Characterizing Energetic Disorder in Organic Solar Cells
Energetic disorder reduces solar cell efficiencies but is difficult to characterize. Hartnagel et al. perform a comparative study of different methods to quantify disorder and show how method-specific bias in the resulting values can modify the results.
The role of energetic disorder in the search for high-efficiency organic solar cells is crucial, and it can be characterized through optical methods that excite electrons from or into defect states by incident photons, such as photothermal deflection spectroscopy, Fourier transform photocurrent spectroscopy, and highly sensitive external quantum efficiency measurements. These absorption-based methods are commonly used in literature. However, the density of defect states can also be probed by varying the quasi-Fermi level splitting with an applied voltage in charge extraction or admittance measurements. In both our experiments and literature, voltage-dependent measurements yield higher Urbach energies than the techniques based on optical excitation. We conducted optical FTPS and PDS measurements and electrical admittance spectroscopy under illumination and in the dark on two nonfullerene acceptor-based material systems, and in all cases, the Urbach energy extracted by the voltage-dependent methods was at least twice as high as their optical counterparts.
We have found that caution must be exercised when analyzing experimental data as different effects can be misinterpreted as features of energetic disorder. Even for purely optical data, we have observed that a low dynamic range in PDS measurements can result in a lower slope at the band edge than in FTPS on the same material. Voltage-dependent admittance measurements are even more sensitive, and the analysis in terms of energetic disorder is based on numerous assumptions. Our experiments and simulations on a solar cell based on PffBT4T-2OD:EH-IDTBR have demonstrated how bad transport properties can result in an internal series resistance overlaying the exponential regime of the capacitance-voltage measurements. This can lead to a discrepancy in the Urbach energy between optical and voltage-dependent measurements, which can be attributed to an overestimation in voltage-dependent measurements due to poor electronic properties.
We acknowledged the limitations of the characterization techniques and pointed out that both optical and electrical methods might still provide accurate measurements of the subband-gap density of states, despite yielding different values for the Urbach energy. Moving away from a strict monoexponential band tail, we demonstrated that the quasi-Fermi level splitting typical for voltage-dependent measurements probes energy ranges of the density of states where the signal of the optical measurements is below its resolution. Hence, different characterization techniques may detect different features of a density of defect states. To avoid unintentionally overlooking the limitations of a method and to maximize the information gained on the energetic disorder in organic solar cells, we recommend combining different techniques.
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