Bagnall, D.M.

Boden, S.A.

Crudgington, L.J.

Rind, M.A.

English

Southampton (e-Prints Soton)

High Performance Hetero-Junction Solar Cells with Plasmonic Light Trapping. L. J. Crudgington*, M.A. Rind, S. A. Boden and Prof. D. M. Bagnall. ● University of Southampton, University Road, Southampton, SO17 1BJ * Corresponding Author ljc1g09@ecs.soton.ac.uk    This  paper  attempts  to  describe  the process  flows  and  the  optimum deposition  parameters  for  high-performance heterojunction solar cells, and  the  proposed  introduction  of plasmonic  nanoparticles.  In  the investigation,  the  optimum  deposition conditions  for  the  crystalline  / amorphous  heterojunction  will  be experimentally determined, and charge-carrier  lifetime  measurements  will  be presented.  Investigations  will  then  be performed  on  self-aligned  Ag nanoparticles deposited on amorphous silicon,  and  the  optimal  particle configuration  based  on  annealing temperature  and  duration  will  be identified. Introduction Hetero-junction  solar  cells  are  currently one  of  the  best  performance  designs  for achieving  high  energy  conversion efficiency in solar cells. This is due to their innovative use of a single-crystal silicon as a  light  absorption  layer,  and  amorphous silicon  intrinsic  and  doped  regions  which provide the charge carrier separation and passivation  of  the  silicon  material.  [1,2] This design prevents much of the charge carrier  recombination  at  the  device surfaces  and  therefore  demonstrates energy  conversion  efficiencies  of  up  to 25%.  The  purpose  of  this  work  is  to investigate  further  enhancement  of  the performance  of  the  devices  achieved  by applying  plasmonic  light-trapping treatments,  which  enhance  the  optical path length of the incoming light photons and  ensure  optimum  light  absorption.  [3] This additionally allows for devices which use  less  silicon  material  as  thinner absorption layers are necessary to absorb incoming  photons,  leading  to  a  reduction in device cost. Device Architecture The  combined  use  of  doped  and intrinsic  amorphous  silicon  layers deposited  at  a  low  temperature  on crystalline  absorber  layers  provides significantly  superior  surface  passivation compared  to  conventional  devices.  This design  eliminates  the  high-temperature dopant  diffusion  and  oxide  passivation steps of conventional solar cells. Because the  amorphous  layers  are  not  used  for absorption  and  carrier  generation,  the device  does  not suffer from light-induced degradation as purely amorphous devices do,  while  the  use  of  n-type  crystalline wafers  ensures  that  no  boron-oxygen reactions occur and create recombination centres  in  this  region  after  exposure. Additionally,  the  improved  surface passivation  reduces  the  temperature dependence of the device and ensures the maximum possible charge carrier lifetimes. [4-5] Due  to  the  significant  performance losses  which  can  result  from  reflection from  the  top  surface,  detailed  attention must be paid to the optical topology. The use  of  anti-refection  coatings,  light-trapping  and  path-length  enhancement technologies can significantly improve the performance of a device, while the use of these  technologies  can  also  reduce  the required  device  thickness  by  several times.  [6-7]  Light-scattering  concepts  in particular are  very  important in achieving high-efficiency  photovoltaic  device designs,  which  cause  optical  path-length enhancement by preferential scattering of the  incident  light  into  the  semiconductor absorber  layer  material.  This  scattering can  be  achieved  via  the  method  of localised  surface  plasmons  by  the positioning  of  sub-wavelength  plasmonic metal nanoparticles within the photovoltaic device. [8]. Experimental Detail The modern device topology  is based upon a low-doped crystalline silicon n-type wafer.  