Novel 2-D bifacial solar cell using large built-in internal electric fields: AP-I-N structure simulation

Solar cells made of single-crystalline silicon, as alternative energy sources, became the most widely used solar cells in recent years. The mainstream manufacturing approach is to process the cells from Si wafers, and then assemble these cells into photovoltaic (PV) modules. However, the direct conversion of solar energy into electricity using the photovoltaic effect suffers from low efficiency. Thus, increasing the conversion efficiency at low production costs becomes the main goal of solar cells manufacturers. One way to increase the efficiency of a solar cell is to use an ultra-wide layer of intrinsic semiconductor as the depletion region of a P-N junction. Computer simulation shows that an intrinsic layer of 1 μm thickness results in an increase of the photovoltaic conversion efficiency by 4.85% for conventional single-crystalline silicon P-N junction. In our work, we present a novel geometrical concept of a PIN structure for PV applications. The width of the intrinsic layer in our construction is 5-20 mm. Moreover, in our novel structure, the light irradiation acts directly on the active region of the PV cell, which enables bi-facial irradiation and results in ~28% conversion efficiency. A low-cost fabrication is ensured in our design due to a new manufacturing technology by eliminating some expensive processes, such as photolithography. The feasibility proof of the novel concept in mono-crystalline silicon solar cells is presented. We demonstrate simulation results and preliminary experimental results confirming our approach. Detailed computer simulations show that the voltage drop and accordingly the high and constant electric field within the intrinsic region of the structure take place for sufficiently small sizes of the region (up to 200 - 300 nm). With extension of the intrinsic region length the field becomes extremely uneven with peak values at the p-i and i-n junctions. The field strength falls from tens of thousands Volts per centimeter to hundreds of Volts per centimeter for 5 μm intrinsic zone and after 10 μm the field practically disappears. Analysis of the simulation results, electrons and holes concentration profiles, space charge and electric field distributions, brings the idea that the uncompensated charges of the donors and acceptors at the n-i and p-i junctions are the source of the built-in field in the intrinsic region. The simulation of the p-i-n structure equilibrium state was done by the Silvaco's ATLAS silicon device simulator.