Over the last decade, perovskite solar cells (PSCs) have emerged as one of the most
promising new photovoltaic technologies in terms of the rapidly rising efficiency and low-cost
fabrication processes. The new record of certified power conversion efficiency (PCE) has
reached 24.2% in 2019. However, the high-performance PSCs are seriously questioned on the
stability issues. The use of vacuum deposition process of noble metal electrodes and
employment of the expensive hole transport material (HTM) are high-cost and high energy
consuming methods, also limiting their practical applications. In this respect, the HTM-free
carbon-based PSCs (C-PSCs) are believed to be the front runner towards commercialization
among all types of PSCs because of the long-term stability and low-cost advantages. However,
the efficiency of C-PSCs is much lower and increases slower than conventional PSCs, steadily
detracting their competitiveness among various kinds of PSCs. The issues in C-PSCs include
the poor perovskite/carbon contact, weak hole selectivity, large V
OC loss and trap-induced
charge recombination, etc. They are undermining the photovoltaic performance of C-PSCs and
are urgent to be solved. In my thesis research, I focused on the understanding and designing of C-PSCs, and developed several effective strategies to improve the performance and push the
development of C-PSCs.
Firstly, to improve the quality of perovskite/carbon contact, I developed a new deposition
method named the ultrasound-spray deposition for carbon electrode (CNTs) with the embedded
structured C-PSCs. This method employs pure CNT ink as carbon source, the naturally aligned
CNT bundles by weak Van der Waals forces can collapse into individual nanotubes with the
help of ultrasonic sound power and high surface tension of solvent, which ensures the
smoothness and compactness of the CNT electrode. This deposition method achieved a high-quality
carbon electrode and seamless contact at perovskite/carbon interface, guaranteed the
hole collection efficiency, thus boosting the PCE to 14.07%. More excitingly, the superiority of
ultrasound-spray deposition method could favor the fabrication of large-scale C-PSCs, PCE as
high as 10.33% in a larger active area of 1 cm
2 was achieved.
Secondly, to enhance the hole selectivity at perovskite/carbon interface in C-PSCs, I
developed a partially embedded perovskite film with nickel oxide nanoparticles (NiO NPs) for
CNT embedded structured PSCs. Detailed characterizations revealed that by planting NiO NPs
into the surface region of CH
3NH
3PbI
3 crystals, the hole extraction efficiency was effectively
enhanced, and interfacial recombination was reduced. The NiO NPs interlayer was found to
favorably bend the energy levels at the interface for selective hole extraction. Finally, this cell
achieved a PCE as high as 15.8%, which is among the highest efficiencies in C-PSCs reported
to date.
Thirdly, to reduce the V
OC loss coming from the missing of HTM in C-PSCs, I reported
a novel strategy based on the ferroelectric perovskite oxide (PbTiO
3) in increasing the V
OC and
PCE hand in hand. An ultrathin layer (~ 1nm) of ferroelectric PbTiO
3 was grown on the TiO
2
scaffold by the in-situ reaction from TiO
2 and lead precursor. This ferroelectric layer in the PN
junction could unambiguously introduce a larger internal electrical field provided by the
permanent electrical polarization, enhancing the built-in potential and improving the V
OC. In
this PbTiO
3 based device, the built-in potential was found to have an increase of 0.05 V, with
the quasi fermi level E
fn in the TiO
2 side lifted up. This strategy further pushes the PCE to 16.3% with a higher V
OC, which is the best recorded efficiency for HTM-free C-PSCs to date. This
work represents a significant step forward in lifting up the currently stagnant efficiency record
of C-PSCs, and will give impetus to push the low-cost and high-performance perovskite solar
cells towards commercialization. It also shed insight into the role of spontaneous polarization
in photovoltaic conversion.
Finally, to suppress the trap-induced charge recombination on perovskite in C-PSCs, I
designed and fabricated a polyethyleneimine functionalized carbon nanotubes (PEI-CNTs) as
the bifunctional interface bridge for all inorganic C-PSCs. Systematic investigations revealed
that the PEI/CNT bridges not only improved the interface charge transport efficiency, but also
reduced the trap states at the perovskite surface due to the coordination ability of the amine
groups and the multifunctionality of the CNTs. This effort led to a champion PCE of 10.58% of
the CsPbI
3 based C-PSCs, with a much higher FF of 0.71 than that without the PEI/CNT bridge.
Moreover, the non-encapsulated PEI/CNT based devices showed higher stability than those
without the PEI/CNT bridge.
In my thesis, different strategies including effective carbon electrode deposition, interface
design and modification have been developed to enhance the photovoltaic performance in C-PSCs.
Chemical and material mechanisms associated with the device performance have been
systematically investigated, and the efficiency of C-PSCs has been improved. This study reveals
the great potential of C-PSCs towards commercialization in the photovoltaic market in the
future.
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