The discovery of conducting polymers 40 years ago paved the way for organic electronics promising exciting new applications. Already a considerable range of electronic devices containing (semi) conducting polymers has been developed like light emitting diodes, non-linear optical devices, artificial muscles, thin film transistors and large area photovoltaic devices. Incorporating organic compounds extracted from plants (so called 'green' materials) in these devices may improve stability, sustainability and manufacturing costs. Green materials are processed under ambient atmosphere using simple deposition techniques like spin and dip coating or drop casting. Even well known organic semiconductors behave differently under these conditions. As a model system and reference for green materials, we reinvestigated the injection of holes into the regioregular semiconducting polymer poly(3-hexylthiophene-2,5-diyl) (P3HT). P3HT is known to be sensitive to ambient dopants, like moisture, oxygen or reminiscent solvents. In the present work, special emphasis is given to understanding the electrical signature of degrading processes, which affect both the bulk and the interfaces of the organic semiconductor. To this extend the charge injection into P3HT was monitored after various drying scenarios, heat treatment (annealing) and aging. Especially fruitful proved the comparison of charge injection across a metal-semiconductor interface and an organic heterojunction in the same sample to distinguish bulk and interface effects. Unintentional doping, the presence of charged impurities, degradation by deep hole traps and non-uniform interfacial layers could be all identified by carefully analysing current-voltage characteristics, partially with the help of numerical simulations solving the coupled nonlinear Poisson and drift-diffusion differential equations. Moreover, charge injection into selected green materials like plasticised films based on Buriti and Annatto oil revealed striking differences compared to the reference material P3HT, most likely related to interface modifications. The given example proves the usefulness of establishing charge injection into casted P3HT film as control when developing green materials for electronic applications.