The  amorphous  silicon  layers  are then  deposited  on  either  side,  which initially  consist  of  a  very  thin  layer  of intrinsic  material,  followed  by  the  doped emitter and back-surface field regions. The deposition  of  the  intrinsic  layers  in  this investigation is performed using an Oxford Instruments PlasmaLab System 100. The excitation  frequency  of  this  system  is 13.56MHz,  which  supplies  ample excitation  energy  at  low  power  whilst minimising  ion  bombardment  damage  to the growing films. The deposition chamber is supplied with source gases via a series of  programmable  mass-flow  controllers (MFCs)  which  combine  and  provide  the atomic  species  for  CVD  via  a  uniform shower-head arrangement.  Deposited layers are then coated with a thin TCO layer, which is necessary due to  the  low  charge  carrier  mobility  of  the doped  layers,  and  then  finally  the metallisation grid is deposited. Texturing of the wafer to improve light absorption and enhances  internal  reflection  and  light trapping, the inclusion of the back-surface field  and  rear  passivation  layers  have been  included  in  recent  devices  to maximise  performance,  and  the  current state-of-the-art  devices  approach efficiencies of 25%. The  highest  quality  of  amorphous silicon  material  is  that  which  has  a  low density  of  stressed  Si  bonds,  fewer defects,  a  smaller  quantity  of  micro-void regions  and  a  slightly  decreased  band-gap. This material has a lower density of bonded hydrogen within the matrix and is therefore  said  to  be  less  susceptible  to light-induced degradation. [9-10] The first step of the fabrication process is  to  determine  the  optimum  deposition conditions for the amorphous silicon. The quality of the amorphous silicon thin films are variable depending on the deposition conditions,  so  these  purposes  it  is essential  to  fabricate  individual  thin  film single  layers  using  varying  deposition conditions  and  thoroughly  examine  their optical  and  electrical  performance  before committing these conditions to a complete heterojunction device. The first step is to examine the dark and photo conductivities (σ),  optical  band  gap  (Eg),  optical absorption  and  reflectivity  at  various wavelengths and thin film thickness. From these  experiments  it  is  possible  to determine  specific  deposition  problems such  as  inefficient  doping  of  layers  or contamination of intrinsic material. Once  optimum  layer  quality  has  been obtained,  deposition  of  heterojunction structures using the optimised single-layer conditions  is  performed,  and  the  device can  be  examined  by  means  of  charge carrier  lifetime  measurements,  which  are fundamental  in  determining  the  energy conversion  efficiency  (η),  fill  factor,  VOC, ISC, and maximum power output of the final device.  Fabrication A.  Intrinsic Layers The intrinsic region of the heterojunction structure  is  fundamental  in  defining  the performance  of  the  solar  device.  The intrinsic  region  performs  surface passivation  of  the  crystalline  substrate absorber layer, and therefore must be free from all contamination which could provide Shockley-Reid-Hall recombination centres.  It is vitally important that the temperature is maintained below 180°C to prevent the amorphous  material  crystallising  on contact  with  the  substrate  material.  The hydrogen  passivation  mechanism  is additionally  very  important  in  this  region, so  while  the  atomic  hydrogen  species  in the  SiH4  source  gas  is  often  sufficient  in PECVD,  dilution  in  additional  hydrogen (H2) has been investigated.  Care must be taken  with  this  dilution  ratio,  as  the restructuring effect of hydrogen can result in  microcrystalline  films  being  formed  at sufficient dilutions. The  complete  set  of  experimental results are published elsewhere, [11] while the  results  of  this  investigation  are presented in table 1. The resultant silicon thin films are investigated electronically via their  photo-  and  dark-conductivity measurements  and  the  optimum  layer conditions are identified. Table 1 - Intrinsic Layer Deposition Parameters Variable  Unit  Value RF Power  (W)  10 Temp  (°C)  180 Chamber Pressure  (mTorr)  350 SiH4 Flow Rate  (sccm)  50 H2 Flow Rate  (sccm)  50 Thickness  I (nm)  375 Deposition Rate (p-type)  (nm/sec)  0.60 Deposition Rate (n and i-type)  (nm/sec)  0.26   B.  Heterojunction Devices Based on the optimum results from the single layer investigations in section A, the deposition  of  the  silicon  heterojunction structures  have  been  fabricated.  