Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells

Although photovoltaic solar energy technology (PV) is not the sole answer to the challenges posed by the ever-growing energy consumption worldwide, this renewable energy option can make an important contribution to the economy of each country. According to the New Policies Scenario of the “World Energy Outlook 2010” published in November 2010 by the International Energy Agency (IEA) [1], it is to be expected that the share of renewable energies in global energy production increases threefold over the period 2008-2035, and that almost one third of global electricity production will come from renewables by 2035, thus catching up with coal. The “Solar Generation 6” report of the European Photovoltaic Industry association published in October 2010 [2] predicts in its Solar Generation Paradigm Shift Scenario that by 2050, PV could generate enough solar electricity to satisfy 21% of the world electricity needs, i.e. a total of up to 6750 TWh of solar PV electricity in 2050, coming from an installed capacity of 4670 GW in 2050. This is to be compared with 40 GW installed in the world at the end of 2010 [3].

In this chapter the best wafer-based homojunction and heterojunction crystalline silicon solar cells are compared, and the advantages of heterojunction silicon solar cells related to the processing of the junction and solar cell operation are explained.

The development and recent status of HIT (Heterojunction with Intrinsic Thinlayer) silicon solar cells at the company Sanyo are presented. In order to reduce cost of the HIT solar cells, Sanyo is focusing on reducing the thickness of the silicon wafer. In 2009 the company demonstrated 22.8% conversion efficiency and record high open circuit voltage of 0.743 V on a solar cell based on a 98 μm thick wafer with a total area of 100.3 cm².

Achievements from other research groups such as Tokyo Institute of Technology (Tokyo Tech) and the National Institute of Advanced Industrial Science and Technology (AIST) in Japan, the National Renewable Energy Laboratory (NREL) in the U.S.A., Helmholtz Zentrum Berlin (HZB) and Frauhofer institute for Solar Energy Systems (Frauhofer ISE) in Germany, L’Institut National de l’Energie Solaire (INES) in France, Neuchatel PV-lab of Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland, National Agency for New Technologies, Energy and the Environmentand (ENEA) in Italy and Mingdao University in China are presented. The research activities and results achieved with heterojunction silicon solar cells in the Netherlands are also reported.

Challenges to further improve the performance of heterojunction silicon solar cells by minimizing the optical, recombination, and resistance losses in heterojunction silicon solar cells are discussed. These challenges deal with wafer cleaning, suppression of epitaxial growth, controlling thin silicon layer thickness, reduction of absorption losses in thin silicon layers and transparent conductive oxide, surface texturing and the improvement of grid electrodes.

The influence of wet-chemical silicon (Si) substrate pre-treatments on surface morphology and electronic interface properties is discussed for various hetero interfaces of crystalline Si (c-Si) and Si oxides (SiO_x), or amorphous materials such as Si (a-Si:H), Si nitride (a-SiN_x:H) and Si carbide (a-SiC:H), which are typically applied in Si heterostructure solar cells. Combined application of surface sensitive techniques, the field-modulated surface photovoltage (SPV), ex-situ and in-situ photoluminescence (PL) measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM), spectroscopic ellipsometry in the ultra-violet and visible region (UV-VIS-SE) and Fourier-Transform infrared ellipsometry (FTIR-SE), total hemispherical UV-NIR-reflectance measurements, microwave detected photo-conductance decay (μW-PCD) and quasi-steady-state photo conductance (QSSPC) provides detailed information about the influence of wet-chemical treatments on preparation induced micro-roughness, surface charge, energetic distribution of interface states D _it(E) and the resulting interface recombination behaviour of wet-chemically passivated Si substrates with special surface morphology. The stability of wet-chemical surface passivation during storage in ambient air is found to be strongly influenced by the preparation-induced surface morphology. As shown for various heterojunction structures, the effect of optimized wet-chemical pre-treatments can be preserved during the subsequent soft plasma enhanced chemical vapour deposition of a-Si:H, a-SiN_x:H or a-SiC:H. As demonstrated for selected examples, the results of these investigations could be successfully used to enhance the energy conversion efficiency of heterojunction solar cells prepared on flat, saw damage etched and textured Si substrates. Implementation of optimised wet-chemical surface pre-treatments prior to a-Si:H deposition in (ZnO/a-Si:H(n)/c-Si(p)/Al) heterojunction solar cells with pyramidal texturisation increased significantly the solar cell parameters I _sc, V _oc, fill factor and enhanced the solar cell efficiency from 17.4% (confirmed) to 18.4%.

