Effects of oxidation state on photovoltaic properties of reactively magnetron sputtered hole-selective WO _x contacts in silicon heterojunction solar cells

Abstract

The stoichiometry value x of WO x , or its oxidation state, is crucial for improving performances of the hole-selective contact heterojunction silicon solar cell. However, it is challenging to tune the films' oxidation state using the well-known evaporation method. In this study, a simulation was performed to analyze the effect of x on short-circuit current (J sc ) loss, attributed to the holeselective contact in the device. Compared to the thickness of WO x layer, x has a more important role in minimizing J sc loss. Based on the simulation, the WO x /c-Si heterojunction solar cells having hole-selective WO x contacts with tuned x to vary its oxidation state were fabricated using reactive magnetron sputtering. The relationships of the open-circuit voltage (V oc ) and J sc with respect to x were similar. The experimentally determined J sc increased from 34.7 to 36.6 mA cm −2 when x was increased from 2.72 to 2.77; this result is consistent with the simulation. Nevertheless, fill factor (FF) reduced with the increase of x, owing to the reduced conductivity of WO x . Both oxidation state and film conductivity must be as high as possible to simultaneously achieve high V oc , J sc , and FF. The lowest x yielded a solar cell efficiency of 13.3%.

Carrier‐selective contacts using metal compounds for crystalline silicon solar cells

“…Sputtered WO 3-x also provides lower V oc and FF compared with the thermally evaporated one. 195,207,224 In the lat- which results in a high work function, a result also reported in a previous study. 225 Again, strong conclusions are hard to make due to the limited amount of published studies on the different ways of depositions, yet forming good contacts with WO 3-x and V 2 O 5-x seems less particular to thermal evaporation compared to the use of MoO 3-x .…”

“…The same tendency is observed for WO 3‐x and V 2 O 5‐x with a predominance of thermal evaporation and few results with alternative deposition techniques. Sputtered WO 3‐x also provides lower V oc and FF compared with the thermally evaporated one 195,207,224 . In the latter work, Mews et al reached 16.6% efficiency and a V oc around 700 mV with front‐contact sputtered WO 3‐x and (i)a‐Si:H passivation layer.…”

Carrier‐selective contacts using metal compounds for crystalline silicon solar cells

“…Sputtered WO 3-x also provides lower V oc and FF compared with the thermally evaporated one. 195,207,224 In the lat- which results in a high work function, a result also reported in a previous study. 225 Again, strong conclusions are hard to make due to the limited amount of published studies on the different ways of depositions, yet forming good contacts with WO 3-x and V 2 O 5-x seems less particular to thermal evaporation compared to the use of MoO 3-x .…”

“…The same tendency is observed for WO 3‐x and V 2 O 5‐x with a predominance of thermal evaporation and few results with alternative deposition techniques. Sputtered WO 3‐x also provides lower V oc and FF compared with the thermally evaporated one 195,207,224 . In the latter work, Mews et al reached 16.6% efficiency and a V oc around 700 mV with front‐contact sputtered WO 3‐x and (i)a‐Si:H passivation layer.…”

Investigation of H2 Plasma Incorporated ALD-TiOx Films as Hole-Selective Passivating Contacts in Crystalline Silicon Solar Cells

“…Compared with other metal oxides which have also been studied as hole-selective contacts for c-Si solar cells including MoO x , NiO x , Cu 2 O, and WO x , the TiO x -based heterojunction proposed in this work shows a J 0 around 45 fA/cm 2 in the best case, placing it between MoO x /a-Si­(i) and MoO x /AlO x structures in terms of passivation quality but with a better thermal stability. , Unlike other metal oxides such as NiO x , Cu 2 O, and WO x which struggle with poor passivation quality with J 0 values often reported in the range of hundreds to thousands of fA/cm 2 , − this work highlights the TiO x -based heterojunction’s favorable balance of performance and stability.…”

Spin Solar Cell Phenomenon on a Single-Molecule Magnet (SMM) Impacted CoFeB-Based Magnetic Tunnel Junctions

“…We also considered the possibility of the MTJMSD bottom electrode becoming oxidized during the fabrication state and subsequently aging with time. According to the literature, oxidized metals such as Cu 2 O, WO x , and FeO can produce the photovoltaic effect and have been used in forming complete solar cells. ,, Oxidized metals’ optical properties, such as light reflection scattering and absorption, can vary significantly from their bare metal counterparts. , Our second photolithography step consists of coating the bottom electrode with a photoresist and then curing the photoresist at 90 °C. Baking of a photoresist-covered bottom electrode with a CoFeB layer on top could cause the CoFeB layer to oxidize and change the bottom electrode’s electric and magnetic properties.…”

“…According to the literature, oxidized metals such as Cu 2 O, WO x , and FeO can produce the photovoltaic effect and have been used in forming complete solar cells. 11,20,21 Oxidized metals' optical properties, such as light reflection scattering and absorption, can vary significantly from their bare metal counterparts. 22,23 Our second photolithography step consists of coating the bottom electrode with a photoresist and then curing the photoresist at 90 °C.…”

Carrier‐selective contacts using metal compounds for crystalline silicon solar cells

“…Sputtered WO 3-x also provides lower V oc and FF compared with the thermally evaporated one. 195,207,224 In the lat- which results in a high work function, a result also reported in a previous study. 225 Again, strong conclusions are hard to make due to the limited amount of published studies on the different ways of depositions, yet forming good contacts with WO 3-x and V 2 O 5-x seems less particular to thermal evaporation compared to the use of MoO 3-x .…”

