High-efficiency Silicon Heterojunction Solar Cells: A Review

Silicon heterojunction solar cells consist of thin amorphous silicon layers deposited on crystalline silicon wafers. This design enables energy conversion efficiencies above 20% at the industrial production level. The key feature of this technology is that the metal contacts, which are highly recombination active in traditional, diffused-junction cells, are electronically separated from the absorber by insertion of a wider bandgap layer. This enables the record open-circuit voltages typically associated with heterojunction devices without the need for expensive patterning techniques. This article reviews the salient points of this technology. First, we briefly elucidate device characteristics. This is followed by a discussion of each processing step, device operation, and device stability and industrial upscaling, including the fabrication of solar cells with energy-conversion efficiencies over 21%. Finally, future trends are pointed ou

Kinetics of a-Si:H bulk defect and a-Si:H/c-Si interface-state reduction

Low-temperature annealing of hydrogenated amorphous silicon (a-Si:H) is investigated. An identical energy barrier is found for the reduction of deep defects in the bulk of a-Si:H films and at the interface such layers form with crystalline Si (c-Si) surfaces. This finding gives direct physical evidence that the defects determining a-Si:H/c-Si interface recombination are silicon dangling bonds and that also kinetically this interface has no unique features compared to the a-Si:H bulk

Impact of TCO Microstructure on the Electronic Properties of Carbazole-based Self-Assembled Monolayers

Carbazole-based self-assembled monolayers (PACz-SAMs), anchored via their phosphonic acid group on a transparent conductive oxide (TCO) have demonstrated excellent performance as hole-selective layers in inverted perovskite solar cells. However, the influence of the TCO microstructure on the work function (WF) shift after SAM anchoring as well as the WF variations at the micro/nanoscale have not been extensively studied yet. Herein, we investigate the effect of the Sn-doped In2O3 (ITO) microstructure on the WF distribution upon 2PACz-SAMs and NiOx/2PACz-SAMs application. For this, ITO substrates with amorphous and polycrystalline (featuring either nanoscale or microscale-sized grains) microstructures are studied. A correlation between the ITO grain orientation and 2PACz-SAMs local potential distribution was found via Kelvin probe force microscopy and electron backscatter diffraction. These variations vanish for amorphous ITO or when adding an amorphous NiOx buffer layer, where a homogeneous surface potential distribution is mapped. Ultraviolet photoelectron spectroscopy confirmed the ITO WF increase after 2PACz-SAMs deposition. Considering the importance of polycrystalline TCOs as high mobility and broadband transparent electrodes, we provide insights to ensure uniform WF distribution upon application of hole transport SAMs, which is critical towards enhanced device performance.Comment: 18 pages, 5 figure

Very fast light-induced degradation of a-Si:H/c-Si(100) interfaces

Light-induced degradation (LID) of crystalline silicon (c-Si) surfaces passivated with hydrogenated amorphous silicon (a-Si:H) is investigated. The initial passivation decays on polished c-Si(100) surfaces on a time scale much faster than usually associated with bulk a-Si:H LID. This phenomenon is absent for the a-Si:H/c-Si(111) interface. We attribute these differences to the allowed reconstructions on the respective surfaces. This may point to a link between the presence of so-called "fast" states and (internal) surface reconstruction in bulk a-Si:H

Amorphous silicon passivated contacts for diffused junction silicon solar cells

Carrier recombination at the metal contacts is a major obstacle in the development of high-performance crystalline silicon homojunction solar cells. To address this issue, we insert thin intrinsic hydrogenated amorphous silicon [a-Si:H(i)] passivating films between the dopant-diffused silicon surface and aluminum contacts. We find that with increasing a-Si:H(i) interlayer thickness (from 0 to 16 nm) the recombination loss at metal-contacted phosphorus (n +) and boron (p+) diffused surfaces decreases by factors of ∼25 and ∼10, respectively. Conversely, the contact resistivity increases in both cases before saturating to still acceptable values of ∼ 50 mΩ cm2 for n+ and ∼100 mΩ cm2 for p+ surfaces. Carrier transport towards the contacts likely occurs by a combination of carrier tunneling and aluminum spiking through the a-Si:H(i) layer, as supported by scanning transmission electron microscopy-energy dispersive x-ray maps. We explain the superior contact selectivity obtained on n+ surfaces by more favorable band offsets and capture cross section ratios of recombination centers at the c-Si/a-Si:H(i) interface

