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

Abstract

The interface energetics and intermixing effects of the donor/acceptor bulk heterojunction (BHJ) blends of poly[2,3‐bis‐(3‐octyloxyphenyl) quinoxaline‐5, 8‐dilyl‐alt‐thiophene‐2, 5‐diyl]: [6,6]‐phenyl C71butyric acid methyl ester (TQ1:PC71BM) have been investigated using ultraviolet photoemission spectroscopy (UPS) in combination with the integer charge transfer model. The TQ1:PC71BM represents the useful model system for BHJ organic photovoltaics featuring effective charge generation and transport. It finds out that the positive integer charge state of TQ1 are equal in energy to the negative integer charge state of PC71BM, leading to a negligible potential step at TQ1:PC71BM interface and thus the vacuum level alignment. It is observed that the TQ1 accumulates on the top of TQ1:PC71BM BHJ and UPS spectra as function of various blend ratios suggest that the TQ1 mixes finely with PC71BM with the little work function modification in a wide range. In addition, no significant influence of the long‐range Coulomb interactions or the intermolecular hybridization on the occupied electronic structures is present for the well‐intermixed TQ1:PC71BM BHJs. These findings provide deep insights into the properties of BHJ blends and are beneficial for the performance optimization in organic photovoltaics.

Enhanced and Balanced Charge Transport Boosting Ternary Solar Cells Over 17% Efficiency

“…[20] Our previous work has demonstrated that the charge recombination loss in operating OSCs can be effectively suppressed via heterojunction ICT states, leading to the enhancement of V oc . [20,21] Blend morphologies that possess fine phase separation and molecular packing can create highly effective charge transfer networks in the ternary system, resulting in improved charge transport and thus high device FF. [11,[22][23][24] Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs).…”

“…It is known that the electronic structure at the donor-acceptor interface affects charge generation and transport, and thus device performance. [21] Hence, ultraviolet photoelectron spectroscopy (UPS) is performed to estimate the energy levels of the donors and acceptor ( Figure S3, Supporting Information). The ionization potential (IP) is directly derived from the secondary electron cut-off and the frontier edge of the occupied density of the states.…”

“…Consequently, there are "dark" occupied charge transfer electron-hole pairs at the interface that enhance the percentage of excitons converted into free charges at the ternary heterojunctions due to the reduced probability of the free charges trapped by Coulomb force during device operation, while minimizing the trapassisted recombination via ICT states and their contribution to V oc loss. [20,21] As the E ICT+ of the second donor DRTB-T-C4 is larger than the E ICT− of Y6, no ground state charge transfer occurs at the DRTB-T-C4:Y6 interface and hence vacuum level alignment (no potential step), which can also avoid the ICT trap-assisted recombination at the interface. [20] The ternary and the binary reference devices with the conventional structure of ITO (indium tin oxide)/poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/ active layer/PFN-Br/Ag are fabricated as illustrated in Figure S4 in the Supporting Information.…”

Uphill and downhill charge generation from charge transfer to charge separated states in organic solar cells

“…By UPS depth profiling not only the compositional profile of donor and acceptor could be optained, but also the depth dependent HOMO-HOMO offsets between them were recorded 27 . 30 ).…”

Recent progress in organic solar cells (Part II device engineering)

“…The energy level alignment at the D:A BHJ interface can be precisely described by the ICT model based on the results from PES (Figure 7) [69] ICT state is related to the smallest energy required to take away one electron (the largest energy gained from adding one electron) from (to) the organic semiconductor molecule at the heterojunction interface producing the fully relaxed state. The values of E ICT+/− are determined by separately conducting UPS measurement on donor and acceptor films deposited on the different substrates with a large range of WFs.…”