Transparent conductive oxide (TCO) substrates are essential components in emerging photovoltaic technologies, yet they are still most commonly used as uniform, single-sided electrodes [1]. This conventional role limits their functionality in architectures that require bifacial illumination, semitransparency, tandem integration, compact monolithic assembly, or application-specific device layouts. Here, we present a substrate-engineering strategy for the fabrication of tailor-made transparent conductive glass platforms, using fluorine-doped tin oxide (FTO) as an initial model TCO.

The developed approach enables double-sided FTO deposition on glass, including electrical continuity between the conductive layers located on opposite faces of the same substrate. In parallel, controlled masking allows selective TCO deposition, defining conductive and non-conductive regions from the micrometer to millimeter scale. Together, these capabilities allow the substrate to function not only as a transparent electrode, but also as an integrated manufacturing platform where optical access, current collection, electrical interconnection, and device layout can be co-designed. In this concept, the transparent conductive substrate becomes the central architectural element of the device rather than a passive support.

This strategy is particularly relevant for monolithic photovoltaic assembly. Instead of constructing devices from separately contacted components or relying on external wiring, the transparent conductive substrate itself can provide internal electrical continuity between functional regions and opposite interfaces. Monolithic dye-sensitized solar cell (DSSC) structures have previously been explored as a route to simplify fabrication compared with conventional sandwich-type configurations [2,3]. The present work extends this principle by proposing the conductive glass substrate itself as the integration layer for compact, stacked, and tandem-relevant photovoltaic architectures.

DSSCs were selected as a first experimental benchmark because they provide a transparent, interface-sensitive platform in which optical access, electrode geometry, and bifacial operation can be directly evaluated [2,4]. As a proof‑of‑concept, two DSSCs employing complementary sensitizers were assembled in a stacked three-glass configuration. Conventional one-sided FTO substrates were used as the external electrodes, while an electrically connected double-sided FTO substrate served as the central shared conductive element. Two electrical configurations were investigated. In the parallel arrangement, both working electrodes shared the central double-sided substrate, enabling the two light-harvesting units to be coupled through a common transparent conductive platform. In the series arrangement, the counter electrode of the first cell and the working electrode of the second cell were integrated through the same central double-sided conductive substrate, demonstrating internal electrical connection between opposite cell interfaces.

These configurations highlight how complementary light-harvesting units can be combined through a shared transparent conductive substrate while maintaining optical access through the stack [5]. The approach is especially relevant for bifacial DSSCs, where device performance depends on illumination from both front and rear directions, and where electrode transparency, geometry, and interconnection strongly affect operation [4,5]. More broadly, the same substrate concept is intended to be adaptable to other TCO materials and glass-based platforms, with potential relevance for semitransparent, bifacial, tandem, and application-specific emerging photovoltaic devices.

Overall, this work introduces double-sided and selectively patterned transparent conductive substrates as a route toward true monolithic solar-cell assembly. By integrating optical access, current collection, and internal electrical interconnection into a single glass platform, conductive substrates can be transformed from passive electrodes into active manufacturing and integration layers for next-generation photovoltaic architectures.