This chapter addresses the electrochemical passivation of Si surfaces by hydrogen, small organic molecules and ultra-thin polymeric layers which is not yet a standard technique in Si solar cell preparation. The electrochemical surface conditioning leads to different surface structures compared to the wet-chemical techniques (Chapter 3). The electronic properties of the interface depend strongly on these surface morphologies and consequently it is important to measure and control their changes during the electrochemical processing. Therefore, pulsed photoluminescence (PL) spectroscopy is applied as fast and non-destructive method to monitor in-situ and ex-situ the electronic surface properties during electrochemical oxidation, hydrogenation, and grafting of organic molecules and ultra-thin polymeric layers. The additionally used in-situ surface photovoltage (SPV) provides information on the surface charge during the wet-chemical and electrochemical processing. Unusual low concentration of recombination active defects at Si:H surfaces, Si(111) and Si(100), can be obtained after electropolishing in the current oscillating regime in diluted HF solutions. The passivation by hydrogen is influenced by the applied potential, the current flow, the temperature and the solution composition, where nitrogen bubbling of the solution is an important step to enhance the surface passivation.

PL investigations of the organically modified surfaces show that a slightly higher defect concentration at the interface (typical by a factor of 2) is usually observed. However, organically modified Si surfaces have extremely long time stability versus oxidation in ambient air, especially after grafting of 10-carboxydecyl groups via hydrosilylation which shows an ideal Si surface passivation with respect to interface recombination losses.

Hydrogenated amorphous and microcrystalline silicon deposition has been a subject of research over the last four decades, supported by its increasing number of applications. Many deposition techniques involving physical (sputtering) or chemical (plasma enhanced chemical vapour deposition) processes have been studied. The choice of the deposition technique may help to favour some type of film precursor, in particular SiH₃ which is often considered as the most suitable to obtain device grade material. However, taking as a general case the growth of μc-Si:H films, we show that the growth process and film properties are mainly controlled by the surface and subsurface reactions. In particular, thanks to in-situ ellipsometry measurements, we demonstrate that there is a growth zone close to the film surface, where cross-linking reactions leading to bulk-like formation take place. In fact, the crystallization front may be located a few tens of nanometers below the surface exposed to the plasma, thus suggesting that the film properties are governed neither by the film precursor, nor by the deposition technique. Finally, we address the issue of the substrate dependence of the growth process, which is fundamental in the case of heterojunction solar cells.

The a-Si:H/c-Si heterojunction constitutes the core building block of a-Si:H/c-Si solar cells. In these cells, a key issue to obtain high efficiencies is the minimization of recombination losses at the a-Si:H/c-Si interfaces: The a-Si:H layers induce the band bending at the p/n-junction, but also passivate the surface of the c-Si, by saturation of dangling Si bonds. This is essential to realize the V _oc potential > 700 mV of this cell type. High defect densities at the a-Si:H/c-Si interfaces lead to a pronounced decrease of the cell efficiency, by ~4% absolute at defect densities of 10¹² cm^− 2 (~ 1 dangling bond per 1000 interface atoms). Thus, it is important to obtain information on recombination-active defects in the ultra-thin a-Si:H layer and at the a-Si:H/c-Si interface. After introducing the basic electronic properties of a-Si:H, this chapter discusses the density of occupied valence band and defect states N _occ(E) and the position of the Fermi level in the band gap of undoped (so called intrinsic) and of doped ultra-thin a-Si:H layers. The measured a-Si:H properties are correlated to the band bending in the c-Si absorber and to charge carrier recombination at the a-Si:H/c-Si interface. The connection to solar cell open circuit voltage V _oc is made, and the current state-of-the-art of c-Si surface passivation by (i)a-Si:H is reviewed. Furthermore, the use of temperaturedependent current-voltage measurements on complete a-Si:H/c-Si solar cells to extract information on recombination and transport is discussed. Finally, the influence of band bending at the TCO/a-Si:H interface on cell performance is outlined briefly.

