Device Architecture for Next Generation CdTe PV (Final Report)
The overall project objective has been to enhance the device architecture of CdTe solar cells in ways that contribute to their performance without compromise of cost-effective manufacturability. The project contributors included James Sites, Jennifer Drayton, Alexandra Bothwell, Tao Song, Andrew Moore, Anna Wojtowicz, Pascal Jundt, Ramesh Pandey, and Darius Kuciauskas (NREL). The primary technical milestones focused on higher cell voltage and efficiency, and the significant contributions to the CdTe device structure, with full references in body of report, are as follows: Thin CdTe Absorbers. Two objectives for thinner CdTe cells were to have fully depleted absorbers to enhance the effectiveness of electron reflection at the back interface and to show that thinner cells with lower materials cost and production time were viable. In comparison to thicker cells with standard 3-µm CdTe, the thin ones also had well-behaved current-voltage characteristics, no reduction in voltage, and very respectable efficiencies: 15% with 1.0-µm CdTe and over 10% with 0.4 µm [Bothwell, Drayton, and Sites, IEEE J. Photovoltaics 10, 259 (2020)]. Te Layer to Enhance the Back Contact. A tellurium layer with a valence-band energy intermediate between the CdTe absorber and Ni used for the back contact was added between the two to replace the CdTe/metal barrier to holes with two smaller ones. This configuration, which is particularly important for the thinner cells, increased cell efficiency and allowed voltage above 1 volt at reduced temperatures [Song, Moore, and Sites, IEEE J. Photovoltaics 8, 293 (2018)]. Bilayer CdSeTe/Te Cells. The thin-absorber approach was extended with a bilayer of a CdSeTe alloy in front of CdTe, a strategy developed by GE and First Solar. Replacing the first 0.5 µm of CdTe with the lower band-gap CdSeTe produced cells with 2 mA/cm2 higher current, only a partial reduction in voltage, and a much larger photoluminescence response consistent with voltage being closer to its ideal value [Bothwell, Drayton, Jundt, and Sites, MRS Advances 4, 37 (2019)]. CdMgTe Layer for Electron Reflection. The purpose was to reflect electrons from the rear of the cell with a layer of CdMgTe, an alloy with the band gap expanded primarily in the conduction-band direction, to direct electrons towards the front for higher current and to reduce the hole barrier. This structure was partially successful; current and voltage were increased, but the voltage did not achieve the project milestone [manuscript led by Bothwell in preparation]. Front-Interface MgZnO Buffer. MgZnO had earlier replaced CdS at Colorado State because its higher band gap transmits shorter-wavelength photons and it has a favorable conduction-band offset with CdTe. Other labs, however, have had mixed results. The reason appears to be been low electron density, partially mitigated in some cases by photogeneration, but now seen to be better addressed by doping the MgZnO with Ga [Pandy, Shimpi, Munshi, and Sites, IEEE J. Photovoltaics, in press]. In addition to the specific cell results, there were two important development projects: The first was professional visual publication of the process for fabricating the cells [Bothwell, Drayton, Jundt, and Sites, J. Visualized Experiments 157 (2020)]. The second was a broader interpretation of time-resolved photoluminescence (TRPL) data by varying the voltage bias across a completed cell. In sufficient forward bias, the diode field is eliminated and the lifetime of the absorber can be reliably measured [manuscript led by Jundtin preparation]. During the course of the project, the research team maintained close contact the industrial CdTe community, especially First Solar, Inc., and with CdTe researchers at several other universities and at NREL. The results summarized above now appear in the published literature as noted above and have been presented at several conferences and workshops. Of the six students involved in the project, three (Song, Moore, and Bothwell) completed their Ph.D., and one (Wojtowicz) her thesis M.S. The other two (Jundt and Pandey) should complete the Ph.D. in 2021.
- Research Organization:
- Colorado State Univ., Fort Collins, CO (United States); National Renewable Energy Lab. (NREL), Golden, CO (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Solar Energy Technologies Office
- DOE Contract Number:
- EE0007543
- OSTI ID:
- 1765132
- Report Number(s):
- DE-EE0007543
- Country of Publication:
- United States
- Language:
- English
Similar Records
Photovoltaics Research and Development: Device Architecture for Next-Generation CdTe PV. Cooperative Research and Development Final Report, CRADA Number CRD-17-00662
Investigating Local Carrier Dynamics in PERC Patterned CdTe Solar Cells