Comparing Methods of Characterizing Energetic Disorder in Organic Solar Cells

Abstract

The energetic disorder has been known for decades to limit the performance of structurally disordered semiconductors such as amorphous silicon and organic semiconductors. However, in the past years, high‐performance organic solar cells have emerged showing a continuously reduced amount of energetic disorder. While searching for future high‐efficiency material systems, it is therefore important to correctly characterize this energetic disorder. While there are several techniques in the literature, the most common approaches to probe the density of defect states are using optical excitation as in external quantum efficiency measurements, or sequential filling of the tail states by applying an external voltage as in admittance spectroscopy. A metanalysis of available literature, as well as the experiments using four characterization techniques on two material systems, reveal that electrical, voltage‐dependent measurements frequently yield higher values of energetic disorder than optical measurements. With drift‐diffusion simulations, it is demonstrated that the approaches probe different energy ranges of the subband‐gap density of states. The limitations of the techniques are further explored and it is found that extraction of information from a capacitance‐voltage curve can be inhibited by internal series resistance. Thereby, the discrepancies between measurement techniques with sensitivity to different energy ranges and electronic parameters are explained.

Discerning rise time constants to quantify charge carrier extraction in perovskite solar cells

“…The large values between 1-1.3 eV followed by the transition at lower DE f has recently been interpreted as originating from SRH recombination via shallow traps. 50,51 The origin of the slope factor at large DE f is unclear, though we speculate it could be influenced by transport effects due to the position-dependent generation profiles of carriers in the perovskite at very early times. In order to compare the time constants obtained from electrical measurements on the full device with those obtained from tr-PL measurements on the glass/ITO/PTAA/Cs 0.05 FA 0.8 MA 0.15 PbI 2.25 -Br 0.75 /C 60 /BCP stack, we assume that the DE f of the stack is identical to the open-circuit voltage of the full device under the same illumination conditions (as has been previously observed 52 ).…”

Understanding Method-Dependent Differences in Urbach Energies in Halide Perovskites

“…Such discrepancies in Urbach energy values have also been observed by Ledinsky et al when comparing temperature-dependent FTPS- and PL-based Urbach energy analyses of MAPbI 3 films . In both works, the discrepancies are assumed to result from considering different analyzed energy ranges with different contribution of band-to-band absorption increasing near the band edge, which causes deviations from the disorder-induced exponential band-edge shape. , Such energy range-dependent discrepancies have also been discussed in the field of organic semiconductors (optical PDS, optical FTPS, and electrical admittance spectroscopy). , However, although the Urbach energy is a popular and important figure of merit for the performance potential of semiconductor materials, a profound understanding of the origin of the prevailing discrepancies in Urbach energy values is still missing.…”

“…The Urbach energy is a popular structural characteristic representing energetic disorder in crystalline (e.g., GaAs, Si, MAPbI 3 , CIGS) , and also semicrystalline (such as organic) semiconductors for optoelectronic applications. , The relevance of the Urbach energy manifests itself in its correlation with structural and optoelectronic properties − and thus also with optoelectronic functionality. ,,− For example, the Urbach energy is related to the minimal open-circuit-voltage deficit achievable when a semiconductor is used for solar cell applications. ,,,,− Accordingly, the Urbach energy is a valuable figure of merit, expressing the performance potential of present and future semiconductor materials in a single number …”

Trap‐Assisted Charge Generation and Recombination in State‐of‐the‐Art Organic Photodetectors

“…Research spanning over more than a decade successfully links the disorder to the photovoltaic parameters of organic BHJ solar cells. [ 36–41 ] With typical E 0 numbers of 20–40 meV, Figure 2b shows that the density of states decreases rapidly into the bandgap. If there were no states in the gap other than these band tails, then the minimum density of states at mid‐gap, using the value of 40 meV for the characteristic energies, N C and N V of 2 × 10 20 cm −3 eV −1 and choosing an effective bandgap of 1.3 eV, which is typical for an OPD, would be ≈3 × 10 13 cm −3 eV −1 .…”