The Shockley-Queisser limit for a single-junction solar cell is approximately 33%. A tandem cell stacking two junctions (1.7 eV top cell on 1.1 eV bottom cell) can exceed this limit. Why?
Think about your answer, then reveal below.
Model answer: A tandem cell splits the solar spectrum between two absorbers. The wide-gap top cell absorbs high-energy photons efficiently (less thermalization loss because the photon energy is closer to the gap). Low-energy photons pass through to the narrow-gap bottom cell, which absorbs them. Each junction operates closer to its optimal voltage, reducing the total energy lost to thermalization. The theoretical limit for an optimal two-junction tandem is about 46%. The materials challenge is finding two absorbers with complementary band gaps and compatible processing — perovskite (1.7 eV) on silicon (1.1 eV) is the leading tandem configuration, with lab efficiencies above 33%.
The Shockley-Queisser limit arises from a fundamental tradeoff: a narrow gap absorbs many photons but with low voltage; a wide gap gives high voltage but absorbs fewer photons. Tandem cells break this tradeoff by using multiple gaps. In the limit of infinite junctions, the theoretical efficiency approaches 68% under concentrated sunlight. Multi-junction III-V cells (GaInP/GaAs/Ge) achieve ~47% under concentration and are used in space and concentrated PV.
Question 2 Multiple Choice
Halide perovskite solar cells (e.g., CH3NH3PbI3) have increased from 3.8% to over 26% efficiency in just 15 years. Which materials properties explain this rapid progress?
APerovskites are cheaper than silicon, so more research funding has been available
BPerovskites have a direct band gap with sharp absorption onset, long carrier diffusion lengths despite being processed from solution, tunable band gap through halide composition, and defect tolerance that maintains performance despite high defect densities
CPerovskites are more stable than silicon, allowing higher operating temperatures
DPerovskites use only abundant, non-toxic elements
The remarkable defect tolerance of halide perovskites — high efficiency despite defect densities millions of times higher than in silicon — is the most surprising property. In silicon, a single deep trap kills carrier lifetime; in perovskites, defects tend to be shallow (near band edges) and relatively benign. The direct band gap provides strong absorption in thin (~500 nm) films, and the tunable band gap (1.2-2.3 eV by mixing I/Br/Cl) makes perovskites ideal for both single-junction and tandem configurations. The main unsolved challenge is long-term operational stability — degradation by moisture, heat, and light.
Question 3 True / False
Silicon solar cells use an indirect band gap material, which means they require thicker absorber layers than direct-gap materials to absorb the same fraction of sunlight.
TTrue
FFalse
Answer: True
Silicon's indirect band gap requires a phonon to assist optical absorption, making the absorption coefficient much lower than for direct-gap materials near the band edge. A crystalline silicon cell needs a ~180 micrometer wafer to absorb most above-gap photons (with light-trapping texturing). A direct-gap material like GaAs or CH3NH3PbI3 absorbs the same light in just 1-2 micrometers. This is why silicon solar cells are relatively thick and rigid, while thin-film technologies (CdTe, CIGS, perovskites) can be deposited as thin layers on flexible substrates. Silicon compensates with mature manufacturing, high material purity, and excellent passivation chemistry.