Photoshop scripting, export to png?

I use the following code:

function SavePNG(saveFile){

  var opts = new ExportOptionsSaveForWeb();

  opts.format = SaveDocumentType.PNG;

  opts.PNGB = true;

  opts.quality = 100;

  pngFile = new File(saveFile);

  opts.includeProfile = true;

  app.activeDocument.exportDocument(pngFile, ExportType.SAVEFORWEB, opts);

}

It almost workable, but it works with fully visible or fully invisible areas only. For example if I have a transparent layer and add a second transparent layer with a red rect with opacity 50%, then I will got an image where the transparency of this rect is 100% (fully visible).

How to solve this problem? I use Photoshop CS6 (Mac OS version).