Date on Master's Thesis/Doctoral Dissertation

12-2013

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Electrical and Computer Engineering

Committee Chair

Alphenaar, Bruce William

Committee Co-Chair (if applicable)

Sumanasekera, Gamini

Committee Member

Walsh, Kevin

Committee Member

Mendes, Sergio

Committee Member

McNamara, Shamus

Author's Keywords

Organic polymers; Red shift; Solar cells; Photocurrent; Bulk heterojunctions; Recombination

Subject

Conjugated polymers; solar cells

Abstract

Charge transfer and charge extraction mechanisms are two prevalent issues in the growing field of organic solar cells. Due to their complexity in nature, new methods need to be involved in addressing the fundamental properties associated with organic polymer solar cells. This dissertation has focused on developing a new method to estimate the charge collection lengths and surface recombination lengths of organic polymer solar cells. Photocurrent spectra have been analyzed systematically to observe the dependence on thickness of the active material. A red shift of the peak of the normalized photocurrent with respect to the device thickness has been further analyzed for two major material systems used in organic polymer solar cells, namely MDMO-PPV: PCBM and P3HT: PCBM. A theoretical model that measures the charge extraction of bulk hetero junction solar cell structures has been used taking into account of three main parameters including charge carrier collection length, absorption variation and surface recombination. This model has led to estimate two important parameters associated with charge transfer, recombination and extraction of organic solar cells which will provide opportunities for improvements in the performance of organic electronic devices. Key results are summarized as follows. A complete analysis of photocurrent spectra has been done to see its variation with active material thickness of well-known two material systems of bulk heterojunction organic solar cells. Results of these preliminary measurements suggest that peak of the photocurrent for both systems red shift with increasing thickness. Charge extraction model is introduced to explain the initial red shift of the photocurrent. This model fits well with the experimental results. Further analysis of the model suggests that the charge collection lengths can be estimated for organic polymer structures. Theoretical model gives higher collections lengths for MDMO-PPV solar cells while a lower collection length for P3HT solar cells. This model also has the capability to estimate the surface recombination length of organic bulk heterojunction solar cells. Different interfacial layers have been used to fit to the model calculation. These results suggest that the least surface recombination lengths were achieved with solar cells of PEDOT-PSS. This method can be used to optimize the interfacial layers to improve the efficiency in organic solar cells. AC photocurrent measurements have been carried out to observe the frequency dependence of organic solar cells. Main results show that increasing response time from the light source increases the performance of the solar cells. Further analysis of these results suggests that thicker devices of organic polymer solar cells can be applied with longer response time light sources. Degradation of organic polymer solar cells over a period of several days has been carried out to observe the stability of solar cells with device thickness. Results suggest that the degradation occurs regardless of device thickness of the active material. A new probing technique called capacitive photocurrent spectroscopy (CPS) has been carried out with P3HT solar cells to observe the charge dissociation efficiency. The CPS measurements suggest that sub band gap states have higher dissociation efficiencies that may not contribute to overall photocurrent of the solar cells.

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