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Investigation of Bismuth Iodine as Light Absorbing Materials for Solar Cell Applications: From Synthesis to XPS Characterisation
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
2017 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

During the last years perovskite materials have taken the photovoltaic community by storm,

bringing promises of solar cells with efficiencies comparable to conventional silicon devices but

at a lower price. However perovskite solar cells so far are facing two main obstacles, they are

unstable in the presence of air, moisture and heat and they are usually toxic due to being

based on lead-halide materials. This has spurred investigations into alternative materials with

similar properties but without the mentioned drawbacks. Just next to Pb in the periodic

table is bismuth (Bi) with just one more electron in its outer-shell, Bi however is less toxic.

In this work the perovskite derived compounds of Ag-Bi-I and Cu-Bi-I are characterized and

their properties as light absorbing material in solar cell devices are investigated. Devices are

prepared by preparing Ag-Bi-I and Cu-Bi-I solutions which are then spin-coated on top of

a mesoporous TiO2. A conducting polymer, P3HT, was then deposited and serve as hole

transport material. For Ag-Bi-I, the molar ratios of AgI:BiI3= 1:2 and 2:1 were observed

with SEM to form homogeneous crystal films with one dominating crystal phase, which by

XRD could be determined to most likely have formed a cubic AgBi2I7 crystal structure for the

1:2 ratio and a hexagonal Ag2BiI5 crystal structure for the 2:1 ratio. The Cu-Bi-I materials

were not successfully synthesized to form homogeneous films with a dominating crystal phase,

although several molar ratios were investigated. All investigated compositions of both Cu and

Ag devices showed to in principle work as light absorbing materials, the best Ag-Bi-I device

showing a PCE of 1.92%, for the 2:1 ratio, while the Cu-Bi-I devices at best reached 0.32%

for a ratio of 1:1.

XPS measurements were carried out with a classical in-house XPS using an Al K X-ray source

of 1486.7 eV as well as at the Diamond Light Source (UK) synchrotron facility using photon

energies of 758 eV and 2200 eV so that a depth resolution of the composition could be observed.

Because of their inhomogeneous crystal formation, XPS couldn’t give much useful quantitative

information regarding the Cu devices. For Ag devices it was observed that the stoichiometry

at the extreme surface deviated from that predicted by XRD, but deeper into the surface the

relative ratio of elements approach the predicted ones, hinting towards a different structure

at the outermost surface or a lot of surface defects. For all samples, two types of bismuth

atoms were observed, metallic (Bi0) as well as a cationic (Bi+x), the later corresponding to

Bi atoms which are partaking in the crystal bond. The ratio of metallic to cationic Bi was

observed to decrease notably just a few nm below the extreme surface. The effect of the high

presence of metallic Bi on final device performance was not concluded with certainty but not

believed to be positive. By varying the annealing temperature, after spin coating the light

absorber solution on the TiO2, it was observed that lower temperature resulted in a lower

ratio of metallic Bi.

As final conclusions, it was said that the synthesis method of Cu-Bi-I needs to be improved

before those materials can be studied further. The synthesis of Ag-Bi-I is showing much more

promise and one can start looking into further optimizing their final device structure to boost

efficiency. Both Cu-Bi-I and Ag-Bi-I devices are relatively simple, cheap and energy efficient

(with annealing temperatures around 150C) to produce, great aspects for solar cells. UVVis

measurements showed they have band gaps around 1.6-1.7 eV which makes them a great

potential material for use in tandem solar cells together with a semiconductor of lower band

gap such as silicon.

Place, publisher, year, edition, pages
2017.
Series
FYSAST ; FYSMAS1063
Keyword [en]
Bismuth, Iodine, Light absorbing, Light absorbing materials, Solar, solar cell, Synthesis, XPS, Characterisation, synchrotron, energy, x-ray photoelectron spectroscopy, perovskite
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-331340OAI: oai:DiVA.org:uu-331340DiVA, id: diva2:1148925
Subject / course
Material
Educational program
Master Programme in Physics
Supervisors
Examiners
Available from: 2017-10-24 Created: 2017-10-12 Last updated: 2017-10-24Bibliographically approved

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