Porous silicon potentiel solar cells EPJAP 2013

Thin layers of nanoporous silicon PS were synthesized by anodic etching, in order to develop photovoltaic cells. We proposed a diluted concentration of hydrofluoric acid with different etching current densities (1, 3, 5 mA/cm 2) on a fairly short
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  Eur. Phys. J. Appl. Phys. (2013) 61: 30102 DOI:  10.1051/epjap/2013120152 Influence of etching parameters on optoelectronic propertiesof c-Si/porous silicon heterojunction – application to solar cells Fatiha Bechiri, Mokhtar Zerdali, Ilham Rahmoun, Saad Hamzaoui, Mohamed Adnane, and Taoufik Sahraoui  Eur. Phys. J. Appl. Phys. (2013) 61: 30102DOI: 10.1051/epjap/2013120152  T HE  E UROPEAN P HYSICAL  J OURNALA PPLIED  P HYSICS Regular Article Influence of etching parameters on optoelectronic propertiesof c-Si/porous silicon heterojunction – application to solar cells Fatiha Bechiri a , Mokhtar Zerdali, Ilham Rahmoun, Saad Hamzaoui, Mohamed Adnane, and Taoufik Sahraoui Laboratoire de Microscopie Electronique & Sciences des Mat´eriaux (LME&SM),Universit´e des Sciences et de la Technologie d’Oran (USTO), BP 1505, El MNaouer, 31100 Oran, AlgeriaReceived: 23 April 2012 / Received in final form: 30 September 2012 / Accepted: 15 January 2013Published online: 8 March 2013 – c   EDP Sciences 2013 Abstract.  Thin layers of nanoporous silicon PS were synthesized by anodic etching, in order to developphotovoltaic cells. We proposed a diluted concentration of hydrofluoric acid with different etching currentdensities (1, 3, 5 mA/cm 2 ) on a fairly short time anodization. Observations by scanning electron microscope,electrical measurements and optical measurements revealed that the structural properties of PS layersdepended on strong conditions of prints. The reverse and forward component of the  I-V   characteristicsshowed an appropriate method to explore and extract the parameters of the diode ideality factor  n  . Theoptimum conditions of formation of PS were: HF concentration of 1% and an etching current density of 1 mA/cm 2 . Unlike silicon, which has a low absorption of short visible wavelengths, it was shown that thePS had wide energy gap of  ≈ 2 eV, and a marked improvement in the absorption between 400 and 600 nm.This property has been used to optimize the response of the solar cell Ni/PS/c-Si. Efficiency performanceclose to 4.2% was obtained with a  V  oc  of 400 mV, and fill factor of 46%. The solar cell exhibited betterresponse than the reference cell Ni/c-Si. These results show that PS/c-Si heterojunction has a potentialfor photovoltaic applications. 1 Introduction Porous silicon (PS) films improve the performance of solarcells by trapping light [1], as antireflection coating (ARC) and significantly increase the efficiency of solar cells [2]. PS thin films are usually made by electrochemical etch-ing, taking silicon as an anode. The solution is composedof hydrofluoric acid (HF) and an organic solvent [3,4]. This method has the advantage of forming porous sili-con to achieve low manufacturing cost to be later used formass production of commercial solar cells [5]. After achiev-ing improvement in the parameters and the photovoltaiccell efficiency, various technologies were proposed such asthe nanostructured antireflection layers [6], textured lay- ers [7], porous silicon (IG-PS structure) [8] and hybrid so- lar cells [9]. These techniques generally require more than two different materials and many manufacturing technol-ogy steps in several stages. Many authors have reportedvarious studies on both types of silicon,  p  and  n  , for thesynthesis of PS by both anodic etching in HF solution andas a result of high etching current density [10]. Generally,these conditions lead to a PS layer thickness of severalmicrons to ten microns (1–50 microns) [11]. These lay-ers are manufactured and used as antireflection coatings(ARC) in photovoltaic devices. The physical phenomenon a e-mail: involved in the trapping of light is the change in refractiveindex that ranges from 1.9 to 3.4 for the PS [12]. On the other hand, the PS layer was identified asa semiconductor material, including the presence of thenanoporous structure of silicon that leads to the formationof