NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Junction Transport in Epitaxial 

Film Silicon Heterojunction 

Solar Cells

Preprint

David L. Young, Jian V. Li, Charles W. Teplin, 

Paul Stradins, and Howard M. Branz

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50712

July 2011
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1

JUNCTION TRANSPORT IN EPITAXIAL FILM SILICON HETEROJUNCTION SOLAR CELLS

David L. Young, Jian V. Li, Charles W. Teplin, Paul Stradins, and Howard M. Branz

National Renewable Energy Laboratory, Golden, CO USA

ABSTRACT

We report our progress toward low-temperature HWCVD 

epitaxial film silicon solar cells on inexpensive seed layers,

with a focus on the junction transport physics exhibited by 

our devices. Heterojunctions of i/p hydrogenated 

amorphous Si (a-Si) on our n-type epitaxial crystal Si on 

n++ Si wafers show space-charge-region recombination, 

tunneling or diffusive transport depending on both epitaxial

Si quality and the applied forward voltage.

INTRODUCTION

Today’s commercial photovoltaic market is dominated by 

crystalline silicon wafer technology, however an increasing 

share of the market is shifting toward thin-film materials 

due to cost advantages in fabricating large area 

semiconductors. Many research groups are working on 

crystalline film-silicon (1-40 μm) solar cells to take 

advantage of the potential cost advantage, high efficiency, 

scientific knowledge base and marketplace inertia toward 

silicon-based semiconductors [1-6]. Our group has been 

developing a low-cost hot wire chemical vapor (HWCVD) 

epitaxial growth technique to form high efficiency epitaxial 

film silicon solar cells at temperatures between 620 and 

830°C on inexpensive substrates coated in crystalline 

seed layers [7-10]. 2-μm-thick devices grown on heavily 

doped wafers have shown Voc values as high as 630 mV 

with fill factors of 78%. Devices grown on seed layers 

(both hetero- and homo-epitaxy) with no Si wafer exhibit 

device parameters that exceed those of fine-grained 

polysilicon devices on display glass.

These high voltages indicate high quality epitaxial material 

with effective minority carrier diffusion length, Leff, more 

than 3 times the absorber thickness.[7] However, our 

epitaxial films have threading dislocation densities in the 

104 – 106 cm-2 range (imaged by electron-beam induced 

current) and crystalline pit and tower structures on the 

surface of the films at densities <104 cm-2 [9, 11] (imaged 

by scanning electron microscopy). The effect of these 

electronic and crystallographic structures can be 

significant [9, 11] and yet the high Voc values our a-Si/c-Si 

heterojunction cells demand a closer look and analysis. 

The question addressed here is whether devices made 

from our low-temperature epitaxial films, despite the 

dislocation, pits and towers, can operate with similar 

transport physics as wafer-based heterojunction devices 

[12]. In other words, can our films form electronically clean 

enough interfaces and provide long enough bulk minority 

carrier lifetimes to allow tunneling and diffusive transport 

across the interface rather than voltage-killing 

recombination in the bulk [12].

In this contribution, we will compare the device physics of

our heterojunction solar cells with that of wafer-based aSi/c-Si heterojunction solar cells. Our study relies on 

analysis of temperature dependent dark current density vs 

voltage (JV(T)) measurements. The transport mechanisms 

measured in our best film-silicon devices are similar to 

those of the wafer-based a-Si/c-Si heterojunction solar 

cells, but further improvement of the Si epitaxy is needed 

to improve Voc values to the 650 mV range.

EXPERIMENT

Epitaxial films are grown by HWCVD using a gas mixture 

of SiH4, H2, and PH3 over a temperature range from 650 to

830  ̊C. Details of the film growth can be found elsewhere

[7, 9, 13]. Films are grown on either heavily doped (2x1019

cm-3

), RCA cleaned, Si:As wafers or on a variety of 

substrates coated with proprietary seed layers to promote 

epitaxial silicon growth. Seed substrates are glass, 

ceramic or metal foils. Heterojunction formation (~5 nm of 

intrinsic a-Si followed by ~15 nm of p-type a-Si:B) is done 

in a separate HWCVD deposition chamber after a 4% HF

etch. Next, a 700 nm thick layer of In2O3:Sn is deposited 

by reactive evaporation to provide a transparent 

conducting layer and a top contact. Small area (0.05 cm2

) 

round mesa devices are isolated by wet and dry chemical 

techniques. Back contacts are formed either to the highly 

conducting wafer or to a grown-in, heavily doped back 

surface field layer (See reference [7] for details).

