# PECVD-ONO: A New Deposited Firing Stable Rear Surface Passivation Layer System for Crystalline Silicon Solar Cells

Authors: M. Hofmann; S. Kambor; C. Schmidt; D. Grambole; J. Rentsch; S. W. Glunz; R. Preu
Journal: Advances in OptoElectronics
 (2008)

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## Abstract

A novel plasma-enhanced chemical vapour deposited (PECVD) stack layer system consisting of a-SiOx:H, a-SiNx:H, and a-SiOx:H is presented for silicon solar cell rear side passivation. Surface recombination velocities below 60 cm/s (after firing) and below 30 cm/s (after forming gas anneal) were achieved. Solar cell precursors without front and rear metallisation showed implied open-circuit voltages Voc values extracted from quasi-steady-state photoconductance (QSSPC) measurements above 680 mV. Fully finished solar cells with up to 20.0% energy conversion efficiency are presented. A fit of the cell's internal quantum efficiency using software tool PC1D and a comparison to a full-area aluminium-back surface field (Al-BSF) and thermal SiO2 is shown. PECVD-ONO was found to be clearly superior to Al-BSF. A separation of recombination at the metallised and the passivated area at the solar cell's rear is presented using the equations of Fischer and Kray. Nuclear reaction analysis (NRA) has been used to evaluate the hydrogen depth profile of the passivation layer system at different
stages.


