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Improved amorphous/crystalline silicon interface passivation for heterojunction solar cells by low-temperature chemical vapor deposition and post-annealing treatment Fengyou Wang,a Xiaodan Zhang,*a Liguo Wang,b Yuanjian Jiang,a Changchun Wei,a Shengzhi Xua and Ying Zhaoa In this study, hydrogenated amorphous silicon (a-Si:H) thin films are deposited using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) system. The Si–H configuration of the a-Si:H/ c-Si interface is regulated by optimizing the deposition temperature and post-annealing duration to improve the minority carrier lifetime (teff) of a commercial Czochralski (Cz) silicon wafer. The mechanism of this improvement involves saturation of the microstructural defects with hydrogen evolved within the a-Si:H films due to the transformation from SiH2 into SiH during the annealing process. The postannealing temperature is controlled to B180 1C so that silicon heterojunction solar cells (SHJ) could be

Received 28th May 2014, Accepted 5th August 2014

prepared without an additional annealing step. To achieve better performance of the SHJ solar cells, we

DOI: 10.1039/c4cp02212b

using different temperatures for the a-Si:H film deposition to study the influence of the deposition

also optimize the thickness of the a-Si:H passivation layer. Finally, complete SHJ solar cells are fabricated temperature on the solar cell parameters. For the optimized a-Si:H deposition conditions, an efficiency of

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18.41% is achieved on a textured Cz silicon wafer.

Introduction Silicon heterojunction (SHJ) solar cells fabricated by deposition of ultrathin hydrogenated amorphous silicon (a-Si:H) on a crystalline silicon (c-Si) absorber layer are promising candidates for high-efficiency, low-cost solar cells.1–5 Compared with the traditional monocrystalline silicon solar cells, SHJ solar cells generally exhibit higher open-circuit voltages (Voc) because the wide-band-gap a-Si:H is used as the emitter layer and the metal contact is separated from the wafer by a-Si:H films and indium tin oxide (ITO), which reduces the amount of interface recombination.6–10 However, the silicon heterojunction structure is also more sensitive to the surface defect density, because the p–n junction is across two different materials.11,12 Silicon surface defects such as dangling bonds or contamination by metallic ions will increase the amount of recombination at the a-Si:H/c-Si interface, increase the diode saturation current density, and deteriorate the output parameters of the SHJ solar cells.13 The key requirement for high efficiency of heterojunction

a

Institute of Photo-electronic Thin Film Devices and Technology of Nankai University, Tianjin 300071, China. E-mail: [email protected]; Fax: +86 22-23499304; Tel: +86 22-23499304 b Institute of Information Functional Materials, Hebei University of Technology, Tianjin 300130, China

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solar cells is a low amount of recombination at the silicon surfaces, which can be achieved by introducing an intrinsic a-Si:H passivation layer.14,15 Nolan et al.16 reported interface models of amorphous–crystalline silicon which is generated on Si(100), (110) and (111) surfaces. In particular, they found that the least stable (100) surface will result in the formation of the thickest amorphous silicon layer with the highest density of co-ordination defects, while the most stable (110) surface formed the smallest amorphous region with the least defects. Lee et al.17 improved the passivation by performing duallayered a-Si:H deposition. They found that a large amount of hydrogen (H2) dilution and a high deposition temperature (Tdep) caused the epitaxial growth of silicon thin films with an admixture of phases, which increased the amount of recombination centers and reduced the teff of c-Si. Furthermore, Schulze et al.18 and Kanevce et al.19 investigated a-Si:H/c-Si heterojunction devices by performing experiments and model simulations, respectively. They found that the bulk defects in the a-Si:H film would increase the amount of defect-assisted tunneling recombination and thus decreased the passivation effect. In summary, it is necessary to improve the a-Si:H film quality but without using common deposition conditions such as a high amount of hydrogen dilution or a high temperature. In this article, we explore a process for a-Si:H preparation that meets both these requirements. Because the deposition temperature directly

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influences the hydrogen content of the a-Si:H films and annealing is an effective way of changing the Si–H configuration of a-Si:H materials.20,21 A process including low-temperature deposition and subsequent annealing treatment has been adopted to fabricate high-quality a-Si:H films without risking the possibility of epitaxial growth. We describe the a-Si:H/c-Si interface in terms of the specific Si–H configurations and their evolution for different deposition temperatures and durations of the annealing treatment. After the thickness of the a-Si:H passivation layer for c-Si is optimized, the optimal process is applied to form a passivation layer on a textured Czochralski (Cz) substrate to fabricate SHJ solar cells.

