No abstract

Quantum dot photovoltaics (QDPV) offer the potential for low-cost solar cells. To develop strategies for continued improvement in QDPVs, a better understanding of the factors that limit their performance is essential. Here, we study carrier recombination processes that limit the power conversion efficiency of PbS QDPVs. We demonstrate the presence of radiative sub-bandgap states and sub-bandgap state filling in operating devices by using photoluminescence (PL) and electroluminescence (EL) spectroscopy. These sub-bandgap states are most likely the origin of the high open-circuit-voltage (VOC) deficit and relatively limited carrier collection that have thus far been observed in QDPVs. Combining these results with our perspectives on recent progress in QDPV, we conclude that eliminating sub-bandgap states in PbS QD films has the potential to show a greater gain than may be attainable by optimization of interfaces between QDs and other materials. We suggest possible future directions that could guide the design of high-performance QDPVs.

This work presents polymer photovoltaic devices based on poly(3-hexylthiophene) (P3HT) and TiO2 nanorod hybrid bulk heterojunctions. Interface modification of a TiO2 nanorod surface is conducted to yield a very promising device performance of 2.20% with a short circuit current density (J(sc)) of 4.33 mA/cm2, an open circuit voltage (V(oc)) of 0.78 V, and a fill factor (FF) of 0.65 under simulated A.M. 1.5 illumination (100 mW/cm2). The suppression of recombination at P3HT/TiO2 nanorod interfaces by the attachment of effective ligand molecules substantially improves device performance. The correlation between surface photovoltage and hybrid morphology is revealed by scanning Kelvin probe microscopy. The proposed method provides a new route for fabricating low-cost, environmentally friendly polymer/inorganic hybrid bulk heterojunction photovoltaic devices.

Chemical oxidation of under‐charged Pb atoms reduces the density of trap states by a factor of 40 in films of colloidal PbS quantum dots for devices. These emissive sub‐bandgap states are a byproduct of several standard ligand‐exchange procedures. X‐ray photoelectron spectro­scopy measurements and density function theory simulations demonstrate that they are associated with under‐charged Pb.

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,...