TECHNICAL FIELD

The present invention relates to a Cu(ln,Ga)Se ₂ (CIGS) photovoltaic module or solar cell. In particular the present invention relates to a barrier layer in a window layer assembly of a CIGS module and the production of a CIGS module with a barrier layer in the window layer assembly.

BACKGROUND

In the design of solar cells much attention has been given to the window layer, also referred to as the transparent conductive layer (TCL). The window layer should provide sufficient conductivity and at the same time has as little absorption as possible in the wavelength range of the incoming light. In addition the window layer should preferably provide mechanical and chemical protection to the solar cell and be compatible with the underlying layers in the solar cell structure.

In order to optimize for the different requirements multilayer structures are commonly used for the window layer. One conventional window layer for monolithically integrated Cu(ln,Ga)Se ₂ (CIGS) photovoltaic modules consists of CdS / ZnO / ZnO:AI (AZO). Due to the required sheet resistance (ca 10 W/D), the AZO suffers some absorption losses. An established alternative to the AZO front electrode is ZnO:B (BZO), which has significant less absorption losses at this rather high conduction level. Although BZO as a front electrode has very promising properties, the full potential of such cells have not been reached due to degradation of the properties in other parts of the cell.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate or at least to minimize the problems discussed above in relation to using a doped window layer, in particular a boron doped window layer such as ZnO:B (BZO). This is achieved by a solar cell as defined in claim 1 produced with the method as defined in claim 7.

The conventional window layer for monolithically integrated Cu(ln,Ga)Se ₂ (CIGS) photovoltaic modules consists of CdS / ZnO / ZnO:AI (AZO). Due to the required sheet resistance (ca 10 W/D), the AZO suffers some absorption losses. An established alternative to the AZO front electrode is ZnO:B (BZO), which has significant less absorption losses at this rather high conduction level. In order to achieve the full potential of the BZO in the device, the inventors have found significant advantages by replacing also the ZnO 2 ^nd buffer layer with a tailored Zni_ _x Mg _x O (ZMO) film.

A solar cell according to the invention comprises a substrate, a back contact provided on top of the substrate, a photovoltaic absorber layer provided on top of the back contact, a first buffer layer provided on top of the photovoltaic absorber layer and a doped window layer which is doped with at least one dopant. According to the invention at least one barrier layer is introduced in the structure and provided between the doped window layer and the photovoltaic absorber layer. The barrier layer is arranged to limit diffusion of the dopant of the doped window layer from diffusing into the underlying layers. According to one aspect of the invention the first barrier layer has a dual function of being a barrier layer for the dopant of the doped window layer and a second buffer layer.

According to another aspect the solar cell comprises a first and second barrier layer wherein the first barrier layer differs in composition and/or thickness from the second barrier layer.

According to another aspect of the present invention the barrier layer comprises Zni_ _x Mg _x O wherein x is between 0.05 and 0.1, preferably between 0.05 and 0.085 and even more preferably between 0.05 and 0.075. The barrier layer should have a thickness above 90nm, and preferably a thickness above lOOnm, and even more preferably a thickness of 105nm.

The method according to the invention of producing the above described solar cell comprises applying the barrier layer by pulsed DC sputtering.

Thanks to the invention the full potential of providing a BZO front electrode may be utilized without degrading the photovoltaic absorber layer of the solar cell. Solar cell efficiencies up to 21% are reached when the optimized ZMO/BZO (10 W/D) window layer is combined with CIGS/CdS from an established production line.

DRAWINGS

The invention will now be described in more detail with reference to the appended drawings, wherein

Fig. 1 illustrates schematically the solar cell according to the invention;

Fig. 2 is a SEM image of a solar cell according to the invention;

Fig. 3 is a graph of a SIMS boron depth profile of a solar cell according to the invention;

Fig. 4 is a graph of Voc for different TCO and 2nd buffer combinations;

Fig. 5 is a graph of relative boron concentration at the depth of the CdS peak intensity;

Fig. 6 is a graph of the external quantum efficiency spectra comparing the ZMO/BZO window layer to the reference IZO/AZO;

Fig. 7 is a graph of the absorptance spectra derived from spectrophotometry of AZO and

BZO glass sandwich structures including a diiodomethane index matching liquid; and

Fig. 8 are graphs showing solar cell IV parameter comparison of the optimized ZMO/BZO window layer and reference IZO/AZO.

DETAILED DESCRIPTION

In order to achieve the full potential of providing the boron doped BZO-layer in the device, the inventors have introduced a barrier layer placed between the photovoltaic absorber layer and the BZO. Without being bound by theory boron appears to disturb the function of the photovoltaic absorber layer, and without the barrier layer boron diffuses into the photovoltaic absorber layer. The barrier layer is chosen to hinder boron diffusion into the photovoltaic absorber layer. Hence the advantages provided by the increased conductivity of the BZO can be utilized without degradation of the photovoltaic absorber layer. Similar conditions may occur for other dopants and/or combinations of base materials and dopants.

The structure of a solar cell according to the invention is schematically depicted in Fig 1. A substrate 11 for example a glass substrate is provided with a back contact 12. Molybdenum is a commonly used back contact material. On top of the back contacts is a photovoltaic absorber layer 13, typically a CIGS layer. A buffer layer 14, most commonly a CdS-layer, is typically provided on top of the CIGS layer 13. A doped window layer 16 provides the functionality of being a transparent top contact and is doped with at least one dopant. According to the invention a barrier layer 15 is provided between the doped window layer 16 and the photovoltaic absorber layer 13, preferably between the buffer layer 14 and the window layer 16.

