Proof of Down-Conversion by CdSe/ZnS Quantum Dots on Silicon Solar Cells

Transcription

1 Proof of Don-Conversion by CdSe/ZnS uantum Dots on Silicon Solar Cells Bahareh Sadeghimakki, Zhen GaO and Siva Sivoththaman Centre for Advanced Photovoltaic Devices and Systems, Electrical and Computer Engineering Department, Waterloo, Ontario, N2L 3G1, Canada Abstract - The performance of the crystalline silicon solar cells can be improved ith spectrally engineered layers. Core/shell CdSe/ZnS quantum dots (Ds) ith tunable absorption-emission avelengths in UV-Vis range can be applied as luminescence centers in luminescence don-conversion layers (LDC). In this ork, CdSe/ZnS Ds ith an emission avelength of 620nm and luminescence quantum efficiency (LE) of 85% ere deployed on planar and textured crystalline silicon solar cells for don-conversion. The structural and optical characteristics of the Ds and the employed layer ere studied. The cell performances before and after LDC layer deployment ere investigated. Experimental verification as obtained on luminescence don-conversion of the incident photons in the cells containing LDC layers. Index Terms - photoluminescence, silicon solar cells. CdSe/ZnS quantum dots, don-conversion, I. INTRODUCTION Lninescence don-conversion (LDC) is one of the spectral engineering approaches [1-4] that can improve solar cell performance by means of absorption and emission in luminescence centers such as rare-earth ions [5], dyes [6] and quantum dots (Ds) [1, 7, 8]. The avelength of the incident photons can be don-converted, from the avelengths here the spectral response of the solar cell is lo, to the avelengths here the spectral response of the cell is high. High brightness, stability and broad absorption band are the advantages of Ds ith respect to other luminescent materials, hich makes them potential candidates for use in donconversion layers [7, 8]. Since LDC is a passive approach, it eliminates any interference ith the active material of a photovoltaic (PY) device hich is technically favorable. Hence, the method does not add any complication to the production of the existing device. One factor that limits the LDC method from being perfectly beneficial is the relatively lo luminescence quantum efficiency (LE) of existing Ds. Exploiting highly luminescent Ds in LDCs needs to be considered in order to gain from this method. In this ork, CdSe/ZnS Ds ith high LE ere deployed on planar and textured crystalline silicon solar cells to examine the don-conversion effect. The morphology and photoluminescence (PL) properties of the Ds and the employed layer ere studied. The performance of cells ith LDC layers as compared ith a reference cell. The improved performance of cells ith LDC layers proved the D donconversion effect. II. METHODOLOGIES FOR LDC LAYER FORMATION A. Structural Characteristics ojcdselzns in Liquid Form Core/shell CdSe/ZnS Ds dispersed in toluene and stabilized in octadecyl amine (ODA) ere purchased from a commercial supplier. Fig. 1 exhibits the Transmission Electron Microscope (TEM) image of the Ds prepared by dropcasting a diluted solution onto a carbon coated copper grid. The Ds average particle size is 6nm and they are ell separated from one another ith no agglomeration. The lattice structure of the Ds in the zone axis is clear in the high resolution (HR)-TEM image, indicating their Wurtzite crystalline structures (see inset in Fig. 1). The thickness of the shells is about 6nm and the unclear interface beteen the CdSe core and ZnS shell is due to the epitaxial groth of the shell and small lattice mismatch beteen the core and the shell. Fig. 1. TEM micrograph of core/shell CdSe/ZnS Ds, inset is the HRTEM image of a single D B. LDC Layer Formation and Optical Characterization For deployment of Ds in the LDC layer, Ds ith a peak emission avelength of 620nm ere used. Ds have broad absorption bands that result in absorption of the entire light ith the avelength smaller than the absorption maximum (See Fig. 2). The absorption and PL spectra in Fig. 2 sho that CdSe/ZnS Ds are appropriate candidates to use as donconverting material for the silicon solar cells /14/$ IEEE 2262

