Publication date: 27th June 2014
Carrier Multiplication (CM), the generation of multiple electron-hole pairs by a single photon, is of great interest for solar cells as it may enhance their photocurrent. This process has been shown to occur efficiently in colloidal quantum dots, however harvesting of the generated multiple charges has proved difficult. Here we show that by tuning the charge carrier mobility in quantum-dot films, CM can be optimized and may show an efficiency as high as in colloidal dispersion.
We vary the charge carrier mobility in PbSe quantum-dot films by using ligands of varying lengths and varying anchor groups as well as infilling with alumina via atomic layer deposition. Carrier multiplication is observed and quantified in both time-resolved microwave photoconductivity studies and utrafast transient absorption measurements. These results indicate that the number of surviving free charge carriers that result from CM increases with increasing charge carrier mobility.
Our results are explained quantitatively by the competition between dissociation of multiple electron-hole pairs and Auger recombination. For low mobility materials CM results in multi-excitons that however decay via Auger recombination before they can dissociate into free charges. In contrast, above a mobility of ~1 cm2 V−1 s−1 multi-excitons dissociate rapidly and are quantitatively converted to free charges before Auger recombination can take place.
In addition we show that the threshold energy for CM is reduced to twice the band gap of the quantum dots in these photoconductive films. This surprising observation could be related to the existence of trap states that participate in the CM process.
C.S. Suchand Sandeep, S.t. Cate, J.M. Schins, T.J. Savenije, Y. Liu, M. Law, S. Kinge, A.J. Houtepen & L.D.A. Siebbeles. High Charge-Carrier Mobility Enables Exploitation of Carrier Multiplication in Quantum-Dot Films. Nature Communications 4, 2360, (2013).
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