Effect of SiO₂ and SiN_x Passivation on CdTe based Solar Cells

Effect of SiO₂ and SiN_x Passivation on CdTe based Solar Cells

Vinay Budhraja

ITM University, Sector 23A, Gurgaon, Harayana 122017, India

Abstract Back passivation of CdTe solar cells was done using silicon di oxide (SiO2) and silicon nitride (SiNx) and its effect was seen on dark I-V, light J-V and C-V characteristics. We also did the annealing of solar cell which further changes the performance of solar cell. Solar cells show enhancement in performance with SiO2 passivation and annealing after SiO2. There is no enhancement in performance with SiNx but it improves by doing annealing after SiNx.

1. INTRODUCTION

   The presence of defects on CdTe degrades [1-3] the performance of CdTe solar cells. One possible way to improve the performance of solar cells is the passivation of CdTe and its effect has been studied by different research groups. Hydrogenation of CdZnTe improved the sample quality and affected the physical properties of material [4]. The results were observed by photoluminescence which tells that there is an enhancement of excitonic photoluminescence intensity and reduction of transition involving defective bonding in the Cd1-xZnxTe (CZT) alloy after hydrogenation. The passivation of Cd1-xZnxTe (CZT) was also tried with oxidation in low energy atomic oxygen [5]. Exposure of oxygen makes the grain size to be more homogenous but does not change the surface roughness. The experiments were done on metal-semiconductormetal devices after oxidation of CZT. The oxidation of CZT increases the resistivity thus the leakage current reduces. Even though some enhancement were shown in results but change in performance was not much significant. Thermal and wet oxidation of CdTe can also passivate [6] the surface of CdTe but its effect was shown on MOS structures. Consonni et. al. [7] investigated the effect of polycrystalline chlorine doped CdTe layer. The chlorine atoms appear to compensate unintentional impurities and give rise to levels within the bandgap. Kang et. al. [8] did the surface passivation by sulfur treatment of CdTe and observed the change in photoluminescence intensity which relates with the minority carrier life time of CdTe. Kim et. al.

   1. also showed the change in photoluminescence intensity by doing surface preparation with ZnS and CdZnTe. Nelson et. al [10] reported in situ H2S plasma processing of CdTe and observed reduction in surface recombination velocity through passivation of surface states.

      In this study we are also passivating the back side of CdTe on the structure of CdTe solar cells. Silicon di oxide (SiO2) and silicon nitride (SiNx) were used for passivation. The deposition of SiO2 increases the value of cell parameters by reducing the surface states and by improving the interface properties. The samples were annealed and further improved

      the performance of CdTe solar cells. The deposition of SiNx did not show as much enhancement as SiO2 on solar cells.

      Fig 1. shows the structure of CdTe solar cells. Fig 1a. shows the structure of CdTe solar cells without passivation and Fig 1b and c. shows the structure of CdTe with SiO2 and SiNx passivation respectively.

      1. (b)

         (c)

         Fig 1. Structure of CdTe solar cells; (a) without; (b) with SiO2 passivation and (c) with SiNx passivation

2. EXPERIMENTAL DETAILS

   The fabrication of planar structure of CdTe/CdS/ITO was done using standard techniques. The deposition of CdTe on CdS/ITO/Glass was done using close space sublimation. Finally graphite paste was used for contact. The samples were characterized after the fabrication of the structure shown in Fig 1a, using light and dark I-V and C-V. A layer of 200nm of (i) SiO2 and (ii) SiNx was deposited using Plasma enhanced chemical vapor deposition in different structures. Samples were again characterized after the deposition of SiO2

   and SiNx Solar cells were annealed at 200oC for 30 minutes and again characterized.

3. RESULTS

   Fig 2. shows the light J-V characteristics of CdTe solar cells under six conditions: (i) without SiO2 (structure of Fig 1a), (ii) with SiO2 (structure of Fig 1b), (iii) annealing after SiO2 deposition, (iv) without SiNx (structure of Fig 1a), (v) with SiNx (structure of Fig 1c) and (vi) annealing after SiNx deposition. Although there is small change in Voc with the deposition of SiO2, Jsc and fill factor improves significantly with the deposition of SiO2. Jsc further improves after annealing. In the case of passivation with SiNx the performance of solar cell first reduced and enhances after annealing.

   Fig 2. Light J-V characteristics of CdTe solar cells under six conditions.

   Fig 3. Dark I-V characteristics of CdTe solar cells under six different conditions.

   The improvement in Jsc and fill factor is due to the fact that surface states reduce with deposition of SiO2 and it also reduces surface recombination by passivating bonds at the surface. The deposition of SiNx cannot reduce the recombination and create defect in CdTe which passivate after annealing.

   Fig 3. shows the behavior of dark I-V under above mentioned six conditions. There is decrease in dark forward current on the samples having the deposition of SiO2 which means that the saturation current density will reduce with the deposition of SiO2. Series resistance will be lower for samples having SiO2 deposition. The curve further shifted downward for the samples on which annealing was done after SiO2. This shows that annealing further passivate the defects present on CdTe which is responsible for enhanced performance.

   Fig 4. shows the C-V characteristics of CdTe solar cells under above mentioned six conditions.

   (a)

   Fig 4. C-V characteristics of CdTe solar cells under six conditions, (a) at 10 KHz and (b) at 100 KHz

   Fig 4 tells that both depletion and diffusion capacitance show its effect with the deposition of SiO2 and after annealing. In the case of CdTe solar cells as fabricated (without SiO2) the recombination effect reduces the capacitance values at different frequencies. The similar study of defects using C-V characteristics on CdTe solar cells has been reported by Poonam et. al. [11].

   In the case of SiNx the deposition of SiNx increases the trap density which is responsible for the reduced value of depletion capacitance (Fig 4b) but after annealing depletion capacitance again increases which tells that traps get filled and reduce the trap density. That is why the deposition of SiNx does not fully passivate the back surface of CdTe solar cells.

4. CONCLUSION

   The back passivation of CdTe solar cells were studied using the deposition of SiO2 and SiNx. The samples were characterized three times after fabrication of planar CdTe solar cells (without dielectric), after dielectric deposition and annealing after dielectric deposition. The changes were observed in dark and light I-V and frequency dependent C-V characteristics.

   The results clearly tell that the performance of CdTe solar cells improve with the deposition of SiO2 and after annealing. There are defects present on back surface of CdTe which get passivated after the deposition of SiO2 and with annealing. With respect to SiNx passivation there is not much enhancement in the performance of solar cells. The deposition of SiO2 reduces the surface recombination and enhances the performance of solar cells. The effect of back surface recombination cannot be neglected in CdTe solar cells because of the less thickness of solar cells [12].

   The scope of this work is extended to the possibilities of other delectrics also like Al2O3 and HfO2.

5. ACKNOWLEDGEMENT

The author thanks to the ECE team of ITM University for technical help and useful discussions.

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