A new triplejunction solar cell (3J) design exploiting the highly absorptive I–III–VI chalcopyrite CuInSe_{2 } material is proposed as an alternative to III–V semiconductor 3J solar cells. The proposed structure consists of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/CuInSe_{2} (1 eV) which can be grown on a GaAs substrate in an inverted manner using epitaxial liftoff techniques. To latticematch epitaxial CuInSe_{2} to Ga(In)As, a compositionally graded buffer region composed of Ga_{x}In_{1−x}P is used. The modeling and simulation of the device include the effects of threading dislocations on minority carrier lifetimes in the metamorphic buffer and bottom subcell active region. Studies focus on device performance under standard testing conditions and concentrated illumination. The results are compared to a reference lattice mismatched 3J composed of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/GaInAs (1 eV) and to a lattice matched 3J composed of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/Ge (0.67 eV). The advantage of CuInSe_{2} is its higher absorption coefficient, which requires only 1
The area of research and development in photovoltaic device technologies has led to significant increases in power conversion efficiencies from 17% to 44.4% since 1983 [
For more ideal current matching, a metamorphic lattice mismatched (LMM) 3J design composed of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/Ga_{0.7}In_{0.3}As (1.0 eV) has been proposed [
Here we suggest a similar approach to integrate a lower cost epitaxial material CuIn_{1−x}Ga_{x}Se_{2} (CIGS) as a bottom subcell in a 3J, as shown in Figure
Proposed (a) growth sequence and (b) final design of the inverted metamorphic GaInP/Ga(In)As/CuInSe_{2} structure after a selective wet etch of an AlAs release layer.
Structural details of the inverted metamorphic GaInP/Ga(In)As/CuInSe_{2} structure.
The aim of this paper is to investigate the potential benefits of this material system as the third subcell of a 3J for concentrated photovoltaic (CPV) applications using TCAD Sentaurus by Synopsys version vG2012 (Mountain View, California) [
The paper is outlined as follows. Section
The growth of epitaxial CuInSe_{2} on GaAs has been demonstrated previously in the literature for photovoltaic applications [
A candidate for the CGB material is the ternary alloy Ga_{x}In_{1−x}As based on its ability to be grown with high quality as demonstrated in the LMM GaInP/Ga(In)As/Ge cell [
The details of the structure are outlined in Figure
Lattice constant as a function of molar fraction for CuIn_{1−x}Ga_{x}Se_{2}, Ga_{x}In_{1−x}P, and Ga_{x}In_{1−x}As based on [
The reference LMM 3J is composed of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/GaInAs (1 eV) and is identical to the structure in Figure
Similarly, the LM design composed of GaInP (1.9 eV)/Ga(In)As (1.4 eV)/Ge(0.67 eV) is identical to Figure
The semiconductor simulation software TCAD Sentaurus is used to specify the 3J structure of interest and solve the set of coupled differential equations consisting of Poisson’s equation and the electron and hole currentcontinuity equations with thermionic emission as boundary conditions for heterointerfaces. These equations consider generation computed using the transfer matrix method, and radiative, ShockleyReadHall and Auger recombination using standard recombination formalisms [
The most important properties for CIGS and Ga_{x}In_{1−x} P as a function of molar fraction include
Material properties for CuI
Parameter  Model for CuI 
Model for G 












[ 

300, 200 [ 
[ 

3, 3 [ 
20, 20 [ 

[ 
[ 
The energy band diagram of the complete CuInSe_{2} 3J structure simulated at equilibrium is shown in Figure
(a) Simulated energy band diagram of the GaInP/Ga(In)As/CuInSe_{2} 3J under illumination conditions and (b) a closeup of the energy band diagram at the compositionally graded buffer region composed of GaInP including the overshoot layer and the CuInSe_{2} based bottom subcell including a highly doped CuInSe_{2} back surface field (BSF).
A closeup of the bottom subcell is shown in Figure
The effects of threading dislocations on current transport are also considered in the model by effectively modifying the minority carrier ShockleyReadHall (SRH) lifetimes parameterized by the threading dislocation density (TDD), as described in [
Minority carrier lifetimes in CuInSe_{2} and GaInAs as a function of threading dislocation densities (TDD). A threshold density of 8.4 × 10^{6} cm^{−2} for CuInSe_{2} can be observed based on the reduction of the lifetime by a factor of 2 (marked by light gray rectangle), whereas the threshold is closer to 2.6 × 10^{4} cm^{−2} for electrons in GaInAs (marked by dark gray rectangle).
Lastly, each interface of the complete structure considers surface recombination which is parameterized using a surface recombination velocity (SRV) according to the standard interface recombination formalism [
The simulated external quantum efficiency (EQE) and currentvoltage (
(a) Simulated EQE and (b)
The EQE of the top GaInP subcell has a comparable albeit high response in the UV to other published data [
Interestingly, one can notice on the
Simulated currentvoltage metrics of the 3Js of interest under 1000 W/m^{2} and at 300 K with TDD_{CGB} and TDD_{bot} of 10^{6} cm^{−2}.
3J structure 





CuInSe_{2}  14.0  2.86  79.0  31.7 
LMM  14.0  2.95  83.2  34.5 
LM  14.3  2.65  87.0  33.0 
Figures
Simulated (a)
With respect to the open circuit voltage, Figure
The
The benefits of replacing a Ge subcell with an epitaxial CuInSe_{2} subcell would thus enable a comparable efficiency at 1 sun operation compared to the LM device. With respect to the LMM device; however, a reduction in efficiency is observed due to the low
The simulated device
Simulated (a)
The
The results of Figure
The simulated performance metrics of the GaInAs LMM 3J, the Ge LM 3J, and the proposed CuInSe_{2} 3J are studied up to concentration ratios of 3000 suns at 300 K. For the LMM and CuInSe_{2} 3J designs, the modeling takes into account the effects of threading dislocations on minority carrier lifetimes in the bottom subcell. Although the bandgap combination of the CuInSe_{2} 3J is more optimal than the LM 3J, their performances are comparable as a result of the low fill factor of CuInSe_{2} 3J cell. With respect to their respective bottom subcells, it is especially important to note that the Ge subcell
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the Photovoltaic Innovation Network funded by the National Science and Engineering Research Council of Canada, the Ontario Graduate Scholarship fund, CMC Microsystems for the licensing of TCAD Sentaurus, the Ministry of Research and Innovation in Ontario, the Canadian Foundation for Innovation, and Canada Research Chair programs for funding.