Research Article  Open Access
Silvio Pierro, Andrea Scuto, Luca Valenti, Marina Foti, Anna Battaglia, Giovanni Mannino, Cosimo Gerardi, Felice Crupi, Salvatore Lombardo, "Measurements and Simulations on the Mechanisms of Efficiency Losses in HIT Solar Cells", International Journal of Photoenergy, vol. 2015, Article ID 515767, 7 pages, 2015. https://doi.org/10.1155/2015/515767
Measurements and Simulations on the Mechanisms of Efficiency Losses in HIT Solar Cells
Abstract
We study the electrical and the optical behavior of HIT solar cell by means of measurements and optoelectrical simulations by TCAD simulations. We compare the HIT solar cell with a conventional crystalline silicon solar cell to identify the strengths and weaknesses of the HIT technology. Results highlight different mechanisms of electrical and optical efficiency losses caused by the presence of the amorphous silicon layer. The higher resistivity of the aSi layers implies a smaller distance between the metal lines that causes a higher shadowing. The worst optical coupling between the amorphous silicon and the antireflective coating implies a slight increase of reflectivity around the 600 nm wavelength.
1. Introduction
The heterostructure with intrinsic thin layer solar cell, so called HIT, is the most promising monocrystalline silicon based technology to enhance the cost/efficiency factor [1]. This assumption is based on the enhancement made by the HIT with respect to a monocrystalline solar cell by the cost reduction introduced by the lower thermal budget, and by the increasing of the open circuit voltage thanks to the heterojunction barrier that decreases the minority current [2]. From the previous considerations we expect a strong increase in the HIT performance with respect to the cSi technology. The base researchcell efficiencies reported in literature tell us that the efficiency of the HIT solar cell is slightly higher than the efficiency of the microcrystalline bulk solar cell (25.6% of HIT against 25% of bulk microcrystalline) [3]. Subsequent improvements have allowed HIT solar cells to reach the efficiency of 25.6% [4]. Actually, the main focus to increase the HIT efficiency is connected to the surface recombination velocity at amorphouscrystalline interface [5]. In order to further increase the cell efficiency we need to understand the main causes of efficiency losses and how to reduce them.
This paper focuses the attention on the physical aspects that affect the HIT solar cells different from technological aspects as surface passivation or defect densities. For this purpose we compare measurements of bulk crystalline silicon solar cell with the equivalent HIT solar cell. Furthermore, by using a commercial TCAD simulator [6] tuned with device measurements, we intend to investigate the main loss mechanisms for both structures and identify the main problems of the HIT technology.
The remainder of this paper is organized as follows. Section 2 describes the process details of the realized samples and the simulation setup. Section 3 reports the experimental and numerical results and discusses the main phenomena affecting the HIT efficiency. Section 4 reports the conclusions of this work.
2. Experimental Details
The HIT structure is presented in Figure 1. Starting from a ptype CZ wafer, by a HF etching process a front random pyramids texturing is built. A highly doped ptype layer is deposited on the back surface, which is crystallized by a thermal process, forming the back surface field (BSF) layer. The intrinsic and the ntype amorphous layers are deposited by inductively coupled plasma chemical vapour deposition (ICPCVD) on the top surface of the wafer. The aluminum doped zinc oxide (AZO) is used as antireflective coating (ARC) layer and is grown by sputtering. To finalize the structure, the back metallization covers the entire device and is a Ti/Pt/Au multilayer; the top metal is made by screen printing of a silver paste. A double HIT structure was not realized, since the amorphous BSF does not add enhancement on a ptype HIT solar cell [7, 8]. For comparison purpose, we realized the crystalline structure shown in Figure 1. The device parameters of the HIT and the reference structures are reported in Table 1. The key difference between the two structures is the emitter region, which is crystallized by a thermal process in the case of the crystalline structure.

