18 research outputs found
Valence and conduction band tuning in halide perovskites for solar cell applications
We performed density functional calculations aimed at identifying the atomistic and electronic structure origin of the valence and conduction band, and band gap tunability of halide perovskites ABX3 upon variations of the monovalent and bivalent cations A and B and the halide anion X. We found that the two key ingredients are the overlap between atomic orbitals of the bivalent cation and the halide anion, and the electronic charge on the metal center. In particular, lower gaps are associated with higher negative antibonding overlap of the states at the valence band maximum (VBM), and higher charge on the bivalent cation in the states at the conduction band minimum (CBM). Both VBM orbital overlap and CBM charge on the metal ion can be tuned over a wide range by changes in the chemical nature of A, B and X, as well as by variations of the crystal structure. On the basis of our results, we provide some practical rules to tune the valence band maximum, respectively the conduction band minimum, and thus the band gap in this class of materials
A theoretical perspective of the ultrafast transient absorption dynamics of CsPbBr3
Transient absorption spectra (TAS) of lead halide perovskites can provide important insights into the nature of the photoexcited state dynamics of this prototypical class of materials. Here, we perform ground and excited state molecular dynamics (MD) simulations within a restricted open shell Kohn-Sham (ROKS) approach in order to interpret the characteristic features of the TAS of CsPbBr3. Our results reveal that properties such as the finite temperature band gap, the Stokes shift, and therefore, also the TAS are strongly size-dependent. Our TAS simulations show an early positive red-shifted feature on the fs scale that can be explained by geometric relaxation in the excited state. As excited-state processes can crucially affect the electronic properties of this class of photoactive materials, our observations are an important ingredient for further optimization of lead halide based optoelectronic devices.LCB
Computational characterization of the dependence of halide perovskite effective masses on chemical composition and structure
Effective masses are calculated for a large variety of perovskites of
the form ABX
3
differing in chemical composition (A= Na, Li, Cs; B = Pb, Sn; X=
Cl, Br, I) and crystal structure. In addition, the effects of some defects and dopants
are assessed. We show that the effective masses are highly correlated with the
energies of the valence-band maximum, conduction-band minimum, and band gap.
Using the
k
·
p
theory for the bottom of the conduction band and a tight-binding
model for the top of the valence band, this trend can be rationalized in terms of the
orbital overlap between halide and metal (B cation). Most of the compounds
studied in this work are good charge-carrier transporters, where the effective masses
of the Pb compounds (0 <
m
h
*
<
m
e
*
< 1) are systematically larger than those of the
Sn-based compounds (0 <
m
h
*
â
m
e
*
< 0.5). The effective masses show anisotropies depending on the crystal symmetry of the
perovskite, whether orthorhombic, tetragonal, or cubic, with the highest anisotropy for the tetragonal phase (ca. 40%). In general,
the effective masses of the perovskites remain low for intrinsic or extrinsic defects, apart from some notable exceptions. Whereas
some dopants, such as Zn(II),
flatten the conduction-band edges (
m
e
*
= 1.7
m
0
) and introduce deep defect states, vacancies, more
specifically Pb
2+
vacancies, make the valence-band edge more shallow (
m
h
*
= 0.9
m
0
). From a device-performance point of view,
introducing modifications that increase the orbital overlap [e.g., more cubic structures, larger halides, smaller (larger) monovalent
cations in cubic (tetragonal/orthorhombic) structures] decreases the band gap and, with it, effective masses of the charge carriers
Effect of graphene oxide nanosheets on visible light-assisted antibacterial activity of vertically-aligned copper oxide nanowire arrays
Entropic stabilization of mixed A-cation ABX3 metal halide perovskites for high performance perovskite solar cells
ABX3-type organic lead halide perovskites currently attract broad attention as light harvesters for solar cells due to their high power conversion efficiency (PCE). Mixtures of formamidinium (FA) with methylammonium (MA) as A-cations show currently the best performance. Apart from offering better solar light harvesting in the near IR the addition of methylammonium stabilizes the perovskite phase of FAPbI3 which in pure form at room temperature converts to the yellow photovoltaically inactive d-phase. We observe a similar phenomenon upon adding Cs+ cations to FAPbI3. CsPbI3 and FAPbI3 both form the undesirable yellow phase under ambient condition while the mixture forms the desired black pervoskite. Solar cells employing the composition Cs0.2FA0.8PbI2.84Br0.16 yield high average PCEs of over 17% exhibiting negligible hysteresis and excellent long term stability in ambient air. We elucidate here this remarkable behavior using first principle computations. These show that the remarkable stabilization of the perovskite phase by mixing the A-cations stems from entropic gains and the small internal energy input required for the formation of their solid solution. By contrast, the energy of formation of the delta-phase containing mixed cations is too large to be compensated by this configurational entropy increase. Our calculations reveal for the first time the optoelectronic properties of such mixed A-cation perovskites and the underlying reasons for their excellent performance and high stability
Synthesis, characterization and ab initio investigation of a panchromatic ullazine-porphyrin photosensitizer for dye-sensitized solar cells.
An ullazine unit was employed as a donor moiety in a donorâÏâacceptor (DâÏâA) motif, employing the porphyrin macrocycle as a Ï-system. Synthesis of this ullazineâporphyrin dyad containing sensitizer (SM63) was achieved and an investigation of the electrochemical and spectroscopic properties of this dye was performed. Introduction of the ullazine donor promoted significant enhancements in long and visible wavelength absorption, leading to panchromatic light harvesting. SM63 demonstrated a maximum absorption approaching 750 nm, a significant improvement compared to the model compound LD14-C8, which features a simpler donor component (4-(N,N-dimethylamino)phenyl) and an absorption onset at âŒ700 nm. The dye SM63 was subjected to a rigorous ab initio investigation to gain further insight into its unique absorption and emission properties. Application of the molecular ullazineâporphyrin dyad SM63 into dye-sensitized solar cells afforded a device with significantly improved light harvesting abilities in both the visible region of the spectrum as well as NIR light (âŒ800 nm), demonstrating the value of ullazine unit in developing panchromatic dyes for light harvesting applications
RuddlesdenâPopper Phases of Methylammonium-Based Two-Dimensional Perovskites with 5-Ammonium Valeric Acid AVA2MAnâ1PbnI3n+1 with n = 1, 2, and 3
In Situ Mapping of the Molecular Arrangement of Amphiphilic Dye Molecules at the TiO<sub>2</sub> Surface of Dye-Sensitized Solar Cells
Amphiphilic
sensitizers are central to the function of dye-sensitized solar cells.
It is known that the cellâs performance depends on the molecular
arrangement and the density of the dye on the semiconductor surface,
but a molecular-level picture of the cellâelectrolyte interface
is still lacking. Here, we present subnanometer in situ atomic force
microscopy images of the Z907 dye at the surface of TiO<sub>2</sub> in a relevant liquid. Our results reveal changes in the conformation
and the lateral arrangement of the dye molecules, depending on their
average packing density on the surface. Complementary quantitative
measurements on the ensemble of the film are obtained by the quartz-crystal
microbalance with dissipation technique. An atomistic picture of the
dye coverage-dependent packing, the effectiveness of the hydrophobic
alkyl chains as blocking layer, and the solvent accessibility is obtained
from molecular dynamics simulations