WO2012028684A2

WO2012028684A2 - Method for thin film silicon photovoltaic cell production

Info

Publication number: WO2012028684A2
Application number: PCT/EP2011/065099
Authority: WO
Inventors: Hanno Goldbach; Tobias Roschek; Roman Kravets
Original assignee: Oerlikon Solar AG
Current assignee: TEL Solar AG
Priority date: 2010-09-03
Filing date: 2011-09-01
Publication date: 2012-03-08
Anticipated expiration: 2013-03-03
Also published as: WO2012028684A3; CN103262263A; TW201216491A

Abstract

This invention relates to a micromorph tandem solar cell and a method of manufacturing said solar cell. It particularly addresses the topic of reducing the active layer thicknesses significantly by advanced light trapping. The invention proposes a silicon -based solar cell with an extent > 1.4 m2 in tandem configuration comprising an a-Si cell (4) and a μc-Si cell (10), the absorber layer of the a- Si cell (4) having a thickness of 210nm ± 20 nm, the absorber layer of the μc-Si cell (10) having a thickness of 900 nm ± 200 nm.

Description

METHOD FOR THIN FILM SILICON PHOTOVOLTAIC CELL PRODUCTION

FIELD OF THE INVENTION

This invention relates to a process for manufacturing thin film silicon-based solar cells. It particular addresses the topic of reducing the active layer thicknesses significantly by advanced light trapping.

BACKGROUND OF THE INVENTION

Photovoltaic solar energy conversion offers the perspective to provide for an environmentally friendly means to generate electricity. However, at the present state, electric energy provided by photovoltaic energy conversion units is still significantly more expensive than electricity provided by conventional power stations.

Therefore, the development of more cost-effective means of producing photovoltaic energy conversion units attracted attention in the recent years. Amongst different approaches of producing low-cost solar cells, thin film silicon solar cells combine several advantageous aspects: firstly, thin-film silicon solar cells can be prepared by known thin-film deposition techniques such as plasma enhanced chemical vapour deposition (PECVD) and thus offer the perspective of synergies to reduce manufacturing cost by using experiences achieved in the past for example on the field of other thin film deposition technologies such as the displays sector. Secondly, thin-film sili- con solar cells can achieve high energy conversion efficiencies striving for 10% and beyond. Thirdly, the main raw materials for the production of thin-film silicon based solar cells are abundant and non-toxic . A thin-film solar cell generally includes a first electrode, one or more semiconductor thin-film p-i-n or n-i-p junctions, and a second electrode, successively stacked on a substrate. Figure 1 shows a tandem-junction silicon thin film solar cell as known in the art. Such a thin-film solar cell 50 usually includes a first or front electrode 42 on a substrate 41, one or more semiconductor thin-film p-i-n junctions (52-54, 51, 44-46, 43), and a second or back electrode 47. Each p-i-n junction 51, 43 or thin-film photoelectric con- version unit includes a substantially intrinsic i-type layer 53, 45 sandwiched between a p-type layer 52, 44 and an n-type layer 54, 46 (p-type = positively doped, n-type = negatively doped) . "Substantially intrinsic" in this context is understood as undoped or exhibiting essentially no resultant doping. Photoelectric conversion occurs primarily in this i-type layer; it is therefore also called "absorber layer".

Depending on the crystalline fraction (crystallinity) of the i-type layer 53, 45 solar cells or photoelectric _. ( conversion) devices are characterized as amorphous (a-Si, 53) or microcrystalline ( c-Si, 45) solar cells, independent of the kind of crystallinity of the adjacent p and n-layers. "Microcrystalline" layers are being understood, as common in the art, as layers comprising of a significant fraction of crystalline silicon - so called micro-crystallites - in an amorphous matrix. Stacks of p-i-n jucnctions are called tandem or triple junction photovoltaic cells. The combination of an amorphous and microcrystalline p-i-n- junction, as shown in Fig. 1, is also called "micromorph tandem cell". Fig. 1 shows a prior art - tandem junction thin film silicon photovoltaic cell. Thicknesses not to scale .

