Solution-Processed Short-Wave Infrared PbS Colloidal Quantum Dot/ZnO Nanowire Solar Cells Giving High Open-Circuit Voltage

A systematic investigation into the performance of PbS quantum dot (QD)/ZnO nanowire (NW) solar cells in the near-infrared (NIR) and short-wave infrared (SWIR) regions was carried out. The solar cells were confirmed to convert a wide range of solar energy (3.54–0.62 eV, corresponding to 0.35–2.0 μm). We found that the solar cells working in the SWIR region had a high open-circuit voltage (<i>V</i>_oc). A relatively high <i>V</i>_oc of 0.25 V was achieved even in solar cells whose photocurrent onsets were at approximately 0.64 eV (1.9 μm); this <i>V</i>_oc is as high as that of Ge solar cells, which have been used for III–V compound semiconductor triple-junction solar cells. Although short-circuit current density and fill factor have to be further increased, these results indicate that solution-processed colloidal QD solar cells with ZnO NWs are promising candidates for the middle and/or bottom subcells of multijunction solar cells

Theoretical Study of the Surface Complex between TiO₂ and TCNQ Showing Interfacial Charge-Transfer Transitions

The surface complex of TiO₂ nanoparticles and TCNQ was studied using density functional theory (DFT) calculations. The structure of the surface complex was optimized, showing an IR spectrum analogous to the experimental spectrum. From time-dependent DFT calculations based on this optimized structure, we demonstrated that the interfacial charge-transfer transitions from the HOMO of the surface-bound TCNQ molecule to the unoccupied levels of the TiO₂ nanocluster occur in the visible to near-IR region

Effects of Chain Orientation in Self-Organized Buffer Layers Based on Poly(3-alkylthiophene)s for Organic Photovoltaics

Surface-segregated monolayers (SSMs) based on two poly­(3-alkylthiophene)­s with semifluoroalkyl groups at either the side chains (P3DDFT) or one end of the main chain (P3BT-F₁₇) were used as self-organized buffer layers at the electrode interfaces in bulk heterojunction (BHJ) organic photovoltaic devices. Both of the SSMs greatly shifted the vacuum levels of the BHJ films at the surface due to the aligned permanent dipole moments of the semifluoroalkyl chains. Hole extraction in the BHJ of poly­(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) became more efficient in the presence of the P3DDFT buffer layer, resulting in an improved power conversion efficiency. In contrast, the SSM of P3BT-F₁₇ induced changes in the chain orientation of P3HT and the morphology of the BHJ films, resulting in decreased performance. These results indicate that the molecular design of polymer-based SSMs can affect not only the energy structure at the interface but also the morphology and the molecular orientations in the BHJs

PbS-Quantum-Dot-Based Heterojunction Solar Cells Utilizing ZnO Nanowires for High External Quantum Efficiency in the Near-Infrared Region

The improvement of solar cell performance in the near-infrared (near-IR) region is an important challenge to increase power conversion efficiency under one-sun illumination. PbS quantum-dot (QD)-based heterojunction solar cells with high efficiency in the near-IR region were constructed by combining ZnO nanowire arrays with PbS QDs, which give a first exciton absorption band centering at wavelengths longer than 1 μm. The morphology of ZnO nanowire arrays was systematically investigated to achieve high light-harvesting efficiency as well as efficient carrier collection. The solar cells with the PbS QD/ZnO nanowire structures made up of densely grown thin ZnO nanowires about 1.2 μm long yielded a maximum incident-photon-to-current conversion efficiency (IPCE) of 58% in the near-IR region (@1020 nm) and over 80% in the visible region (shorter than 670 nm). The power conversion efficiency obtained on the solar cell reached about 6.0% under simulated one-sun illumination

Enhancement of Near-IR Photoelectric Conversion in Dye-Sensitized Solar Cells Using an Osmium Sensitizer with Strong Spin-Forbidden Transition

A new osmium (Os) complex of the [Os­(tcterpy)-(4,4′-bis­(<i>p</i>-butoxystyryl)-2,2′-bipyridine)­Cl]­PF₆ (Os-stbpy) has been synthesized and characterized for dye-sensitized solar cells (DSSCs). The Os-stbpy dye shows enhanced spin-forbidden absorptions around 900 nm. The DSSCs with Os-stbpy show a wide-band spectral response up to 1100 nm with high overall conversion efficiency of 6.1% under standard solar illumination

Nanostructured Two-Component Liquid-Crystalline Electrolytes for High-Temperature Dye-Sensitized Solar Cells

Nanostructured liquid-crystalline (LC) ion transporters have been developed and applied as new electrolytes for dye-sensitized solar cells (DSSCs). The new electrolytes are two-component liquid crystals consisting of a carbonate-based mesogen and an ionic liquid that self-assemble into two-dimensional (2D) nanosegregated structures forming well-defined ionic pathways suitable for the I^–/I₃^– redox couple transportation. These electrolytes are nonvolatile and they show LC phases over wide temperature ranges. The DSSCs containing these electrolytes exhibit exceptional open-circuit voltages (<i>V</i>_oc) and improved power conversion efficiencies with increasing temperature. Remarkably, these solar cells operate at temperatures up to 120 °C, which is, to the best of our knowledge, the highest working temperature reported for a DSSC. The nature of the LC electrolyte and the interactions at the TiO₂ electrode/electrolyte interface lead to a partial suppression of electron recombination reactions, which is key in the exceptional features of these LC-DSSCs. Thus, this type of solar cells are of interest, because they can produce electricity efficiently from light at elevated temperatures

