National Formosa University
NFU
ξPhotodetector
Contents
1. Photoconductor :
--Structure, Operation…
2. Photodiode
--Characteristics, Photodiode types…
Day-Shan Liu
Institute of Electro-Optical and Material Science
1
National Formosa University
NFU
ξPhotodetector
Achieved to a Photodetector
1. Photon incident and generate electron-hole pairs  Free carriers
2. Free carrier  Current and multiplication by current gain
3. Current  interacts with external circuit  Output signal
Day-Shan Liu
Institute of Electro-Optical and Material Science
2
NFU
National Formosa University
ξPhotodetector
& Photoconductor – Structure and Operation
• Structure : a photoconductor consists a piece of semiconductor with
“ohmic contacts” at both ends
• Operation : the incident light generates electron-hole pairs and results
in an enhancement on conductivity as following 
σ = q (μnn0 + μpp0)  σ = q (μn (n0 + ∆n) + μp (p0 + ∆p))
Ohmic contact
Ohmic contact
Day-Shan Liu
Institute of Electro-Optical and Material Science
3
National Formosa University
ξPhotodetector
NFU
& Photoconductor – Photocurrent gain and Response time
• Influenced on Current gain and response time :
-- Long minority-carrier lifetime or short electrode spacing  G↑
-- Short electrode spacing or high electric field  t respon. ↓
• carriers migration in photoconductor  t (electron) << t (hole) (b)
(a)
Maintain neutral
(d)
(c)
Day-Shan Liu
(e)
Institute of Electro-Optical and Material Science
4
National Formosa University
ξPhotodetector
NFU
& Photodiode – Characteristics
• Structure : a photodiode consists a p-n junction or metal-semiconductor
contact that operated under “reverse bias”
• Operation :
1. p-n junction or m-s contact induce depletion region 
2. optical signal impinges on photodiode and generates ehps 
3. ehps are separated by electric field induced from depletion region and
result in photocurrent 
4. the induced photocurrent are conducted by external circuit
• Key issues and trade-off consideration :
-- Transit time shortened  depletion region must be narrowed and
results in higher electric field  high frequency achieved
-- Quantum efficiency increased a wider depletion region is needed to
enhance the optical absorption
Day-Shan Liu
Institute of Electro-Optical and Material Science
5
NFU
National Formosa University
ξPhotodetector
& Photodiode – Characteristics
• Quantum efficiency ηint and ηext
-- Internal quantum efficiency ηint : the ratio that ehps (generation
rate G (number/s) absorb from incident optical power (Pabs (J/s))
ehps(G )

ηint =
Pabs / hν
-- External quantum efficiency ηext : the ratio that photocurrent (Ip
(C/s)) generated from incident optical power (Popt (J/s)) 
 I p   Popt 
1

