UPS and XPS Case Study: Multi technique characterisation of semi transparent perovskite solar cells

Figure 1. Mo 3d XPS high resolution spectra, UPS spectra and band diagrams of semi-transparent perovskite solar cells

Semi-transparent perovskite solar cells (ST-PeSCs) have received great attention because of their excellent performance and promising application in areas such as tandem devices and building integrated photovoltaics (BIPVs). Critical across all these applications is achieving both high efficiency and stable photovoltaic performance through the use of different materials than the archetypal doped Spiro-OMeTAD as a hole transport layer (HTL). In this work, a thermally cross-linked poly-VNPB material was used as the HTL and displayed significantly enhanced stability under humid and elevated temperature conditions.

The Monash X-ray Platform’s Thermo Nexsa Surface Analysis system can perform a range of integrated characterisation techniques on solar devices, with X-ray photoelectron spectroscopy (XPS), cluster gun elemental depth profiling, ultraviolet photoelectron spectroscopy (UPS) and angle resolved XPS (ARXPS) regularly being used by groups from Monash and their academic and commercial partners.

In this case, XPS and UPS were used to analyse the effect of a post-annealing thermal (PAT) treatment on the elemental composition and work function (WF) of the poly-VNPB/MoO_x material, with confidence in the fast and correlative data collection afforded by the integrated multi-technique approach running on the same sample area in the same experiment. UPS spectra are collected and WF measured using specialised sample bias stage which allows small bias (typically 5-10 V) to be applied the sample surface, so as to deconvolute the true workfunction of the surface from the internal workfunction of the spectrometer.

Comparison of the high-resolution Mo 3d XPS spectra of with and without PAT treatment provided information on the stoichiometry of the deposited MoO_x. The combination of differing Mo 3d chemical shifts observed via XPS and the UPS spectra and WF analysis allowed comparative insights into the electronic properties of both materials to be made.

Data and images courtesy of Jae Choul Yu, Jingsong Sun, Naresh Chandrasekaran, Christopher J. Dunn, Anthony S.R. Chesman, Jacek J. Jasieniak. 'Semi-transparent perovskite solar cells with a cross-linked hole transport layer', Nano Energy, Volume 71, 2020,
104635, https://doi.org/10.1016/j.nanoen.2020.104635.