Universal and Versatile MoO3-Based Hole Transport Layers for Efficient and Stable Polymer Solar Cells

Two solution-processed and highly dispersed MoO3 called d-(MoO3)(120) and d-(MoO3)(15) with sizes of 120 nm and extremely smaller 15 nm, respectively, are applied into polymer solar cells, and the evaporated MoO3 as hole transport layers (HTLs) in devices is also compared. It is the first time it has been found that the different size of MoO3 can induce the quite different morphologies of the HTLs and their upper active layers due to the unexpectedly caused difference in the surface energy levels. It is worthy to note that the performance of the device with solution-processed d-(MoO3)(15) is higher than that of the device with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTLs and even comparable to that of the device with optimized evaporated-MoO3. Simulated by the transfer matrix method, the light intensity and the exciton generation rate in the active layer are found to be greatly enhanced by incorporation of an ultrathin MoO3 combined with PEDOT:PSS. As a result, by inserting a layer of evaporated MoO3 (e-MoO3) between the ITO and PEDOT:PSS, power conversion efficiency (PCE) can be dramatically improved to 7.10% for PBDTTT-C-T:PC71BM. Moreover, the e-MoO3/PEDOT:PSS bilayer also ensures good stability for the devices, due to the MoO3 preventing moisture and oxygen attack and protecting ITO from corrosion caused by the acid PEDOT:PSS.