Enhanced Photocurrent in PbS Quantum Dot Photovoltaics via ZnO Nanowires and Band Alignment Engineering

Abstract

is sufficiently thick to harvest all near-infrared photons. This limitation can be relaxed by employing an ordered bulk heterojunction (OBHJ) architecture in which the n-type metal oxide acceptor is composed of 1D nanostructures such that the directions of photon absorption and charge collection are decoupled, allowing for absorption in an optically dense film while maintaining efficient charge collection. PbS QD PVs employing OBHJs of TiO 2 nanopillars [24] and ZnO nanowire arrays [25] have previously been shown to enhance short-circuit current density (J SC ) by more than 20% over planar counterparts. J SC above 30 mA cm −2 has been achieved in PbS QD PVs by employing relatively long ZnO nanowire arrays (length greater than 1 μm) and QDs with smaller band gap (first exciton absorption at approximately 1030 nm), but PCE was limited to 6.1% due to relatively low V OC and poor fill factor (FF). [26] In this work, we demonstrate a device architecture that combines a ZnO nanowire array OBHJ architecture with band alignment engineering of the PbS QD film in an effort to maximize J SC while preserving the V OC and FF achieved by optimized planar devices (Figure 1a). Previous PbS QD OBHJ devices have employed QD layers with uniform energy levels using a single ligand exchange treatment. However, a multistep ligand exchange process can be used to alter the conduction and valence band energy levels of the QD layers [27] (Figure 1b) and consequently promote charge extraction and reduce carrier recombination at the anode. [8,28] We confirm that this multi-step ligand exchange process produces discrete functional layers through nanoscale cross-sectional elemental analysis. The resulting combination of band alignment engineering and nanostructured heterojunction approaches yields devices with improved J SC and PCE compared to planar devices that utilize band alignment engineering alone. Champion nanowire devices achieve J SC above 30 mA cm −2 and 9.6% PCE under 100 mW cm −2 AM1.5G illumination. Our photocurrent density is the record for a PbS QD PV device with an excitonic absorption peak of at least 1.3 eV, [23,26] which is significant because a band gap of 1.3 eV maximizes the efficiency potential of a PV device under terrestrial illumination. [29,30] Finally, we demonstrate that this enhanced performance is a result of both improved light harvesting due to the presence of the ZnO nanowire array and improved carrier collection due to the 3D junction formed by the OBHJ; these effects are generalizable to other thin film PV absorber materials with transport lengths that are incommensurate with their absorption coefficients.ZnO nanowire arrays were synthesized on indium tin oxide (ITO) via a hydrothermal method, as follows. A 40 nm thick textured polycrystalline seed film of ZnO was first deposited directly on ITO by a sol-gel process. Next, ZnO nanowires were Colloidal quantum dots (QDs) have gained attention for a range of optoelectronic device applications, including photodetectors, [1,2] light-emitting diodes, [3] lasers,...

Overcoming the Ambient Manufacturability‐Scalability‐Performance Bottleneck in Colloidal Quantum Dot Photovoltaics

“…For these reasons, CQD solar cells fabricated under humid ambient conditions are of particular interest, as are those translated into large‐area compatible, high‐throughput fabrication techniques. A low‐cost PV technology aiming at industrial implementation in ambient air will need to be invariant under moisture conditions during manufacture and utilization . In contrast with this requirement, today's best cells—if they are to be manufactured year‐round in a wide range of geographies—require careful and costly environment control during manufacture .…”

“…Inspecting the as‐prepared device's figure‐of‐merit reveals that the short‐circuit current ( J SC ) is reasonably high, but open‐circuit voltage ( V OC ) and fill factor (FF) are well below expectations (see Table 1 ), providing the first clue into charge extraction issues. Since it is known that CQD layer fabrication in a N 2 glove box, followed by device fabrication and testing without further exposure to ambient air, also results in low PCE, we infer that oxygen doping is important . We took the view that humidity interferes with the oxygen doping process of one or more of the CQD layers in the device.…”

Enhancing Electron and Hole Extractions for Efficient PbS Quantum Dot Solar Cells

“…For instance, boron, nitrogen, indium, and magnesium were incorporated into the ZnO film to adjust its band gap structure and strengthen the depletion region at the ZnO/PbS interface. Nanowire array ZnO was adopted to shorten the transfer path and speed up photoelectron extraction; Modifications of the ZnO film surface using chalcogenides, conjugated polyelectrolyte, thin oxide, and CdSe quantum dot were implemented to facilitate carrier transfer from the PbS QDs to the ZnO film. In the anode side, graphdiyne was introduced in‐between the PbS‐EDT QDs film and the Au anode to accelerate photohole extraction …”

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

“…[12] However, there is still room for further improvement in the QDSC performance. [18][19][20] However, flexible QDSCs are more susceptible to the loss of charge carriers at the junction due to recombination pathways that arise from the straining of the semiconducting layers during the fabrication process, which is a major limitation that has to be dealt with for an improvement in the performance of flexible QDSCs to be realized. [13][14][15][16] In particular, the heterojunction between the electron transport layer (ETL) and the PbS QD layer plays a key role in governing the overall performance of the PbS QDSCs because charge trapping and recombination occurs much faster at this heterojunction than at other locations between and within the PbS QD layers.…”