Oerlikon Solar TCO 1200 System

Description

TCO films have been used within Thin Film PV panels since the beginning of the technology. The TCO film needs to: allow full spectrum light into the panel for generation of energy by the PV Absorber, act as a Current Collector to efficiently harvest electricity fabricated within the panel and act as a light scattering and trapping layer which forces key wavelengths to traverse the PV Absorber stack for multiple reflections, increasing the chance for the light to create electricity. The standard technology for deposition of TCO films has been sputtering, where a plasma in a high vacuum chamber knocks molecules of material off a powered target and they strike with high energy the surface of the Absorber stack. The sputtering process allows only a small window of process adjustment and no optimization of material composition or light scattering is practically possible. Another popular approach to TCO fabrication, especially by glass manufacturers, is Atmospheric Pressure CVD (APCVD). APCVD techniques typically require very high deposition temperatures, >500 C, and so forces the films to be applied to the glass directly without any PV Absorber films present. This is ok for the initial film on glass at the beginning of Module manufacture, however, the back Contact TCO film cannot be deposited by APCVD directly onto the PV Absorber. The TCO 1200 LPCVD Reactor has innovated several hardware and chemistry innovations, allowing a high heat up rate, a highly uniform IR Camera controlled heating within a Load Lock and Process Module, a low temperature process (<200 C), and controllable conductivity and crystal morphology through adjustable doping gas delivery. All these innovation features allow the film to be adjusted to optimize panel power output.

Innovative Aspect

In order to develop the next generation of thin film silicon PV cells, light trapping by advanced TCO is essential. ZnO deposited by LPCVD process has demonstrated higher light trapping abilities (ref). This is especially true with respect to a-Si/ucSi Tandem solar cells where the Si absorber layers are engineered to be as thin as possible while keeping the electricity generating potential (efficiency) high. In addition, amorphous silicon single junction cells have an enhanced susceptibility to Stabler Wronski degradation, so efficient light management allows a reduction in layer thickness, fundamental for high performance in these material systems. Therefore, the LPCVD ZnO TCO film in use by the TCO 1200 is truly an innovative development in Thin Film PV technology. Figure 1 demonstrates the I/V characteristics of 2 x stabilized tandem Junction a=Si/ucSi cells. The cells were prepared identically except for the Front Side TCO layers; clearly, the output current for the LPCVD TCO film is significantly higher than a high quality commercial APCVD film, accounting for an additional 0.5% improvement in stabilized efficiency. The high Innovation resulting from the TCO 1200 ZnO manufacturing system allows a single layer to optimize a Tandem absorber stack with very high haze light scattering (see Figure 2 for opaqueness of glass / TCO stack), very low sheet resistance and excellent transmission characteristics. Also, the low temperature nature of the deposition allows this same TCO layer to be used very successfully for the TCO back Contact, again increasing light trapping within the cell.

Economic benefits

TCO 1200 LPCVD Front and Back Contacts are shown to be able to increase cell efficiencies by ~0.5% over standard commercial APCVD TCO films (Figure 1). Initial production runs using this TCO material (200,000 panels) have shown an increase in efficiency consistent with our data; also important is that thisimprovement is achievable in high throughput manufacturing. The economic benefit to the customer can be significant:

Assume a 30 MW Solar farm utilizing 100 W/Modules. 30 MW requires 300,000 panels at 100W output. If 0.5% efficiency increase, power output is increased nominally 5% to 105 W. The extra 5 Watts output allows 5% less panels to be installed, for a direct Module savings of 5% to the Solar farm developer. Typical system pricing in US of $4/Wp, translating to Module Price of nominally $2/Wp assuming 50% BOS costs 5% of 300,000 Modules is 15000 Modules, nominally producing 100 Watts, yielding 1.5 Million Watts, allowing $3.0 Million saving for only one utility scale installation. All this for a single TCO film optimization.