3.1. Study of MoSe2 Interfacial Layer in CIGS Thin Film Solar Cell Structure
It was stated in the
Section 2 that ZnO/CdS/CIGS/Mo device structure without any interfacial MoSe
2 layer was used as the baseline case in this study. The photovoltaic performance parameters of J
SC = 35.75 mA/cm
2, V
OC = 0.681 V, FF = 82.88%, and PCE = 20.19% for this baseline configuration are considerably lower than all the simulated cases with MoSe
2 layer in the structure, which will be shown later. The significantly low cell performance is primarily due to large work function of CIGS,
in comparison to that of Mo,
(
), which results in high barrier for holes (
) at the back contact. This barrier at the back contact interface (CIGS/Mo) is recognized to be an inherent loss mechanism due to the formation of a second junction, but with opposite polarity [
35]. It affects other parameters of the solar cell, such as FF and V
OC of the device, and hence, the power conversion efficiency (PCE). The emphasis of this part of the numerical analysis is the CIGS/MoSe
2/Mo section, which is a semiconductor-semiconductor-metal structure with two interfaces: CIGS/MoSe
2 and MoSe
2/Mo.
Figure 3a shows the band profile of conventional CIGS structure with CIGS/Mo interface in comparison to our proposed structure with CIGS/MoSe
2/Mo interface in
Figure 3b. The Schottky contact at the CIGS/Mo interface indicated by the large downward bending of the valence band,
shown in
Figure 3a, becomes a quasi-Ohmic contact with the inclusion of a thin MoSe
2 semiconducting layer as in
Figure 3b. Higher bandgap of MoSe
2 leads to upward shift of the conduction band,
, creating a
barrier impeding the recombination of minority carriers (electrons) at the back contact that is detrimental to the solar cell performance. In the case of majority carriers (holes) collection, although MoSe
2/Mo interface is still a Schottky contact, the formation of the MoSe
2 layer shifts both the maxima of the,
and the minima of the
upwards, as evident in
Figure 3b. This upward shift of the bands results in the elimination of the barrier for holes between the CIGS and Mo layers that is present in
Figure 3a. Nevertheless, the collection rate of holes will be suppressed when the MoSe
2 interfacial layer is very thick.
In the next stage, optimization of interfacial MoSe
2 layer in terms of layer thickness, bandgap energy, electron affinity, and acceptor carrier concentration has been studied to choose the most effective MoSe
2 layer properties for the proposed CIGS solar cell design. The effects of acceptor carrier concentration (
) from 1.0 × 10
14 to around 1.0 × 10
20 cm
−3 and MoSe
2 layer thickness from 0.01 to 0.1 μm on the photovoltaic performance of the device were investigated.
Figure 4 shows that the overall performance of the device improved when the thickness of MoSe
2 layer was increased. This finding arises from the fact that increasing the thickness leads to an increase of photon absorption rate in the near infrared region resulting in higher photogenerated charge carriers, as the MoSe
2 layer is effectively extending the width of the p-type region in the solar cell [
36]. Additionally, a thinner MoSe
2 layer causes lower shunt resistance which is evident from the significantly lower V
OC value as can be seen from
Figure 4b. This graph depicts the significant dependency of V
OC on the two variables of MoSe
2;
and thickness. At
larger than 1.0 × 10
16 cm
−3, V
OC minutely increased with the increase in thickness until
1.0 × 10
18 cm
−3, at which point V
OC starts to saturate for MoSe
2 layer thickness above 0.04 μm, hence the thickness must be maintained above
0.04 μm to obtain the maximum attainable V
OC. On the other hand, a thicker MoSe
2 layer induces higher series resistance as apparent from the decreased FF in
Figure 4c. As such, carrier collection efficiency will be limited due to the increased resistivity of a thick MoSe
2 layer in comparison to pure Mo [
37]. Therefore, the thickness of MoSe
2 layer needs to be carefully controlled to turn the back contact from a rectifying Schottky contact to a low resistance quasi-ohmic contact [
38].