Measurements of light current density vs voltage are done 

with a calibrated AM1.5 solar simulator to give device 

operating parameters. JV(T) measurements are made 

between 77 K – 350 K, at ~10°C steps, with a Linkam 

Scientific cryostat and a Keithley 6517B electrometer. 

DLTS measurements are made with custom equipment.

Table 1, Device parameters.
2 

RESULTS AND DISCUSSION 

Samples of different quality were selected for this study to 

determine the main junction transport mechanism 

responsible for limiting Voc in our films. Table 1 shows 

device parameters for these samples. Note that devices 

1-4 have Voc ranging from 555 to 627 mV while the 

difference between devices 4 and 5 is the Jsc value. 

Device 1 reached the highest Voc through a hydrogen 

passivation treatment that will be described elsewhere. 

Here, we will show detailed data for Device 2, with Voc = 

600 mV, and compare them to key results and conclusions 

for the other devices. Figure 1 shows dark JV(T) data for 

Device 2 over the temperature range of 250 – 350 K and 

voltage range of -2 to 1.2 Volts. 

The forward bias data of Fig. 1 can be fit with a simplified 

diode relationship 

J = Jo exp[AV] [1] 

where 

A = q

nkT [2] 

for both diffusion and space-charge-region recombination 

(SCRR) models. Here, k is Boltzmann’s constant, T is the 

measurement temperature, n is the diode ideality factor 

and q is the elementary charge. However, if transport is 

instead governed by tunneling, A becomes a temperatureindependent constant. The pre-exponential factor Jo can 

be described by 

Jo ∝ exp − ΔEa

kT





 





 [3] 

where ΔEa is an activation energy associated with the rate 

limiting recombination process [14]. Analysis of the 

forward bias data in Fig. 1 reveals four voltage regions 

characterized by a significant change in slope (different A

values in Eq. 1) or some curvature. These regions, 

identified in Figure 2, each indicate a change in transport 

mechanism. A 2-parameter fit of the forward bias data of 

Fig. 2 to Eq. 1 gives the temperature dependence of the A

factor (see Fig. 3) and Jo (see Fig.4) in voltage regions 1-

3. In contrast, region 4 shows the J ~ V2

 dependence 

characteristic of space-charge-limited transport in high 

forward bias [15]. 

Fig. 3 clearly shows that A depends strongly on T in 

regions 1 - 2 but is nearly T-independent in Region 3. 

In regions 1 and 2, Eqn. (2) yields n values above 2, 

consistent with a space-charge-region recombination 

(SCRR) mechanism [16]. In Region 3, the A factor is 

Figure 3. A value (Eq. 1) vs 1/kT for different bias regions 

from device 1. 

Figure 2 Forward bias temperature dependent dark current 

density vs voltages for device 1 showing the 4 bias regions 

discussed in the text. [ I’d add the T arrow again] 

Figure 1. Temperature dependent dark current density vs 

voltage for device 1. Currents decrease monotonically as 

measurement T decreases as indicated by the arrow. 
3 

nearly constant over the temperature range measured 

indicating tunneling transport as the rate limiting step [15]. 

Figure 4 shows ln(Jo) vs. 1/kT, that we use to find the 

slope Ea, which is the transport activation energy 

according to Eq. (3). Bias regions 1 and 2 give lower Ea

values (0.19 eV and 0.14 eV, respectively), while Ea in 

region 3 is much higher at 0.37 eV. The lack of Tdependence of A in Region 3 is consistent with the 

multitunneling capture-emission model of Matsuura et al 

[14]. The activation energy of Jo in Region 3 is consistent 

with the conductivity activation energy in the doped p-type 

a-Si:B layer ( ~ 0.4 eV) as noted elsewhere [12, 14]. 

Table 2 Identified transport mechanisms and associated Ea

for low and high forward bias regions. 