DEFICIENCIES IN THE ART

Despite the low thicknesses used one of the main cost factors for thin film silicon solar cells is still the thickness of the layers (in particular of the i-layer) . E.g. for micromorph tandem cells the microcrystalline (μσ) bottom cell is often in the range of 1.5μπι or even more. This thickness is influencing the Cost of Ownership for a producer of thin film solar cells in many ways:

The high thicknesses require a long process duration of the silicon deposition systems (e.g. PECVD) both for the deposition itself and for the following plasma cleaning. Since such production systems are generally a major investment, processing time is a major contribu- tion to the cost of ownership.

Further, also the gases used for deposition and cleaning are a big contributor, in particular silane SiH and F-source gas (like NF₃, SF₆, F₂) . All these factors can be substantially reduced when the layer thickness can be lowered.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide high performance solar cells with reduced layer thicknesses. This is achieved by a solar cell arrangement according to claim 1 and a method of manufacturing such a solar cell arrangement according to claim 8. Further embodiments of the invention are specified in the dependent claims. The invention involves a solar cell arrangement with an extent > 1.4 m² in tandem configuration comprising an a-Si cell and a μο-εί cell, the absorber layer of the a-Si cell having a thickness of 210nm ± 20 nm, the absorber layer of the μο-Si cell having a thickness of 900 nm ± 200 nm.

There are two key criteria for realizing high performance modules with reduced layer thicknesses. One is a high uniformity of the layers over the whole glass, because at low thicknesses of the layers the photovoltaically generated current becomes much more sensitive to thickness variations, the cell being more far away from the saturation current. This way small fluctuations in thickness can lead to large variations in current and that way to reduced overall efficiency by current limitation. This is shown in Fig. 2 (simulated for a-Si) .

The other criterion is the realisation of a good light trapping. "Light trapping" means a solar cell's ability to exploit impinging light. Measures improving the light trapping behaviour are antire- flective coatings, textured or as-grown rough TCO layers and further any steps taken e. g. to extend the effective path of the light in the absorber layers. Generally, the optimum layer thickness is determined by two factors, the light trapping and the layer quality. With a good light trapping a significant absorption of the light can be achieved at lower thicknesses. Limitations in the material quality cause that generated charge carriers cannot be extracted at higher thicknesses, since the charge carriers recombine before they reach the electrode layers. Fig. 3 illustrates how improved light trapping is shifting the efficiency optimum to lower thicknesses (also simulated for a-Si) .

While the above mentioned was shown for a-Si the principal effects are the same for tandem junction solar cells, e. g. of the micro- morph type as shown in Fig. 1.

Large solar modules with thin layers in industrial scale at high performance are being produced nowadays by using high uniformity deposition equipments like the deposition tools TCO1200 and KAI1200 manufactured by Oerlikon Solar, Triibbach, Switzerland. For optimum light trapping a combination of front and back contact had to be optimised. Additional an improved back reflector (white foil) was introduced. This white foil replaces white paint as diffuse back reflector in the thin film solar cell stack: see Fig. 4.

That way without loss in stabilised efficiency at bottom cell thickness could be realised: see Fig. 5.

In one variant of the solar cell arrangement according to the inven- tion, which may be combined with any variant to be addressed unless in contradiction, the absorber layers have a homogeneity of ± 5%.

In one variant, which may be combined with any variant addressed or to be addressed unless in contradiction, the solar cell arrangement according to the invention comprises a transparent conductive oxide layer of ZnO with a haze of 25%, preferably deposited by LPCVD.

In one variant, which may be combined with any variant addressed or to be addressed unless in contradiction, the solar cell arrangement according to the invention comprises a transparent conductive oxide layer as a back electrode of ZnO having a homogeneity of haze of 10% and preferably deposited by LPCVD. In one variant, which may be combined with any variant addressed or to be addressed unless in contradiction, the solar cell arrangement according to the invention comprises a white foil as a back reflec- tor, preferably of polyvinylbutyral, preferably equipped with white reflecting particles and preferably of 0.5 mm thickness.

