Monolithic Perovskite-Silicon Tandem Solar Cells: From the Lab to Fab?

This review focuses on monolithic 2-terminal perovskite-silicon tandem solar cells and discusses key scientific and technological challenges to address in view of an industrial implementation of this technology. The authors start by examining the different crystalline silicon (c-Si) technologies suitable for pairing with perovskites, followed by reviewing recent developments in the field of monolithic 2-terminal perovskite-silicon tandems. Factors limiting the power conversion efficiency of these tandem devices are then evaluated, before discussing pathways to achieve an efficiency of >32%, a value that small-scale devices will likely need to achieve to make tandems competitive. Aspects related to the upscaling of these device active areas to industry-relevant ones are reviewed, followed by a short discussion on module integration aspects. The review then focuses on stability issues, likely the most challenging task that will eventually determine the economic viability of this technology. The final part of this review discusses alternative monolithic perovskite-silicon tandem designs. Finally, key areas of research that should be addressed to bring this technology from the lab to the fab are highlighted.

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

Perovskite photovoltaics is one of the fastest growing research fields in materials science. Apart from photovoltaics, a wide range of energy-related research fields have gained substan-tial new momentum owing to the fantastic optoelectronic properties of hybrid lead halide per-ovskites. Starting with solar cells showing a mere 3% power conversion efficiency in 2009, researchers have been able to increase this value to over 22% in 2017, comparable to the best monocrystalline silicon solar cells. These improvements are chiefly due to the compositional tuning and mixing of the ABX3 perovskite structure. However, this strategy increases not only the efficiency but also in the same time the complexity of the perovskite formulation. Differ-ent phases, from the nano- to macroscale, can potentially form. In order to rationalize the effi-ciency progress, one needs to understand and characterize the complex perovskite composi-tion and crystal structure in great detail. The main focus of this thesis lies in the in-depth compositional and phase analysis of perovskite thin films as well as full perovskite solar cells, trying to rationalize the consequences of A cation and X anion tuning and mixing. In the first part, it is shown that fabricating solar cells from a non-stoichiometric perovskite precursor composition, namely one with PbI2 excess, leads to increased grain size, enhanced cristallinity, reduced recombination as well as an improved TiO2/perovskite interface. As op-posed to lead iodide phases resulting from film degradation, which is hampering device per-formance, unreacted excess PbI2 phases originating from an excess of PbI2 in the precursor solution are very beneficial for the final solar cell efficiency, reproducibility and stability. The next scientific challenge adressed in this thesis is related to the nano- and microscale composition of the record-breaking mixed cation/mixed anion perovskite composition. Mac-roscale investigations have previously suggested that this formulation results in a single ho-mogenous phase. However, using nanoscale elemental and charge carrier distribution mapping as well as microscale structural and optical film analysis, it is here found that in high efficient solar cells partial phase segregation does take place at the micro- and nanoscale. Moreover, it is shown that mixed cation/anion formulations allow the formation of never re-ported 3D hexagonal lead halide perovskite polytypes, namely 4H and 6H. These polytypes are shown to play a key role during the crystallization process, which is also revealed for the first time here. Indeed, mixed cation/mixed anion perovskite films are fabricated via the crys-tallization sequence 2Hï 4Hï 6Hï 3R(3C). It is demonstrated that the complex crystalliza-tion via these defect-prone hexagonal intermediates can be by-passed by the incorporaton of low amounts of Cs+ cations into the structure. This can explain the improved stability as well as the increased power conversion efficiency and device reproducibility. It gives for the first time a rational explanation as to why the halide perovskite community rapidly adopted the incorporation of Cs+ in mixed perovskites. The last results presented in this thesis are related to the hole-transporting materials (HTM) used in perovskite solar cells. It is shown that carbazole-based small molecule HTMs are a cheap and efficient alternative to the much costlier commercial spiro-OMeTAD.

EPFL2018

Extraction of p-n junction properties and series resistance in GaAs nanowire-based solar cells using light concentration

Anna Fontcuberta i Morral, Dmitry Mikulik, Ian Nachemson, Pablo Romero Gómez

Series resistance in solar cells limit their maximum conversion efficiency and thus should be minimized. Generally, such losses originate from deficiencies at the contact or absorber level. Quantifying them is the first step for tackling its reduction. In this work, we provide a new way to assess the series resistance in nanowire-based solar cells, which significantly underperforms predicted theoretical efficiency. We illuminate the devices at different levels of light intensity (from 1 to 1000 suns), which gives us insight in the carrier transport and series losses mechanism. We demonstrate the method on a device obtained by self-assembled GaAs nanowire p-n junction arrays on silicon. This analysis method provides a platform to distinguish the intrinsic response of the nanowire p-n junction from the series resistance effects. More generally, we provide a means of optimizing the efficiency in next generation solar cells, where contacts still have to be developed.

IOP PUBLISHING LTD2019

SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells

Xiaofei Dong, Ulf Anders Hagfeldt, Yang Li, Yuhang Liu, Dan Ren, Peng Wang, Zaiwei Wang, Shaik Mohammed Zakeeruddin

Perovskite solar cells have achieved comparable power conversion efficiencies as commercial silicon cells, making the issue of long-term operation stability as the most critical scientific factor toward industrial realization. In this work, we introduce SnS quantum dots (QDs) as a new inorganic hole transporting material (HTM) to perovskite solar cells. The SnS QDs decorated with the oleylamine (OAm), oleic acid (OA), and trioctylphosphine (TOP) ligands are prepared through the traditional nonaqueous solvothermal method. Therefore, the as-synthesized SnS QDs can be orthogonally processed onto the top of a triple cation perovskite film, exhibiting a good surface coverage and an excellent hole extraction ability. With careful device engineering towards film thickness, annealing procedure, and ligand exchange on the SnS layer, we have obtained a power conversion efficiency (PCE) of 13.7%. Compared with the 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD)-based control device, the SnS-based perovskite solar cell presents a better air stability, showing unaltered device performance after 1000 h storage under ambient conditions.

AMER CHEMICAL SOC2019

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

EPFL2018