Three-dimensional simulations of a thin film heterojunction solar cell

APPLIED PHYSICS LETTERS 95, 122108 共2009兲

Three-dimensional simulations of a thin film heterojunction solar cell with a point contact/defect passivation structure at the heterointerface N. Allsop,1 R. Nürnberg,2 M. Ch. Lux-Steiner,1 and Th. Schedel-Niedrig1,a兲 1

Helmholtz Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany Weierstraß Institut für Angewandte Analysis und Stochastik, 10117 Berlin, Germany

2

共Received 16 April 2009; accepted 11 August 2009; published online 24 September 2009兲 Thin film heterojunction solar cells such as those based on the chalcopyrites or amorphous silicon are often limited by interface recombination at the active heterointerface. A new strategy to overcome this limitation is described, replacing the conventional wider band gap contact material with a combination of a passivation layer plus the conventional contact in a point contact type structure. This is similar to the established method to minimize rear contact recombination in crystalline silicon solar cells. Here point contacts at the heterointerface of a CuInS2 based solar cell are modeled using the WIAS-TeSCA code. The importance of the donor defect energy level at the absorber/passivation interface is shown, and a way to improve the cell efficiency by ⬎25% 共relative兲 is outlined. © 2009 American Institute of Physics. 关doi:10.1063/1.3233962兴 Thin film solar cells based on the chalcopyrite materials have reached efficiencies of 20% in the laboratory using a simple heterostructure consisting of ZnO/ CdS/ Cu共In, Ga兲Se2 / Mo/ glass. These high-efficiency devices are based on absorbers with a Ga/ 共In+ Ga兲 ratio of approximately 0.25–0.35, corresponding to a band gap energy 共Eg兲 of approximately 1.1–1.2 eV.1 One approach to further improve device efficiencies is to increase the absorber band gap in order to better match the AM 1.5 solar spectrum. Another reason to pursue the development of so called wider-band gap chalcopyrite solar cells is for use in a chalcopyrite-based tandem solar cell. However, the performance of solar cell devices based on wider-band gap chalcopyrites tends to fall behind those based on low-band gap chalcopyrites. It is believed that one of the reasons for the poor performance of these wider-band gap chalcopyrite cells is an increased recombination rate at the absorber/CdS interface and Fermi level pinning further from the conduction band edge, compared to the low-gap chalcopyrites.2–4 Research efforts to find a better alternative to the CdS contact 共buffer layer兲 for the wide-gap chalcopyrites have not been successful. This is partly because the requirements of such a layer are quite severe. It must be transparent to most of the light, have a very close conduction band alignment to the absorber layer, be n-doped, and most importantly form an interface with the chalcopyrite, which results in a low recombination rate. This low recombination rate can result from either a low concentration of defects at the interface or from the presence of donor defects which help to pin the Fermi level close to the conduction band.3 We propose here a point-contact geometry at the front side of thin film heterojunction chalcopyrite solar cell devices, which is similar to the concept of contacts at the rear side of high-efficiency silicon based solar cells.5 In this case, we consider replacing the CdS buffer layer, with a combination of a passivation layer and the normal CdS layer 共Fig. 1兲. The introduction of a point contact geometry allows the relaxation of the requirements at the absorber/buffer interface.

The passivation layer keeps the total interface recombination rate low, however, the choice of the passivation layer is no longer restricted to those materials which have a good conduction band alignment to the absorber layer. Structuring the solar cells in this way can only be fully understood by undertaking two-dimensional 共2D兲 or threedimensional 共3D兲 modeling of the semiconductor device. For this we used the WIAS-TeSCA code, which solves the semiconductor drift/diffusion equations in 2D or 3D.6–8 In this case, a structural unit with cylindrical symmetry was chosen so that the calculation is performed in a 2D parameter space but provides results for a 3D cylindrical structure, which will be a good approximation to a close packed hexagonal array of contacts. A CuInS2 solar cell was chosen as our model system because these devices show good properties with respect to carrier collection but are believed to be limited by the CdS/ CuInS2 interface. The structure of a standard CuInS2 solar cell was modeled after Klenk3 with an additional thin wide band gap passivation layer interserted between the absorber and the CdS buffer layer 共see Table I兲. Circular holes in the passivation layer where the absorber directly contacts the CdS buffer layer provide a path for the flow of charge carriers through the cell. The interface between the passivation layer and the CuInS2 was assumed to contain the same concentration of donor defects as those present at the CdS/ CuInS2 interface. The case with no donors present at the CuInS2 / passivation interface was also considered. The scale of the structure is achievable with conventional photoresist based lithograpy, however the cost require-

a兲

FIG. 1. 共Color online兲 The point contact structure and the structural unit modeled using the TeSCA program.

