Identifying recombination mechanisms in glass- and metal-mounted perovskite solar cells

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Matt Carnie, 255
Publication date: 5th February 2015

The lab-scale efficiency of all solid-state perovskite solar cells (PSCs) has taken an extraordinary trajectory since their introduction to the research literature in 2012. Initial efficiencies reported were 9.7 % ¹and 10.9 % ²and in less than three years, the NREL certified power conversion efficiency stands at an astonishing 20.1 % ³.  Since their introduction, researchers have begun to probe and elucidate the recombination mechanisms than can lead to voltage losses in perovskite devices. Approaches have included using transient photovoltage  (TPV) decays ^4,5, laser transient absorption spectroscopy (L-TAS) ⁵and impedance spectroscopy (EIS) ⁶. Our work so far in the emerging field of PSCs primarily concerns materials and process developments but by applying some of the techniques mentioned above, to our developments in materials and device architectures, we have observed changes to recombination kinetics, which have given insight in to the nature of the recombination mechanisms. Most significant of our recent advancements is the development of an indium-free, transparent, self-adhesive, laminate electrode to replace the evaporated gold cathode ⁷. This has allowed us to characterize devices whilst under illumination from both the photo-anode side of the device (forward illumination) and the counter-electrode side of the device (reverse illumination). When measuring the TPV decay in such a way (Figure 1), we consistently observe marginally faster recombination (for a given V_OC) when illuminated in reverse from the counter electrode side of the device. In this instance, the charges are generated closer to the perovskite/Spiro-OMeTAD interface and so the faster recombination indicates that interfacial charge recombination at the perovskite/Spiro-OMeTAD interface is a significant contributor to the overall charge recombination in these devices. The transparent, self-adhesive electrode has also allowed us to fabricate devices on titanium foil substrates where we observe significantly slower recombination when compared to, otherwise identical, FTO-glass devices (Figure 1). This is due to a secondary TiO₂ electron collection layer formed via thermal oxidation of the Ti substrate during heat treatment of the compact (c-TiO₂) layer precursor. In this instance, due to the slower recombination compared to glass devices, we observe that interfacial recombination at the c-TiO₂ interface also contributes significantly to overall charge recombination in the devices. This presentation will explore these ideas further and will show that modifications to device architecture and materials can identify recombination mechanisms and reveal insights into device operating physics.
Recombination lifetime vs. open circuit voltage of a glass based device measured in both forward and reverse illumination and a metal mounted device.
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