Characterization of silicon heterojunctions for solar cells
- Jean-Paul Kleider1Email author,
- Jose Alvarez1,
- Alexander Vitalievitch Ankudinov2,
- Alexander Sergeevitch Gudovskikh3,
- Ekaterina Vladimirovna Gushchina2,
- Martin Labrune4, 5,
- Olga Alexandrovna Maslova1, 3,
- Wilfried Favre1,
- Marie-Estelle Gueunier-Farret1,
- Pere Roca i Cabarrocas4 and
- Eugene Ivanovitch Terukov2
© Kleider et al; licensee Springer. 2011
Received: 6 September 2010
Accepted: 16 February 2011
Published: 16 February 2011
Conductive-probe atomic force microscopy (CP-AFM) measurements reveal the existence of a conductive channel at the interface between p-type hydrogenated amorphous silicon (a-Si:H) and n-type crystalline silicon (c-Si) as well as at the interface between n-type a-Si:H and p-type c-Si. This is in good agreement with planar conductance measurements that show a large interface conductance. It is demonstrated that these features are related to the existence of a strong inversion layer of holes at the c-Si surface of (p) a-Si:H/(n) c-Si structures, and to a strong inversion layer of electrons at the c-Si surface of (n) a-Si:H/(p) c-Si heterojunctions. These are intimately related to the band offsets, which allows us to determine these parameters with good precision.
In the field of silicon solar cells, recent progress has been achieved in two directions: silicon heterojunctions and silicon nanowires. These two topics are briefly addressed here and we show some new characterization results that use conductive-probe atomic force microscopy (CP-AFM) measurements.
Silicon heterojunctions are formed between crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H). Solar cell efficiencies of up to 23% have been demonstrated on high quality n-type c-Si wafers with layers of p-type a-Si:H deposited at the front (as the emitter) and n-type a-Si:H deposited at the back (as the back surface field), respectively . Since transport properties are quite poor in a-Si:H due to the large amount of defects and band gap states and low carrier mobilities, the doped a-Si:H layers are used to form the junctions, but their thickness has to be kept very low. The front a-Si:H layer has to be very thin in order to minimize absorption of incoming photons and to privilege absorption in c-Si. One key feature of the Si heterojunctions is the very good passivation property of the c-Si surface by a-Si:H. This is even improved by inserting a thin undoped a-Si:H layer (so-called "intrinsic" layer, which leads to the "HIT"-heterojunction with intrinsic thin layer denomination ). This limits interface recombination and leads to very high open circuit voltages . Band offsets between a-Si:H and c-Si also play a crucial role because they determine the band bending, which influences the carrier collection. We here demonstrate the existence of a conduction channel along both the (n) a-Si:H/(p) c-Si and the (p) a-Si:H/(n) c-Si interfaces from direct CP-AFM measurements performed on cleaved sections of solar cells. We show from additional planar conductance measurements and simulations that these are related to strong inversion regions at the interfaces. From the temperature dependence, we determine the values of band offsets.
Solar cell structure
The sample structure for these measurements is shown in Figure 3b for p-doped a-Si:H. The a-Si:H layer was deposited in the same run on both n-type c-Si and glass (Corning 1737). Top coplanar aluminum electrodes were then deposited on the top of a-Si:H. We measured the DC current, I, resulting from application of a DC bias, V, between two adjacent electrodes. We had several electrode designs with various gap distances between them. We checked that the current scaled with the inter-electrode gap distance. We also checked that the current was linearly dependent on the DC voltage, so that we defined the conductance G = I/V. This was then measured as a function of temperature between 150 and 300 K in a cryostat chamber pumped down to 10- 5 mbar.
The same kind of measurements were also performed on series of samples with n-doped a-Si:H deposited onto p-type c-Si and glass.
Results and discussion
However, the quantitative results of the interface layer conductivity deduced from CP-AFM measurements have to be considered carefully. Indeed, the reliability of the latter is affected by the quality and nature of the contact between the conductive tip and the sample surface. The sample surface roughness, the AFM tip radius, shape and pressure are well-known factors driving local electrical measurements. Moreover, surface states can induce additional band bending at the tip-surface junction modifying significantly the conductance values . The CP-AFM scanning measurements can also be influenced by the oxidation process after cleaving the sample and the presence of a water meniscus between the tip and the surface that can also lead to tip-induced oxidation or trapping of carriers in localized states [6, 7]. The contact between the tip and the cleaved surface can behave as a metal-oxide interface that then determines the current flowing through the tip.