2 Fig. 2. D layer...j CL 0.2 s ).t; 0.08 " g 0.06 jg -"' L'O!!!!!'1!!!... --'-----1!1...-'--_-.J Absorption/emission spectra of 6nm core/shell CdSe/ZnS Introducing the 6nm Ds in the LDC layer results in absorption of photons ith avelengths smaller than 620nm, here the response of the cell is lo; and don-converting them to the longer avelength (620nm), here the response of the cell is high. Hence the strongest LDC effects ere expected to be seen. For layer formation, CdSe/ZnS Ds from the colloidal solution ere spin cast on the substrate in a glove box under controlled ambient. Spin coating conditions ere controlled for the concentrated and diluted ensemble of CdSe/ZnS quantum dots to obtain thin films containing CdSe/ZnS ith a specific thickness, D concentration and uniformity for achieving the highest gain from the LDC layer. Fig. 3 demonstrates the TEM image of a D layer formed by spin casting from a 5mglmL colloidal solution onto a silicon substrate at 1000 rpm for 30s. The substrate is isolated from the Ds ith a thin silicon nitride film. The layer as then capped ith a spin on glass (SOG) film..c «:J ;?: 'iii c.$ c -l a... Fig Ex::300nm Wavelength (nm) :tj-oo"' 4OOIXIO x. Max 3! Wrrelenglh{nm) 700 CdSe/ZnS Ds emission map at excitation avelengths in the 300-0nm range. emission maximum. Inset is the D excitation spectrum at The spectrum depicts that the emission intensity is about to-fold higher at the excitation avelengths, in the range of 300-3nm, hich is about to times the bandgap of crystalline silicon. The inset shos the D excitation spectrum at D emission maximum hich also shos that the Ds have the highest efficiency at nm ith the excitation peak intensity at 320nm. Therefore the DC effect should be more pronounced in this range. III. DEVICE CONFIGURATION AND PERFORMANCE A. Device Configurations ith LDC Layers The device configurations that ere studied are depicted in Fig. 5. In the fust architecture, Ds ere spin-cast on top of the c-si solar cell ith an nm silicon nitride layer used as anti-reflection coating (ARC) and for isolation of the Ds from the cell. In the second architecture, the Ds ere fully buried in an SOG transparent layer. The thickness and transparency of the SOG layer needs also to be optimized for effective LDC performance. Photons Incident photons Incident photons escape Fig. 3. HRTEM image of a crystalline structure of the Ds hich uniformly deposited on a thin transparent silicon nitride film and capped ith SOG The excitation and emission scans ere performed on the D solution in an integrating sphere. The absolute value of 85% for the quantum efficiency as determined. Fig. 4 demonstrates the emission map of the LDC layer at different excitation avelengths ranging from 300-0nm. LDCt Cell t Fig. 5. (a) LDCt cell I Schematic diagrams of cells ith a) a D layer and b) D layer capped ith SOG used as LDC layers (b) 2263

3 For the LDC layer configuration ith no capping layer a large proportion of the don-converted photons ill reflect back from the surface and scatter to the top escape cone. A cap layer, ith refractive index larger than the air, allos a large proportion of the luminescent don-converted photons to be transmitted to the underlying cell, either directly or folloing internal reflection. B. Cell Characteristics after Deployment of the LDC Layer: Proof of Don-Conversion The effect of don-conversion on the cell performance as examined by exploiting a D layer on top of a planar c-si cell fabricated in our lab. In Fig. 6, the Internal uantum Efficiency (IE) of the cell structures ith LDC layer as compared ith the bare cell ith no luminescent materials. The IE higher than 100% in the avelength range of nm verifies the don-conversion effect. The cell ith the LDC layer shos higher reflectance ith respect to the bare cell (see Fig. 7). This indicates that some of the don-converted photons are reflecting from the surface, therefore not being collected by the underlying cell. This is additional evidence proves that the don-conversion is taking place in the cell architecture. Higher EE response of the cell ith the LDC layer as compared to the bare cell demonstrates that even though some 140 i-----;:====;_] 120 -PlanarceU - Planar eell+ldc la er Fig r----, -Planareell - Planar eell+ldc la or SOD Wavelength(nm} EE of a planar cell ith