Effect of heat treatments on the properties of hydrogenated amorphous silicon for PV and PVT applications

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ScienceDirect Solar Energy 119 (2015) 225–232 www.elsevier.com/locate/solener

Effect of heat treatments on the properties of hydrogenated amorphous silicon for PV and PVT applications C. Frigeri a,⇑, M. Sere´nyi b, Zs. Szekre´nyes c, K. Kamara´s c, A. Csik d, N.Q. Kha´nh b a CNR-IMEM Institute, Parco Area delle Scienze 37/A, 43100 Parma, Italy Institute of Technical Physics and Materials Science, Centre for Energy Research, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary c Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, Hungary d Institute for Nuclear Research, Hungarian Academy of Sciences, P.O. Box 51, H-4001 Debrecen, Hungary b

Received 3 February 2015; received in revised form 1 July 2015; accepted 2 July 2015

Communicated by: Associate Editor Nik Romeo

Abstract Photovoltaic (PV) solar cells and photovoltaic thermal (PVT) hybrid devices very often employ hydrogenated a-Si (a-Si:H) because of its cost effectiveness and better performance as light absorber. The properties of a-Si:H can be improved by heat treatments that also help to recover the Staebler–Wronski effect. Here the effect of heat treatments on the behavior of H is discussed. It is shown that, upon annealing, the grown-in SiH monohydride groups are partially transformed into SiH2 dihydrides and polysilane chains which have been reported to impair the performance of a-Si:H based PV(T) devices. Since the polyhydrides reside on the surface of voids the increase of their density also affects the a-Si:H layer morphology by the formation of blisters due to the increased volume of the voids. The influence of such changes of the H bonding configuration and of the morphological structure on the performance of PV(T) devices is discussed. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Amorphous Si; Hydrogenation; Heat treatments; Solar cells; IR Spectroscopy; Voids

1. Introduction Photovoltaic (PV) modules using hydrogenated a-Si (a-Si:H) are known to have a good rate of cost per watt and the a-Si technology is now the most studied and used technology to fabricate solar cells (Mun˜oz-Garcı´a et al., 2012). Hydrogenated amorphous Si is used for solar energy conversion in several ways. It is employed to replace c-Si in conventional single junction photovoltaic cells mostly because of its cost effectiveness (Myong and Jeon, 2014), and in intrinsic a-Si/c-Si (Myong and Jeon, 2014) or doped ⇑ Corresponding author. Tel.: +39 0521 269235.

E-mail address: [email protected] (C. Frigeri). http://dx.doi.org/10.1016/j.solener.2015.07.004 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.

a-Si:H/intrinsic a-Si/c-Si heterojunction PV cells (Mitchell et al., 2009; Mishima et al., 2011; Fujiwara and Kondo, 2007; Wen et al., 2013) again with higher efficiency with respect to the c-Si homojunction thanks to its wider band-gap, i.e. greater total energy range of collected sunlight. The use of a-Si:H also has the beneficial effect of passivating the surface of c-Si by H saturation of its dangling bonds which results in an efficiency increase in both planar c-Si substrate based solar cells (Mitchell et al., 2009; Mishima et al., 2011) and c-Si nanowire based solar cells (Li et al., 2013). Post-deposition annealing of the a-Si:H/c-Si system at temperatures as high as 225–275 °C was seen to improve significantly the quality of the surface passivation provided by the intrinsic a-Si:H as confirmed

