Indoor photovoltaic (IPV) devices are poised to make a significant contribution to the proliferation of the “Internet of Things” (IoT). For the accurate intercomparison of IPVs (and, hence, to advance the rational development of the technology), lighting conditions representative of those in typical indoor settings must be created reproducibly. As indoor lighting is invariably broadband, this will typically require the use of optical attenuation to achieve varying irradiance conditions at the device under test location. However, most forms of optical attenuation will suffer from some degree of spectral dispersion, creating sources of uncertainty for key figures of merit, such as power conversion efficiency. In this work, we examine the contribution of the mode of optical attenuation to the accurate characterization of IPV systems. We discuss requirements for broadband light source attenuation for the accurate characterization of photovoltaic devices under indoor illumination and consider the importance of using suitable reference devices for light intensity calibration. Furthermore, we experimentally verify attenuation methods typically used, including power control of the light source itself, use of neutral density filters, and advanced attenuation based on tandem prism attenuators. Finally, spectral shape alteration-induced uncertainties in performance parameter determination of photovoltaic cells under indoor illumination are quantified for three common broadband light attenuation methods, where we found , , and up to ambiguity in photovoltaic device efficiency when using LED power control, prism attenuators, and neutral density filter-based broadband light attenuation, respectively.
Over the past few years, indoor photovoltaic (IPV) technology has witnessed notable advancements in device efficiency and fabrication development.1–5 As a result, IPV has emerged as a promising solution for powering Internet of Things (IoT) devices that are widely utilized in various fields such as electronics, sensing, and machine learning via the harvesting and utilization of ambient light.3,5–8 Compared to outdoor photovoltaics, IPV technology presents fewer complexities thanks to the relatively lower light levels and gentler operating conditions typically encountered indoors. These favorable conditions contribute to enhanced device longevity, which opens up new possibilities for solution-processed semiconductors such as perovskites and organic materials.8–13 The tunability of the energy bandgap of these materials and their low embodied energy make them particularly well-suited for IPV applications, thus driving further interest and exploration in this domain.
As new PV materials and devices are being developed for IPV use, the lack of standardized light sources (LSs) (in terms of both spectrum and irradiance) for IPV testing complicates their development as different loss mechanisms dominate at different intensities.14,15 Real-world IPV devices operate under variable conditions, requiring benchmarking with a range of lighting conditions.16 Accurate characterization of IPV devices is necessary for their development, with research focusing on experimental factors such as spectral validation, illumination masks, stray light, and radiation uniformity.17,18 Subsequently, measuring IPV devices under a range of irradiances reflects real-world scenarios and helps to identify dominant power conversion efficiency (PCE) loss mechanisms essential for optimizing IPV development.19
Light intensity-dependent photocurrent (IPC) measurement has been previously reported as a powerful tool for understanding photovoltaic device fundamentals.20,21 In particular, this experimental technique can identify different photocurrent loss mechanisms that are effective at different intensities. IPC provides insight into the varying performance-limiting factors, including trap-assisted recombination, bimolecular recombination, the build-up of space-charge due to imbalanced mobility of charge carriers, and series resistance limitations.22 Experimentally, IPC measurements are typically conducted by varying the irradiance at a single wavelength over orders of magnitude. This can be achieved using a laser light source with variable output power in combination with an attenuator, for example, a series of neutral density (ND) filters, resulting in a quasi-continuum of points in the irradiance regime of interest. Prior to an IPC measurement, a photodiode of known responsivity is typically used as a reference device for light calibration. Similarly, the measurement of the open circuit voltage’s (Voc) dependence on irradiance can also yield insights into the operation of a PV system.23,24 The IPC measurement technique discussed above, utilizing a laser light source with variable output power, offers a straightforward approach to investigating the light intensity dependence of photocurrent. However, it is important to note that this simple approach may not be as straightforward when working with broadband sources.
