Reducing the voc loss of hole transport layer-free carbon-based perovskite solar cells via dual interfacial passivation

A breakthrough study in Nano-Micro Letters led by Xian Zhang, Cuncun Wu and colleagues from Hebei University of Technology and Peking University introduces a simple, scalable Li₂CO₃ surface-modification protocol that simultaneously tames defects at both the SnO₂/perovskite bottom interface and the perovskite/carbon top interface of HTL-free, carbon-electrode perovskite solar cells (C-PSCs). The result is a champion device delivering 19.1 % power-conversion efficiency under AM 1.5 G and a record 33.2 % under 2000-lx LED light—while retaining >90 % of its initial performance after 360 h continuous operation without encapsulation.

Why This Research Matters
• Bridging the Voc Gap: HTL-free C-PSCs are intrinsically stable and low-cost, yet their open-circuit voltage (Voc) lags >150 mV behind metal-electrode counterparts owing to rampant non-radiative recombination at grain boundaries and mismatched energy levels. Li₂CO₃ acts as a dual passivator: (i) it boosts SnO₂ electron-transport-layer conductivity and lowers its work function for smoother electron extraction, and (ii) it induces controlled PbI₂ crystallites at perovskite grain boundaries that plug surface traps and block parasitic hole back-flow, elevating Voc from 1.085 to 1.142 V.
• Indoor PV Champion: Under weak 3000-K LED light, the same device architecture delivers 33.2 % PCE—among the highest ever reported for carbon-based cells—making it ideal for powering IoT sensors, wearables and indoor energy-harvesting systems.
• Ambient Durability: Unencapsulated devices retain 90 % of initial efficiency after 360 h 1-sun maximum-power-point tracking and show negligible degradation after 30 days in 15–30 °C, 20–30 % RH air, underscoring the robustness of the carbonate-induced passivation.

Innovative Design and Mechanisms
• Bottom Interface Optimization: Li₂CO₃ spin-coated on conformal SnO₂ increases carrier density and flattens the energy landscape, evidenced by a 0.15-eV larger light/dark surface-potential contrast in KPFM and a 20 % reduction in trap density via SCLC measurements.
• Top Interface Grain Engineering: Thermal annealing drives Li₂CO₃-released CO₃^2- to selectively react with MA⁺, forming insulating PbI₂ crystallites that decorate grain boundaries without disrupting bulk perovskite integrity—confirmed by SEM, GIWAXS and ¹H NMR. The PbI₂ layer simultaneously passivates surface traps and acts as an electron-blocking layer, cutting recombination and dark saturation current.
• Scalable, Ambient Process: All steps—CBD SnO₂, spin-cast Li₂CO₃, low-pressure perovskite crystallization and blade-coated carbon—are performed in air below 120 °C, fully compatible with roll-to-roll manufacturing.

Applications and Future Outlook
With earth-abundant reagents, low-temperature processing and proven upscaling to 10 × 10 cm² modules, Li₂CO₃-dual-passivated C-PSCs provide a cost-effective pathway to both rooftop and indoor photovoltaics. The team is now integrating the strategy into flexible substrates and tandem architectures, aiming for >25 % outdoor and >35 % indoor efficiencies while maintaining the intrinsic stability that makes carbon-based perovskites the front-runner for next-generation, mass-deployable solar technologies.