Bulk heterojunction optimization

In the following, we discuss strategies for optimizing the power efficiency of polymeric solar cells based upon bulk heterojunctions. 

Optimization of Bulk Heterojunction Composite Nanostructures 175 Pressure... 

Conventional organic bulk heterojunction solar cells do not have hole-blocking electron transport layers (HBETL) at the cathode/active layer interface. Metals with reasonably low-work function (e.g., calcium, aluminium, magnesium, or barium) are typically evaporated directly onto fullerene/polymer blend forming electronic contacts with both donor and acceptor materials under optimal conditions. This situation leads to significant recombination of positive and negative charge carriers at the active layer/cathode interface. 

Attempts have been made to deposit TIPS-pentacene from solution as the functional layer in a pentacene/C60 bilayer photovoltaic device. Careful optimization of deposition conditions, optimal concentration of mobile ion dopants, thermal postfabrication annealing, and the addition of an exciton-blocking layer yielded a device with a moderate white-light PCE of 0.52% [41]. Since TIPS-pentacene derivatives rapidly undergo a Diels-Alder reaction with fiillerene, the assembly of potentially more efficient bulk-heterojunction photovoltaic devices from TIPS-pentacene and fiillerene derivatives were not possible [42]. The energy levels of the TIPS-pentacene-PCBM adduct (PCBM is [6,6]-phenyl C61-butyric acid methyl ester) ineffectively supports the photoinduced charge transfer. 

Lin C, Pan W-C, Tsai F-Y. Optimization of the active-layer morphology with a non-halogenic solvent for bulk-heterojunction polymer solar cells. Synth Met 2010 160(23-24) 2643-2647. 

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