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## Body

## 1. Introduction

Electrical rear  surface passivation is becoming increasingly important in crystalline silicon
solar cell technology. The main reason  for that is the need for a cost decrease
in the photovoltaic sector. This leads to wafers that are becoming thinner and
thinner and a steadily increasing need for higher energy conversion
efficiencies. The result of both trends is an increased importance of an
effectively passivated rear surface. In Figures1 to 3, simulations on
rear surface passivation and cell thickness are presented using PC1D [1]. Open-circuit voltage Voc and energy conversion efficiency ηdepend
strongly on the rear surface recombination velocity (SRV) Seff when
high-quality silicon substrates are taken into account (τbulk=750 μs, excellent optics,
front emitter, and passivation, Figure 1). For Seff values in the
range of 5∗101 cm/s to 5∗103 cm/s, the
strongest interdependence was found. The thinner the solar cells the stronger
this effect appears. Note that very thin solar cells (50 μm) show a higher
Voc level at very low Seff values while the 50 μm
cells suffer from a decreased light trapping in our simulation and convert the
light a little less efficiently to electricity at the maximum power point. When
looking at medium-efficient solar cells (τbulk=100 μs, excellent optics,
front emitter, and passivation, Figure 2) the influence of the rear passivation
increases when the cells are getting thinner. We found that the thinnest cells
show even better efficiencies than thicker cells when an excellent rear
passivation is applied. Low-efficiency cells (τbulk=10 μs, excellent optics,
standard front emitter 50 Ω/sq, and low-level front passivation, Figure 3)
show only a slight dependence on the rear passivation due to a low diffusion
length (τbulk=10 μs → approximately Leff=170 μm).
Hence, most minority carriers do not profit from a low rear recombination
velocity but recombine in the bulk silicon. The thinner the low-efficient cells
become, the more pronounced will be the influence of a good rear passivation.Simulated open-circuit voltageVoc (left) and simulated energy conversion efficiency η versus the effective rear surface recombination velocity Seff for high-efficiency solar cells. Calculated using PC1D.
Parameters: NA=3∗1016 cm−3, τbulk=750 μs (approx. Auger limit), textured front, front emitter: 120 Ω/sq, Sfront=1000 cm/s.
(a)(b)Simulated open-circuit voltageVoc (left) and simulated energy conversion efficiency η versus the effective rear surface recombination velocity Seff for medium-efficiency solar cells. Calculated using PC1D.
Parameters: NA=7∗1015 cm−3, τbulk=100 μs, textured front, front emitter: 120 Ω/sq, Sfront=1000 cm/s.
(a)(b)Simulated open-circuit voltageVoc (left) and simulated energy conversion efficiency η versus the effective rear surface recombination velocity Seff for low-efficiency solar cells. Calculated using PC1D.
Parameters: NA=7∗1015 cm−3, τbulk=10 μs, textured front, front emitter: 50 Ω/sq, Sfront=106 cm/s.
(a)(b)Agostinelli etal. [2] have shown that silicon solar cells on very thin substrates (down to
105 μm) can reach higher efficiencies compared to a standard Al-BSF rear
structure when an adapted rear surface passivation is applied. Kray [3] has presented  energy conversion efficiencies above 20% for
substrates with less than 50 μm thickness. Both publications make use of
the passivated emitter and rear cell (PERC) concept [4].Fraunhofer ISE has developed an economically feasible technology for mass production of
the local rear contacts of PERCs [4] by implementing a laser technology. This patented laser-fired contacts
(LFCs) approach [5] allows the local aluminium contact firing through a passivating—and in most cases
isolating—layer system in approximately 1 to 3 seconds per cell. Within the same step, aluminium is
alloyed into the silicon forming a p+ regime underneath the local
contact creating a very good ohmic contact at the cell’s rear [6, 7].Rear side surface passivation schemes need to be adapted to the LFC process and to the
industrial production environment. The most common layers are thermally grown
silicon dioxide (SiO2), plasma-enhanced chemical vapour deposited
(PECVD) hydrogenated amorphous silicon nitride (a-SiNx:H, in short:
SiN), and PECVD hydrogenated amorphous silicon (a-Si:H, in short: a-Si). As
thermally grown silicon dioxide (SiO2) layers are manufactured by a
time- and energy-intensive high temperature process, they are not the first
choice for mass production, although they possibly provide a very good and
thermally stable passivation [8]. Unfortunately, the temperature level is not suitable for every
material quality. Especially, low-cost wafers might degrade during an oxidation
process.SiN films are fabricated by a low temperature processing step (approximately 300–400°C).
They also provide a very good passivation quality [9–15] but are not as thermally stable as SiO2 layers [8]. The thermal stability of optimised SiN layers can be improved to a
degree that is sufficient for the typical thermal budget of the solar cell
production line excluding emitter diffusion. Additionally, the cells passivated
with SiN films typically suffer from an inversion layer which is shunted by the
local rear contacts [16, 17]. The inversion layer is due to a high density of fixed positive charges
which typically can be found in SiN films.Good a-Si films provide a very efficient surface passivation without forming such an
inversion layer as a-Si films do not mainly passivate via a field effect but
through the lowering of the density of states at the interfaceDit.
Different groups have been using a-Si as passivation, either as single layer a-Si
passivation [18], a-Si stacks [19, 20] or a-Si stacked with SiN [21, 22], or PECVD silicon oxide (SiOx) [23]. Not only intrinsic but also doped a-Si layers have been used. The
major drawback when using a-Si passivation layers in crystalline silicon solar
cell technology is the relatively low thermal stability of the passivation
properties. After a typical screen printed contact firing step with
temperatures of up to ∼850°C the passivation properties are strongly reduced [24].Therefore, an alternative passivation layer system that(i)
can be deposited quickly and easily at low temperatures, preferably
using PECVD,(ii)
provides good passivation without a significant shunting,(iii)
is thermally stable in a contact firing process at ∼850°C,(iv)
provides a compatible internal rear surface reflectance
would be a very good way to incorporate an effective surface passivation layer system in
today’s industrial solar cells.


