Next Generation Solar Cells Based on Graded Bandgap Device Structures Utilising Rod-Type Nano-Materials

Abstract

Current solar cells under research and development utilise mainly one absorber layer limiting the photon harvesting capabilities. In order to develop next generation solar cells, research should move towards effective photon harvesting methods utilising low-cost solar energy materials. This will lead to reduce the $W −1 figure for direct solar energy conversion to electrical energy. In this work, a graded bandgap solar cell has been designed to absorb all photons from the UV, visible and IR regions. In addition, impurity PV effect and impact ionisation have been incorporated to enhance charge carrier creation within the same device. This new design has been experimentally tested using the most researched MOCVD grown GaAs/AlGaAs system, in order to confirm its validity. Devices with high Voc ~ 1175 mV and the highest possible FF ~ (0.85-0.87) have been produced, increasing the conversion efficiency to ~20% within only two growth runs. These devices were also experimentally tested for the existence of impurity PV effect and impact ionisation. The devices are PV active in complete darkness producing over 800 mV, Voc indicating the harvesting of IR radiation from the surroundings through impurity PV effect. The quantum efficiency measurements show over 140% signal confirming the contribution to PV action from impact ionisation. Since the concept is successfully proven, the low-cost and scalable electrodeposited semiconducting layers are used to produce graded bandgap solar cell structures. The utilisation of nano-and micro-rod type materials in graded bandgap devices are also presented and discussed in this paper. Preliminary work on glass/FTO/n-ZnS/n-CdS/n-CdTe/Au , showing the high potential of these devices in achieving higher efficiencies. The detailed results on these low-cost and novel graded bandgap devices are presented in a separate publication.

Electroplating of CdTe Thin Films from Cadmium Sulphate Precursor and Comparison of Layers Grown by 3-Electrode and 2-Electrode Systems

“…All possible device properties have been extracted from these I-V characteristics and summarized in Table 3. These excellent properties, including very high short circuit current densities, show the high potential of graded bandgap device structures [36,37]. These high efficiencies have been observed for CdTe materials grown using 2E systems.…”

Unravelling complex nature of CdS/CdTe based thin film solar cells

“…This idea was experimentally tested during the same year [73] using wellestablished metal organic vapour phase epitaxy (MOVPE) grown semiconductor system, GaAs/AlGaAs. Using only two growth runs, the experimental results demonstrated high device parameters such as V oc = 1175 mV, FF = 0.85 [73] and experimental evidence for impurity PV effect [74] and impact ionisation [75]. These devices were capable of producing V oc values in excess of 750 mV in complete darkness [74], demonstrating the harvesting of IR radiation from the surroundings.…”

“…Therefore, the electric currents produced in all three junctions add up to yield large short circuit current density. These devices are capable of harvesting IR radiation from the solar spectrum and from the surroundings, and enhances the number of charge carriers using impurity PV effect and impact ionisation [59,75].…”

Graded-Bandgap Solar Cells Using All-Electrodeposited ZnS, CdS and CdTe Thin-Films

“…This is possible due to the position of the major depletion region in the device (near the n-CdTe/Au interface) and the overall shape of the band diagram of the devices. Full description of next generation solar cells based on graded bandgap devices, and the reasons for observing high current densities are reported in [36]. These effects may not be as easy in a p-n junction-type device structure, primarily due to the position of the depletion region.…”