Introduction to Solar Cell Materials-I

23 July 2012

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Photovoltaic cell: short history

Russell Ohl (Bell Labs) discovered the silicon p-n junction and the effect of light on the junction

Bell Labs researchers Pearson, Chapin, Fuller demonstrated the photovoltaic cell with 4.5% efficiency

1941

1954

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Photovoltaic cell: working principle

“Conventional” photovoltaic cells are based p-n junction between semiconductors.

N-type silicon P-type silicon

Continuous Current

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

What limits the efficiency of a p-n solar cell?

Solar Cell Efficiency Limits

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Modern solar cell

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Materials for photovoltaic cells Bulk semiconductors

– Silicon • Single crystal • Multi crystalline

– Gallium arsenide (GaAs) – Other III-V semiconductors

Thin Films semiconductors – Amorphous silicon (a-Si) – Cadmium telluride (CdTe) – Copper-Indium diselenide (CuInSe2, o CIS) – Coper-Gallium-Indium diselenide (CIGS)

Organic and hybrid materials - Small molecules - Polymers - Dye Sensitized Solal Cell

CdTe

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Solar Spectrum Sp

ectr

al p

ower

den

sity

[(W

/m2 )

/nm

]

Wavelength [nm]

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Efficiency One of the most important parameters of the photovoltaic cell is the efficiency defined as:

EFFICIENCY = η = Max electrical power produced by the cell

Total solar power impinging on the cell

10 W/dm2

Example:

1dm

1dm

η = 10% 1 W

η = 20% 2 W

It is important to increase as much as possbile the efficiency.

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Figures of merit Important features of the I-V curves · The intersection of the curve with the y-axis (current) is referred to as the short circuit current ISC. ISC is the maximum current the solar cell can put out under a given illumination power without an external voltage source connected. · The intersection with the x-axis (voltage) is called the open circuit voltage (VOC). VOC is the maximum voltage a solar cell can put out. · IMP and VMP are the current and voltage at the point of maximum power output of the solar cell. IMP and VMP can be determined by calculating the power output P of the solar cell (P=I*V) at each point between ISC and VOC and finding the maximum of P.

Fill form factor OCSC

MPMP

OCSC VIVI

VIPFF == max

The overall efficiency of a solar cell is larger for larger FF

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

PHOTORESPONSIVITY

EXTERNAL QUANTUM EFFICIENCY

POWER CONVERSION EFFICIENCY

The photoresponsivity is defined as the photocurrent extracted from the solar cell divided by the incident power of the light at a certain wavelength.

The external quantum efficiency is defined as the number of charges Ne extracted at the electrodes divided by the number of photons Nph of a certain wavelength incident on the solar cell

The power conversion efficiency is defined as the ratio of the electric power output of the cell at the maximum power point to the incident optical power.

Figures of merit

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Which are the factors influencing the cell efficiency ?

EFFICIENCY

MATERIALS

Silicon GaAs CdTe Organic …..

TECHNOLOGY

Single junctions Multiple junctions Hybrid cell Up/Down conversion

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Solar Energy Map

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Solar Cell Spectral Response

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

High Efficiency Solar Cell: GaAs

High conversion efficiency: 25-27% at 1000X

High throughput manufacturing process (MOCVD) Present technology for space application

Small size (1mm2 active area) allows for...

reduced series resistance losses efficient heat extraction

High cost affordable by means of high

concentration level

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Max and module level efficiencies

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Beyond the Shockley-Queisser limit The maximum thermodynamic efficiency for the conversion of unconcentrated solar irradiance into electrical free energy in the radiative limit, assuming detailed balance, a single threshold absorber, and thermal equilibrium between electrons and phonons, was calculated by Shockley and Queisser in 1961 to be about 31%.

W. Shockley and H. J. Queisser. J. Appl. Phys. 32 (1961) 510.

What do we do to achieve efficiencies > 31 % ?

• Concentration

• Multijunction

• Up/Down Conversion Nanotecnology

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

The thermalisation loss can be largely eliminated if the energy of the absorbed photon is just a little higher than the cell bandgap.

Tandem or multijunction cells tackle this problem by stacking the cells with the highest bandgap uppermost to achieve the desired filtering effect.

Increasing the number of cells in the stack improves the performance. The limiting conversion efficiency for direct sunlight amounts to 86.8% for an infinite stack of independently operated cells

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Multijunctions

Cell 1

Cella 2

Cella 3

Eg1

Eg2<Eg1

Eg3<Eg2

Eg=1.9eV

Eg=1.42eV

Eg=0.7eV

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I

Introduction to Solar Cell Materials Continue next lecture

P.Ravindran, Elective course on Solar Rnergy and its Applications Auguest 2012 Introduction to Solar Cell Materials-I