## 2. Lifetime Investigation

### 2.1. Symmetric Sample Structure

As described above, thermally grown silicon dioxide layers provide all major requirements to
be suitable for the rear passivation for silicon solar cells: good surface
passivation, good rear reflectance, and thermal stability in solar cell
production processes. Only the process throughput and cost and the possible
degradation of the solar cell precursors seem to be the major and crucial
problem.PECVD silicon oxide layers (SiOx) look like a possible alternative to
combine the good layer characteristics of an SiO2 film with the
benefits of PECVD. Regrettably, no PECVD SiOx layers could be found by
the authors that show a good surface passivation and thermal stability, no
matter if the layer has been annealed in different gas ambient (e.g., forming
gas) or not. Hence, different stack layers have been under investigation with
the PECVD SiOx layer being the first layer deposited (sitting
directly on the wafer’s surface).It should be noted that an investigation of deposited oxide layers stacked with PECVD SiNx has been presented by Agostinelli etal. [25].Hoex etal. have presented well-passivating SiOx films deposited by the expanding thermal plasma
(ETP) technique [26].The lifetime investigation has been performed using silicon wafers as substrates with the
following properties: float zone, p-type, boron doped, 1 Ω cm, 250μm thick, and shiny etched
surfaces with a crystal orientation of (100) [27]. The wafers were cleaned with a wet chemical RCA clean bath sequence [28] including a final dip in hydrofluoric acid (HF) and subsequently, the
deposition on both surfaces of the wafers has been performed. Two different
PECVD stack layer systems have been investigated: (i) SiOx and SiNx and (ii) SiOx, SiNx, and SiOx. SiOx always has been the first layer deposited. The deposition took place in a
parallel plate PECVD reactor using a plasma excitation frequency of
13.56 MHz and a gas mixture of monosilane (SiH4) and nitrous
oxide (N2O) for the SiOx layer and monosilane, hydrogen
(H2) and nitrogen (N2) for the SiNx layer. The
deposition temperature for all layers was 350°C. The oxide layers can be
deposited very quickly at a deposition rate of ∼120 nm/min. The nitride
layer takes longer, at a rate of ∼8 nm/min.
Thicknesses of 120 nm for the oxide and 70 nm for the nitride layers have been applied.
See Figure 4
for the process sequence and Figure 5 for the sample structures.Figure 4
Work flow of the lifetime experiment on symmetric sample structures.Figure 5
Symmetrical lifetime sample structures. Left: double layer sample structure SiOx + SiNx. Right: triple layer sample structure SiOx, SiNx and SiOx (PECVD-ONO).Minority carrier lifetimes were measured using the quasi-steady-state photoconductance
(QSSPC) method [29]. The surface recombination velocity has been calculated from the
measured lifetimes assuming that only Auger recombination took place in the
sample’s bulk (i.e., “perfect bulk”). The Auger  model by Glunz etal. [30] has been applied for the calculation using the following equation [31]:(1)1τeff=1τb+(W2Seff+1Dn(Wπ)2)−1, with the silicon wafer’s bulk lifetime τb, the wafer thickness W, the effective surface recombination
velocity (SRV) Seff, and the diffusion constant for
electrons Dn.The stack system of PECVD SiOx and SiNx led to as-deposited Seff values below 700 cm/s. Subsequently, an annealing in forming gas at 425°C
for 15 minutes has been performed. Minority carrier lifetime measurements
at this stage showed Seff values below 50 cm/s. Next, a thermal
treatment comparable to firing of screen printed front contacts with a peak wafer
temperature of approximately 850°C for about 3 seconds was done with Seff increasing to <70 cm/s.For the triple layer system (SiOx, SiNx, and SiOx, in short: PECVD-ONO), the same thermal
processing as for the two layer sample structure has been applied. This led to
the following surface recombination velocities: as-deposited: below 240 cm/s, after annealing in forming
gas at 425°C for 15 minutes: below 30 cm/s, after thermal processing
comparable to firing of screen printed front contacts with a peak wafer
temperature of approximately 850°C for about 3 seconds: below 60 cm/s.TheSeff results are summarised in Figure 6.Figure 6
Effective surface recombination velocities of PECVD stacks of SiOx + SiNx (left red columns) and SiOx, SiNx and SiOx (right green striped columns, PECVD-ONO) as-deposited, after annealing (425°C, 15 min, forming gas) or firing (850°C, ∼3 s).Both stack systems are suitable for the surface passivation of p-type Si wafers before and
after a thermal treatment like the contact firing. After deposition and after annealing, a stronger passivation
effect was found for the triple stack structure SiOx + SiNx + SiOx (PECVD-ONO, Patent pending). This effect will be investigated
further in the future. However, one beneficial effect is the increased overall
layer thickness providing a higher probability for hydrogen to diffuse to the
silicon/silicon oxide interface and lowering the amount of recombinatively
active interface traps.


