Abstract
AbstractFree carrier photogeneration in bulk‐heterojunction solar cells composed of blends of acceptor and donor organic semiconductors proceeds via intermolecular charge transfer (CT) states. Non‐adiabatic Marcus theory has proven valid to explain the absorption and emission of these sub‐gap states which have extremely weak emission probabilities and absorption cross sections making them difficult to probe directly using optical spectroscopy. Therefore, the CT state parameters involved in the Marcus model are often extracted from fittings on the photovoltaic external quantum efficiency (EQEPV) and electroluminescence. These two spectra are (ideally) interrelated via the so‐called reciprocity principle. In this paper, the limitations of such an approach are demonstrated, in particular the impact of simple low finesse cavity interference effects acting as an uneven spectral filter for emission and absorption. This can produce almost spurious CT state parameterization with, for example, relative errors as large as 90% in absorption coefficients obtained from EQEPV. It is shown how these limitations can be partially lifted using an iterative transfer matrix approach applied to the EQEPV.
Highlights
Free carrier photogeneration in bulk-heterojunction solar cells composed of tion (BHJ) organic solar cell results in new blends of acceptor and donor organic semiconductors proceeds via intermolecular charge transfer (CT) states
These limitations arise from how light absorption and emission are affected differently by the low-finesse cavity mode whose energy is dependent upon the thickness of the junction. This is probed and quantified in detail for one given system, and we demonstrate that the absorption pre-factor obtained from fittings on the absorption coefficient estimated from the EQEPV via Equation (4) contains a large relative error which is thickness dependent
Parameterization of the CT states of acceptor: donor organic semiconducting combinations based upon Gaussian fittings to sub-gap absorption or EQEPV and EL spectra has become a standard tool in organic solar cells
Summary
To exemplify the accuracy of the fittings and CT analysis based upon Equations (1) and (2) on EQEPV and EL spectra we selected six different BHJ solar cell material combinations. In order to further probe the potential impact of interference effects on the EQEPV spectra we analyzed PCDTBT:PC70BM devices with multiple different junction thicknesses This provided a means to evaluate the absorption pre-factor (fα) from the absorption coefficient spectra of the CT states. Oscillations in the EQEPV spectra are often not observed in devices with standard active layer thicknesses of order of 100s of nm Such thin active layers form extremely low-finesse cavities with large mode separation and spectrally broad modes as seen from Figure 4 and the slowly varying sine function in Equation (6) when d is small. To unambiguously demonstrate the role of cavity interference in the subband gap EQEPV spectral shape, we fabricated an extremely thick PCDTBT:PC70BM device with an active layer thickness of 1500 nm For this device, a shorter mode separation is expected in accordance with Figure 4.
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