TECHNOLOGY I MANUFACTURING
ABOUT 90% OF THE SILICON solar cells commonly manufactured are based on screen-printed Silver (Ag) front contacts and an aluminium-BSF (Back Surface Field) as rear passivation. The success of this type of cell lies in the relatively low manufacturing cost and straightforward production technology.
But they also have a downside: the typical efficiency on monocrystalline silicon is limited to about 19%. To overcome this hurdle, alternative cell concepts are being developed, such as PERx-type solar cells promising efficiencies of 22% and higher. Researchers at imec have developed n-PERT solar cells, a specific version of a PERx-type cell, with unique properties, achieving as much as 21.5% with a clear roadmap to reach 22.5% .
The PERx family
The main silicon solar cell structure used in industry today is based on the Aluminum Back Surface Field (Al-BSF). It typically consists of a p-type multi- or monocrystalline silicon substrate doped with phosphor on the front side to create an n-type emitter. Silver contacts are applied as front metallization by screen- printing on top of a silicon nitride layer (SiNx) which serves as anti-reflection coating. The rear side is covered completely with screen-printed aluminum, used as contact. Subsequently, a firing step is applied to make good electrical contact between the screen-printed metals and the silicon substrate. During this process, the aluminum alloys with silicon forming a highly doped BSF. This layer passivates the rear side of the cell by means of an electric field and thus reduces recombination of free carriers.
The drawback of this kind of solar cells is that the efficiency is limited to about 19% on monocrystalline wafers. This is due to the moderate passivation at the rear side which is not sufficient to prevent some recombination losses. Also, the internal
Figure1: Structure of an Al-BSF solar cell; a PERC, PERL and PERT cell
reflection at the rear surface is limited to values around 70% so a considerable part of the light is lost by parasitic absorption. Another issue of Al-BSF cells is warping due to a mismatch in thermal expansion coefficient between silicon and aluminum. This will cause problems as substrates get thinner, which is the expected trend in the PV industry in order to save on silicon material costs.
66
www.solar-international.net I Issue IV 2014
For all these reasons researchers are studying PERx-type cells which promise efficiencies of more than 22%. These cells come in different ‘flavours’ but all have two points in common: local contacts next to a dielectric passivation layer (e.g. SiO2
, Al2 O3 ) at the rear side. In PERC
(Passivated Emitter and Rear Contact cell) cells there is no additional passivation present around the local contact openings
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96