**3. Front a-Si:H/c-Si heterointerface**

#### **3.1. Front a-Si:H/c-Si: influence of interface defect states**

The front a-Si:H/c-Si heterointerface is a key part of the SHJ solar cell which has the main influence on the recombination processes in the structure and thus the output performance. The connection of two materials with different band gaps, lattice and electrical properties results into the formation of band discontinuity and defect states at the interface. Such properties are strongly affecting the carrier transport through that interface. In order to investigate the influence of *D*it on the carrier inversion and hence recombination activity at the interface, numerical calculations using the program ASA was carried out. **Figure 3(a)** shows *V*OC and *η* calculated as a function of *D*it for SHJp and SHJn solar cell structures. As can be seen, *V*OC and hence *η* exhibit a decrease upon the increase of *D*it for both SHJp and SHJn structures. To explain the recombination processes at the a-Si:H/c-Si interfaces connected with the presence of defect states, it is necessary to consider the band diagram. Analysis will be provided for SHJp structure, however, conclusions are applicable also to SHJn structure. **Figure 3(b)** shows the band diagram of SHJp structure calculated for two values of *D*it. In the case of SHJp structure with negligibly low *D*it = 109 cm–2, the rectification behaviour of the junction is formed due to the presence of the negative and positive space charges in the space charge region (SCR) of the c-Si and a-Si:H part of the junction, respectively. The SHJ solar structure exhibits a high asymmetry of doping, which shifts the SCR into the c-Si part of the junction, leaving a negligible part of the diffusion voltage *V*d in the a-Si:H. Due to the presence of band discontinuity in the SHJp solar cell structure, the bands in the c-Si bend downwards and a high concentration of minority electrons is formed at the c-Si surface at the a-Si:H/c-Si interface. Such a layer with a high concentration of minority carriers is called an inversion layer. In the case of high inversion, the concentration of minority electrons is high and the concentration of majority holes is low at the c-Si surface of the heterointerface. For SHJp structure, the photo-generated electrons are collected by the front electrode and transferred through the front heterointerface. High carrier inversion and thus a low concentration of holes results into a low probability of photo-generated electrons to recombine with them. Such a behaviour is characterized by the barrier for interface recombination *Φ*B, which is for SHJp expressed by Eq. (6). The high carrier inversion, in other words high value of *Φ*B, leads to low interface recombination. From this it is obvious that carrier inversion plays a crucial role in the *V*OC of SHJ solar cell. The carrier inversion is changed by introducing a high value of defect states at the interface, *D*it = 6 × 1012 cm–2. Such defect states affect the charge conditions in the SCR of junction. The defects states at the heterointerface represent traps which are for SHJp structure occupied by electrons forming negative charge *Q*<sup>i</sup> in the SCR at the c-Si part of the junction. Such negative *Q*<sup>i</sup> screens the positive charge in the a-Si:H part of the junction and thus hinders the extension of the SCR in the c-Si. As results, the band bending and electric field in the c-Si part of the junction are lowered, which decreases the carrier inversion at the interface and causes decrease of *Φ*B followed by an increase of interfacial recombination. Moreover, due to the lower electric field, the majority of holes have a high probability to diffuse to the interface and contribute to the recombination at the interface [28]. The same mechanism of carrier inversion decrease caused by the presence of *Q*<sup>i</sup> is presented in case of SHJn solar cells (not shown here). However, due to the n-type silicon used in SHJn solar cells and holes collected through the front contact, the *Q*<sup>i</sup> has a positive charge. Comparing both structures (**Figure 3a**), the SHJn structure exhibits a lower sensitivity to *D*it and a higher efficiency. There are two main sources of such a higher efficiency for the SHJn structure. The first source is the higher FF (not shown here) of SHJn structure compared to SHJp structure. The second reason is the collection of holes through the front heterointerface of SHJn structure, which due to the band alignment exhibits lower interface recombination. The impact of the band alignment on the carrier inversion of SHJ structures is further discussed in Section 3.2.

