Korean Researchers Lift Efficiency Ceiling For Tin-Based Thin-Film Solar Cells

Researchers at Chonnam National University have overcome a long-standing performance bottleneck in tin monosulfide (SnS) thin-film solar cells by introducing an ultra-thin germanium oxide layer at the device’s rear contact, significantly improving efficiency and stability.

SnS is widely regarded as a promising earth-abundant and non-toxic material for low-cost thin-film photovoltaics, but progress has been hampered by defects and parasitic chemical reactions at the interface between the absorber layer and the metal back contact. These issues have limited charge collection and kept device performance well below theoretical limits.

Breaking The Bottleneck

In a study published online in Small, a team led by Professor Jaeyeong Heo and Dr Rahul Kumar Yadav demonstrated that inserting a 7-nanometre-thick germanium oxide (GeOx) interlayer between the molybdenum back contact and the SnS absorber mitigates these problems.

The GeOx layer was formed through the controlled oxidation of an ultra-thin germanium film during a vapour transport deposition process, a technique the researchers say is scalable and compatible with industrial manufacturing.

“Despite its nanoscale thickness, the interlayer resolves several long-standing challenges at the rear interface,” Heo said. “It suppresses deep-level defects, blocks unwanted sodium diffusion, and prevents the formation of resistive molybdenum disulphide phases during high-temperature processing.”

Enhancing Charge Support 

As a result, the SnS absorber showed improved crystallinity, with larger and more uniform grains, enhanced charge transport and lower electrical losses.

Devices incorporating the GeOx interlayer achieved a power conversion efficiency of 4.81%, up from 3.71% in standard SnS cells. The result ranks among the highest efficiencies reported for vapour-deposited SnS solar cells, the researchers said.

Beyond photovoltaics, the findings underscore the importance of interface engineering across a range of electronic and energy technologies, including thin-film transistors, thermoelectric devices, sensors, flexible electronics, photodetectors and memory devices.

“Mastering the metal–semiconductor interface is central to advancing next-generation devices,” Heo said, adding that the approach could open new research pathways for improving performance across multiple applications.