Triple-junction solar cells from III-V compound semiconductors have thus far delivered the highest solar-electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four-junction solar cell architectures with optimum bandgap combination are found for lattice-mismatched III-V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs-based top tandem solar cell structure was bonded to an InP-based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four-junction solar cell with a new record efficiency of 44.7% at 297-times concentration of the AM1.5d (ASTM G173-03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III-V multi-junction solar cells having four and in the future even more junctions.

The essential role of the dark equilibrium potential is discussed for charge separation and the photovoltaic functioning of the title cell. A quantitative model is presented for the potential distribution in the sponge-type title cell. The unique screening process for the photogenerated electrons is discussed that facilitates their extremely long lifetime since the screening ions cannot function as recombination centers. A general analogy is pointed out for the photovoltaic functioning of the sponge-type electrochemical solar cell and of a conventional single-crystal solid-state solar cell. † Parts of this letter have been presented as an invited talk at the Spring

Electron injection from optically excited dye molecules into the depletion layer of polycrystalline n-TiO2 electrodes is measured as photocurrent. The characteristic shape of the photocurrent transients has a point of inflection and is controlled by the complete filling of deep traps. The rate equations for the Shockley–Read trapping process are solved numerically for the case of high injection levels, and the shape of the transients is simulated.

A comprehensive investigation of the luminescent properties of carbon nitride polymers, based on tri-s-triazine units, has been conducted. Steady-state temperature-and excitation-power-dependent as well as time-resolved measurements with near-UV excitation (λ = 325 nm and 405 nm) yield strong photoluminescence, covering the visible spectrum. The spectral, thermal, and temporal features of the photoluminescence can be satisfactorily described by the excitation and radiative recombination of molecular excitons, localized at single tri-s-triazine units. The discussed model is in accordance with the recently reported absorption features of carbon nitride polymers. Thus, from the point of view of optical spectroscopy, the material effectively behaves as a monomer.

The ultrafast injection dynamics, early recombination dynamics, and spectral signatures of four systematically varied dye-metal oxide hybrid systems were investigated using transient absorption spectroscopy techniques. First, photoinduced electron transfer from two different perylene derivatives into zinc oxide (ZnO) colloidal films is reported. Here, the electronic coupling of the perylene chromophore 2,5-Di-tert-butyl-perylene-9-yl-propionic acid (1) to the ZnO colloids was weaker than the electronic coupling of the chromophore 2,5-Di-tert-butyl-perylene-9-yl-acrylic acid (2). Second, the photoinduced electron transfer of the same two molecules attached to TiO2 colloids was measured and compared to the results for the ZnO colloids using the same techniques. The temporal traces at both the excited-state and the cationic state of the chromophores attached to the semiconductor surfaces were measured simultaneously and showed very good agreement, which indicated a direct injection into the semiconductor. The overall injection times for the ZnO samples was as short as 190 fs, which suggested a strong electronic coupling element for these systems. This injection time is short compared to reports on similar ZnO hybrid systems, but it is still longer than the injection times reported for the TiO2 hybrid systems. The transient absorption spectra of molecule 2 attached to TiO2 showed a large negative signal at 530–550 nm, which indicated the presence of a direct charge transfer state contribution in this system.