nanocrystalline silicon elements. Therefore, an offset of the two conduction bands and valence band was observed.In a narrow space, the quantum confinement leads to theshift of the energy gap of silicon ranging from 1.12 eVto 2 eV for PS film [13]. A direct band transition was observed [14,15] along with the presence of photolumines- cence properties [16].In this study, we focus on the production of layers of nanoporous PS whose property is to improve the absorp-tion of photons of short wavelengths near the blue, unlikethe effect of trapping by the antireflection layers [17]. Our interest was focused on the  n  -type silicon, as itwas reported that the anodic etching was driving at orbefore the formation of nano- and macroporous silicon,and this mixture improves the current density in the solarcell [18]. To this end, we propose a concentration of di-luted hydrofluoric acid to etch silicon surface with differ-ent current densities (1, 3, 5 mA/cm 2 ) in order to obtainoptimum conditions to form a nanoporous layer.We have demonstrated in this study that the optimumcurrent density of etch is 1 mA/cm 2 when considering ashort anodizing time of 10 min to adjust the thickness 30102-p1  The European Physical Journal Applied Physics of PS to about  ≈  100 nm. A range in thickness has theadvantage of having a PS layer whose resistance is lowerfor the contribution to the series resistance of the solarcell [19,20]. The presence of PS film as a nanostructure is inferredindirectly from the shift of the photoluminescence peakobserved; the measurements indicate a high energy gap of 2 eV for the sample PS, in contrast to silicon where theenergy gap is 1.12 eV. Measurements of photoconductivityconfirmed that the short wavelengths in the visible regionexhibited strong absorption and a maximum absorptionclose to PL peaks.These measurements were carried out with a metalcomb structure to take into account only the current pic-ture of the thin PS.In this work, we propose that the solar cellNi/PS/c-Si require only two manufacturing steps: electro-chemical etching followed by deposition of metal contacts.The srcin of the photovoltaic effect is attributed tothe effect in terms of the Schottky junction between themetal and the PS layer, in addition to the contribution of PS with Si heterostructure.The photovoltaic cell based on nanoporous siliconworks in tandem, thus promoting the absorption of shortwavelengths, in contrast to silicon where the absorptionof long wavelengths is favorable [21]. 2 Experimental Layers of porous silicon were prepared using anodic etch-ing in HF electrolyte solution. The electrolyte solution washeld in the electrochemical cell with a molybdenum foil asa cathode and polished silicon, whose properties are asfollows:  n  -type oriented (1 0 0) and resistivity 1 Ω cm.To ensure electrical contact with the substrate, we intro-duced nickel on the front using radio frequency sputtering(RF) (ULVAC RFS-200). The etching solution consistedof 1:1:50 parts HF (50%), C 2 H 5 OH (99%) and H 2 O re-spectively.PS layers were then etched by a DC voltage under dif-ferent current densities 1, 3 and 5 mA/cm 2 . The anodizingtime was adjusted in order to keep the thickness of the PSlayer to 100 nm.The porous silicon surface was observed by scanningelectron microscope (SEM-S2500C Hitachi).  I-V   charac-teristics were recorded by an Advantest TR8652 electrom-eter connected to the devices. Photoconductivity measure-ments were performed using a 50 W halogen lamp, coupledwith optical interference filters as a source of monochro-matic light. The photoluminescence spectra of PS filmswere examined at room temperature. Photo-excitation forthe PL experiment was done with a (325 nm) He-Cd laser,incident power of 22.3 mW/cm 2 , with a Hamamatsu Pho-tonics spectrophotometer, and a multi-Channel C10027analyzer as a detector in the range of 350–1100 nm. Thephotovoltaic characteristics were studied under the AM1.5condition, using a halogen lamp (Osram, 24 V/250 W) asthe light source. Fig. 1.  SEM micrographs of PS layers prepared by etching;(a) observation taken by a tilt angle of 30 ◦ for sample pre-pared by anodic current density of 1 mA/cm 2 ; (b) top surfaceprofile for sample prepared with 1 mA/cm 2 ; (c) 3 mA/cm 2 ;(d) 5 mA/cm 2 . 