Stanislau Y. Herasimenka

750 mV open circuit voltage measured on 50 μm thick silicon heterojunction solar cell

This paper presents experimental evidence that silicon solar cells can achieve >750 mV open circuit voltage at 1 Sun illumination providing very good surface passivation is present. 753 mV local open circuit voltage was measured on a 50 μm thick non-metalized silicon heterojunction solar cell. The paper also considers a recombination model at open circuit based on the recent Auger and radiative recombination parameterization and the measured surface saturation current density. The loss mechanisms at open circuit and several practical pathways to achieve >760 mV open circuit voltage in silicon heterojunction solar cells are discussed.

Inter-laboratory study of eddy-current measurement of excess-carrier recombination lifetime

Excess-carrier recombination lifetime is a key parameter in silicon solar cell design and production. With the vast international use and recent standardization (SEMI PV13) of eddy-current wafer and brick silicon lifetime test instruments, it is important to quantify the inter- and intra-laboratory repeatability. This paper presents results of an international inter-laboratory study conducted with 24 participants to determine the precision of the SEMI PV13 eddy-current carrier lifetime measurement test method. Overall, the carrier recombination lifetime between-laboratory reproducibility was found to be within ±11% for quasi-steady-state (QSS) mode and ±8% for transient mode for wafer samples and within ±4% for bulk samples.

Surface passivation of n-type c-Si wafers by a-Si/SiO2/SiNx stack with <1 cm/s effective surface recombination velocity

The passivation quality of an a-Si/SiO2/SiNx (aSON) stack deposited by conventional PECVD at 60 ms on 5000 Ω-cm and 20.9 ms on 1.7 Ω-cm mirror polished float zone (FZ) material passivated with aSON stacks.

Explanation of the device operation principle of amorphous silicon/ crystalline silicon heterojunction solar cell and role of the inversion of crystalline silicon surface

The device operation principle of amorphous silicon/crystalline silicon heterojunction solar cell is discussed. The band diagram obtained by the computer model developed in the commercial simulator Sentaurus shows that the c-Si surface is inverted at the interface between a-Si and c-Si (heterointerface). A strong inversion gives a strong electric field at the c-Si surface, which in turn facilitates the transport of minority carriers across the heterointerface. A high performance device requires a strongly inverted c-Si surface. Calculations are performed to show that the doping of the doped a-Si layer, the thickness of the intrinsic layer, and the defect state density at the heterointerface all affect the inversion of the crystalline silicon surface. Unlike homojunction devices, the defects in heterojunction devices have a greater role in transport mechanism than in recombination mechanism. The results show that in devices with a large number of defects at the interface, the fill factor degrades with little change in open circuit voltage. This explains why it is relatively easy to obtain VOCs approaching 700 mV with heterojunctions but often with low fill factors.

Manipulation of K center charge states in silicon nitride films to achieve excellent surface passivation for silicon solar cells

High quality surface passivation (Seff  ±8 × 1012 cm−2) into a dual layer stack of Plasma Enhanced Chemical Vapor Deposition (PECVD) Silicon Nitride (SiNx)/PECVD Silicon Oxide (SiO2) films using a corona charging tool. We demonstrate long term stability and uniform charge distribution in the SiNx film by manipulating the charge on K center defects while negating the requirement of a high temperature thermal oxide step.

2D modeling of Silicon Heterojunction Interdigitated Back Contact solar cells

Silicon Heterojunction Interdigitated Back Contact (SHJ-IBC) solar cells were studied by two dimensional modeling using Sentaurus TCAD tools. It was shown that low fill factor caused by the S-shape behavior of experimental J-V curves of standard interdigitated back contact cells can be recovered by making small openings in the intrinsic buffer layer. The small openings in the buffer layer also substantially reduce the influence of the relative dimensions of the silicon strips as when compared to cells with a continuous buffer layer.

A simplified process flow for silicon heterojunction interdigitated back contact solar cells: Using shadow masks and tunnel junctions

A novel process flow, which can allow the formation of interdigitated p- and n-type a-Si strips and corresponding transparent conductive oxide (TCO) and metal layers for silicon heterojunction interdigitated back contact (SHJ-IBC) solar cells using only a single alignment step and without using any resist patterning is presented. The flow is based on the deposition of a-Si, TCO and metal layers through a stack of shadow masks. Three variation of the flow are described. Several key process components to include a-Si deposition and H2 plasma etch through the shadow mask are demonstrated and described.

Development of Cu plating for silicon heterojunction solar cells

This paper reports the results of the study comparing various patterning and plating methods for the deposition of Cu electrodes on transparent conductive oxides for silicon heterojunction solar cells. We compared direct electroplating of Cu on different metal seeds (Ag, Ni, Cr and Ti deposited on transparent conductive oxide by physical vapor deposition) to the light induced plating of Ni/Cu directly on transparent conductive oxide. Patterning was done either using photoresists (formed by spin-on, screen printing or lamination) or lift-off of the PECVD dielectric using screen printed resist. The geometry of the fingers, line resistance, contact resistance and adhesion were used as comparative parameters. We identified direct electroplating of Cu on the sputtered Ag seed to achieve the lowest contact resistance and the best adhesion. All photoresists were able to achieve less than 60 micron resolution and could produce the fingers with the sought height (some, however, having a characteristic mushroom shape). The best silicon heterojunction cell with Cu contacts directly electroplated on the sputtered Ag seed achieved 21.9% efficiency on 153 cm2 area.

Analysis of the recombination mechanisms of a silicon solar cell with low bandgap-voltage offset

The mathematical dependence of bandgap-voltage offset on Auger and radiative recombination is derived. To study the recombination near the intrinsic limit, we manufacture thin silicon heterojunction test structures designed to minimize surface recombination, and to measure voltages and effective lifetimes near the Auger and radiative limit. Open-circuit voltages over 760 mV were measured on 50-μm-thick structures, leading to bandgap-voltage offsets at open-circuit down to 0.35 V. The Auger and radiative recombination represents over 90% of the recombination at open-circuit. This dominance also holds at the maximum power point, giving pseudo-fill factors of 86%. We demonstrate the potential of thin silicon devices to reach high voltages, and bandgap-voltage offsets in line with the best reported for direct bandgap materials such as gallium indium phosphide and gallium arsenide.

Series connection front-to-front and back-to-back of silicon heterojunction solar cells

Alternating cells with p- and n-type emitters enables direct series connection of equivalent sides, i.e. front-to-front and back-to-back connection of adjacent cells. The challenge is to match the current of cells with p- and n-type emitters. The electrical properties of silicon heterojunction solar cells with front and rear junctions are remarkably similar. The short-circuit current density mismatch between front and rear junction cells is as low as 0.1 mAcm-2. The cells are connected using thin indium coated wires. One-cell and two-cells modules were manufactured, and efficiencies up to 21.2% were reached for one-cell modules. Electroluminescence of the two-cells module is a good indication about the quality of the direct series connection between front and rear junction cells.