The performance of crystalline silicon (c-Si) heterojunction (SHJ) solar cells critically depends on the properties of the deposited hydrogenated amorphous silicon (a-Si:H) films. Surface passivation is an important role they need to fulfill. Additionally, the a-Si:H films should also act as efficient emitter and back surface field (BSF). In this chapter, we focus on the electronic passivation properties of thea-Si:H/c-Si interface. First, relevant literature on c-Si surfaces is briefly reviewed, including the effect of hydrogenation of surface states. This is followed by a discussion of how electronic surface recombination is calculated and measured. Recombination is mainly determined by electronic gap-states. The precise nature of these states is discussed both for the c-Si surface and for the a-Si:H bulk. Next, the physical passivation mechanism of intrinsic a-Si:H is elucidated. It is concluded that it stems from chemical surface state passivation by hydrogen, similar to defect passivation in the a-Si:H bulk. For these films, it is also argued how epitaxial growth may detrimentally influence the passivation quality. For heterojunction devices this has its importance, as the deposition of device-grade a-Si:H is often very close to the transition to epitaxial growth. A following section focuses on the effect of doping of the amorphous films. Doping is principally expected to improve the passivation quality further, as it should give rise to additional field-effect passivation. Here, it is discussed why this is not necessarily the case, as doping is also linked to Fermi-level dependent Si–H bond rupture in the films. A compromise between doping and surface-passivation may be obtained by employing an intrinsic buffer layer between the doped film and the wafer. By using intrinsic buffer layers, values for the energy conversion efficiency as high as 23% were reported to date for SHJ devices.

Photoluminescence and electroluminescence from amorphous silicon / crystalline silicon heterostructures is a measure of the radiative band-to-band recombination, described in terms of the quasi-Fermi level splitting according to Planck’s generalised law, and the experiments thus probe the excess carrier densities in the crystalline silicon. Depending on the layer structure of the investigated sample, the contact-less photoluminescence experiment allows the characterisation of precursor structures for solar cell optimisation and for the study of related physical aspects like interface recombination. Both photoluminescence and electroluminescence experiments can be applied to solar cells for which the luminescence yield, or more precisely the deduced quasi-Fermi level splitting, can be related to the open-circuit voltage of the device which itself is limited by factors like the interface recombination rate. The coverage of luminescence techniques is complemented here by an account of modulated photoluminescence, a variant of the experiment, which may be used for the lifetime determination in wafer structures. Numerical modelling provides additional insight into the physics of interface recombination and its impact on the quasi-Fermi level splitting and thus the luminescence yield and the open-circuit voltage.

a-Si:H/c-Si heterojunction solar cells require different contacting schemes as compared to conventional solar cells with diffused emitters due to the low emitter conductivity. Apart from back-contacted solar cells it is common to use a transparent conducting oxide (TCO) instead of silicon nitride as an anti-reflection (AR) layer. The choice of materials is vast, with materials based on indium oxide and zinc oxide being the most prominent choice. The optical and electrical properties of these films both play a significant role for the solar cell but they are strongly related, meaning that one cannot optimize them independently. Too high carrier concentrations for instance lead to a lower refractive index of the TCO even for light with a wavelength well below 1100 nm, which results in a worsened AR effect. It is therefore advantageous to use materials with moderate carrier concentrations. The challenges for the deposition of these materials are mainly the low thickness required for an optimum AR effect, for which properties are still influenced by inferior film growth during the nucleation phase, and the allowed substrate temperature of around 200 °C which is limited by the thermal stability of the a-Si:H/c-Si interface.

