2D IBC-SHJ Solar Cell Simulation and Optimization

 

Introduction

Interdigitated back contact silicon heterojunction (IBC-SHJ) combines the advantages of interdigitated back contact (IBC) and silicon heterojunction (SHJ) solar cells. Having all the contacts at the back of the cell eliminates contact shading, leading to a higher short-circuit current (JSC). Being a heterojunction device, IBC-SHJ also has the potential of higher open circuit voltage (VOC) due to the better surface passivation of the deposited amorphous silicon (a-Si) layer. The low temperature deposition, instead of high temperature diffusion, decreases thermal stress, which is the trend of future silicon solar cells. Rear surface passivation by deposited intrinsic amorphous silicon (a-Si) buffer layer in IBC-SHJ solar cells significantly improves open circuit voltage (VOC) and short circuit current (JSC) but can lead to very low fill factor (FF) with an “S” shape J-V curve. In this paper, methods to optimize IBC-SHJ solar cell with improved FF are discussed and guided by two-dimensional numerical simulation. The modeling of the IBC-SHJ solar cell requires device simulation software operating in two dimensions and incorporating amorphous silicon. To study this innovative structure, we use ATLAS two-dimensional (2-D) device simulation software that provides accurate bulk and interface defects needed to model amorphous silicon. We first present the geometrical structure of the solar cell, then review the general framework of the simulation by specifying the different physical models, and finally examine the simulation results in order to determine the important parameters to achieve high efficiency.

 

Device Structure

Back contact cells differ from conventional structures in that all contacts are on the back side (not illuminated side) of the cell. The front surface is subject to illumination. On the rear side, we have an interdigitated structure of hydrogenated amorphous silicon layers alternately n-type and p-type to play the role of emitter or back surface field (BSF) according to the c-Si substrate doping. We used an n-type c-Si substrate in this Device. The emitter (p-stripe) and the BSF (n-stripe) are covered by metal contacts. An intrinsic a-Si buffer layer was deposited over the entire rear surface of the IBC-SHJ solar cell. The IBC-SHJ solar cell presents a periodic structure. The periodicity of the structure allows us to use an elementary structure, shown in Figure 1, that will serve as a basis for optimizing the performance of this type of cell. The geometrical and material parameters of the simulated structure were chosen according to [1].

Figure 1. IBC-SHJ solar cell elementary structure.

 

Physical Models

The simulation is based on the solution of three governing semiconductor equations: Poisson’s equation, electron and hole continuity equations. Fermi statistic was used for carriers with drift-diffusion combined with Bohm Quantum Potential for quantum correction. Fermi model and Recombination models (i.e srh, auger and surface recombination) were also included into the simulation. For a-Si layers, critical parameters like band gap, doping and defect distribution are defined in the input deck. The critical parameters for accurate simulation are energy distribution of the exponential band tails, and the Gaussian distribution of the mid-gap trap states. They were chosen according to reference [1] and shown in Figure 2. For c-Si/a-Si interfaces at the back surface a thermionic emission model was used. For even more realistic modeling of this interface we have introduced defect states at the hetero-interface by putting a very thin defective layer of c-Si [2]. An AM1.5G solar spectrum is used for the optical generation to simulate the J-V curve under standard one-sun illumination conditions. A Sopra database is used for a-Si index of refraction.

Figure 2. Density of States (DOS) in a-Si material.

 

To properly simulate the behavior of the structure, it is essential to apply an adapted mesh. A mesh as thin as possible applied to the whole structure ensures good accuracy of calculations but requires greater computation time to simulate the behavior of this structure. It is therefore necessary to find a compromise between computational time and accuracy of the calculation. To reach this compromise, we applied a fine mesh only in areas where changes in physical quantities are important and a coarse mesh in areas where these quantities are quasi static. Thus, the mesh is refined in the critical areas that are the front surface (strong absorption), the c-Si/a-Si:H hetero-interfaces and the areas around frontiers of the various layers in which the variations of physical quantities are important. In the middle of the c-Si substrate, a coarse mesh is used as the physical quantities do not vary significantly. Figure 3 represents the mesh used to simulate IBC-SHJ solar cell.

Figure 3. Mesh of the elementary IBC-SHJ solar cell.

 

Simulation Results

A complete parametrized input deck, including geometry and mesh, was created not only to optimize simulation time and accuracy but also for solar cell optimization purposes. This input deck was used in DBinternal to vary different parameters in order to optimize the solar cell efficiency. DBINTERNAL is a simple but powerful DECKBUILD tool that allows you to create a Design Of Experiments (DOE) from a pair of input files. Amongst other things, you can create corner models for process parameters or device characteristics or both. Any parameters that are to be used as variables must be specified as set statements in a template file. Any results of interest should be calculated using extract statements. The DOE is specified with simple sweep statements in a separate design file. The sweep statement defines which variables are required in the DOE, and the range of values these variables are to take. The parameter values and the results of each simulation can be stored in a file that can be viewed in TonyPlot or used as a database for input to a statistical analysis tool such as SPAYN. We have chosen 4 parameters to optimize efficiency. These parameters are intrinsic a-Si thickness, n-stripe and p-strip width and gap width.

It was experimentally observed that an intrinsic a-Si layer increases Voc and Jsc but also decreases FF and leads to a “S” shape IV curve. Simulation results, shown in Figure 4, confirm the reduction of FF and “S” shape IV curve.

Figure 4. IV curves as a function of a-Si thickness.

 

The results of the DoE are shown in Figures 5, 6, 7 and 8. Figure 5 shows an increase of Voc when the thickness of the intrinsic a-Si layer increases as reported in [3], leading to an optimum efficiency of around 10nm for the intrinsic a-Si layer. n-stripe, p-stripe and gap width were also optimized. Figures 6 and 7 show that Jsc decreases when n-stripe width and gap width increases. As a consequence n-stripe and gap width have to be chosen as narrow as possible.

Figure 5. IBC-SHJ solar cell Key figure of merits as a function of a-Si thickness.

 

Figure 6. IBC-SHJ solar cell Key figure of merits as a function of n-stripe width.

 

Figure 7. IBC-SHJ solar cell Key figure of merits as a function of gap width.

 

Figure 8. IBC-SHJ solar cell key figure of merits as a function of p-stripe width.

 

It is interesting to notice that Jsc increases and FF decreases when p-stripe increases as shown in Figure 8. The different evolution of Jsc and FF leads to an optimum for the efficiency of around 1mm for p-stripe width.

 

Conclusion

2D numerical simulations were performed in order to study and optimize IBC-SHJ solar cells. The impact of several geometrical parameters were studied and their impact was shown on the IBC-SHJ solar cell output characteristics. This gives the designer guidelines to achieve high efficiency IBD-SHJ solar cells.

 

References

  1. M.Lu et al “Optimization of interdigital back contact silicon solar cells by two-dimensional numerical simulation” IEEE 2009.
  2. D. Diouf, J.-P. Kleider, and C. Longeaud, “Two-Dimensional Simulations of interdigitated ack Contact Silicon Heterojunctions Solar Cells”, Chapter 15 of the book “Physics and Technology of Amorphous-Crystalline Silicon Heterostructure Solar Cells”, Springer 2011.
  3. D.Munos et al. INES-CEA, “Key features of highly efficient a-Si:Hc-Si heterojunction solar cells” 7th Workshop on the future Direction of Photovoltaics (by JSPS 175th committee), march, 2011.

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