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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© 2009 American Institute of Physics

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Allsop et al.

TABLE I. Layer properties. Defect levels are specified from the valence band edge of the CuInS2. Layer

CuInS2

Passivation

CdS

ZnO

Interface

CuInS2 / passivation

CuInS2 / CdS

Thickness 共␮m兲 Band gap 共eV兲 Electron affinity 共eV兲 Donor conc. 共cm−3兲 Acceptor conc. 共cm−3兲

2.0 1.5 3.6 0 5 ⫻ 1016

0.002 Infinite ¯ 0 0

0.05 2.4 3.8 1 ⫻ 1015 0

0.6 3.2 4.1 1 ⫻ 1018 0

Neutral defect conc. 共cm−2兲 Neutral defect level 共eV兲 Donor defect conc. 共cm−2兲 Donor defect level 共eV兲 Capture cross sections 共cm2兲

0 ¯ 1 ⫻ 1013 1.1/1.3 1 ⫻ 10−15

1 ⫻ 1014 0.75 1 ⫻ 1013 1.1 1 ⫻ 10−15

ments of thin film solar cells mean that other low cost structuring methods should be considered. Alternatively the passivation layer could be deposited in such a way that the absorber layer is only partially covered due to the natural deposition mechanism. This is believed to be the case in the ZnS layer deposited by the ILGAR method.9 In Fig. 2共a兲 the current-voltage 共IV兲 curves are displayed for a device with the radius of the point contact 共r兲 varying from 250 to 5 nm. The distance between the center of the point contacts has been kept constant at 1 ␮m 共d / 2 = 0.5 ␮m兲 and therefore the area ratio of contact area to total area varies from 1:4 to 1:10 000. At a distance between the points of 1 ␮m and less the distance between the points does not have a large influence on the IV curves. The onedimensional 共1D兲 curve represents the standard model with-

out any passivation layer, it overestimates the efficiency of the best CuInS2 cell as no reflection loss, ZnO optical loss, series resistance or rear contact recombination is modeled. The parameter of curves shown in Fig. 2 are given in Table II. In Fig. 2共b兲 the effect of changing the properties of the donor defects at the CuInS2 / passivation interface is shown. In curves F and H in Fig. 2共b兲, the donor defect level is at the same level as the donor at the CuInS2 / CdS interface, 1.1eV above the CuInS2 valence band. However for curve H the donor is not an active recombination center, which would be a good approximation if donor/positive charge is inside the passivation layer. In curve F the donor defect is an active recombination center with a capture cross section of 10−15 cm2, the same as the donors at the CdS interface, resulting in a dramatic loss of fill factor. In Fig. 2共b兲 curves C and I the energy of the donor level has been raised to 1.3 eV with cross sections of zero and 10−15 cm2, respectively. The influence of the cross section is now drastically reduced due to the better type inversion at the interface. The prospect of raising the energy level of the donor at the normal CuInS2 / CdS interface is limited by the level of the CdS conduction band. The wider band gap passivation material has much greater energy range for the donor level. Curve G is the case where no donors are present at the passivation layer, in this case although the defect passivation is perfect the cell becomes much worse because the total number of interface donors in the CdS-passivation system is reduced 共see Table II兲. Figure 3 shows the potential and electron current flow in

0.08 0.06

Depth (µm)

0.04 0.02 0 -0.02 -0.04 -0.06 -0.08 -0.1

FIG. 2. 共Color online兲 IV curves calculated for points contact with a 1 ␮m spacing. 共a兲 Radius variation and 共b兲 radius 50 nm with varying donor defect properties and modeling of the interfacial layer. In curves A to E and curve I the passivation has donor defects with an energy 1.3 eV above the VB, curves F and H have donor defects 1.1 eV above the VB and curve G has no donor defects. In curves H and I the donors have a capture cross section of zero, in curves A to F the capture cross section is equal to those at the CdS/CIS interface.

