MAE 212 Final Summary

7/25/2019 MAE 212 Final Summary

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Liquid Junction Solar Cells

1. Charge Carriers in Electrode Materials

Metal (Pt) Semiconductors (n

Si)

Solid Electrolytes

(LaF3)

Insulators

(SiO2)

Electrons Electrons and holes Ions No chargecarriers

2. Electrodes in solution

For current to pass through the interface Metal/Solution, an electrochemical reaction

must occur, for example a reduction on the cathode or!ing Electrode "E#$ Fe%& &

e Fe2&. For a complete circuit, the cell also needs a Counter Electrode "CE# for the

re'erse anodic reaction on the anode.

Metals Semiconductors Solid Electrolytes Insulators

ithout applied (ias te

!otential dro! occurs

across te "elmolt#

(dou$le) layer

Most of the !otential

dro! is in te

semiconductor

instead of in the

solution.

No electrons

exchange occurs at

the surface, )ust ions

e%can&e*ith the

solid often *ith 'er+

high selecti'el+.

'o electron

e%can&e

and no ion

e%can&e.

he potential on the E is

determined (+ the redox

species *ith the largest

electron exchange current

densit+ i-,e "the rate of

electrons going (ac! and

forth (et*een redoxspecies and electrode in

euili(rium i.e. at ero

current#. e.g. 0t electrode

senses 2 in a ri'er

(ecause 2 has the higher

i-,e *ith 0t

ransport of charges to

and from solution is

limited to those redox

s+stems that ha'e

states that o'erlap

*ith the

semiconductor (ands

he reaction *ith

fastest ion exchange

current, i-,idetermines the

potential. In the case

of aF%, that is F3. In

the case of glass, thatis 4&. It is an ion3

selecti'e sensor

If it is an oxide

insulator it *ill

exhi(it p4

sensiti'it+ li!e

an oxide

semiconductor


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0otential drop

occurs in the

insulator

ften there are di5erent redox species in'ol'ed in esta(lishing the euili(rium

potential in *hich case *e spea! a(out a mi%ed !otentialero external current

(ut no net reaction "e.g. corrosion#

%. Semiconductors 6and 6ending

hen the semiconductor is in contact *ith the solution a (and (ending occurs as its

Fermi le'els euili(rates *ith a redox couple.

he initial di5erence (et*een the SC EFand the solution Fermi e'el "i.e., itselectrochemical potential#, determines the extent of (and (ending at the interface

SC/liuid )unction *hen the interface reaches euili(rium. his di5erence is also the

maximum theoreticall+ attaina(le photo3'oltage. he photo3'oltage can thus (e

manipulated (+ 'ar+ing the redox couple in the electrol+te.

A/A

V=EF

he region of the 6and 6ending is referred as the S!ace Car&e la+er, *hich

usuall+ has -.1 31 7m

8uring (and gap excitation, the space charge la+er assists in charge separation asthe electrons are dri'en into the (ul! semiconductor and holes to the electrol+te

interface "for p3t+pe semiconductor, this situation is re'ersed9#.

:nder open circuit conditions, electrons accumulate *ithin the conduction (and,

resulting in the ;attening of (ands.


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he ;at3(and potential "< F6# is that potential one needs to appl+ to ma!e the (ands

;at in the semiconductor all the *a+ to the surface "it can (e deduced from a

capacitance measurement of the interface#

For a semiconductor co'ered *ith an oxide "e.g. Si *ith Si2, i2# the ;at (and

potential is a function of p4 "ioniation of the surface 4 groups changes *ith p4#

and is often independent of redox s+stems. e more te !" is increased temore te conduction $and ed&e is si*ted to more ne&ati+e !otentials,

=. Semiconductors$ >uantiation concept

Max 0lan! discrete energ+ uanta ? !otons. he energ+ of each photon is

related to the *a'elength of the radiation$

E=h=hc

[ J],[eV]

@ freuenc+ "4 @ s1#

h=Plan k'sconstant:6.631034Js

c=Speed of light(3108 m /s)

@ *a'elength "m#

hese energies are 'er+ small and hence are usuall+ measured using a ne* energ+

unit called electron3'olts. ne e< is the energ+ acuired (+ an electron *hen

accelerated (+ a 1.- < potential di5erence. Energ+ acuired (+ the electron is qV.

Since qis 1.A 1-1B C, the energ+ acuired is 1.A 1-1B. *hich is deDned as 1 e


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Metals Semiconductors Insulators

Either$


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6ohr model he energ+ of electrons in atomic s+stems is restricted to a limited

set of 'alues.

In silicon$ 1- of the 1= Si3atom electrons occup+ 'er+ deep l+ing energ+ le'els

and are tightl+ (ound to the nucleus

he remaining = electrons, called 'alence electrons are not 'er+ strongl+(ound and occup+ = of the allo*ed slots. Je "Jermanium# and C ha'e the

exact same conDguration, except that its cores ha'e 2 and 12 electrons

respecti'el+.

Co'alent (onding

each atom shares its electrons *ith its nearest neigh(or.-t ./0In Si, no electrons area'aila(le for conduction in this

co'alent structure, so the

material is and should (e an

insulator.

E'er+ 'alence site is

occupied (+ an electron,

thus, it does not contri(ute to

current. No electrons allo*ed in (and

gap No electrons *ith enough

energ+ to populate the

conduction (and

-$o+e ./$ Enough thermal energ+ G!

"[email protected]


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A. 8oping

8onor$ creates extra electron cceptor$ creates a'aila(le hole0hosphor "0# atom$ H 'alence electrons

0 atoms @ free elect

6oron "0# atom$ % 'alence electrons

6 atoms @ holes


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Ioniation energ+ of donor$

Ei@ Ec3EdG =- me

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MAE 212 Final Summary