Solar Cells Flashcards
How do solar cells generally generate electricity?
- absorb photons
- excite e-
- create a Pd
- current flows around external circuit
How do semiconductor solar cells work?
UV/visible light photons are absorbed, if these have more energy than Eb - e- is excited to conduction band
Irradiation penetration causes this in p and n layers, creates Pd between conduction band (filled with e-) and valence band (holes), attach an external circuit causes current to flow = electricity
What is the structure of a Si solar cell?
- anti reflective layer and n+type contacts
- insulator and etched surface (to absorb more photons)
- diffused n-type (same structure as above)
- p-type
- p+-type contacts
- insulator and conductor layer
How are heavily doped (n+/p+) regions doped?
Using ion implantation so that they are heavily focused (diffusion would spread particles out along surface)
Explain the ion implantation process
- Gas passed through a high electric field (strips e- and creates ions)
- ions move down tube & vacuum pump removes neutral atoms
- magnetic field aligns particles according to mass/charge
- selector picks ion to implant
- ion passes through accelerator & electric field widens the beam = impregnate target
How do you control the depth of impregnation during the ion implantation process?
Accelerating voltage dictates depth
Higher energy ions penetrate further means can get areas underneath surface without implanting on surface
Where do ions end up after they have been implanted and what does his mean?
Most end up in interstitial sites, but some create defects so material must be annealed to remove defects and improve efficiency (although causes some diffusion)
What are photoresists, how are they made and what do they do?
- shade some regions from implantation
- positive photoresist covers majority of area
- negative photoresist covers minority of area
- polymer based material spread over surface and irradiated using a patterned mask to remove some (regions left shield material)
Describe the surface layers of a Si solar cell
- glass layer for protection
- anti-reflective layer sprayed on top to finish
- crystal surface with tetrahedral gaps (maximise trapping), etched to give required finish
- Conductor (Al) and insulator added so current only travels to wanted areas
Why is some of the Si shaded?
Narrow connectors added to top of cell to join conduction bands to other cells, shadows underneath (want surface area to be small)
What are the problems and solutions of Si solar farms?
Low efficiency - improve photon absorption (thin film retrofitted)
High cost - poly Si systems
High area - focus light intensity using mirrors/guides
How are poly Si solar created?
- graphite connectors transfer heat from induction coils to the crucible & melt
- crucible removes vertically (and slowly) from heating -> causes directional solidification and verticals grain boundaries
- no rotation = rectangles cast
What grain boundaries are wanted in poly Si and why?
Vertical so that charge carrier trapping is minimised
Describe vapour deposition
- PVD - vapour condensates onto substrate to form a thin film
- CVD - chemical reactions cause vapour to condensate (slow but bonding good)
MBE And sputtering also used
Which vapour deposition is better for thin films and why?
CVD is used because Tm is too high for PVD to be used
What microstructure is wanted for SI based thin films and how is this achieved?
Want lack of long range order as this means dislocations and grain boundaries aren’t present - amorphous material
By depositing silane into a substrate before it has time to crystallise means a silicate glass is formed (vacancies still present to act as charge traps)
How can amorphous Si be adapted to become a semiconductor?
A-Si has high defect density, adding H to the structure neutralises vacancies (charge carrier traps) and creates a semi conductor with Eb roughly 1.5Ev
However still has poor conductivity = poor solar cell
What mechanism is used to dope A-Si?
Dopant hydrides are added to the CVD process and doped material is generated (diffusion is too slow in amorphous materials - no fast paths)
What are the uses and drawbacks of A-Si?
Very poor efficiency (low conductivity) so used for low power app (calculators, watches etc)
CVD is low temp so can use a thermoplastic polymer as substrate = cheap and large quantity production
Charge carrier speed is slow so greater field is required - PIN junction used
What is a PIN junction?
P-type, intrinsic, N-type
- Results in sloping band gaps (Ef still equalises) with depletion zones at both interfaces = field throughout cell = e- travel further
- most carriers travel as e-, hole pairs and are separated by field
Draw the structure of a A-Si semiconductor and discuss light trapping needs
Glass layer, transparent conducting oxide, ZnO layer, NIP junction, ZnO layer, contacts and connectors
Thinness of cell means photon absorption time is limited, ZnO layer stops excess energy being turned into heat = higher absorption rate
What is the Shockley-queisser limit?
Theoretical PV cell limit of 33.7% efficiency - due to distance light travels through atmosphere
Assumes: Eb = 1.5Ev, only photons E>Eb create e- (19% spectrum), 1 photon = 1 pair rest E released as heat (33% spectrum)
What does the Shockley-quester limit mean for PV materials?
Says ideal band gap = 1.5Ev = GaAs, CdTe, LnP are ideal
Draw a CdTe PV cells structure and explain the processing
Glass, CdS,CdTe, Ohmic conductor, Metal conductor, Glass
CdS naturally n-type, CdTe naturally p-type = no doping needed
Can be electrically deposited (PVD as similar vaporisation temps) as a thin film = low energy (quickest payback)
Powder quickly condensates on substrate
