Photovoltaic materials: Present efficiencies and future challenges

BACKGROUND: Photovoltaics, which directly convert solar energy into electricity, offer a practical and sustainable solution to the challenge of meeting the increasing global energy demand. According to the Shockley-Queisser (S-Q) detailed-balance model, the limiting photovoltaic energy conversion efficiency for a single-junction solar cell is 33.7%, for an optimum semiconductor band gap of 1.34 eV. Parallel to the development of wafer-based Si solar cells, for which the record efficiency has continually increased during recent decades, a large range of thin-film materials have been developed with the aim to approach the S-Q limit. These materials can potentially be deposited at low cost, in flexible geometries, and using relatively small material quantities.

ADVANCES: We review the electrical characteristics of record-efficiency cells made from 16 widely studied photovoltaic material geometries and illuminated under the standard AM1.5 solar spectrum, and compare these to the fundamental limits based on the S-Q model. Cells that show a short-circuit current (J_sc) lower than the S-Q limit suffer from incomplete light absorption or incomplete collection of generated carriers, whereas a reduced open-circuit voltage (V_oc) or fill factor (FF) reflects unwanted bulk or interfacial carrier recombination, parasitic resistance, or other electrical nonidealities. The figure shows the experimental values for J_sc and the V_oc × FF product relative to the S-Q limiting values for the different materials. This graph enables a direct identification of each material in terms of unoptimized light management and carrier collection (J_sc/J_SQ < 1) or carrier management (V_oc × FF/V_SQ × FF_SQ < 1).
Monocrystalline Si cells (record efficiency 25.6%) have reached near-complete light trapping and carrier collection and are mostly limited by remaining carrier recombination losses. In contrast, thin-film single-crystalline GaAs cells (28.8%) show only minimal recombination losses but can be improved by better light management. Polycrystalline CdTe thin-film cells (21.5%) offer excellent light absorption but have relatively high recombination losses; perovskite cells (21.0%) and Cu(In,Ga)(Se,S)2 (CIGS) cells (21.7%) have poorer light management, although CIGS displays higher electrical quality.
Aside from these five materials (Si, GaAs, CdTe, CIGS, perovskite) with efficiencies of >20%, a broad range of other thin-film materials have been developed with efficiencies of 10 to 12%: micro/nanocrystalline and amorphous Si, Cu(Zn,Sn)(Se,S)₂ (CZTS), dye-sensitized TiO₂, organic polymer materials, and quantum dot solids. So far, cell designs based on these materials all suffer from both light management and carrier management problems. Organic and quantum dot solar cells have shown substantial efficiency improvements in recent years.

OUTLOOK: The record-efficiency single-crystalline materials (Si, GaAs) have room for efficiency improvements by a few absolute percent. The future will tell whether the high-efficiency polycrystalline thin films (CdTe, CIGS, perovskite) can rival the efficiencies of Si and GaAs. Because the cost of photovoltaic systems is only partly determined by the cost of the solar cells, efficiency is a key driver to reduce the cost of solar energy, and therefore large-area photovoltaic systems require high-efficiency (>20%), low-cost solar cells. The lower-efficiency (flexible) materials can find applications in building-integrated PV systems, flexible electronics, flexible power generation systems, and many other (sometimes niche) markets. High-efficiency (>20%) materials find applications in large-area photovoltaic power generation for the utility grid as well as in small and medium-sized systems for the built environment. They will enable very large-scale penetration into our energy system, starting now and growing as the cost per kilowatt-hour is reduced further by a factor of 2 to 3. This can be achieved by nanophotonic cell designs, in which optically resonant and nonresonant structures are integrated with the solar cell architecture to enhance light coupling and trapping, in combination with continued materials engineering to further optimize cell voltage. Making big steps forward in these areas will require a coordinated international materials science and engineering effort.