Experimental methods Preparation and characterization of a-Si:H passivation layers For the experiments, 250 mm-thick bifacial mirror-polished 1–3 O cm phosphorus-doped commercial Cz(111) Si wafers were used to optimize the a-Si:H deposition process. For predeposition surface cleaning, the samples were treated with acetone–methanol–deionized water and then subjected to the RCA cleaning procedure. Then, the samples were immersed in a (H2SO4 : H2O2) = (3 : 1) solution for 5 min to grow a chemical oxide and rinsed in deionized water for storage. Before a-Si:H deposition, the native oxide was removed by dipping them into 1% hydrofluoric acid for 3 min. After blow-drying with nitrogen, the samples were transported to the load lock of the deposition system immediately in order to avoid oxidation or other contamination. The a-Si:H layers were then deposited on both sides of the Cz silicon wafer using a multichamber cluster PECVD system. The deposition details are listed in Table 1. To determine the heterointerface quality, teff was measured using the quasi-steady-state photoconductance (QSSPC) method with a commercial photoconductance setup from Sinton Consulting (WCT-120). The optical bandgap of the thin a-Si:H films was calculated from Tauc’s plot22 of the reflectance–transmittance results. Reflectance–transmittance measurements were performed using a Varian-Cary 5000.

60 min under a H2 atmosphere to study its effect on the samples and to determine the optimized Tdep. After that, another series of samples were fabricated with the optimized Tdep to optimize the annealing duration in the range from 20 min to 80 min. SHJ solar cell fabrication SHJ solar cells were fabricated on n-type tetramethylammonium hydroxide (TMAH)-textured 280 mm-thick Cz silicon wafers. The cell structure is Ag grid/ITO/p-a-Si:H/i-a-Si:H/n-cSi/i-a-Si:H/n-a-Si:H/Ag back contact. p-a-Si:H and n-a-Si:H films were prepared in two separation chambers of the PECVD system at 180 1C. The p-a-Si:H films were prepared using gaseous mixtures of silane (SiH4), trimethylboron (TMB), and H2. The n-a-Si:H films were prepared using gaseous mixtures of SiH4, phosphine (PH3), and H2. The Ag grid (B600 nm thick), Ag back contact (B600 nm thick), and ITO layers (B80 nm thick) were prepared by physical vapor deposition (PVD) and patterned onto the 1 cm2 pad area. The J–V characteristics of solar cells were measured at 25 1C under 1 sun (AM1.5, 100 mW cm2) simulator radiation.

Results and discussion Regulation of the intrinsic a-Si:H microstructure for silicon wafer surface passivation Fig. 1 depicts the teff values of passivated c-Si wafers obtained after a-Si:H deposition at different temperatures. The teff value increases from 213 ms to 692 ms (injection level at 1015 cm3) when Tdep increases from 100 1C to 190 1C. Further increases in Tdep cause a drastic decrease in teff to values lower than those obtained at a Tdep of 100 1C. Fig. 2(a) presents the Si–H wagging band observed at 640 cm1 in the Fourier transform infrared (FTIR) spectrum recorded when a-Si:H is deposited at various temperatures. It is obvious that a lower Tdep leads to a stronger absorption peak at 640 cm1, which indicates higher hydrogen content. Fig. 2(b) shows the stretching bands of monohydride (SiH) bonds at 1980– 2030 cm1 and higher hydride (SiH2) bonds at 2060–2160 cm1.

Modification of the a-Si:H passivation layer microstructure The deposition temperatures were varied from 100 1C to 230 1C to investigate the effect of the deposition temperature on the a-Si:H passivation. a-Si:H was deposited bifacially on c-Si at different temperatures with all other parameters being the same for each side, and the degree of passivation was evaluated by determining teff. Then, an annealing treatment was performed at a constant annealing temperature of 180 1C for

Table 1

Deposition conditions for a-Si:H films

Deposition parameter Temperature (1C) Power density (mW cm2) SiH4 : H2 (sccm) Pressure (Torr) Electrode distance (mm)

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100–230 25 20 : 50 1 20

Fig. 1 teff vs. excess carrier density measured for passivated wafers fabricated at different Tdep.

