3.1. Diode Current-Voltage Characteristics
Typical measurements of the J-V characteristics of a diode based on an a-GaAs/c-Si heterocontact reveal an explicit rectification trend as shown in
Figure 1 where forward and reverse bias regions can be clearly distinguished.
Further J-V curves of a-GaAs/c-Si heterojunction were also measured at different temperatures ranging from 286.8 K to 448 K using an HP4124B DC source/monitor system. The results are shown in a semilogarithmic plot in
Figure 2 where the slope of the forward current remains constant for all temperatures in the low voltage regime. This behavior indicates that for low voltages, the forward current is dominated by a tunneling mechanism [
21]. For a larger forward bias voltage, the dependence of the current can be written as follows [
21]:
where A is a constant that is independent on the temperature. In
Figure 3, the dark saturation current density Jo determined from the data shown in
Figure 2 is plotted as a function of 1000/T. The linear behavior in this semilogarithmic plot indicates that Jo can be modeled by [
21]:
where ΔE
af as activation energy.
Figure 1.
J-V characteristics of an a-GaAs/c-Si heterocontact diode measured at 300 K.
Figure 1.
J-V characteristics of an a-GaAs/c-Si heterocontact diode measured at 300 K.
Figure 2.
J-V characteristics of an a-GaAs/c-Si heterocontact diode measured at different temperatures in the forward bias voltage regime.
Figure 2.
J-V characteristics of an a-GaAs/c-Si heterocontact diode measured at different temperatures in the forward bias voltage regime.
Figure 3.
Jo of an a-GaAs/c-Si heterocontact diode as a function of 1000/T.
Figure 3.
Jo of an a-GaAs/c-Si heterocontact diode as a function of 1000/T.
A least square fit provides a value of the activation energy of 0.18 eV ± 0.02 eV. The result is considerably smaller than previously reported values for GaAs/c-Si heterojunctions fabricated by sputtering [
19,
20], which range from 0.34 to 0.43 eV. This can be explained by different deposition processes: Our samples were deposited at room temperature and sintered later in N
2 atmosphere at 425 °C for 20 min, while in the cited works GaAs was deposited at substrate temperatures of 150 °C [
20,
21].
Figure 4.
Reverse saturation current density of an a-GaAs/c-Si heterocontact diode measured for different temperatures as a function of V1/2.
Figure 4.
Reverse saturation current density of an a-GaAs/c-Si heterocontact diode measured for different temperatures as a function of V1/2.
Figure 4 shows the reverse saturation current density plotted as a function of (V)
1/2 for several temperatures with some kind of a linear behavior on the semilogarithmic scale in the reverse bias voltage regime (–2 V to −0.4 V). This result indicates that the reverse saturation current density is probably limited by generation-recombination processes [
29,
30].
3.2. Solar Cell Photovoltaic Characteristics
The HF treatment has a strong effect on the interface quality of the heterojunction as it can be seen in
Figure 5a. The device without HF treatment (sample B) shows a dark saturation current density value that is roughly two orders of magnitude lower than that of the device where the HF treatment was applied (sample A).
Figure 5.
J-V characteristics of an a-GaAs/c-Si heterocontact solar cell under dark conditions (a) and under standard illumination conditions (b).
Figure 5.
J-V characteristics of an a-GaAs/c-Si heterocontact solar cell under dark conditions (a) and under standard illumination conditions (b).
J-V measurements under STC are shown in
Figure 5b. For the HF treated device an increase of the short circuit current density is observed whereas the open circuit voltage is decreased. The open circuit voltages under illumination are 173 mV and 234 mV for samples A and B, respectively, which is consistent with the values of the saturation current density.
The values of the short circuit current density are 2 × 10
−5 A/cm
2 and 3.7 × 10
−6 A/cm
2 for samples A and B, showing that the HF treatment improves the photocurrent. This result can be explained by the quality of the GaAs/c-Si interface. In
Figure 6, a TEM picture of the GaAs/c-Si heterocontact region of a sample prepared without HF treatment is shown in cross section geometry. A thin amorphous SiO
x can be observed at the GaAs/c-Si interface, which is due to the native oxide at the surface of the Si. Moreover, the GaAs film appears to be amorphous as well. The fill factors (FF), series (Rs), and shunt resistances (Rp) of samples A and B are given in
Table 1.