In this chapter a description of the contact formation in a-Si:H/c-Si heterojunction solar cell is detailed. Firstly the doping of amorphous films is reported together with the possibility to enhance the amorphous film conductivity by using Chromium Silicide formation on top of the doped films. Then a finite difference numerical model is used to describe the a-Si:H/c-Si heterojunction solar cell in which both contacts are made by amorphous films. In particular to evaluate the effect of the bandgap mismatch between amorphous and crystalline silicon at the base contact a detailed investigation is presented comparing experimental current voltage characteristics of heterojunction contacts with the results of a simulation based on numerical model. Subsequently, details about formation and properties of a transparent conductive oxide and a screen printing procedure to form metallic grids are presented as a common way to form the heterojunction solar cell electrodes. Finally three examples of heterojunction solar cells are proposed using different approaches to form the contacts. In particular a double side heterojunction cell fabricated on multicrystalline silicon is presented, a laser fired local contact for the rear side of the cell is shown and finally an interdigitated back contact is described. All the investigations are based on our experience on heterostructure solar cells developed in the past years.

The silicon heterojunction solar cell (SHJ) has made rapid progress in reaching high efficiency and it is already developed as an industrially viable product. However, much of its progress has come through process development while there is scarce knowledge on the microscopic nature of the functioning of this device. Although this device as a whole can be considered as bulk type, the parts of a SHJ solar cell that control the charge transport behavior are limited to very thin regions, either interface or a very thin layer. This poses problems on accurate determination of the physical quantities, such as defect densities and energetic positions, conductivity, carrier recombination and the overall charge transport behavior. This chapter gives the present understanding of electrical characterization of SHJ solar cells and provides a study of defects in the interesting regions of the device.

Semiconductor heterojunctions have been used in the last decades to build devices with enhanced electrical or optoelectrical properties compared to those of equivalent homojunction devices. Examples of heterojunction devices are encountered in laser applications using band gap engineering possibilities in crystalline III-V compounds, and in bipolar transistors in crystalline silicon based electronics. More recently, heterojunctions formed between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) were introduced for the fabrication of silicon solar cells. The main advantage of heterojunctions over homojunctions is due to the band offsets that can provide selective barriers for one type of carriers. The determination of band offsets and band lineup at the interface is thus of crucial importance. A lot of theoretical work has been devoted to this issue. In parallel, various characterization techniques have been developed to provide experimental insight into band offsets. In this chapter the principal models for band lineup at interfaces are recalled, with particular emphasis on Anderson’s electron affinity rule and Tersoff’s branch-point energy alignment theory. The application to the a-Si:H/c-Si system is discussed. Then, the principal electrical characterization tools based on capacitance and admittance measurements are presented. After a general overview of the widely used capacitance versus bias voltage technique (so-called C-V or 1/C² method), the main potential problems and sources of uncertainty when applying this technique to the a-Si:H/c-Si system are addressed. Some features specific to the a-Si:H/c-Si interface are identified and illustrated using both numerical simulations and experimental data. These features are related to the amorphous nature of a-Si:H, e.g. the high density of band gap states, and to the existence of a strong inversion regime at the c-Si surface that can lead to two dimensional electron or hole gases. A simple technique based on the measurement of the planar conductance of a-Si:H/c-Si structures is presented. The determination of band offsets from such measurements and related modelling on both (p) a-Si:H / (n) c-Si and (n) a-Si:H / (p) c-Si structures is discussed. For interfaces used in high efficiency solar cells the band offsets are found to be 0.15 eV for the conduction band and 0.40 eV for the valence band.

The principles of numerical solar cell simulation are described, using AFORS-HET (automat for simulation of heterostructures) which is a device simulator program for modelling multi layer homo- or heterojunction solar cells and typical characterization methods in one dimension. The basic equations for the optical and electrical calculations used in AFORS-HET are explained including a detailed description of the equations needed to calculate the recombination via defects in the semiconductor layers.

