The Donor–Supply Electrode Enhances Performance in Colloidal Quantum Dot Solar Cells

Abstract

Colloidal quantum dot (CQD) solar cells combine solution-processability with quantum-size-effect tunability for low-cost harvesting of the sun's broad visible and infrared spectrum. The highest-performing colloidal quantum dot solar cells have, to date, relied on a depleted-heterojunction architecture in which an n-type transparent metal oxide such as TiO2 induces a depletion region in the p-type CQD solid. These devices have, until now, been limited by a modest depletion region depth produced in the CQD solid owing to limitations in the doping available in TiO2. Herein we report a new device geometry-one based on a donor-supply electrode (DSE)-that leads to record-performing CQD photovoltaic devices. Only by employing this new charge-extracting approach do we deepen the depletion region in the CQD solid and thereby extract notably more photocarriers, the key element in achieving record photocurrent and device performance. With the use of optoelectronic modeling corroborated by experiment, we develop the guidelines for building a superior CQD solar cell based on the DSE concept. We confirm that using a shallow-work-function terminal electrode is essential to producing improved charge extraction and enhanced performance.

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

“…4b) under 1 sun illumination for the best obtained devices for each class of materials show that the sequential passivation strategy, with chlorothiols followed by a CdCl 2 treatment, yielded the best performance of 8.5%. Although previous optimization schemes achieved similar performance by exploiting improved electrode design 59 , this work does so through CQD solid engineering alone. Interestingly, chloro-propanethioltreated CQDs can reach B6% PCE on their own without any CdCl 2 treatment, compared with the B3% obtained for the nonchlorothiol-treated CQDs.…”

To Be or not to Be: Band-Like Transport in Quantum Dot Solids

“…2 /V ⋅ s, carrier diffusion lengths of several hundred nanometers, low excitonic binding energies, size-tunable bandgaps and large persistence to photo-bleaching have paved the way to successful implementation into solar cells, light-emitting diodes, photodetectors and field-effect transistors [10][11][12][13].…”

Multiple exciton generation in quantum dot-based solar cells

“…TiO 2 ), show ultrafast, hot electron transfer to the MO [93]. Considering the similar device architecture in common QD-based PV devices where a MO is used as electron acceptor [80,94,95], this transfer constitutes another potential hot carrier decay channel competing with MEG and hot carrier cooling (see Figure 2). …”

“…While this structure has been shown to provide an acceptable compromise between the absorbance-and extraction-limited regime in conventional QD solar cells, different device architectures need to be considered to optimise MEG in PV devices [82,95,103,129]. For instance, the high absorption cross-section of QDs in the energy range relevant for MEG (>2E g ) does not require a QD-film thickness of hundreds of nanometres as is common in efficient, conventional QD devices [95,103,130]. Furthermore, the major loss processes in conventional QD solar cells (i.e.…”

“…That is due to recently emerging concepts based on rigorous light-trapping techniques [140], solar photon concentration [141] and highly luminescent semiconductors [132,142], which are already being introduced in present day's PV market [143]. Similarly, techniques to reduce photovoltage losses in QD-based solar cells are put in the research focus of an increasing number of groups [95,104]. Interestingly, the resulting highly efficient solar cells do not necessarily rely on a fully depleted QD film for charge extraction but function similar to conventional bulk semiconductor devices where carrier diffusion is responsible for efficient charge transport to the electrodes.…”