The Efficency Of Hit Solar Cell

Published Date: 02 Nov 2017

Abstract:For heterojunction with intrinsic thin-layer (HIT) solar cell on n-type flat c-Si substrate,the roles of fermi level of doped a-Si:H and valence band offsets at the a-Si/c-Si interface in solar cell performance were studyed through computer simulation. Our results show that the fermi level(Ef) of a-Si:H(p) or a-Si:H(n) needn't to get very close to the conduction-band edge(Ec) or the valence-band edge(Ev) to enhance the solar cell efficiency.Interesitingly,when Ef-Ev��250meV for a-Si:H(p), it had little effect on solar cell efficiency in this study.Meanwhile, the solar cell performance become unchanged as Ec-Ef��350meV for a-Si:H(n).in addition,the band discontinuities at the a-Si:H/c-Si interface could lead to severe decline of the fill factor when ?Ev>450meV in our simulation.As a result ,we obtain the highest solar cell efficiency of 25.51% with the valence band offset aroud 450meV.While considering the a-Si/c-Si interface defects, the optimal valence-band offset become bigger as the density of interface state increases.

1.Introduction

Heterojunction with intrinsic thin-layer (HIT) solar cell both has the advantages of crystalline silicon solar cells and thin film solar cells,because it is a hybrid composed of a n-type single crystal silicon wafer covered by layers of ultrathin amorphous silicon. High open-circuit voltage (Voc) is one of the most striking characteristics of HIT solar cells[1].In order to obtain a higher Voc,the excellent surface passivation is essential for silicon wafers.Generally,ultrathin a-Si:H layers are deposited on both sides of wafers by PECVD (plasma enhanced chemical vapor deposition) and then high-quality a-Si:H/c-Si interfaces can help to enhance open-circuit voltage[2]. With the superiority of low processing temperature��high conversion efficiency and low temperature coefficient,HIT solar cells has been attracting much attention all over the world since it was developed by Sanyo Ltd. in 1992[3].Recently, the Panasonic Corporation (Sanyo was acquired by this company) has demonstrated a new record conversion efficiency of 24.7%(Voc:0.750V, Isc: 39.5mA/cm2,FF: 83.2%, 101.8 cm2), using a thin wafer of practical size at 98��m thickness[4].

Although many groups who work on HIT solar cells in the world have achieved over 20% conversion efficiency,none have yet surpassed Sanyo��s level[5-7]. Nonetheless,a lot of simulations have been done to investigate the physical mechanism and the design optimization of HIT solar cells[8-11]. Role of the work function of transparent conductive oxide (TCO) was investigated in detail with numerical simulation[12-14].Considering the density of oxygen defects in c-Si bulk and interface defects,Zhao et al.proposed an optimal substrate resistance to obtain the maximal solar cell efficiency[15].High conversion efficiency of 27% was realized in bifacial HIT solar cell via computer simulation[16]. Apparently,it is a convenient way to understand the HIT solar cells.

Besides the quality of a-Si:H/c-Si interfaces,the doping concentration and defect density in doped a-Si:H layers also affect Voc largely. In previous simulation, defects density were constant with doping in a-Si:H layers.Ignoring the variation of defect density with the increasing of doping concentration, the simulation results show that the Voc rises to a saturation value for the doping concentration exceeds 2��1020cm-3[17]. However,the defect density of a-Si:H increases strongly with doping[18].As we konwn,the Fermi level position in a-Si:H is not only determined by the doping concentration ,but also the defect density. Therefore the fermi level position is a more effective parameter to evaluate the solar cell performance.Further more it can be directly measured by using near-UV-photoelectron spectroscopy[19, 20].

For the heterojunction solar cell,band discontinuities at the a-Si:H/c-Si interface have a significant impact on the fill factor (FF) in solar cells. High band offset in the minority carrier band can help to suppress the interface recombination,which may be a substaintial advantage for a-Si:H(p)/c-Si(n) structure compared to a-Si:Hn)/c-Si(p) structure[21].It was reported that the valence band offset (?Ev) of n-type HIT solar cell should be kept between 400 meV and 500 meV to get high conversion efficiency[22].In this paper, the influences of fermi level of doped a-Si:H and valence-band offsets at a-Si/c-Si interface on the HIT solar cell performance are studied by using AFORS-HET software.

2.Modelling

The HIT solar cell structure designed in our simulation is TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n)/TCO with layer thicknesses of 80nm/5nm/4nm/100um/4nm/5nm/80nm typically.The TCO layers on both sides are treated as optical layers and the contacts of TCO/a-Si:H(p or n) are assumed as flat band to neglect the contact potential influence.The crystalline silicon layer and the amorphous silicon layers are treated as electric layers with the parameters shown in table 1.In our simulation process, the solar AM1.5 radiation was adopted as the illumination source with a power density of 100mW/cm2.

