Dear prof. Faisal,

you can raise the efficiency of solar cell from many ways such as: good choose for thin film that deposited on silicon wafer, suitable method for deposition of thin film, suitable annealing temperature after deposition the film, doping, and more other things.

Best regards.

While solar energy holds the promise of long-term sustainable energy, current systems are too inefficient to make mass installations financially viable.

Most solar energy costs originate from the price of installing the panels and the land they sit on. The cells themselves only contribute approximately 20 percent of the cost of solar energy. Due to this, researchers have been attempting to help improve the efficiency of the cells, so that less land may be used to generate electricity from the sun.

Efficiency Improvements on Perovskite Solar Cells

Perovskite solar cells are potentially the most promising solar power technology for researchers right now. First appearing in 2009 with an efficiency of just 3.8 percent, engineers around the world has been developing new ways to increase its energy conversion efficiency.

In April, researchers from Hong Kong Polytechnic University announced the development of perovskite-silicon tandem solar cells, with the world's highest conversion efficiency of 25.5 percent.

The study team noted that a calibrated mix of solar cell materials would be ideal for energy absorption because different materials absorb different wavelengths of solar energy. For instance, methylammonium lead tri-halide perovskite and silicon solar cells are a complementary pair. With the perovskite as a top layer, it can collect the short wavelength photons while the bottom silicon layer absorbs long wavelength photons.

Latest Developments in Photovoltaics

The team was able to boost the efficiency of this hybrid cell, in three different steps. To begin with, the team used a low-temperature chemical method to lower the impact created by perovskite defects.

Next, the team made a three-layer sandwich of molybdenum trioxide/gold/molybdenum trioxide, rendering it transparent, so light could penetrate to the bottom silicon layer. Finally, a rose petal-like film was placed as the top layer of the solar panel to capture even more light.

The study team said they are continuing to search for ways to refine their cell and facilitate mass fabrication.

Putting the Pressure on Perovskite

At Stanford University, researchers have taken a different approach to perovskite solar cells, applying intense pressure to increase solar absorption.

In a study published in April, Stanford researchers showed how pressure impacts the way hybrid perovskite cells respond to light. To reach their conclusion, the scientists loaded perovskite samples into a high-pressure “anvil” device made up of two opposing diamonds. Each small sample was placed between the diamonds and then compressed at very high pressures.

The outcomes were obvious, the researchers said in a press release. One sample, which is commonly orange, became a lighter color under pressure, a sign the perovskite was soaking up higher-energy light waves than normal. However, as the stress heightened further, the sample darkened, implying that lower-energy light was then being drawn in.

The Stanford researchers said that their results indicate pressure could be used to manipulate the wavelengths of light a perovskite cell absorbs.

Improving Conventional Photovoltaics

In addition to making progress on state-of-the-art technologies, researchers are still looking for ways to continue boosting the efficiency of conventional photovoltaics.

In August 2015, engineers from the University of California Riverside announced the successful development of a coating that could potentially boost the efficiency of commercial solar cells by around 30 percent.

Conventional solar cells aren’t able to capture the infrared part of the sun’s rays, allowing it to pass right through. However, the newly-developed coating essentially reshuffles the infrared part of solar radiation to allow conventional photovoltaics to harvest this previously untapped energy. The coating takes two infrared photons and combines their energies to make one higher energy photon. This photon is then readily absorbed by conventional photovoltaic cells.

The UC Riverside engineers said this reshuffling of the infrared spectrum could have implications far beyond that of photovoltaics.

On the other hand, researchers have been able to improve the efficiency of solar cells by coating the cell surface with extremely small nanoscale structures. The new technology has been shown to nearly eliminate the reflection losses of solar radiation. Cost-effective solar photovoltaic materials are being developed within the Academy of Finland's research programme Photonics and Modern Imaging Techniques.

The nanostructured black silicon coating features very low reflectivity, meaning that a larger portion of the Sun's radiation can be exploited. At Aalto University, a research team led by Assistant Professor Hele Savin is conducting studies on crystalline silicon solar cells, which are the main type of solar cells that are currently on the market.

"The advantages of silicon include the long-term stability, sufficiency, low-cost and non-toxicity of the element, as well as the advanced production technology. Another benefit of these solar cells is their relatively high efficiency and technological compatibility with the manufacturing technologies currently used by the semiconductor industry," Savin explains. The technology, however, still calls for development and improvement.

According to Savin, the researchers are motivated in their work by the fact that the manufacturing technology players are major corporations in the industry, which allows for immediate large-scale experimenting of new ideas and improved methods in collaboration with the partner corporations.

Promising techniques in the test bench

The efficient operation of solar cells may be compromised by impurities in silicon. In solar cells, it is not possible to use as pure a form of silicon as in, for example, microelectronics, and thus they exhibit a higher degree of impurities. Moreover, the solar cell utilizes the silicon disk in its entirety, whereas transistors, for example, are located on the surface of the silicon disk, and accordingly, impurities cannot be controlled in solar cells by means of the same methods as those used in microelectronics.

One goal of the research led by Savin is to find ways to produce equally efficient solar cells using the less expensive but impure silicon rather than the more expensive purified silica. "We've already succeeded rather well in this respect. Certain promising techniques are currently being tested in production by a leading European solar cell manufacturer."

Another new research topic involves the so-called light-induced degradation of silicon solar cells. Light degradation is a harmful effect that reduces the solar cell efficiency by several percentage units during the first 24 hours of use, after which the situation becomes stabilized. The aim is to gain an understanding of the phenomenon itself and its causes. Understanding the phenomenon will contribute to the development of methods to minimise the harmful degradation and, possibly, to eliminate it completely.

The comment that solar cells only contribute about 20%  of the cost of the generated electricity is well mentioned! However, the obvious corollary is surely to reduce the cost of everything else rather than try to increase the efficiency of the cells. Sadly, doing the latter seems far more exciting than working on the former!

Also, twin photon processes, while attractive for improved light harvesting, tend to work only at very high power densities due to exciton life time effects. It remains to be seen if they can be made to work at light intensities you get in typical daylight.

The material and fabrication, the new material like CdTe are the new solar cell with hight efficiency. At first you need to choose a material suitable band gap and light emission after choosing the contact... 

There is continuous improvements in the efficiency of Si solar cells such that they crossed the 24 percent. This is made by mostly increasing the light absorption and decreasing the recombination losses in the volume and at the surfaces. In addition to optimizing the doping of the different layers.

I invite you to see my book chapter: Chapter Solar cells and arrays: Principles, analysis and design

Best wishes

The efficiencies of some techniques for solar cells:

•Multijunction (3-junction, 2-junction, Concentrator)(40.7%)

•GaAs (single crystal, Thin Film, Concentrator)(33.3)%

•Silicon crystals (single crystal, multicrystalline, Thick Film)(24.7)%

•Thin Film (CdTe, CuInSe2, Amorphous Si, nano-,micro-, polycrystalline Si)(19.9)% (Multijunction poly (CdTe/CIS)(15)%

•Emerging (Dye sensitized, Organic cells)(11.1)-(5.4)%

The efficiency of such solar cells can be increased by hybrid silicon with germanium or another in the field of hetero-structure synthesis. This technique will tailor and tuning the band gap of the cell. In general, now nanomaterials are used.