rier inversion is linked with *Φ*B thus directly related to *V*OC. For both SHJ, high carrier inversion is required to obtain high value of *Φ*<sup>B</sup> and thus high *V*OC. Eqs. (1) and (2) do not take into account the influence of a-Si:H layer and indicate that *V*OC depends on the illumination intensity, recombination properties at the heterointerface, recombination at the rear surface and in the c-Si substrate, and on the dopant activation energy of the c-Si substrate. The properties of the emitter seem to play no role in the *V*OC. In fact, the parameters like doping, defect density and affinity (or band offset with c-Si) have no direct influence on the *V*OC. However, all of these parameters affect the charge properties of the space charge region (SCR) of SHJ junction and thus carrier inversion at the heterointerface and consequently *V*OC. In the following sections, we will describe by means of simulation various SHJ solar

The front a-Si:H/c-Si heterointerface is a key part of the SHJ solar cell which has the main influence on the recombination processes in the structure and thus the output performance. The connection of two materials with different band gaps, lattice and electrical properties results into the formation of band discontinuity and defect states at the interface. Such properties are strongly affecting the carrier transport through that interface. In order to investigate the influence of *D*it on the carrier inversion and hence recombination activity at the interface, numerical calculations using the program ASA was carried out. **Figure 3(a)** shows *V*OC and *η* calculated as a function of *D*it for SHJp and SHJn solar cell structures. As can be seen, *V*OC and hence *η* exhibit a decrease upon the increase of *D*it for both SHJp and SHJn structures. To explain the recombination processes at the a-Si:H/c-Si interfaces connected with the presence of defect states, it is necessary to consider the band diagram. Analysis will be provided for SHJp structure, however, conclusions are applicable also to SHJn structure. **Figure 3(b)** shows the band diagram of SHJp structure calculated for two values of *D*it. In the case of SHJp structure with negligibly low *D*it = 109 cm–2, the rectification behaviour of the junction is formed due to the presence of the negative and positive space charges in the space charge region (SCR) of the c-Si and a-Si:H part of the junction, respectively. The SHJ solar structure exhibits a high asymmetry of doping, which shifts the SCR into the c-Si part of the junction, leaving a negligible part of the diffusion voltage *V*d in the a-Si:H. Due to the presence of band discontinuity in the SHJp solar cell structure, the bands in the c-Si bend downwards and a high concentration of minority electrons is formed at the c-Si surface at the a-Si:H/c-Si interface. Such a layer with a high concentration of minority carriers is called an inversion layer. In the case of high inversion, the concentration of minority electrons is high and the concentration of majority holes is low at the c-Si surface of the heterointerface. For SHJp structure, the photo-generated electrons are collected by the front electrode and transferred through the front heterointerface. High carrier inversion and thus a low concentration of holes results into a low probability of photo-generated electrons to recombine with them. Such a behaviour is characterized by the barrier for interface recombination *Φ*B, which is for SHJp

cell properties which affect carrier inversion and *V*OC.

**3.1. Front a-Si:H/c-Si: influence of interface defect states**

**3. Front a-Si:H/c-Si heterointerface**

76 Nanostructured Solar Cells

**Figure 3.** (a) The *V*OC and *η* calculated in dependence on *D*it for SHJn and SHJp structures. (b) Band diagrams calculated for two values of *D*it for SHJp structure. The inset shows the change of carrier inversion (change in the distance of the conduction band level from the Fermi level at the heterointerface) with the change of *D*it.

From the above discussion it is clear that the change of the charge properties in the SCR plays the key role for the carrier inversion at the heterointerface and strongly affects *V*OC. The *D*it are formed by acceptor and donor types of defects which form negative and positive charges in the c-Si part of SCR, respectively. Our recent study shows that the band bending at the c-Si part of the structure is lowered mainly due to the presence of *Q*<sup>i</sup> with negative charge and *Q*<sup>i</sup> with positive charge for SHJp and SHJn solar cells, respectively [22]. Because of this, the defect asymmetry at the interface plays also an important role for the recombination processes at the interface [22]. The presence of acceptor defects at the heterointerface is more detrimental for the function of SHJn, while in the case of SHJp structure the donor defects are more affecting the performance of solar cell.

