1. Terawatt challenges in Photovoltaics
Challenges of coming decades are as follows:
Sustainable energy system
Providing terawatts (TWs) of electricity for global demands
Terawatt challenge : The difficulty of developing an energy source which can replace the existing electrical infrastructure is termed as terawatt challange

2. Terawatt challenges in Photovoltaics
Issues behind the Terawatt challenge Global Warming Shortage of electricity Uneven geographical distribution in sense of resources Cost of conventional resources e.g. oil Aged existing infrastructure Need of local electricity production Use of renewable technologies

3. Photovoltaic energy conversion is the newest energy conversion mechanism for large scale production of electricity
In other words, photovoltaic energy conversion mechanism is the best solution for Terawatt challenge

4. Limits of Conversion Efficiency
The principal losses are due to
Loss of photons with energy below the band gap
loss of excess energy of photon above the band gap in terms of energy relaxation of photo excited carriers back to band edges
Fig. The black shaded region represents the calculate conversion efficiency as a function of band gap, while the
other shaded regions represent the losses

5. Existing Solar cell Technology
Commercial solar cells (80%) consists of Silicon solar cells
Front junction
Screen printed device
Silicon nitride surface passivation
The record Silicon solar cells is 25 %
From which 14 % for multi crystalline and 17% for monocrystalline
Thin film solar cells (CdTe and CIGS (CuInGaSe2)) results in
Lower fabrication and Lower $/W cost
Lower efficiency

6. Advanced Concept Solar cells
Fig. Module cost as a function of levelized cost of electricity (COE) for different conversion efficiencies
showing the advantage of high efficiency solar cells in terms of reduced COE

7. Nanostructured materials and Devices
Quantum wells and superlattices
Sandwich composed of narrower band gap material with larger band gap material (e.g. GaAs and AlxGa1-xAs)
If thickness of barriers separating large and small band gap materials is reduced so that electronic states overlap, the system referred to as superlattice
Nanowires
Silicon nanowires
Fig. (a) Schematic of the self-assembled growth of nanowires using vapor–liquid–solid (VLS) epitaxy. (b) Scanning electron microscopic picture of the vertical growth of Si nanowires
by VLS epitaxy (from T. Picraux and J. Drucker, unpublished)

8. It introduces an intermediate band (IB) energy level in between the valence and conduction bands. Theoretically, introducing an IB allows two photons with energy less than the bandgap to excite an electron from the valence band to the conduction band. This increases the induced photocurrent and thereby efficiency.
Fig. Band diagram illustrating the realization of an intermediate band solar cell using
quantum dots

9. Hot carrier solar cells
Fig. Schematic of a hot-carrier solar cell consisting of an ideal absorber with energy
selective contacts

10. solar cell, and a hybrid thermo-photonic device,
Hybrid Concepts
Hybrid of several devices to exceed potential and overcome material specific limitation
Fig. Comparison of the detailed balance calculation of the 1 sun efficiency for a hot carrier
solar cell, and a hybrid thermo-photonic device,

11. Some Benefits of Solar Electricity
Energy independence
Environmentally friendly
“Fuel” is already delivered free everywhere
Minimal maintenance
Maximum reliability
Reduce vulnerability to power loss
Systems are easily expanded

12. Semiconductor and Optical absorption
All Photovoltaic devices are based on the absorption of photon of sufficient energy by semiconductors.
at normal temperature it has only a very few mobile charges able to conduct electricity, but on absorption of a photon of appropriate energy an electron can be promoted from immobility in the valence band to mobility in the conduction band, leaving a corresponding hole, equivalent to a mobile positive charge, in the valence band.

13. Status of light on the sample
The light enters the sample at x=0 is
I(0)=I0 (1-R)
I0 is the total incident light intensity and R is the fraction of light reflected at the front surface
The light intensity, which is a measure of the flux of photons in the optical beam, decreases through the sample as the photons are absorbed.
I(x)=I0 e-αx
α is the optical absorption coefficient

14. The minimum energy for optical absorption by silicon, for example is 1
The minimum energy for optical absorption by silicon, for example is 1.1 eV, corresponding to the bandgap between its valence and conduction states. It can therefore absorb the entire visible spectrum and that part of the near infrared with wavelength shorter than μ m. Narrower bandgap materials absorb a wider section of the solar spectrum, and as a consequence of absorbing more photons, can provide a higher current density.

15. Dye-sensitized solar cells
Photovoltaic devices are based on the concept of charge separation at an interface of two materials of different conduction mechanism
Fig. Principle of operation and energy level scheme of the dye-sensitized nanocrystalline solar cell. Photo-excitation of the sensitizer (S) is followed by electron injection into the conduction band of the mesoporous oxide semiconductor. The dye molecule is regenerated by the redox system, which itself is regenerated at the counter electrode by electrons passed through the load. Potentials are referred to the normal hydrogen electrode (NHE). The open-circuit voltage of the solar cell corresponds to the difference between the redox potential of the mediator and the Fermi level of the nanocrystallline film indicated with a dashed line.

16. Operational Principle of DSSCs
charge transfer dye attached to surface of nanocrystalline film.
Photo excitation of the dye results in the injection of an electron into the conduction band of the oxide.
The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an organic solvent containing redox system, such as the iodide/triiodide couple.

17. Dye molecular engineering
Choice of suitable electroactive dye
titanium dioxide semiconductor
using a ruthenium tris - bipyridyl complexwith this trisbipyridyl molecular sensitizer, the HOMO – LUMO gap is about 2.0 eV, and as a result the photovoltaic response is limited to wavelengths below 600 nm
Nature’s solar energy conversion dye, chlorophyll c, the version of the molecule found in green algae.

18. Figure The photocurrent action spectra of cells containing various sensitizers, where the incident photon to current conversion efficiency is plotted as a function of wavelength.
1, Terpyridyl panchromatic “ black dye ” ;
2, standard N3 dye;
3, cyanide – substituted trimer dye;
4, original tris – bipyridyl ruthenium [11] dye;
5, unsensitised titanium dioxide

19. Stable self assembling dye monomolecular layer
Optically excited dye is required to be in contact with titanium dioxide semiconductor
Dye transfer an electron and relax to lower energy charged cation state
Only monolayer of dye can be photoactive
Dye aggregates or multilayers can reduce efficiency of a cell
Stability of molecular monolayer is necessary for life of DSSCS

20. Two bipyridal ligands bonded o ruthenium to establish photosensitivity
Two thiocyanide ligands to modify the spectral response
Two carboxyl groups to chemically adsorb to the semiconductor oxide surface
Two hydrocarbon chains to have hydrophobicity to electrolyte
Fig. Structure of the Z – 907 amphiphilic dye
Advantages
Otical absorption coefficient increases
Thinner photoanode with enhanced electrode kinetics
Better performance

21. the segregation of atoms to particular atom sites takes place with little or no deformation of lattice , creating an ordered solid solution is termed as superlattice