# Space Solar Cover Glass: Why Cerium, and How to Read It

> Space solar cells sit under cerium-doped cover glass. What cerium absorbs, how to read the datasheet, and how a spectral loss becomes a current loss.

![Space Solar Cover Glass: Why Cerium, and How to Read It](https://cdn.ooitech.com/static/upload/image/20261017/2026101712121212.webp)

- ** Ooitech Editorial Team
- ** 2026-10-29
- ** 2 Views
- ** [Blog](./Blog.html)

## Space Solar Cover Glass: Why Cerium, and How to Read It

**Quick answer:** Space solar cells are covered by a tenth of a millimetre of glass, and the glass is part of the optical design rather than its packaging. Cerium is added to it because it absorbs the short-wavelength ultraviolet before that light reaches the adhesive underneath, and because it helps the glass keep transmitting after years of irradiation. Reading a cover glass datasheet means reading three different kinds of number: a band average, a single-wavelength value, and a defined cutoff.

A solar array in orbit has one job: convert sunlight into current for as long as the mission lasts. Everything between the sun and the semiconductor is either protecting the cell or costing the array power, and the cover glass does both at once. It is the first optical element in the system and the only one that has to survive the environment in direct contact with it.

That double role is what makes cover glass selection awkward. Thicker glass shields the cell from charged particles and adds mass to a structure that is usually mass-limited. Absorbing ultraviolet protects the adhesive below and removes light that a cell would not have used efficiently anyway, but the same absorption mechanism can trim useful wavelengths if the design is pushed too far.

This page follows the light path from the front surface of the glass to the I-V curve of the assembled cell. It covers what cerium does inside the glass, how to read the datasheet numbers that describe it, and how a loss at a specific wavelength turns into a loss of current in a cell with more than one absorbing layer.

## What the light crosses before it reaches the cell

Sunlight arriving at a solar array meets the cover glass first, then a transparent adhesive, then the cell. The glass is thin, measured in tenths of a millimetre, and it performs two functions at once: it withstands the space environment on the outside, and it lets through the light the cell is designed to use. How much short-wavelength ultraviolet it removes and how much useful light it keeps are both visible in the performance of the finished assembly.

The adhesive is the reason the ultraviolet question matters. It has to stay bonded and transparent for the life of the mission, and under strong illumination and the rest of the space environment it can darken. A darkened adhesive layer reduces what reaches the cell, which shows up as a drop in current. Adhesive candidates have been screened by holding them between two glass plates and running long-duration environmental simulation, with changes in colour and transmission as the evaluation criteria. That kind of test measures the adhesive, not the glass.

Two processes then have to be kept apart. Filtering happens from the first day the cover glass is bonded: it changes the spectrum reaching the adhesive, immediately and permanently. Ageing happens on a slower clock: the adhesive may lose transmission over years of illumination, and the glass itself may darken. The same glass-adhesive-cell combination therefore needs an initial measurement and a retained-performance measurement after a defined environmental test, and the two are not interchangeable.

## Why cerium, and where the ultraviolet is stopped

Sunlight covers a wide range of wavelengths, and the short end of it carries more energy per photon. Two bands matter here: UV-B runs from 280 to 315 nanometres and UV-A from 315 to 400 nanometres. The approximate relation between wavelength and photon energy is E ≈ 1240/λ, with E in electron volts and λ in nanometres, which puts a 300 nanometre photon at about 4.13 eV and a 400 nanometre photon at about 3.10 eV.

That extra energy per photon is what drives photochemistry in materials. The adhesive is the component most exposed to it, and there are two established engineering routes to keep it away. One is a fused silica cover glass, which transmits well into the ultraviolet, with a coating on its surface that reflects the short wavelengths back out. The other is a cerium-doped borosilicate glass, which absorbs those wavelengths inside the material before they reach the adhesive. Both approaches solve the same problem on the same optical path, at different positions in the stack.

