Kyungsun Ryu

Chemical etching of boron-rich layer and its impact on high efficiency n-type silicon solar cells

This paper reports on an effective chemical etching treatment to remove a boron-rich layer which has a significant negative impact on n-type silicon (Si) solar cells with boron emitter. A nitric acid-grown oxide/silicon nitride stack passivation on the boron-rich layer-etched boron emitter markedly decreases the emitter saturation current density J0e from 430 to 100 fA/cm2. This led to 1.6% increase in absolute cell efficiency including 22 mV increase in open-circuit voltage Voc and 1.9 mA/cm2 increase in short-circuit current density Jsc. This resulted in screen-printed large area (239 cm2) n-type Si solar cells with efficiency of 19.0%.

20% Efficient Screen-Printed n-Type Solar Cells Using a Spin-On Source and Thermal Oxide/Silicon Nitride Passivation

Due partly to the fact that n-type solar cells do not suffer from light-induced degradation, there has been a recent surge in research and industrial interest in n-type silicon solar cells. However, the most common dielectric materials that are used to passivate the surface of p-type cells — thermal oxide and silicon nitride — are well known to have problems with providing adequate passivation of heavily boron doped surfaces, especially on textured surfaces. As a result, the highest efficiency n-type cells have thus far relied on Al<inf>2</inf>O<inf>3</inf> whose high negative charge density results in good passivation on both planar and textured boron diffused surfaces. We show here that though thermal SiO<inf>2</inf> and SiN<inf>x</inf> alone provide poor passivation on p+ surfaces, a thin (15 nm) thermal-SiO<inf>2</inf> layer, capped with SiN<inf>x</inf> can, after firing, provide high-quality passivation on a textured, boron emitter. Firing at ∼ 750°C was found to result in a threefold improvement in the surface passivation quality with a final J<inf>0</inf> of ∼ 150 fA/cm² being achieved. Additionally, a spin-on boric acid source was used to form a uniform, textured emitter as verified on completed solar cells which did not demonstrate junction shunting. Using a symmetric SiO<inf>2</inf>/SiN<inf>x</inf> stack passivation and screen-printed contacts, a verified n-type cell efficiency of 20.3% was achieved with V<inf>OC</inf> up to 650mV and J<inf>SC</inf> over 40mA/cm². The stability of this structure was also tested, with no degradation being observed over 7 months of dark storage in air.

High-Efficiency n-Type Si Solar Cells With Novel Inkjet-Printed Boron Emitters

Formation of a well-passivated boron emitter for mass production of low-cost and high-efficiency n-type silicon solar cells is a major challenge in the photovoltaic industry. In this letter, we report on a novel and commercially viable method, inkjet printing, to create boron emitters. Phosphorus diffusion was used on the rear to form a back-surface held in conjunction with chemically grown oxide/silicon nitride (SiNx) stack on the front and back for surface passivation. Finally, front and back screen-printed contacts were formed through the dielectric stacks to fabricate large-area (239 cm2) n-type cells. This technology resulted in 19.0%-efficient p+-n-n+ cells with a Voc of 644 mV, a Jsc of 38.6 mA/cm2, and a fill factor of 76.3%. This demonstrates for the hrst time the promise of boron-inkjet-printing technology for low-cost and high-performance n-type Si cells.

Comparison of POCl 3 diffusion with phosphorus ion implantation for Czochralski and Quasi-mono silicon solar cells

Both ion implanted and POCl3 diffused emitters are used for industrial production of p-type Si solar cells. Formation of phosphorus doped emitter is known to perform gettering of impurities, however, gettering efficiency of these two diffusion techniques is not well quantified for single crystal Cz Si and Cast multi or Quasi-mono Si wafers. In addition, ion implantation can provide higher quality emitter with in situ oxide passivation[1]. This paper compares the performance of Quasi-mono and Cz Si cells fabricated with POCl3 and ion-implanted emitters. Quasi-mono wafers have more defects and are expected to benefit from gettering. Large area screen printed p-type industrial cells with full Al-BSF cell structure were fabricated on commercial grade single crystal Cz Si and two different Cast Quasi-mono Si wafers with 50% and 80% mono-crystalline regions. Bulk lifetime was measured to evaluate the gettering efficiency of each technology before and after each emitter formation[2]. POCl3 diffusion gave greater improvement in bulk lifetime of Quasi-mono wafers compared to ion implanted wafers, resulting in 0.7 ~ 1% higher absolute efficiency and over 5 ~ 15mV higher Voc compared to the implanted cells. However, in the case of Cz cells, bulk lifetime remained high and comparable for the two emitters. Therefore, Cz cells with implanted emitter gave 0.4% higher efficiency and 7mV higher Voc due to a higher quality emitter with in situ front oxide passivation.

