SUN IN CITY : 2019

Utility companies are doing a great job but one must understand that anticipating the load demand is a critical task. Complexity comes in with the increasing production of electricity from solar energy. Curve shown below represents the load demand curve of electricity in California on 31 March 2012. The blue line in the curve represents the actual load demand in California on 31 March 2012. Least power is required over midnight. During the morning, curve ramps up when offices and schools get started, during evening time curve ramps up to the peak load and then again goes down.

Duck Curve illustration by CAISO (California Independent System Operator) with "Net Load" versus "Time of Day" plot. Predictions for Net Load are made for different Years.

As electricity from solar is produced only during the day time and grid gets flooded with solar energy, a dip is observed in the demand curve (red lines) during the day time with an increase in solar energy production. With further increment in solar energy production, dip observed in the curve goes on lowering down during the day time (red lines 2014-2020). The graph curve resembles a "Duck" so it was given the name "Duck Curve" by CAISO (California Independent System Operator).

In the graph with increased solar capacity, it looks like demand has been dropped during the day time. Now the problem arises due to intense ramps in the new graph. As the sun sets, utilities have to suddenly ramp up the demand to meet the peak load which is not possible as it destroys the economics of power plant. Nuclear and Coal power plants are baseload power plants, so they need to be turned on 24X7 and can't be turned on and off every day for economic load dispatch with solar power plant. To avoid this problem curtailment of solar energy must not be considered as a solution.

Some of the solutions are:

Integrating solar energy with energy storage technologies (batteries) could possibly eliminate the risk of overgeneration of electricity during day time.
Switching to electric vehicles will enhance the usage of solar energy.
Encouraging the use of innovative solutions that are cost effective such as Tesla Powerwall.
Using Machine Learning to predict load demands much more accurately leading to lower losses.

These are essentially thin film solar cells. So, thin film of CIGS is used as primary absorber. These are essentially called chalcopyrite based solar cells. CuInSe2 ( Copper Indium Selenide ), CuInS2 ( Copper Indium Sulphide ) and CuGaSe2 ( Copper Gallium Selenide ) with band gap of 1 eV, 1.5 eV and 1.7 eV are the major chalcopyrite compounds that are being used for solar cell applications. With a band gap of 1 eV CuInSe2 ( Copper Indium Selenide ) provides the heighest efficiency and its efficiency can be further increased by addition of Gallium, thus making it Cu(InGa)Se2 (Copper Indium Gallium Selenide) with a band gap of 1.15 eV.

Note :

Band gap of 1.45 eV is most appropriate for solar cell applications because wavelength corresponding to 1.45 eV comes out to be 855 nm (λ = hc/E = 1240/1.45 = 855 nm). Thus, whole visible region will be absorbed and wavelength above 855 nm will not be absorbed.

Absorption coefficient of chalcopyrites is quite high; therefore, a few micron thick active layer is required for the absorption of visible region of sunlight. CIGS is an interesting material as by varying the ratio of Indium to Gallium, one can change the bandgap. As we can vary this ratio with large tolerance CIGS band gap is tuned efficiently. Commercially bandgap of CIGS is between 1.01 eV to 1 eV. CIGS is used as P-type semiconductor and CdS is used as N-type semiconductor leading to the formation of junction and depletion region. Band gap of CIGS and CdS is 1.1 eV and 2.42 eV. As the major portion of the solar spectrum is absorbed by CIGS, CdS is mainly used as partner layer to form the cell and is referred as "Buffer Layer".

CIGS Solar Cell Parameters

Efficiency (η) : 20 %

Open Circuit voltage (V_OC) : 0..69 V

Short Circuit Current Density (J_sc) : 35.5 mA/cm²

Fill Factor (FF) : 81 %

Mobility of Electrons (μ_e) : 90-900 cm² / Vs

Mobility of Holes (μ_h) : 5-50 cm² / Vs

Melting Temperature : 1259 K
Density : 5.75 g / cm³

Fabrication

For substrate, Mo coated glass or Soda Lime glass is used because sodium helps in surface passivation by controlling dangling bond density. Moreover coefficient of thermal expansion of soda lime glass matches with that of CIGS leading to lower thermal stress and prevents cracking in thin film. CIGS is the heart of this solar cell. CIGS is commonly deposited with co-evaporation technique. In this process temperature of around 550 degree celsius is required and all the elements ( Copper, Indium, Gallium, Arsenide ) are evaporated together. CdS is deposited using Chemical Bath Deposition (CBD) method. In CIGS solar cell substrate configuration is used instead of superstrate configuration as in CdTe solar cell. Since CIGS deposition requires high temperature conditions so, CIGS is deposited before CdS.

