Water/Alcohol Soluble Small Molecule Electrolyte As Electron Buffer Layer For Organic Solar Cell

With respect to the commercialized but expensive inorganic silicon solar cells, as a new type of solar photovoltaic device, the organic solar cells has attracted more and more attention, which is due to that it can obtain a flexible devices by solution processible, using the roll to roll printing process. Although the power conversion efficiency of organic solar cells is exceed 11%, it still far away from to the organic solar cell commercial production. In order to further improve the performance of the organic solar cell, the design and synthesis of the high efficient donor/acceptor material, and optimize their phase separation morphology are needed. Meanwhile, the design of suitable interface material which can effectively modifiy the work function and improve charge transportation is significant. Moreover, increasing its own charge conductivity can avoid the thickness-sensitivity issue and facilitate its large-scale printing preparation.Currently, the electrolyte interface materials with conjugated backbone and polar side chains are being increasingly applied to the polymer solar cell(PSC), which mainly due to the electrolyte can soluble in environment friendly solution and effectively refine the interface work function, simultaneously improve the efficiency and stability of the PSC. However, the mechanism of the modification in electrolyte is still not clear. In this paper, a variety of n-type small molecules electrolyte(SME) were applied to study the effect of pendant polar groups in their side chain on the device performance when act as the electron buffer layer.Firstly, Three novel n-type alcohol-soluble small molecule electrolytes(n-SMEs) based on diketopyrrolopyrrole(DPP) backbone functionalized with different polar group(diethylamino, diethanolamino, quaternary ammonium), as called DPPN, DPPNOH and DPPNBr, have been designed and synthesized, respectively. The n-SMEs can be easily deposited on the zinc oxide(ZnO) surface to modify Zn O defects, simultaneously fine-tune the work function of ZnO and improve compatible interaction at the ZnO/active layer inorganic/organic interface. Inspiringly, the device based on P3 HT with ZnO/DPPNOH as ETL delivers a notable power conversion efficiency(PCE) of 4.0%, which is 38% enhancement than the control device with pure ZnO as ETL. The devices based ZnO/DPPNBr and ZnO/DPPN as ETLs also display the improved PCE of 3.7% and 3.1%, respectively. The enhancement of the PCE at different degree is mainly caused by the various interactions between ZnO and the small molecule electrolytes. Notably, the device based on PTB7 with ZnO/DPPNOH as ETL can dramatically improved the PCE to 8.0%. In addition, ZnO/n-SMEs ETLs also ensure the devices with long-term stability.Secondly, on the basis of the first work, the bromide CA in DPPN-Br were exchanged with the larger size tetrafluoroborate anion(BF4-) and tetraphenylborate anion(BPh4-) to achieve DPPN-BF4 and DPPN-BPh4, respectively. Intriguingly, an unexpected n-type self-doping was observed in DPPN-BF4 and DPPN-Ph4 films at ambient, in sharp contrast to the DPPN-Br film without doping effect. A more pronounced effect occurred in DPPN-BPh4 relative to DPPN-BF4, showing a strong CA size dependence of self-doping. Such stable and intrinsic n-type doping is ascribed to the synergistic function of n-type DPP backbone and bulky CAs that substantially increase the electron affinity(EA) for more easily reduction. Meanwhile, the bulky size favors CAs to escape from the pendant cations for more actively doping on the DPP skeleton. More interestingly, the tunable dipoles were achieved on the ITO surface modified with electrolytes by simply varying the size of CAs, as verified by the significantly reduced effective work function(?eff), resulting from the combined contributions of ions-induced dipoles and doping-induce dipoles. Consequently, these n-type self-doped electrolytes possessed a higher electron mobility and created Ohmic contact between the ITO and active layer. Notably, the device based on PTB7-Th and PC71 BM with DPPN-BPh4 interlayer further improved the PCE to 9.0%.