You don't need to install an app. Unless you want all the additional bloat, Alfred looks like a massive overkill solution to simply launching Photoshop via a key combination.
Use the Automator app that comes with OS X to create a simple Service that launches Photoshop, and make that Service available to all apps.
In all your apps, including Finder (which includes the Desktop), the Services menu will contain the Launch Photoshop item. So use the system Keyboard Preferences to assign a Cmd-key combination to that Services menu item. The screenshot shows Opt+Cmd+P.
You'll be able to launch Photoshop (or bring it to front) at any time by pressing the key combination.