#### **3.2. Front a-Si:H/c-Si: influence of band alignment**

Comparing with the standard c-Si-based solar cells, the SHJs are characterized by the formation of a carrier inversion layer of minority carriers at the c-Si surface. The origin of this inversion layer steams from the presence of the band discontinuity at the interface and is the main factor for higher *V*OC compared to the standard c-Si-based solar cells. In order to describe the impact of band alignment on the *V*OC, simulation of SHJp solar cells with a varied conduction band offset Δ*E*C is presented in **Figure 4(a)**. The impact of non-ideal a-Si:H/c-Si interface is shown as well by using four different values of *D*it. Clearly, the decrease of Δ*E*C results in the decreases of *V*OC. This effect is stronger, when higher *D*it is present at the interface. On the other hand, for high Δ*E*C, the *D*it has a weaker impact on the *V*OC. Such a behaviour can be explained by considering the band bending and carrier inversion in the structure. **Figure 4(b)** shows band diagrams of SHJp solar cells for *D*it = 5 × 1011 cm–2 and two values of Δ*E*C. As can be seen, higher Δ*E*C results into higher band banding in the c-Si part of the junction and stronger carrier inversion at the heterointerface. Because of this, *V*OC exhibits higher values for structures with high Δ*E*C. Moreover, the strong inversion causes a pronounced suppression of interface recombination since only few majority carries are available for recombination. As a result, the negative impact of *D*it is less serious for structures with high values of Δ*E*C. From this it is obvious that the ability to prepare a-Si:H/c-Si with high Δ*E*C should be the way how to suppress the influence of *D*it and how to attain high *V*OC and thus the efficiency of SHJ solar cells. However, there are only limited possibilities to modify the band alignment of a-Si:H/c-Si heterointerface based on tuning the hydrogen content in the a-Si:H layer [33]. The literature presents a consensus that Δ*E*<sup>C</sup> at a-Si:H(n)/c-Si(p) heterointerface is below 0.30 eV [23, 34–36].

**Figure 4.** (a) *V*OC calculated as a function of Δ*E*C at the front a-Si:H/c-Si of SHJp solar cell structure. *D*it is varied as a parameter in the simulations. (b) Band diagrams calculated for two values of Δ*E*<sup>C</sup> and *D*it = 5 × 1011 cm-2 of SHJp solar cell structure. The inset shows the change in the carrier inversion (change in the distance of the conduction band level from the Fermi level at the heterointerface).

Therefore, the critical aspect to obtain high *V*OC of SHJ structures remains the suppression of defect states at the interface.

In the case of the SHJn structure, the transport of photo-generated minority holes is affected by the valence band offset Δ*E*V which, due to the band alignment, has a higher value compared to the Δ*E*<sup>C</sup> of SHJp. Because of this, the SHJn solar cell structures have higher carrier inversion at the interface as well as higher *Φ*<sup>B</sup> and exhibit higher *V*OC compared to the SHJp structures. Moreover, due to the higher carrier inversion the SHJn structure exhibits a lower sensitivity to *D*it compared to the SHJp structure (**Figure 3a**).

#### **3.3. Front a-Si:H/c-Si: influence of a-Si:H(i) passivation layer**

asymmetry at the interface plays also an important role for the recombination processes at the interface [22]. The presence of acceptor defects at the heterointerface is more detrimental for the function of SHJn, while in the case of SHJp structure the donor defects are more affecting

Comparing with the standard c-Si-based solar cells, the SHJs are characterized by the formation of a carrier inversion layer of minority carriers at the c-Si surface. The origin of this inversion layer steams from the presence of the band discontinuity at the interface and is the main factor for higher *V*OC compared to the standard c-Si-based solar cells. In order to describe the impact of band alignment on the *V*OC, simulation of SHJp solar cells with a varied conduction band offset Δ*E*C is presented in **Figure 4(a)**. The impact of non-ideal a-Si:H/c-Si interface is shown as well by using four different values of *D*it. Clearly, the decrease of Δ*E*C results in the decreases of *V*OC. This effect is stronger, when higher *D*it is present at the interface. On the other hand, for high Δ*E*C, the *D*it has a weaker impact on the *V*OC. Such a behaviour can be explained by considering the band bending and carrier inversion in the structure. **Figure 4(b)** shows band diagrams of SHJp solar cells for *D*it = 5 × 1011 cm–2 and two values of Δ*E*C. As can be seen, higher Δ*E*C results into higher band banding in the c-Si part of the junction and stronger carrier inversion at the heterointerface. Because of this, *V*OC exhibits higher values for structures with high Δ*E*C. Moreover, the strong inversion causes a pronounced suppression of interface recombination since only few majority carries are available for recombination. As a result, the negative impact of *D*it is less serious for structures with high values of Δ*E*C. From this it is obvious that the ability to prepare a-Si:H/c-Si with high Δ*E*C should be the way how to suppress the influence of *D*it and how to attain high *V*OC and thus the efficiency of SHJ solar cells. However, there are only limited possibilities to modify the band alignment of a-Si:H/c-Si heterointerface based on tuning the hydrogen content in the a-Si:H layer [33]. The literature presents a consensus that Δ*E*<sup>C</sup> at a-Si:H(n)/c-Si(p) heterointerface is below 0.30 eV [23, 34–36].

