Laboratoire de Photoniques et Interfaces, Institut des Sciences et Ingénierie Chimiques, école Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
Energy crisis and environment pollution are the main challenges of human beings in the 21th century. Developing solar cells with high efficiency and stability at low production cost is an effective way to achieve “price parity” with fossil-fuel-based grid power and large-scale use of solar energy. Since the breakthrough of dye-sensitized solar cells (DSSCs) in 1991, mesoscopic solar cells have been developing fast. Especially in the past three years, the power conversion efficiency (PCE) of perovskite solar cells based on mesoporous structure increased at an amazing rate. Recently, a certificated PCE of 21.0% was reached, which is higher than the certificated record PCE of polycrystalline silicon solar cells, indicating a competitive advantage. However, as new type of photovoltaics, solar cells based on dyes, quantum dots or perovskite pigments as light harvesters, still have many unknowns to be explored. I am delighted to be invited by Frontiers of Optoelectronics, as the guest editor of the first issue in 2016, to arrange the special edition on Mesoscopic Solar Cells. This issue contains 7 review and research papers, all invited written by international well known scientists who are active in the field of mesoscopic solar cells. It covers research frontiers from DSSCs, quantum-dot-sensitized solar cells to mesoscopic perovskite solar cells, from fabrication, characterization to optimization of different components of mesoscopic solar cells. This special issue offers a general view of the development of mesoscopic solar cells. One of the most important components in DSSCs is the sensitizing dye. The donor-π bridge-acceptor (D-π-A) structure is the mainstream in the design of organic sensitizers. Hua et al. discussed the donor design and modification strategies of metal-free sensitizers for highly-efficient DSSCs, however, some sensitizers that do not follow D-π-A structure also present good efficiency when applied in DSSC devices. Robertson and Hu summarized the atypical dyes in order to inspire more diverse designs towards highly efficient DSSCs. The effect of the perovskite morphology on the photovoltaic performance is a critical factor. Etgar and Cohen reviewed various parameters influence and control the organo-metal halide perovskite crystallization and morphology, such as the annealing process, the precursor solvent, anti-solvent treatment and additives to the perovskite solution. Except the comprehensive reviews of organic sensitizers in DSSC, some research results of mesoscopic solar cells with novel photoelectrode, sensitizers, and cathode layers were presented as well. Zhao et al. proposed a new architecture design, micro-scale hierarchical TiO2 nanowires, for nanowire-based quantum-dot-sensitized solar cells to improve the photovoltaic performance. Li et al. designed and synthesized a series of metal-free organic sensitizers with D-A′- π-A configurations, in which quninoxaline or benzoxadiazole moiety was incorporated into the conjugated bridge as the auxiliary electron acceptor (A′) to extend the absorption spectra and broaden the light-harvesting region. Yang et al. explored the use of P3HT modified carbon nanotubes (CNTs@P3HT) (P3HT = poly(3-hexylthiophene)) for the cathodes of hole transporter free, mesoscopic perovskite solar cells, simultaneously achieving high-performance, high stability and low-cost perovskite solar cells. Despite of the increasing importance of CH3NH3PbI3based organicinorganic hybrid perovskites, studies relating to basic structural characterization of these powerful light-harvesting materials remains sparse. Padture et al. conducted a systematic ambient Raman spectroscopy characterization study of MAPbI3 thin films to elucidate the possible sources of artifacts in the Raman spectra, and raised the awareness of the challenges in the ambient Raman spectroscopy of MAPbI3 perovskites. I would like to thank the authors for their excellent works, which set a new landmark in this rapidly evolving and fascinating field. I am particularly grateful to the strong support from Prof. Dexiu Huang and the assistance from Prof. Hongwei Han for this special issue.