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

Demonstrating the high Voc potential of PEDOT:PSS/c-Si heterojunctions on solar cells

In this study, we demonstrate the high surface passivation quality of PEDOT:PSS/c-Si junctions for the first time on solar cell level, reaching a record high Voc value of 688 mV after full-area metallization of the PEDOT:PSS. We achieve this by combining the PEDOT:PSS hole-selective layer at the rear of the crystalline silicon wafer with a well-passivating electron-selective a-Si:H(i/n) layer stack at the front. Our results clearly prove the excellent hole selectivity of PEDOT:PSS on crystalline silicon. © 2017 The Authors. Published by Elsevier Ltd

Effect of calcium-channel blockers on calcium—phosphate metabolism in patients with end-stage renal disease

Background After EDTA-induced hypocalcaemia, healthy volunteers treated with diltiazem display more severe hyperparathyroidism than subjects on felodipine studied under identical conditions. Therefore patients with end-stage renal disease (ESRD) and severe secondary hyperparathyroidism might be particularly sensitive to this side-effect. Methods To test this hypothesis, seven patients with ESRD on chronic haemodialysis (3 women and 4 men) with serum levels of intact PTH ranging from 204 to 675 pg/ml were studied both before and during the first 180 min of haemodialysis against a dialysate with low calcium concentration (0.75 mmol/1, n=6 and 1 mmol/1, n=1) under the following three experimental conditions: control, felodipine (10 mg/day) and diltiazem (120 mg b.i.d.). Results At onset of dialysis, plasma phosphorus level was higher on diltiazem (2.03±0.08 mM) than on felodipine (1.64±0.10, P<0.02), and on the latter it was lower than in control condition (1.88±0.16, P<0.02). As a probable consequence, blood ionized calcium concentration was lower on diltiazem (1.14 mM±0.02, mean±SEM) than on felodipine (1.2±0.03, P<0.05) or in control condition (1.17±0.01, NS). There was a trend for intact PTH to be higher on diltiazem (324±47 pg/ml) than on felodipine (246±55) or in control condition (305±49) and 1,25-dihydroxyvitamin D was higher indeed on diltiazem (6.70±0.92 pg/ml) than on felodipine (4.75±0.91, P<0.02) or control (3.87±0.62, P<0.05). Area under the curve PTH over the first 60 min of dialysis was higher by 16±7% on diltiazem than on felodipine (P<0.05). Conclusions While on diltiazem rather than on felodipine, patients with ESRD display higher plasma phosphorus levels, and slightly aggravate the degree of severity of hyperparathyroidism recorded during haemodialysis against low-calcium dialysate. The longterm effect of this new observation remains to be evaluate

Empirical comparison of high gradient achievement for different metals in DC and pulsed mode

For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an UV laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a DC breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. In addition high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns FWHM, 500 kV electron gun and tested for high gradient breakdown and for quantum efficiency using an ultra-violet laser.Comment: 25 pages, 13 figures, 5 tables. Follow up from FEL 2008 conference (Geyongju Korea 2008) New Title in JVST A (2010) : Vacuum breakdown limit and quantum efficiency obtained for various technical metals using DC and pulsed voltage source

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

>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

The silane depletion fraction as an indicator for the amorphous/crystalline silicon interface passivation quality

In silicon heterojunction solar cells, thin amorphous silicon layers passivate the crystalline silicon wafer surfaces. By using in situ diagnostics during plasma-enhanced chemical vapor deposition (PECVD), the authors report how the passivation quality of such layers directly relate to the plasma conditions. Good interface passivation is obtained from highly depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells, yielding aperture efficiencies up to 20.3% on 4 cm

Infrared light management in high-efficiency silicon heterojunction and rear-passivated solar cells

Silicon heterojunction solar cells have record-high open-circuit voltages but suffer from reduced short-circuit currents due in large part to parasitic absorption in the amorphous silicon, transparent conductive oxide (TCO), and metal layers. We previously identified and quantified visible and ultraviolet parasitic absorption in heterojunctions; here, we extend the analysis to infrared light in heterojunction solar cells with efficiencies exceeding 20%. An extensive experimental investigation of the TCO layers indicates that the rear layer serves as a crucial electrical contact between amorphous silicon and the metal reflector. If very transparent and at least 150 nm thick, the rear TCO layer also maximizes infrared response. An optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why: parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector, and this parasitic absorption in the metal can exceed the absorption in the TCO layer itself. Thick TCO layers-or dielectric layers, in rear-passivated diffused-junction silicon solar cells-reduce the penetration of the evanescent waves to the metal, thereby increasing internal reflectance at the rear surface. With an optimized rear TCO layer, the front TCO dominates the infrared losses in heterojunction solar cells. As its thickness and carrier density are constrained by anti-reflection and lateral conduction requirements, the front TCO can be improved only by increasing its electron mobility. Cell results attest to the power of TCO optimization: With a high-mobility front TCO and a 150-nm-thick, highly transparent rear ITO layer, we recently reported a 4-cm(2) silicon heterojunction solar cell with an active-area short-circuit current density of nearly 39 mA/cm(2) and a certified efficiency of over 22%. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772975

Current Losses at the Front of Silicon Heterojunction Solar Cells

The current losses due to parasitic absorption in the indium tin oxide (ITO) and amorphous silicon (a-Si:H) layers at the front of silicon heterojunction solar cells are isolated and quantified. Quantum efficiency spectra of cells in which select layers are omitted reveal that the collection efficiency of carriers generated in the ITO and doped a-Si:H layers is zero, and only 30% of light absorbed in the intrinsic a-Si:H layer contributes to the shortcircuit current. Using the optical constants of each layer acquired from ellipsometry as inputs in a model, the quantum efficiency and short-wavelength current loss of a heterojunction cell with arbitrary a-Si:H layer thicknesses and arbitrary ITO doping can be correctly predicted. A 4 cm2 solar cell in which these parameters have been optimized exhibits a short-circuit current density of 38.1 mA/cm2 and an efficiency of 20.8%