Analyzing local exciton generation profiles as a means to extract transport lengths in organic solar cells

Abstract

In this work, we determine the carrier-transport lengths of electrons and holes ͑L e,h ͒ for bulk heterojunction ͑BHJ͒ organic solar cells using a method applicable to functional devices. By linking the local exciton generation profile ͓G͑x͔͒ in the photoactive layer to photocurrent losses, we are able to determine the onset of bimolecular recombination, which is the dominate loss process of free carrier transport. Even though many factors affect photocurrent generation, we single out bimolecular recombination by measuring the scaling of photocurrent with light intensity as a function of applied voltage. For the common BHJ system, annealed poly-3-hexylthiophene:͓6,6͔-phenyl-C61-butyric acid methyl ester ͑P3HT:PCBM͒, a minimum for L e in PCBM is found to be 340 nm while L h is estimated to be 90 nm for P3HT. The relationship between G͑x͒ and carrier transport is further exemplified by demonstrating a scaling exponent below that for traditional space-chargelimited photocurrent. Likewise, by incorporating a drift/diffusion model, an intuitive link between G͑x͒ and charge transport is established where recombination is shown to occur in regions of the photoactive layer far from the electrode of the slowest carrier species. Finally, the consequences of L e,h on device design for operation under 1 Sun conditions are described.

What parameters can be reliably deduced from the current-voltage characteristics of an organic bulk-heterojunction solar cell?

“…The sublinear behavior of the short-circuit current of BHJ devices (a < 1) has been suggested to be indicative of the dominance of the bimolecular recombination. 4,8,13 From the perspective of the analysis of scales, the choice of focusing on the shortcircuit current seems arbitrary; there is no physical justification for why the current should be evaluated at the bias voltage of 0 V, as opposed to 0.1 V or À0.1 V. Given this understanding, we note that the current scale in the operating regime of a solar cell (from the voltage regimes I to II) can exhibit a power law relationship with the photogeneration rate given by ½j tot / G a , where a can take on values ranging from 0.5 to 1 (see Table I). The a value assumed by a device at bias voltage of 0 V depends on the critical potential scale of the transition region, Du crit bulk .…”

“…Similar models have been employed to evaluate recombination mechanisms, 2-7 quantify losses, [8][9][10] and investigate the relationships between the j-V characteristics and the various aspects of device physics. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Often, analyses based on the device models involve comparing current-voltage curves; a good agreement between the model-predicted current-voltage curves and the measured ones validates a device model; 1-4, [9][10][11][12][13][14][15][16] conversely, the failure of a model to capture certain current-voltage characteristics warrants the modification of existing formulations or the inclusion of additional mechanisms or mathematical relations. 1-4, 14 Frequently, the same procedure is also applied to extracting model parameters; with an optimization algorithm, the parameters are varied until a consistent fit between the model predictions and measurements is obtained; the fitted ones are then used for a further analysis.…”

Comparison of conventional and inverted organic photovoltaic devices with controlled illumination area and extraction layers

“…Extraction layers (ELs) are often inserted between the active layer and the electrodes of OPVs to determine polarity and improve performance [13,14]. OPVs with conventional versus inverted architectures, where the holes collected by the front versus back electrode, have been shown to exhibit discrepancy in performance that have been attributed to vertical phase separation during active layer drying [15][16][17], mismatched electron and hole mobility in the active layer [18][19][20][21], resistive losses between different ELs and electrode [22], and unwanted absorption of incident light by the EL [23][24][25].…”

Poly-(3)hexylthiophene nanowire networks for versatile fabrication of bulk heterojunctions with increased active volume

“…11,12 One strategy to avoid the aggregation of NCs (or any other complementary phase) when utilized in a composite material is to structure one phase of the composite first and then backfill the second phase. 13 For PV devices, it is particularly desirable for the donor phase (where the majority of optical absorption occurs) to have a nanowire morphology which results in a high volume fraction of material within an exciton diffusion length 14 of a donor-acceptor interface (i.e., active volume).…”