On the variations in the MoSe
2 carrier concentration, it can be observed that, in general, increasing the
leads to an increase in the Jsc, Voc, FF as well as the conversion efficiency of the cell, hence higher
of MoSe
2 layer is favorable. MoSe
2 layer is found to perform better in terms of J
SC and V
OC at
1.0 × 10
16 cm
−3. This finding can be attributed to the fact that
and
rise as
increases according to [
39]. The beneficial band bending facilitates the transport of holes from MoSe
2 to Mo, hence enhancing photogenerated carrier collection. At the same time, electrons are hindered from flowing to the back contact thereby inhibiting back surface carrier recombination, enhancing both J
SC and V
OC. Though the reduced back surface recombination due to band bending with an increase in
attributes to Voc change, this factor is insufficient to account for observed increase. The significant improvement in V
OC for high carrier concentration,
of MoSe
2 can be correlated to the formation of p
+-layer at the interface between p-type CIGS and Mo back contact, creating what can be considered as back surface field (BSF) [
40]. On the other hand, when the
of MoSe
2 is
1.0 × 10
16 cm
−3 which is the value of
defined for CIGS absorber layer, the overall cell performance degrades. This effect can be described as the phototransistor effect which involves the formation of a second junction, but with opposite polarity to the pn-junction caused by the formation of a Schottky contact in the interfacial region between the MoSe
2 with low carrier concentration and the CIGS absorber with higher concentration. As a result, the V
OC in the device is impaired [
41]. Though phototransistor effect is commonly observed in the J-V curves measured at low temperatures in the range from 100 to 200 K, it was reported that a decrease in the effective doping level of the MoSe
2 layer might induce this effect even at room temperature [
35,
41].
The change in FF with MoSe
2 carrier concentration might be discussed and analyzed from the interface properties standpoint. One of the band alignment-related electronic parameter according to Anderson-energy band rule [
42] that is significantly affected by the variation in MoSe
2 carrier concentration is equilibrium contact potential (
) that determines potential barrier retarding hole transport from the semiconductor valence band to metal in a Mo/p-type semiconductor structure [
43]. Generally, lower
is desirable for the hole transport. Increment in the carrier concentration,
in p-type MoSe
2 layer shifts the Fermi level downwards closer to the valence band edge and subsequently increases the semiconductor work function (
) and
between MoSe
2 and Mo as determined by the following equations [
42]:
Calculated interface electronic parameter values (
and
) as a function of
in p-type MoSe
2 layer is presented in
Table 3 in order to verify the claim. The unfavorable high
with the increase in
forms a larger downward bending barrier for hole transport from the valence band of MoSe
2 to the Fermi level of Mo. With barriers larger than 0.2753 eV when
of MoSe
2 was set to 1.0 × 10
18 cm
−3, the FF starts to degrade.
In terms of different combinations of carrier concentration and thickness, as it is clear in
Figure 4a, with high MoSe
2 carrier concentration, thin MoSe
2 layer is sufficient to achieve high J
SC. This is in accordance to the reported findings by Hossain et al. [
44] and Ferdaous et al. [
45] that even small changes in the J
SC values, the phenomenon of higher carrier concentration and lower thickness are preferred for p-type MoS
2 (p-MoS
2 yields similar benefits as p-MoSe
2 as interfacial layer at the back contact). However,
Figure 4c shows that when
is
1.0 × 10
18 cm
−3, the trend in FF changes from favoring larger thickness to favoring lesser thickness. Nevertheless, the difference in the electrical characteristics of thin and thick MoSe
2 layers are not very significant if the carrier concentration is sufficiently high. Hence, it is more favorable to have a thinner MoSe
2 layer with high carrier concentration in order to avoid a jump in series resistance due to the high resistivity nature of MoSe
2.
The aforementioned results indicate that significant changes in the performance parameters of the cell were observed when the
of MoSe
2 layer was around 10
16, 10
18, and 10
19 cm
−3. Hence, the effects of bandgap energy (
) and electron affinity (
) on the conversion efficiency were investigated in the range from 1.0 to 1.5 eV and 3.5 to 4.5 eV, respectively, at these three domains of
and are illustrated in
Figure 5a,b. As shown, with MoSe
2 layer
of 10
18 and 10
19 cm
−3, CIGS device exhibits similar and almost identical patterns in the PCE with the increase in
and
, while at
of 10
16 cm
−3, the PCE variation demonstrates a distinguish phenomenon of nearly linear decrease from 20% to 2% and from around 32% to 2% with increasing values of
and
, respectively. This result verifies our earlier findings that higher doping level of the MoSe
2 layer is more beneficial for the overall cell performance. Therefore, based on this preliminary investigation,
of MoSe
2 layer in the CIGS model was then fixed at 10
19 cm
−3 to perform further simulations for different combinations of
and
for the MoSe
2 layer.
