Although their spectral features demonstrate the potential of outperform-ing any other solar simulator radiation source, the output power of nowa-days commercially available supercontinuum lasers impedes their usage in solar simulators for large area industrial solar cells. In order to over-come this limitation a supercontinuum-based differential measurement ap-proach is presented in this chapter that has been developed in the course of this work [120].
The new approach takes advantage of combining a chopped and low powered, but (nearly) perfectly shaped supercontinuum spectrum with high powered bias irradiation of a much less demanding spectrum. Mea-suring the differential current response of the device under test, theISCis directly measured as no spectral mismatch correction has to be applied.1 In Fig. 7.1a the experimental setup for generation and spectral shaping of supercontinuum radiation is shown that has been developed based on the work presented byDennis et al. in [122].
In the following subsections the individual parts of the experimental approach are explained in more detail.
1Naturally, this does only hold for a shaped supercontinuum spectrum that per-fectly resembles the considered standard solar spectrum. Moreover, the simplification of linear cells is underlying this statement. The case of nonlinear cells is briefly ad-dressed in Section 7.4.
(a) (b)
Figure 7.1: (a) Schematic setup for generation and shaping of supercontinuum radiation. The ultrashort pulses originate from the same laser system that is being used for the measurement facility presented in Chapter 6 (compare Fig. 6.1). The supercontinuum is generated inside a photonic crystal fiber (PCF) and spectrally shaped by a spatial amplitude mask and a grating light valve (GLV). (b) Illustration of a single GLV pixel with some voltage applied to the deflected ribbons resulting in an optical path length difference for radiation incident from top (image from [123]).
7.2.1 Generation of Supercontinuum Radiation
Instead of applying a commercially available supercontinuum laser, the developed approach takes advantage of the laser radiation emitted from the measurement facility presented in Chapter 6. Coupling ultrashort laser pulses of approximately 100 fs pulse duration into a photonic crystal fiber (PCF,FemtoWHITE 800/NKT Photonics) broadband supercontin-uum radiation is generated by highly nonlinear optical effects.2 Amplitude and spectral shape of the generated supercontinuum vary with incident pulse intensity and wavelength. For typical input parameters applied in this work, a supercontinuum ranging from 450 to more than 1600 nm is generated. A typical supercontinuum spectrum generated with the setup
2In Section 3.2.5 a brief description of the physical processes underlying supercon-tinuum generation is given. For a more profound discussion the interested reader is referred to [29].
is given as dashed blue line in Fig. 7.2a for the spectral range up to 1200 nm, that is most relevant for silicon solar cell measurements.
7.2.2 Spectral Shaping of Supercontinuum Radiation
The supercontinuum radiation generated by the PCF is collected by a microscope objective (not shown in Fig. 7.1a) and spectrally dispersed by a prism made of F2 flint glass. In analogy to the prism monochroma-tor described in Section 6.2.3 and Fig. 6.4, a concave mirror is applied to collimate different spectral components while simultaneously focusing identical ones in the focal plane of the mirror. The spectral shaping it-self is conducted in that focal plane by a combination of a Grating Light ValveTM(GLV) from Silicon Light Machines and a spatial amplitude mask.
The GLV is a dynamically adjustable phase grating that consists of an array of aluminum coated silicon nitride ribbons of 2-4µm width and 100-300µm length. Six of these form a single pixel of approximately 25.5µm pitch that is located over a common electrode as shown in Fig. 7.1b [123].
If a voltage is applied to the ribbons they are deflected towards the sub-strate by electrostatic force. Applying a voltage to every second ribbon induces an optical path length difference between neighboring ribbons re-sulting in a (partial) diffraction of the incident radiation. Setting the voltage to zero, the ribbons move back to the non-deflected state and the GLV acts as a mirror. By variation of the applied voltage the optical path length difference can be tuned between 0 and approximately 240 nm in 1024 steps allowing for a pixel-wise control of diffraction efficiency. As the setup shown in Fig. 7.1a exhibits a distinct wavelength-to-pixel rela-tion the GLV therefore allows for spectrally resolved amplitude variarela-tions of the back-reflected 0th order beam. Slightly tilting the GLV and im-plementing an additional mirror separates the input radiation from its back-reflected counterpart and completes this fully computer-controlled approach for real-time variations of the shaped supercontinuum output.
Full extinction requires perfectly destructive interference and, thus, fulfillment of theλ/4-condition. As a consequence, the full extinction po-tential of the GLV is limited to wavelengths below approximately 950 nm.
Therefore, a specifically designed spatial amplitude mask is additionally
(a) (b)
Figure 7.2: (a) Unshaped (dashed blue line) and shaped (solid blue) supercontinuum (SC) spectrum that mimics the AM1.5g standard so-lar spectrum (black). The red line indicates a 5 nm bandwidth moving average of the AM1.5g standard spectrum. Published in [120]. (b) Comparison of computed spectra combining SC with additional UV radiation to the AM1.5g standard solar spectrum (black lines). UV sources are either frequency-doubled SHG radiation (bottom, blue line, 425 nm center wavelength), a single LED at 425 nm center wave-length (middle, red line) or two LEDs at 365 and 425 nm center wavelengths (top, green line).
inserted close to the GLV to allow for more efficient extinction of wave-lengths longer than 950 nm by (rather) fixed losses.
In Fig. 7.2a a shaped supercontinuum (solid blue line) that mimics the AM1.5g (black line) is shown, demonstrating the excellent spectral match of the shaped supercontinuum obtained from the presented setup. It is noteworthy, that this shaped spectrum rather represents a first shaping attempt with the presented setup and that substantial improvements in spectral match are expected to be realized in near future. Especially the development of a refined control of the setup’s spectral output, being conducted in the course ofX. Wang’s master thesis that is supervised by the author of this work [124], might improve spectral shaping capabilities significantly.
7.2.3 Prospects for Tackling the Lack of UV Radiation
In addition to the excellent spectral match of shaped supercontinuum and AM1.5g standard spectrum, the lack of UV radiation below approximately 460 nm is apparent from Fig. 7.2a. As this lack induces a non-vanishing spectral mismatch correction, following prospects for tackling that missing spectral part are being discussed in the next section:
• add frequency-doubled radiation of the incident laser pulse (SHG)
• add one or more LEDs in the UV spectral range
• apply a PCF generating supercontinuum radiation in the entire spec-tral range
The first two options might be realized by combining the additional radiation from frequency-doubling or LEDs with the shaped supercon-tinuum radiation by the optical mixing rod applied in the measurement system (see e.g. Figs. 6.6 and 6.7). Moreover, the frequency-doubled ra-diation is directly available from the SHG-unit of the same measurement facility (compare Fig. 6.1). UV-LEDs are commercially available at sev-eral center wavelengths. Spectral shaping of either additional radiation source by the presented shaping setup is unfavorable due to their small bandwidth (SHG, see Fig. 6.2a) or their incoherent nature (LEDs). In Fig. 7.2b computed spectra obtained when adding SHG or LED radiation to the supercontinuum spectrum are shown.
The third option requires application of a new PCF that is capable of covering the entire relevant spectral range by supercontinuum radiation.
In fact, researchers have recently demonstrated that they are capable of producing a PCF that exhibits this feature [125]. Assuming application of such a PCF allows for a virtually perfect imitation of a standard solar spectrum and, thus, appears to be most promising.
The potential of either of these approaches regarding spectral mismatch and its uncertainty is assessed in the next section.