Researchers report positive results from tests with low-cost fluorene-xantene-based HTM

Researchers at the University of Rome Tor Vergata's Centre for Hybrid and Organic Solar Energy (CHOSE) and ISM-CNR have tested a commercially available HTM with a new core made by low-cost fluorene'xantene units. The experimentation was conducted on small (0.09 cm²) and large area (1.01 cm²) cells.

The one-pot synthesis of this compound is said to drastically reduce its cost compared with the commonly used Spiro-OMeTAD. The optoelectronic performances and properties were characterized through JV measurement, IPCE (incident photon to current efficiency), steady-state photoluminescence and ISOS stability test. SEM (scanning electron microscope) images reveal a uniform and pinhole free coverage of the X55 HTM surface, which reduces the charge recombination losses and improves the device performance relatively to Spiro-OMeTAD from 16% to 17%. The ISOS-D-1 stability test on large area cells without any encapsulation reports an efficiency drop of about 15% after 1000 h compared to 30% for the reference case.

The HTL plays a crucial role in terms of efficiency, stability and cost of PSCs. Devices based on the expensive Spiro-OMeTAD HTM have very high efficiency but poor stability in presence of humidity. In this work, the researchers experimented with the cost-effective X55 HTM, which has a much deeper HOMO level and a higher hole mobility and conductivity than Spiro-OMeTAD.

The results show a higher efficiency (17% vs. 16%), film homogeneity and better hole transport for the X55 with respect to the reference Spiro-OMeTAD. Then, the team tested the promising X55 HTM on large area devices during MPP tracking and under ISOS-D-1 stability test for more than 1000 h. On large area cells, the best efficiency is 16.05% and 14.9% for X55 and Spiro-OMeTAD HTM, respectively.

They then found a stabilized efficiency at MPP of 15% for X55 and 13.9% for Spiro-OMeTAD.

The efficiency drop was about 15% and 30% for the X55- and Spiro-OMeTAD-based device after 1000 h at ISOS-D-1 condition, respectively. The sturdiness relative to the moisture and the low upscaling losses make the X55 HTM a potential candidate for stable large area modules.