| AVS 60th International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Monday Sessions |
| Session EN+PS+TF-MoM |
| Session: | Thin Film, Organic, and Chalcogenide Solar Cells |
| Presenter: | J.J. Li, Colorado School of Mines |
| Authors: | J. Beach, Colorado School of Mines C.A. Wolden, Colorado School of Mines J.J. Li, Colorado School of Mines |
| Correspondent: | Click to Email |
Development of plasma-enhanced vapor transport deposition of CdTe absorbers
Jiaojiao Li, Joseph Beach, and Colin A Wolden
Colorado School of Mines, Golden, CO 80401
Cadmium telluride is a leading absorber material for thin-film solar cells. However, state-of-the art open circuit voltages (Voc) of CdTe thin film solar cells fall ~350 mV below the value expected based on the band gap of CdTe. Voc is mainly compromised by the structural defects, low carrier density and low recombination lifetime. Conventional methods to eliminate defects require the use of high temperature deposition or annealing processes which are incompatible with low cost soda-lime glass. The low hole density is because CdTe is difficult to dope due to numerous self-compensation mechanisms. CdCl2 treatment is always used to passivate the grain boundaries and increase carrier lifetime, but additional Cd may induce Te vacancies. In this work we introduce plasma-enhanced vapor transport deposition (PEVTD) as an alternative technique for thin film CdTe synthesis. The viability of vapor transport deposition (VTD) is demonstrated by First Solar's use of this approach to produce both the lowest cost modules. Plasma deposition techniques are able to introduce energy to films at low thermal budget. PEVTD is a hybrid of these two technologies, and we expect it to also be amenable to scale-up. In this method, high energy electrons dissociate Te2 dimers into reactive Te species, so a high quality Te rich film may be achieved at low temperature. Moreover, plasma enhancement may activate group V dopants to dope CdTe at a higher concentration. Finally, active gases such as H2S or SO2 are added to the carrier gas stream to passivate interface defects and increase minority carrier lifetime. In this talk we discuss the design, construction, and operation of this reactor. We describe the dependence of the CdTe film quality on the kind of gases we use, plasma power, and substrate temperature. Films are characterized using XRD, SEM, UV-Vis-NIR spectrophotometry, Hall and light and dark current-voltage measurements.