The material composition and/or the thickness of the barrier layer 15 is selected so that it limits diffusion of the dopant or dopants into the underlaying layers. In particular, and preferably, no significant diffusion of dopants should have occurred into the photovoltaic absorber layer 13. "no significant diffusion" should here be interpreted as the function of the photovoltaic absorber layer not being effected as compared to a reference solar cell. Suitable materials include, but are not limited to ZnMgO (ZMO), ZnSnO, ZnOS and GaO. The barrier layer may comprise a combination of materials or itself be a multilayer structure of different materials. It could also be a material with varying composition in the vertical direction of the structure. According to one aspect of the invention the barrier layer 15 has a dual function as a barrier layer and a buffer layer.

According to one embodiment of the invention the barrier layer comprises Zni_ _x Mg _x O wherein 0.05<x<0.1, preferably 0.05<x<0.085 and even more preferably 0.05<x<0.075. Increasing the Mg content further may impede the electric properties of the cell. Hence the interval represents an optimization with regards to the diffusion barrier properties and the electric properties. In order to achieve the positive effects of not having any substantially amounts of boron entering the CIGS-layer the thickness of the ZMO should be in the order of 90nm or thicker.

One embodiment of the invention is illustrated in the SEM-image of Fig. 2. The substrate 11 is a glass substrate (not shown) followed by a Mo back contact 12, a CIGS layer 13 and a CdS buffer layer 14. The doped window layer 16 is ZnO:B (BZO). The barrier layer is a tailored Zni_ _x Mg _x O (ZMO) film, 0.05<x<0.1. Compared to the common window layer structure of CdS / ZnO / ZnO:AI (AZO), it can be seen as replacing the second buffer layer, the ZnO-layer with ZMO. In that regard the ZMO functions both as buffer layer and a barrier layer.

The skilled person realize that other buffer layers may be included in the structure. Also the layers here described could be multilayer structures or layers having compositional gradients. Such variations are known in the art and may be combined with the barrier layer introduced in the solar cell according to the invention.

Results

A number of solar cell structures with a barrier layer according to the invention were prepared and categorized. The barrier layer is Zni_ _x Mg _x O with varying composition (x varied from 0.05 to 0.20) and of different thickness (from 35nm to 200nm).

Fig. 3 is a graph showing SIMS boron depth profile over 3 BZO/ZMO/CdS/CIGS/Mo cell stacks with varied ZMO film thickness. The 105nm ZMO layer effectively blocks B diffusing to the CIGS.

Fig.4 is a graph showing Voc for different TCO and 2 ^nd buffer/barrier layer (ZMO) combinations. 2 ^nd buffer/barrier layer thickness is stated in nm. All devices have the same CIGS and CdS (cut from 0.75 m ² substrate). A ZMO thickness > 90nm, preferably > lOOnm and even more preferably at around 105nm, is required for the BZO device.

Fig. 5 is a graph showing the relative boron concentration at the depth of the CdS peak intensity, related to maximal B cone., for each SIMS measurement and as a function of ZMO thickness. The 105nm ZMO layer blocks B diffusing through the CdS buffer.

Fig. 6 is a graph showing an external quantum efficiency spectra comparing the ZMO/BZO window layer to the reference IZO/AZO. Both devices have the same CIGS, CdS (35nm) and MgF ₂ AR-coating. Calculated short circuit current density for each device is stated within parentheses. Fig. 7 is a graph showing absorptance spectra derived from spectrophotometry of AZO and BZO glass sandwich structures including a diiodomethane index matching liquid. The absorptance of the substrate and cover glass is also included. BZO has a sharper absorption edge than AZO.

Fig. 8 is a graph showing solar cell IV parameter comparison of the optimized ZMO/BZO window layer and reference IZO/AZO. The solar cells were probed directly on the TCO and the short circuit current was obtained by EQE. The cells are MgF ₂ AR-coated and have an area of 0.11 cm ² .

The exemplary solar cells according to the invention and references cells were produced according to:

Mo: DC sputtered

CIGS: Co-evaporation

CdS: deposited via chemical bath deposition (CDB)

ZMO: Pulsed DC sputtering from compound target in 02/Ar atmosphere (0.016 m ² )

^■ Tested Zni_ _x Mg _x O Mg-fractions (X=): 0.05, 0.07, 0.10, 0.15, 0.20

^■ Power density 3.4 W/cm ² , substrate temperature 120 ^° C, multiple passes over the target

BZO: Low pressure chemical vapor deposition prototype production tool

^■ 1600 nm (10 W/D), substrate temperature 170 ^° C, 0.5 mbar process pressure

AZO: 1 wt%, DC sputtering, 1000 nm (10 W/D), substrate temperature 175 ^° C.

Conclusions

Zni_ _x Mg _x O is successfully deposited via pulsed DC sputtering of compound target

Zni_ _x Mg _x O blocks boron diffusion from the BZO TCO (ZMO thickness dependent)

A ZMO thickness > lOOnm eliminates voltage losses induced by the BZO TCO

The BZO TCO is optically superior to AZO at a sheet resistance of 10 W/D

Solar cell efficiencies up to 21% are reached when the optimized ZMO/BZO (10 W/D) window layer is combined with CIGS/CdS.

For simplicity a single solar cell has been described. As apparent for the skilled person the novel structure is advantageous also for interconnected cells, or solar cell modules, which is how solar cells typically are utilized in commercial products.

It is to be noted that elements of different embodiments described herein may freely be combined with each other unless such a combination is expressly stated as unsuitable, as will be readily understood by the person skilled in the art.