**1.1. Current trends in SHJ solar cell development**

fabrication [1]. However, this trend was stopped due to the bending of thin wafers during high temperature processing of standard silicon solar cells, which results into the increasing efforts focused on the technologies with lower silicon usage. Among them, the silicon heterojunction solar cells (SHJ) provide both high performance together with a perspective of low-cost fabrication and decrease of silicon wafers thickness bellow 100 µm [2]. The advantages of heterojunction between amorphous and crystalline silicon were first introduced into the socalled HIT concept (Hetero-junction with Intrinsic Thin-layer) by former company SANYO (currently SANYO is part of the company Panasonic) in 1992 [3]. The SHJ HIT solar cell is composed of a single thin crystalline silicon wafer, c-Si surrounded by ultra-thin intrinsic silicon layers, a-Si:H(i) and n-type and p-type doped amorphous silicon layers, a-Si:H (**Figure 1**), which can be deposited at temperature below 200°C and so can be used in processing of thin wafers. On the two doped layers, transparent conducting oxide (TCO) layers and metal electrodes are formed with sputtering and screen-printing methods, respectively. The TCO

**Figure 1.** Silicon heterojunction solar cells with on n-type silicon (SHJn) and n-type silicon (SHJp) hetero-junction with

Since the first introduction, the HIT solar cells have been the subject of extensive research. Recently, the record efficiency *η* = 25.6% with open-circuit voltage *V*OC = 0.74 V, short-circuit

solar cell by Panasonic, which makes this technology currently the most efficient among siliconbased solar cells [4]. Current strong interest in SHJ concept is motivated by the high conversion efficiency as well as further possibilities for decreasing the fabrication cost. SHJ can be prepared by simple and low temperature fabrication processes, which decreases the thermal budget and thus the cost of the cell. Since the base material of the structure is crystalline silicon, the typical degradation due to the Staebler-Wronski effect observed in amorphous silicon solar cells does not take place in SHJ solar cells, where the base material of the structure is crystalline silicon [5]. Moreover, the HIT cell shows a better temperature coefficient (<–0.25%/K) compared to standard c-Si solar cells (–0.45%/K), which means more power generated in outdoor conditions

and fill factor *FF* = 82.7% were achieved on the rear junction HIT

layer on the top also works as an anti-reflection layer.

intrinsic thin-layer (HIT) solar cell.

70 Nanostructured Solar Cells

current *J*SC = 41.8 mA/cm2

To make the SHJ solar cells more economically attractive, current efforts are focused on the development of technologies and approaches focused on two main objectives (i) to increase the efficiency and (ii) to decrease the fabrication costs. The utilization of emitters with a large band gap such as amorphous silicon carbide a-SiC:H [8], nanocrystalline silicon oxide nc-SiOx:H [9] or micro-crystalline silicon oxide µc-SiOx:H [10], thus lowering light absorption is a common approach on how to increase *J*SC and hence the performance of such solar cells. The advantage of this approach is that only low adjustment of production lines is required for replacements of a-Si:H emitter by a-SiC:H or SiOx:H emitter layers. An increase in *J*SC by about 1 mA/cm2 was demonstrated by replacing a-Si:H by a-SiC:H [8] or by µc-SiOx:H [10]. However, also in this case the heterojunction with a c-Si substrate plays a crucial role and its fabrication has to be well mastered to benefit from the lower parasitic absorption of light. Another way on how to decrease absorption losses is based on the preparation of the two collection contacts at the bottom side of the silicon substrate forming an inter-digitated back contact silicon heterojunction (IBC-SHJ) solar cells. The beneficial effects of collection electrodes at the bottom of the cell are demonstrated by the best efficiency of 25.6% currently achieved at SHJ solar cells [4]. High *J*SC = 41.8 mA/cm2 in such solar cells is attained due to the eliminated absorption losses of a-Si:H layers as well as losses in TCO.

The decrease of fabrication cost can be realized through the replacement of expensive materials by cheaper alternatives. Several groups have investigated alternative materials such as zinc oxide, ZnO [11], and indium zinc oxide, IZO [12], as a replacement of expensive indium tin oxide, ITO. Replacement of silver used in the collection electrodes by copper [1, 13] is another way, and is currently highly investigated to decrease SHJ cost.

Another approach to make SHJ cells more economically attractive is based on the reduction of silicon wafer thickness. The ability of HIT structure to use silicon wafers of low thicknesses and to achieve high performance at the same time was demonstrated already in 2009, when the SHJ HIT solar cell with a conversion efficiency of 22.8% prepared on a 98 µm thick n-type silicon wafer was introduced by former company Sanyo (currently Panasonic) [2].

