(193d) Interface Engineering in Solid-State Quantum Dot-Sensitized Solar Cells: Strategies to Improve Charge Collection

Solid-state dye-sensitized solar cells (ss-DSSCs) are composed of cheap, abundant materials, and have record power conversion efficiencies now at 6.1%.¹  In this work, the dye in ss-DSSCs was replaced with inorganic quantum dots as the light-absorbing material, creating solid-state quantum dot-sensitized solar cells (ss-QDSSCs).  Quantum dots (QDs) show favorable absorption properties due to their size-dependent band gap, as well as higher molar extinction coefficients than commonly-used dyes.  Increased absorption is important as the active layer thickness in the devices is limited by charge recombination.

Despite the promise of quantum dots as a light absorbing material, QDSSC efficiencies remain low – especially in solid-state devices – due to high rates of recombination and low surface coverages of the photoanode by QDs.  A recent study places QD surface coverages below 14% of the photoanode surface area.²  The record efficiency for liquid QDSSCs is 5.6%,³ and only 1.5% for QDSSCs with a solid-state hole-transport material (HTM).⁴  In this work, we examine two strategies for improving efficiency in QDSSCs: inserting barrier layers to reduce recombination, and increasing QD surface coverage.

We first demonstrate that ultra-thin recombination barrier layers of Al₂O₃ deposited by atomic layer deposition (ALD) can improve the performance of cadmium sulfide (CdS) quantum dot-sensitized solar cells (QDSSCs) with spiro-OMeTAD as the solid-state HTM.  CdS QDs were grown directly on nanoporous TiO₂ substrates by successive ion layer absorption and reaction (SILAR).  The deposition of CdS quantum dots was confirmed with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy.  We explored depositing the Al₂O₃ barrier layers by ALD either before or after the QDs, resulting in TiO₂/Al₂O₃/QD and TiO₂/QD/Al₂O₃ configurations.  The effects of barrier layer configuration and thickness were tracked through current-voltage (J-V) measurements of device performance and transient photovoltage measurements (TPV) of electron lifetimes.  

The Al₂O₃ barrier layers were found to suppress dark current and increase electron lifetimes with increasing Al₂O₃ thickness in both barrier layer configurations in CdS ss-QDSSCs.  For thin barrier layers, gains in open-circuit voltage (V_OC) and concomitant increases in efficiency were observed, although at greater thicknesses, losses in short-circuit current (J_SC) caused net decreases in efficiency.  Devices with 1 Al₂O₃ ALD cycle deposited before the QDs had the highest average device parameters, with an efficiency of 0.35%, J_SC of 0.83 mA/cm², V_OC of 0.70 V, and fill factors of 0.60.

A close comparison of the electron lifetimes in TiO₂ in the TiO₂/Al₂O₃/QD and TiO₂/QD/Al₂O₃ configurations suggests that electron transfer from TiO₂ to spiro-OMeTAD is a major source of recombination in ss-QDSSCs, although recombination of TiO₂ electrons with oxidized QDs can also limit electron lifetimes, particularly if the regeneration of oxidized QDs is hindered by a too-thick coating of the barrier layer.

However, the low efficiencies of CdS ss-QDSSCs, due in part to the large, non-ideal band gap of CdS, motivated the development of a lead sulfide (PbS) ss-QDSSC system. PbS QDs were grown in situ on nanoporous TiO₂ by SILAR as well as by ALD.  With the ultimate goal of increasing QD surface coverage, we compare the impact of these two synthetic routes on the absorption and device electrical properties.  Thus far, the highest efficiencies were observed in devices with ALD-grown QDs: 0.30 % at 10 ALD cycles, although the devices with SILAR-grown QDs had the highest J_SC's observed in our work to date, of 0.98 mA/cm², and efficiencies of 0.23 %.  We hypothesize that the higher J_SC's and the higher reverse-bias photocurrent observed in the case of SILAR-grown QDs is due to a higher number of defect states in these solution-deposited QDs, as compared to the vapor-deposited QDs grown by ALD.

To understand the effects of QD surface coverage on device performance, particularly interfacial recombination, TPV measurements were carried out to determine electron lifetimes, and external quantum efficiency (EQE) measurements were used to track charge collection efficiencies.  These measurements were taken for varying QD deposition cycles.  Electron lifetimes were found to decrease with increasing SILAR cycles. We hypothesize that recombination to oxidized QDs is the critical source of recombination in these devices.  We are currently investigating methods to increase QD surface coverage and improve electron injection by pre-deposition chemical treatments of the QD surface.

1.         Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G., All-Solid-State Dye-Sensitized Solar Cells with High Efficiency, Nature 2012, 485 (7399), 486-489.

2.         Guijarro, N. S.; Lana-Villarreal, T.; Mora-Seró, I. N.; Bisquert, J.; Gómez, R., CdSe Quantum Dot-Sensitized TiO₂ Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment, J. Phys. Chem. C 2009, 113 (10), 4208-4214.

3.         Lee, J.-W.; Son, D.-Y.; Ahn, T. K.; Shin, H.-W.; Kim, I. Y.; Hwang, S.-J.; Ko, M. J.; Sul, S.; Han, H.; Park, N.-G., Quantum-Dot-Sensitized Solar Cell with Unprecedentedly High Photocurrent, Scientific Reports 2013, 3.

4.         Lee, H.; Leventis, H. C.; Moon, S. J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nuesch, F.; Geiger, T.; Zakeeruddin, S. M., et al., PbS and CdS Quantum Dot-Sensitized Solid-State Solar Cells: ‘‘Old Concepts, New Results,’’ Adv. Func. Mater. 2009, 19, 2735-2742.