**2.4. Optical, resistive and recombination losses in a solar cell**

Theoretical maximum *J*sc of ∼44 mA/cm2 can be achieved for a single‐junction crystalline silicon solar cell, if all the striking photons with energy higher than silicon band gap (*E*photon ≥ *Eg*) are absorbed and all the electron‐hole pairs generated contribute to the cell current [3]. However, several optical losses can cause lower current density in real cells, including reflection at the front side (metal gridline and front surface), absorption in the metal and dielectric layers on both sides, absorption via free‐carrier absorption in highly doped regions and transmission (without being absorbed in the cell). It is important that the front metal shading should be as low as possible without contributing to significant increase in series resistance. Therefore, grid pattern needs to be optimized to minimize the resistive and optical losses. Finally, the size of pyramid on the front side should be as small as possible after the saw damage removal and texturing process to minimize the line width due to spreading of screen‐printed metal.

Actual silicon solar cells also suffer from ohmic losses due to parasitic resistance *Rs* and *R*sh. *Rs* is mainly attributed to the sheet resistance of doped regions (*p+* emitter and *n+* back surface field) in the case of two dimensional current flow, bulk resistance of silicon substrate, metallic resistance of gridline and specific contact resistance between silicon and metal. Each of these resistive components can be approximately estimated by using a model and calculation approach [30] to minimize total *Rs* and achieve high *FF*. *R*sh is a factor monitoring the non‐ ideality of the *pn*‐junction and some defects near junction, especially edge shunting. Both high *Rs* and low *Rsh* can result in low *FF* and cause power loss.

Apart from optical and ohmic losses, recombination of generated carriers can reduce *J*sc and *V*oc and limit the cell performance. There are two different intrinsic recombinations in semi‐ conductor materials, namely Auger recombination and radiative recombination. Auger recombination is dominant in the heavily doped *p+* and *n+* regions. Auger recombination represents the process in which electron and hole recombine first by band‐to‐band radiative recombination and use the excess energy to excite another majority carrier (electron) in the conduction band for *n+* silicon or hole in the valence band in the case of *p+* silicon. Then, this excited carrier thermalizes towards the band edge by emitting phonons. Auger recombination strongly depends on majority carrier density; hence it is very effective in the heavily doped regions (*p+* emitter and *n+* back surface field). Radiative recombination refers to a process in which electron makes direct band‐to‐band transition to recombine with a hole in the valence band while emitting light. Because silicon is an indirect band gap material and a phonon is required for the band‐to‐band transition, this recombination mechanism is often neglected in silicon solar cell. A third and important bulk recombination mechanism is called SRH recom‐ bination (named after Shockley, Read and Hall [31, 32]), which is initiated by the energy levels created within the forbidden gap by impurities or defects. These energy levels form a stepping stair to facilitate the recombination of holes and electrons, which is a function of energy level location, trap density and its capture cross‐sections [31, 32]. Generally, mid‐gap or deeper traps are more efficient recombination centres. Because all three recombination mechanisms occur in parallel, the silicon substrate bulk lifetime (bulk) is expressed as

High‐Efficiency Front Junction *n*‐Type Crystalline Silicon Solar Cells http://dx.doi.org/10.5772/65023 101

$$\frac{1}{\tau\_{\text{bulk}}} = \frac{1}{\tau\_{\text{Auger}}} + \frac{1}{\tau\_{\text{radiative}}} + \frac{1}{\tau\_{\text{SRH}}} \tag{6}$$

where Auger is the Auger lifetime, radiative is the radiative lifetime and SRH is the SRH lifetime.

**2.4. Optical, resistive and recombination losses in a solar cell**

*Rs* and low *Rsh* can result in low *FF* and cause power loss.

in parallel, the silicon substrate bulk lifetime (bulk) is expressed as

regions (*p+* emitter and *n+*

solar cell, if all the striking photons with energy higher than silicon band gap (*E*photon ≥ *Eg*) are absorbed and all the electron‐hole pairs generated contribute to the cell current [3]. However, several optical losses can cause lower current density in real cells, including reflection at the front side (metal gridline and front surface), absorption in the metal and dielectric layers on both sides, absorption via free‐carrier absorption in highly doped regions and transmission (without being absorbed in the cell). It is important that the front metal shading should be as low as possible without contributing to significant increase in series resistance. Therefore, grid pattern needs to be optimized to minimize the resistive and optical losses. Finally, the size of pyramid on the front side should be as small as possible after the saw damage removal and texturing process to minimize the line width due to spreading of screen‐printed metal.