The physics models and material parameters needed to simulate an a-Si:H/c-Si solar cell with AFORS-HET are discussed and a simulation study showing solar cell characteristics subject to emitter doping, i-layer thickness and interface quality is presented. The AFORS-HET user interface is introduced so that the interested reader can repeat the simulation study. It is explained in detail how to define a structure and how to simulate a solar cell under different external conditions such as external current, voltage and illumination and how to calculate I-V curves to obtain solar cell characteristics.

Interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cells that combine the amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction- and interdigitated back contact (IBC) concepts are very promising in order to reach the highest one-junction efficiencies. In this chapter, a comparative two-dimensional simulation study has been done on the IBC-SiHJ structure based on n-type and p-type crystalline silicon by varying the values of the following parameters: minority carrier lifetime in c-Si, c-Si thickness, c-Si doping concentration, surface recombination velocity, density of defect states at the a-Si:H/c-Si hetero-interface and rear side geometry. The influence of these parameters has been tested by generating the current-voltage characteristics under illumination. Results indicate that the key parameters to achieve high efficiency are high c-Si substrate quality, low surface recombination velocity especially at the front surface, and a low recombining a-Si:H/c-Si interface. The width of the gap region (spacing between the back-surface field (BSF) and the emitter) must be kept as small as possible to avoid recombination of minority carriers in the bulk c-Si. For IBC-SiHJ based on n-type c-Si, the optimum geometry corresponds to a minimum size BSF region and a maximum size emitter region while for IBC-SiHJ based on p-type c-Si a BSF width equivalent to around 30% of the pitch is an optimum.

Ever since the first proposal of Interdigitated Back Contact (IBC) silicon solar cells in 1975, this type of cell has been under development as a means to reach high energy conversion efficiencies. Since no metal contacts are present on the front of the cell, IBC cells in general have a high generated current density (J _sc ). Apart from this obvious advantage, IBC cells also have advantages related to the integration in modules. The series interconnection between various cells can be done at module level, without the need for connecting the front of one cell to the rear of the next one, as is the case in two-side contacted cells. IBC solar cell efficiencies of 21 to 24 percent have been shown on large area industrially produced cells. Another successful high efficiency concept is the heterojunction emitter solar cell, where the junctions are realized by application of intrinsic and doped amorphous silicon (a-Si) layers on high quality mono-crystalline silicon bulk material. The cells realized with a-Si heterojunctions have high open-circuit voltage values thanks to the excellent passivating properties of the a-Si layers. Combining the IBC concept with heterojunction junctions using thin high quality substrates has the potential of reaching solar cell efficiencies over 25 percent.

Classical IBC cells have been studied for many years. Some of the more important aspects include substrate quality, front and rear surface passivation, rearjunction design and design and structure of the metallization. In literature several types of IBC cells have been reported, where a large variety of processes are described utilizing technologies ranging from lab-scale to industrially applicable methods. In recent years IBC research has focused on development of low cost and industrial technologies suited for IBC cell production on large scale and exploring the way towards the use of thinner and thinner silicon substrates.

Although results have been reported on heterojunction emitter structures for almost twenty years, it is only in the last five years that the implementation of the heterojunction emitter at the rear of the wafer has received much research interest. For this reason, many of the papers that have been published on this subject display cell structures and processing that are not optimized, and are typically fabricated on small area cells. Efficiencies currently are in the order of 12 to 16 percent. These proof-of-concept cells are just the start of a new development and a rapid evolution in the efficiencies is expected in the months and years ahead.

In this chapter, we start by a short presentation of the state-of-the art of the energy market to understand the evolution of the energetic demand and the role of photovoltaic technology in the near future. Moreover, we present all the actual industrial high efficiency solar cells among which is located the heterojunction technology. Then, we talk about the key points that define the technology, the main bottlenecks and the main solutions found at INES research group on heterojunction devices. Also, we show our best results obtained recently and some guidelines to improve still more the efficiency of the devices. Finally, we finish by a summary of the main advantages of this technology taking into account all the parameters described above.