules" for the selection of molecules to achieve electronic control over semiconductor surfaces, using a simple molecular orbital model. The performance of most electronic devices depends critically on their surface electronic properties, i.e., surface band-bending and surface recombination velocity. For semiconductors, these properties depend on the density and energy distribution of surface states. The model is based on a surface state-molecule, HOMO-LUMO-like interaction between molecule and semiconductor. We test it by using a combination of contact potential difference, surface photovoltage spectroscopy, and time-and intensity-resolved photoluminescence measurements. With these, we characterize the interaction of two types of bifunctional dicarboxylic acids, the frontier orbital energy levels of which can be changed systematically, with air-exposed CdTe, CdSe, InP, and GaAs surfaces. The molecules are chemisorbed as monolayers onto the semiconductors. This model explains the widely varying electronic consequences of such interaction and shows them to be determined by the surface state energy position and the strength of the molecule-surface state coupling. The present findings can thus be used as guidelines for molecule-aided surface engineering of semiconductors.

Photosystem I (PS I) is a transmembrane, multi-subunit protein-chlorophyll complex that mediates vectorial, light-induced electron transfer. The nanometer-sized dimensions, an energy yield of approximately 58 %, and the quantum efficiency of almost 1 [1] make the reaction center a promising unit for applications in molecular nanoelectronics. PS I is located in the thylakoid membranes of chloroplasts and cyanobacteria. It mediates light-induced electron transfer from plastocyanin or cytochrome C 553 to ferredoxin. [2,3] The crystalline structure of PS I from Synechococus elongatus and from plants' chloroplast was resolved to 2.5 and 4.4 Å, respectively. [4,5] In cyanobacteria, the complex consists of at least 12 polypeptides, some of which bind 96 light-harvesting chlorophyll molecules. The electron-transport chain contains P700, A 0 , A 1 , F X , F A , and F B , representing a chlorophyll a dimer, a monomeric chlorophyll a, two phylloquinones, and three [4Fe-4S] iron-sulfur centers, respectively. The reaction-center core complex is made up of the heterodimeric PsaA and PsaB subunits, containing the primary electron donor, P700, which undergoes light-induced charge separation and transfers an electron through the sequential carriers A 0 , A 1 , and F X . The final acceptors, F A and F B , are located on another subunit, PsaC. The redox potential of the primary donor, P700, is +0.43 V and that of the final acceptor, F B , is -0.53 V, producing a redox difference of -1.0 V. The charge separation spans about 5 nm of the height of the protein, representing the center-tocenter distance between the primary donor and the final acceptor. The protein complex is 9 nm in height and has a diameter of 21 nm and 15 nm for the trimer and the monomer, respectively.[4] The photoactivity and the nanometer-sized dimensions make this complex a promising unit for applications in molecular nanoelectronics. In earlier works, care was taken to indirectly attach plant PS I [6,7] and bacterial reaction centers [8,9] to solid surfaces in attempts avoid inactivation of selfassembled monolayers.In this work, we devised a system that overcame the problems arising from direct covalent binding of proteins to metal surfaces. We selected the robust PS I reaction centers from the cyanobacteria Synechocystis sp. PCC 6803. The main reason for the structural stability of this PS I is due to the fact that all chlorophyll molecules and carotenoids are integrated into the complex of core subunits, while, in plant and bacterial reaction centers, the antenna chlorophylls are bound to chlorophyll-protein complexes that are attached to the core subunits. Indeed, there was no need to use peptide surfactants, which were essential for stabilization of plant PS I and the bacterial reaction centers. [7] A careful selection of the amino acids, which were modified to cysteines for covalent attachment of the PS I to the gold surface, was the second factor that insured structural and functional stability of the self-assembled, oriented PS I. The rational design was b...

Hot-electron cooling dynamics in photoexcited bulk and quantum-well GaAs structures were determined using time-correlated single-photon counting of photoluminescence (PL) decay. Hot-electron cooling curves were generated from analyses of the time-resolved PL spectra. The time constant characterizing the hot-electron energy-loss rate,~" g, was then determined, taking into account electron degeneracy and the time dependence of the quasi-Fermi-level. This analysis was also applied to earlier data