Silicon solar cells with nano-crystalline silicon down shifter: experiment and modeling

Yoann Jestina,*, Georg Puckera, Mher Ghulinyana , Lorenza Ferrariob, Pierluigi Belluttib, Antonio Picciottob, Amos Collinib, Alessandro Marconic, Aleksei Anopchenkoc, Zhizhong Yuanc, Lorenzo

Pavesic

a Advanced Photonics and Photovoltaics Group, Bruno Kessler Foundation, Via Sommarive 18, 38121 Povo (Trento) Italy;

b Microtechnologies Laboratory, Bruno Kessler Foundation, Via Sommarive 18, 38121 Povo (Trento) Italy

c Nanoscience Laboratory, Department of Physics, University of Trento, Via Sommarive 14, 38121 Povo (Trento) Italy

ABSTRACT

Materials used as luminescent down shifters (LDS) have to absorb light effectively in the spectral area where solar cells have poor internal quantum efficiency. At the same time these materials have to emit most of the absorbed spectral powers at lower energies where the internal quantum efficiency of the solar cell is close to the maximum. The effects of silicon nanocrystals prepared by thermal treatment of a silicon-rich-oxide (SRO) layer on the efficiency of c-Si cells are investigated in this paper. The SRO layer is characterized by a high photoluminescence peak at around 800 nm. Influence of the active layer on light transmission and on the modification of the optical spectra due to photoluminescence generation has been determined with the help of optical measurements and transfer matrix simulations. The solar cell efficiency for cells with and without down-shifting layer were measured under illumination with AM1.5G solar spectrum and compared with the simulations. Finally, we model the behavior of cells with and without LDS layer showing that a cell with LDS suffers less from bad surface passivation.

Keywords: Silicon solar cells, PECVD, silicon nanocrystals, silicon rich oxide, spectral down shifting, photoluminescence, power conversion efficiency, modeling.

1. INTRODUCTION Fundamental spectral losses limit the efficiency of single junction solar cells1 with an band gap Eg = 1.1 eV to η≈31%. Photons with energy lower than Eg are not absorbed, while the excess energy of photons (E>Eg) is lost via non-radiative relaxation of the excited electrons to the conduction band, in the form of heat. Different concepts and ideas are currently investigated to overcome these fundamental limits of solar cells2 (e.g. hot carrier cells, up- and down-conversion, intermediate band gap cells). Beside these concepts there are two relatively simple principle approaches to achieve more efficient utilization of the short-λ part of the solar spectrum. The first is to improve the electronic properties of existing devices by using very narrow junctions or low doping levels3, which is not always easily to implement or too expensive for use in production. A second approach is the use of luminescence down shifting (LDS) of the short-λ part of the solar spectrum. Thus, a layer of luminescent material absorbs the short-wavelength photons, where the solar cell has a low internal quantum efficiency (IQE), and re-emits photons of a longer wavelength, where the IQE of the solar cell is high. This application of luminescent materials for overcoming the poor blue response of solar cells was first described by Hovel et al.3 in the late 1970s. Simulations4 predict that when applied to CdS/CdTe solar cells, organic LDS layers could result in an increase in conversion efficiency from η=9.6% to 11.2% which corresponds to an increase in efficiency of nearly 17%. An advantage of this concept is the fact that the LDS is only optically coupled to the solar cell. This down shifter concept mainly shifts the photons from the blue to the red.

*jestin@fbk.eu; phone 39 0461-314188; fax 39 0461-302040; http://www.fbk.eu

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In the past two decades silicon nanocrystals (Si-nc) fabricated by different ways attracted considerable attention, mainly because of its potential applications in optoelectronics5. In recent years Si-nc in dielectric matrices have also been studied for use in solar cells6. There they might be used as material in the anti-reflective coating, as photoactive material, or due to their emission properties as down-shifter or even down-converter. All these applications are of considerable academic interest, but have still to proof to be of technological importance for solar cell fabrication. Recently, Si-nc incorporated into spin-on-glass have been tested as a down-shifter of high energetic photons7. In these studies an improvement in the spectral response in the region where the Si-nc photoluminescence excitation occurs was observed together with an slight enhancement of the short-circuit current with respect to cells without Si-nc on top of the cell.

Si-nc have a series of properties which makes them very attractive as candidate material for down-shifters: (i) the quantum-efficiency of the photoluminescence can be very high8 achieving even 40% to 60%, (ii) Si-nc show strong absorption in the UV (below 450 nm), and exact position and intensity of the absorption band can be tuned by controlling size and density of the Si-nc9, (iii) the photoluminescence of Si-nc is typically between 700 and 900 nm, a range in which the internal quantum efficiency of Si- solar cells is generally close to unity.

