Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading. Part II: finite-difference algorithm and double-layer antireflection coatings

Abstract

In Part I [Appl. Opt. 58, 6067 (2019)APOPAI003-693510.1364/AO.58.006067], we used a coupled optoelectronic model to optimize a thin-film C u I n 1 − ξ G a ξ S e 2 (CIGS) solar cell with a graded-bandgap photon-absorbing layer and a periodically corrugated backreflector. The increase in efficiency due to the periodic corrugation was found to be tiny and that, too, only for very thin CIGS layers. Also, it was predicted that linear bandgap-grading enhances the efficiency of the CIGS solar cells. However, a significant improvement in solar cell efficiency was found using a nonlinearly (sinusoidally) graded-bandgap CIGS photon-absorbing layer. The optoelectronic model comprised two submodels: optical and electrical. The electrical submodel applied the hybridizable discontinuous Galerkin (HDG) scheme directly to equations for the drift and diffusion of charge carriers. As our HDG scheme sometimes fails due to negative carrier densities arising during the solution process, we devised a new, to the best of our knowledge, computational scheme using the finite-difference method, which also reduces the overall computational cost of optimization. An unfortunate normalization error in the electrical submodel in Part I came to light. This normalization error did not change the overall conclusions reported in Part I; however, some specifics did change. The new algorithm for the electrical submodel is reported here along with updated numerical results. We re-optimized the solar cells containing a CIGS photon-absorbing layer with either (i) a homogeneous bandgap, (ii) a linearly graded bandgap, or (iii) a nonlinearly graded bandgap. Considering the meager increase in efficiency with the periodic corrugation and additional complexity in the fabrication process, we opted for a flat backreflector. The new algorithm is significantly faster than the previous algorithm. Our new results confirm efficiency enhancement of 84% (resp. 63%) when the thickness of the CIGS layer is 600 nm (resp. 2200 nm), similarly to Part I. A hundredfold concentration of sunlight can increase the efficiency by an additional 27%. Finally, the currently used 110-nm-thick layer of M g F 2 performs almost as well as optimal single- and double-layer antireflection coatings.

Effects of defect density, minority carrier lifetime, doping density, and absorber-layer thickness in CIGS and CZTSSe thin-film solar cells

“…(16) conforms to the experimental data reasonably well without using surface defects, 69 so all performance parameters of CIGS thin-film PVSCs can be predicted using only the bulk material-response parameters. Equation (16) helps model the defect density in graded-bandgap CIGS solar cells 69 and can be expected to do the same for quantum-well-based CIGS solar cells, 50 thereby assisting experimentalists.…”

“…As our focus is on modeling the electrical characteristics of the solar cell, not on how it interfaces with an external circuit, both terminals were assumed to be ideal ohmic contacts. We used a one-dimensional drift-diffusion model 33 , 34 , 68 , 69 to investigate the transport of electrons and holes for z∈[Lnormalw,Lnormald]. The solution of the drift-diffusion system yields the electron current density Jnormaln(z), the hole current density Jnormalp(z), and the device current density Jdev=Jn(z)+Jp(z),z∈[Lw,Ld],which is independent of z but is a function of the voltage Vext applied across the planes z=Lnormalw and z=Lnormald.…”

Bifacial flexible CIGS thin-film solar cells with nonlinearly graded-bandgap photon-absorbing layers

“…The schematic of a standard MgF 2 /AZO/od-ZnO/CdS/CIGS/Al 2 O 3 /Mo thin-film solar cell is shown in figure 2(a) [18]. The solar cell is homogeneous in any xy plane, the z axis of the Cartesian coordinate system being aligned parallel to the thickness direction.…”

“…A recent theoretical investigation demonstrated that judiciously selected compositional-grading profiles (i.e. ξ as a function of z) leading to bandgap grading [18] in the CIGS photon-absorbing layer can enhance the efficiency. Furthermore, it has been experimentally observed that, despite the induction of higher defect density by significant compositional grading in the CIGS layer, there is a concurrent reduction in open-circuit voltage loss.…”

“…In the optical step of this model, the transfer-matrix method [21,22] is used to determine the electron-hole-pair (EHP) generation rate G inside the solar cell [18], assuming normal illumination by unpolarized polychromatic light endowed with the AM1.5G solar spectrum [23]. In turn, the EHP generation rate is used in the electrical step as an input to the drift-diffusion system of differential equations to obtain the charge-carrier fluxes and, hence, the current density J dev generated by the solar cell as well as the electrical power density P = J dev V ext as functions of the bias voltage V ext under steady-state conditions [18]. In turn, the J dev -V ext and the P-V ext curves yield the efficiency η, short-circuit current density J sc , open-circuit voltage V oc , and fill factor FF [24,25].…”