Lithium ion batteries have reshaped our life with their omnipresence in portable electronics. However, increasing the specific energy of these batteries is reaching its limit and high-profile fire accidents (e.g. cell phones spontaneously combusting) cast doubt of their applications in electric vehicles and large-scale energy storage. Intrinsically safe batteries such as aqueous batteries and all-solid-state batteries are being actively studied in the battery community but also faced with several challenges. In this paper, we review our recent progress on the electrolyte-dictated materials design of organic redox materials as potent enablers for aqueous and solid-state electrolytes/batteries.
State of art III-V multi-junction solar cells have demonstrated a record high efficiency of 43.5%. However, these cells
are only applicable to high concentration systems due to their high cost of substrates and epitaxial growth. We
demonstrate thin film flexible nanostructure arrays for III-V solar cell applications. Such nanostructure arrays allow
substrate recycling and much thinner epitaxial layer thus could significantly reduce the cost of traditional III-V solar
cells. We fabricate the GaAs thin film nanostructure arrays by conformally growing GaAs thin film on nanostructured
template followed by epitaxial lift-off. We demonstrate broadband optical absorption enhancement of a film of GaAs
nanostructure arrays over a planar thin film with equal thickness. The absorption enhancement is about 300% at long
wavelengths due to significant light trapping effect and about 30% at short wavelengths due to antireflection effect from
tapered geometry. Optical simulation shows the physical mechanisms of the absorption enhancement. Using thin film
nanostructure arrays, the III-V solar system cost could be greatly reduced, leading to low $/W and high kW/kg flexible
solar systems.
The effect of side-chains on the molecular weight and the optical and electrical property of a low band gap copolymer
poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5',5"-diyl} (PF-co-DTB)
was studied. The decyloxy side-chains help to increase molecular weight (Mw = 115,000) and decrease the band gap
(1.78 eV) as well as the oxidation potential (-5.4 eV). Zero-field mobility of 2×10-5 cm2/Vs is measured in hole-only devices. Photovoltaic devices based on PF-co-DTB/fullerene bulk-heterojunction show power conversion efficiency of
up to 1.6% under air mass 1.5G, 100 mW/cm2 illumination. Side-chains effect on the photovoltaic devices studies show
the trade-off between short circuit current increase and open-circuit voltage drop. Thermal annealing on device
performance is also discussed.
To increase the absorption of sunlight in polymer solar cells a large active layer thickness is desired. This, however, is
limited by the short charge carrier diffusion lengths in the active organic materials. Efficient light harvesting can be
achieved in organic solar cells by using a tandem structure. However, fabricating a tandem structure for polymer solar
cells presents its own difficulties. Since the polymer film is solution processed, spin-coating multiple layers in tandem
can result in significant damage to the underlying layers. This problem can be overcome by fabricating separate PV cells
and stacking them in tandem. Here, we report a multiple-device stacked structure where two polymer photovoltaic cells
are stacked together with the help of a multi-layer semi-transparent electrode, made of lithium fluoride (LiF) / aluminum
(Al) / gold (Au) metal layers. The semi-transparent electrode is used as the top contact in the bottom cell to efficiently
transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent
cathode. In the stacked structure, the open circuit voltage and the short circuit current are twice those of a
single cell. As a result, power conversion efficiency of up to 2.6% is achieved, which is double than that of a single cell.
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