Conductivity‐Engineered Polymeric Rear Contacts for Bifacial Sb 2 (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 Sb 2 (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 Sb 2 (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 Sb 2 (S,Se) 3 /CuInSe 2 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 Sb 2 (S,Se) 3 photovoltaics.