Low-energy, dark excitonic states have recently been predicted to lie below the first bright (E11) exciton in semiconducting single-walled carbon nanotubes Phys. Rev. Lett. 93, 157402 ...(2004)10.1103/PhysRevLett.93.157402. Decay into such deep excitonic states is implicated as a mechanism which reduces photoluminescence quantum yields. In this study we report the first direct observation of deep excitons in SWNTs. Photoluminescence (PL) microscopy of suspended semiconducting single-walled carbon nanotubes (SWNTs) reveals weak emission satellites redshifted by approximately 38-45 and approximately 100-130 meV relative to the main E11 PL emission peaks. Similar satellites, redshifted by 95-145 meV depending on nanotube species, were also found in PL measurements of ensembles of SWNTs in water-surfactant dispersions. The relative intensities of these deep exciton emission features depend on the nanotube surroundings.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Photoluminescence spectra of water−surfactant dispersions of semiconducting single-walled carbon nanotubes (SWNTs) show large shifts of interband transition energies upon freezing and cooling the ...dispersions to 16 K. This is accompanied by an increase of the emission intensities up to ∼10 times in the presence of poly(vinylpyrrolidone). The shifts develop mainly in the temperature interval of ∼100−240 K and are reversible by cycling the temperature. Two groups of nanotubes classified by the value of (n-m) mod 3, where n,m are structure indices, demonstrate opposite shifts, the largest ones from nanotubes with small chiral angles. The experimental data agree well with calculations of Yang et al. Phys. Rev. B 1999, 60, 13874 for SWNTs under axial compression and indicate that large stresses of up to ∼5 GPa are generated in individual nanotubes by thermal contraction of the ice matrix.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
New processing developments in the Cu(In,Ga)Se 2 (CIGS)-based solar cell technology have enabled best cell efficiencies to exceed 21%. The key innovation involves the alkali post-deposition treatment ...(PDT) of the CIGS film. Furthermore, the range of optimal CIGS growth parameters and the minimal thickness of the CdS buffer layer is affected by the process modifications. In 2013, we reported a 20.8% record device with PDT. Later optimizations, e.g., in the composition profile and CdS buffer layer thickness as discussed in this study, enabled us to increase the photocurrent density with only a slight loss in open-circuit voltage and unchanged fill factor, resulting in the current world record of 21.7% efficiency. Furthermore, a record efficiency of 21.0% could be achieved with a Cd-free Zn(O,S) buffer layer. This contribution presents measurements, simulations, and a discussion of the photocurrent increase.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
10.
High-efficiency Cu(In,Ga)Se2 solar cells Friedlmeier, Theresa Magorian; Jackson, Philip; Bauer, Andreas ...
Thin solid films,
07/2017, Volume:
633
Journal Article
Peer reviewed
Recent progress in the development of high-efficiency solar cells based on Cu(In,Ga)Se2 at our institute is reviewed. The post-deposition treatment with alkali elements (PDT) has been found to ...improve device quality, mostly through reduced recombination, with the effect of increasing the open-circuit voltage. At the same time, PDT is shown to improve initial growth properties of the chemical-bath-deposited buffer layer, so that this layer may be made thinner and the corresponding losses are reduced. Further optimization by replacing the non-doped ZnO resistive layer by (Zn,Mg)O enables gains in photocurrent for the ultraviolet region. A certified efficiency of 22.0% with a CdS/(Zn,Mg)O buffer/resistive layer combination is presented here. Furthermore, the effect of band gap grading with gallium in the absorber layer is explored with respect to the buffer layer properties by means of device simulation.
•Best Cu(In,Ga)Se2-based solar cell efficiencies have surpassed 22%.•Key developments are briefly reviewed.•Developments in buffer/resistive layer combinations are presented.•Ga gradient profiles are investigated via device simulation with SCAPS.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