The crystalline silicon layer has the defect density of 1��1010cm-3 located in the middle of the energy band.For the density of localized states in the band gap of amorphous silicon, it has been assumed that there are both acceptor-like states and donor-like states modeled by exponential band tails (Urbach tails) and Gaussian mid-gap states (associated to silicon dangling bonds)[23]. The fermi level position is achieved by changing the doping concentration and defect density in a-Si:H layer. It is in the range of 100 meV to 600 meV away from the band edge in doped amorphous silicon.Through varing the electron affinity and the band gap in a-Si:H layers,we can get various band offsets at a-Si/c-Si interface.

Table 1

Parameters set for the simulation of HIT solar cells with AFORS-HET in this work

In order to study the influnence of a-Si:H(i)/c-Si(n) interface state density on solar cell,a very thin layer (~1nm) is inserted between the a-Si:H(i) layer and the c-Si(n) layer.This layer is c-Si like with respect to the band gap but comprises Gaussian distributed dangling bond defects within the bandgap.Obviously it is a approximation to the real interface of a-Si:H(i)/c-Si(n).Regarding to the n-type HIT solar cell,the impacts of fermi level and band offsets on solar cell were explored with the variation of the defect density of a-Si:H(i)/c-Si(n) interface.

3.Results and Discussion

3.1 Influence of fermi level (Ef) in doped a-Si:H

In this section,the influence of fermi level (Ef) position in the doped a-Si:H(p or n) layer on solar cell is described. During the simulation,the electron affinity and bandgap in doped or intrinsic a-Si:H layers were all setted as 3.9eV and 1.72eV.At first,no interface defects at a-Si/c-Si were taken into consideration.

Fig.1(a) gives the variation of open circuit voltage (Voc),short circuit current density (Jsc), ?ll factor (FF) and conversion efficiency (��) as the function of Ef in a-Si:H(p) with different Gaussian defect density(Dp) of a-Si:H(p).In this simulation ,the fermi level position in a-Si:H(n) was Ec-Ef=250meV and the defect density Dn=6.9��1019cm-3.It is evident that initially Voc remains constant at 772.3 mV for different Dp when Ef-Ev��300meV.However,beyond 300meV (upto 600meV),Voc continuously decreased with the increasing of Ef-Ev.It��s worth noting that the rates of descent are not the same for various Dp.In the case of the higher defect density in a-Si:H(p), Voc is reduced slowly.On the other hand,FF shows a quite similar trend to Voc.With the increasing of Ef-Ev,FF almost kept the constant when Ef-Ev is less than 350meV,and droped quickly when Ef-Ev>350meV.But the rates of descent for various Dp are opposite to the case on Voc. As for the Jsc,there are low points at Ef-Ev =400meV for different Dp.Nonetheless, the Jsc was enhanced with the decreasing of Dp and the low of lines become not evident.As a result,the maximum value of �� is 25.58% at Ef-Ev =100meV with Dp=6.9��1019cm-3,25.81% at Ef-Ev =150meV with Dp=1.4��1019cm-3 and 25.91% at Ef-Ev =200meV with Dp=2.8��1018cm-3.The value of Ef-Ev to obtain the highest �� becomes larger with the decreasing of Dp .

The solar cell parameters as a function of Ef in a-Si:H(n) with different Gaussian defect density(Dn) of a-Si:H(n) were depicted in Fig.1(b).In this case,the fermi level position in a-Si:H(p) was Ef-Ev=250meV and the defect density Dn=6.9��1019cm-3. Apparently,the solar cell output is less sensitive to the fermi level position of a-Si:H(n) layer compared to the case in a-Si:H(p).Only when Ec-Ef>450meV,Voc and FF show droped obviously with the increasing of Ec-Ef.However,Jsc is very stable in spite of the fermi level and defect density in a-Si:H(n) layer.Consequently,the conversion efficiency maintained the highest value of 25.51% for Ec-Ef��350meV,then decreased slightly in the region of 350meV<Ec-Ef��450meV and jumped down rapidly if Ec-Ef beyond 450meV.

Fig. 1. The Voc,Jsc,FF and �� of HIT solar cell as the function of fermi level in (a) a-Si:H(p) layers and (b) a-Si:H(n) layers with different defect density.

When Ef��Ev >350meV in a-Si:H(p),the Voc fall dramatically with the increase of Ef - Ev due to the decline of the built-in voltage,which is equal to the bias of the ferimi levels in a-Si:H(p) and c-Si(n). On the other hand ,Voc also largely depends on the recombination at the interface of a-Si:H/c-Si. The Voc limited by interface recombination is given by Jensen et al.[24]:

V_oc=��_B/q-nkT/q ln((qN_V S_it)/j_sc ) (1)

where Sit is the interface recombination velocity, ��B is the effective barrier height in c-Si, Nv is the effective density of states in the valence band, kT is the thermal energy, n is the diode ideality factor, jsc is the short circuit current density, and q denotes the elementary charge.From Eq.1,it can be deduced that a lower carrier recombination velocity at the interface contribute to enhance the Voc.

