We attempted to clarify the impact of the compositional distribution on recently reported improvement in the conversion efficiency of solar cells based on bulk multicrystalline SiGe. For this purpose, Si1−xGex/Si1−yGey multiple quantum well structures on heavily doped Si-on-insulator were employed as model crystals. The combination of x and y, the width of each layer, and the number of repetitions were systematically changed to study the influence of the introduction of Ge on photocarrier generation and carrier transport while keeping the average Ge composition as 0.03. Spatial modulation of the band structure leads to formation of quantum wells for holes and gives negative impact especially in the photocarrier collection from the n-type region. When the depth of wells was designed to be constant, short-circuit carrier density was found to show a maximum at appropriate compositional distribution due to the competition between the increase in the photocarrier generation and the decrease in the minority carrier diffusion length. Within a limited compositional range, the overall performance of the solar cell was revealed to be improved by the introduction of the compositional distribution compared with that based on uniform Si0.97Ge0.03. Therefore, intentional introduction of the compositional distribution is concluded to be useful for improvement in the solar cell performance if appropriate dispersion is chosen.
Kinetic measurements have been made on heterogeneous nucleation of long-chain n-alcohol crystals (herewith referred to as guest crystals) from solution, which was accelerated by the presence of vapor-deposited thin films (referred to as templates) of monoacylglycerols and long chain fatty acids. The host templates were put in slightly supersaturated solution (decane solvent), in which no crystallization occurred over several hours without the template films, yet the template films accelerated the crystallization within several minutes of induction times. The crystallization behavior was examined by microscopic observation and induction time measurements, with specific attention to the template−guest relationships in terms of polymorphism, molecular orientation, and chain length limitation. It was confirmed that the host template films of the monoacylglycerols and the fatty acids accelerated the nucleation of the n-alcohol crystals, exhibiting the polymorphic matching, preservation of the molecular orientation, and the chain length matching. However, remarkable differences were revealed in the kinetics of heterogeneous nucleation between the host templates of monoacylglycerols and fatty acids; e.g., the monoacylglycerol templates accelerated two or three times more than the fatty acid templates under the same supersaturation conditions. This difference was ascribed to the specific host−guest interactions between the template and the guest materials as revealed in the phase behavior of the binary mixtures in the bulk state: monoacylglycerols and n-alcohols exhibited molecular compound formation, whereas eutectic phases are formed between fatty acids and n-alcohols. It was inferred that the template−solute interactions through van der Waals forces and hydrogen bonding at the interfaces of the template films and solution are differently operative in the two sets of the thin film templates and the guest materials.
A simple, low-cost, and non-vacuum epitaxial growth method to realize large-area semiconductors on crystalline silicon will become the game-changer for various applications. For example, we can expect the disruptive effect on the cost of large-scale III-V multi-junction solar cells if we could replace the high-cost germanium substrate with silicon-germanium (SiGe) on Si. For SiGe epitaxial growth, we attempted to develop a process using original Al-Ge pastes for screen printing and subsequent annealing. We compare two pastes including Al-Ge alloyed pastes with compositional uniformity in each particle and Al-Ge mixed pastes. We revealed that Al-Ge alloyed paste could form flatter SiGe film with much less residual pastes, supported by in-situ observations. The uniform and sufficient dissolution of the alloyed paste is responsible for these and led to higher average Ge-composition by annealing at 500 °C. The composition in SiGe was vertically graded up to ~ 90% at the topmost surface. These results show that printing and firing of Al-Ge alloyed paste on Si is the desirable, simple, and high-speed process for epitaxial growth of SiGe, which could be potentially used as the lattice-matched virtual substrate with III-V semiconductors.
We present device performance of solar cells based on multicrystalline SiGe (mc-SiGe) bulk crystal with microscopic compositional distribution grown by the casting method. The average Ge composition was systematically changed in the range between 0% and 10%. A small addition of Ge to multicrystalline Si (mc-Si) was found to be very effective to increase the short-circuit current density without affecting the open-circuit voltage. As a consequence, the overall efficiency of a solar cell based on mc-SiGe was improved compared with that based on mc-Si. This result demonstrates that mc-SiGe is a promising candidate to replace mc-Si since it could achieve higher conversion efficiency without drastic increase of the production cost.
Multicrystalline SiGe (mc-SiGe) with microscopic compositional distribution, which has been proposed as a promising material for solar cell applications, was characterized by microscopic Raman spectroscopy. As expected, a strong spatial variation of Raman spectra was observed. However, the compositional distribution obtained by a separate energy-dispersive X-ray analysis did not fully explain the observed spatial variation of Raman spectra. A plausible explanation of the inconsistency is the existence of built-in strain, which originates from the lattice mismatch between Si and Ge.