In one variant, which may be combined with any variant addressed or to be addressed unless in contradiction, the solar cell arrangement according to the invention has a light induced degradation of less than 10 %.

In one variant of the solar cell arrangement according to the invention, which may be combined with any variant addressed or to be ad- dressed unless in contradiction, the a-Si cell comprises a 10 nm silicon p-layer, a 210 nm a-Si:H absorber layer, a 30 nm n-layer and the μο-Ξί cell comprises a 24 nm p-layer, a 900 nm absorber layer of μσ-3ί:Η and a 36 nm n-layer. The invention further involves a method of manufacturing a solar cell arrangement with an extent > 1.4 m² in tandem configuration comprising an a-Si cell and a μο-Si cell, the absorber layer of the a- Si cell having a thickness of 210nm ± 20 nm, the absorber layer of the μσ-εί cell having a thickness of 900 nm ± 200 nm and the method comprising:

- PECVD depositing the absorber layer of the a-Si cell with the following deposition parameters:

• Flow of SiH4: 10.4 slm

• Flow of H2: 10.4 slm

· Deposition rate: 3.35 A/s

• Deposition time: 634 s

• Pressure: 0.5 mbar

• Temperature: 200° C

• Power: 380 W, and

- PECVD depositing the absorber layer of the μο-Si cell with the following deposition parameters:

• Flow of SiH4 : 7.7 slm • Flow of H2: 170 slm

• Deposition rate: 5 A/s

• Deposition time: 1830 s

• Pressure: 2.5 mbar

• Temperature: 160° C

• Power: 3500 W.

In one variant the method according to the invention, which may be combined with any variant to be addressed unless in contradiction, the arrangement further comprises a transparent conductive oxide layer of ZnO with a haze of 25%, and comprises LPCVD depositing the layer of ZnO with the following deposition parameters:

• Temperature: 180° C

• Flow of H2: 577 seem

• Flow of B2H6: 400sccm

• Flow of H20: 2460 seem

• Flow of DEZ: 2200 seem

• Pressure: 0.5 mbar.

In one variant, which may be combined with any variant addressed or to be addressed unless in contradiction, the method according to the invention, the a-Si cell comprises a 10 nm silicon p-layer, a 210 nm a-Si:H absorber layer, a 30 nm n-layer, the μο-Si cell comprises a 24 nm p-layer, a 900 nm absorber layer of μο-3ί:Η and a 36 nm n- layer, and the method comprises:

- depositing the p-layer of the a-Si cell with the following deposition parameters:

• Flow of SiH4: 5.64 slm

• Flow of H2: 10.58 slm

• Flow of Trimethylboron (TMB) : 6.45 slm

• Flow of CH4: 10.26 slm

• Deposition rate: 2.6 A/s

• Deposition time: 39 s

• Pressure: 0.5 mbar

• Temperature: 200° C

• Power: 295W, depositing the n-layer of the a-Si cell with the following deposi tion parameters :

• Flow of SiH4: 0.86 slm

• Flow of H2: 95 slm

• Flow of PH3: 1.02 slm

• Deposition rate: 1 A/s

• Deposition time: 300 s

• Pressure: 2 mbar

• Temperature: 200° C

• Power: 1800 W,

depositing the p-layer of the c-Si cell with the following deposition parameters:

Flow of SiH4 : 1.6 slm

Flow of H2: 200 slm

• Flow of Trimethylboron (T B) : 0.5 slm

Deposition rate: 1.4 A/s

Deposition time: 175 s

Pressure: 2.5 mbar

Temperature: 160° C

• Power: 3100 W, and

depositing the n-layer of the μο-Si cell with the following deposition parameters:

Flow of SiH4: 6.21 slm

Flow of H2: 14.36 slm

• Flow of PH3: 4 slm

Deposition rate: 2 A/s

Deposition time: 180 s

Pressure: 0.5 mbar

Temperature: 160° C

• Power: 700 W.