The design question is which wavelengths are removed, and which are kept. Visible and near-infrared light carries most of the photocurrent in a silicon or gallium arsenide cell, so the glass has to stay transparent there, and the position of the filtering edge moves with the glass composition, the thickness and the process. A cover glass that absorbs too far into the visible range protects the adhesive and costs the array power.

## Two effects of cerium, on two time scales

Cerium does a second job that has nothing to do with the spectrum at delivery. High-energy particles and intense ultraviolet can create new absorbing defects inside glass, which gradually darkens and is often called radiation darkening or solarization. The cerium-related components help suppress that change, so the glass keeps more of its transmission after irradiation than an undoped equivalent would.

The two effects are measured differently. The initial filtering is read from the transmission spectrum of the glass as manufactured, including the position where it starts to pass short wavelengths. The retention is read by measuring the same glass before and after a defined irradiation, in the band the cell actually uses. A cover glass can be strong on one and weak on the other, which is why both numbers belong in the comparison.

## What cerium is doing inside the glass

Cerium enters the glass melt with the other raw materials and can end up in more than one oxidation state. The two that matter are Ce3+ and Ce4+. The valence state changes the energy levels available to the electrons, and therefore which photons the material can absorb. Glass composition, the redox conditions during melting and later processing all influence the balance between the two, which is why two cerium-doped glasses can have different transmission curves.

One absorption path is an electron in a Ce3+ ion absorbing a photon and moving from a lower 4f level to a 5d level. A second contribution comes from charge transfer involving Ce4+, where an electron moves between states associated with different ions. Both consume photons of a corresponding energy, and both produce absorption bands whose position and strength depend on the glass host. For a commercial cover glass, the filtering performance is judged on the measured transmission spectrum of the finished product rather than on the recipe.

The recovery behaviour has a mechanism of its own. After irradiation, electrons and the vacancies they leave behind can be trapped by local structures in the glass and form new absorbing centres, which is what the eye sees as darkening. An experiment on potassium barium silicate glass reported in 1962 irradiated samples to the same degree of colouration with gamma rays and then compared how quickly they faded. Samples with more Ce3+ faded faster, and the proposed explanation was that cerium promotes recombination of the electron and the hole, so the absorbing defects are progressively removed. That is one mechanism behind recovery; what a space cover glass is specified on is how much transmission it retains after a defined irradiation, measured before and after on the same glass.

Material datasheets and product datasheets also operate at two different levels. A composition table may state the mass fraction of cerium oxide in the batch of raw oxides, which describes the material system. The transmission values state the optical result after the glass has been made. For comparing two finished cover glasses, the thickness, the transmission spectrum and the change after irradiation are what the comparison should rest on.

## How to read one row of a datasheet

Transmittance at a given wavelength is the ratio of the optical power leaving the sample to the power entering it, in the measurement arrangement the datasheet defines. A reading of 91.3% at 400 nanometres means that, in that arrangement, of 100 parts of light entering at that wavelength about 91.3 reach the exit; the rest is accounted for by surface reflection and absorption in the glass.

A representative cerium-doped borosilicate cover glass at 0.100 millimetres without a coating is published with two readings on the same row: an arithmetic average of 11.4% across 300 to 315 nanometres, and a single point of 91.3% at 400 nanometres. The first summarises several measured points across a band; the second describes one wavelength. Both are correct, and neither can be used in place of the other.

