(a) Schematic cross-section of the planar PSC architecture: ITO/NiOx/Perovskite/PCBM/PEOz/Ag; (b) Molecular structure of PEOz; (c) Schematic representation of band energy diagrams for the PSC. (d) Band alignment in the energy diagrams at the PCBM/Ag interface with (right) and without (left) PEOz. (e) Cross-sectional SEM image of a typical intact planar PSC device. The scale bar of SEM image is 500 nm.
(a) Schematic cross-section of the planar PSC architecture: ITO/NiOx/Perovskite/PCBM/PEOz/Ag; (b) Molecular structure of PEOz; (c) Schematic representation of band energy diagrams for the PSC. (d) Band alignment in the energy diagrams at the PCBM/Ag interface with (right) and without (left) PEOz. (e) Cross-sectional SEM image of a typical intact planar PSC device. The scale bar of SEM image is 500 nm.

The mineral perovskite is an attractive material for solar cells because of its unique light-absorbing properties and high conversion efficiencies. Prototype designs use layered structures of different organic and inorganic materials, but this makes the interface between layers – particularly at the point where electrons are extracted – crucially important to the overall efficiency and stability.

Now researchers from the Southern University of Science and Technology in China have demonstrated how these interfaces can boost the performance of ‘inverted’ perovskite solar cells (PSCs) simply by introducing an additional polymer layer [Chen et al., Materials Today Energy 1-2 (2016) 1-10].

So-called ‘inverted’ PSCs have a structure comprising electron and hole transporting layers (usually phenyl-C61-butyric acid methyl ester or PCBM and NiOx, respectively) separated by a perovskite layer and a metal electrode such as Ag or Au. The interface between the organic conductor and the metal electrode can lead to poor performance.

To overcome this shortcoming, Zhu-bing He and his colleagues inserted a thin film of the low-cost, highly stable polymer poly(2-ethyl-2-oxazoline) – or PEOz – in between PCBM and Ag as a cathode electron extraction layer. The device also contains a nanostructured NiOx layer as the hole transport layer.

“Both of [the layers] led to remarkable enhancements in conversion efficiency and stability of MAPbI3 solar cells,” says He.

In fact, the inclusion of a PEOz layer boosts the power conversion efficiency of an ITO/NiOx/perovskite/PCMB/Ag device from just under 12% to over 18%. The researchers believe that the PEOz layer acts to eliminate the interface barrier between the PCBM and Ag layers, improving the electron extraction. Simultaneously, the NiOx layer improves hole extraction. Moreover, the results suggest that N-groups in the PEOz layer boost the overall durability and stability of devices.

“One unique point of our discovery is that N-elements in the backbone of PEOz can lock penetrating Ag and I ions by forming chemical bonds, which enhances the stability of MAPbI3 solar cells to a large extent,” explains He.

In addition, both the PEOz and NiOx layers are low cost and easy to fabricate at low temperatures, say the researchers, making them straightforward to incorporate into PSCs.

“With a certified 22.1% solar conversion efficiency, PSCs have reached a performance level that is comparable to state-of-the-art copper indium gallium diselenide (CIGS) solar cells and approaching commercial monocrystalline silicon solar cells,” says He. “There is no sign that PSC efficiency has reached a bottleneck.”

He hopes that these new findings will support the commercialization of PSCs by improving device stability and making processing easier.