Solar PV Stand Alone Design Project

1

Solar Energy: Overview of Photvoltaics

Renewable Energy Technology and

Systems

1

Solar Energy: Outline

• Section 2: Solar Energy

o 2.1 Solar resource

• Solar radiation

• Solar radiation on the earth’s surface

• Effects of green house gasses on earth’s temperature

o 2.2. Solar thermal energy

• Solar thermal heat transfer

• Low temperature solar power

• Concentrating solar power (CSP)

o 2.3 Photovoltaics

• PV basics, different technologies

• PV systems and applications

2

Overview of Photovoltaics

• Direct conversion of sunlight into electricity via the

photovoltaic effect

• Photovoltaic effect first discovered by Bequerel

(1839); Se/Au solar cell (C. Fritts, 1883)

• Modern junction solar cell (R. Ohl, 1946)

• Silicon junction formation allowed formation of first

practical devices, at Bell Labs (1954)

3

4

Why and Where to use Photovoltaics

• Features of Photovoltaics:

o High efficiency

o Distributed energy source

o Low energy payback time

o Clean energy source

o Low water usage

o Modular

• Markets

o Remote area power

o Grid-connected: residential

and utility

o Niche markets

5

Solar Electricity Opportunity

Solar energy is a unique source of

energy:

o Large resource & renewable

o Distributed generation

30% US Electricity from PV

6

2-Level System and Optical Absorption

Most optical

absorption processes

involve excitation of

an electron from a

filled state, across an

energy gap to an

unoccupied state

LUMO; Conduction Band

HOMO; Valence Band

hE ph 

Photon

7

Solar Energy Conversion Efficiencies

• Losses primarily arise from large range of photon

energies in incident spectrum and ability to only

utilize energy = band gap.

• In a solar cell, detailed balance calculations quantify

these losses, giving single junction efficiency =

33.7% under one sun AM1.5 spectrum (Shockley-

Queisser), versus 86.5% thermodynamic limit

8

Photovoltaic Operation

I

V

I sc

V oc

C

BA

I D

  

   

  1ln

o

sc

oc I

I

q

nkT V

I D

= I 0 e qVD/nkT -1( )

in

scoc

P

FFIV 

C E

V E

9

Photovoltaic Energy Conversion

‘Fill Factor’ (FF) strongly

affected by parasitic series and

shunt resistances.

in

scoc

P

FFIV 

 1/ 0

 nkTqV

D

DeII

ID

Energy conversion

efficiency

  

   

  1ln

o

sc

oc I

I

q

nkT V

Solar Cell Technologies Established technologies

• First Generation: Silicon

(single /polycrystalline)

III-V solar cells

o GaAs/AlGaAs

o GaAs/InGaAsP

o InP

• Second Generation: Thin

Film

o CuInSe2 (CIS)

o CuInGaSe2 (CIGS)

o CdTe

o Amorphous Si (a-Si)

o Organic

• Third Generation

– Multi-junction

– Nanotechnology advanced concept

– Organic (advanced concept)

– Dye sensitized solar cells 10

Solar Cell Efficiencies

11

Summary: Overview of Photovoltaics

• Main CSP designs based on parabolic trough, linear

Fresnel, power tower and parabolic concentrators

• Efficiency of CSP receiver general nonlinear with

operating temperature due to radiant heat loss

• Linear Fresnel has similar efficiency to parabolic trough

but better space utilization

• Power towers are generally utility scale and use large

heliostat fields focused on a central receiver

• Efficiency of the receiver degrades with temperature due

to radiative losses

• Parabolic dish coupled with a Stirling heat engine can

directly produce electricity at very high efficiency

12

1

Solar Energy: Photovoltaic Operation

Renewable Energy Technology and

Systems

1

Photovoltaic Operation

• Basic operation:

o Generation of light generated current

• Absorption of light to raise energy of a carrier

• Collection of carriers to prevent loss of carriers.

