In the summer of 2010, I worked with Dr. Bob Street at Palo Alto Research Center on a few projects related to understanding the physics of bulk heterojunction (BHJ) organic solar cells. In one project, we created an empirical model to understand how series resistance (e.g. from junctions between layers of the thin film stack) affects the solar cell’s measured photocurrent, which is an important measure for deriving electronic properties and analyzing electron-hole recombination effects that affect the solar cell’s overall performance. I also conducted low-level measurements of the photoconductivity spectral response of BHJ solar cells to better understand the cells’ electronic structure and density of states.
My internship was funded by the Princeton Environmental Institute’s (now the High Meadows Environmental Institute) Energy Grand Challenges program, and solar cells used for our study were designed and fabricated by Dr. Sarah Cowan in Professor Alan Heeger’s lab at UC-Santa Barbara.
publications
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Measurements of the localized state electronic structure of organic solar cells
R. A. Street and K. W Song
In 2011 37th IEEE Photovoltaic Specialists Conference, Jun 2011
Experimental measurements of the band tail density of states distribution in bulk heterojunction solar cells, are described, with measurements made on P3HT:PCBM and PCDTBT:PCBM. The transient photoconductivity experiment measures the electron and hole drift mobility in the solar cells. The dispersive nature of the electronic transport is directly related to the band tail density of states distribution. The technique is extended to measure the band tail DOS over a range of energies. In each type of cell we observe an approximately exponential band tail, but also find an additional broad distribution of deep states. Measurements of the photocurrent spectral response to low energy provide an alternative measure of the optical absorption spectrum for excitations from the polymer HOMO to the PCBM LUMO. These data also reveal an exponential distribution of band tail states with a slope that is the same as the electron transport measurements.
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Influence of series resistance on the photocurrent analysis of organic solar cells
R.A. Street, K.W. Song, and S. Cowan
Organic Electronics, Feb 2011
The series resistance of a bulk heterojunction solar cell diode affects the measurement of the voltage dependence of the diode photocurrent, P(V). An empirical model for the effect is described and shows that P(V) can be significantly modified, particularly at high bias voltages. Experimental measurements on PCDTBT:PCBM solar cells with added series resistance demonstrate the effect and confirm the model. The model provides a method to correct the photoconductivity data using measured quantities.
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Photoconductivity measurements of the electronic structure of organic solar cells
R. A. Street, K. W. Song, J. E. Northrup, and S. Cowan
Physical Review B, Apr 2011
Experimental and theoretical studies of the electronic structure of bulk heterojunction (BHJ) organic solar cells are reported. The photoconductivity spectral response of the solar cells has a weak absorption band extending from the band-gap energy down to <1 eV due to charge-transfer optical excitation at the interface between the polymer and the fullerene. The low-energy absorption indicates an exponential band tail of localized states and an absorption model based on the one-electron joint density of electronic states accounts for the data. Transient photoconductivity measurements of the carrier mobility exhibit a temperature-dependent carrier dispersion. Data analysis for the particular case of transport in the BHJ structure is developed. A multiple trapping model of the dispersive transport is consistent with localized band tail states having a comparable density-of-states distribution to those observed by optical absorption. Theoretical calculations of the density of states including disorder in the π-π spacing of the polymer chains also shows exponential band tailing. A density-of-states model is developed from the data and is discussed.