In this study, we present the experimental results obtained on c-Si solar cells covered by a SiO2 layer containing Si-nc, where the Si-nc are grown by phase separation of silicon-rich silicon-oxide during the cell fabrication process. The experimental data are explained by comparison with calculations regarding both the effect of the luminescence down-shifting layer on the solar spectrum and the electro-optical properties of the c-Si solar cell.

2. METHODOLOGY 2.1 Description of Si-nc growth and solar cells

The silicon rich oxide (SRO) was deposited by plasma enhanced chemical vapor deposition (PECVD) from N2O, SiH4 and nitrogen. The ratio of N2O/SiH4 (in the following called Γ) defines the excess amount of silicon in the film, while nitrogen is used for dilution. Films with different composition (Γ=10,12,15) with a thickness of about 100 nm were deposited on c-Si silicon substrates and annealed afterwards at 1000 ºC in nitrogen atmosphere. Optical properties of these films were investigated with variable-angle ellipsometric spectroscopy and photoluminescence spectroscopy. The ellipsometric spectra were measured from 300 nm to 1700 nm at different angles and the dielectric function obtained by a standard least square regression analysis. All photoluminescence spectra were measured at room - with an excitation wavelength of 532 nm and the spectra have been normalized to the spectral response of the detector.

The solar cells were realized within a study for the realization of c-Si silicon solar cells for a concentrator system. 3 wafers with solar cells were dedicated to our experiment: A reference wafer with an ARC consisting of a single layer of SiO2 of 106 nm (the cells are in the following labeled as Ref-cell) and two wafers on which we deposited SRO-layer (Γ=15) of different thickness, one with 235 nm (labeled as LDS 235nm) the other with 677 nm of SRO (labeled as LDS 677nm), the thickness was measured after the thermal annealing with interferometry. Solar cells were realized using floating zone substrates (280 µm,<100>,front-side polished, resistivity 0.3-07 Ωcm, p-type), the back-side was boron implanted resulting in a sheet resistance of 10 ohm/ measured at the end of the processing. The top-emitter was realized by phosphor diffusion (sheet resistance 80 ohm/). The activation of boron and phosphor together with the growth of Si-nc was performed by thermal annealing at 900º C for 30 minutes. The SRO on top of the emitter was etched by reactive ion etching before metallization (Al deposited by DC-sputtering). The front metal is formed of metal fingers with two larger bus bars for current collection. The back-side of the cell is completely covered by Al. The size of the cells of this study is 4.25mm x 4.25mm, with an active area of 3.70mm x 4.25mm.

Photoluminescence excitation (PLE) spectra were measured on cells from LDS 235nm and LDS 677nm with a Cary Eclipse Fluorescence spectrometer. The wavelength of incident light was scanned from 400 nm to 680 nm for the PLE measurement observing the emission at 800 nm. A 550 W solar simulator (ABET technology Sun 2000) with AM 1.5G spectrum filter was used to measure the efficiency of the solar cells under 1 sun illumination.

2.2 Calculation of spectral downshifting and simulation of solar cell efficiency

Our numerical calculations are divided into two main parts: 1st the calculation of the modification of the AM1.5G spectrum transmitted into the silicon part of the solar cell for different cells (with and without down shifting layer), and 2nd the calculation of the solar cell efficiency and internal quantum-efficiency for the different cell geometries and for different surface recombination velocities. The effects of the presence of the LDS or ARC on the solar spectrum where calculated by means of transfer-matrix methods using the commercial software SCOUT, while the electrical properties of

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the solar cells were simulated using the software PC1D5 developed by the University of New South Wales, Australia, which allows the simulation of solar cells in 1 dimension. In the following we describe the procedure of the calculations in more detail:

We calculate the transmitted, reflected and absorbed fraction of light for normal incidence over the whole wavelength range of the AM1.5G spectrum (from 300 nm to 4025 nm) for the different downshifting layers and anti-reflective coatings. For the dielectric functions of the LDS layers we used the ones measured by ellipsometry for SRO (Γ=15) annealed at 1000º C, while we used tabulated dielectric functions for the other materials. For the thickness of the LDS layers we took the values from interferometry. When we calculate the absorbed, reflected and transmitted fraction of the light we assume that the whole AM1.5G spectrum arrives perpendicular with respect to the cell surface. The absorbed fraction of high energy photons gives the maximum number of photons which can be emitted by the LDS. In the estimation of the contribution of the re-emitted photons, we assume that all the absorbed photons are emitted (internal quantum efficiency of 100%). In this way, we obtain a upper limit for the effect of the LDS. The photoluminescence properties of the Si-nc are simulated with the photoluminescence tool of SCOUT, representing the Si-nc emission band by an oscillator with the maximum at 800 nm and a full-with half maximum of about 200 nm, approximately the same as observed in the PL-spectra from the Si-nc. Next we calculate the PL emission patterns for different angles from 0 to 90º for both light emitted into the silicon and emitted into air. Integration of the emission patterns over the solid angles gives the fraction of light emitted into the silicon. Interestingly these fraction, for all the geometries studied, never exceeds 50%. For this reason, we assumed that 50% of the re-emitted light is transmitted into the solar cell, which is again a upper limit. Then, we assume that the whole photoluminescence arrives perpendicular to the cell surface to simplify the calculations. Finally, we obtain the modified AM1.5G spectrum by adding the PL-contribution to the part of the AM1.5G spectrum, which is directly transmitted into silicon. These modified AM1.5G spectra were used as input files in the simulations performed next with the software PC1D5. In Figure 4, we show the modified AM1.5G spectrum for the best-configuration (Best-conf) of a LDS layer with Si-nc, which we could identify.

The parameters for the simulation of internal quantum efficiency and power efficiency – performed with PC1D5 were chosen to be as close as possible to the experimental ones of the fabricated solar cells. The cell thickness was 280 µm, front, backside and substrate doping were set to reflect as close as possible the measured sheet-resistances and the resistivity of the substrate. The depth of the junction was about 1.8 µm. The bulk recombination lifetime was set to τp = τn = 50 µs. The surface recombination velocity was varied between 104 and 106 cm/s to obtain correspondence with the experimental data and to explore the effect of the LDS for different IQE in the UV-blue spectral region. Surface reflectance was set to 0%, since the effects of ARC and LDS are included in the modified AM1.5G spectrum.

Figure 1. Dependence of the absorption and photoluminescence of annealed SRO on composition (excess-silicon

content).

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3. RESULTS AND DISCUSSIONS In Fig.1 we show the results from ellipsometric and PL measurements, which were performed on annealed SRO with different compositions. All the samples shown in Fig. 1 show strong absorption below 450 nm. The shape of the absorption bands is similar to the one of bulk silicon, while the characteristic peaks and shoulders are slightly blurred out due to disorder and quantum-size effects. In addition, we observe a blue-shift of the absorption bands with decreasing excess silicon content (increasing Γ). The PL emission bands are centered around 800 and 850 nm showing a shift to shorter wavelengths for lower excess Si-content. The lower excess Si-content results generally in smaller Si-nc and explains the blueshift of both the absorption and the PL bands [reference]. Similar results are generally reported for Si-nc independent of the method of growth. Considering the excess-Si content the sample with SRO composition Γ=15 shows the brightest PL and this composition was therefore chosen for the LDS-layers on the solar cells.

Figure 2. Room temperature photoluminescence excitation spectra (a), and photoluminescence spectra (b) of annealed

SRO deposited on top of solar cells.

From the PLE spectra, (see Fig. 2a) performed on the solar cells with LDS, we see that absorption in Si-nc is strong below 400 nm. The absorption is ~2 times stronger for LDS 677nm. The shape of absorption band is different from the one shown in Fig. 1, since it is also effected by multiple-interference effects. In addition we observe also week PLE above 400 nm a region in which the absorption coefficients were too low to be measured with ellipsometry. The photoluminescence spectra measured on the cells with LDS (Fig. 2b) are very similar as the ones obtained on the annealed SRO samples in Fig.1 and confirm that the properties of the Si-nc are not altered by the solar cell fabrication process.

Table 1. Results obtained on the solar cells by testing under AM1.5 illumination together with predicted PCE from simulation.

Solar cell Voc [V] Isc [mA] FF PCE [%] PCE-Sim [%]

Ref-cell 0.61 4.38 80.6 15 15.5

LDS 235nm 0.57 2.97 80.2 9.5 14.3

LDS 677nm 0.57 3.01 80.6 9.9 14.7

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Table 1 summarizes the results obtained from the measurements with the solar simulator and compares with the expected efficiency (PCE-Sim) from our simulations. The measured power conversion efficiencies (PCE) for the cells with LDS are considerably lower than the ones of the reference cell (Ref-cell). Comparison of open-circuit voltage (Voc), short-circuit current (Isc) and fill-factor (FF) shows that the difference in PCE is mainly caused by a low Isc for the cells with LDS. A low Isc means that either less photons arrive into silicon creating less current or that electron-hole pairs are lost by recombination. The calculated power conversion efficiencies (PCE-Sim) are only slightly lower for the cells with LDS, which means that the down-shifting effect does not overcome the additional losses due to higher reflectance and absorption, with respect to the reference cell with a single layer ARC. The large difference in PCE observed for cells with and without LDS has to have another origin. Indeed, the presence of the annealed SRO layer might dramatically increase the surface recombination velocity, an explanation which will need further experimental evidence.