o Generation of photovoltage

from light generated carriers

o Minimization of parasitic losses

o Match of solar cell properties

to load characteristics

o Low temperature operation

o Minimization of interconnect

losses

Light Generated Current

• Light generated current

o Collection of carrier by necessary to keep light generate carriers

from relaxing (recombining) to their original lower energy

o Collection depends on diffusion length, 𝑳𝑫, i.e. how far carriers travel before recombining on average

o Carrier loose some energy in crossing junction, and so

maximum voltage from solar cell < band gap p-n junction

p-side n-side

Light Generated Current

• Light generated current depends on:

o Wavelength of incident light, i.e. the photon energy must be

greater than the semiconductor bandgap, 𝒉𝝂 > 𝑬𝑮

o Reflection as a function of wavelength and shading

o Absorption coefficient, 𝜶 𝝀 , of material compared to thickness

N ph x( ) = Nse

-ax

Solar Cell Structure

• The basic solar cell is a p-n diode that is illuminated

• It consists of a thin ‘emitter’ and a thick ‘base’, with a back contact

(with back surface field), and a top grid contact which minimizes

shading, and an antireflection coating

B lu

e

~ 1

 m

G re

e n

~ 2

 m

R e

d

~ 1

0 

m

Diffusion

length

n+

p

p+

Anti reflection (SiN)

Contact: Contact resistance

Metal resistance

Light blocking

p+/p:

Reduce surface

recombination

N D

high:

Low resistance

Low  Auger

Reduced E G

Si

Top View

busbars

Equivalent Circuit Model

• The p-n junction without illumination is a diode with ‘dark’ current

𝑰𝑫 = 𝑰𝟎 𝒆 𝒒𝑽/𝒏𝒌𝑻 − 𝟏

where 𝑰𝟎 is the reverse saturation current (typically very small) which decreases exponentially with the bandgap (𝒏=ideality factor)

• The equivalent circuit for a solar cell can through superposition be

represented by a current source in parallel with the diode (𝑰𝒑𝒉= 𝑰𝒔𝒄)

I

V

I sc

V oc

C

BA

I D

sc I

nkT

qV II 



  

  

  

  1exp

0

𝑅𝐿 = 0;𝑠ℎ𝑜𝑟𝑡 𝑐𝑖𝑟𝑐𝑢𝑖𝑡 𝑅𝐿 = ∞;𝑜𝑝𝑒𝑛 𝑐𝑖𝑟𝑐𝑢𝑖𝑡

Output Power and FF

• At maximum voltage, the current is zero (hence zero power), while

at maximum current the voltage is zero (and hence zero power)

• Obtain the maximum power from the product of the voltage and the

current

• Current given by diode

equation:

• Power given by:

• No closed form solution

𝐼 = 𝐼𝑠𝑐 − 𝐼0 exp 𝑞𝑉

𝑛𝑘𝑇 − 1

 

  

  

  

  

  

 

sc I

nk T

qV IVIVP 1exp

0

Open Circuit Voltage

• Open circuit voltage

o Photovoltage arises the photocurrent flowing through a non-

zero load, which create a voltage across the device

o This voltage forward biases the device, causing a current to

flow in the opposite direction of the light generated current

o We want to minimize the ‘dark’ current that flows in forward

bias, characterized by the diode parameter, I0

o Setting the current equal to zero in the circuit model gives

V oc

= nkT

q ln I sc

I 0

+ 1 æ

è ç

ö

ø ÷

o Since I0 decreases exponentially with bandgap, 𝑽𝒐𝒄 increases

linearly with bandgap. Empirically, 𝑽𝒐𝒄 = 𝑬𝒈

𝒒 − 𝟎.𝟒 𝑽

Fill Factor and Efficiency

• The maximum power points are related to the maximum voltage

and current by defining a parameter called fill factor

• The fill factor (FF) is the maximum rectangle that can fit under the

solar cell IV curve

• FF is a measured

or numerically

calculated parameter.

• Efficiency for solar cell:

h = V oc I sc FF

P in

Parasitic Losses

• Solar cells are high current devices, and hence suffer from

significant resistive losses, particularly in top metal contacts.

• Resistances reduce the FF and hence the solar cell efficiency.

Resistive and reflective losses are coupled, and the degree

to which these can be minimized depend on fabrication technology.

Model of Solar Cell

• For analysis, solar cell is

modeled as a lumped

parameter model.

o Light source

represents light

generated current.

o Resistances

correspond to

parasitic resistances.

o Voltage determined

from diode equation.

