Solar costs: four horsemen?

Solar costs have deflated by an incredible 90% in the past decade to 4-7c/kWh. Some commentators now hope for 2c/kWh by 2050. Further innovations are doubtless. But there are four challenges, which could stifle future deflation or even re-inflate solar. Most debilitating would be a re-doubling of CO2-intensive PV-silicon?