Development of emitter layers for cadmium selenide telluride-based solar cells

Abstract

CdTe-based solar cells are the leading thin-film photovoltaic technology due to the low material and manufacturing cost. Recent improvement in the current collection and device efficiency is achieved by replacing the conventional CdS emitter with more transparent magnesium zinc oxide (MgxZn1-xO; MZO) and alloying Se into CdTe absorber to form a graded CdSeyTe1-y (CST) layer. Both MZO and CST have tunable band gap by controlling Mg/Zn and Se/Te ratios, respectively. Furthermore, recent studies have focused on improving device performance and stability by replacing Cu with Group V (GrV) dopants. MZO is highly insulating, which is problematic for pairing it with emerging GrV doped CdTe-based absorbers that have substantially higher carrier concentration. This thesis focused on two research objectives: identifying the optimal band gap of MZO for CST-based devices and developing doped MZO emitters suitable for emerging GrV device architectures. Graded CST absorbers are formed by depositing a thin CdSe or CST bilayer followed by a relatively thick CdTe layer. A continuous graded Se layer is then formed by interdiffusion during the subsequent cadmium chloride treatment. The Se content and energy level of conduction band at the front interface varies with specific device processing conditions. MZO has been a promising emitter for CdTe and CST devices. MZO is fully transparent with tunable band gap to engineer a proper conduction band offset for emitter|absorber interface in CdTe-based solar cells. To date high efficiency devices have been reported using MZO|CST structure, but the optimal value remains unknown. MZO is nominally prepared by sputtering a single ceramic target, limiting exploration of composition to discrete values and the stability of the resulting MZO has been a concern. In this thesis reactive co-sputtering of MZO deposited was used to determine the optimal MZO composition for CST devices. The champion MZO|CST device obtained 19.5% efficiency using 3.70 eV MZO without anti-reflective coating, the highest value reported to date for this architecture. It also displayed insensitivity to the specific MZO composition (3.68 – 3.92 eV). This is attributed to the formation of oxygen vacancies in MZO driven by Se content in absorber. This hypothesis was supported by time-of-flight secondary-ion-mass-spectroscopy and filtered current-voltage measurements suggest that MZO improves its carrier concentration and conductivity during subsequent device processing. While independent of MZO composition, performance was highly dependent on the quality of the as-deposited MZO which was shown to be a complex function of the sputter ambient and target aging. The optimal MZO quality is achieved by operating in the compound mode of reactive sputtering but approaching the transition zone. As-deposited MZO by reactive co-sputtered is highly insulating (<1014 cm-3), which appears to be sufficient for the use with conventional Cu-doped absorbers with low carrier concentration (~1014 cm-3). Doping CdTe-based absorbers with GrV elements improves the carrier concentration of absorber (~1016 cm-3). Insulating MZO is problematic for these absorbers due to a diminished depletion zone within the absorber. To date researchers have been successfully fabricating degenerately doped GMZO films (ND> 1019 cm-3) at relatively low Mg content with a small variation in carrier concentration. Doping MZO with gallium (GMZO) is studied in this thesis to improve the variation in MZO properties for emerging CdTe-based absorbers. GMZO is reactively co-sputtered from Mg and Zn metal targets and a Ga2O3 ceramic target in Ar/O2 ambient. As-deposited GMZO is fairly insulating. Upon annealing in high vacuum (<10-4 Torr) at 500 °C, the Ga dopants are activated and develop semiconductor or degenerately doped conductivities in GMZO depending on the Ga content in the film. Similar to MZO, GMZO is fully transparent with adjustable band gap by controlling Mg/Zn ratio. In addition, the carrier concentration of GMZO can be controlled by Ga content. Co-sputtering GMZO can deposit both uniform and combinatorial libraries with both band gap and carrier concentration gradients. GMZO offers a wide range of band gap (3.3 – 4 eV) and donor concentration (1016-20 cm-3). This covers the ideal emitter properties of conduction band offset (+0.2 eV) and carrier concentration (1018 cm-3) for CdTe-based:GrV devices determined from simulation. The stability of these GMZO through CdTe device processing appears to be promising and this is a promising emitter for GrV devices.