Temperature coefficient of photovoltaic devices
Imaging of temperature coefficients
In this project, we are developing a novel method to obtain implied open-circuit voltage images of silicon wafers and cells at different temperatures. Using the proposed method, we investigate the temperature coefficients of various regions across p-type multi-crystalline silicon wafers and cells from different heights of two bricks with different dislocation densities. Interestingly, both low and high temperature coefficients are found in dislocated regions on the wafers. A large spread of temperature coefficient is observed at regions with similar performance at 298 K. Reduced temperature sensitivity is found to be correlated with increasing brick height and is exhibited by both wafers and cells. This may indicate that cells made from the top of the brick, although having higher defect concentration, actually suffer relatively less degradation in performance at higher temperatures.
Spatially resolved implied open-circuit voltage (iVOC) at 298 K (a, d), temperature coefficient (TC) of the iVOC (b, e) and the gamma factor (γ) (c, f) of two wafers with low (a-c) and high (d-f) dislocation densities under 0.5 sun illumination.
Temperature coefficients of advanced solar cell structures
The temperature coefficient (TC) is a critical figure of merit to accurately evaluate the performance of solar cells at various operating temperatures, and hence, enabling the comparison between different cell technologies. In this project, we investigate the temperature dependence of advanced solar cell structures, such as tunnel oxide passivated contact (TOPCon) solar cells.
Regarding TOPCon, to gain a better understanding regarding the temperature-dependent behaviour of their performance, the passivation quality and the contact resistivity of polysilicon (poly-Si) passivating contacts as a function of temperature are investigated. We observe a gain in effective lifetime over the entire measured injection level range and a small improvement of the passivation quality of these contacts with increasing temperature. However, it seems this improvement does not have a strong impact on the open-circuit voltage TC. The cell performance at elevated temperatures is dominated by the drop in the open-circuit voltage, associated with the intrinsic carrier concentration related to band gap narrowing. The fill factor TC (TCFF) is superior to those of other cell structures reported in the literature. We attribute this favorable TCFF to the fact that some of the fill factor losses are compensated by the decrease in contact resistivity of the poly-Si passivating contacts at elevated temperatures. The relative TC of the cell efficiency of the investigated TOPCon solar cells is −0.285±0.005 %/°C. This value is comparable to the TC of silicon heterojunction cells and it is superior to those of cell structures without passivating contacts.
(a) TCVoc, (b) TCJsc, (c) TCFF and TCpFF, and (d) TCη of UMG-Cz TOPCon and Cz monoPolyTM solar cells, these TCs are normalised to the value at 25 °C. Error bars are obtained from the linear fits. TCs of other solar cell structures reported in the literture are also shown for comparison. (e) Percentage contribution of TCVoc, TCJsc, and TCFF to TCη of the UMG-Cz TOPCon solar cells and other cell structures.