Substrate configuration, bifacial CdTe solar cells grown directly on transparent single wall carbon nanotube back contacts

Abstract

Substrate configuration CdTe/CdS solar cells were fabricated directly on a 25 nm thick single wall carbon nanotube (SWCNT) back contacts using a high temperature, commercial deposition process. No copper was introduced to facilitate the formation of an ohmic back contact at the back of the device. The champion device had an efficiency of 6.5%. The thin SWCNT layer allowed the device to be illuminated through the back contact, and a comparison of the device performance with front and backside illumination revealed several novel features of the SWCNT back contact. Our findings indicate that SWCNT films may be useful as a recombination layer contact for bifacial solar cells, photovoltaic windows, or two-terminal thin-film tandem solar cells.

Understanding and Advancing Bifacial Thin Film Solar Cells

“…Due to the potential for a significant increase in energy yield, there has been substantial interest in developing bifacial devices. Most of this work has been on high efficiency crystalline Si devices, which are now deployed in the market, but research into bifacial thin film (BTF) PV has begun. − In Si PV devices, a long bulk carrier lifetime (hundreds of microseconds to milliseconds) , and long diffusion length coupled with low surface recombination velocities (SRVs; ∼10 cm s –1 ) means that carriers can reach the carrier selective contacts before recombining, regardless of the direction of the incoming light. When paired with the development of transparent back contacts, bifacial Si devices efficiencies of 19.5–22% and 17–19% have been achieved when illuminated through the front and rear of the device, respectively .…”

“…Recent improvements in the carrier lifetime with Se incorporated into CdTe , and companion reports of diffusion lengths much greater than the typical device thickness suggest that it should be possible to fabricate high performance BTF CdTe devices. To date, however, understanding of the deficiencies of bifacial thin film devices has been limited to generalizations about absorber thickness , or analysis of the PV parameters . Recent modeling indicates that a highly doped back buffer layer or reduction in back surface recombination velocity (BSRV) can lead to improved device performance for bifacial CIGS, but there is still a lack of general understanding.…”

“…This configuration has been shown to produce very low front surface recombination currents that can be neglected for all conditions examined here. , The back buffer layer was chosen with a valence band 0.2 eV higher in energy than the valence band of CdTe (valence band offset of −0.2 eV) and a bandgap of 3.3 eV to be a transparent contact. Typically, in BTF devices, an n-type transparent conducting oxide (TCO) is deposited at the back of the device. − However, implementation of a traditional n-type TCO on both sides of the device is not permitted in the SCAPS software, so the bands between the buffer and transparent back contact were fixed to be flat. While the effects of the n-type TCO at the back of a p-type absorber and buffer on the band diagram are not well-understood in BTF devices, we expect this to induce downward band-bending in the buffer at the buffer/TCO interface.…”

“…We first investigated the performance limitations of the best reported BTF devices with back side illumination by considering the band diagrams and the controlling recombination currents. , To understand previously fabricated devices, we used values that are representative of the materials properties of those devices. The initial carrier lifetimes were set to 1 ns .…”

Assessment of Optimum Installation and Power Injection Parameters for a Bifacial Rooftop System

“…A prominent target for bifacial dyesensitized photovoltaic cells has been to pursue high energy transformation efficiency without compromising effectiveness or productivity concerning its cost [6 -9]. GaAs thin film [10][11][12] and CdTe [13][14][15][16][17][18][19][20][21] are increase attention as they provide a technically and economically convincing alternative concept. The weight of bifacial photovoltaic is considered too low, flexible, and semi-transparent.…”