Intensity-modulated photocurrent spectroscopy (IMPS) characterizes the charge carrier transport in a solar cell by subjecting it to a sinusoidally varying illumination and measuring the resultant photocurrent. The IMPS spectra of certain P3HT:PCBM bulk heterojunction solar cells exhibit a poorly understood feature: a quadrant I semicircle, which implies that the sinusoidal variation of the photocurrent leads that of the illumination in a certain frequency range. To understand the mechanisms underlying this feature, we have adopted a mathematical framework based on drift-diffusion modeling instead of the conventional equivalent circuit approach. By incorporating the effects of traps into the mathematical model and conducting targeted experiments, we have demonstrated that the quadrant I semicircle arises from degradation-related trap states behaving as recombination centers. Our analysis of the device's intrinsic mechanisms indicates that the trap-assisted recombination can have a time scale that is at least one order of magnitude slower than that of the carrier extraction mechanism; such a mismatch can generate the observed quadrant I semicircle.

The device lifetime of an inverted organic solar cell (IOSC) is significantly better compared to standard-architecture OSC under ambient conditions. However, various studies have shown that when an n-type oxide is used as a selective electron transport layer (ETL) in the IOSC, a reversible light-soaking treatment is required. This reversible treatment largely hampers the practicality of the device, especially in outdoor applications, in which the light-soaking time may take hours every morning. In this work, fluorinated TiOx (F-TiOx), prepared by low-temperature solution-processed chemical bath deposition technique, was used as the ETL to significantly reduce the light-soaking time for a P3HT:PCBM based IOSC. Without affecting the device efficiency, more than ten-fold reduction in light-soaking time was observed for fluorinated TiOx (F-TiOx) when compared with conventional sol-gel TiOx. Ultraviolet photoelectron spectroscopy (UPS) and UV photoconductivity measurements were used to understand the light-soaking time reducing mechanism. From the perspective of ITO/TiOx interface, shift in work function was observed in F-TiOx due to the partial filling of its defective sites by fluorine atoms. Consequently, this process reduces its intrinsic trap state density compared to sol-gel TiOx even before the light-soaking treatment. As a result, the trap filling action can be completed in a shorter time upon illumination, and thus significantly reduce the duration of the necessary light-soaking

We have derived a closed-form analytical expression for intensity-modulated photovoltage spectroscopic (IMVS) responses of organic bulk-heterojunction (BHJ) solar cells. With the expression, we have predicted the features of the IMVS spectra, extracted physical parameters from the IMVS measurements, quantified the recombination characteristics of the BHJ devices, and compared the IMVS with the transient photovoltage responses. The analytical model has enabled the utilization of the quantitative information that is inherent in the IMVS measurements

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Through the analysis of scales and simplification of the drift-diffusion device model, we have obtained a quantitative description of the mechanisms underlying the current-voltage (j-V) characteristics of organic bulk-heterojunction solar cells. The mechanisms have been resolved into the competition between the photogeneration, recombination, and extraction/injection rates, which determines the bulk charge carrier concentration; and the combined effect of the built-in field and the boundary layers in shaping the electric potential distribution, which determines the bulk field. The relationships between the j-V characteristics and standard model parameters have been captured with analytical expressions and verified through 1-D numerical simulations. We have determined that while the charge carrier generation rate can be reliably extracted with the device model from j-V measurements alone, the effective density of states and built-in potential, and the mobility and recombination prefactor are clustered pairs that can only be decoupled through other characterization techniques