Solar Cell Efficiency

• Two key determinants of solar cell efficiency:

o Theoretical efficiency determined by material band gap: High

band gap has high voltage and low current

o Practical efficiency depends on technology and cost of

fabrication methods, with high cost processes giving higher

efficiency

0

10

20

30

1940 1950 1960 1970 1980 1990 2000

Efficiency of silicon solar cells (%)

Typical commercial solar cell efficiencies

Characteristic Resistance

• The power produced by the solar cell, and hence its efficiency is

determined by the operating point of the solar cell

• In order to operate at maximum efficiency, the load must be

matched to the characteristic resistance of the solar cell

• Characteristic load

depends on light

intensity

• Typically accomplished

in systems by using

electronics between

system and load

Effect of Temperature

• The performance of solar cells is affected by temperature, and solar cell

voltage will decrease as the temperature increases

• The temperature decrease depends on the open circuit voltage and the

band gap of the material used to make the solar cell

• Higher voltage solar cells

are less sensitive to

higher temperatures

• Thin-film solar cells

can also be relatively

temperature insensitive

due to improvement of

material parameters

• Silicon solar cells loose

2.2 mV/ °C

Summary: Solar Cell Operation

• Solar cells operate a p-n diodes under illumination

• The photocurrent depends on photons with energy

above 𝑬𝑮, reflection, absorption, and collection

• Equivalent circuit model used to describe solar cell as

power source

• The maximum power is defined as the product of the

open circuit voltage, short circuit current, and fill factor

• The open circuit voltage increase with material bandgap,

while photocurrent decreases

• In circuit applications, the characteristic resistance gives

maximum power

• Most solar cell performance degrades with temperature

15

1

Solar Energy: Photovoltaic Technologies

Renewable Energy Technology and

Systems

1

Solar Cell/Photovoltaic Technologies Established technologies

• First Generation: Silicon

(single /polycrystalline)

III-V solar cells

o GaAs/AlGaAs

o GaAs/InGaAsP

o InP

• Second Generation: Thin

Film

o CuInSe2 (CIS)

o CuInGaSe2 (CIGS)

o CdTe

o Amorphous Si (a-Si)

o Organic

• Third Generation

– Multi-junction

– Nanotechnology advanced concept

– Organic (advanced concept)

– Dye sensitized solar cells 2

Si Technology

• Dominant technology is silicon, due to its abundance, ease of

processing and use by the Integrated Circuit (IC) industry.

• Two dominant silicon materials used: single crystalline (higher

cost, higher efficiency) and multicrystalline (lower efficiency, lower

cost).

3

Single Crystal Silicon

Czochralski growth is the standard

process used to grow high quality

single crystal Si.

A single crystal ‘seed’ is lowered

into a molten bath of Si (~1500 C)

and slowly drawn up (few cms per

hour) as it is rotated. Precise

control the temperature gradients,

pulling rate and speed of rotation

allow the growth of large, single-

crystal, cylindrical ingots (boule)

from the melt.

n and p doping can be added to melt

to provide background doping

4

Single crystal solar cells can be fabricated on wafers cut

directly from the ingot (left). Semi-square cells can be

manufactured by cutting off the edges of round wafers, to

increase module packing density (right). Typical wafer

thicknesses in 2014 around 175 microns.

5

Single Crystal Silicon

Polycrystalline Silicon

• Grown from a melt of molten Si,

where multicrystalline Si forms

• Process is much cheaper than CZ

grown

• Material is lower quality (i.e. lower

lifetime) due to grain boundaries

Source: Kyocera Co. Japan

6

Si Technology

• Production technology determines how close to theoretical

efficiency a material can reach

• Fabrication technology developed for other electronic devices

(integrated circuits (ICs), optical devices) gives high efficiency, but

also high cost due to high area-based costs

7

Si Technology

• IC processing techniques typically too expensive due to batch

nature, high capital costs and time consuming nature.

• Commercial solar cells use reduced cost technologies, particularly

regarding how to get thin, narrow metal lines.

• Dominant technology is screen printing on belt furnaces.

p-type

n ++

p +

metal

metal

8

9

Interdigitated back contact (IBC) PERL/PERC Cell (UNSW)

• Heterojunction with intrinsic thin

layer (HIT) solar cell has record

Si cell efficiency > 26.7%

• Uses thin amorphous Si (a-Si)

with bandgap of 1.7 eV, forming

Si heterojunction (SHJ) cell

• Record open circuit voltage of

0.75 V

High Efficiency Si Technology

Si Photovoltaics

ITRPV 2019

 Currently 95% of the world market

 Recent advances involve c-Si/a-Si heterostructures 𝜂 > 26.5%  Approaching its theoretical efficiency limit including Auger