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by the increase of the effective carrier lifetime (Mitchell et al., 2009). Similar improvement was also observed in finished ITO/a-Si:H(n+)/a-Si:H(i)/c-Si(p) solar cell devices by annealing at 400 °C in N2:H2 = 95:5 atmosphere (Alnuaimi et al., 2013) because of the H passivation. Annealing at 150 °C for 8 h was equally applied by Fujiwara and Kondo (2007) to improve the performance of ITO/a-Si:H/c-Si/Al solar cells and at 200 °C by Li et al. (2013) for their c-Si/a-Si:H core–shell nanowires. Improvement of the solar cell efficiency was pursued by employing a-Si:H/lc-Si:H double-junction tandem structures as well (Matsui and Kondo, 2013). Another application of a-Si:H is as a replacement of c-Si in photovoltaic solar thermal (PVT) hybrid systems thanks to its superior temperature coefficient (0.1%/°C) (Pathak et al., 2012a,b). Having c-Si a temperature coefficient 4 times greater, the performance of the c-Si based PV component of a PVT system decreases rapidly as the temperature increases. To keep the electrical gain high the system is then cooled. In so doing, however, the heat (thermal) gain drops (Pathak et al., 2012a,b). Using a-Si:H as the absorber material would guarantee better electrical and thermal performance of the PVT system as it can more safely work at high temperatures which can reach values over 100 °C and also 200 °C if the system is stagnated (Pathak et al., 2012b) by the application of highly selective coatings (Kalogirou, 2003). The performance of a-Si:H as PV material can however be impaired by the Staebler–Wronski effect (SWE) that causes reduction of the solar cell efficiency (Matsui and Kondo, 2013; Pathak et al., 2012a; Mun˜oz-Garcı´a et al., 2012; Fanni et al., 2011; Staebler and Wronski, 1977). This is due to the formation of defect states in a-Si:H upon sunlight illumination which bring about degradation of its electrical and optical properties with consequent reduction of the lifetime of the photogenerated minority carriers. However, also in this case annealing, at 150–200 °C depending on the thickness (Pathak et al., 2012a,b), or even at 100 °C (Fanni et al., 2011), can be highly beneficial as it helps to eliminate the unwanted defect states created upon exposure to the sunlight thus making the SWE reversible. Since in a PVT system under operation the temperature can raise up to 100 and even 200 °C, as said above, the system itself assures conditions suitable for, or close to, annealing so that the SWE is easily recovered and the PV efficiency is little affected. The effectiveness of applying high-temperature annealing pulses in improving the efficiency of PVT hybrid devices was demonstrated experimentally (Pathak et al., 2012b). From the above the conclusion can be drawn that the electro-optical properties of hydrogenated a-Si for PV(T) devices can be optimized by submitting it to appropriate annealing. This, however, may raise the question of whether the H is stable against heat treatments, more specifically whether the H bonding configuration and the structural characteristics of the a-Si:H layer undergo modifications upon annealing. Here we address in detail this

issue. In a previous work (Frigeri et al., 2013) there was evidence of the formation of bubbles and surface blisters in hydrogenated a-Si/a-Ge multilayers and a-Si layers upon annealing which suggested there could be some relationship between those structural features and the behavior of H as a function of annealing. On the other hand, in the literature several papers suggested the presence of molecular H in voids embedded in a-Si:H (Acco et al., 1996; Beyer, 2003; Chabal and Patel, 1987; Jackson and Tsai, 1992; Street, 1991; Manfredotti et al., 1994). Stimulated by such findings in this paper we report on an FTIR (Fourier transform infrared) spectroscopy investigation of the evolution of the Si–H bonding configuration in annealed a-Si with different initial H content with the aim to establish how hydrogen influences the formation of the bubbles and blisters. The a-Si:H samples were prepared by radio frequency sputtering. In the last part of the Results and Discussion section the possible negative effects on the performance of PV(T) devices of the changes of the Si–H bonds and of the morphological structure produced by the heat treatments are discussed.

2. Experimental The a-Si layers have been deposited on polished (1 0 0) silicon wafers by radio frequency (RF) sputtering from a high purity crystalline silicon target in a Leybold Z400 apparatus evacuated at 5  105 Pa. The target was coupled to the RF generator operating at 13.56 MHz. The average layer thickness was 400 nm obtained with a sputtering rate of 6.3 nm/min. Sputtering has been performed under a mixture of high purity argon and hydrogen gases with an applied wall potential of 1500 V dc yielding a plasma pressure of 2 Pa. Hydrogen was incorporated into the layers by flowing it continuously into the sputtering chamber at three different flow rates, namely 0.4, 0.8 and 1.5 ml/min. The corresponding effective H incorporation in the as-deposited layers was 10.8, 14.7 and 17.6 at.%, respectively, as determined by ERDA (Elastic Recoil Detection Analysis). Annealing of the hydrogenated layers was carried out at 350 °C for 1 and 4 h in high purity (99.999%) argon. By transmission electron microscopy it was seen that the samples remained amorphous after annealing (Frigeri et al., 2008), as it could be expected because the annealing temperature of 350 °C is much lower than the crystallization temperature of Si, which is 700–725 °C (Cziga´ny et al., 1997). The ERDA measurements were carried out at the 1.6 MeV 4He+ beam at the 5 MeV Van de Graaff accelerator of Budapest on a-Si layers 40 nm thick grown under the same H flow rates and overall sputtering conditions as the 400 nm thick layers. The recoiled H signal was collected by an Si detector placed at 10° detecting angle to the beam direction, with the sample tilted 85° to the normal. In order to obtain almost background-free ERDA spectra for H the forward scattered He ions were stopped by putting a 6 lm