Figure 6 illustrates the solar photovoltaic parameters as a function of
of MoSe
2 layer, with bandgap energy
ranging from 1.0 eV to 1.5 eV. It can be observed that, in general, increasing the
leads to an increase in the conversion efficiency of the cell over the whole investigated range of
. Hence, MoSe
2 layer of higher
is favorable. As depicted in
Figure 6a, for
1.1 eV, J
SC of the device is almost constant for all values of
, indicating that beyond a minimum value or threshold,
and
do not notably affect the current collection. This may be explained by the fact that the thin p-type MoSe
2 layer in this CIGS solar cell structure does not play an important role in the photon absorption, hence, other than layer thickness as discussed earlier, variation in other material parameters of this layer does not contribute to the change in J
SC of the whole device. On the other hand, V
OC and PCE exhibit similar behavior where for the values of
defined earlier for the MoSe
2 layer, the V
OC and PCE were almost saturated for all values of
, provided that
5.4 eV. If the total of
rises beyond 5.4 eV, V
OC and PCE start to deteriorate and further increase will result in non-working device as can be seen in
Figure 6b,d. These observations indicate that optimum range of electron affinity for a functional MoSe
2 interface layer is dependent on the value of its bandgap energy. Improvement in V
OC with the increase in
of MoSe
2 layer can be analyzed based on the band offsets at CIGS/MoSe
2 and MoSe
2/Mo interfaces. The features of band alignment are determined based on the Anderson’s rule or known as electron affinity model as mentioned earlier.
and
χ of MoSe
2 layer crucially determine
,
, back contact barrier height with respect to
(
), back contact barrier height with respect to
of the MoSe
2/Mo metal-semiconductor junction and conduction band offset (
), valence band offset (
), and back diode built-in voltage (
) of the CIGS/MoSe
2 heterojunction [
46] as given in Equations (4)–(10).
Increment/decrement in the MoSe
2 and
values shifts the whole band structure of the layer upward or downward, which in turn has an effect on the mentioned band alignment related parameters. Nevertheless, since no significant effect on the cell performance is found in the variation of
in comparison to
from our earlier discussion on
Figure 6,
is defined as 3.9 eV in order to calculate the interface electronic parameter values as a function of
as tabulated in
Table 4. Changes in
are usually reflected as simultaneous change in
and
. Thus, any increase in
can be seen as upshifting of
and/or downshifting of
. However, since the value of
was kept constant, the increase in
is assumed to cause the downshifting of
, while
does not change, thus the values of
and
are constant with the increase in
, as shown in
Table 4. Therefore, the resulting device performance is solely a composite function of
,
,
and
. In general, for CIGS/MoSe
2/Mo band alignment, lower
and
along with higher positive
(or lower negative
) are desirable for greater hole transport across the valence band of CIGS to valence band of MoSe
2 and from the valence band of MoSe
2 to Mo metal. Increase of MoSe
2 results in an increase in MoSe
2 work function (
) due to the downward shift of
. This will lead to larger values of
,
and
, compounding the inhibition of hole flow to the back contact. Nevertheless, this drawback can be negated by carrier tunneling through a very thin MoSe
2 layer, which can be related back to the improved conversion efficiency obtained with the increase in
as illustrated in preceding
Figure 6d. Additionally, increased hole population by means of higher doping concentration can yield slightly better performance for MoSe
2 with higher
as previously demonstrated in
Figure 5.
