David Beynon (left) and Ershad Parvazian hold a sample of the fully printable perovskite solar cell. [Image: Swansea University]
Researchers at Swansea University, UK, say they have created a low-cost carbon ink that, for the first time, allows the fabrication of complete perovskite solar cells using roll-to-roll printing (Adv. Mat., doi: 10.1002/adma.202208561). With the fully printed devices achieving a conversion efficiency of close to 11%, the single-step process offers a high-speed solution for manufacturing perovskite devices at scale.
While previous studies have shown that high-performance perovskite solar cells can be fabricated by coating the device layers onto flexible substrates, a gold electrode must still be carefully evaporated onto the surface of each device after the printing process. In contrast, the conductive carbon ink can be applied at the same time as the other materials, forming an electrode that completely covers the device surface. “This innovative layer can be applied continuously and compatibly with the underlying layers at a low temperature and high speed,” comments lead author David Beynon.
The challenge for the team was to formulate a carbon ink that produces a conductive coating while still retaining the integrity of the other materials in the device stack. “The key was identifying a solvent mix that dries as a film without dissolving the underlying layer,” adds Beynon. This required determining a suitable orthogonal solvent: one that effectively dissolves the conductive graphite components without affecting the performance of the active device layers.
A single-step process
To test their formulation, the researchers first fabricated small-scale perovskite devices on a glass substrate. However, when using the most popular high-performance material—spiro-MeOTAD—as the hole-transport layer between the electrode and the perovskite, the conversion efficiency struggled to reach 3%. This result suggested that the solvent in the carbon ink had degraded this interlayer. The researchers achieved much better results when they swapped the specialist material for cheaper, widely available PEDOT, with conversion efficiencies reaching between 13 and 14%.
The team then used a sequential roll-to-roll process to print the complete device structure onto a flexible substrate about 20 m long. All the device layers were printed at a continuous rate of one meter per minute, with tests showing that the carbon ink forms a compact coating that has minimal impact on the properties of the perovskite or the other device layers.
Solar cells produced with the single-step process achieved a stabilized power-conversion efficiency of 10.8%, compared with 12 to 13% for equivalent devices fabricated with a gold electrode. The researchers believe that further performance improvements could be achieved by optimizing the material used for the interlayer between the carbon electrode and the perovskite.