Developmentstatusofhigh-efficiencyHITsolarcells

Development status of high-ef?ciency HIT solar cellsTakahiro Mishima n , Mikio Taguchi, Hitoshi Sakata, Eiji MaruyamaAdvanced Energy Research Center, SANYO Electric Co., Ltd., Japan, 7-3-2 Ibukidai-higashimachi, Nishi-ku, Kobe City, Hyogo 651-2242, Japana r t i c l e i n f oAvailable online 28 April 2010Keywords:High-ef?ciency solar cellCrystalline siliconHeterojunctiona b s t r a c tThis paper describes the development status of high-ef?ciency heterojunction with intrinsic thin-layer(HIT) solar cells at SANYO Electric. Presently, the conversion ef?ciency of our standard HIT solar cell hasreached a level of 23.0% for a practical size of (100.4 cm 2 ) substrate. On the other hand, we havedeveloped special technologies for effectively using thinner substrates for HIT solar cells. Surprisingly,we have achieved a quite high open circuit voltage ( Voc ) of 743 mV, and a high conversion ef?ciency of22.8% using only a 98- mm-thick substrate. A 98- mm-thick cell also exhibits a good temperaturecoef?cient, and allows the thickness of the substrate to be reduced by more than 50% while maintainingits ef?ciency. These results suggest that the HIT solar cell has the potential to further improvecost-performance.b) effectively use thinner substrates;c) improve the temperature coef?cient of ef?ciency;d) develop a bifacial module.Some of the above points are related to special features of theHIT solar cell, such as its ability to use thinner substrates and itssymmetrical device structure.We have recently succeeded in raising the maximum conver-sion ef?ciency for a practical sized standard HIT solar cell to23.0%, measured at National Institute of Advanced IndustrialScience and Technology (AIST) Japan [1] , which was achieved byoptimizing a variety of processes at our laboratory [1] . Now, wehave set our new target for conversion ef?ciency to 25.0%. In thispaper, recent SANYO Electric activities for reducing the cost of theHIT solar cell will be described.2. High-ef?ciency HIT solar cell2.1. HIT solar cell structureThe structure of the HIT solar cell is shown in Fig. 1 . Anintrinsic amorphous-Si layer, a doped amorphous-Si layer, and aTCO layer are deposited on both sides of a crystalline-Si substrate.Grid electrodes are also fabricated on both sides of the dopedamorphous-Si layer. Therefore, the HIT solar cell has asymmetrical structure, and does not use high-hardness alloymetals as electrodes. As a result, it achieves a stress-free cellstructure, which is well suited to thinner-substrate solar cells.The HIT solar cell uses high-quality and very-thin amorphoussilicon (a-Si) layers to form the Hetero-Junction. In addition, ahigh-quality intrinsic a-Si layer can effectively passivate thesurface dangling bonds of the crystalline silicon substrate. As aresult, this cell has achieved a high Voc of more than 700 mV [2] .Further, all of the process temperatures for HIT solar cells arebelow 200 1C, which allows it to preserve the initial high-levelquality of the crystalline silicon substrate. We have improved thea-Si/c-Si hetero-interface structure of the HIT solar cell throughfurther process optimization, to reach a Voc of more than720 mV [3] . The higher V oc offers not only a high conversionef?ciency but also a better temperature coef?cient, which iscomparable to amorphous silicon solar cells ( Fig. 2 ). Of course, abetter temperature coef?cient of conversion ef?ciency improvesthe output power under high temperature operating conditions.The symmetrical structure of the HIT solar cell is alsosuitable for bifacial modules, which are explained in detail [4]in Section 4.Contents lists available at ScienceDirectjournal homepage: www.elsevier.com/locate/solmatSolar Energy Materials fax: +81 78 993 1096.E-mail address: takahiro.mishima@sanyo.com (T. Mishima).Solar Energy Materials & Solar Cells 95 (2011) 18– 212.2. 