Widely Controllable Electronic Energy Structure of ZnSe–AgInSe₂ Solid Solution Nanocrystals for Quantum-Dot-Sensitized Solar Cells

I–III–VI₂-semiconductor-based nanocrystals of ZnSe–AgInSe₂ solid solution ((AgIn)_<i>x</i>Zn_{2(1‑<i>x</i>)}Se₂, ZAISe) with average sizes of 3.5–6.2 nm were successfully synthesized through thermal reaction of corresponding metal acetates and selenourea in a hot oleylamine solution. The optical property of ZAISe solid solution nanocrystals was tunable in a broad wavelength region from visible to near-infrared light by changing the composition of solid solution, where the energy gap of ZAISe nanocrystals was enlarged from 1.44 to 3.00 eV with an increase in the fraction of ZnSe in ZAISe, that is, with a decrease in <i>x</i> from 1.0 to 0. Both levels of conduction band and valence band edges, determined by photoelectron spectroscopy in air, were monotonously shifted to higher levels with an increase in the fraction of ZnSe. Quantum-dot-sensitized solar cells were fabricated with porous TiO₂ film electrodes immobilized with ZAISe nanocrystals using 3-mercaptopropionic acid as a cross-linking agent. The light conversion efficiency of the thus-obtained cells was enhanced by covering ZAISe nanocrystals with a CdS thin layer by the SILAR method. The photocurrent action spectra agreed well with absorption spectra of ZAISe nanocrystals immobilized on TiO₂ electrodes. Maximum energy conversion efficiency of 1.9% was obtained for the cell fabricated with ZAISe nanocrystals with <i>x</i> = 0.5 as a sensitizer under irradiation with simulated solar light of AM 1.5G

Phase Control of Organometal Halide Perovskites for Development of Highly Efficient Solar Cells

To develop a highly efficient solar cell using organometal halide perovskites, its microscale structure control is one of the most important factors because the microstructural defects inside the organometal halide perovskite are harmful to charge carrier flow and, thus, degrade device performance. In this study, we confirmed the existence of large physical gaps at the grain boundary in a methylammonium iodide (MAPbI3, MA = CH3NH3) perovskite with transmission electron microscopy (TEM) analysis and revealed that the physical gap prevents charge carrier flow in the MAPbI3 perovskite. To minimize the physical gap and its negative influences, the grain size of the MAPbI3 perovskite was optimized by increasing the portion of the cubic phase via microstructural phase control using liquid nitrogen (LN2). Through microstructural phase control of the MAPbI3 perovskite, its grain boundaries and physical gap were significantly decreased, and 20.23% power conversion efficiency (PCE) was achieved with a single cation MAPbI3 perovskite solar cell

Adjustment of Conduction Band Edge of Compact TiO₂ Layer in Perovskite Solar Cells Through TiCl₄ Treatment

Perovskite solar cells (PSCs) without a mesoporous TiO₂ layer, that is, planar-type PSCs exhibit poorer cell performance as compared to PSCs with a porous TiO₂ layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO₂ layer in the former case. The matching of the conduction band levels of perovskite and the compact TiO₂ layer is thus essential for enhancing PSC performance. In this study, we demonstrate the shifting of the conduction band edge (CBE) of the compact TiO₂ layer through a TiCl₄ treatment, with the aim of improving PSC performance. The CBE of the compact TiO₂ layer was shifted to a higher level through the TiCl₄ treatment and then shifted in the opposite direction, that is, to a lower level, through a subsequent heat treatment. These shifts in the CBE were reflected in the PSC performance. The TiCl₄-treated PSC showed an increase in the open-circuit voltage of more than 150 mV, as well as a decrease of 100 mV after being heated at 450 °C. On the other hand, the short-circuit current decreased after the treatment but increased after heating at temperatures higher than 300 °C. The treated PSC subjected to subsequent heating at 300 °C exhibited the best performance, with the power conversion efficiency of the PSC being 17% under optimized conditions

Liquid-Crystalline Dye-Sensitized Solar Cells: Design of Two-Dimensional Molecular Assemblies for Efficient Ion Transport and Thermal Stability

Nanostructured liquid-crystalline (LC) electrolytes have been developed for efficient and stable <i>quasi</i>-solid-state dye-sensitized solar cells (DSSCs). Two types of ionic LC assemblies for electrolytes have been designed: (i) noncovalent assemblies of two-component mixtures consisting of I₂-doped imidazolium ionic liquids and carbonate-terminated mesogenic compounds (noncovalent type) and (ii) single-component mesogenic compounds covalently bonding an imidazolium moiety doped with I₂ (covalent type). These mesogenic compounds are designed with flexible oligooxyethylene spacers connecting the mesogenic and the polar moieties. The oligooxyethylene-based material design inhibits crystallization and leads to enhanced ion transport as compared to alkyl-linked analogues due to the higher flexibility of the oligooxyethylene spacer. The noncovalent type mixtures exhibit a more than 10 times higher I₃^– diffusion coefficient compared to the covalent type assemblies. DSSCs containing the noncovalent type liquid crystals show power conversion efficiencies (PCEs) of up to 5.8 ± 0.2% at 30 °C and 0.9 ± 0.1% at 120 °C. In contrast, solar cells containing the covalent type electrolytes show significant increase in PCE up to 2.4 ± 0.1% at 120 °C and show superior performance to the noncovalent type-based devices at temperature above 90 °C. Furthermore, the LC-DSSCs exhibit excellent long-term stability over 1000 h. These novel electrolyte designs open unexplored paths for the development of DSSCs capable of efficient conversion of light to electricity in a wide range of temperatures