To produce printable, portable, and fl exible bulk heterojunction (BHJ) solar cells, the development of effi cient, inexpensive, and high-throughput fabrication methods is critically important. A number of methods to fabricate BHJ solar cells already exist, including highvacuum deposition systems, solution processing, and direct chemical deposition on device substrates. [ 5,[14][15][16] Because solution processing is amenable to the formation of an interpenetrating donoracceptor network, while also being a costeffective approach, it is one of the most promising approaches to large-scale BHJ solar cell modules. [ 15,16 ] Among various solution processable donor-acceptor systems, polymer BHJ solar cells based on interpenetrating networks of conjugated polymer and fullerene derivatives as donor and acceptor materials have exhibited a power conversion effi ciency (PCE) up to ≈10%. [17][18][19][20] This dramatic increase in photovoltaic performance is caused by the optimization of the morphology of the active layer, the device architecture, and the interface control of the electron donors and acceptors. The other important approach to optimizing the interpenetrating networks and interfacial contacts between the donor and acceptor components is the use of hybrid polymer-metal oxides, in which the polymer acts as the light-absorbing component. [ 15,[21][22][23][24] One of the key issues associated with the polymer-metal oxide BHJ solar cells is the high electron mobilities in the inorganic component compared with the modest hole mobilities in the polymer. Effi cient polymer-metal oxide BHJ solar cells have been demonstrated using ZnO nanoparticles and a conducting polymer, such as a poly-1,4-phenylenevinylene derivative. [ 15,22 ] For instance, under AM1.5 conditions, polymer/ZnO solar cells with short circuit current densities ( J SC ) of 3.3 mA cm −2 , open circuit voltages ( V OC ) of 0.81 V, fi ll factors (FF) of ≈60%, and overall PCE of 1.6% have been reported. [ 15 ] This fi nding indicates that ZnO, an n -type metal oxide, possessing a wide direct bandgap (3.37 eV), an appropriate conduction band, and high electrontransporting properties, is an effective electron transport layer (ETL) for inverted polymer solar cells. [25][26][27] The strong absorption of ZnO in the UV region with a band edge cut-off at 370 nm is also important to blocking UV light and protecting the photoactive layer.