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.

In the optical packet switching network, optical buffer is an important device. Microring resonator optical buffers provide good delay performance and flexibility in design. By cascading multiple microring resonators, higher delay-bandwidth product is obtained, but the requirements of high integration and low dispersion are hard to satisfy simultaneously. Double-slot waveguide was proposed to construct highly integrated racetrack microring resonators in this study. Based on dispersion analysis of the thickness of each layer of a waveguide, the structure of waveguide was optimized to reach flat and low dispersion. Average dispersions of straight and 3 μm bend waveguides were 5.1 ps/(nm?km) and 4.4 ps/(nm?km), respectively. Besides, the additional loss from coupling was greatly reduced when applying proper relative displacement between straight and bend waveguides. Theoretical and design basis provided in this paper will help to develop multi-microring optical buffers in the future.

A highly efficient tunable optical filter of liquid crystal (LC) optical micro-ring resonator (MRR) was proposed. The 4-μm-radius ring consists of a silicon-oninsulator (SOI) asymmetric bent slot waveguide with a LC cladding. The geometry of the slot waveguide resulted in the strong electro-optic effect of the LC, and therefore induced an increase in effective refractive index by 0.0720 for the quasi-TE mode light in the slot-waveguide. The ultra-wide tuning range (56.0 nm) and large free spectral range (FSR) (~28.0 nm) of the optical filters enabled wavelength reconfigurable multiplexing devices with a drive voltage of only 5 V. The influences of parameters, such as the slot width, total width of Si rails and slot shift on the device’s performance, were analyzed and the optimal design was given. Moreover, the influence of fabrication tolerances and the loss of device were both investigated. Compared with state-of-the-art tunable MRRs, the proposed electrically tunable micro-ring resonator owns the excellent features of wider tuning ranges, larger FSRs and ultralow voltages.

Dye-sensitized solar cells (DSSCs) cannot be developed without the research on sensitizers. As the key of light harvesting and electron generation, thousands of sensitizers have been designed for the application in DSSC devices. Among them, organic sensitizers have drawn a lot of attention because of the flexible molecular design, easy synthesis and good photovoltaic performance. Recently, new record photovoltaic conversion efficiencies of 11.5% for DSSCs with iodide electrolyte and 14.3% for DSSCs with cobalt electrolyte and co-sensitization have been achieved with organic sensitizers. Here we focus on the donor design and modification of organic sensitizers. Several useful strategies and corresponding typical examples are presented.

Since their invention, dye-sensitized solar cells (DSSCs) have received enormous research attention from scientists with different background due to the possibility of low-cost production and fabrication on flexible substrates. One of the most important components in DSSCs is the sensitizing dye, including metal-complexes and metal-free organic dyes. The donor-π bridge-acceptor (D-π-A) structure is the mainstream in the design of organic sensitizers because it facilitates efficient chargetransfer toward the TiO2 upon excitation. However, some sensitizers that do not follow this structure, or have some modification upon this structure, also present good efficiency when applied in DSSC devices. This review summarizes these atypical dyes in order to inspire more diverse designs toward highly efficient DSSCs.

This review discusses various parameters that influence and control the organo-metal halide perovskite crystallization process. The effect of the perovskite morphology on the photovoltaic performance is a critical factor. Moreover, it has a dramatic effect on the stability of the perovskite, which has significant importance for later use of the organo-metal perovskite in assorted applications. In this review, we brought together several research investigations that describe the main parameters that significantly influence perovskite crystallization, for example, the annealing process, the precursor solvent, anti-solvent treatment, and additives to the iteite solutions.

This paper proposed a new architecture design for nanowire-based quantum-dot-sensitized solar cells to improve the photovoltaic performance. Microstructured rough substrate was used to increase the surface area of the photoanode without influence on charge carrier transport in the system. Compared to conventional devices, the short circuit current density and power conversion efficiency were enhanced by 50%. And the technology can be widely used in the photoelectrochemical (PEC) field, and it can be combined with other hierarchical nanostructures.

Four organic sensitizers containing quinoxaline or benzoxadiazole as an auxiliary electron acceptor in conjugated bridge were synthesized and utilized for dyesensitized solar cells (DSSCs). It was found that the incorporation of different electron-withdrawing moieties can affect the absorption spectra, electronic properties, the interfacial interactions and then the overall conversion efficiencies significantly. Therefore, the appropriate selection of the auxiliary acceptor was important to optimize the photovoltaic performance of solar cells. Among these sensitizers, LI-44 based solar cell showed the best photovoltaic performance: a shortcircuit photocurrent density (J_sc) of 13.90 mA/cm², an open-circuit photovoltage (V_oc) of 0.66 V, and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency of 6.10% under standard global AM 1.5 solar light conditions.