η ext = ⋅
=   ⋅ 
q Popt / hν  q   hν 
Ip
−1
-- (1). ηext is influenced by absorption or loss in the structure or circuit
(2). ηint is influenced by absorption and thickness of depletion layer
(2). ηint >> ηext (for which usually simply signed asη)
Day-Shan Liu
Institute of Electro-Optical and Material Science
6
National Formosa University
ξPhotodetector
NFU
& Photodiode – Characteristics
•Quantum efficiency at long- and short-wavelength :
-- Long-wavelength cutoff λc is determined by energy bandgap of
semiconductor (ex. Ge ~ 1.8μm ; Si ~ 1.1μm ; GaAs ~ 870 nm…)
-- Short-wavelength cutoff λc is
related to the absorption
coefficient α Since αis
very large at short-wavelength
 the incident light was mostly
absorbed near surface and
degraded quantum efficiency
Day-Shan Liu
Institute of Electro-Optical and Material Science
7
National Formosa University
ξPhotodetector
NFU
& Photodiode – Characteristics
• Responsivity R : The photocurrent (Ip) generated from incident optical
power (Popt) at a specific wavelength
A
I
)= P
W
Popt
• Relationship between responsivity
and wavelength
IP
q
qλ
⇒R=
=η ⋅
=η ⋅
Popt
hν
hc
⇒ R(
Here, R is called “spectral responsivity”
or “radiant sensitivity”
Day-Shan Liu
Institute of Electro-Optical and Material Science
8
National Formosa University
Day-Shan Liu
NFU
Institute of Electro-Optical and Material Science
9
National Formosa University
ξPhotodetector
NFU
& Photodiode – Photodiode types
• p-i (intrinsic)-n Photodiode :
(1). Structure : p-i-n with ohmic contact on each side  band diagram
(2). Bias : the p-i-n photodiode is operated under reverse bias
(3). Why p-i-n structure ?  the depletion region (almost at “intrinsic
layer” at all  the depletion region is tunable by varying the
thickness and concentration of intrinsic layer
Day-Shan Liu
Institute of Electro-Optical and Material Science
10
National Formosa University
ξPhotodetector
NFU
& Photodiode – Photodiode types
(4). Operation : Light absorption
 ehps generation  “Diffusion”
into depletion region  “Drift”
in depletion layer  “Drift” by
external reverse bias
(5). Designation :
-- AR coating + depletion region
close to surface  enhance
quantum efficiency
-- Control the thickness of depletion
layer  shorter response time
Day-Shan Liu
Institute of Electro-Optical and Material Science
11
NFU
National Formosa University
ξPhotodetector
& Photodiode – Characteristics
• Response speed -- response speed is limited by :
(1). Diffusion of carriers  carriers diffusion into depletion layer 
depletion close to surface  reduce response time
(2). Drift time in depletion layer  carriers migrate in the electric field
result from the depletion region  reduce depletion layer 
reduce response time and decrease quantum efficiency
(3). Capacitance originated from depletion layer  the capacitance
results in a large “RC time constant” for depletion layer (d) is too
thin (C ∞ A / d)
Day-Shan Liu
Institute of Electro-Optical and Material Science
12
National Formosa University
ξPhotodetector
NFU
& Photodiode – Photodiode types
• Metal-Semiconductor Photodiode :
(1). Structure : metal-semiconductormetal with Schottky contact
(2). Bias : one side at “reverse”
and the other at “forward” bias
(3). Why m-s structure ?  a
simplest structure and useful
for ultra-violet and visible
wavelength (α is very high
for semiconductor and ohmic contact is hardly achieved)
Day-Shan Liu
Institute of Electro-Optical and Material Science
13
NFU
National Formosa University
ξPhotodetector
& Photodiode – Photodiode types
Structure
Under bias at reach-through voltage
VRT (the region between two
electrode is completely depleted)
Day-Shan Liu
Band diagram (at thermal equilibrium)
Under bias at flat-band voltage VFB (the conduction
band at forward bias side is flattened due to the
depletion region resulted from the reverse side had
reached the forward-side electrode) device operated
at this bias till V is too large to breakdown
Institute of Electro-Optical and Material Science
14
National Formosa University
ξPhotodetector
NFU
& Photodiode – Photodiode types
• Heterojunction Photodiode :
(1). Structure : a large bandgap semiconductor
epitaxially on a small bandgap semiconductor
(2). Why heterojunction structure ?
(a). Optical window  quantum efficiency does
not depended critically on the distance of
the junction from the surface
(b). Schottky barrier enhanced  avoid leakage
current with and without illuminating
(c). Materials select-able to optimized the
quantum efficiency and response speed
Day-Shan Liu
Institute of Electro-Optical and Material Science
15
National Formosa University
ξPhotodetector
NFU
& Photodiode – Photodiode types
• Avalanche Photodiode :
(1).Structure : n+-p-π(light p-doped)-p+ with ohmic contact
(2).Operation : under “reverse bias” to enable “avalanche multiplication”
