Effect of Surface Passivation on Si Heterojunction and Interdigitated Back Contact Solar Cells

Silicon surface passivation of hydrogenated silicon (Si:H) thin films deposited by RF and DC plasma process was investigated by measuring effective minority carrier lifetime (τeff) on Si <100> and <111> wafers and correlated with the silicon heterojunction (SHJ) cell performances in front emitter structure and interdigitated back contact (IBC) structure. Excellent surface passivation (τeff > 1 msec) and high efficiency front emitter SHJ cells are obtained by both RF and DC plasma deposited intrinsic a-Si:H buffer layer. High efficiency with open circuit voltage (VOC) of 694 mV was achieved on n-type textured Cz wafer using DC plasma deposited buffer layer. High VOC of 683 mV is also achieved in an exploratory IBC heterojunction structure and needs further optimization for improved fill factor. Introduction Fabrication of crystalline silicon (c-Si) heterojunction (SHJ) devices by the deposition of wide band gap semiconductors such as amorphous hydrogenated silicon (a-Si:H) are a demonstrated way to fabricate high efficiency c-Si solar cells [1-3]. High efficiency of a-Si:H/c-Si SHJ solar cells is primarily due to increased open circuit voltages (VOC). The increase in VOC is achieved by reduced surface recombination and low emitter saturation currents using a thin intrinsic a-Si:H buffer (i-layer) on both surfaces of c-Si wafer. However, the a-Si:H layer absorbs short wavelength (<600 nm) light and the best passivation is achieved with thicker layers, resulting in greater absorption losses and reduced short circuit currents (JSC) in the front emitter SHJ cells. Formation of heterojunction with both a-Si:H emitter and base contact on the back side in an interdigitated pattern would hence improve the cell blue response and JSC and offer a pathway to realize the full potential of heterojunction devices. Furthermore, a good front surface passivation quality is absolutely necessary for interdigitated back contact (IBC) structure, since the carriers that are generated near the front surface need to diffuse the entire wafer thickness to be collected in the rear. In this work, we studied the surface passivation quality of a-Si:H i-layers deposited with varying hydrogen dilution on n-type float zone (FZ) Si wafers with two different crystallographic orientation (<100> and <111>) and correlate with the device performances of front emitter SHJ and IBC cells. The SHJ cells were also fabricated on n-type textured Czochralski (Cz) Si wafers, where <100> surface changes to <111> after texturing to realize the benefit of light scattering. Additionally, the a-Si:H layers were deposited by standard 13.56 MHz RF plasma and by DC glow discharge process. Comparison between the two processes elucidates the effect of ion bombardment on Si surface passivation and cell performances during growth of these thin layers. Experimental The n-type Si wafers (~300 μm) with resistivity of 1.0 Ω.cm for <100> and 2.5 Ω.cm for <111> were cleaned for 5 mins in a mixture of H2SO4:H2O2 (4:1) followed by 5 mins rinse in de-ionized water and in 10% HF for 30 sec prior to each Si:H deposition. The Si:H i-layers (10 nm) were deposited on both sides by RF and DC plasma process using a six-chamber large area (30 × 35 cm2) deposition system. The substrate temperature, deposition pressure and SiH4 flow rate were fixed at 200°C, 1250 mTorr and 20 sccm respectively. The primary variable in i-layer process is the H2 flow rate, which was varied from 0 to 200 sccm and characterized by the dilution ratio R = H2/SiH4. The RF power of 30 W and a DC plasma current of 123 mA were maintained constant. The quality of surface passivation was determined by measuring effective minority carrier lifetime (τeff) using photoconductive decay method [4]. Thin layers Presented at the 17 Workshop on Crystalline Silicon Solar Cells & Modules: Materials and Processes (Vail Cascade Resort, Vail, CO, Aug. 5 – 8, 2007). (10 nm) of p-type a-Si:H emitter followed by 70 nm Indium Tin Oxide (ITO) with metal grids in the front and n-type a-Si:H followed by evaporated Al contact on the rear were deposited to fabricate front emitter SHJ cells. The interdigitated pattern of deposited pand n-type a-Si:H layers (20 nm) and evaporated Al contacts were created for IBC structure by two step photolithography processing, where the p-region has lateral dimension of 1.2 mm and n-region is 0.5 mm wide. The front surface of IBC cells were passivated by a 20 nm a-Si:H i-layer followed by antireflection (AR) coatings composed of ITO and MgF2 layers. This surface passivation/AR structure is not ideal for high JSC but provided a means of evaluating the device performance and the utility of the structure as an alternate diagnostic tool to evaluate surface recombination. Results and Discussions Figure 1 shows the τeff as a function of R for i-layers deposited on <100> and <111> wafers by DC and RF plasma after annealing the samples at 280°C for 10 mins. It is worth noting that the DC plasma process will have a significantly higher ion bombardment compared to RF in otherwise similar plasma parameters due to higher plasma potential. Three distinct regions can be identified in the variation of τeff with R (Fig. 1). In region I (R < 2), τeff shows plasma process dependence, namely, DC plasma deposited i-layer at R = 0 show lower lifetime (~ 500 μsec) compared to RF plasma deposited i-layers (> 1 msec) irrespective of the surface orientation. With R = 0 at 1250 mTorr, it is likely to have significant polysilane contribution in the film deposition for both DC and RF plasma with comparatively higher in DC due to larger amount of ionic species, resulting in a more defective film in DC process, thereby reduces τeff. Formation of poly-silane decreases with an increase of R in DC plasma [5] and improves film quality. Hence, τeff becomes similar for both RF and DC plasma deposited i-layers at R > 2 (region II and III in Fig.1), implying little / no deleterious effect of ion damage in DC process on Si surface passivation. In region III (R > 4), however, the measured τeff exhibits a pronounced Si surface orientation dependence. The values of τeff sharply decreases to < 10 μsec on <100> wafers for i-layers deposited with R > 4, while on <111> wafers remains > 1 msec even at R = 10 for both RF and DC process. It was found out from variable angle spectroscopic ellipsometry (VASE) measurement that i-layer at R > 4 on <100> wafers grow epitaxial or nanocrystalline, while the films remain amorphous on <111> wafers. Low value of τeff and loss of surface passivation on <100> wafers by R > 4 i-layers are likely due to the low band gap of epitaxial or nanocrystalline Si layers with larger amount of defects at the interface. 0 2 4 6 8 10 10 0 10 10 10 10