Phase Engineering of High Efficiency aSi:H Solar Cells

1
Phase Engineering of High Efficiency a-SiH Solar
Cells
Center for Thin Film Devices The Pennsylvania
State University

• C. Wronski, R. Collins1, J. Pearce2, J. Deng, G.
  Ferreira, N. Podraza, M. Albert, C. Chen
• Center for Thin Film Devices, The Pennsylvania
  State University, University Park, PA 16802
• 1Department of Physics and Astronomy, University
  of Toledo, Toledo, OH 43606
• 2106 Pierce Science Center, Clarion University of
  Pennsylvania, Clarion, PA 16214

Acknowledgements - NREL
2
Introduction

• High performance a-SiH based solar cells
  obtained with hydrogen dilution
• In systematic optimization
• Take into account protocrystalline nature of
  materials during growth
• Improve understanding of mechanisms limiting cell
  performance and stability
• Develop and apply deposition phase diagrams
  guided by RTSE
• Clearly identify characterize contributions
  ofcontacts, p/i interface and bulk
  recombination, and light induced defects

3
Real Time Optics of Silicon Film PECVD

• Developed at PSU being applied in other
  laboratories
• Allows in situ characterization of growth
  (surface roughness) microstructure, optical
  properties 1.5 to 4.5eV
• Acquisition time 50ms allows monolayer growth to
  be characterized

Suitable for analysis of inhomogeneous films with
micro/macro/geometric scale structure
4
Extended Phase DiagramSiH Growth on c-Si/oxide
Substrates
Obtained from the three transitions detected
during SiH film growth

• (1) a ? a surface roughening transition
• (2) a ? (amc) surface roughening transition
• (amc) ? mc surface smoothening transition
• These transitions provide insights into materials
  and device optimization

Narrow window for protocrystalline SiH growth in
a thick layer is centered at R10here the film
surface is stable throughout deposition.
Microstructure and its evolution is strongly
dependent on substrate
5
Deposition Phase Diagram for rf PECVD SiH Film
Growth on R0 a-SiH Film Surfaces

• Phase diagrams
• describe regimes of db
• and R within which
• a-SiH, (amc)-SiH,
• and mc-SiH films are
• obtained in the growth
• process
• The a ? (amc) transition
• thickness is deduced
• by RTSE from the
• roughening transition
• The (amc) ? mc transition
• thickness is deduced
• by RTSE from a
• smoothening transition

AFM study
XTEM study
rf PECVD 13.56 MHz Subst. temp. T200C rf
power P0.08 W/cm2 SiH4 partial pressure
p(SiH4) 0.05 Torr
Phase diagrams depend on the substrate and
deposition conditions
6
Simple Phase DiagramsSubstrate Dependence of a
? (a?c) Transition

• Maximum H2-dilution levels for a 200 Å thick
  layer (e.g., for a protocrystalline interface
  i-layer) while remainingbelow the a ? (amc)
  transition
• ?c-SiH substrate R 7.5
• c-Si/oxide substrate R 20
• a-SiH substrate (R0) R 40

• Under protocrystalline growth conditions, the
  phase of the nucleating film depends on the
  substrate
• Local epitaxy occurs formc-SiH substrates
• Barrier to mc-nucleation exists for a-SiH
  substrates

rf plasma power P0.08 W/cm2 Substrate temp.
T200C
7
TEM of SiH Deposited with R20 on c-Si/SiO2
8
TEM of SiH Deposited with R10 on Cr (evap)
9
Optimization Principle for Two-Step i-Layer of
a-SiH p-i-n Solar Cell

• Optimization principle
• Prepare interface and bulk
• i-layers with the maximum
• RH2/SiH4 values possible
• without crossing the a?(amc)
• transition for the desired
• thickness ? concept of
• protocrystallinity is useful
• Difficulties
• If the a?(amc) transition is
• crossed accidentally in this
• process, one must decrease R
• below 10 (below protocrystalline
• regime) to suppress continued
• growth of the microcrystallites ?
• real time monitoring and
• control are needed

• Two step optimization of R10 bulk i-layers with
  R40 p/i 200Å layer
• Improvement
• Voc 0.88 to 0.92V
• Annealed FF same 0.72
• R0 p/I Voc 0.88 to 0.84

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Nature of (amc) Phase and Its Effect on Solar
Cell Performance

• From RTSE and AFM for R40 on R0 film onset of
  mc nucleation occurs at thickness of 200Å, with
  complete coalescence of mc nuclei within d400Å.
• Increase in recombination due to reduction in the
  mobility gaps in R40 layer to 1.62eV at 300Å and
  1.22eV at 400Å.
• Presence of such an a?(amc) transition greatly
  increases carrier recombination and has profound
  effect on cell characteristics.

4000Å p(a-SiCH)-i-n R10 i-layer
Increase in d
Such phase transitions are even more critical in
n-i-p structures where the (amc) phase is in
direct contact with the p-layer.
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Phase Boundary Determination for the i-Layer
ofn-i-p's from Voc ( J. Yang et al., MRS Proc.
2000)
R1.2R "the edge" maximum H2-dilution
sustainable without generating crystallites at
the i-p interface for given thickness
1.2R lt R lt 2.0R mixed phase (amc) region at
the i-p interface (irreproducible vol. fraction)
d 2500 Å
R ? 2.0R single-phase mc-SiH at i-p interface
The "edge" is thickness-specific it shifts to
lower R with increasing d.
12
Protocrystallinity Concept Applied to p-SiH
Contacts in n-i-p Cells

• Phase diagrams of p-SiH layers on R10 a-SiH
  were used in optimizing VOC in n-i-p cells.
• The maximum VOC occurs with R150 and corresponds
  to a protocrystalline layer terminated at 200Å or
  close to the (aµc) phase.
• The lowest VOC is obtained with R200 where the
  layer has evolved into a purely µc-SiH phase
  because i/p recombination increases significantly.