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Fig. 2

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FTIR spectra of the passivation layers deposited at different temperatures in the range of (a) 500–806 cm1 and (b) 1840–2250 cm1.

These two stretching modes are referred to as the low stretching mode (LSM) and the high stretching mode (HSM), respectively. Generally, a double-Gaussian-function fitting is used to analyze such FTIR spectra. The ratio of the intensity of the Gaussian peak of LSM to that near HSM is calculated to determine the chemical bonding state of the hydrogen atoms in the films. The larger the intensity ratio is, the higher the Si–H bond content is.23 Fig. 2(b) shows that the intensity of HSM of the sample deposited at low Tdep is stronger than that of the samples deposited at higher temperatures, whereas the intensity of LSM has the opposite tendency. The stronger HSM with large hydrogen is always assigned as containing amounts of microscopic voids in the a-Si:H thin films. Therefore, by analysing the LSM and HSM of Fig. 2(b), we can conclude that the microstructure of the a-Si:H thin films deposited at lower temperature is relatively poor even if its hydrogen content is high enough. Fig. 3 shows the optical band gaps of the a-Si:H films deposited at different Tdep. With increasing Tdep from 100 1C to 230 1C, the optical band gap of the a-Si:H films decreases by B0.2 eV, which indicates that the a-Si:H films deposited at

Fig. 3 Plots of (ahn)1/2 vs. hn for the intrinsic a-Si:H films deposited at different Tdep.

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lower temperatures have larger hydrogen contents due to the larger SiH2 fraction. All these results can be explained by the kinetics of film growth at different Tdep. In order to suppress the epitaxial growth of a-Si:H films on the c-Si substrate, the hydrogen dilution and glow discharge power density must be kept low, which results in the formation of higher hydride precursors such as Si-H2, Si-H3, etc. in the plasma.20,24 When the Tdep is low, large amounts of higher hydride precursors with low kinetic energies will lead to more disorder in the film network and higher amount of microvoids, which increase the number of recombination centers and widen the band gap of the a-Si:H films.25 Therefore, a low Tdep leads to a high hydrogen content of a-Si:H but simultaneously a larger defect density of the film. For this reason, the passivation effect is poor. At higher Tdep, the kinetic energy of the precursors is higher and the hydrogen effusion environment is significantly improved, which decreases the SiH2 fraction and the defect density in a-Si:H. However, when the Tdep is further increased to 230 1C, there is too little hydrogen in a-Si:H to saturate the dangling bonds of the thin films, resulting in an increased defect density of the film and poorer passivation. In addition, Si–H bond rupture within the films deposited at high Tdep can cause epitaxial growth of silicon with a porous and defect-rich structure,26 which is also detrimental to the a-Si:H/c-Si heterojunction solar cell performance. The passivation layers deposited at different Tdep are annealed in a hydrogen atmosphere for 60 min, and the resulting teff (injection level at 1015 cm3) is presented in Fig. 4. The teff values of the samples deposited at 100 1C, 140 1C, and 190 1C all apparently increase after post-annealing treatment, and the sample deposited at Tdep = 140 1C, in particular, show a remarkable B115% enhancement in teff. However, the samples deposited at Tdep = 230 1C show a lower teff after post-annealing treatment. Initially, to understand the mechanism for the increase in the teff after post-annealing treatment, we considered the FTIR spectra of the samples deposited at 140 1C, which showed the largest change after post-annealing treatment.

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Fig. 4 Influence of post-annealing treatment on the surface passivation of silicon films deposited at different Tdep. The circle symbols with error bars are the percentage increase in teff after treatment.

Fig. 5 shows the FTIR spectra of the samples obtained before and after post-annealing treatment in the range of 1840– 2250 cm1. The untreated sample is passivated with a-Si:H only on a single side in order to avoid any annealing during deposition on the second side. The sample used for the postannealing treatment is passivated bifacially with a-Si:H. The microstructure factor of a-Si:H defined as R = I2100/I2000 + I2100, where I2000 and I2100 are the intensities of the Gaussian peaks at 2000 cm1 and 2100 cm1, respectively, is decreased from 0.54 to 0.17 by the post-annealing treatment. This means that the network matrix of the a-Si:H film is more ordered with less microvoids after the annealing treatment, and the SiH2 transforms into an Si–H configuration. The hydrogen released during this transformation process saturates the defects at the a-Si:H/c-Si interface, improving the passivation performance. Compared with the samples deposited at 140 1C, the samples fabricated at 100 1C have larger SiH2 fractions within the a-Si:H films because of their larger hydrogen contents, but they

Fig. 5 FTIR spectra of the as-deposited and annealed samples in the range of 1840–2250 cm1. The inset figures show schematics of the threedimensional a-Si:H film networks before and after annealing treatment.

