N-type Organic Electron Transfer Layer For High Performance Polymer Solar Cells

Polymer solar cells(PSCs) have attracted a great deal of attention due to their lowcost, large area, easy solution-based fabrication with mechanical flexibility and the general applicability of organic materials. During the past decades, PSCs have achieved tremendous progress and the state-of-the-art devices have already exceeded 10%. To obtain a more brilliant performance, the most common and successful strategy is developing an original photoactive layer material and fabricating a desirable device structure. Nevertheless, the interface engineering in the device also plays a significant role for a breakup in the superior efficiency and stability. An excellent interlayer can not only build a tunable energy level alignment between the contact interface to facilitate the charge extraction and transportation but also promote interfacial stability for a high-efficiency and stable device eventually.In this paper, we report the design and synthesis of two novel N type organic interfacial materials to improve the interface contact and tune the contact interface energy alignment on the premise of a superior electron mobility, giving rise to an enhancing of the PSCs device performance eventually. Firstly, A novel polymerizable fullerene derivative, 2-(2-(2-methoxyethoxy) ethoxy) ethyl undec-10-enyl malonate C60(EEMC), functionalized with ethenyl and tri-(ethylene oxide)(TEO), has been rationally designed and synthesized. The functionalized fullerene can easily selfassemble on the surface of ZnO to form an immobilized and robust fullerene/ZnO electron-transporting layer(ETL) for the inverted solar cells. The oxygen groups of TEO anchoring on ZnO effectively passivate the surface traps of ZnO and induce a smoother and more hydrophobic surface. Additionally, the solution processed and assembled fullerene/Zn O ETL can be further stabilized by a polymerized network resulting from ethenyl groups undergoing subsequent thermal annealing. Moreover, the EEMC is a typical N type organic semiconductor endowing itself a superior conductivity and electron mobility. As a result, the performance and stability of the inverted solar cell device is improved by incorporation of such novel ETL; Furthermore, A novel alcohol-soluble n-type conjugated polyelectrolyte(n-CPE) poly-2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-2,5-bis[6-(N,N,N-trimethylammonium)hexyl]-3,6-bis(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione(PDPPNBr) is synthesized to replace the common ZnO as an ETL in an inverted PSCs device. Because of the electron-deficient nature of diketopyrrolopyrrole(DPP) backbone and its planar structure, PDPPNBr is endowed with high conductivity and electron mobility. Additionally, the pendant ionic functionalities not only ensures CPEs dissolve in water/alcohol solution but also can act as a remarkable interfacial material to establish a connection between hydrophilic electrode and the hydrophobic organic layer in PSCs. And the interfacial dipole moment created by n-CPE PDPPNBr can substantially reduce the work function of ITO and induce a better energy alignment in the device, facilitating electron extraction and decreasing exactions recombination at active layer/cathode interface. As a result, the power conversion efficiency(PCE) of the inverted devices based PDPPNBr as cathode interface layer could improve the device performance from 7.15%(ITO/ZnO/PTB7: PC71BM/MoO3/Ag) to 8.02%(ITO/PDPPNBr/PTB7: PC71BM/MoO3/Ag). It's noteworthy that this n-CPE exists a characteristic with a thickness of around 30 nm, which is quite different from the traditional p-type conjugated and non-conjugated polyelectrolytes ETLs with only several nanometers.Above all, the successful application of these solution processed N-type organic ETLs attributed to the improved the surface contact between the cathode and the organic active layer and a better energy alignment in the device as well as an excellent electron mobility would be beneficial to a further optimizing PSCs device and be anticipated to be used for fabrication of printable and large scale PSCs.