Molecularly Engineered Functional Materials for High Performance Perovskite Solar Cells

Over the last few years, thanks to rapid development of molecular engineering and perovskite composition, it has allowed extremely promising solar energy conversion to achieve a certified power conversion efficiency (PCE) of over 25% for perovskite solar cells (PSCs). However, there is always room for improvement in research for the long-term stability and high cost of PSCs, which are the major obstacles to the commercialization process, which can also be caused by organic materials and dopants. Thus, the work presented in this thesis is focused on synthesis, profound characterization and use of novel organic functional materials for PSCs. In addition, I aim to find synthetic guidelines that show how thoughtful molecular design, such as different atoms in molecular structure, different functional groups can have different effects on photovoltaic performance and long-term stability of PSCs.In this work, I introduced three novel hole-transporting materials (HTMs) with donor-bridge-acceptor design principle. Triazatruxene-based donor units were functionalized with terthiophene conjugated arms, then differentiated into CI-B1, CI-B2 and CI-B3 HTMs by linking strong electron-accepting units. All three HTMs showed intermolecular aggregation, especially CI-B3 molecules are packed in favor of well-ordered edge-on stacking. PSCs with triple cation perovskite in n-i-p architecture with dopant-free HTM CI-B3 showed an impressive PCE of 17.54% with a small hysteresis and improved long-term stability.Following up this work, the novel dopant-free HTM, CI-TTIN-2F, based on donor-bridge-acceptor, have been demonstrated for all-inorganic CsPbI3 PSCs. CI-TTIN-2F features intermolecular aggregation formed both face-on and edge-on orientation. Molecular dynamics (MD) calculations revealed the passivation effect of CI-TTIN-2F with Pb on the perovskite surface with different contact types. All-inorganic CsPbI3 PSCs with dopant-free CI-TTIN-2F HTM demonstrate a high PCE of 15.9%, along with 86% efficiency retention after 1000 hours. Moreover, the largest all-inorganic perovskite solar module was fabricated using the CI-TTIN-2F HTM, and exhibited PCE of 11.0% with an area of 27 cm2.As the next part of our work, three benzodipyrrole-based organic small-molecules functionalized with 4-methoxyphenyl (CB-1), 3-fluorophenyl (CB-2), and 3-trifluoromethylphenyl (CB-3) have been introduced as HTMs for PSCs. PL characteristics and MD simulations revealed the passivation effect of the CB-2 and CB-3 molecules on top of perovskite surface. The PSC using the highly planar CB-2 achieved the highest PCE of 18.23% with excellent long-term storage stability in the air without encapsulation, and the molecular hydrophobicity of the fluorinated HTMs ensured that the devices showed no degradation of their PCEs over 6 months. Finally, a novel passivation molecule with a phosphine oxide group providing a Lewis base, tris(5-((tetrahydro-2Hpyran-2-yl)oxy)pentyl)phosphine oxide (THPPO), has been employed at the HTM/perovskite interface. THPPO effectively passivated the undercoordinated Pb defect on the surface of the perovskite layer by forming a coordinate bond with P=O functional Lewis base. The HTM-free devices have confirmed significant increase in PCE from 5.84% to 13.31% with a significant improvement of the stability. Further, THPPO boosted PCE of the devices from 19.87% to 20.70% when combining the device with spiro-OMeTAD as an HTM.