300 to 315 nanometres is the long-wavelength end of UV-B, and 400 nanometres sits at the long-wavelength edge of UV-A, close to where visible light begins. A band average answers how much the glass transmits across that range on average. It does not say how many photons the sun delivers there, and the two are different questions. Estimating current requires multiplying the transmission at each wavelength by the solar photon count and the cell response at that wavelength, which is why the same band average can produce different current losses under different spectra.

| Number in the row | What it describes | How to use it |
| --- | --- | --- |
| 11.4% across 300-315 nm | An arithmetic mean over a band, at 0.100 mm uncoated | As a summary of the ultraviolet blocking, not as a wavelength-resolved value |
| 91.3% at 400 nm | One wavelength, one thickness, one coating state | Only against the same conditions elsewhere in the comparison |
| 332 nm cutoff | A defined position: where transmission reaches 46% at 0.100 mm | As a marker of where the filtering edge sits, not as a physical threshold |

The cutoff is worth reading carefully. The product defines it as the position where transmission reaches 46%, at the stated thickness. That is a convention chosen by the manufacturer, not a law of the material. In the same datasheet, 332 nanometres sits on the short-wavelength side of UV-A, and transmission then rises through a transition region towards the visible. Moving along the spectrum, the number that changes is the one the convention defines.

## The same glass, three numbers at one wavelength

The same datasheet lists three values at 400 nanometres: 91.3% for uncoated glass, 92.3% for glass with a single-side anti-reflective coating, and 95.8% for the case where the light goes on into the adhesive. The third column is the one that usually needs explaining, because a higher number sounds like a better glass and it is really a different measurement.

When light crosses from one material into another, part of it is reflected, and how much depends on the refractive indices on the two sides. The refractive index of an adhesive is much closer to that of glass than air is, so replacing a glass-air interface with a glass-adhesive interface reduces the reflection at the back of the glass, and more light continues into the adhesive. The measurement is made by simulating that contact: an index-matching liquid of about 1.41 and a backing plate stand in for the adhesive, and the reflection from the outer face of the backing plate is corrected out of the result. The 95.8% value ends where the light enters the adhesive, so absorption inside the real adhesive and reflection at the adhesive-cell interface are still to come.

It also helps to keep the coatings apart. An anti-reflective coating exists to reduce reflection of the light the cell wants, which raises transmission in the useful bands. A ultraviolet-reflective coating on the fused silica route exists to send the short wavelengths back out, which lowers transmission there. They sit on the same surface and do opposite things at opposite ends of the spectrum. When comparing candidate cover glasses, the comparison has to fix the thickness, the coating state and the exit plane first, and then read transmission in the band the cell responds to.

## Thickness changes three things at once

Thickness is the parameter that couples optics, mass and radiation shielding. Light travels further through the material, so bands that are absorbed attenuate more. Areal density is simply material density multiplied by thickness: at a density of 2.51 grams per cubic centimetre, 0.100 millimetres of glass works out to about 0.251 kilograms per square metre. On a mass-limited array, every additional tenth of a millimetre has to earn its place in shielding or in optical performance.

Datasheets often separate thicknesses for different measurements, and it is worth noticing. The continuous spectra before and after irradiation published for the same glass are taken on a 0.150 millimetre sample. The exposure is a stated solarization condition in vacuum, integrated to the equivalent of 2000 sun-hours. The three single values above belong to a 0.100 millimetre sample. Both are useful, and they answer different questions: one shows how the material evolves under irradiation, the other gives a defined reading at a defined thickness.

## From lost photons to lost current

The path from a transmittance figure to a current figure can be followed with a simple count. Take a narrow band and 100 photons arriving at the cover glass. Suppose 80 of them cross the glass and 90% of those cross the adhesive, so 72 reach the front of the cell. Suppose that in this packaging state 60% of the photons reaching the cell correspond to charge collected in the external circuit. The collected charge then averages out at 72 × 0.60 = 43.2 electron charges, and because it is an average it can be fractional.

Dividing 43.2 by the 100 photons that arrived gives the external quantum efficiency of that band for that assembly: of every photon entering the reference surface, how much charge is collected externally on average. The word external matters, because the reference surface is where counting starts; on a bare cell it is the front of the cell, and on an encapsulated assembly it is the outside of the cover glass. Bonding the glass replaces an air-cell interface with an adhesive-cell interface and changes the reflection there, which is one reason the assembly is better measured as an assembly.

