Using modular components and instruments available on the market enables building well customized QE configurations that offer great flexibility needed by solar cell researchers for testing existing and new solar cells.
Quantum efficiency (QE) measurement is one of the most significant characterization tools for solar cells, allowing for quantifying the efficiency of the conversion of light to electrons as a function of wavelength of the impinging light. The measurement is used to test new cell structures and materials as well as to verify the reproducible production of solar cells and modules. There are only a few turn-key QE measurement systems available and by definition these systems have defined limits. Building a measurement system based on modular components offers great flexibility and customization possibilities to meet the specific and changing demands of researchers, but several aspects have to be considered for choosing the proper components for a high accuracy measurement system. This article is a discussion of needed hardware for QE measurement from light beam generation to its absorption by the cell under test.
Theoretical background of quantum efficiency
The QE measurement is an indicator of how good the solar cell is at converting sun light to electricity. A solar cell should optimally have high spectral response at the point at which the spectral component of the sunlight is abundant. Quantum efficiency is the ratio of the number of charge carriers that are collected by the solar cell to the number of photons of a given wavelength shining on the solar cell (Fig. 1).
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| Figure 1. External QE measurement obtained on a mono-crystalline Si solar cell. |
If all the photons of a certain wavelength are absorbed and the resulting carriers are collected, then the QE at that particular wavelength has a value of 100%. The QE for photons with energy below the bandgap is zero. Hence the quantum ideal efficiency has a square shape (Fig. 1). However, the QE for most solar cells is reduced because of the effects of recombination, where charge carriers are not able to move into an external circuit. The same mechanisms that affect the collection probability also affect the QE. For example, modifying the cell front surface can affect carriers generated near the surface. And because high-energy (blue) light is absorbed very close to the surface, considerable recombination at the front surface will affect the “blue” portion of the QE.
Similarly, lower energy (green) light is absorbed in the bulk of a solar cell, and a low diffusion length will affect the collection probability from the solar cell bulk, reducing the QE in the green portion of the spectrum. At lower energies QE is rapidly reduced due to passivation of rear cell surface, reduced absorption, and low diffusion length at long wavelengths. Two types of QE of a solar cell are often considered:
- External QE (EQE) includes the effect of optical losses such as transmission through the cell and reflection of light away from the cell.
- Internal QE(IQE) refers to the efficiency under exclusion of transmitted or reflected light by the cell. Only the absorbed portion of light can generate charge carriers that can generate current.
Principle of QE measurement
The band gap structure in a semiconductor device introduces wavelength dependent absorptivity. A photon with energy larger than the band gap is typically absorbed by the material, while a photon with energy smaller than the band gap is not. The absorbed photon creates an electron-hole pair charge, which leads to creation of electricity. To determine a device’s QE, one must know the power reaching the cell and it’s produced current at each wavelength. During a test, the computer records the currents produced by the test cell and by a calibrated photo detector. The ratio of cell current Icell(λ) to beam power is the cell responsivity, which is converted to units of quantum efficiency by using the following equation:
where h is Planck’s constant, c is the speed of light, e is the electron charge, Iref(λ) is the current of the calibrated reference detector and R(λ) is the known responsivity value of the reference detector (in A/W).
Instrumentation
A typical setup for QE measurement consists of a tunable light source, detection system, bias light source and accessories for proper beam manipulation and delivery (Fig. 2). The main task of the system is to provide a nearly monochromatic light source with a typical bandpass of 1-10nm, tunable over the whole wavelength range at which the solar cell is active.
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| Figure 2. Typical setup for EQE measurement of crystalline silicon cells. |
Tunable light source
Figure 3 shows a scheme of a typical tunable light source. The choice of the monochromator depends on the needed bandpass, wave length range and power throughput. For a narrow bandpass (0.1-0.5nm) a monochromator of large focal length (250mm or larger) is needed, while for a wider bandpass (0.5-10nm) a monochromator with a short focal length (about 125mm) is a good choice for saving costs and space. On the other hand, even if a narrow bandpass is not needed, a monochromator with a large focal length may be preferred, especially for QE measurement of solar cells with low optical response. This is because such a monochromator allows higher power throughput at a given bandpass. The bandpass can be adjusted through variation of monochromator slit width. Today’s monochromators are equipped with micrometer driven slits that can be motorized for fully automation of the setup. Since the bandpass depends on the wavelength and thus for a constant bandpass one needs to adjust the slit width during a wavelength scan, the slit motorization allows fully automation of this function for easier and more precise scans.
In most QE applications, a very broad wavelength range is needed, typically wider than the range of the monochromator grating efficiency. A motorized monochromator equipped with two gratings or more should be chosen for covering a broad wavelength range. The gratings should have high efficiency over the whole range and the line density should be as high as possible for achieving a moderate to high resolution and high power throughput. Common line densities are 600 – 3000 lines/mm for UV-VIS, NIR- and IR wavelength ranges.
