The advantages and disadvantages of thin film solar cells

The principle of thin film solar battery

In the chemical battery, chemical energy directly gets into electrical energy by the battery internal spontaneous oxidation, reduction, etc. The result of a chemical reaction respectively  is the reaction on the two electrodes,. The cathode active material in the electrolyte by potential is negative and the stability of the reducing agent, such as zinc, cadmium, lead, and other active metal and hydrogen and hydrocarbons. The positive active material in the electrolyte by potential is more positive and stable oxidant, such as manganese dioxide, lead dioxide, nickel oxide, such as metal oxide, oxygen or air, halogen and its salts, oxygen-containing acid and its salts, etc.

Electrolyte is has good ionic conductivity of materials, such as acid, alkali, salt aqueous solution, organic or inorganic nonaqueous solution, molten salt and solid electrolyte. When open circuit disconnection, although there is a potential difference between the poles (open circuit voltage), but there is no current, the chemical energy stored in the battery is not converted to electricity. When open circuit closed, under the action of two electrode potential difference is a current through the external circuit.

Inside the battery at the same time, due to there is no free electrons in the electrolyte, the charge transfer must accompany the active material and electrolyte interface between oxidation and reduction reaction, and shifting of reactants and reaction products of the material. Charge transfer in electrolyte are done by ion migration. Therefore, inside the battery normal charge transfer and mass transfer process is the necessary condition to ensure the normal output power. When charging, the battery inside the transmission of power and the direction of the mass transfer process in contrast to the discharge; Must be a reversible electrode reaction, to ensure that the mass transfer and transmission of power in the process of normal direction.

Electrode reaction is reversible, therefore, constitutes a necessary condition for storage battery. To response the gibbs free energy increment (coke)F = 96500 Faraday constant library Ann = 26.8 hours; N for equivalent number of battery reaction. This is battery electromotive force and the reaction between the basic thermodynamic equation also calculates the battery energy conversion efficiency of the basic thermodynamic equations. In fact, when current flows through the electrode, electrode potential is deviating from the thermodynamic equilibrium electrode potential, this phenomenon is called polarization. The greater current density (unit electrode area through the current), the more serious the polarization is. Polarization is one of the important cause of the battery energy loss. Polarization of three reasons: (1) caused by battery resistance parts of the polarization is called ohm polarization;(2) by the charge transfer process in electrode, electrolyte interface layer block caused by the polarization called activation;(3) by the electrode, electrolyte interface layer and mass transfer process in the delay caused by the polarization is called the concentration polarization. Electrode reaction area, increase with the decrease of polarization method is to reduce the current density, improve the reaction temperature and improving the catalytic activity of the electrode surface.

Thin film solar cells and disadvantages

Thin film solar cells due to the use of material is less, in terms of cost of each module has decreased significantly than accumulation type solar cell, on the manufacturing process needs less accumulation type solar energy batteries to small, it also has integration type of connection module, so you can save the independent modules needed in the cost of the fixed and internal connection.

Future thin film solar cells will be likely to replace the current commonly used silicon solar cells, to become the market mainstream. Amorphous silicon solar cells with monocrystalline silicon solar energy cell or polycrystalline silicon solar battery materials are the main differences of the different, monocrystalline silicon solar energy cell or polycrystalline silicon solar battery material is thin, and amorphous silicon solar battery material is SiH4, different because the material and make the structure of the amorphous silicon solar cells and crystalline silicon solar cell is a little different.

The advantage of SiH4 light effect and optical absorption effect is very good, but the electrical insulators, similar characteristics with silicon semiconductor features far, therefore initially think SiH4 is unsuitable material. But in the 1970 s scientists overcame this problem, not long after the RCA, creating the first amorphous silicon solar cells. Although SiH4 light effect and optical absorption effect is very good, but because of its crystal structure worse than polysilicon solar cell, so suspension key problem is more severe than polycrystalline silicon solar cells, free electrons and holes of composite rate very fast; In addition the crystallization of SiH4 irregular structure will hinder the electron and hole move made shorter diffusion area.

Based on the above two factors, so when light on SiH4 holes for electrons, must separate the electrons and holes, as soon as possible to effectively produce the photoelectric effect. Mostly very thin amorphous silicon solar cells reduce free electron and hole composite. Due to SiH4 suction light effect is very good, although very thin amorphous silicon solar cells still can absorb most of the light.

Amorphous silicon thin film solar cell structure is different from general silicon solar cell, as shown in figure 9, the main can be divided into three layers, the upper layer is very thin (about 0.008 microns) and high doping concentrations of P +;Intermediate layer is a thicker (0.5 ~ 1 micron) of pure quality layer (Intrinsiclayer), but in general are not generally pure mass layer is completely pure mass (Intrinsic), but low doping concentrations of n type materials; The bottom layer is thin (0.02 microns) n. And this kind of p + - p - I - n the structure of the more traditional n structure has a large electric field, the pure mass layer generated in the electronic holes can be quickly after the electric field separation. And a thin layer of oxide film on the P + for transparent conductive film (Transparent Conducting Oxide: TCO), it can prevent the sunlight, to effectively absorb sunlight, usually is the use of silica (SnO2).

The advantage of amorphous silicon solar cells cost is low, and the disadvantage is low efficiency and photoelectric conversion efficiency with the using time of recession. So the amorphous silicon solar cells are widely used in small power market, but in the power market is less competitive.

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