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also have more defects in their microstructures. Hence, the improvement in teff due to post-annealing treatment is limited by the amount of defects and disorder in the network. For samples deposited at 190 1C, the microvoid and hydrogen contents are lower than those of the films deposited at Tdep = 140 1C. Post-annealing treatment increases the teff, but only by 14.9%, which is lower than the percentage increase obtained for the samples deposited at 140 1C. For the samples deposited at 230 1C, the teff of the samples after post-annealing treatment is even lower than that of the as-deposited samples. This can be ascribed to the lower hydrogen content within the as-deposited a-Si:H films and the epitaxial growth of silicon films. To further optimize the annealing process, a set of samples were deposited at a temperature of 140 1C and then annealed for different durations: 20 min, 40 min, 60 min, and 80 min. Fig. 6 presents the teff as a function of the excess carrier density. The excess carrier density is artificially divided into high- and low-injection-level regions at 1015 cm1. The teff increases with an increase in the post-annealing treatment duration from 20 min to 60 min. Because of the Auger recombination limits of the silicon wafer,27 the teff converges in the high-injectionlevel region. When the annealing treatment duration is further increased to 80 min, teff is almost the same in the highinjection-level region as for the other samples, but it is obviously reduced in the low-injection-level region. Note that the behaviour of teff in the low-injection-level region mainly depends on the interface recombination.28,29 Thus, this behaviour can be interpreted as a decline in the silicon surface passivation quality. The overly long post-annealing treatment causes excess effusion of the hydrogen atoms in the a-Si:H films, rupture of the hydrogenated dangling bonds at the a-Si:H/c-Si interface, and an increase in interfacial recombination. Consequently, a 60 min post-annealing treatment was selected as the appropriate duration for the passivation process and was applied to prepare the SHJ solar cells. The above results demonstrate that a low Tdep can be used to prevent epitaxial growth and achieve higher hydrogen content, but it is also associated with higher SiH2 fractions and more microvoids in the silicon films. A high hydrogen content and a low electronic defect density could be achieved simultaneously by integrating deposition with a low Tdep and a post-annealing process. Furthermore, because the fabrication of silicon heterojunction solar cells typically involves thermal processes carried out at B200 1C in subsequent steps, this low-temperature method for the deposition of a-Si could be integrated with these thermal processes to enhance the teff, and thus no additional annealing step would be needed for cell integration. Because of the low conductivity and the high absorption coefficient of intrinsic a-Si:H, it is important to investigate the surface passivation quality of a-Si:H films as a function of the film thickness to validate their applicability as buffer layers in SHJ solar cells. Therefore, the thickness of the deposited films was varied from 20 nm down to 2 nm in this study. Fig. 7 shows the variation in teff with a-Si:H film thickness before and after post-annealing treatment (injection level at 1015 cm3). The implied Voc which is the theoretical maximum open-circuit

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Fig. 6

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teff vs. excess carrier density measured on passivated wafers post-annealed for different durations.

Fig. 7 teff as a function of a-Si:H film thickness before and after postannealing treatment.

voltage is given by the splitting of the electrons and holes at quasi-Fermi levels Fn and Fp. It is given by:   kT ðn0 þ DnÞðp0 þ DpÞ ln (1) implied Voc ¼ q ni2 where K is Boltzmann’s constant, T is the absolute temperature, q is the electric charge, n0 and p0 are the electron and hole densities, respectively, at thermal equilibrium, Dn and Dp are the excess electron and hole densities, respectively, and ni is the intrinsic carrier density. In a semiconductor under steady-state illumination, the effective minority carrier lifetime teff is given by: teff ¼

Dnav GL

(2)

where Dnav is the average excess carrier density and GL is the generation rate. From eqn (1) and (2) we can see that the value of Voc-imp and that of the lifetime have a dependence. Actually, their values are calculated and given by the WCT-120 lifetime tester automatically after finishing the QSSPC measurement.

