I've been at Clover since January. Cannot recommend enough.
If you want to learn why, I'm always open and willing to chat about it.
Hardware heavy companies include places like SolarCity (solar power) and Tesla (battery company with a car side business :)). These tend to be more direct in their impact by actively replacing "dirty" technology.
More on the software / data side include places like PlanetLabs (satellite imagery with data) and Aclima (dense sensor networks) which tend to be more indirect by helping people make informed decisions.
Full disclaimer and shameless plug: I work at Aclima and we are totally hiring :) https://boards.greenhouse.io/aclima
The Financial Solutions Lab has tons of startups interested in tackling large financial problems. Even works on income smoothing to make hourly and seasonal workers have steady paychecks. Digit helps with automated saving. Tala uses machine learning to create credit scores for people in Kenya, Tanzania, and other developing countries. Nova Credit helps immigrant gain better credit. There are dozens more that are all helping people get access to better/fairer financial products.
If you've always been wondering if you should join a company that is trying to help people, send me an email at vincent [at] omadahealth.com, we'll grab a coffee!
If you want to work on big, meaningful problems, here are some compelling options.
Do you think our government is broken? Join Nava (a new type of government contractor working on helping the VA fix its backlog and helping CMS move our healthcare system toward value-based-care) or 18F (a new agency within the government).
Want to save lives? There's Syapse (precision medicine to cure cancer), Clover (building a Medicare Advantage health insurance company from scratch), Omada Health (diabetes prevention through behavior change), or Grail or Freenome (liquid biopsy).
Even well-known startups are helping to solve important problems: think about the impact of Uber and Lyft on drunk driving, or of Airbnb on foreclosures. It wasn't a coincidence Airbnb was founded in the middle of a housing crisis.
I don't know green tech well, but surely somebody here will have good suggestions on what technologists can do about the environment.