In one variant the method according to the invention, which may be combined with any variant addressed or to be addressed unless in contradiction, the method comprises a water vapour flush treatment between deposition of the p-layers and deposition of the absorber layers . In one variant, which may be combined with any variant addressed or to be addressed unless in contradiction, the method according to the invention, the water vapour flush treatment is performed at 1.2 mbar vapour pressure during 120 s.

In one variant, which may be combined with any variant addressed or to be addressed unless in contradiction, the method according to the invention, the water vapour flush treatment between depositing of the p-layer and the absorber layer of the a-Si cell is performed at 2 mbar vapour pressure during 120 s and the water vapour flush treatment between depositing of the p-layer and the absorber layer of the μο-Ξί is performed at 1.2 mbar vapour pressure during 120 s.

DETAILED DESCRIPTION OF THE INVENTION

The invention shall further be exemplified with the help of the figures. These figures show: Fig. 1: an illustration of a tandem junction thin film silicon

photovoltaic cell according to the prior art;

Fig. 2: a diagram illustrating Jsc in relation to thickness of i- layer;

Fig. 3: a diagram illustrating stable Eta in relation to thickness;

Fig. 4: a diagram illustrating Total Reflectance in relation to

Wavelength;

Fig. 5: a diagram illustrating Pmpp in relation to bottom cell

Thickness;

Fig. 6: an illustration of a tandem junction according to the invention . Fig. 1 shows a prior art tandem junction thin film silicon photo voltaic cell 50 (thicknesses not to scale) . The arrows indicate the direction of impinging light. The tandem junction comprises a substrate 41, a front electrode 42, a bottom cell 43, a p-doped Si layer (p μο-3ί:Η) 44, a i-layer μο-3ί:Η 45, a n-doped Si layer (n a- Si:H / n μο-3ί:Η) 46, a back electrode 47, a back reflector 48, a top cell 51, a p-doped Si layer (p a-Si:H / p μο-είιΗ) 52, a i-layer a-Si:H 53, a n-doped Si layer (n a-Si:H / n μο-3ί:Η) 54. Fig. 6 shows a tandem junction according to the invention in the form of a so called micromorph junction. The tandem junction comprises a front contact 3 having a thickness of 1.55 um, a a-Si layer 4 having a thickness of 210 nm, a μο-3ί:Η layer 10 having a thickness of 900 nm, a back contact 11 having a thickness 1.55 um and a white foil 13.

The present invention addresses a method for producing thin film cells with reduced absorber layer thickness at high stabilized power on large area > 1.4 m² by

a) enhancing the light trapping capability over the complete deposition area utilising a highly homogenous high haze front contact; and b) a white foil with improved reflection properties as back reflector;

c) by increased homogeneity of the semiconductor layers reducing the effect of thickness variations of said layers; and

d) by reduction of thicknesses of the Si layer thus reducing the susceptibility to a-Si degradation.

The absorber layers (i-layers 53, 45) in a tandem junction configu- ration as shown in Fig. 1 can be reduced to 210nm ±20nm for the top- cell 51 and 900±200nm for the bottom cell 43. This can be achieved without loss in stabilized power. In Prior Art a top cell 51 is realized with around 300nm thickness and about 1.45 μπι for a bottom cell 43. With the aid of the invention a reduction of 1/3 for the top cell in material cost and time is possible. The bottom cell thickness may be reduced almost by a factor 2 ! This is essentially due to (i) the improvement in homogeneity of the semiconductor layers to +-5% in the active part of a solar panel and (ii) the improved homogeneity of the LPCVD ZnO layers to 10% haze uniformity and (iii) the high haze of 25%. All these achievements allow for a good light confinement or light trapping. The white foil as back reflector layer 48 provides for an extreme good back reflection and scattering of the light thus further contributing to good light confinement. The improved reflection supplies more light to the top cell and thus thinner cells are possible.

