Band gap grading of a–SiO_x:H and its effect on the performance of thin film solar cells

Abstract

Solar cells are showing significant promise to become the solution for the growing energy needs of our world. However for this to happen, new disruptive technologies with high efficiency and low cost are needed in the market. One possibility comes from multijunction thin film solar cells based on a-Si alloys and nc-Si. For this purpose a–SiO_x:H is an interesting material since it can have V_OC values above 1 V and FF above 0.7. However when paired in a tandem structure with a material of an advantageous bandgap, like nc-Si:H. The performance of the tandem solar is limited in output current by the a–SiO_x:H layer.



Research to increase J_SC by increasing thickness of the absorber layer show it is not practical to increase the thickness of i-a–SiO_x:H above 250 nm. Spillover knowledge from other thin film solar cells (GaAs, a-SiGe_x and CIGS), showed bandgap grading was able to increase performance of the electrical parameters. Grading in a solar cell means that in one of the layers a material property is varied continuously in concentration in order to achieve a different performance.

Our aim was to experiment with bandgap grading in the absorber layer of a–SiO_x:H solar cells, to try to achieve a higher J_SC while still retaining the high V_OC x FF product. Test layers were deposited at different CO₂/SiH₄ ratios to determine the dependence of the bandgap (E₀₄) and the deposition rate on the CO₂/SiH₄ ratio. With the experimental data and fitted polynomial equations a method was devised to vary continuously the bandgap in a step wise manner. Using this grading method, experiments were designed where the intrinsic a–SiO_x:H layer of a total length of 200 nm was subdivided in 3 graded bandgap regions. The first graded region named p-i started from the end of the p-layer with a high bandgap (2.1 eV). Decreasing the bandgap over a certain length until reaching a region with no added oxygen with a low bandgap (1.96 eV.) From here the central i region started, maintaining a constant bandgap for a certain length until the bandgap starts increasing again. This marks the 3rd region called i-n, where the bandgap continues to increase over a certain width until reaching 2.1 eV at the beginning of the n-layer.



Graded experimental cells results showed that it was beneficial to have a small graded region width in the p-i and i-n region (10-30 nm). Since with this grading length J_SC could be increased significantly (8% increase) from reference cell values without grading performed. However a small compromise in a drop of V_OC and FF values around (1-2%) was observed at the same time from reference cell values. The gains in J_SC are bigger than the loss of V_OC x FF product and result in a relative efficiency gain of (4-5%) from reference cell. This technique paired with other cutting edge techniques to increase photocurrent can lead to a new record for an a–SiO_x:H thin film solar cell. Or it can lead to better current matching in tandem or triple junction solar cells.