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Following a previously introduced entropy approach and reviewing experimental measurements, we find a similarity option between photoelectric effects, photovoltaic effects and thermoelectric effects. The photovoltaic effect and the thermoelectric effect are proved in this study to be driven by a Seebeck effect which depends mainly on the thermal potential of the incident radiation and the interacting materials. Hence, we apply such exciting conclusion to derive an advanced efficiency limit of the developed and multijunction solar cells that exceed the previously derived limit by Shockley and Queisser.

According to the similarity and analogy of laws that govern the flow of heat and electric current and reviewing their common features as a flow of energy and entropy [

The thermoelectric effect is defined in literature as conversion of thermal energy into electrical energy due to replacing thermal potential of the flowing energy by electric potential or vice-versa [

The presented study will consider such postulated definition of electric current as a base to prove the similarity of the Seebeck effect, the photoelectric effect and the photovoltaic effect. In the second section of the presented study the postulated definition of electric current will be considered to introduce a modified definition of the Seebeck effect. In the third section, experimental results of measurements of Planck’s constant by a photocell will be reviewed to show an option of similarity between the photoelectric effect and the defined Seebeck effect. Accordingly, experimental measurements of the performance of photovoltaic cells will be analyzed in the fourth and fifth sections to prove the similarity between the photovoltaic effect and the defined Seebeck effect. In the sixth section, the found conclusion of the photovoltaic effect as a thermoelectric effect will be applied to derive through an entropy approach a practical limit of the efficiency of the photovoltaic cells. The found limit adopts the measured efficiency of the recently produced photovoltaic cells that exceed the previously found limit by Shockley and Queisser.

Seebeck effect, as a thermoelectric effect, is defined in literature as production of an electromotive force, or potential difference, and consequently an electric current in a loop of materials consisting of at least two dissimilar conductors when its two junctions are maintained at different temperatures [

Accordingly, when the junctions of such loop have a temperature difference “ΔT”, it is generated an electricpotential difference “

In Equation (1), the term “

rise of the incident radiation in the photocell measurements, or the measured slope of the found dependence, to the definition and the values of the Seebeck coefficient of metals and semiconductors, they have the same definition and are in the same order of magnitudes. Such results mean the conversion of the incident radiation into electrical potential in photocells may be due to a similar thermoelectric effect as the Seebeck effect that converts the heat flow into electric current. According to the postulated definition of electric current, it is possible to explain the incident heat waves from a high source temperature will gain upon reflection on a low temperature cathode an electric potential which is proportional to the thermal potential of the incident radiation by a similar effect as the Seebeck effect. Accordingly; it is possible to define the thermoelectric effect as conversion of the incident radiation into electric current in photocells due to replacing the thermal potential of the incident heat by electric potential by a similar Seebeck effect as in thermocouples [

According to the found similarity between the photoelectric effect and the defined Seebeck effect; it is expected to find a similarity option between the photovoltaic effect and Seebeck effect as both the photoelectric effect and the photovoltaic effect are explained as emission of electrons by bouncing photons. So, the measured performance of a typical silicon solar photovoltaic cell,

The value of Seebeck coefficient in Equation (2) as defined according to Equation (1) is identical to tabulated values of the Seebeck coefficient of Silicon [

The application of the same multijunction technique for magnifying the electrical potential difference in multijunction solar cells and in thermopiles, or multijunction thermocouples, represents also a proof of a similarity option between the photovoltaic effect and the Seebeck effect [

GaInP subcell, 1.04 V for the GaInAs subcell and 0.25 V for the Ge subcell and a total open circuit potential of 2.5 Volt [

A rather modified analysis for prediction of such multijunction solar cells that considers the previously attained conclusions will be tried in this study. So, the potential rises through the considered multijunction solar cell will be estimated as the sum of the potential rises in the involved subcells by applying Equation (2) for each subcell. Then, it is possible to consider the multijunction as a thermopile formed of three junctions and to apply a similar relation as that applied on thermopiles to find the electromotive force or potential “ ” [

In Equation (3), the flowing radiation gains a potential rise when crossing each PV junction by the Seebeck coeffi- cient of the corresponding junction times the same thermal potential of the flowing radiation “

Such value is identical to the measured “

Regarding the previous conclusions of sections 2, 3 and 4, it is possible to consider the photovoltaic cell as a thermoelectrical generator driven by Seebeck effect. Hence, the efficiency of a photovoltaic cell can be expressed as a thermoelectric generator as follows [

According to literature, the charges transport in conductors is characterized by energy and entropy transport [

In (6), represents a scale constant for conversion the temperature from the Kelvin scale into volts [

Substituting the electric potential in “E” in Equation (6) in terms of the temperature difference “

So, the cell’s efficiency may attain the Carnot cycle efficiency by increasing the Seebeck coefficient of the junction. Accordingly, the introduced multijunction technology that accumulates the resultant Seebeck coefficient of many subcells, as found in Equation (3), may lead to reach the Carnot cycle efficiency if the resultant Seebeck coefficient reaches the value of the conversion constant which is theoretically possible [

Starting from a previously postulated definition of electric current as a flow of electromagnetic waves that have a specified, positive or negative, potential, it was possible to prove that the classical definition of thermoelectric effect as conversion of thermal energy into electrical energy implies the truth of such postulated definition of electric current. So, it was possible also to prove that the photoelectric and the photovoltaic effects are driven by a similar effect as the Seebeck effect which depends mainly on the thermal potential of the incident radiation and the interacting materials. We use such conclusion to deal with the photovoltaic cells as a thermoelectric power generator and to find a new limit of the efficiency of advanced and the multijunction solar cells that exceed the broken limit of Shockley and Queisser. The found limit proves the conversion of the incident thermal radiation into electric current by the photovoltaic effect is a reversible process whose efficiency equals the efficiency of a Carnot cycle operating between the temperatures of the source of radiation and of the photovoltaic junctions.