23.0% Standard HIT solar cellPresently, the conversion ef?ciency of the standard HIT solarcell has reached a level of 23.0% for a 100.4 cm 2 practical sizecrystalline silicon substrate ( Voc : 729 mV, short circuit current(Jsc): 39.52 mA/cm 2 , ?ll factor: 80.0%). There are three keytechnologies for improving the conversion ef?ciency of HIT solarcells. The ?rst point is to improve the HIT structure, which isrelated to improvement of heterojunction. The second point is tooptimize the grid electrode. And the third point is to improve theoptical con?nement structure, which is related to light absorptionin the cell [5] .2.3. Loss analysis of the 23.0% cellAiming for even higher ef?ciency, we have analyzed variouslosses of the 23.0% HIT solar cell from experimental data (EQE,IQE, re?ection spectra, etc.). From this analysis, the total powerloss is more than 20 percent of the current output power. Amongthe estimated losses, the loss by light re?ection and transmittanceaccounts for about 1/3. The light absorption loss in the TCO anda-Si, and the carrier recombination loss also account for about 1/3.And the remaining 1/3 consists of losses by the electrode shadeand series resistance.The majority of the power loss is related to the loss of incidentlight, and carrier recombination in the cell. Therefore, we willcontinue to make better components (for e. g., TCO, a-Si, etc.) forthe HIT solar cell. If we can lower these losses in the future, wewill be able to improve the conversion ef?ciency.3. Thinner-substrate HIT solar cell3.1. Unique Voc propertyIn conventional cells, the output power generally decreaseswhen the cell thickness is reduced. This is mainly due to alowering of the Isc because of less light absorption, and due to alowering of the V oc caused by the surface recombination loss thatbecomes predominant in the total recombination loss of the solarcell. As a result, the output power decreases with cell thinning.However, high-performance HIT solar cells have differentcharacteristics. Fig. 3 shows the thickness dependence of HITsolar cell output parameters in R&D. In Fig. 3, the Isc decreaseswith cell thinning, but the V oc increases with cell thinning. Weattribute these phenomena as follows. Total recombinationvelocity was determined only by bulk recombination due toextremely low surface recombination velocity of the HIT solarcells [6] . This makes it possible to maintain higher conversionef?ciency in the cell thickness region below 100 mm.3.2. New technologies for thinner cellsWe have customized our high-ef?ciency technologies toimprove thinner-substrate HIT solar cells. There are two keytechnologies. The ?rst technology is related to a higher V oc. Itconsists of a new device design to obtain a higher Voc , and animproved junction formation process. This technology directlyimproves the diode characteristics of the HIT junction. The secondtechnology is related to a higher Jsc . We have optimized thetexture structure specially for thinner substrates, which leads toimproved optical con?nement. We have also reduced the opticalabsorption loss of the a-Si and TCO by using wide-gap a-Si ?lmsand high transparency TCO. As a result, a relatively high Jsc wasobtained with thinner substrates.Fig. 4 shows the optimized IQE spectra of HIT solar celloptimized for thinner-substrate cells. Here, the upper curveindicates the data of our newly developed cell, and the lowercurve indicates the data of a typical cell. In Fig. 4 , the shortwavelength response is improved by using a newly developed a-Si?lm, and the long wavelength response is related to a new TCOand improved texture structure.i-type a-Si ~ 0.01 μ m c-Si (CZ, n-type) Grid electrode n-type a-Si ~ 0.01 μ m TCO p-type a-Si ~ 0.01 μ m Fig. 1. Structure of a HIT solar cell.Fig. 2. Temperature coef?cient of the conversion ef?ciency of HIT solar cells.Fig. 3. Thickness dependence of HIT ( Isc, Voc , Eff.).Fig. 4. IQE spectra of improved HIT solar cell with new a-Si, new TCO, and texturestructure optimized for a thinner-substrate. (Upper curve).T. Mishima et al. / Solar Energy Materials & Solar Cells 95 (2011) 18– 21 193.3. 