Influence of transport-related material parameters on the I–V characteristic of inorganic–organic hybrid solar cells

The carrier transport in inorganic–organic hybrid cells based on small band gap inorganic semiconductors is theoretically studied. In such cells, photo-carriers are generated at the heterojunction (due to dissociation of the donor excitons) and in the bulk of the acceptor material due to direct generation in the small band gap inorganic material. By means of a two-dimensional drift–diffusion model, we demonstrate that material properties directly related to transport, i.e., (i) the carrier mobilities, (ii) the static dielectric constants, and (iii) transport level offsets give, essentially, rise to marked variations in the open circuit voltage, VocVoc, and the shape of the I–V curve. As the most striking consequence of the differing generation mechanisms, the role of the electron and hole level offsets entirely differ. A lack of a hole transport level offset represents the most serious loss factor for the power conversion efficiency (PCE) due to a pronounced sensitivity of the shape of the I–V   curve towards the carrier mobilities and the dielectric constant. On the other hand, degenerate electron transport levels give rise to (i) appreciably large fill factors and (ii) values of Voc,jsc, and PCE that are insensitive towards the ratio   of mobility and ϵrϵr values. However, a non-vanishing electron transport level offset is necessary to achieve a VocVoc beyond Vbuilt-inVbuilt-in at all. Then, VocVoc is further (i) increased by an amount that corresponds to the smaller value of the transport level offsets and (ii) reduced due to a diffusion term that depends on the ratio of the mobility values in the organic and inorganic phase. The latter detrimental term is the smaller, the less the mobility ratio differs from unity, i.e., the smaller the mismatch in the mobility values becomes. Moreover, any mismatch in the carrier mobilities causes an effective mobility-induced diffusion of holes from the organic to the inorganic layer that gives rise to a s-shaped section in the I–V curve.

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►  Heterojunction formed by an organic and a small-band-gap inorganic semiconductor.
► Charge carriers generated at the heterojunction and within entire inorganic phase.
► Voc and shape of I–V curves determined by charges generated within the inorganic phase.
► Ratio of mobilities and dielectric constants in the two materials is required to be unity.
► Minimal electron transport level offset not required for efficient transport.