Effects Of Electrode Modification With Conjugated Polyelectrolytes And MoO₃ On The Performance Of Polymer Solar Cells

The thesis focuses on interface engineering between active layer and electrodes for polymer solar cells (PSCs). After a brief review of the interlayer materials and their functions, we carried a systematic study on the effects of interlayers from such as conjugated polyelectrolytes and MoO₃ on the performances of PSCs, especially on the open circuit voltage (Voc). We successfully increased the Voc via electrode modification and analyzed the origin of its enhancement. In addition, we proposed a novel intermediate layer for polymer tandem solar cells with the structure of PFN/Al/MoO₃, which provides an opportunity for further improvement of the performance of PSCs.Conjugated polyelectrolytes have been widely used as electron injection layer in PLEDs, but not very well investigated in PSCs so far. It was first time reported by our group that polyelectrolyte as cathode interlayer could significantly enhance Voc in PSCs. In this study we found out that not only Voc, both the short circuit current (Isc) and fill factor (FF) of PSCs can be also enhanced with proper polyelectrolytes of right thickness for certain donor polymers such as PFO-DBT35 when proper polyelectrolytes were used as cathode interlayer. After a careful analysis about the function of polyelectrolytes in the device and the dark I-V curves of devices, we interpreted the origin of the Voc enhancement can be explained based on Shockley's equation analysis.MoO₃ is an excellent anode interlayer. There are plenty of reports about using MoO₃ to increase the efficiency of PSCs, but much less investigated its effect on Voc enhancement. We replaced PEDOT:PSS with MoO₃ in devices of P3HT, MEH-PPV and PFO-DBT35. We found an increase in Voc of PFO-DBT35 (with a deeper HOMO) devices. With all the data considered, we attributed the increase in Voc of PFO-DBT35 devices not to a better ohmic contact between anode and active layer, but to a vertical phase separation caused by MoO₃.Finally, we proposed a new intermediate layer for polymer tandem solar cells with the structure of PFN/Al/MoO₃. This novel intermediate layer for tandem cell is chemically stable in air and can be easily processed. The intermediate layer shows good optical and electrical properties. With devices of a novel low band-gap conjugated polymer PBDT-DTNT as top cells, we gained a Voc as high as 1.65V, ECE exceeding 3%, and FF exceeding 50%. We believe that the performance of polymer tandem solar cells with PFN/Al/MoO₃ as intermediate layer will be enormously improved once a more suitable top cell is introduced.