You did specify that ideally you didn't want a hub, but I think you'll either have to compromise on that if you want IFTTT to be able to integrate with the sensor.
If you're open to a Z-Wave sensor, the Everspring ST815 seems relatively suitable:
The Z-Wave Wireless Illumination Sensor with LCD Screen from Everspring activates programs or devices automatically based on illumination settings chosen in advance by the user. This sensor has a backlit LCD that displays a timer, illumination, transmission status and battery status. It has a lux (measurement relating to amount of visible light present) detecting range from 0 (darkness) to 3000 and a countdown timer that is adjustable from one minute up to eight hours. The sensor has a 60dB (decibel) built-in beeping tone/alert and has a wall-mountable design for use indoors or outdoors. The Z-Wave Wireless Illumination Sensor can be enrolled into any Z-Wave network to begin communicating with other Z-Wave enabled devices. It has an operating range up to 100 feet and is powered by three 1.5V AA batteries.
So, this device seems suitable because:
- it is rated for both indoor and outdoor use
- it can detect from 0 - 3000 lux, so although it won't detect bright sunlight, it should be able to tell between overcast and sunny conditions
- configurable lux settings so that you can reduce false positive detections.
The Vera Hub seems to have relatively good support for the sensor, so it's an option if you can't find any Wi-Fi sensors that meet your requirements. Alternatively, you could consider making a sensor yourself if you're open to that; if you do decide to go down that route, you're welcome to edit the question.