Electron-Affinity-Triggered Variations on the Optical and Electrical Properties of Dye Molecules Enabling Highly Efficient Dye-Sensitized Solar Cells

Solar Cells for Indoor Applications: Progress and Development

The Internet of things (IoT) has been rapidly growing in the past few years. IoT connects numerous devices, such as wireless sensors, actuators, and wearable devices, to optimize and monitor daily activities. Most of these devices require power in the microwatt range and operate indoors. To this end, a self-sustainable power source, such as a photovoltaic (PV) cell, which can harvest low-intensity indoor light, is appropriate. Recently, the development of highly efficient PV cells for indoor applications has attracted tremendous attention. Therefore, different types of PV materials, such as inorganic, dye-sensitized, organic, and perovskite materials, have been employed for harvesting low-intensity indoor light energy. Although considerable efforts have been made by researchers to develop low-cost, stable, and efficient PV cells for indoor applications, Extensive investigation is necessary to resolve some critical issues concerning PV cells, such as environmental stability, lifetime, large-area fabrication, mechanical flexibility, and production cost. To address these issues, a systematic review of these aspects will be highly useful to the research community. This study discusses the current status of the development of indoor PV cells based on previous reports. First, we have provided relevant background information. Then, we have described the different indoor light sources, and subsequently critically reviewed previous reports regarding indoor solar cells based on different active materials such as inorganic, dye-sensitized, organic, and perovskite. Finally, we have placed an attempt to provide insight into factors needed to further improve the feasibility of PV technology for indoor applications.

Phenanthrene-Fused-Quinoxaline as a Key Building Block for Highly Efficient and Stable Sensitizers in Copper-Electrolyte-Based Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) based on Cu^II/I bipyridyl or phenanthroline complexes as redox shuttles have achieved very high open-circuit voltages (V_OC , more than 1 V). However, their short-circuit photocurrent density (J_SC ) has remained modest. Increasing the J_SC is expected to extend the spectral response of sensitizers to the red or NIR region while maintaining efficient electron injection in the mesoscopic TiO₂ film and fast regeneration by the Cu^I complex. Herein, we report two new D-A-π-A-featured sensitizers termed HY63 and HY64, which employ benzothiadiazole (BT) or phenanthrene-fused-quinoxaline (PFQ), respectively, as the auxiliary electron-withdrawing acceptor moiety. Despite their very similar energy levels and absorption onsets, HY64-based DSSCs outperform their HY63 counterparts, achieving a power conversion efficiency (PCE) of 12.5 %. PFQ is superior to BT in reducing charge recombination resulting in the near-quantitative collection of photogenerated charge carriers.

Molecular Engineering of Simple Metal-Free Organic Dyes Derived from Triphenylamine for Dye-Sensitized Solar Cell Applications

Two new metal-free organic sensitizers, L156 and L224, were designed, synthesized, and characterized for application in dye-sensitized solar cells (DSCs). The structures of the dyes contain a triphenylamine (TPA) segment and 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid as electron-rich and -deficient moieties, respectively. Two different π bridges, thiophene and 4,8-bis(4-hexylphenyl)benzo[1,2-b:4,5-b']dithiophene, were used for L156 and L224, respectively. The influence of iodide/triiodide, [Co(bpy)₃ ]^2+/3+ (bpy=2,2'-bipyridine), and [Cu(tmby)₂ ]^2+/+ (tmby=4,4',6,6'-tetramethyl-2,2'-bipyridine) complexes as redox electrolytes and 18 NR-T and 30 NR-D transparent TiO₂ films on the DSC device performance was investigated. The L156-based DSC with [Cu(tmby)₂ ]^2+/+ complexes as the redox electrolyte resulted in the best performance of 9.26 % and a remarkably high open-circuit voltage value of 1.1 V (1.096 V), with a short-circuit current of 12.2 mA cm^-2 and a fill factor of 0.692, by using 30 NR-D TiO₂ films. An efficiency of up to 21.9 % was achieved under a 1000 lx indoor light source, which proved that dye L156 was also an excellent candidate for indoor applications. The maximal monochromatic incident-photon-to-current conversion efficiency of L156-30 NR-D reached up to 70 %.

Metal Coordination Complexes as Redox Mediators in Regenerative Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attracted a substantial interest in the last 30 years for the conversion of solar power to electricity. An important component is the redox mediator effecting the transport of charge between the photoelectrode and the dark counter electrode (CE). Among the possible mediators, metal coordination complexes play a prominent role and at present are incorporated in several types of devices with a power conversion efficiency exceeding 10%. The present review, after a brief introduction to the operation of DSSCs, discusses at first the requirements for a successful mediator. Subsequently, the properties of various classes of inorganic coordination complexes functioning as mediators relevant to DSSC operation are presented and the operational characteristics of DSSC devices analyzed. Particular emphasis is paid to the two main classes of efficient redox mediators, the coordination complexes of cobalt and copper; however other less efficient but promising classes of mediators, notably complexes of iron, nickel, manganese and vanadium, are also presented.