LIU Renjie,YIN Lu,ZHOU Yichun

(Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education,School of Materials Science and Engineering,Xiangtan University,Xiangtan 411105,China)

Abstract: The bifacial n-PERT (Passivated Emitter Rear Totally diffused) solar cells were fabricated using a simplified process in which the activation of ion-implanted phosphorus and boron diffusion were performed simultaneously in a high-temperature process.For further efficiency improvement,the rear side doping level was regulated by applying two different implantation doses and the chemical etching step of boron rich layer (BRL) was added,and their effects on cell performance were investigated.The solar cells average efficiency reaches 20.35% with a bifaciality factor of 90% by optimizing rear side doping level,which can be explained by the decrease of Auger recombination.And it is further enhanced to 20.74% by removing the front side BRL due to the improvement of surface passivation and bulk lifetime.The improved fabrication process possesses the advantages of low complexity and cost and high cell efficiency and bifaciality factor which could provide a promising way to the commercial production of bifacial n-PERT solar cells.

Key words: n-PERT solar cells;ion implantation;diffusion;bifacial structure

In recent years,n-type Si wafer solar cells have attracted more and more interest in the photovoltaic(PV) industry because of the potential of high efficiency.As predicted by the latest International Technology Roadmap for Photovoltaics (ITRPV,2020),n-type Si wafer solar cells will gain significant market share,which is expected to be greater than 40% by 2030[1].N-type solar cells have been developed during the last decade because of their advantages compared to conventionalp-type solar cells,such as high minority lifetime,free of light induced degradation,low sensitivity to metallic impurities and potential for efficiency enhancement[2-4].As a result,n-type Si wafers are being used in most of high-efficiency solar cells.

To achieve higher conversion efficiency,lots of explorations have been carried out on the solar cell materials,structure,fabrication processes and technologies[5,6].As forn-type solar cells,some advanced cell architectures have been designed,such as passivated emitter and rear totally diffused (PERT)[7],heterojunction (HJT)[8],and interdigitated back contact(IBC)[9].Among of them,n-PERT solar cells have been regarded as the most prominent candidate which would replace the current dominantp-type solar cell due to relatively simpler process and moderate cost.Additionally,n-PERT solar cells with bifacial structure can achieve higher energy yield because of the additional power generated from the rear side[10].However,n-PERT solar cells also have some disadvantages in comparison with thep-type,and more process steps is an important aspect.At present,monofacial passivated emitter and rear cell (PERC) is a common structure applied inp-type solar cells in which just one high-temperature process for phosphorus (P)diffusion is needed to formn+-emitter[11].While for the traditional bifacialn-PERT solar cells,an extra P diffusion[7]or implantation[12]on rear side is essential to form then+-back surface field (BSF) in addition to the formation ofp+-emitter through boron (B) doping on front side,which leads to a more complex process and higher production cost.

To realize commercial application of bifacialn-PERT solar cells,some efforts have been made in the simplification of production process for the last few years.Two typical technologies for B/P doping,namely co-diffusion[13-18]and co-annealing[19-23]after ion implantation,have been developed to reduce the amount of high temperature steps.Another promising approach to simplify the process procedure is the combination of B emitter diffusion and the activation of the ion-implanted P BSF,which has already been proven to work well by Kaniaet al[24].The novelty of this process is that high temperature diffusion of B is employed to activate the prior implanted P,and in this way one single high temperature step can be realized,and a median cell efficiency of 20.4% is achieved.However,to our knowledge,the reports on this new process for bifacialn-PERT solar cells are very few[24].And Kaniaet aldid not reveal the detailed fabrication process of bifacialn-PERT solar cells,especially whether the boron-rich layer (BRL) was removed.During the B diffusion,the BRL is unexpectedly formed on the emitter surface,which can act as a recombination site and interfere with subsequent surface passivation[25,26].Therefore,the removal of the BRL is extremely necessary in the cell processing to achieve high performance photovoltaic devices.

