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 adva...[ Read more ]

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 cm² 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 CH₃NH₃PbI₃ 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 (PbTiO₃) in increasing the V_OC and PCE hand in hand. An ultrathin layer (~ 1nm) of ferroelectric PbTiO₃ was grown on the TiO₂ scaffold by the in-situ reaction from TiO₂ 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 PbTiO₃ 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 TiO₂ 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 CsPbI₃ 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.