Unique Degradation Signatures of Organic Solar Cells with Nonfullerene Electron Acceptors

• DOI: 10.1021/acsami.0c21367
• Publication Date: 2021-01-23T04:56:11Z
• Authors: Pranab Deb; Ryan T. Grimm; John K. Grey
• Keywords: Chemical Sciences not elsewhere classified; device optimization strategies; NFA; Biophysics; 8DFIC; 02 engineering and technology; Microbiology; 7. Clean energy; k rec values; Unique Degradation Signatures; Space Science; device contact interfaces; Nonfullerene Electron Acceptors; Molecular Biology; degradation trends; density; Organic Solar Cells; excitation intensity; NIR; trap densities; contrast; display; recombination; PTB; quadrant IMPS component; intensity-modulated photocurrent sp.; Medicine; 0210 nano-technology; Physical Sciences not elsewhere classified; performance; Biotechnology; Developmental Biology; Biological Sciences not elsewhere classified

We investigate the degradation phenomena of organic solar cells based on nonfullerene electron acceptors (NFA) using intensity-modulated photocurrent spectroscopy (IMPS). Devices composed of NIR absorbing blends of a polymer (PTB7) and NFA molecules (COi8DFIC) were operated in air for varying periods of time that display unusual degradation trends. Light aging (e.g., ∼3 days) results in a characteristic first quadrant (positive phase shifts) degradation feature in IMPS Nyquist (Bode) plots that grow in amplitude and frequency with increasing excitation intensity and then subsequently turns over and vanishes. By contrast, devices aged and operated in air for longer times (>5 days) display poor photovoltaic performance and have a dominant first quadrant IMPS component that grows nonlinearly with excitation intensity. We analyze these degradation trends using a simple model with descriptors underlying the first quadrant feature (i.e., trap lifetime and occupancy). The results indicate that the quasi first-order recombination rate constant, krec, is significantly slower in addition to lower trap densities in devices exhibiting light aging effects that are overcome by increasing carrier densities (viz. excitation intensity). By contrast, larger trap densities and distributions coupled with larger krec values are found to be responsible for the continuous growth of the first quadrant with light intensity. We believe that defect formation and charge recombination at device contact interfaces is chiefly responsible for performance degradation, which offers several directions for materials and device optimization strategies to minimize long-term detrimental factors.