(3).Why avalanche photodiode ?
-- Avalanche multiplication results in internal current gain
-- Since a avalanche photodiode has the advantage of “current gain”, a
external current multiplication is absent  the response time is
shortened  achieved for detecting the light modulated at higher
frequency such as microwave frequency
(4).Weakness : Avalanche is random and experience different
multiplication, an APD is need to minimize avalanche noise
Day-Shan Liu
Institute of Electro-Optical and Material Science
16
National Formosa University
ξPhotodetector
NFU
& Photodiode – Photodiode types
(5).Schematic for APD :
-- Avalanche region : p-semi.
 at high reverse bias  a high
electric-field exist in n+-p
depletion region
-- absorption and generation region
: πlayer  ehps generated and
diffused into p-semi. region and
therefore impact-ionized in
depletion region
Day-Shan Liu
Institute of Electro-Optical and Material Science
17
National Formosa University
NFU
ξSolar Cell
Contents
1. Solar Radiation :
--Definition…
2. p-n junction Solar Cell
--Structure, Operation…
3. Conversion Efficiency :
--Parameters, Characteristics…
4. Advanced Solar Cells
--Silicon, Compound-Semiconductor Solar cell
Day-Shan Liu
Institute of Electro-Optical and Material Science
18
National Formosa University
Day-Shan Liu
NFU
Institute of Electro-Optical and Material Science
19
National Formosa University
NFU
產業定義
太陽光電是個技術多元的產業，且各技術間關聯性不高，因此範圍廣泛。廣義而言，只
要是利用太陽光激發電子流動而產生發電機制之裝置，皆稱為太陽光電產品。
依型態來分，太陽光電可分為平板型與聚光型兩大類。而目前典型之產品集中在平板型，
其中包含矽晶、矽薄膜、化合物薄膜（含CdTe與CIGS技術）、及較新的有機型產品（染料
敏化電池與有機薄膜電池）；在平板型的產品中又以矽晶型產品產業規模較大，產業鏈結
構較為完整，由上游至下游包含多晶矽、晶碇/晶圓、電池、模組、系統五大部份
矽晶太陽能電池為利用高純度矽材料經長晶、切晶並生成P/N極表面之零組件，為太陽
光電模組板之重要原料，占太陽光電模組成本六成以上，其製程特性與半導體產品類似
太陽光電產業範疇
Day-Shan Liu
Institute of Electro-Optical and Material Science
20
National Formosa University
Day-Shan Liu
NFU
Institute of Electro-Optical and Material Science
21
National Formosa University
Day-Shan Liu
NFU
Institute of Electro-Optical and Material Science
22
NFU
National Formosa University
ξSolar Cell
Why Solar Cell (also called Photovoltaic Device) ?
Sunlight  Electricity
• Good conversion efficiency  multi-layer absorbed sunlight
• Sunlight is permanent power and low cost
• Virtually nonpolluting
Day-Shan Liu
Institute of Electro-Optical and Material Science
23
National Formosa University
NFU
科學發展電子報
Day-Shan Liu
Institute of Electro-Optical and Material Science
24
National Formosa University
Day-Shan Liu
NFU
Institute of Electro-Optical and Material Science
25
National Formosa University
Day-Shan Liu
NFU
Institute of Electro-Optical and Material Science
26
National Formosa University
Day-Shan Liu
NFU
Institute of Electro-Optical and Material Science
27
National Formosa University
ξSolar Cell
NFU
& Solar Radiation
• Definition
-- Air mass = h / h0 = 1 / cosφ; φ is the angle between the vertical and
the sun’s position  AM0 > AM1  Enhance absorption efficiency
-- Solar constant : the intensity of solar radiation outside the earth’s
atmosphere perpendicular to the solar direction with air mass = “0”
Day-Shan Liu
Institute of Electro-Optical and Material Science
28
National Formosa University
NFU
ξSolar Cell
& p-n Junction Solar Cell – Structure
• p-n Junction Solar Cell :
(1). Structure :
--Semiconductor : p-n junction
--Electrodes :
n-contact  a ohmic contact with interfigure electrodes
p-contact  a ohmic contact covers
entire back surface
--why inter-figure electrodes ?
 increases expose area and reduces
capacitance
Day-Shan Liu
Institute of Electro-Optical and Material Science
29
National Formosa University
NFU
ξSolar Cell
& p-n Junction Solar Cell – Operation
(2). Operation :
--Sunlight  generated ehps
--A built-in voltage induces from p-n
 Electrons migrate to n-side
Holes migrate to p-side
--Excess electrons and holes accumulate
at ohmic contact 
Open  a photovoltage of Voc induced
Short  a photocurrent of Iph induced
--Key issue  Built-in voltage is needed
Day-Shan Liu
Institute of Electro-Optical and Material Science
30
NFU
National Formosa University
ξSolar Cell
& p-n Junction Solar Cell – Characteristics
• Current characteristic :
n
p
(a). A idealized equivalent circuit of a solar cell
(b). Under short circuit (no Voc induce):
--a short circuit current Isc is formed from
p
n
p
n
sunlight exposure and results in the
photocurrent Iph  Isc = - Iph
(c). With a load of R (with Voc present):
--a photocurrent (Iph) is conducted from
sunlight exposure
Day-Shan Liu
Institute of Electro-Optical and Material Science
31
National Formosa University
NFU
ξSolar Cell
& p-n Junction Solar Cell – Characteristics
--The built-in voltage is reduces by the open circuit voltage Voc  this
solar cell is similar to a diode  increases minority “diffusion” and
results in a “diode saturation current” Id, where Id is expressed as :
  qV  
 − 1
I d = I s  exp

