Abstract
In recent years, the main topics for research in organic solar cells are not only in boosting the efficiency, but also in improvement of the device lifetime. Many degradation pathways have been found that cause efficiency and lifetime losses. The charge traps might play a decisive role, as they affect the charge carrier mobility, the recombination dynamics of charge carriers as well as internal field distribution. Due to charge trapping leaves many issues to be clarified, such as the assignment of trap polarity (hole or electron), the influence of electrode on charge traps, etc. Herein we use thermally stimulated current to investigate the origins and characteristics of charge traps. By using time of flight (TOF)-based TSC, the trap states of hole in the device ITO/P3HT(drop-casting)/Al can be clearly assigned. The hole trap is located at about 190 K with an activation energy of 0.37 eV. In the device ITO/P3HT(spin-coating)/Al, three distinct peaks (125 K, 225 K and >275 K) can been seen in the TSC spectrum after exposing to dry oxygen. The TSC distribution of the 225 K trap current is in good agreement with the hole trap distribution in drop-casting P3HT, implying that this trap could be originated from the same source. It is not affected by the presence of oxygen but slightly changes with different morphology. By further comparison between optical and electrical trap-fillings, we confirm that the trap current at about 125 K is caused by hole trap, and the trap current at temperature higher than 275 K is due to electron trap. Both of them increase dramatically after exposing the device to dry oxygen. There are just one peak at about 115 K in the trap current of P3HT in the device ITO/PEDOT:PSS/P3HT/Al. The TSC peak at 225 K would disappear after coating PEDOT:PSS as a hole transport layer, so the origin of the trap current at 225K is hole trap caused by ITO/P3HT interface. After exposure of the device to dry oxygen and combine the results of optical and electrical trap filling approaches, we confirm that the trap current at about 115 K is caused by hole trap, and the trap current at temperature higher than 250 K is due to electron. This result is in accordance with the measurement from the above-mentioned ITO/P3HT/Al. Applying vacuum and thermal treatment to the sample leads to a decrease in trap current, exhibiting a partial reversibility of the influence by oxygen. Thereafter, we replace the hole transport layer with SPAN and the device structure is ITO/SPAN/P3HT/Al. The trap current of this device is larger than that by using PEDOT:PSS. We also found that the trap current distribution in electrical trap filling case is in agreement with that by using PEDOT:PSS, but the trap current distribution between them in optical trap filling case is quite different. The TSC peaks of electrical and optical trap filling cases are 127 K and 150 K, respectively. Since both hole transport materials have the same work function, the different doping mechanisms might provide different effect on charge transport. In addition to the investigations of the above-mentioned RR-P3HT, we also performed TSC measurements on regio-random P3HT (RRa-P3HT). Two maxima can be seen in the TSC spectrum, one at about 130 K and the other broader one at 190 K. Both of them increase dramatically after exposing to dry oxygen, but only the broad trap current near 200K could be detected at electrical filling case. This trap current might originate from higher energetic disorder caused by larger amorphous phase content which makes a broader hole trap distribution. In the blend system of P3HT:PCBM, the TSC distribution of pristine sample is broader than pure P3HT and pure PCBM, which suggest an appearance of additional deeper traps in the blend. The TSC distributions of the weight ratios 1:1 and 1:4 blends are narrower after thermal annealing. By comparing the results of various conditions, we found that the deeper traps can be attributed to shallower HOMO level which would be hole trap as compared with deeper HOMO level caused by less order of P3HT chains. From microscopic view point, the order of molecular chains in the active layer is not homogeneous and less order P3HT chains could have higher HOMO level which causes additional deeper hole traps. On exposure of pure P3HT or blend P3HT with PCBM to dry oxygen, the trap current at about 120 K (that exists in pristine P3HT film) increases dramatically. Since both cases give fluorescence quenching, suggesting that excitons are dissociated into free electrons and holes and leave only a little excitons to be radiative. Therefore, in the result of TSC measurement, apparently oxygen seems to increase hole traps and electron traps of P3HT simultaneously; actually it just increase the electron traps (caught by oxygen) and produce holes for promoting exciton dissociation, which makes more holes to fill into the hole traps. In the device ITO/PEDOT:PSS/P3HT:PCBM(4:1)/Al, the trap current strongly increase at 150 K after annealing, indicating that some electrons are trapped in PCBM when the content of PCBM in the blend is too low to form sufficient continuous channels to conducing the electrons to the cathode. Therefore, we must take blend ratio and annealing condition of active layer into account for better charge transport.