Unraveling the Role of Substrates on Interface Energetics and Morphology of PCDTBT:PC₇₀BM Bulk Heterojunction

Abstract

still some obstacles for holding back the further development.The state-of-the-art device structure of PSCs is based on the bulk heterojunction (BHJ) formed by conjugated polymers (electron donor) and fullerene derivatives (electron acceptor). It is commonly recognized that the interfacial properties (e.g., energetics and morphology) between the BHJ and electrodes are decisive for charge transport and extraction process, which determine the device performance and long-term stability. [12][13][14][15] For instance, the energy offset between the lowest unoccupied molecular orbital level (LUMO) of the acceptor and the highest occupied molecular orbital level (HOMO) of the donor sets an upper limit for the open-circuit voltage ( V oc ) in PSCs. [16][17][18][19][20][21][22][23] However, for various organic/substrate contacts and organic donor/acceptor heterostructures with weak interfacial interaction and electronic coupling, energy level alignments (ELAs) at the interfaces are highly dependent on the substrates, revealing a clear transition from the traditionally used vacuum level alignment (VLA) mode to the Fermi level pinning (FLP) mode. [ 12,15,[22][23][24][25][26][27] When the substrate work function (Φ SUB ) lies between the ionization potential (IP) and electron affi nity (EA) values of the organic overlayer, VLA holds for the interface energetics and the work function of the organic overlayer (Φ ORG/SUB ) equals to the Φ SUB . If Φ SUB is larger (or lower) than IP (or EA) of the organic overlayer, spontaneous charge transfer occurs from the organic layer to the substrate (or from the substrate to the organic layer), creating an interface dipole equivalent to Φ SUB -Φ ORG/SUB . The resultant ELAs are determined by the FLP to the HOMO edge (or LUMO edge) of the organic layer. Recently, several models have been proposed to interpret the energetics transition between VLA and FLP. For example, Fahlman and coworkers proposed the integer charge transfer (ICT) model to describe organic/organic interfaces that are characterized by a negligible hybridization of π-electronic molecular orbitals and substrate wave functions. [ 12,28 ] Koch and co-workers used the numerical electrostatic model to reproduce the alignment between the electrode Fermi energy and the transport states in organic semiconductors. [ 26 ] Similarly, the universal ELAs of organic molecules on metal oxides have been experimentally summarized, [ 29,30 ] and theoretically calculated based on Fermi statistics. [ 31 ] In addition, surface properties of the substrates have a substantial infl uence on the fi lm morphology of

Intermixing Effect on Electronic Structures of TQ1:PC71BM Bulk Heterojunction in Organic Photovoltaics

“…It is evident in Figure a and b that the frontier electronic features of TQ1:PC 71 BM blend films in low binding energy are dominated by the TQ1 component. This characteristic is consistent with previous reports of polymer: fullerene derivative blends, which has been ascribed to the accumulation of the donor polymer at the top surface of the BHJ blend due to its lower surface energy . Upon increasing the TQ1 concentration from 1:10 to 10:1, the onsets of the secondary electron cutoff and the HOMO edges gradually shift toward higher binding energy, implying a smaller work function.…”

“…This characteristic is consistent with previous reports of polymer: fullerene derivative blends, which has been ascribed to the accumulation of the donor polymer at the top surface of the BHJ blend due to its lower surface energy. [35][36][37] Upon increasing the TQ1 concentration from 1:10 to 10:1, the onsets of the secondary electron cutoff and the HOMO edges gradually shift toward higher binding energy, implying a smaller work function. However, as shown in Figure 4c, the IP is not obviously affected by the ratios, which keep almost constant.…”

Efficient Color-Stable Inverted White Organic Light-Emitting Diodes with Outcoupling-Enhanced ZnO Layer

“…Figure b displays the UPS spectra of the secondary electron cutoff (SECO) and the highest occupied molecular orbital (HOMO) regions of ZnO substrates with various surface modification and 10 nm-thick TmPyPb layers deposited onto various substrates, respectively (see the details of UPS measurements in the Experimental Section). , Vertical bars in Figure b mark the SECO and HOMO edge of the corresponding layers, which can be determined by the intercept between the background level and the extrapolation of the cutoffs. As displayed in Figure b, a PEI layer on ZnO leads to the SECO shift toward the high binding energy region, corresponding to the vacuum level (VL) shift toward E F and the lowering of the ZnO work function from 4.0 to 3.1 eV.…”

“…Optical transmittance spectra were recorded by a UV–vis/near-IR spectrophotometer (PerkinElmer Lambda 750) with an integrating sphere. The ultraviolet photoemission spectroscopy (UPS) measurement was conducted to characterize the valence states and vacuum level (VL) in a Kratos AXIS Ultra-DLD ultrahigh vacuum system, which consists of an analysis chamber and an evaporation chamber for in situ material deposition. , The UPS measurements were performed with an unfiltered He I ( h ν = 21.22 eV) gas discharge lamp and a total instrumental energy resolution of 100 meV. The 1,3,5-tri­[(3-pyridyl)-phen-3-yl]­benzene (TmPyPb) and Cs 2 CO 3 -doped tris­(8-hydroxyquinolate) aluminum (Alq 3 ) layers with a thickness of 10 nm were in situ deposited on pristine ZnO and PEI-coated ZnO substrates in the evaporation chamber, and transferred to the analysis chamber for UPS measurements without breaking the vacuum.…”

Fabrication Processes to Generate Concentration Gradients in Polymer Solar Cell Active Layers

“…Note that the effect of annealing and interfacial materials on the active layer morphology and vertical distribution is not limited to the P3HT:PC 61 BM pair but has also been studied in active layers based on highly efficient solution-processed EDs [ 36 , 37 , 38 , 39 , 40 ]. Poly[ N -9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and its derivatives have been increasingly studied since 2007 and devices based on PCDTBT paired with fullerene derivatives now often exhibit PCEs over 6% [ 41 , 42 , 43 , 44 ].…”