Nowadays, new advance concepts are emerging based on the replacement of the amorphous emitter by metal oxides [14–16]. Such a concept has the ability to provide both an increase of efficiency as well as a decrease of fabrication cost. Metal oxides provide advantages of large band gaps, thus lower parasitic absorption in the emitter, simpler deposition by thermal evaporation [13] and no requirements of toxic dopant gases during fabrication. Moreover, the deposition of such oxides can be carried out at low temperatures leading to a further decrease of the thermal budget and hence fabrication cost. Metal oxides are widely used as a hole transport layers in organic solar cells [16, 17]. Current attempts to transfer them into the SHJn technology show very promising results with achieved efficiency of *η* = 22.5% for molybdenum oxide hole collector MoOx-based SHJ solar cell [18]. The progress in the development of electron selective contacts based on lithium fluoride (LiFx) allows fabrication of dopant-free asymmetric heterocontacts cell (DASH) with conversion efficiency approaching 20% [19].

#### **1.2. Aim of this chapter**

Two targets have to be attained for the good performance of solar cells: (i) light has to be absorbed in the absorption layer of the solar cell and (ii) the photo-generated carriers have to be effectively collected by the top and bottom collection electrodes. The first target is focused on the improvement of light management, which with the decreasing of the c-Si substrate thickness starts to be important also for SHJ solar cell. The optimization of TCO [12, 20], tuning of emitter layer band gap [8] and texturization of c-Si [21] are crucial to achieve high *J*SC. The second target, which is described in detail in this chapter, is focused on the recombination and carrier transport processes in the structure. Such processes determine the collection of the photo-generated carriers and thus the performance of the solar cell. Since the SHJ is formed as a stack of various layers surrounding the absorber c-Si layer, the current transport of photogenerated electron/hole pairs to the collection electrodes is highly influenced by the interfaces between the neighbouring layers. Due to the connection of various materials with different lattice parameters, defect states can be formed at the interface. The difference in the band gap, affinity, doping level and type of adjacent layers results into the formation of heterojunctions/ carrier transport barrier. Application of a-Si:H or alternative emitter (such as a-SiC:H, nc-SiOx or metal oxides) in the SHJ solar cell is linked with several challenging requests concerning the quality of this layer and its interfaces with c-Si and TCO. On the one hand, the defect states and band alignment at the a-Si:H/c-Si interface determine the band bending at the c-Si surface and hence recombination and collection of photo-generated carriers [6, 22, 23]. Good quality of the a-Si:H(i) layer as well as a-Si:H/c-Si interface is one of the main challenges in order to achieve high SHJ solar cell efficiency. On the other hand, due to the low specific conductivity of a-Si:H it is required to use conductive TCO as a collection electrode. When the TCO is not chosen carefully regarding the proper work function, or is not properly prepared, the parasitic Schottky barrier can arise at the TCO/a-Si:H interface [24–26]. This parasitic Schottky barrier has an opposite diffusion potential compared to the a-SiH/c-Si junction and thus hinders the collection of photo-generated carriers. As a result, the performance of the SHJ deteriorates.

The aim of this chapter is to explore the processes connected with the collection of photogenerated carriers and to explain the key role of the front a-Si:H/c-Si and TCO/a-Si:H interfaces for carrier recombination processes. ASA simulation is carried out to provide an insight into the charge properties of both a-Si:H/c-Si and TCO/a-Si:H junctions forming the front emitter stack of the SHJ solar cell and to explore their interconnection. Strong emphasis is focused on the presence of carrier inversion at the a-Si:H/c-Si, which is the most determining factor for *V*OC of SHJ cell. The alternative approaches to obtain high carrier inversion based on field effect passivation and metal oxides are described in the chapter as well. The study is extended by simulation of the SHJ under concentrated light and varied temperatures to explore the perspective and limitations of n- and p-type silicon-based SHJ structures for utilization in light concentrated applications.

#### **1.3. Simulation set-up**

molybdenum oxide hole collector MoOx-based SHJ solar cell [18]. The progress in the development of electron selective contacts based on lithium fluoride (LiFx) allows fabrication of dopant-free asymmetric heterocontacts cell (DASH) with conversion efficiency approach-

