N型太阳能电池研究

Presented at the 23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, SpainTOWARDS 20 EFFICIENT N-TYPE SILICON SOLAR CELLS WITH SCREEN-PRINTED ALUMINIUM-ALLOYED REAR EMITTER Christian Schmiga, Martin Hermle and Stefan W. Glunz Fraunhofer Institute for Solar Energy Systems ISE Heidenhofstrae 2, 79110 Freiburg, Germany Phone 490761/4588-5201, Fax 490761/4588-9250, E-mail christian.schmigaise.fraunhofer.de ABSTRACT We present n-type silicon solar cells featuring an effectively passivated full-area screen-printed aluminium-alloyed rear emitter. Two different passivation stacks for Al- p emitters are investigated The first one consists of a plasma-enhanced-chemical-vapour-deposited amorphous silicon film covered by a plasma silicon oxide layer, the second one of a plasma-assisted atomic-layer-deposited aluminium oxide also covered by a plasma silicon oxide. For our a-Si/SiO x-passivated back junction n np solar cells 4 cm2 we achieve an increase in the open-circuit voltage of 15 – 20 mV compared to the non-passivated emitter cells, for our Al 2O3/SiOx-passivated cells the shift amounts to 25 – 30 mV, resulting in Voc values up to 655 mV. This leads to record-high efficiencies for solar cells with aluminium-doped emitter of 19.5 and 20.1 , respectively, on n-type phosphorus-doped 10 Ω cm float-zone silicon material. Keywords n-type Solar Cells, Aluminium-alloyed emitter, Passivation 1 INTRODUCTION n-type silicon n-Si has been proven to be a high-quality silicon material due to its larger tolerance to most common impurities e. g. Fe compared to p-type Si, resulting in higher minority carrier diffusion lenghts [1]. Additionally, n-Si is free of light-induced degradation related to boron-oxygen complexes. However, recently it has been reported that n-type multicrystalline mc Si appears to be less attractive for the application to rearjunction cells than previously assumed [2, 3]. From this, it can be concluded that i the production of n-type rear junction cells has to be restricted to monocrystalline mono Si because of its lower impurity concentration or that ii an adequate processing technology for frontjunction n-type mc Si cells has to be developed. In this paper, we focus on n-type solar cells fabricated on mono Si substrates. The two only companies, SunPower and Sanyo, which produce high-efficiency solar cells today are using n-type mono Si wafers which further supports the large potential of this material for the application to industrial high-efficiency cells. In the following, a short review of monocrystalline n-type silicon solar cells is given separated by the three different techniques for the p emitter formation 1.1 Boron-diffused Emitters Recently, Benick et al. [4] have achieved an efficiency of 23.2 for a front junction passivated emitter with rear locally diffused PERL solar cell 4 cm2 on 1 Ω cm n-type float-zone FZ Si. Mihailetchi et al. [5] reported an efficiency of 18.3 for a large-area 156 cm2 screen-printed Cz Si 1.5 Ω cm solar cell. n-type cells 146 cm2 with front boron emitter have also been fabricated by Buck et al. [6], reaching efficiencies of 17.1 on 2 Ω cm Cz Si. For passivated emitter with rear totally-diffused solar cells 22 cm2 featuring a boron-doped rear emitter re-PERT, Zhao et al. [7, 8] demonstrated efficiencies of 22.7 on 1.5 Ω cm FZ Si substrates and 20.8 on 5 Ω cm Czochralski-grown Cz Si wafers. Guo et al. [9] presented laser-grooved interdigitated backside buried contact IBBC cells 8 cm 2 with efficiencies of 19.2 on 1 Ω cm FZ Si and 16.8 on Cz Si. Froitzheim et al.have obtained efficiencies up to 17.4 for large-area 149 cm2 screen-printed n-type Cz Si cells with a boron-diffused back junction [10]. Conversion efficiencies of 22.7 have been reported by De Ceuster et al. [11] for SunPower’ s back-contact solar cell on n-type FZ Si material. Cells 149 and 155 cm2 with an efficiency mode of 22.4 are manufactured in the latest production line. 1.2 Amorphous Si/Crystalline Si Heterojunctions Sanyo’ s heterojunction with intrinsic thin layer HIT solar cell reaches efficiencies of 22.3 on n-type Cz Si wafers 100.5 cm 2 which has been published by Taira et al. [12]. In mass production, an averaged cell efficiency of 19.5 is obtained [13]. Conrad et al. [14] have fabricated 19.8 efficient a-Si/c-Si cells without additional intrinsic buffer layer on n-type Si substrates. 