It is convenient to define that ��f=Ef-Ev at the absorber side of the front a-Si:H(i)/c-Si(n) interface,or ��b=Ec-Ef at the absorber side of the back c-Si(n)/a-Si:H(i) interface. The band diagram under thermal equilibrium of a-Si/c-Si heterostructure solar cell (no illumination) is sketched in Figure 2. Due to the fact that the fermi level is much closer to the valence-band edge than the conduction-band edge,a high electric feild inversion region is present at the c-Si surface.Thus the hole generated in the absorber have to cross the inversion region before being collected at the front electode[25].As the a-Si/c-Si interface defects was not taken into consideration in this step,the main recombination taken place at the interface was band-to-band recombination.With the formation of the inversion region,there are less majority carriers compared to the minority carriers so that the band-to-band recombination can be suppressed well.

Fig. 2.Band diagram of HIT solar cells in our simulations.The symbols are explained in the text.

The dependence of the ��1 on the position of Ef in a-Si:H(p) is depicted in Figure 3(a). The value of ��1 grows slowly when Ef��Ev<300meV,otherwise it goes up quickly.For a-Si:H(p),low ��1 can enlarge the band banding of the absorber layer and enhance the electric field strength of inversiton region .Therefore the holes could be accelerated to across the heterojunction of a-Si:H(i)/c-Si(n).Furthermore,low ��1 can reduce the majority carrier(electron) at the interface of a-Si:H(i)/c-Si(n) and hence the interface recombination of holes can be suppressed effectively .Consequently,we can get high Voc of 772.3.6 mV as the value of ��1 decreases. However,when the interface recombination is well suppressed,the further shrink of ��1 can not enhance the Voc any more and result in a saturated value of Voc.

The back surface field refers to a region which will be a barrier for minority carrier, thereby reducing the recombination velocity of photo-generated carriers. The electrical field direction should agree with the one in p-n junction to reflect the minority carrier electrically.So the ��2 of the rear interface of c-Si(n)/a-Si:H(i) should satisfy the equation ��2>Ef-?Ec.As Fig 3(b) shown ,when Ec-Ef<350meV,the ��2 is smaller than the value of Ef-?Ec. Accordingly,the solar cell output parameters keep the constant in the case of Ec-Ef<350meV.

Fig. 3.the value of �� at (a) front and (b) back a-Si/c-Si interface as a function of Ef in doped a-Si:H layers with different interface defect density.Dit=0 means that interface defects were not taken into consideration.the inset to (a) gives the corresponding band diagram. the dash line in (b) means the equation ��=Ec-Ef-��Ec2.

When the defect density (Dp) in a-Si:H (p) goes up,only the Jsc goes down slightly.This is mainly due to the loss of recombination in the emitter. Howerver,the Jsc keeps constant with the variety of defect density (Dn) of a-Si:H(n).Moreover,the influence of the defect density in doped a-Si:H layers is not so obvious with the fermi level position close to the corresponding band edge.But for Ef-Ev>450meV or Ec-Ef>450meV,the Voc decreases so slowly with high defect density in doped a-Si:H layers. In order to maintain the same fermi level ,a-Si:H layers with high defect density are highly doped, which maybe help to form more effective back surface field(BSF),resulting in a higher open voltage..

In addition ,we also discussed the role of the defect density of a-Si:H(i) in solar cells.when the defect density of a-Si:H(i) increases to one order of magnitude lower than that in adjacent doped a-Si:H,there is little impacts on the solar cell output parameters with different defect density of doped a-Si:H layers(not shown here).Since the defect density of a-Si:H(i) is about three or two orders of magnitude lower than that of doped a-Si:H[26],it makes little influence on solar cell performence in this simulation.

In the above discussion, the interface state density is not taken into consideration for simplicity. The additional influence of the interface state density of a-Si:H(i)/c-Si is explicitly considered below.We inserted a ultrathin(~1nm) layer to the interface of a-Si:H(i)/c-Si(n).The inserted layer has the c-Si like properties to simulate the interface defects.The defect density , electron affinity and bandgap in doped a-Si:H(p or n) layers are set as 6.9��1019cm-3,3.9eV and 1.72eV.

