Molybdenum oxide and molybdenum oxide-nitride back contacts for CdTe solar cells

Abstract

Molybdenum oxide (MoOx) and molybdenum oxynitride (MoON) thin film back contacts were formed by a unique ion-beam sputtering and ion-beam-assisted deposition process onto CdTe solar cells and compared to back contacts made using carbon–nickel (C/Ni) paint. Glancing-incidence x-ray diffraction and x-ray photoelectron spectroscopy measurements show that partially crystalline MoOx films are created with a mixture of Mo, MoO2, and MoO3 components. Lower crystallinity content is observed in the MoON films, with an additional component of molybdenum nitride present. Three different film thicknesses of MoOx and MoON were investigated that were capped in situ in Ni. Small area devices were delineated and characterized using current–voltage (J-V), capacitance–frequency, capacitance–voltage, electroluminescence, and light beam-induced current techniques. In addition, J-V data measured as a function of temperature (JVT) were used to estimate back barrier heights for each thickness of MoOx and MoON and for the C/Ni paint. Characterization prior to stressing indicated the devices were similar in performance. Characterization after stress testing indicated little change to cells with 120 and 180-nm thick MoOx and MoON films. However, moderate-to-large cell degradation was observed for 60-nm thick MoOx and MoON films and for C/Ni painted back contacts.

Band Alignment for Rectification and Tunneling Effects in Al2O3 Atomic-Layer-Deposited on Back Contact for CdTe Solar Cell

“…Although Si-based solar cell devices currently dominate photovoltaic power production, thin-film solar cell devices based on CdTe have achieved a notable market share as a cost-effective alternative. − For thin-film CdTe solar cells, efficiency improvement is always the main goal. Generally, the common methods for improving efficiencies include designing new device structures, − improving the material composition and related crystalline quality of thin films, , and optimizing the contact interfaces between thin films. − The back contact is a key issue for the performance of CdTe thin-film solar cells because it is challenging to attain low resistance and a stable electrical contact. − Many efforts have been devoted to optimize the back contact based on physical concepts and nanoengineering. − …”

Back contacts materials used in thin film CdTe solar cells—A review

“…MoO 2 is expected to have a higher work function and more metallic conduction than MoO 3 . Drayton (2015)159 also found that low-temperature deposition MoO x deposition gave MoO 2 XPS peak positions. Ta 2 O 5 has been successfully used as a high resistance layer between CdS and CdTe…”

CdTe Photovoltaics in Superstrate Geometry–Back-Contact Materials: A Review

“…Utilizing metal nitride-based back contacts is still in the infant stage, and an advantage over well-established back contacts is yet to be achieved. The ZrN-based back contact achieved a PCE of 1.68% with a fill factor of 27.83% and an efficiency of 11.9% with a 120-nm-thick layer of MoN x with a fill factor of 68.6% …”