Progress and Challenges in Absorber and Interface Fabrication of Polycrystalline CdTe Photovoltaics
Polycrystalline thin-film CdTe device efficiency of over 19% has been achieved at Colorado State University using a highly repeatable and scalable sublimation process. Record efficiency for research scale CdTe photovoltaics has been reported to be over 22% and highest reported module efficiency is over 18% by First Solar Inc. Use of Se alloying with CdTe to form a lower band-gap CdSexTe1-x (CST) alloy has led to higher devices efficiencies with better current collection. Recent materials characterization has revealed that Se may be responsible for more than just reduction of band-gap. With CdCl2 treatment Cl is known to very effectively passivate defects at CdTe grain boundaries. Recent materials and electrical characterization have shown that Se has a very similar behavior to Cl at the grain boundaries of polycrystalline CdTe. These include Se decorating the grain boundaries like Cl in the diffusion region between CST and CdTe while cross-section cathodoluminescence maps show substantially higher signal intensity in regions with Se. Double heterostructures fabricated using CdSeTe with Al2O3 as a passivating oxide have been measured to have several hundred nanosecond bulk recombination lifetime. These films were also measured to have surface recombination velocity of under 100 cm/s. These results among others emphasis that Se also helps in passivation of grain boundaries. High absorber quality may help in mitigating the voltage deficit that has traditionally been the largest limiting factor in CdTe photovoltaics in achieving efficiency closer to Shockley-Queisser limit. In addition to graded absorber devices and double heterostructures, replacing the traditional Cu doping with more advanced group V doping is an important research effort that maybe critical for future improvements in device efficiency. Nitrogen, Phosphorous, Arsenic and Antimony doping has been investigated by several research groups. Advanced co-sublimation hardware developed at Colorado State University provides researchers with a unique capability of depositing films for group V doping with Cd overpressure. Such deposition under Cd overpressure causes formation of CdV which is known to be favorable for incorporation of group V dopant and activation. Utilizing the high absorber and interface quality material and higher doping, efforts are in progress to fabricate polycrystalline devices with open-circuit voltage over 1V and device efficiency over 24%.
- Research Organization:
- Colorado State Univ., Fort Collins, CO (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Solar Energy Technologies Office
- DOE Contract Number:
- EE0008557
- OSTI ID:
- 1670795
- Report Number(s):
- NoneES20.05.01
- Resource Relation:
- Conference: Materials Research Society Spring Meeting 2019
- Country of Publication:
- United States
- Language:
- English
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