Indium Phosphide (InP)

2015

ACS Photonics

III−V semiconductor nanowire arrays show promise as a platform for next-generation solar cells. However, the theoretical efficiency limit for converting the energy of sunlight into electrical energy in such solar cells is unknown. Here, we calculate through electromagnetic modeling the Shockley−Queisser efficiency limit for an InP nanowire array solar cell. In this analysis, we calculate first from the absorption of sunlight the short-circuit current. Next, we calculate the voltage-dependent emission characteristics of the nanowire array. From these processes, we identify how much current we can extract at a given voltage. Finally, after constructing this current−voltage (IV) curve of the nanowire solar cell, we identify from the maximum power output the maximum efficiency. We compare this efficiency of the nanowire array with the 31.0% efficiency limit of the conventional InP bulk solar cell with an inactive substrate underneath. We consider a nanowire array of 400 nm in period, which shows a high short-circuit current. We optimize both the nanowire length and diameter in our analysis. For example, nanowires of 4 μm in length and 170 nm in diameter produce 96% of the short-circuit current obtainable in the perfectly absorbing InP bulk cell. However, the nanowire solar cell emits fewer photons than the bulk cell at thermal equilibrium, especially into the substrate. This weaker emission allows for a higher open circuit-voltage for the nanowire cell. As an end result, nanowires longer than 4 μm can actually show, despite producing a lower short-circuit current, a higher efficiency limit, of up to 32.5%, than the bulk cell. B oth InP 1−3 and GaAs 4,5 nanowire array solar cells have shown very promising performance with a measured highest efficiency of 13.8% when converting the energy in sunlight into electrical energy. 1 Nanowires open up for combining lattice mismatched materials due to strain relaxation in the radial direction. 6 Thus, the nanowire geometry gives a great freedom for the choice of lattice-mismatched materials 6−11 to create heterostructures in the active region of a solar cell. In addition, the expensive III−V material can be fabricated epitaxially in nanowire form on a cheaper but latticemismatched substrate. 11,12 Furthermore, the nanophotonic properties of nanowire arrays can be used for tuning and designing the absorption of light more distinctively than in bulk-like devices, 13−16 and absorption resonances can show up in the individual nanowires. 17−24 For example, a nanowire array, where the nanowires covered only 10% of the substrate surface, absorbed more than 90% of the incident light. 25 A lot of work has been done to optimize the geometry of III−V nanowire arrays 18,24,26,27 in order to maximize the short-circuit current, which is the current extracted under zero voltage bias on the solar cell. However, the efficiency, which depends on the current−voltage relationship of the solar cell, has not been studied with the Shockley− Queisser detailed balance analysis.Under the conditions where...

Section: Resultsmentioning

confidence: 99%

Shockley–Queisser Detailed Balance Efficiency Limit for Nanowire Solar Cells

2015

ACS Photonics

“…Also here, for accurate results, we use such optics modeling [26]. We use tabulated values for the refractive index of InP [27], and for the TCO, we use the values shown in the supplementary information figure S3 of [18]. For the polymer between the nanowires, we assume a typical value of n=1.5.…”

Section: Optical Considerationsmentioning

confidence: 99%

Physics and design for 20% and 25% efficiency nanowire array solar cells

2018

Nanotechnology

Bottom-up fabricated single-junction III-V nanowire array solar cells have shown efficiency up to 15.3 %, which is approximately half of the conventional Shockley-Queisser detailed balance efficiency limit of 33.6 %. Here, based on numerical and analytical opto-electronics modeling and analysis, we give guidelines for (i) geometry that gives strong absorption as well as (ii) the design of efficient p-n junction and electrical contacts in the nanowires to reach 20 % and 25 % efficiency. We exemplify the impact of 8 different optical and electrical loss mechanisms in a 15 % and a 25 % design. We also provide an analytical equation for estimating the efficiency drop due to resistive losses in the top contact layer for varying cell size.

“…We chose to employ the scattering matrix method to solve the Maxwell equations for normally incident light in order to calculate the absorption spectrum A 1(2) (λ) of the nanowire top and bottom cells. We use tabulated refractive index values n ( λ ) for the Al 0.10 Ga 0.90 As28, In 0.34 Ga 0.66 As29, Ga 0.51 In 0.49 P31, and InP34. We then calculate the Shockley-Queisser detailed balance efficiency (see Supplementary Information Eqs.…”

Section: Geometry Designmentioning

confidence: 99%

Design for strong absorption in a nanowire array tandem solar cell

Chen

Pistol

2016

Sci Rep

Semiconductor nanowires are a promising candidate for next-generation solar cells. However, the optical response of nanowires is, due to diffraction effects, complicated to optimize. Here, we optimize through optical modeling the absorption in a dual-junction nanowire-array solar cell in terms of the Shockley-Quessier detailed balance efficiency limit. We identify efficiency maxima that originate from resonant absorption of photons through the HE11 and the HE12 waveguide modes in the top cell. An efficiency limit above 40% is reached in the band gap optimized Al0.10Ga0.90As/In0.34Ga0.66As system when we allow for different diameter for the top and the bottom nanowire subcell. However, for experiments, equal diameter for the top and the bottom cell might be easier to realize. In this case, we find in our modeling a modest 1–2% drop in the efficiency limit. In the Ga0.51In0.49P/InP system, an efficiency limit of η = 37.3% could be reached. These efficiencies, which include reflection losses and sub-optimal absorption, are well above the 31.0% limit of a perfectly-absorbing, idealized single-junction bulk cell, and close to the 42.0% limit of the idealized dual-junction bulk cell. Our results offer guidance in the choice of materials and dimensions for nanowires with potential for high efficiency tandem solar cells.