Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We detect and measure the trap charges in a small molecular bulk heterojunction solar cell under operating conditions. The trap-charge density is estimated from capacitance measurements with light incident on the sample. At high intensities (∼1sun, 100mW/ cm²), the trapped charge concentration leads to a spatial distortion of the electric field in the device. The lower limit of the trap-charge density is estimated to be 6× 10 ¹⁶ cm^-3. The frequency dependence of the capacitance suggests that the charges are trapped in a manifold of deep states present in the energy gap of the semiconductors. The distortion of the electric field by this trap charge affects the charge-carrier collection efficiency. © 2010 The American Physical Society.

Abstract:

We present small molecule solar cells with α,ω-bis- (dicyanovinylene)-sexithiophene: C60 mixed heterojunctions, reaching power conversion efficiencies of 4.9±0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%. © 2010 American Institute of Physics.

Abstract:

A key feature of stacked organic solar cells is an efficient recombination contact at the interface between the solar cells in the stack. Here, an electron current has to be converted into a hole current without loss of energy. Furthermore, the recombination contact has to be highly transparent. We present a new approach for small molecule organic solar cells using highly doped organic layers. Our approach adapts the use of tunnel diodes known from inorganic tandem solar cells. We compare a metal cluster based recombination contact reported in literature to the new approach using different organic tandem solar cell structures. For this purpose, current-voltage characteristics of adequate solar cells are measured. The experiments show that highly doped layers as recombination contacts in tandem organic solar cells are superior to the metal cluster based approach. The proposed concept allows an addition of the open circuit voltages of the subcells of a tandem solar cell, without absorption or reflection at the recombination contact. The results further show that our concept does not depend on the specific choice of materials as it is seen for metal cluster based recombination contacts. It therefore represents a general approach which is compatible to mass manufacturing. © 2010 American Institute of Physics.