Actual silicon solar cells also suffer from ohmic losses due to parasitic resistance *Rs* and *R*sh. *Rs* is mainly attributed to the sheet resistance of doped regions (*p+* emitter and *n+* back surface field) in the case of two dimensional current flow, bulk resistance of silicon substrate, metallic resistance of gridline and specific contact resistance between silicon and metal. Each of these resistive components can be approximately estimated by using a model and calculation approach [30] to minimize total *Rs* and achieve high *FF*. *R*sh is a factor monitoring the non‐ ideality of the *pn*‐junction and some defects near junction, especially edge shunting. Both high

Apart from optical and ohmic losses, recombination of generated carriers can reduce *J*sc and *V*oc and limit the cell performance. There are two different intrinsic recombinations in semi‐ conductor materials, namely Auger recombination and radiative recombination. Auger recombination is dominant in the heavily doped *p+* and *n+* regions. Auger recombination represents the process in which electron and hole recombine first by band‐to‐band radiative recombination and use the excess energy to excite another majority carrier (electron) in the conduction band for *n+* silicon or hole in the valence band in the case of *p+* silicon. Then, this excited carrier thermalizes towards the band edge by emitting phonons. Auger recombination strongly depends on majority carrier density; hence it is very effective in the heavily doped

which electron makes direct band‐to‐band transition to recombine with a hole in the valence band while emitting light. Because silicon is an indirect band gap material and a phonon is required for the band‐to‐band transition, this recombination mechanism is often neglected in silicon solar cell. A third and important bulk recombination mechanism is called SRH recom‐ bination (named after Shockley, Read and Hall [31, 32]), which is initiated by the energy levels created within the forbidden gap by impurities or defects. These energy levels form a stepping stair to facilitate the recombination of holes and electrons, which is a function of energy level location, trap density and its capture cross‐sections [31, 32]. Generally, mid‐gap or deeper traps are more efficient recombination centres. Because all three recombination mechanisms occur

back surface field). Radiative recombination refers to a process in

can be achieved for a single‐junction crystalline silicon

Theoretical maximum *J*sc of ∼44 mA/cm2

100 Nanostructured Solar Cells

In addition to these three bulk recombination mechanisms, surface recombination is also very critical for cell performance. This is because dangling bonds present at both surfaces of the wafer induce defect levels within the forbidden band gap. Surface recombination is charac‐ terized by a surface recombination velocity that is a function of surface state density and cross‐ section of surface traps [31]. To account for all the four recombination mechanisms, an effective lifetime (eff) is used, and given by

$$\frac{1}{\tau\_{\text{eff}}} = \frac{1}{\tau\_{\text{bulk}}} + \frac{2s}{d} \tag{7}$$

where *s* is the surface recombination velocity (SRV) and *d* is the silicon wafer thickness. All recombination processes not only reduce the maximum short circuit current density (*J*sc) but also diminish the maximum *V*oc. According to Eq. (8), the total saturation current density (*J*0,total) in Eq. (1) strongly influences *V*oc because

$$V\_{\rm oc} = \frac{nkT}{q} \ln\left(\frac{J\_{\rm sc}}{J\_{0,\rm total}} + 1\right) \tag{8}$$

$$J\_{0, \text{total}} = J\_{0a} + J\_{0b, \text{bulk}} + J\_{0b} \text{ \textdegree} \tag{9}$$

where *J*0*<sup>e</sup>*, *J*0*b,*bulk and *J*0*<sup>b</sup>*' are the saturation current density contributions from front emitter, bulk wafer and back side, respectively. These parameter values can be directly measured and extracted from the quasi‐steady‐state photoconductance (QSSPC) technique [33]. Due to the progress and availability of high lifetime wafers and current photovoltaic industry trend towards thinner wafers for cost reduction, passivation of front and rear surfaces is becoming vitally important for achieving higher efficiency silicon solar cells.