**Figure 4.** (a) *V*OC calculated as a function of Δ*E*C at the front a-Si:H/c-Si of SHJp solar cell structure. *D*it is varied as a parameter in the simulations. (b) Band diagrams calculated for two values of Δ*E*<sup>C</sup> and *D*it = 5 × 1011 cm-2 of SHJp solar cell structure. The inset shows the change in the carrier inversion (change in the distance of the conduction band level

the performance of solar cell.

78 Nanostructured Solar Cells

from the Fermi level at the heterointerface).

**3.2. Front a-Si:H/c-Si: influence of band alignment**

The most straightforward way to increase the carrier inversion at the c-Si surface is to decrease *D*it. The a-Si:H emitter with p- or n-type doping is characterized by a high concentration of defects resulting in a high *D*it at the a-Si:H/c-Si interface. Because of this a thin intrinsic passivation layer of a-Si:H(i) with a significantly lower defect concentration ~5 × 1021 m–3 [36] compared to doped a-Si:H layer [27] is inserted at the interface. The quality of the a-Si:H(i) and thus passivation effect increases with the increase of a-Si:H(i) thickness *d*a-Si:H(i). However, high *d*a-Si:H(i) results in a decrease of FF and performance of SHJ solar cell [37].

A simulation study with a-Si:H(i) inserted at the heterointerface was carried out to describe the impact of *d*a-Si:H(i) on the carrier inversion at the c-Si surface and consequently on *V*OC and the output performance. **Figure 5(a)** shows *V*OC simulated as a function of *d*a-Si:H(i). Three values of *D*it were used in the simulation as a parameter reflecting the possible passivation effect of a-Si:H(i) layer. In the case of low *D*it = 109 cm–2, the change of *V*OC with *d*a-Si:H(i) is negligible. On the other hand, for higher value of *D*it the decrease of *V*OC with increase in *d*a-Si:H(i) is more relevant. *V*OC is less sensitive to the presence of *D*it for low *d*a-Si:H(i). This sensitivity to *D*it increases with increasing *d*a-Si:H(i). The band diagrams for *D*it = 5 × 1011 cm–2 and with *d*a-Si:H(i) of 10 and 50 nm were calculated to explain the impact of *d*a-Si:H(i) on *V*OC at high *D*it (**Figure 5b**). The band lines of a-Si:H(i) were aligned for both thicknesses to have the heterointerface at the same place. **Figure 5(b)** shows the decreases of band bending in the c-Si, thus the decrease of the carrier inversion at the heterointerface upon the increase of *d*a-Si:H(i) for *D*it = 5 × 1011 cm–2 resulting in the decreases of *V*OC. The a-Si:H(i) layer has a low concentration of free carriers and thus is a source of a potential drop across this layer, which affects the charge distribution and electric field in the SCR. This potential drop increases with the increase of *d*a-Si:H(i). In the case of low *D*it = 109 cm–2 the strong carrier inversion occurs, in other words a high minority carrier concentration at the c-Si surface screens the potential drop over the a-Si:H(i) layer. Consequently, the potential drop over the a-Si:H(i) layer has a negligible influence on the carrier inversion and thus causes a negligible change of *V*OC even at high *d*a-Si:H(i). In the case of high *D*it = 5 × 1011 cm–2 the carrier inversion is much weaker due to the presence of trapped charge *Q*<sup>i</sup> . Such trapped charge lowers the electric field in the c-Si, hence lowers band bending and decreases the carrier inversion at the c-Si surface. Due to the high *Q*<sup>i</sup> the higher concentration of localized charge in the a-Si:H part of the junction is required to screen the charge in the c-Si. Because of this the potential drop over the a-Si:H(i) becomes more important for the distribution of the diffusion potential in the junction and with an increase of the *d*a-Si:H(i) the SCR is more widened in the amorphous emitter (formed by the intrinsic and doped parts) resulting in an increase of the diffusion voltage in a-Si:H part of the junction and in a decrease of carrier inversion at c-Si surface of the a-Si:H/c-Si interface with increased *d*a-Si:H(i). This conclusion is in accordance with experimental observation [38]. *V*OC decreases as a consequence of weaker carrier inversion. In accordance with this explanation, **Figure 5(b)** shows a more significant decrease of band banding in the c-Si and an increase of the band banding in the a-Si:H followed by a decrease of the carrier inversion at the interface for *d*a-Si:H(i) = 50 nm compared to the sample with *d*a-Si:H(i) = 10 nm. While the quality and thus passivation properties of the a-Si:H(i) layer increase with the thickness, careful tuning of the thickness and passivation ability is required to achieve high *V*OC and high output performance. The same principle can be applied to the SHJn structure.