 Si based multijunction cells required to exceed 𝜂 = 30%

• Thin films photovoltaics describe a class of solar cells in which a

thin layer(s) of semiconductor material is deposited on to a low

cost inactive substrate

Thin Film Solar Cells

11

• Material can be silicon, amorphous

silicon, or combinations primarily

from groups II & VI of the

periodic table, including CdTe and

CIGS (copper indium gallium

diselenide), CdS (cadmium sulfide)

• Typically heterojunctions with wider

bandgap window layer

• Thin film cells offer potential for low

cost, but due to non-uniform material,

larger devices suffer from low

efficiency

Thin Film Solar Cells

• In large scale production, cost of the materials

dominates the overall solar cells cost

• Goal of thin film approaches is to reduce the

materials and processing costs while retaining

acceptable efficiency

• CdTe is dominant technology presently holding 5-

7% of world PV market (Si is the other 95%)

12

CdTe Modules

From First Solar,

Blythe Solar Project

Flexible thin film

Perovskite Solar Cells

• Hybrid organic metal halide

perovskites: ABX3- A=CH3NH3, B=Pb,

X=I or Cl

• Bandgap of 1.55 eV

• Low cost materials, thin film processing

• Lifetime and reliability are main barriers

to commercialization

13

PV Market

• In 2020, mono-Si passed multi-Si, total market 95% for Si

14

Summary: Photovoltaic Technology

• Solar cell (photovoltaic, PV) technology roughly grouped

historically as 1st(Si), 2nd(Thin film), and 3rd generation

• Si technology is based on either single crystal or

polycrystalline wafers

• Si solar cell manufacture based on low cost variants of

methods used in IC manufacture

• Thin film solar is based on thin layers (a few microns

thick) on glass of high absorption semiconductors

• Trade off between reduced manufacture cost and poorer

efficiency due to higher defect levels than Si

• Perovskite solar cells are the most recent thin film

technology with rapid increase in efficiency with time,

but degradation issues

15

1

Solar Energy: Third Generation PV Technologies

Renewable Energy Technology and

Systems

1

Solar Cell/Photovoltaic Technologies Established technologies

• First Generation: Silicon

(single /polycrystalline)

III-V solar cells

o GaAs/AlGaAs

o GaAs/InGaAsP

o InP

• Second Generation: Thin

Film

o CuInSe2 (CIS)

o CuInGaSe2 (CIGS)

o CdTe

o Amorphous Si (a-Si)

o Organic

• Third Generation

– Multi-junction

– Nanotechnology advanced concept

– Organic (advanced concept)

– Dye sensitized solar cells 2

*W. Shockley and H. Queisser, JAP32, 510 (1961)

Shockley-Queisser Limit*

Single Bandgap Cell Efficiencies

• Limits of solar cell

efficiency assume a

single bandgap, and

1 photon = 1

electron+hole

• Using multiple

bandgaps in a single

cell can overcome

this limit

• Producing more than

1 electron/hole per

photon another

• Not losing the

energy to heat

(hybrid PV/thermal)

Multijunction/Tandem Solar Cells

• Multijunction (tandem) solar cells overcome

the single gap efficiency limit using multiple

bandgaps

• Each solar cell absorbs light with energy

above its band gap

• In series connected structure, the voltage is

the sum of the voltages of each cell

• Maximum thermodynamic efficiency is 86.8%,

but material limitations give maximum

efficiencies of just over 46% with

concentration with 3-4 junctions

• Growth of high quality material is very

expensive

• Therefor used primarily in space markets

4

Concentrating PV (CPV) Systems

• Concentration improves the performance of

PV, and allows the use less cell area

• CPV systems are potential candidate for low

cost terrestrial power, but require two axis

tracking and high optical efficiency (similar

performance and cost to Stirling CSP)

• Theoretical efficiency with concentration can

approach thermodynamic limit as number of

subcells becomes large; difficult in practice

5

Two Junction Si Tandems

 Calculation of the two-junction series connected efficiency shown

 Si (1.1 eV) + 1.7 eV top cell can exceed 45% efficiency (33% is 1J max)

 Si tandem cells expected to be future technology advance

• New physics concepts to take PV efficiencies closer

to thermodynamic limits

• Nanotechnology solutions

Third Generation (3G) Photovoltaics

7

3G Solar Cell Technologies

• Emerging technologies include both approaches for ultra-high efficiency

and very low cost

• Organic solar cells: Offer potential for lower cost, flexible substrates, but

presently have low efficiencies and stability issues

• Dye-sensitized solar cells: use an

dye to absorb light, embedded in

a titanium dioxide base. Can

theoretically achieve good control

over absorption and low cost.

