Cell and module construction

Photovoltaic effect and basic solar cell parameters To obtain a potential difference that may be used as a source of electrical energy, an inhomogeneous structure with internal electric field is necessary. Suitable structures may be: PN junction heterojunction (contact of different materials).

V mp V OC open circuit voltage V OC, short circuit current I SC maximum output power V mp I mp fill factor efficiency Parameters V OC, I SC, FF and η are usually given for standard conditions: spectrum AM 1.5 radiation power 1000 W/m 2 cell temperature 25°C. Important solar cell electrical parameters

Modelling I-V characteristics of a solar cell Series resistance R S Parallel resistance R p PN junction I-V characteristics A ill – illuminated cell area A - total cell area Output cell voltage V = V j - R s I

To maximise current density J PV it is necessary maximise generation rate G minimise losses losses opticalrecombination electrical reflection shadowing not absorbed radiation emitter region base region surface series resistance parallel resistance

Series resistance R s consists of: ·R 1 – contact metal-semiconductor on the back surface ·R 2 – base semiconductor material ·R 3 – lateral emitter resistance between two contact grid fingers · R 4 – contact metal-semiconductor on the grid fingers ·R 5 – resistance of the grid finger ·R 6 – resistance of the collector bus Series resistance R s influences strongly solar cells efficiency

R 3 – lateral emitter resistance between two contact grid fingers Decrease of ρ N is connected with increasing N D  Auger recombination rate increases Decrease of the finger distance d results in a decrease of illuminated area Aill It is very important to optimise x j

Optimising PN junction depth The photo-current density J PV consists from carriers collected by the electric field in the space charge region of the PN junction, i.e. from carriers generated in a distance of about diffusion length from the PN junction. The PN junction depth x j should be less than 0.5 μm (0.2  m is desirable).

Construction of solar cells To obtain high efficient solar cell it is necessary maximise I PV and R p minimise R s In the illuminated area generated excess carriers diffuse towards the PN junction. The density J FV is created by carriers collected by the junction space charge region in the N-type region in the P-type region in the PN junction space charge region

More detail information can be obtain from solution of continuity equations. J p - current density generated in N-type layer J n - current density generated in P-type base J OPN - generated in the PN junction space charge layer

Results of simulations for different wavelength of incident light Losses due to recombination may considerably limit the cell parameters, especially, losses connected with the recombination in the P-type region.

To maximise current density J PV it is necessary maximise generation rate G minimise recombination rate To maximise generation rate it is necessary to minimise surface reflection, especially in the visible region of solar spectrum Φ 0 = Φ in (1 – R ) R is the surface reflexivity

For a monochromatic light, the minimum reflexivity R min occurs when the optical path is equal to a quarter of wavelength, i.e. the layer thickness is. Antireflection coating From that follows for Silicon refraction index Thin film material of n 2  2 is desirable for silicon solar cells (Si 3 N 4 or TiO 2 layers of d  75 nm are usually used for antireflection coating).

Theoretical efficiency limit – 18% - plane surface - SiO 2 (TiO 2 ) antireflection layer - photolithography - vacuum contact deposition

Surface texturing If the surface has a pyramidal structure it is possible to decrease reflection on about one third of that on a plane surface. texturised Both principles (surface texturing and antireflection coating) can be combined to minimise losses by surface reflection

PEARL structure (1994) New construction principles - antireflection coating improvement - high contact quality - high quality starting (FZ) Si - minimising the structure thickness microelectronics technology with several photolithographic processes principles of construction and technology were simplified for mass production

Present construction used in mass production - etching monocrystalline (1,0,0) Si in KOH -Surface texturing without photolithography - acid etching in the case of other crystallographic orientation of Si

A single solar cell……~0.5 V, about 30 mA/cm 2 For practical use it is necessary connect cells in series to obtain a source of higher voltage and in parallel to obtain a higher current Solar cell PV module PV field

Optimum situation: all cells have the same V MP If characteristics of individual cells in parallel differ, efficiency decreases Cell connection in parallel

Cells in series….. the same current flows through all cells voltage does sums I V n cells n + 1 cells Optimum situation: all cells have the same I MP If characteristics of individual cells in series differ, efficiency decreases

Effect of partial shading – one cell (a decrease of illuminated area A ill ) In the case of one cell a decrease of the output current (proportional to area illuminated) a decrease of the output voltage a decrease of the output power

Effect of partial shading – cells in series In the case of cells in series increases series resistance a decrease of the output current a decrease of the output voltage a considerable decrease of the output power One cell shaded

A practical example of the shading effect – a bottom of the field 2 is shaded in January by the field 1

Crystalline silicon cellsThin film cells Suitable materials CuInSe 2 amorphous silicon amorphous SiGe CdTe/CdS Basic types of solar cells: Basic problem: cost…...

Glass Molybdenum Cu(InGa)Se 2 CdS ITO/ZnO MgF 2 NiAl  m  m  m 0.1  m  m CIS Cadmium Telluride (Copper Indium Selenide) Contact CdTe CdS TCO Glass  m  m

A simple structure of a thin film solar cell from amorphous silicon transparent substrate transparent conducting electrode a-Si:H p+ layer ( nm) a-Si:H (non-doped layer – 250 nm) a-Si:H n+ layer (20 nm) Transparent conducting electrode (diffusion barrier) Silver or aluminium back reflector

Light trapping

Tandem cells irradiation W g1 > W g2

“Micromorf” cells

A III B V triple cells The highest efficiency from all solar cell types   30%

High efficiency solar cell application Concentrator modules