Using nanowires to enhance light trapping in solar cells
The photovoltaic (PV) industry reduced the cost of silicon solar panels by 50% between 2006 and 2011, thanks to modifications that improved panel efficiency.1, 2 Moreover, the cost per watt of solar power is now below $0.75, and prices per kilowatt-hour are fast approaching parity with electricity available on the grid.3 It is possible to further increase panel efficiency using thin-film silicon (Si) that incorporates nanometer-sized features (nanostructures) to improve light trapping, thus achieving greater efficiency without requiring more of the active material. Random texturing or unevenly distributed pyramid structures within the silicon can enhance light gathering up to the theoretical limit described by Yablonovitch.4,5 However, we can overcome this limit using concepts based on wave optics.6, 7
Solar cells featuring nanowires with p-n junctions constructed in a radial direction have high light-trapping efficiency8 and enhanced charge collection. This is because radial charge separation requires a shorter carrier diffusion length than for planar junction arrangements.9, 10 Individual nanowires exhibit high absorption by coupling incoming light into their localized resonances.11 By changing the distance between nanowires without changing any other parameter, we showed that this coupling does not require periodic arrangement.11, 12 For the best performance, we needed to carefully adjust the nanowire diameters and the silicon (the volume fraction). We confirmed that vertical nanowire arrays have high tolerances to changes in geometry without causing a drastic reduction in device performance (see Figure 1). This is important for many low-cost fabrication processes, which cannot guarantee exact geometrical dimensions for every nanowire in the array. We also showed that optimal performance does not depend heavily on whether the arrangements are square or hexagonal (see Figure 1), given the same volume fraction, and that the light absorption improves logarithmically with nanowire heights (see Figure 2).
Furthermore, we improved the performance of already optimized arrays by combining two types of nanowires with different diameters in closely packed hexagonal periodic arrangements. This created a cross section of wires much larger than the geometric dimensions of a single nanowire, with the benefit that wires with smaller diameters are more efficient in trapping light at short wavelengths, while larger wires absorb better at longer wavelengths.13 We optimized dual-diameter nanowire arrays for fixed distances between centers of the neighboring nanowires. The results indicate that combining wires of two different diameters is superior to the standard single-diameter configuration (see Figure 3). The highest short-circuit current-density value obtained from calculations on a 5μm-long dual-diameter array is close to the value for an optimized 7.5μm single-diameter array, representing a 33% reduction in material used.
We fabricated radial junction solar cells using hydrogenated amorphous silicon (aSi:H) as an active material (see Figure 4). First, we prepared p-doped crystalline silicon nanowire cores using a low-temperature plasma-enhanced vapor-liquid-solid process.14 Next, we deposited a 100nm-thick intrinsic aSi:H coating, followed by a thin coating of n-doped aSi:H, and sputtered a transparent indium tin oxide electrode on top of the nanowires.9 Figure 5 shows intermediate steps in scanning electron microscopy images.
Strong light scattering, combined with coupling of light into resonances inside the nanowires, enhances the absorption. Samples prepared with different nanowire densities (from 110 to 370 million per cm2) achieved energy conversion efficiencies of up to 8.1%, with the highest-performing cell reaching an open-circuit voltage of 0.80V, a short-circuit current of 16.1mA/cm2, and a fill factor of 0.628.15 Figure 6 shows an example of density-voltage characteristics for a solar cell with energy conversion efficiency of greater than 8%. Relatively high short-circuit current (Jsc) values measured on radial junction-based solar cells are mainly due to the efficient light scattering between nanowires (see Figure 7).
Our future work will focus on further improving device performance using extensive numerical modeling and by optimizing the cell's transparent conductive coating (TCO). We plan to further increase Jsc density by switching from amorphous silicon to micro-crystalline for the active absorber layer. Using atomic layer deposition techniques, we aim to achieve improved uniformity of the TCO layer.
CNRS École Polytechnique
Martin Foldyna is a tenured CNRS researcher. His work focuses on optical properties of periodic and non-periodic nanostructures for photovoltaic and semiconductor applications, and optimization of solar cell performance.