U[V] -1 -0.8 -0.6 -0.4 -0.2 JNX JNY

0 WIAS-TeSCA

0.05

0.1 0.15 0.2 Radial Distance (µm)

0.25

FIG. 3. 共Color online兲 2D cross section showing the potential distribution 共color scale兲 and the electron flow 共arrows兲 at the maximum power point of curve C from Fig. 2. The CdS layer extends from 0 to 0.05 ␮m and the position of the passivation layer is indicated by the black line. The lateral current at a depth of around 0.05 ␮m is due to the accumulation layer at the CdS/ZnO interface.


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Allsop et al. TABLE II. Parameters of the curves displayed in Fig. 2. Curve

1D

A

C

E

F

G

H

I

Radius 共nm兲 Open circuit voltage 共mV兲 Fill factor 共%兲 Efficiency 共%兲 Efficiency gain relative to 1D 共%兲

¯ 721 82 14.4 ¯

250 766 83 15.5 7.8

50 891 84 18.2 26

12.5 941 84 19.3 34

50 793 62 11.8 ⫺18

50 805 62 12.0 ⫺17

50 842 82 16.8 17

50 895 84 18.3 27

the region around the CdS contact, for a cell with a passivation donor at 1.3 eV. It can be seen that the space charge region extends deeper into the absorber near to the passivation layer due to the pinning of the Fermi level at the higher energy donors. The model shows that devices can be improved considerably with the most dramatic improvements in systems which are strongly limited by the interface, such as the wide band gap solar cells discussed previously. However, the standard Cu共In, Ga兲Se2 will also be affected by a finite amount of interface recombination. Further detailed studies of the variation of the conditions at the heterointerface will be published elsewhere. We conclude that introducing a point contact type structure into an appropriate defect passivation layer at the interface of heterojunction thin film solar cells based on CuInS2 could dramatically increase cell performance. If the function of the CdS layer is replaced by two different materials the extra degree of freedom will allow improved combined properties. 3D simulations are crucial to understanding the way in which such a system functions. As in the case of the CdS/ CuInS2 interface the presence of donor levels are critical to the performance. New passivation layers may have an

increased density of such donor defects, or donors which have a higher energetic level relative to the bands in the absorber and therefore behave as stable positive charges 共as in the SiNx / Si interface兲. Increasing the donor energy level at the CuInS2 / passivation interface by just 0.2 eV leads to an improvement of 26% compared to the 1D case. The authors would like to thank Dr. Reiner Klenk for fruitful discussions. 1

I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To, and R. Noufi, Prog. Photovoltaics 16, 235 共2008兲. 2 M. Turcu and U. Rau, J. Phys. Chem. Solids 64, 1591 共2003兲. 3 R. Klenk, Thin Solid Films 387, 135 共2001兲. 4 M. Gloeckler and J. R. Sites, Thin Solid Films 480, 241 共2005兲. 5 R. R. King, R. A. Sinton, and R. M. Swanson, Appl. Phys. Lett. 54, 1460 共1989兲. 6 WIAS-TeSCA—Modeling and Simulation of Semiconductor Devices, http://www.wias-berlin.de/software/tesca. 7 H. Gajewski, Mitt. Ges. Angew. Math. Mech. 16, 35 共1993兲. 8 H. Gajewski, H.-Chr. Kaiser, H. Langmach, R. Nürnberg, and R. H. Richter, in Mathematical Modeling and Numerical Simulation of Semiconductor Detectors, Mathematics-Key Technology for the Future, edited by W. Jäger and H.-J. Krebs, 共Springer, New York, 2003兲, pp. 355–364. 9 N. A. Allsop, C. Camus, A. Hänsel, S. E. Gledhill, I. Lauermann, M. C. Lux-Steiner, and Ch.-H. Fischer, Thin Solid Films 515, 6068 共2007兲.
