HIGH EFFICIENCE PHOTOVOLTAIC CELLS

High Efficiency Photovoltaic Cells

If solar energy is to become a practical alternative to fossil fuels, we must have efficient ways to con-
vert photons into electricity, fuel, and heat. The need for better conversion technologies is a driving

force behind many recent developments in biology, materials, and especially nanoscience.

Different engineering approaches have been used to raise collection and distribution efficiency, though perovskite-based cells have recently captured the most attention at major research facilities.

A number of solar cells electrically connected to each other and mounted in a support structure are called a photovoltaic module. Modules are designed to supply electricity at a certain DC voltages such as 12, 24 or 48 volts. The current produced is directly dependent on how much light hits the module. Multiple modules can be wired together to form an array. A larger area of a module or array will produce more electricity. PV 
modules are rated on the basis of the power delivered under Standard Testing Conditions (STC) of 1 kW/m² of sunlight and a PV cell temperature of 25 degrees 
Celsius (°C). Their output measured under STC is expressed in terms of “peak Watt” or Wp nominal capacity [54]. A typical crystalline silicon module consists of a series circuit of 36 cells, encapsulated in a glass and plastic package for protection from the 
environment. Although PV modules are warranted for power output for periods from 10-25 years, they can be expected to deliver amounts of energy (voltage and current) for periods of 40 to 50 years [53]. Typical electrical information supplied by the manufacturer includes: 
• Polarity of output terminals or leads 
• Maximum series fuse for module protection 
• Rated open-circuit voltage 
• Rated operating voltage 
• Rated operating current 
• Rated short-circuit current 
• Rated maximum power 

• Maximum permissible system voltage

Guide to Photovoltaic Cell (PV) Technology

Many industries have found applications for inverters; hence design requirements tend to be specific to the needs of a particular application. A whole new industry has evolved around the need of a proper inverter to accommodate the needs of the relatively new solar industry, with both big and small manufacturers entering the market. PV modules produce DC outputs which are dependent on the irradiance, temperature and load operation. Stand-alone inverters operating with energy storage or batteries need a small DC voltage operating range to allow for voltage differences due to battery state of charge, and surge capacity to allow for safe and uninterrupted transient event operation. Grid-tied systems do not normally incorporate energy storage; hence larger DC voltage operating ranges are needed to accommodate both the varying operating conditions and module configurations. Maximum power point tracking control algorithms are normally included to take full advantage of the PV module energy production capabilities. Advanced protection functions are normally also included in order to guarantee safe operation in parallel with the distribution grid. These are just 
examples of specific requirements for PV inverters in their specific applications. The following section shall summarize current PV inverter characteristics, industry status and trends, especially in the grid-tied market, which is currently of most public interest. The industry challenges attended include:
1. Reliability 
2. Inverter lifetime improvements 
3. Higher inverter efficiencies 
4. Production cost reduction 
5. System and installation cost reduction 
6. Unreliable or inadequate components or parts 
7. Safety 
8. Grid connection issues 
9. Optimal circuit topologies, etc.


A wide variety of solar technologies have the potential to become a large component 
of the future energy portfolio. Passive technologies are used for indoor lighting and 
heating of buildings and water for domestic use. Also, various active technologies are used to convert solar energy into various energy carriers for further utilization: 

• Photovoltaics directly convert photon energy into electricity. These devices use 
inorganic or organic semiconductor materials that absorb photons with energy 
greater than their bandgap to promote energy carriers into their conduction band. 
Electron-hole pairs, or excitons for organic semiconductors, are subsequently 
separated and charges are collected at the electrodes for electricity generation. 

• Solar thermal technologies convert the energy of direct light into thermal energy using concentrator devices. These systems reach temperatures of several hundred degrees with high associated exergy. Electricity can then be produced using various strategies including thermal engines (e.g. Stirling engines) and alternators, direct electron extraction from thermionic devices, Seebeck effect in thermoelectric generators, conversion of IR light radiated by hot bodies through thermophotovoltaic devices, and conversion of the kinetic energy of ionized gases through magnetohydrodynamic converters. 

• Photosynthetic, photo(electro)chemical, thermal, and thermochemical processes are used to convert solar energy into chemical energy for energy storage in the form of chemical fuels, particularly hydrogen. Among the most significant processes for hydrogen production are direct solar water splitting in photoelectrochemical cells or various thermochemical cycles such as the two-step water-splitting cycle using the Zn/ZnO redox system. This document focuses on active solar technologies which are organized according to the energy conversion paths that they use to convert the energy of photons into a usable energy form: photons-to-electricity, photons-to-chemical energy, photons-to-heat-to-
electricity, and photons-to-heat-to-chemical energy (see Fig. 6). This analysis focuses on 
the fundamental physical processes that govern the operation of the solar devices with the 
intent of identifying common trends or synergies between different technologies that 
could help identify novel research opportunities.

Photon-to-Electric Energy Conversion 
Photovoltaic devices allow the direct production of electricity from light absorption. The active material in a photovoltaic system is a semiconductor capable of absorbing photons with energies equal to or greater than its bandgap. Upon photon absorption, an electron of the valence band is promoted to the conduction band and is free to move through the bulk of the semiconductor. In order for this free charge to be captured for current generation, decay to the lower energy state, i.e. recombination with the hole in the valence band, has to be prevented through charge separation. 
In photovoltaic devices made of inorganic semiconductors, charge separation is driven 
by the built-in electric field at the p-n junction. As a consequence, their efficiency is 
determined by the ability of photogenerated minority carriers to reach the p-n junction 
before recombining with the majority carriers in the bulk of the material. Thus, bulk properties such as crystallinity and chemical purity often control the device efficiency.