22.8% Thin (98 mm) cellBy applying newly developed technologies for thinner-substrate HIT solar cells, we have obtained a conversion ef?ciencyof 22.8% and a V oc of 743 mV for a 98- mm-thick practical size cell(Fig. 5 , certi?ed by AIST). Other cell parameters are Jsc : 38.84mA/cm 2 , ?ll factor: 79.1%, and cell size: 100.3 cm 2. In particular,the Voc value of 743 mV is quite high for a crystalline silicon solarcell. Our new technologies enable us to obtain a higher Voc withthinner crystalline silicon substrates. A higher Voc also maintainsrelatively high conversion ef?ciency with a thin substrate.Additionally, we have con?rmed very low ( o 0.3%/ 1C)temperature coef?cient of conversion ef?ciency for 98 mm cell.3.4. Conversion ef?ciency comparisonFig. 6 shows the history of conversion ef?ciency for HIT solar cellsat our laboratory. During the last few years, we have accelerated theimprovement of the conversion ef?ciency. At the same time, wehave started developing high-ef?ciency thinner-substrate HIT solarcells, which today have reached an ef?ciency of 22.8%. Surprisingly,this 22.8% is comparable to our standard cell ’s best ef?ciency of23.0%. Indeed, our thinner HIT solar cell can reduce the thickness ofthe substrate by more than 50% while maintaining its ef?ciency. Webelieve this will prove the possibility of further improving the cost-performance of HIT solar cells.4. HIT Double: bifacial PV moduleThe HIT Double module uses the symmetrical structure featureof the HIT solar cell. This feature makes it possible to create abifacial module. Fig. 7 shows the normalized output power of HITDouble and single-sided HIT modules. The output power of theHIT Double is higher than that of the single-sided HIT modulethroughout the year. In this case, the HIT Double produces 10.9%more output power than the single-sided HIT module. We believethe HIT Double module is proof of another possibility forimproving the performance of HIT solar cells.5. SummaryWe have achieved of?cially certi?ed world ’s highest conver-sion ef?ciency of 23.0% for a practical size (100.4 cm 2 ) crystallinesilicon solar cell [1,7] . In addition, we have con?rmed thepotential of the HIT solar cell for thinner-substrate use byobtaining a conversion ef?ciency of 22.8% and a Voc of 743 mVFig. 5. I– V characteristics of the 22.8% ef?ciency HIT solar cell with 98- mm-thick substrate. (Certi?ed by AIST).Fig. 6. History of the HIT cell ’s conversion ef?ciency (R&D).Fig. 7. Output power trends of single-sided HIT and bifacial HIT Double TMmodules throughout the year.T. Mishima et al. / Solar Energy Materials & Solar Cells 95 (2011) 18– 2120for a 98- mm-thick cell. Indeed, the increased Voc successfullycompensates for the decreased Jsc in thinner-substrate HIT solarcells. Our thinner cells offer the advantage of not only a higher Vocbut also a better temperature coef?cient. This suggests that theHIT solar cell has the potential of an improved overall perfor-mance in the near future. The HIT Double module can boost theoutput power by more than 10% over the single-sided HIT moduleby using a bifacial structure. We believe that all of these effortswill be linked to the further improvement of HIT solar cells.Acknowledgements‘‘ HIT’’and ‘‘ HITDouble ’’ are trademarks of SANYO Electric Co., Ltd.References[1] Mikio Taguchi, Yasufumi Tsunomura, Hirotada Inoue, Shigeharu Taira,Takeshi Nakashima, Toshiaki Baba, Hitoshi Sakata, Eiji Maruyama, High-ef?ciency HIT solar cell on thin ( o 100 mm) silicon wafer, in: Proceedingsof the 24th European Photovoltaic Solar Energy Conference, 2009,pp. 1690– 1693.[2] M. Taguchi, M. Tanaka, T. Matsuyama, T. Matsuoka, S. 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Green, Keith Emery, Yoshihiro Hishikawa, Wilhelm Warta, Solar cellef?ciency tables (Version 34), Progress in Photovoltaics: Research andApplications 17 (2009) 320– 326.T. Mishima et al. / Solar Energy Materials & Solar Cells 95 (2011) 18– 21 21