Oleamide as a self-assembled cathode buffer layer for polymer solar cells: the role of the terminal group on the function of the surfactant

Abstract

An amphiphilic surfactant oleamide was incorporated into P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs) as a novel cathode buffer layer (CBL) for the first time by doping in the P3HT:PCBM photoactive layer followed by self-assembly. The power conversion efficiency (PCE) of the annealed P3HT:PCBM/oleamide BHJ-PSC device is enhanced by $28% at the optimum oleamide doping ratio of 2.5%, which is primarily due to the increase of fill factor (FF) by $22%. The surface morphologies of the oleamide-incorporated P3HT:PCBM photoactive films were studied by transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning Kelvin probe microscopy (SKPM), revealing that oleamide molecules initially doped in the P3HT:PCBM layer may undergo self-assembly and migrate to the top surface of the P3HT:PCBM layer, leading to the formation of a cathode buffer layer (CBL) as an interfacial dipole layer between the photoactive layer and Al cathode electrode. Such an oleamide interfacial dipole layer lowers the work function of Al, thus the energy level offset between the work function of Al and the LUMO level of the PCBM acceptor is decreased, facilitating the electron extraction by the Al cathode. Furthermore, we found that the crystallinity of P3HT upon the incorporation of oleamide was almost unchanged according to X-ray diffraction (XRD) characterization. It is noteworthy that, this phenomenon is completely different from the case of the previously reported analogous surfactant oleic acid, which was doped in the P3HT:PCBM photoactive layer and led to the efficiency enhancement as well due to the increased crystallinity of P3HT, suggesting the strong influence of the terminal group of the surfactant on its function in P3HT:PCBM BHJ-PSC devices.

An alcohol-soluble perylene diimide derivative as cathode interfacial layer for PDI-based nonfullerene organic solar cells

“…The stark difference in surface potential suggests that PDIP interlayer could induce a microscopic electric dipole moment with the positive charge end pointing toward Al electrode and the negative charge end pointing toward the active layer (as shown in Fig. 5C) [38,40]. The direction of the dipole moment formed at the interface between active layer and electrode is identical to the built-in potential, which implies that the actual built-in potential across the device is reinforced.…”

Solution-processed cathode interfacial layer materials for high-efficiency polymer solar cells

“…Later, similar to the previous work, oleamide was introduced into the donor/accepter blend to act as an efficient cathode interfacial layer by Zhang et al [45]. The authors found that oleamide molecules tend to self-assemble and migrate to the [53][54][55].…”

Efficient Polymer Solar Cells with Alcohol-Soluble Zirconium(IV) Isopropoxide Cathode Buffer Layer

“…PBDTBDD was synthesized according to previous report [14]. All the commercially available materials were used as received without further purification.…”

“…Unfortunately, large scale practical application has been inhibited by the poor stability of the devices because the commonly adopted cathode buffer layer (CBL, e.g., calcium (Ca) or barium (Ba)) is prone to oxidative degradation as well as the instable photoactive layer and anode modification layer [9][10][11]. In addition, it is difficult to avoid the interdiffusion of active metal atoms of CBL at the photoactive-layer/metal interface during the thermal evaporation of the CBL, and thus, this leads to an evident performance decrease for PSCs [12] Undoubtedly, the alternative CBL candidates of alkali metal compounds (such as LiF, CsF and Cs 2 CO 3 ) suffer from both of the vacuum involved deposition techniques, which are incompatible with all solution processed PSCs [13,14]. Therefore, considerable efforts have been directed to exploring a stable and solution-processable CBL between an active layer and a cathode since interfacial engineering is a critical factor for the stability improvement of PSCs [15].…”