The characterization of device performance under various indoor illumination intensities is, in turn, complicated due to the tendency of most attenuation techniques to change the spectral shape of broadband light sources. Such an erratic phenomenon can affect the beam quality (i.e., beam homogeneity and spectral validation) and the linearity of the calibration.25 While the latter will inevitably be linked to errors in light intensity determination using reference devices, the former will ultimately affect IPV cell and module performance characterization. Such links between attenuator-induced spectral effects of broadband light sources and related uncertainties in device PV parameters probed under indoor illumination have not been established yet but are of utmost importance for driving development and research in the IPV community—this was indeed the case in the development of outdoor PV standards.
In this work, we outline the importance of considering spectral-dependent effects when attenuating broadband light sources for IPV characterization. We experimentally verified the attenuation methods typically used. We further consider the importance of using suitable reference devices and demonstrate the need for spectrally stable light attenuation from broadband light sources for IPV device characterization. Following this, we determine the uncertainties in device PV parameters as associated with different broadband light source attenuation techniques.
RESULTS AND DISCUSSION
Optical attenuation of broadband light sources with constant radiant power
Another method of optical attenuation with a low degree of spectral dispersion is the use of arrays of micromirrors, such as those found in a digital micromirror device (DMD),25 which can reflect the incident beam at varying angles. The irradiance at a given location can then be controlled by selecting the number of “pixels” that are steered toward the DUT or toward baffles.26
Varying the radiant power of an LED source
The radiant power of LED sources can be controlled by varying their driving current. However, this will cause a spectral shift in the output spectrum. For most white (e.g., phosphor-coated blue) LEDs, this will manifest primarily as a blueshift and increase in full width at half maximum (FWHM) of the phosphor-related emission peak. Figure 2 shows the emission spectrum of a laboratory LED source (Prizmatix UHP-T-LED-White) plotted as a function of wavelength. The inset shows the corresponding blueshift in emission peak when increasing the LED-driving current. The radiant power of LED sources can be controlled by varying their current, although this approach has limitations in terms of its dynamic range. Operating the LED at extremely low or high currents may not be feasible, which restricts the range of achievable power levels. Typically, pulse-width modulation (PWM) is employed for dimming purposes, but this method is not suitable for accurate and precise testing of indoor photovoltaics (IPV). Therefore, while controlling LED current alone may not be sufficient for IPV testing, it can still be a valuable tool when used in conjunction with other techniques.
Next, the responsivity of all four reference devices was measured using our IPV test setup, as described in detail in the supplementary material. Here, the 818-UV Si photodiode was used for light intensity calibration, assuming intensity-dependent spectral response and broad wavelength linearity [see Fig. 3(a)]. Figure 3(b) shows the corresponding total (i.e., wavelength-integrated) responsivity as a function of irradiance and is compared for all four reference devices. As shown, all four devices show a non-constant responsivity over the probed intensity regime, indicative of attenuator-induced spectral changes. In this regard, the absolute change in normalized responsivity, as an estimate for attenuator-induced spectral changes, ΔRNorm = RNorm,max − RNorm,min [see Fig. 3(c)], was determined for all four reference systems, from which the GaAs shows the highest ΔRNorm with , as compared to the other three Si-based reference devices: (Thorlabs, FDS1010), (ScienceTech RefQ) and (Newport, 818-UV). We note that attenuator-induced spectral changes of the input light should be in general avoided; the corresponding attenuator can, however, be quite expensive and complex to program.25 Thus, when using conventional and often spectral change-inducing attenuators, such as ND filters, tandem prisms, or mesh filters, it is recommended to use a reference device for the light intensity calibration process that is sensitive enough to detect those spectral changes (i.e., a reference device with ΔRNorm as large as possible).
