Solar Spectrum Splitting Parallel Junction High ...

Mater. Res. Soc. Symp. Proc. Vol. 1391 © 2012 Materials Research Society DOI: 10.1557/opl.2012.690

Solar Spectrum Splitting Parallel Junction High Efficiency Concentrating Photovoltaics Lirong Z. Broderick1, Marco Stefancich2, Dario Roncati3, Brian R. Albert1, Xing Sheng1, Lionel C. Kimerling1, and Jurgen Michel1 1

Massachusetts Institute of Technology, Cambridge, MA, USA, 02139 Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates 3 Istituto dei Materiali per l' Elettronica ed il Magnetismo, Consiglio Nazionale delle Ricerche, Parco Area delle Scienze 37/A – 43124, Parma, Italy 2

ABSTRACT A compact, single element concentrator comprising a near linear array of prisms has been designed to simultaneously split and concentrate the solar spectrum. Laterally aligned solar cells with different bandgaps are devised to be fabricated on a common Si substrate, with each cell absorbing a different spectral band optimized for highest overall power conversion efficiency. Epitaxial Ge on Si is used as a low cost virtual substrate for III-V materials growth. Assuming no optical loss for the prism concentrator, no shadowing and perfect carrier collection for the solar cells, simulations show that 39% efficiency can be achieved for a parallel four-junction (4PJ) InGaP-GaAs-Si-Ge cell under 200X concentration, and higher efficiency is possible with more junctions. INTRODUCTION Concentrating photovoltaics (CPV) is a promising method to increase efficiency and reduce cost by utilizing expensive semiconductor materials more efficiently using inexpensive optical elements. Traditional CPV system uses vertically stacked III-V multijunctions to absorb different spectral bands. It necessitates expensive substrates and suffers from current matching constraints. An improvement to direct vertical stacking is splitting the solar spectrum into several bands directed towards discrete solar cells with spectrally matched bandgaps, allowing cells to be optimized independently, eliminating the requirement for current matching, and enabling freedom in materials choices based on bandgap considerations. A schematic of a typical spectrum splitting system is shown in Figure 1(a) [1]. To concentrate and split the spectrum simultaneously, however, discrete optical components comprised of complex lenses and dichroic elements or diffraction gratings are usually used [2], which increases optical loss and adds to the complexity of mounting and manufacturing. Meanwhile, expensive substrates such as GaAs, Ge, or InP are typically used for individual cells. Compact optical system development is still in its infancy. A recently published article reported a compact optical system [3], consisting of an assembly of lenses, concentrators and a dichroic mirror made of fused silica and dozens of layers of five types of thin oxide coatings. In this paper, we designed a significantly simplified compact optical system for spectrum splitting and concentration as well as inexpensive, Si-based parallel junction solar cells to capture these different spectral ranges (Figure 1(b)). PRISM CONCENTRATOR DESIGN Our optical system design is based on simulation results from Matlab programming and ray tracing. It is comprised of a single element prism concentrator using a near linear array of

Figure 1. (a) Schematic of a typical spectrum splitting system with discrete optical components [1]; (b) schematic of the design discussed in this paper: a single optical element concentrates and splits solar spectrum simultaneously, and parallel junction solar cells with differing bandgaps are integrated on a common Si substrate.

Figure 2. Schematic of the novel prism concentrator that concentrates and splits simultaneously the incident solar radiation [4]. prisms, which splits and concentrates the solar spectrum. The same wavelength refracted by different prisms will be projected onto the same point, where a bandgap-appropriate solar cell can be located, and other bandgap-matched cells of higher and lower energies would be located adjacently, where higher and lower energy photons are directed. Each prism by itself only diffracts light, and the concentration level depends on the number of prisms used. A schematic is shown in Figure 2. In the initial design of the 1-D prism concentrator, the system includes 200

prisms, each 700 µm long in the direction perpendicular to the beam, and the average entrance surface radius of curvature is around 180 mm. The receiver is based on three 10 mm wide cells with 200 µm separation. Further improvements to the design and prototype fabrication using polycarbonate are underway [4]. INTEGRATED SOLAR CELL DESIGN Instead of using discrete solar cells made on expensive substrates, we adopt an integrated approach where a common Si substrate is used, and laterally aligned III-V, Si and Ge cells receive different spectral bands optimized for highest efficiency. This architecture allows easy mounting of the cells with respect to the spectrum splitter /concentrator, and using the Si substrate significantly reduces cost. Despite of the 4.2% lattice constant mismatch between Si (5.431A) and Ge (5.657A), high quality single crystalline Ge thin film can be epitaxially grown on top of a Si substrate by using a graded Si1-xGex buffer layer [5] or a constant composition buffer through a two-step deposition, where a thin Ge buffer layer is deposited on Si at low temperature (< 400°C), followed by a higher temperature (600-700°C) device quality film deposition and thermal annealing [6]. This Ge film can be used to fabricate a Ge solar cell, and more importantly, it can be subsequently used as a virtual substrate for III-V semiconductor film epitaxy [7], because of their very close lattice constants (In0.51Ga0.49P: 5.656 A; GaAs: 5.653 A). Here we focus our attention on four types of solar cells, commonly used in CPV systems, that can be easily integrated on a Si substrate: In0.51Ga0.49P (InGaP), GaAs, Ge and Si. Individual solar cell efficiency simulation The first step is to make realistic design of individual cells on a Si wafer substrate. Simple planar AR coatings are used. It is important to choose an appropriate absorber thickness J V FF using cell efficiency η as the figure of merit, which can be calculated from η = sc oc , where Pin Jsc is the short circuit current density, Voc the open circuit voltage, FF the fill factor, and Pin the incident solar power per unit area under AM1.5 conditions. Using a commercial Finite Difference Time Domain (FDTD) simulation software package, the absorption spectra under AM1.5G for the aforementioned four types of solar cells were calculated. Jsc was then acquired from ∞