This work explores the use of poly(3- hexylthiophene) (P3HT) modified carbon nanotubes (CNTs@P3HT) for the cathodes of hole transporter free, mesoscopic perovskite (CH₃NH₃PbI₃) solar cells (PSCs), simultaneously achieving high-performance, high stability and low-cost PSCs. Here the thin P3HT modifier acts as an electron blocker to inhibit electron transfer into CNTs and a hydrophobic polymer binder to tightly cross-link the CNTs together to compact the carbon electrode film and greatly stabilize the solar cell. On the other hand, the presence of CNTs greatly improve the conductivity of P3HT. By optimizing the concentration of the P3HT modifier (2 mg/mL), we have improved the power conversion efficiencies (PCEs) of CNTs@P3HT based PSCs up to 13.43% with an average efficiency of 12.54%, which is much higher than the pure CNTs based PSCs (best PCE 10.59%) and the sandwich-type P3HT/CNTs based PSCs (best PCE 9.50%). In addition, the hysteresis of the CNTs@P3HT based PSCs is remarkably reduced due to the intimate interface between the perovskite and CNTs@P3HT electrodes. Degradation of the CNTs@ P3HT based PSCs is also strongly retarded as compared to cells employing the pure CNTs electrode when exposed to the ambient condition of 20%– 40% humidity.

The importance of methylammonium lead triiodide (CH3NH3PbI3 or MAPbI₃) organic-inorganic hybrid perovskites has shot up dramatically since their use in highly efficient thin-film perovskite solar cells (PSCs). However, the basic structural characterization of these fascinating materials remains sparse. In particular, Raman spectroscopy, which is a powerful vibrational spectroscopy characterization tool and complements other characterization methods, of MAPbI₃ under ambient conditions is plagued with difficulties. Here, a systematic ambient Raman spectroscopy characterization study of MAPbI₃ thin films is conducted under different conditions (excitation laser wavelength, integration time, filter characteristic). The results from this study help elucidate the possible sources of artifacts in the Raman spectra, and raise the awareness of the challenges in the ambient Raman spectroscopy of MAPbI₃ perovskites. Approaches to overcome these challenges are suggested.

The rectifying behavior between graphene and semiconductors makes novel type of solar cells, photodetectors and light emitting diodes (LEDs). The interface between graphene and ZnO is the key for the performance of the optoelectronic devices. Herein, we find that green light emission is very strong for the forward biased graphene/ZnO nanowire van derWaals heterostructure.We correlated the green light emission with the surface defects locating at the ZnO nanowire surface through the detailed high resolution transmission electron microscopy and photoluminescence measurements. We pointed out engineering the surface of ZnO nanowires could bring a dimension of designing graphene/ZnO LEDs, which could be extended to other types of graphene/semiconductor heterostructure based optoelectronic devices.

In this paper, via numerical simulation we designed the geometry of solar cell made by onedimensional (1D) and two-dimensional (2D) photonic crystals with two kinds of materiel (silicon (Si) and hydrogenated amorphous silicon (a-Si:H)) in order to enhance its absorption. The absorption characteristics of light in the solar cell structures are simulated by using finite-difference time-domain (FDTD) method. The calculation results show that the enhancement of absorption in patterned structure is apparent comparing to the unpatterned one, which proves the ability of the structure to produce photonic crystal solar cell. We found solar cell geometries as a 2D photonic crystal enable to increase the absorption between 380 and 750 nm.

A soluble low bandgap poly (pyrrole methine) with alkoxyl substituent, poly {(3-hexanoyl)pyrrole-[2,5- diyl(p-tetradecyloxybenzylidene)]} (PHPDTBE), was synthesized and characterized by 1H nuclear magnetic resonance (1H-NMR), Fourier transform-infrared (FT-IR), elemental analysis (EA) and gel permeation chromatography (GPC). PHPDTBE was readily soluble in weak polar organic solvents. The absorption peaks of PHPDTBE solution and film were located at around 458 and 484 nm, respectively. The optical bandgaps of PHPDTBE film for indirect allowed and direct allowed transitions were measured to be 1.66 and 2.35 eV, respectively. PHPDTBE film had few defects in the energy band and the Urbach energy of PHPDTBE film was calculated to be about 0.19 eV. The resonant third-order nonlinear optical susceptibilities of PHPDTBE solution and film measured by degenerate four-wave mixing (DFWM) technique at 532 nm were all in the order of 10^–8 esu, which was about 1~3 orders of magnitude larger than that of the other ordinary π-conjugation polymers.