Free carrier generation and recombination in PbS quantum dot solar cells

Abstract

Time Delayed Collection Field (TDCF) and Bias Assisted Charge Extraction (BACE) experiments are used to investigate the charge carrier dynamics in PbS colloidal quantum dot solar cells. We find that free charge carrier creation is slightly field dependent, thus providing an upper limit to the fill factor. BACE measurements reveal a rather high effective mobility of 2 × 10 cm²/Vs, meaning that charge extraction is efficient. On the other hand, a rather high steady state non-geminate recombination coefficient of 3 × 10 cm³/s is measured. We, therefore, propose rapid free charge recombination to constitute the main origin for the limited efficiency of PbS colloidal quantum dots cells.Colloidal quantum dots (CQDs) are a promising class of materials for optoelectronic applications due to their well-tunable optical and electronic properties. In the last years improvement in the quality of the CQDs, as a consequence of better and cleaner synthetic processes, has stimulated their use in a variety of applications from light emitting diodes 1 to solar cells 2,3 , field effect transistors 4-6 and photocatalytic devices 7,8 . The size-induced quantum confinement in the quantum dots can be tuned via precise tuning of the QDs dimensions. This quantum confinement can be partially maintained even in strongly coupled CQD solids 9 . Strong coupling is achieved through the ligand exchange process, by which the original, bulky organic ligands are replaced with shorter ones, enhancing the electronic coupling. Lead chalcogenide quantum dot solids exhibit a very broad absorption range, extending into the near infrared region 10 . This makes them particularly attractive for photovoltaic devices with high photocurrents 11 . From the early Schottky devices 9,11 , which show limited open circuit voltage (V oc ) and fill factors (FF), great improvement in the device structure and in the choice of ligands led to a record PbS CQD solar cell with efficiencies up to 10%. That record device showed a short circuit current of up to 22 mA/cm 2 , V oc of 0.63 V and FF of 0.7 12 . Still, the device performance seems to be limited by the V OC and the FF, purportedly due to recombination

Efficient and Stable PbS Quantum Dot Solar Cells by Triple-Cation Perovskite Passivation

“…voltage (Figure 4a) shows only a minor decrease for increased active layer thickness which originates from a higher recombination, 36 yet does not strongly influence the optimal thickness for the maximum PCE (Figure 4d). It is interesting to compare the optimal thickness of the CsMAFA-…”

“…A further increase of the thickness drastically reduces the charge extraction, meaning that the charge carriers recombine prior to extraction due to the longer distance they must overcome before reaching the extraction layers. We note that the open-circuit voltage (Figure a) shows only a minor decrease for increased active layer thickness which originates from higher recombination yet does not strongly influence the optimal thickness for the maximum PCE (Figure d). It is interesting to compare the optimal thickness of the CsMAFA–PbS devices to that of the reference devices: 250 and 325 nm for MAPbI 3 –PbS and PbX 2 –PbS devices, respectively. , Yang et al reported that the short carrier diffusion length within the MAPbI 3 –PbS active layer is responsible for the decrease in photovoltaic performance for thicknesses above 250 nm .…”

Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend

“…The much more pronounced initial recombination loss in the thin sample is, therefore, clearly revealed by the dependence of Q tot on fluence at even short delays. As attempts to fit these traces with the established iterative approach 25 33 , using a constant, time independent recombination coefficient, turned out to be unsuccessful, We therefore analyzed these data with a new approach as outlined in refs 35 and 36 . In short, the incremental change of the total extracted charge density per time increment, Δ n tot /Δ t , with n tot = Q tot ( eAd ), is plotted versus n coll = Q coll ( eAd ) for different delay times and fluences.…”

Orders of Recombination in Complete Perovskite Solar Cells – Linking Time‐Resolved and Steady‐State Measurements

“…After a given delay time, the carriers which survived recombination are extracted from the device by applying a reverse bias V coll . The technique has been used in the past to study free charge generation and recombination in organic, [34] inorganic [47] and hybrid devices. [48] A fluence series is shown in Figure 2D (see Figures S5 and S6 (Supporting Information) for additional data).…”