High Performance and Stable Perovskite Solar Cells Based on Vertically Aligned Carbon Nanotube Arrays

Exploring clean and sustainable energy resources to meet the ever-increasing global energy demand becomes one of the biggest challenges in this century. This is due to the depletion of fossil fuels within the next 50 years and public concern on the environmental and climate change related to the consumption of fossil fuels. Solar energy is one of the most important renewable energy resources, due to its wide availability and low environmental impact. Photovoltaic (PV) solar cells that can directly convert photons into electricity present an ideal solution to harvest solar energy. A recent forecast predicts that solar PVs will contribute nearly a third of newly installed electricity generation capacity worldwide between now and 2030. Although crystalline silicon solar cells still dominate the PV market due to high module efficiency and mature techniques, they are still less competitive in cost to the traditional energy resources, which calls for the development of novel PV technologies with the highest performance and the lowest cost. Perovskite solar cells (PSCs) have emerged as a new class of thin film solar cells based on earth-abundant materials and cheap deposition techniques. The unexpected boosting of device performance in terms of power conversion efficiency (PCE) has rocketed up from an initial 3.8% to a certified 22.7% within a few years' research efforts, which is unprecedented in the history of PV technologies. Although PSCs are very promising to take a significant PV market share in the next few years, their commercialization is still hampered by the relatively poor material stability under ambient conditions. Moreover, the cost of solar power is determined not only by the PV modules themselves but also by the fixed costs of frames, inverters, installation and land, etc. Because the fixed costs are not reduced as fast as the cost of PV modules, the key route to continuously reduce the cost of solar powers is to enhance the absolute PCE of the PV modules, without overtly increasing their cost.

In this proposal, we aim to provide a solution to these challenges of large-scale deployment of PSCs, by further pushing the PCE of state-of-the-art PSCs toward their theoretical limit, and simultaneously improving their long-term stability. Our methodologies largely rely on the combination of new materials and innovation of device structure. In particular, we will employ carbon nanotube (CNT) arrays and fullerenes as the charge collection layers in a new device structure termed as "vertical heterojunction". This "full carbon" based PSCs are expected to exhibit improved PCE and stability beyond the-state-of-the-art devices. This is because both CNT arrays and fullerenes are good charge carrier conductors, and vertically aligned CNT arrays will further enhance the charge collection efficiency due to the direct charge transport pathways toward the conductive substrates and much larger contact areas between perovskite and CNTs. Another important innovation of this project is that the carbon nanomaterials work simultaneously as the encapsulating materials that protect perovskite from moisture and heat, so as to improve the device long-term stability without increasing production cost. This study will provide new insights into the development of novel interfacial materials and device structures towards more efficient and stable PSCs for their future commercialisation. Whilst this proposal primarily responds to calls within the PSC community for detailed investigations on device efficiency and stability, it naturally supports the domestic research based on solution-processed thin film PVs in general, thereby helping to maintain the U.K.'s leading position in advanced solar cell concepts and technology development.

The most immediate beneficiaries of the proposed research will be the research community working on emerging thin film photovoltaics, in particular perovskite solar cells (PSCs) and carbon nanotechnologies. This community, which comprises materials scientist, chemist, physicist and electrical engineer, spans both academia and industry. The proposed research will lead to a deeper understanding of limiting factors that restrict the PSC performance and stability, and the role of charge collection materials and nanostructures to enhance both. While the focus of the proposed research is on photovoltaic applications, the new material interfaces and new device structure to be developed may also be usefully exploited by researchers investigating other applications including light emitting diodes, lasers, photodetectors and optical sensors.

In a broad sense, the proposed research is associated with the exploitation of renewable energy resources, to drive the sustainable economic growth of societies, and mitigate the adverse effects of on environmental and climate change due to the consumption of fossil fuels. The economy will benefit not only with the supply chain to produce the solar PVs (from materials production, device fabrication, to encapsulation and final application), but also their deployment and management. Other industrial sectors such as ICT will also benefit from the proposed research, as lightweight PSCs fabricated on the flexible substrate will provide power to the sensors, communication, and man-machine interaction systems. This will also enable remote and wireless sensors powered by PSCs in the future.

The impact of the proposed research will be fostered by the participation of world-leading industrial partners in the project (Canadian Solar and Yingli Solar) after IP protection by the project team, the academic collaboration with researchers in the Surrey and overseas, and through the connection to the Supergen Supersolar Hub (the biggest solar PV network in UK). In order to maximise the impact of the findings, key outcomes will be publicised through journal publications and conference presentations, advertised on department websites, and facilitated by the University Communications and Marketing Services. To enhance the public engagement in particular young people, outreach activities such as school visits, talks on clean energy and PV technology at Surrey Research Festival will also be included as part of this proposal. These activities are vitally important to educate young people who have interest in the physical science and engineering.