Stuart Bowden

Interdigitated back contact silicon heterojunction solar cell and the effect of front surface passivation

This letter reports interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells which combine the performance benefits of both back contact and heterojunction technologies while reducing their limitations. Low temperature (<200°C) deposited p- and n-type amorphous silicon used to form interdigitated heteroemitter and contacts in the rear preserves substrate lifetime while minimizes optical losses in the front. The IBC-SHJ structure is ideal for diagnosing surface passivation quality, which is analyzed and measured by internal quantum efficiency and minority carrier lifetime measurements. Initial cells have independently confirmed efficiency of 11.8% under AM1.5 illumination. Simulations indicate efficiencies greater than 20% after optimization.

Surface passivation and heterojunction cells on Si (100) and (111) wafers using dc and rf plasma deposited Si:H thin films

The structure of hydrogenated silicon (Si:H) films deposited by rf and dc plasma process on Si (100) and (111) wafers is correlated with the surface passivation quality and heterojunction cell performance. Microstructural defects associated with SiH2 bonding and apparent ion bombardment in dc plasmas have little or no adverse effect on passivation or cell properties, while presence of crystallinity in Si:H i layer severely deteriorates surface passivation and cell open circuit voltage (Voc). Excellent surface passivation (lifetime of >1ms) and high efficiency cells (>18%) with Voc of 694mV are demonstrated on n-type textured Czochralski wafer using dc plasma process.

50% Efficient Solar Cell Architectures and Designs

Very high efficiency solar cells (VHESC) for portable applications that operate at greater than 55 percent efficiency in the laboratory and 50 percent in production are being created. We are integrating the optical design with the solar cell design, and have entered previously unoccupied design space that leads to a new paradigm. This project requires us to invent, develop and transfer to production these new solar cells. Our approach is driven by proven quantitative models for the solar cell design, the optical design and the integration of these designs. We start with a very high performance crystalline silicon solar cell platform. Examples will be presented. Initial solar cell device results are shown for devices fabricated in geometries designed for this VHESC program

Determining lifetime in silicon blocks and wafers with accurate expressions for carrier density

In recent years, many studies of silicon minority-carrier lifetime have been performed with quasi-steady-state photoconductance measurements. The method has been used to characterize the defect levels, surface passivation, and trapping effects in wafers by using absolutely calibrated data for minority-carrier lifetime versus minority-carrier injection level. This paper generalizes the quasi-steady-state photoconductance technique for use in thick wafers or blocks of silicon where the diffusion length or light absorption depth is much less than the sample thickness. The photogeneration and carrier diffusion profiles are calculated for special cases of interest. The measured effective lifetimes can then be used to estimate the bulk lifetime in the material, and report this lifetime at an appropriate average minority-carrier density for the measurement conditions. In this way, the results and measurement methodologies previously developed for use on wafers can be applied to single- or multi-crystalline silic...

High effective minority carrier lifetime on silicon substrates using quinhydrone-methanol passivation

Iodine-methanol (I2/ME), a chemical passivation method, is extensively used in silicon (Si) solar cell fabrication for measuring minority carrier lifetime in bulk regions. We demonstrate that quinhydrone-methanol (QHY/ME) provides higher lifetimes than I2/ME. For 0.01u2002mol/dm−3 QHY/ME on float-zone (FZ) wafers at 1×1015u2002cm−3 injection level, a high lifetime of 3.3 ms and surface recombination velocity of 7 cm/sec on n-type (100u2002Ωu2009cm) and 1.1 ms on p-type (2u2002Ωu2009cm) is reported. The surface recombination velocity is also measured out of solution for several days. Chemical characterization results indicate increasing surface oxidation with decreasing passivation, consistent with proposed bonding mechanism.

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 >750u2009mV open circuit voltage at 1 Sun illumination providing very good surface passivation is present. 753u2009mV 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 >760u2009mV open circuit voltage in silicon heterojunction solar cells are discussed.

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 60u2009ms on 5000 Ω-cm and 20.9u2009ms on 1.7 Ω-cm mirror polished float zone (FZ) material passivated with aSON stacks.

Optimization of interdigitated back contact silicon heterojunction solar cells by two-dimensional numerical simulation

In this paper, two-dimensional (2D) simulation of interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells is presented using Sentaurus Device, a software package of Synopsys TCAD. A model is established incorporating a distribution of trap states of amorphous-silicon material and thermionic emission across the amorphous-silicon / crystalline-silicon hetero-interface. The 2D nature of IBC-SHJ device is evaluated and current density-voltage (J-V) curves are generated. Optimization of IBC-SHJ solar cells is then discussed through simulation. It is shown that the open circuit voltage (VOC) and short circuit current density (JSC) of IBC-SHJ solar cells increase with decreasing front surface recombination velocity. The JSC improves further with the increase of relative coverage of p-type emitter contacts, which is explained by the simulated and measured position dependent laser beam induced current (LBIC) line scan. The S-shaped J-V curves with low fill factor (FF) observed in experiments are also simulated, and three methods to improve FF by modifying the intrinsic a-Si buffer layer are suggested: (i) decreased thickness, (ii) increased conductivity, and (iii) reduced band gap. With all these optimizations, an efficiency of 26% for IBC-SHJ solar cells is potentially achievable.

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

High quality surface passivation (Seffu2009 ±8 × 1012u2009cm−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.

Stability of amorphous/crystalline silicon heterojunctions

Silicon heterojunction solar cells have demonstrated high efficiencies through excellent surface passivation. However, the use of amorphous silicon (a-Si) in the structure raises the question of long term stability as a-Si solar cells suffer degradation due to the Staebler-Wronski effect. The stability of a-Si in terms of its ability to passivate silicon wafers is evaluated by measuring the minority carrier lifetime and it is found to be unstable over time. The instability is most evident in thin, ∼ 10 nm, intrinsic a-Si only layers where lifetime falls from over 1ms immediately after annealing, to 300 μs one month later. The change in lifetime occurs even in the dark at room temperature with the largest fall in the first few days after deposition. The lifetime can be recovered close to its initial level by annealing the sample in air, indicating that the lifetime change is not due to a simple oxidation of the outer layer of a-Si. However, device measurements where an intrinsic/doped a-Si structure is used for the emitter and contact show no change in open circuit voltage or fill factor after one year. The changes in the passivation during processing has important implications for the sequence of device fabrication, the use of lifetime testing as a diagnostic tool and the passivation of all back contact devices.