### 2.2. Solar Cell Precursors

An investigation of the suitability of the newly developed PECVD-ONO stack system
for passivation of solar cell rear sides is presented here. A short summary has
been shown in [27].Float zone wafers with the following characteristics were used: p-type, boron doped,
0.5 Ω cm, 250μm thick, and shiny etched
surface with a crystal orientation of (100). The wafers were oxidised, front
side windows were opened by photolithography. Then, an emitter was diffused
into the front (120 Ω/sq.) followed by a wet chemical etch of the
phosphorous silicate glass (PSG). In a second oxidation step, an antireflecting
SiO2 layer was grown on the front (and the oxide on the back was
thickened). After removing the oxide on the rear and cleaning the surface in a
wet chemical bath sequence of (i) HNO3 and (ii) HF, the novel
passivation stack PECVD-ONO has been deposited on the rear and the lifetime was
measured. Subsequently, the samples were annealed at 425°C for 15 minutes
in forming gas and/or a thermal treatment comparable to the firing of screen
printed front contacts was performed. After each thermal process, the lifetime
was measured using the QSSPC technique. The sample structure can be found in Figure 7 and the process sequence in Figure 8.Figure 7
Sample structure of the solar cell precursors.Figure 8
Process sequence of the solar cell precursor experiment. The colours correspond to the colours in Figure9.It can be summarised that the sample structure equals a finished solar cell except the metal
contacts on front and rear.Cuevas and Sinton have shown that impliedVoc values can be extracted from the
measured effective minority carrier lifetimes [32, 33] using an approximation equation for Voc:(2)Voc=kTqln⁡(npni2), with Boltzmann’s constant k, temperature T, elementary charge q, free-electron density
n, free-hole density p, and intrinsic free-electron density ni.For p-type wafers and long diffusion lengths,
(2) can be written as [32, 33](3)Voc=kTqln⁡(Δn(Δp+NA)ni2), with NA being the density of
acceptors.ni can be calculated from(4)ni=NCNVe−Eg/kT, with silicon’s density  of states in the
conduction band NC and in the valence band NV and the
silicon band gap energy Eg. Hence, one can calculate Implied Voc values from QSSPC measurements. The implied Voc was extracted at
1 sun. Initially, Voc values of approximately 631 mV have
been observed. After annealing or firing, the implied Voc values
increased strongly to values above 680 mV. Here, the results after firing
were very similar regardless of whether an annealing was done prior to the
firing step or not. The effect is suspected to be due to an enhancement of the
front and rear surface passivation. See Figure 9.Figure 9
Solar cell precursors. ImpliedVoc values extracted from QSSPC measurements at 1 sun show a strong increase after thermal treatment. Colours correspond to Figure 8.


### 3. Hydrogen Depth Profiling

For the passivation effect, the diffusion of hydrogen from the strongly hydrogenated silicon
nitride layer to the SiOx/bulk Si interface might be responsible. Typically, a high density of recombinatively active states at this interface can be found. These states, mostly Si dangling bonds, can be saturated by hydrogen and implicitly made recombinatively inactive. Therefore, the hydrogen concentration and the change
in hydrogen concentration in different layers within the passivation stack system is of great interest.Nuclear reaction analysis (NRA) measurements have been performed at Forschungszentrum Dresden-Rossendorf, Germany [34]. NRA is based on a reaction between a nitrogen isotope N15
which is accelerated to several MeV and a hydrogen atom H1 in the
sample. The reaction only takes place when the nitrogen atom has a specific kinetic
resonance energy (6.385 MeV). At lower energy, no nuclear reaction will
happen. At higher energy, the nitrogen atom looses speed (energy) while passing
through the first atomic layers of the film. When the nitrogen is decelerated
to the resonance energy, the reaction will happen if there is a hydrogen atom
to react with (Figure 10). The reaction of hydrogen and nitrogen will finally
lead to a C12 atom and an alpha particle and a gamma quantum that are
radiated. The reaction equations are as follows:(5)N15+H1→O*→16C*+α,12C*12→C12+γ.Counting the measured gamma quanta for a certain nitrogen kinetic leads to the hydrogen
concentration and the depth profile [34].Measurement principle of the nuclear reaction analysis (NRA) technique (after [35]). The intensity of gamma radiation for a known kinetic energy of N15 above the threshold energy of 6.385 MeV is correlated with the depth of hydrogen.
(a)(b)Silicon wafer samples comparable to the symmetrical lifetime samples were used for the
NRA measurement [27]. The process state of the samples under investigation was as-deposited,
after annealing (425°C, 15 min) and after firing (850°C, ∼3 s).
The hydrogen depth profiling of the as-deposited sample showed a hydrogen
concentration of approximately 8 at% in the PECVD SiOx films
and approximately 16 at% in the SiNx film. Almost no hydrogen
could be  found within the crystalline silicon bulk. After annealing the
hydrogen depth profile did not change much. Hence, no strong hydrogen diffusion
could be observed. The sample that was subjected to the “contact firing” showed
a significantly modified hydrogen profile. As expected at this temperature
range, hydrogen was very mobile. The overall hydrogen concentration was
lowered, within the SiOx layers approximately 5 at% hydrogen
could be found and a peak concentration of approximately 8 at% was
observed within the SiNx layer. This shows that a strong
out-diffusion (i) from the hydrogenated layers to the ambient and (ii) from the
nitride film to the surrounding oxide layers took place. No significant effect
can be observed at the SiOx/c-Si interface (see Figure 11).Figure 11
Results of the hydrogen depth profiling using the NRA method. Samples with PECVD-ONO after a variation of thermal treatment are shown: as-deposited, after annealing (425°C, 15 min), after firing (850°C, ∼3 s, sample not annealed prior to firing).