  nk BT  
  qV  
 − 1
∴ I = I sc + I d = − I ph + I s  exp

  nk BT  
(a). When V = 0  I = - Iph ; V > 0  I = Isc + Id > Isc > or < 0
(b). When V > 0 and I < 0  operated at fourth quadrant and Q =
I · V < 0  Power is extracted from the solar cell device
Day-Shan Liu
Institute of Electro-Optical and Material Science
32
NFU
National Formosa University
ξSolar Cell
& p-n Junction Solar Cell – Current-Voltage Curve
• Since the current I of a solar cell is I = I sc + I d = − I ph

 qV

+ I s  exp
 nk BT

 
 − 1

 
 I-V characteristics located at fourth quadrant :
-- Intercept at “y axis” is Iph and proportion to the sunlight intensity
-- Intercept at “x axis” is Voc  at what voltage that the built-in voltage
is compensated
-- Exponential term for I-V curve
-- At V = 0  I = - Iph = Isc
-- At I = 0  V = Voc


 qV  
 qV  
 − 1 ⇒ I ph = I s  exp
 − 1
I = 0 = − I ph + I s  exp



 nk BT  
 nk BT  


I

I 
 qV  I ph
qV
nk T
 =
+1 ⇒
= l n ph + 1 , and I ph >> I s ∴V ( I = 0) ≡ Voc ≅ B ⋅ l n ph 
⇒ exp
q
nk BT
 nk BT  I s
 Is

 Is 
Institute of Electro-Optical and Material Science
Day-Shan Liu
33
National Formosa University
NFU
ξSolar Cell
& p-n Junction Solar Cell – Output Power
• The solar cell support power to a load with Q = I · V from currentvoltage relationship  a maximum power is limited
 Pm = Im × Vm
Day-Shan Liu
Institute of Electro-Optical and Material Science
34
National Formosa University
ξSolar Cell
NFU
& p-n Junction Solar Cell – Output Power
Maximum output power : Pm = I m ⋅ Vm
qV

  qV nkT

since P = I ⋅ V =  I s  e
− 1 − I ph  ⋅ V = I sVe nkT − I sV − I phV


 
qVm
dP
q

 qVm nkT
nkT
I
V
e
I
I
I
e
while at max. ⇒
0
=
⇔
+
−
−

s
s m
s
ph  V =Vm = 0 − − ( A)
V =Vm
dV
nkT


qVm
 I s + I ph 
q
q
q
 I s + I ph

nkT 
 − l n1 +
Vm  =
Vm = l n
Vm 
⇔
⇒e
1 +
Is
nkT


 nkT
 nkT
 Is 

  I s + I ph 
q
nkT
q





 l n

n
V
V
n
V
l
1
l
1
≅
−
⋅
+
−
+


m 
OC
m

  I
nkT
q
nkT




s

 
qVm
qVm
q
 qVm nkT

nkT
Im = Is  e
I sVm e nkT
− 1 − I ph V =Vm , with (A) ⇒ I s + I ph = I s e
+
nkT


