Photovoltaic Absorption and Band Gaps

Introduction

A solar cell needs an absorber that turns incident photons into mobile charge carriers. A semiconductor band gap helps determine which wavelengths can create electron–hole pairs and sets an important scale for possible voltage. Choosing a smaller gap collects more of the spectrum but generally lowers attainable voltage; choosing a larger gap loses more low-energy photons. Absorption strength, film thickness, defects and material availability then decide whether the theoretical trade-off becomes a useful device.

Core explanation

In a simplified semiconductor picture, electrons occupy a valence band and can be excited into a conduction band. The minimum energy for a band-to-band transition is the band gap E g, although real absorption can include defect or excitonic features. A photon has energy E = hc/λ, so shorter wavelength means higher energy. In convenient units, E(eV) ≈ 1240/λ(nm). A 620 nm photon has about 2.0 eV; a 1,240 nm photon has about 1.0 eV. DOE's solar-energy basic-research report explains how above-gap absorption creates electron and hole populations for photovoltaic conversion.

A photon with energy below E g generally cannot drive the desired direct band-to-band excitation and may pass through or be lost elsewhere. A photon well above E g can be absorbed, but its excess energy usually relaxes as heat before charge extraction in an ordinary cell. This thermalization loss means a cell cannot turn the entire energy of each high-energy photon into electrical work. DOE's photovoltaic efficiency overview identifies unabsorbed and heat-lost photon energy as basic efficiency losses.

The absorption edge is not always a perfect vertical step. Absorption coefficient α describes how strongly a material absorbs at each wavelength. For a uniform film with no reflection and a simple single-pass approximation, transmitted intensity is I = I₀e^(−αd), where d is thickness. Strongly absorbing materials can use thin layers; weaker near-edge absorption may require thicker material or optical trapping. Reflection, parasitic absorption in contacts and texture complicate real devices. A nominal gap alone does not tell how much sunlight a practical thickness absorbs.

Semiconductor selection also depends on band structure . Direct-gap materials can absorb near their edge strongly; indirect-gap silicon needs phonon assistance for some near-edge transitions and often uses a relatively thick wafer with light trapping. Some thin-film absorbers achieve strong absorption in much thinner layers. Thickness affects material consumption, mechanical flexibility and carrier travel distance, but thinner is not automatically better if photons pass through unabsorbed. DOE's photovoltaic-cell primer places silicon's absorption and band-gap constraints in the broader device context.

After a photon is absorbed, the generated electron and hole still must be separated and collected before recombination. An absorber with a well-chosen E g but severe defects may have poor voltage and current. Interfaces and contacts can also parasitically absorb or reflect light. Semiconductor choice thus balances spectral absorption, attainable voltage, carrier lifetime, stability, manufacturability and resource use. A tandem cell can stack different gaps to use the spectrum more efficiently, but it adds interface and manufacturing challenges.

The solar spectrum varies with time of day, atmosphere and illumination source. Standardized test spectra allow comparison, but a measured spectral response is needed to predict behavior under different light. A lab lamp with a mismatched spectrum can favor one absorber artificially. NREL calibration services show why reference cells and spectral calibration matter in quantitative photovoltaic testing.

Step-by-step reasoning

Convert wavelengths to photon energies using E(eV) ≈ 1240/λ(nm). Compare each energy with the absorber's E g to classify sub-gap or potentially absorbed photons. Next examine absorption coefficient and layer thickness rather than assuming every above-gap photon is captured. Account for reflection and parasitic absorption. Finally ask whether generated carriers survive and reach contacts, and whether the absorber's stability and manufacture suit the intended device.

Visual explanation

Draw valence and conduction bands separated by E g. A below-gap arrow fails to reach the conduction band; an above-gap photon arrow reaches it and then a downward squiggle represents thermalization of excess energy. Beneath, plot sunlight photon flux against wavelength and mark the absorption edge. A second plot of absorption coefficient versus wavelength shows that the edge may be gradual. The diagrams explain why changing E g affects both current opportunity and voltage scale.