J sc = q

∫ A(λ )s(λ )dλ

(1),

λ =300 nm

where q is the electronic charge, A(λ) is the absorption, and s(λ) is incident solar photon flux density from the AM1.5 spectrum. Voc is then obtained from J kT ln( sc + 1) Voc = (2), q J s0

where k is the Boltzmann’s constant, T = 300 K, and Js0 is the diode reverse bias saturation current taken from experimental values. Fill factor is taken as 0.86 for ideal diodes. Note that for simplicity and in order to estimate the efficiency limit, no shadowing is considered, and 100%

carrier collection is assumed. Solar cell efficiency under X times concentration is calculated by kT ln X . multiplying the efficiency under one sun by the factor 1 + qVoc Based on calculated efficiency versus thickness relations, active layer thicknesses of 2 µm, 2 µm, and 2.5 µm are chosen for InGaP, GaAs and Ge cells, respectively, because thicker absorbers render diminishing increase in cell efficiency and complicate processing. The Si cell is made out of the common 6 inch diameter wafers with thickness 675 µm. Cell performance is listed in Table I for individual cells. Table I. Simulated individual solar cell performance under AM1.5 one sun conditions Cell

InGaP GaAs Si Ge

Absorber thickness (µm) 2 2 675 2.5

Jsc (mA/cm2) 14.86 23.94 35.42 42.19

Voc (V)

FF

η (%)

1.40 1.04 0.70 0.19

0.86 0.86 0.86 0.86

17.90 21.34 21.28 6.81

Spectrum splitting optimization

Given the individual cell’s absorption spectrum obtained above, Matlab simulation was performed to determine the optimal cell combination and spectrum splitting wavelengths to achieve the highest overall system efficiency. For each cell, Jsc was obtained by modifying the integration limits in Equation (1) to the optimized wavelength range for it. Other parameters were calculated according to the method in the section above. It was found that for a specific parallel junction cell, the optimal splitting points barely change with the concentration level, and they are always near the band edge of each individual cell, e.g., for the four parallel junction (4PJ) InGaP-GaAs-Si-Ge cell, splitting the solar spectrum at λ1=660 nm, λ2=840 nm and λ3=1110 nm renders the highest efficiency, η=33.6% under one sun, and 38.6% under 200X concentration. The results are shown in Figure 3 for various combinations of 3PJ and the 4PJ cells. The 4PJ cell has the highest η, and among all the 3 PJ cells, InGaP-GaAs-Si cell ranked the highest, whereas GaAs-Si-Ge cell positioned the lowest. The high efficiency achievable by our parallel junction cells stems from the optimal utilization of the solar spectrum, which can be easily understood by the aid of Figure 4. It illustrates the absorption spectra of the 3PJ InGaP-GaAs-Ge solar cell and the highest efficiency 4PJ cell. The advantage of spectrum splitting is obvious: a bandgap appropriate cell is chosen for each spectral range, and when a higher bandgap cell becomes weakly absorbing, another cell with a smaller bandgap will take over and boost the absorption. Through this "relaying" action, absorption is kept high within the whole solar spectrum while Voc is maintained as high as possible for each range. Furthermore, there is no requirement on current matching, therefore each absorbed photon counts, leading to high overall efficiency. DISCUSSION The only difference between the two absorption spectra in Figure 4 is in the range of 840

Ge Si G aAs In G a P

0 .4

Efficiency

0 .3

0 .2

0 .1

0 .0 1 sun

2 0 0 X c o n c e n tra tio n

T h re e a n d fo u r p a ra lle l ju n c tio n s o la r c e lls

Figure 3. Maximum efficiency achievable from parallel junction solar cells integrated on a common Si substrate under one sun and 200X concentration. 1.0

5.00E+018

4.00E+018

Absorption

2

0.8

Photon Number (/s/m /nm)

4&3 PJ InGaP 4&3 PJ GaAs 4PJ Si 4PJ Ge 3PJ Ge

0.6

3.00E+018

0.4

2.00E+018

0.2

1.00E+018

0.0

0.00E+000

400

600

800

1000 1200 1400 1600 1800

Wavelength (um)

Figure 4. Absorption spectra of integrated three parallel junction InGaP-GaAs-Ge cell and four parallel junction InGaP-GaAs-Si-Ge cell. Also shown is the solar spectrum on the right axis.

-1110 nm, where the 3PJ cell absorbs more in most of this band with high solar photon flux. Therefore, the higher efficiency of the 4PJ cell is due to the higher Voc of Si originating from its bigger bandgap. Hence it can be inferred that including higher bandgap cells than InGaP that are still lattice matched to Ge would render higher overall efficiency. Another interesting observation is about Ge. Although it absorbs well within most of the solar spectrum, Figure 3 clearly shows that it contributes negligibly (