Based on the aforesaid results, variations in the bandgap, electron affinity, thickness and carrier concentration of the MoSe
2 layer mainly affected the band alignment at the CIGS/MoSe
2 and MoSe
2/Mo junctions, which subsequently influences the overall performance of the CIGS solar cell. It was identified that a MoSe
2 layer with bandgap in the range of 1.2 eV to 1.4 eV and high carrier concentration above 1.0 × 10
18 cm
−3 is beneficial for the PV cell. As predicted, the MoSe
2 layer needs to be quite thin, but not less than 0.04 μm thickness in order to be effective or operational. These findings provide a limiting condition for MoSe
2 layer in CIGS solar cell since the formation is absolutely unavoidable during the growth process. Our CIGS model with optimized MoSe
2 layer is then validated by comparison with extant experimental results for the conventional ZnO/CdS/CIGS/Mo solar cells considering that MoSe
2 is naturally formed as opposed to being a deliberately added layer in the device stack during fabrication. The 23.3% efficiency of our simulated CIGS cell is significantly higher with the addition of MoSe
2 layer than the recorded efficiency of 21.7% [
24]. This is apparently due to the fact that there are many other factors during the fabrication process that could affect the cell performance that cannot be replicated by the simulation software. Therefore, it is impossible to simulate these realistic experimental conditions due to the limitations of the SCAPS software. In addition, not many details were provided in the report about the actual steps involved in the making of these record-breaking solar cells. Nevertheless, the adequately high efficiency of the actual solar cells suggests the presence of MoSe
2 layer in the structure as the simulated structure with MoSe
2 is closer to the experimental value. The resulting performance parameters of the J
SC, V
OC, FF and PCE are displayed in
Table 5.
3.2. Study of Absorber Layer Bandgap Grading in CIGS Solar Cell Structure
In this study, bandgap grading was introduced into the model by including a Ga concentration profile in the absorber layer, that is the variation of GGI ratio throughout the depth of the absorber layer. Double grading (DG) profile was defined within the absorber layer, which corresponds to a minimum amount of Ga in the middle layer region and a higher amount toward the CIGS/MoSe
2 and the CdS/CIGS interfaces. These bandgap variations are described in this work by the parameters: (i)
and
(corresponding to the Ga concentration at the back and front sides of the absorber layer); (ii)
(notch point which represents the lowest composition value); (iii)
(position of the notch point across the depth of absorber layer) through the use of parabolic grading [
28]. Thus,
and
define the
(maximum bandgap value) while
defines
(minimum bandgap value) in the bandgap grading profiles. With the purpose of determining the best grading configuration for this notch type double graded bandgap structure in our CIGS model, different values for these related bandgap grading parameters were simulated and the effects on cell performance were analyzed. In order to incorporate an optimum DG bandgap profile, the effects of separate grading at either ends of the absorber must be first understood. Thereby, optimization of single graded Ga profiles (back and front grading) was conducted prior to the introduction of DG and the results obtained are illustrated in
Figures S2–S4 in the supporting materials section.
Table 6 shows the optimum values for Ga composition (GGI ratio) at the back contact and at the CdS/CIGS interface and the lowest bandgap (
) in the CIGS absorber layer obtained from the optimization of single graded Ga profiles.
Conceptually, double graded (DG) bandgap absorbers promote photovoltaic performance by increasing J
SC, which is a consequence of the minimum bandgap introduced in the absorber layer of the device, and at the same time increased V
OC due to the increased bandgap in the SCR. This principal of two bandgaps was experimentally verified by Dullweber et al. [
47]. In DG bandgap profile, the position of notch (
) is a significant parameter that will influence the effectiveness of bandgap grading in the absorber layer. Hence,
is varied in the range from 0 to 2.5
m to observe the effects mainly on J
SC and V
OC, keeping the notch value (
) constant at 0.3. From
Figure 7a, it can be observed that J
SC significantly improved when the
value is shifted to the front junction of the CIGS layer. As mentioned earlier in
Section 1, absorption in the frontal part is reduced as the bandgap in this region becomes larger. However, if the front grading with wider bandgap is restricted within the SCR by placing the notch (
) inside the width of SCR, the loss in J
SC can be compensated by an increased absorption further into the CIGS absorber layer, where the band gap is minimum. Moreover, the rise of
in the SCR will generate an additional electric field that will oppose the transport of electrons. Hence, it is important to confine the front grading only in this region. As
shifts further towards the backside of the absorber layer (from the distance of 2.25 to 0.25
m), the decrease in J
SC is more pronounced (from 38.97 to 36.37 mA/cm
2). The graph in
Figure 7b depicts V
OC dependency on the notch position (
) that is fairly contrasting to J
SC. V
OC does not seem to be substantially affected by the variation in
, though small increment is observed as
shifts towards the front side of the absorber layer.