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thick Mylar foil in front of the detector. Further details can be found in reference (Kha´nh et al., 2012). The surface morphology of unannealed and annealed samples was checked by a Hitachi S-4300 CFE Scanning Electron Microscope (SEM). To avoid any change of the surface the samples were not covered with carbon or gold before the SEM investigation. A 15 kV accelerating voltage has been used which was found suitable to guarantee good images and to avoid charge accumulation, too. Fourier transform infrared (FTIR) spectroscopy was employed to study the type and evolution of the Si–H bonds in both annealed and unannealed layers. A Bruker Tensor 37 spectrometer with 2 cm1 resolution was used. The spectra were taken in the 400–4000 cm1 range with a Ge/KBr beam splitter while the baseline was corrected by an adjusted polynomial function. For the determination of the absorption coefficient a(x) the formula for the transmittance T of the film with thickness d was used (Brodsky et al., 1977). T ðxÞ ¼

4T 20 ead 2

2

ð1 þ T 0 Þ  ð1  T 0 Þ e2ad

ð1Þ

where T0 is the transmittance of the crystalline silicon substrate. The equation is correct only in a restricted film thickness range (Brodsky et al., 1977; Langford et al., 1992); our films are around the lower limit of this range as determined by Langford et al. (1992). T0 of the single side polished substrate was determined experimentally as the ratio of the transmission through two wafers compared to a single one. It was found that T 0 monotonically decreases from 23% to 16% in the wavenumber region of 3000–500 cm1. This behavior can be ascribed to the wavelength dependent light scattering of the rough back side of the wafer. 3. Results and discussion Heat treatment causes two modifications in the hydrogenated a-Si layers, namely change of their morphology and change of the configuration of the Si–H bonds. As regards the former issue it consists in the formation of blisters on the sample surface. By way of example, Fig. 1 shows how the surface morphology changes as a function of the annealing time for a hydrogen content of 14.7 at.%. For all H concentrations blisters are not visible in the unannealed samples. They appear after annealing with an average size that increases with increasing annealing time as seen in Fig. 1, while their density changes only by some percent. This same behavior is also exhibited for increasing initial H content. In the samples annealed for 4 h also craters appear like those indicated by A in Fig. 1c). The crater size is almost the same as the one of the largest blisters. Their density increases with increasing H content. The changes of the configuration of the Si–H bonds have been studied by FTIR. For quantitative estimation

Fig. 1. SEM pictures of hydrogenated a-Si layers with H content of 14.7 at.%. (a) Unannealed layer, (b) annealed for 1 h (average blister size 7.5 lm, density 1.78  104 cm2), and (c) annealed for 4 h (average blister size 11.7 lm, density 1.69  104 cm2). Two craters are indicated by A.

we use the stretching mode since this mode occurs in all SiHn configurations independent of n (Brodsky et al., 1977). The FTIR spectra allow to calculate the concentration NH (cm3) of the Si–H bonds by integrating the peaks in the IR spectrum of the absorption coefficient a(x) through the formula (Brodsky et al., 1977; Amato et al., 1991; Langford et al., 1992; Manfredotti et al., 1994). Z ð2Þ N H ¼ A ½aðxÞ=xdx ¼ A  I: A (cm2) = cnxl/(2p2es*2), with c the velocity of light, n the refractive index, x the vibration frequency, or wavenumber (cm1), l the reduced mass of the dipole,

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respectively, while es* is the effective charge of the Si–H oscillator dependent on the dielectric constant. I (cm1) is the value of the integral, i.e. the integrated absorption intensity. The integral is calculated only in the absorption mode region of interest. The total NH is calculated either from the wagging mode (at 640 cm1 for the Si/H system) or from the stretching mode. In the latter case, since in the Si/H system the stretching mode often consists of two peaks at 2000 and 2100 cm1, NH is given by Amato et al. (1991) and Langford et al. (1992). N H ¼ A2000  I 2000 þ A2100  I 2100 :