Fig.4(a) shows the dependence of Ef in a-Si:H(p) layers on the solar cell performance with different interface state density(Dit). With the increasing of interface defect density, the interface recombination velocity Sit rises accordingly. On the other hand, the effective barrier height ��B in c-Si increases with interface defect density goes down,which can be seen from the inset to Fig.3(a).It can be deduced that Voc is reduced due to the increased Dit.Because the existence of interface defects,interface recombination can not be suppressed completly with the decreasing of ��(see Fig.3(a)),Therefore the Voc continuously increased instead of reaching a saturated value.When Ef-Ev>300meV,the �� for Dit=1��1012 cm-2 gets larger than the other two , resulting in a lower barrier at the interface which can help more carriers to transport.So higher Jsc is observed for Dit=1��1012 cm-2 when Ef-Ev>300meV. In a word ,with the Dit increase from 1��1010 cm-2 to 1��1012 cm-2,the maximal efficiency decreases from 25.44% to 21.72% accordingly.

It is interesting to find that the Voc becomes nearly unchanged with different back interface defect density when 400meV<Ec-Ef<500meV.Compared to front interface defect density,back inteferce defect density seems have little impact on solar cell performance in this region. Nonetheless,when Ec-Ef<350meV in a-Si:H(n),the solar cell with low back interface defect density shows high Voc due to strong BSF effect.

Fig. 4. The Efficency of HIT solar cell as a function of fermi level of a-Si:H for different a-Si:H(i)/c-Si(n) interface state density.

3.2 Influence of band offsets at a-Si/c-Si interface

In order to understand the influence of the band offsets on the solar cell performence ,We changed the electron affinity and band gap of a-Si:H.The fermi level positon of a-Si:H(p) and a-Si:H(n) are both set as 250meV apart from the corresponding band edge.Dependence of the solar cell performence on the ?Ev at a-Si:H(i)/c-Si(n) interface is shown in Fig 5.The results indicate that ?Ev should be about 450meV to obtain the highest efficiency of 25.21% with the interface defect density of 1��1010cm-2. With the increase of ?Ev1,the Voc and Jsc present a slight climbing up but the FF decreases slowly when the ?Ev1 smaller than 500 meV.However,the FF drops rapidly after the ?Ev exceeds 500meV due to the formation of S-shaped J-V (see Fig.6)characteristics[27].Additionally,the ?Ec1 (from 150 meV to 350 meV)nearly makes no impacts on the solar cell.

Fig. 5. The Voc,Jsc,FF and �� of HIT solar cell as the function of valence band offsets(?Ev1) with different a-Si:H(i)/c-Si(n) interface defect density(Dit).

Since the ?Ec1 is a constant with the variation of ?Ev,the band gap of a-Si:H(p) get larger as the ?Ev1 increase. Thereby more photons can reach the absorber layer and more photon-generated carriers lead to larger Jsc.Although the higher valence band offset means a barrier for the minority carrier to flow across the heterojunction,this can be compensated by a stronger band bending in c-Si layer.Briefly,the reason is that the stronger electric feild contribute to the collection of carriers.However ,it can no longer be compensated if the ?Ev1 exceeds 500 meV, especially above 550meV. In this case,the energy barrier is so high that only a small number of the holes can get over the barrier.Apparently,Jsc and FF drop dramatically and so does the efficiency.

Fig. 6. J-V characteristics for simulated solar cells with different valence band offsets.

For Dit=1��1010cm-2,the �� can reach 25.15%~25.21% in the region 420meV��?EV1��500meV;but for Dit=1��1011cm-2, �� can reach 23.52%~23.6% in the region 440meV��?EV1��510meV;And for Dit=1��1012cm-2, �� can reach 21.05%~21.14% in the region 460meV��?EV1��510meV.It is worth to note that with the increase of interface defect density, the suitable ?EV1 to get high �� become bigger.Briefly ,the larger ?EV1 can contribute to degrade the interface recombination,but it should not be larger than 550meV.

4.Conclusions

In summary,The n type crystaline silicon HIT solar cell with the structure of TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n)/TCO is studyed by AFORS-HET.

The results indicate that Ef-Ev of a-Si:H(p) layer is required to be smaller than 350meV ,preferably around 200meV to reach over 25% conversion efficiency. For the a-Si:H(n) layer, the solar cell efficiency can be maintained above 25% at the condition of Ec��Ef��450meV.However,if the interface state density is taken into consideration, Only Ef-Ev<250meV for a-Si:H(p) is acceptable for high solar cell efficiency.And for a-Si:H(n),the fermi level position should be as close as possible to the conduction band.Hence,controlling the doping concentration of a-Si:H to obtain suitable fermi level is very important for fabricating the solar cell with high performance.

For the band offsets of a-Si:H/c-Si interface,the values of the conductive band offset from 150 meV to 350 meV make little impacts on the solar cell performance.Referring to the valence band offset,the value aroud 450 meV can obtain maximal efficiency of 25.21% at Dit=1��1010cm-2,(Voc: 0.748V, Isc:40.28 mA/cm2, FF: 83.7%).Interestingly,the value turn to 500 meV with the interface density of 1��1012cm-2.