Date Published: December 01, 2016
Publisher: John Wiley and Sons Inc.
Author(s): Christian Kästner, Koen Vandewal, Daniel Ayuk Mbi Egbe, Harald Hoppe.
Efficient charge generation via exciton dissociation in organic bulk heterojunctions necessitates donor–acceptor interfaces, e.g., between a conjugated polymer and a fullerene derivative. Furthermore, aggregation and corresponding structural order of polymer and fullerene domains result in energetic relaxations of molecular energy levels toward smaller energy gaps as compared to the situation for amorphous phases existing in homogeneously intermixed polymer:fullerene blends. Here it is shown that these molecular energy level shifts are reflected in interfacial charge transfer (CT) transitions and depending on the existence of disordered or ordered interfacial domains. It can be done so by systematically controlling the order at the donor–acceptor interface via ternary blending of semicrystalline and amorphous model polymers with a fullerene acceptor. These variations in interfacial domain order are probed with luminescence spectroscopy, yielding various transition energies due to activation of different recombination channels at the interface. Finally, it is shown that via this analysis the energy landscape at the organic heterojunction interface can be obtained.
Photon absorption in organic semiconductors generally leads to excitons, i.e., bound electron–hole pairs, exhibiting a binding energy in the range of 0.1–1 eV.1 This is in contrast to inorganic semiconductors, where exciton binding energies are below the thermal energy (kBT), leading to free charge carriers at room temperature upon photon absorption as in silicon solar cells.2 These high exciton binding energies in organic semiconductors can be largely assigned to be due to comparatively small dielectric constants lately leading to a quest for obtaining high‐permittivity organic semiconductors by intelligent material design.3, 4, 5
In order to address the potential recombination channels depicted in Figure 1, electroluminescence spectra were recorded on complete solar cells.33 The results of these measurements are depicted in Figure2 in two representations: as obtained under constant current density of 200 mA cm−2 (left) and normalized to the CT‐emission peaks (right). From the left graph in Figure 2 it is obvious that the relative intensity of the different CT peaks is strongly varying over two orders of magnitude, emphasizing the necessity of highly sensitive detection setups having a low noise level. To the naked eye three different CT transitions with varying oscillator strength, depending on the composition ratio of semicrystalline and amorphous polymer, are visible in the right graph in Figure 2 and represent a novelty. A first transition is seen at around 859 nm (CTαEL), a second transition at around 942 nm (CTβEL) and a third transition at around 1007 nm (CTγEL). Even though already three CT transitions are clearly visible, this observation seems to be in contradiction to our expectation of four different recombination channels, as depicted in Figure 1. In order to gain higher confidence, the respective CT emission spectra were subdued to a quantitative analysis of fitting them by either Gaussian, Lorentzian, or their convolutions, Voigt profiles.
In summary, we have demonstrated the existence of several CT transitions within one single organic semiconductor bulk heterojunction. By systematic variation of the order within the conjugated polymer phase through application of mixtures of semicrystalline and amorphous representatives of one and the same polymer backbone within ternary blends with the fullerene derivative PCBM, the domain order at the heterojunction interface could be tuned from ordered–ordered, over ordered–disordered, up to disordered–disordered. In direct relation with the respective interfacial area, the strength of the corresponding CT transition varied systematically and could be therefore unambiguously assigned via electroluminescence characterization. Via photoluminescence also fullerene‐intercalated amorphous polymer phases could be detected. Thus it could be shown that the CT transition can indeed be used to probe the interfacial order at the heterojunction, allowing deep insights into the internal structure of the bulk heterojunction by simple means.
Ternary polymer:polymer:PCBM blends were prepared from mixtures of semicrystalline AnE‐PVab and amorphous AnE‐PVba with concentrations running from 0% to 100% AnE‐PVba in steps of 10%. The global polymer:PCBM weight ratio was held constant at 2:3. The synthesis of the polymers is described elsewhere.85 PCBM was used as received from supplier Nano‐C. Donors and acceptors were dissolved in a 1:1 blend of chloroform and chlorobenzene assigned to be the optimal mixture promoting phase separation in AnE‐PV:PCBM blends.86 The solution concentration was 0.4 wt% of polymer part. Thin films of polymer:PCBM blends were spin cast on glass substrates for photoluminescence spectroscopy. Solar cell device preparation for electroluminescence spectroscopy on glass involved etching part of the ITO‐layer for selective contacting of the back electrode, followed by spin coating of PEDOT:PSS (Clevios PH, Heraeus). Deposited PEDOT:PSS films were annealed at 170 °C for 15 min to release residual moisture and immediately transferred to a nitrogen filled glovebox. The top aluminium electrode was deposited by physical vapor deposition yielding an active area of 0.5 cm2. Thin film steady‐state PL spectra and solar cell EL spectra were recorded with an Avantes AvaSpec ULS‐2048 fiber spectrometer. PL excitation was applied with a laser diode emitting at 405 nm. EL was conducted at an injection current of 100 mA applied with a Keithley 2601 Source Measure Unit. For PL normalization and evaluation of optical band‐gap, thin film transmission, and reflection spectra were recorded with a Varian Cary 5000 spectrophotometer under VW condition87 and reassembled to the thin film absorption spectra. For reference, all photovoltaic parameters on this set of devices have been published in ref. 33.