Crystalline versus Amorphous Donor-Acceptor Blends: Influence of Layer Morphology on the Charge-Transfer Density of States

Abstract

Organic small molecule solar cells are used as a test bed to investigate the influence of film morphology on the density of charge-transfer (CT) states. CT states are considered as precursors for charge generation and their energy as the upper limit for the open-circuit voltage in organic donor-acceptor solar cells. In this study the influence of morphology for two perylene donors [crystalline diindenoperylene (DIP) versus amorphous tetraphenyldibenzoperiflanthene (DBP)] with almost identical ionization energy is investigated. As acceptor material, the fullerene C 60 is used. By combining device measurements with optical and low-energy ultraviolet photoelectron spectroscopy, a comprehensive picture is obtained that describes how morphology and the connected density of states (DOS) affect device performance and the spectroscopic signature of CT states. Especially for the crystalline donor material DIP, strong exponential tail states reaching far into the gap are observed, which can be related to the presence of grain boundaries. A voltage-dependent filling of these states is identified as the origin of a blue shift of electroluminescence spectra with increasing applied voltage. Different approaches are compared to study the influence of static and dynamic disorder in the description of CT emission and absorption spectra of organic solar cells. Despite the fact that both donors yield almost identical CT energy (and, thus, the same open-circuit voltage) the Stokes shift between photocurrent and electroluminescence spectra and, concomitantly, the width of the CT DOS varies by more than a factor of 2. We discuss this observation in terms of the donoracceptor reorganization energy as well as an additional line broadening by static disorder. Remarkably, the more crystalline donor DIP shows a significant deviation from a Marcus-type description, while this is not the case for the amorphous DBP. This highlights the importance of film morphology in organic solar cells.

Temperature dependence of the spectral line-width of charge-transfer state emission in organic solar cells; staticvs.dynamic disorder

“…Our experimental observations on the other hand do not corroborate Kahle's claim on the strong influence of static disorder. We finally note the very recent manuscript by Linderl et al, 58 who observed a very similar behavior in DBP:C 60 devices as we have done herein.…”

Influence of static disorder of charge transfer state on voltage loss in organic photovoltaics

“…This type of broadening is referred to as dynamic disorder 13 . Whilst dynamic disorder is temperature ( T ) dependent, static disorder is not and T-dependent CT absorption or emission measurements have therefore often been used to differentiate them 11 , 14 – 17 . Using T-dependent measurements, recent studies by Ortmann, Vandewal, Deibel and co-workers 14 , 16 , 18 have suggested that static disorder is less important in OPV than dynamic disorder, but other studies have observed the opposite 9 , 11 , 15 , 17 .…”

“…Whilst dynamic disorder is temperature ( T ) dependent, static disorder is not and T-dependent CT absorption or emission measurements have therefore often been used to differentiate them 11 , 14 – 17 . Using T-dependent measurements, recent studies by Ortmann, Vandewal, Deibel and co-workers 14 , 16 , 18 have suggested that static disorder is less important in OPV than dynamic disorder, but other studies have observed the opposite 9 , 11 , 15 , 17 . Although this debate is still ongoing, there is a large amount of computational and experimental evidence that the CT-state energy is sensitive to the microstructure, composition, and interfacial properties of organic semiconductor blends 19 – 31 , and multiple CT states have also been observed in several studies 26 , 29 , 32 , 33 .…”

“…A model that includes static disorder would allow us to account for the impact on voltage losses of phenomena that are not currently included, such as microstructure and conformational disorder of the molecules. Several studies to date have incorporated static disorder in order to model the luminescence behaviour 11 , 17 , 42 of OPV. Those models are however all limited to a gaussian distribution of CT states (CT-DoS) and therefore cannot be applied to the general cases where a number of different CT states are present; 26 , 29 , 32 , 33 moreover, these models usually (ref.…”

“…Those models are however all limited to a gaussian distribution of CT states (CT-DoS) and therefore cannot be applied to the general cases where a number of different CT states are present; 26 , 29 , 32 , 33 moreover, these models usually (ref. 11 , 17 , 42 ) fail to obey the principle of detailed balance (see Supplementary Fig. 1 and Supplementary Note 1 ).…”