The substrate material is a 625µm double-side polished  n-type  crystalline  <100>  silicon wafer, with resistivity between 1-30 Ω-sq. In  order  to  prepare  both  sides  of  the device, an oxide cap is required to protect one  side  of  the  silicon  surface.  This  is deposited  using  PECVD  to  a  thickness depth  of  300nm,  after  thorough  RCA cleaning using the sequence. After these steps, it is necessary to perform a further cleaning  step  immediately  before deposition  of  the  first  amorphous  silicon layers.  The  first  side  of  the  device  to  be fabricated  is  the  intrinsic  and  n-type regions, which are performed in sequence using  PECVD  based  on  the  optimum conditions determined in section A. 15nm of  intrinsic  material  is  first  deposited followed  by  15nm  of  n-type  doped material.  This  is  then  immediately transferred to a Leybold Optics LAB700 e-beam assisted evaporation system for the deposition  of  200nm  of  indium-doped  tin oxide  to  enhance  the  charge  carrier collection from the silicon surface. The ITO layers are deposited to a resistivity of 20 Ω-sq using a rate of 2.5Å/s. After this step it  is  advisable  to  perform  a  chamber ‘recondition’ on the plasma deposition tool, in order to mask any dopants which may be  on  the  chamber  surfaces  and  affect subsequent films. The  next  stage  in  the  deposition  is  to protect  the  silicon  and  ITO  layers  whilst the sample is once again cleaned and the other  side  is  deposited.  This  is  achieved by  the  spinning  of  6µm  AZ9260  photo-resist,  chosen  for  its  resistance  to  high temperatures  and  cleaning  solution, followed by a subsequent hard-bake. After this  step  it  is  necessary  to  perform  the cleaning  routine  once  more  and additionally  perform  an  extended  oxide etch for around 60 seconds in hydrofluoric acid  (HF)  to  remove  the  protective  oxide on the second side  deposited in the first step. This step can be continued until the surface  becomes  hydrophobic  and  the pure silicon surface is exposed. The second side of the device can now be  fabricated,  which  is  performed  under the same conditions as the first apart from the  doped  layer,  which  is  p-type  in  this instance. Once again the newly deposited silicon  material  is  transferred  to  the  e-beam evaporator which deposits a further 200nm to the p-type side. This side is now able  to  be  metallised  with  the  aluminium contact  grid.  Once  the  p-type  side  is completed, the  photoresist layer must be removed  using  a  plasma  ashing  step,  at 800W  2.45GHz  RF  in  an  environment  of oxygen  /  nitrogen.  Following  this,  the aluminium  metallisation  grid  can  be deposited on the n-type side using e-beam evaporation finalising the completed cell. Results The  performance  of  the  heterojunction  is in part proportional to the minority charge carrier  lifetime.  A  high  charge  carrier lifetime  increases  the  likelihood  that collection of a charged particle will occur and will and is therefore proportional to the efficiency of the solar cell. Initial  results  are  concerned  with devices which have a silicon nitride cap as opposed  to  the  indium-doped  tin  oxide layer. Figure 1  depicts the charge carrier lifetime measurements, performed using a calibrated  Sinton  WCT-120  Photo-conductance Lifetime Tester.  Figure 1 - Charge Carrier Lifetime C.  Self-Aligned Nano-particles In order to enhance the optical absorption within  the heterojunction  device, a series of self-aligned silver nanoparticles will be deposited  within  the  surface  transparent-conductive oxide layer. Particle arrays are fabricated  by  the  e-beam  assisted evaporation  of  silver  granules  on  to  the substrate material, which are subsequently annealed  for  a  specific  time  period  and temperature.  The shape, dimensions and periodicity  of such particles is dependent on these conditions of fabrication, such as the  material,  layer  thickness,  and  anneal time and duration. These attributes define the extent to which light is scattered within the  absorber  layer,  and  at  which wavelengths.  