Silicon Research at IEC

The production of silicon solar cells continues to grow at rates of ~30 % per year and forms the basis of a multi-billion industry. At IEC we are implementing new device structures for high efficiency, low cost photovoltaic modules. The new device structures combine the high efficiency and large manufacturing capacity of crystalline silicon photovoltaics, with the low cost of thin film solar cells.

Silicon Research Group at IEC.

Area 1. Silicon heterojunction devices.

The ultimate efficiency of current silicon devices is limited by recombination and bandgap narrowing in the diffused regions. A new approach to the fabrication of crystalline silicon solar cells is the deposition of wide-band gap semiconductors to from a heterojunction and achieve higher efficiencies. Furthermore, devices fabricated by deposition use lower processing temperatures and so at a lower cost. The Institute of Energy Conversion has extensive experience with the deposition of films and the development of low cost heterojunctions through work on cadmium telluride (CdTe) and copper-indium-gallium-diselenide (CIGS) device. We are applying knowledge gained through this work to silicon heterojunctions.

The silicon heterojunction technology has considerable flexibility that has allowed IEC to implement several new structures with record efficiencies.

Heterojunction Solar Cells
We have also use the heterojunction approach to design more traditional front junction devices. Here we have achieved efficiencies of 18.8 % (independently confirmed by NREL) on Cz substrates. We have also achieved open circuit voltages of 702 mV (also confirmed by NREL and also on Cz substrates)

First published efficiency of an interdigitated back contact (IBC) solar cell using silicon heterojunction(Lu).
We demonstrated that heterojunctions solve many of the problems with the IBC approach and achieved a high open circuit voltage of 691 mV indicating the future potential of the device structure. We have also implemented the first two-dimensional device modeling of a silicon heterojunction device.

Rear junction, interdigitated back contact (IBC) solar cells have several advantages over the more common front junction solar cell with contacts on either side Moving all the contacts to the back of the cell eliminates contact shading, leading to a high short-circuit current (JSC). With all the contacts on the back of the cell, series resistance losses are reduced as the trade-off between series resistance and reflectance is avoided and contacts can be made far larger. Having all the contacts on the one side simplifies cell stringing during module fabrication and improves the packing factor. The reduced stress on the wafers during interconnection improves yields, especially for large thin wafers. The more uniform appearance of the PV module is desirable for architectural applications.

While the advantages of rear junctions are well known, their implementation is hindered by several design constraints, which are circumvented by amorphous silicon on crystalline silicon heterojunctions. First, since rear junction cells require diffusion lengths greater than twice the device thickness, thin wafers are attractive. Using low temperature depositions rather than high temperature diffusions decreases the thermal stress and reduces the bowing in thin wafers. Deposition temperatures are also low enough to prevent impurity indiffusion and maintain high initial lifetime of n-type substrates. Second, rear junction designs require low front surface recombination velocities, which can be provided by deposited passivation layers. Thirdly, the central challenge in rear junction solar cells, patterning the rear, is easier in silicon heterojunctions since it is much easier to mask and etch depositions than diffusions, and further isolation between p/n a-Si layers is not always necessary.

All back contact cells are particularly suited to heterojunctions as they eliminate the costly front transparent conductive oxide (typically indium-tin-oxide) along with its absorption losses. Interestingly, with the heterojunction material on the rear, it no longer needs to be transparent and thus there is a wider flexibility in choosing heterojunction partner materials as not all wide band gap semiconductors are completely transparent.

NREL confirmed efficiency of a rear junction cell. This is the first report in the literature of an independently confirmed efficiency result for an inter-digitated all rear contact cell utilizing silicon heterojunctions.

Silicon Solar Cell for 42.8% efficiency
A silicon concentrator device was also developed at IEC for use in the DARPA very high efficiency program [Ref]. The silicon solar cell was measured in conjunction with a GaInP/GaAs cell and a GaInAsP/GaInAs cell. The devices were measured separately by restricting the illumination to a specific wavelength range. The cumulative efficiency of the three devices was 42.8%. The silicon device operated under concentration and with the illumination restricted to wavelengths greater than 870 nm. When combined with the short development time, it indicates the flexibility of our approach to use new device structures to solve problems in the photovoltaics field.

* fabricated at IEC

Area 2. Amorphous Silicon and Thin Silicon Solar Cells.

The IEC has over 25 years of continuous funding in support of developing amorphous Si and thin Si solar cell processing and device design. In the 1980’s and 1990’s, IEC’s thin Si film work focused on single junction a-Si solar cells. We developed a novel Hg sensitized photo-chemical vapor deposition (photo-CVD) reaction system to deposit high quality a-Si and a-SiGe films and devices. This lead to fabrication of the first 10% efficient single junction a-Si solar cell [Ref] by a US university. Photo-CVD [Ref] was useful to produce very low defect a-SiGe films [Ref]. This approach was abandoned due to chronically low growth rates. Next, a standard plasma-CVD system was designed and built. It also allowed IEC to produce single junction devices with initial efficiencies of 10% [Ref]. Thus, the IEC is the only US university to achieve this level of NREL-confirmed efficiencies with 2 completely different growth deposition methods. We worked closely with a major US manufacturer to optimize their interconnect, or shorting, junction in their a-Si tandem product [Ref]. Working with a different US manufacturer, we developed a new method to characterize the transparent conductive oxide (TCO)/p contact in cells or modules [Ref] and showed that the reason for the lower FF and VOC widely observed when SnO₂ is replaced with ZnO is due to increased recombination not contact resistance as was commonly assumed. Analysis of optical enhancement using both empirical [Ref] and theoretical [Ref] methods identified parasitic absorption in the textured SnO₂ as the source of the unexplained optical loss in textured a-Si devices. A study of the a-SiC/nc-SiC alloy system found that over a wide range of PECVD conditions, the films were a-SiC with nc-Si regions [Ref]. We have developed characterization techniques (VASE, glancing incidence XRD) and understanding of the nucleation and grain enhancement mechanisms. IEC has been active in tandem cell analysis. We developed an a-Si/mc-Si tandem cell under the NREL High Performance program, and also conducted extensive device analysis of micromorph a-Si/nc-Si tandem cells fabricated by a US manufacturer. In 2004, IEC installed a multi-chamber plasma-CVD system capable of deposition over 1 square foot areas. Plasma diagnostics such as optical emission spectroscopy have been applied. Both a-Si and nanocrystalline Si films have been used as low temperature deposited emitters and passivation layers for heterojunction a-Si/c-Si devices leading to NREL-confrmed efficiencies over 18%. Presently, we are assisting several US companies to develop new materials, deposition methods, or device concepts for a-Si based solar cells. These efforts have given IEC considerable experience in deposition, fabrication and characterization of a-Si films and single junction solar cells, and nc-Si (or mc-Si) tandem junction solar cells. Evidence that IEC works well with industrial members is given by our numerous joint publications with industry, and our active participation and leadership in the NREL Thin Film Partnership Teams.