1. Field of the Invention
The present invention is directed to novel aluminum-based pastes with improved properties and solar cells including novel aluminum-based paste compositions.
2. Description of the Related Art
Solar cells generally consist of two differently doped silicon layers allowing for a separation of charge carriers at the top and bottom surfaces thereof. A standard crystalline silicon solar cell begins with a p-type wafer which is doped with a thin layer of n-type dopants on the front surface or sunlight receiving side.
A pn junction is created where the n and p type layers meet to separate positive (holes) and negative (electrons) charge carriers. When photons from sunlight or artificial sources enter the solar cell, an electron may be promoted from the valence band to the conduction band resulting in a hole in the valence band and an electron in the conduction band. As holes flow to the back surface while electrons flow to the front surface, a current is produced when placed in a circuit.
During production of silicon solar cells, a coating of an aluminum metallization paste is deposited on a back side of the p-type wafer to form a back-surface electrode. Aluminum metallization pastes typically include an aluminum powder, a glass frit and an organic vehicle. While many printing methods may be used to print aluminum metallization pastes, screen printing is considered the most popular method by those skilled in the art. Once the paste is applied on the back surface of the cell, the semiconductor substrate undergoes a drying and firing process. During firing, the cell is heated to temperatures above the melting point of aluminum. An aluminum-silicon alloy layer (Al—Si) is thus formed between an aluminum back side electrode and the p-type silicon semiconductor wafer. Firing also causes a p+ layer (i.e., impurity layer) to form by diffusion of aluminum atoms into the p-type wafer. The Al—Si layer and p+ layer secures ohmic contact between the p-type wafer and the back-side electrode. The Al—Si layer also prevents recombination of electrons since excited electrons in the conduction band of the semiconductor are prevented from falling back into empty states in the valence band. By so doing, a back surface field (BSF) effect improves energy conversion efficiency of the solar cell by increasing the collection efficiency of generated carriers.
Pastes containing heavy metals generally are used to increase solar cell efficiency and open circuit voltage. Heavy metals are natural elements with low densities, such as mercury, cadmium, lead, beryllium and arsenic. Heavy metals are considered toxic or poisonous to humans and animals when concentrated. Thus, most heavy metal elements are regulated by environment regulations at least in Europe and North America. Cadmium, for example, may pose significant health risks including kidney damage/failure and increased likelihood of bone defects and fractures. Lead also poses many health risks including but not limited to negative effects on the gastrointestinal tract, joints, kidneys and the reproductive system. Acute nerve damage may also occur.
Based on the Restrictions of Hazardous Substances Directive (RoHS), heavy metals are generally restricted to 0.1% or 1000 ppm by weight of the homogenous material understood as being a single mechanically separable substance. Cadmium is further restricted to 0.01% or 100 ppm in most applications, however, cadmium presently is not restricted in solar cell applications as of the most recent 2011 RoHS. Values below the RoHS maximums generally are considered heavy metal-free. A need therefore exists to produce pastes and solar cells which are essentially free of lead and other heavy metals while still maintaining high open circuit voltages.
Solar cells also have different sizes based on their intended use. Thus, different amounts of aluminum paste are required to cover solar cells of varying sizes. A problem of maintaining fairly constant and reliable open circuit voltages (Voc) at different applied aluminum pastes weights is apparent in the art. A need exists for an aluminum paste composition capable of minimizing or eliminating fluctuations in open circuit voltage when different aluminum paste amounts are applied on a solar cell.
Reducing cost and increasing throughput of solar cells has always been a goal for manufacturers in the solar cell industry. Manufacturers have attempted to reduce costs by making thinner semiconductor substrates. However, when aluminum paste compositions are applied on the backside of thinner wafers, and later fired to a firing temperature, deformation occurs. Specifically, the difference in thermal expansion coefficients between silicon and aluminum results in a concave-shaped cell understood by those in the art as “bowing”. Excessive bowing causes a reduction in structural rigidity and in some cases, breakage of the solar cell. On the other hand, too little aluminum paste applied on the back surface of the cell may cause blisters and/or bubbles on the surface after firing. A need therefore exists for an improved aluminum paste composition and solar cell that minimizes bowing to increase product yield. A need also exists for a solar cell with minimal defects such as blisters and/or bumps to improve the aesthetic appeal to customers in the field.
In recent years, there has been a sharp increase in demand for using solar cells as a primary or secondary source of electricity for residential and/or commercial use. Specifically, solar cells have been mounted on roofs or other key areas of homes to absorb maximum amounts of natural light. Constant contact with harsh environmental considerations such as rain, snow, heat and humidity may likely degrade the life expectancy of solar cells. Therefore, a need exists for a solar cell capable of withstanding harsh and/or variable weather conditions and still provide good open circuit voltage (Voc).