an LDC layer photons ere lost, due to higher reflectance, the cell still benefits from some don-converted photons (see Fig. 8). LDC layers ith different thicknesses ere also deployed on textured c-si cells fabricated in our lab. The same trend as observed for these cells. IE higher than 100% in the avelength range of nm as obtained (see Fig. 9) hich is proof of don-conversion. C r====== Texture cell - Relcell layer bilayers r---:s;;o la ers Fig. 6. Fig Wavelength (nm) IE of a planar cell ith an LDC layer <) c: t5 c:: == ======== - Planar cell - Planar cell+ldc la er O -L Wavetength (nm) Reflectance of a planar cell ith an LDC layer Fig. 9. IE of a textured cell ith different LDC thicknesses In these studies, highly concentrated Ds (Smg/ml) ere used to form the LDC layers. The re-absorption in a closelypacked D structure loers the luminescent quantum efficiency of the Ds in the layer. This causes non-desired absorption in the LDC layer in the avelength range here Ds can potentially emit (300nm<A<0nm). High D concentration also results in the increased reflectance. In order to loer the high reflectance and re-absorption in the LDC layer, a loer concentration of Ds (O.Smg/ml) as used. The LDC layers ere deployed on the solar cells using the same method as discussed above. In Fig. 10, the EE, IE and reflectance characteristics of a 2x2 cm 2 planar cell ith an LDC layer containing loer D concentration ere compared ith a reference cell. The solar cell ith the LDC layer shos a higher response hen compared to the bare cell in the part of the avelength range here the reflectance is higher. This indicates that the cell 2264

4 benefits from some don-converted photons. Also, the reflectance and absorption is less for the LDC layer ith loer D concentration. The illuminated current-voltage (IV) characteristic of the cells ith and ithout the LDC layer is also compared in Fig. 11. From the IV result, it is apparent that the current density and the overall cell efficiency are improved for the cell ith the LDC layer containing a loer D concentration. 100,-----, 0: 0/5 0/5 40 o EElBare planar cell - EOE/Cell+LOC layer... RlBare cell... RlCell+LOC layer -IOE/bare cell -IOE/Cell+LOC laver sho loer reflectance in the cell ith LDC as compared to reflectance previously obtained for the LDC containing highly concentrated Ds. Hoever, no improvement as observed in the overall efficiency of the device EOE/ Bar. planar cell - EOE/ Cell WIth LOC layer 50 _. - RI Bare cell 0/5 RI Cell ith LOC layer - toe/ Bar. cell o \ - toe/ Cell WIth LOC laver 25 ' '.;\.. - -,.,....." "./.... O L-----'- Fig. 12. EE, IE and reflectance characteristics of a 1 xl cm 2 planar cell ith an LDC layer containing loer D concentration Fig. 10. EE, IE and reflectance characteristics of 2x2 cm 2 planar cells ith an LDC layer containing loer D concentration Fig Bare planar cell - Cell ith LOC la er L---'-----' -'----'- J...lJ V (v) IV characteristics of 2x2 cm 2 planar cells ith an LDC layer containing loer D concentration In order to determine the effect of the cell size on the LDC, the D layer as deployed on the 1 x 1 cm 2 cell. Figures 12 and 13 sho improvements in the quantum efficiency and illuminated IV characteristics of the cell ith an LDC layer, respectively. The results shon in Fig. 12 also exhibit that the absorption and reflectance of the LDC layer ere better controlled in the smaller cells. Table 1 demonstrates the illuminated IV characteristics of the planar cells ith small and big dimensions and high and lo D concentrations. 2 rna and 0.31% improvement in current density and cell efficiency ere obtained respectively for the cells ith LDC layer containing a loer concentration of Ds. An LDC layer containing lo D concentration as also deployed on 1 x 1 cm 2 textured cells (see Fig. 14). The results Fig Bare planar cell - Planar cell ith LOC la er 0.00 L- --'-.l l.l ' V(v) IV characteristics of 1 x 1 cm 2 planar cells ith an LDC layer containing loer D concentration TABLE I ILLUMINATED IV CHARACTERISTICS OF LABORATORY SCALE PLANAR SOLAR CELLS WITH AND WITHOUT LDC LAYER Planar solar cells Voc Jsc FF '1 (V) (rna) (%) (%) Bare cell (small size) high D concentration Cell+LDC(small size) high D concentration Bare cell (big size) high D concentration Cell+LDC (big size) high D concentration Bare cell (small size) Lo D concentration Cell+LDC(small size) Lo D concentration Bare cell (big size) Lo D concentration Cell+LDC (big size) Lo D concentration 2265

