| AVS 63rd International Symposium & Exhibition | |
| Thin Film | Tuesday Sessions |
| Session TF-TuA |
| Session: | Thin Film Photovoltaics |
| Presenter: | Diane K. Lancaster, University of Colorado, Boulder |
| Authors: | D.K. Lancaster, University of Colorado, Boulder H. Sun, University of Colorado, Boulder S.M. George, University of Colorado at Boulder |
| Correspondent: | Click to Email |
Zn(O,S) alloys are promising conduction band buffers for solar cells. The tunable conduction band of Zn(O,S) alloys improves electron transfer between electron absorber and electron transport materials. Zn(O,S) alloys are produced with atomic layer deposition (ALD) by alternating between ZnO ALD and ZnS ALD using diethylzinc (DEZ) and H2O or H2S as the reactants. However, controlling the composition of Zn(O,S) alloys is complicated by an efficient exchange reaction between H2S and ZnO to produce H2O and ZnS. This H2S + ZnO -> H2O + ZnS exchange reaction dramatically increases the sulfur content of the Zn(O,S) alloys. Because of this exchange reaction, the growth temperature and the H2S exposure have an effect on the Zn(O,S) alloy composition that is nearly equivalent to the ratio between the alternating numbers of ZnO ALD and ZnS cycles. For example, the exchange reaction is much more efficient at higher temperatures. Increasing the growth temperature from 100°C to 225°C changed the composition for films grown with an alternating sequence of 3 ZnO ALD cycles and one ZnS ALD cycle from 65% ZnS to 90% ZnS, respectively. To overcome the complexity of the exchange reaction, we have developed a new method of growing Zn(O,S) alloy films that avoids alternating between numbers of ZnO ALD and ZnS ALD cycles. This new growth method exposes the H2O and H2S simultaneously in sequence with the DEZ exposures. The simultaneous H2O and H2S exposures allow competition between the forward exchange reaction (H2S + ZnO -> H2O + ZnS) to produce ZnS and the reverse exchange reaction (H2O + ZnS -> H2S + ZnO) to recreate ZnO. Film composition using this method was determined by the mole fraction of the H2S in the dosing mixture. The bandgaps of the Zn(O,S) alloys were measured versus film composition. Composition determined the bandgaps of the Zn(O,S) alloys regardless of growth conditions. Controlling the Zn(O,S) band gap was also much more reproducible using the simultaneous H2O/H2S exposures. Bandgaps varied linearly from 3.06 eV to 3.39 eV for H2S mole fractions in the H2O/H2S dosing mixture from 0.03 to 0.39, respectively. We are now able to prepare tunable conduction band buffers for solar cell applications.