#### **3. Surface passivation of crystalline silicon solar cells**

At the silicon wafer surface, the covalent silicon‐silicon bonds of crystal lattice are broken during wafer slicing, which creates non‐saturated ('dangling') bonds that are often referred as 'surface states' and can easily trap electrons from the conduction band or holes from the valence band as some of the energy levels are located near mid‐gap. In order to keep surface recom‐ bination losses at a tolerable level, the wafer surfaces must be electronically well passivated. According to the Shockley‐Read‐Hall theory [31], the SRV depends on several features, including the properties of the surface states, state density, their capture cross‐sections for electrons and holes, the injection level at the surface and the wafer doping level [34]. Therefore, SRV can be decreased by two technical approaches: (1) the chemical passivation by reducing the surface state density via depositing or growing a passivating dielectric film on the silicon surface and (2) the field‐effect passivation by reducing the concentration of one charge carrier type (either electrons or holes) at the surface via forming an internal electric field below the silicon surface with doping profile or electrical charges in dielectric insulator. Practically, these two fundamental passivation approaches are often applied together to minimize the SRV. For front junction *n*‐type crystalline silicon solar cells, the electronic quality of front boron emitter and back surface are usually expressed and quantified by *J*0*<sup>e</sup>* and *J*0*<sup>b</sup>*'. In a typical solar cell, there are two fundamentally different types of surface regions: metallized and non‐metallized. Non‐ metallized surface regions are usually covered with dielectrics and are referred to as passivated regions. In order to achieve high cell efficiency, both *J*0*<sup>e</sup>* and *J*0*<sup>b</sup>*' should be as low as possible.

#### **3.1. Front boron emitter passivation**

In order to take the advantages of high bulk lifetime *n*‐type crystalline silicon wafers, excellent passivation of the boron emitter is essential to reduce the SRV or the emitter saturation current density of the passivated region (*J*0*e,* pass). It is well known that aluminium oxide (Al2O3) contains a high density of built‐in negative charges on the order of ∼1013 cm‐2 [35], so the majority carrier (hole) is accumulated and minority carrier (electron) concentration is effectively reduced at the Al2O3‐passivated boron emitter surface, and consequently, the SRH recombination on the surface or the SRV is reduced. Experimentally, very low *J*0*e,* pass in the range of 10∼30 fA/cm2 has been demonstrated on 150 and 54 Ω/□ is the standard unit to describe a sheet resistivity for emitter or back surface field for silicon solar cells in the photovoltaic field. sheet resistance *p+* emitters with surface doping concentrations around 1019 cm‐3 (prepared by BBr3) and Al2O3 passivation synthesized by plasma‐assisted atomic layer deposition (ALD) (**Figure 6**) [36].

It is also shown that with ALD‐Al2O3 passivation, ion‐implanted boron emitters (post‐implant anneal at 1040°C for 1 h) demonstrate noticeably higher *J*0*e,* pass at <150 Ω/□ sheet resistance, but similar passivation performance for ≥150 Ω/□ sheet resistance, compared to the BBr3‐prepared emitters (oxidation anneal at 1050°C). This is due to higher surface doping concentration (∼5 × 1019 cm‐3) resulted from lower oxidation temperature and shorter oxidation time. In addition, thermally grown silicon oxide (SiO2) also demonstrates decent surface passivation for the boron emitters with *J*0*e,* pass in the range of 60∼110 fA/cm2 . However, silicon nitride (SiNx) deposited by plasma‐enhanced chemical vapour deposition (PECVD) does not effectively passivate boron‐doped emitters (**Figure 6**) due to the presence of positive built‐in charge. Furthermore, hydrogen‐rich amorphous silicon yields the passivation performance compara‐ ble to SiO2 [37].

**Figure 6.** Measured emitter saturation current density on passivated region *J*0*e,*pass as a function of the sheet resistance for ion‐implanted boron emitters passivated by SiNx, thermal SiO2 and Al2O3. BBr3‐prepared and Al2O3‐passivated bor‐ on emitter reported in Ref. [36] is cited for comparison purpose.