There are certain conditions related to the grading parameters that must be satisfied in order to completely benefit from bandgap grading. In the case of a p-type CIGS layer, the needed quasi-electric field arising from an absorber material with graded bandgap structure should be directed towards the back contact so that electrons will drift in the opposite direction to reduce back surface recombination. In other words, in this case, the bandgap must be larger towards the back contact. The second condition is that the formation of wider bandgap region at the front junction must be confined within SCR since this quasi-electric field would oppose the motion of photo-generated carriers from the bulk of the semiconductor towards the collecting junction. From the above considerations, different magnitudes, shapes, and depths of double graded (DG) bandgap structure were tested in order to find its relevance in the solar cell performance and to determine the best grading configuration for an optimized CIGS device.
Figure 8a illustrates the simulated compositional ratios of five models with different bandgap configurations and the resulting current density-voltage (J-V) curves obtained are shown in
Figure 8b. A summary of the solar cell parameters for the different Ga profiles is presented in
Table 7.
As can be observed in
Table 7, the best PCE is obtained from the simulated Model 2 that presents a front grading and back grading composition of 0.8 and 0.7, respectively. The notch for this model is positioned at a distance of 1.25 μm from the CdS/CIGS interface. Although it is mentioned earlier that the notch should be placed close to the junction and restricted within the SCR for improved J
SC as evident from previous
Figure 7a, 1.25 μm was found to be the optimum notch position for the DG absorber bandgap profile developed in this study. The slight loss in J
SC in Model 3 in comparison to Model 2, with Model 3 having a steeper Ga grading profile at the frontside of the absorber, can be attributed to the fact that the notch acts like a confinement region for electrons decreasing photogenerated carrier collection probability. Similar observation can be made for Model 4 and Model 5. In line with the reflection that improved J
SC in DG absorber bandgap structure is controlled by the minimum bandgap value in the layer, it can be discerned that J
SC values are higher when
was set at GGI = 0.2 as compared to 0.3 in all bandgap grading models. With regard to V
OC, Model 4 presents the highest value of 0.856 V that can be attributed to the highest GGI ratio of 0.8 (
= 1.5 eV) at the backside of the absorber layer. The enhanced V
OC is directly correlated to the reduced back surface recombination as discussed and presented in the earlier section. FF is the highest in device Model 5 with DG absorber bandgap value of 1.28 eV corresponding to GGI (=0.5) at both the frontside and backside of the absorber layer. Ideally, FF is only a function of V
OC. However, practically, FF does not only depend on V
OC, but also on the recombination process in the depletion region [
45], which explains the difference in the trend of FF in comparison to V
OC as can be seen in
Table 7. The highest PCE gain from the ungraded bandgap cell is over 8.8% as seen in Model 2.
3.3. Optimizing Bandgap Energy of MoSe2 Layer for Compatibility with Double Graded (DG) Bandgap Profile of CIGS Absorber Layer
For the purpose of associating the bandgap grading study with MoSe
2 layer properties, optimum DG bandgap profile with front and back GGI ratio of 0.8 and 0.7, respectively,
= 1.25 μm, and
= 0.2 as previously determined was employed with MoSe
2 layers with varying bandgaps of 1.0 to 1.4 eV. The resulting solar photovoltaic parameters (J
SC, V
OC, FF and PCE) obtained are tabulated in
Table 8. In
Section 3.1, it has been shown that
and
χ of both MoSe
2 and CIGS layers are vital in determining the
,
, and
of the CIGS/MoSe
2 heterojunction [
43]. Lower
and higher positive
(or lower negative
) are desirable for the ease of hole transport across the valence band of the CIGS layer to the valence band of MoSe
2. Hence, the resulting device performance is partly contributed by the composite function of
and
. The effects of MoSe
2 bandgap on these parameters have been discussed and presented in
Section 3.1. Based on the values of J
SC, V
OC, FF and PCE in
Table 8, MoSe
2 bandgap of 1.3 eV is the best match with
of 1.43 eV at the backside of CIGS absorber layer for devices with DG absorber bandgap profile.