In 1972, first CdTe/CdS solar cell was reported. It is a type of photovoltaic that uses Cadmium Telluride (CdTe) and Cadmium Sulphide (CdS) thin film for light absorption and charge extraction for generating electricity using the photovoltaic effect. Two major companies that are manufacturing CdTe solar cells are "Antec" in Germany and "First Solar" in US.

CdTe (Cadmium Telluride) solar cell belongs to the family of chalcogenides (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe). CdTe is formed by reaction of Cd and Te vapours and then deposition on surface using sputtering, thermal evaporation and other physical vapor deposition techniques. CdTe is a direct band gap semiconductor and has high absorption coefficient. As absorption coefficient is inversely proportional to absorption length, only a few micron thick layer of cadmium telluride is required to absorb visible spectrum of sunlight. Band gap of CdTe is 1.45 eV which is most appropriate for solar cell applications because wavelength corresponding to 1.45 eV comes out to be 855 nm (λ = hc/E = 1240/1.45 = 855). Thus, whole visible region will be absorbed and wavelength above 855 nm will not be absorbed.

CdTe Solar Cell Parameters

Efficiency (η) : 21 %

Open Circuit voltage (V_OC) : 0.88 V

Short Circuit Current Density (J_sc) : 30 mA/cm²

Fill Factor (FF) : 80 %

Mobility of Electrons (μ_e) : 500-1000 cm² / Vs

Mobility of Holes (μ_h) : 50-80 cm² / Vs

Melting Temperature : 1365 K
Absorption Coefficient (600 nm) : >5 * 10⁵ / cm
Density : 6.2 g / cm³

CdTe Solar Cell Structure

Figure 1. CdTe Solar Cell Structure

Figure 1 represents superstrate structure of CdTe solar cell. Superstrate structure is the one in which light enters through the substrate itself. In CdTe solar cell front contact is made by TCO (Transparent Conducting Oxide) and is also called (Window Layer). TCO layer should be thick enough so that it can prevent the impurities from the substrate. On TCO CdS layer is deposited (100 nm) followed by deposition of CdTe layer (5 micro meter). Once the junction is formed device needs to be activated by chlorine treatment using CdCl2. In this treatment CdCl2 is dripped on to CdTe coated substrate and then heating it at 450 degree Celsius for 15 minutes. CdCl2 treatment lowers down the defect density and give rise to holes, promoting P-type character. The last step involves the formation of back contact. Figure 2 represents sequence wise manufacturing process of CdTe solar cell.

Figure 2. Manufacturing process of CdTe Solar Cell

CdTe is used as P-type semiconductor and CdS is used as N-type semiconductor leading to the formation of junction and depletion region as shown in figure. Band gap of CdTe and CdS is 1.45 eV and 2.42 eV. As the major portion of the solar spectrum is absorbed by CdTe, CdS is mainly used as partner layer to form the cell and is referred as "Buffer Layer". To enhance the efficiency of solar cell active layer band gap should align with that of buffer layer band gap. In cadmium telluride solar cell, CdS is highly doped and CdTe is lightly doped so that depletion layer portion is extended deep inside CdTe side and recombination will be less probable due to the presence of an electric field in the depletion region as shown in figure 3.

Figure 3. CdTe Solar Cell Energy Band Diagram

Issues with CdTe Solar Cell

1.) Intermixing of CdS-CdTe layer.
2.) Toxic nature of Cd.
3.) Bandgap can not be tailored.

In the early stage of technology, amorphous silicon was found to be an interesting material because with doping it can be made P-type or N-type and provided an opportunity for the formation of junction and leading to the formation of solar cells. But after introducing sunlight radiation on solar cell, efficiency decreases and this effect is called Staebler Wronski effect.

Large number of defects around 10²³ / cm³is present in amorphous silicon due to large number of dangling bonds. There are several missing bonds and these missing bonds are called dangling bonds. Dangling bond reduces the performance because they act as traps and they absorb the carriers. This problem was solved by making hydrogenated amorphous silicon (a-Si: H). Hydrogen passivation during the manufacturing process creates hydrogen bond (with dangling bond) and reduces the defect densities ( to around 10¹⁶/ cm³ and improves the device performance. With this improvement, a lot of interest began in building in amorphous silicon and the manufacturing process started.

For solar applications hydrogenated amorphous silicon (a-Si: H) showed high absorption coefficient in the visible range of solar spectrum (AM 1.5G). As the absorption coefficient is inversely proportional to the absorption length, only a few micron layer thickness of hydrogenated amorphous silicon is required to absorb 90% of the visible spectrum. In 1976 first hydrogenated amorphous Silicon (a-Si: H) solar cell was developed by “Carlson and Wronski” with an efficiency of 2.4 %.

