Defence of doctoral thesis: Qilun Zhang – Materials and interfaces for sustainable organic solar cells

Photovoltaics, the apparatus utilizing green solar power to generate electricity, is one of the efficient measures to the continuously increasing energy demand and exacerbated carbon emission of the human civilization. As a candidate with the potential of providing the cheapest and greenest form of electricity, organic solar cells (OSCs) received tremendous scientific interest, resulting in a significant power conversion efficiency (PCE) boosting to above 19% recently. Despite the impressive achievements in PCE, it’s alone not enough for the commercialization of OSCs. Sufficient lifetime and scalability at competitive cost are necessary as well. A better understanding of the interface energetic properties and materials electronic structures in the multilayer stacked OSCs are needed to improve the inherent instability of the devices. In addition, exploration of the sustainable and low-cost materials for OSCs are crucial. In this thesis, we carefully investigated dipole-induced energy level matching at the OSC interfaces, and the oxygen/water caused electronic structure evolution of the OSC materials via various of spectroscopic characterizations, and introduced natural wood-based materials to achieve highly efficient and stable OSCs.

We employed ultraviolet photoelectron spectroscopy (UPS) to investigate the interface energetic properties of a commercially available cathode interface layer (CIL) material polyvinylpyrrolidone (PVP) in OSCs and proposed a “double dipole” model to explain the work function modification properties of PVP on several substrates. Then we used the large-area compatible immersion method to obtain the ultrathin PVP layer on the ITO substrate, the fabricated OSCs have a comparable efficiency to the traditional Zinc Oxide (ZnO) CIL based devices. We further use photoelectron spectroscopy (PES) to investigate the electronic structures of advanced OSC materials, i.e., PM6 and Y6. To better understand the degradation mechanism caused by water and oxygen in these materials, the electronic structures of the materials were in-situ characterized in near-ambient pressure with controllable water and oxygen dosing. We carefully analyzed the evolution of the PES spectra during the water and oxygen dosing, and unveiled that oxygen affected backbone sulfur in PM6 and a weak interaction between cyano groups in Y6 with water. Furthermore, the enhanced stability of the Y6 was observed in the blend films as the electronic structures in the PES spectra, which matched the device results of PM6 and Y6 based OSCs that the blend photoactive layers show better stability in air atmosphere than bilayer.

Lastly, we presented the application feasibility of the natural wood-based materials in state of art OSCs to achieve better stability and lower cost. We firstly introduced an insulating polymer of natural betulin into the active layer, following the “filler strategy”, resulting in an improved open circuit voltage (V_oc) in donor-acceptor-insulator ternary OSCs. We attribute this improvement to the decreased trap-assisted recombination, however, we simultaneously found reduced charge collection in the devices caused by the penetration of the filler materials at the bottom, forming insulator interface to block the charge transfer. The present work expands the range of filler materials in OSCs to include biomass, with the aim of developing highly efficient, environmentally friendly, and cost-effective OSCs. We further extended the utilization of natural wood-based materials to cathode interface layer (CIL). Kraft lignin (KL), the most abundant natural source of aromatic material constituents, has potential compatibility to various of traditional CIL materials, owing to the chemical activity of phenolic functionalities. In this work, we successfully combined the traditional CIL materials, i.e., PFN-Br and bathocuproine (BCP), with large ratio (30%-50% in weight ratio) of industrial solvent fractionated KL, obtained binary CILs with tuneable WF. The binary CILs with suitable KL ratio worked well in OSCs, exhibited equivalent or even higher efficiency to the traditional CILs. In addition, the combination of KL and BCP significantly enhanced the stability of the devices, which mainly ascribed to the protection from KL to block the reaction between BCP and fused-ring electron acceptors.

The author hopes the findings in this thesis can contribute to the industrialization of OSCs, especially from the aspect of the sustainability, cost and stability.