Engineering of Materials for Hole Transport Applications in Perovskite Solar Cells

Ghosh, Paheli (2019). Engineering of Materials for Hole Transport Applications in Perovskite Solar Cells. PhD thesis The Open University.

Abstract

Immense interest has been generated in organic-inorganic perovskite solar cells (PSCs) due to their favourable attributes. PSCs show promise to overcome the trade-off between high efficiency and low-cost device fabrication. Perovskites, being ambipolar in nature, can conduct both electrons and holes. But since holes are present in low concentration, a hole transport material/layer (HTM/HTL) becomes essential for enhanced device performance, resilience and reproducibility.

This thesis explores different hole transport materials, namely spiro-OMeTAD, SFX-OMeTAD, SFX-TAD and graphene oxide. In this thesis chemical and plasma doping of HTMs have been studied. Doping with Li-TFSI is commonly undertaken to improve the hole conductivity and mobility in spiro-OMeTAD. Atmospheric pressure plasma jet (APPJ) based functionalization techniques have been proposed as inexpensive and environmentally friendly alternative to chemical p-type doping for improving the hole conductivity of spiro-OMeTAD and graphene oxide hole conductors. X-ray spectroscopic techniques have been extensively used for understanding the chemical structure and electronic properties of the HTMs.

The understanding of structural variation and its correlation with electronic properties is limited in Li-TFSI doped spiro-OMeTAD. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) have been demonstrated as effective techniques for probing the surface chemistry and electronic properties of spiro-OMeTAD. The findings provide insight into the electronic properties as a result of oxidation of spiro-OMeTAD and provide guidelines for the optimised doping levels suitable for enhanced device performance. The understanding gained from spiro-OMeTAD using NEXAFS has been extended to probe the electronic properties of two novel HTMs, SFX-OMeTAD and SFX-TAD.

P-type dopant, Li-TFSI being hygroscopic in nature, affects the stability of PSCs. A novel alternative in the form of functionalization with low-power (10 W) APPJ has been proposed. The conductivity after 5 minutes of plasma treatment was comparable to spiro-OMeTAD doped with 10-25% Li-TFSI. XPS shows the variation in chemical structure and electronic properties. KPFM measurements demonstrate an increase in work function which is indicative of p-type doping using helium and oxygen plasma jet.

Graphene oxide (GO) has found applications as charge transport layer and electrode due to the fact that it’s work function can be tailored to device specification. A low power (3 W) APPJ was used to functionalize GO sheets with nitrogen, hydrogen and oxygen with helium as carrier gas. The variation in surface chemistry and electronic properties was probed using XPS. Ultraviolet photoelectron spectroscopy (UPS) was used to determine the tuning of work function of the plasma-functionalized GO films for targeted applications as HTL in PSCs.

A novel aerosol-based APPJ deposition technique has been developed for dopant-free spiro-OMeTAD with in situ modification of electronic properties. XPS gave insight into the variation in structural chemistry and electronic properties. Plasma-deposited spiro-OMeTAD showed x4.5 higher conductivity compared to aerosolized films. UPS reported increase in density of states at the top of the valence band, indicating the change in conductivity and the tuning of work function of the functionalized spiro-OMeTAD films.

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