Bifacial amorphous Si quintuple‐junction solar cells for IoT devices with high open‐circuit voltage of 3.5V under low illuminance

Abstract

Hydrogenated amorphous Si(a-Si:H) quintuple-junction solar cells, which consist of a-SiO x :H/a-SiO x :H/a-Si:H/a-SiO x :H/a-SiO x :H, were fabricated by plasma CVD method. The total thickness was 0.6-0.8 μm. Irradiation intensity (Pin) dependence of the open circuit voltage (V oc ) of quintuple-junction solar cells was measured. The decreasing amount ΔV oc (1/10) of the open-circuit voltage when the irradiation intensity became 1/10 was 62mV/cell. Voc drops rapidly from around the irradiation intensity of 1mW/cm 2 (approximately 1,000 lux). This large Voc reduction is due to leakage current. Then, we discussed the origin of the leakage current, and, finally, by improving the leakage current, a very high open-circuit voltage V oc of 3.5 V was demonstrated under LED light illumination. Furthermore, we theoretically analyzed Voc as a function of the irradiation intensity, including effects of the leakage current and the film quality of i-a-Si(O):H. It was found from the simulation results that it is necessary to increase the shunt resistance Rsh and to lower the defect density of i-a-Si(O):H in order to obtain a sufficient Voc-Pin characteristics for IoT devices application under low illuminance. KEYWORDS amorphous silicon, quintuple-junction, solar cell, low illuminance 1 | INTRODUCTION Over the past decades, solar cell development for power applications has been vigorously promoted all over the world. As a result, the production of solar cells in the world in 2017 has reached over 100 GW. 1 Also, the application fields of solar cells are widely spread from smallscale residential use for kW level to large-scale Mega-solar over 100 MW. Recently, in view of new application, developments of photovoltaic system on the ocean, on roads, in mobilities, and in flying vehicles are expected. On the other hand, the Internet of Things (IoT) has caused a new technological paradigm shift around the world, and attention has been focused on solar cell development as a self-sustaining power source for application to IoT devices. 2-8 The IoT promises a vast network of interconnected devices and sensors to monitor the world. As more objects are embedded with wireless sensors and communication systems the challenge of powering devices not connected to an electrical-network has emerged. Energy harvesting has become a key enabler of IoT. Self-powered sensor systems use a variety of energysavings, such as solar energy using solar cells, mechanical energy and so on. As provided in ref. 2, solar cells are the easiest-to-use energy sources. The achieved reduction of the electric power demand for small electronic devices to the range of microwatts has led to the concept of "micro-energy harvesting".Looking at solar cells that have been put into practical use as power sources for consumer equipment, most are crystalline Si solar

Flexible bifacial amorphous Si quintuple‐ and sextuple‐junction solar cells for Internet of Things devices

“…In our previous research, a rapid decrease in V oc was observed in the low irradiance region of approximately 200 lux or less. 5 The main causes of the leakage current are the following three items.…”

“…The deposition conditions for each layer are shown in Konagai and Sasaki. 5 The first cell, the second cell, the fifth cell, and the sixth cell consist of wide bandgap a-SiO x :H (T sub = 120 C, E opt = 1.85 eV).…”

“…The effect of n-layer thickness has already been discussed. 5 On the other hand, although it is possible to completely isolate the cells by laser processing, etc., there is also a report that a new leakage current path occurs at the laser processing edge. 19 2.…”

“…A part of the analysis results of the effect of D-states density on V oc -log (I P ) characteristics of a-Si:H single-junction solar cells using AFORS-HET has already been described in Konagai and Sasaki. 5 Figure 11 shows the effect of N D and i-layer thickness on the irradiance dependence of V oc . According to this analysis result, it can be seen that there is no great effect at N D : 5 × 10 15 /cm 3 , which is known to be a good film quality, but when N D is 5 × 10 16 /cm 3 , the V oc decreases drastically from around P in of 1 mW/cm 2 .…”

“…Energy conversion of single-junction solar cells under low illuminance is described in detail by Russo et al 3 We have already developed quintuple-junction amorphous Si solar cells as a solar cell that can generate a high voltage of 3 V or more without an interconnection. 4,5 Since wiring for integration is not required, it is possible to easily manufacture a cell having an extremely small area of mm size and further a cell having a large area of several tens of cm 2 depending on the output required. The feature of quintuple-junction amorphous Si solar cells is that it is composed of 5-junctions based on amorphous Si(O):H material in order to obtain an operating voltage of 3 V or more and has a bifacial device structure capable of receiving light on both sides so that the bottom cell can be designed to be thin.…”