Furthermore, the spatial uniformity of the probe light at the position of the reference cell and DUT (preferably the same) needs to be considered. As such, photodiodes with areas significantly smaller than those of the DUT can lead to drastic errors in the estimation of total irradiance if the spatial uniformity is poor. On the other hand, if bus-barred reference cells are used, it may be challenging to measure their absolute photovoltaic external quantum efficiency (EQEPV). In practical terms, it is, therefore, recommended to have good spatial probe beam uniformity and to measure multiple reference devices at the location of the DUT to minimize overall uncertainty. The selection of appropriate reference devices requires attention to attenuator-induced spectral changes, the selection of sensitive reference devices, consideration of spatial probe beam uniformity, and multiple measurements to minimize uncertainty.
Comparison of IPV test setups
To demonstrate the various approaches to broadband attenuation and setup calibration outlined above, an IPV characterization apparatus comprising a 4000 K LED (Prizmatix, UHP-T-LED-White) was built. Figure 4(a) shows a schematic of the IPV test apparatus. To illuminate the device under test (DUT), a liquid light guide (LLD; Prizmatix) was used to transfer the attenuated probe light from the light source (LS) to a collimator (C; Prizmatix) directly attached to a collimation tube (CT). Here, we consider three types of LS attenuation that would be representative of typical equipment available in optoelectronic characterization laboratories, including (i) ND filters mounted onto a motor-controlled wheel with six positions in combination with LED power control, (ii) a tandem prism attenuator (Standa), which is also motor-controlled in combination with LED power control, and finally (iii) the LED only. Note that the LED driver uses an internal PID controller to stabilize the output power.
Figure 4(b) shows the result of the initial calibration with a NIST-calibrated silicon reference photodiode (Newport 818-UV) with known responsivity. For comparison, the normalized responsivity (R/R0) (calculated via R = Iref/IL, where Iref denotes the reference photodiode current and IL denotes the input light intensity) points are plotted against the light intensity. As the only form of attenuation, the range of irradiances achievable by varying the LED output power (blue symbols) is only one order of magnitude (i.e., ∼10−3 < IL < 10−2 W/cm2), thus much smaller in comparison to the prism coupler (green symbols) (about 2 orders of magnitude; ∼10−5 < IL < 10−3 W/cm2) and ND filter (red symbols) (four orders of magnitude; ∼10−6 < IL < 10−2 W/cm2). Despite the increased irradiance window available to probe the photovoltaic performance under indoor illumination, the ND filter wheel attenuation contributes the largest source of spectral-induced error with a maximum change of responsivity of up to 15%. In comparison, attenuation induced responsivity changes, when using the LED only, are minimized to % only. As mentioned above, lower irradiances can also be achieved with the LED power control alone, e.g., by increasing the distance between the collimation tube and the DUT. However, this approach may require a lot of space and thus have limited practicality.
Uncertainty and spectral deviation
Changes in the spectral shape of indoor light broadband sources will inevitably influence the accuracy of light intensity calibration and the performance characterization of IPV devices. To this end, we simulated the indoor PCE intensity dependence of a nm tick, 1.79 eV bandgap FA0.85Cs0.15Pb(I0.6Br0.4)3 perovskite PV device, comparing different degrees of attenuator-induced changes in the spectral shape of a broadband light source. Details of the perovskite device fabrication and performance characterization under AM1.5 G conditions are provided elsewhere.27 For the simulations, we used a recently introduced approach, which estimates realistic limits for PCEs under any input spectrum while accounting for radiative and non-radiative losses.28
In our calculations, we compared the above three IPV test setups with indoor light attenuation via (i) LED power control only, (ii) prism attenuator, and (iii) ND filters. The corresponding light spectra at different light attenuation settings were recorded manually using a photonic multichannel analyzer (PMA-12, Hamamatsu) and used as input spectra. The recorded spectra are shown in the supplementary material, Fig. S1. Figures 5(a)–5(c) show the relative changes in output spectra (I/Io) for the three IPV apparatus setups. Here, only minor changes are observed for light attenuation via LED power control only [see Fig. 5(a)], characterized by I/Io ≈ 1 across the wavelength regime. Minor changes at ∼450 nm are caused by the shift in the LED emission spectrum associated with a change in input current. The prism attenuator, on the other hand, clearly affects the shape of the broadband LED spectrum for wavelengths nm—a maximum I/Io ≈ 2 is observed for the strongest attenuation [see Fig. 5(b)]. Finally, as shown in Fig. 5(c), light attenuation via ND filters causes drastic changes in the shape of the output spectrum. In particular, the transmission of high wavelengths (i.e., nm) increases at higher attenuations, noticeably disturbing the shape of the broadband LED spectrum (I/Io > 10).
