Nanoimprinted Silicon Nanowire Solar Cells

The central achievement of this work is the demonstration of highly efficient silicon nanowire (Si NW) solar cells with axial p-i-n junctions, fabricated using nanoimprint lithography (NIL) to achieve dense, periodic NW arrays. This study systematically investigates the effects of surface passivation, nanowire density, and light trapping on photovoltaic performance, showing that carefully engineered NW arrays can significantly improve carrier collection and optical absorption. Through a combination of experimental measurements and finite-difference time-domain (FDTD) simulations, we demonstrate that nanoimprinted Si NW arrays outperform conventional sparse NW arrays and planar Si films in terms of external quantum efficiency (EQE) and internal quantum efficiency (IQE). The findings highlight the importance of precise nanowire arrangement and surface passivation in optimizing photovoltaic performance, paving the way for more efficient next-generation solar cells.

A key breakthrough of this work is the realization that nanoimprinted Si NW arrays dramatically enhance light absorption through diffractive scattering and improved carrier collection. The study shows that denser NW arrays (with an array pitch P = 250 nm) achieve nearly 100% absorption efficiency in the 400–650 nm spectral range, significantly outperforming conventional thin-film Si solar cells. Additionally, surface passivation with a 12 nm SiO₂ layer was found to increase the open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF), leading to improved overall photovoltaic performance. Compared to unpassivated NW arrays, passivated arrays exhibited a higher VOC (250 mV vs. 170 mV) and improved charge carrier lifetime, confirming the effectiveness of oxide passivation in reducing surface recombination losses.

The effects explored in this study extend beyond simple NW solar cell fabrication; we systematically analyze nanowire geometry, density, and contact efficiency to determine their impact on light absorption and charge collection. We find that axial p-i-n junctions, despite being less studied than radial junctions, offer distinct advantages, such as the ability to accommodate longer intrinsic regions, which reduces leakage current and improves carrier separation. By comparing the performance of individual NW devices and dense NW arrays, we confirm that the collective behavior of an ordered NW array results in stronger light trapping and enhanced quantum efficiency. Moreover, real-time photocurrent measurements demonstrate that passivated NWs exhibit a 50% longer carrier recombination time, reinforcing the role of surface treatments in device optimization.

The innovation in this work lies in integrating nanoimprint lithography (NIL) with vapor-liquid-solid (VLS) epitaxy to precisely control nanowire arrangement and improve photovoltaic efficiency. While previous research on Si NW solar cells primarily focused on randomly distributed or large-diameter nanowires, this study establishes that sub-200 nm axial p-i-n NW arrays can achieve superior optical and electrical properties when properly structured. The use of FDTD simulations to optimize NW pitch and array configuration is another innovative aspect, providing a predictive model for tailoring NW-based solar cells. Additionally, the authors successfully demonstrate a scalable, cost-effective approach for producing high-performance NW arrays, bridging the gap between fundamental research and practical solar energy applications.

Future research could build upon these findings by exploring alternative passivation techniques, further reducing nanowire pitch below 250 nm, and integrating heterojunction materials such as Si/Ge to broaden spectral absorption. Another promising direction is enhancing carrier extraction efficiency by developing more effective electrical contact methods to ensure that all NWs contribute to the photocurrent. Additionally, multi-junction or tandem solar cell architectures leveraging these optimized Si NW arrays could push photovoltaic efficiency even further. Overall, this work establishes a strong foundation for next-generation, high-efficiency NW solar cells and highlights the potential of nanoimprinted Si NW arrays in commercial-scale photovoltaic applications.