3 Results and discussions 3.1 Surface morphology Figure 1a shows a view of PS layer under a view angle of 30 ◦ for PS samples realized for 1 mA/cm 2 etching currentdensities.The etching time was fixed at 10 min. We have usedSEM observations to estimate the PS film thickness. Weset the thickness of 100 nm PS layers to keep a close com-parison between the PS samples produced with differentcurrent densities. Figures 1b–1d show the surface mor- phology of PS layers anodically biased with different cur-rent densities of 1, 3 and 5 mA/cm 2 . The best arrangementof pores is obtained for  J   = 1 mA/cm 2 . In the anodizingprocess, the formation of mesoporous silicon (pore sizeof about 100 nm or less) is recorded. Samples biased for3 and 5 mA/cm 2 showed inhomogeneous distribution of pores. We distinguish the presence of both mesoporous,and macroporous silicon (pore size greater than 100 nm).However, a low current density appears to be favorablefor uniform distribution and homogeneous structure of thePS. 3.2 Optical characterization Figure 2 shows typical curves of photoluminescence (PL)of PS layers. A current density of 1 mA/cm 2 and a con-centration of HF (1%) appear to be sufficient to induce anintensive PL. 30102-p2  F. Bechiri et al.: Influence of etching parameters on optoelectronic properties of c-Si/porous silicon heterojunction Fig. 2.  Photoluminescence spectra of PS samples preparedunder different anodic current densities. We note that the energy of PL peak correlated withthe energy gap for the PS material. The gap of siliconchanges from 1.12 to a higher value in the presence of porous silicon. Figure 2 shows that, when the current den-sity increases, the PL peak shifts to longer wavelengths.We observe from the figure that the value of thechanged energy gap of PS can be from 1.71 eV (728 nm)to 1.98 eV (628 nm) by simply adjusting the current den-sity  J   from 5 to 1 mA/cm 2 . PS layers made at differentcurrent density must have different sizes of nanocrystal-lites leading to a shift in PL peaks [22]. On the other hand, the samples made with current densities of 3 and5 mA/cm 2 have rather broad peaks around wavelengths645 and 728 nm, respectively. A wide band around thepeak is generally attributed to the presence of nanocrys-tallites of different sizes in which they modulate the widthof the strips. It has been reported through the conceptof quantum confinement that the conduction band andthe valence of porous silicon are inversely proportional tothe rays of nanoelements [23]. The SEM measurementsrevealed that the PS etched by 1 mA/cm 2 presented ahomogeneous distribution of pores; the samples preparedwith 3 and 5 mA/cm 2 presented a mixture of mesoporousand macroporous silicon structure. 3.3 Electricals measurements Figure 3 shows the experimental semi-logarithmic  I-V  characteristics of the reference structure Ni/c-Si and thestructures Ni/PS/c-Si PS layers etched with currents den-sities of 1, 3 and 5 mA/cm 2 .The electrical bias is applied to the diodes in either di-rection, reverse, forward direction. The forward bias corre-sponds to the case where the electrode metal is connectedto the positive output voltage source.The reference diode exhibits Schottky contact behav-ior between the nickel contact and the silicon substrate. Fig. 3.  Semilogarithmic  I-V   characteristics of PS samples pre-pared by current density of 1 mA/cm 2 , 3 mA/cm 2 , 5 mA/cm 2 and reference sample Ni/c-Si structure. Fig. 4.  Logarithmic plots of   I/ [1 − exp( − qV/kT  )] character-istic as a function of bias voltage at room temperature. The same behavior is observed for Ni deposited on poroussilicon samples. For reverse bias, the current reaches thesaturation considering the limitation by thermionic effect.The  I-V   relationship of the Schottky contact includingseries resistance is conventionally written as [24]: I   =  I  s e ( q ( V   − IR s) / ( nkT  ))  1 − e − q ( V   − IR s) /kT   ,  (1)where  I  s is the saturation current,  R s is the series resis-tance of the device,  k   is the Boltzmann constant,  T  ( K  ) isthe absolute temperature,  n   is the ideality factor and  V   isthe bias voltage supply.Figure 4 shows the characteristic of ln( I/ [1 − exp( − qV/kT  )]) versus  V   for different diodes. For all de-vices, the measured current-voltage  I-V   characteristics atroom temperature have two distinct regions,  − 2 V  ≤  V  ≤  0 . 