A further important point is that thinner top cells (amorphous silicon) exhibit lower degradation (Staebler-Wronski effect) . By this the overall degradation of a tandem junction cell is significantly reduced, below 10%.

By an improved treatment of the interface between p- and i-layer, a water vapour flush treatment of 1.2mbar for 120s, the quality of the bottom cell is improved. The performance of the bottom cell is enhanced improved light trapping allows for very thin cells.

A cell with a structure according to Figure 1 with the advantages according to the invention comprises a front electrode 42 made from LPCVD ZnO, a pin-pin top/bottom cell structure from Si and a back electrode 47 made from LPCVD ZnO, followed by a reflector which is a white foil. This white foil is preferably a polyvinylbutyral foil equipped with white reflecting particles or equivalent.

The front 42 electrode has an average haze of 25% with homogeneity of +/- 10%. The excellent light trapping is facilitated by the improved load lock system of of the TCO deposition tool. The relevant features of siad load Lock have been described in US 61/367,910, which is incorporated herin by reference. The layer is deposited at a load lock temperature of 184°C, 200sccm B₂H₆, 2200sccm DEZ (diethylzinc) and 2460sccm H₂0 at a process pressure of 0.5 mbar. The pin structure exhibits a top cell with a lOnm silicon p-layer, a 210nm i-layer (A- Si:H), an 30nm n-layer . The bottom cell exhibits a 24nm p-layer , a 900nm intrinsic silicon (μο-3ί:Η) layer and finally a 36nm n-layer. The variation of thickness of the absorber layer is as mentioned above, the variation of the p/n layer thicknesses is +/- 20%. Table 1 summarizes the deposition parameters of the pin structures. Between the p and i-layers a water vapour treatment is performed (water flush). Said process is addressed in US 7,504,279, therefore this disclosure is incorporated herein by reference. For the first p-i junction the vapour pressure is 2mbar for 120s. At the second p- i interface the pressure is 1.2mbar for 120s. The pin Si structures are followed by a TCO ZnO back electrode 47 made by LPCVD. The deposition parameters are: load lock temperature 180°C, H₂ 577sccm, B₂H₆ 400sccm, H₂0 2460sccm, DEZ 2200 seem, process pressure 0.5mbar.

The whole stack is finished by a white foil with a thickness of 0.5 mm.

Table 1

TMB = trimethylboron, DR = deposition rate, Dep t = deposition time, all values given refer to a 1.4m² substrate to be coated.

Order of layers described is as deposited and shown in Figure 1.

Claims

Claims :

A solar cell arrangement with an extent > 1.4 m² in tandem configuration comprising an a-Si Cell (4) and a μο-εί cell (10), the absorber layer of the a-Si cell (4) having a thickness of 210nm ± 20 nm, the absorber layer of the μο-Si cell (10) having a thickness of 900 nm ± 200 nm.

2) The solar cell arrangement of claim 1, the absorber layers having a homogeneity of ± 5%.

3) The solar cell arrangement of claim 1 or 2 comprising a transparent conductive oxide layer (3) of ZnO with a haze of 25%, preferably deposited by LPCVD.

4) The solar cell arrangement of one of claims 1 to 3 comprising a transparent conductive oxide layer (11) as a back electrode of ZnO having a homogeneity of haze of 10% and preferably deposited by LPCVD.

5) The solar cell arrangement of one of claims 1 to 4 comprising a white foil (13) as a back reflector, preferably of polyvinylbu- tyral, preferably equipped with white reflecting particles and preferably of 0.5 mm thickness.

6) The solar cell arrangement of one of claims 1 to 5 having a light induced degradation of less than 10 %.

7) The solar cell arrangement of one of claims 1 to 6 said a-Si cell (4) comprising a 10 nm silicon p-layer, a 210 nm a-Si:H absorber layer, a 30 nm n-layer and said μσ-εί cell (10) comprising a 24 nm p-layer, a 900 nm absorber layer of μο-3ί:Η and a 36 nm n-layer.