Real-world analogy

Imagine a turnstile that requires a minimum ticket value. Tickets below the threshold cannot enter; tickets above it get in, but extra value beyond the entrance fee is not paid back. The band gap is the threshold and thermalization is the lost excess in the simplified analogy. Real solar cells are more complex: voltage also depends on recombination and contacts, so the threshold is not equal to delivered electrical energy per photon.

Real-world example

A designer lowers an absorber's gap from 1.8 to 1.4 eV. More red and near-infrared photons can now contribute to current, but the maximum voltage scale decreases, and the actual voltage may fall further if the new composition has more recombination defects. A spectral response measurement can show the extra collected current; an illuminated current–voltage curve tests whether the added current outweighs the voltage cost. Band-gap tuning is therefore an optimization, not an instruction always to choose the smallest gap.

Why?

Why does a high-energy photon not necessarily produce more charge than a near-gap photon in a conventional single-junction cell? Usually each absorbed photon makes one electron–hole pair. Energy above the gap relaxes rapidly as heat, rather than producing extra extracted electrons. Specialized multiple-exciton or hot-carrier concepts seek alternatives, but they should not be assumed for an ordinary cell.

Common misconception

“Any photon brighter than the band gap becomes entirely electrical energy.” Excess energy is commonly thermalized, and carriers may recombine. Another misconception says the band gap alone sets efficiency; absorption strength, carrier lifetime, interfaces and contacts matter. A third treats absorption edge as an exact all-or-nothing wavelength boundary in a real film, ignoring weak absorption tails and finite thickness.

Worked example

An absorber has E g = 1.55 eV. Its simple band-to-band edge wavelength is λ g ≈ 1240/1.55 = 800 nm . A 700 nm photon has E ≈ 1240/700 = 1.77 eV , above the gap and potentially absorbable; about 0.22 eV of energy exceeds the gap and can thermalize. A 900 nm photon has E ≈ 1.38 eV , below the idealized gap and generally cannot make a band-to-band electron–hole pair. Whether the 700 nm photon is actually absorbed depends on α, film thickness, optics and defects.

Quick check

1. Which has greater photon energy, 500 nm or 1,000 nm light? Answer: The 500 nm photon has twice the energy because E is inversely proportional to wavelength.

Exam focus

Use E = hc/λ or 1240 eV·nm/λ and keep units consistent. Explain sub-gap transmission and above-gap thermalization. Distinguish a gap threshold from actual voltage and a nominally absorbable photon from a collected electron. Include absorption coefficient and thickness when discussing thin-film absorber selection.

Advanced insight

Near-edge absorption differs between direct and indirect semiconductors, and disorder can create band-tail states that blur the edge. These states may increase weak sub-gap absorption while also enabling recombination; more optical absorption is not automatically beneficial. Tandems address spectral mismatch by placing a high-gap absorber above a lower-gap one so each photon is used closer to its energy, as illustrated by NREL multijunction work. The full optimization also accounts for current matching and optical losses in the stack.

Summary

The band gap sets a threshold for ordinary band-to-band absorption and an important voltage scale. Sub-gap photons are usually lost, while excess energy of above-gap photons commonly becomes heat. Real absorption depends on wavelength-dependent α, thickness and optics, and useful electricity further requires low recombination and effective contacts.

Practice questions

1. Estimate the photon energy at 400 nm. Answer: 1240/400 = 3.10 eV.

2. What is the approximate edge wavelength for a 2.0 eV band gap? Answer: 1240/2.0 = 620 nm.

3. Why might a 1.1 eV absorber collect more photons but provide lower voltage than a 1.8 eV absorber? Answer: Its smaller gap admits more low-energy photons but sets a smaller energy scale for carrier separation and attainable voltage.

4. Why can two materials with equal band gaps need different film thicknesses? Answer: They can have different absorption coefficients and band structures, so light attenuates at different rates near the edge.

5. Does absorption of a photon guarantee electrical current? Answer: No. The generated carriers may recombine or fail to reach selective contacts; absorption is only the first step.