Researchers at Ajou University's Department of Molecular Science and Technology, working with co-first author Chanhyeok Kim of POSTECH's Department of Chemical Engineering, have engineered two dopant-free derivatives of PTAA, a widely used hole transport layer (HTL) polymer for inverted perovskite solar cells, by copolymerizing carbazole units into its backbone. The carboxylic-acid-terminated version, CA-PTAA, reached a power conversion efficiency (PCE) of 22.8%, up from 20.0% for undoped PTAA, while retaining long-term ambient stability comparable to standard PTAA despite a more hydrophilic surface.
In inverted (p-i-n) perovskite solar cells, the HTL sits at the bottom contact and shapes both charge extraction and the growth of the perovskite film above it. PTAA is a favored choice for its chemical stability, energy-level alignment, and solution processability, but it has two persistent limitations. Its intrinsic conductivity is low enough that high-performing devices typically rely on p-doping, and because most molecular dopants aren't covalently bound to the polymer backbone, they can diffuse or be washed out by the polar solvents used in perovskite precursor solutions, introducing mobile ionic species and compositional inhomogeneity that hurt reproducibility and stability. Separately, PTAA's surface is chemically inert and hydrophobic, which causes poor wetting of the perovskite precursor and provides no chemical anchor points for the growing film, leading to smaller grains, interfacial voids, and higher defect densities that cap the achievable open-circuit voltage.
The team's fix targets both problems within the polymer itself rather than through external dopants or additives. Carbazole, a rigid, fully planar tricyclic unit that is electronically compatible with PTAA's triarylamine backbone, was incorporated as a comonomer to act as a localized π–π contact point between adjacent polymer chains, boosting interchain hole transport without inducing unwanted crystallization. The researchers first built Cz-PTAA, bearing alkyl-substituted carbazole side chains, to isolate the effect of this backbone planarization on charge transport alone. They then extended the design to CA-PTAA, which places carboxylic acid termini on those same carbazole side chains, turning the buried interface itself into an active surface: the carboxylic acid groups improve precursor wettability and coordinate directly with otherwise-uncoordinated Pb2+ ions at the interface, passivating defects through direct chemical bonding rather than a separate interlayer.
CA-PTAA-based devices delivered the 22.8% PCE headline figure alongside improved short-circuit current density and fill factor and a tighter device-to-device performance spread than the undoped PTAA baseline. The more hydrophilic CA-PTAA surface might be expected to compromise ambient stability, but the researchers report long-term stability on par with standard PTAA devices, which they attribute to the same carboxylate-Pb2+ coordination that passivates defects also locking the buried interface against moisture ingress.
By splitting the design into Cz-PTAA and CA-PTAA, the team was able to separate the contribution of backbone planarization from that of interfacial carboxylic acid functionalization. The researchers frame the combined result as a robust, scalable route to high-performance inverted perovskite solar cells that doesn't depend on external dopants, additives, or separate interfacial layers to activate the HTL surface.