• Ultra-high efficiency approaches:

based on advanced microelectronic

concepts, such as quantum wells

and quantum dots

8

Solar Cell Efficiencies

9

Summary: Third Generation PV Technology

• Maximum efficiency of a solar cell limited by photons

lost below bandgap and thermal loss above to 33.7%

• The analysis assumes a single bandgap, 1 photon= 1

electron/hole, all energy lost to heat above bandgap

• Third generation technology focused on high efficiency

above S-Q limit

• Multi-junction or tandem cells exceed limit using multiple

bandgap cells

• High cost limits commercial use to space applications

while concentrating PV used for terrestrial applications

• Other 3G technologies use advanced concepts to

circumvent S-Q assumptions usual using

nanotechnology based approaches

10

1

Solar Energy: Photovoltaic Modules

Renewable Energy Technology and

Systems

1

Solar Cell Modules • Individual solar cells are encapsulated into a module in order to

increase power, increase voltage, protect solar cells from

environment, and protect people

• Solar cells are nearly always connected in series in the modules,

and they historically consist of ~36 solar cells and ~100 W

• Current trend towards large modules, 72 cells, P>300 W

Module Materials

• Front Surface Materials (acrylic, glass)

o High transmission in the needed wavelengths

o Low reflection.

o Impervious to water

o Good impact resistance

o Stable under prolonged UV exposure

o Low thermal resistivity

o Mechanically rigid

• Encapsulant (EVA (ethyl vinyl acetate))

o Adheres cells to top and rear surface

o Stable at elevated temperatures and under high UV exposure.

o Optically transparent

o Low thermal resistance

• Rear Surface (Tedlar)

o Low thermal resistance

o Prevent the ingress of water or water vapor

• Frame (aluminum)

o Structural component

o Should not trap water or dirt

Cell Characteristics: 225 cm2

Vm≈ 0.6 volts @25 C, AM1.5

Im ≈ 30 to 35 mA/cm 2

Power ≈ 4 watts

Combine cells into modules:

Series for voltage

Parallel for current

Single modules typically designed with series

connected cells to drive a 12 volt battery load (100’s

of watts)

Arrays of modules (parallel or series connected) used

to deliver power up to 100s of MWs

Solar Cell Modules

• A conventional module consists of 36 series connected

cells for battery charging (15-16V required):

• V≈ 36x0.6 = 21 volts max, and 17-18V at max power

and operating temperature

• I ≈ 30 to 36 mA/cm2 x 225 cm2 = 6.7-7.8A

• Power ≈ 150 watts

Solar Cell Modules

• Cells in general (and modules) can be either series or

parallel connected

• In the ideal case, assume identical cells with I-V

characteristic given by

 1/  nkTqV oL

eIII

• Series connected cells have the same current, total voltage

is sum of voltages

• Parallel connected cells have the same voltage, total

current is sum of the individual cell currents

• For M parallel connected cells and N series connected

 1//  nkTNqV oLT

TeMIMII

Solar Cell Connections

ITotal is the total circuit current, and VTotal is the total

voltage

Solar Cell Connections

Interconnection

• The interconnection of solar cells can cause significant power losses if the

operating points of all modules are not identical

• Mismatch occurs a solar cells is shaded

• Since solar cells are

series connected,

reduction of current in

one solar cell limits

the current in the

series connected

string

• Instead of all solar cell

being at short circuit,

a voltage is generated

Mismatch losses

• The voltage across the unshaded solar cells is dropped across the

shaded solar cell.

• The power dissipation in the shaded solar cell creates local

heating, which can permanently damage to module or solar cell.

Bypass Diodes

• Mismatch losses can be minimized by inserting a by-pass diode

across several of the solar cells.

Solar Panel Efficiencies

Panel efficiencies<cell due to mismatch, losses, but increasing with

year for all technologies

Summary: Solar Modules

• Solar modules consist of an array of individual solar

cells connect in series and packaged in an Al framed,

sealed, structure for durability and long life

• The module electrical characteristics can be modeled by

a distributed equivalent circuit model of individual cells

• Mismatch of the individual cells causes a reduction in

output power and non-optimum performance

• Effects of shading can cause failure of the module

• Bypass diodes are usually included to short out failed or

shade parts of the module

12