“…The same tendency is observed for WO 3‐x and V 2 O 5‐x with a predominance of thermal evaporation and few results with alternative deposition techniques. Sputtered WO 3‐x also provides lower V oc and FF compared with the thermally evaporated one 195,207,224 . In the latter work, Mews et al reached 16.6% efficiency and a V oc around 700 mV with front‐contact sputtered WO 3‐x and (i)a‐Si:H passivation layer.…”

Investigation of H2 Plasma Incorporated ALD-TiOx Films as Hole-Selective Passivating Contacts in Crystalline Silicon Solar Cells

“…Compared with other metal oxides which have also been studied as hole-selective contacts for c-Si solar cells including MoO x , NiO x , Cu 2 O, and WO x , the TiO x -based heterojunction proposed in this work shows a J 0 around 45 fA/cm 2 in the best case, placing it between MoO x /a-Si­(i) and MoO x /AlO x structures in terms of passivation quality but with a better thermal stability. , Unlike other metal oxides such as NiO x , Cu 2 O, and WO x which struggle with poor passivation quality with J 0 values often reported in the range of hundreds to thousands of fA/cm 2 , − this work highlights the TiO x -based heterojunction’s favorable balance of performance and stability.…”

Spin Solar Cell Phenomenon on a Single-Molecule Magnet (SMM) Impacted CoFeB-Based Magnetic Tunnel Junctions

“…We also considered the possibility of the MTJMSD bottom electrode becoming oxidized during the fabrication state and subsequently aging with time. According to the literature, oxidized metals such as Cu 2 O, WO x , and FeO can produce the photovoltaic effect and have been used in forming complete solar cells. ,, Oxidized metals’ optical properties, such as light reflection scattering and absorption, can vary significantly from their bare metal counterparts. , Our second photolithography step consists of coating the bottom electrode with a photoresist and then curing the photoresist at 90 °C. Baking of a photoresist-covered bottom electrode with a CoFeB layer on top could cause the CoFeB layer to oxidize and change the bottom electrode’s electric and magnetic properties.…”

“…According to the literature, oxidized metals such as Cu 2 O, WO x , and FeO can produce the photovoltaic effect and have been used in forming complete solar cells. 11,20,21 Oxidized metals' optical properties, such as light reflection scattering and absorption, can vary significantly from their bare metal counterparts. 22,23 Our second photolithography step consists of coating the bottom electrode with a photoresist and then curing the photoresist at 90 °C.…”

Carrier‐selective contacts using metal compounds for crystalline silicon solar cells

“…Sputtered WO 3-x also provides lower V oc and FF compared with the thermally evaporated one. 195,207,224 In the lat- which results in a high work function, a result also reported in a previous study. 225 Again, strong conclusions are hard to make due to the limited amount of published studies on the different ways of depositions, yet forming good contacts with WO 3-x and V 2 O 5-x seems less particular to thermal evaporation compared to the use of MoO 3-x .…”

“…The same tendency is observed for WO 3‐x and V 2 O 5‐x with a predominance of thermal evaporation and few results with alternative deposition techniques. Sputtered WO 3‐x also provides lower V oc and FF compared with the thermally evaporated one 195,207,224 . In the latter work, Mews et al reached 16.6% efficiency and a V oc around 700 mV with front‐contact sputtered WO 3‐x and (i)a‐Si:H passivation layer.…”

Investigation of H2 Plasma Incorporated ALD-TiOx Films as Hole-Selective Passivating Contacts in Crystalline Silicon Solar Cells

“…Compared with other metal oxides which have also been studied as hole-selective contacts for c-Si solar cells including MoO x , NiO x , Cu 2 O, and WO x , the TiO x -based heterojunction proposed in this work shows a J 0 around 45 fA/cm 2 in the best case, placing it between MoO x /a-Si­(i) and MoO x /AlO x structures in terms of passivation quality but with a better thermal stability. , Unlike other metal oxides such as NiO x , Cu 2 O, and WO x which struggle with poor passivation quality with J 0 values often reported in the range of hundreds to thousands of fA/cm 2 , − this work highlights the TiO x -based heterojunction’s favorable balance of performance and stability.…”

Spin Solar Cell Phenomenon on a Single-Molecule Magnet (SMM) Impacted CoFeB-Based Magnetic Tunnel Junctions

“…We also considered the possibility of the MTJMSD bottom electrode becoming oxidized during the fabrication state and subsequently aging with time. According to the literature, oxidized metals such as Cu 2 O, WO x , and FeO can produce the photovoltaic effect and have been used in forming complete solar cells. ,, Oxidized metals’ optical properties, such as light reflection scattering and absorption, can vary significantly from their bare metal counterparts. , Our second photolithography step consists of coating the bottom electrode with a photoresist and then curing the photoresist at 90 °C. Baking of a photoresist-covered bottom electrode with a CoFeB layer on top could cause the CoFeB layer to oxidize and change the bottom electrode’s electric and magnetic properties.…”

“…According to the literature, oxidized metals such as Cu 2 O, WO x , and FeO can produce the photovoltaic effect and have been used in forming complete solar cells. 11,20,21 Oxidized metals' optical properties, such as light reflection scattering and absorption, can vary significantly from their bare metal counterparts. 22,23 Our second photolithography step consists of coating the bottom electrode with a photoresist and then curing the photoresist at 90 °C.…”