Abstract
Spectroscopic measurements of charge transfer (CT) states provide valuable insight into the voltage losses in organic photovoltaics (OPVs). Correct interpretation of CT-state spectra depends on knowledge of the underlying broadening mechanisms, and the relative importance of molecular vibrational broadening and variations in the CT-state energy (static disorder). Here, we present a physical model, that obeys the principle of detailed balance between photon absorption and emission, of the impact of CT-state static disorder on voltage losses in OPVs. We demonstrate that neglect of CT-state disorder in the analysis of spectra may lead to incorrect estimation of voltage losses in OPV devices. We show, using measurements of polymer:non-fullerene blends of different composition, how our model can be used to infer variations in CT-state energy distribution that result from variations in film microstructure. This work highlights the potential impact of static disorder on the characteristics of disordered organic blend devices.
Highlights
Spectroscopic measurements of charge transfer (CT) states provide valuable insight into the voltage losses in organic photovoltaics (OPVs)
A charge-transfer (CT) state at a donor–acceptor (D–A) interface in an organic photovoltaic (OPV) device is an intermediate state present after a charge transfer transition in which the electron and the hole reside on either side of the interface[1,2,3,4,5,6]. The properties of this CT-state have been shown to largely determine the open-circuit voltage loss (Vloss) of OPV devices[7,8], which is defined by Vloss 1⁄4 Eg=q À Voc, where Voc is the opencircuit voltage of the solar cell, Eg is its optical gap and q is the elementary charge
We demonstrated that static disorder tends to increase voltage losses in OPV devices, as previously reported, and that it is important in certain well-studied polymer: molecular acceptor material systems, in agreement with ref. 9,11,17
Summary
FiguÀre 1aÁ, b illustrates a general energetic distribution of CT states, ECT , that might result at a D–A heterointerface in which a donor domain is surrounded by acceptor domains of different sizes and strength of interaction with the donor[25]. ECT;Ct À 5σCT;t and b 1⁄4 ECT;Ct þ 5σCT;t: As in previous models[7,8,38], we assume that radiative and nonradiative recombination occur only via the CT states, either directly after exciton dissociation or by reformation of the CT state from free charges. The radiative recombination (kabsð_ωÞ) rate constants per photon energy (s−1 eV−1) and absorption (krð_ωÞ) (s−1 eV−1) can be expressed as the sum of the contribution from all CT manifolds using a similar expression and all depend on the FCWD8, via kabsð_ωÞ
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