Charge transport and trapping in bulk-heterojunction solar cells

Area of Science:

• Organic electronics
• Photovoltaics
• Materials science

Background:

• Organic solar cells (OSCs) offer a promising alternative to silicon-based photovoltaics.
• Poly-3-hexylthiophene (P3HT) and fullerene derivatives like [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are common materials in OSCs.
• Understanding charge carrier dynamics is crucial for optimizing OSC performance.

Purpose of the Study:

• To investigate charge carrier transport and trapping mechanisms in P3HT:PCBM organic solar cells.
• To correlate trapping phenomena with device performance metrics like fill factor and efficiency.
• To quantify the density and energy of trapping states and their impact on carrier mobility.

Main Methods:

• Fabrication of inverted organic solar cells using P3HT:PCBM (6:5 weight ratio).
• Analysis of current-voltage (IV) characteristics to determine fill factor and efficiency.
• Evaluation of trapping state density and activation energy through charge transport analysis.
• Application of the Gaussian disorder model to analyze carrier mobility.

Main Results:

• Achieved a solar efficiency of 3.7% and a fill factor as high as 68% in the inverted devices.
• Demonstrated significant involvement of carrier trapping in charge transport, despite the high fill factor.
• Quantified trapping state density up to 10^20 / (7 x 10^21) cm^-3 with an activation energy of approximately 0.18 eV.
• Identified trapping states at high densities potentially acting as transport states, limiting carrier mobility.

Conclusions:

• Carrier trapping plays a critical role in charge transport within P3HT:PCBM organic solar cells.
• The quantified trapping states can influence carrier mobility, even at high fill factors.
• The Gaussian disorder model provides consistent mobility parameters when considering thermal generation from traps.