Unique Degradation Signatures of Organic Solar Cells with Nonfullerene Electron Acceptors

Abstract

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, k rec, 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 k rec 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.

Correcting unintended changes in electroluminescence perturbation for reliable light intensity modulated spectroscopies

“…d due to the unintended modulation observed in Q Id . Subsequently, one can substitute equation (10) in (7) for correcting the IMPS from the sample of area A s , whose uncalibrated ratio is…”

“…Light-perturbated transient techniques [1,2] and spectroscopic technique methods [3] are commonly used to characterize photosensitive devices, such as photoelectrodes [4], photodetectors [5] and photovoltaic (PV) cells [6], based on a broad range of materials, including silicon [3,7] to chalcopyrites [8], dyes [9], organic semiconductors [10,11], perovskites [12,13], and ternary oxide semiconductors [14]. Among the spectroscopic techniques, the intensity-modulated photocurrent and photovoltage spectroscopies, IMPS and IMVS, respectively, are commonly used to approach the transport properties and dissipation mechanisms [15,16] in photosensitive samples.…”

“…Measurement of the IMPS of a reference photodiode for estimating the frequency-dependent values η EL ( f ) of the electroluminescence rate, using the expression in equation (10). In case of varying modulation effects (e.g.…”

Degradation through Directional Self‐Doping and Homogeneous Density of Recombination Centers Hindered by 1,8‐Diiodooctane Additive in Non‐Fullerene Organic Solar Cells

“…Sudakov et al [ 35 ] highlighted the importance of the reactive oxygen species in P3HT:FBR blends and found that the generation of singlet oxygen by energy transfer from the FBR triplet state is responsible for the poor stability of FBR and for the accelerated photodegradation at later times of the entire blend. Deb et al [ 36 ] presented an intensity‐modulated photocurrent spectroscopy study suggesting that defect formation and charge recombination at device contact interfaces is chiefly responsible for performance degradation. Wang et al [ 37 ] measured the built‐in potential via transient photocurrent measurement, and found that retaining high values of this parameter mitigates degradation mechanisms.…”