Recently, a new field in photovoltaics (PV) has emerged, focusing on solar cells that are entirely based on metal oxide semiconductors. The all-oxide PV approach is very attractive due to the chemical stability, nontoxicity, and abundance of many metal oxides that potentially allow manufacturing under ambient conditions. Already today, metal oxides (MOs) are widely used as components in PV cells such as transparent conducting front electrodes or electron-transport layers, while only very few MOs have been used as light absorbers. In this Perspective, we review recent developments of all-oxide PV systems, which until today were mostly based on Cu2O as an absorber. Furthermore, ferroelectric BiFeO3-based PV systems are discussed, which have recently attracted considerable attention. The performance of all-oxide PV cells is discussed in terms of general PV principles, and directions for progress are proposed, pointing toward the development of novel metal oxide semiconductors using combinatorial methods.

All-oxide-based photovoltaics (PVs) encompass the potential for extremely low cost solar cells, provided they can obtain an order of magnitude improvement in their power conversion efficiencies. To achieve this goal, we perform a combinatorial materials study of metal oxide based light absorbers, charge transporters, junctions between them, and PV devices. Here we report the development of a combinatorial internal quantum efficiency (IQE) method. IQE measures the efficiency associated with the charge separation and collection processes, and thus is a proxy for PV activity of materials once placed into devices, discarding optical properties that cause uncontrolled light harvesting. The IQE is supported by high-throughput techniques for bandgap fitting, composition analysis, and thickness mapping, which are also crucial parameters for the combinatorial investigation cycle of photovoltaics. As a model system we use a library of 169 solar cells with a varying thickness of sprayed titanium dioxide (TiO2) as the window layer, and covarying thickness and composition of binary compounds of copper oxides (Cu–O) as the light absorber, fabricated by Pulsed Laser Deposition (PLD). The analysis on the combinatorial devices shows the correlation between compositions and bandgap, and their effect on PV activity within several device configurations. The analysis suggests that the presence of Cu4O3 plays a significant role in the PV activity of binary Cu–O compounds.

can be produced via RF sputtering, [ 8a ] spray pyrolysis, [ 6a ] chemical vapor deposition [ 11 ] or electrodeposition techniques. [ 12 ] Here we investigate, for the fi rst time, Co 3 O 4 as an MO semiconductor light absorber for all-oxide PV cells. Thin fi lm heterojunction cells, [ 13 ] based on a compact TiO 2 layer produced by spray pyrolysis onto a fl uorine-doped tin oxide (FTO) substrate, followed by pulsed laser deposition of Co 3 O 4 , were investigated as a function of both their layer thicknesses and the Co 3 O 4 deposition temperature. To do this in an effi cient way, combinatorial device libraries [ 14 ] were produced with a thickness gradient ( Figure 1 d). An array of 169 round metal back-contacts, defi ning the cells, were deposited onto the library, allowing for PV performance characterization via automated high throughput methods. [ 15 ] We show that the pulsed laser deposition temperatures of the Co 3 O 4 absorber have a strong impact on device performance due to improved crystallinity and morphology. The library approach proved to be an effi cient method for determining the optimal layer thicknesses; the best performing cells (maximum power) were selected from each library created. Co 3 O 4 is investigated as a light absorber for all-oxide thin-fi lm photovoltaic cells because of its nearly ideal optical bandgap of around 1.5 eV. Thin fi lm TiO 2 /Co 3 O 4 heterojunctions are produced by spray pyrolysis of TiO 2 as a window layer, followed by pulsed laser deposition of Co 3 O 4 as a light absorbing layer. The photovoltaic performance is investigated as a function of the Co 3 O 4 deposition temperature and a direct correlation is found. The deposition temperature seems to affect both the crystallinity and the morphology of the absorber, which affects device performance. A maximum power of 22.7 µW cm −2 is obtained at the highest deposition temperature (600 °C)with an open circuit photovoltage of 430 mV and a short circuit photocurrent density of 0.2 mA cm −2 . Performing deposition at 600 °C instead of room temperature improves power by an order of magnitude and reduces the tail states (Urbach edge energy). These phenomena can be explained by larger grains that grows at high temperature, as opposed to many nucleation events that occur at lower temperature.

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In article number 1401007, Assaf Y. Anderson, Arie Zaban, and co‐workers use combinatorial material science to develop an all oxide photovoltaic (PV) heterojunction consisting of anatase TiO2 and spinel Co3O4. This finding is a critical milestone in ongoing research towards all‐oxide photovoltaics, which hold the potential to become the most ecofriendly and cost‐effective renewable energy source.