Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been recognized as a promising solution to energy and environmental issues. Recently, signifi cant progress has resulted in power conversion effi ciencies (PCEs) of more than 7% in PSCs containing single active layers fabricated by physically blending p-type conjugated polymers and n-type fullerenederivatives. [ 1 ] Even higher PCEs have been reached with improved device architectures and by using interfacial layer modifi cations. [ 2 ] Along with good performance, the practical use of PSCs requires consideration of device stability and fabrication costs. The stability of conventional PSCs using a device architecture of indium tin oxide (ITO)/anodic buffer layer (ABL)/active layer/Ca/Al suffers from rapid oxidation of the low-work-function metal cathodes and the unstable activelayer morphology. Extended device lifetimes were reached via the design of an inverted device architecture (ITO/ZnO/active layer/ABL/Ag), which precludes the use of easily oxidized metal electrodes, [ 3 ] and via developments in thermally stable polymeric active layers. [ 4 ] As for fabrication costs, because of the potential of utilizing low-cost solution and roll-to-roll processes, PSCs are advantageous for the mass production of large-area, light-weight, and fl exible photovoltaic devices with very attractive energy payback time. [ 5 ] Recent breakthroughs in the usage of physisorbed polyethylenimine-modifi ed conductors as low-work-function cathodic materials further paves the way to stable, low-cost, and large-area cathodes; these conductors greatly simplify the production of inverted PSCs. [ 6 ] In addition to the active material, morphology, and device architecture, the performance and stability of PSCs are strongly related to the interfacial properties between the active layers and electrodes. [ 7 ] ABLs have been used to improve anode contact, hole collectionb and device performance in both conventional and inverted PSCs. Because the acidic and hygroscopic nature of the widely used ABL, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), are detrimental to device stability, great efforts have been devoted to developing highwork-function and air-stable transition metal oxides (TMOs)such as VO x , NiO x , MoO 3 , WO 3 , and ReO 3 [ 8-12 ] -as substitutes for PEDOT:PSS. [ 7a , 13 ] The tunability in the chemical and electronic properties of the TMOs also enables good energy-level alignment between the TMOs and a wide variety of organic molecules, resulting in low contact resistance and high device effi ciency. [ 14 ] Numerous solution-processable TMO (sTMO) ABLs have shown their effectiveness in conventional cells. The highest PCE values of 7.75% and 7.62% have been reported in single-layer PSCs using solution-processable hydrogen molybdenum bronze and hydrogen vanadium bronze as the ABL; [ 15 ] however, success in inverted PSCs remains limited. Although thermally evaporated TMO (eTMO) ABLs, such as evaporated MoO 3 (eMoO 3 ), have proven their effi cacy in...

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