Nanostructured Black Silicon for Efficient Thin Silicon Solar Cells: Potential and Challenges

Abstract

The world’s energy system is at crossroads as the natural fossil fuels are becoming increasingly unavailable and more expensive. Thus, the usage of various renewable energy (RE) sources to provide environmentally benign, economically feasible and sustainable energy supply is drawing more attention to meet ever-increasing energy demands. Among the various RE sources, solar energy has the potential to provide energy independence and security of supply to every economy. Among the possible solutions, the deployment of photovoltaic (PV) modules to directly convert solar radiation into electricity is one of the best choices. The PV is one of the promising technologies to provide a feasible carbon-free route to replacing nonrenewable power sources worldwide. However, the limited performance to cost ratio for the present market-dominating silicon (Si) wafer PV modules restricts large-scale civil utilization of solar electricity. One of the basic costs for Si PV cells is the starting Si wafer itself, which requires several extensive purification to maintain the reasonable performance of the device. Thus, developing PV devices of high performance to cost ratio has always been in demand. Researchers are trying to explore solar cell designs via an unconventional method, aiming at cost reduction and performance improvement. One of the growing potential approaches is PV designs based on nano-architectured materials with low cost and advanced optical and electrical management properties. Micro-and nanostructured Si surfaces are well known for their applications in Si micro-and optoelectronic devices, particularly in solar PV. A particular class of nanostructured silicon is called black silicon. The black Si concept is a promising approach to eliminate front surface reflection (<2% in broad spectral range) omnidirectionally in PV devices without the need for a conventional anti-reflection coatings (ARC). Besides, strong light-trapping can also be achieved for weakly absorbed photons with energies close to the absorption edge of Si and might lead to both an increase in efficiency as well as a reduction in the fabrication costs of solar cells. The nanostructured black Si surfaces, suitable for solar cell applications, have been fabricated by various methods such as metal-assisted wet-chemical etching (MACE), dry reactive ion etching (RIE), etc. In the chapter, a brief introduction of PV technology is presented along with the current status of Si wafer based solar cells. Various challenges/losses associated with conventional solar cell technology with an emphasis on addressing the optical/reflection losses are also discussed. Efforts made to minimize these losses with an emphasis on minimization of reflection losses through nanostructuring schemes are discussed. Moreover, an attempt is made to present a review on the recent progress of black Si research for solar cell technologies. First, MACE technique for the preparation of black Si is discussed in detail along with a brief critical analysis with respect to advantages and disadvantages for solar PV applications. The applications of black Si in solar cells and the progress over the years are then summarized. In addition, one of the major challenges of black Si cell is enhanced surface recombination which imposes a critical limit to solar cell efficiency especially in thinner solar cells. Hence, an efficient black Si solar cell can only be obtained if an optimal trade-off between light absorption gain and electrical losses (due to recombination) is achieved for the black Si-based PV technology. The dielectric thin films play a critical role in effective surface passivation of the black Si surfaces. Therefore, the current status of passivation schemes and challenges for black Si solar cell technology is also highlighted. Moreover, the current status of black Si solar cells concepts employing both monocrystalline, multicrystalline Si as well as the concept of thin/flexible silicon solar cells towards high-efficiency solar cells is reviewed critically. Finally, conclusions and future prospects of the black Si concept is outlined wherein it is envisaged that nanostructured black Si will play a key role in cost-effective and efficient solar photovoltaic devices in days to come.