The electrical simulation setup uses a driftdiffusion model with Fermi statistics, with the Schenk bandgap narrowing model [9] and the Slotboom model for free mobility carrier degradation [10]. The Auger and Radiative recombination has been added as far as the SRH recombination for both bulk and surfaces. For the crystalline solar cell, the surface recombination velocity of cSi/AZO interface uses the SRH surface recombination model, tuned with literature results [11]; the doping level for both emitter and BSF layers is tuned with spreading resistance measurements on our devices. The HIT solar cell is made by replacing the emitter with the i/n aSi:H layers. The HIT physical model requires adding the thermionic current and the surface recombination at the amorphouscrystalline interface [12, 13]. The amorphous silicon density of states (DOS) is modeled by three Gaussian distributions, two for the conduction and valence band tails and one for the mid gap defect concentration [13]. The most relevant aSi:H electrical parameters used in the simulations are reported in Table 2. The optical simulation solves an extended version of the Transfer Matrix Method with diffused and direct light to take into account surface texturing [14, 15]. The coherent light follows the direct path, while the diffused ray follows a scattered function, the socalled angular distribution function [16]. The ratio between the scattered and direct ray is the haze parameter. In the simulation above, the haze parameter has been set to 0.73 and the haze profile follows a square cosine law. The values of refractive index and absorption coefficient for the materials used in the solar cell are consistent with literature data [17].

3. Results and Discussion
Figure 2 shows the main electrical parameters of HIT and crystalline (subsequently cSi) solar cells as a function of pitch. According to literature, the open circuit voltage of the HIT cell is greater than cSi [2]; on the other hand, the short circuit current and the fill factor are lower. This translates into a lower efficiency value of the HIT compared to cSi, and the difference increases as the pitch increases. Figure 3 shows the optimized pitch distance that reaches the highest efficiency; this value reaches a tradeoff between the shadowing losses and the fill factor that show opposite trends as a function of pitch length. Increasing the sun concentration, the pitch distance decreases faster in the cSi solar cell compared to HIT, thus increasing the efficiency of HIT compared to cSi. In order to understand the fill factor loss we analyze simulation results for both HIT and cSi structures at maximum power peak; results are shown in Figure 4, where the black lines are the current path and the color is the current density. It is worth noting that the current flows vertically inside the bulk, while it goes in direction of the contact in both the emitter and the AZO regions. The current direction into the emitter causes an increase of the density current near to the contact, thus increasing the electrostatic potential loss inside the emitter. This effect depends on the pitch width and the emitter resistivity. In order to gain insight on the lateral loss effect, Figure 5 compares the electrostatic potential along the cSi/aSi heterointerface for the HIT structure and the electrostatic potential at the pn junction for the cSi structure. Moving away from the contact, the potential decreases due to the emitter and AZO resistivity. The potential loss is highly close to the contact due to the higher current density and is higher in the HIT compared to the reference one, due to the higher resistivity of aSi compared to cSi. The electrostatic potential at the interface causes an increase in the dark current that decreases the photogenerated current. This effect depends on the operation condition. At short circuit current condition, the dark current is low with respect to the photogenerated current, so the effect can be neglected. By increasing the voltage, the diode current becomes relevant with respect to the photogenerated current, and the effect cannot be neglected anymore. At open circuit voltage, the diode dark current is equal to the photogenerated current, since the dark current depends on the lateral effect; there is an open circuit voltage reduction. This phenomenon is present in both HIT and crystalline devices. As can be seen from Figure 5, the potential variation in the HIT structure is higher than the crystalline solar cell because of the higher emitter resistivity; the higher potential variation produces a decrease in the fill factor and in the open circuit voltage. In order to reduce this effect we need to shrink the pitch, thus causing a higher shadowing that decreases efficiency. The optimum value between lateral losses and shadowing is shown in Figure 3. Because of the previous effect, the HIT solar cell needs a smaller pitch than the cSi, but with increasing sun concentration this effect tends to decrease, since the cSi solar cell pitch decreases faster than the HIT and when the two pitch values are the same the HIT efficiency becomes greater than the cSi one.