**Figure 5.** (a) *V*OC calculated as a function of a-Si:H(i) thickness, *d*a-Si:H(i) inserted at the front a-Si:H/c-Si of SHJp solar cell structure. *D*it is varied as a parameter in the simulations. (b) Band diagrams calculated for two values of *d*a-Si:H(i) and *D*it = 5 × 1011 cm–2 for SHJp solar cell structure. The inset shows the change in the carrier inversion (change in the distance of the conduction band level from the Fermi level at the heterointerface).

#### **3.4. Alternative concepts to obtain carrier inversion at emitter/c-Si interface**

From the above discussion it is clear that high carrier inversion at the emitter/c-Si interface is crucial for high *V*OC and high output performance of the SHJ solar cell. The high carrier inversion in the SHJ solar cells can be attained through (i) modification of band alignment at the heterointerface or (ii) by a decrease of *D*it by optimizing the cleaning process or by insertion of a thin passivation a-Si:H(i) layer [6]. In following, we will discuss two alternative concepts of emitters which allow formation of high inversion at the emitter/c-Si interface and offer perspective to achieve high performance. The first one is the hetero-homojunction concept based on the field passivation effect [39, 40] and the second one is the use of alternative emitters based on transition metal oxides TMO with high *W*<sup>f</sup> , which form the hole transport layers in SHJn structures [41].

The first alternative approach is based on the insertion of a highly doped c-Si layer of n+ - and p+ -type doping at the a-Si:H/c-Si interface of SHJp and SHJn solar structure, respectively [39, 40]. Such a highly doped layer with opposite doping of c-Si provides field passivation, and causes a shift of the Fermi level, which leads to an increase in carrier inversion at the c-Si surface. Our recent simulation study shows that by using the field effect passivation it is possible to decrease the sensitivity of *V*OC to *D*it and Δ*E*C at the a-Si:H/c-Si interface [40]. The main drawback of this approach is, however, the additional technological steps required for preparation of a thin highly doped c-Si layer [42].

TMO with a high work function *W*<sup>f</sup> such as MoOx, V2O5 and WO3 represent alternative materials which can replace the a-Si:H emitter and can provide high carrier inversion at the c-Si surface [41]. The work function of these oxides changes according to the presence of adjacent environment or layer and varies in the range from 6 to 7 eV for as deposited layers and from 5 to 5.3 eV for oxides exposed to air [41]. Due to the intrinsic oxygen vacancies in their structure TMO are acting as n-type semiconductors [41, 43]. However, due to the high *W*<sup>f</sup> TMO provides band alignment with c-Si in the way that acts as a p-contact and allows formation of a depletion silicon surface and strong carrier inversion at the interface in connection with n-type c-Si. Also in the case of SHJ with TMO, the carrier inversion is strongly affected by the defect states at the heterointerface, thus passivation a-Si:H(i) layer is required to insert at the TMO/c-Si interface to provide high performance of such SHJ solar cell structures. The efficiency of 22.5% was obtained for MoOx based on SHJ cell [18]. Despite the high efficiency obtained on TMObased SHJ, the carrier transport mechanism and collection of photo-generated carriers are still not fully understood. Recent results suggest that regardless of the rectification behaviour caused by the high *W*<sup>f</sup> , classical depletion approximation can be used to describe the rectification behaviour of TMO/c-Si junction [41]. It was shown that measured *I*-*V* curves can be described by a two-diode model with current transport limited by the recombination in the SCR of c-Si and diffusion of injected minority carriers [41]. However, further research is required to understand the extraction mechanism of photo-generated holes assisted by the gap states in the emitter based on the metal oxide.