It  is  therefore  essential  to accurately  tailor  the  response  of  the particle  array  to  a  particular  type  of photovoltaic  device,  to  ensure  that  the optimum  performance  enhancement  is achieved.  Fabrication The deposition of the particle arrays is performed  on  375nm  of  intrinsic amorphous  silicon,  deposited  on  a  plain glass  substrate  using  PECVD  with  the optimum  electrical  and  optical  quality  for photon absorption as described in section A.  A  layer  of  silver  is  subsequently deposited directly on to the silicon using a Leybold  Optics  BAK600  e-beam  assisted evaporation  system.  The  depth  of  these layers is varied between 5, 10 and 15nm thicknesses  although  15nm  only  is presented  here.  The  layers  are subsequently  transferred  to  a  Jipelec JetFirst 200C rapid thermal annealer and optically  heated  to  temperatures  ranging from 225°C to 300°C for between 30 and 90 minutes. Measurement  is  performed  using  a Bentham PVE300 quantum efficiency and integrating  sphere  system,  first  by measuring  the  optical  transmission  over the  wavelength  range  400  –  1100nm followed by the optical reflection over the same range. The optical absorption of the device can therefore be calculated which will  illustrate  the  performance  of  the device. Results The data presented in figure 2 shows the optimum  light  absorption  values  for  the best performing durations for annealing at temperatures  ranging  from  225°C  – 300°C. Wavelength (nm)400 500 600 700 800 900 1000Absorption (%)01020304050607080Silicon300°C275°C250°C225°C Figure 2 - Optical Absorption of Silver Arrays  The  results  from  the  investigation illustrate  the  extent  to  which  the  self-aligned nanoparticle arrays can affect the absorption of light within a silicon thin film. For this data set of 15nm films, it can be determined  that  an  annealing  step  at  a temperature of 300°C for a total duration of  70  minutes  provides  the  optimum absorption  of  photons.  Figure  3  below illustrates  an  SEM  image  of  the nanoparticles,  which  confirms  their  size, shape and diameter.  Figure 3 - SEM Image of Silver  Nano-Particles Summary  It  was  experimentally  determined  that excellent silicon wafer passivation can be achieved using amorphous silicon within a hetero-junction structure, based on charge carrier  lifetime  measurements. Subsequently,  the  optimum  light absorption  conditions  were  found  by  the use  of  self-aligned  silver  nanoparticle arrays  on  amorphous  silicon.  The continuation of this work is the fabrication of a functional heterojunction solar cell as per the process flow defined in this paper, and to deposit the silver arrays on to the device  while  observing  the  optical  and electrical performance compared to a cell with no such nanoparticle coating and / or conventional silicon wafer texturing steps. References [1]  Mishima, Takahiro, et al. Solar Energy Materials and Solar Cells 95.1 (2011): 18-21 [2]  Tanaka,  Makoto,  et  al.  Photovoltaic  Energy Conversion,  2003.  Proceedings  of  3rd  World Conference on. Vol. 1. IEEE, (2003). [3]  Tristan  L.  Temple  and  Darren  M.  Bagnall Progress  in  Photovoltaics:  Research  and Applications (2012). [4]  Sark, Wilfried G. J. H. M.; Korte, Lars; Roca, Francesco, Springer, (2011).  [5]  Mishima, T., Taguchi, M., Sakata, H., Maruyama, E.: Sol. Energ. Mat. Sol. 95, 18 (2010).  [6]  R.S.A.  Sesuraj,  T.L.  Temple,  D.M.  Bagnall, Solar Energy Materials and Solar Cells, Volume 111, (2013), Pages 23-30, [7]  Catchpole,  K.  R.,  and  Albert  Polman,  Optics express 16.26 (2008): 21793-21800.  [8]  Hairen  Tan,  Rudi  Santbergen,  Arno  H.  M. Smets,  and  Miro  Zeman,  Nano Letters (2012) 12 (8), 4070-4076 [9]  Fehr, M., Schnegg, A., Rech, B., Astakhov, O., Finger, F., Bittl, R., Teutloff, C., Lips, K.. Rev. Lett., (2014)  [10]  A. H. Mahan, J. Carapella, B. P. Nelson, R. S. Crandall, and I. Balberg, J. Appl. Phys. 69, 6728 (1991).  [11]  L. J. Crudgington et al. Molecular Crystals and Liquid Crystals, 589 (2014) 

Molecular Crystals and Liquid Crystals,

Photovoltaic Energy Conversion,

Solar Energy Materials and Solar Cells, Volume 111,

High performance hetero-junction solar cells with plasmonic light trapping

High performance hetero-junction solar cells with plasmonic light trapping

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