ð3Þ

I2000 and I2100 are the integrated absorption intensities of the low- (2000 cm1) and high-frequency (2100 cm1) stretching mode (LSM and HSM) peaks, respectively. Several values for the A’s of the stretching mode to be included in Eqs. (2) and (3) have appeared in the literature (Brodsky et al., 1977; Amato et al., 1991; Langford et al., 1992; Smets et al., 2003; Nadzhafov and Isakov, 2005). The most accepted values are those of Amato et al. (1991) and Langford et al. (1992) who determined them through calibration of the FTIR data versus hydrogen concentrations measured by ERDA and NRA (nuclear-reaction analysis), respectively. They also suggested that instead of 2 different values A2000 and A2100 an average of them can be used, Aav = 1.4  1020 cm2 (Amato et al., 1991; Langford et al., 1992), connecting NH to the integral over the whole stretching frequency range. The integrated intensity of each FTIR peak is therefore proportional to the amount of hydrogen taking part in the Si–H bond responsible for that peak. According to these considerations, we performed a two-component Gaussian fit to the baseline-corrected spectra in the wavenumber region 1875–2250 cm1. As we are only interested in the change of the relative concentrations of the Si–H bonds when changing the initial hydrogen content and the annealing time, in the following we compare only the integrated intensities of these Gaussians. Fig. 2 shows the fitted stretching mode regions of the FTIR spectra for the three hydrogen contents. For each H concentration the spectra for the unannealed, annealed for 1 h and for 4 h samples are given. For every H concentration both the peaks at low wavenumber (1991 ± 5 in our case) and at high wavenumber (2094 ± 7 cm1 in our case) are present. However, as the heat treatment time increases the intensity of the peak at 1991 cm1 decreases while the one of the 2094 cm1 peak increases. The peak at 2094 cm1 becomes stronger and stronger as the H concentration increases. Eventually it dominates over the one at 1991 cm1 for the highest H content of 17.6 at.%. This result from Fig. 2 is plotted in Fig. 3 reporting the so-called microstructure factor R = I2094/(I1991 + I2094) as a function of the H content. Following the literature, the SM vibration mode at low wavenumber of 1991 cm1 (LSM) has to be ascribed to the presence of monohydride (SiH) bonds. They are generally isolated network sites and are associated with H bonded

Fig. 2. Typical IR absorption spectra in the stretching mode range of the wavenumber for the H content of (a) 10.8 at.% (H flow rate: 0.4 ml/min), (b) 14.7 at.% (H flow rate: 0.8 ml/min), and (c) 17.6 at.% (H flow rate: 1.5 ml/min). For each plot the spectra for the unannealed (solid curve), annealed for 1 h (dash curve) and for 4 h (dot curve) samples are reported.

at isolated dangling bonds and vacancies (Brodsky et al., 1977; Langford et al., 1992; Amato et al., 1991; Manfredotti et al., 1994; Von Keudell and Abelson, 1998; Lucovsky et al., 1979; Touir et al., 1999; Jackson and Tsai, 1992; Daouahi et al., 2000). The vibration mode at the high wavenumber of 2094 cm1 (HSM) is instead due to the SiH2 dihydrides and polysilane chains (SiH2)n, n P 2 (Brodsky et al., 1977; Langford et al., 1992; Amato et al., 1991; Manfredotti et al., 1994; Nadzhafov and Isakov, 2005; Von Keudell and Abelson, 1998; Lucovsky et al., 1979; Touir et al., 1999; Daouahi et al., 2000).

C. Frigeri et al. / Solar Energy 119 (2015) 225–232 0,8

t=4

0,7

t=1

R

0,6 0,5

t=0

0,4 0,3 10

12

14

16

18

20

H content (at %) Fig. 3. Plot of R = I2094/(I1991 + I2094) as a function of the H content for the three annealing times.

