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Sanyo Pushes c-Si Solar Cell to 23% Conversion Efficiency

by Aaron Hand, Executive Editor, Electronic Media
Source: Semiconductor International, 22 May 2009

Sanyo Electric Co. Ltd. (Tokyo) announced today that it has broken its own record for the highest energy conversion efficiency for a crystalline silicon (c-Si) solar cell in a practical size (≥100 cm2). Sanyo’s proprietary HIT (heterojunction with intrinsic thin layer) photovoltaic cell conversion efficiency has been boosted from 22.3% to 23.0% at the research level, as validated by the National Institute of Advanced Industrial Science and Technology (AIST).

The HIT solar cell, which measures 100.4 cm2, is a hybrid cell composed of a single thin c-Si wafer sandwiched by ultrathin amorphous silicon (a-Si) layers. The structure minimizes defects within the p/n junction of the cell, producing higher-efficiency cells while producing more power even at higher temperatures.

Reaching the higher efficiency in its latest research involved improvements in several areas, including the quality of the heterojunction, the laminated structure formed by the c-Si and a-Si. The structure of the HIT cell reduces recombination loss of the electrical element (charged carrier) by surrounding the energy-generation layer of single thin c-Si with high-quality ultrathin a-Si layers. The latest improvement comes from developing a technology to deposit a higher-quality a-Si layer over the c-Si substrate while protecting the c-Si surface from being damaged. This resulted in increasing the open circuit voltage (Voc, the maximum voltage produced by the solar cell) from 0.725 V to 0.729 V.

Reducing optical absorption loss in the a-Si and transparent conductive layers, which is key to collecting as much sunlight as possible for energy use, has proven challenging. The a-Si layers tend to absorb short-wavelength radiation, while the transparent conductive layer absorbs longer wavelengths. In its latest research, Sanyo has managed to reduce optical absorption loss in both the a-Si and the transparent conductive layers, leading to improving the short circuit current (Isc, the maximum current produced by the solar cell) from 39.2 mA/cm2 to 39.5 mA/cm2.

Sanyo has also improved the resistance loss of its surface grid electrode, which is what carries the generated electric current from the solar cell to the electrical system. This is achieved through a lower-resistance electrode material as well as higher-aspect-ratio printing technology. Through these modifications, the fill factor has improved from 0.791 to 0.80.

Along with increasing conversion efficiency, Sanyo has also made significant advances in lowering the production cost of the photovoltaic system and reducing the use of raw materials such as silicon. Moving forward, the cell maker is working to move the research into mass production, along with the reductions in cost and materials.