Cadmium telluride is currently one of the most promising materials for high efficiency, low-cost thin film solar cells. CdTe is a direct gap semiconductor with the nearly optimum band gap (1.4 eV) for photovoltaic solar energy conversion. Cadmium sulfide is a commonly used heterojunction window layer in CdTe solar cells due to its similarity to CdTe in electron affinity. Transparent-conductive-oxide (TCO) front electrode is also studied to enlarge the active area for terrestrial application of CdTe solar cells. Therefore thin film solar cells with a TCO/CdS/CdTe structure have been in the spot light for high efficiency low cost solar cell. High optical transmittance and no pinholes are required for CdS. Low bulk resistivity $(＜10^4Ωㆍcm)$ and compact microstructure with large grain size are required for CdTe films.
In this work, CdS films were prepared by a chemical bath deposition (CBD) method on ITO/glass substrate, where the thickness of ITO layer was 150 nm. The CBD solution contained $Cd(COOH)_2$thiouria=0.01 M/0.02 M and $NH_4COOH$=0.02 M. The pH and solution temperature were 12 and 90℃, respectively. Then, CdS films were annealed in $H_2$ for 30 min.
10㎛ thick CdTe films were deposited on CdS/ITO/glass substrates by close spaced sublimation (CSS) with screen printed CdTe layer. A screen printed CdTe layer on glass substrate was proposed as a new CSS source, which could lower source cost and provide a reproducible composition. Screen printed source can be easily prepared from the mixture of Cd and Te. By using these films, Ag/Carbon /CdTe/CdS/ITO/glass thin film solar cells were fabricated and photovoltaic properties were investigated.
In the CSS process, the condition of substrate temperature played an important role in determining the properties of CdTe films and the parameters of solar cells. The grain size and surface morphology were strongly enhanced by increasing substrate temperature. In addition, higher substrate temperature led to improved cell efficiency up to 6.1 % with active area of 0.5 ㎠ due to increased short circuit current $(J_sc)$ and fill factor. However, the open-circuit voltage $(V_oc)$ decreased above 600℃ due to leakage current.
To obtain large grain size and compact microstructure, CdTe films were initially deposited at high substrate temperature (620℃) and then deposited at lower temperature (540℃). After the two step deposition process, the films were annealed at 620℃ for 5 min. The photovoltaic cell efficiency was improved to 7.2 % with active area of 0.5 ㎠ under 100 mW/㎠. The CdTe cell fabricated on the CdS film with larger grains and higher transmittance further increased the efficiency up to 7.8 %.
Since the bulk resistivity of CdTe was about $3×10^4Ωㆍcm$, the 10㎛ thick CdTe film approximately provided the series resistance of 30Ωㆍ㎠, which should be lowered. The bulk CdTe thickness was controlled by changing the deposition time during CSS to examine the effect of CdTe film thickness. The cell efficiency of active area of 1 ㎠ increased from 4 % to 6 % with decreasing the thickness of CdTe from 10 to 5-6 ㎛. The thickness decrease of CdTe film played a role in reducing the series resistance. However, fabrication of uniform CdTe film below 4㎛ remains to be solved.
As the active area increased from 0.25 to 1 ㎠, the efficiency of CdS/CdTe cells degraded to 4 % due to the increase of series resistance. $_oc$ did not degrade with large area, suggesting junction property was not degraded.
The thickness effect and area effect suggest the origin of series resistance come from both the CdTe bulk resistivity and the lateral resistivity of ITO film. For further experiment ITO resistance should be reduced first. And then further reduction of CdTe thickness and the application of new back contact materials should be considered for high $J_sc$ and high efficiency.
