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

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

Enhanced Carrier Transport Distance in Colloidal PbS Quantum-Dot-Based Solar Cells Using ZnO Nanowires

Nanostructured solar cells are a promising area of research for the production of low cost devices that may eventually be capable of complementing or even replacing present technologies in the field of solar power generation. The use of quantum dots (QDs) in solar cells has evolved from being simple absorbers in dye-sensitized solar cells to sustaining the double functions of absorbers and carrier transporters in full solid state devices. In this work, we use both optical and electrical measurements to explore the diffusion limitations of carrier transport in cells made of a heterostructure combining lead sulfide (PbS) QDs as absorbers and hole carrier and zinc oxide nanowires as electron carrier material. The results show efficient charge collection along the PbS-QD/ZnO nanowire (NW) hybrid structure. This is because of the formation of band bending in the ZnO collector, allowing efficient charge separation and spatially well-separated carrier pathways, yielding a hole transportation of over 1 μm. We have also found a limitation in open-circuit voltage (<i>V</i>_oc) associated with band bending in the ZnO collector

Kinetics versus Energetics in Dye-Sensitized Solar Cells Based on an Ethynyl-Linked Porphyrin Heterodimer

Out of the scientific concern on the kinetics versus energetics for rational understanding and optimization of near-IR dye-sensitized solar cells (DSCs), an <i>N</i>-fused carbazole-substituted ethynyl-linked porphyrin heterodimer (<b>DTBC</b>) reported previously by our group was focused upon in terms of photovoltaic, photoelectrochemical, and steady-state and time-resolved photophysical properties in varied electrolyte environments. A primitive attempt to balance the photocurrent against the photovoltage by varying the concentration of the common coadsorbent 4-<i>tert</i>-butylpyridine (TBP) revealed that TBP continuously suppressed injection but provided inadequate compensation in open-circuit voltage (<i>V</i>_oc). This further drew out the perspective of the widely ignored dye–electrolyte interaction in DSCs, specifically the axial coordination of TBP to the central zinc cation in porphyrin sensitizers that may retard electron injection. As an alternative, a TBP-free electrolyte containing guanidinium thiocyanate was developed to realize greatly promoted <i>V</i>_oc with little current sacrifice, thus significantly enhancing overall energy conversion efficiencies. The excited state was protracted to counteract the injection retardation caused by much reduced driving force, setting a successful example of bilateral compromise between kinetics and energetics in near-IR DSCs

Photosensitized Protein-Damaging Activity, Cytotoxicity, and Antitumor Effects of P(V)porphyrins Using Long-Wavelength Visible Light through Electron Transfer

Photodynamic therapy (PDT) is a less-invasive treatment for cancer through the administration of less-toxic porphyrins and visible-light irradiation. Photosensitized damage of biomacromolecules through singlet oxygen (¹O₂) generation induces cancer cell death. However, a large quantity of porphyrin photosensitizer is required, and the treatment effect is restricted under a hypoxic cellular condition. Here we report the phototoxic activity of P­(V)­porphyrins: dichloroP­(V)­tetrakis­(4-methoxyphenyl)­porphyrin (CLP­(V)­TMPP), dimethoxyP­(V)­tetrakis­(4-methoxyphenyl)­porphyrin (MEP­(V)­TMPP), and diethyleneglycoxyP­(V)­tetrakis­(4-methoxyphenyl)­porphyrin (EGP­(V)­TMPP). These P­(V)­porphyrins damaged the tryptophan residue of human serum albumin (HSA) under the irradiation of long-wavelength visible light (>630 nm). This protein photodamage was barely inhibited by sodium azide, a quencher of ¹O₂. Fluorescence lifetimes of P­(V)­porphyrins with or without HSA and their redox potentials supported the electron-transfer-mediated oxidation of protein. The photocytotoxicity of these P­(V)­porphyrins to HeLa cells was also demonstrated. CLP­(V)­TMPP did not exhibit photocytotoxicity to HaCaT, a cultured human skin cell, and MEP­(V)­TMPP and EGP­(V)­TMPP did; however, cellular DNA damage was barely observed. In addition, a significant PDT effect of these P­(V) porphyrins on a mouse tumor model comparable with the traditional photosensitizer was also demonstrated. These findings suggest the cancer selectivity of these P­(V)­porphyrins and lower carcinogenic risk to normal cells. Electron-transfer-mediated oxidation of biomacromolecules by P­(V)­porphyrins using long-wavelength visible light should be advantageous for PDT of hypoxic tumor