CBD CdS films: 550 ml 10M DI water, 8 ml aqueous cadmium acetate solution (0033M), and 47 ml aqueous ammonium acetate (10M). This solution is heated to 363K with the tin-oxide-coated glass substrates placed vertically into the bath at 353K to allow for thermal equilibrium. Thiourea (0067M) is dispensed into the Cd-containing bath at 2 ml every 5 min for a total of 8 ml. The substrates are immersed in this bath for 37 min upon which about 80 nm of CdS is deposited. Film thickness is varied by varying deposition time. 2. CdS growth methods 2.1, Chemical bath deposition (CBD)
Three steps: (i) nucleation, (ii) coalescence (iii) vertical growth. The effect of temperature to CBD CdS
CdS function: window layer; CdTe1xSx mixed-crystal layer; for the crystallinity of the CdTe layer. CdTe 1-x S x =F(d) P(111) and grain size of CdTe as function of d CdS Influence of CdS thickness
The optimal conditions for CBD–CdS: 75 with S(thio)/Cd(CdCl2) = 6 CdS window layers with a large sulfur vacancy band (at 1.8 eV) More sulfur vacancies decrease the minority carrier recombination at the CdS–CdTe interface and at the grain boundaries 80nm CdS is good Effect of CdS thickness and S/Cd ratio
CdS on surface treated ITO had a better transmission( 550nm ) CdS deposited on two types ITO: (1) an insulating ITO overcoat (2) an ion-irradiated. Influence of ITO surface
Result: Voc=858mv and Jsc=23mAcm2 with a fill factor (ff) of 74%, efficiency of 14.6%. Area:0.64 cm2 2.2, CdS by Sputtering Grain size of the CdS film after treatment depends strongly on the SnO2 film thicknes. Grain size(Na) of CdS decrease(less Na) with increase of SnO2 thickness(block Na diffuse from glass) low cost soda-lime glass as substrate The efficience is strongly dependent on the interaction among the different layers. CdS mixes very easily with CdTe at high deposition temperature of 500 and this can remove the mismatch and the interface energy states. This is due to the presence of a smooth transition through a mixed compound CdS 1X Te X.
CSS CdS: T substrates= 748K, d=5 mm,T source =923K, in 133 kPa Helium. The CdS powder consisted of pulverized 99999% CdS crystallites which are annealed in H 2 S prior to use to partially sinter the CdS source and thus minimize source spitting. Under these conditions, a CdS film of about 80 nm could be deposited in about 7 min. A consists large grains and more pinholes. B is denser and contains fewer pinholes. 2.3, CdS growth by CSS
CBD CSVT Both samples has 90% above 550 nm S/Cd=5 by CBD and the best CdS by CSVT Compare CBD with CSVT
CBD (top 40–50 nm) and CSS (bottom 100–300 nm) CdS films. (grain size and structure, pinholes)
CdS by CBD can make a very compact film, but CBD is not suitable for large- scale production since it is not fast and gives a waste. CdS by sputtering was grown in presence of fluorine by introducing in the sputtering chamber Ar containing 3% of CHF3. Despite fluorine is a donor for CdS, we did not see any lowering of the CdS resistivity when it was grown in presence of fluorine. The CSS(500, sticking of CdS by gas pressure and close to source ) allows substrate deposition temperature much higher than that of vacuum evaporation( 150 S will re-evaporated before combining with Cd ). Both CdS(F) by sputtering or CdS(O) by CSS are suitable to make high efficiency cells if they are treated for 20 min, in Ar containing 20% of H 2 at 400 substrate temperature because CdS(F) contains CdF 2 segregated in the grain boundaries and surface. CdF2 is useful to passivate the grain boundaries, but it block CdS to interact with CdTe. The same happens for CdS, because CdO and/or CdSO3 formed in the grain boundaries and surface. They can be removed by the H 2 treatment. The thickness of the CdS layer is, in both cases, 80 nm. 3, CdS improvement method 3.1, H2 treatment of CdS
CdTe(10 nm) has a blue shift in the absorption with an effective band gap of 2.8 eV(Bulk CdTe, 1.44 eV at 300K); CdS (15 nm) has an effective band gap of 2.98 eV(better than Bulk CdS, 2.4ev): Nano-crystalline CdTe has potential applications as n-type window layers in CdTe solar cell 3.2, Nano-crystalline CdS
Patterned growth mask on CdS Patterned growth (left) and planar growth (right) of CdTe The disadvantage of polycrystalline films: First, the grain boundaries result in enhanced migration of dopants, device shunting, and high recombination; Second, inhomogeneity results in efficiency losses. 3.3, Patterned growth
References: 1. Fabrication Procedures and Process Sensitivities for CdS/CdTe Solar Cells 2. Growth and physical properties of CdS thin films prepared by chemical bath deposition 3. Influence of CdS window layer on 2-mm thick CdS/CdTe thin film solar cells 4. Photoluminescence characteristics of CdS layers deposited in a chemical bath 5. High throughput processing of CdTe/CdS solar cells 6. Influence of ITO surface modification on the growth of CdS 7. Growth of polycrystalline CdS and CdTe thin layers for high efficiency thin film solar cells 8. CdS films prepared by the close-spaced sublimation 9. Effects of thiourea concentration on CdS thin films grown by chemical bath deposition 10.The Effect of Oxygen on Interface Microstructure Evolution in CdS/CdTe Solar Cells 11. Recent progress on CdTe/CdS thin film solar cells 12. Nano-structured CdTe, CdS and TiO2 for thin film solar cell applications 13. Ordered CdTe/CdS Arrays for High-Performance Solar Cells
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