>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

Back-Contacted Silicon Heterojunction Solar Cells: Optical-Loss Analysis and Mitigation

We analyze the optical losses that occur in interdigitated back-contacted amorphous/crystalline silicon heterojunction solar cells. We show that in our devices, the main loss mechanisms are similar to those of two-side contacted heterojunction solar cells. These include reflection and escape-light losses, as well as parasitic absorption in the front passivation layers and rear contact stacks. We then provide practical guidelines to mitigate such reflection and parasitic absorption losses at the front side of our solar cells, aiming at increasing the short-circuit current density in actual devices. Applying these rules, we processed a back-contacted silicon heterojunction solar cell featuring a short-circuit current density of 40.9 mA/cm(2) and a conversion efficiency of 22.0%. Finally, we show that further progress will require addressing the optical losses occurring at the rear electrodes of the back-contacted devices

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%

Silane plasma diagnostics for high-efficiency silicon heterojunction solar cells

In silicon heterojunction solar cells, the passivation of the crystalline silicon wafer surfaces and fabrication of emitter and back surface field are all performed by intrinsic and doped amorphous silicon thin layers, usually deposited by plasma-enhanced chemical vapor deposition (PECVD). By using in-situ diagnostics during PECVD, it is found that the passivation quality of such layers directly relate to the plasma conditions, especially on the silane depletion fraction. Good interface passivation is indeed 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 Voc’s up to 727 mV and aperture efficiencies up to 20.7% on 4 cm2 cells

Optimization of high efficiency silicon heterojunction solar cells using silane-plasma diagnostics

In silicon heterojunction solar cells, the passivation of the crystalline silicon wafer surfaces and fabrication of emitter and back surface field are all performed by intrinsic and doped amorphous silicon thin layers, usually deposited by plasma-enhanced chemical vapor deposition (PECVD). Since the properties of materials deposited by PEVCD are directly linked to the plasma properties, plasma diagnostics are very useful tools to optimize such devices. A novel diagnostic has been developed to measure in-situ the molecular silane depletion fraction in the plasma during deposition. It is found that the silane depletion strongly depends on the process parameters, and appears to be a relevant parameter for the quality of the passivating layers. Good passivation is indeed obtained from highly depleted silane plasmas. Based on this, layers deposited in a large-area PECVD reactor working at very high frequency (40.68 MHz) were optimized for heterojunction solar cells. All other fabrication steps were also fully industry compatible, using sputtering for transparent conductive oxide layers and screenprinting for the front grid. The best 2 x 2 cm2 cell shows a high open-circuit voltage of 717 mV, yielding a conversion efficiency of 20.3% (aperture area). Keywords: Heterojunction, PECVD, High-Efficienc

Silicon heterojunction solar cells on n- and p-type wafers with efficiencies above 20%

A systematic comparison of front- and rear-emitter silicon heterojunction solar cells produced on nand p-type wafers was performed, in order to investigate their potential and limitations for high efficiencies. Cells on p-type wafers suffer from reduced minority carrier lifetime in the low-carrier-injection range, mainly due to the asymmetry in interface defect capture cross sections. This leads to slightly lower fill factors than for n-type cells. However, these losses can be minimized by using high-quality passivation layers. High Vocs were obtained on both types of FZ wafers: up to 735 mV on n- and 726 mV on p-type. The best Voc measured on CZ p-type wafers was only 692 mV, whereas it reached 732 mV 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, demonstrating that heterojunction schemes can perform almost as well on high-quality p-type as on ntype wafers. To our knowledge, this is the highest efficiency 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

21% efficiency silicon heterojunction solar cells produced with very high frequency PECVD

Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition (PECVD). By using in-situ plasma diagnostics and ex-situ film characterization, we show that the best a-Si:H films for passivation are produced from deposition regimes close to the amorphous-to-crystalline transition. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells. 4 cm2 solar cells were produced with fully industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%

Micromorph thin-film silicon solar cells with transparent high-mobility hydrogenated indium oxide front electrodes

We investigate the performance of hydrogenated indium oxide as a transparent front electrode for micromorph thin-film silicon solar cells on glass. Light trapping is achieved by replicating the morphology of state-of-the-art zinc oxide electrodes, known for their outstanding light trapping properties, via ultraviolet nanoimprint lithography. As a result of the high electron mobility and excellent near-infrared transparency of hydrogenated indium oxide, the short-circuit current density of the cells is improved with respect to indium tin oxide and zinc oxide electrodes. We assess the potential for further current gains by identifying remaining sources of parasitic absorption and evaluate the light trapping capacity of each electrode. We further present a method, based on nonabsorbing insulating silicon nitride electrodes, allowing one to directly relate the optical reflectance to the external quantum efficiency. Our method provides a useful experimental tool to evaluate the light trapping potential of novel photonic nanostructures by a simple optical reflectance measurement, avoiding complications with electrical cell performance

Nanoimprint Lithography for High-Efficiency Thin-Film Silicon Solar Cells

We demonstrate high-efficiency thin-film silicon solar cells with transparent nanotextured front electrodes fabricated via ultraviolet nanoimprint lithography on glass substrates. By replicating the morphology of state-of-the-art nanotextured zinc oxide front electrodes known for their exceptional light trapping properties, conversion efficiencies of up to 12.0% are achieved for micromorph tandem junction cells. Excellent light incoupling results in a remarkable summed short-circuit current density of 25.9 mA/cm2 for amorphous top cell and microcrystalline bottom cell thicknesses of only 250 and 1100 nm, respectively. As efforts to maximize light harvesting continue, our study validates nanoimprinting as a versatile tool to investigate nanophotonic effects of a large variety of nanostructures directly on device performance