Extending one band to the whole spectrum gives the photocurrent density as an integral over wavelength. It is Jph ≈ q ∫ Φ(λ) EQEassembly(λ) dλ. Here Φ is the solar photon flux in front of the cover glass, per unit area, time and wavelength. EQEassembly is measured with the cover glass as the reference surface, and q is the elementary charge, about 1.602 × 10-19 coulombs. The integral is a sum: photon flux times quantum efficiency times charge, added up across wavelengths.

This is why 91.3% at 400 nanometres still has to be put back into the spectrum before it means anything for power. The sun does not deliver the same number of photons in every band, and the cell does not respond equally to every band. Losing 10% in a band where the cell responds weakly costs little current; losing it in a band that a specific sub-cell depends on costs much more. The calculation should use the spectrum, the temperature and the irradiated response data that match the mission.

## Stacked cells: where the loss lands matters

A multi-junction cell stacks several sub-cells that absorb different parts of the spectrum, and they are connected in series, so one current flows through all of them. Each junction therefore has to be measured on its own. While one junction is being measured, the others are biased with light in their own bands so that they supply enough photocurrent and do not limit the junction under test. A suitable voltage bias may be applied as well. Sweeping the wavelength of the probe light then gives the response of the target sub-cell point by point.

In a current-matching analysis the weakest junction sets the string current. Take a hypothetical three-junction cell with photocurrent densities of 18, 17 and 20 milliamps per square centimetre for the top, middle and bottom junction. The middle junction is the limiting one. Change the cover glass so that the top junction receives less of the short-wavelength light, and the three values become 16, 17 and 20: now the top junction limits. The two situations have the same kind of loss in magnitude, and they have different consequences.

Improving the light reaching the limiting junction, or one close to it, is usually the most direct way to raise the series current. Changes in the other junctions can still affect total power through voltage, so the analysis does not end at the sub-cell currents. The full current-voltage curve of the cell is measured next: at each terminal voltage the current is recorded, the power at each point is the product of the two, and the highest of those products is the maximum power point. Since the cover glass changes the incoming spectrum, the spectrum changes the sub-cell photocurrents, and the series connection and the voltage together decide the output power, the loop has to be closed with a measurement rather than with a datasheet value alone.

One caution belongs here. Reports on multi-junction measurement practice point out that a mismatch between the simulator spectrum and the sub-cell responses changes the measured curve and therefore the measured efficiency. A cell that looks better under one simulator spectrum can look worse under another, and the cover glass is part of what sets the spectrum the cell sees.

## How a cover glass choice is demonstrated

Selection starts with the mission rather than the material: the spectrum the array will see, the cell type, the expected temperature and the required life. From that come the cover glass thickness and the radiation shielding target. Candidates are then compared at the same thickness, the same coating state and the same measurement plane, initially on transmission, and afterwards with the real adhesive and cells by measuring assembly quantum efficiency and the cell-level current-voltage curve.

| Measurement | Question it answers | What it cannot answer |
| --- | --- | --- |
| Transmission spectrum | Which wavelengths the glass passes, and where the filtering edge sits | Nothing about current, because the sun and the cell are not in the measurement |
| Transmission after irradiation | How much transmission the glass retains under a defined exposure | How the adhesive underneath will age |
| Assembly EQE | How many externally collected charges each wavelength yields | Which internal layer caused a loss |
| Cell current-voltage curve | The power the assembled cell actually delivers, including the limiting junction | How the result changes under a different spectrum or temperature |

Environmental testing needs witness samples that can be compared with one another. A coated glass coupon shows what happens to the glass and the coating; a glass-adhesive sandwich isolates the adhesive; a complete cell assembly measures the total output. Their materials and processes should correspond to the flight configuration, so that when the spectrum and the electrical results are compared before and after exposure, the loss can be attributed to a layer rather than only quantified as a total. An assembly that has lost power is, on its own, a measurement of overall degradation.