qVm
qVm

qVm
qV
nkT
q


m
I s e nkT ⇔ Pm = I m ⋅ Vm ≅
I s e nkT ⋅ VOC −
Vm  
⇒ Im =
⋅ l n1 +
nkT
nkT
q

 nkT

nkT
∴Vm =
q
Day-Shan Liu
Institute of Electro-Optical and Material Science
35
NFU
National Formosa University
ξSolar Cell
& Conversion Efficiency – Parameters
•An important parameter of solar cell is “fill factor FF”  the ratio of
the max power rectangle to the rectangle of Iph × Voc :
FF ≡
I mVm
I phVoc
 FF ↑  solar cell efficiency↑)
•The ideal “power conversion efficiency”
of a solar cell η the ratio of the max.
power rectangle to the incident sunlight
power : η = I mVm = FF ⋅ I phVoc
Pin
Pin
Day-Shan Liu
Institute of Electro-Optical and Material Science
36
NFU
National Formosa University
ξSolar Cell
& Conversion Efficiency – Enhance Efficiency
•Spectrum Splitting :
--Why spectrum splitting ?  achieved high conversion efficiency of
sunlight at various wavelength
--What spectrum splitting ?  the sunlight is split into difference
wavelength and absorbed by solar cells
--How spectrum splitting ?
(1).Outside-splitting  sunlight is split by suitable “absorbed mirror”
before sunlight emitting into solar cells
 Each solar cell is independent
Day-Shan Liu
Institute of Electro-Optical and Material Science
37
National Formosa University
NFU
ξSolar Cell
& Conversion Efficiency – Enhance Efficiency
(2).Inside-splitting  sunlight is split and absorbed simultaneously by
solar cells with semiconductor possesses
different bandgap
 Solar cell is monolithic-able
Day-Shan Liu
Institute of Electro-Optical and Material Science
38
National Formosa University
NFU
ξSolar Cell
& Conversion Efficiency – Degrade Efficiency
•Series Resistance and Recombination Current
--Why and where these phenomena appear ?
(a). From the ohmic contact resistance
 series resistance
(b). Carriers recombine in depletion
region  recombination current
--How these degradations affect on
solar cell ?  “Series resistance”
and “recombination current”
degrade the ideal efficiency
Day-Shan Liu
Institute of Electro-Optical and Material Science
39
National Formosa University
NFU
ξSolar Cell
& Advanced Solar Cells – Materials
•Recently, the materials of solar cells are mostly employed Si base,
III-V compound or II-VI compound semiconductor :
Total efficiency for a Si-base solar cell
-- Si material  nontoxic, almost
inexhaustible in earth (ex.
SiO2), mature fabrication technology
 suitable for “large scale”
--III-V or II-VI compound  wide
choices of bandgaps with closely
lattice constant, monolithic-able
 enhance “conversion efficiency”
Day-Shan Liu
Institute of Electro-Optical and Material Science
40
National Formosa University
NFU
ξSolar Cell
& Advanced Solar Cells – Tandem Solar Cell
•Tandem Solar Cell :
 monolithic structure with semiconductor materials of various
bandgaps  reduce the related
series resistance and recombination
current and enhance the employ
of sunlight  higher conversion
efficiency  η~ 30 %
weakness  hard to epitaxial
Day-Shan Liu
Institute of Electro-Optical and Material Science
41
National Formosa University
NFU
ξSolar Cell
& Advanced Solar Cells – Optical Concentration
•Methods to achieved high conversion efficiency :
--Inside the solar cell structures  PERL cells,
Tandem cells..  cost high
--Outside the solar cell structure  low cost
(1). Optical concentrated achieved by optical lens
(2). Design an AR coating
Day-Shan Liu
Institute of Electro-Optical and Material Science
42
National Formosa University
NFU
ξSolar Cell
& Advanced Solar Cells – PERL Cell
•PERL Cell (passivated emitted rear locally-diffused cell):
 Special design of “inverted pyramids” on the top by anisotropic
etch process  reduce reflection of sunlight incident ηbest = 24 %
Day-Shan Liu
Institute of Electro-Optical and Material Science
43
National Formosa University
NFU
ξSolar Cell
& Advanced Solar Cells – Amorphous Si Solar Cell
•Amorphous Si Solar Cell :
 Easy and low cost fabrication  η~ 5 %
Patterned by laser
Day-Shan Liu
Institute of Electro-Optical and Material Science
44
National Formosa University
NFU
Dye sensitized solar cell (DSSC)
科學發展電子報
Day-Shan Liu
Institute of Electro-Optical and Material Science
45
National Formosa University
NFU
第140期國科會工程科技E-paper:共鍍法製備複合薄膜
電極應用於有機太陽能電池之研製 - 魏慶華
Day-Shan Liu
Institute of Electro-Optical and Material Science
46