The efficiency of silicon solar cells has a large influence on the cost of most photovoltaics panels. Here, researchers from Kaneka present a silicon heterojunction with interdigitated back contacts reaching an efficiency of 26.3% and provide a detailed loss analysis to guide further developments.

Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26

The crystalline silicon heterojunction structure adopted in photovoltaic modules commercialized as Panasonic's HIT has significantly reduced recombination loss, resulting in greater conversion efficiency. The structure of an interdigitated back contact was adopted with our crystalline silicon heterojunction solar cells to reduce optical loss from a front grid electrode, a transparent conducting oxide (TCO) layer, and a-Si:H layers as an approach for exceeding the conversion efficiency of 25%. As a result of the improved short-circuit current (J
 sc
), we achieved the world's highest efficiency of 25.6% for crystalline silicon-based solar cells under 1-sun illumination (designated area: 143.7 cm
 2
).

Achievement of More Than 25% Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell

Excellent surface passivation of c-Si has been achieved by Al2O3 films prepared by plasma-assisted atomic layer deposition, yielding effective surface recombination velocities of 2 and 13cm∕s on low resistivity n- and p-type c-Si, respectively. These results obtained for ∼30nm thick Al2O3 films are comparable to state-of-the-art results when employing thermal oxide as used in record-efficiency c-Si solar cells. A 7nm thin Al2O3 film still yields an effective surface recombination velocity of 5cm∕s on n-type silicon.

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Ultralow surface recombination of c-Si substrates passivated by plasma-assisted atomic layer deposited Al2O3

The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.

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Status and prospects of Al2O3-based surface passivation schemes for silicon 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 short-circuit 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%.

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Current Losses at the Front of Silicon Heterojunction Solar Cells

We present back-contacted amorphous/crystalline silicon heterojunction solar cells (IBC-SHJ) on n-type substrates with fill factors exceeding 78% and high current densities, the latter enabled by a SiNx /SiO2 passivated phosphorus-diffused front surface field. Voc calculations based on carrier lifetime data of reference samples indicate that for the IBC architecture and the given amorphous silicon layer qualities an emitter buffer layer is crucial to reach a high Voc, as known for both-side contacted silicon heterojunction solar cells. A back surface field buffer layer has a minor influence. We observe a boost in solar cell Voc of 40 mV and a simultaneous fill factor reduction introducing the buffer layer. The aperture-area efficiency increases from 19.8 ± 0.4% to 20.2 ± 0.4%. Both, efficiencies and fill factors constitute a significant improvement over previously reported values. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient interdigitated back-contacted silicon heterojunction solar cells

The shortage of the p-type silicon (Si) feedstock and the high minority carrier lifetimes in multicrystalline (mc) n-type Si reported by different authors ([1]-[3]) make n-type mc-Si solar cell fabrication more and more interesting. Given the high electronic quality of the material – that is confirmed in our studies again – the task remains to develop an adapted solar cell process. A key feature of the concept presented here is the BBr3diffused emitter on the front side and the surface passivation of this emitter. We show that BBr3 emitter-diffusion is possible without degradation of the high initial carrier lifetimes in the n-type mc-Si material on contrary the diffusion even improves the average lifetime to a large extend. SiO2 provides an excellent surface passivation of the p-Si surface. Application of PECVD SiNx resulted in a decrease of the (implied) Voc measured on lifetime teststructures as well as on solar cell level. As an alternative, a low temperature surface passivation process by PECVD SiCx is investigated. First trials resulted in a very promising value for the emitter saturation current Joe: 180 fA/cm for a 90 Ω/sq emitter. N-type Si solar cells with SiO2-passivated BBr3-emitter were processed in laboratory scale (area of 4 cm) with an efficiency of 15.2% on mc and 16.4% on Cz-Si. With an industrial screen printing process 14.1% and 14.8% were obtained on an area of 12.5 x 12.5 cm on n-type mc-Si and Cz-Si respectively.

N-type multicrystalline silicon solar cells : BBr3-diffusion and passivation of p+-diffused silicon surfaces

We present an experimental method to quantify the series resistance R
 a-Si/ITO
 through the a-Si:H layers and the a-Si:H/ITO interface on test structures In order to optimize R
 a-Si/ITO
, we apply different a-Si:H and ITO deposition parameters We find the best value for R
 (p)-a-Si/ITO
 of 042 Ω·cm
 2
 for an ITO double layer with a 10-nm-thin starting layer that provides good contact resistance and an additional 90-nm top layer that provides good conductivity For R
 (n)-a-Si/ITO
, we reach values below 01 Ω·cm
 2
 We present an analysis of the series resistance and shading losses of our 100-cm
 2
 bifacial screen-printed a-Si:H/cSi heterojunction solar cells, which show an open-circuit voltage of V
 oc
 = 733 mV, demonstrating the excellent level of interface passivation The efficiency of 202% is limited by a low short-circuit current density of 371 mA/cm
 2
 and fill factor of 76%

Analysis of Series Resistance Losses in a-Si:H/c-Si Heterojunction Solar Cells

We demonstrate the processing of a heterojunction solar cell from a purely macroporous silicon (MacPSi) absorber that is generated and separated from a monocrystalline n-type Cz silicon wafer by means of electrochemical etching. The etching procedure results in straight pores with a diameter of (4.7 ± 0.2) µm and a distance of 8.3 µm. An intrinsic amorphous Si (a-Si)/p+-type a-Si/indium tin oxide (ITO) layer stack is on the front side and an intrinsic a-Si/n+-type a-Si/ITO layer stack is on the rear side. The pores are open when depositing the layers onto the 3.92 cm2-sized cell. The conductive layers do not cause shunting through the pores. A silicon oxide layer passivates the pore walls. The energy-conversion efficiency of the (33 ± 2) µm thick cell is 7.2%. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thin macroporous silicon heterojunction solar cells

Surface passivation of p-type crystalline silicon wafers by means of phosphorus-doped hydrogenated amorphous silicon carbide films [a-SiCx(n):H] has been investigated. Particularly, we focused on the effects of layer thickness on the c-Si surface passivation quality resulting in the determination of the fixed charge density, Qf, within the a-SiCx(n):H film and the fundamental recombination of holes, Sp0. The main result is that surface recombination velocity decreases with film thickness up to 40nm and then saturates. The evolution of the interface parameters indicates that Qf could be located in a layer less than 10nm thick. In addition, Sp0 increases with thinner films probably due to different hydrogenation and saturation of interface dangling bonds during forming gas annealing.

R. Ferre

Papers

Efficient interdigitated back-contacted silicon heterojunction solar cells

N-type multicrystalline silicon solar cells : BBr3-diffusion and passivation of p+-diffused silicon surfaces

Analysis of Series Resistance Losses in a-Si:H/c-Si Heterojunction Solar Cells

Thin macroporous silicon heterojunction solar cells

Effect of amorphous silicon carbide layer thickness on the passivation quality of crystalline silicon surface