In this paper,the bifacialn-PERT solar cells were fabricated using a simplified process based on phosphorus activation and boron diffusion in one-step high temperature.For further efficiency improvement,the rear side doping level was optimized and the chemical etching step of BRL was added.The solar cells average efficiency fabricated in our work reaches 20.35% with a bifaciality factor of 90%,and it is further increased to 20.74% by removing the BRL.

The typical process to fabricate then-PERT solar cells is described in Fig.1(a)[27-30].Firstly,the textured surface was formed on both sides of the silicon wafer.Then a B diffusion step was adapted to form the B emitter.A substantial single side etch step was employed to remove the unexpected B diffused into the rear surface.Subsequently,the BSF was fabricated by a P diffusion step or an ion implantation step followed by a high temperature annealing.And edge isolation was needed to prevent shunting of the junction.After the fabrication of the emitter and BSF,a passivation step was adapted in which amorphous silicon nitride (SiNx) was deposited on both surfaces by plasma enhanced chemical vapor deposition (PECVD).At last the screening printing and firing through were adapted to form the front and rear contacts.Obviously,at least two high temperature steps are necessary in the manufacturing process of conventionaln-PERT solar cells.In this work,a simplified process is presented as described in Fig.1(b),and the schematic ofn-PERT solar cells is displayed in Fig.1(c).Firstly,both sides of then-type Czochralski-grown silicon (Cz-Si) wafers with the size of 156.75 mm×156.75 mm and thickness of 180 μm were textured in an alkali solution.Then an ion implantation step was adapted to implant the P ions into the rear surface of the wafer.The implantation energy was 10 keV and the dose were 4.5×1015and 4.0×1015cm-2for two divided groups of samples with 50 pieces of wafers in each group,which were marked as group 1 and group 2,respectively.After the ion implantation,a B diffusion step was carried out at 950 ℃ in an industrial diffusion tube,by using boron tribromide (BBr3) as the precursor.During B diffusion,the damage caused by the ion implantation at the rear side could be recovered and P would diffuse into the Si to formn+-BSF simultaneously.After the diffusion step,edge isolation was adapted with a plasma etch step and the boron silicon glass (BSG) at the front surface was removed by hydrofluoric acid (HF) cleaning in a diluted HF solution at room temperature.After that,both surfaces were passivated by a SiO2/SiNxstack layer.And an industrial screen printing and firing through step was adapted to form the front and rear contacts.For further efficiency enhancement,an alkali etch step can be added after the BSG removal step to remove the BRL at the front surface.The solar cells for group 2 were etched in 1% tetramethylammonium hydroxide (TMAH) solution for different time and then continued the passivation and metallization steps.

Fig.1 Process flow for bifacial n-PERT solar cells fabrication: (a) The typical flow with phosphorus implantation;(b) The simplified flow;(c) The schematic of n-PERT solar cells fabricated in this work

I-V parameters were measured by an IUCTMeasurement-Equipment (halm,Germany) under standard testing conditions (AM1.5G 0.1 W/cm2solar spectrum,T=25 ℃) ofn-PERT solar cells.The active boron concentration profiles were measured by electrochemical capacitance-voltage analysis (ECV).

As described in Fig.1(b),the bifacialn-PERT solar cells were prepared with only one high temperature step in this work.One concern is the diffusion of B into the rear surface which may destroy the BSF during the B diffusion step.The P implanted silicon layer might suppress the diffusion of B into the silicon[31].This can be explained by the occupation of the interstitial sites in the silicon lattice by the implanted P,but the mechanism has not been fully understood yet.Compared to the conventionaln-PERT fabrication process with ion implantation,the simplified process eliminates one step of high temperature annealing.As a result,the non-silicon cost is reduced by 1.4 cents/pcs and 0.33% of the total.In addition to the cost down,the manufacture flow with the simplified process has less process steps which is beneficial for mass production control and yield enhancement.

