| AVS 66th International Symposium & Exhibition | |
| Chemical Analysis and Imaging Interfaces Focus Topic | Friday Sessions |
| Session CA+AS+NS+SE+SS-FrM |
| Session: | Novel Applications and Approaches in Interfacial Analysis |
| Presenter: | Deborah McGott, Colorado School of Mines |
| Authors: | D.L. McGott, Colorado School of Mines C.L. Perkins, National Renewable Energy Laboratory W.K. Metzger, National Renewable Energy Laboratory C.A. Wolden, Colorado School of Mines M.O. Reese, National Renewable Energy Laboratory |
| Correspondent: | Click to Email |
Thin film solar cells, such as cadmium telluride (CdTe) and Cu(In,Ga)Se2 (CIGS), contain buried interfaces that are critical to carrier transport, recombination, and device performance, yet are poorly understood due to their inaccessibility within the device stack. In particular, accessing the interface in a way that preserves the chemical structure has historically been extremely difficult. Here, we describe an innovative technique to expose buried interfaces through a two-step thermo-mechanical cleaving process. First, a stressor layer (typically an epoxy or commercially available polymeric backsheet) is applied to the solar cell. Then, the stack is submerged in a cold bath (T ≤ -30˚C) to thermally shock the system. This causes the stressor to contract quickly and pull the polycrystalline film cleanly off of its substrate at an interface that is weakened by a monolayer accumulation of 2-D material (CdCl2 in CdTe and MoS2 in CIGS) .
Focusing on CdTe solar cells, we then use X-ray photoelectron spectroscopy to probe the oxidation states at the newly exposed SnO2 interface. We show that the tin oxide front electrode promotes the formation of nanometer-scale oxides of tellurium and sulfur. Most oxidation occurs during CdCl2/O2 activation. Surprisingly, we show that relatively low-temperature anneals (180−260°C) used to diffuse and activate copper acceptors in a doping/back contact process also cause significant changes in oxidation at the front of the cell, providing a heretofore missing aspect of how back contact processes can modify device transport, recombination, and performance. Device performance is shown to correlate with the extent of tellurium and sulfur oxidation within this nanometer-scale region. Mechanisms responsible for these beneficial effects are proposed.