## 2.1. Symmetric Sample Structure

As described above, thermally grown silicon dioxide layers provide all major requirements to
be suitable for the rear passivation for silicon solar cells: good surface
passivation, good rear reflectance, and thermal stability in solar cell
production processes. Only the process throughput and cost and the possible
degradation of the solar cell precursors seem to be the major and crucial
problem.PECVD silicon oxide layers (SiOx) look like a possible alternative to
combine the good layer characteristics of an SiO2 film with the
benefits of PECVD. Regrettably, no PECVD SiOx layers could be found by
the authors that show a good surface passivation and thermal stability, no
matter if the layer has been annealed in different gas ambient (e.g., forming
gas) or not. Hence, different stack layers have been under investigation with
the PECVD SiOx layer being the first layer deposited (sitting
directly on the wafer’s surface).It should be noted that an investigation of deposited oxide layers stacked with PECVD SiNx has been presented by Agostinelli etal. [25].Hoex etal. have presented well-passivating SiOx films deposited by the expanding thermal plasma
(ETP) technique [26].The lifetime investigation has been performed using silicon wafers as substrates with the
following properties: float zone, p-type, boron doped, 1 Ω cm, 250μm thick, and shiny etched
surfaces with a crystal orientation of (100) [27]. The wafers were cleaned with a wet chemical RCA clean bath sequence [28] including a final dip in hydrofluoric acid (HF) and subsequently, the
deposition on both surfaces of the wafers has been performed. Two different
PECVD stack layer systems have been investigated: (i) SiOx and SiNx and (ii) SiOx, SiNx, and SiOx. SiOx always has been the first layer deposited. The deposition took place in a
parallel plate PECVD reactor using a plasma excitation frequency of
13.56 MHz and a gas mixture of monosilane (SiH4) and nitrous
oxide (N2O) for the SiOx layer and monosilane, hydrogen
(H2) and nitrogen (N2) for the SiNx layer. The
deposition temperature for all layers was 350°C. The oxide layers can be
deposited very quickly at a deposition rate of ∼120 nm/min. The nitride
layer takes longer, at a rate of ∼8 nm/min.
Thicknesses of 120 nm for the oxide and 70 nm for the nitride layers have been applied.
See Figure 4
for the process sequence and Figure 5 for the sample structures.Figure 4
Work flow of the lifetime experiment on symmetric sample structures.Figure 5
Symmetrical lifetime sample structures. Left: double layer sample structure SiOx + SiNx. Right: triple layer sample structure SiOx, SiNx and SiOx (PECVD-ONO).Minority carrier lifetimes were measured using the quasi-steady-state photoconductance
(QSSPC) method [29]. The surface recombination velocity has been calculated from the
measured lifetimes assuming that only Auger recombination took place in the
sample’s bulk (i.e., “perfect bulk”). The Auger  model by Glunz etal. [30] has been applied for the calculation using the following equation [31]:(1)1τeff=1τb+(W2Seff+1Dn(Wπ)2)−1, with the silicon wafer’s bulk lifetime τb, the wafer thickness W, the effective surface recombination
velocity (SRV) Seff, and the diffusion constant for
electrons Dn.The stack system of PECVD SiOx and SiNx led to as-deposited Seff values below 700 cm/s. Subsequently, an annealing in forming gas at 425°C
for 15 minutes has been performed. Minority carrier lifetime measurements
at this stage showed Seff values below 50 cm/s. Next, a thermal
treatment comparable to firing of screen printed front contacts with a peak wafer
temperature of approximately 850°C for about 3 seconds was done with Seff increasing to <70 cm/s.For the triple layer system (SiOx, SiNx, and SiOx, in short: PECVD-ONO), the same thermal
processing as for the two layer sample structure has been applied. This led to
the following surface recombination velocities: as-deposited: below 240 cm/s, after annealing in forming
gas at 425°C for 15 minutes: below 30 cm/s, after thermal processing
comparable to firing of screen printed front contacts with a peak wafer
temperature of approximately 850°C for about 3 seconds: below 60 cm/s.TheSeff results are summarised in Figure 6.Figure 6
Effective surface recombination velocities of PECVD stacks of SiOx + SiNx (left red columns) and SiOx, SiNx and SiOx (right green striped columns, PECVD-ONO) as-deposited, after annealing (425°C, 15 min, forming gas) or firing (850°C, ∼3 s).Both stack systems are suitable for the surface passivation of p-type Si wafers before and
after a thermal treatment like the contact firing. After deposition and after annealing, a stronger passivation
effect was found for the triple stack structure SiOx + SiNx + SiOx (PECVD-ONO, Patent pending). This effect will be investigated
further in the future. However, one beneficial effect is the increased overall
layer thickness providing a higher probability for hydrogen to diffuse to the
silicon/silicon oxide interface and lowering the amount of recombinatively
active interface traps.