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Figure 6 shows the reflectivity of the devices for both simulations and measurements. As can be seen, the HIT structure shows a higher reflectivity from 300 nm to 600 nm with respect to the cSi structure. This reflectivity increase translates into a lower energy absorbed by the solar cell and less energy. Figure 7 shows the difference of spectral power density absorbed by the HIT and cSi solar cell with respect to the AM1.5G spectral density; this is an optical analysis derived from the overall reflectivity and transmittance; we cannot notice electrical issues and cannot determine which layer adsorbs the light. The optical stack of HIT structure differs from the cSi structure by the addiction of the two aSi:H layers, the ndoped and intrinsic layers (stack successfully called aSi:H) in between the silicon bulk and the AZO layer. From literature we know that a good antireflective layer in between two materials must obey some rules; in particular, the thickness of the layer must ensure the destructive interference of the reflected wave and the refractive index must be as close as possible to the square root of the product of the refractive indices of the two materials. If the layers are more than one, we must use the same rule for the two neighbor materials. Figure 8 shows the refractive index of the cSi, aSi:H, and AZO material compared with the optimum refractive index for the case of 1 layer in between cSi and the air and the case of 2 layers (like HIT structure). As can be seen, the AZO is a good material to use as single inner layer for cSi solar cell; this ensures a low reflectivity in cSi solar cell. The aSi:H shows a refractive index similar to the cSi for wavelength higher than 550 nm; then the reflectivity in this wavelength range should be approximated to the cSi solar cell. For smaller frequencies, the aSi:H and cSi refractive indices are no more similar; this implies that the optical behavior must follow the model with 2 layers in between the silicon and the air. Under this range, the AZO is still a good material, but the aSi:H is close enough to the optimum level for wavelength smaller than 400 nm; before this value the worst index matching does not ensure a low reflectivity. This analysis is in good agreement with the optical measurements shown in Figure 6. The integral of the reflectivity value is used as a figure of merit for the system and Figure 9 shows this integral as a function of the aSi:H layer thickness compared to the structure with no aSi:H layer. The inset figure shows that the AZO reaches a minimum of reflectivity when its thickness is 91 nm and this value is not dependent on the aSi:H thickness. The aSi:H thickness affects the reflectance and there is a maximum of reflectance at 20 nm. This result tells us to use a very thin aSi:H layer or a greater one. But using a thick layer implies a bigger series resistance and a layer thinner than 5 nm can be a problem for quantum effects [18].
4. Conclusions
In this paper we compared the HIT solar cell with a cSi solar cell to identify the main problems of the HIT technology. We made measurements and simulation and by comparing the optimized structure for both technologies we noticed two losses differences into HIT. A lower fill factor and a higher reflectivity for the HIT solar cell. The fill factor decrease is due to a higher potential loss along the pn junction that causes a higher diode current. To prevent this we need to shrink the pitch, causing a higher shadowing. The lower short circuit current is caused by higher reflection losses into HIT. This is caused by the worst optical coupling between the amorphous silicon and the antireflective coating in the wavelength range from 400 nm to 600 nm; we cannot reduce this effect since the reduction is related to optical properties of the amorphous layer; to reduce the reflectivity we must shrink the aSi layer, but the thickness cannot be smaller than the value in use to prevent quantum effects.
Conflict of Interests
The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interests; and expert testimony or patentlicensing arrangements) or nonfinancial interest (such as personal or professional relationships, affiliations, knowledge, or beliefs) in the subject matter or materials discussed in this paper.