The increase of the HSM 2094 cm1 peak in the annealed samples can be due to the thermal activation of H atoms that have occupied interstitial sites, i.e. shallow traps, during sputtering. As their binding energy is quite low (0.2–0.5 eV) (Acco et al., 1996) such H atoms very likely may locally rearrange their positions, upon annealing, by breaking weak Si–Si bonds and forming additional Si–H bonds. The latter ones could be of the dihydride type, SiH2, if the rearrangement involves near-neighboring H atoms. Besides the presence of the dihydrides, also polysilane chains (SiH2)n, n P 2, do exist in the annealed layers, as it could be concluded from the analysis (not shown here) of the bending mode peak of the annealed layers by deconvolution. The bending mode was in fact seen to contain the peak at 853 cm1, usually assigned to the chain (SiH2)n, n P 2, (Brodsky et al., 1977; Langford et al., 1992; Amato et al., 1991; Nadzhafov and Isakov, 2005; Lucovsky et al., 1979; Touir et al., 1999; Daouahi et al., 2000; Tsai and Fritzsche, 1979), beside the one at 887 cm1 typical of the SiH2 dihydride (Brodsky et al., 1977; Langford et al., 1992; Amato et al., 1991; Nadzhafov and Isakov, 2005; Lucovsky et al., 1979; Touir et al., 1999; Daouahi et al., 2000; Tsai and Fritzsche, 1979). The simultaneous decrease of the LSM 1994 cm1 peak, assigned to isolated SiH monohydrides (Manfredotti et al., 1994; Von Keudell and Abelson, 1998; Lucovsky et al., 1979; Touir et al., 1999), would also suggest that previously isolated SiH bonds may have undergone clustering with formation of (SiH)n groups. The clustered (SiH)n groups also vibrate at about 2100 cm1 (Brodsky et al., 1977; Langford et al., 1992; Amato et al., 1991; Manfredotti et al., 1994; Lucovsky et al., 1979; Touir et al., 1999; Jackson and Tsai, 1992; Daouahi et al., 2000). The increase of the intensity of the HSM vibration at 2091 cm1 in the annealed hydrogenated layers is therefore indicative of the increase of the density of clusters of monohydrides, (SiH)n, n P 2, and mostly of the polyhydrides, i.e. SiH2 and polysilane chains (SiH2)n, n P 2. Differently from the monohydrides, all such polyhydrides reside on the surfaces of (micro-)voids (Brodsky et al., 1977; Langford et al., 1992; Amato et al., 1991;

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Manfredotti et al., 1994; Lucovsky et al., 1979; Touir et al., 1999). The FTIR results help understanding the evolution of the Si–H bonds upon annealing. In the as-deposited hydrogenated Si layers hydrogen is mostly incorporated at the Si dangling bonds or vacancies as said earlier. Some hydrogen is however also bonded in dihydrides, that are expected to reside on the inner surfaces of nano-voids that typically exist in amorphous Si (Touir et al., 1999; Acco et al., 1996; Mahan et al., 2001; Beyer, 2003; Beyer et al., 2012; Mu¨llerova´ et al., 2010; Lin and Chen, 2013). The remarkable density increase of the polyhydrides after heat treatment suggests that the nano-voids have become quite greater in size as they have to accommodate a higher number of polyhydrides. Further increase of the void size from the nanometer to the micrometer scale may occur by the coalescence of near-neighboring voids. Similarly to the behavior of the polyhydride density, hence of the void size, the size of the surface blisters also increases with increasing annealing time (Fig. 1) which suggests that the blisters could have originated from the voids whose walls are decorated by the polyhydrides. The occurrence of craters and their size nearly equal to that of the largest blisters in the a-Si layers with the H content of 14.7 and 17.6 at.% submitted to the longest (4 h) heat treatment (Fig. 1c) suggest that the craters are blisters that have undergone bursting as a result of a too high internal pressure and associated stress. The blisters are thus bubbles containing a gas. It is suggested that the gas is molecular H2. This hypothesis is supported by a wide number of papers that have shown both experimentally and theoretically the formation of H2 molecules in a-Si:H, regardless of its preparation method and H incorporation process, by breaking of Si–H bonds and reaction of two H atoms to form H2 molecules which then gather into microvoids (e.g. Acco et al., 1996; Beyer, 2003; Chabal and Patel, 1987; Jackson and Tsai, 1992; Manfredotti et al., 1994; Street, 1991). Chabal and Patel (1987) observed that H2 is incorporated in high-pressure bubbles. In our case the formation of H2 gas in the voids can be explained by analysing the behavior of the total concentration of the Si–H bonds. Since all the Si–H bonds contribute to the stretching mode, such behavior is easily evaluated by using Eq. (3) by summing up only the integrated intensities of the deconvoluted LSM and HSM peaks, i.e. by neglecting the two constants as said above when discussing Eq. (3), as it is not necessary to know the absolute concentrations. The total integrated intensity of the IR stretching mode is plotted in Fig. 4 which shows that upon annealing it decreases with respect to the no-anneal case for the two highest H concentrations. This means that the total amount of the Si–H bonds decreases, i.e. that a certain number of the bonds have broken with consequent release of atomic H. The rupture of the bonds is driven by the thermal energy supplied by the annealing process. Hydrogen is expected to be primarily released from the (SiH2)n, n P 1, polyhydrides decorating the walls of the