It is a fact that recombination of photo-generated carriers is the main detrimental factor affecting the performance of CIGS solar cells [
46,
47,
48]. Extraction of interface recombination rates allows detailed analysis for material and device engineering to reduce interface recombination leading to an improved V
OC. Though experimental V
OC analysis is compulsory in understanding the physics of recombination mechanism, previous investigation has demonstrated the possibility of utilizing numerical simulation for extracting recombination rates in the complete width of the absorber layer (from the buffer/absorber interface [
48] to absorber/back contact interface [
49]). The principle of temperature- and illumination-dependent V
OC analysis is to identify and quantify Shockley–Read–Hall (SRH) as well as radiative recombination rates in different regions of the CIGS absorber-namely, the heterointerface, depletion region and quasi-neutral region (QNR). A study conducted by Paul et al. [
50] depicted that most of the recombination occurs near the CdS buffer/CIGS absorber interface and in the depletion region, with Shockley–Read–Hall (SRH) recombination dominating over radiative recombination when operating temperature, T is set at 300 K. By emulating the investigation carried out by Paul et al., simulation study on the relationship of recombination rate and the depth of the absorber layer was carried out using the baseline CIGS model at V = V
OC. The MoSe
2 interface layer and absorber bandgap grading was not incorporated in this baseline CIGS model. Uniform mid-gap defect level for all layers in the structure were considered, with radiative recombination coefficient defined as 1.0 × 10
−10 cm
3/s based on [
50].
Figure 9 shows the generated SRH and radiative recombination profiles (for T = 300 K) at V = V
OC. It was found that the interface of CdS/CIGS and SCR act as recombination centers in the structure, matching the observations made by [
50]. However, in this study radiative recombination rate seems to be higher at the depletion region and QNR while SRH is significantly more dominant at the front interface. There was not much difference in the recombination rates with the variation in operating temperature, T = 200 K and 300 K.
Controlling bandgap grading within the CIGS absorber is claimed to be one way of improving band alignment and suppressing recombination at the interface and in the bulk of the layer. It is said that in a uniform bandgap absorber with a low Ga content, recombination in the quasi-neutral region dominates, while in the high Ga-absorber, interface recombination dominates [
51]. However, the effect of bandgap grading on the recombination process has not yet been fully understood. Hence, one of the objectives for implementing a graded structure in this study is to rearrange the recombination throughout the cell, by optimizing CdS/CIGS and CIGS/MoSe
2 band offsets to reduce surface recombination at the interface and by utilizing the built-in potential that causes drift of the photogenerated electrons and holes towards the right direction. In order to understand the role that bandgap grading has in suppressing carrier recombination, recombination profiles for the simulated CIGS device with MoSe
2 interfacial layer and with/without graded structure (DG) in the absorber layer were analyzed and presented in
Figure 10 (left inset). There was clear benefit to be seen in recombination yields with the introduction of bandgap grading as suggested by earlier studies, with a significant decrease in back surface and front surface recombination illustrated in
Figure 10 left and right inset respectively. Additional electric field known as quasi-electric field (
) is formed due to the change in bandgap,
over the distance
and can be described by Equation (11) [
20]. This quasi-electric field directed towards the back contact with the introduction of back grading will drift the electrons in the opposite direction to reduce back surface recombination. Hence. Ga-rich structure at the backside of absorber layer (towards CIGS/MoSe
2 interface) is beneficial and effective in suppressing electrons from combining with holes.
With respect to the front surface recombination, recombination rate in the SCR can be reduced by increasing the barrier height via the increase of bandgap at the junction region (CdS/CIGS interface). This is due to the fact that the CdS buffer/CIGS absorber conduction-band offset,
significantly influences the band bending and thereby affecting interface recombination where stronger band bending and larger hole barrier are induced by a positive
at the CdS/CIGS interface [
49].
By comparing the J-V characteristic and quantum efficiency (QE) curves of a uniform bandgap device with and without MoSe
2 layer to the device with DG absorber profile, shown in
Figure 11, the composite benefit of the MoSe
2 layer and DG absorber layer on the performance of CIGS solar cells, is remarkable and was clearly demonstrated in this simulation study, as seen in the overall cell performance parameters. This enhancement in current density is attributed to the increased absorption of photons in the longer wavelength region of above 1000 nm, as can be seen from the QE curve of
Figure 11b. A reasonable explanation for the enhanced photon absorption is the lower
of 1.10 eV in the underlying bulk region of DG CIGS absorber layer in comparison to the device with uniform absorber bandgap of 1.20 eV. This observation is in agreement with the inferred statement earlier that this notch value and its position is crucial for the device to benefit from bandgap grading of the absorber layer.
Table 9 below shows the performance parameters of all the devices investigated in this section.