Electron Extraction Optimization for Carbon-Based Hole-Conductor-Free Perovskite Photovoltaics With Record 1.41 V VOC

Abstract

Carbon-based CsPbI₂Br perovskite solar cells (PSCs) free of a hole-transport layer (HTL) have emerged as promising photovoltaics due to their low processing cost and superior stability. However, the voltage deficit resulting from inefficient carrier extraction causes insufficient power conversion efficiency (PCE), severely hindering their progress. Here, a gradient electron energy level modulation strategy proves effective in reducing voltage losses through the rapid extraction of photogenerated electrons. This process enhances carrier separation/collection and reduces recombination at the back contact, thereby achieving high-performance photovoltaics. It is demonstrated that the front electron extraction, equally critical as the prevailing back perovskite/carbon contact, accounts for the significant contributing factor of voltage deficit in carbon-based HTL-free PSCs. The resulting PSCs deliver a record open-circuit voltage (V_OC) of 1.41 V and a PCE of 17.42% and retain more than 92% of their initial efficiency after 1, 000 h. These results highlight the significant potential of carbon-based HTL-free perovskite photovoltaics.

Keywords: carbon‐based perovskite solar cells; electron extraction; gradient electron energy level; hole‐transport layer free; voltage deficit.

Figure 1

Performance of PSCs. a) J–V curves. b) EQE spectra. c) Statistical V _OC distribution of PSCs. d) Steady‐state current density at maximum point and corresponding PCE. e) J–V curves of PSCs under reverse and forward scans. f) Stability test of PSCs under simulated solar light illumination (AM 1.5, 100 mW cm⁻², open‐circuit condition). g) A comparison of device PCE & V _OC with the state‐of‐the‐art HTL‐free CsPbI₂Br C‐PSCs reported in literature (Table S1, Supporting Information).

Figure 2

Energy level and carrier transportation in PSCs. a) Energy level diagram of PSCs at electron selective contact and their corresponding thermal equilibrium band structures. b) Calculated energy levels of PSCs with different ETL. High‐resolution XPS results of Sn 3d c) and O 1s d) signals for L─TS and L─TMS films. e) Schematic illustration of carrier transport from perovskite to SnO₂ or Mg‐SnO₂. f) The J–V curves of the ITO/ETL/Al films.

Figure 3

Carrier dynamics. a) The distributions of photo‐generated carrier. b) device configuration model for simulation and the proposed carrier behavior. c) The distributions of electron‐hole recombination rate in PSCs with different ETL substrates. I) TiO₂, II) SnO₂/TiO₂, and III) Mg‐ SnO₂/TiO₂. d) TA spectra of the perovskite films deposited on various ETL substrates. I) TiO₂, II) SnO₂/TiO₂ and III) Mg‐SnO₂/TiO₂. e) Derived values of monomolecular recombination rate constant k₁, and bimolecular recombination rate constant k₂ for the perovskite films on different substrates. The error bars indicate the confidence intervals of the fitted rates (Table S4, Supporting Information).

Figure 4

Perovskite film characteristics. a) Contact angle testing with a water droplet on the surface of different ETL substrates. b) XRD patterns of perovskite films. c) GIWAXS patterns and SEM images of the perovskite films deposited on various ETL substrates. d) V _OC dependence on light intensities of C‐PSCs. e) TRPL curves of CsPbI₂Br films. f) Nyquist plots from EIS measurements of C‐PSCs.