The test programme divides by what is being tested. Ultraviolet exposure addresses filtering, glass darkening and adhesive transmission loss. Electron and proton irradiation address the corresponding particle conditions for the material and the cell. Measurements before and after use consistent spectrum, temperature and reference surface, so that a change in transmission can be lined up with a change in electrical output. Where the applicable standard is concerned, its scope covers crystalline silicon and single or multi-junction gallium arsenide cells above 50 micrometres in thickness. Ultra-thin flexible and newer routes need their own applicability assessment against their device and packaging characteristics. For established material combinations the standard ultraviolet verification can build on existing long-term degradation data. A new combination has to establish its basis through a longer engineering test before qualification. During that test the short-circuit current is monitored through the exposure, and current-voltage curves are measured in stages, so the progression is recorded rather than only the end point.

## FAQ

### Why not just use thicker glass and be done with it?

Because thickness is mass, and on a spacecraft mass is the budget everything else competes for. Thicker glass also absorbs more of the light that passes through it, so the shielding gain has to be weighed against an optical cost. The design point is a balance between transmission, shielding and areal density rather than a maximum.

### Is cerium in the glass the same as a ultraviolet filter?

Functionally yes, mechanically no. A filter is an additional optical element that removes a band; cerium doping makes the glass itself absorb that band. Both protect the layer underneath, and they differ in what they add to the stack and in how they behave after irradiation.

### Why does a datasheet give a band average instead of a curve?

Because a single number is easier to specify and to compare, and the average is a compact way to state how well the glass blocks a range. A curve is more informative, and many datasheets provide both: a set of defined readings for specification, and continuous spectra for engineering work.

### What does a higher transmittance figure always mean?

Only that, at the stated wavelength and in the stated measurement arrangement, more light passed the exit plane. The measurement includes the thickness, the coating state and how the back of the sample was treated. Comparing a measurement into the adhesive with one into air is not comparing two glasses, it is comparing two arrangements.

### Why measure a stacked cell junction by junction?

Because the junctions are in series and the weakest one limits the current, and because a single measurement of the whole cell cannot say which junction is short of current. Junction-by-junction quantum efficiency shows where the response is missing, which is the measurement that connects a spectral change in the cover glass to a change in output.

### Does the same reasoning apply to terrestrial modules?

The optics apply, the numbers do not. A terrestrial module also has a glass front, an encapsulant and a cell, and the same logic about interfaces and spectra holds. What differs is the environment: the encapsulant is chosen against humidity and thermal cycling rather than vacuum ultraviolet and charged particles, and the qualification tests are correspondingly different.

## Final thoughts

Cerium-doped cover glass is common on space solar arrays for a reason that can be checked at every step. It reduces the short-wavelength ultraviolet reaching the adhesive, and it helps the glass keep its transmission in the generation bands after irradiation. Neither claim is useful as a slogan. What makes them useful is that they can be measured, before and after a defined exposure, on the same optical path that the array actually uses.

The chain from a glass absorption band to a change in array output runs through the adhesive, the interfaces, the sub-cell responses and the series connection. Each link has its own measurement, and each measurement has its own conditions. A cover glass is chosen well when that whole chain has been walked, and when the numbers being compared were taken at the same thickness, on the same plane, under the same spectrum.

Data note: the transmittance values quoted here, 11.4% across 300-315 nm, 91.3% and 92.3% and 95.8% at 400 nm, the 332 nm cutoff at 46%, the 2.51 g/cm3 density and the 0.150 mm irradiation sample, are published values for a commercial cerium-doped borosilicate cover glass, cited as an example of how a datasheet is structured. They are third-party data rather than measurements made by us, they belong to the thickness and coating state stated with them, and they should not be transferred to another product. The photon-counting ratios used to illustrate external quantum efficiency are teaching assumptions. The UV-B and UV-A band limits are the standard wavelength divisions. The assembly standard referred to in the text is ECSS-E-ST-20-08C Rev.2.

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