Optical and electrical design guidelines of nanostructured CdTe solar cells

---

Tezin Türü: Yüksek Lisans

Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Fen Bilimleri Enstitüsü, Fen Bilimleri Enstitüsü, Türkiye

Tezin Onay Tarihi: 2018

Öğrenci: CAN ÖZCAN

Danışman: SELÇUK YERCİ

Özet:

Cadmium Telluride (CdTe) is a direct band gap material with an almost optimum band gap of 1.5 eV and has a high absorption coefficient at its band edge (>10^4 cm^-1). In the last decade, an alternative to the conventional planar structure of CdTe solar cells has been realized by coating the CdTe absorber around ZnO nanorods (NRs). The NRs serve as the front contact of the solar cells with radial charge collection and vertical light absorption at the same time, allowing efficient charge collection and enhanced light trapping properties. Prior to CdTe coating, a thin conformal interfacial CdS layer is usually deposited around NRs which provides a graded band alignment between CdTe and ZnO and passivation of CdTe surfaces. In the NR-based solar cells, the ZnO NR dimension, CdTe and CdS thicknesses, material and interfacial qualities (i.e doping density and surface recombination velocities) are determining factors in the optical absorption and charge collection. In this context, the effects of length, density and angular deviation of the NRs are analyzed through optical and electrical simulations for two types of CdTe solar cell structures, namely the extremely thin absorber (ETA) and buried. Optical simulations are validated in comparison with the experimental haze measurements of CdS/ZnO NRs. While ETA CdTe solar cell uses a thin (~40 nm) absorbing layer around CdS-coated ZnO NRs, CdTe fills the space between CdS-coated ZnO NRs in the buried solar cells. The short circuit current densities are calculated at various CdTe doping densities and CdS/CdTe surface recombination velocities by electrical simulations, and are compared to the conventional planar CdTe solar cells. It was shown that with a moderate doping density of 10^16 cm^-3 and relatively high CdS/CdTe surface recombination velocities of 10^4 cm/s, NR-based CdTe solar cells can outperform their planar counterpart by achieving Jsc values as high as 27 mA/cm^2. Characterization of CdTe thin films by secondary ion mass spectroscopy, Raman spectroscopies and scanning electron microscope demonstrates that doping densities required to boost the efficiency of NRbased CdTe solar cells can be achieved experimentally. Finally, measured solar cell efficiency of buried structures are also presented.