Organic solar cells make use of low-cost organic polymers for photovoltaics. Although these solar cells may appear to be quite different from solar cells made with conventional inorganic semiconductors (e.g. they make use of exciton generation rather than electron-hole generation), this tutorial shows that the essential device physics is readily understood in terms of familiar solar cell fundamentals. The use of phase-segregated "bulk heterojunction" structures to enhance the short-circuit current is described as well.