1.3 Aluminium-alloyed Emitters For nnp solar cells 4 cm 2 featuring an aluminium-doped emitter on the rear side formed by annealing of evaporated high-purity aluminium, Cuevas et al. [15] have achieved efficiencies up to 16.9 on 80 Ω cm FZ Si material. Using laser-fired local Al emitters LFE, Glunz et al. [16] have obtained 19.4 on 100 Ω cm FZ Si 4 cm 2. By applying a full-area screen-printed Al-p emitter, Schmiga et al. [17] demonstrated efficiencies of 18.9 on 4 Ω cm Cz Si 4 cm 2. For front and rear screen-printed n-type cells with Al- p back junction, several results have been published during recent years Hacke et al. [18] reported an efficiency of 15.0 for the PhosTop cell 100 cm2 on 1 Ω cm FZ Si. Buck et al. [19] and Kopecek et al. [20] attained efficiencies of 15.3 on 5 Ω cm Cz Si 4 cm2 and 16.4 on 20 Ω cm FZ Si 150 cm2, respectively. Schmiga et al. [21] and Nagel et al. [22] achieved an efficiency of 17.0 on 4 Ω cm Cz Si material 100 cm 2. Mihailetchi et al. [23] presented 17.4 efficient nnpsolar cells 140 cm 2 made on 31 Ω cm FZ Si wafers. Presented at the 23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain1.4 This work In this work, we focus on back junction n np solar cells featuring a full-area screen-printed aluminium-alloyed rear p emitter. In order to exploit the advantages of the excellent electrical properties of the n-type Si bulk material, an adequate passivation of the Al-doped emitter is essential. Therefore, we investigate two different passivation layers i Amorphous silicon layers formed by means of plasma-enhanced chemical vapour deposition PECVD Recently, excellent passivation properties of a-Si films have been proven on boron-diffused and aluminium-alloyed emitters. Emitter saturation current densities J0eof lower than 100 fA/cm 2 have been achieved on 30 –225 Ω /sq boron-doped p emitters with a minimum J0evalue of 24 fA/cm 2 for a sheet resistance of 225 Ω /sq by applying amorphous silicon/silicon nitride double layers [24, 25]. On etched 70 Ω /sq Al-doped p emitters, J0e values of 250 fA/cm 2 have been obtained using single a-Si layers, corresponding to implied open-circuit voltages Voc.impl above 660 mV [26]. ii Aluminium oxide layers prepared by plasma-assisted atomic layer deposition ALD Al 2O3 has been demonstrated to create a high-quality field effect passivation as it contains a high fixed negative charge density up to 10 13 cm-2 which effectively shields electrons from the Si surface [27]. The Al 2O3films limit the emitter saturation current density of B-diffused p emitters to 10 and 30 fA/cm 2 on 100 and 54 Ω /sq emitters [28]. Applying ALD Al 2O3 layers to boron-doped p emitters, Benick et al. achieved an efficiency of 23.2 for a front junction n-type PERL solar cell with an open-circuit voltage of 704 mV [4]. As for our n np solar cells the Al- p emitter is located at the cell ’ s rear, we cover both passivation layers by PECVD silicon oxide to increase the internal reflectance. Additionally, the thermal stability of a-Si films is improved by a covering SiO x layer [29]. The two main challenges of this work are i implementation of an effective passivation for Al-doped emitters into our nnp cell process and ii demonstration of the high potential of Al- p emitters for n-type Si solar cells. We first of all describe in detail our nnp solar cell structure and the processing sequence. Subsequently, we present results and characteristics for our n-type cells featuring full-area screen-printed aluminium-alloyed rear p emitters passivated by a-Si/SiO x and Al 2O3/SiO x stacks, respectively. 2 SOLAR CELL FABRICATION 2.1 Cell Structure We fabricated n np solar cells with a high-efficiency front side consisting of i a textured surface with inverted pyramids, ii a phosphorus-diffused nregion with a sheet resistance of 120 Ω /sq, acting as a front-surface field FSF and providing a good electrical contact to the base, iii a thermally grown silicon oxide layer as surface passivation and antireflection coating and iv a contact grid formed by evaporation of a TiPdAg seed-layer followed by silver plating. On the rear, we investigate and compare three different kinds of passivation for full-area Al- p emitters i without additional passivation layer, emitter entirely contacted, ii with a-Si/SiO x passivation stack, contacted via point contacts and iii with Al 2O3/SiOx passivation stack, contacted via point contacts. Figure 1 shows the schematic cross sections of the realised nnp rear junction cell structures with non-passivated and passivated aluminium emitters, respectively. a TiPdAg metal gridn-type Si baseEvaporatedAl rear contactSiO2 AR coatingAl- p rear emittern front surface fieldInverted pyramidsb TiPdAg metal gridn-type Si baseEvaporatedAl rear contactSiO2 AR coatingAl- p rear emittern front surface fieldInverted pyramidsSiOxa-Si or Al 2O 3Point contactsFigure 1 Schematic cross sections of nnp n-type Si solar cells with full-area screen-printed aluminium-alloyed rear p emitter a non-passivated Al- p emitter, b a-Si/SiO x- and Al 2O3/SiO x-passivated Al- p emitter, respectively. 