The best article I could find which addresses this question is "Life‐Cycle Assessment of Solar Charger with Integrated Organic Photovoltaics".
Their conclusion: Because of the complicated construction process and short-life span of cell phone chargers, solar chargers are less sustainable than traditional grid-charged battery banks, unless the grid where you live uses a high percentage of fossil fuel generation.
The study compared three different types of phone chargers:
- A cutting-edge charger, HeLi-On, using organic photovoltaics (OPV) with 2600 mAh battery
- A more tradititional amorphous silicon charger (a-Si), the SolarWrap mini with 2200 mAh battery
- A conventional 2600 mAh battery power bank charged from the grid
The authors looked at production, use, and disposal of all three in Denmark (DK) and China (CN). Here are a few of the graphs from the results:
Here, and in all the other charts, you can see that in Denmark the conventional battery bank is usually better. In China, the conventional bank is sometimes worse and sometimes about the same as the a-Si charger, depending on which impact category you're looking at.
From the conclusions:
Looking at the impact scores for the production individually, the regular power bank had the lowest impact across all impact categories. The reason lies in the fact that it is a simpler device that only includes a battery and not a solar panel. [...] For the two power bank products that include solar panel, the difference between the production only and the production plus the use phase is minimal and it corresponds to the transport in the distribution of the product. Production and use phase for the regular power bank showed increased scores when compared to production only due to the electricity needed for the 300 charges specified in the [functional unit].