5 V and  V   ≥  0 . 5, indicating different conductionmechanisms. 30102-p3  The European Physical Journal Applied Physics Current-voltage characteristics In the interval region of   − 2 V  ≤  V   ≤  0 . 5 V, the current I   should be expressed with a negligible series resistance R s. This approximation is sufficient to find an adequateideality factor  n  .Such an interval shows a straight line from  V   = − 2 Vto  V   = 0 . 5 V, covering a wider range of the curve fromwhich the ideality factor  n   can be determined.The ideality factor  n   is determined from the experi-mental plot of ln( I/ [1 − exp( − qV/kT  )]) versus  V   and tak-ing the slope of the linear region. The ideality factor isgiven by: n  =  q/kT  d V/ dln( I  ) .  (2)The  n   value is equal to 1.01 and confirms thermionic-diffusion mechanisms in reverse bias ( − ( − 2 V  ≤  V   ≤ 0 V)) and small interval for forward directions (0 ≤  V   ≤ 0 . 5 V).On the other hand, the diodes exhibit a rectificationbehavior for bias voltage  V   ≥  0 . 5 V; the curves deviateconsiderably from linearity due to the effect of series re-sistance  R s, interface states, and interlayer film. The se-ries resistance  R s is significant in the downward curvature(nonlinear region) of the forward bias  I-V   characteristicsand cannot be neglected unlike the linear region.The voltage  V d  =  V   − IR s across the Schottky diodecan be expressed in terms of the total voltage drop  V  across the series combination of the Schottky diode andthe series resistance. Therefore, the ideality factor andthe series resistance are calculated from the forward bias I-V   data using the method developed by Cheung andCheung [25]. The forward bias current-voltage character- istics due to thermionic emission of Schottky contact withseries resistance can be expressed as [26]: d V/ dln( I  ) =  n ( kT/q  ) +  IR s .  (3)Figure 5 shows the experimental d V/ dln( I  ) versus  I   plotfor various diodes.Equation (3) should give straight line for the data of downward curvature region in the forward bias  I-V   char-acteristics. Thus, a plot will give  R s as the slope, and n ( kT/q  ) as the  y  -axis intercept.The ideality factor  n   obtained using equation (3) is3.83, 4.08, 4.62. We observed that the  n   value increaseswith increasing anodic current density.The obtained  n   value is higher than unity; this is at-tributed to the series resistance. Beside the presence of theporous silicon layer, the Ni/c-Si interface causes a nonidealbehavior and an ideality factor  n   higher than unity.The obtained series resistances values  R s are quiteclose; with  R s values of 120, 122 and 126 Ω respectivelyat 300 K. 3.4 Photoconductivity properties Figure 6 shows measurements of photoconductivity (PC)of the PS layer formed by the current densities 1, 3 and5 mA/cm 2 . We have deposited on the PS layer, two comb-shaped electrodes using micro-photolithography. Fig. 5.  Experimental d V/ dln( I  ) versus  I   plot for different PSdiodes structures. Fig. 6.  Photoconductivity spectra ratio of typical PS filmsunder different DC bias. The spectral response of PC had maxima at  ∼ 546 nmequivalent to 2.27 eV. We clearly see that the response of the cell in the presence of PS layer significantly improvesthe absorption in the short wavelength.However, the reference device based on silicon showsonly a small absorption at short wavelengths. The interestin the PS layer is to improve the absorption spectrum to-ward the blue. It should be noted that the peak at 2.27 eVof PC is quite close to the photoluminescence peak of 1.98 eV for the same PS layer produced at a drive currentof 1 mA/cm 2 . According to Ozaki et al. [27], the difference between the peak in photoluminescence and photoconduc-tivity is the fluctuation of the electric potential along thedirection of the PS layer depth during etching. 3.5 Photovoltaic properties Figure 7 shows the  I-V   characteristics of solar cells madeby different etching current densities 1, 3 and 5 mA/cm 2 ,and the solar cell formed by only bulk silicon. 30102-p4
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