5 0::: a EOEI Bare textured celt - EOEI Celt+LDC tayer... RJ Bare cetl RJ celt+ldc layer - IOEI Bare celt - IOEI Celt+LDC layer reflectance characteristics for the planar and textured cells ere obtained and shon in Fig. 15 and Fig. 16, respectively. The results sho that although the SOG cap layer loered the reflectance, the LDC layer ith the SOG cap layer suffers from high absorption at longer spectrum avelengths (0nm<A<1000nm). In order to benefit from the capping layer and therefore better cell perfonnance ith LDC, the thickness of the SOG needs to be optimized. It can also be replaced ith another cap layer ith loer absorption at this avelength range. IV. CONCLUSIONS Fig. 14. IV characteristics of 1 x 1 cm 2 textured cells ith and ithout an LDC layer containing lo D concentration As mentioned earlier, some of the don-converted photons reflect from the surface, therefore are not collected by the underlying cell. An LDC layer capped ith a transparent layer, ith a refractive index higher than air, can benefit from the total reflection of the emitted photons at the layer/air interface. As a result the emitted photons can be directed toard the cell more efficiently. A thin layer of SOG as spin-coated onto the D layer at 4000 rpm for 30s to fonn a capping layer. The EE, IE and 100 0::: 40 a 20 \ - :':' " " " " : - EOEI Bare planar cell - E.EI CeI+lOC - EOEI CeI+lOC+SOG.._.- RI Bare P'anar cell -RlCeIl+lOC -'RlCeII+LDC+$OG -IOEI Bare planar cell, r Fig. 15. EE, IE and reflectance characteristics of a 1 x 1 cm 2 planar cell ith LDC and SOG capping layer <F. Ii' 200 r ,========== &1 50,\ Textured cell - EEI Bare cell - EOEf CeU+LDC - EEI Cel1+lOC+SOG _._._.. RI Bare cell _._.- RI Cell+LDC _._._.. RI Cell+LDC+SOG -ICEI Bare cell -IOE! Cell+LDC - IOE! Cell+LDC+SOG Core/shell CdSe/ZnS Ds ith high LE ere exploited as luminescent centers on planar and textured crystalline silicon solar cells for don-conversion. The structural and optical characteristics of the Ds and the employed layer ere studied. Cell performance characteristics verified the luminescence don-conversion of the incident photons in both of the cell structures containing LDC layers. REFERENCES [I] B. Sadeghimakki, N. M. S. lahed and S. Sivoththaman "Spectrally resolved dynamics of synthesized CdSe/ZnS quantum dot/silica nanocrystals for photonic don-shifting applications," ieee Transactions on Nanotechnology (in press), 2014 ( rrNANO ). [2] E. Klampaftis, D. Ross, K. Mcintosh, B. Richards, "Enhancing the performance of solar cells via luminescent don-shifting of the incident spectrum: A revie," Sol. Energ. Mat. Sol. c., vol. 93, pp , [3] Z. Cheng, F. Su, L. Pan, M. Cao, and Z. Sun, "CdS quantum dot embedded silica film as luminescent don-shifting layer for crystalline Si solar cells," J. Alloy Compd., vol. 494, pp. L7-LlO, [4] E. Klampaftis, M. Congiu, N. Robertson, and B. Richards, "Luminescent ethylene vinyl acetate encapsulation layers for enhancing the short avelength spectral response and efficiency of silicon photovoitaic modules," IEEE J. Photovoltaics, vol. I, no. I, pp , [5] R. Nakata, N. Hashimoto, and K. Kaano, "High conversion efficiency solar cell using fluorescence of rare earth ions," Jpn. J. Appl. Phys., vol. 35, pp. L90-93, [6] L. Danos, T. Parel, T. Markvart, V. Barrioz, W. Brooks, and S. Irvine, "Increased efficiencies on CdTe solar cells via luminescence donshifting ith excitation energy transfer beteen dyes," Sol. Energ. Mat. Sol. c., vol. 98, pp , [7] T. Trupke, M.A. Green, and P. WOrfel, "Improving solar cell efficiencies by don-conversion of high-energy photons," J. Appl. Phys., vol. 92, no. 3, pp ,2002. Fig. 16. EE, IE and reflectance characteristics of a 1 xl cm 2 textured cell ith LDC and SOG capping layer [8] K. R. McIntosh, G. Lau, J. N. Cotsell, K. Hanton, D. L. Batzner, F. Bettiol, and B. S. Richards, "Increase in external quantum efficiency of encapsulated silicon solar cells from a luminescent don-shifting layer," Prog. Photovolt: Res. Appl., vol. 17, no. 3, pp , May