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Fig. 4. Plot of the total integrated absorption intensity of the stretching mode ISM total as a function of annealing time for the three different H contents: H = 10.8, 14.7 and 17.6 at.%.

voids as they have a smaller binding energy than the SiH monohydrides (Jackson and Tsai, 1992). The H atoms are liberated into the voids themselves where they may react to form molecular H2. The break of two Si–H bonds with simultaneous formation of a hydrogen molecule has been discussed by Beyer (1991) in a-Si:H. The transformation is energetically favored since the binding energy of SiH is 3.2 eV (Beyer, 1991) so that the break of two SiH bonds makes available 6.4 eV which is higher than the binding energy of H–H in the molecule which is 4.5 eV (Beyer, 1991). The transformation has been invoked to account for the effusion out of molecular H2 in annealed a-Si:H (Beyer, 2003). In hydrogenated porous Si Gupta et al. (1988) reported the same binding energy for the H molecule (4.52 eV) while the one of the SiH species was higher (3.58 eV). They also gave the binding energy of the SiH2 dihydrides, i.e. 3.1 eV, which is a factor 0.866 smaller than the SiH one. Assuming that the SiH2 binding energy in a-Si is smaller than the SiH one by the same factor one gets 2.77 eV for it. Also the rupture of two SiH2 can thus make available sufficient energy to form the H2 molecule. The main supply of H to form H2 in the voids is expected to be the dissociation of the dihydrides and their chains as they preferentially reside on the walls of voids. H release from the isolated monohydrides outside the voids is expected to be less likely as they represent the deepest binding sites at about 2.4 eV (Jackson and Tsai, 1992). If release occurred, H atoms would occupy interstitial positions wherefrom they might diffuse toward nearby voids. H evolution, i.e. breaking of the Si–H bonds, has been reported to already start at temperatures as low as 150 °C (Carlson, 1986) or 250 °C (Tsai and Fritzsche, 1979), i.e. smaller than 350 °C used here. The molecular H2 in the gas state inside the voids expands by the gas law upon annealing with consequent increase of the volume of the voids, which would favor their coalescence leading to larger and larger voids. Such bigger voids offer larger inner surfaces for the formation of additional (SiH2)n, n P 1, polymers which will further contribute to the release of additional H to be transformed into H2. Eventually, the voids will reach such a big size to

