Main aspects of the back side of amorphous/crystalline silicon heterojunction solar cells physics 

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The solar cell and the impact of the BS on device characteristics

Silicon heterojunction device structure uses c-Si absorber layer. However, the p/n junction is no longer achieved by contact of opposite doping types within a crystal lattice as it is been done in homojunction solar cells. Rather, the junction is made through deposition of thin a-Si:H film doped with the opposite polarity to that of the absorber. Then, the heterojunction is done between crystalline and amorphous phases, which have a band gap energy difference of approximately 0.6 eV.

Current density and open-circuit voltage

The effects of the p/n junction can be described with the diode equation, an expression for the current density at the junction as a function of voltage. When a load is present, a potential difference develops between the terminals of the cell. A current which acts in the opposite direction to the photocurrent is generated and the net current is reduced from its short circuit value. This called dark current density (Jdark) varies like:  where J0 is the saturation current density, q is the charge of an electron, nd is the diode ideality factor, k is the Boltzmann’s constant and T is the temperature.
As inferred from equation (2.26), the p/n junction diode admits a much larger dark current under forward bias (V > 0) than under reverse bias (V < 0). As early reviewed, the behaviour of the device is strongly affected by the recombination properties of the material. Generated minority carriers need to diffuse to the junction to be collected and thereby, be able to contribute to the current. Therefore, diffusion length Ldiff [cm] being characterised by the diffusion coefficient D [cm2/s] and the lifetime τ according to Ldiff=Dτ, is a valuable indication of recombination properties of material (and it is taken into account in the calculation of J0). In order to extract the charge carriers, Ldiff should be in the order of wafer thickness, W.
Under typical illumination conditions, photons with energy larger than the band gap are absorbed creating electron and hole pairs which are separated in the space charge region with electrons being attracted to the n-type region (positively charged) and vice versa for holes (p-type region). If an external circuit is connected with the front contact held at voltage V, dark current is shifted by the value of the photogenerated current (Jph): phBd0J1TknqVexpJJ.
where Jph can be calculated using the approximation Jph = qG(ddepl+ Ln+ Lp), being Ln and Lp the diffusion lengths of the minority carriers in their respective regions and ddepl the width of the depletion region [107].

Band diagram analysis and carrier transport at the rear side of HJ solar cells

The amorphous/crystalline heterojunction results from the contact between these two materials, which have a different energy band gap (Eg) of approximately 0.6 eV (Eg, c-Si = 1.12 eV and Eg, a-Si:H = 1.75 eV at 300 K). According to the Anderson’s model [126], which is the most widely used to represent the band diagram of the HJ cells, the difference of band gaps and electron affinities entails the presence of band offsets at the conduction and valence band edges (ΔEC, ΔEV respectively).

Back side optical considerations

As it has been previously said, optical issues concerning the back side of HJ solar cells are clearly determined by the way layers at the front side behave. The a-Si:H and TCO films at the front of silicon heterojunction solar cells absorb light parasitically. As presented in Figure 2.13, the a-Si:H stack at the front of the cell is very absorbing at short wavelengths (< 600 nm), which is characterised by the increased extinction coefficient. Moreover, ITO absorbs both in the UV and also in the IR due to excitation of free carriers. Then, all of the short wavelength losses and some of the IR losses occur at the front of the cell, thus governing the attainable JSC of the cell. For this reason it can be considered that the back side of solar cells will not play a decisive role on the optical losses of the device.

Amorphous silicon layers applied to back side of HJ devices

Hydrogenated amorphous silicon has demonstrated to be a high performing material in a-Si:H/c-Si heterojunction solar cells, leading to conversion efficiencies over 23% [64]. Intrinsic and doped layers applied to the emitter of devices have been widely studied and their influence on the solar cell conversion efficiencies has been reported. By contrast, little research has been done on amorphous silicon at the rear side of HJ cells, since its impact on the overall device performance depends on the front side events, and therefore it is difficult to quantify. Nevertheless, this amorphous back side is a key part of the solar cell and plays a valuable role to obtain high efficiencies.
In the present chapter, some fundamental aspects of amorphous silicon on which the analysis of experimental data is based will be introduced. The experimental set-up used for the deposition and characterization of studied amorphous layers will be briefly described. Moreover, the technological steps taken in order to optimise the properties of (n)a-Si:H layers will be detailed. Finally, optimised amorphous layers will be used at the BSF of heterojunction solar cells and their effects on the device will be investigated. The fundamental parameters and the main requirements at the BSF to achieve high efficiencies will be assessed.