2.2 Processing Sequence In this study, we used n-type phosphorus-doped float-zone silicon wafers with a resistivity of 10 Ω cm as base material for our n np solar cells. For this material we have measured extremely high effective lifetime values τ eff up to 10 ms using quasi-steady-state photoconductance QSSPC and photoluminescence QSSPL methods, see Figure 2 [30]. Both surfaces of these lifetime samples are passivated by a 120 Ω /sq ndiffusion and 105 nm thick thermally grown SiO 2 layers. After removal of the saw damage from the starting wafer, a silicon oxide etching mask for inverted pyramids is grown by a dry thermal oxidation in an open quartz-tube furnace. After photolithographically structuring the oxide on the front surface and texturing the wafer in KOH solution, this oxide layer acts as diffusion barrier on the rear side during the subsequent POCl 3 diffusion to form a phosphorus-doped n front-surface field with asheet resistance of about 120 Ω /sq. In the next step, a short dip in HF solution removes the phosphorus glass on the front and the oxide mask on the rear. After that, a 105 nm thick antireflection oxide layer is thermally grown and removed from the rear. Now, a non-fritted Presented at the 23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain10 13 10 14 10 15 10 16 10 1710 110 210 310 410 5PLn-type P-doped FZ Si, 10 Ω cm Starting materialQSSPCEffectivecarrierlifetimeτ eff[μs]Excess carrier density [cm -3 ]Figure 2 Measured QSSPC and QSSPL lifetime τ eff as a function of the excess carrier density of the n-type phosphorus-doped FZ Si starting material used for cell processing in this work. aluminium paste is screen-printed onto the entire rear surface and, subsequently, the p emitter is alloyed in a conveyor belt furnace at peak temperatures around 900 C. After firing, the residue of the aluminium paste and the eutectic layer are etched off in HCl. At this point of the process, the batch is split up into three parts i cells with non-passivated Al- p emitter, ii cells with a-Si/SiO x-passivated emitter and iii cells with Al 2O3/SiOx-passivated emitter. To prepare the rear p emitter surface for an effective passivation, we perform a short KOH dip, see section 2.3. After an additional RCA cleaning, for the cells of part ii a 70 nm thick PECVD amorphous silicon layer is deposited on the rear Al- p emitter surface at 250 C, and the cells of part iii receive a 30 nm thick ALD Al 2O3 layer at 200 C at Eindhoven University of Technology, The Nether-lands [27]. Then, an additional 150 nm thick PECVD silicon oxide layer is deposited on the rear of the cells of both parts ii and iii at 260 C. After that, rear contact points in the SiO x layer are photolithographically opened followed by plasma etching for part ii and an HF dip for iii to open the a-Si and Al 2O3 layer, respectively. For the following steps, all cells are processed together in one batch again. Now, the full-area aluminium contact is evaporated on the entire rear, and the front contact grid is formed by evaporating Ti, Pd and Ag and a photolithographic lift-off process. After that, the front contacts are thickened by light-induced Ag plating [31] and, finally, the solar cells are annealed in a forming gas ambient at 350 – 425 C. 2.3 Preparation of Aluminium Emitter Surface The doping profile of the aluminium-alloyed emitter was detected by electrochemical capacitance voltage ECV measurements, see Figure 3. Therefore, we removed the paste matrix consisting of Al-Si particles from the surface and the Al-Si eutectic layer using HCl solution [32]. Subsequently, we cleaned the surface by a short KOH dip to etch off aluminium-rich structures which otherwise would create an Al concentration peak in the doping profile of about 10 19 cm-3 close to the surface [26]. A well prepared surface is essential for an effective passivation of the Al- p emitter. 0 2 4 6 8 10 1210 1510 1610 1710 1810 1910 20After removal of paste matrixand Al-Si eutectic layerand KOH dip for surface cleaningAluminiumdopantdensity[cm-3]Depth [ μ m]Figure 3 ECV doping profile measurement of the screen-printed aluminium-alloyed rear p emitter of an nnp Si solar cell. The thickness of the Al- p region of about 10 μ m obtained from the doping profile has also been verified by a scanning electron microscope picture of the cross-section. The interface between the Al-doped p emitter and the n-type Si bulk clearly appears due to the potential contrast, see Figure 4. n-type Si bulkAl-p emitterRear surfaceFigure 4 SEM picture of a cross-section of a screen-printed aluminium-alloyed p emitter fired under the same conditions as the sample of Figure 3. The Al-doped p region with a thickness of 10 – 12 μ m is clearly visible. 3 SOLAR CELL CHARACTERISATION 3.1 Solar Cell ResultsTable I summarises the electrical parameters of our back junction n-type silicon solar cells featuring differently passivated aluminium-alloyed p rear emitters, fabricated in the course of this work. Figure 5 shows the IV curves of these cells measured at Fraunhofer ISE CalLab. The effectiven