Measuring and controlling exciton diffusion, charge generation and charge extraction in organic and hybrid semiconductors for photovoltaic applications
Date
06/2020Author
Supervisor
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Abstract
The growing demand for energy and the need for renewables as well as the advent of the internet of
things increase the demand for more versatile, efficient, cheap and environmentally friendly solar cells
based on organic and hybrid semiconductors. Understanding and controlling the underlying processes
that govern exciton diffusion, charge generation, charge extraction and domain size is of vital
importance to the efficiency, stability and scalability of these devices.
This thesis examines in detail the exciton and charge carrier behaviour in, and the characteristics of
new materials for organic and hybrid solar cell applications. A particular focus is put on measuring,
understanding and controlling hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ to hole extracting layers and
overcoming the trade-off between exciton harvesting and charge extraction in small molecule bulk
heterojunction organic solar cells. The main method used for this investigation was ultrafast timeresolved
spectroscopy,
specifically
ultrafast
optical
transient
absorption
and
ultrafast
time-resolved
fluorescence
decay
with a
streak
camera.
A study of hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ was carried out for two different hole extracting layers,
the standard PEDOT:PSS polymer used in the inverted p-i-n perovskite solar cells and a new nanoparticle NiO low temperature solution processed thin film. The two extraction layers and the
𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ perovskite active layer were first characterized using optical and physical methods such
as UV-Vis spectroscopy and atomic force microscopy as well as air photoemission spectroscopy to
confirm that the same perovskite was grown on top of both PEDOT:PSS and NiO and to investigate
energy level alignment. A new method based on the ultrafast photoluminescence surface quenching
experiment was developed and introduced which allows for the separation of bulk and interfacial
effects on charge extraction from thin films by illuminating the samples from opposite sides. This new
method was used to compare hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ to NiO and PEDOT:PSS. It was found
that NiO shows faster hole extraction from the 300 nm thick perovskite film than PEDOT:PSS on the
time scale of 300 ps, which is independent of charge carrier density in the region of 10¹⁶-10¹⁷ cm⁻³. The interface with PEDOT:PSS was found to severely limit charge extraction rate at charge densities
exceeding 10¹⁶ cm⁻³. Furthermore, the transfer rate was found to decrease with time and to be
dependent on charge density in the region 10¹⁶-10¹⁷ cm⁻³ which we interpreted as charge
accumulation. These findings were confirmed by transient absorption spectroscopy. Hole diffusion
coefficient 𝐷 = 2.2 cm²/s ± 0.4 cm²/s and quenching rate k=3.6 × 10⁵ m/s ±0.2 m/s were
determined in the perovskite film that were independent of charge density. This indicates a band-like
hole transport regime, not observed for solution processed CH₃NH₃PbI₃ films before. Our findings
stress the importance of interface optimization in devices based on perovskite active layers as even in the case of the superior quencher, NiO, there is still room for improvement of the interfacial transfer
rate.
The trade-off between exciton harvesting and charge extraction in small molecule bulk heterojunction
organic solar cells was tackled by employing a post processing method of solvent vapour annealing on
thin films of DR3TBDTT:PC₇₁BM and SMPV1:PC₇₁BM. It was found that as a result of annealing with
carbon disulfide, the UV-Vis absorption spectrum changes which indicates changes to the structure of
the film. It was further revealed, using exciton-exciton annihilation, that the exciton diffusion
coefficient and exciton diffusion length are increased (almost 3-fold for the best case) as a result of
solvent vapour annealing. Furthermore, enhanced device performance after treatment with carbon
disulfide was recorded; this is explained by better charge extraction, an insight revealed by a transient
absorption study. Finally, using an all optical method for domain size determination it was found that
solvent vapour annealing can be used to increase domain size. Normally increased domain size would
have a detrimental effect on device performance due to a loss in the number of excitons which can
reach the donor-acceptor interface, but the increased exciton diffusion length allows us to overcome
this trade-off and achieve better device performance.
Type
Thesis, PhD Doctor of Philosophy
Rights
Embargo Date: 2021-04-30
Embargo Reason: Thesis restricted in accordance with University regulations. Print and electronic copy restricted until 30th April 2021
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Measuring and controlling exciton diffusion, charge generation and charge extraction in organic and hybrid semiconductors for photovoltaic applications (Thesis data) Blaszczyk, O., University of St Andrews. DOI: https://doi.org/10.17630/2476208c-9e44-480f-b60c-9ec5ccd553bbRelated resources
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