Indolocarbazole (Ic) conjugated small molecular electrolytes (CSMEs) with different amine side chains are synthesized and applied to the formation of an electron transporting layer (ETL), via spontaneous phase separation, in bulk heterojunction (BHJ) polymer solar cells (PSCs). The self-assembled bilayer with a BHJ-active layer on top and a CSME layer at the bottom is generated by a single coating step of one solution of CSME and BHJ-active layer materials, CSME:PTB7-Th:PC71BM. The location of the CSME layer is confirmed by time-of-flight secondary ion mass spectroscopy. The CSME layer plays the role of an ETL, which reduces the work function of indium tin oxide (ITO) effectively and leads to high power conversion efficiency (PCE) of the CSME-modified PSC. We also use X-ray photoelectron spectroscopy to test the relative intensity of hydrogen bonds between the CSME and ITO. With the increased relative intensity of hydrogen bonds between the CSME and ITO, more effective spontaneous phase separation of the CSME/BHJ bilayer can be formed to generate the CSME layer on top of ITO, and the photovoltaic performance of the PSC enhanced. Out of the synthesized series of CSMEs (IcL4N, IcL6N, IcL8N, IcL10N, and IcB8N), the IcB8N-based device shows the highest relative intensity of hydrogen bonding and the highest PCE with a value of 7.09% without using any other ETL materials, which is the highest PCE value without using any other ETL in a separate processing step, in I-PSCs utilizing PTB7-Th:PC71BM.

We report the design and synthesis of phenothiazine-based conjugated small-molecular electrolytes as an ETL that could be applied to provide spontaneous phase separation, to reduce the number of steps required for device fabrication.