Erroneous conclusions that highest VOCs are
obtained with µc p-SiH is due to characterizing
microstructure on layers gtgt200Å.
13
Two-Transition Phase Diagram Effect of rf Plasma
Power

• Phase diagrams depend on deposition conditions
  other than R
• Identify effect of deposition parameters on a ? a
  and a ? (amc) transitions regimes of
  protocrystalline SiH growth
• Large shifts in transitions when the plasma power
  is increased

Phase diagrams are a powerful guide in optimizing
deposition conditions for fast growth.
14
Protocrystalline a-Si1-xGexH Alloys
c-Si substrate, Si/Ge 5, RH2/(SiGe) Ts 290oC

• Presence of protocrystalline growth regime
  identified for first time in evolution of
  Si1-xGexH thin films prepared with H2 dilution
• Development of deposition phase diagrams enhances
  systematic improvement of their absorber layers

Amorphous phase at R50 with Eg 1.4eV
15
Characterization of Recombination in Solar Cells
From Dark Current-Voltage Characteristics

• Clear separation of p/i interface recombination
  from that in the bulk of a-SiH solar cells
• Bulk recombination identified and quantified by
  systematic reduction of p/i contributions in cell
  structures
• Cell structures studied where the two components
  of carrier recombination are clearly separated.

p(a-SiCH)-i-n 4000Å R10 i-layers
Cody Gap of 200 Å p/i interface layer
Information about the gap states in the intrinsic
layers can be obtained directly from the bulk
recombination.
16
Forward Bias Dark Current

• Two regimes exponential dependence of JD on V
  effective series resistance limited by limited
  by carrier injection over Vn, Vp
• From analytical approach and SRH recombination
  in bulk and p/i interface region ?
• p/i interface recombination diode quality
  factor independent of V, value very close to 1
• Bulk recombination through continuous
  distribution of gap state differential diode
  quality factor n(V) kT/q(d(lnJD)/dv)-1
• n(V) bias dependence related to energy
  distribution of gap states with values between 1
  and 2 values related to rate of changes in
  distribution

17
Importance of Vn, Vp in Determining Cell
Characteristics

• Vn potential barrier in i-layer at n contact
• Vn determined by space charge of electron filled
  gap states due to large n0 not large densities
  of states
• n0 depends on alignment of EF in n layer with Ec
  in i-layer
• Alignment different for p-i-n cells with a-SiH
  and ?c-SiH n contacts
• Bulk recombination same
• Carrier injection clearly lower for n-a-SiH
  higher Vn
• No effect on VOC zero current flow
• Large effect on FF
• Similar results for Vp

Limitations of Carrier Injection
Bulk
18
Defect States in the Intrinsic Layers of a-SiH
Solar Cells with Low p/i Interface Recombination

• From clearly identified bulk contributions to
  JD-V characteristics no evidence found for highly
  non-uniform distributions of gap states across
  i-layers in p-i-n and n-i-p cell structures
• Clear dependence on thickness of i-layers
• Recombination consistent with essentially uniform
  defect states densities
• n(V) characteristics consistent with
  recombination through these gap states
• Limitations of this recombination on 1 sun Voc
  can be identified

19
Light Induced Defect States in i-layers

• Systematic changes in JD-V characteristics with
  introduction of light-induced defects
• Densities of defect states reflected in increases
  of JD
• Changes in energy distribution of defect states
  reflected in evolution of n(V)
• n(V) characteristics can be related to two
  Gaussian-like distributions one at midgap the
  other away from midgap
• Distinctly different evolution in the broadening
  of these distributions

20
n(V) Characteristics for R0 and R10 in Degraded
States

• Different gap state distributions in annealed
  state identified
• Evolutions distinctly different in
  protocrystalline R10 and undiluted R0
• Distribution away from midgap much broader in R0
  and R10
• The difference in these states accompanied by
  higher stability of protocrystalline a-SiH
  material and cells

21
Importance of Non-midgap Light-Induced Defect
States in Solar Cells
Fill Factor

• Degradation of same 1 sun FF in annealed state
• Dramatic difference in degradation in FF
• Critical role of non-midgap states clearly
  indicated

• 1 sun Voc quasi-Fermi level splitting
  encompasses recombination through states up to
  0.45eV from midgap
• Very big difference in the stability of 1 sun Voc

Reduction of these states key to further
optimization of stability with microstructure in
high performance cells
22
Summary

• Deposition phase diagrams extremely useful in
  systematic optimization of SiH solar cells.
• Concept of protocrystallinity useful in
• Systematic improvement of cell structures
• Controlling deleterious effects of a ? (amc)
  transition on solar cell characteristics
• Identifying p/i recombination key to
  characterizing i-layers and their contributions
  to cell characteristics.
• JD-V characteristics a new probe for
  characterizing recombination and defect states in
  i layers of solar cells.
• Spatially uniform densities of defects in the
  i-layers allow correlations with corresponding
  thin film materials.
• New insights into mechanisms limiting performance
  and stability of high quality a-SiH solar cells
  offer guidance in optimizing a-SiGeH,
  microcrystalline/nanocrystalline solar cells

23
Schematic of the structure of SiH films on
a-SiH (R0)
Despite evolutionary nature, protocrystalline
a-SiH has uniform properties over extended
regions of thickness
R

• Great attention must be given to the transition
  a?(aµc) and its thickness dependence on R- films
  and cells
• Phase diagrams are a powerful guide in optimizing
  deposition conditions for fast growth