Brandon Dunham – Christos Dimitrakopoulos Lab
Optimization of Solution-Processable, Low-Temperature Planar p-i-n Perovskite Solar Cells
Brandon Dunham, Drake Bal, and Christos Dimitrakopoulos
Solution-processable organic-inorganic perovskites have emerged as one of the most promising technologies for the future of photovoltaics due to their exceptional intrinsic optoelectronic properties. Perovskite solar cells achieving over 20% power conversion efficiency (PCE) often adopt an n-i-p architecture with TiO2 scaffolds requiring processing at T > 450 ℃, which render the devices incompatible with flexible substrates and roll-to-roll manufacturing. Alternatively, organic materials have been used in p-i-n perovskite solar cells as the hole and electron transport layers. Despite their lower PCE compared to TiO2-containing devices, p-i-n devices demonstrate low hysteresis, low processing temperatures, and compatibility with flexible substrates. In an effort to make p-i-n devices more competitive with their n-i-p counterparts, we have taken steps to understand the origins of their relatively lower PCE, and have developed optimized processing schemes to improve their performance and stability. We do this without any compositional or interfacial engineering, which improve charge transport or mitigate the poor ohmic contact at the electrode interfaces, an approach investigated separately[[i]].
Whereas denser, larger-grained perovskite films can be easily grown on crystalline surfaces like TiO2, those grown on amorphous surfaces like PEDOT:PSS show numerous pinholes and poor surface coverage, a main reason for lower PCE in p-i-n devices. To combat this disparity, we developed an evaporation-induced self-assembly processing technique that yields pin-hole free lead iodide (PbI2) complex intermediate films with excellent surface coverage on PEDOT:PSS surfaces. These qualities were maintained upon further conversion of the intermediate film to the desired methylammonium lead iodide (MAPbI3) perovskite, and thick films with crystal grain sizes of up to 1 μm were produced. Incorporating this higher quality perovskite film into a completed solar cell led to PCE up to 16.72%, compared to a PCE up to ~11% for our control devices made with a standard sequential deposition process.
In an effort to prolong the ambient lifetime of our perovskite devices, we developed a process to transfer large-area graphene cap layers on top of the PCBM layers of our p-i-n perovskite devices for encapsulation. It is well known that solar cells comprising ABX3 perovskite active layers exhibit very poor moisture stability. However, due to its inherent barrier properties and conductivity, graphene effectively prevents water from penetrating our devices while simultaneously allowing for electron transfer to the top electrode. The incorporation of the graphene barrier avoids the need for expensive, post-processing encapsulation, and represents a key step towards the realization of low temperature roll-to-roll manufacturing of stable, efficient, and flexible perovskite solar cells. We are currently implementing the transfer of graphene onto the cell avoiding the use of chemicals that deteriorate the active layers of the cell.
 Duzhko, et al. ACS Energy Lett., (2017), 2, 957–963.
Yalin Liu – Sarah Perry Lab
Designing Material Dynamics in Polyelectrolyte Complexes
Yalin Liu and Sarah Perry
Polyelectrolyte complexes are formed through the electrostatic interaction of oppositely-charged polymers. Depending on the solution conditions and choice of polymers, polyelectrolyte complexation can result in the formation of solid precipitates and/or a liquid-liquid phase separation, known as complex coacervation. Interestingly, recent reports have demonstrated that salt can serve as a plasticizing agent, allowing for a continuous transition between solid complexes, liquid coacervates, and ultimately a single-phase solution. However, a universal understanding of how various parameters affect the material dynamics in these systems is still missing, including the mechanism of the saloplastic solid-to-liquid phase transition. We have recently used linear viscoelasticity to explore the nature of this liquid-to-solid transition for the model system of poly(4-styrenesulfonic acid, sodium salt) (PSS) and poly(diallyldimethyl ammonium chloride) (PDADMAC) in the presence of potassium bromide (KBr). The use of time-salt superposition facilitated the unambiguous characterization of this transition as a salt-driven physical gelation. These results form the basis of a systematic investigation into the effect of polymer chemistry and molecular weight on the resulting material dynamics, the goal of which is to establish a molecular-level understanding of the dynamic mechanical re