Graphene Oxide:Single‐Walled Carbon Nanotube‐Based Interfacial Layer for All‐Solution‐Processed Multijunction Solar Cells in Both Regular and Inverted Geometries

Abstract

Over the past decade, advances in material synthesis and interfacial engineering have produced more efficient polymer solar cells with improved absorption of the solar irradiation and charge carrier transport that have led to continuously increased power conversion efficiencies (PCE), recently surpassing 8%.[1] In parallel to the material design and engineering in the active layer, novel organic photovoltaic architectures have emerged. A representative example is tandem photovoltaics in which subcells are stacked along the optical path to increase optical absorption.[2–5] When the subcells are connected in series, the ideal open-circuit voltage (VOC) is equivalent to the sum of the VOCs of the individual subcells. PCE enhancement is most significant when high-efficiency polymers with complementary absorption profiles are stacked. Central to the construction of tandem devices is the interconnect layer that spatially separates but electrically connects the two subcells. Ideal candidates for interconnect layers require (a) high optical transparency over the entire solar spectrum; (b) sufficient vertical conductivity; (c) complete coverage to avoid intermixing of the two subcells; (d) low surface roughness to avoid interrupting the deposition of neighboring subcells; and (e) appropriate energetics to facilitate efficient charge recombination. For solution-processable interconnect materials, there are additional challenges such as orthogonal solubility, chemical and mechanical stability, and so on. Thus far, the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is predominantly used as a component in solution-processable interconnect layers.[3–7] However, low transmittance of PEDOT:PSS in the red to near infrared (NIR) region results in a loss of available solar flux, which could adversely affect the overall photocurrent. In addition, PEDOT:PSS could also react with some common interfacial materials such as metal oxides, thus degrading the quality of interfaces over time.[6] Recently, graphene oxide (GO) was found to be capable of replacing PEDOT:PSS as a hole transporting layer in polymer solar cells.[8,9] Since GO is an insulating soft carbon sheet[10,11] with thickness of around 1 nm and lateral dimension extends readily into tens of micrometers, an effective GO modifying layer needs to have as small thickness