5 – La meta de la sabiduría Las Cuatro Nobles Verdades no sólo sirven como el

According to the SRH theory outlined above, the diverse behavior seen in Fig. 4.9B with re- spect to preparation conditions likely results from variations in the band offsets between the TiO2 conduction-band edge and the Si valence band (∆), the TiO2 doping density, and the

interfacial defect properties. Consistently, the highest-performing TDMAT-ALD TiO2 samples

exhibited relatively large electron affinities for anatase. The vacuum levels are only an approxi- mate indicator of the actual band alignment, but the data suggest that a small ∆, in addition to a conductive TiO2 layer, may be important to obtain a high-conductivity contact between p+-Si

work function of the n-type TiO2 selective contacts (corrected for surface dipole contributions),

and thus has an important effect on the maximum open-circuit voltage. Larger n-type selective contact work functions reduce the built-in voltage and therefore the achievable quasi-Fermi level splitting, in contrast to the experimental observations that the perovskite cells on TDMAT-ALD TiO2 function without substantial losses. The observed cell performance is thus indicative of

an additional role for the mesoporous TiO2 layer that is inserted in the standard cell architec-

ture between the compact TiO2 layer and the perovskite, to improve film quality and electron

extraction. Single-junction perovskite cells fabricated without a mesoporous TiO2 layer (i.e.

directly on the TDMAT-ALD TiO2 compact layer) exhibited drastically reduced open-circuit

voltages (note that this does not indicate an intrinsic limit of planar cells, as solution-processed compact layers yielded uncompromised, high open-circuit voltages [78]), supporting our measure- ments which indicated that the conduction band in compact TDMAT-ALD TiO2 is energetically

lower-lying than in other TiO2 preparations. Due to its smaller work function, inclusion of the

solution-processed mesoporous layer therefore appears to maintain the built-in voltage of the cell [55]. A simulated band diagram of the complete tandem structure (Fig. 4.13) summarizes the key findings that contact between the high-work function cp-TiO2 layer and p+-Si is facilitated

by interface defects, and that high cell voltages for the complete device rely crucially on the inclusion of the lower work function mesoporous TiO2 to maintain the built-in voltage of the

perovskite sub-cell. This latter point is substantiated by I-V simulations of a single-junction perovskite cell with and without a smaller work-function “mesoporous-titania” layer included between the perovskite and the “ALD-titania” layer with larger work-function (Fig. 4.13).

4.3

Summary and Conclusions

In this final chapter we have discussed the steps necessary to integrate perovskite and Si cells into a tandem arrangement, as motivated in Chapter 1. The requirements for integration hinge on whether the tandem is to be operated as two independent but stacked sub-cells (4-T), or as one monolithic (2-T) cell. In the former case no less than 3 transparent conductors are required to contact the two sub-cells, placing considerable weight on the properties of these. The use of metallization relieves the burden on a transparent conductor’s sheet resistance, leveling the competition between technologies and allowing for other factors such as material compatibility or ease of processing to take precedence. We have shown that whenever it is possible to incor- porate a narrow metallic grid (w < 30µm), the requirements for a transparent conductor shift dramatically towards achieving ultra-high transparency (> 95%). Wire widths in the range of 20−30µm already relax the conventional sheet resistance requirement of 10Ω/sq by more than an order of magnitude, encompassing a wide range of emerging technologies, whilst widths

0 50 100 150 200 250 300 350 400 Depth (nm) -3 -2 -1 0 1 2 3 Energy (eV) 0 0.2 0.4 0.6 0.8 1 1.2 Voltage (V) -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 Current (mA cm -2) meso no meso meso no meso

Figure 4.13: Simulated band diagram at 0V (left) and J-V behavior (right) of a single-junction perovskite solar cell with and without an additional “mesoporous” titania layer inserted between the perovskite and compact titania. This represents a 1D simplification of the complex 3- dimensional mesoporous structure present in our tandem cell design, but captures the qualitative effect of including titania layers with contrasting work functions. The mesoporous layer functions primarily to maintain the built-in voltage in the perovskite cell, as seen in the band diagram (left) and consequently discrepant open-circuit voltages (right). Here electron affinities of 4.1eV and 4.5 eV were used for the compact and mesoporous titania layers respectively.

approaching 5µm will allow for several thousands of ohms per square in the transparent layer. Furthermore, we have shown that metallized transparent layers generally exceed the perfor- mance of the best bare transparent conductors due to a combined reduction of losses at the cell and module level. Narrow-width metallization therefore provides a clear route for approaching ideal electrode performance in thin-film solar cells. These findings demonstrate the potential for scalable methods of incorporating thin metal wires into cell substrates to disrupt the transparent conductor landscape.

For sensitive cell architectures embedding of the wires may be necessary to avoid drastic nonuniformities, and this is likely to be simpler for flexible polymer substrates than for glass. However even in the latter case one may look to the literature of microfluidics where the problem of structuring glass has received considerable attention [79, 80], or to well-established sol-gel techniques [12, 13] for imprinting patterns onto glass. Once embedded, these metal wires could be made very much taller than otherwise in order to accommodate larger cell widths, providing a reduction in module dead-space. Electrodes which are naturally exposed, such as the rear electrode in a perovskite 4-T tandem device, can be easily metallized and researchers should be aware that this obviates the widely quoted requirement of 10Ω/sq.

Monolithic 2-T tandems also require at least one transparent electrode, and for these the considerations above apply equally. Furthermore, regarding the interconnection of sub-cells, we have demonstrated a proof-of-concept 2-T perovskite-Si tandem device that functions without a conventional interlayer. An enabling feature of the device is the formation of an atomic-scale recombination layer between a compact TDMAT-ALD TiO2 layer and the p+-Si emitter region,

which produces a highly conductive Ohmic contact between the two materials. The contact resistance, interfacial band offsets, defect densities, and doping densities are strongly dependent on processing, but therefore present several handles for tunability. The twin advantage of this design in reducing optical losses and in reducing processing steps brings the perovskite-Si pairing tandem structure one step closer to delivering its full potential.

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