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When the film thickness is 4.3 nm, the teff of the sample after post-annealing treatment is much lower than 20 nm, but the implied Voc is just 9 mV smaller. For a film thickness of 2 nm, the a-Si:H films could not be deposited homogeneously across the whole wafer surface, and the teff is therefore very small. The a-Si:H passivation layer with the optimal thickness in a SHJ solar cell should balance the passivation quality with the current losses due to low conductivity. Here, the appropriate thickness is considered to be 4.3 nm, for which the deposition duration is 30 s. It is also worth noting that the samples covered with thicker a-Si:H films (longer deposition durations) show a more significant improvement in teff after annealing because their larger hydrogen capacities result in more passivation of interface defects by the hydrogen released during annealing. We finally fabricated complete SHJ devices on textured substrates using different Tdep values for deposition of the a-Si:H passivation layer while keeping all the other fabrication parameters constant. Because the substrates of solar cells are covered with textured pyramids, the deposition duration used to form the intrinsic a-Si:H is 51 s rather than 30 s, as a film deposited on a flat surface is generally 1.7 times thicker than that deposited on a textured surface for the same duration.30 Fig. 8(a) shows the structure of the SHJ solar cells and Fig. 8(b) shows the obtained output parameters of the solar cells fabricated with various Tdep. We have fabricated at least three solar cells under each condition to ensure the repeatability of our proposed process. So the deviation in output parameters of the solar cells prepared under different conditions has some differences. To make the error bar more accurately reveal the output characters of the devices, we set different lengths of the bars for different conditions according to the data deviation. The shortcircuit current density ( Jsc) values were similar for all the cells, but the cell efficiency changes with Tdep because of changes in Voc and the fill factor (FF). This clearly demonstrates the necessity of a high teff and good surface passivation to achieve a high Voc and FF. The FF only decreases drastically for the

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project (12ZCZDGX03600), the Major science and technology support project in Tianjin (Grant No. 11TXSYGX22100), and the specialized research fund for the doctoral program of higher education (20120031110039).

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Notes and references

Fig. 8 (a) Schematic drawings of the heterojunction solar cell structures used in this work. (b) Performance of the SHJ solar cell as a function of Tdep. (c) Illuminated J–V characteristics of the SHJ solar cells when the Tdep of a-Si:H was 140 1C.

intrinsic a-Si:H films grown at 230 1C, although a reduction in teff was observed when Tdep was 190 1C. This is because the high Tdep leads to epitaxial growth of an intrinsic layer with a mixture of amorphous and crystalline phases, which increases the number of interface recombination centers and shunting channels and severely decreases the FF of the solar cells. Fig. 8(b) shows the J–V characteristics of the solar cells at a deposition temperature of 140 1C, exhibiting an 18.41% of conversion efficiency.

Conclusions In summary, intrinsic a-Si:H thin films deposited on commercial n-type Cz wafers at different deposition temperatures and subjected to post-annealing treatment of different durations were investigated in order to improve the silicon surface passivation. The a-Si:H films deposited at low temperatures did not exhibit epitaxial growth and showed enhanced hydrogen contents, and post-annealing treatment could be performed to change the Si–H configuration of the a-Si:H films. By combining low-temperature deposition (140 1C) and postannealing treatment (180 1C, 60 min), a very low interface recombination defect density of the intrinsic a-Si:H film on a Cz silicon wafer was achieved, leading to a teff of more than 1 ms. When this process was used to fabricate SHJ solar cells, a conversion efficiency of 18.41% was obtained under the optimized experimental conditions. This combination of low-temperature deposition with postannealing treatment is a very promising technique for improving the surface passivation quality in SHJ solar cells, because the thermal processes in SHJ fabrication can be used for the postannealing to enhance the minority carrier lifetime, and thus no additional annealing step would be needed for cell integration.

Acknowledgements This work was supported by the National High Technology Research and Development Program of China (Grant No. 2013AA050302), the Tianjin science and technology support

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crystalline silicon interface passivation for heterojunction solar cells by low-temperature chemical vapor deposition and post-annealing treatment.

In this study, hydrogenated amorphous silicon (a-Si:H) thin films are deposited using a radio-frequency plasma-enhanced chemical vapor deposition (RF-...
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