Weekly Intelligence Brief
IN-DEPTH: Evaluating the role of TCO layers in improving the efficacy of thin film solar modules
14 April 2009
Transparent conducting oxides (TCOs) can serve a variety of important functions in thin film photovoltaics such as the ones related to transparent electrical contacts and anti-reflection coatings.
TCOs are considered to be the materials which in film-form possess high electrical conductivity together with retaining high transparency to visible light. Their function in solar cells is to provide a means of transporting photo-generated current from the active layer/layers to an external circuit. In the past, it has been found out that differences between the various TCO materials appear in terms of their transparency and light scattering ability, the latter one being determined by the structure and feature sizes of the TCO surface. Furthermore, this surface also influences the electrical properties of the solar cell, as it determines the interface area and chemistry of the TCO/Si-p-layer interface.
The industry has already witnessed significant developments in this arena, especially Oerlikon Solar making progress with the same over the years.
Recently, SCI Engineered Materials also indicated that it is focusing on developing transparent conductive oxides target materials for the thin film solar market.
For the same, the company is working in conjunction with both original equipment manufacturers and thin film solar cell panel fabricators. The company says due to its flexibility in manufacturing both planar and rotatable target material designs, it is able to provide TCO compositions tailored to customer requests.
It was in September 2006 when Oerlikon claimed to be the first manufacturer to cover the entire production process from untreated glass to finished solar module and that time the company had mentioned that the first process step – the coating of the glass with TCO – was hitherto carried out by the glass supplier as an additional service.
Moving on, Oerlikon Solar, which has implemented a special high performance front and back contact method in its production lines, has worked on a Low Pressure Chemical Vapor Deposition (LPCVD) process. The process is used to generate a transparent and conductive TCO layer which is said to be superior to conventional methods. The light transmission and scattering properties of this layer are very important for the efficiency, with which the solar module can convert sunlight into electric power.
Oerlikon Solar’s applied technologies for the mass production of thin-film silicon solar modules consist of different processes to deposit TCO layers for front and back contacts, plasma enhanced chemical vapor deposition (PECVD) to build the photosensitive layers and laser patterning to generate the serially connected cells.
For the photosensitive layer, Oerlikon offers two production technologies, including: Amorphous Solar Modules - this solar cell is made by depositing amorphous silicon (a-Si) on the TCO layer; Micromorph Solar Modules - in addition to the a-Si layer, the micromorph cell has a tandem structure with an additional microcrystalline absorber. This layer converts the energy of the red and near infrared spectrum, allowing an efficiency increase of approximately 30%. The Oerlikon production solution permits a modular upgrade from the single a-Si cell to the micromorph tandem cell.
Gaining a competitive advantage
Elaborating on the TCO’s role, Jim Flanary, COO, HelioSphera, refers to two major reasons for increasing efficiency.
“First of all, this layer acts as a metallic electrode that conducts the generated current from the cells to the junction box. For this purpose, TCO layer has to have a low resistivity to prevent ohmic losses on the cell. The TCO also acts as a light trapping layer that increases the probability for photons to be trapped within the absorbing layer and thus contributes to the photocurrent,” said Flanary, who is scheduled to speak during Thin Film Solar Summit Europe (on 19-20 May in Berlin this year).
Flanary added that because of the specific properties of the micromorph cells and its future development, the in-house TCO deposition improves the possibility to perfectly match and relate the TCO with the micromorph cell, that will result in an increased overall efficiency.
“In other words, in-house development of the TCO can successfully lead to a perfect matching between the absorbing material and the TCO,” said Flanary.
“The bottom line to a technology like micromorph is we feel that TCO is one of the core competencies we must develop in-house to gain competitive advantage in the future,” he said.
HelioSphera believes that TCO is one of the levers which can help in improving the company’s process and product performance in the long-run.
“However, we will also be open to other opportunities of partnerships and other outside sources to enhance our expertise,” said Flanary.
Thin Film Solar Summit Europe 2009
HelioSphera’s COO Jim Flanary is scheduled to speak during the forthcoming Thin Film Solar Summit Europe. It will take place on 19-20 May in Berlin this year.
For information, click here: http://www.thinfilmtoday.com/eu/agenda.shtml
Or Contact: Joshua Bull at +44 (0) 207 3757227 or email josh@cpvtoday.com