## 2.2. Solar Cell Precursors

An investigation of the suitability of the newly developed PECVD-ONO stack system
for passivation of solar cell rear sides is presented here. A short summary has
been shown in [27].Float zone wafers with the following characteristics were used: p-type, boron doped,
0.5 Ω cm, 250μm thick, and shiny etched
surface with a crystal orientation of (100). The wafers were oxidised, front
side windows were opened by photolithography. Then, an emitter was diffused
into the front (120 Ω/sq.) followed by a wet chemical etch of the
phosphorous silicate glass (PSG). In a second oxidation step, an antireflecting
SiO2 layer was grown on the front (and the oxide on the back was
thickened). After removing the oxide on the rear and cleaning the surface in a
wet chemical bath sequence of (i) HNO3 and (ii) HF, the novel
passivation stack PECVD-ONO has been deposited on the rear and the lifetime was
measured. Subsequently, the samples were annealed at 425°C for 15 minutes
in forming gas and/or a thermal treatment comparable to the firing of screen
printed front contacts was performed. After each thermal process, the lifetime
was measured using the QSSPC technique. The sample structure can be found in Figure 7 and the process sequence in Figure 8.Figure 7
Sample structure of the solar cell precursors.Figure 8
Process sequence of the solar cell precursor experiment. The colours correspond to the colours in Figure9.It can be summarised that the sample structure equals a finished solar cell except the metal
contacts on front and rear.Cuevas and Sinton have shown that impliedVoc values can be extracted from the
measured effective minority carrier lifetimes [32, 33] using an approximation equation for Voc:(2)Voc=kTqln⁡(npni2), with Boltzmann’s constant k, temperature T, elementary charge q, free-electron density
n, free-hole density p, and intrinsic free-electron density ni.For p-type wafers and long diffusion lengths,
(2) can be written as [32, 33](3)Voc=kTqln⁡(Δn(Δp+NA)ni2), with NA being the density of
acceptors.ni can be calculated from(4)ni=NCNVe−Eg/kT, with silicon’s density  of states in the
conduction band NC and in the valence band NV and the
silicon band gap energy Eg. Hence, one can calculate Implied Voc values from QSSPC measurements. The implied Voc was extracted at
1 sun. Initially, Voc values of approximately 631 mV have
been observed. After annealing or firing, the implied Voc values
increased strongly to values above 680 mV. Here, the results after firing
were very similar regardless of whether an annealing was done prior to the
firing step or not. The effect is suspected to be due to an enhancement of the
front and rear surface passivation. See Figure 9.Figure 9
Solar cell precursors. ImpliedVoc values extracted from QSSPC measurements at 1 sun show a strong increase after thermal treatment. Colours correspond to Figure 8.