We attribute this thin conductive interface channel along with the low conductance activation energy to a strong inversion layer at the c-Si surface that is related to the band offset at the heterojunction.
In order to further demonstrate the existence of the strong interface inversion layer and the related contribution to the conductance, we used the AFORS-HET software  to evaluate the free carrier profiles. We introduced the density of states (DOS) typical for n-type a-Si:H (band gap E g = 1.75 eV) consisting of two exponential band tails with characteristic energies k B T C and k B T V of 0.055 and 0.12 eV for the conduction and valence band, respectively, and with a pre-exponential factor of 2 × 1021 cm-3 eV-1, and two Gaussian deep defect distributions of donor and acceptor nature being located at 0.58 and 0.78 eV above the top of the valence band, respectively, with a maximum value of 8.7 × 1019 cm-3 eV-1 and a standard deviation of 0.23 eV. A doping density of N d = 5.34 × 1019 cm-3 was also introduced, setting the Fermi level E F at 0.2 eV below the conduction band at 300 K, as suggested from the activation energy of the conductance data measured on (n) a-Si:H samples deposited on glass. The doping density in the crystalline silicon was set at N a = 7 × 1014 cm-3, as found from capacitance versus bias measurements , and in agreement with the resistivity of our CZ c-Si p-type wafers.
where q is the elementary charge, h the length of the coplanar electrodes, L the gap between them, μ the mobility of the carriers in the strong inversion region, and N the sheet carrier density, i.e., the integral over the c-Si thickness of the carrier concentration. Carriers to be considered are the electrons for the (n) a-Si:H/(p) c-Si interface and the holes for the (p) a-Si:H/(n) c-Si interface. We calculated the values of N as a function of the band offset and of the temperature. We thus were able to compute the planar conductance and compare it to the experimental data. This proved to be a very precise way to determine the band offsets in the (n) a-Si:H/(p) c-Si system , where a value of ΔE C = 0.15 eV was found. In the (p) a-Si:H/(n) c-Si system, the measured resistance profile was compared to the calculated resistivity profile across the heterojunction. Both profiles have very similar shapes, and the thickness of the strong inversion layer is of the same order of magnitude (50-100 nm). Further analysis of the CP-AFM measurements shows that a strong inversion layer only exists if the valence band offset is large enough, ΔE V > 0.25 eV . A more detailed theoretical and modeling study including the effect of temperature dependence of the band gaps and of the DOS parameters in a-Si:H is under way. It confirms our previous determination of conduction band offset and indicates that the value of valence band offset that best reproduces our experimental data is around ΔE V = 0.4 eV.
Silicon heterojunctions were characterized by the CP-AFM technique. A conductive channel between a-Si:H layer and c-Si substrate was detected in both (n) a-Si:H/(p) c-Si and (p) a-Si:H/(n) c-Si heterostructures. This conductive channel was attributed to the existence of a strong inversion layer that was also suggested by planar conductance measurements. The existence of this layer can be explained by relatively large band offsets at the heterojunction, as we demonstrated by numerical calculations of the carrier concentration profiles. Comparison with our experimental data allowed us to deduce values of the conduction and valence band offsets.
conductive-probe atomic force microscopy
density of states
indium tin oxide
plasma-enhanced chemical vapor deposition
transparent conductive oxide.
This study was partly supported by European Community's Seventh Framework Programme (FP7/2007-2013) under Grant agreement no. 211821 (HETSI project), by OSEO's Solar Nanocrystal project as well as by CNRS and the Russian Foundation for Basic Research in the framework of a joint Russian-French project (07-08-92163), and by the Programme of Fundamental Research of Russian Academy of Sciences (Programme No. 27). Two of the authors, W. Favre and O.A. Maslova, would like to thank ADEME and SUPELEC, and the French embassy in Russia, respectively, for their grants.
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