#### **3.2. Rear surface passivation**

band as some of the energy levels are located near mid‐gap. In order to keep surface recom‐ bination losses at a tolerable level, the wafer surfaces must be electronically well passivated. According to the Shockley‐Read‐Hall theory [31], the SRV depends on several features, including the properties of the surface states, state density, their capture cross‐sections for electrons and holes, the injection level at the surface and the wafer doping level [34]. Therefore, SRV can be decreased by two technical approaches: (1) the chemical passivation by reducing the surface state density via depositing or growing a passivating dielectric film on the silicon surface and (2) the field‐effect passivation by reducing the concentration of one charge carrier type (either electrons or holes) at the surface via forming an internal electric field below the silicon surface with doping profile or electrical charges in dielectric insulator. Practically, these two fundamental passivation approaches are often applied together to minimize the SRV. For front junction *n*‐type crystalline silicon solar cells, the electronic quality of front boron emitter and back surface are usually expressed and quantified by *J*0*<sup>e</sup>* and *J*0*<sup>b</sup>*'. In a typical solar cell, there are two fundamentally different types of surface regions: metallized and non‐metallized. Non‐ metallized surface regions are usually covered with dielectrics and are referred to as passivated regions. In order to achieve high cell efficiency, both *J*0*<sup>e</sup>* and *J*0*<sup>b</sup>*' should be as low as possible.

In order to take the advantages of high bulk lifetime *n*‐type crystalline silicon wafers, excellent passivation of the boron emitter is essential to reduce the SRV or the emitter saturation current density of the passivated region (*J*0*e,* pass). It is well known that aluminium oxide (Al2O3) contains a high density of built‐in negative charges on the order of ∼1013 cm‐2 [35], so the majority carrier (hole) is accumulated and minority carrier (electron) concentration is effectively reduced at the Al2O3‐passivated boron emitter surface, and consequently, the SRH recombination on the surface or the SRV is reduced. Experimentally, very low *J*0*e,* pass in the range of 10∼30 fA/cm2 has been demonstrated on 150 and 54 Ω/□ is the standard unit to describe a sheet resistivity for emitter or back surface field for silicon solar cells in the photovoltaic field. sheet resistance *p+* emitters with surface doping concentrations around 1019 cm‐3 (prepared by BBr3) and Al2O3 passivation synthesized by plasma‐assisted atomic layer deposition (ALD) (**Figure 6**) [36].

It is also shown that with ALD‐Al2O3 passivation, ion‐implanted boron emitters (post‐implant anneal at 1040°C for 1 h) demonstrate noticeably higher *J*0*e,* pass at <150 Ω/□ sheet resistance, but similar passivation performance for ≥150 Ω/□ sheet resistance, compared to the BBr3‐prepared emitters (oxidation anneal at 1050°C). This is due to higher surface doping concentration (∼5 × 1019 cm‐3) resulted from lower oxidation temperature and shorter oxidation time. In addition, thermally grown silicon oxide (SiO2) also demonstrates decent surface passivation for the

deposited by plasma‐enhanced chemical vapour deposition (PECVD) does not effectively passivate boron‐doped emitters (**Figure 6**) due to the presence of positive built‐in charge. Furthermore, hydrogen‐rich amorphous silicon yields the passivation performance compara‐

. However, silicon nitride (SiNx)

boron emitters with *J*0*e,* pass in the range of 60∼110 fA/cm2

ble to SiO2 [37].

**3.1. Front boron emitter passivation**

102 Nanostructured Solar Cells

In practice, there are two different types of surfaces on the rear side of front junction *n*‐type silicon solar cells: diffused and non‐diffused. Thermal SiO2 film is very effective in passivating *n*‐type silicon surface because it not only reduces the surface state density but also leads to a field‐effect passivation due to positive fixed oxide charge [38]. It can reduce non‐diffused *n*‐ type silicon surface SRV values below 10 cm/s [39]. Thermally grown SiO2 is also very effective in passivating phosphorus‐doped silicon surfaces. Since the phosphorus dopants in the diffused back surface of a *n*‐type cell have the same polarity as the *n*‐type silicon wafer, this doping profile creates a high‐low junction (*n+ n*), the so‐called back surface field (BSF). The corresponding electric field formed by the rear high‐low junction of a BSF can very effectively shield holes from the rear surface, dramatically reducing rear surface recombination losses.

**Figure 7.** Saturation current density of phosphorus‐doped and planarized rear surface *J*0*b,*pass as a function of the ion‐ implanted phosphorus dose [41, 42].