Two targets have to be attained for the good performance of solar cells: (i) light has to be absorbed in the absorption layer of the solar cell and (ii) the photo-generated carriers have to be effectively collected by the top and bottom collection electrodes. The first target is focused on the improvement of light management, which with the decreasing of the c-Si substrate thickness starts to be important also for SHJ solar cell. The optimization of TCO [12, 20], tuning of emitter layer band gap [8] and texturization of c-Si [21] are crucial to achieve high *J*SC. The second target, which is described in detail in this chapter, is focused on the recombination and carrier transport processes in the structure. Such processes determine the collection of the photo-generated carriers and thus the performance of the solar cell. Since the SHJ is formed as a stack of various layers surrounding the absorber c-Si layer, the current transport of photogenerated electron/hole pairs to the collection electrodes is highly influenced by the interfaces between the neighbouring layers. Due to the connection of various materials with different lattice parameters, defect states can be formed at the interface. The difference in the band gap, affinity, doping level and type of adjacent layers results into the formation of heterojunctions/ carrier transport barrier. Application of a-Si:H or alternative emitter (such as a-SiC:H, nc-SiOx or metal oxides) in the SHJ solar cell is linked with several challenging requests concerning the quality of this layer and its interfaces with c-Si and TCO. On the one hand, the defect states and band alignment at the a-Si:H/c-Si interface determine the band bending at the c-Si surface and hence recombination and collection of photo-generated carriers [6, 22, 23]. Good quality of the a-Si:H(i) layer as well as a-Si:H/c-Si interface is one of the main challenges in order to achieve high SHJ solar cell efficiency. On the other hand, due to the low specific conductivity of a-Si:H it is required to use conductive TCO as a collection electrode. When the TCO is not chosen carefully regarding the proper work function, or is not properly prepared, the parasitic Schottky barrier can arise at the TCO/a-Si:H interface [24–26]. This parasitic Schottky barrier has an opposite diffusion potential compared to the a-SiH/c-Si junction and thus hinders the collection of photo-generated carriers. As a result, the performance of the SHJ deteriorates.

The aim of this chapter is to explore the processes connected with the collection of photogenerated carriers and to explain the key role of the front a-Si:H/c-Si and TCO/a-Si:H interfaces for carrier recombination processes. ASA simulation is carried out to provide an insight into the charge properties of both a-Si:H/c-Si and TCO/a-Si:H junctions forming the front emitter stack of the SHJ solar cell and to explore their interconnection. Strong emphasis is focused on the presence of carrier inversion at the a-Si:H/c-Si, which is the most determining factor for *V*OC of SHJ cell. The alternative approaches to obtain high carrier inversion based on field effect passivation and metal oxides are described in the chapter as well. The study is extended by simulation of the SHJ under concentrated light and varied temperatures to explore the

ing 20% [19].

72 Nanostructured Solar Cells

**1.2. Aim of this chapter**

The ASA simulation program was used for characterization of recombination processes in the SHJ structure. This program is designed for the simulation of solar cells based on a-Si:H and c-Si semiconductors. ASA program solves the Poisson equation and continuity equations for electrons and holes in one dimension and includes several physical models which describe the trapping and generation/recombination processes in the structures with consideration of spatial disorder of amorphous silicon [26]. The simulated solar cell structures have the following layer sequence: TCO/a-Si:H(n)/a-Si:H(i)/c-Si(p)/a-Si:H(i)/a-Si:H(p)/TCO/Metal and TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n)/TCO/Metal denoted as SHJp and SHJn, respectively. In the simulated models, the thicknesses of 5 and 10 nm were used for a-Si:H(i) and doped a-Si:H(n) and a-Si:H(p) layers, respectively. The band gap of a-Si:H(p) was set to 1.95 eV and the band gaps of a-Si:H(i) and c-Si(n) were set to 1.76 eV in accordance to [27]. The doping activation energies of 0.2 and 0.4 eV were used for a-Si:H(n) and a-Si:H(p) layers, respectively. The gap state densities of amorphous layers have a Gaussian distribution of dangling bonds and an exponential distribution of band tails was set together with additional parameters according to the literature [27]. While the main aim of the simulation is to describe recombination processes in the structure, flat silicon substrate conditions were used in the models. The silicon substrates with thickness of 200 µm, lifetime, *τ* = 1 ms and concentration of dopants, *N*dop = 5 × 1021 m–3 were used for both SHJp and SHJn structures. TCO was adopted as an optical layer with a thickness of 80 nm. The defect states at the front a-Si:H/c-Si interface was modelled by inserted 1 nm thick highly defective c-Si layer. Flat band conditions at the TCO/a-Si:H interface were used in the initial simulations focused on the study of a-Si:H/c-Si properties. The negligible defect state density of 109 cm–2 were set at the back c-Si/a-Si:H contact for all simulations. The conduction band offset, Δ*E*<sup>C</sup> = 0.15 eV, and valence band offset, Δ*E*V = 0.55 eV, were used as an initial values determining band alignments in SHJp and SHJn structures, respectively. As an illumination source the light with power density of 100 mW/cm2 and spectrum AM1.5 was used for the output performance simulations.