Since the cells with LDS showed lower PCE than the Ref-cell we searched for a configuration for the LDS which improves the efficiency. This best configuration (Best-conf), - which we found for a planar cell - consisted of a SRO-layer (Γ=10) of 4 µm covered by 80 nm of SiO2. The SRO-layer has to be very thick to optimize the absorption in the UV-blue region of the solar spectrum and to produce very narrow interference fringes to avoid important reflection losses in spectral regions were the Si-cell has a large IQE. The silicon-dioxide layer helps to further reduces the reflection losses. In Figure 3 we show how the LDS of this best-conf influences the AM1.5G spectrum: The LDS absorbs all the photons below 300 nm and most of the AM1.5G spectrum below 500 nm. The fraction of light re-emitted from the LDS (black filled PL-band centered at 800 nm in Figure 3) is lower than one might expect comparing with the fraction of a absorbed light, this is due to the fact that we lose at least half of the absorbed photons and due to the loss of energy of the single photon. Therefore, the modified AM1.5G spectrum with contributions from the LDS results only slightly more intense than the calculated spectrum with the LDS working just as ARC-coating (curve with black squares in Figure 3). In this best configuration the cell with the LDS has a efficiency of 15.0% close to the one of the reference cell (15.5%) for surface recombination velocity of 104 cm/s.

Figure 3. Calculated modification of the AM1.5G spectrum by the presence of a LDS consisting of 0.08µm SiO2 and

4.0 µm of annealed SRO (Γ=10): AM1.5G spectrum (black triangles), fraction absorbed by LDS (grey filled), fraction of solar spectrum downshifted end re-emitted into solar cell (black filled), fraction of solar spectrum transmitted into solar cell (black squares), and fraction of PL re-emitted into solar cell + transmitted fraction of solar spectrum (black solid line).

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Finally, we performed simulations for both the reference cell and the best configuration of the LDS varying the surface velocity to understand, if and under what circumstances, we can observe a benefit from the presence of the LDS-layer. These results are summarized in Figure 4 and Table 2. The IQE, which is the same for the Ref-cell and cell with LDS in our simulations, depends strongly on the surface recombination velocity for wavelengths lower than 450 nm (see Figure 4). This is the region where the Si-nc strongly absorb, while the IQE remains nearly constant for the spectral interval of the Si-nc photoluminescence.

Figure 4. Calculated internal quantum efficiency for the silicon solar cells as a function of surface recombination

velocity together with absorption and emission spectra of annealed SRO.

From Table 2 we see how the variation of the surface recombination velocity modifies the spectral dependence of the IQE and finally the PCE for the two types of cells. For the Ref-cell the efficiency is reduced from 15.5% to 12.4% by an increase of the surface recombination velocity by two orders of magnitude. For the Best-conf of the LDS-cell the efficiency changes only from 15.0% to 12.8%, which shows that the LDS-cell is less sensitive to low IQE in the UV-blue region of the solar spectrum.

Table 2. Results obtained on the solar cells by testing under AM1.5 illumination together with predicted power conversion efficiency from simulation.

Surface recombination velocity [cm/s]

Ref-cell

PCE-Sim [%]

Best-conf

PCE-Sim [%]

104 15.5 15.0

105 13.4 13.6

106 12.4 12.8

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4. CONCLUSIONS The effect of LDS-layers containing Si-nc on the PCE of Si-based solar cells was studied by realization of solar cells with and without LDS and further investigated by simulations. The results show that LDS based on Si-nc increase the cell performance only in case of very poor performance in the UV-blue region (e.g. very high surface recombination velocity). In this case solar cells with LDS can outperform normal c-Si based solar cells.

The main problem of successful application of LDS on silicon solar cells is the large IQE of these kind of solar cells also for low wavelengths.

ACKNOWLEDGMENTS This work has been partially supported by: i) OPTOI through the project HCSC (Legge 6, Provincia Autonoma di Trento, Italy) and ii) EU 6th framework program through the project ANNA and by EU 7th framework program through the project LIMA.

.

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