For commercialisation fully low-temperature solution processible perovskite solar cells (PSC) are required to make them compatible with low-cost processing techniques such as roll-to-roll printing. Inverted PSCs (p-i-n devices) have received great interest recently (despite their inferior PCE to normal PSC devices) due to the plethora of low-temperature solution processable materials available. Furthermore, low hysteresis and high devices stability are hallmarks of inverted PSC devices – which are two other critical problems that need addressing for the commercialisation of PSCs.[1] Inverted PSC devices typically utilise organic materials as electron transport materials (ETM).  These materials allow for a bottom-up design approach with a wide degree of design space to utilise. This allows for key aspects of the materials to be tailored towards having desirable ETM properties: efficient electron extraction, high electron mobility, surface passivation, and overall device stability.

Naphthalene diimides (NDI) are a class of organic small molecule which have seen success in use as PSC ETLs due to their well-aligned LUMO level (with typical lead hybrid halide perovskites) and reasonable electron transport. The N-group of NDIs provide an area of synthetic modification which can alter solubility and thermal stability properties of these molecules without largely impacting the favourable electronic properties of the core.[2] It is well-established that Lewis bases provide surface passivation to the perovskite surface supressing photogenerated charge recombination and thus, improves overall PSC performances. This is postulated to be due to the Lewis base electron lone pair coordinating to uncoordinated Pb²⁺ ions at the perovskite surface.[3]

With all this in mind, two NDI derivatives (one novel) have been synthesised for use as ETMs for PSCs. The N-groups have been picked to introduce sulfur atoms with available electron lone pairs which are envisioned to passivate the perovskite surface while also providing sufficient solubilisation for good film formation. The difference in the two molecules will provide design insight with respect to electronic nature of the sulfur atoms for surface passivation and impact of the annulated benzene moiety on packing (i.e., charge transport) and thermal stability. The synthesis of these derivatives will be presented along with basic characterisation related to their applicability for use as ETMs: such as cyclic voltammetry for demonstrating energy level alignment and differential scanning calorimetry for demonstrating potential thermal stability. Furthermore, preliminary electron conductivity and PSC device data using these molecules will be presented to give insight into their potential suitability as ETMS.