KEYWORDS: Thin films, Thin film solar cells, Silicon, Tandem solar cells, Solar cells, Modeling, Thin film devices, Simulations, Photovoltaics, Performance modeling
Dedicated optical models are crucial for advancing the modelling of next-generation solar cells. Incorporating various textures of different shapes and sizes into solar cells significantly improves light management. This study optimizes the optical design and predicts the performance of a novel thin-film tandem solar cell device. The top cell features a hydrogenated amorphous silicon (a-Si:H) absorber layer, while the bottom cell incorporates a low-bandgap tin-lead-based perovskite (Sn-Pb PVK) absorber layer, all supported on a flexible aluminum substrate Optical simulations exhibit 24 mA/cm² as total implied photo-current density when the absorber layer thicknesses are current-matched. The maximum absorptance reaches 80% at 500 nm for a-Si:H and only 62% at 800 nm for Sn-Pb PVK sub-cells. Experimental results show open-circuit voltages of 0.9 V for a-Si:H and 0.85 V for Sn-Pb PVK solar cells. Based on the highest achieved fill factor of 0.77, the researchers estimated a power conversion efficiency exceeding 16%.
Thin-film silicon single- and multi-junctions are a viable option to manufacture lightweight, flexible solar modules via high-throughput roll-to-roll (R2R) processes, starting from earth-abundant, non-toxic raw materials, at very cost competitive levels and with a range of application spanning from large area solar plants to portable devices. Nevertheless, flexible thin-film silicon modules have currently lower power conversion efficiency (PCE) compared to modules fabricated on glass substrates. Here, we focus on improving the efficiency of flexible single-junction modules by changing the chemical composition and the growth conditions of the p-doped window layer. Highly efficient devices require a window layer with excellent optical and electronic properties so that incoming light photons can easily reach the absorber layer, while photogenerated holes can be promptly extracted from the device. Our baseline modules have a p-doped hydrogenated silicon carbide (p-SiC:H) window layer. In order to simultaneously reduce optical losses and improve the charge collection, we reduced the thickness of p-SiC:H by modifying the plasma enhanced chemical vapor deposition tool, and we inserted a layer of p-doped nanocrystalline silicon oxide (p-nc-SiOx:H) in between p-SiC:H and TCO. The double p-layer modules that we obtained showed a 2% increase in the open circuit voltage compared to the single p-layer modules. Fine-tuning the deposition conditions for both p-layers will further reduce optical and resistive losses and improve the PCE of the modules; additionally, the double p-layer architecture will allow for an accurate control of the light transmission through the window layer, facilitating the current matching for multi-junction modules.
Introduction
In order to increase efficiency in flexible thin-film solar cells, it is crucial to employ advanced light trapping schemes [1]. Such light-trapping schemes must be coupled with high quality absorber layers to ensure high open-circuit voltages in multi-junction solar cells [2][3][4]. The challenge is that the introduction of textured interfaces that facilitate enhanced light trapping competes with the ability to process high quality PV materials on top of it. To cope with these limitations, modulated surface texturing (MST) approach has been employed [5]. It consists of superimposing the micro-sized craters induced on the substrate with naturally nano-texturing of a TCO [6]. The embodiment of this approach has lead very high efficiency for tandem micro-morph solar cells on glass substrates [7].
In this work, we show the development of such MST approach on temporary Al foil substrates. We first investigate and characterize different texturing techniques, then we grow nc-Si:H absorber layer to investigate its quality.
Experimental details
Bare Al foils are divided in two different categories; i) direct etching and ii) sacrificial layer etching. The first samples are etched directly in KOH diluted in H2O at T > 30 °C. The second category samples instead undergo a sputtering deposition of ITO/AZO as sacrificial layers. This layer is subsequently etched in HF:H2O2:H2O or KOH diluted in H2O at T > 30 °C. The samples are characterized by AFM, SEM and angular intensity diffraction spectrophotometer.
Results and discussion
Aluminum samples etched in KOH result in a relatively deep craters with an aspect ratio of 10% (RRMS = 322 nm, LC = 3.02 μm). Al samples etched via sacrificial layer have a similar aspect ratio to the one of direct etching (~10%). The difference between ITO and AZO sacrificial is the craters’ size and depth. The different etching solutions (HF:H2O2:H2O or KOH/H2O) have also an impact on craters’ distribution in Al textured samples. The physical mechanism of this etching is that the porous sacrificial layers deposited allow an anisotropic etching that will induce craters in the Al substrate once the etching is completed. A further etching with diluted H3PO4 leads a cleaning of solid precipitates after texturing process. All these process conditions set, it is possible to grow high quality, >2 μm-thick nc-Si absorber layers in combination with excellent light trapping for micro-morph tandem application.
References
[1] A. Shah et. al., Prog. Photovolt: Res. Appl. 2004; 12:113–142 (DOI: 10.1002/pip.533).
[2] H. Sai et. al., Appl. Phys. Lett 101, 173901 (2012).
[3] M. Kambe et al., doi: 10.1109/PVSC.2009.5411411.
[4] J. Bailat et al., JAP 2003; 93: 5727–5732.
[5] H. Tan, et al, Appl Phys. Lett. 103, 173905 (2013).
[6] J. Müller et. al., Solar Energy, doi.org/10.1016/j.solener.2004.03.015.
[7] H.Tan et. al., doi: 10.1002/pip.
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