## 3. Hydrogen Depth Profiling

For the passivation effect, the diffusion of hydrogen from the strongly hydrogenated silicon
nitride layer to the SiOx/bulk Si interface might be responsible. Typically, a high density of recombinatively active states at this interface can be found. These states, mostly Si dangling bonds, can be saturated by hydrogen and implicitly made recombinatively inactive. Therefore, the hydrogen concentration and the change
in hydrogen concentration in different layers within the passivation stack system is of great interest.Nuclear reaction analysis (NRA) measurements have been performed at Forschungszentrum Dresden-Rossendorf, Germany [34]. NRA is based on a reaction between a nitrogen isotope N15
which is accelerated to several MeV and a hydrogen atom H1 in the
sample. The reaction only takes place when the nitrogen atom has a specific kinetic
resonance energy (6.385 MeV). At lower energy, no nuclear reaction will
happen. At higher energy, the nitrogen atom looses speed (energy) while passing
through the first atomic layers of the film. When the nitrogen is decelerated
to the resonance energy, the reaction will happen if there is a hydrogen atom
to react with (Figure 10). The reaction of hydrogen and nitrogen will finally
lead to a C12 atom and an alpha particle and a gamma quantum that are
radiated. The reaction equations are as follows:(5)N15+H1→O*→16C*+α,12C*12→C12+γ.Counting the measured gamma quanta for a certain nitrogen kinetic leads to the hydrogen
concentration and the depth profile [34].Measurement principle of the nuclear reaction analysis (NRA) technique (after [35]). The intensity of gamma radiation for a known kinetic energy of N15 above the threshold energy of 6.385 MeV is correlated with the depth of hydrogen.
(a)(b)Silicon wafer samples comparable to the symmetrical lifetime samples were used for the
NRA measurement [27]. The process state of the samples under investigation was as-deposited,
after annealing (425°C, 15 min) and after firing (850°C, ∼3 s).
The hydrogen depth profiling of the as-deposited sample showed a hydrogen
concentration of approximately 8 at% in the PECVD SiOx films
and approximately 16 at% in the SiNx film. Almost no hydrogen
could be  found within the crystalline silicon bulk. After annealing the
hydrogen depth profile did not change much. Hence, no strong hydrogen diffusion
could be observed. The sample that was subjected to the “contact firing” showed
a significantly modified hydrogen profile. As expected at this temperature
range, hydrogen was very mobile. The overall hydrogen concentration was
lowered, within the SiOx layers approximately 5 at% hydrogen
could be found and a peak concentration of approximately 8 at% was
observed within the SiNx layer. This shows that a strong
out-diffusion (i) from the hydrogenated layers to the ambient and (ii) from the
nitride film to the surrounding oxide layers took place. No significant effect
can be observed at the SiOx/c-Si interface (see Figure 11).Figure 11
Results of the hydrogen depth profiling using the NRA method. Samples with PECVD-ONO after a variation of thermal treatment are shown: as-deposited, after annealing (425°C, 15 min), after firing (850°C, ∼3 s, sample not annealed prior to firing).


