High Efficiency Monolithic Solar Cell with Reduced Forward Bias Current: Theory, Design and Application

As the single-junction silicon solar cell is approaching its theoretical efficiency limits, the loss from shading and resistance is gaining increasing attention. The metal grid pattern may cause an efficiency loss up to 1–3%abs (absolute percentage) depending on the grid’s materials and structure.Many attempts have been proposed to reduce the loss caused by the contacts and module. Among them, the monolithic solar cell, which is a solar cell with multiple string cells on the same wafer and connected in a series, presents advantages of low output current, busbar-free contact, minimized interconnection space, and ohmic loss reduction. However, this structure also introduces a lateral forward bias current through the base region, which severely degrades the cell’s performance. In addition, this interconnection in the base region has partially shunted certain solar cells in the monolithic cell, which created a mismatch between string cells. For the last few decades, researchers have used different methods such as etching trenches or enlarging the distance between the neighboring string cells to solve this problem. However, these methods were both ineffective and defective. In this work, a novel method of suppressing the lateral forward bias current is proposed. By adding a very high surface recombination to the mid-region between the string cells, the carrier density in the mid-region can be decreased close to the doping density. Thus, the resistivity in the mid-region can be increased tenfold or more. As a result, the lateral forward bias current is greatly reduced. Other methods to reduce lateral forward bias current include optimizing the width of the mid-region, shading the mid-region, reducing the base doping and base thickness which can be used to reduce the mismatch as well. Another method has been proposed to calculate the minimum efficiency loss of a monolithic cell compared to the baseline solar cell. As a result, the monolithic cell could potentially gain more advantages over the baseline solar cells with a thinner and low-doped wafer. A monolithic solar cell with innovative designs is presented in this work which shows an efficiency that is potentially higher than that of normal solar cells.