Optimizing the Top Sub-Cell Design
Amorphous silicon (a-Si:H) and organic semiconductor-based thin-film solar cells have emerged as promising renewable energy sources due to their low manufacturing costs and lightweight construction. However, the efficiency of single-junction a-Si:H or organic solar cells is typically limited to around 6-7.5%. To overcome this limitation, researchers have explored tandem solar cell architectures that stack multiple junctions with different bandgaps in series.
One such approach is the hybrid inorganic-organic tandem solar cell, which combines an a-Si:H top sub-cell with a polymer-based bottom sub-cell. This combination allows leveraging the strengths of each material – the high open-circuit voltage (Voc) of a-Si:H and the broad absorption spectrum of the polymer. By optimizing the design of the a-Si:H top sub-cell, significant improvements in the overall tandem cell efficiency can be achieved.
Controlling the Photocurrent in the Top Sub-Cell
The short-circuit current density (Jsc) of the top sub-cell is a critical parameter that limits the overall Jsc of the tandem device, as the sub-cells are connected in series. Researchers have found that the optimal a-Si:H absorber thickness to reach the maximum photovoltaic conversion efficiency is around 200 nm. Thinner films limit the photocurrent, while thicker films can reduce the fill factor (FF) due to increased recombination.
For the polymer-based bottom sub-cell, the optimal thickness was determined to be around 120 nm, as carrier mobility in organic photovoltaic materials is limited by the short conjugated length and large energetic disorder. By carefully matching the photocurrent generation between the top and bottom sub-cells, the tandem device can achieve high efficiency.
Interface Engineering for Improved Charge Transport
Forming an effective series connection between the a-Si:H top sub-cell and the polymer bottom sub-cell is essential for high-performing tandem devices. Researchers have explored the use of transparent conducting oxide (TCO) interlayers, such as indium tin oxide (ITO) or aluminum-doped zinc oxide (ZnO:Al), to promote electrical conduction between the sub-cells.
These TCO interlayers work as a cathode for the a-Si:H front sub-cell and an anode for the polymer back sub-cell, seamlessly connecting the two without the need for a tunneling junction. The vertically oriented charge transport through the 100 nm-thick TCO layer also ensures negligible electrical resistance.
The choice of hole collection layer (HCL) for the polymer sub-cell, such as molybdenum oxide (MoO3) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), is also crucial. The HCL must be compatible with the TCO interlayer to maintain a high-quality series connection. For example, PEDOT:PSS was paired with ITO, while MoO3 was used with ZnO:Al, as the acidic PEDOT:PSS can etch the ZnO:Al layer during deposition.
Enhancing Light Absorption in the Top Sub-Cell
To further boost the Jsc of the top a-Si:H sub-cell, researchers have explored strategies to enhance light absorption, particularly in the long-wavelength region. One approach is to use a graded forward-profile (f-p) bandgap hydrogenated amorphous silicon germanium (a-SiGe:H) active layer in the top sub-cell.
The a-SiGe:H alloy has a higher absorption coefficient in the long-wavelength region compared to standard a-Si:H. By grading the bandgap of the a-SiGe:H layer, the absorption spectrum can be tuned to better match the polymer bottom sub-cell, improving the current matching between the two.
Additionally, the use of buffer layers at the p/i and i/n interfaces of the a-SiGe:H top sub-cell can help mitigate the detrimental effects of band gap discontinuities, maintaining high Voc and FF while enhancing Jsc.
Improving Light Management for the Tandem Structure
Alongside the optimization of the top sub-cell design, effective light management strategies can further enhance the efficiency of the tandem solar cell.
Textured Substrate Morphology
Researchers have fabricated the tandem devices on textured ZnO:Al/glass substrates to harness the light-trapping benefits of surface texturing. The ZnO:Al layer was textured by dipping it in a diluted hydrochloric acid (HCl) solution, creating a rough surface morphology.
The textured tandem cells showed a substantial enhancement in Jsc compared to the planar devices, due to the improved light absorption. However, the fill factor (FF) was degraded in the heavily textured cells, likely due to non-conformal deposition of the polymer layer, leading to shunt paths.
While the textured tandem cells did not outperform the planar devices in overall efficiency, the high Jsc and Voc product suggests that further improvements in the polymer layer deposition process could unlock the full potential of the textured architecture.
Double-Layer Anti-Reflective Coating (DL-ARC)
To minimize optical losses at the front surface, a double-layer anti-reflective coating (DL-ARC) consisting of magnesium fluoride (MgF2) and indium tin oxide (ITO) was employed on the tandem devices.
The DL-ARC significantly reduced the broadband reflectance on the front surface, enhancing the light absorption in both the a-Si:H top sub-cell and the polymer bottom sub-cell. This approach, combined with the use of a graded f-p bandgap a-SiGe:H active layer in the top sub-cell, resulted in a remarkable increase in the tandem cell’s Jsc, ultimately boosting the power conversion efficiency up to 16.04% – the highest reported to date for inorganic-inorganic c-Si-based tandem solar cells.
Conclusion
The development of high-efficiency hybrid inorganic-organic tandem solar cells, specifically the combination of a-Si:H and polymer-based sub-cells, has been an active area of research. By carefully optimizing the design of the a-Si:H top sub-cell, including the absorber thickness, bandgap engineering, and interface engineering, researchers have been able to achieve record-breaking efficiencies exceeding 16%.
Strategies such as using graded a-SiGe:H active layers, buffer layers at interfaces, and advanced light management techniques like textured substrates and DL-ARC have all contributed to the enhanced performance of these tandem solar cells. As the field continues to evolve, further advancements in materials, device architectures, and manufacturing processes will undoubtedly lead to even more efficient and cost-effective hybrid tandem solar technologies.
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