Suppression of interfacial oxygen vacancies for efficient charge extraction at CZTS/TiO2 heterojunction

Abstract

Earth abundant CZTS (Cu2ZnSnS4) absorber layers are promising for the development of cost-effective and large area photovoltaics; however, interfacial nonradiative recombination is a major obstruction to the pathways toward high performing CZTS devices. Elimination of interfacial recombination losses via interface engineering is paramount to obtain efficient CZTS solar cells. Herein, we report a systematic investigation of the influence of oxygen vacancies (OV) settled at the CZTS/TiO2 interface on the charge transfer rate in heterostructures. Modulation of OV by varying oxygen flow rate during TiO2 deposition was confirmed by x-ray photoelectron spectroscopy. Lower OV concentration shifted the conduction band offset from negative to positive at the CZTS/TiO2 heterojunction, which is essential for efficient charge transportation through the interface. Photoluminescence quenching of the CZTS/TiO2 heterojunction also showed a strong correlation between charge dynamics and OV at the interface. Finally, we found the fast decay response of photogenerated charge carriers for the CZTS/TiO2 device with lower OV strongly favors the suppression of carrier trapping at the interface. This work provides a critical insight into interface engineering in CZTS solar cells through regulating interfacial OV, particularly when an oxide electron transport layer is applied.

Interface Engineering of CZTS/TiO2 Heterojunction Using Wide‐Bandgap Ga2O3 Passivation Interlayer for Efficient Charge Extraction

“…A 1.5 μm-thick CZTS absorber layer was fabricated by magnetron sputtering technique using a seed layer approach. [22,24] A molybdenum (Mo) layer of %500 nm was deposited using DC sputtering source at 120 W for 20 min on cleaned soda lime glass (SLG) substrates. After that, initially, a seed layer of CZTS was deposited at a substrate temperature of 400 °C by cosputtering of Zn, Cu, and SnS 2 targets at 6 mTorr growth pressure with 20 sccm Ar flow, followed by deposition of ordinary CZTS at substrate temperature of 200 °C for 25 min.…”

“…X-ray diffraction and Raman spectra for CZTS film can be found in our previous literature. [7,22] Following sulfurization, the amorphous TiO 2 film was deposited by radio frequency (RF) sputtering at room temperature for 60 min at 60 W. For Ga 2 O 3 -passivated samples, Ga 2 O 3 was deposited prior to TiO 2 deposition by RF magnetron sputtering at 100 W for three different deposition times, that is, 30, 60, and 90 s. The deposition was carried out at room temperature at a deposition pressure of 3 mTorr. [25] Henceforth, samples were recognized as G:0, G:30, G:60, and G:90 for CZTS/TiO 2 heterojunctions according to Ga 2 O 3 deposition time.…”

“…Our previous reports provide clear evidence that TiO 2 can be a potential alternative buffer layer in CZTS solar cells. [7,21,22] It has been widely accepted by the researchers that metal oxide is a more promising buffer layer choice compared with nonoxide materials because they reduce the shunting paths between the absorber film and the top electrode and act as an excellent hole-blocking material. [22,23] The current investigation analyzed the electrical properties of passivated and nonpassivated CZTS/TiO 2 heterojunction so that this configuration can be used for PV or other optoelectronic device applications.…”

“…[7,21,22] It has been widely accepted by the researchers that metal oxide is a more promising buffer layer choice compared with nonoxide materials because they reduce the shunting paths between the absorber film and the top electrode and act as an excellent hole-blocking material. [22,23] The current investigation analyzed the electrical properties of passivated and nonpassivated CZTS/TiO 2 heterojunction so that this configuration can be used for PV or other optoelectronic device applications. An optimized thickness of Ga 2 O 3 thin film reduces the interfacial trap states and significantly quenches the photoluminescence (PL) intensity, which signifies the efficient charge separation and low recombination at CZTS/TiO 2 heterojunction.…”

Efficiency Improvement of Flexible Cu2ZnSn(S,Se)4 Solar Cells by Window Layer Interface Engineering

“…Kesterite Cu 2 ZnSn­(S,Se) 4 (CZTSSe) is a potential absorber material to replace a similarly structured chalcopyrite counterpart Cu­(In,Ga)­Se 2 (CIGS) due to its rich elemental composition, excellent optoelectronic properties, tunable band gaps, and higher theoretical efficiency. − Flexible CZTSSe solar cells have great application potentials such as in the field of building-integrated photovoltaic, portable equipment and indoor photovoltaic owing to their nontoxicity, excellent mechanical flexibility, and high stability. − Mo foil is a good flexible substrate for CZTSSe solar cells . It can reduce process cost without sputtering the Mo layer in the device preparation.…”