There are various types of current inside solar cells, such as dark current, reverse current, and leakage current. These currents have varying degrees of impact on the power output of solar modules. Distinguishing the characteristics of these currents can help identify the causes of abnormal module power output, contributing to a thorough resolution of the problems.
Dark Current
Definition
Dark current, also known as reverse saturation current under no illumination, refers to the reverse DC current generated in a P-N junction under reverse bias conditions when there is no incident light. It is generally caused by carrier diffusion or defects on the surface and inside the device, as well as harmful impurities.
Formation
(1). Diffusion Process: Inside a P-N junction, there are more electrons in the N-region and more holes in the P-region. Due to the concentration difference, electrons in the N-region diffuse towards the P-region, and holes in the P-region diffuse towards the N-region. Although the built-in electric field of the P-N junction resists this diffusion, it still occurs until a dynamic equilibrium is achieved, forming a diffusion current.
(2). Defects and Impurities: When defects exist on the surface or inside the device, they act as recombination centers, capturing electrons and holes and facilitating recombination. Harmful impurities play a similar role, contributing to the formation of dark current.
Impact
Dark current is often considered during silicon wafer sorting. Excessive dark current indicates poor wafer quality, such as many surface states, numerous lattice defects, harmful impurities, or overly high doping concentrations. Solar cells made from such wafers usually exhibit low minority carrier lifetimes, directly leading to low conversion efficiency.
Dark Current in Solar Cells
In simple diodes, dark current corresponds to reverse saturation current. In solar cells, however, dark current includes reverse saturation current, thin-layer leakage current, and bulk leakage current.
Reverse Saturation Current
Definition
Reverse saturation current refers to the current in a P-N junction when reverse bias is applied. The reverse voltage widens the depletion layer, increasing the electric field and the potential energy of electrons. This makes it difficult for majority carriers to cross the barrier, reducing diffusion current to near zero.
Formation
1. Drift Current: The increased electric field makes it easier for minority carriers in the N and P regions to drift, forming a reverse current.
2. Temperature Dependence: Since minority carriers are thermally generated, their number is constant at a given temperature, and so is the reverse current.
Leakage Current
Definition
Solar cells can be divided into three regions: thin layer (N-region), depletion layer (P-N junction), and bulk region (P-region). Defects and impurities in these regions act as recombination centers, capturing electrons and holes to facilitate recombination. This process generates small currents, contributing to the measured dark current.
Types
· Thin-Layer Leakage Current: Caused by defects and impurities in the thin layer.
· Bulk Leakage Current: Caused by defects and impurities in the bulk region.
Purpose of Dark Current Testing
1. Preventing Breakdown
When the cell is reverse-biased or module polarity is reversed, excessive dark current can lead to rapid cell breakdown. Although rare, testing dark current helps prevent such occurrences.
2. Monitoring Production Processes
Dark current testing helps identify potential process issues. Dark current is composed of reverse saturation current, thin-layer leakage current, and bulk leakage current, represented by J1J_1J1, J2J_2J2, and J3J_3J3, respectively.
When reverse voltage is applied:
· Region 1: Dominated by J2J_2J2 (thin-layer leakage current).
· Region 2: Dominated by J3J_3J3 (bulk leakage current).
· Region 3: Dominated by J1J_1J1 (reverse saturation current).
The boundaries of these regions are determined by specific test voltages.
Voltage Effects
When a voltage is applied to the cell, it causes electrical injection into the silicon wafer, exciting non-equilibrium carriers. The higher the voltage, the more carriers are excited, leading to higher dark current. However, the growth rate slows as voltage increases until the cell is broken down.
Standard Testing
Dark current is typically tested at 12V. By comparing the test results to standard curves, the state of the cell can be assessed:
· Excessive dark current in Region 1 indicates issues in the thin layer.
· Excessive dark current in Region 2 suggests problems in the bulk region.
· Excessive dark current in Region 3 indicates issues with the P-N junction, such as diffusion, screen printing, or temperature inconsistencies.
Conclusion
Testing dark current is crucial for identifying process-related issues and improving solar cell production.
