Comparison of Rear Surface Passivation Strategies via Surface Photovoltage Spectroscopy

Cadmium Telluride photovoltaics have advanced considerably over the past decade, achieving certified efficiencies greater than 22%. However, deficiencies in open current voltage continue to plague this architecture. A primary reason for this is incompletely passivated interfaces caused by surface defects and suboptimal band alignment with contacts, especially at the rear contact.
We will demonstrate the passivating effects of diamond like carbon on CdTe surface by means of surface photovoltage spectroscopy. The diamond like carbon films are produced using highly scalable pulsed DC sputter deposition methods, using earth abundant carbon.
The surface photovoltage spectroscopy technique is highly sensitive to defect states, especially at incident photon energies well above band gap, where the majority of character generation happens near the surface. We will show significantly higher surface photovoltage signal for photon energies well above band gap, compared to bare CdTe as well as to state of the art electron reflectors such as ZnTe. Other passivation strategies such as PTAA will also be evaluated.
Using full device modeling in SCAPS-1D, we will demonstrate the changes in the surface photovoltage spectrum are consistent with decreased surface recombination velocity and decreased surface potential barrier at the rear contact. By modeling a wide variety of conditions, we will build confidence windows key parameters such as surface recombination velocity, bulk lifetime and rear surface potential barrier height.