Figure 6.
Transmission electron microscopy TEM micrograph of GaAs/Si interface.
Figure 6.
Transmission electron microscopy TEM micrograph of GaAs/Si interface.
Table 1.
Summary of solar cell parameters of samples A and B.
Table 1.
Summary of solar cell parameters of samples A and B.
| Sample A (with HF) | Sample B (without HF) |
---|
Rs (Ω) | 1880 | 4390 |
Rp (Ω) | 112 | 240 |
FF (%) | 25.6 | 10.36 |
We have also analyzed the effect of the annealing step performed before or after the ITO deposition on samples that were treated with HF prior to the GaAs sputtering.
Figure 7 shows a typical image of the sputtered GaAs surface after the heat treatment without any ITO layer using a VEECO Mic Interferometer.
Figure 7.
Front surface details of the GaAs absorber of an a-GaAs/c-Si heterocontact solar cell after an annealing (details in the text).
Figure 7.
Front surface details of the GaAs absorber of an a-GaAs/c-Si heterocontact solar cell after an annealing (details in the text).
As it can be seen in
Figure 7, many structures up to a diameter of 25 µm are visible on the front side of the a-GaAs/c-Si heterocontact solar cell similar to oval defects and mounds which are observed on MBE grown GaAs where gallium droplets are formed [
31,
32,
33]. Even under arsenic-rich growth conditions gallium droplets can be seen in the GaAs surface [
21,
33]. Furthermore, the thermal treatment causes a reduction of the As concentration in the GaAs film favoring its recrystallization, which can promote the apparition of gallium droplets due to stoichiometry changes [
25,
26,
27,
28]. Excessive As tends to migrate to the surface of the sample and sublimates during the thermal step [
27].
In our research, we also focused on the effect of an annealing step before or after the ITO deposition procedure. The results are shown in
Figure 8a,b. As it can be seen, the density of defect like structures on the front side of the solar cell is considerably reduced in case the ITO layer is deposited prior to the annealing. This fact has also implications on the electrical performance of the solar cells as shown in
Figure 9. There, an increase of the open circuit voltage to a value of 250 mV is observed along with an increase of the short circuit current density to 4.2 × 10
−5 A/mm
2 and an improved fill factor. We conclude that both, the saturation current density and the layer resistance are significantly reduced by protecting the GaAs layer with the ITO layer prior to the heat treatment. As it can be seen in
Figure 9 the series resistance of sample 1 is lower than the series resistance of sample 2. The solar conversion efficiencies are 0.26% and 0.09% for samples 1 and 2, respectively. These values, even if they are low, exceed efficiencies that were recently reported for heterojunction solar cells on a highly doped GaAs Substrate with an intermediate undoped epitaxial Si layer followed by a doped amorphous Si absorber both deposited by PECVD [
19].
The spectral response (SR) measurements of these samples are shown in
Figure 10. The photogenerated current is clearly increased on the sample with the ITO layer deposited before the thermal treatment.
Figure 8.
Details of the front contact region of the a-GaAs/c-Si heterocontact solar cell.(a) Thermal treatment before ITO deposition. (b) Thermal treatment after ITO deposition. The vertical bars in the figure are Au collecting fingers.
Figure 8.
Details of the front contact region of the a-GaAs/c-Si heterocontact solar cell.(a) Thermal treatment before ITO deposition. (b) Thermal treatment after ITO deposition. The vertical bars in the figure are Au collecting fingers.
Figure 9.
J-V characteristics of an a-GaAs/c-Si heterocontact solar cell under standard illumination conditions. Sample 1: Annealing after ITO deposition; Sample 2: Annealing before ITO deposition.
Figure 9.
J-V characteristics of an a-GaAs/c-Si heterocontact solar cell under standard illumination conditions. Sample 1: Annealing after ITO deposition; Sample 2: Annealing before ITO deposition.
Figure 10.
Spectral response (SR) of different samples with the thermal treatment taken place before (sample 1) and after (sample 2) the ITO deposition.
Figure 10.
Spectral response (SR) of different samples with the thermal treatment taken place before (sample 1) and after (sample 2) the ITO deposition.