Evaluation of ALD-Grown Metal Oxide Tunnel Junction Layer for Organic Tandem Cells

Since their inception, organic solar cells have been a cynosure of photovoltaic (PV) community, owing to their simplicity, affordability and mass-production capabilities. Research on organic solar cells has been propelled by advances in nanotechnology, polymer synthesis, and semiconductor processing; efforts to achieve high power conversion efficiencies (PCE) through refined device geometry, tailored synthesis and/or combination of new low-bandgap materials, and heuristic process optimization have fructified as ~10 % efficiency in single-junction organic solar cells. Recent PCE enhancement in organic solar cells has mostly attributed to the development of small bandgap materials that extend absorption of the sun into the red and infrared wavelengths.

Utilizing tandem cells comprised of both large and small bandgap sub-cells enables light harvesting of different parts of the solar spectrum to be separately optimized, yielding higher PCE than single junction solar cells. But the challenges in achieving highly efficient organic tandem solar cells include optimal combination of materials for each sub-cell and interconnecting layers between sub-cells that are process compatible. Optimal combination of materials can be achieved by selection of two materials having complementary absorption windows. The interconnecting layer provides electrical and optical coupling between the two sub-cells and plays a key role in the performance of tandem cells. For the design of the interconnecting layer, electrical, optical, chemical and structural characteristics should be considered. Ideally, it should effectively facilitate charge transport between two sub-cells. Moreover, high transmittance and negligible absorption in the spectral region where two sub-cells absorb is desirable. Last but not least, the interconnecting layer should be designed to survive the subsequent solution coating and provide conformal surface coating for tandem cells with solution processed top cells.       

Here we evaluate some of candidate metal oxides for tunnel junction layers for organic tandem solar cells. Metal oxides are grown by Atomic Layer Deposition (ALD) for thin defect-free conformal layer to circumvent miscibility issues and negligible absorption. Normal and inverted interfaces are examined. The influence of annealing under different conditions and methods will be presented. Tunneling properties are qualified by J-V characteristics and optical properties evaluated by UV-Vis are contrasted to suggest optimal design tradeoffs of tunnel junctions for solution processed organic tandem solar cells.