Figure 1 Energy Band Structure

Figure 1 shows energy band of crystalline silicon and hydrogenated amorphous silicon (a-Si: H). It is observed that in case of crystalline silicon there is tetrahedral configuration with SP³ hybridization which indicates “no trap states”. But in case of hydrogenated amorphous silicon (a-Si: H) instead of tetrahedral configuration, bonds with different length and angles are present which are less stable and are called as “weak bonds”. When energy is given to material in the form of solar radiation, these week bonds break and causes degradation in inefficiency. This effect is called Staebler Wronski effect. It is fortunate enough that efficiency does not drop continuously after light exposure. After some time, it gets saturated to a value of 80 to 90% of the initial value.

Gallium Arsenide (GaAs) is a compound of two basic elements; Gallium and Arsenic. It is III-V direct bandgap semiconductor with 1.41 eV band gap, so relatively it can absorb (AM 1.5G spectrum) light more efficiently. This band gap can be varied by Aluminium (Al) or Indium (In) like AlGaAs or InGaAs. First GaAs solar cell was used in space application for Venera 3 mission and then till now, its efficiency has reached up to 29.1 % by Alta Devices. To check the efficiency of all types of solar cells, click here.

GaAs has several advantages over Silicon such as higher temperature applications and higher carrier mobility. But is 5 to 10 times expensive than silicon and due to direct bandgap radiative recombination is also dominating.

Gallium Arsenide (GaAs) unit cell representation

In other semiconducting materials that are being used in solar applications, efficiency reduces with increase in temperature. But GaAs has a low-temperature coefficient, thus it is not much affected by increasing temperature. In GaAs elements used for N-type doping are Selenium (Se), Silicon (Si) and Tin (Sn) while elements used for P-type doping are Zinc (Zn), Magnesium (Mg) and Carbon (C). With lower resistance and higher carrier mobility, GaAs creates less noise in electronic circuits at high frequencies. Although being expensive GaAs finds it's applications in other technologies like satellites, military applications and ICs.

Note

Mobility
Mobility is defined as how quickly the charge carrier can move within the semiconductor. In GaAs electron mobility (μ_e) and hole mobility (μ_h) is in the range of
(μ_e) = 5000 to 10000 V/cm²
(μ_h) = 100 to 400 V/cm²

Carrier Life Time
It is the average time taken by the minority carrier in the excited state to recombine. In GaAs electron lifetime (T_e) and hole lifetime (T_h) is in the range of
(T_e) = 1 microsecond to few nanoseconds
(T_h) = few nanoseconds.

For solar applications, GaAs solar cell provides high mobility and high carrier lifetime which cause lower recombination possibilities and higher efficiency is obtained.

Solar radiation that earth receives varies inversely with the square of the distance between the sun and earth. Due to continuous motion of the earth in the orbit, radiation coming on earth also varies.

Extra-Terrestrial Solar Radiation

It is solar radiation just outside the earth’s atmosphere. The figure shown below represents extra-terrestrial solar irradiation.

Two terms are commonly used while studying solar spectrum namely “Irradiance” and “Insolation”

Irradiance: It refers to the solar power received by the collector per unit area and its unit will be kW/m²/nm.

Insolation: It refers to the energy received by the collector per unit area over a given period of time and its unit will be kWhr/m²/nm.

Figure: Reduction in radiation at the earth's surface because of absorbption and scattering of radiation by the ozone layer, CO₂, water vapour and dust particles present in the atmosphere. (Source: https://rredc.nrel.gov/solar//spectra/am1.5/)

Solar Spectrum atEarth’s Surface

After entering into the atmosphere about 6% of the irradiation is reflected and around 16% irradiation is absorbed and scattered by the ozone layer, CO₂, water vapour and dust particles present in the atmosphere. Radiations that gets scattered in the atmosphere are called Diffused Radiation and radiation that gets reflected from the ground is called Albedo Radiation.

Airmass

Airmass gives an idea of the path length through which spectral irradiation passes through the atmosphere before reaching the earth’s surface. This length varies continuously due to the varying position of the sun with respect to earth. To avoid confusions with varying path lengths, standard Airmass coefficients (AM0, AM1, AM1.5, AM2) are made to compare different solar panels and same is used worldwide.

Airmass Coefficient

It is defined as the direct optical path length of the insolation through the atmosphere. It is expressed relative to the zenith path length which is normal to the horizon plane at sea level.

Air mass approximation is given by

AM = 1/Cos(z)

Where z is zenith angle in degree.

AM0 represents radiation spectrum just outside the earth’s atmosphere.

When sun is exactly at overhead position, spectrum is referred to as AM1. In this case, sun radiations travel the shortest distance and is considered unity(1).

When sun is at 48.5 degree from zenith, spectrum is referred to as AM1.5. In this case obviously, the radiations will have to travel a longer distance in the atmosphere.