Transient photocurrent measurements on Polymer Solar Cells

Organic solar cells made of polymers are an attractive alternative to the conventional solar cells which are commercially used today. The polymer solar cell is cheap to produce, due to the high absorption coefficients which limits the needed thickness of the material and due to the simple manufacturing process, which doesn’t demand high temperatures. However, the efficiencies of polymer solar cells are yet too low for commercial production. To optimize the efficiency a critical step is to understand the overall mechanism of the conversion from photon energy into electric and what factors limit the conversion. In this report studies on the device physics of two different polymer solar cells were performed by time resolved electrical traces in terms of EQE*. Pulsed laser was used as a light source. The aim with the study was to investigate how dynamical excitation conditions as repetition rate and pulse energy influence on the solar cell device physics. To specify the investigation the following questions were asked: Are there any limits of solar cell performance regarding concentration of photons incident on the solar cell? If yes, what are the limits? And in regarding repetition rate; does the solar cell need to establish some quasi-equilibrium distribution of photo-generated charges in order to optimize the solar cell performance? A substantial part of this project involved the set up of a program which would enable the analyses of the solar cell response and put the data into a fit. It is shown that the pulse energy is substantial for the conversion of photons into electrons. For high pulse energies (~10-10 hv) the EQE* starts to decreases and this is attributed to an increased contribution from non-geminate recombination. Shortly above the pulse energy 10 hv the limit for solar cell operation is reached. No lower limit for solar cell operation regarding pulse energy was found in the energy range measured. As for the repetition rate, it’s shown that no significant change in the physical processes of the solar cell can be deduced when the frequency is varied in a range of 1000 Hz to 25000 Hz. This is interpreted as there is probably no need to establish a quasi-equilibrium of photo generated charges to optimize the solar cell efficiency. *EQE is the external quantum efficiency which is the ratio between incoming photons and extracted electrons from the solar cell. See also 2.1.2 External quantum efficiency in the theory section.