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This review highlights recent advancements in the zinc oxide electron transport layer for PbS colloidal quantum dot solar cells. 

We present a method for designing spectrally- selective optoelectronic films with a finite absorption bandwidth. We demonstrate the process by designing a film composed of lead sulfide colloidal quantum dots (PbS-CQDs). Designs incorporate the patterning of absorbing PbS-CQD films into photonic crystal- like slabs which couple incident light into leaky modes within the plane of the absorbing films, modulating the absorption spectrum. Computational times required to calculate optical spectra are drastically decreased by implementing the Fourier Modal Method. Furthermore, a supervised machine-learning-based inverse design methodology is presented which allows tailoring of the PbS-CQD film optical properties for use in a variety of photovoltaic applications, such as tandem cells in which spectral tailoring can enable current-matching flexibility. 

Lead Sulfide (PbS) colloidal quantum dots (CQDs) are promising materials for flexible and wearable photovoltaic devices and technologies due to their low cost, solution processibility and bandgap tunability with quantum dot size. However, PbS CQD solar cells have limitations on performance efficiency due to charge transport losses in the CQD layers and hole transport layer (HTL). This study pursues two promising techniques in parallel to address these challenges. Solution-phase annealing of the absorbing PbS-PbX2 (X = Br, I) layer can reduce charge transport losses by removing oleic acid and parasitic hydroxyl ligands. Additionally, optoelectronic simulations are used to show that HTL performance can be improved by the addition of a 2D transition metal dichalcogenide (TMD) layer to the PbS CQD-based HTL. We use solution-phase exfoliation to produce and incorporate 2D WSe2 nanoflakes into the HTL. We report a power conversion efficiency (PCE) increase of up to 3.4% for the solution-phase-annealed devices and up to 1% for the 2D WSe2 HTL augmented devices. A combination of these two techniques should result in high-performing PbS CQD solar cells, paving the way for further advancements in flexible photovoltaics. 

Colloidal quantum dots (CQDs) are promising materials for photovoltaic applications due to their solution processibility and size-dependent band gap tunability. The electron transport layer (ETL) is an important component of PbS CQD solar cells, and the quality of the zinc oxide nanoparticle (ZnO NP) ETL film significantly impacts both the power conversion efficiency (PCE) and fabrication yield of CQD solar cells. We report on multiple methods to improve the quality of ZnO NP ETL films and demonstrate increased PCE and device yield in standard CQD solar cells employing optimized ZnO NP films. We also discuss the application of these methods in an inverted CQD solar cell architecture. 

The inverse design of photovoltaic 2D photonic crystals using machine learning will be presented. The technique bypasses calculation of photonic bandstructure in favor of directly computing designer-friendly properties such as spectral transmission. 

Colloidal quantum dots (CQDs) are of interest for photovoltaic applications such as flexible and multijunction solar cells, where solution processability and infrared absorption are crucial; however, current CQD solar cell performance is limited by the hole transport layers (HTLs) used in the cells. We report on a method to develop new HTLs for the highest-performing PbS CQD solar cell architecture by tuning the stoichiometry via sulfur infiltration of the p-type CQD HTL to increase its doping density and carrier mobility. Using SCAPS simulations, we predict that increased doping density and mobility should improve the performance of the solar cells. We show that sulfur doping of the current HTL is a facile and effective method to boost the performance of CQD photovoltaics. 

Spectral selectivity is of interest for many photovoltaic applications, such as in multijunction and transparent solar cells, where wavelength-selectivity of the photoactive material is necessary. We investigate using artificial photonic band engineering as a method for achieving spectral selectivity in an absorbing material such as PbS CQD thin films. Using FDTD simulations, we find that a CQD-based photonic crystal (CQD-PC) is able to maintain its photonic band structure, including the existence of a reduced photonic density of states, in the presence of weak material absorption. This shows that CQD-PCs are a promising material for photovoltaic applications that require spectral selectivity. 

The most common solution for achieving arbitrary spectral selectivity in optoelectronic devices is adding external filters. Here we propose using semiconductor thin film photonic crystals with relevant photonic bands that fall within the absorbing frequency range of the material for spectral selectivity. Optical simulations show that the in-plane photonic bands couple strongly to normal-incidence external fields, inducing tunable resonance features in the out-of-plane transmission and reflection spectra. Experimentally, we fabricate a proof-of-principle photonic structure with enhanced visible transparency, consisting of a self-assembled polystyrene bead array infiltrated with colloidal quantum dots, showing promise for multijunction and transparent photovoltaics.