Metal Oxide Nanoparticles as an Electron‐Transport Layer in High‐Performance and Stable Inverted Polymer Solar Cells

Polymer solar cells have many advantages, including transparency, aesthetically pleasing, fl exibility, and light weight. They are particularly compatible with high throughput and low-cost fabrication processes, which make them a promising photovoltaic technology. [ 1–5 ] These properties enable a wide range of potential applications, even for outer space. [ 6 , 7 ] In the last few years, many high-performance polymers with high solar-cell effi ciency have been reported. [ 8–16 ] Among those, benzodithiophene (BDT) and thionothiophene (TT)-based polymers were the fi rst polymer family to break the 7% and 8% effi ciency barriers. [ 8–12 ] Poly{2,6 ′ -4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b] dithiophenealt -5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl) pyrrolo[3,4-c]pyrrole-1,4-dione} (PBDTT-DPP) with a lower bandgap ( ≈ 1.4 eV) showed superior performance in long wavelength regions, which enabled signifi cant progress in tandem solar cells with effi ciency close to 9%. [ 14 ] For historical reasons, these high-effi ciency low-bandgap polymers were mostly evaluated based on standard structures, typically with poly(3,4-ethy lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a hole-transport layer (HTL) and a low-work-function metal such as Ca as the electron-transport layer (ETL). Inverted polymer solar cells have been developed and continue to grow particularly due to their potential for superior device stability and manufacturing compatibility. [ 17–24 ] In the inverted architecture with the classical poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) active layer, several successful n -type buffer layers such as cesium carbonate (Cs 2 CO 3 ), [ 17 ] titanium oxide (TiO 2 ), [ 22 ] Cs-doped TiO 2 , [ 25 ] zinc oxide (ZnO), [ 18 ] and a combination of ZnO and self-assembled monolayers [ 19 ] have been shown to be able to alter the carrier selectivity of the indium tin oxide (ITO) electrode and convert it to a cathode contact. On the anode side, the most widely used are transition metal oxides