Annealing at moderate temperatures is required to activate the silicon surface passivation by Al2O3 thin films while also the thermal stability at higher temperatures is important when Al2O3 is implemented in solar cells with screenprinted metallization. In this paper, the relationship between the microstructure of the Al2O3 film, hydrogen diffusion, and defect passivation is explored in detail for a wide range of annealing temperatures. The chemical passivation was studied using stacks of thermally-grown SiO2 and Al2O3 synthesized by atomic layer deposition. Thermal effusion measurements of hydrogen and implanted He and Ne atoms were used to elucidate the role of hydrogen during annealing. We show that the passivation properties were strongly dependent on the annealing temperature and time and were significantly influenced by the Al2O3 microstructure. The latter was tailored by variation of the deposition temperature (Tdep = 50 °C–400 °C) with hydrogen concentration [H] between 1 and 13 at.% and mass density ρmass between 2.7 and 3.2 g/cm3. In contrast to films with intermediate material properties, the passivation by low- and high density films showed a reduced thermal stability at relatively high annealing temperatures (∼600 °C). These observations proved to be in good agreement with thermal effusion results of hydrogen and inert gas atoms that were also strongly dependent on film microstructure. We demonstrate that the temperature of maximum effusion decreased for films with progressively lower density (i.e., with increasing [H]). Therefore, the reduced thermal stability of the passivation for low-density hydrogen-rich ([H] >∼5 at. %) films can be attributed to a loss of hydrogen at relatively low annealing temperatures. In contrast, the lower initial [H] for dense Al2O3 films can likely explain the lower thermal stability associated with these films. The effusion measurements also allowed us to discuss the role of molecular- and atomic hydrogen during annealing.

The thermal and ultraviolet (UV) stability of crystalline silicon (c-Si) surface passivation provided by atomic layer deposited Al2O3 was compared with results for thermal SiO2. For Al2O3 and Al2O3/a-SiNx:H stacks on 2 Ω cm n-type c-Si, ultralow surface recombination velocities of Seff<3 cm/s were obtained and the passivation proved sufficiently stable (Seff<14 cm/s) against a high temperature “firing” process (>800 °C) used for screen printed c-Si solar cells. Effusion measurements revealed the loss of hydrogen and oxygen during firing through the detection of H2 and H2O. Al2O3 also demonstrated UV stability with the surface passivation improving during UV irradiation.

This paper examines the thermodynamic limits of photovoltaic solar energy conversion by fluorescent collectors. The maximum efficiency of a fluorescent collector corresponds to the Shockley–Queisser limit for a nonconcentrating solar cell with a single bandgap energy. To achieve this efficiency, the collector requires a photonic structure at its surface that acts as an omnidirectional spectral band stop filter. The large potential of photonic structures for the efficiency enhancement of idealized as well as real fluorescent collectors is highlighted.

In this paper, we report on the outcome of a German network project conducted with 12 partners from universities and research institutes on the material development of dye solar cells (DSC). We give an overview in the field and evaluate the concept of monolithic DSC further with respect to upscaling and producibility on glass substrates. We have developed a manufacturing process for monolithic DSC modules which is entirely based on screen printing. Similar to our previous experience gained in the sealing of standard DSC, the encapsulation of the modules is achieved in a fusing step by soldering of glass frit layers. For use in monolithic DSC, a platinum free, conductive counter electrode layer, showing a charge transfer resistance of R CT < 1Á5 V cm 2 , has been realized by firing a graphite/carbon black composite under an inert atmosphere. Glass frit sealed monolithic test cells have been prepared using this platinum-free material. A solar efficiency of 6% on a 2Á0 cm 2 active cell area has been achieved in this case. Various types of non-volatile imidazolium-based binary ionic liquid electrolytes have been synthesized and optimized with respect to diffusion-limited currents and charge transfer resistances in DSC. In addition, quasi-solid-state electrolytes have been successfully tested by applying inorganic (SiO 2 ) physical gelators. For the use in semi-transparent DSC modules, a polyol process has been developed which resulted in the preparation of screen printed, transparent catalytic platinum layers showing an extremely low charge transfer resistance (0Á25 V cm 2 ).

We investigate resistive losses at p-type crystalline Si∕hydrogen passivated Si:H∕ZnO:Al heterojunction back contacts for high efficiency silicon solar cells. A low tunneling resistance for the (p-type) Si:H∕(n-type) ZnO part of the junction requires deposition of Si:H with a high hydrogen dilution rate RH&gt;40 resulting in a highly doped microcrystalline (μc) Si:H layer. Such a μc-Si:H layer if deposited directly on a Si wafer yields a surface recombination velocity of S≈180cm∕s. Using the same layer as part of a (p-type) c-Si∕Si:H∕ZnO:Al back contact in a solar cell results in an open circuit voltage VOC=640mV and a fill factor FF=80%. Insertion of an undoped amorphous (i) a-Si:H layer between the μc-Si:H and the wafer leads to a further decrease of S and, for the solar cells, to an increase of VOC. However, if the thickness of this intrinsic layer exceeds a threshold value of 4–5nm, resistive losses degrade the fill factor FF of the solar cells. Temperature dependent measurements of the contact resistance unveil activation energies in a range of 0.49–0.65eV, which we attribute to the valence band offset between a-Si:H and c-Si. The balance of FF losses and VOC gains determines the optimum (i) a-Si:H interlayer thickness for (i) a-Si:H∕(p) μc-Si:H double layer or (i) a-Si:H∕(p) a-Si:H∕(p) μc-Si:H triple layer back contacts.