Implications of Trap-Assisted Nongeminate Charge Recombination on Time- and Frequency-Domain Photocurrent Degradation Signatures of Organic Solar Cells

Abstract

Charge transport and collection in organic solar cells are heavily influenced by traps which ultimately limit the ability to harvest all photogenerated carriers. We investigate photocurrent responses of organic solar cells subjected to varying degrees of aging from time-and frequency-domain perspectives. Intensitymodulated photocurrent spectroscopy (IMPS) is primarily used here to resolve the effect of trap-assisted nongeminate charge recombination over a broad frequency range (e.g., ∼1 mHz−1 MHz). We use a combination of IMPS and time-dependent photocurrent transients to understand characteristic degradation signatures (i.e., positive, low-frequency imaginary component and "gain peak" where the real photocurrent exhibits a characteristic maximum, I max , at high frequencies) unique to organic solar cells. As trap densities and occupation increase with aging and light intensity, the photocurrent contrast (i.e., maximum/steady-state photocurrent, I max /I DC ) and the size of the low-frequency imaginary contribution increase. Substantial harmonic content underlies this trend which becomes more prominent as modulation frequencies and trap levels increase. We then use drift-diffusion simulations to describe IMPS responses and photocurrent transient signals over the entire frequency sampling window for aged devices that show excellent agreement with experiment. The results provide deeper insights into trap-related phenomena over a larger frequency bandwidth and further demonstrate the effectiveness of IMPS in its ability to identify mechanistic and kinetic details of degradation.

Dominant Effect of UV-Light-Induced “Burn-in” Degradation in Non-Fullerene Acceptor Based Organic Solar Cells

“…42,56 To gain further insight into the charge transport and trapassisted recombination processes in fresh and aged devices, intensity-modulated photocurrent spectroscopy (IMPS) measurements were performed. 57,58 Contrary to the IS, in IMPS, light intensity modulation at reference frequency (500 Hz−50 kHz) generates (ac) steady-state photocurrent. 57 Figure 7 shows the photocurrent response I ph (ω) of the fresh devices and devices exposed to different accelerated aging conditions for 120 h. The photocurrent response in general consists of three regions: a low-frequency region in which I ph (ω) is nearly independent of frequency, after which I ph (ω) starts increasing with frequency and reaches a distinct maximum of I max at a frequency ω max followed by a rapid decrease at a high frequency beyond the distinct maxima.…”

Unique Degradation Signatures of Organic Solar Cells with Nonfullerene Electron Acceptors

“…The large increase in ΔRe[Pc] at higher modulation frequencies is indicative of the relative amounts of carriers shelved in traps that are subsequently re-emitted and collected. 37 The trends in photocurrent contrast with aging duration are consistent with expected trap contributions and kinetics 37,50 though direct estimates of densities are not possible solely with IMPS. The Bode plot of Im[Pc] also shows significant changes in behavior compared to pristine and lightly aged devices where a larger positive-valued component emerges at lower modulation frequencies.…”

“…The large increase in ΔRe­[Pc] at higher modulation frequencies is indicative of the relative amounts of carriers shelved in traps that are subsequently re-emitted and collected . The trends in photocurrent contrast with aging duration are consistent with expected trap contributions and kinetics , though direct estimates of densities are not possible solely with IMPS.…”

“…The emergence of a first quadrant IMPS feature is rather unique to organic solar cells and underlying kinetic information surrounding trap-mediated recombination, and carrier extraction can be inferred from the positive imaginary photocurrent signal. ,,, In particular, the maximum frequency of Im­[Pc] in the low-frequency regime (i.e., ω max Im ) has been related to the rate constant of interfacial, nongeminate charge recombination, k rec . ,,, Importantly, this relationship assumes that recombination is a first-order process consistent with expectations from the Shockley–Read–Hall mechanism of trap-assisted charge recombination. ,, However, some studies have shown that higher-order processes can become important that also vary with temperature and active layer morphology. ,,, It is possible to deduce the nature of these mechanisms by varying the total excitation intensity and the AC modulation depth, where k rec should not change if a purely linear process dominates this signature . This condition may be readily assessed using ω max Im from IMPS to estimate the recombination rate constant that has been confirmed using intensity-modulated photovoltage spectroscopy (IMVS), where no carrier extraction is possible and responses only reveal charge accumulation and recombination .…”

“…The emergence of a first quadrant IMPS feature is rather unique to organic solar cells and underlying kinetic information surrounding trap-mediated recombination, and carrier extraction can be inferred from the positive imaginary photocurrent signal. 36,37,40,50 In particular, the maximum frequency of Im[Pc] in the low-frequency regime (i.e., ω max Im ) has been related to the rate constant of interfacial, nongeminate charge recombination, k rec . 36,37,40,50 Importantly, this relationship assumes that recombination is a first-order process consistent with expectations from the Shockley−Read−Hall mechanism of trap-assisted charge recombination.…”