THESIS
2020
xvii, 110 pages : illustrations ; 30 cm
Abstract
Recent developments in organic solar cells (OSCs) have achieved over 17% power
conversion efficiency (PCE) under AM1.5G conditions. However, OSCs still suffers from
voltage loss and instability compare to inorganic solar cells as well as perovskite solar cell
counterparts. Recently, an organic solar cell with a small driving force has been reported. The
small driving force minimizes the voltage loss and achieves high open-circuit voltage (Voc)
~1.1V. However, the physics behind the OSCs still not clear. In this thesis, I will focus on
studying the charge transport mechanism of small driving force OSCs, the impact of charge
transport interlayers on device performance under indoor conditions, and the origin of the good
performance of thick film non-fullerene-based OSCs under indoo...[
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Recent developments in organic solar cells (OSCs) have achieved over 17% power
conversion efficiency (PCE) under AM1.5G conditions. However, OSCs still suffers from
voltage loss and instability compare to inorganic solar cells as well as perovskite solar cell
counterparts. Recently, an organic solar cell with a small driving force has been reported. The
small driving force minimizes the voltage loss and achieves high open-circuit voltage (Voc)
~1.1V. However, the physics behind the OSCs still not clear. In this thesis, I will focus on
studying the charge transport mechanism of small driving force OSCs, the impact of charge
transport interlayers on device performance under indoor conditions, and the origin of the good
performance of thick film non-fullerene-based OSCs under indoor conditions.
In chapter 2, the charge transport property of two small driving force system bulk
heterojunction (BHJ) OSCs is studied. In our study, we observed a surprisingly large increase
(up to 30x) of electron mobility in an OSC blend when introducing polymer donor into small
molecular acceptor. By ruling out the morphology reasons, we show that the donor polymer
can assist the electron transport by providing “bridges” or “shortcut” for electron transport
across the domains of small molecular acceptors and thus facilitates the overall electron
transport in the blend. This can happen because for these systems the LUMO offset is small.
Our study shows the benefits of donor assisting electron transport in BHJ systems with small
energetic offsets.
Chapter 3 and 4 concentrate on the performance of OSCs under indoor conditions. In
chapter 3, the small driving OSCs and a new BHJ system are studied under AM 1.5G and
indoor conditions. We successfully demonstrated a high-efficiency non-fullerene organic
photovoltaic (OPV) cells with over 30% power conversion efficiency (PCE) under indoor
conditions. Our results show that the choice of electron transporting layer (ETL) is critically
important to enable such performance. The use of an ETL (named PDI-NO) with a deep highest
occupied molecular orbital (HOMO) level can effectively suppress leakage current and reduce
trap assisted recombination of the devices. When applying this ETL to a bulk-heterojunction
(BHJ) blend based on a low-bandgap, acceptor molecule, we achieve a PCE of 31%, which is
the best indoor OPV efficiency reported to date. We also show that this ETL can be applied to
another BHJ system based on a wide bandgap PDI acceptor, which also leads to efficient indoor
OPV devices with a PCE of 26% and a high open-circuit voltage (V
oc) of over 1V. Our study
paves the way towards high-performance indoor OPV devices for powering IoT electronics.
Chapter 4 is based on the findings in Chapter 2 and 3. In this chapter, a non-fullerene based
OSC under indoor conditions can achieve over 20% of PCE even increase the active layer
thickness to 200nm. We find that it is attributed to 1.) the small LUMO offset BHJ systems
(chapter 2), where the donor polymer can assist electrons to hop back and forth between the
donor and acceptor domains, 2.) the deep HOMO level ETL, which can suppress the leakage
current, and 3.) low energetic disorder and Urbach energy. Therefore, the devices can still
maintain good performance under the indoor conditions (chapter 3).
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