Conductivity-Engineered Polymeric Rear Contacts for Bifacial Sb₂(S,Se)₃ Solar Cells and 4T Tandem Photovoltaics

Transparent rear contacts that combine efficient charge extraction with low optical loss are essential for extending Sb₂(S,Se)₃ photovoltaics toward bifacial and tandem applications. Here, a solution-processed 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) bilayer is introduced as a polymeric transparent rear-contact architecture for bifacial Sb₂(S,Se)₃ solar cells. Established dimethyl sulfoxide (DMSO) solvent engineering reduces the sheet resistance of PEDOT:PSS by nearly four orders of magnitude while preserving its high transparency, enabling front- and rear-side power conversion efficiencies (PCEs) of 8.2% and 3.3%, respectively. Under 1-sun front plus 0.3-sun rear illumination, the bifacial device delivers a power generation density of 9.7 mW cm^-2. Combined optical and electrical analyses reveal that the lower rear-side response arises from asymmetric photogeneration, direction-dependent carrier transport, depth-dependent absorber inhomogeneity, and enhanced recombination associated with the transparent rear interface. The device is further integrated as the top cell in an electrically independent Sb₂(S,Se)₃/CuInSe₂ four-terminal tandem, achieving a combined standard-condition PCE of 12.32%. This work demonstrates a solution-processable polymeric rear-contact strategy complementary to conventional inorganic transparent contacts and identifies the optical and interfacial factors governing bifacial Sb₂(S,Se)₃ photovoltaics.