Solar energy products naturally come in many types of batteries, generally including chemical batteries, dry electrolyte batteries, rechargeable batteries, and thin-film batteries. Today, we'll introduce thin-film solar cells and their parameters.
What are thin-film solar cells?
Thin-film solar cells, as the name suggests, are solar cells made by fabricating a thin film. They use very little silicon, making them easier to reduce costs. Furthermore, they are both a high-efficiency energy product and a new type of building material, making them easier to integrate with buildings. Against the backdrop of a continued shortage of silicon raw materials in the international market, thin-film solar cells have become a new trend and a hot topic in the international photovoltaic market.
Parameters of thin-film solar cells
The performance and lifespan of thin-film solar cells are mainly determined by their parameters, including rated capacity, rated voltage, charge/discharge rate, impedance, lifespan, and self-discharge rate.
1. Rated capacity
The minimum capacity a battery should be able to discharge under the specified design conditions (such as temperature, discharge rate, and termination voltage), expressed in ampere-hours (Ah-hours), is represented by the symbol C. Capacity is significantly affected by the discharge rate, so the discharge rate is often indicated by an Arabic numeral in the lower right corner of the letter C, such as C20=50, which indicates a capacity of 50 Ah at a discharge rate of 20 Ah. The theoretical capacity of a battery can be precisely calculated based on the amount of active material used in the electrode reaction formula and the electrochemical equivalent of the active material calculated according to Faraday's law. Due to potential side reactions in the battery and special design requirements, the actual capacity of a battery is often lower than its theoretical capacity.
2. Rated voltage
The typical operating voltage of a battery at room temperature, also known as the nominal voltage, serves as a reference when selecting different types of batteries. The actual operating voltage of a battery varies depending on the usage conditions. The open-circuit voltage of a battery is equal to the difference in equilibrium electrode potential between the positive and negative electrodes. It depends only on the type of active material in the electrodes, and not on the quantity of active material. Battery voltage is essentially a direct current voltage, but under certain special conditions, phase transitions in metal crystals or certain phase-forming films caused by electrode reactions can result in minute voltage fluctuations; this phenomenon is called noise. The amplitude of the fluctuations is small, but the frequency range is wide, thus distinguishing it from self-excited noise in a circuit.
3. Charge and discharge rate
There are two ways to express the charge/discharge rate: time rate and multiplier rate. The time rate is the charge/discharge rate expressed as the charging/discharge time, numerically equal to the battery's rated capacity (ampere-hours) divided by the specified charging/discharging current (amperes) in hours. The multiplier rate is another way to express the charge/discharge rate, and its value is the reciprocal of the time rate. The discharge rate of a primary battery is expressed as the time it takes to discharge to the termination voltage through a fixed resistor. The discharge rate has a significant impact on battery performance.
4. Impedance
A battery has a large electrode-electrolyte interface area, so it can be considered equivalent to a series circuit of a large capacitor and a small resistor and inductor. However, the actual situation is much more complex, especially since the battery impedance changes with time and DC level, and the measured impedance is only valid for the specific measurement state.
5. Lifespan
Storage life refers to the maximum permissible storage time, measured in years, from battery manufacturing to initial use. The total lifespan, including both storage and usage periods, is called the battery's effective life. Battery life is categorized into dry storage life and wet storage life. Cycle life is the maximum number of charge-discharge cycles a battery can achieve under specified conditions. Specifying cycle life must simultaneously define the charge-discharge cycle test protocol, including charge/discharge rates, depth of discharge, and ambient temperature range.
6. Self-discharge rate
The rate at which a battery loses capacity during storage. Expressed as the percentage of capacity lost through self-discharge per unit storage time relative to the initial capacity before storage.