High efficiency n-type solar cells with screen-printed boron emitters and ion-implanted back surface field

Formation of low-cost boron-doped emitters for mass production of n-type silicon solar cells is a major challenge in the PV industry. In this paper, we report on commercially viable screen printing technology to create boron emitters. A screen-printed boron emitter and phosphorus implanted back surface field were formed simultaneously by a co-annealing process. Front and back surfaces were passivated by chemically-grown oxide/PECVD silicon nitride stack. Front and back contacts were formed by traditional screen printing and firing processes with silver/aluminum grid on front and local silver contacts on the rear. This resulted in 19.3 % high efficient large are (239cm2) n-type solar cells with an open-circuit voltage Voc of 653 mV, short-circuit current density Jsc of 37.7 mA/cm2, and fill factor FF of 78.3 %. Co-diffusion and co-firing reduced the number of processing steps compared to the traditional technologies like BBr3 diffusion. Detailed cell analysis gave a bulk lifetime of over 1 ms, the emitter saturation current density J0e of 101 fA/cm2, and base saturation current density J0b of 259 fA/cm2 respectively. This demonstrates the potential of this novel technology for production of low-cost high-efficiency cells.

Mass production of low-cost screen-printed bifacial N-type Si solar cells with BBr 3 -diffused front emitter and ion-implanted back surface field

Formation of boron (B) emitter and phosphorus back surface field (BSF) is a major challenge in mass production of low-cost and high efficiency n-type Si solar cells. This paper reports on the successful mass production of industrially cost-effective 239cm2 n-type Si solar cells. Conventional BBr3 thermal diffusion formed a heavily B-doped emitter, and the phosphorus BSF was formed by ion-implantation followed an annealing, which simultaneously provides dopant activation and SiO2 growth. For metallization, a conventional screen-printing technique was utilized. This process flow in industrial mass production yielded a median efficiency 20.2% from ~6,000 cells. The champion cell had an efficiency of 20.73% with a Voc of 638mV, JSc of 39.49mA/cm2, and FF of 82.28%.

High Implied Voc (>715 mV) and low emitter saturation current density (∼10fA/cm2) from a lightly B doped implanted emitter

In this paper, we demonstrate a very low emitter saturation current density (J_0e) of ~10 fA/cm² from an implanted lightly doped B emitter (>150 ohm/□) passivated with Al₂O₃/SiNx stack. The test cell structure with lightly B doped emitter passivated with Al₂O₃/SiNx on front and tunnel oxide/n+ poly silicon passivated back gave a high implied V_oc of 715~722 mV on ~5 Ω-cm n-type Cz wafers. It is also shown that Ti/Pd/Ag contact resistance on the lightly doped B emitter was ~2 mΩ-cm², but screen printed Ag/Al contact gave a high contact resistance of 25 mΩ-cm². Therefore, a selective B emitter was used in this study, which gave ~21.0% efficiency with V_oc of 689 mV.

High efficiency n-type silicon solar cell with a novel inkjet-printed boron emitter

Formation of boron emitters for mass production of low-cost and high efficiency n-type silicon solar cells is a major challenge in the PV industry. In this paper, we report on the successful fabrication of high efficiency screen-printed 19.3% n-type silicon cell with Voc of 646 mV, Jsc of 39.4 mA/cm2, and FF of 75.6 %, using boron dopant ink applied by inkjet printing to create boron-doped emitter. The detailed internal quantum efficiency (IQE) analysis showed excellent front surface recombination velocity (FSRV) of 15,000 cm/s and back surface recombination velocity (BSRV) of 66 cm/s. This demonstrates for the first time the promise of boron dopant ink for high performance n-type silicon solar cells.

Study of lifetime degradation in n-type silicon due to oxidation of boron-rich layer

Various boron diffusion techniques are being investigated to fabricate n-type Si cells. Thermal oxidation is often used in photovoltaic to remove boron-rich layer (BRL) formed as a byproduct of boron diffusion because it interferes with surface passivation. However, oxidizing the BRL can cause a severe degradation in bulk lifetime. In this paper, high resolution electron microscopy (HREM) was performed to detect the presence of BRL after B diffusion and its removal after subsequent oxidation. In addition, bulk lifetime of n-type Si with BRL was measured after various oxidation conditions to systematically investigate the BRL-induced lifetime degradation mechanism in n-Si. The primary metal impurity responsible for the bulk lifetime degradation was concluded to be Fe in this study.