## 4. Solar Cell Fabrication

Finally, solar cells with the newly developed passivation stack system PECVD-ONO were
fabricated to show the quality of the passivation on a final cell.Starting the cell fabrication using float-zone Si substrates with the same characteristics
as in the above experiments, the final cells exhibit evaporated TiPdAg front
contacts, a thermally oxidised antireflection coating that also serves as front
passivation layer, a 120 Ω/sq n-type  emitter, fabricated by diffusion
using a POCl3 ambient, a 1 Ω cm p-type bulk, the newly developed
PECVD-ONO stack passivation system, and an evaporated Al layer at the back and
laser-fired contacts that led to a local Al-BSF underneath (above if looking at
Figure 12) the point contacts. The rear surface was cleaned in a wet chemical
bath sequence of (i) HNO3 and (ii) HF.Figure 12
Structure of the fabricated solar cells with the newly developed passivation stack system PECVD-ONO on the rear and LFC contacts.The cell
scheme can be found in Figure12.The final
cell was annealed at 425°C in forming gas for 15 minutes. A thermal
process comparable to the firing of screen printed front contacts (as used in
the lifetime experiments described above) was not applied. This process would
harm the solar cell structure used in this batch.The best
solar cell with the novel PECVD-ONO stack system as rear passivation led to a
cell efficiency of 20.0% (see Table1).Table 1
Best solar cells of this investigation.
Rear passivationAreaVocJscFFηPECVD-ONO4.0 cm2664 mV38.2 mA/cm278.7%20.0%Thermal SiO24.0 cm2676 mV38.3 mA/cm280.4%20.8%For comparison also thermally grown silicon dioxide was used for rear passivation within
the same solar cell batch that is known for its excellent rear passivation
quality. 20.8% peak efficiency could be obtained for these reference cells.Internal quantum efficiency (IQE) measurements (Figure13) allow for a comparison of the quality
of the novel rear passivation PECVD-ONO stack with thermally grown SiO2 as
well as with full area high-quality Al-BSF (the Al-BSF cells originated
from a different but similar solar cell batch). It could be proven that PECVD-ONO
is superior to Al-BSF rear sides but shows lower quality than thermally grown
SiO2.Figure 13
Internal quantum efficiency for cells of this batch with thermal SiO2 and PECVD-ONO rear passivation and high-quality Al-BSF rear passivation of another batch.To quantify
the rear passivation quality in the finished solar cell, a numerical fit to the
experimental external quantum efficiency (EQE), IQE, and reflection properties
was prepared using PC1D [1]. The graphical results can be found in Figure 14. The total front and
rear surface recombination velocities can be derived from the PC1D fit as
3200 cm/s ± 500 cm/s
(front) and 550 cm/s ± 50 cm/s (rear), respectively. This includes the recombination
at the passivated area as well as the contact area. Fischer
presented an equation describing the SRV of locally contacted solar cell rear
sides [36](6)Seff=DW(Lp2Wπfarctan(2WLpπf)−e−W/Lp+DfWSmet)−1+Spass1−f, with the minority carrier diffusion constant D, the wafer thickness W, the contact pitch Lp, the metallisation fraction f, the SRV of the metallised surface Smet, and the SRV of the passivated surface Spass of the rear side. For the cells presented here, the following values are valid: Dn = 27.1 cm2/s, W = 240 μm, Lp = 1 mm, and f = 1%. Kray and Glunz have experimentally investigated and modelled the recombination velocity at the laser-fired contacts leading to the expression [37](7)Smet(NA)=S0+αeβ(NA+N0), with S0=−900 cm/s,
α = 22.1 cm/s, β=1.29×10−16 cm3, and
N0=3.40×1016 cm−3 and NA being
the density of acceptors in the substrate (in our case 1.5×1016 cm−3).Figure 14
External quantum efficiency, IQE and reflection values for the solar cell using PECVD-ONO rear passivation. Experimental data (symbols) and calculated values (lines) are displayed. The calculation was done using PC1D [1].Hence, by using the above equations and theSeff value extracted from PC1D, Spass can be calculated to 500 cm/s ± 50 cm/s.


## 5. Conclusion

A novel surface passivation stack comprising of three PECVD layers was presented. The
stack consists of a-SiOx:H, a-SiNx:H, and a-SiOx:H.
Lifetime measurements with the novel stack system on both surfaces of p-type
silicon wafers showed surface recombination velocities of <240 cm/s,
after annealing at 425°C: <30 cm/s, after firing: <60 cm/s.
Solar cell precursors without metallisation but with PECVD-ONO rear passivation
exhibited implied Voc values extracted from the QSSPC measurements
of >680 mV. Hydrogen depth profiling using the NRA method was conducted
resulting in hydrogen concentrations of approximately 8 at% in the PECVD
SiOx films and approximately 16 at% in the SiNx film
in the as-deposited and annealed state. After firing, the hydrogen
concentrations were strongly lowered to 5 at% in the PECVD SiOx films and approximately 8 at% in the SiNx film.Solar cells
with the novel passivation stack were processed and showed efficiencies of up
to 20.0%. IQE characterisation and modelling using PC1D led to a total
SRV at the rear surface of 550 cm/s ± 50 cm/s. Additionally, the model for the
recombination at locally contacted rear sides by Fischer and Kray led to an SRV
of 500 cm/s ± 50 cm/s
at the passivated area.A patent on the
novel PECVD-ONO passivation stack system is pending.



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*Source: 485467-2008-06-04.xml*