SiNx formed by PECVD provides excellent passivation for *n*‐type surfaces because of its field‐ effect passivation provided by positive interface charges and the properties of the capture cross‐sections of the dominant interface defects. Although the surface state density at Si‐SiNx interface is much higher than that in the case of thermally grown Si‐SiO2 interfaces, SiNx demonstrates additional advantages for silicon solar cells: (1) excellent anti‐reflection coating (ARC) on the rear side, which is suitable for bifacial architecture and (2) releasing large amounts of hydrogen during a high temperature (∼800°C) contact firing process to passivate interface and bulk defects ('hydrogen passivation' [40]).

In front junction *n*‐type silicon solar cells, phosphorus‐doped rear surfaces are typically passivated by the combination of thermal SiO2 and SiNx stacks. **Figure 7** shows that very low saturation current density has been demonstrated on planarized back surface with ion‐ implanted BSF *J*0*b,*pass of ≤20 fA/cm2 for low phosphorus dose and *J*0*b,*pass of ∼80 fA/cm2 for high phosphorus dose with doping concentration of ∼2 × 1020 cm‐3, which is compatible with screen printing [41, 42]. The *J*0*b,*pass of ≤100 fA/cm2 has also been reported for SiO2/SiNx‐passivated ion‐ implanted and POCl3 formed BSF on textured surface [42, 43], which are suitable for fully screen‐printed bifacial front junction *n*‐type silicon solar cells [17].

#### **3.3. Carrier selective tunnel oxide passivated rear contact**

Current high‐efficiency front junction *n*‐type silicon solar cells are often limited by the recombination in the heavily doped regions and at the metal/silicon contacts. A possible solution for minimizing doping and contact recombination is to insert a passivating material with offset bands between the metal and silicon, also known as passivated contact. One approach to accomplish this is to use an amorphous silicon‐based heterojunction (HIT solar cell), which suppresses recombination effectively and which has resulted in outstanding cell *V*oc of 750 mV [44]. However, this passivation scheme can withstand only low temperature (≤250°C) back‐end process, hence is not compatible with the industry standard screen‐printing and firing metallization technology [45]. Another approach to achieve a passivated contact involves a chemically grown ultra‐thin (∼15 Å) tunnel oxide capped with phosphorus‐doped *n+* polycrystalline silicon (as shown in **Figure 8(A)**) and metal contact on the entire back side

**Figure 8.** (A) Transmission electron microscopy image and (B) schematic band diagram of tunnel oxide passivated con‐ tact structure [45].

of *n*‐type silicon cell [45], which is referred to as the tunnel oxide passivated contact (TOPCON) [46, 47].

SiNx formed by PECVD provides excellent passivation for *n*‐type surfaces because of its field‐ effect passivation provided by positive interface charges and the properties of the capture cross‐sections of the dominant interface defects. Although the surface state density at Si‐SiNx interface is much higher than that in the case of thermally grown Si‐SiO2 interfaces, SiNx demonstrates additional advantages for silicon solar cells: (1) excellent anti‐reflection coating (ARC) on the rear side, which is suitable for bifacial architecture and (2) releasing large amounts of hydrogen during a high temperature (∼800°C) contact firing process to passivate

In front junction *n*‐type silicon solar cells, phosphorus‐doped rear surfaces are typically passivated by the combination of thermal SiO2 and SiNx stacks. **Figure 7** shows that very low saturation current density has been demonstrated on planarized back surface with ion‐

phosphorus dose with doping concentration of ∼2 × 1020 cm‐3, which is compatible with screen

implanted and POCl3 formed BSF on textured surface [42, 43], which are suitable for fully

Current high‐efficiency front junction *n*‐type silicon solar cells are often limited by the recombination in the heavily doped regions and at the metal/silicon contacts. A possible solution for minimizing doping and contact recombination is to insert a passivating material with offset bands between the metal and silicon, also known as passivated contact. One approach to accomplish this is to use an amorphous silicon‐based heterojunction (HIT solar cell), which suppresses recombination effectively and which has resulted in outstanding cell *V*oc of 750 mV [44]. However, this passivation scheme can withstand only low temperature (≤250°C) back‐end process, hence is not compatible with the industry standard screen‐printing and firing metallization technology [45]. Another approach to achieve a passivated contact involves a chemically grown ultra‐thin (∼15 Å) tunnel oxide capped with phosphorus‐doped *n+* polycrystalline silicon (as shown in **Figure 8(A)**) and metal contact on the entire back side

**Figure 8.** (A) Transmission electron microscopy image and (B) schematic band diagram of tunnel oxide passivated con‐

has also been reported for SiO2/SiNx‐passivated ion‐

for high

implanted BSF *J*0*b,*pass of ≤20 fA/cm2 for low phosphorus dose and *J*0*b,*pass of ∼80 fA/cm2

interface and bulk defects ('hydrogen passivation' [40]).

screen‐printed bifacial front junction *n*‐type silicon solar cells [17].