It is well known that rear side doping is needed for good contact with metal electrode and the passivation at silicon-metal contact area,but heavy doping is also detrimental for solar cells efficiency due to auger recombination.Therefore,the rear side doping level is regulated by applying two different implantation doses for two groups of samples.Fig.2 shows the distribution of open circuit voltage (Voc),short circuit current (Jsc),fill factor (FF) and efficiency(Eff.) of the two groups of samples.By reducing the BSF doping level through lowering the implantation dose of P,all of the four parameters are improved which lead to an enhanced cell performance.Table 1 displays the cell results of two groups of samples prepared by the simplified process.It is observed that by optimizing the BSF doping level,the average cell efficiency of the group 2 is 20.35% and the best cell efficiency is up to 20.46%.This can be explained by the decrease of Auger recombination from the doped P in the BSF.In principle,Auger recombination depends on the doping level[32,33].A lower doping level of P in the silicon leads to a lower Auger recombination which in turn results in a higherVocand cell efficiency.The bifacial property is also explored by calculating the bifaciality factor,which is defined as the ratio of rear to front side efficiency under standard test conditions.Therefore,the front and rear efficiency of the solar cells for group 2 are normalized,as shown in Fig.3.The bifaciality factor of the solar cells fabricated by the simplified process reaches as high as 90%,which will increase the power generated by the rear side and be a benefit for the bifacial modules[34].

Fig.2 I-V characteristics of two groups of solar cells fabricated with the simplified process: (a) Voc;(b) Jsc;(c) FF;(d) Eff.

Table 1 I-V parameters of two groups of solar cells fabricated with the simplified process

Fig.3 The bifacial property of the solar cells for group 2 prepared with the simplified process

In order to enhance the cell efficiency,the solar cells for group 2 were etched in 1% TMAH solution.Fig.4 shows the B concentration profile with different etching times.It is observed that B surface concentration is 3×1019cm-3without etching which is increased to 5×1019cm-3by 180 s etching of front B emitter with an estimated etching thickness of 20 to 30 nm.The implied open-circuit voltage (iVoc) is improved from 650 to 665 mV after etching for 180 s,as shown in Fig.5.The above findings indicate that the etching step not only removes the BRL at the front surface,but also reduces the negative effect of the high-low junction produced by the B concentration difference at the surface and in the bulk[35].Fig.6 shows the cell efficiency with two different etching times.The average cell efficiency is observed to be enhanced from 20.64%for 150 s to 20.74% for 180 s,and the corresponding best efficiency is 20.79% and 20.91%,respectively.Compared with the cell performance reported by Kaniaet al[24],the average and best efficiency of bifacialn-PERT solar cells with TMAH etching in our work are both improved,further verifying the positive effect of the removal of the BRL.In addition to interfering with surface passivation,the BRL has been suggested to induce the degradation of bulk lifetime because of crystal defects caused by the different thermal expansion behavior between BRL and Si[25,26].Therefore,the increase of cell efficiency with etching time can be attributed to the improvement of surface passivation and bulk lifetime.And the solar cells efficiency can be further improved by utilizing an Al2O3passivation layer for the front side B emitter[36]and applying the selective emitter or BSF structures[37,38].

Fig.4 Boron concentration profiles with different etching times measured by ECV

Fig.5 The implied open-circuit voltages of the solar cells for group 2 with different etching time

Fig.6 The efficiency of the solar cells for group 2 with two different etching times

A simplified fabrication process for the bifacialn-PERT solar cells was presented,including only one high temperature step by performing simultaneously the activation of ion-implanted phosphorus and boron diffusion,which reduces the process complexity and the cost of the manufacturing ofn-PERT solar cells.To further enhance the cell efficiency,the process was improved in terms of the BSF doping level and the removal of BRL.By reducing the BSF doping level,the average efficiency reaches 20.35% with a bifaciality factor of 90%.And the solar cells average efficiency is enhanced to 20.74% by adding a TMAH etching step of BRL after BSG removal.The improved process with low complexity and cost and high cell efficiency and bifaciality factor provides a promising way to the mass production of bifacialn-PERT solar cells.

推薦訪問:Solar Cells Fabricated