Sequentially Deposited versus Conventional Nonfullerene Organic Solar Cells: Interfacial Trap States, Vertical Stratification, and Exciton Dissociation

TL;DR

This study compares sequentially deposited and conventional non-fullerene organic solar cells, revealing that interfacial trap states and vertical heterogeneity significantly influence exciton dissociation and device efficiency.

Contribution

It provides detailed insights into interfacial trap states and vertical stratification in sequentially deposited solar cells, highlighting their impact on performance and potential improvements.

Findings

Interfacial trap states are about 50% more populated in sq-BHJ than c-BHJ.

Vertical donor-acceptor stratification is visualized with 1-2 nm precision.

Sequential deposition enhances exciton dissociation efficiency.

Abstract

Bulk-heterojunction (BHJ) non-fullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq-BHJ) have recently been promising to be highly efficient, environmentally friendly, and compatible with large area and roll-to-toll fabrication. However, the related photophysics at donor-acceptor interface and the vertical heterogeneity of donor-acceptor distribution, critical for exciton dissociation and device performance, are largely unexplored. Herein, steady-state and time-resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlation with the luminescent efficiency of interfacial states and its non-radiative recombination, interfacial trap states are characterized to be about 50% more populated in the sq-BHJ than as-cast BHJ (c-BHJ), which probably limits the device voltage output. Cross-sectional energy-dispersive X-ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly vizualize the donor-acceptor vertical stratification with a precision of 1-2 nm. From the proposed "needle" model, the high exciton dissociation efficiency is rationalized. Our study highlights the promise of sequential deposition to fabricate efficient solar cells, and points towards improving the voltage output and overall device performance via eliminating interfacial trap states.