As an upgraded version of passivated emitter and back-side cell (PERC) solar cells, the performance of tunnel oxide passivation contact (TOPCon) solar cells is very dependent on the silicon oxide layer and polysilicon layer. We found that different crystallization rates in polysilicon or doping of elements such as germanium and carbon can change the band gap of polysilicon, thereby affecting the efficiency (Eff) of TOPCon solar cells. Therefore, a suitable band gap is particularly important. At the same time, the internal defects of polysilicon also have a great impact on the performance of solar cells. Using the simulated band diagram, carrier concentration and recombination rate, the defects in polysilicon and the influence of different bandgap polysilicon on the performance of TOPCon solar cells were deeply explored.

 Effect of band gap width on TOPCon solar cells

  In TOPCon solar cells, the polysilicon layer has a great influence on the quality of the cell. Current research mainly focuses on phosphorus doping concentration and tunneling oxide thickness, and often ignores the influence of polysilicon bandgap width and defects on TOPCon solar cells.

  At present, the main manufacturing technology for polysilicon in the back field layer of TOPCon solar cells is LPCVD or PECVD. For polysilicon prepared by LPCVD method, the band gap is difficult to adjust. However, with PECVD, the band gap of polysilicon can be adjusted by high-temperature annealing or doping of C and O. Under different annealing conditions, the crystallization rate of PECVD-grown amorphous silicon films tends to be very different, while the band gap of polysilicon is closely related to the crystallization rate of polysilicon. The higher the crystallization rate, the smaller the band gap of poly-Si. In addition to the crystallization rate, doping also affects the band gap of polysilicon, for example, when germanium is doped in amorphous silicon, the band gap of amorphous silicon decreases; When doped with carbon, the band gap increases. With the change of band gap, the Voc, Jsc, FF and Eff of TOPCon solar cells have all changed a lot.

  In polysilicon, defect states include valence band tail local state, conduction band tail local state, and four Gaussian distribution energy gap states. Band-tail localization is caused by bond angle deformation, and gap localization is caused by floating bonds. The energy gap states in polysilicon can be divided into donor-like defect states and acceptance-like defect states. As the concentration of defective states increases, the performance of TOPCon solar cells decreases

   Polysilicon on the back has a great influence on the performance of TOPCon solar cells. In the case of narrowband systems, the formation of quantum wells at the polysilicon interface valence band leads to the accumulation of holes, which leads to a surge in the recombination rate, resulting in poor device performance. When the band gap increases to 1.34 eV, the quantum well disappears, the recombination rate decreases by two orders of magnitude, and the performance improves. Therefore, in order to obtain good TOPCon solar cells, attention should be paid to controlling the width of the band gap when preparing the polysilicon layer. At the same time, we should pay attention to reducing the concentration of defective states in the polysilicon layer, especially the concentration of the class-accepting defect state. Doping concentrations should be controlled to be less than 1-2 orders of magnitude.

 Main conclusions:

  The band gap of polysilicon can significantly affect the performance of TOPCon solar cells, and larger bandgaps can improve open-circuit voltage (Voc), short-circuit current density (Jsc), and efficiency (Eff).

  An increase in the concentration of defective states of polysilicon will lead to the deterioration of solar cell performance, among which acid defective states have a more obvious effect on the cell.

  When preparing polysilicon layers, it is necessary to control the width of the band gap, especially in the control of the receptive defect state, which should be controlled at a doping concentration of less than 1-2 orders of magnitude.