Improving a Silicon Based Triple Junction Cell for Solar Water Splitting

Abstract

In the transition towards a renewable based energy supply, one of the main challenges is the intermittent nature of most renewable energy sources. In order to ensure a stable energy supply, different storage technologies attract a lot of research attention. For long term storage, using hydrogen as artificial fuel is a promising option. However, in order for hydrogen storage to be renewable, the hydrogen must be produced using renewable energy. A possible option is to use solar energy to drive the electrolysis of water. In this way, hydrogen is produced directly from sunlight by driving the redox reaction with a reversible potential of 1.23 V. However, due to overpotentials and possible losses a voltage of 1.6-2 V is needed form the used device to drive the reaction. Recently, a wafer based triple junction silicon solar cell was proposed to generate the voltage needed to drive the water splitting reaction. The cell uses an n-type crystalline silicon wafer as substrate, and has an a-Si/nc-Si/c-Si structure. Although this cell demonstrates the feasibility of this technology, the performance is not good enough yet to drive the water splitting reaction efficiently.
In this work, several ways to optimize the triple junction solar cell have been studied. First, optimising the tunnel recombination junctions improves the open circuit voltage of the device. For the tunnel recombination junction between the top a-Si and the middle nc-Si cell, this is done by using a combination of a thin n-SiOx layer and a thin n-a-Si layer. Apart from a small improvement in optical performance this configuration improves the conductivity of the junction and also slightly enhances charge separation and collection in both sub-cells. For the junction between the middle nc-Si and the bottom c-Si cell, the same result was achieved by increasing the doping in the n-layer. Subsequently, to improve current matching of the device, the nc-Si layer thickness can be increased to 3500 nm without sacrificing the electrical performance. In addition, optimizing the interface between the top p-nc-SiOx layer and the front transparent conductive oxide (TCO) also improves the electrical performance of the device. Moreover, it was found that using hydrogenated indium oxide as front TCO instead of indium doped tin oxide increases the current by decreasing the reflection in the wavelength region of the current limiting nc-Si middle cell by having reflection, and also increases the open circuit voltage of the device by 0.3 Volts. The final triple junction cell achieved an open circuit voltage of 1.93 V and a short circuit current density of 8.5 mA/cm2, with a fill factor of 0.65. With these improvements, it is estimated that a solar to hydrogen efficiency of 6.2 % can be reached by using an IrOx counter electrode.