References
 M. Tanaka, M. Taguchi, T. Matsuyama et al., “Development of new aSi/cSi heterojunction solar cells: ACJHIT (Artificially Constructed JunctionHeterojunction with Intrinsic Thinlayer),” Japanese Journal of Applied Physics, vol. 31, no. 11, pp. 3518–3522, 1992. View at: Publisher Site  Google Scholar
 M. Nath, P. Chatterjee, J. DamonLacoste, and P. Roca i Cabarrocas, “Criteria for improved opencircuit voltage in aSi:H(N)/cSi(P) front heterojunction with intrinsic thin layer solar cells,” Journal of Applied Physics, vol. 103, no. 3, Article ID 034506, 2008. View at: Publisher Site  Google Scholar
 M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (version 39),” Progress in Photovoltaics: Research and Applications, vol. 20, no. 1, pp. 12–20, 2012. View at: Publisher Site  Google Scholar
 S. Tohoda, D. Fujishima, A. Yano et al., “Future directions for higherefficiency HIT solar cells using a Thin Silicon Wafer,” Journal of NonCrystalline Solids, vol. 358, no. 17, pp. 2219–2222, 2012. View at: Publisher Site  Google Scholar
 T. Mishima, M. Taguchi, H. Sakata, and E. Maruyama, “Development status of highefficiency HIT solar cells,” Solar Energy Materials & Solar Cells, vol. 95, no. 1, pp. 18–21, 2011. View at: Publisher Site  Google Scholar
 SYNOPSYS, Sentaurus Device User Manual, chapter 21, 2013.
 L. Zhao, C. L. Zhou, H. L. Li, H. W. Diao, and W. J. Wang, “Design optimization of bifacial HIT solar cells on ptype silicon substrates by simulation,” Solar Energy Materials and Solar Cells, vol. 92, no. 6, pp. 673–681, 2008. View at: Publisher Site  Google Scholar
 A. Datta and P. Chatterjee, “Computer modeling of heterojunction with intrinsic thin layer ‘HIT’ solar cells: sensitivity issues and insights gained,” in Solar Cells—ThinFilm Technologies, L. A. Kosyachenko, Ed., InTech, 2011. View at: Publisher Site  Google Scholar
 A. Schenk, “Finitetemperature full randomphase approximation model of band gap narrowing for silicon device simulation,” Journal of Applied Physics, vol. 84, no. 7, pp. 3684–3695, 1998. View at: Publisher Site  Google Scholar
 D. B. M. Klaassen, “A unified mobility model for device simulation—I. Model equations and concentration dependence,” SolidState Electronics, vol. 35, no. 7, pp. 953–959, 1992. View at: Publisher Site  Google Scholar
 M. J. Kerr, J. Schmidt, A. Cuevas, and J. H. Bultman, “Surface recombination velocity of phosphorusdiffused silicon solar cell emitters passivated with plasma enhanced chemical vapor deposited silicon nitride and thermal silicon oxide,” Journal of Applied Physics, vol. 89, no. 7, pp. 3821–3826, 2001. View at: Publisher Site  Google Scholar
 A. Kanevce and W. K. Metzger, “The role of amorphous silicon and tunneling in heterojunction with intrinsic thin layer (HIT) solar cells,” Journal of Applied Physics, vol. 105, no. 9, Article ID 094507, 2009. View at: Publisher Site  Google Scholar
 K. Horio and H. Yanai, “Numerical modeling of heterojunctions including the thermionic emission mechanism at the heterojunction interface,” IEEE Transactions on Electron Devices, vol. 37, no. 4, pp. 1093–1098, 1990. View at: Publisher Site  Google Scholar
 A. Gerrard and J. M. Burch, Introduction to Matrix Methods in Optics, Dover Publications, 2012.
 J. Krč, F. Smole, and M. Topič, “Analysis of light scattering in amorphous Si:H solar cells by a onedimensional semicoherent optical model,” Progress in Photovoltaics: Research and Applications, vol. 11, no. 1, pp. 15–26, 2003. View at: Publisher Site  Google Scholar
 P. Beckmann and A. Spizzichino, The Scattering of Electromagnetic Waves from Rough Surfaces, Artech House, Norwood, Mass, USA, 1987.
 E. D. Palik, Handbook of Optical Constants of Solids, Academic Press, Boston, Mass, USA, 1985.
 A. Muoz, N. Chetty, and R. M. Martin, “Modification of heterojunction band offsets by thin layers at interfaces: role of the interface dipole,” Physical Review B, vol. 41, no. 5, pp. 2976–2981, 1990. View at: Publisher Site  Google Scholar
Copyright
Copyright © 2015 Silvio Pierro et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.