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Properties of hydrogenated amorphous silicon

In single crystal silicon each atom is covalently bonded to four neighbouring atoms through equal bonding lengths and angles. A unit cell can be defined, from which the crystal lattice is identically reproduced by stacking it one next to each other. A regular atomic arrangement results in a structure with a long range order that differs from that of hydrogenated amorphous silicon, as illustrated in Figure 3.1. Though most silicon atoms are four-fold coordinated, a-Si:H does not exhibit a structural order over a long range due to small deviations in bond lengths and angles. The larger deviations lead to weak bonds that can easily be broken, resulting in defect formation within the atomic network. In a-Si:H, defects are mainly silicon atoms that are covalently bonded to only three silicon atoms and have one unpaired electron [136]. These unsaturated bonds are the so-called dangling bonds.

Table of contents :

Table of contents
Table of contents
1. Introduction 
1.1. Status of world’s photovoltaics
1.2. Solar cell technologies
1.3. State of the art in surface passivation
1.3.1. High-temperature approach
1.3.2. Low-temperature approach Hydrogenated amorphous silicon nitride Hydrogenated amorphous silicon oxide Hydrogenated amorphous silicon carbide Aluminium oxide Hydrogenated amorphous silicon
1.4. a-Si:H/c-Si heterojunction solar cells
1.5. Structure and fabrication process of heterojunction solar cells at INES.
1.6. Specific research objectives: back side of heterojunction solar cells
1.7. Aim of this work
2. Main aspects of the back side of amorphous/crystalline silicon heterojunction solar cells physics 
2.1. n/n+ junction: recombination and passivation
2.1.1. Basics of carrier recombination mechanisms in crystalline silicon Bulk recombination Surface recombination
2.1.2. Effective lifetime and surface recombination velocity
2.1.3. Surface passivation techniques Field effect passivation Chemical passivation: saturation of defects
2.2. n/n+ band energy diagram analysis
2.3. The solar cell and the impact of the BS on device characteristics
2.3.1. Current density and open-circuit voltage
2.3.2. Fill factor and efficiency
2.3.3. Equivalent circuit of heterojunction solar cells
2.4. Band diagram analysis and carrier transport at the rear side of HJ solar cells
2.5. Back side optical considerations
3. Amorphous silicon layers applied to back side of HJ devices 
3.1. Properties of hydrogenated amorphous silicon
3.2. Experimental details for PECVD a-Si:H
3.2.1. PECVD set-up
3.2.2. Sample preparation and standard characterisation techniques
3.3. Advanced electrical characterisation
3.4. Influence of deposition parameters
3.4.1. Process pressure
3.4.2. Inter-electrode distance
3.4.3. PH3 concentration
3.4.4. Hydrogen dilution ratio
3.4.5. Influence of c-Si wafer orientation
3.5. Integration of (n)a-Si:H single layers at the BSF of HJ devices
3.6. Integration of (n)a-Si:H double-layer stacks at the BSF of HJ devices
3.6.1. Buffer layer doping influence Solar cells results Solar cells modelling
3.6.2. Buffer layer thickness influence
3.6.3. N+ doping influence
3.6.4. Buffer layer and N+ layer hydrogen dilution
3.6.5. N+ layer thickness
4. Back contact of heterojunction solar cells 
4.1. Transparent conductive oxides in HJ solar cells: Why ZnO:B?
4.2. Fabrication of ZnO:B layers by low pressure CVD
4.2.1. Relevant properties of ZnO films
4.2.2. Low-pressure chemical vapour deposition system
4.2.3. Influence of deposition parameters Diborane to DEZ gas flow ratio Water vapour to DEZ gas flow ratio Increasing thickness Heater temperature variation
4.3. Post-H-plasma treatment
4.4. Stability analysis under atmosphere exposure
4.5. Laser annealed ZnO: a novel approach for high-efficiency cost-effective HJ solar cells
5. Solar cell and module integration 
5.1. Back-contact comparison
5.2. Diverse technological issues
5.3. Record efficiency solar cells
5.4. Industrial module integration
Conclusions and outlook
A. (n)a-Si:H layers modelling
List of Figures
List of Tables
List of symbols and abbreviations


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