8) A method of manufacturing a solar cell arrangement with an extent > 1.4 m² in tandem configuration comprising an a-Si cell (4) and a μο-Si cell (10) , the absorber layer of the a-Si cell (4) having a thickness of 210nm ± 20 nm, the absorber layer of the c-Si cell (10) having a thickness of 900 nm + 200 nm, and the method comprising:

- PECVD depositing said absorber layer of the a-Si cell (4) with the following deposition parameters:

Flow of SiH4: 10.4 slm

Flow of H2: 10.4 slm

• Deposition rate: 3.35 A/s

• Deposition time: 634 s

• Pressure: 0.5 mbar

Temperature: 200° C

Power: 380 W, and

PECVD depositing said absorber layer of the c-Si cell (10) with the following deposition parameters:

Flow of SiH4: 7.7 slm

Flow of H2: 170 slm

• Deposition rate: 5 A/s

• Deposition time: 1830 s

• Pressure: 2.5 mbar

Temperature: 160° C

Power: 3500 W.

The method of claim 8 for manufacturing said arrangement further comprising a transparent conductive oxide layer (3) of ZnO with a haze of 25%, and comprising LPCVD depositing said layer of ZnO with the following deposition parameters:

Temperature: 180° C

Flow of H2: 577 seem

Flow of B2H6: 400sccm

Flow of H20: 2460 seem

Flow of DEZ: 2200 seem

• Pressure: 0.5 mbar.

The method of claim 8 or 9 said a-Si cell (4) comprising a 10 nm silicon p-layer, a 210 nm a-Si:H absorber layer, a 30 nm n- layer, said pc-Si cell (10) comprising a 24 nm p-layer, a 900 nm absorber layer of μο-3ί:Η and a 36 nm n-layer, and the method comprising:

- depositing said p-layer of said a-Si cell (4) with the following deposition parameters:

Flow of SiH4 : 5.64 slm

Flow of H2: 10.58 slm

Flow of Trimethylboron (TMB) : 6.45 slm

Flow of CH4: 10.26 slm

• Deposition rate: 2.6 A/s

Deposition time: 39 s

Pressure: 0.5 mbar

Temperature: 200° C

Power: 295 ,

- depositing said n-layer of said a-Si cell (4) with the following deposition parameters:

Flow of SiH4: 0.86 slm

Flow of H2: 95 slm

Flow of PH3: 1.02 slm

• Deposition rate: 1 A/s

Deposition time: 300 s

• Pressure: 2 mbar

Temperature: 200° C

Power: 1800 W,

- depositing said p-layer of said μο-Si cell (10) with the following deposition parameters:

Flow of SiH4: 1.6 slm

Flow of H2: 200 slm

Flow of Trimethylboron (TMB) : 0.5 slm

• Deposition rate: 1.4 A/s

• Deposition time: 175 s

• Pressure: 2.5 mbar

Temperature: 160° C

Power: 3100 W, and - depositing said n-layer of said μο-Si cell (10) with the following deposition parameters:

Flow of SiH4: 6.21 slm

Flow of H2: 14.36 slm

Flow of PH3: 4 slm

Deposition rate: 2 A/s

• Deposition time: 180 s

• Pressure: 0.5 mbar

• Temperature: 160° C

Power: 700 W.

The method of one of claims 8 to 10 comprising a water vapour flush treatment between deposition of the p-layers and deposition of the absorber layers.

The method of claim 11 said water vapour flush treatment being performed at 1.2 mbar vapour pressure during 120 s.

The method of claim 11 said water vapour flush treatment between depositing of said p-layer and said absorber layer of said a-Si cell (4) being performed at 2 mbar vapour pressure during 120 s and said water vapour flush treatment between depositing of said p-layer and said absorber layer of said μσ-3ί (10) being performed at 1.2 mbar vapour pressure during 120 s.