Tuning Self-Assembly of Hole-Selective Monolayers for Reproducible Perovskite/Silicon Tandem Solar Cells

Abstract

Self-assemble monolayers (SAMs) have become state-of-the-art hole-selective contacts for high-efficiency perovskite-based solar cells due to their easy processing, passivation capability, and low parasitic absorption. Nevertheless, for the deposition of SAMs with a monolayer thickness and a high packing density on metal oxide substrates, critical challenges persist. To overcome these, the study focuses on the impact of annealing temperature - an intrinsic yet so far unexplored process parameter - during the formation of SAMs. By performing in situ angle-resolved X-ray photoelectron spectroscopy combined with advanced data analysis routines, it is revealed that increasing the annealing temperature reduces the formed SAM layer thickness from a multilayer stack of ≈5 nm at 100 °C (conventional temperature employed in literature) to a monolayer at 150 °C. Furthermore, denser adsorption of the SAM to the metal oxide surface is promoted at high temperatures, which enhances the interfacial SAM/perovskite passivation quality. With this strategy, a 1.3%_abs power conversion efficiency (PCE) increment is obtained in fully-textured perovskite/silicon tandem solar cells, with improved reproducibility, and a champion device approaching 30% PCE. This study advances the understanding of SAMs formation and presents a promising strategy for further progress in high-efficiency perovskite-based solar cells.

Keywords: Photovoltaics; hole transport layers; perovskite silicon tandem solar cells; reproducibility; self‐assembled monolayers.

Figure 1

Schematics of potential defects in self‐assembled monolayers (SAMs) and characterization method for assessment. a) Potential defects in self‐assembled monolayers deposited on a metal oxide substrate: (1) molecules where the anchoring group is not covalently bounded to the substrate and thus adsorb on the formed monolayer leading to the formation of a “multi”‐layer stack of SAMs instead of a monolayer, (2) tilted or misaligned molecules, (3) aggregated molecules leading to the formation of dimers trimers etc., and (4) voids created by molecules’ interaction on the substrate which lead to non‐full coverage (adopted from Hinckley et al.).^{[ 39 ]} b) Schematic depicting the employed in situ angle‐resolved X‐ray photoelectron spectroscopy (AR‐XPS) characterization method and the data evaluation routine to investigate the formation of SAMs.

Figure 2

Impact of 2PACz annealing temperature on the formed layer's properties. a) Contact angle measurements of glass/ITO/2PACz substrates with a variation in 2PACz annealing temperature and images of a representative water droplet at b) 100 °C, c) 125 °C, and d) 150 °C. e) X‐ray photoelectron spectroscopy (XPS) survey of an uncoated glass/ITO substrate reference, and glass/ITO/2PACz samples after ex situ annealing at 100, 125, and 150 °C. f) Fitted 2PACz thickness using scattered electron background analysis on insitu annealed samples (in green) and comparison to ex situ annealed samples (in purple). g) Atomic composition change as a function of 2PACz annealing temperature (in situ, calculated assuming a homogeneous medium). With increasing annealing temperature, the thickness of the 2PACz layer decreases.

Figure 3

Understanding the impact of increased 2PACz annealing temperature through analysis of the detailed XPS spectrum. Detailed XPS spectrum of the a) O 1s, b) C 1s and c) N 1s of ex situ annealed 2PACz/ITO samples with increasing annealing temperature. With higher temperature, a reduced P─O contribution to the O 1s peak is observed together with a shift of C 1s and N 1s peaks to higher energies.

Figure 4

Comparison between flat and textured substrates. Change in a) carbon to indium (C/In) and b) phosphorus to indium (P/In) as a function of annealing temperature, using a sample structure consisting of silicon substrate/ITO/2PACz, with the substrate being flat (red, circle symbols) versus textured (gray, square symbols). The heating was done in situ. A higher impact of 2PACz annealing temperature can be observed on flat substrates.

Figure 5

Impact of 2PACz annealing temperature on perovskite and 2PACz/perovskite interfacial properties. a) Absolute photoluminescence (PL) response, b) implied open‐circuit voltage (iV _OC) determined from PLQY analysis, and c) PL response tracked under 1‐sun over 300 s in the air (unencapsulated samples) of stacks based on textured silicon/ITO/2PACz/perovskite with a variation in 2PACz annealing temperature. d) Work function of 2PACz‐modified ITO substrates annealed at different temperatures compared to the ITO reference (from KPFM measurements). f) Energy band offset (∆E) between the HOMO level of 2PACz‐modified ITO substrates annealed at different temperatures and the VBM of the perovskite (from PESA measurements). f) X‐ray diffraction pattern and g) morphology (top‐view and cross‐sectional SEM images) of stacks based on textured silicon/ITO/2PACz/perovskite with a variation in 2PACz annealing temperature. Increased SAM annealing temperature results in enhanced iV _OC, improved perovskite photostability, and better energetic alignment between 2PACz and perovskite, while no changes are observed in the structural and morphological properties of the perovskite.

Figure 6

Impact of 2PACz annealing temperature on perovskite/silicon tandem solar cell performance. a) Schematic of the fully‐textured perovskite/silicon tandem solar cell structure studied. Photovoltaic parameters presented in a box plot showing the b) open‐circuit voltage (V _OC), c) fill factor (FF), d) short‐circuit current density (j _SC), and e) power conversion efficiency (PCE) of the tandem solar cells as a function of 2PACz annealing temperature both in the forward and reverse scans. f) Cross‐sectional SEM image of a representative tandem solar cell. g) Current–density voltage (J–V) curves of the best devices (forward in continuous line, reverse in dashed line) and a photograph of a representative solar cell with a 1 cm² active area. Increasing the annealing temperature of 2PACz from 100 to 125 °C improves the performance of the tandem solar cells.