Publication date: 4th October 2024
In this work we show the results obtained in the manufacture of bifacial semitransparent solar cells based on hydrogenated amorphous silicon (a-Si:H).
Amorphous silicon solar cells were studied for many years (1990) due to their ease of being deposited over a large area and at moderate temperatures (<200 C). Due to their limited efficiency (record 14% according to NREL chart) they were no longer investigated. However, a-Si:H technology continued to be used for the manufacture of TFTs. These solar cells used ETL and HTL based on dopant compounds such as diborane and phosphine, which resulted in appreciable optical absorptions.
In recent years, the introduction of alternative ETL and HTL, and the possibility offered by a-Si:H to fabricate semi-transparent solar cells over a large surface area, has led to a-Si:H being studied again for photovoltaic integration applications. Furthermore, a-Si:H has a gap of 1.8 eV and can be increased by incorporating carbon (a-SiC:H).
In this work we show the results of semitransparent cells with a bifacial structure using a-Si:H as an absorber. Two geometries have been studied: glass/FTO+AZO/HTL/a-Si:H/ETL/AZO and glass/FTO+AZO/ETL/a-Si:H/HTL/AZO. MoOx (thermally evaporated) and VOx (deposited by atomic layer deposition (ALD)) have been used as hole-selective layers (HTL), while phosphine-doped a-SiC:H has been used as electron-selective layer (ETL). The amorphous silicon layers have been deposited using plasma-enhanced chemical vapor deposition (PECVD). The AZO layer was deposited by ALD.
This innovative approach led to a Power Conversion Efficiency (PCE) above 2%, Voc=830 mV and an Average Photopic Transmittance (APT) of 52%, resulting in a Light Utilization Efficiency (LUE) of 1.1%. Device characterization includes optical spectrophotometry, J-V measurements under standard test conditions (AM1.5G solar spectrum 100 mWcm-2, T=25ºC), Spectral Response measurements and STEM-EDX cross-sectional measurements. The bifacial behavior of this architecture is highlighted, demonstrating a bifacial factor close to unity after measuring J-V characteristics from both sides of the device. This bifacial study is complemented with spectral response measurements from both sides. Additionally, J-V measurements done with a white reflective surface on the backside of the devices further increase the PCE of devices up to 2.9%, which is an interesting feature when considering the use of these devices on alternative substrates such as ceramics or metals, for example.
Additionally, the ability of a-Si:H cells to maintain stability over time, along with their suitability for deposition on large surfaces and their controllable bandgap, makes this technology attractive for integration into tandem structures with other semitransparent cell technologies.
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