THIN-FILM SILICON TANDEM CELL
A thin-film tandem photovoltaic cell comprises on a glass substrate a front TCO ( 3 ), an amorphous silicon cell as top cell ( 5 ), a semi-transparent reflector layer ( 7 ), a microcrystalline silicon bottom cell ( 9 ). Thereby, the semi-transparent reflector layer ( 7 ) is of n-doped silicon oxide with an index of refraction below 1.7. The thickness of the amorphous silicon top cell ( 5 ) is below 200 nm.
1 . A thin-film silicon tandem cell comprising:
a glass substrate, transparent to impinging light to be photovoltaically converted by the cell; thereon
a top layer of a transparent conductive oxide; thereon
an amorphous silicon top cell; thereon
a semi-transparent reflector layer, reflecting light in the wavelength spectrum of 500 to 750 nm; thereon
a microcrystalline bottom cell; thereon
a bottom layer of a transparent, conductive oxide; thereon a white light reflector
wherein
said semi-transparent reflector layer is of an n-doped silicon oxide and has a index of refraction n below 1.7 and
the thickness of said amorphous silicon top cell is below 200 nm.
2 . The cell of claim 1 , wherein said n is 1.68.
3 . The cell of one of claim 1 or 2 , wherein thickness d of said semi-transparent reflector layer is 30 nm or 50 nm and the current of said bottom cell is lower than the current of said top cell by a mismatch current JΔ of 0.5 mA/cm2 and 1.5 mA/cm2 respectively and efficiency of said tandem cell after 1000 h of light soaking under standard test conditions at AM 1.5 sun spectrum is between 10 and 10.5%.
4 . The cell of claim 3 , wherein light-induced degradation is below 8.2%.
5 . The cell of one of claim 3 or 4 , wherein light-induced degradation of V OC is between 1 and 2%.
6 . The cell of claim 1 , wherein said silicon oxide has an oxygen content of 57 to 65%.
7 . The cell of claim 1 , wherein said top layer of a transparent, conductive oxide is of LPCVD-deposited ZnO and has a haze factor higher than 7%.
8 . The cell of claim 7 , wherein said haze factor is 13%.
9 . The cell of claim 8 , wherein the thickness of said semi-transparent reflector layer is 30 nm or 50 nm.
10 . The cell of one of claims 7 to 9 , wherein thickness d of said semi-transparent reflector layer is between 5 and 150 nm, preferably between 20 and 100 nm.
11 . A method for manufacturing a semi-transparent reflector layer between an amorphous silicon cell and a microcrystalline silicon cell of a tandem solar cell, comprising depositing said reflector layer as a layer of n-doped hydrogenated silicon oxide by a 40 MHz Rf PECVD process with the following settings:
Temperature: 150 to 250° C.,
pressure: 0.3 to 5 mbar
power density: 60 to 250 mW/cm2
time: 100 to 4000 sec
and, if making use of an Oerlikon KAI-M plasma enhanced chemical vapor deposition reactor as deposition tool, setting the following gas flows:
SiH 4 : 3 to 30 sccm
H 2 : 300 to 3000 sccm
PH 3 in H 2 , 2 ppm: 50 to 500 sccm
CO 2 : 10 to 50 sccm,
thereby adapting said gas flows, if a different deposition tool is used, to such different deposition tool.
12 . The method of claim 11 , wherein there is valid:
Pressure: 1 to 4 mbar
power density: 100 to 200 mW/cm2
and, if making use of an Oerlikon KAI-M plasma enhanced chemical vapor deposition reactor as deposition tool:
SiH 4 : 4 to 15 sccm
H 2 : 500 to 2000 sccm
PH 3 in H 2 , 2 ppm: 100 to 300 sccm
CO 2 : 10 to 30 sccm.
13 . The method of claim 11 , wherein there is valid:
Pressure: 2 to 3 mbar
power density: 100 to 170 mW/cm2
time: 500 to 2000 sec
and, if making use of the Oerlikon KAI-M plasma enhanced chemical vapor deposition reactor as deposition tool:
SiH 4 : 4 to 8 sccm
H 2 : 1400 to 1700 sccm
PH 3 in H2, 2 ppm: 150 to 300 sccm
CO 2 : 15 to 25 sccm.