Damage at hydrogenated amorphous/crystalline silicon interfaces by indium tin oxide overlayer sputtering

Damage of the hydrogenated amorphous/crystalline silicon interface passivation during transparent conductive oxide sputtering is reported. This occurs in the fabrication process of silicon heterojunction solar cells. We observe that this damage is at least partially caused by luminescence of the sputter plasma. Following low-temperature annealing, the electronic interface properties are recovered. However, the silicon-hydrogen configuration of the amorphous silicon film is permanently changed, as observed from infra-red absorbance spectra. In silicon heterojunction solar cells, although the as-deposited film’s microstructure cannot be restored after sputtering, no significant losses are observed in their open-circuit voltag

Analysis of lateral transport through the inversion layer in amorphous silicon/crystalline silicon heterojunction solar cells

In amorphous/crystalline silicon heterojunction solar cells, an inversion layer is present at the front interface. By combining numerical simulations and experiments, we examine the contribution of the inversion layer to lateral transport and assess whether this layer can be exploited to replace the front transparent conductive oxide (TCO) in devices. For this, heterojunction solar cells of different areas (2 x 2, 4 x 4, and 6 x 6 mm(2)) with and without TCO layers on the front side were prepared. Laser-beam-induced current measurements are compared with simulation results from the ASPIN2 semiconductor simulator. Current collection is constant across millimeter distances for cells with TCO; however, carriers traveling more than a few hundred microns in cells without TCO recombine before they can be collected. Simulations show that increasing the valence band offset increases the concentration of holes under the surface of n-type crystalline silicon, which increases the conductivity of the inversion layer. Unfortunately, this also impedes transport across the barrier to the emitter. We conclude that the lateral conductivity of the inversion layer may not suffice to fully replace the front TCO in heterojunction devices. (C) 2013 AIP Publishing LLC

>21% Efficient Silicon Heterojunction Solar Cells on n- and p-Type Wafers Compared

The properties and high-efficiency potential of frontand rear-emitter silicon heterojunction solar cells on n- and p-type wafers were experimentally investigated. In the low-carrierinjection range, cells on p-type wafers suffer from reduced minority carrier lifetime, mainly due to the asymmetry in interface defect capture cross sections. This leads to slightly lower fill factors than for n-type cells. By using high-quality passivation layers, however, these losses can be minimized. High open-circuit voltages (Voc s) were obtained on both types of float zone (FZ) wafers: up to 735mV on n-type and 726mV on p-type. The best Voc measured on Czochralski (CZ) p-type wafers was only 692mV, whereas it reached 732mV on CZ n-type. The highest aperture-area certified efficiencies obtained on 4 cm2 cells were 22.14% (Voc = 727 mV, FF = 78.4%) and 21.38% (Voc = 722 mV, FF = 77.1%) on n- and p-type FZ wafers, respectively, proving that heterojunction schemes can perform almost as well on high-quality p-type as on n-type wafers. To our knowledge, this is the highest efficiency ever reported for a full silicon heterojunction solar cell on a p-type wafer, and the highest Voc on any p-type crystalline silicon device with reasonable FF

A-Si:H/c-Si heterojunctions: a future mainstream technology for high-efficiency crystalline silicon solar cells ?

In this contribution, we shortly review the main features of amorphous/crystalline silicon heterojunction (SHJ) solar cells, including interface defects and requirements for high quality interfaces. We show how a process flow with a limited number of process steps leads to screen printed solar cells of 2x2cm(2) with 21.8% efficiency and of 10x10cm(2) with 20.9% efficiency (n-type FZ). We show that the devices work in high injection conditions of 3x10(15)cm(-3) at the maximum power point, a factor two higher than the base doping. Several research labs and companies can now produce large area 6 '' cells well over 20% on CZ wafers and some of the critical cost factors, such a metallization can be overcome with suitable strategies. Based on the high quality coating tools and processes developed for thin films used for flat panel display or thin film solar cell coatings, the deposition of the layers required to make SHJ cells has the potential to be performed in a controlled way at low cost. Considering the few process steps required, the high quality n-type Cz wafers that can be obtained by proper crystal growth control, SHJ technology has several assets that could make it become a widespread PV technology