cause a plastic deformation of the layers with the formation of the observed blisters. The blisters thus correspond to bubbles containing molecular H2. They have developed from nano- and micro-sized voids, decorated by SiH2 dihydrides and associated chains, which have increased their volume because of the increase of the inside pressure due to the thermal expansion of the H2 gas upon annealing. The inside blister pressure was calculated to be on the order of 50 MPa (Frigeri et al., 2013). Beside the stress associated with the pressurization of the blisters it has to be noticed that their formation could also be favored by intrinsic stresses, generated for instance by the Si–H bonds rupture. However, according to Nickel and Jackson (1995) the strain released as a consequence of the break of the Si–H bonds can be re-created by its propagation through the amorphous network to the neighboring atoms and reconstruction of strained Si–Si bonds. They also concluded that the average network strain remains independent of the H concentration and annealing as well. It can thus be assumed that the annealed hydrogenated samples do not change significantly their intrinsic strain status with respect to that of the as-deposited ones. Stress measurements in samples grown and annealed under the same conditions as here are given in Frigeri et al. (2011). It has been reported that the performance of solar cells containing a-Si:H is adversely affected by the presence of SiHn polyhydrides, in particular SiH2 (Matsui and Kondo, 2013; Matsui et al., 2013). Such polyhydrides were thought to be the cause of enhanced light induced degradation. In our opinion, very likely this occurs because, as said earlier, the (SiH2)n, n P 1, polysilanes on the inner surface of (micro-)voids have a lower binding energy than the isolated monohydrides so that they release H more easily and earlier upon supply of external energy, either by heat treatment or by illumination, with consequent formation of unwanted dangling bonds that recombine the photogenerated minority carriers. Most often the presence of the polyhydrides has been ascribed to the growth conditions, as in the case of PECVD (Plasma Enhanced Chemical Vapour Deposition) for which the density of the SiH2 dihydrides was drastically reduced by preventing the incorporation of reactive species generated in the plasma, like SiH2 and higher silane radicals, by adopting the triode electrode configuration instead of the conventional diode one, with a factor 3 decrease of the concentration of SiH2 (Matsui et al., 2013). We have shown here that the polyhydrides can also be produced upon annealing. As recalled in the Introduction, annealing has been shown to improve the surface passivation properties of a-Si:H (Mitchell et al., 2009; Mishima et al., 2011; Li et al., 2013; Matsui and Kondo, 2013) and the performance of the solar cells under light illumination mostly through a recover of the SWE (Pathak et al., 2012a; Sakata et al., 2012). However, it can also cause the generation of SiH2 as documented here. Although a direct comparison with the references (Myong and Jeon, 2014; Mitchell et al.,

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2009; Mishima et al., 2011; Fujiwara and Kondo, 2007; Li et al., 2013; Matsui and Kondo, 2013; Pathak et al., 2012a,b; Kalogirou, 2003; Sakata et al., 2012) cited in this work is not possible as they do not report the actual H concentration in at.%, our results show that in order to keep short (1 h) the heat treatments at relatively high temperatures (350 °C) the H concentration in the hydrogenated a-Si must be smaller than 15–15.3 at.% since for greater values the ratio [SiH2]/[SiH] rapidly increases over 1 (R > 0.5 in Fig. 3). High concentrations of SiH2, in fact, deteriorate the light-soaking stability of a-Si:H (Li et al., 2013; Matsui and Kondo, 2013; Matsui et al., 2013) and would make annealing not effective to recover the SWE. Moreover, a high density of SiH2 and polysilane chains causes a severe degradation of the layer structure as discussed earlier (Fig. 1) that produces irregular interfaces and possible short circuit channels in the case of craters. If the voids are interconnected the a-Si:H very likely is not of the expected device grade quality since loss of H, by out-diffusion of atomic or molecular hydrogen, and possible in-diffusion of water and ambient molecules may occur depending on the void size (Beyer et al., 2012). Furthermore, the accumulation of the polyhydrides in (micro-)voids determines an inhomogeneous distribution of H that causes elastic potential fluctuations in the amorphous network resulting in a local variation in energy band gap (Agarwal et al., 2007). In particular, an increase of the optical band gap with increasing concentration of SiH2 was observed (Manfredotti et al., 1994). The blisters formation mechanism suggested here, i.e. that they are bubbles filled with H2 that had formed by reaction of atomic H released by the polyhydrides in the interior of the (micro-)voids, also implies that several dangling bonds form that act as non-radiative recombination centers that potentially deteriorate the electro-optical characteristics of solar cell devices.

4. Conclusions The effects of heat treatments on the properties of hydrogenated a-Si to be used in PV and PVT devices have been investigated. To better study them the annealings have been performed at temperatures (350 °C) higher than those (200–275 °C) usually applied to improve the performance of a-Si:H employed in PV and PVT technologies since those effects were expected to be stronger and more easily detectable. It has been seen that the heat treatments cause changes of the Si–H bonding configuration with the transformation of the SiH monohydrides to the less stable SiH2 dihydrides and polysilane chains. The latter polyhydrides, in their turn, cause a strong degradation of the a-Si layer morphology with the formation of surface blisters with size of some microns as well as craters depending on the H content and annealing time. Although such big surface features are not expected to occur for heat treatments at 200–275 °C the micro-cavities filled with H2 gas from which they originate are expected to do. How

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