Illuminating pathways to long-lived organic solar cells

Next-generation solar cells made from organic materials could soon contribute to real-world renewable energy generation, suggests an outdoor stress test conducted under intense Saudi Arabian sunlight. The long-term study showed that certain organic light-capturing materials are surprisingly resilient to light-induced ‘photodegradation’ and revealed new ways to further optimize the cells’ longevity in realistic conditions^[1].

Lightweight, semi-transparent and flexible, organic solar cells (OSCs) could potentially be used in a range of situations where conventional silicon solar panels would be too heavy, rigid or opaque to be deployed. “Recently, OSCs’ solar power conversion efficiencies (PCEs) have improved rapidly, surpassing 20 percent in laboratory settings,” says Han Xu, a postdoc in the KAUST lab of Derya Baran, who led the research with then postdoc Jianhua Han. “However, there has been much less focus on improving long-term stability of OSCs, which remains a major bottleneck hindering commercial viability,” he says.

The performance of an OSC can decline precipitously when exposed to the heat, light and moisture of outdoor environments; this occurs via degradation pathways that are poorly understood. “To bridge this knowledge gap, we systematically investigated the degradation behavior of various OSCs under light, thermal stress, and outdoor conditions,” Xu says.

The researchers focused on a component of the OSC’s light-harvesting core called the polymer donor. These materials’ photodegradation pathways have rarely been studied, despite their crucial role in OSC light absorption, charge generation and transport.

The team made a series of OSCs incorporating different polymer donors and studied the impact of factors such as polymer molecular structure on OSC longevity.

The weak spot of the polymer donors’ key photodegradation, it turned out, was the side chains that branch from the polymers’ central molecular backbone. The team showed that light could knock a hydrogen atom from a side chain or break off a side chain entirely, initiating cascading damage. “This can result in by-products such as cleaved side chains, radicals, twisted polymer backbones, and cross-linked structures,” Xu says.

Some side chains, however, were far less susceptible to this form of light-driven damage than others, the team showed. “In our study, OSCs incorporating the polymer donor PCE10, which features robust side chains, achieved 91 percent of retained PCE even after seven months of outdoor stability testing,” Xu says.

“Also notable is that some OSCs can survive harsh environmental conditions over a long period of time,” Baran says. PCE10’s class-leading stability was a surprise, she adds, because previous results have shown that PCE10 is photo-unstable in air. For their outdoor testing, the team encapsulated their OSCs to exclude air and moisture from the device. Under these conditions, PCE10 proved to be remarkably resilient to degradation — despite experiencing intense sunlight and peak temperatures of over 65 degrees Celsius.

The study highlights that OSCs should be optimized not only for their initial power conversion efficiency, but also for how well that efficiency is maintained over time in outdoor environments, Baran says.

Based on their findings, the team’s next step will be to test a new set of OSCs, trialing them under various conditions that solar cells might be exposed to in different outdoor environments around the world.