Pure sulfide kesterite solar cells with cation substituted absorber and back contact intermediate layer

• Main Author: Zhuk, Siarhei
• Other Authors: Wong Kin Shun, Terence
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics; Engineering::Electrical and electronic engineering::Semiconductors
• Online Access: https://hdl.handle.net/10356/138577
• Institution: Nanyang Technological University

Chalcogenide Cu(In,Ga)Se2 (CIGS) and CdTe thin film solar cells have the potential to reduce the cost of the photovoltaic (PV) technology. They showed high power conversion efficiency (PCE) of more than 20% shrinking the gap between them and champion crystalline Si solar cell. Despite of high efficiency and ease of fabrication, application of CdTe PV technology is limited on terawatt (TW) scale owing to scarcity of Te and because there is only one supplier. As for CIGS, a high demand for In from flat panel display industry along with its limited availability pushes the price of In up to several hundred USD/kg. Therefore, tremendous success of CIGS solar cells brought much attention to earth abundant, non-toxic and low cost kesterite Cu2ZnSnS4 (CZTS) where In and Ga are replaced with Zn and Sn. Similar electronic and optical properties of the materials enables CZTS solar cells to inherit the device structure of CIGS solar cells. CZTS is an intrinsic p-type semiconductor with a direct bandgap of about 1.5 eV and high absorption coefficient of >104 cm^(-1) for visible wavelengths. The bandgap of kesterite can be tuned from 1 eV to 1.5 eV by changing S/(S+Se) ratio from 0 to 1 which provides a degree of flexibility in device fabrication with the material. There is still large gap in PCE between the best chalcogenide and kesterite solar cells. Firstly, solid state reaction occurs at the back interface between CZTS absorber layer and Mo back contact during high temperature sulfurization. S diffuses into Mo to form resistive MoS2. In addition, as a result of the reaction, CZTS may decompose into secondary phases such as ZnS, Cu2S and volatile SnS causing void formation. Secondly, large open-circuit voltage deficit limits performance of the kesterite solar cells. Similar size of cations in CZTS facilitates cation disordering and formation of detrimental defects causing charge recombination. In this thesis, the effect of sputter of TaN, ZnS and CuO intermediate layers (IL) on the quality of the back interface, pure sulfide kesterite absorber layer and photovoltaic properties of the solar cells was studied in detail. The novel IL were used to slow down the back interface reaction and control crystal quality of kesterite absorber layer. It has been found that interfacial MoS2 thickness between sputtered Cu-poor CZTS and Mo can be effectively controlled using TaN IL. When 12 nm TaN IL is present, no continuous MoS2 is observed using scanning transmission electron microscopy after sulfurization at 600°C for 10 min. Incorporation of 10 nm ZnS IL at the interface between solution-processed Cd-substituted CZTS (CZCTS) and Mo enhances CZCTS grain growth and suppresses void formation. Moreover, deposition of the ZnS IL does not cause increase of MoS2 thickness. PCE of solution-processed CZCTS solar cells was enhanced from 9.5% to 10% as a result of 10 nm ZnS IL layer insertion. A higher PCE of 10.8% was achieved for solution processed CZCTS solar cells using CuO IL incorporation. Insertion of the optimized 4 nm CuO IL resulted in enhancement of short-circuit current density (Jsc) due to the increased width of depletion region while open-circuit voltage (Voc) and fill factor (FF) were almost constant. Moreover, further increase of CuO IL thickness enables to get as high Jsc as 28.5 mA/cm2. To the best of my knowledge, this is the highest reported Jsc for pure sulfide CZCTS solar cells. To address the problem of the absorber layer quality, impact of Mo on sputtered CZTS thin films was studied. Mo was found to be a dopant for CZTS enabling to tune CZTS resistivity in a wide range. In addition, it has been revealed that Mo incorporation enables to enhance absorbance of CZTS thin films while not affecting bandgap of CZTS based on measured external quantum efficiency spectra. Furthermore, Mo incorporation in CZTS demonstrates a substantial effect on the photovoltaic performance of the solar cells prepared by sputtering of quaternary compound CZTS target. When optimized Mo co-sputtering power is applied, PCE of CZTS solar cell is increased from 1.6% to 5.5%. In addition, effect of carbon (C) incorporation in sputtered CZTS and Cu doped ZnS (ZnS:Cu) thin films was investigated. It has been found that optimized content of C facilitates growth of CZTS crystallites and reduces void formation. Furthermore, the addition of C in sputtered CZTS and ZnS:Cu thin films enables reduction of tail states through defects passivation, decrease resistivity and tune bandgap. Moreover, formation of copper sulfide phase is suppressed while formation of ZnS is enhanced with increase of C content in p-type ZnS:Cu thin films resulting in increase of its bandgap from 3.2 eV to 3.6 eV.