From the data of Figures 1- 4, it is clear that the operating 

transport mechanism within Device 2 shifts with increasing 

forward bias from an SCRR capture/emission process 

to/from increasing shallow defects, followed by an abrupt 

change to tunneling-like behavior as the forward bias 

approaches Voc. The Ea ~0.4 value of Jo in the tunneling 

region suggests [12] that the dominate dark current arises 

from an electron in the conduction band of the n-type epi 

c-Si tunneling through states in the i-a-Si layer, separated 

by energy < kT, to recombine with a hole from the a-Si:B 

layer. The hole transport through the p-layer is 

presumably the rate-limiting step. 

Similar analysis of dark JV(T) data were completed for 

devices 1-5. The results from these devices are shown in 

Table 2. At low bias (typically less than 0.4 V) the lowervoltage devices 3 and 4 show tunneling behavior with 

similar Ea values to that of Device 2. However, at high 

bias (greater than 0.4 V) the A value in Eq. 1 becomes 

temperature dependent with an n value greater than 2 and 

an Ea value around 0.5 eV. These data indicate SCR 

recombination to a mid-gap level in the c-Si epitaxy layer 

near the operating voltage of the device. The worstperforming Device 5 showed SCR recombination with Ea ~ 

0.3 eV across all forward biases. No tunneling-like 

transport was observed for this sample. It should be noted 

that device 5 showed a mid-gap minority carrier trap by 

DLTS. The origin of this trap is not yet known. 

Device 1, with the highest Voc of 627 mV, has very 

different transport behavior with increasing forward bias 

compared to the other epi/a-Si devices. At low bias, it 

shows tunneling transport with an activation energy of 

0.34 eV. As forward bias increases, however, the A value 

becomes temperature dependent with an activation 

energy for Jo of 0.92 eV and an n values less than 2. 

These values point toward diffusive transport of electrons 

over the heterojunction barrier rather than tunneling or 

SCRR in the base. 

For perspective on the data of Table 2 we compare our 

data to a wafer-based heterojunction solar cell with Voc > 

710 mV fabricated by Sanyo [12]. From Table 2, we see 

that at low bias, the Sanyo HIT cell exhibits tunneling 

transport with an Ea of 0.41 eV. However, above about 0.4 

V forward bias the Sanyo device switches to diffusive

transport (n=1.2) with an Ea of 1.13 eV – the bandgap of 

the c-Si wafer absorber layer. This very significant change 

– apparently an optimized transport mechanism, allows 

the Sanyo device to have record high Voc values. The 

Sanyo HIT cell acts like a diffused junction device near the 

maximum power point, but benefits from the band offsets 

between crystal and amorphous Si to increase Voc. 

Our epi/a-Si Device 1 shares the Sanyo cell’s transport 

regimes despite having dislocations and crystallographic 

defects. From Table 2 we see that increasing Voc in our 

devices is accompanied by a dark current transport 

mechanism shift from SCRR recombination to tunneling 

and finally to diffusion over the junction barriers. The 

particular transport reveals much about the bulk and 

surface quality. Device 2 is limited by tunneling at the 

interface, however, Device 1 reached diffusive transport 

by the addition of a hydrogen passivation processing step 

that also increased the voltage. 

Figure 4. Ln(Jo) vs 1/kT for different bias regions from device 

1. Ea values (Eq. 3) are shown in the legend. 
4

CONCLUSION

Our study of c-Si/a-Si junction physics on lowtemperature, HWCVD epitaxial films by dark JV(T) data 

indicate that space-charge-region recombination and 

tunneling are the main transport mechanisms operating in 

devices with Voc < 600 mV. Devices that are hydrogen 

passivated prior to growth of the c-Si/a-Si heterojunction 

show diffusive transport over the junction barrier which 

allows Voc values as high as 630 mV.

ACKNOWLEDGEMENT

We thank Falah Hasoon, Russell Bauer, Lorenzo Roybal, 

Kirstin Alberi, Max Shub, Harvey Guthrey and Vincenzo 

LaSalvia for assistance in sample preparation and Richard 

Crandall for helpful discussions. This work was supported 

by the U.S. Department of Energy under Contract No. DEAC36-08-GO28308 with the National Renewable Energy 

Laboratory.

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