**3.3. Carrier selective tunnel oxide passivated rear contact**

printing [41, 42]. The *J*0*b,*pass of ≤100 fA/cm2

104 Nanostructured Solar Cells

tact structure [45].

In this TOPCON structure, four parallel mechanisms contribute to carrier selectivity (as shown in **Figure 8(B)**) [45]. (1) Heavily doped *n+* polycrystalline silicon creates an accumulation layer at the absorber surface due to the work function difference between the *n+* polycrystalline silicon and the *n‐* silicon absorber. This accumulation layer or band bending provides a barrier for holes to get to the tunnel oxide while electrons can migrate easily to the oxide/Si interface. (2) Tunnel oxide itself provides the second level of carrier selectivity because it presents a 4.5 eV barrier for holes to tunnel relative to 3.1 eV for electrons [48]. (3) There are very few or no states on the other side of the dielectric for holes to tunnel through because the valence band edge of *n‐* silicon absorber is facing the forbidden gap of *n+* polycrystalline silicon. (4) Even if a hole is able to tunnel through the oxide, it sees much higher resistance due to the *n+* polycrystalline silicon regions to get to the metal contact for recombination while the majority carrier electrons can easily get there. Last but not least, due to the full area metal contact on the back, there is one‐dimensional (1D) current flow. This eliminates the lateral transport resistance (2D carrier flow), resulting in much higher *FF*. Therefore, this passivated contact is highly carrier selective and allows the flow of majority carriers via tunnelling while blocking minority carriers.

**Figure 9.** (A) Implied *V*oc and *J*0*<sup>b</sup>*' as a function of the PH3/SiH4 ratio during PECVD. (B) Implied *V*oc and *J*0*<sup>b</sup>*' as a func‐ tion of the polysilicon anneal temperature [45].

To obtain an efficiently doped *n+* polycrystalline silicon layer to maintain the quasi‐Femi level splitting in silicon absorber (high *V*oc), a proper PH3/SiH4 ratio during PECVD is required for forming the phosphorus‐doped amorphous silicon layer [45]. **Figure 9(A)** displays that if the PH3/SiH4 ratio is too high (high doping level in as‐deposited amorphous silicon layer), the surface passivation quality degrades as indicated by low implied *V*oc (*iV*oc) and high saturation current density *J*0*<sup>b</sup>*' obtained on symmetrical TOPCON structure. This is because more phosphorus dopants diffuse from the *n+* polycrystalline silicon layer through the tunnel oxide into the silicon absorber, resulting in high Auger recombination, as well as high recombination at the silicon/SiO2 interface [49]. However, very low PH3/SiH4 ratio also causes inferior surface passivation due to the small band bending or weak accumulation layer created by the reduced doping in the *n+* silicon layer, resulting in very weak field‐effect passivation. In addition, to facilitate the solid‐phase crystallization and activate the phosphorus dopants in the as‐ deposited *n+* amorphous silicon layer, a proper polysilicon anneal temperature is necessary as shown in **Figure 9(B)**. However, a strong degradation in the interface passivation quality is observed if the anneal temperature is too high (≥900°C), again due to more dopant diffusion into the silicon base causing high Auger recombination and possible local disruption of tunnel oxide due to polycrystalline silicon grain growth and more interface defects [45, 46]. It has also been demonstrated that both the tunnel oxide growth temperature in nitric acid and the high temperature firing process can affect the passivation quality. A very low *J*0*b*' of ≤5 fA/cm2 has been achieved by optimizing the TOPCON fabrication processes [50]. Similar passivation performance has also been achieved by depositing intrinsic amorphous silicon layer on top of the tunnel oxide layer followed by ion implantation of phosphorus and thermal annealing [51].