During a scan of a broad wave range, high order intensities of short wavelengths are present in the “monochromatic” beam at longer wavelengths; e. g., tuning the monochromator to 800nm, the 800nm beam is “contaminated” with light of 400nm (second order reflection of 400nm), in case the source emits at 400 nm. To eliminate this effect, order sorting filters are applied for cutting off the entire short wave range of the source. For example, in a wavelength scan of 350-1100nm with a Xe light source (200-2400nm), two filters are needed with cut-on wavelengths of about 550 and 300nm. A filter wheel is typically used in front of the monochromator, which is typically commanded by the monochromator software to use the proper filters over the entire wave ranges during a scan.
Three groups of light sources are commonly used for monochromator illumination, deuterium, quartz-tungsten-halogen (QTH) or xenon (Xe) arc sources. The choice of a proper light source depends on the needed wavelength range and power throughput. Deuterium sources are useful in UV range, while QTH sources emit in a very broad range from visible to IR. In cases where the tunable light source should cover UV- and VIS-NIR range, two light sources deuterium and QTH are coupled to the input of the monochromator (Fig. 3). Both sources emit a very smooth spectrum, free of irradiance minima and maxima, which is whenever possible preferred, since the smooth output spectrum enables high repeatability and accuracy of the wavelength scans and thus of the measured QE.
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| Figure 3. Scheme of a typical tunable light source. |
An additional advantage of deuterium and QTH sources is the excellent temporal beam stability. A drawback of QTH sources is the very limited lamp life time and moreover the filament size is typically very large, compared with the width of the monochromator input slit. As a result of this, it is impossible to tightly focus the output beam of a QTH source to a small spot on the slit, needed for high throughput into the monochromator. This effect is more pronounced for high power sources. A trade off between lamp power and filament size is considered for a given monochromator and resolution, which is about 50-100W lamp operation power for a small monochromator and 100-300W for a large one.
Xenon arc sources emit much brighter than QTH sources and the arc source is markedly smaller so that they are clearly preferred for high throughput. On the other hand the output spectrum of Xe sources is not smooth. The power distribution above the wavelength of the monochromatic beam has power maxima, which lead to higher measurements uncertainties, especially in the wavelength range 800-1000nm. In order to minimize this effect, the resolution of the wavelength scans should be set as low as possible. An additional difficulty of using a Xe source is its high temporal instability, which is a general problem of arc sources. Overcoming this difficulty requires stabilizing the beam intensity through a feedback system that senses the lamp intensity and controls its operation power, which allows maintaining a constant intensity.
Source/monochromator coupling optics
The easiest way for coupling a source to a monochromator is using fiber optics. Unfortunately, this coupling method suffers under very large power loss. Thus, in most QE applications, it is preferred to use free space-based optics, which requires maintaining an optical axis for the system and special care in choosing the right optics for high power throughput and low level of stray light. For collecting as much power as possible from the light source, a lens condenser with low F-number (F/#c) can be used. The collected beam from the condenser is then a parallel beam that needs to be re-focused on the monochromator slit using a plano-convex lens (F/#re).
The ratio (F/#re)/F/#c is equal to the image magnification of the lamp arc or filament and thus in order to keep its image as small as possible, a high F/#c has to be chosen; a trade off between F/#c and power throughput of the whole system has to be considered. For an F/4 monochromator an optimal F/#c is about F/1-F/1.5. The choice of the re-focusing lens (F/#re) depends only on the monochromator F/# (F/#m). Ideally, F/#re and F/#m should be equal for best illumination of the monochromator. In this case, lowest stray light and maximum throughput is achieved. Figure 4 illustrates why this F/# matching is important for proper monochromator illumination. The solid angle of the acceptance pyramid becomes too large at lower F/# of the re-focussing lens (F/#re) which leads to overfilling the monochromator grating and thus to a high level of stray light. At larger F/#re, the beam would under fill the grating, which leads to loss of resolution. The ideal situation is given when F/#re= F/#c.
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| Figure 4. Optical scheme of monochromator illumination. |
For simplifying the choice of proper optics for a given light source-monochromator combination optical kits are commercially available with proper optics and optomechanical parts for maintaining the equipments on one optical height. Using such kits it becomes easy to get the maximum possible throughput of the system.
Conclusion
Using modular components and instruments available on the market enables building well customized QE configurations that offer great flexibility needed by solar cell researchers for testing existing and new solar cells. Only when components are carefully chosen to fit to each other and fulfill the requirements of the application, the full potential of available components is used and highly repeatable QE measurements can be made under high signal to noise ratio leading to very accurate measurement results.
References
1. ASTM Standard E 973M – 96, West Conshohocken, PA: American Society for Testing and Materials, 1996.
Adnan Adla, PhD is a product manager at Newport Spectra-Physics GmbH, Guerickeweg 7, 64291 Darmstadt, Germany; ph.: +49 (0)6151/708-355; email [email protected]