Finally, we want to emphasize that one can, in principle, also correct for spectral changes induced through the IPV attenuator system using a correction function. However, doing so in a dynamic way (i.e., measuring at different light intensities) necessarily increases the uncertainty of the measurement. As such, the propagation of the measurement uncertainty of (i) the spectrum at each attenuator condition and (ii) the additional uncertainty in returning to that specific condition would increase.
In practical terms, it is, therefore, recommended to use an IPV test apparatus with light attenuation characterized by as few changes in spectral shape as possible. In particular, the high wavelength regimes are expected to contribute to the spectrum at high light attenuation when using conventional prism and ND filter attenuators [see Figs. 5(b) and 5(c)]. For light attenuation in an IPV test apparatus (additionally) controlled via LED power control, one needs to consider changes in the low wavelength regime as associated with the LED pump peak emission [see Fig. 5(a)]. Both, spectral deviations of the IPV test apparatus light source to a standard (e.g., CIE LED-B4) and attenuator-induced spectral changes must be considered when probing PV figures of merit.
With the convergence of advancements in low-power electronics, wireless communications, automation, big data, and sensors, the emergence of ambient light harvesting via indoor photovoltaics is set to create fresh prospects in areas such as wireless sensing and the Internet of Things (IoT). To enable the growth of this field, accurate characterization methods for IPV devices are required. Measuring IPV devices over a broad irradiance range provides both practical and theoretical insights that will prove invaluable in scientific and technical development. The spectral dependence of the attenuator systems used to reproducibly create varying irradiance can lead to sources of uncertainty if not properly considered. Further, suitable reference devices with good linearity should be used that are optimal for the light source and bandgap of the PV system being characterized. Here, reference devices that are sensitive to spectral changes (i.e., large ΔRNorm) are recommended. The origin of spectral changes in the output spectrum can be, inter alia, related to LED-driving current variations and attenuator-induced wavelength transmission changes. For our IPV test apparatus, we determined those non-linear phenomena to cause uncertainty in responsivity (and, thus, power conversion efficiency) as high as % when using conventional ND filters.
The derivation of the expression for spatial non-uniformity, a description of the calibration procedure, and data obtained under additional experimental conditions are given in the supplementary material.
This work was funded through the Welsh Government’s Sêr Cymru II Program “Sustainable Advanced Materials” (Welsh European Funding Office—European Regional Development Fund). P.M. is a Sêr Cymru II Research Chair funded through the Welsh Government’s Sêr Cymru II “Sustainable Advanced Materials” Program (European Regional Development Fund, Welsh European Funding Office, and Swansea University Strategic Initiative). This work was also funded by the UKRI through the EPSRC Program Grant No. EP/T028513/1 Application Targeted and Integrated Photovoltaics. The authors wish to thank George Koutsourakis and James C. Blakesley from National Physical Laboratory (NPL, United Kingdom) for fruitful discussions and Pietro Caprioglio and Michael Farrar for providing perovskite photovoltaic devices.
Conflict of Interest
The authors have no conflicts of disclose.
S.Z. performed the experiments. P.M. and A.A. assisted with analyzing and interpreting the data. G.B. supervised the work. All authors contributed to development of the manuscript first drafted by S.Z. together with G.B.
Stefan Zeiske: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Software (lead); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Paul Meredith: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (supporting); Methodology (supporting); Project administration (equal); Resources (equal); Software (lead); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Ardalan Armin: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Writing – review & editing (equal). Gregory Burwell: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (lead); Writing – review & editing (equal).
The data that support the findings of this study are available within the supplementary material.