Cadmium Tin Oxide and Zinc Magnesium Oxide Prepared by Hollow Cathode Sputtering for CdTe Photovoltaics

Abstract

This work reports the fabrication and characterization of superstrate-type Zn1-xMgxO/CdTe heterojunction solar cells on both CdxSnyO and commercial SnO2:F transparent conducting oxides (TCOs) in which the ZMO and CTO layers are produced for the first time by hollow cathode sputtering. The sputtering is conducted in a reactive mode using metal or alloyed metal targets fitted to a custom-made linear cathode. It is notable that the CdS buffer layer conventionally employed in CdTe solar cells is entirely replaced by the ZMO window layer. The use of ZMO is found to eliminate the blue loss associated with CdS optical absorption and further results in a higher open-circuit voltage. Key parameters were found to be the conduction band offset at the ZMO/CdTe interface and the ZMO thickness. It was discovered that the ZMO exhibits intense photoluminescence even at room temperature. Most of the solar cells were fabricated in the FTO/ZMO/CdTe configuration although CTO/ZMO/CdTe solar cells were also demonstrated. The CTO was produced with an electron mobility of 46 cm2 V-1s-1 without any post-deposition annealing or treatment.

Eliminating S-Kink To Maximize the Performance of MgZnO/CdTe Solar Cells

“…In 2017, Delahoy et al reported the fabrication of ZMO/CdTe solar cells, but the devices showed unsatisfactory efficiencies (∼10.5%) accompanied by low fill factors (FF) due to the existence of an S-kink in the current density−voltage (J−V) curves. 11 In 2018, Bittau et al reported ZMO/CdTe solar cells, but their best-performing cell showed an efficiency of 13.5% with a low FF around 65%. 12 S-kink in J−V curves has been observed in other solar cells including organic, 14 copper indium gallium selenide (CIGS, without blue photons), 15 and crystalline silicon 16 cells.…”

“…15−17 For ZMO/CdTe solar cells, it is generally attributed to the high CBO spike at the ZMO/ CdTe interface. 11,18 Device modeling has shown that a high CBO spike, large interface recombination velocity, and low doping density in absorber and buffer layers can induce a significant barrier at the ZMO/CdTe interface that blocks electrons transferring from CdTe into fluorine doped tin oxide (FTO) electrode, causing the S-kink in the J−V curves. 6 However, keeping the CBO spike small and fully realizing the benefits of the CBO spike are challenging.…”

“…However, the success of fabricating high efficiency ZMO/CdTe solar cells has not been reproduced by other research groups, though significant efforts have been paid. − Bittau et al improved their CdS/CdTe solar cell performance from 8.2% to 10.5% when ZMO was deposited prior to the CdS layer . Wang et al obtained an efficiency of 13.1% and 14.7% for ZMO/CdS/CdTe and ZMO/CdS/CdSe/CdTe stack, respectively .…”

“…In all the studies, ZMO was used as a high resistive transparent layer rather than a buffer layer to improve their device performance. In 2017, Delahoy et al reported the fabrication of ZMO/CdTe solar cells, but the devices showed unsatisfactory efficiencies (∼10.5%) accompanied by low fill factors (FF) due to the existence of an S-kink in the current density–voltage ( J – V ) curves . In 2018, Bittau et al reported ZMO/CdTe solar cells, but their best-performing cell showed an efficiency of 13.5% with a low FF around 65% …”

“…S-kink in J – V curves has been observed in other solar cells including organic, copper indium gallium selenide (CIGS, without blue photons), and crystalline silicon cells. The S-kink is often attributed to the poor charge extract at the absorber/buffer interfaces. − For ZMO/CdTe solar cells, it is generally attributed to the high CBO spike at the ZMO/CdTe interface. , Device modeling has shown that a high CBO spike, large interface recombination velocity, and low doping density in absorber and buffer layers can induce a significant barrier at the ZMO/CdTe interface that blocks electrons transferring from CdTe into fluorine doped tin oxide (FTO) electrode, causing the S-kink in the J – V curves . However, keeping the CBO spike small and fully realizing the benefits of the CBO spike are challenging.…”