Nanoscale Physics

Charge carrier spatial trapping limits all-polymer solar cell performance
Jasiunas R., Gulbinas V.
Center for Physical Sciences and Technology, Sauletekio av. 3, LT-10257, Vilnius, Lithuania. 
E-mail: rokas.jasiunas@ftmc.lt

Fig. 1 Charge carrier spatial trapping contributes strongly to photocurrent decrease (1), reversing electric field polarity results in increased photocurrent due to carrier detrapping (2)

Extraction of photogenerated charge carriers from organic bulk heterojunction (BHJ) solar cells is one of the most important processes determining device performance. Slow carrier extraction causes their accumulation in the active layer and, as a consequence, an increased bimolecular recombination, which reduces the short-circuit current and particularly the fill factor. It is commonly assumed that the optimal morphology for efficient BHJ solar cells is one where photogenerated electrons and holes avoid recombination by moving via spatially separated percolating domains consisting (predominantly) of electron accepting material in one phase and donating material in the other. However, such strongly phase separated morphologies are prone to cause extraction problems due to the formation of dead ends and/or isolated domains. 
 We have investigated charge carrier motion in an archetypical all-polymer organic solar cell by a combination of carrier extraction experiments and numerical modeling. We find that in the as-cast active layer, the bulk heterojunction morphology contains a significant concentration of spatial traps that we tentatively associate with individual polymer chain ends. The presence of these spatial traps manifests in a suppressed yield of free charge generation due to one of the charges in the splitting CT pair ‘getting stuck’ close to its countercharge, leading to enhanced geminate recombination. While these findings highlight a morphological problem that seems most likely to arise in all-polymer BHJ (in BHJ based on the same polymer donor with a small molecule (PCBM) acceptor spatial trapping is much weaker) they also show that the number of spatial traps can, at least in the present case, be drastically reduced by thermal annealing.